40. geochemical investigations of volcanic ash ......geochemical investigations of volcanic ash...

Ciesielski, P. F., Kristoffersen, Y., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 114

40. GEOCHEMICAL INVESTIGATIONS OF VOLCANIC ASH LAYERS FROM SOUTHERNATLANTIC LEGS 113 AND 1141

Hans-W. Hubberten,2 Wolfgang Morche,2 Frances Westall,2 Dieter K. Fütterer,2 and Jorg Keller3

ABSTRACT

Petrographic and geochemical investigations were carried out on 21 ash layers from four sites of ODP Legs 113and 114 in the southern Atlantic Ocean. With the help of geochemical data and petrographic characterization threerock series can be distinguished for stratigraphically different ash layers from Site 701 (Leg 114) located east of theSouth Sandwich Island Arc, whereas the Leg 113 tephras from the southern slope of the South OrkneyMicrocontinent belong to another magmatic series. Geochemical correlation of the Leg 113 tephras with possiblesource areas indicates that they were probably erupted from the Antarctic Peninsula.

The Miocene ashes from Site 701 are probably derived from the now-extinct Discovery Arc, the precursor of theSouth Sandwich Islands. The Pliocene ashes from the site show some affinity with the South Shetland Islands,although the available data do not permit a clear correlation. The Quaternary ashes from Site 701 display a chemistrytypical of island-arc tholeiites and are therefore most probably derived from eruptions on the South SandwichIslands. Because of their distant position the southern Andes seem to be rather improbable as a potential sourceregion for the tephra layers investigated.

INTRODUCTION

Eight holes from four Ocean Drilling Program (ODP) drillsites in the western Antarctic-South Atlantic Ocean wereselected for studies of volcanic ash geochemistry. Holes695A, 696A, 696B, 697A, and 697B from ODP Leg 113 arelocated between 40° and 44°W and 61° and 63°S on thesouthwestern slope of the South Orkney Microcontinent (Fig.1; Barker, Kennett, et al., 1988). The large number of green,devitrified volcanic ash layers—in addition to several thin (

H.-W. HUBBERTEN ET AL.

L,.>

- 60°S

a

45°W

SCOTIA SEA

^ Λ SOUTH ORKNEY 1.

eV̂ • " I £Ofi 697*., "E L ANTARCTICW^ PENINSULA

||v WEDDELL SEA

25°W

• 701

50°S —

>( SOUTH, SANDWICH• ISLANDS

,* 60°S —

DISCOVERY ARCAND JANE BANK

25°W

7 0 ° S -

Figure 1. Principal source regions for pyroclastic products in thesouthwestern Atlantic Ocean. Ash-bearing sediments were investi-gated from ODP Sites 695, 696, and 697 of Leg 113 and Site 701 of Leg114.

the lower Pliocene were sampled in Hole 701B. The one lowerMiocene and two upper Miocene ash layers from Hole 701Crepresent the oldest volcanic products analyzed in this inves-tigation (Fig. 2).

The examined samples are identified by Alfred-Wegener-Institute (AWI) laboratory sample numbers corresponding toODP sample designations (Table 1).

All samples were dried at 50°C and weighed. Carbonatewas removed by adding 10% hydrochloric acid. The carbon-ate-free residue was rinsed with distilled water, dried, andreweighed. The samples were then sieved in order to obtain200-µm fractions. Only the 63-200-µmfraction was investigated.

The samples were described using a petrographic micro-scope. Special textural phenomena were documented withhelp of a scanning electron microscope (SEM). The glassshards were separated using heavy liquids and a FrantzIsodynamic magnetic separator, with the magnetic fieldstrength varied to obtain the different glass fractions.

Polished grain mounts were prepared from the separatedvolcanic glasses and minerals of each ash layer for microprobeanalyses and petrographic investigations. Geochemical analy-sis was performed with energy-dispersive KEVEX equipment(EDX) attached to an SEM at Freiburg University using themethod described by Morche (1988). To avoid uncontrolledloss of Na2O, glass particles were measured under a defocusedelectron beam of 10 × 10 µtn minimum size (acceleratingvoltage 15 kV, beam current approximately 5 nA, and count-

Table 1. Investigated ash layers from ODP Legs 113 and 114.

Age

QuaternaryQuaternaryQuaternarylate Pliocene

early Plioceneearly Pliocene

late Miocenelate Mioceneearly Miocene

late Pliocenelate Pliocenelate Pliocene

early Plioceneearly Plioceneearly Plioceneearly Plioceneearly Plioceneearly Plioceneearly Plioceneearly Plioceneearly Pliocene

early Plioceneearly Plioceneearly Pliocene

early Pliocene

early Plioceneearly Plioceneearly Plioceneearly Plioceneearly Plioceneearly Pliocene

Sample (Core,section, interval in

cm)

114-701A-1H-4, 58-602H-4, 17-185H-5, 58-598H-4, 55-56

114-701B-3H-4, 84-864H-1, 132-134

114-701C-24H-7, 13-1525X-1, 143-14537X-1, 126-128

113-696A-7H-3, 5-77H-7, 53-559H-2, 4-6

113-695 A-1H-1, 50-519H-2, 5-714H-CC, 34-3618X-4, 120-12121X-2, 68-7023X-5, 61-6325X-1, 89-9125X-6, 3-526X-2, 19-21

113-696B-3R-4, 143-1455R-6, 135-13749R-1, 65-67

113-697A-1H-2, 52-54

113-697B-14X-1, 40-4214X-1, 80-8214X-1, 110-11215X-2, 74-7532X-2, 73-7532X-2, 146-148

AWI sampledesignation

1-11-2

23

45

678

161718

9

1011121413

15

1920

Thickness ofash layer (cm)

7742

107

364

Disseminated21

No glass5

No glass41232

No glass

No glass1

No glass

No glass

No glassNo glassNo glassNo glass

3Disseminated

Chemicalanalysis

YesYesYesYes

YesYes

YesYesYes

YesYesYes

NoYesNoYesYesYesYesYesNo

NoYesNo

No

NoNoNoNoYesYes

734

GEOCHEMICAL INVESTIGATIONS OF VOLCANIC ASH LAYERS

Leg 113 Leg 114

mbsfHole

695 AHole

696 AHole

697 A

5 0 -

100 -

2 0 0 -

300 -

3ua1

PUö

§oS1JS

ε

early

Plio

cene

—

1 1

III

9(5)

3uai

1K

J

Plid

Φ

—

j P

leis

t

Hole697 B

Hole701 A

Hole701 C

16(diss)17 Hole

180) 696 B

I. P

lioea

rly P

lioce

ne

— 15(1)

10(4)

11(1)

—Tephra Layers recorded

— investigated

AWI sample numbers

(thickness in cm)

diss = disseminated

ary

c8aδ

Φ

8

j 1.

Plio

— 1-1(7)

1-2(7)

2(4)

3(2)

Hole701 B

ΦC

8o

n

e ea

rly 1

c

ü8JS

mmm 4(10)5(7)

19(3)20(diss)

jrnar

y

a

late

Plio

cene

Q

early

Plio

cene

early

Mio

cene

la

te M

ioce

neSt

6(3)7(6)

8(4)

Figure 2. Schematic stratigraphy of the cored sections investigated in Holes 695A, 696A, 697A, and 697B (Barker, Kennett, et al., 1988) and701A-701C (Ciesielski, Kristoffersen, et al., 1988).

735


Table 2. Analytical results obtained by EDX at Freiburg University with Smithsonian Institution reference glassand mineral standards.

Referencesample0

Glass111240

Glass72854

Plagioclase115900

Diopside117733

Cr-Augite164905

Hypersthene746

Hornblende143965

Analysis

Smithsonian (mean)Freiburg (mean)

(SD)Smithsonian (mean)Freiburg (mean)


(SD)Smithsonian (mean)Frieburg (mean)




(SD)

SiO2

51.1150.700.42

77.1977.240.17

51.2152.110.26

55.1355.460.07

50.4150.830.23

53.9653.820.13

40.8841.020.16

TiO2

1.861.870.070.12000.05000000.510.400.060.160.030.074.784.780.17

A12O3

14.1414.180.20

12.1412.310.10

30.8930.300.060.110.090.198.027.920.071.230.970.09

15.0915.030.10

Cr2O3

0000000000000.850.820.090.750.600.11000

FeO

11.9112.220.071.241.070.070.460.400.030.240.190.134.684.560.14

15.1815.060.42

11.0610.740.16

MnO

0.220.300.070.030000.040.080.04000.12000.490.560.110.090.080.10

MgO

6.756.460.080.09000.1400

18.3918.120.28

17.3017.720.16

26.7227.670.46

12.9613.100.24

CaO

11.1811.300.10.500.430.04

13.6313.480.17

25.7526.140.28

17.2817.050.11.521.290.14

10.4310.310.18

K2O

0.190.200.074.925.040.090.180.090.060000000002.082.180.02

Na2O

2.642.630.213.773.930.063.453.580.150.34000.840.720.160002.632.780.26

a Reference analysis number = USNM standard (Jarosewich et al., 1980).b Freiburg mean and standard deviation are based on four analyses.

Calc-alkaline orTholeiite Series

Leg 113 Tephra

53 55 57 59

oLeg 113 . L e g 114

61 63 65

% SiO2

73

Figure 3. Total alkali-silica diagram of analyzed samples from Legs113 and 114. The separation line of alkaline and calc-alkaline/tholeiiticrock series is according to MacDonald (1968).

ing time 100 s). Matrix correction was calculated according toa modified routine of Ware (1981). For calibration, standardsfrom the Smithsonian Institution (Jarosewich et al., 1980)were used. Selected analyses are given in Table 2.

RESULTSAll the tephra layers investigated are of calc-alkaline

composition ranging from island-arc tholeiites to high-potas-sium calc-alkaline series (Figs. 3-5).

PetrographyA summary of the important petrographic features of the

ash layers is given in Table 3. Most of the glass shardsinvestigated are very dark to pale brown in color, althoughsome samples consist of at least two populations of colorlessand brown glass. Glass textures range from dense, poorlyvesicular blocky shards to highly vesicular particles with a

Figure 4. K2O vs. SiO2 of analyzed Leg 113 ashes. Individual ashlayers are marked by their AWI sample numbers to show the range ofvariation for single layers (classification after Taylor et al., 1981). HK= high-potassium calc-alkaline rocks; LK = low-potassium calc-alkaline rocks.

pumiceous or elongated, fibrous habit. Some extremely vesic-ular types exhibit a bubble-wall texture (platy, cuspateshards). Fusiform (melt) structures were also found, indicat-ing an overheated, low-viscosity magma during the eruptionphase. Plates 1 and 2 show the structures most commonlyobserved. Some of the tephra layers contain glass shardsshattered by numerous cracks (see Table 3), which are inter-preted as the effect of shock cooling of the magma on contactwith water or ice. The blocky to cuspate shards suggestphreatomagmatic to Plinian styles of eruption (cf. Heiken andWohletz, 1985), in the latter case admitting the possibility ofdistant source areas for the ash layers. Tiny mineral inclu-sions, such as Fe-Ti oxides and pyroxenes, are commonlypresent, especially in the dark brown glasses.

736


Leg 114Tephra

61 63 65% SiO2

Figure 5. K2O vs. SiO2 of analyzed Leg 114 ashes. Individual ashlayers are marked by their A WI sample numbers (classification afterTaylor et al., 1981). HK = high-potassium calc-alkaline rocks; LK =low-potassium calc-alkaline rocks.

Leg 113

The mineral assemblages from Leg 113 are quite uniform,consisting of Plagioclase, clinopyroxene, opaque minerals, andvery rare orthopyroxene or pigeonite, all of which are thought tobe phenocrysts. According to the microanalyses, An contents ofthe weakly zoned plagioclases are between An]3 and An^,reflecting a long range of fractionation from a similar parentmagma composition for all the Leg 113 ash layers. The clinopy-roxenes can be characterized as diopsidic augites and augites.No olivine was detected in either the Leg 113 or the Leg 114tephras, despite their partially basic geochemistry.

Additional accessory minerals such as quartz, orthoclase,zircon, sphene, garnet, and amphibole are interpreted eitheras xenocrysts (originating from the wall rock during erup-tion) or, more likely, as terrigenous detritus derived from thesouthern slopes of the South Orkney Microcontinent.

Leg 114Despite their overall similar mineralogy (Plagioclase, clinopy-

roxene, opaque minerals, ± pigeonite and/or orthopyroxene;Table 3), three groups of tephras can be distinguished in the Leg114 samples on the basis of their mineral assemblages. The fouryoungest ash layers (samples AWI1-1, AWI1-2, AWK, andAWI3; late Pliocene-Quaternary) are characterized by the com-mon occurrence of pigeonite, ± orthopyroxene, and relativelyAn-rich plagioclases (An45_g5). In contrast, two of the lowerMiocene layers (samples AWI6 and AWI7) contain no orthopy-roxenes and clearly have lower An contents in the plagioclases(An25_83), as well as additional sanidine as a minor constituent,indicating a more highly evolved (high-potassium) calc-alkalinerock series. The mineralogy of the remaining ash layers variesbetween these two extremes (Table 3).

The very low abundance of detrital minerals at Site 701, aswell as the different mineralogy of this fraction (quartz,garnet, and orthoclase), contrasts with that of the samplesfrom Leg 113 and reflects the intraoceanic position of this site,far from terrigenous influence.

GeochemistryTable 4 presents the mean values of chemical data from

about 400 analyses of individual glass shards from 12 Leg 113

samples and nine Leg 114 samples. The complete data set islisted in the Appendix.

The geochemistry of the ash layers shows a large varia-tion in most of the major elements, both within and betweenthe different layers. As shown in the plots of total alkalis orpotassium against silica (Figs. 3-5) all the ashes investigatedbelong to the island arc series characterized by subduction-related calc-alkaline magma compositions and follow theclassical fractionation trend. This trend, caused by a cli-nopyroxene/plagioclase/magnetite fractionation, is also seenin the other major elements; magnesium, iron, calcium, andtitanium decrease with increasing silica contents, whereassodium and potassium increase, as does the potassium/sodium ratio. Because potassium as a representative of theincompatible elements is the most useful parameter todiscriminate between the ash layers investigated, we willfocus on the potassium/silica relationship in the followingdiscussion.

Leg 113

The ash samples from Leg 113 (Fig. 4) are all of Plioceneage and plot in a narrow field that classifies them as typicalof a calc-alkaline series (using the potassium/silica classifi-cation scheme of Taylor et al., 1981). According to thisclassification, they range from basaltic andesite to rhyo-dacite in composition. Individual samples exhibit somevariability within this field. Samples AWI9, AWI12, andAWI17 show a chemical variance that covers almost theentire field of the Leg 113 ashes. The ashes in samplesAWI11 and AWI14 are distinctly bimodal in their composi-tion, with one group of glass shards plotting at the basalticandesite end of the field whereas the other populationcorresponds to a rather highly evolved rhyodacitic type. Theremaining ashes have basic or intermediate compositionsand are less variable. Ashes from samples AWI10, AWI19,and AWI20 fall within a field of somewhat higher potassiumcontents, with silica contents between 57% and 65%.

Leg 114

In contrast, the ash samples from Leg 114 have distinctlydifferent geochemical patterns. Although similar in silicacontent to the Leg 113 samples, they display a broad rangeof potassium contents (Fig. 5). Three different fields can beobserved, from which the ash layers of Site 701 are classifiedas island-arc tholeiitic, calc-alkaline, and high-potassiumcalc-alkaline series (according to Taylor et al., 1981). Thesecorrespond to three age groups, Quaternary, Pliocene, andMiocene, respectively, with the exception of the lowerMiocene tephra layer (AWI8), which has a typical calc-alkaline composition. The youngest (Quaternary) ash layersof Hole 701A in samples AWI1-1, AWI1-2, and AWI2 showa very narrow range in composition and are represented bythe island-arc tholeiitic field. The upper Pliocene ash layer(sample AWI3) in Hole 701A shows higher potassium con-tents and plots in a field within the calc-alkaline series. Thetwo lower Pliocene ash layers from Hole 701B (samplesAWI4 and AWI5) fall in the same field as ash layer sampleAWI3. One of the two upper Miocene ash layers from Hole701C (sample AWI7) displays a bimodal distribution, withone population of ashes belonging to the high-potassiumcalc-alkaline series (AWI7B) and the other population(AWI7A) plotting together with ash layer sample AWI6 in arather narrow calc-alkaline field with intermediate potassiumcontent. The oldest ash layer encountered from Legs 113and 114, sample AWI8 of early Miocene age, shows acontinuous calc-alkaline trend with silica contents from 54%to 68%.

737


Table 3. Petrographic summary of ashes studied from Legs 113 and 114.

Sample (interval in cm) andsize fraction (/xm)

Thickness(cm) Crystals

0 Glass Morphology* Colorc

113-AWI9: 695A-9H2, 5-7

63-125

AWI10: 695A-18X-4, 120-12163-125

AWIll: 695A-21X-2, 68-7063-125

AWI12: 695A-23X-5, 61-6363-125

AWI13: 695A-25X-6, 3-563-125

AWI14: 695A-25X-1, 89-9163-125

AWI15: 696B-5R-6, 135-13763-125

AWI16: 696A-7H-3, 5-763-125

AWI17: 696-7H-7, 53-5563-125

ANI18: 696A-9H-2, 4-663-125

AWI19: 697B-32X-2, 73-7563-125

AWI20: 697B-32X-2, 146-14863-125

114-AWI1-1: 701A-1H-4, 58-60

total sampleAWI1-2: 701A-2H-4, 17-18

total sample

AWK: 701A-5H-5, 58-5963-125

AWI3: 701A-8H-4, 55-5663-125

AWI4: 701B-3H-4, 84-8663-125

AWI5: 701B-4H-1, 132-13463-125

AWI6: 701C-24H-7, 13-1563-125

AWI7: 701C-25X-1, 143-14563-125

AWI8: 701C-37X-1, 126-12863-125

DISCUSSION

5

4

1

2

2

3

1

Disseminated

2

1

3

Disseminated

7

7

4

2

10

7

3

6

4

+lith

**

*

+ +

+

+lith

+lith

+lith

+ + +

+ + +

+

+lith

+

+ +

+

the

The chemical compositions of the ash samples from Sites695, 696, and 697 close to the South Orkney Microcontinentare typically calc-alkaline, ranging from basalt-andesiticthrough dacitic to rhyolitic in composition (Table 4 and Figs.3 and 4). The small amount of variation in the potassium/silicaplot (Fig. 4) makes distinction between individual samplesfrom the whole group of Leg 113 ashes difficult. This arguesfor magma sources that resemble each other, in which magmageneration occurred under comparable conditions (i.e., atrather similar depths and with similar degrees of partialmelting in the mantle), although not necessarily erupted from

hv; +inclblockyfibr,

bubble-rich,tub

v; +incl"cracked"

blockylv+incl to hv

fibr to bwplaty

hv;+bubblesfibr, tub

v;crackedblocky

lv to hv;blocky

lv; blockyfibr, +bubbles

crackedhv;

pum, fusiv to hv;

block fibr,pum

hv; +incl(blocky)

fibr, + bubblespum, bw

hv; blockycracked

fibrhv; blocky

crackedpum, fibr

hv; +inclpum

hv; pumfibr

bubble-richv; +inclblocky

lv; blockyhv; fibr

hv;fibr, fusi bw

v; +inclblocky(fibr)

v;blocky

palag (fibr)lv;

blockypalag

v; +inclblocky

brpbrcl

dbrpbr

dbrpbr to cl

dbrpbr, cl

dbrpbr

dbrpbr to cl

(br)lbr

brpbrdbrpbr

brpbr

(br)pbrclbr

pbrcl

(br)pbr

(dbr)pbr

blackgrbr pbr

rbrpbr-cl

rbrpbrdgrbr

pbr(br)pbr

grbrpbr

br(pbr)

the same volcanic area. The geochemistry is typical formagma generation in connection with a subduction zone.Within the magma chambers, various degrees of fractionalcrystallization led either to continuous compositions of theeruption products (zoned magma chambers), as observed insamples AWI9, AWI17, and AWI19, or to smaller chemicalvariabilities, as in samples AWI10, AWI12, AWI13, AWI15,AWI16, AWI18, and AWI20. Two distinctly different mag-matic compositions must have coexisted at the same time forthe formation of the two bimodal ash layers in samples AWIlland AWI14. This can be explained by a compositionally zonedmagma chamber with sharp boundaries or by eruptions trig-gered by magma mixing.

738


Table 3 (continued).

Clino-pyroxene0

Ortho- OpaquePigeonite pyroxene minerals Accessories

PlagioclaseAn content

Alkalifeldspar >200µm

53-59Orthoclase

+glass incl

dim)

SpheneGarnet

AmphiboleGarnet

Zircon

GarnetAmphiboleGlauconiteAmphibole

Garnet

GarnetNot

determinedGarnet

Amphibole

13-57

21-86

13-60

57-71

54-70

56-69

28-79

Orthoclase(Albite)

(Sanidine)Orthoclase

Albite?

OrthoclaseAlbite?

OrthoclaseAlbite?

Orthoclase

OrthoclaseSanidine

(Ti-Mt)(Dm)

(incl) (Ti-Mt)

(Ti-Mt)

(Ti-Mt)

(Dm)

GarnetZircon

Amphibole

GarnetSphene

AmphiboleGarnet

Amphibole(bl-gr)

Garnet

Biotite

Garnet

42-68

Not determined

Not determined

65-69

64-73

45-88

58-81

Not determined

65-75

25-82

30-83

54-55

OrthoclaseSanidineAlbite?

SanidineAlbite?

Orthoclase

Orthoclase

Sanidine

SanidineAlbite?

Note: Mineralogy and An content determined by EDX analysis. ( ) = minor. Relative abundance of glass andmineral phases: — = not found; + = present; ++ = abundant; + + + = highly abundant.a +lith = rich in lithic material.

Iv = low vesicularity; v = vesicular; hv = high vesicularity; +incl = rich in inclusions; palag = palagonitized; pum= pumiceous; fibr = fibrous; tub = tubular; fusi = fusiform; bw = bubble wall; "cracked" = shattered by polygonalcracks.c dbr = dark brown; rbr = red brown; grbr = gray brown; br = brown; lbr = light brown; pbr = pale brown; cl =colorless.

2 = two different clinopyroxenes.

In determining possible sources for the ashes, it should betaken into account that the analyses were performed on the glassfractions of the samples, without considering the phenocrystcontent of the bulk sample. Variable amounts of mostly Plagio-clase, pyroxene, and opaque phenocrysts have been observed inmost samples (Table 3). In consequence, the glass compositions

presented here should be slightly more evolved than the bulk-rock analyses taken from the literature for comparison.

Comparisons of the potassium/silica data from the Leg 113ashes with published analyses for the Antarctic Peninsula andthe South Shetland Islands over the time interval concerned(Fig. 6; data from Saunders et al., 1980; Smellie et al., 1984)

739


Leg 113

53 55 57 59 61 63 65%SiO2

67 69

Figure 6. K20 vs. SiO2 of the Leg 113 analyses. The shaded fieldsindicate the composition of magmatic products from the SouthShetland Islands (data from Smellie et al., 1984; Weaver et al., 1982),the Antarctic Peninsula (data from Saunders et al., 1980), and theSouth Sandwich Islands, Discovery Arc, and Jane Bank (data fromBarker et al., 1984; Barker and Griffiths, 1972; Tomblin, 1979). Onlythose rocks that were formed during the same time period as the Leg113 ashes are plotted.

show that there is only a weak correlation between the Leg113 samples and the South Shetland Island analyses, but amuch better correlation exists with those from the AntarcticPeninsula. It is not possible, however, to correlate individualash layers with specific eruption sequences from the peninsulabecause of the sparse information on volcanism in this prov-ince.

Our data indicate that in a few cases compositional andstratigraphic correlation between holes is possible (e.g., theearly Pliocene age layer AWI10 from Hole 695A and layersAWI19/AWI20 from Hole 697B). Furthermore, the layers insamples AWI12 and AWI13 (Hole 695A) correspond compo-sitionally with layer AWI15 (Hole 696B).

The chemical analyses show that the Leg 114 samples arevery different from those from Leg 113. Three differentfractionation trends can be observed in the time span of thefour stratigraphically defined groups (Fig. 5 and Table 1):

1. The Quaternary ash layers from Hole 701A (AWI1-1,AWI1-2, and AWI2) plot in the island-arc tholeiitic field withpotassium contents of less than 0.5% (K2O).

2. The Pliocene tephras from Holes 701A (AWI3) and 701B(AWI4 and AWI5) are typically calc-alkaline and are con-nected by a similar fractionation trend.

3. The upper Miocene ash layers from Hole 701C (AWI6and AWI7) represent a unique high-potassium calc-alkalinefractionation trend.

4. The lower Miocene age tephra from Hole 701C (AWI8)also shows the typical calc-alkaline composition similar to thePliocene ash layers.

It is obvious from these data that the various ash layersstudied from Leg 114 cannot have formed from a commonsource of magma. The potassium-enriched calc-alkalinegeochemistry of the Miocene ashes is believed to be typical ofmagma formation associated with subduction. The fact thatpotassium contents in the ashes of late Miocene age are higherthan those of the early Miocene implies, however, differentconditions of generation, possibly caused by a change in the

inclination angle of the subducting plate. The composition ofthe late Miocene ash layer in sample AWI7 has a bimodaldistribution that is believed to result from a change in magmacompositions rather than from mixing of ashes from differenteruptions. This sample was taken from a discrete ash layer,and it would have been rather fortuitous to have two volca-noes erupting at the same time from different locations.

The Pliocene ash layers from Holes 701A and 701B (AWI3,AWI4, and AWI5) plot within a common calc-alkaline fieldand were probably formed during the same mobilizationprocess. Chemical compositions for the youngest ash layers(AWI1-1, AWI1-2, and AWK) are different, indicating either adifferent geographic origin or a change in the magma-genera-tion process.

Correlation of the Leg 114 ashes is somewhat difficultbecause there are only a few published geochemical analysesof the possible source areas. However, comparison with theavailable data (Fig. 7) shows that there is, in fact, a goodcorrelation of the Quaternary ashes with analyses from theSouth Sandwich Islands, which form an island arc less than 4(8?) Ma old (Baker, 1978; Barker and Griffiths, 1972; Tarney etal., 1982; Tomblin, 1979). The older calc-alkaline ashes(Pliocene samples AWI3, AWI4, and AWI5 and early Mi-ocene sample AWI8) plot in the same field as the SouthShetland Island volcanics, for which two cycles of activity inthe Miocene and Pliocene have been reported (Smellie, 1983;Weaver et al., 1982). Our data provide supporting evidence ofthis.

No such correlation for the other Leg 114 ash layers (lateMiocene age high-potassium calc-alkaline tephras) is possible atpresent, and determination of source areas must, therefore,await further detailed investigations. On the other hand, strati-graphic correlations suggest that the Pliocene and Miocenetephra layers may be derived from the now-extinct structures ofthe Discovery Arc and Jane Bank, respectively (Barker et al.,1982,1984), taking into account the thickness of the layers (3-10cm!). These structures are regarded as precursors of the SouthSandwich Island Arc, now separated by the Southern ScotiaRidge transform fault. Unfortunately there are no representativereference samples from these arcs to test this hypothesis. Nev-

•if)

4.5.

4.0.

3.5.

3.0.

2.5.

2.0.

1.5.

1.0.

0.5.

Leg 114

s×~i ( Antarctic Peninsula )

•

(South Shetland) . ^ j β ^ ^ 3 ^ ^

47 49 51 53 55 57 59 61 63 65 67 69 71 73

%SiO 2

Figure 7. K2O vs. SiO2 of the Leg 114 analyses. The shaded fieldsindicate the composition of magmatic products from the SouthSandwich Islands, Discovery Arc, and Jane Bank (data from Barker etal., 1984; Barker and Griffiths, 1972; Tomblin, 1979), the SouthShetland Islands (data from Smellie et al., 1984; Weaver et al., 1982),and the Antarctic Peninsula (data from Saunders et al., 1980).

740


Table 4. Mean values of the chemical data from the ash layers studied from Legs 113 and114.

Sam-pie*

AWI1-1N = 15AWI1-2N = 21AWKN = 20AWI3N = 21AWI4AN = 5AWI4BN = 9AWI5N = 19AWI6N = 17AWI7AN = 29AWI7BN = 6AWI8N = 22AWI9N = 23AWI10N = 19AWI11AN = 13AWI11BN = 11AWI12AN = 13AWI12BN = 5AWI13N = 8AWI14AN = 6AWI14BN = 8AWI15N = 10AWI16N = 12AWI17N = 18AWI18N = 12AWI19N = 17AWI20N = 11

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

MeanSD

SiO2

56.640.79

56.210.84

56.670.97

67.481.79

61.523.33

69.000.18

57.570.93

58.391.32

55.860.82

62.233.62

59.094.11

63.633.98

60.130.51

56.350.92

70.420.57

56.651.58

64.502.20

57.450.68

64.511.00

70.270.41

59.481.24

55.090.49

60.174.65

59.541.41

61.362.09

60.911.08

TiO2

1.090.101.140.081.120.080.770.131.020.090.660.081.320.101.230.081.110.131.180.141.460.201.040.391.400.101.450.150.330.111.500.170.920.231.440.090.860.060.350.031.160.071.440.081.120.341.370.231.070.121.260.12

A12O3

15.560.41

15.040.71

15.361.05

14.770.87

15.090.20

13.970.17

15.490.61

15.810.39

16.230.58

15.790.71

15.870.45

16.280.86

16.210.40

16.990.46

14.550.13

16.620.29

16.270.61

16.560.47

15.270.36

14.470.07

16.060.42

16.540.16

16.200.67

16.170.50

15.930.58

15.900.22

FeO

11.170.62

11.690.66

11.640.906.060.869.381.316.220.15

11.320.88

10.0o40.59

10.380.727.532.639.161.826.141.538.080.449.280.723.750.339.340.835.941.819.010.576.430.794.010.108.740.51

10.120.297.821.698.431.207.590.757.930.60

MnO

0.250.090.300.250.270.120.110.100.230.060.030.070.260.100.240.080.200.110.080.130.270.050.070.110.150.100.190.100.080.120.180.110.070.100.220.100.110.120.090.100.220.090.150.120.160.090.200.110.240.080.190.10

MgO

3.410.433.670.733.130.840.280.381.761.260.040.102.440.461.930.483.080.421.071.002.271.060.850.971.630.282.790.370.000.002.740.410.730.522.540.301.140.340.000.002.110.363.460.162.091.451.820.541.540.531.510.27

CaO

8.270.258.460.617.900.584.440.606.181.523.830.187.510.236.490.568.030.404.530.866.661.514.171.615.540.207.420.301.570.177.140.594.320.856.770.244.060.271.730.095.850.598.050.205.742.155.840.455.390.875.310.51

K2O

0.360.060.390.090.410.061.180.210.820.201.290.100.760.081.990.221.750.343.500.901.060.302.120.632.070.201.360.113.890.271.310.212.400.271.380.062.510.113.340.191.650.221.110.121.710.641.570.282.040.322.280.15

Na2O

3.200.322.990.463.450.424.830.433.920.664.820.283.220.263.750.183.290.263.990.634.080.575.580.524.710.364.140.345.210.264.490.404.670.844.530.264.990.355.580.174.640.464.030.344.940.645.010.364.680.344.560.27

a Two different populations in an ash layer are labeled A and B. N = number of glass shards analyzedfrom a single ash layer.

ertheless, the tephra samples studied plot in the field of thepublished analyses (Fig. 7).

Ciesielski, Kristoffersen, et al. (1988) suggested eruptionsin the southern Andes also as potential sources for the tephralayers. Correlation of such thick and distinct airborne(?)tephra layers with eruptions from the calc-alkaline volcaniccenters of South America seems rather unlikely because of thelarge distances involved (approximately 3500 km). Only minoror trace amounts of disseminated volcanic ash of Oligocene toHolocene age are reported from the Falkland Plateau area,which is in an even closer position with respect to SouthAmerica (Deep Sea Drilling Project Leg 36; Elliot and Emer-ick, 1977). Of course, additional distribution of tephra by icerafting has to be considered, but cannot be evaluated here.

CONCLUSIONS

The volcanic ash samples studied from ODP Legs 113 and114 in the southern Atlantic-Antarctic cover the whole fieldof the calc-alkaline suite, ranging from basaltic andesites toandesites, dacites, and rhyolites, and are classified as island-arc tholeiitic, calc-alkaline, and high-potassium calc-alkalinerock series on the basis of their potassium/silica ratios.

On a stratigraphic basis three age groups are recognized:

1. Quaternary ashes (Leg 114);2. Pliocene ashes (Legs 113 and 114);3. Miocene ashes (Leg 114).

741


Ash samples from Leg 113 Sites 695, 696, and 697, close tothe South Orkney Microcontinent, are Pliocene in age andbelong to a normal calc-alkaline suite ranging from basalticandesites to dacites and rhyolites. Their restriction to a verynarrow field of evolution argues for a common subduction-related origin. The source volcanoes for these ashes arebelieved to be on the Antarctic Peninsula.

The variable potassium contents of the ashes from Site 701of Leg 114 allow them to be grouped in three separatemagmatic series. The oldest investigated tephra, of earlyMiocene age, has a calc-alkaline composition and could bederived from the Miocene activity of the South ShetlandIsland Arc. Late Miocene tephra layers have the highestpotassium contents and plot in the high-potassium calc-alka-line field. The Miocene tephras could alternatively stem fromthe now-extinct Discovery Arc or Jane Bank, the ancestors ofthe South Sandwich Islands Arc. It is not possible at thepresent stage of investigation to determine a distinct sourceregion for these ashes, because of the lack of representativereference samples. The Pliocene ashes can be separated intodifferent geochemical fields indicating different origins. Earlyand late Pliocene age ashes having a classic calc-alkalinecomposition probably originate from the South Shetland Is-lands. The Quaternary samples, displaying low potassiumcontents that classify them as island-arc tholeiites, are mostlikely derived from the South Sandwich Islands.

ACKNOWLEDGMENTS

We wish to thank Heike Ostermann (Alfred-Wegener-Institute) and Erika Lutz (Freiburg University) for technicalhelp and assistance during sample processing and microprobemeasurements. We are grateful to Dr. W. Ehrmann (AWI) forcritically reviewing this manuscript, and we address ourcompliments to two reviewers and to P. Ciesielski for theeditorial handling. Financial support by the Deutsche Fors-chungsgemeinschaft (Grant Fu 119/14) is acknowledged. Thisis A WI publication no. 187.

REFERENCES

Baker, P. E., 1978. The South Sandwich Islands: III. Petrology of theVolcanic Rocks: Sci. Rept. Br. Antarct. Surv., 93.

Barker, P. F., Barber, P. L., and King, E. C , 1984. An early Mioceneridge-crest collision on the South Scotia Ridge near 36°W. Tec-tonophysics, 102:315-332.

Barker, P. F., and Griffiths, D. H., 1972. The evolution of the ScotiaRidge and Scotia Sea. Philos. Trans. R. Soc. London, A, 271:151-183.

Barker, P. F., Hill, A., Weaver, S., and Pankhurst, R., 1982. Theorigin of the eastern South Scotia Ridge as an intra-oceanicisland-arc. In Craddock, C. (Ed.), Antarctic Geoscience: Madison(Univ. Wisconsin Press), 203-211.

Barker, P. F., Kennett, J. P., et al., 1988. Proc. ODP, Init. Repts.,113: College Station, TX (Ocean Drilling Program).

Ciesielski, P., Kristoffersen, Y., et al., 1988. Proc. ODP, Init. Repts.,114: College Station, TX (Ocean Drilling Program).

Dalziel, I.W.D., 1983. The evolution of the Scotia arc: a review. InOliver, R. L., James, P. R., and Jago, J. B. (Eds.), Antarctic EarthScience: Canberra (Austral. Acad. Sci.), 283-288.

Elliot, D. H., and Emerick, C. M., 1976. Volcanic glass in someDSDP Leg 36 cores. In Barker, P. F., Dalziel, I.W.D., et al., Init.Repts. DSDP, 36: Washington (U.S. Govt. Printing Office), 871-876.

Federman, A. N., Watkins, N. D., and Sigurdsson, H., 1982. ScotiaArc volcanism recorded in abyssal piston cores downwind fromthe islands. In Craddock, C. (Ed.), Antarctic Geoscience: Madi-son (Univ. Wisconsin Press), 223-238.

Gonzalez-Ferran, O., 1982. The Antarctic Cenozoic tectonic pro-cesses. In Craddock, C. (Ed.), Antarctic Geoscience: Madison(Univ. Wisconsin Press), 687-694.

Heiken, G., and Wohletz, K., 1985. Volcanic Ash: Berkeley (Univ. ofCalifornia Press).

Jarosewich, E., Nelen, J. A., and Norberg, J. A., 1980. Referencesamples for electron microprobe analysis. Geostandards News-lett., 4:43-47.

Keller, R. A., Fisk, M. R., White, W. M., and Birkenmajer, K., 1988.Late Tertiary-Quaternary transition from arc to back-arc volca-nism, South Shetland Islands and Bransfield Strait, Antarctica.EOS, Trans. Am. Geophys. Union, 44:1471.

MacDonald, G. A., 1968. Composition and origin of Hawaiian lavas.In Coats, R. R., Hay, R. L., and Anderson, C. A. (Eds.), Studiesin Volcanology: A Memoir in Honor of How el Williams. Geol.Soc. Am. Mem., 116:477-522.

Morche, W., 1988. Tephrochronologie der Åolischen Inseln [Ph.D.dissert.]. Mineralogisch-Petrographisches Institut der UniversitàtFreiburg/Br.

Ninkovich, D., Heezen, B. C , Conolly, J. R., and Burckle, L. H.,1964. South Sandwich tephra in deep-sea sediments. Deep-SeaRes., Part A, 11:605-619.

Saunders, A. D., Tarney, J., and Weaver, S. D., 1980. Transversegeochemical variations across the Antarctic Peninsula: implica-tions for the genesis of calc-alkaline magmas. Earth Planet. Sci.Lett., 46:344-360.

Smellie, J. L., 1983. A geochemical overview of subduction-relatedigneous activity in the South Shetland Islands, Lesser Antarctica.In Oliver, R. L., James, P. R., Jago, J. B. (Eds.), Antarctic EarthScience: Canberra (Austral. Acad. Sci.), 352-356.

Smellie, J. L., Pankhurst, R. J., Hole, M. J., and Thomson, J. W.,1988. Age, distribution and eruptive conditions of late Cenozoicalkaline volcanism in the Antarctic Peninsula and Eastern Ells-worth Land: a review. Bull. Br. Antarct. Surv., 80:21-49.

Smellie, J. L., Pankhurst, R. J., Thomson, M.R.A., and Davies,R.E.S., 1984. The Geology of the South Shetland Islands: VI.Stratigraphy, Geochemistry and Evolution. Sci. Rept. Br. Ant-arct. Surv., 87.

Smith, D. G., Ledbetter, M. T., and Ciesielski, P. F., 1983. Ice-raftedvolcanic ash in the South Atlantic sector of the Southern Oceanduring the last 100,000 years. Mar. Geol., 53:291-312

Tarney, J., Weaver, S. D., Saunders, A. D., Pankhurst, R. J., andBarker, P. F., 1982. Volcanic evolution of the northern AntarcticPeninsula and the Scotia Arc. In Thorpe, R. S. (Ed.), Andesites:Orogenic Andesites and Related Rocks: Chichester (Wiley), 371—400.

Taylor, S. R., Arculus, R., Perfit, M. R., and Johnson, R. W., 1981.Island arc basalts. In Basaltic Volcanism on the Terrestrial Plan-ets: New York (Pergamon Press), 193-213.

Tomblin, J. F., 1979. The South Sandwich Islands: II. The Geology ofCandlemas Islands. Sci. Rept. Br. Antarct. Surv., 92.

Ware, N. G., 1981. Computer programs and calibration with PIBStechnique for quantitative electron probe analysis using a lithium-drifted silicon detector. Comput. Geosci., 7:167-184.

Weaver, S. D., Saunders, A. D., Pankhurst, R. J., and Tarney, J.,1979. A geochemical study of magmatism associated with theinitial stages of back-arc spreading: the Quaternary rocks ofBransfield Strait, South Shetland Islands. Contrib. Mineral.Petrol., 68:151-169.

Weaver, S. D., Saunders, A. D., and Tarney, J., 1982. Mesozoic-Cenozoic volcanism in the South Shetland Islands and the Ant-arctic Peninsula: geochemical nature and plate-tectonic signifi-cance. In Craddock, C. (Ed.), Antarctic Geoscience: Madison(Univ. Wisconsin Press), 263-273.

Date of initial receipt: 24 March 1989Date of acceptance: 27 October 1989Ms 114B-182

742


APPENDIX

Chemical Elements of Analyzed Glass Samples, Legs 113 and 114

Data set is normalized to 100% water free. Original sums range approximatelybetween 93% and 98%, depending on the volatile content and measured bubble-free surface.

SiO2 TiO 2Sample (%) (%)

Leg 114AWI1-1 55.60

55.7955.9756.1356.3056.3756.4356.4756.5256.5556.70 (56.91 (57.1457.77 (58.89

AWI1-2 54.4854.6355.1655.1855.2255.8255.9655.9856.2756.2956.3256.4256.5356.6156.6756.8556.9757.0157.2257.2257.56

AWK 54.1654.8855.8056.0356.0556.3056.39 (56.4256.8156.8556.9556.9557.0357.0457.0457.0757.7157.90 (58.36

AWI3 59.0159.8062.17 (62.21

1.161.081.14.15.15

L291.11LOO.26.09

).96).961.10

.02

.02

.24

.08

.23

.21

.23

.14

.18

.271.15.34.06.08.15

1.04.02.10.12.16.15.03.12.23.14.12.02.21

.12

.20

.12[.17.14.16.15.18.18.12

).94.09.53.24

).87.01

62.29 0.9762.32 .0163.32 0.8863.74 0.8864.12 0.9965.95 0.5466.89 0.6866.96 0.6867.06 0.68

A12O3

15.4715.3815.3215.4915.1816.2415.2915.3815.2315.3016.7415.4215.7315.7415.5413.4814.0913.9714.9415.3115.0315.3415.4615.1115.3913.1015.3615.3215.5115.5315.6715.5115.5415.6115.2715.3315.7513.9216.6715.2916.0713.7716.5413.3215.3814.9115.8015.2914.9015.3516.5614.8413.5816.3916.9214.4715.2816.4715.2615.3715.3915.5615.6915.0716.7716.1014.6115.91

FeO

11.6111.6411.3412.0211.3911.0411.6311.5711.1111.329.84

11.5611.0210.729.77

12.5012.5712.6411.9111.5911.8911.4911.7911.8611.0013.6511.1511.5111.1511.5511.2211.1911.1610.9611.6211.1411.7212.7911.2711.8211.1612.5710.8313.2712.1812.2511.1811.6912.0711.9210.8312.0812.6710.419.60

12.1510.617.618.628.348.177.747.667.585.085.326.575.27

MnO MgO(%) (%)

0.040.300.360.270.33 :

o.i8 :0.23 :0.180.340.26 :0.21 :

o :0.200.38 ;0.210.23 (0.380.231.32 :0.340.37 :o.3θ :0.29 :0.22 :0.360.26o.3θ :0.25 :0.32 :0.30 :

o :o.i9 :o :o.2θ :0.28 :0.22 :

5.795.645.855.415.785.275.575.475.565.645.115.695.57>.745.03i.261.751.545.675.915.345.655.505.215.635.985.585.48(.135.355.255.465.045.25>.995.19

0.29 4.140.33 4.930.16 ;0.42 :0.24 ;

>.6O5.55

0.41 4.630.32 :>.760.24 4.04

o :0.38 :0.22 :

1.405.16>.77

0.35 2.750.19 :0.42 :0.240.36 :

'.89i.08.77

5.060.35 4.010 2.810.180.20 :0.180.280.2400.190.210

.55L33.70.44.57.86.51.08.17

0.18 0.960.18 ()0 00 0.690 0

CaO

8.528.528.368.528.208.498.128.318.268.328.298.258.348.117.499.709.13

10.558.148.488.668.328.398.388.148.128.328.218.148.328.198.318.138.298.017.829.928.248.417.538.227.557.978.277.827.377.917.617.487.638.157.257.277.498.125.486.556.105.905.975.745.555.455.395.124.724.435.00

K 2 O

0.350.380.350.380.310.500.340.370.350.340.320.370.220.390.420.240.420.150.360.400.480.380.470.450.430.520.370.350.420.320.370.310.400.450.490.460.450.380.410.400.350.390.340.470.480.440.340.440.430.390.490.450.460.300.460.890.880.780.890.771.030.930.920.970.930.991.100.79

N a 2 O

3.263.183.242.633.303.553.273.253.283.183.762.742.613.163.642.062.611.683.253.392.993.352.803.143.442.563.343.153.462.933.352.963.512.792.823.252.363.233.433.783.933.173.952.733.633.453.653.703.753.023.643.712.823.663.583.953.774.294.294.324.584.684.494.755.335.294.865.22

743


Appendix (continued).

SiO2 TiO2 A12O3 FeO MnO MgO CaO K2O Na2O

AWI4A

AWI4B

AWI5

AWI6

67.3367.3967.4868.0768.2768.3568.5568.5668.7668.7669.1469.2269.2469.8155.9361.6362.4662.8364.7668.7468.8468.9368.9868.9869.0469.0969.0969.3254.9456.2956.7556.8957.0457.0457.3157.4957.5557.5857.9257.9758.0458.1158.2558.3758.4558.8059.0056.3156.3756.7256.9757.2357.5357.9658.0558.3458.54 159.36 159.66 159.75 159.85 159.85 159.87 160.18 154.47 154.71 154.73 154.76 154.91 154.92 155.00 155.27 1

1.050.800.570.920.660.690.950.760.730.740.640.840.78D.761.041.071.021.100.870.68D.673.643.643.613.733.603.783.501.441.401.241.411.361.401.191.271.331.601.311.23.27.30.30.34.25.16.35.25.24.25.33.26.31.13.30.34.10.20.26.16.01.19.28.24.55.29.20.15.03.13.16.17

55.44 0.9255.62 155.68 1

.25

.1155.77 0.9955.78 1 .08

14.4314.5916.1813.9414.2014.1814.1014.1614.1913.9914.1814.1114.2713.8715.3514.8915.1215.2114.8913.7414.0414.0914.0415.6013.8814.2113.7514.0313.9015.0515.7016.0115.3115.5815.9115.4514.9214.8215.6216.1116.1315.5415.0016.9015.5615.5915.2116.1815.6916.2316.0015.2815.5416.1015.9615.6515.6515.6715.5115.4216.9415.8015.5115.5714.7215.9315.7316.5316.0017.0115.5915.8217.6716.6015.8716.3416.26

7.676.044.966.016.085.965.995.925.605.615.755.415.275.70

11.339.868.729.167.856.376.026.306.194.436.296.116.426.03

14.2812.1711.3510.7211.7211.0511.1011.1211.3112.1411.3410.6610.8711.1611.579.62

10.7611.1211.0810.8410.9210.7710.6310.6510.369.989.89

10.0310.219.579.979.858.709.539.559.27

12.5910.9011.4310.8410.0410.3011.1010.689.33

10.1211.1410.059.87

00.26000.200.1700.170.190.210.190.2300.160.220.180.320.240.1800000000.2000.280.310.420.370.320.290.270.290.320.340.310.28000.250.260.190.200.200.210.280.2200.290.290.290.310.350.340.200.200.170.250.170.320.2600.180.240.280.190.190.210.260.240.1900.270.19

00.5600.330.330.29000.2300.300003.981.371.480.991.0000.2800000003.383.052.982.472.562.682.472.513.081.622.422.371.812.492.511.802.342.071.822.482.722.392.132.792.172.031.992.021.981.641.381.570.991.501.511.603.153.463.373.064.363.103.743.422.752.973.063.403.32

4.704.144.774.744.033.964.344.323.783.933.763.524.043.658.885.685.585.575.174.173.603.783.853.923.993.693.833.768.047.787.417.517.617.687.637.517.137.257.507.387.737.637.567.607.447.287.067.387.117.177.286.996.766.756.496.466.555.855.855.996.455.855.615.787.958.268.367.959.208.728.168.308.46I.Ti7.918.428.40

1.331.151.021.471.161.171.281.221.321.311.201.601.421.430.480.830.960.950.891.381.281.281.181.311.421.181.411.210.520.750.690.730.760.810.730.750.870.940.780.670.730.770.770.760.760.790.861.691.701.561.671.912.011.942.011.992.022.222.072.212.002.252.322.312.201.711.661.721.251.521.791.751.581.781.911.611.69

3.335.025.014.324.995.164.654.905.055.334.764.874.834.502.794.384.243.864.314.715.134.875.055.004.474.954.365.023.163.083.303.833.223.303.293.503.353.542.723.273.332.932.743.253.252.873.323.583.873.683.823.443.853.653.863.583.474.183.983.763.703.773.923.613.243.443.163.623.022.983.253.223.523.673.223.093.27

744



Sample

AWI7B

AWI8

Leg 113AWI9

AWI10

SiO2

55.8055.8355.8655.9555.9956.1856.2456.2556.3356.4556.5156.7656.7656.8657.0158.1458.3658.4661.0962.7066.2666.5254.1054.4154.4954.6254.7254.8255.3055.6556.6457.7058.1458.5959.5960.2561.3362.0863.1963.7164.0764.1864.2268.07

57.0057.5858.9458.9759.1860.0160.5661.1861.2461.9562.6765.3965.5965.6665.6766.0466.2166.7566.7967.4669.4269.5969.6059.3059.4759.5759.6659.7759.7759.9459.96

TiO2

0.901.271.051.100.971.181.031.151.161.161.221.011.050.940.990.99

(

.28

.19

.28

.25

.14

.57

.61

.68

.52

.46

.55

.53

.58

.68

.601.541.551.711.301.621.401.321.281.141.311.150.93

1.821.371.561.381.531.521.191.271.221.091.260.950.580.910.810.880.660.860.670.820.570.390.531.581.431.301.491.321.421.461.15

A12O3

16.8316.2116.3116.2316.7716.0816.2516.0316.1115.0316.1616.4915.9816.5417.1916.4516.9716.1615.1915.8215.0315.5416.2516.0516.0416.4616.3416.3116.2416.2515.9715.8715.9516.3015.9216.0415.7915.6515.6515.4315.2015.5615.2914.50

15.8516.8017.3116.2316.2217.6816.9217.6617.3316.4316.4415.9316.9416.0616.1415.9317.0915.5515.7815.3115.5614.6014.5915.9916.2316.4816.1516.2915.4415.9117.07

FeO

9.4110.6310.2310.769.76

11.0710.0110.309.979.91

11.1510.1810.2510.329.629.059.62

10.219.097.554.654.03

11.4411.2211.3611.0111.4110.9410.9610.6010.139.599.579.598.468.647.718.047.487.056.906.557.285.55

9.538.327.868.058.246.447.296.066.716.366.945.775.055.145.525.444.355.405.255.043.334.644.548.648.698.108.148.098.458.857.37

MnO MgO

0.220.340.310.340.3200.250.290.290.160.2400.26000.230.2100.270000.310.240.390.340.270.330.230.220.290.260.320.180.290.210.270.200.34 10.240.27 10.280.32 10.26 (

0.32o :0.200.180.28 :00000.200 (0 (0 (0.16 (0 (0.24 (0 (0 (0 (0 (0 (

3.652.922.872.563.172.553.283.083.063.162.562.922.792.542.582.411.732.491.473.71333.553.433.373.193.453.233.373.132.922.782.552.462.042.321.681.673.791.103.871.193.873

».99'.851.24

>.231.101.461.061.14.15

).94).5233.32).343)))))

0 00 00.170.2100.220.200.23 :0.160.15

.75

.731.801.86.69

!.2O.63.39

CaO K2O

8.697.618.107.687.927.787.567.827.698.14 :7.627.797.637.877.757.525.04 :5.28 :5.36 :4.45 '

1.221.691.891.971.541.821.601.80.83

(.22.93.46.74

1.52.60.75

J.78>.96>.54t.00

3.26 4.903.77 :5.818.46 0.808.52 0.828.47 0.808.44 0.718.14 0.818.19 0.808.03 0.777.86 0.977.58 0.947.12 0.867.15 0.876.72 0.866.276.315.825.625.164.814.734.864.853.49

6.206.915.926.125.85 15.89 15.71 15.73 15.05

.05

.00

.16

.20

.36

.41

.41

.47

.47

.82

.53

.12

.44

.43

.48

.62

.87

.67

.574.72 2.274.45 1 .963.66 2.333.45 13.13 :2.95 :2.99 :3.18 :

.96..17..12.33.16

2.89 2.672.58 : .352.61 2.922.06 3.082.02 3.341.83 3.375.76 A5.68 :5.76 1

.17

.14

.935.84 2.025.45 15.47 :5.68 A

.68

.19

.135.93 2.09

Na2O

3.283.443.323.323.563.263.723.203.482.772.473.393.363.413.263.473.953.253.613.684.474.963.543.703.413.593.283.763.503.683.744.223.853.754.553.854.484.054.584.885.304.504.445.20

4.675.015.405.344.955.544.915.235.685.735.255.216.366.336.336.046.275.636.445.565.875.175.464.514.355.074.475.524.654.154.76

745



SiO2 TiO2 A12O3 FeO MnO MgO CaO K2O Na2O

AWI11A

AWI11B

AWI12A

AWI12B

AWI13

AWI14A

AWI14B

AWI15

59.9760.0860.1160.2360.3560.3960.3960.6560.6761.0561.1355.4355.4555.5355.7155.9555.9955.9956.3656.4056.4757.0957.8858.3569.8069.8969.8969.9970.2070.2970.5370.6670.6770.9271.7254.7555.0255.4955.5655.5855.8656.0856.5356.8757.6858.3558.6959.9362.0463.1264.4565.0667.8356.5857.0057.2157.2757.3557.5257.7658.8855.6255.6656.9957.2157.5558.0369.6670.2370.4970.6869.6670.2370.4970.6857.4658.03

1.401.281.421.551.471.321.331.471.381.351.451.631.701.471.611.481.481.541.311.391.441.361.281.190.420.270.410.440.480.320.220.290.400.170.211.741.451.581.581.651.691.401.501.571.561.341.271.141.131.130.890.910.561.441.501.471.361.481.561.441.251.621.561.521.551.451.510.340.400.310.340.340.400.310.341.161.20

15.8016.9116.0115.7116.0615.9516.7116.4016.4216.2616.1716.4916.5216.7516.6117.2616.7916.8117.5616.4417.1617.7916.9717.6714.5014.7714.6114.3114.5914.4014.7014.5614.5614.5914.4816.7516.6017.0616.7816.3816.2116.8316.6616.5317.0316.5016.0616.6316.0516.1315.9115.9017.3516.5716.2516.0216.3616.5616.5516.5617.6316.2816.1116.2715.9616.9916.4314.5714.3814.4914.4414.5714.3814.4914.4416.5015.96

8.597.867.868.358.148.057.507.797.947.467.60

10.0110.1910.2010.059.299.379.468.489.438.898.538.698.013.824.123.754.024.043.903.683.623.743.632.91

10.0910.249.469.669.73

10.549.669.419.658.488.248.437.847.367.086.146.282.839.169.139.749.468.759.029.037.81

10.2610.689.919.388.459.314.153.934.043.924.153.934.043.929.098.94

0.1600.240.300.2000.21000.200.190.1800.160.260.260.180.190.370.300.220.1700.160.3200.180.2000.16000000.2100.230.200.350.2400.3100.200.250.190.180.18000.1900.240.300.2700.280.250.230.190.170.1700.260.2500.180.18000.180.18000.320.26

1.561.681.811.561.611.851.591.32

1.381.670.843.172.953.112.942.672.853.252.653.172.762.132.412.17000000000002.923.323.163.082.842.833.092.762.492.412.662.041.981.031.400.600.6402.762.862.542.422.702.482.651.892.883.172.652.642.302.29000000002.722.38

5.785.585.255.365.445.405.405.455.415.215.487.857.747.277.817.697.367.517.257.387.517.096.857.181.541.961.631.671.621.641.501.431.471.541.267.767.617.297.427.847.307.477.347.096.956.426.425.855.075.154.084.223.076.746.836.967.236.646.726.506.567.207.557.317.187.056.751.821.641.651.821.821.641.651.826.616.41

2.221.741.942.212.371.901.842.282.281.962.291.451.451.201.421.451.351.281.421.181.261.371.541.364.443.623.644.214.014.053.893.923.663.653.721.301.061.161.031.231.560.981.381.451.331.351.611.542.102.222.402.492.801.431.371.501.421.301.341.361.351.521.361.321.431.291.573.313.633.203.213.313.633.203.211.371.49

4.374.885.284.634.285.064.974.584.404.854.773.733.904.303.573.864.633.974.604.314.304.384.363.904.935.165.684.914.864.985.285.315.325.365.484.374.694.454.704.383.704.504.034.224.264.885.234.924.893.555.374.065.484.934.674.114.344.784.524.414.444.383.644.034.384.664.115.805.425.665.445.805.425.665.444.605.22

746



SiO2Sample (%)

59.0659.2159.2959.4559.7859.9960.7561.79

AWI16 54.5054.6254.7354.8154.9354.9554.9955.0855.0855.3755.8356.18

AWI17 53.9154.6456.4257.0758.2158.3058.3658.3758.5958.6658.8359.1759.8362.3063.9467.1167.%71.34

AWI18 57.9258.1158.2758.6958.7759.1759.4059.7159.9160.5461.2562.70

AWI19 57.6557.9358.7859.4160.5060.6360.9661.1561.3561.3861.9262.6263.1363.2663.9964.2064.29

AWI20 58.6659.9960.3060.4560.7860.8461.1661.2661.9062.1262.53

TiO2(%)

::

1.10.17.12.26.05.11.26.14.53

1.561.351.36.36.47.46.42.55.33

1.381.451.151.030.981.011.421.491.331.391.431.401.500.841.231.271.070.550.640.351.771.511.571.341.211.381.511.371.421.101.320.901.181.241.221.191.131.071.171.041.011.211.070.971.101.010.900.840.881.331.261.381.361.481.211.201.281.101.191.06

A12O3(%)

15.8816.3816.6015.4216.0316.5215.7615.5816.4316.3216.5316.5716.6016.7216.8216.5116.2716.4116.6516.6816.1216.5717.3316.8116.0216.1516.2916.1916.4916.3416.3816.4315.9916.3017.1515.1515.1714.6415.1815.9816.0116.0016.6315.9515.9115.9216.2616.4616.5217.2116.6216.8016.6416.2616.0615.9116.0816.0615.9515.7615.8116.2515.0814.3615.8115.6615.7415.9716.2215.6215.9215.7016.2016.0616.0615.7015.6315.84

FeO(%)

9.098.808.459.738.698.488.197.98

10.4610.5410.0210.139.98

10.2710.0810.1910.2010.329.749.499.618.728.578.798.958.908.738.858.488.828.768.078.497.005.615.675.343.42

10.49.329.358.978.349.028.738.537.967.387.465.759.068.368.328.157.588.107.687.677.637.657.246.677.557.966.676.376.309.268.528.318.187.827.407.707.807.617.317.32

MnO MgO(%)

0.260.160.180.200.290.260.2300.330.210.250.160.2300.250.180000.2300.230.220.190.260.220.240.180.230.2400.170.180.2100.160.2100.160.160.3300.320.170.190.300.280.290.1700.260.330.260.220.340.270.230.230.220.250.230.320.240.190.20 (0 (0.30 (

o :0.170.250.180.220.280.240.260.290.250

(%)

2.332.192.151.952.311.891.691.523.453.593.703.473.643.473.583.443.293.313.423.111.931.653.352.992.212.362.282.532.252.362.172.501.863.95).22)))2.252.292.002.292.022.142.101.91L.811.43L.00).582.492.442.151.841.661.74L.781.641.58.23.28

1.27[.061.67).73).9O).72'.04.48

1.731.72.75.34.41.49.29.21.20

CaO K2O(%)

5.886.015.986.165.975.615.43 :4.488.358.337.897.827.978.097.958.09 (8.348.097.807.87

10.079.187.227.186.216.206.036.045.996.225.896.285.504.22 :4.57 :2.68 ;2.46 I1.40 :6.156.566.446.026.075.625.865.465.65

(%)

1.55L.551.451.881.56.65

2.021.951.24.21.05.08

1.081.01.01

).99.21.01.03.37.07

Lu.11.19.38.42

L46.43.46.33.57.48.72

>.39'.37>.45!.51S.37.55.28.60.30.25.40.37.54.53

5.90 2.005.275.096.966.686.435.90

.99

.99

.43

.65

.72

.985.53 2.075.65 1 .975.55 2.025.41 2.065.44 2.025.23 I5.30 :4.67 2

.18

.17

.234.19 2.495.98 1 .484.18 2.394.36 2.384.14 2.426.27 2.086.02 2.075.42 :5.50 :

.27

.195.30 2.265.33 2.205.35 2.275.00 2.374.79 2.444.79 2.474.61 2.50

Na2O(%)

4.754.474.783.844.244.504.565.463.623.534.504.604.204.023.864.104.064.164.163.563.143.884.714.785.294.965.165.015.094.624.834.975.205.214.956.135.655.354.614.804.305.335.285.164.855.175.124.904.965.664.254.464.384.864.984.494.264.594.635.034.814.824.983.974.985.105.024.294.174.594.374.515.064.484.334.714.844.77

747


Plate 1. SEM images of glass shards from Site 701, showing observed variability of shard morphology in all samples investigated. Bar = 0.1 mm.1. Equant, blocky-shaped grain with no vesicles. The cracks are presumably due to shock cooling in contact with water or ice. 2 and 3. Shattered,equant, blocky grains with low to medium vesicularity. 4. Highly vesicular, strongly elongated, tubular structures caused by a high eruption rate.The bubble caves are partly collapsed or deformed.

748


Plate 2. SEM images of glass shards from Site 701, showing observed variability of shard morphology in all samples investigated. Bar = 0.1 mm.1. Very strongly vesicular, pumiceous particle with typical Y-shaped vesicle junctions, indicating low viscosity and/or high volatile contents ofthe magma. 2. Strongly elongated, fibrous structure with delicate collapsed bubble walls. 3. Fragment of extremely elongated bubble cavities,with a very low width/length ratio. 4. Platy to cuspate structures caused by fragmentation of bubble walls.

749

40. geochemical investigations of volcanic ash ......geochemical investigations of volcanic ash...

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