mafic magmas from mount baker in the northern cascade arc, washington

ORIGINAL PAPER

Mafic magmas from Mount Baker in the northern Cascade arc,Washington: probes into mantle and crustal processes

Nicole E. Moore • Susan M. DeBari

Received: 23 February 2011 / Accepted: 26 August 2011

� Springer-Verlag 2011

Abstract Five mafic lava flows located on the southern

flank of Mount Baker are among the most primitive in the

volcanic field. A comprehensive dataset of whole rock and

mineral chemistry reveals the diversity of these mafic lavas

that come from distinct sources and have been variably

affected by ascent through the crust. Disequilibrium tex-

tures present in all of the lavas indicate that crustal pro-

cesses have affected the magmas. Despite this evidence,

mantle source characteristics have been retained and three

primitive endmember lava types are represented. These

include (1) modified low-K tholeiitic basalt (LKOT-like),

(2) typical calc-alkaline (CA) lavas, and (3) high-Mg

basaltic andesite and andesite (HMBA and HMA). The

Type 1 endmember, the basalt of Park Butte (49.3–50.3

wt% SiO2, Mg# 64–65), has major element chemistry

similar to LKOT found elsewhere in the Cascades. Park

Butte also has the lowest overall abundances of trace ele-

ments (with the exception of the HREE), indicating it is

either derived from the most depleted mantle source or has

undergone the largest degree of partial melting. The Type 2

endmember is represented by the basalts of Lake Shannon

(50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek

(51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are

comparable to calc-alkaline rocks found in arcs worldwide

and have similar trace element patterns; however, they

differ from each other in abundances of REE, indicating

variation in degree of partial melting or fractionation. The

Type 3 endmember is represented by the HMBA of Tarn

Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA

of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The

strongly depleted HREE nature of these Type 3 units and

their decreasing Mg# with increasing SiO2 suggests frac-

tionation from a high-Mg basaltic parent derived from a

source with residual garnet. Another basaltic andesite unit,

Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an

Mg-poor differentiate of the Type 3 endmember. The calc-

alkaline lavas are least enriched in a subduction component

(lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are

intermediate (moderate Sr/PN and Ba/Nb), and the HMBA

are most enriched (highest H2O, Sr/PN and Ba/Nb). The

generation of the LKOT-like and calc-alkaline lavas can be

successfully modeled by partial melting of a spinel lherz-

olite with variability in composition of slab flux and/or

mantle source depletion. The HMBA lavas can be suc-

cessfully modeled by partial melting of a garnet lherzolite

with slab flux compositionally similar to the other lava

types, or less likely by partial melting of a spinel lherzolite

with a distinctly different, HREE-depleted slab flux.

Keywords Cascade arc � Mount Baker � Basalt �Geochemistry � Petrology � Mantle � Slab

Introduction

Primitive basalts in volcanic arc settings are valuable

recorders of magmatic flux from the mantle to the crust. By

examining the mass balance of input and output from

subduction zones, we can determine both how magma is

generated in these settings, and how this balance affects the

Communicated by T. L. Grove.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-011-0686-4) contains supplementarymaterial, which is available to authorized users.

N. E. Moore (&) � S. M. DeBari

Geology Department, Western Washington University,

MS 9080, Bellingham, WA 98225, USA

e-mail: [email protected]

123

Contrib Mineral Petrol

DOI 10.1007/s00410-011-0686-4

http://dx.doi.org/10.1007/s00410-011-0686-4

growth and evolution of continental crust (Hirschmann

et al. 2000). Primitive magmas from around the world have

been used to probe the mantle input into magmatic arcs

(e.g., Hildreth and Moorbath 1988; Sisson and Bronto

1998; Class et al. 2000; Grove et al. 2002; Walker et al.

2003).

The processes of magma generation in arcs may vary

depending on the age of the subducting slab (cf. Hirsch-

mann et al. 2000). The Cascade magmatic arc (Fig. 1) is

one of the hottest endmembers of arcs worldwide, with a

subducting plate that is only a few million years old at the

trench. It is a continental arc built on juvenile accreted

terranes in a manner similar to the Southern Volcanic Zone

(SVZ) of the Andes (Hildreth 2007; Hildreth and Moorbath

1988). As such, the lack of old continental basement allows

mafic magmas to be effective probes into magma genera-

tion processes.

Studies of primitive magmas in the Cascade arc have

been concentrated in the middle and southern Cascades

where mafic magmas are abundant (e.g., Bacon et al. 1997;

Borg et al. 1997; 2000; Conrey et al. 1997; Reiners et al.

2000; Elkins-Tanton et al. 2001; Grove et al. 2002; Strong

and Wolff 2003; Leeman et al. 1990; 2005; Smith and

Leeman 2005; Schmidt et al. 2008). The earliest studies by

Leeman et al. (1990) and Bacon et al. (1997) distinguished

three endmember types of primitive magma, interpreted to

represent three different mantle sources for the Cascade

arc. These three magma types consist of hydrous calc-

alkaline basalts (CAB), alkalic ocean-island-type basalts

(OIB), and dry, decompression-related low-potassium

olivine tholeiites (LKOT, aka HAOT of Bacon et al. 1997

and Grove et al. 2002), which share some characteristics

with mid-ocean ridge basalts.

Equivalent studies of primitive magmas are less com-

mon in the northern part of the Cascade arc. In this portion

of the arc, known as the Garibaldi Belt (Fig. 1), the volume

of mafic lavas is dramatically less than in the south. Green

and coworkers analyzed a restricted sample set of primitive

lavas from a transect of the Garibaldi Belt (Green and

Harry 1999; Green and Sinha 2005; Green 2006) and

determined that the slab age, depth of fluid loss, and degree

of partial melting in the mantle wedge all decrease north-

ward into this segment. In their studies, compositions

ranged from calc-alkaline basalts in the southern part of the

Belt (Glacier Peak and Mount Baker) to alkalic basalts in

the north. Green and Sinha (2005) also determined that the

component responsible for the slab signature in the calc-

alkaline basalts is from fluid input, not from partial melting

of sediment. However, a more detailed study of mafic lavas

from Glacier Peak showed that there are multiple mantle

sources for the basalts, including LKOT and calc-alkaline

endmembers (Taylor 2001), with strong isotopic hetero-

geneity (e.g., DeBari et al. 2005 and unpublished

manuscript).

In this study, our aim was to see whether some of the

same variability existed in primitive lavas from Mount

Baker, and whether these lavas could provide insight into

processes of magma generation beneath this part of the arc.

This study presents detailed petrologic and geochemical

analyses of the most mafic lavas erupted from Mount Baker

throughout its history.

Tectonic and geologic setting

The Cascade volcanic arc extends from northern California

to southern British Columbia along the west coast of North

America. The volcanic centers begin in the south at Lassen

Peak and extend northward through Mount Meager in

Canada (Fig. 1). Arc volcanism is generated by the slightly

oblique subduction of the young and hot Juan de Fuca Plate

under the North American Plate at about 45 mm/year

(Riddihough 1984). The dip of the subducting slab beneath

Fig. 1 The Cascade volcanic arc and Mount Baker (MB) study area.

Orange shaded regions and circles represent areas of mafic volcanism

after Borg et al. (1997). Black lines indicate segments of the Cascade

arc as described by Guffanti and Weaver (1988). Triangles represent

major volcanic centers, abbreviated as follows: MM Mount Meager,

MC Mount Cayley, MG Mount Garibaldi, MB Mount Baker, GPGlacier Peak, MR Mount Rainier, MSH Mount St. Helens, MA Mount

Adams, SVF Simcoe Volcanic Field, MH Mount Hood, MJ Mount

Jefferson, TS Three Sisters, NV Newberry Volcano, CLV Crater Lake

Volcano, MMc Mount McLoughlin, MLV Medicine Lake Volcano,

MS Mount Shasta, LVC Lassen Volcanic Center. Rates of subduction

are from McCrory et al. (2004)


123

Washington and northern Oregon is estimated to be *20�(McCrory et al. 2004). Along-strike variations in magma-

tism and structure are the basis for a subdivision of the arc

into five major segments (Guffanti and Weaver 1988;

Schmidt et al. 2008). The northernmost segment is the

Garibaldi Belt, which is separated from the rest of the

Cascades by a 90-km-wide volcanic gap. This segment is

distinct because a structural change in the Juan de Fuca

plate off the coast of central Washington causes the overall

trend of the arc to change from N–S to NW–SE. A marked

decrease in mafic volcanism occurs northward along the

arc, resulting in few isolated mafic centers in the Garibaldi

Belt (Fig. 1).

Mount Baker (3,286 m) is the northernmost volcano of

the Cascade arc in the United States. It is one of only two

U.S. volcanoes located within the Garibaldi Belt (Fig. 1),

situated between Glacier Peak to the south and Mount

Garibaldi to the north. This primarily andesitic stratovol-

cano is part of a volcanic field that has been active since

about 1.3 Ma, with the most recent magmatic eruption

occurring at 6.5 ka (Hildreth et al. 2003). Mount Baker

is *350 km east of the trench above the subducting Juan

de Fuca plate. The top of the plate lies at depths

of *80–90 km beneath Mount Baker (McCrory et al.

2004) and is dipping at *15� (Hyndman et al. 1990). The

subducting plate age is *18 million years at this location

(Green and Harry 1999). Beneath the volcanic center, the

crust is approximately 40–45 km thick (Mooney and

Weaver 1989), and the basement is composed of accreted

Paleozoic and Mesozoic oceanic rocks. The eruptions in

the volcanic field have largely taken place through the

metamorphosed Nooksack Group and Chilliwack Group,

which are primarily composed of alternating volcanic and

sedimentary rocks that were generated in an oceanic setting

(Tabor 1994; Tabor et al. 2003).

Although Mount Baker is an andesitic volcano, about

1% of the total eruptive products in the volcanic field are

basaltic (Hildreth et al. 2003). Sources of mafic volcanism

at Mount Baker are primarily monogenetic outlier vents

and cinder cones and are generally restricted to the

southern flank of the volcano. This study focuses on five

units identified as the most mafic at Mount Baker, based on

major element data and mapping by Hildreth et al. (2003).

The MgO content reported by Hildreth et al. (2003) for

these five units is 4.0–8.4 wt%, indicative of potentially

primitive magmas (Mg# [ 60, where Mg# = 100*[Mg/

(Mg ? FeT)]). The units include the (1) basalt of Park

Butte, (2) basaltic andesite of Cathedral Crag, (3) basaltic

andesite of Tarn Plateau, (4) basalt of Lake Shannon, and

(5) basalt of Sulphur Creek (Fig. 2; italicized unit names

are from Hildreth et al. 2003). These flows range in age

from Middle Pleistocene to Holocene and K–Ar ages listed

below are from Hildreth et al. (2003).

The basalt of Park Butte (716 ± 45 ka) is a flow rem-

nant from an unknown vent, located 7.5 km southwest of

Grant Peak, the true summit of Mount Baker (Fig. 2). It is

located *400 m east of the Park Butte Lookout. The

remnant is *30 m thick, defined by a thin, discontinuous

red basal layer of flow breccia. The majority of the remnant

is talus, but some basalt columns are found in place

overlain by till. The columns are weathered to a charac-

teristic buff color, but internally reveal a coarse, inter-

granular texture. The lavas contain abundant olivine and

plagioclase (Table 1). In some boulders near the base of the

talus, mingling textures are observed between this and

another mafic, oxidized, and vesicular lava.

Less than 0.5 km to the north and slightly east of the

Park Butte unit lies the glacially eroded basaltic andesite of

Cathedral Crag (Fig. 2; 331 ± 18 ka). The unit crops out

in a large knob *300 m in length and contains both

prismatic vertical and irregular horizontal columns. No

vent has been located and it is unclear whether this unit

contains multiple flows (Hildreth et al. 2003). Plagioclase,

clinopyroxene, and olivine are abundant (Table 1), and

many samples also contain large (4–22 mm) felsic xeno-

liths with surrounding reaction rims.

The basaltic andesite of Tarn Plateau (203 ± 25 ka) is

a mesa-shaped unit that nearly banks against the eastern

portion of the Park Butte lavas (Fig. 2). It rises approxi-

mately 150 m above the floor of the valley that separates it

from Cathedral Crag. The unit crops out in a nearly square

fashion, *0.5 km on a side. Irregular columnar joints are

visible in cross section from across the valley, but the flat

top surface has been glacially scoured and fractured. As

with the Park Butte and Cathedral Crag units, the location

of the source vent is unknown. Samples are rich in clino-

pyroxene, plagioclase, and olivine (Table 1).

The southernmost and farthest outlying mafic unit in this

study is the basalt of Lake Shannon (94 ± 21 ka), located

1 km west of Lake Shannon, and more than 15 km

southeast of the summit of Mount Baker (Fig. 2). The most

prominent feature of this flow unit is a conical knob that

rises about 400 m above Baker Lake Road. The eruption

may have been englacial (not entirely subglacial, Hildreth

et al. 2003), as the majority of the outcrop is composed of

hyaloclastite tuff (*100 m thick). Where they occur, lavas

are vesicular and contain abundant plagioclase with lesser

olivine (Table 1).

The basalt of Sulphur Creek is a compositionally zoned

flow unit (basalt and basaltic andesite) that erupted from a

cinder cone in Schreibers Meadow *8 km south of the main

edifice (Fig. 2). The age of the unit is constrained by 14C ages

from carbon buried by scoria near the vent (8,750 ± 50 to

8,850 ± 50 years BP, Tucker et al. 2007; radiocarbon cali-

brated to 9.8 ka by Scott et al. 2001). Exposures are typically

massive or blocky, but sometimes irregularly jointed.


123

Although lavas proximal and medial to the cone are basaltic

andesite (up to 55 wt% SiO2 and *4.5 wt. % MgO), the most

distal portions (those that form a lava fan near Baker Lake)

are generally basaltic (51 wt% SiO2 and 5.5 wt% MgO;

Green 1988; Hildreth et al. 2003; Tucker and Scott 2009;

Baggerman and DeBari 2011). The basalt is rich in plagio-

clase and olivine, and the basaltic andesite also contains

abundant clinopyroxene (Table 1).

Analytical methods

A total of 38 samples were collected from the 5 flow units

described above (see Electronic Supplement 1 for sample

locations). Sample collection for the Sulphur Creek basalt

was limited to the mafic toe of the flow.

Whole rock analyses

Abundances of major element oxides and 6 trace elements

(Ni, Cr, V, Ga, Cu, and Zn) were determined by X-ray

fluorescence (XRF) analysis at the Washington State Uni-

versity (WSU) GeoAnalytical Laboratory. Rock powders

were prepared at Western Washington University (WWU)

by grinding select rock chips in a tungsten carbide SPEX

Shatterbox grinding mill, following the method of Johnson

et al. (1999). Powders were fused into beads using a 2:1

mix of dry dilithium tetraborate flux to dry rock powder, in

a 1,000�C furnace for 10 min. Beads were then re-ground

and re-fused by the above method to ensure homogeneity

of each sample. The samples were analyzed at WSU using

a ThermoARL Advant’XP ? sequential XRF spectrome-

ter. A thorough description of sample preparation as well as

Fig. 2 Geologic map of Mount Baker from Hildreth et al. (2003).

The target mafic lavas for this study are labeled with their respective

ages. Each box is color coded to match the symbol color used for each

unit throughout this study. Early Pleistocene units are in brown,

except for the rhyodacites of the Kulshan caldera in red. Middle

Pleistocene units are in green, Late Pleistocene units in purple.

Holocene eruptive unit is in orange


123

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analytical procedure, accuracy, and precision are detailed

by Johnson et al. (1999).

For XRF analysis, the GeoAnalytical Lab at WSU esti-

mates accuracy by comparing the known values of nine

USGS standard samples with compositions determined by

preparation and analysis at WSU for each of these USGS

standard samples. For all major elements except Na, the

difference between the known values and WSU results are

less than the difference expected between two random

samples selected from the same rock unit. Maximum mea-

sured differences (in weight %) between known values and

WSU results for major elements are as follows: \0.60% for

SiO2 and FeO; \0.40% for MgO, B 0.20% for Al2O3 and

Na2O; \0.10% for TiO2, CaO, K2O, and P2O5; and \0.01%

for MnO. For trace elements, values obtained by XRF for Ni,

Cr, and V can be considered only semiquantitative below

30 ppm. The remainder of trace elements analyzed by XRF

are satisfactorily precise and accurate down to values from 1

to 3 ppm.

All 14 naturally occurring rare earth elements (REE)

along with an additional 13 trace elements (Ba, Rb, Y, Nb,

Cs, Hf, Ta, Pb, Th, U, Sr, Sc, Zr) were analyzed in 32

samples by high-precision inductively coupled plasma

source mass spectrometry (ICP-MS) analysis at the WSU

GeoAnalytical Laboratory. Rock powders were prepared at

WWU by grinding select rock chips in an alumina ceramic

SPEX Shatterbox grinding mill. Powders were fused into

beads using a 1:1 mix of dry dilithium tetraborate flux to

dry rock powder, in a 1,000�C furnace for 10 min. Beads

were then re-ground using the above method to ensure

homogeneity of each sample. The resulting powders were

sent to WSU and analyzed using an Agilent 4500 ? ICP-

MS. Analytical procedure as well as accuracy and precision

are detailed by Knaack et al. (1994).

For ICP-MS analyses, the GeoAnalytical Lab at WSU

estimates accuracy by comparing the known values of fif-

teen USGS standard samples with results from two beads

of each of the USGS standard samples that have been

prepared and analyzed at WSU. Maximum measured dif-

ferences (in ppm) between known values and WSU

results for ICP-MS-analyzed trace elements are as fol-

lows: \40 ppm for Ba; \30 ppm for Sr and Zr; B 5 ppm

for La, Ce Pr, Nd, Sm, Dy, Hf, Th, Pb, and Sc; \3 for Rb,

Y, and Nb; \1 ppm for Gd, Er, and U; \0.5 ppm for Eu,

Tm, Yb, Ta, Ho, and Cs; and B 0.1 ppm for Tb and Lu.

Precision is generally better than 5% for the REE and 10%

for the other trace elements.

Isotopic ratios for Pb, Sr, and Nd were measured for 6

samples, 2 each from the following units: the Tarn Plateau,

Park Butte, and Lake Shannon. The most primitive and

most differentiated samples in each flow unit were chosen

to represent the probable isotopic compositional range of

these individual flows. Field samples were reduced to chips

by the method described above and then ground into

powders using an alumina ceramic SPEX Shatterbox

grinding mill. Powders were sent to the University of

Washington Isotope Geochemistry Lab and analyzed using

a Nu Instruments multiple collector inductively coupled

plasma mass spectrometer (MC-ICP-MS).

Samples measured for Pb isotopes were dissolved in

concentrated HF ? 8 N HNO3, dried, and re-dissolved. An

aliquot of 2 samples, NM-LS6 and NM-TP6, were

reserved, dried, and passed twice through ion-exchange

columns (for detailed procedure of entire Pb separation

chemistry, see Nelson 1995). Both aliquots (simply dis-

solved and purified) of samples NM-LS6 and NM-TP6

were analyzed, as well as a second aliquot of sample NM-

LS2 (run as a repeat) to test for reproducibility. Analytical

procedure for Pb analysis is fully described in Harkins et al.

(2008). Pb isotopic compositions are normalized to NIST

981, with isotopic standards of 16.9356, 15.4891, and

36.7006 for 206Pb/204, 207Pb/204Pb, and 208Pb/204Pb,

respectively. Error on Pb analyses is ± 150 ppm (2r)

or ± 0.015%.

Samples measured for Sr and Nd isotopes were dis-

solved in a 10:1 mixture of concentrated HF-HNO3. After

drying, samples were fumed with HClO4 to decompose

fluorides. Final dissolution and sample equilibration were

achieved by adding 6 M HCl. Column separation was

conducted first to partition the light rare earth elements

(LREE) from Sm and Nd and second to partition Sm from

Nd. Analytical procedure for Sr isotope analysis is fully

described in Nelson (1995) and in Gaffney et al. (2007) for

Nd. Sr isotopic compositions are normalized to87Sr/86Sr = 0.710240 for NIST 987, which was the aver-

age value yielded for the standard. Nd isotopic composi-

tions are normalized to the La Jolla value of143Nd/144Nd = 0.511843. Error on Sr and Nd analyses

is ± 30 ppm (2r) or ± 0.3 epsilon units. The 145Nd/144Nd

was monitored as an additional accuracy check. This

yielded a value of 0.348413 for all samples of the standard,

which is within ± 17 ppm (2r) of the accepted value of

0.348417.

Mineral analyses

Compositions of olivine, pyroxenes, plagioclase, Fe–Ti

oxides, and chromium spinel were obtained using a JEOL

Superprobe 733 Electron Probe Microanalyzer (EPMA)

equipped with 4 wavelength-dispersive X-ray spectrome-

ters and an energy-dispersive X-ray spectrometer at the

University of Washington. A standardized set of natural

and synthetic minerals were used for calibration. Acceler-

ating voltage was set to 15 keV, with a beam current of 10

nA for plagioclase and 15 nA for olivine, pyroxene, and

oxides. Beam diameter was \1 lm for all minerals except


123

plagioclase, which was analyzed at 3 lm. Element peaks

were counted for a minimum of 20 s, and for a maximum

of 40 s, or when 0.4% statistical error was achieved.

Analytical error is \2% for major elements and \12% for

trace elements. Data corrections were performed using the

CITZAF X-ray Correction Program based on the method of

Armstrong (1988).

Summary of petrography and mineral chemistry

Petrographic characteristics and mineral chemistry are

described in detail for each unit in Electronic Supplement

2. These data are summarized below and in Table 1, and

compositional data for all mineral phases are listed in

Tables 2 and 3 (olivine and pyroxene) and Electronic

Supplements 3 and 4 (plagioclase and oxide minerals).

The mafic flows from Mount Baker are dominated by

either Mg-rich clinopyroxene (basaltic andesites of Tarn

Plateau and Cathedral Crag; Mg# 76–88) or olivine (basalts

of Park Butte, Lake Shannon, Sulphur Creek; *Fo66–85)

with variable amounts of plagioclase and Fe–Ti oxides. In

the basaltic andesites of Tarn Plateau and Cathedral Crag,

olivine phenocrysts are also common (Fo70–85), but in

lesser abundance than clinopyroxene and plagioclase. In

contrast, the basalts of Park Butte, Lake Shannon, and

Sulphur Creek contain only rare phenocrysts of clinopy-

roxene (\1%, Mg# \ 75). The Park Butte basalt has

olivine as the most abundant phenocryst phase, followed by

plagioclase, whereas in basalts of Lake Shannon and Sul-

phur Creek, plagioclase is most abundant, followed by

olivine.

Phenocryst and microphenocryst textures from all of the

units at Mount Baker show that disequilibrium conditions

existed prior to their eruption. Disequilibrium textures are

pervasive in phenocrysts, common in microphenocrysts, but

absent in the groundmass. The most mafic units do not

always contain the most mafic phenocrysts. For example,

Tarn Plateau basaltic andesites contain the most magnesian

olivine phenocrysts and most anorthitic plagioclase pheno-

crysts (Electronic Supplement 3), but Cathedral Crag

basaltic andesite contains the most magnesian clinopyroxene

Table 2 Representative olivine compositions

Sample SiO2 MgO FeO* CaO MnO NiO Total Fo

Park Butte

PB5_Ph_c 37.32 31.98 28.67 0.24 0.47 0.03 98.71 67

PB5_Ma_c 36.72 27.49 34.95 0.13 0.54 0.06 99.90 58

PB8_Ph_c 40.10 42.64 16.80 0.11 0.26 0.13 100.04 82

PB8_Ph_r 37.44 32.35 29.99 0.19 0.48 0.03 100.48 66

PB8_Ma_c 36.35 26.60 36.37 0.09 0.53 0.00 99.94 57

Cathedral Crag

CC1_Mph_c 38.25 35.28 25.65 0.16 0.57 0.05 99.95 71

CC1_Ph_c 38.01 34.87 26.11 0.19 0.52 0.05 99.75 70

CC7_Ph_c 38.45 37.12 23.45 0.13 0.37 0.09 99.61 74

Tarn Plateau

TP3_Ph_c 40.08 44.90 13.98 0.11 0.22 0.12 99.41 85

TP3_Ph_r 39.17 39.26 21.15 0.15 0.30 0.10 100.13 77

TP4_Ph_c 39.15 41.89 17.79 0.15 0.28 0.12 99.39 81

Lake Shannon

LS1_Mph_c 39.47 41.69 17.28 0.21 0.28 0.21 99.14 81

LS4_Ma_c 37.55 34.15 25.35 0.30 0.40 0.04 97.80 71

LS4_Mph_c 39.04 40.18 19.20 0.21 0.29 0.19 99.12 79

LS4_Ph_c 39.11 39.69 19.78 0.21 0.33 0.15 99.25 78

Sulphur Creek

SC1_Mph_c 38.82 37.93 22.12 0.19 0.41 0.07 99.55 75

SC1_Mph_r 37.84 33.93 25.99 0.33 0.55 0.02 98.66 70

SC4_Ph_c 40.72 44.15 13.54 0.16 0.20 0.20 98.97 85

SC4_Ph_r 38.57 36.29 23.63 0.21 0.53 0.10 99.34 73

SC4_Ma_c 37.40 30.96 30.56 0.31 0.66 0.02 99.91 64

Individual mineral grains analyzed in each sample number are designated as Ph for phenocryst, Mph for microphenocryst, and Ma for matrix.

Each is also assigned a letter to indicate location of the analysis, c core or r rim, Fo forsterite content


123

phenocrysts with less forsteritic olivine (Tables 1, 2). While

mineral chemistry can vary greatly within units, the range in

Mg# of olivine and clinopyroxene, as well as anorthite

content of plagioclase, is fairly consistent throughout all of

the flow units (Table 1, Electronic Supplement 5).

Chromite occurs only as inclusions in olivine in all lava

flows, indicating a restricted period of crystallization at

high temperature. The most forsteritic olivines (Fo [ 80)

contain chromite with Cr# (100*Cr/[Cr ? Al]) that are

distinct for each flow. Tarn Plateau chromites have the

highest Cr# (Cr# *70), whereas Lake Shannon chromites

have the lowest (Cr# *30).

Abundant Fe–Ti oxides are present in the groundmass

of all units in this study. Coexisting mineral pairs of

magnetite and ilmenite in the Park Butte, Cathedral Crag,

and Tarn Plateau lavas were used to calculate pre-eruptive

oxygen fugacity (Electronic Supplement 4). Equilibrium

of coexisting pairs was determined using the Bacon and

Hirschmann (1988) test. Oxygen fugacity for all three of

these units lies near or within 1 log unit above the nickel–

nickel oxide (NNO) buffer and ranges from -11.6 to

-12.9 log units for Park Butte, -12.4 to -14.2 for

Cathedral Crag, and -9.4 to -11.9 for Tarn Plateau. Low

equilibration temperatures were obtained by the coexis-

ting pairs (Electronic Supplement 4), thus the fugacity

values could be considered suspect. However, Shaw

(2011) determined similar values (DQFM ?1.0 to ?1.3)

using sulfur speciation of melt inclusions in olivine from

Table 3 Representative pyroxene compositions

Sample SiO2 Al2O3 TiO2 FeO* MnO MgO CaO Na2O Cr2O3 Total En Fs Wo Mg#

Clinopyroxene

Park Butte

PB5_Ma_c 51.65 1.75 0.82 9.18 0.24 15.56 19.76 0.35 0.06 99.36 45 15 41 75

PB8_Mph_r 51.61 1.45 0.90 11.30 0.30 14.35 18.92 0.47 0.06 99.35 42 18 40 69

PB8_Ma_c 51.15 2.53 0.75 11.01 0.33 15.03 18.08 0.55 0.74 100.16 44 18 38 71

Cathedral Crag

CC1_Mph_c 51.21 3.29 0.64 6.70 0.21 15.89 21.03 0.38 0.01 99.38 46 11 43 81

CC1_Ph_c 51.93 2.72 0.55 7.19 0.19 15.55 20.97 0.33 0.01 99.44 45 12 43 79

CC1_Ph_r 51.56 1.54 0.91 9.67 0.39 15.74 18.69 0.50 0.02 99.01 45 16 39 74

CC7_Ph_c 49.84 4.96 1.20 6.04 0.14 14.97 21.55 0.50 0.45 99.66 44 10 46 82

CC7_Ph_r 52.54 3.24 0.33 4.22 0.11 17.27 21.08 0.35 0.93 100.07 50 7 44 88

CC7_Ma_c 51.89 2.03 0.94 9.17 0.22 16.06 19.33 0.38 0.09 100.11 46 15 40 76

Tarn Plateau

TP3_Mph_c 52.46 2.51 0.35 4.36 0.10 16.78 22.28 0.24 0.58 99.66 48 7 45 87

TP3_Ma_c 50.74 2.00 1.09 11.62 0.29 15.05 18.08 0.33 0.01 99.23 44 19 38 70

TP4_Mph_c 51.33 2.51 0.52 5.90 0.17 16.07 21.20 0.22 0.16 98.09 46 10 44 83

Sulphur Creek

SC1_Ma_c 50.21 2.65 1.13 9.30 0.29 15.29 19.21 0.46 0.06 98.60 45 15 40 75

SC4_Ma_c 50.78 2.66 1.46 9.77 0.27 14.42 19.78 0.58 0.01 99.73 42 16 42 72

Orthopyroxene

Park Butte

PB5_Ma_c 52.05 0.52 0.35 20.48 0.57 20.56 4.18 0.12 0.03 98.86 59 33 9 64

PB8_Ma_r 52.71 0.47 0.49 20.43 0.47 20.60 3.76 0.11 0.00 99.04 59 33 8 64

PB8_Ma_c 50.39 0.63 0.29 23.53 0.55 20.08 2.80 0.12 0.00 98.39 57 37 6 60

Cathedral Crag

CC7_Ma_c 53.85 1.16 0.46 14.91 0.37 26.21 2.28 0.03 0.06 99.31 72 23 5 76

Tarn Plateau

TP3_Ma_c 52.90 1.52 0.52 19.03 0.41 23.12 2.15 0.06 0.02 99.72 65 30 4 68

TP4_Ma_c 52.27 1.13 0.58 18.54 0.60 23.40 2.18 0.09 0.00 98.78 66 29 4 69

Sulphur Creek

SC4_Ma_c 35.78 1.53 0.11 32.80 0.77 27.34 0.97 0.32 0.00 99.63 59 40 2 60

Abbreviations as in Table 2. En enstatite content, Fs ferrosilite content, Wo wollastonite content

Mg# = Mg/(Mg ? Fe*)


123

tephra associated with the Sulphur Creek lava flow

(equates to Fe2?/FeT of 0.83). Similar values were also

determined for other Mount Baker units (Baggerman and

DeBari 2011).

Whole rock chemistry

Major elements

Representative major element chemical data for the five

mafic lava flow units from Mount Baker are presented in

Table 4. All units are subalkaline and according to the

AFM diagram of Irvine and Baragar (1971), all are calc-

alkaline (not shown). However, the Park Butte lavas plot

directly on the dividing line between the calc-alkaline and

tholeiitic fields. Using the classification of Le Maitre et al.

(1989), the Cathedral Crag, Tarn Plateau, Lake Shannon,

and Sulphur Creek lavas are all categorized as medium-K,

while the Park Butte unit falls just into the low-K field

(Fig. 3).

Harker variation diagrams for major elements are pre-

sented in Fig. 4. Park Butte lavas have the most Si-poor

compositions and are exclusively basaltic (49.3–50.3 wt%

SiO2) with 7.9–8.4 wt% MgO. Basaltic andesite is the sole

rock type found at Cathedral Crag (52.1–52.9 wt% SiO2)

with *4 wt% MgO. Tarn Plateau is basaltic andesite

(53.6–54.0 wt% SiO2), with the exception of one sample

that borderline qualifies as basalt (51.8 wt% SiO2). This

unit is relatively Mg rich for its SiO2 content, with 7.0–7.8

wt% MgO, and we designate the unit as high-Mg basaltic

andesite (HMBA; cf. Grove et al. 2002; 2005; Tatsumi

2006). This high-Mg character is shared by another Mt.

Baker lava, the Glacier Creek andesite (plotted on Fig. 4 as

Mt. Baker HMA for comparison; data from Baggerman and

DeBari 2011). The Lake Shannon and Sulphur Creek units

both include basalt and basaltic andesite (50.7–52.6%, and

51.2–54.6 wt% SiO2, respectively). However, there is a

distinct compositional gap in Sulphur Creek lavas between

basalts with B52 wt% SiO2 and basaltic andesites

with C54.5 wt% SiO2 (cf. Hildreth et al. 2003; Baggerman

and DeBari 2011). Basalts in both of these units are poorer

in MgO than Park Butte low-K lavas and Tarn Plateau

HMBA.

There are no clear trends in major elements between the

different units. Interestingly, Mg# is not correlated with

silica content (Fig. 4), as Tarn Plateau basaltic andesites

have both the highest silica content and the highest Mg#

(68–70) of all units in the study. The most Si-poor unit,

Park Butte, has the second highest Mg# (63–65). The other

basaltic andesite unit, Cathedral Crag, has the lowest Mg#

(55–58), with the exception of one sample (NM-CC7) that

has Mg# 72 due to olivine accumulation. Both MgO

(Fig. 4) and FeO (Table 4) are highest in Park Butte

(7.9–8.4 wt% MgO, 8.9–9.7 wt% FeO).

There is a relatively small range in Al2O3 across four of

the units (15.7–17.9 wt%), but the fifth, Cathedral Crag, is

notably enriched in Al2O3 (20–20.2 wt%, Fig. 4). This is

reflected in the high abundance of plagioclase phenocrysts

in Cathedral Crag lavas (see Table 1) but not in positive Eu

anomalies, as discussed below.

The abundances of TiO2, CaO, and Na2O are distinctly

different between units (Fig. 4). The Sulphur Creek and

Lake Shannon samples have the highest TiO2 ([1.4 wt%)

and Na2O ([4 wt%), and Tarn Plateau and Park Butte have

the lowest (\0.95 wt% TiO2, \3 wt% Na2O). Conversely,

CaO is greatest in the Park Butte and Tarn Plateau samples

(abnormally high for the HMBA of Tarn Plateau) and

lowest in Sulphur Creek and Lake Shannon. The high-Mg

basaltic andesites from Tarn Plateau have the lowest TiO2

(*0.7 wt%).

Although major element compositions vary significantly

between the units, variations within units are limited,

especially for Al2O3, TiO2, and Na2O. Lake Shannon is the

only flow unit that shows a trend in Mg#, MgO and FeO.

Sulphur Creek mafic lavas (B52 wt% SiO2) show no var-

iation, but there is a trend across the compositional gap to

basaltic andesite (Fig. 4). In contrast, K2O content does

generally increase with silica within each individual unit

and across the range of the flow units, forming a sublinear

trend (Fig. 3).

In summary, major elements distinguish three types of

primitive magmas at Mount Baker: low-K basalt (Park

Butte, similar to low-K olivine tholeiites elsewhere in the

Cascades), calc-alkaline basalts (Lake Shannon and Sul-

phur Creek), and high-Mg basaltic andesites and andesites

(Tarn Plateau and Glacier Creek). Major elements also

distinguish between groups of less primitive basaltic

andesites (Cathedral Crag and Sulphur Creek).

Trace elements

Compatible trace element compositions vary widely among

the units, and in some cases within units. In this study,

abundances of both Ni and Cr are relatively low compared

to other primitive Cascade calc-alkaline and LKOT basalts

(Fig. 5). Park Butte LKOT and Tarn Plateau HMBA have

similar Ni content (52–69 ppm), but Park Butte has higher

Cr than Tarn Plateau at a given Ni abundance. Lake

Shannon basalt has the highest Ni content of all the flow

units (62–90 ppm) and is the only lava that exhibits a

predictable decrease in both Ni and Cr as silica decreases.

However, this unit has much lower Cr relative to the Tarn

Plateau HMBA and Park Butte low-K basalt.

Other trace element compositions differ greatly between

the units in this study, and trends within units are variably


123

Ta

ble

4R

epre

sen

tati

ve

wh

ole

rock

maj

or

and

trac

eel

emen

tco

mp

osi

tio

ns

Unit

Par

kB

utt

eC

athed

ral

Cra

gT

arn

Pla

teau

Lak

eS

han

non

Sulp

hur

Cre

ek

Sam

ple

NM

-PB

2N

M-P

B5

NM

-PB

6N

M-P

B8

NM

-CC

1N

M-C

C5

NM

-CC

6N

M-T

P2

NM

-TP

3N

M-T

P4

NM

-TP

6N

M-L

S1

NM

-LS

2N

M-L

S6

NM

-SC

1N

M-S

C2

NM

-SC

3N

M-S

C6

Majo

rel

emen

ts(w

t%)

SiO

249.4

549.9

150.0

549.5

952.4

652.1

652.4

853.9

753.6

053.9

053.9

051.3

152.4

850.7

051.2

451.5

551.4

951.4

4

TiO

21.0

64

1.0

63

1.0

41

1.0

13

1.1

34

1.0

53

1.1

35

0.9

19

0.9

05

0.9

22

0.8

95

1.4

63

1.4

39

1.4

20

1.6

20

1.6

35

1.6

34

1.6

37

Al 2

O3

17.3

016.9

317.1

017.3

220.1

020.0

320.0

916.1

715.8

416.1

515.7

417.7

617.8

717.9

117.3

417.5

517.5

217.4

7

FeO

*9.6

69.1

59.3

98.9

06.4

26.3

26.4

06.9

27.0

26.9

56.9

48.5

08.0

88.3

38.5

08.8

08.7

48.7

7

MnO

0.1

68

0.1

66

0.1

65

0.1

61

0.1

13

0.1

12

0.1

13

0.1

29

0.1

34

0.1

31

0.1

32

0.1

57

0.1

50

0.1

55

0.1

64

0.1

66

0.1

65

0.1

65

MgO

8.1

48.1

68.3

88.1

83.9

74.1

73.8

17.3

57.7

97.0

37.8

86.3

65.2

86.4

25.3

95.4

85.3

75.4

0

CaO

9.1

79.1

99.2

19.2

99.3

29.5

19.3

39.4

79.7

69.4

19.6

68.8

48.0

28.8

18.2

48.4

08.2

98.3

3

Na 2

O3.2

13.1

63.2

33.2

03.8

33.7

03.8

72.8

92.8

02.9

42.8

34.0

84.2

43.9

14.3

64.4

14.3

74.3

9

K2O

0.4

50.4

50.4

40.4

20.9

80.9

40.9

80.9

50.8

60.9

50.9

10.6

40.8

80.5

50.8

60.8

50.8

50.8

6

P2O

50.1

50

0.1

60

0.1

55

0.1

49

0.3

32

0.3

15

0.3

34

0.1

84

0.1

78

0.1

84

0.1

76

0.2

81

0.3

06

0.2

73

0.4

29

0.4

27

0.4

29

0.4

31

Tota

l98.7

698.3

399.1

798.2

498.6

698.3

198.5

498.9

598.8

998.5

799.0

699.4

198.7

498.4

898.1

599.2

798.8

598.8

9

Mg#

63.8

65.1

65.2

65.8

56.4

58.1

55.5

69.0

69.9

68.0

70.4

61.1

57.8

61.7

757.1

56.6

56.3

56.3

Tra

ceel

emen

tsby

XR

F(p

pm

)

Ni

67

65

69

65

20

25

19

56

61

52

63

85

62

90

38

38

37

37

Cr

253

235

257

243

23

28

22

198

233

176

234

164

115

170

93

93

91

92

V174

192

185

182

201

191

205

180

181

181

176

178

166

173

202

203

202

201

Ga

17

18

18

17

20

19

19

16

17

17

17

18

18

17

17

18

18

18

Cu

24

26

25

26

44

45

31

40

43

36

40

60

58

69

38

34

40

37

Zn

82

79

82

77

69

68

70

65

67

65

65

76

77

75

85

85

86

84

Tra

ceel

emen

tsby

ICP

-MS

(ppm

)

La

8.0

47.8

18.0

87.6

214.8

116.1

117.0

612.8

112.6

712.4

012.3

411.4

313.2

611.5

016.2

115.8

216.3

716.0

6

Ce

18.4

918.4

418.2

117.6

733.0

536.4

538.2

329.2

428.6

128.3

427.4

326.8

630.4

226.5

438.8

537.8

038.9

838.3

6

Pr

2.6

32.6

02.6

62.4

94.5

34.9

75.2

44.0

03.9

63.8

63.8

23.7

24.1

23.7

25.3

55.2

05.3

95.3

2

Nd

11.8

811.6

611.9

711.1

719.1

421.1

321.9

416.8

416.7

716.2

416.1

516.3

517.7

316.3

023.1

422.9

423.0

823.0

9

Sm

3.1

63.0

83.0

62.9

34.1

14.4

84.7

53.7

63.7

43.6

23.6

24.2

74.3

94.1

25.6

75.6

15.6

25.5

7

Eu

1.1

71.1

51.1

71.1

11.3

31.4

61.5

21.2

31.2

11.2

21.1

71.5

51.5

31.5

01.8

61.8

81.8

71.8

9

Gd

3.5

33.5

33.5

73.3

13.5

54.0

04.2

23.6

03.6

53.4

63.4

44.7

34.7

84.6

25.9

15.8

45.9

55.8

4

Tb

0.6

30.6

20.6

30.5

80.5

40.6

00.6

30.5

80.5

70.5

60.5

50.8

20.8

10.8

00.9

80.9

70.9

90.9

7

Dy

4.0

13.9

84.0

33.6

43.1

13.4

53.6

43.4

73.4

73.3

43.3

35.1

65.0

05.0

86.0

65.9

86.0

35.9

7

Ho

0.8

50.8

50.8

60.7

90.6

10.6

80.7

10.7

10.7

00.6

60.6

81.0

81.0

51.0

71.2

41.2

21.2

71.2

2

Er

2.3

62.3

42.3

22.1

71.6

21.8

01.8

51.8

91.8

91.8

11.7

92.9

22.8

92.9

13.3

73.3

33.4

23.3

2

Tm

0.3

40.3

50.3

40.3

20.2

30.2

50.2

60.2

70.2

70.2

60.2

60.4

40.4

30.4

30.4

90.4

80.4

90.4

9

Yb

2.1

12.1

32.1

62.0

41.3

81.5

31.6

21.6

81.6

51.6

41.5

92.6

82.6

22.6

53.0

32.9

92.9

93.0

0

Lu

0.3

30.3

40.3

30.3

20.2

10.2

40.2

50.2

60.2

60.2

50.2

50.4

20.4

20.4

20.4

70.4

70.4

80.4

8

Ba

207

202

207

203

403

405

431

350

322

333

315

212

283

212

284

279

287

280

Th

0.6

30.6

40.6

00.6

61.6

31.8

41.9

02.1

62.0

82.1

22.0

51.0

81.5

11.1

51.3

91.2

81.4

51.3

6


123

developed (Fig. 6). Park Butte LKOT has the lowest

abundances of both large-ion lithophile elements (LILE,

Ba, Rb, and Pb) and high-field-strength elements (HFSE,

Zr, Th, and Ce) compared to the other four flow units

(Table 4). Abundances of Ba, Sr, Th, Yb, Pb, and Ce in the

Park Butte basalt are comparable to values published for

LKOT elsewhere in the Cascade arc (Bacon et al. 1997).

Values of Rb, Ba, Pb, and Th generally increase with silica

between units but do not show much variation within units

(except for Lake Shannon and the shift across the com-

positional gap of Sulphur Creek). Zr, Nb, and Y values

vary widely between units, but are constant for all samples

within a given flow unit. Sr values in the Tarn Plateau

HMBA and Cathedral Crag lavas vary widely within those

units and are much higher than in the other units

(750–900 ppm and 1,000–1,200 ppm, respectively). Nb

and Ta display incompatible behavior in Lake Shannon

samples, but values within the other units are relatively

constant (Table 4). Sulphur Creek lavas have anomalously

high Zr, Nb, Y, and Yb (Figs. 5, 6). These lavas show a

slight decrease in Y and Yb with silica (Fig. 7). Overall,

most trace elements have relatively clustered values within

each flow unit (with the exception of Sr in Cathedral Crag

samples, see Fig. 5).

The NMORB-normalized trace element diagram

(Fig. 6) reveals patterns very typical of calc-alkaline arc

lavas. All the samples are enriched in LILE and exhibit

distinctive negative Nb and Ta anomalies that are repre-

sentative of arc magmas whose generation has been

influenced by a subduction component. The lavas all have

pronounced enrichment in Ba, U, Pb, and Sr. Park Butte

LKOT is the least enriched in these trace elements and

shifts to a flatter, more MORB-like pattern on the right side

of the diagram, with moderate enrichment compared to the

other units. This is similar to patterns for LKOT from other

Cascade volcanoes (Bacon et al. 1997). The Tarn Plateau

HMBA and Cathedral Crag basaltic andesites have the

lowest abundances of HFSE on the right of the diagram,

but have intermediate Nb and Ta relative to the other flow

units. This type of pattern is similar to high magnesium

andesites (HMA) from Mount Shasta (Grove et al. 2002).

The Sulphur Creek and Lake Shannon calc-alkaline basalts

display a third type of trend, with intermediate HFSE and

the highest Nb and Ta; however, the relative enrichment of

the Sulphur Creek samples is slightly higher than those

from Lake Shannon.

On the chondrite-normalized REE diagram (Fig. 7),

Park Butte LKOT samples have the least enriched LREE

signature and flattest overall patterns, with (LREE)N

*20–35 and (La/Sm)N B 1.7. Tarn Plateau HMBA has

moderate LREE enrichment and a relatively steep REE

pattern, with (LREE)N values from 40 to 55 and (La/Sm)N

at *2.2. Cathedral Crag, the most Mg-poor lava, has highTa

ble

4co

nti

nu

ed

Unit

Par

kB

utt

eC

athed

ral

Cra

gT

arn

Pla

teau

Lak

eS

han

non

Sulp

hur

Cre

ek

Sam

ple

NM

-PB

2N

M-P

B5

NM

-PB

6N

M-P

B8

NM

-CC

1N

M-C

C5

NM

-CC

6N

M-T

P2

NM

-TP

3N

M-T

P4

NM

-TP

6N

M-L

S1

NM

-LS

2N

M-L

S6

NM

-SC

1N

M-S

C2

NM

-SC

3N

M-S

C6

Nb

2.7

42.7

82.6

42.6

44.2

34.6

14.8

73.5

83.3

93.5

03.3

75.1

06.3

34.9

97.2

17.0

97.3

17.2

2

Y21.3

821.2

021.2

619.9

815.6

517.1

618.0

718.0

518.1

617.4

017.3

527.2

026.2

826.7

731.2

230.9

931.4

731.0

6

Hf

2.0

32.0

71.9

61.9

22.2

82.5

12.6

12.9

82.8

32.8

82.8

03.3

13.5

93.2

04.3

74.2

64.3

64.3

0

Ta

0.1

70.1

80.1

80.1

70.2

50.2

70.2

70.2

40.2

30.2

30.2

20.3

60.4

60.3

60.4

80.4

60.4

90.4

8

U0.2

40.2

50.2

40.2

70.5

50.6

30.6

50.7

80.7

20.7

00.7

30.4

80.6

70.5

00.6

10.5

80.6

30.6

0

Pb

1.6

41.6

81.7

91.7

92.9

43.3

63.7

43.5

13.3

13.1

63.1

62.7

03.6

42.8

33.4

23.2

63.4

93.4

0

Rb

4.8

5.4

5.6

5.7

9.9

12.1

11.6

12.4

10.1

11.6

11.7

8.1

12.1

7.7

9.8

8.3

9.2

9.0

Cs

0.0

50.0

60.0

50.0

70.0

90.1

50.1

20.2

00.2

20.1

70.1

70.1

80.2

70.1

90.2

60.2

00.2

30.2

5

Sr

521

491

499

499

1,0

51

1,2

06

1,2

27

854

853

858

833

477

506

490

559

563

560

562

Sc

28.3

30.1

30.0

28.3

17.7

20.7

21.2

25.8

26.7

26.4

28.2

26.6

21.4

25.7

25.4

25.9

26.2

26.1

Zr

76

77

73

72

87

97

100

107

103

105

102

145

157

141

195

192

198

195

All

anal

yse

sper

form

edat

the

WS

UG

eoA

nal

yti

cal

Lab

.F

eis

report

edas

all

FeO

*.

Mg#

=M

g/(

Mg

?F

e2?

),F

e2?

isca

lcula

ted

as0.8

5F

eO*.

For

det

ails

on

accu

racy

,se

e‘‘

Anal

yti

cal

met

hods’

’se

ctio

n


123

LREE enrichment and the steepest REE pattern, with

(LREE)N values from 45 to 70 and (La/Sm)N at *2.3. All

of the samples from both Park Butte and Lake Shannon

have minor positive Eu anomalies, and one Sulphur Creek

sample appears to have a very slight negative Eu anomaly.

Notably, the Cathedral Crag samples with high modal

0

0.5

1

1.5

2

2.5

3

45 47 49 51 53 55 57 59 61 63

K2O

(wt.

%)

SiO2 (wt.%)

Park Butte (LKOT-like)

Cathedral Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Creek (CA)

Mt. Baker HMA

High-K(Calc-Alkaline)

Medium-K(Calc-Alkaline)

Low-K(Tholeiitic)

Shoshonitic

Fig. 3 K2O versus SiO2

classification diagram. Fields

from Le Maitre et al. (1989).

Abbreviations for lava types are

indicated in parentheses next to

their respective unit names:

basaltic andesite (BA), high-Mg

basaltic andesite (HMBA), high-

Mg andesite (HMA), low-K

olivine tholeiite (LKOT-like),

and calc-alkaline (CA). Mt.

Baker HMA from the Glacier

Creek unit of Baggerman and

DeBari (2011). Black and browndashed fields encompass fields

of other Cascade primitive calc-

alkaline basalt (CAB) and

LKOT (data compiled from

GEOROC database 2008)

15

16

17

18

19

20

21

48 50 52 54 56 58 60

Al 2

O3 (w

t.%

)

Uncertainty

0.5

1.0

1.5

2.0

48 50 52 54 56 58 60

TiO

2(w

t.%

)

SiO2 (wt.%)

Uncertainty

52

56

60

64

68

72

48 50 52 54 56 58 60 M

g #

6

7

8

9

10

11

48 50 52 54 56 58 60

CaO

(w

t.%

)

Uncertainty

2

4

6

8

10

48 50 52 54 56 58 60

Mg

O (w

t.%

)


Cath. Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Cr. (CA)

Felsic SC

Mt. Baker HMA

Uncertainty

2

3

4

5

48 50 52 54 56 58 60

Na 2

O (

wt.

%)

SiO2 (wt.%)

Uncertainty

Fig. 4 Select major element

variation diagrams. Symbols,

data sources, and abbreviations

as in Fig. 3. Cathedral Crag

sample circled in black is

accumulative in phenocrysts

(see ‘‘Whole rock chemistry’’).

Open purple triangles are more

felsic Sulphur Creek samples

from Hildreth et al. (2003).

Maximum uncertainty in

analyses shown at bottom left.Mg# = Mg/(Mg ? Fe2?), Fe2?

is calculated as 0.85 FeO*


123

plagioclase, high Al2O3, and high Sr do not have significant

positive Eu anomalies. This suggests that high modal pla-

gioclase is not accumulative, but is a result of high alumina

content in the melt.

Isotopic composition

Isotopic compositions for Sr, Nd, and Pb were analyzed for

the three most primitive flow units: Park Butte, Tarn Pla-

teau, and Lake Shannon (Table 5). The most mafic and

most felsic sample in the Park Butte and Lake Shannon

units were selected to represent the probable isotopic

compositional range of each (note that the range of silica

content is *1 and 2%, respectively, in these units). The

range in silica content for Tarn Plateau is less than the error

associated with XRF analysis, and thus two samples with

identical silica values were analyzed. The samples fall

within or just outside the mantle array on the eNd versus87Sr/86Sr (Fig. 8a) and plot within the field for primitive

lavas from other parts of the Cascade arc.

Measured 87Sr/86Sr values in the analyzed units have a

range of 0.70309–0.70335 (Table 5; Fig. 8a). This range is

wider within the Lake Shannon and Park Butte lavas than

between the units. In the Lake Shannon samples, 87Sr/86Sr

300

500

700

900

1100

1300

48 50 52 54 56 58 60

Sr

(pp

m)

0

4

8

12

16

20

24

48 50 52 54 56 58 60

Rb

(pp

m)

SiO2 (wt.%)

0

50

100

150

200

250

48 50 52 54 56 58 60

Zr

(pp

m)

100

200

300

400

500

48 50 52 54 56 58 60

Ba

(pp

m)

10

20

30

40

48 50 52 54 56 58 60

Y (p

pm

)

0

2

4

6

8

10

48 50 52 54 56 58 60

Nb

(pp

m)

SiO2 (wt.%)

0

50

100

150

200

250

300

350

45 50 55 60

Ni (

pp

m)


Cath. Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Cr. (CA)

Felsic SC

Mt. Baker HMA

Cascades CABCascades LKOT

0

100

200

300

400

500

600

45 50 55 60

Cr

(pp

m)

Cascades LKOT

Cascades CAB

Fig. 5 Select minor and trace

element variation diagrams.

Symbols, data sources, and

abbreviations as in Figs. 3, 5.

More felsic Sulphur Creek

samples are from Baggerman

and DeBari (2011). Black and

brown dashed lines in Ni and Cr

diagrams encompass fields of

other Cascade primitive calc-

alkaline basalts (CAB) and low-

K olivine tholeiites (LKOT; data

compiled from GEOROC

database 2008)


123

0.1

1

10

100

Cs Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Zr Hf Sm Eu Ti Tb Y Yb Lu

Sam

ple

/NM

OR

B


Cathedral Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Creek (CA)

Diff. SC (Mixed)

Mt. Baker HMA

Mt. Shasta LKOT

Mt. Shasta PMA

Fig. 6 NMORB-normalized

trace element diagram for

representative samples.

Normalization values from Sun

and McDonough (1989). Data

sources and abbreviations as in

Fig. 3. Diff. SC (mixed) is a

Sulphur Creek basaltic andesite

sample calculated by mixing of

mafic and felsic endmember

magmas (Baggerman and

DeBari 2011). Mt. Shasta

LKOT and PMA (Primitive

magnesian andesites) from

Grove et al. (2002). The high-

Mg lavas and derivatives (Tarn

Plateau HMBA, Cathedral Crag

BA, and Mt. Baker HMA) are

most depleted in HREE and plot

nearly atop one another

10

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Sam

ple

/Ch

on

dri

te


Cathedral Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Creek (CA)

Diff. SC (Mixed)

Mt. Baker HMA

NMORB

Mt. Shasta LKOT

Mt. Shasta PMA

4

8

12

16

20

48 50 52 54 56 58 60

La

(pp

m)

SiO2 (wt.%)

PBCCTPLSSCMB HMA

0

1

2

3

4

48 50 52 54 56 58 60

Yb

(pp

m)

SiO2 (wt.%)

Fig. 7 Chondrite-normalized

rare earth element (REE)

diagram. Normalization values

and NMORB from Sun and

McDonough (1989). Data

sources and abbreviations as in

Figs. 3 and 6. Select REE (La

and Yb) versus SiO2 are plotted

to show changes in pattern

across the range of each flow

unit


123

is higher in the more felsic samples, but in the Park Butte

samples, this ratio is higher in the more mafic samples.

Tarn Plateau samples have constant 87Sr/86Sr. The range of87Sr/86Sr does not extend to the low values seen in the

Columbia segment of the Cascade arc (Schmidt et al.

2008), nor does it extend to the high values seen in the

Central segment of the arc. The values are typical of the

range seen in the southern Cascades of California (Schmidt

et al. 2008).

Measured eNd values in the analyzed units are between

?6.8 and ?8.0 (Table 5; Fig. 8a). In contrast to 87Sr/86Sr,

the range is wider between units than within units, and

within flow unit variation is within analytical error.

All three units plot just above the NHRL (Northern

Hemisphere Reference Line) on the 207Pb/204Pb versus206Pb/204Pb diagram (Fig. 8b), but plot just below the

NHRL in the 208Pb/204Pb versus 206Pb/204Pb diagram (not

shown). Pb isotopic ratios range from 18.75 to 18.86 for206Pb/204Pb, 15.54–15.56 for 207Pb/204Pb, and 38.20–38.29

for 208Pb/204Pb (Table 5). All Pb isotope ratios increase

with silica in the Lake Shannon lavas, and the same is true

for 206Pb/204Pb and 208Pb/204Pb in the Park Butte samples;

however, 207Pb/204Pb decreases as silica increases in Park

Butte samples.

Summary of chemical characteristics

Overall, major and trace element chemistry reveals that the

flow units can be categorized as three distinct primitive

endmember magma types: calc-alkaline basalts, LKOT-

like basalts, and high-Mg basaltic andesite (with probable

lower-Mg derivatives).

The calc-alkaline lavas are represented by the basalts of

Lake Shannon and Sulphur Creek. They exhibit charac-

teristic enrichment in large-ion lithophile elements (LILE:

Ba, Rb, K, Pb) and relative depletions in Nb and Ta

(Fig. 6). However, they display the highest overall abun-

dances of Nb, Ta, Zr, and middle REE among the units.

The LKOT-like endmember is represented by the basalt

of Park Butte, which has major element chemistry similar

to LKOT found elsewhere in the Cascades (Bacon et al.

1997, Conrey et al. 1997; Leeman et al. 1990). Park Butte

also has the lowest abundances of Nb, Ta, Zr, and middle

REE among the units.

The third magma type, high-Mg basaltic andesite

(HMBA), is represented by the basaltic andesite of Tarn

Plateau. It has high MgO for a given silica content, and

Mg# higher than the basalts of this study (Fig. 4). This flow

unit is strongly depleted in heavy rare earth elements

(HREE; Fig. 7), but has intermediate Nb, Ta, Zr, and

middle REE compared to the other lavas (Fig. 6). Other

high-Mg andesites with similar trace element characteris-

tics are present at Mount Baker (the Glacier Creek unit, or

Mount Baker HMA, see Baggerman and DeBari 2011;

Figs. 5, 6, 7). The basaltic andesite of Cathedral Crag is not

a high-Mg lava, but it shares the same depletion in heavy

REE, with even higher (La/Yb)N.

Isotopic compositions do not show any systematic

trends for these lava types. Within flow unit variability in87Sr/86Sr is as great as the variability between units,

Table 5 Sr, Nd, and Pb isotopic compositions

Sample 87Sr/86Sr eSr143Nd/144Nd eNd

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb SiO2

Park Butte

NM-PB3 0.703347 -16.4 0.512997 7.0 18.748 15.552 38.215 49.25

NM-PB6 0.703157 -19.1 0.512988 6.8 18.806 15.547 38.238 50.05

Tarn Plateau

NM-TP4 0.703090 -20.0 0.513011 7.3 18.778 15.547 38.224 53.90

NM-TP6 0.703094 -20.0 0.513027 7.6 18.771 15.543 38.212 53.90

NM-TP6 (2nd solution) – – – – 18.771 15.542 38.205 53.90

Lake Shannon

NM-LS2 0.703291 -17.2 0.513026 7.6 18.861 15.554 38.283 52.48

NM-LS2 (repeat) – – – – 18.861 15.555 38.286 52.48

NM-LS6 0.703193 -18.6 0.513049 8.0 18.837 15.548 38.249 50.70

NM-LS6 (2nd solution) – – – – 18.833 15.548 38.244 50.70

Analyses performed at the University of Washington Isotope Geochemistry Laboratory by Dr. Bruce Nelson. Error on Nd and Sr analyses

is ± 30 ppm (2r) or ± 0.3 epsilon units; error on Pb analyses is ± 150 ppm (2r) or ± 0.015%. NM-LS2 (repeat) is a second analyses of the

same solution

NM-TP6 (2nd solution) and NM-LS6 (2nd solution) are second aliquots of the dissolved sample that went through the entire Pb separation

chemistry


123

whereas the compositional range of Nd isotopes within

each unit is negligible (well within analytical error), and

the spread between the units is very small as well.

H2O contents

Sisson and Grove (1993) presented a method for estimating

H2O content of magmas. By calculating the exchange of Ca

and Na in plagioclase versus liquid, the resulting Kd rela-

tionship (defined as [Ca/Na]plag/[Ca/Na]liquid) can be cor-

related with H2O content. Since high water content leads to

more calcic plagioclase, as H2O of the magma increases, so

does KdCa-Na of plagioclase–liquid.

Figure 9 shows Ca/Na relationships and resulting esti-

mates of H2O contents using the whole rock compositions

of CaO and Na2O as a proxy for liquid and associated

plagioclase grains. The plagioclase grains selected for use

with this hygrometer include phenocrysts and micro-

phenocrysts with the highest An content to represent the

earliest-formed grains. Only the unresorbed portions of the

cores were analyzed. Estimated H2O content of the units

are as follows: *2% for Lake Shannon and Sulphur

Creek, *3 wt% H2O for Park Butte, *4 wt% H2O for

Tarn Plateau, and *4.5 wt% H2O for Cathedral Crag.

These estimated H2O contents are used in the following

sections for interpreting eruption temperatures, contribu-

tions from the subducting slab, and for modeling fluid

compositions. The results of the hygrometer are supported

by the following arguments. First, the unit that gives the

highest H2O content estimate, the Cathedral Crag basaltic

andesite, also has the highest Al2O3 with no associated Eu

anomaly. Gaetani et al. (1993) demonstrated that plagio-

clase crystallization is suppressed in H2O-rich magmas,

causing elevated Al2O3 content. Second, H2O content

estimates calculated for the Sulphur Creek basalt (2%) are

consistent with the 2.3% H2O measurement obtained by

Shaw (2011) using FTIR of olivine-hosted melt inclusions

from tephra associated with the Sulphur Creek lava.

Finally, measured H2O contents (by FTIR) for LKOT-like

lavas at Glacier Peak are also not very different from calc-

alkaline lavas (2.0 wt% as compared to 2.2 wt%; Shaw

2011).

Magmatic temperatures and equilibration pressures

Magma equilibration temperatures can be determined from

major element compositions using the olivine–liquid ther-

mometer of Suguwara (2000) and H2O liquidus depres-

sions from Medard and Grove (2008). Using the estimated

H2O contents from Fig. 9, the LKOT-like lavas of Park

Butte record the highest liquidus temperatures of

1,110–1,120�C. The calc-alkaline lavas record magmatic

temperatures of 1,070–1,100�, with the more evolved

Sulphur Creek lavas recording temperatures on the cooler

end of this range. The high-Mg basaltic andesites of Tarn

Plateau record similar temperatures (1,065–1,085�), a

15.48

15.52

15.56

15.60

15.64

15.68

15.72

19.0018.50

207P

b /2

04P

b

206Pb/ 204Pb

(b)

1

2

3

4

5

6

7

8

9

0.7028 0.7032 0.7036 0.7040 0.7044

εNd

87Sr/ 86Sr


Tarn Plateau (HMBA)

Lake Shannon (CA)

(a)

Fig. 8 Select isotopic compositions. Abbreviations as in Fig. 3. a eNd

versus 87Sr/86Sr and b 207Pb/204Pb versus 206Pb/204Pb. Black dashedfields indicate the range of compositions for Cascade primitive lavas

as compiled in Schmidt et al. 2008. Error bars are symbol sized or

smaller where not visible on the diagrams. NHRL from Hart (1984),

Bulk Cascadia subducted sediment from Plank and Langmuir (1998),

and Juan de Fuca MORB from White et al. (1987). ‘‘M’’ indicates the

most mafic sample analyzed from the Park Butte and Lake Shannon

flow units

0

5

10

15

20

0 1 2 3 4 5

Ca/

Na

Pla

gio

clas

e

Ca/Na Liquid (WR)

Park Butte (LKOT-like)Cath. Crag (BA)Tarn Plateau (HMBA)Lake Shannon (CA)Sulphur Creek (CA)

Fig. 9 Ca–Na exchange in plagioclase and liquid. Ca/Na (molar)

liquid is from whole rock compositions. Estimates of H2O content (in

wt%) are based on Kd after the method of Sisson and Grove (1993).

Note that two points exist for the Park Butte unit that plot nearly atop

one another. See text for details


123

result of a much larger liquidus depression because of

higher H2O contents. The more differentiated Cathedral

Crag lavas record significantly lower temperatures

of *985�. Equilibration pressures using the Si-activity

barometers of Albarede (1992) and Putirka (2008) are

variable, but yield the lowest pressures for Tarn Plateau

and Cathedral Crag lavas (5–8 kbar), intermediate pres-

sures for the calc-alkaline lavas of Lake Shannon and

Sulphur Creek (6–10 kbar), and the highest pressures for

the LKOT-like lavas of Park Butte (9.5–10.5 kbar).

Discussion

The focus of this discussion is the characterization of

subducted slab and mantle inputs for the production of

mafic lavas (Mg# 55–70) beneath the northern Cascade arc

at Mount Baker. We begin the discussion by first estab-

lishing that these whole rock compositions are indeed

representative of mantle-derived liquid compositions. This

requires the assessment of mineral–whole rock equilibria

and effects of crustal processes such as assimilation and

magma mixing.

Assessment of equilibrium and crustal effects

Because the primitive lavas in this study are moderately to

strongly phenocryst rich, we used olivine–liquid distribu-

tion coefficients to ascertain whether olivine compositions

are in equilibrium with whole rock compositions (i.e.,

whether whole rocks represent equilibrium liquids). Oliv-

ine core compositions from Tarn Plateau and Lake Shan-

non lavas, as well as select samples from Park Butte,

represent near-equilibrium conditions (Electronic Supple-

ment 6). In contrast, olivine compositions from Cathedral

Crag, Sulphur Creek, and most (but not all) from Park

Butte, are more Fe rich than would be expected from their

whole rock Fe/Mg compositions. Hence, whole rock

compositions are too mafic to be in equilibrium with their

enclosed olivine, suggesting that these three units may have

experienced some (few %) accumulation of olivine.

Olivine accumulation will produce variation in the

whole rock composition of lavas, specifically in MgO,

SiO2, and Ni, and has been documented for other flows at

Mount Baker (Baggerman and DeBari 2011). Within flow

unit variations in MgO and SiO2 exist within flow units,

some of which cannot be ascribed to the standard error in

XRF analysis (Fig. 4). Thus, SiO2, MgO, and Ni contents

of the Park Butte, Cathedral Crag, and Sulphur Creek lavas

may not be reflective of liquids, and magmatic tempera-

tures described above may be maxima. However, olivine in

the Tarn Plateau high-Mg basaltic andesite does record

near-equilibrium conditions (Electronic Supplement 6),

thus their high-Mg nature is a liquid characteristic not

reflective of olivine accumulation.

Although some of the units in this study may have been

affected by olivine accumulation, the most incompatible

element abundances, those which clearly define mantle

source compositions, will be unaffected by this process.

The Cathedral Crag sample that has anomalous major

element chemistry and petrography (i.e., much higher

modal olivine than the rest of the unit) has a REE pattern

similar to the other samples in that unit (Fig. 7), indicating

that the REE budget is controlled by the melt rather than

mineral accumulation.

Xenolith textures described earlier suggest that assimi-

lation of exotic material plays an insignificant role in

generating the geochemical signature of the flow units. As

previously mentioned, xenoliths have been noted in only

one unit of this study, the basaltic andesite of Cathedral

Crag. The two types of xenoliths in this lava contain thin

reaction rims; however, the boundaries are very sharp,

indicating minimal chemical exchange between the xeno-

liths and the magma. Additionally, isotopic ratios do not

reflect assimilation of crustal material. The analyzed flows

have relatively low Sr and Pb isotopic ratios, relatively

high Nd isotopic ratios, and are comparable to other Cas-

cade primitive lavas (Fig. 8).

Magma mixing and mingling seem to be likely causes of

pervasive disequilibrium textures found in all of the units

(e.g., Green 1988; Baggerman and DeBari 2011), but has

not dramatically altered trace, REE, and some major ele-

ment abundances. Baggerman and DeBari (2011) showed

that mixing between Sulphur Creek basalt and Mount

Baker dacite can effectively produce the basaltic andesites

on the Si-rich side of the Sulphur Creek compositional gap.

In this scenario, 70% basalt mixed with 30% dacite yields a

close match with most major, trace, and REE compositions

of the basaltic andesite. However, even after this mixing,

the trace element chemistry of the Sulphur Creek basaltic

andesite samples is comparable to that of the Sulphur

Creek basalt (Figs. 6, 7). This suggests that magma mixing

had a minimal impact on the trace element signature of the

lavas, and thus mantle source characteristics are still

discernable.

In summary, despite pervasive textural evidence for

disequilibrium, these lavas appear to retain the trace ele-

ment signature of their mantle-derived parents. This is

demonstrated by: (1) the lack of variation in many of the

most highly incompatible elements, even in the units that

have experienced olivine accumulation; (2) insignificant

addition of crustal material and lack of an obvious crustal

radiogenic source; and (3) minimal difference in trace

element chemistry between mixed lavas and their mafic

endmembers, even with significant addition by a felsic

endmember. Together, these observations suggest that


123

distinct mantle source characteristics are still interpretable

from the mafic units of this study.

Characterization of mantle and slab sources beneath

the Mount Baker volcanic field

Endmember magma types

Three endmember primitive magma types were originally

described for the middle and southern Cascades: (1) a

typical hydrous, arc-like calc-alkaline basalt; (2) a depleted

and anhydrous low-K olivine tholeiite (LKOT), alterna-

tively called high alumina olivine tholeiite (HAOT); and

(3) an enriched, intraplate-type ocean island basalt (OIB;

e.g., Bacon et al. 1997; Borg et al. 1997; 2000; Conrey

et al. 1997; Grove et al. 2002; Leeman et al. 1990; 2005;

Reiners et al. 2000; Smith and Leeman 2005). More

recently, primary high-Mg andesites have been described at

Mount Shasta (Grove et al. 2002). Major and trace element

criteria are specific to each endmember group.

In this study, we present evidence for three endmember

magma types that bear resemblance to the types described

above. These include a calc-alkaline endmember, a LKOT-

like endmember (with modified trace element characteris-

tics), and a high-Mg basaltic andesite endmember. None of

the lavas of this study resemble OIB.

Calc-alkaline lavas in the Cascade arc are generally

medium-K (Fig. 3) with higher silica content than LKOT

magmas. The Lake Shannon and Sulphur Creek lavas from

Mount Baker are typical examples and are characterized by

enrichment of LREE relative to HREE, and enrichment of

LILE (Ba, Sr, Pb, Rb) relative to HFSE (Nb, Ta, Zr, Hf).

These lavas have nearly identical incompatible trace ele-

ment patterns (Figs. 6, 7), but abundances are higher in the

Sulphur Creek samples for a given Si content (Fig. 5).

These units were probably derived from similar sources,

with variable amounts of partial melting and/or fractional

crystallization.

The LKOT magmas in the Cascade arc are characterized

by [17 wt% Al2O3, generally \0.1 wt% K2O, and *2.5

wt% Na2O (Bacon et al. 1997). The Park Butte basalt fits

these criteria, containing high Al, but low K and Na

compared to other Mount Baker units (Figs. 3, 4). The

incompatible trace element abundances that are particularly

reflective of mantle source (i.e., Nb, Ta, Zr, and Hf) are

similar in Cascade LKOT, MORB, and the Park Butte

basalts. Cascade LKOT have distinct negative Nb and Ta

anomalies, as well as depleted Zr and Hf (Bacon et al.

1997, see Mount Shasta LKOT in Fig. 6), suggesting that a

depleted mantle source is responsible for their chemical

signature. However, Cascade LKOT show more enriched

LREE and LILE (e.g., Ba, Sr, Rb, Pb) than MORB, more

typical of back-arc basin basalts. The Park Butte basalt is

even more enriched than typical Cascade LKOT in these

elements (Figs. 6, 7), suggesting that magma generation

processes for these ‘‘LKOT-like’’ magmas are distinct in

this northern part of the arc.

High-Mg lavas occur throughout the Cascade arc

(Bacon et al. 1997), but have been studied in detail in the

southern Cascade arc at Mount Shasta (Grove et al. 2002;

2005) and in the northern Cascade arc at Glacier Peak

(Taylor 2001). In these studies, high-Mg lavas are thought

to be derived by fluid-induced melting of depleted mantle

with significant subduction component. The high-Mg lavas,

including Mount Baker’s Tarn Plateau basaltic andesite

(HMBA, this study) and associated Glacier Creek andesite

(HMA, Baggerman and DeBari 2011), have high Mg#

([60), high MgO wt% ([7%) and higher Ni and Cr than

other lavas with similar Si contents. In addition, Sr/Y ratios

are higher in the high-Mg lavas from Shasta and Mount

Baker ([40) than in typical arc andesites/basaltic andesites

(\30, see Electronic Supplement 7). Their La/Yb ratios are

also high ([La/Yb]N [5), reflective of low heavy REE

(Fig. 7) with higher Dy/Yb than the other units.

One distinct feature of northern Cascade arc high-Mg

lavas (Mt. Baker and Glacier Peak) compared to Mt. Shasta

is that basaltic andesites have the highest Mg# and MgO

contents (Mg# 68–70; 52–54 wt% SiO2) rather than

andesites (Mg #63–64; *58% SiO2). The HMAs are

clearly differentiates of the HMBAs (Figs. 4, 5) and cannot

represent slab melts as has been suggested for HMAs in

some arcs (e.g., Kelemen et al. 2003). Mount Baker’s

Cathedral Crag basaltic andesites share the characteristic

trace element trends of the Tarn Plateau HMBA end-

member, but not the high-Mg character. They are most

likely low-Mg differentiates of that endmember.

Slab components responsible for magma generation

The extent to which each mafic flow unit at Mount Baker

has been affected by a subduction component can be

assessed by utilizing LILE/HFSE ratios (e.g., Ba/Nb),

proxies such as primitive-mantle-normalized Sr/P ratios

([Sr/P]N; Borg et al. 1997), and estimated H2O contents.

The HFSE and P are fluid-immobile and are left behind

when a hydrous phase is transferred from the slab (e.g.,

Brenan et al. 1994), whereas LILE (Rb, Sr, Ba, K) are fluid

mobile and are enriched in slab fluids. Figure 10a shows a

clear correlation between Ba/Nb and (Sr/P)N. The calc-

alkaline Lake Shannon and Sulphur Creek basalts appear to

have been least modified by a subduction component, while

Tarn Plateau high-Mg basaltic andesites and Cathedral

Crag derivatives appear to have been most modified by slab

flux. Interestingly, the lava with the most LKOT-like sig-

nature, Park Butte basalt, falls between the other units.

Calculated H2O contents also generally increase with (Sr/


123

P)N (Fig. 10b) and coincide with the endmember lava

types. These results are unusual, since LKOT lavas would

be expected to have the weakest slab flux signature. It is

evident that variable influence of the subduction compo-

nent is partially responsible for the distinct character of

each endmember magma type.

Modeling mantle and slab contributions

In this section, we model melting to match conditions in

the mantle wedge (mineralogy and % melting) that gen-

erate geochemical characteristics of the various endmem-

ber magmas at Mount Baker. In the first model, the source

for Park Butte LKOT and the calc-alkaline lavas (Lake

Shannon and Sulphur Creek) is assumed to be spinel

lherzolite (5% cpx). However, due to the strongly depleted

HREE character of the Tarn Plateau HMBA and Cathedral

Crag lavas, garnet must be present in their source. Garnet

can contribute to the depleted HREE signature of these

lavas by several possible mechanisms: melting of a mantle

source containing garnet (e.g., model of Borg et al. 1997;

supported by experiments of Gaetani and Grove 1998),

fluids/melt contributed from a subducting slab containing

garnet (e.g., Kelemen et al. 2003), or sub-Moho garnet

fractionation (e.g., Macpherson 2008). Since there is no

textural evidence that garnet fractionation has occurred in

any of the lavas of this study, we prefer either of the first

two options. However, given that we cannot distinguish

whether the garnet source is from the slab or the mantle, we

model both possibilities. Matching the partial melts that

result from these models against lava compositions tests

the validity of these hypotheses.

Equilibrium, or batch melting, is assumed in this model

because it is consistent with a fluid flux–induced melting

process (Grove et al. 2002). In this type of melting, the

partial melt continuously reacts and re-equilibrates with the

residual crystalline solid, and melt is segregated in a single

event. Modal melting (where minerals contribute elements

to partial melts in the same proportions as they appear in

the solid) is also assumed in this case to reduce the number

of unknown variables. We use the batch modal melting

equation defined by Shaw (1970) for determining trace

element concentrations at various melt fractions (F).

Trace element concentrations of depleted MORB mantle

(DMM) used in the model are from Workman and Hart

(2005) and partition coefficients were compiled from sev-

eral sources (Electronic Supplement 8). Partial melting was

modeled at various melt fractions to determine the best fit

to the most primitive REE pattern (middle to heavy REE)

from each magma type (sample NM-PB8 for the LKOT

type, samples NM-LS6 and NM-SC1 for the calc-alkaline

type, and sample NM-TP6 for the high-Mg basaltic

andesite). Best fit was determined by dividing the modeled

HREE concentrations by the measured HREE values from

each sample, with the best fit approaching modelHREE/

measuredHREE = 1 (Figs. 11, 12). Modeled compositions

and parameters are reported in Table 6.

The focus of this melt modeling is to most closely

approximate the middle and heavy REE compositional

patterns of the lavas, since these are dominantly mantle-

derived, whereas LREE are largely slab-derived. Thus, the

results presented in this step are expected to show poor

matches with LREE. This poor match will be resolved in

the next step where subduction fluids are added.

Melting of a spinel lherzolite mantle source with the

assemblage Ol ? Cpx ? Opx ? Spl in the proportions

76:5:15:4 can effectively model the average HREE pattern

of the LKOT-like lava (Park Butte) and the calc-alkaline

lavas (Lake Shannon and Sulphur Creek; Fig. 11). Melting

percentages required to produce these HREE abundances

are 4.2% for Sulphur Creek, 6% for Lake Shannon, and

10% for Park Butte. Variation in melt fraction is the most

logical explanation for the similarly shaped HREE pattern

at different HREE abundances seen in these three units;

however, a variably depleted mantle source cannot be ruled

0

20

40

60

80

100

120

Ba/

Nb


Cathedral Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Creek (CA)

Mt. Baker HMA

(a)

0

1

2

3

4

5

6

0 1 2 3 4 5 6

0 1 2 3 4 5 6

H2O

wt.

%

(Sr/P)N

(b)

Fig. 10 Trace element ratios and estimated H2O wt% versus

primitive-mantle-normalized Sr/P [(Sr/P)N]. Abbreviations and data

sources as in Fig. 3. a Ba/Nb versus (Sr/P)N. b Estimated H2O wt%

versus (Sr/P)N. Normalization values from Sun and McDonough

(1989). Estimated H2O wt% from Fig. 9 and for Sulphur Creek from

Shaw (2011)


123

out. Using enriched MORB (E-DMM of Workman and

Hart 2005) mantle instead of DMM increases calculated

melt fractions by only a small amount (\0.05%), but does

significantly change required slab flux (see Discussion

below).

Partial melting of a garnet lherzolite source with the

assemblage Ol ? Cpx ? Opx ? Grt in the proportions

73:5:20:2 at 7% melt fraction can effectively model the

average HREE pattern of the Tarn Plateau high-Mg

basaltic andesite (Fig. 12). Melting of a spinel lherzolite

source with the assemblage Ol ? Cpx ? Opx ? Spl in the

proportions 76:5:15:4 at 16% melt fraction produces a less

robust match for the majority of the HREE (Fig. 12).

Garnet, rather than spinel, is a necessary component in this

model to successfully reproduce the majority of the

observed HREE (the spinel melt only closely approximates

Yb and Lu, see Fig. 12b, c). Cathedral Crag lavas are not

included in this model since we assume that they are

derivatives of Tarn Plateau-like magmas after crustal

differentiation.

In the following discussion, we model the REE com-

position of the subduction component, following the

assumption that the trace element composition of the

magma is derived from only two components: a mantle

1

10

100

1000

10000


Sam

ple

/Ch

on

dri

te

PB Lava

LS Lava

SC Lava

PB Melt (F=0.10)

LS Melt (F=0.06)

SC Melt (F=0.042)

PB Fluid

LS Fluid

SC Fluid

Mt. Shasta Fluid 85-15


DMM

(a)

0.00.51.01.5

Dy

Ho Er

Yb Lu

CL/C

Ob

(c)

0.0

0.5

1.0

1.5

Dy

Ho Er

Yb Lu

CL/C

Ob

(d)

0.0

0.5

1.0

1.5

Dy

Ho Er

Yb Lu

CL/C

Ob

(b)

Fig. 11 Mantle partial melt and slab fluid models for Park Butte

(PB), Lake Shannon (LS), and Sulphur Creek (SC) lavas. See text for

discussion. a Chondrite-normalized REE abundances of representa-

tive lavas, modeled melt, and modeled slab fluids. Normalization

values from Sun and McDonough (1989), depleted MORB mantle

(DMM) from Workman and Hart (2005). Melts are from a depleted

spinel lherzolite source with the proportions Ol ? Cpx ? Opx ?

Spl = 76:5:15:4 at F = 0.10 for Park Butte, F = 0.06 for Lake

Shannon, and F = 0.042 for Sulphur Creek. Mt. Shasta calculated

fluids from Grove et al. (2002). Plots b–d are ratios of modeled melt

HREE abundances to measured HREE abundances to demonstrate

best-fit melt fraction for each unit. Fit is perfect where the ratio of

(CL)/(COb) or modeled abundance to observed abundance equals 1


123

partial melt and a hydrous subduction component. This

subduction component may either be a supercritical fluid or

a small degree, H2O-rich partial melt. Following the

method of Grove et al. (2002), a mass balance equation can

be used to calculate the concentration of each trace element

contributed by the subduction component and is defined as

Cf ¼ Clava � XmCmð Þ= Xfð Þ

where Cf is the concentration of the trace element con-

tributed by the subduction component, Clava is the con-

centration of the trace element in the lava (from Table 4),

Cm is the concentration of the trace element contributed by

the mantle partial melt (from melt modeling above), Xm is

the weight fraction of the lava derived from the mantle

melt, and Xf is the weight fraction of the lava that was

contributed by a subduction component. Results are

presented in Table 6. The H2O contents estimated by

Ca–Na exchange in plagioclase–liquid (Fig. 9) are used as

an approximation of Xf (2% for Lake Shannon and Sulphur

Creek, 3% for Park Butte, 4% for Tarn Plateau).

The REE abundances of the subduction component for

each of the units are illustrated in Figs. 11 and 12. They

display a broad range of compositions, but all are enriched

in LREE and depleted in HREE. The calc-alkaline lavas

are distinct in that their fluid components have much higher

overall REE abundances than both the LKOT and HMBA.

Adding the appropriate proportions (2–4%) of each of these

fluids to the corresponding melts of the mantle wedge

successfully produces the range of primitive compositions

seen at Mount Baker. Similar results are obtained when the

subduction flux model of Portnyagin et al. (2007) is used

(not shown).

1

10

100

1000

10000


Sam

ple

/Ch

on

dri

te

TP Lava

TP Garnet Source Melt (F=0.07)

TP Garnet Source Fluid

TP Spinel Source Melt (F=0.16)

TP Spinel Source Fluid



DMM

(a)

0.0

0.5

1.0

1.5

Dy

Ho Er

Yb

Lu

CL/

CO

b

Garnet Melt Fit(b)

0.0

0.5

1.0

1.5

Dy

Ho Er

Yb

Lu

CL/

CO

b

Spinel Melt Fit(c)

Fig. 12 Mantle partial melt and slab fluid models for Tarn Plateau

(TP) lavas. See text for discussion. a Chondrite-normalized REE

abundances of representative lava, modeled melt, and modeled slab

fluids. Normalization values from Sun and McDonough (1989),

depleted MORB mantle (DMM) from Workman and Hart (2005).

Melts are from a depleted garnet lherzolite source with the

proportions Ol ? Cpx ? Opx ? Grt = 73:5:20:2 at F = 0.07, and

a depleted spinel lherzolite source with the proportions Ol ? Cpx ?

Opx ? Spl = 76:5:15:4 at F = 0.16. Mt. Shasta calculated fluids

from Grove et al. (2002). Plots b, c are ratios of modeled melt HREE

abundances to measured HREE abundances to demonstrate best-fit

melt fraction for each unit. Fit is perfect where the ratio of (CL)/(COb)

or modeled abundance to observed abundance equals 1


123

An important caveat to the model results presented in

Figs. 11, 12 is that we have assumed that both LKOT-like

lavas and calc-alkaline lavas come from a similarly

depleted source (DMM). If we instead assume that calc-

alkaline lavas come from a more enriched mantle source

(E-DMM of Workman and Hart 2005) than the LKOT-like

lavas, then the calculated subduction components are less

enriched and more comparable to that calculated for the

LKOT-like lavas (Electronic Supplement 9). Given the

data we have, we cannot distinguish between these models.

Another important point is that the HMBA lavas require

some residual garnet for their generation, either in the

mantle wedge as a garnet lherzolite source (producing an

HREE-depleted mantle melt) or in the slab (producing an

HREE-depleted subduction component). The models pre-

sented cannot distinguish between them. However, if one

magma type at Mt. Baker requires a strongly HREE-

depleted subduction flux, why isn’t this flux evident in all

the lavas? If we assume a garnet lherzolite source, the

subduction component is similar in HREE character to that

of the other lavas (cf. Figures 11, 12) as is the degree of

melting (7% as compared to 16%). It seems more plausible

to envision a mantle wedge comprising both spinel lherz-

olite (shallow) and garnet lherzolite (deep) producing

similar degree partial melts with variable HREE than a

subducting slab producing such diverse fluid components.

Summary model of mafic magma evolution at Mount

Baker

The schematic illustration in Fig. 13 summarizes one

possible scenario for the evolution of mafic magmas at

Mount Baker. As the Juan de Fuca plate subducts, varying

degrees and compositions of a subduction component

infiltrate the mantle wedge beneath the volcano. This slab

flux assists in melting two distinct mantle regions: spinel

lherzolite and garnet lherzolite. This melting occurs at

various fractions, and the resulting melts stall at different

levels in the crust as they ascend. Differentiation occurs in

the crust at a range of pressures and to differing degrees,

producing magmas with diverse chemistry, resulting in the

three endmember lava types observed at Mount Baker:

LKOT-like, calc-alkaline, and high-Mg basaltic andesite.

LKOT-like lavas are the result of the highest degrees of

partial melting from a spinel lherzolite source. However,

they are atypical of LKOTs from elsewhere in the arc due

to their greater slab signature (both H2O content and trace

element characteristics). This may be due to the lack of an

active back-arc spreading system at this latitude. Their

distinct compositional characteristics compared to calc-

alkaline lavas can be attributed either to a more depleted

mantle source with a compositionally similar slab flux

(Electronic Supplement 9) or to a similar mantle source

Table 6 Modeled mantle partial melt and slab fluid REE compositions

DMM Spinel source Garnet source Fluid composition

4.2% 6% 10% 16% 7% SC LS PB TP Sp TP Grt

La 0.192 3.77 2.79 1.77 1.14 2.43 650.6 437.9 196.8 281.1 250.3

Ce 0.550 10.24 7.69 4.95 3.23 6.68 1,505.1 950.2 429.0 608.3 525.5

Pr 0.107 1.86 1.42 0.93 0.62 1.24 187.3 116.4 52.8 80.6 65.7

Nd 0.581 9.69 7.48 4.97 3.31 6.23 737.4 448.5 211.6 324.5 254.4

Sm 0.239 3.48 2.78 1.91 1.30 2.33 130.1 69.9 35.9 59.1 34.5

Eu 0.096 1.27 1.03 0.73 0.51 0.85 36.6 24.1 13.1 17.2 9.0

Gd 0.358 4.28 3.55 2.57 1.82 2.71 103.5 57.2 27.1 42.2 20.9

Tb 0.070 0.80 0.67 0.49 0.35 0.50 12.8 7.0 3.5 5.4 1.9

Dy 0.505 5.34 4.53 3.38 2.45 3.07 60.9 32.2 12.1 24.3 9.4

Ho 0.115 1.13 0.97 0.74 0.54 0.63 10.7 6.3 2.2 4.1 2.0

Er 0.348 3.09 2.69 2.09 1.57 1.74 26.5 13.5 4.6 7.2 3.1

Yb 0.365 2.82 2.51 2.01 1.54 1.51 20.6 9.7 3.0 2.8 3.6

Lu 0.058 0.46 0.41 0.32 0.25 0.25 2.4 0.9 0.4 0.3 0.3

Xf – – – – – 2.0% 2.0% 3.0% 4.0% 4.0%

Xm – – – – – 98.0% 98.0% 97.0% 96.0% 96.0%

Depleted MORB mantle (DMM) starting composition from Workman and Hart (2005)

PB Park Butte basalt, TP Tarn Plateau basaltic andesite, LS Lake Shannon basalt, SC Sulphur Creek basalt. TP Sp and TP Grt are calculated

fluids based on spinel (16%) and garnet melts, respectively. Modeled compositions of partial melts (4.2, 6, 10, and 16%) from spinel lherzolite

(‘‘spinel source’’) are from a mantle source with Ol ? Cpx ? Opx ? Spl in the proportions 76:5:15:4. Modeled composition of partial melt (7%)

from garnet lherzolite (‘‘garnet source’’) are from a mantle source with Ol ? Cpx ? Opx ? Grt in the proportions 73:5:20:2. Xf is the weight

fraction of lava contributed by fluid in the fluid composition model and is based on estimated H2O content of the Mt. Baker mafic magmas (see

Fig. 9). Xm is the weight fraction of lava contributed by the mantle melt in the fluid composition model


123

fluxed by a less enriched subduction component (Fig. 11).

This latter model is comparable to recent studies in the

central Oregon Cascades (Rowe et al. 2009), where melt

modeling suggested that a fertile mantle source (more

enriched than depleted MORB) can produce LKOT, calc-

alkaline, and OIB endmember primitive magmas, and

called upon variability in a subduction component to pro-

duce the different endmembers.

Flow age correlation with mantle and slab indicators

There is no correlation between age of the units and

petrogenetic indicators such as Sr/Y and (Sr/P)N (Fig. 14).

The LKOT-like Park Butte is the oldest unit and has

intermediate values of both mantle- and slab-influenced

trace element ratios. The Tarn Plateau and Cathedral Crag

basaltic andesites are intermediate in age among the units,

but have the highest values of these ratios (Fig. 14). The

Lake Shannon and Sulphur Creek lavas are youngest and

have the lowest values. This lack of correlation suggests

that different mantle and slab sources are being tapped

over the life of the Mount Baker volcanic field. Initially, a

depleted spinel lherzolite mantle with a moderate sub-

duction component was responsible for the generation of

mafic magma (LKOT-like Park Butte). Over time, an

additional mantle source with residual garnet and a larger

contribution from a subduction component became the

primary input for magma petrogenesis (Tarn Plateau

HMBA and Cathedral Crag basaltic andesite). This source

was tapped as recently as 14,000 years ago with the

eruption of the high-Mg andesites of Glacier Creek

(Baggerman and DeBari 2011). Spinel lherzolite contin-

ued to contribute to magma generation in the Lake

Shannon and Sulphur Creek units in the last

100,000 years, but with the lowest degree of slab input

over the life of mafic volcanism (Fig. 14). This suggests

that both mantle sources are being tapped simultaneously

at different depths in the mantle wedge beneath Mount

Baker, with variable amounts and compositions of slab

input.

Fig. 13 Schematic illustration of mafic magma evolution at Mt.

Baker. Mafic magmas are generated at various melt fractions when

slab fluid flux contributes to melting of mantle with different residual

mineralogy: a depleted spinel lherzolite and a depleted garnet

lherzolite. The resulting melts stall at different levels in the crust as

they ascend, where differentiation occurs at a range of pressures and

to varying degrees. This produces magmas with diverse chemistry,

resulting in the three endmember lava types observed at Mount Baker.

Depth estimates from McCrory et al. (2004), Robinson and Wood

(1998), and Thompson (1975)

0

20

40

60

80

Sr/

Y


Cathedral Crag (BA)

Tarn Plateau (HMBA)

Lake Shannon (CA)

Sulphur Creek (CA)

Mt. Baker HMA

0

1

2

3

4

5

6

0 200 400 600 800

0 200 400 600 800

(Sr/

P) N

Age (ka)

Fig. 14 Sr/Y and (Sr/P)N versus flow unit age. Symbols and

abbreviations as in Fig. 3. Mt. Baker HMA from Baggerman and

DeBari (2011). Ages from Hildreth et al. (2003)


123

Summary and conclusions

The chemical and petrologic characteristics of five mafic

lava units from the Mount Baker volcanic field in the

northern Cascade arc reveal a diversity of near-primitive

compositions that require distinct mantle sources and

varying subducting slab influence to explain their petro-

genesis. Three distinct endmember magma types are rep-

resented that cannot be related by fractional crystallization

or other crustal processes. These include LKOT-like, calc-

alkaline, and high-Mg basaltic andesite.

The LKOT-like basalt of Park Butte is the result of partial

melting of a spinel lherzolite mantle that was moderately

fluxed by a subduction component. The calc-alkaline basalts

of Lake Shannon and Sulphur Creek may have been derived

from the same mantle source (DMM) or a more enriched one

(E-DMM), but at lower degrees of partial melting (discerned

from higher trace element abundances). They also have a

smaller contribution from the subducting slab (as evidenced

by lower H2O contents and lower values of slab fluid indi-

cators such as Ba/Nb and [Sr/P]N).

High-Mg basaltic andesite (HMBA) of Tarn Plateau was

likely derived by partial melting of a garnet lherzolite

mantle source that had the highest flux of subduction

component (highest H2O contents and [Sr/P]N). However,

we cannot rule out a spinel lherzolite mantle source with a

subduction component that is more HREE-depleted than

that required for the other lava types. Other units on Mount

Baker with similar trace element patterns must be derived

from this same source (e.g., high-Mg andesite of Glacier

Creek from Baggerman and DeBari (2011) and basaltic

andesite of Cathedral Crag). High-Mg andesites at Mt.

Baker have lower MgO and Mg# than high-Mg basaltic

andesite and are fractionated versions of the latter.

The age of the various endmembers lacks correlation

with petrogenetic indicators. This suggests that both mantle

sources have been tapped simultaneously throughout the

history of mafic volcanism in this region and that the

mantle has been variably fluxed by a subduction compo-

nent over time as well.

Acknowledgments Funding for this project was provided by the

Geological Society of America Parke D. Snavely, Jr. Cascadia

Research Award, the USGS Cascades Volcano Observatory Kleinman

Grant for Volcano Research, and Western Washington University.

The manuscript benefited from reviews by Pete Stelling, Elizabeth

Schermer, and two anonymous reviewers.

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