calibration of andesitic whole-rock trace-element ... · calibration of andesitic whole-rock...

CALIBRATION OF ANDESITIC

WHOLE-ROCK COMPOSITIONS

AGAINST MOHO DEPTH

Glen McIlwain

October 2003

Thesis submitted as partial fulfilment for requirement of the Bachelor of Science

(Honours) degree, Discipline of Geology, University of Newcastle.

ACKNOWLEDEMENTS

Firstly, I would like to thank my supervisor Bill Collins for an exciting project and

valuable assistance throughout the year.

To my fellow honours students who made this year enjoyable, especially to Gavin

Mantle where we formed a great team in field of New Zealand and in the lab.

Finally, to the Geology department staff who made this possible.

i

TABLE OF CONTENTS

LIST OF FIGURES .........................................................................................................iii

LIST OF TABLES ...........................................................................................................vi

LIST OF TABLES ...........................................................................................................vi

ABSTRACT....................................................................................................................vii

CHAPTER 1 INTRODUCTION, BACKGROUND & AIMS ...................................1

1.1 Introduction.......................................................................................................1

1.2 Background .......................................................................................................2

1.2.1 Petrogenesis of Andesitic Magmas ...........................................................2

1.2.2 Geochemical Theory of Andesitic Magmas..............................................4

1.2.2.1 Solid Residue and Melt Products Formed at the Moho ........................4

1.2.2.2 Pressure Controls on the Stability of Minerals .....................................4

1.2.2.3 H2O Concentration Controls on the Stability of Minerals ....................5

1.2.2.4 Temperature Controls on the Stability of Minerals ..............................6

1.2.2.5 Trace Elements in Minerals ..................................................................6

1.2.2.6 Calcium, Sodium and Strontium Partitioning in Minerals....................7

1.2.2.7 Base Level geochemical signature of andesitic magmas ......................8

1.3 Aims and Objectives .........................................................................................9

CHAPTER 2 APPROACH AND METHODS .........................................................10

2.1 Approach.........................................................................................................10

2.2 Methods...........................................................................................................10

2.2.1 Collection of Geochemical Data and Moho Depths ...............................10

2.2.2 Geochemical Trends................................................................................11

2.2.3 Andesitic Standardisation........................................................................11

2.2.4 Proxy Ratios versus Moho Depth ...........................................................11

2.2.5 Field, Sample preparation and Lab Work ...............................................11

ii

CHAPTER 3 VOLCANO LOCATIONS AND GENERAL PROPERTIES............13

3.1 Arc Volcanoes of the World ...........................................................................13

3.2 Moho Depths...................................................................................................14

3.3 Volcano Properties ..........................................................................................15

3.3.1 Major Element Discrimination Diagrams ...............................................15

3.3.2 Isotopic Ratio Correlations .....................................................................19

3.3.3 Temperature and H2O Content of Andesitic Magmas ............................21

3.3.4 MORB Spider Diagrams for Each Volcano............................................22

CHAPTER 4 PROXY RATIOS VERSUS MOHO DEPTH AND CHONDRITE-

NORMALISED SPIDER-DIAGRAMS .........................................................................27

4.1 Introduction.....................................................................................................27

4.2 Proxy Ratios versus Moho Depth ...................................................................27

4.2.1 Sr/Y versus Moho Depth (Figure 16) .....................................................27

4.2.2 LREE/HREE or Y versus Moho Depth (Figures 13, 14 & 15)...............30

4.2.3 Ca/Sr versus Moho depth (Figure 20).....................................................30

4.3 Chondrite-Normalised REE+Sr+Y Spider-Diagrams.....................................32

4.4 Anomalous Groups .........................................................................................33

CHAPTER 5 DISCUSSION .....................................................................................35

5.1 Shoshonites .....................................................................................................35

5.2 Effect of H2O...................................................................................................35

5.3 H2O Andesite Categories ................................................................................38

5.4 Effect of P-T and H2O on the Residual Mineral Assemblage.........................39

CHAPTER 6 ADAKITES, PRODUCTS OF SLAB MELT?...................................43

CONCLUSIONS.............................................................................................................45

REFERENCES................................................................................................................48

APPENDIX A GEOCHEMICAL RATIO PLOTS .....................................................53

iii

LIST OF FIGURES

Figure 1. Schematic cross section of an arc subduction zone, showing the dehydration

of the subduction slab (grey), hydration and melting of the mantle wedge (purple),

underplating of the mantle derived melts at the base of the crust and the MASH

zone (yellow). Figure from Winter (2001)................................................................3

Figure 2. P-T diagram for picritic tholeiite (anhydrous, 2.5-3% H2O and H2O saturated)

showing mineral phase relations of plagioclase, hornblende and garnet. Redrawn

from Loucks & Ballard (2002)..................................................................................5

Figure 3. Partition Coefficients for REEs + Sr + Y between minerals and hydrous

basaltic melt at 10 kbar. Data from Loucks & Ballard (2002)..................................7

Figure 4. Chondrite-normalised REEs+Sr+Y plots that show the different dominant

residual mineral phases for andesites from three volcanoes. Mt Shasta (green)

(Grove et al. 2002), Adatara (blue) (Fujinawa 1992) and Tata Sabaya (red) (De

Silva et al 1993). .......................................................................................................9

Figure 5. Map of the world showing active volcano locations (red triangles) and the

locations of the studied volcanoes and Moho depth within each volcanic arc.

Adapted from (Simkin & Siebert 2002)..................................................................13

Figure 6. Histogram showing the Moho depth beneath each volcano and the type of

setting each one belongs to. Arc setting classification from Gill (1981). ...............14

Figure 7. Total alkalies-silica diagram using IUGS volcanic rock-type classifications.

Symbols represent different ranges of Moho depth except for the high-K

shoshonitic rocks that have been grouped together. ...............................................17

Figure 8. K2O-silica andesitic types diagram. Symbols represent different ranges of

Moho depth except for the high-K shoshonitic rocks that have been grouped

together. Divisions from Gill (1981).......................................................................18

iv

Figure 9. Andesitic K2O-FeO*/MgO diagram showing dicriminations between calc-

alkaline and tholeiitic series, and between low-k, medium-K, high-K and

shoshonitic rocks with 60% SiO2. Discrimation lines from Gill (1981). Equations:

FeO*/MgO = 0.1562 x SiO2 - 6.685; K2O = 0.145 x SiO2 - 5.135; K2O = 0.0818 x

SiO2 - 2.745; K2O = 0.0454 x SiO2 - 1.864. ...........................................................19

Figure 10. Sr-Nd isotope ratio correlation diagram. Showing mid-ocean ridge basalt

(MORB) and primordial bulk silicate Earth (BSE) regions....................................20

Figure 11. 87Sr/86Sri-silica diagram...........................................................................21

Figure 12. MORB spider diagrams for White Island (Whakaari) 17.5 km, Ruapehu

22.5 km, Santorini 23 km, Campi Flegrei 25 km, Egmont (Taranaki) 30 km,

Atacazo 32.5 km, Puyehue 32.5 km & Pinatubo 32.5 km ......................................23

Figure 13. MORB spider diagrams for Parícutin 33 km, Bakening 34 km, Taal 34

km, Vesuvius 35 km, Mt Shasta 35 km, Adatara 35 km, Nekoma 35 km and

Towada 35 km.......................................................................................................24

Figure 14. MORB spider diagrams for San Pedro-Pellado 37.5 km, Ichinsky 38 km,

Galeras 40 km, Chichinautzin 42.5 km, San Jeronimo 45 km, Cerro Tuzgle 45 km,

Quimsacocha 50 km & Sangay III 50 km...............................................................25

Figure 15. Morb spider diagrams for Cerro Leon Muerto 55 km, Antisana 60 km,

Nevado Solimana 62.5 km, Quillacas 65 km, Ollagüe 65 km, Parinacota 70 km &

Tata Sabaya 70 km..................................................................................................26

Figure 16. Sr/Y versus Moho depth diagram for average andesite composition

(60% SiO2). .............................................................................................................29

Figure 17. La/Yb versus Moho depth diagram for average andesite composition

(60% SiO2). .............................................................................................................29

v

Figure 18. La/Y versus Moho depth diagram for average andesite composition

(60% SiO2). .............................................................................................................31

Figure 19. Ce/Y versus Moho depth diagram for average andesite composition

(60% SiO2). .............................................................................................................31

Figure 20. Ca/Sr versus Moho depth diagram for average andesite composition

(60% SiO2). .............................................................................................................32

Figure 21. Chondrite-normalised REE+Sr+Y spider-diagrams divided into groups

of similar shapes and slopes. (A) plagioclase + hornblende signature, (B) garnet

signature, (c) plagioclase signature, (D) hornblende signature, (E) shoshonitic, (F)

Garnet + Plagioclase. ..............................................................................................34

Figure 22. Ca/Sr versus La/Yb diagram, representing changes in residual

plagioclase versus hornblende and garnet with respect to Moho depth..................37

Figure 23. La/Sm versus La/Yb diagram representing changes in dominant residual

minerals and H2O. ...................................................................................................37

Figure 24. Sr/Ce (Sr anomaly) versus La/Yb (pressure signature) diagram, showing

H2O Andesite category divisions. ...........................................................................38

Figure 25. P-T diagrams for average, high and low H2O system showing six

volcanoes that represent H2O andesite categories from section 5.2. P & T

conditions are measured for all the volcanoes, except White Island and Ollagüe. In

the White Island case, the T field represents the typical range for andesites,

whereas with Ollagüe, the T field is an estimate for a thick crust, dry system.......40

vi

LIST OF TABLES

Table 1. Volcano Locations, Moho depth, Properties and Classifications. ..................16

Table 2. Major elements, REE, Sr, Y and proxy ratios for each volcano averaged to

60% SiO2.................................................................................................................28

vii

ABSTRACT

An analysis of the global database of volcanoes, for which Moho depth is known, was

undertaken to determine if a relation exists between andesitic composition and crustal thickness.

One thousand and sixty seven samples from 31 active volcanoes from 11 volcanic arcs were

examined, where the Moho depths ranged from 17 km to 70 km. With increasing Moho depth,

chondrite normalised spider diagrams become progressively steeper, which is best quantified by

increasing LREE/HREE ratios. Only shoshonites and some low-K calc-alkaline andesites do not

follow this trend.

The correlation of composition with Moho depth is best explained if it is considered

andesites acquire their geochemical character at the Moho, which supports the MASH

hypothesis of Hildreth & Moorbath (1988). At the Moho, the composition of andesites is

controlled by the major residual minerals, plagioclase, hornblende and garnet. At shallow Moho

depth (<30 km), plagioclase is the dominant mineral, at intermediate depth it is hornblende, and

at greater depth (>45 km) it is garnet. The geochemical signature of andesitic magmas formed at

the Moho within volcanic arc environments shows a characteristic mirror image of the dominant

residual minerals, when plotted on Chondrite-normalised REE+Sr+Y graphs. Therefore, the

elements that partition within these three minerals can be used as proxies for Moho depth. The

Sr/Y, La/Yb, La/Y and Ce/Y ratios are particularly useful in this regard.

H2O content affects the residual mineral assemblage. For a given pressure, increasing

H2O content destabilises plagioclase whereas hornblende and garnet becomes increasingly

stable. This effect causes the LREE/HREE ratio to be modified. Andesites with high H2O

content (>4%) crystallise hornblende at an early stage, whereas relatively anhydrous andesites

(<1%) crystallise plagioclase at an early stage. Garnet crystallises at much greater depths in

anhydrous systems (40 km versus ~65 km for anhydrous versus “average” andesites with 1-4%

H2O). The different andesite categories (low-, average-, high-H2O) can be distinguished on

Sr/Ce versus La/Yb plots. This effect must be considered when applying HREE/LREE or Sr/Y

to establish Moho depths.

Based on this work, it appears that the composition of andesites is independent of

whether it formed either by, (1) differentiation from basaltic arc magmas, (2) partial melting of

under-plated basaltic arc magmas, or other crustal material, or (3) mixing of these crustal and

mantle derived magmas. Adakites are formed at the Moho under unusually high-H2O contents

and/or high-pressure conditions. Finally, the data suggest that the mantle lithosphere beneath

arcs is very thin or non-existent.

Chapter 1 Introduction, Background & Aims

CHAPTER 1 INTRODUCTION, BACKGROUND & AIMS

1

1.1 Introduction

In the past, geologists have looked at the structural evolution of the crust to

qualitatively determine crustal thickness variations in orogenic systems, but the

composition of andesitic magmas may also show this variation.

A global compilation of active volcanoes (Plank & Langmuir 1988) shows that

Mohorovičić discontinuity (Moho) depths beneath arcs varies between 15-70 km. The

Moho represents the boundary where the velocity of seismic P-waves increases abruptly

from 7 km per second to 8 km per second for the deeper rocks. The velocity of 8 km per

second indicates that the deeper rocks are probably dense ultramafic rock, peridotite,

whereas the above rocks are the less dense buoyant gabbro (Best 2003). Thus, the Moho

is interpreted to reflect the boundary between the lower crust and the mantle and hence

the thickness of the crust.

Most workers consider that andesites form at the base of the crust, either by (1)

differentiation from basaltic arc magmas, (2) partial melting of under-plated basaltic arc

magmas, or other crustal material, or (3) mixing of these crustal and mantle derived

magmas (e.g.Tatsumi & Eggins 1995). These processes occur at varying depth at the

base of arcs, either within the garnet stability field (>50 km), hornblende stability field

(30-50 km), or the plagioclase stability field (<30 km) (e.g. Kay & Mpodozis 2001).

The Sr/Y and La/Yb ratios are particularly sensitive to the presence of these minerals,

so it is possible that such ratios can be used to calibrate the Moho depth to andesitic

Chapter 1 Introduction, Background & Aims 2

compositions. This approach could be used to monitor the long-term crustal thickness

variations in accretionary orogenic systems, which has not been attempted before.

1.2 Background

1.2.1 Petrogenesis of Andesitic Magmas

Within accretionary orogens, subduction zones are formed when two tectonic

plates converge (Figure 1). Subduction zones have three main components: (1) a

relatively, cold and dense subducting oceanic plate; (2) overriding buoyant continental

plate; (3) a mantle wedge between the two. The crust of the oceanic plate contains a

large proportion of hydrous minerals, which have formed during hydration and

greenschist facies metamorphism at the mid-ocean ridge. As the oceanic slab is

subducted, the crust dehydrates and releases H2O into the mantle wedge to form a

hydrous peridotite layer, which is dragged down by the subducting slab. When a depth

of ~110 km is reached, H2O is released from this layer by pressure-sensitive

dehydration reactions (Tatsumi & Eggins 1995). The influx of free H2O into the mantle

wedge causes partial melting and initiates rise of a melt column, which undergoes

further melting during decompression in the mantle wedge peridotite. The resultant

partially melted product a hot (>1100oC), dense, hydrous basalt, which ponds at and

commonly underplates the base of the overlying (more buoyant) crust, with only a small

proportion of magma rising to higher crustal levels (Tatsumi & Eggins 1995). At the

base of the crust (the Moho), combined processes of crustal partial melting,

assimilation, storage, and homogenisation (MASH) takes place to produce andesitic

magmas with a “base level” geochemical signature reflecting equilibration with residual

3 Chapter 1 Introduction, Background & Aims

minerals which form at specific Moho depths (Hildreth & Moorbath 1988). Over time,

these MASH zone magmas become buoyant as they become more silica rich, and may

ascend to upper levels of the crust when they experience shallow fractional

crystallisation, magma mingling and wall rock contamination. Such processes will

change the concentration of many elements and the magmas will evolve to more silicic

compositions. For this reason, this work is restricted to examination of intermediate-

composition rocks, between 57-63% SiO2.

Figure 1. Schematic cross section of an arc subduction zone, showing the dehydration of the

subduction slab (grey), hydration and melting of the mantle wedge (purple), underplating of the

mantle derived melts at the base of the crust and the MASH zone (yellow). Figure from Winter

(2001).


1.2.2 Geochemical Theory of Andesitic Magmas

1.2.2.1 Solid Residue and Melt Products Formed at the Moho

At the Moho within volcanic arcs, three processes may be operating to control

the “base level” geochemical signature of andesitic magmas: partial melting of the

lower crust; fractional crystallization of under-plated hydrous mantle derived mafic

magmas; and magma mixing (Winter 2001). Rocks partially melt to form two

components, a buoyant silicic melt and a dense solid residue. The silicic melt is formed

where low temperature minerals are initially melted (eg. biotite), whereas the solid

residue consists of stable high temperature minerals (eg. pyroxene). In contrast, during

fractional crystallization the stable minerals drop out of the melt and become part of the

solid residue. The elemental abundances within the melt and residue component are

mirror images to each other, where the elements that concentrate in the residue are

depleted from the melt. The mineral composition of the residue is dependent on the

environment (temperature, pressure and H2O content), which therefore controls the

“base level” geochemical signature within the magma.

1.2.2.2 Pressure Controls on the Stability of Minerals

Different minerals are stable at different pressures (e.g. Kay & Abbruzzi 1996).

Changes in mineral stability change the crystallization or partial melting order. For

example, with increasing pressure plagioclase becomes increasingly unstable, whereas

hornblende and garnet become increasingly stable. Therefore, plagioclase diminishes in

modal abundance and eventually vanishes from the mineral assemblage of the solid

residue with increasing pressure, whereas the modal abundances of hornblende and

garnet increases. This can be seen in Figure (2). By monitoring the modal abundance of


the minerals within the solid residue, through systematic changes in trace element

ratios, the depth of crystallisation might be determined.

Figure 2. P-T diagram for picritic tholeiite (anhydrous, 2.5-3% H2O and H2O saturated)

showing mineral phase relations of plagioclase, hornblende and garnet. Redrawn from Loucks &

Ballard (2002).

1.2.2.3 H2O Concentration Controls on the Stability of Minerals

Increasing the H2O concentrations has similar effects of increasing pressure on

the crystallisation order and stability of minerals (Figure 2). High H2O contents (>3

wt%) (Müntener et al. 2001) suppresses plagioclase and enhances the crystallisation of

hornblende and garnet, to produce a corundum normative andesitic melt. Low H2O (<1


wt%) concentrations stabilises plagioclase earlier than garnet and hornblende, so the

melt retains its quartz normative character (Müntener et al. 2001).

1.2.2.4 Temperature Controls on the Stability of Minerals

Changes in temperature of andesitic magmas have a minimal effect on the nature

of the residual mineral assemblage when compared to pressure and H2O content. This is

because the majority of andesites have a narrow pre-eruption temperature range

between ~1000 and 1100 °C (Gill 1981).

1.2.2.5 Trace Elements in Minerals

Different minerals concentrate different proportions of trace elements within

their crystal structure. The major residual minerals in andesites (plagioclase, hornblende

and garnet) all show contrasting trace elemental abundance characteristics, as shown in

the partition coefficient plots (Figure 3). The plagioclase plot shows a flat gradient,

large strontium (Sr) and europium (Eu2+) positive anomalies, but with low overall

concentrations of rare earth elements (REE) and yttrium (Y). The REE and Y are more

strongly partitioned within hornblende, and a small negative Sr anomaly exists. The

concave-down curved pattern maximising at the middle heavy rare earth element region,

is a mirror image of the plagioclase curve. Garnet shows a steep gradient, with the

heavy rare earth elements (HREE) strongly partitioned and the LREE strongly excluded

from the mineral lattice. The remaining major rock forming minerals (eg. pyroxene ±

olivine) have flat patterns and very low partition coefficients in andesites (Best 2003).

This indicates that these other basalt-hosting minerals do not influence the behaviour of

REE and Sr during andesite petrogenesis.


Figure 3. Partition Coefficients for REEs + Sr + Y between minerals and hydrous basaltic

melt at 10 kbar. Data from Loucks & Ballard (2002).

1.2.2.6 Calcium, Sodium and Strontium Partitioning in Minerals

Increasing pressure and/or H2O content of a magma causes crystallising

plagioclase to become more calcic and less sodic (Blundy & Shimizu 1991). Sr2+ is

more strongly partitioned into sodic plagioclase than calcic plagioclase, because the

relatively weak Na-Al bonds give the albite lattice the flexibility to accommodate the

large Sr ion (Best 2003).

The decreasing partitioning of Sr within hornblende and garnet can be explained

by the following mineral reactions where the plagioclase component breaks down to

form these minerals:

2((Ca,Na) Al2Si2O8) + 3(MgSiO3) → (Na,Ca)2Mg3Al4SiO6O22(OH)2 + SiO2

Plagioclase (An > Ab) + OPX → Hornblende + Quartz


CaAl2Si2O8 + (Ca,Mg)2Si2O6 → (Ca,Mg)3Al2Si3O12 + SiO2

Anorthite + CPX → Garnet + Quartz

Therefore, since the increasing role of calcic plagioclase in these reactions the

partitioning of Sr in hornblende and garnet would decrease. This also strongly implicit

on the Ca/Sr ratios of the melt, and decreasing Ca/Sr ratios in andesites would indicate

higher pressure and/or H2O content.

1.2.2.7 Base Level geochemical signature of andesitic magmas

The geochemical signature of andesitic magmas formed at the Moho within

volcanic arc environments shows a characteristic mirror image of the dominant residual

minerals. This is regarded as their “base level” signature and can be seen in Figure (4),

where three contrasting chondrite-normalised plots display mirror images to the

partition coefficient plots shown in Figure (3). Therefore, the composition of the

andesitic magmas appears to reflect the presence of the pressure-dependent residual

mineral assemblage. This compositional variation is well characterised by the REE, Sr

and Y.

It therefore might be possible to determine the stable mineral phases during

formation of most andesite magmas. By using contrasting elemental ratios that are

controlled by these minerals, the depth to the Moho could be determined. For example,

andesites formed under thick crustal conditions (>50 km) show a mirror image of garnet

(eg. high Sr/Y, La/Yb), whereas andesites formed during thin crust conditions (<30 km)


show a mirror image of plagioclase or a mixture of plagioclase and hornblende (eg. low

Sr/Y, La/Yb) (Kay & Mpodozis 2001).

Figure 4. Chondrite-normalised REEs+Sr+Y plots that show the different dominant

residual mineral phases for andesites from three volcanoes. Mt Shasta (green) (Grove et al. 2002),

Adatara (blue) (Fujinawa 1992) and Tata Sabaya (red) (De Silva et al 1993).

1.3 Aims and Objectives

• Given the theoretical considerations outlined above, the aim of this thesis is to

determine if a relation exists between andesite composition and Moho depth

from active volcanoes.

• The objective is then to calibrate the trace element ratios of andesites with

Moho depth. If successful, a proxy indicator for crustal thickness will exist,

which can be applied to those orogens that contain abundant intermediate-

composition rocks.

Chapter 2 Approach And Methods

CHAPTER 2 APPROACH AND METHODS

10

2.1 Approach

The approach is to compile a database using existing whole-rock geochemical

data from active volcanoes from around the world, where Moho depths have been

determined. Properties (geochemistry, isotopic characteristics, location and type) for

each volcano were studied to investigate whether the volcanos can be grouped into

similar petrological types. This database was used to determine which geochemical

parameters vary consistently with Moho depth.

2.2 Methods

2.2.1 Collection of Geochemical Data and Moho Depths

Before any volcanic arc whole-rock trace-element geochemical data was entered

into a database, the Moho depth and the age of the rocks had to be known. The age of

the rocks was restricted to <1 Ma, because the thickness of the crust can rapidly change

over a short period of time (Gutscher et al. 2000). Only those volcanoes with at least

five analysis of andesite composition were considered, to avoid analytical bias. The

majority of the data was collected from journals and a large proportion is stored in the

web-based databank GEOROC (http://georoc.mpch-mainz.gwdg.de). All volcanoes that

contained andesites, where Moho depths were known, were included in this study.

Chapter 2 Approach And Methods 11

2.2.2 Geochemical Trends

For each volcanic dataset, graphs of element abundance ratios were plotted

against SiO2 to investigate what petrological processes were involved in their genesis.

These processes could be fractional crystallisation, partial melting of the lower crust, or

mixing of both components. 143Nd/144Ndi versus 87Sr/86Sri isotopic ratios were also

plotted to determine if multiple source components were involved in magma genesis.

2.2.3 Andesitic Standardisation

For the purposes of direct comparison of data, the elemental compositions for

each volcano were restricted to andesitic compositions (57-63% SiO2) and then

averaged to 60% SiO2. These averaged elemental compositions were plotted as

chondrite-normalised spider-diagrams to compare data from each volcano.

2.2.4 Proxy Ratios versus Moho Depth

Trace element ratios were plotted against Moho depth and those with the highest

degree of correlation were identified for each volcanic suite. These were the values used

to calibrate andesite composition against Moho depth, and thus can be considered

“proxy” ratios of crustal thickness. Finally, proxy values were calculated for each

Moho.

2.2.5 Field, Sample preparation and Lab Work

A major part of my honours requirement involved collection and processing of

~150 samples of basaltic dykes from the South Island of New Zealand, with Gavin

Chapter 2 Approach And Methods 12

Mantle. Processing involved crushing and pulverising samples, producing XRF disks

and preparing rock samples for thin section. This aspect of the work provided

experience in field and laboratory work that would have been otherwise lacking in this

thesis.

Chapter 3 Volcano Locations And General Properties

CHAPTER 3 VOLCANO LOCATIONS AND GENERAL

PROPERTIES

13

3.1 Arc Volcanoes of the World

Figure 5. Map of the world showing active volcano locations (red triangles) and the locations

of the studied volcanoes and Moho depth within each volcanic arc. Adapted from (Simkin &

Siebert 2002)

Thirty-one active volcanoes from eleven different arcs around the world were

studied in this thesis. It can be seen in Figure (5) that the majority of volcanoes are

located within the Pacific Rim, with only three from Aeolian and Aegean arcs of

Europe. The Andean continental arc system of South America is highly represented

because this is where Moho depths vary most, and where a large body of geochemical

data exists. The thirty one volcanoes studied is only a small proportion of the >1500

volcanoes that have erupted during the past 10,000 years (Gill 1981). This restriction is

14 Chapter 3 Volcano Locations And General Properties

due to the lack of geochemical analyses of rocks with an andesitic composition, where

accurate Moho depth measurements have been determined.

Island ArcsContinental FragmentsContinental PeninsulasContinental MarginsContinental Interiors

Moh

o De

pth

Volcanoes

Figure 6. Histogram showing the Moho depth beneath each volcano and the type of setting

each one belongs to. Arc setting classification from Gill (1981).

3.2 Moho Depths

The range in Moho depths of the volcanoes studied (Figure 6) range from ~17

km at White Island (Whakaari) within the Taupo Volcanic Zone of New Zealand to ~70

km at Parinacota and Tata Sabaya within the Central Andean Volcanic Zone of South

America. Gill (1981) divided each volcanic arc into geographical categories; mainland

continental margins (e.g. Chile); peninsular continental margins (e.g. Kamchatka);

detached continental fragments (e.g. Japan); island arcs (e.g. Tonga); or continental

interiors (e.g. Eastern Colorado). These categories are used in Figure (6). This shows

Chapter 3 Volcano Locations And General Properties 15

that the majority of volcanoes have Moho depths between 30 and 40 km. Those

volcanos located at continental margins generally have Moho depths >35 km. The

deficiency of island arc volcanoes in the compilation is because these arcs do not

commonly contain andesites.

3.3 Volcano Properties

The general geochemical properties of each volcano are presented in table (1)

and geochemical plots for each volcano exist in Appendix (A).

3.3.1 Major Element Discrimination Diagrams

The Total alkalies-silica diagram (Figure 7) shows that the majority of rocks fall

into the basalt to dacite fields, with some extending to rhyolite. Those that fall in the

basaltic trachyandesite to trachydacite fields are typically the high-K suites (Figure 8).

A group with anomalously high K2O also exist, which typically have a phonolitic

character (Figure 7). Such volcanoes exist in the Aeolian arc and in Eastern Central

Andes (Cerro Tuzgle and San Jeronimo). These are grouped as shoshonitic in this

thesis. Figures (7 & 8) also show that the majority of the rocks show a linear trend for

each volcano to high K2O with increasing SiO2. Some volcanoes also show a general

trend from low to high K2O and total alkalies increasing Moho depth (Figures 7 & 8).


Table 1. Volcano Locations, Moho depth, Properties and Classifications.

Volcano Volcanic Arc Latitude Longitude Moho (km) SiO2 Range (%) 87Sr/86Sr Range Magma Processes Rock Type (60% SiO2)

1 White Island (Whakaari)New Zealand / Taupo Volcanic Zone -37.52 177.18 17.50 43.92 64.40 0.70511 0.70548 Differentiation Andesite

2 RuapehuNew Zealand / Taupo Volcanic Zone -39.77 175.57 22.50 52.18 63.62 0.70435 0.70574 Mixing Andesite

3 Santorini Aegean Arc 36.40 25.45 23.00 52.03 70.00 0.70472 0.70573 Mixing Trachyandesite

4 Campi Flegrei Aeolian Arc 40.83 14.14 25.00 50.94 61.70 0.70681 0.70861 Mixing Trachyte

5 Egmont (Taranaki) New Zealand / Taupo Volcanic Zone -39.30 174.07 30.00 48.92 59.25 0.70378 0.705041 Mixing Trachyandesite

6 Atacazo Andean Arc / Northern Andean Volcanic Zone -0.35 -78.62 32.50 57.74 66.40 0.70419 0.704304 Mixing Andesite

7 Puyehue Andean Arc / Southern Andean Volcanic Zone -40.50 -71.00 32.50 48.39 71.65 0.70378 0.70433 Differentiation Andesite

8 Pinatubo Luzon Arc 15.14 120.33 32.50 50.50 65.39 0.70419 0.70433 Mixing Andesite

9 Parícutin Mexican Volcanic Belt 19.49 -102.25 33.00 52.10 72.40 0.7037 0.7056 Mixing Andesite

10 Bakening Kamchatka Arc 53.93 158.07 34.00 48.70 75.06 0.70311 0.70337 Mixing Andesite

11 Taal Luzon Arc 14.00 120.99 34.00 47.42 68.69 0.70443 0.70478 Differentiation Trachyandesite

12 Vesuvius Aeolian Arc 40.82 14.43 35.00 47.60 63.20 0.70711 Differentiation Trachyte

13 Mt ShastaCascades / Southern Cascades 41.46 -122.17 35.00 48.51 66.38 0.70275 0.70434 Differentiation Andesite

14 Adatara Honshu Arc 37.62 140.28 35.00 51.99 61.74 0.70477 0.705836 Mixing Andesite

15 Nekoma Honshu Arc 36.83 140.00 35.00 54.15 66.62 0.70491 0.70542 Mixing Andesite

16 Towada Honshu Arc 40.56 140.88 35.00 49.51 68.15 0.70392 0.704382 Mixing Andesite

17 San Pedro-Pellado Andean Arc / Southern Andean Volcanic Zone -36.00 -70.64 37.50 48.99 75.40 0.70371 0.704109 Mixing Andesite

18 Ichinsky Kamchatka Arc 55.72 157.75 38.00 47.40 68.30 0.70332 0.703405 Mixing Andesite

19 Galeras Andean Arc / Northern Andean Volcanic Zone 1.22 -77.37 40.00 53.63 61.57 Mixing Andesite

20 Chichinautzin Mexican Volcanic Belt 19.13 -99.33 42.50 49.90 66.40 Differentiation Andesite

21 San Jeronimo Andean Arc / Central Andean Volcanic Zone -23.90 -66.50 45.00 55.28 60.73 0.70688 0.70758 Differentiation Trachyandesite

22 Cerro Tuzgle Andean Arc / Central Andean Volcanic Zone -24.05 -66.48 45.00 56.41 70.72 0.70624 0.709691 Mixing Trachyandesite

23 Quimsacocha Andean Arc / Northern Andean Volcanic Zone -3.03 -79.23 50.00 58.80 71.70 Mixing Andesite

24 Sangay III Andean Arc / Northern Andean Volcanic Zone -2.03 -78.33 50.00 49.79 65.47 0.7041 Mixing Trachyandesite

25 Cerro Leon Muerto Andean Arc / Central Andean Volcanic Zone -25.45 -68.47 55.00 50.89 60.85 0.70529 0.70679 Mixing Andesite

26 AntisanaAndean Arc / Northern Andean Volcanic Zone -0.05 -78.15 60.00 53.20 67.00 0.70419 0.704507 Mixing Trachyandesite

27 Nevado Solimana Andean Arc / Central Andean Volcanic Zone -15.42 -72.90 62.50 51.96 67.84 0.70588 0.706214 Mixing Trachyandesite

28 Quillacas Andean Arc / Central Andean Volcanic Zone -19.00 -67.25 65.00 56.40 62.58 0.70704 0.713657 Mixing Trachyandesite

29 Ollagüe Andean Arc / Central Andean Volcanic Zone -21.30 -68.18 65.00 52.90 67.00 0.70537 0.708319 Mixing Trachyandesite

30 Parinacota Andean Arc / Central Andean Volcanic Zone -18.15 -69.16 70.00 51.40 68.20 0.70612 0.70692 Mixing Trachyandesite

31 Tata SabayaAndean Arc / Central Andean Volcanic Zone -19.13 -68.53 70.00 52.13 62.32 0.70516 0.706908 Mixing Trachyandesite


Figure 7. Total alkalies-silica diagram using IUGS volcanic rock-type classifications.

Symbols represent different ranges of Moho depth except for the high-K shoshonitic rocks that

have been grouped together.

The K2O-FeO*/MgO diagram (Figure 9) discriminates at 60% SiO2 whether the

volcano is calc-alkaline or tholeiitic, and whether the volcano is low-K, medium-K,

high-K or shoshonitic (adapted from Gill (1981)). The majority of volcanoes belong to

the medium-K calc-alkaline group, although the high-K calc-alkaline group is also well

represented. The few tholeiitic volcanoes are Taal (high-K), Santorini, Cerro Leon

Muerto, Puyehue and Quimsacocha (medium-K) and Nekoma (low-K). One volcano is

low-K, calc-alkaline (Towada) and few are shoshonitic (Campi Flegrei, Vesuvius, Cerro

Tuzgle and San Jeronimo).

The medium-K calc-alkaline group have Moho depths <45 km, whereas the

volcanoes that belong to the high-K calc-alkaline group have Moho depths >50 km or


are located at the back-arc side of volcanic arc systems (e.g. Egmont (Taranaki) 30 km).

The tholeiitic groups show no trend with Moho depth.

Figure 8. K2O-silica andesitic types diagram. Symbols represent different ranges of Moho

depth except for the high-K shoshonitic rocks that have been grouped together. Divisions from Gill

(1981).


Figure 9. Andesitic K2O-FeO*/MgO diagram showing dicriminations between calc-alkaline

and tholeiitic series, and between low-k, medium-K, high-K and shoshonitic rocks with 60% SiO2.

Discrimation lines from Gill (1981). Equations: FeO*/MgO = 0.1562 x SiO2 - 6.685; K2O = 0.145 x

SiO2 - 5.135; K2O = 0.0818 x SiO2 - 2.745; K2O = 0.0454 x SiO2 - 1.864.

3.3.2 Isotopic Ratio Correlations

Isotopic studies of arc andesites are used primarily to evaluate whether the

magma source is crustal or mantle-derived or whether the magma is a crust-mantle mix

(Gill 1981). The Sr-Nd isotope diagram (Figure 10) shows that the majority of

volcanoes trend between mid-ocean ridge basalt (MORB) and the primordial bulk

silicate Earth (BSE). These volcanoes generally have formed at low Moho depths <40

km, and show little scatter in isotope compositions. In contrast, the volcanoes formed at

greater Moho depths show a large scatter towards high 87Sr/86Sri and lower 143Nd/144Ndi,

suggesting that the magma is contaminated by radiogenic crustal sources. This can be


seen also in Figure (11) where the volcanoes with low Moho depths show very little

change in 87Sr/86Sri with respect to silica, but the volcanoes with high Moho depths

show a large scatter.

Figure 10. Sr-Nd isotope ratio correlation diagram. Showing mid-ocean ridge basalt (MORB)

and primordial bulk silicate Earth (BSE) regions.

The shoshonitic volcanoes also form a distinct group with 143Nd/144Ndi ratios

close to BSE, but the 87Sr/86Sri ratios are generally elevated. However, they show a

small scatter with respect to silica (Figure 11). This relation could be explained if the

major source region for these magmas is enriched lithospheric mantle.

Only those volcanoes formed above thick crust (i.e. those with isotopic scatter)

show obvious isotopic evidence for magma mixing. Nonetheless, the wide silica range

for some volcanoes, suggest they may also be crust-mantle mixes, even though they

show a limited isotopic range. For these volcanoes, the crustal source must be juvenile

(i.e. non-radiogenic).


Figure 11. 87Sr/86Sri-silica diagram.

3.3.3 Temperature and H2O Content of Andesitic Magmas

The temperature of andesite at eruption is a function of volatile composition,

particularly water content, degree of crystallisation, and heat liberated during eruption

(Gill 1981). Field measurements during the 1944, 1945 and 1946 andesitic eruptions at

Parícutin (Mexican Volcanic Belt) yielded maximum temperatures of 1100°C, 1070°C

and 1040°C respectively (Luhr 2001). Phase-equilibrium and mineral geothermometry

experiments on Parícutin andesite have predicted that the pre-eruption temperatures

were 1110±40°C. Three other volcanoes, Mt Shasta (Southern Cascades), Adatara

(Honshu Arc) and Tata Sabaya (Central Andean Volcanic Zone) also show similar pre-

eruption temperatures of 1200-1020°C (Grove et al. 2002), 1250-875°C (Hunter &

Blake 1995) and 1021-996°C (De Silva et al. 1993), respectively.

Measurements from Parícutin glass inclusions within olivine indicate the H2O

content of the melt ranged from 1.8 to 4.0 wt% (Luhr 2001). Mt Shasta and the trench-

Chapter 3 Volcano Locations And General Properties 22

side Honshu arc volcanoes (e.g. Adatara) show contrasting H2O concentrations of 4.5-

>8.0 wt% (Grove et al. 2002) and <0.5 wt% (Tatsumi & Eggins 1995), respectively.

The H2O content for Tata Sabaya is unknown. These results show that H2O content does

not affect pre-eruption temperatures. The pre-eruption temperature for Tata Sabaya may

be low due large crustal thickness (70 km).

3.3.4 MORB Spider Diagrams for Each Volcano

MORB normalised spider diagrams (Figures 12–15) are plotted for each volcano

in Moho depth order. All of the plots show similar patterns, except the volcanoes with

insufficient data (e.g. Santorini has only 9 out of 28 elements available). These similar

patterns are positive Pb and negative Nb, Ta and Ti anomalies. Some of the volcanoes

show large positive Sr anomalies (Mt Shasta, Quimsacocha and Sangay III) whereas the

majority have small or no anomaly (Adatara, Puyehue, Towada and the shoshonites).

The majority have large ion lithophile (LIL) element normalised abundances

(particularly Rb & Ba) 70-100, except for the volcanoes with Moho depths over 60 km,

which have values 100-200, the shoshonitic volcanoes with values up to 700.

The high field strength (HFS) elements are more variable, with the slope

generally steepening with increasing Moho Depth reflecting decreasing HREE (0.5-0.2).

The exceptions are the Honshu arc volcanoes (~35 km depth) that show a relatively flat

HFS slope, similar to White Island (17.5 km). In addition, the shoshonitic volcanoes

have a steep HFS slope compared with volcanoes with similar Moho depths. This

reflects elevated LILE for Campi Flegrei and Vesuvius, but decreased HREE for Cerro

Tuzgle and San Jeronimo.


Figure 12. MORB spider diagrams for White Island (Whakaari) 17.5 km, Ruapehu 22.5 km,

Santorini 23 km, Campi Flegrei 25 km, Egmont (Taranaki) 30 km, Atacazo 32.5 km, Puyehue 32.5

km & Pinatubo 32.5 km


Figure 13. MORB spider diagrams for Parícutin 33 km, Bakening 34 km, Taal 34 km,

Vesuvius 35 km, Mt Shasta 35 km, Adatara 35 km, Nekoma 35 km and Towada 35 km


Figure 14. MORB spider diagrams for San Pedro-Pellado 37.5 km, Ichinsky 38 km, Galeras

40 km, Chichinautzin 42.5 km, San Jeronimo 45 km, Cerro Tuzgle 45 km, Quimsacocha 50 km &

Sangay III 50 km


Figure 15. Morb spider diagrams for Cerro Leon Muerto 55 km, Antisana 60 km, Nevado

Solimana 62.5 km, Quillacas 65 km, Ollagüe 65 km, Parinacota 70 km & Tata Sabaya 70 km

Chapter 4 Proxy Ratios Versus Moho Depth And Chondrite-Normalised Spider-Diagrams

CHAPTER 4 PROXY RATIOS VERSUS MOHO DEPTH

AND CHONDRITE-NORMALISED

SPIDER-DIAGRAMS

27

4.1 Introduction

It was suggested in chapter 1 that the best trace element ratios to discriminate

changes in the Moho depth are those that represent the properties of the residual

minerals of plagioclase, hornblende and garnet. These ratios are Sr/Y, LREE/(HREE or

Y) and Ca/Sr. The averaged (to 60% SiO2) elemental compositions are shown in Table

(2) with the major elements, REE, Sr and Y. Chondrite-normalised REE+Sr+Y spider-

diagrams are also plotted for those volcanoes of similar properties and sufficient data.

4.2 Proxy Ratios versus Moho Depth

4.2.1 Sr/Y versus Moho Depth (Figure 16)

It can be seen in Figure (16) that a positive correlation exists of increasing Moho

depth with increasing Sr/Y, for many volcanoes. At low Moho depths (White Island

(Whakaari) ~17.5 km), average Moho depths (Parícutin ~33 km) and high Moho depths

(Tata Sabaya ~70 km), Sr/Y is ~10, 25 and 70 respectively. Two main groups do not

follow this trend: Mt Shasta (blue crosses) and the Adatara (red dots) groups. The Mt

Shasta group (Mt Shasta, Quimsacocha and Sangay III) has very high Sr/Y values (>70)

with respect to Moho depth (35 to 50 km), whereas the Adatara group (Adatara,

Nekoma, Towada, Puyehue and Taal) has very low Sr/Y values (<10) at ~35 km Moho

28 Chapter 4 Proxy Ratios Versus Moho Depth And Chondrite-Normalised Spider-Diagrams

depths. Omitting the Mt Shasta, Adatara and the shoshonitic groups the trend can be

expressed by the equation Sr/Y = 0.2542 x (Moho)1.3097 with a R2 value of 0.8517.

Table 2. Major elements, REE, Sr, Y and proxy ratios for each volcano averaged to 60% SiO2.


Figure 16. Sr/Y versus Moho depth diagram for average andesite composition (60% SiO2).

Figure 17. La/Yb versus Moho depth diagram for average andesite composition (60% SiO2).

Chapter 4 Proxy Ratios Versus Moho Depth And Chondrite-Normalised Spider-Diagrams 30

4.2.2 LREE/HREE or Y versus Moho Depth (Figures 13, 14 & 15)

The diagrams that are used to represent LREE/HREE ratios are La/Yb (Figure

17), La/Y (Figure 18) and Ce/Y (Figure 19). These diagrams show similar trends to

each other and all three are used because certain elements are preferred with different

analytical techniques (e.g. INAA=Yb and XRF=Y). Like Sr/Y ratios, LREE/HREE

ratios also show a positive trend with increasing Moho depth. La/Yb, La/Y and Ce/Y

values for White Island (Whakaari) (17.5 km) are 4.5, 0.5 and 1.0; Parícutin (33 km) are

11, 1.0 and 2.2; and Tata Sabaya (70 km) are 40, 7.75 and 7.0, respectively. The major

difference between these diagrams and the Sr/Y diagram is that the Mt Shasta group

now fits the average trend, but the Adatara group is anomalously low and displays low

values similar to that of White Island, where the Moho is extremely shallow. Omitting

the Adatara and shoshonitic groups each trend can be expressed by the equations:

La/Yb = 2.7257 e0.0392 x (Moho), R2 = 0.9027; La/Y = 0.3295 e0.0321 x (Moho), R2 = 0.8039;

Ce/Y = 0.6497 e0.0324 x (Moho), R2 = 0.9024.

4.2.3 Ca/Sr versus Moho depth (Figure 20)

It can be seen in Figure (20) that Ca/Sr has a negative correlation with Moho

depth where White Island (Whakaari) has Ca/Sr values of 250, Adatara of 200 and Tata

Sabaya of 40. The major difference between the Ca/Sr diagram and the previous plots is

that the Adatara group now fits a linear trend (Ca/Sr = -4.1956 x (Moho) + 326.01, R2

= 0.8984) between White Island and Tata Sabaya, and a hyperbolic trend exists for the

volcanoes that followed the normal trend in those plots (pink line).


Figure 18. La/Y versus Moho depth diagram for average andesite composition (60% SiO2).

Figure 19. Ce/Y versus Moho depth diagram for average andesite composition (60% SiO2).


Figure 20. Ca/Sr versus Moho depth diagram for average andesite composition (60% SiO2).

4.3 Chondrite-Normalised REE+Sr+Y Spider-Diagrams

The Chondrite-normalised REE+Sr+Y spider-diagrams (Figure 21) have been

divided into groups of similar shapes and slopes to help distinguish the different

residual mineral phases existing at the base of the crust. To understand the data,

reference should be made to Figures (3 & 4) in chapter 1.

The volcanoes plotted in Figure (21A) have a moderate negative slope and a

small positive Sr anomaly. This suggests that the main residual phases are plagioclase

and hornblende. The plots in Figure (21B) show a very steep negative slope indicating

that garnet is the main residual mineral. The plots in Figure (21C) have a very flat slope,

with no or negative Sr and Eu anomalies. This suggests that plagioclase is the main

residual mineral, with no or very little hornblende involved. The plots of Figure (21D)

Chapter 4 Proxy Ratios Versus Moho Depth And Chondrite-Normalised Spider-Diagrams 33

show the complete opposite to Figure (21C), where there is a very large positive Sr

anomaly, a steep negative slope and an upward concavity shape within the HREE

region. This suggests that hornblende is the main residual phase, with very little

plagioclase. The plots of shoshonites (Figure 21E) show high levels of LREE, a very

steep slope, and a negative Sr anomaly. They show similarity to the garnet signatures of

Figure (21B), but the negative Sr anomaly shows that plagioclase was fractionating as a

primary phase, not garnet. The plot within Figure (21F) shows a steep slope and high

LREE, which indicates that garnet is the main residual mineral, but it also has a

negative Eu anomaly, which may suggest that plagioclase was also involved, perhaps

during fractionation at upper crustal levels.

4.4 Anomalous Groups

The two groups that do not follow normal trends in the Sr/Y and LREE/HREE

versus Moho diagrams are the same groups of volcanoes that can be separated by the

Chondrite-normalised REE+Sr+Y spider-diagrams. The volcanoes that belong to the Mt

Shasta group are those that contain hornblende as a residual mineral phase (Figure

21D). The volcanoes that belong to the Adatara group are those that contain plagioclase

as the residual phase (Figure 21C). Therefore, when proxy ratios are being used to

determine the Moho depth, Chondrite-normalised REE+Sr+Y spider-diagrams should

also be used to determine what residual phases are present at some stage in the partial

melting or fractionation history. This is important, for these mineral assemblages may

not be present in the sampled rocks, yet their presence is as crucial for determining the

shape of the REE+Sr+Y patterns. Without this information, the proxy values for Moho

depths could be wrong.


Figure 21. Chondrite-normalised REE+Sr+Y spider-diagrams divided into groups of similar

shapes and slopes. (A) plagioclase + hornblende signature, (B) garnet signature, (c) plagioclase

signature, (D) hornblende signature, (E) shoshonitic, (F) Garnet + Plagioclase.

Chapter 5 Discussion

CHAPTER 5 DISCUSSION

35

It was shown in chapter 4 that a systematic trend exists between trace element

ratios (Sr/Y, HREE/LREE and Ca/Sr) and Moho depth, reflecting changes in the

residual mineral assemblage (plagioclase, hornblende and garnet) for the majority of

andesitic volcanoes. As with most natural systems, there are usually anomalous results.

In this case, the volcanoes do not follow the average trend. These include Mt Shasta,

Adatara and the Shoshonitic groups. The processes that generate these anomalous

groups are discussed below.

5.1 Shoshonites

The shoshonitic group of volcanoes do not show many common properties with

the typical calc-alkaline/tholeiitic andesites. Shoshonites can be distinguished from

common calc-alkaline rocks by very high K2O, higher Ba, Rb, Sr, V, Cr, Ni and

LREE/HREE (Rock et al. 1991). This group are located inboard from the typical arc

system and their geochemical properties could result from partial melting of an enriched

subcontinental lithospheric mantle (e.g. Déruelle 1991). Since shoshonitic magmas are

already highly concentrated in LREE and Sr, fractionation at the Moho would have a

minimal effect on the total abundance of these elements. Therefore, the volcanic

products of lithospheric melting cannot be used to calculate Moho depths.

5.2 Effect of H2O

In chapter 4, it was shown that the anomalous groups of calc-alkaline rocks (Mt

Shasta and Adatara) have unusual H2O contents. The Mt Shasta group contains 4.5 to

>8.0 wt% H2O, whereas the Adatara group from the trench-side Honshu arc have <0.5

Chapter 5 Discussion 36

wt% H2O. The average is 2 to 4 wt% H2O, such as Parícutin (Gill 1981). Since H2O has

a similar effect to pressure on the stability of plagioclase and hornblende in andesites

(Müntener et al. 2001), the H2O content should be also be considered in determining the

Moho depth. Figure (22) shows increasing Ca/Sr, with decreasing La/Yb that reflects

increasing residual plagioclase involvement, and decreasing residual hornblende and

garnet involvement at shallow Moho depths. This is predicted in Chapter 1. However

whereas the graph also shows a general trend from low Moho to high Moho with

increasing La/Yb the increase is not systematic. For example, those volcanoes defined

by open red circles (Figure 22) do not follow the pattern, yet they follow the hyperbolic

trend. Because these volcanoes have similar Moho depths (pressures) and since the

processes that destabilise plagioclase are increasing pressure and/or H2O, this variability

could be due to changes in H2O. Evidence for this possibility is that all the volcanoes

with anomalously high Ca/Sr are those with anomalously low H2O (Adatara group). As

all volcanoes show a systematic hyperbolic trend from high La/Yb and Low Ca/Sr to

low La/Yb and high Ca/Sr, this plot suggests that low H2O enhances plagioclase

stability over that of hornblende.

The La/Sm against La/Yb diagram (Figure 23) also shows a systematic trend

that generally reflects the transition from residual plagioclase to hornblende to garnet,

where increasing La/Sm represents an increasing role for hornblende and increasing

La/Yb represents an increasing role for garnet. However, like Figure (22), a general

increase in La/Yb reflects higher Moho, but the anomalous volcanoes are those with

high and low H2O, as well as the shoshonites.

37 Chapter 5 Discussion

Figure 22. Ca/Sr versus La/Yb diagram, representing changes in residual plagioclase versus

hornblende and garnet with respect to Moho depth.

Figure 23. La/Sm versus La/Yb diagram representing changes in dominant residual minerals

and H2O.


5.3 H2O Andesite Categories

By using the observation that both H2O content and pressure have a controlling

influence on the nature of the residual andesite mineral assemblage, andesites can be

grouped into high-H2O (Mt Shasta group), average-H20 and low-H2O (Adatara group)

types by using the relative size of the Sr anomaly on chondrite-normalised REE+Sr+Y

spider-diagrams. This reflects the differences of residual hornblende and plagioclase.

Therefore, by plotting Sr/Ce (Sr anomaly) against La/Yb (pressure signature) (Figure

24) these different H2O andesite categories divisions can be shown graphically. Placing

andesites with these categories is very important before any correlation with the Moho

depth can be attempted.

Figure 24. Sr/Ce (Sr anomaly) versus La/Yb (pressure signature) diagram, showing H2O

Andesite category divisions.


5.4 Effect of P-T and H2O on the Residual Mineral

Assemblage

By using the trace element characteristics of andesites, it is possible to show the

effect of P-T and H2O on the stability of the residual mineral assemblages. This can be

seen in Figure (25) where six volcanoes representing different environments are plotted

in P-T diagrams for high, average and low H2O contents. Figure (25) can be compared

with the partition coefficients diagram (Figure 3), with the dominant residual mineral

phase diagram (Figure 4), and with the Chondrite-normalised REE+Sr+Y spider-

diagram (Figure 21).

The average-H2O group of volcanoes White Island (17.5 km), Parícutin (33 km)

and Tata Sabaya (70 km) are plotted in Figure (25a), representing typical (2.5-3% H2O)

content. The volcanoes illustrate the effects of changing pressure (Moho depth) with

respect to residual mineral assemblage. White Island has the lowest Moho depth and its

dominant residual mineral assemblage is plagioclase, with a small hornblende

component. Parícutin represents a typical andesitic volcano with average Moho depth

and H2O content. The residual mineral assemblage of Parícutin is dominated by

hornblende, with a small plagioclase component. Tata Sabaya is a volcano that has one

of the greatest Moho depths in the world, and its dominant residual assemblage is

garnet, with subordinate hornblende. The H2O content is unknown for Tata Sabaya, but

if it were higher than ~3%, it would still show a similar residual mineral assemblage.

This trend of increasing pressure (Moho depth) with the residual mineral assemblage

changing from plagioclase to hornblende to garnet confirms what was stated in Chapter

1.


Figure 25. P-T diagrams for average, high and low H2O system showing six volcanoes that

represent H2O andesite categories from section 5.2. P & T conditions are measured for all the

volcanoes, except White Island and Ollagüe. In the White Island case, the T field represents the

typical range for andesites, whereas with Ollagüe, the T field is an estimate for a thick crust, dry

system.

The high H2O group (>4% H2O) is represented by Mt Shasta (35 km), and is

plotted in the H2O saturated diagram (Figure 25b). Mt Shasta has a dominant residual

assemblage of hornblende and no plagioclase or garnet component. A lack of residual

plagioclase explains why the Sr/Y ratio is very high, a lack of garnet is consistent with

the average Moho depth (~35 km) beneath Mt Shasta.


The low H2O group is represented by Adatara (35 km) and Ollagüe (65 km) and

are plotted in the anhydrous P-T diagram (Figure 25c). Arc magmas are very unlikely to

be anhydrous, so this diagram provides the extreme situation for magmas with very

little H2O (<0.5 wt%). Adatara plots in the plagioclase stability field and outside the

garnet field. Hornblende is not stable in anhydrous magmas. This explains the relatively

low HREE/LREE and Sr/Y ratios of Adatara-type magmas, which have average Moho

depths. Ollagüe has a dominant residual assemblage of garnet, but may also have a

plagioclase component, depending on the H2O content. If the magma has a low H2O

content, as shown in the anhydrous diagram (Figure 25c), then the unusually flat REE +

Y pattern for Ollagüe can be explained by residual plagioclase component. Alternately,

the pattern for Ollagüe could be explained if the magmas had at least two stages of

ponding, one at Moho and this magma has moved and ponded at a mid crustal level.

The first alternative is preferred, as the composition of no other volcano requires

ponding of mid-crustal magmas.

The P-T diagrams show the same dominant residual mineral assembles as in the

REE+Sr+Y spider-diagrams (i.e. White Island: plagioclase>hornblende; Parícutin:

hornblende>plagioclase; Tata Sabaya: Garnet; Mt Shasta: Hornblende dominant;

Adatara: plagioclase dominant; Ollagüe: Garnet + plagioclase). Therefore, it is possible

to identify from the REE+Sr+Y diagrams, and from the Sr/Ce (Sr anomaly) versus

La/Yb (pressure signature) diagram (Figure 24), which volcanoes have high or low H2O

contents. Therefore, these plots are able to distinguish the volcanoes that will not fit the

Sr/Y, HREE/LREE or Ca/Sr against Moho depth calibrations. For each H2O andesite

category, the following criteria should be applied for the determination which proxy

calibration for Moho depth should be used:


High H2O group → HREE/LREE v Moho depth only (Figures 17-19)

Low H2O group → Ca/Sr v Moho depth only (Figure 20)

Average group → HREE/LREE or Sr/Y v Moho depth (Figures 16-19)

Chapter 6 Adakites, Products Of Slab Melt?

CHAPTER 6 ADAKITES, PRODUCTS OF SLAB MELT?

43

Drummond & Defant (1990) identified a group of high Sr/Y andesites, dacites

and rhyolites in Cenozoic arcs, which they called adakites, after Adak Island in the

Aleutians. Adakites are characterized by >56 wt% SiO2, >15 wt% Al2O3, MgO usually

>3% (rarely > 6 wt%), low Y (<20 ppm), high Sr (>400) and low HFSE showing

negative Nb and Ta anomalies (Drummond & Defant 1990).

The andesites from Mt Shasta and Tata Sabaya also show adakitic

characteristics: 16.08 and 16.27 wt% Al2O3, 5.01 and 2.53 wt% MgO, 12 and 13 ppm

Y, 884 and 852 ppm Sr and negative Nb anomalies (Chapter 3).

The two andesite suites from Mt Shasta and Tata Sabaya are classed as adakitic,

but have different origins. Mt Shasta magmas formed under average crust (35 km), high

H2O content and have hornblende as the dominant residual mineral. Tata Sabaya formed

under thick crust (70 km) and shows garnet as the dominant residual mineral. The

absence of plagioclase from the residual mineral assemblage is the major common

factor between the two. This absence of plagioclase and the presence of silica poor

minerals (hornblende and/or garnet) drives the resultant magma towards a corundum

normative composition. (Müntener et al. 2001).

Many researchers (e.g. Drummond & Defant 1990; Drummond et al. 1996;

Kamber et al. 2002; Mahlburg Kay et al. 1993; Martin 1999) inferred that these

magmas most likely represent eclogite partial melts from the subducting slab, which

ascended through and reacted with peridotite in the mantle wedge to form the general

characteristics of adakites. To partially melt the subducting slab, it was suggested by

Drummond & Defant (1990) that it should be young (<30 Ma) and therefore still warm.

Chapter 6 Adakites, Products Of Slab Melt? 44

However, numerical models of slab and mantle wedge thermal structure, have shown

that it is only possible that the slab could partially melt after initiation of a new

subduction zone at ages <3 Ma (Peacock et al. 1994). Since many adakites are formed

with a subducted slab with age >3 Ma and even >30 Ma. The slab under Adak Island is

~39 Ma old and is improbable that adakites are formed from subducted slabs. Using the

information contained in this thesis, adakites are considered to have formed at the Moho

under unusually high-H2O contents and/or high-pressures, producing a corundum

normative magma, caused by the absence of plagioclase in the residual mineral

assemblage.

Conclusions

CONCLUSIONS

45

• A relation exists between andesite composition and Moho depth.

• Andesite composition is controlled by the residual mineral assemblage at the

Moho. This is it’s “base level” composition.

• This base level composition is independent of the andesite genesis processes,

either by (1) differentiation from basaltic arc magmas, (2) partial melting of

under-plated basaltic arc magmas, or other crustal material, or (3) mixing of

these crustal and mantle derived magmas.

• The major residual minerals in andesites (plagioclase, hornblende and garnet) all

show contrasting trace elemental abundance characteristics. Sr is strongly

partitioned within plagioclase, whereas HREE and Y are strongly partitioned

within hornblende and garnet. These features are mirrored by the andesitic melt

compositions.

• A systematic trend exists between trace element ratios (Sr/Y, HREE/LREE and

Ca/Sr) and Moho depth, reflecting changes in the residual mineral assemblage

(plagioclase, hornblende and garnet) for the majority of andesitic volcanoes.

• With increasing pressure, the stability field of minerals vary. For temperatures

between 1100-1200oC, plagioclase is stable up to 70 km depth under anhydrous

conditions. The stability field for plagioclase progressively diminishes to <10

km for H2O saturated conditions. At normal H2O contents, (~2.5-3%)

plagioclase is stable up to ~30 km. Average to high H2O content has minimal

effect to the stability of hornblende which is stable for depths up to 70 km, but

under anhydrous conditions, it does not exist in the melt. Garnet becomes

Conclusions 46

increasingly stable with increasing pressure but with increasing H2O contents, its

stability changes from 45 km to 70 km for anhydrous to saturated H2O contents.

• Andesites can be grouped into high-H2O, average-H20 and low-H2O categories

by using the relative size of the Sr anomaly on chondrite-normalised REE+Sr+Y

spider-diagrams. A large Sr anomaly and concave up HREE pattern indicates

hornblende is the dominant residual mineral and hence a high H2O content.

Whereas, no or a small Sr anomaly and flat REE pattern indicates plagioclase is

the dominant residual mineral and hence a low H2O content. The combination of

the two patterns indicates average H2O content. A very steep REE pattern

indicates garnet is the dominant residual phase and is grouped as average H2O.

• H2O andesite categories divisions can be identified by plotting Sr/Ce (Sr

anomaly) against La/Yb (pressure signature) (Figure 24). Placing andesites

within these categories is very important before any correlation with the Moho

depth can be attempted.

• For each H2O andesite category, the following criteria should be applied for the

determination which proxy calibration for Moho depth should be used:

High H2O group → HREE/LREE v Moho depth only (Figures 17-19)

Low H2O group → Ca/Sr v Moho depth only (Figure 20)

Average group → HREE/LREE or Sr/Y v Moho depth (Figures 16-19)

• Using these criteria and the Moho depth proxy calibrations, changes in the

crustal thickness of orogenic system could be determined.

Conclusions 47

• Shoshonites, the products of lithospheric melting rather than mantle wedge

(asthenospheric melting) cannot be used to calculate Moho depths. Medium to

high-K calc-alkaline rocks best fit the calibration.

• Adakites are formed at the Moho under unusually high-H2O contents and/or

high-pressure conditions producing a corundum normative magma caused by the

absence of plagioclase in the residual mineral assemblage.

• The lack of an obvious contribution from the mantle lithosphere for andesite

genesis suggests that it is very thin or non-existent beneath arc volcanoes.

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Appendix A Geochemical Ratio Plots

APPENDIX A GEOCHEMICAL RATIO PLOTS

calibration of andesitic whole-rock trace-element ... · calibration of andesitic whole-rock...

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