Compositional Gradients and Gaps in High-silica Rhyolites of the Rattlesnake Tuff, Oregon

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Article

Reference

Compositional Gradients and Gaps in High-silica Rhyolites of the Rattlesnake Tuff, Oregon

STRECK, Martin, GRUNDER, Anita L.

Abstract

The Rattlesnake Tuff of eastern Oregon comprises >99% of high-silica rhyolite glass shards and pumices representing ∼280 km³ of magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass shards cluster in five chemical groups that range in color from white to dark gray with increasing Fe concentration. Compositional clusters are defined by Fe, Ti, LREE, Ba, Eu, Rb, Zr, Hf, Ta, and Th. Progressive changes with increasing degree of evolution of the magma occur in modal mineralogy, mineral composition, and partition coefficients. Partition coefficients are reported for alkali feldspar, clinopyroxene, and titanomagnetite. Models of modal crystal fractionation, assimilation, successive partial melting, and mixing of end members cannot account for the chemical variations among rhyolite compositions. On the other hand, ∼50% fractionation of observed phenocryst compositions in non-modal proportions agrees with chemical variations among rhyolite compositions. Such non-modal fractionation might occur along the roof and margins of a magma chamber and would yield compositions of removed solids ranging from syenitic to [...]

STRECK, Martin, GRUNDER, Anita L. Compositional Gradients and Gaps in High-silica Rhyolites of the Rattlesnake Tuff, Oregon. Journal of Petrology , 1997, vol. 38, no. 1, p.

133-163

DOI : 10.1093/petroj/38.1.133

Available at:

http://archive-ouverte.unige.ch/unige:154098

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Compositional Gradients and Gaps in

High-silica Rhyolites of the Rattlesnake Tu ff , Oregon

MARTIN J. STRECK ∗ AND ANITA L. GRUNDER

DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331-5506, USA

RECEIVED MAY 12, 1995 ACCEPTED AUGUST 22, 1996

The Rattlesnake Tuffof eastern Oregon comprises >99% of high- INTRODUCTION

silica rhyolite glass shards and pumices representing ~280 km³ of The origin, structure, and differentiation processes in magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass zoned magma chambers have been approached by many shards cluster in five chemical groups that range in color from white

workers through the study of ignimbrites since the to dark gray with increasing Fe concentration. Compositional clusters

ground-breaking work of Smith (1960) and Smith &

are defined by Fe, Ti, LREE, Ba, Eu, Rb, Zr, Hf, Ta, and Th.

Bailey (1966). Zoned high-silica rhyolite chambers have Progressive changes with increasing degree of evolution of the magma

been of particular interest, because the high viscosity occur in modal mineralogy, mineral composition, and partition

of such magmas inhibits crystal–liquid separation and coefficients. Partition coefficients are reported for alkali feldspar, because extreme trace-element gradients, relative to clinopyroxene, and titanomagnetite. Models of modal crystal frac-

major element variations, are not easily reconciled (Hil- tionation, assimilation, successive partial melting, and mixing of

dreth, 1979, 1981; Michael, 1983; Miller & Mittlefehldt, end members cannot account for the chemical variations among

1984). The generally high trace-element partition co- rhyolite compositions. On the other hand, ~50% fractionation of efficients in silica-rich magmas and the presence of ac- observed phenocryst compositions in non-modal proportions agrees

cessory phases with exceedingly high partition with chemical variations among rhyolite compositions. Such non- coefficients, coupled with low crystal contents, has made modal fractionation might occur along the roof and margins of a

the quantitative modeling of high-silica rhyolites exacting magma chamber and would yield compositions of removed solids

(see Mahood & Hildreth, 1983; Michael, 1988). In ad- ranging from syenitic to granitic. A differentiation sequence is

dition to compositional zonation, compositional gaps in proposed by which each more evolved composition is derived from

ignimbrites have been documented with implications for the previous, less evolved liquid by fractionation and accumulation,

chamber configuration and tapping mechanisms (e.g.

occurring mainly along the roof of a slab-like magma chamber. As

Blake, 1981; Blake & Ivey, 1986; Fridrich & Mahood, a layer of derivative magma reaches a critical thickness, a new layer

1987).

is formed, generating a compositionally and density stratified magma

The Rattlesnake Tuffof southeastern Oregon is com- chamber.

posed nearly entirely of high-silica rhyolite that defines five distinct compositional and mineralogic clusters. The differentiation of the different rhyolite compositions, using newly derived partition coefficients, the configuration of the pre-eruptive chamber, and the origin of the compositional gaps are presented here. The petro- genesis of the least evolved rhyolite and the evolution of

KEY WORDS:high-silica rhyolite; partition coefficients; rhyolite differ-

entiation; zoned ash-flow tuff; layered convection the mafic underpinning of the Rattlesnake Tuff system

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Fig. 1. Regional setting and outcrop pattern of the Rattlesnake Tuffin Oregon. Light stipple represents Blue Mountains Province, dense stipple shows Rattlesnake Tuff;Χ, proposed source area. Dashed lines with numbers are simplified isochrons in millions of years for NW-migrating

silicic volcanism, after MacLeod et al. (1976). Continuous lines indicate faults;_, Cascade composite volcanoes.

have been discussed by Streck (1994) and will be treated for spectacular banded pumices and a salt and pepper in detail in forthcoming papers. matrix of white and gray glass shards (Fig. 2). Exclusively white tephra occurs in some basal and distal sections or in rare basal fallout deposits, suggesting that white tephra represents magma from the top of the magma chamber THE RATTLESNAKE TUFF (see Smith, 1979). The tuff is fresh and little welded at many places, facilitating the sampling of individual vitric The 7·05 Ma Rattlesnake Tufferupted from the Harney

pumices. High-silica rhyolite (>75 wt % SiO2) makes up Basin, a center of Late Miocene silicic magmatism in

>99% of the tuff. Dacite pumices are minor (<1 vol. %) southeastern Oregon (Fig. 1). It is part of a north-

and cognate mafic inclusions, mainly in dacite, are rare westward-younging trend of silicic domes and tuffs

(p0·1 vol. %) (Streck, 1994).

(MacLeod et al., 1976) associated with widespread high-

Five distinct high-silica rhyolite compositions are rep- alumina olivine tholeiite lavas that define the High Lava

resented by pumice clasts and shards. From most to least Plains. Intermediate compositions are scarce.

differentiated, these are referred to as Groups A, B, C, The tuff consists of a single cooling unit, 10–30 m

D, and E. The rhyolites are metaluminous to slightly thick, that probably covered an area of ~35 000 km²and

peralkaline, with molar ratios of alkalis to aluminum of represents ~280 km³of magma (Streck & Grunder, 1995).

0·88–1·03 (Table 1). All pumices are crystal poor (0–1·3 The tuff typically has few lithic fragments and ranges

wt % crystals) but mineralogically distinct. Group A and from pumice rich, with pumice clasts as large as 60 cm

B pumices are white and A is essentially aphyric (Fig. 2, near the source, to pumice poor with distance from the

vent (Streck & Grunder, 1995). The tuff is remarkable Table 2). Pumices of the other groups range from beige

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Fig. 2. (a) Banded pumice block in the center of picture is 50 cm across and consists of two to three different high-silica rhyolites. Rhyolite ranges from gray to white with decreasing Fe concentration. (Note also small black dacite pumice at upper left of banded block.) Tuffis glassy throughout. (b) Typical vitrophyre of Rattlesnake Tuffcontaining differently colored high-silica rhyolite glass shards under plane-polarized light.

Color variation is related to Fe concentration as indicated by average FeO∗(n=3±1 SD) of individual shards obtained by electron microprobe analysis. Shard compositions correspond well to observed high-silica rhyolite pumice clusters. Horizontal field of view is ~3·5 mm.

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Table1:ChemicalcompositionofRattlesnakeTuffpumicesandglassshards GroupAGroupB RT14FRT55BRT165ERT173DRT173ERT173HMean±1rRT34DRT34ERT120ART220ART219AMean±1r XRF(wt%) SiO276·0077·4477·6077·7777·8277·6777·4±0·776·5377·1877·0277·0677·0±0·3 TiO20·120·110·110·120·110·110·11±0·010·120·120·110·120·12±0·01 Al2O311·7212·1012·0711·9711·9111·9612·0±0·111·8211·7711·8011·8211·8±0·02 Fe2O3∗0·860·860·860·890·870·870·87±0·011·321·371·411·391·37±0·04 MnO0·080·070·080·090·080·080·08±0·010·090·090·090·100·09±0·01 MgO0·100·13n.d.0·030·040·050·07±0·040·100·090·020·080·07±0·04 CaO(2·32)0·300·300·280·260·260·28±0·020·350·330·390·260·33±0·06 Na2O3·333·353·893·683·813·463·59±0·243·193·063·363·723·3±0·3 K2O5·475·625·065·175·085·535·32±0·256·435·955·745·405·9±0·4 P2O5n.d.n.d.0·020·010·020·020·02±0·010·050·030·050·050·05±0·01 prn.total99·799·699·699·2100·299·699·399·199·497·5 AI0·970·960·980·970·990·980·98±0·011·030·971·001·011·00±0·03 XRF(p.p.m.) Rb116121123123122122121±39191949492±2 Ba19402518103925±12122132131115125±8 Sr(36·8)3·33·42·01·12·12±15·43·712·5128±5 Zr169180175174175175175±4304308302258293±23 Nb37·438·840·340·639·339·439±131·532·033·032·132·2±0·6 Y969810110110010099±29396988994±4 Pb17·918·318·921·117·220·019±118·019·316·91918±2 Zn53798891878881±1499106101101102±3 Ga17·218·417·817·817·917·617·8±0·417·117·918·021·019±2 V6·42·42·5n.d.n.d.n.d.4±25·65·81·4n.d.4±2 Cu1·90·81·22·01·41·01·4±0·52·11·94·674±2 Ni10·911·114·012·612·614·313±19·89·515·01412±3 Cr1·52·51·6n.d.n.d.n.d.1·9±0·62·22·40·621·8±0·8 Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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GroupAGroupB RT14FRT55BRT165ERT173DRT173ERT173HMean±1rRT34DRT34ERT120ART220ART219AMean±1r INAA(p.p.m.) FeO∗(wt%)0·750·740·800·760·780·780·77±0·021·141·221·291·241·271·23±0·06 Na2O(wt%)3·403·113·863·583·623·353·5±0·33·533·423·303·441·943·1±0·7 Cs4·104·234·544·284·504·484·4±0·23·073·193·303·182·883·1±0·2 U4·614·654·484·434·774·944·7±0·23·263·523·623·633·173·4±0·2 Th8·869·939·879·299·629·479·5±0·47·157·717·697·847·577·6±0·3 Hf6·756·957·247·127·157·137·1±0·29·099·619·419·689·799·5±0·3 Ta2·052·152·282·142·152·162·16±0·071·681·751·811·711·621·71±0·07 Sb1·321·431·421·501·521·551·46±0·081·261·331·411·391·701·4±0·2 As4·83·55·33·94·44·44·4±0·64·7n.d.5·74·55·35·1±0·6 Sc3·763·964·053·903·893·933·9±0·13·473·693·843·773·973·8±0·2 Co0·420·080·050·060·050·080·1±0·20·180·320·280·140·580·3±0·2 La19·919·121·720·219·019·920±137·438·939·438·737·538±1 Ce46495455514951±3859097998892±6 Nd29233031322829±3474553504849±3 Sm9·098·839·589·359·069·579·3±0·312·6312·8913·5713·4512·1112·9±0·6 Eu0·650·670·650·640·650·650·65±0·011·071·221·161·211·271·19±0·08 Tb2·182·232·302·242·172·222·22±0·052·532·702·682·642·142·54±0·23 Yb9·509·6210·5510·3510·2210·5410·1±0·59·489·669·799·618·489·4±0·5 Lu1·451·491·621·621·541·611·56±0·071·351·381·421·371·231·35±0·07 Eu/Eu∗0·190·200·180·180·190·180·19±0·010·240·260·240·260·310·26±0·03 Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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Table1:continued GroupCGroupD RT165ART120BRT80ART62AMean±1rRT50DRT173BRT173IRT173LRT4AMean±1r XRF(wt%) SiO277·1176·8276·6376·9±0·276·1276·5975·3176·0±0·6 TiO20·120·130·140·13±0·010·140·140·140·14±0·00 Al2O311·7711·6712·0611·8±0·211·8811·9712·1712·0±0·2 Fe2O3∗1·611·641·911·72±0·21·811·931·951·89±0·07 MnO0·090·080·080·08±0·010·080·090·090·09±0·01 MgOn.d.n.d.0·040·060·05(0·50)0·06±0·01 CaO0·350·430·320·37±0·060·620·450·650·57±0·11 Na2O3·703·743·013·5±0·42·973·842·353·1±0·8 K2O5·235·435·815·5±0·36·294·926·836·02±1 P2O50·010·06n.d.0·040·030·020·010·02±0·01 prn.total99·799·399·299·599·699·2 AI1·001·030·930·99±0·050·990·970·930·96±0·03 XRF(p.p.m.) Rb80787778±2676763688169±7 Ba379491664511±144120112301120114011161161±51 Sr6·512·56·89±318·812·424·319±6 Zr371372404382±19426433432430±4 Nb30·529·028·329±126·027·225·626·3±0·8 Y90858286±476797175±4 Pb16·413·617·416±216·114·816·916±1 Zn106103113107±5106115107109±5 Ga17·718·118·218·0±0·318·017·617·917·8±0·2 V1·50·13·62±24·21·83·73±1 Cu1·94·22·93±13·34·06·35±2 Ni9·911·610·111±19·19·78·49·0±0·7 Cr0·9n.d.1·31·1±0·32·5n.d.43±1 Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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GroupCGroupD RT165ART120BRT80ART62AMean±1rRT50DRT173BRT173IRT173LRT4AMean±1r INAA(p.p.m.) FeO∗(wt%)1·371·441·681·441·48±0·141·571·691·691·721·711·68±0·06 Na2O(wt%)3·683·622·763·083·3±0·42·853·933·283·472·353·2±0·6 Cs2·802·642·713·032·8±0·22·272·382·172·203·402·5±0·5 U3·122·672·863·183·0±0·22·413·162·492·442·332·6±0·3 Th6·876·806·937·947·1±0·56·406·475·876·026·156·2±0·3 Hf9·8610·0410·7310·5010·3±0·410·3910·5510·3010·6010·7210·5±0·2 Ta1·561·521·511·811·60±0·141·341·341·291·381·401·35±0·04 Sb1·161·171·281·331·2±0·11·161·191·141·121·051·13±0·05 As5·24·5n.d.4·84·8±0·4n.d.3·93·73·13·63·6±0·3 Sc3·553·584·054·343·9±0·43·573·763·653·573·733·66±0·09 Co0·080·170·390·360·3±0·20·400·110·120·240·260·2±0·1 La49·951·750·445·149±353·054·351·052·949·352±2 Ce113124112107114±7117130117118113119±6 Nd595949·459·457±5536858595158±7 Sm14·5514·4412·2614·6014·0±1·112·1614·0113·1013·8012·4213·1±0·8 Eu1·471·541·811·341·54±0·201·972·032·012·041·942·00±0·04 Tb2·422·322·392·642·44±0·142·172·232·122·182·062·15±0·06 Yb8·388·588·229·988·8±0·87·907·957·327·587·157·6±0·4 Lu1·281·331·231·451·32±0·091·201·221·081·151·071·14±0·07 Eu/Eu∗0·300·320·420·270·33±0·070·480·440·460·450·470·46±0·01 Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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Table1:continued GroupEBanded RT34ART34BRT34CRT55ART140ART173ART173CMean±1rRT17ART55CRT127A XRF(wt%) SiO275·3275·4275·0875·7374·5275·7075·4775·3±0·475·9876·2475·23 TiO20·170·180·180·160·180·160·160·17±0·010·140·150·16 Al2O312·4212·1912·3212·1913·0412·4712·2612·4±0·312·2312·0712·11 Fe2O3∗2·232·072·342·072·372·232·182·21±0·121·671·872·09 MnO0·090·070·090·070·110·100·100·09±0·020·090·090·09 MgO0·120·150·070·080·10n.d.0·010·09±0·050·070·130·10 CaO0·490·550·630·630·370·540·500·53±0·090·870·430·69 Na2O3·573·563·882·642·544·184·613·6±0·82·722·222·84 K2O5·575·775·386·346·754·594·695·6±0·86·206·776·62 P2O50·020·040·030·080·010·020·010·03±0·020·030·010·06 prn.total99·598·899·799·1100·399·599·599·799·398·9 AI0·960·990·990·920·880·951·030·96±0·050·910·910·98 XRF(p.p.m.) Rb6364636271626464±3847770 Ba19471898191419991839203118351923±758439981825 Sr23·922·826·329·218·422·523·424±319·013·625·4 Zr457469460464488474457467±11355395444 Nb25·625·225·425·826·726·826·426·0±0·629·927·726·8 Y7675757478777676±1867974 Pb15·414·714·915·49·713·110·313±28·915·415·9 Zn117106117108110120113113±560115116 Ga1918·318·318·319·919·018·618·8±0·617·917·817·5 V7·720·916·84·4n.d.3·71·99±82·11·94·0 Cu3·14·13·03·58·34·34·74±23·38·94·3 Ni8·410·69·48·210·710·08·89±19·310·610·0 Cr4·23·42·91·0n.d.0·30·32±23·14·0n.d. Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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GroupEBanded RT34ART34BRT34CRT55ART140ART173ART173CMean±1rRT17ART55CRT127A INAA(p.p.m.) FeO∗(wt%)2·001·872·091·832·002·021·911·96±0·091·481·711·82 Na2O(wt%)3·863·903·712·482·424·044·503·6±0·82·962·282·79 Cs2·282·322·332·252·442·332·352·33±0·062·962·632·43 U2·302·442·353·102·412·682·182·5±0·33·502·602·70 Th5·635·696·125·475·335·75·675·7±0·27·116·725·64 Hf11·1711·3111·1911·1110·7611·2011·0711·1±0·210·0510·4110·63 Ta1·331·361·351·341·371·361·351·35±0·011·621·531·41 Sb1·181·281·341·081·161·141·171·19±0·091·281·221·19 As4·95·44·53·44·34·24·04·4±0·64·03·93·5 Sc4·294·334·834·125·024·514·324·5±0·34·074·854·18 Co0·260·500·590·500·220·140·130·3±0·20·180·212·19 La51·151·049·049·853·551·951·751±144·549·950·4 Ce112110106108126120112113±710293126 Nd5250515065615855±6485645 Sm12·8512·7612·2611·7514·1613·5613·3913·0±0·813·0414·0213·41 Eu2·562·952·712·502·672·602·482·64±0·161·711·942·26 Tb2·172·222·162·112·122·162·132·15±0·042·332·222·06 Yb7·687·657·517·567·997·977·917·8±0·28·898·247·91 Lu1·161·141·081·161·251·241·231·18±0·061·321·251·19 Eu/Eu∗0·600·680·650·620·590·580·560·61±0·040·390·420·52 Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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Table1:continued FalloutShardmatrixsampleRT75 AF1AAF1BAF1CMean±1rGlassshardpopulationsBulktuff INAA(p.p.m.) FeO∗(wt%)0·880·880·880·88±0·000·891·161·391·491·791·791·22 Na2O(wt%)2·782·112·302·4±0·43·493·413·343·323·403·643·51 Cs3·844·454·014·1±0·33·343·112·972·953·012·873·14 U6·835·737·396·7±0·84·13·72·63·22·42·73·38 Th10·6911·2010·8010·9±0·39·167·396·857·026·135·637·39 Hf8·078·158·088·10±0·048·059·3610·1510·4511·3011·159·02 Ta2·572·612·582·59±0·022·041·661·461·551·511·321·68 Sb1·611·501·481·53±0·071·751·261·832·591·411·251·32 As6·13·84·24·7±1·24·14·04·73·592·14·53·7 Sc4·604·714·524·6±0·14·363·764·074·104·644·314·15 Co0·240·250·170·22±0·040·210·170·310·310·510·310·23 La20·919·017·819±225·645·051·855·654·455·840·1 Ce51534650±47011112213513312894 Nd33322831±335536267716848 Sm10·588·998·549·4±1·110·4513·2514·7014·514·2313·8312·4 Eu0·770·650·560·66±0·110·871·311·661·762·442·421·45 Tb2·432·191·922·18±0·262·392·512·322·392·172·072·26 Yb11·269·588·839·9±19·999·579·028·848·368·179·26 Lu1·661·501·341·50±0·161·511·421·361·351·261·251·38 Eu/Eu∗0·200·190·180·19±0·010·220·280·350·370·530·540·34 RattlesnakeTuffpumiceanalysesarearrangedingroups(A–E)accordingtoclustersobservedinscatterdiagrams(Fig.3),plusagroupofsmallpumices(Ø~1cm) fromathinprecursorfalloutdeposit.MajorelementXRFdataarenormalizedto100%,volatilefree;prenormalization(prn.)totalsarelisted.AIismolarratio(Na+K)/ Al.ItalicizednumbersarevaluesfromINAA;valuesinparenthesesareanomalouslyhighandarenotused.SampleAF1cispumicecompositeofsixsmallpumices. Fe2O3∗andFeO∗aretotalironconcentrationsexpressedasFe2O3andFeO,respectively. Downloaded from https://academic.oup.com/petrology/article/38/1/133/1425908 by guest on 23 August 2021

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to be representative for regional, local and stratigraphic Table 2: Modal mineral abundances of

chemical variations.

representative samples from Rattlesnake Major-element analyses were made on fused glass disks, Tuff pumice clusters derived by heavy with 5:1 flux to rock powder, and selected trace elements on pressed powder pellets by X-ray fluorescence (XRF ) liquid mineral separation

at Stanford University using a Rigaku instrument, except for sample RT220A, which was analyzed at Washington

Sample: RT34E RT165A RT173B RT173C

State University, Washington. Trace element con-

Pumice group: B C D E

centrations were determined by instrumental neutron activation analysis (INAA) at the Radiation Center, Or-

Minerals (wt %)

egon State University, using a 1 MW Triga reactor.

alkali-fsp 0·025 0·736 0·874 0·522

Analytical uncertainties for XRF trace elements, based

quartz 0·032 0·536 0·022 0·002

on replicate analyses of US Geological Survey (USGS)

cpx 0·007 0·028 0·055 0·062

standards G2 and AGV1, are: <5% for Nb, Zr, Y, Sr,

magnetite 0·027 0·026 0·041 0·043

Rb, Ga and Zn; <10% for Cu, V and Ba; and <5–15%

fayalite — — 0·001 0·007

for Ni, Cr and Pb. Uncertainties for INAA trace elements

biotite trace — trace trace

(also based on replicate analyses of in-house standards,

total min. % 0·091 1·326 0·993 0·636

CRBIV and SPGa) are: <5% for Fe, Na, Co, Eu, Hf,

Glass 99·910 98·674 99·007 99·364

La, Sc, Sm and Yb; <5–10% for Ce, Cr, Lu, Ta, Tb

total sample wt (g) 477 1022 752 1154

and Th; <5–15% for Ba, Cs, Nd, Rb and Zn; and <15%

Accessory minerals

for Ni and U. The range of uncertainties for single

(× =present)

elements is based on the concentration range observed

zircon × × × × in standards used as monitors.

apatite × × × × Mineral separates were obtained from five rep-

pyrrhotite × × × resentative pumices. After crushing with hammer and

chevkinite × × jaw crusher, pumices were sieved. Sieving was done

sequentially with the following mesh size: 2 mm, 1·7

Pumice from cluster A (RT173H) had a total weight of 580 g, mm, 990lm, 750lm, 500 lm, 300 lm, 180 lm, and

but only eight crystals (feldspar and quartz) were retrieved,

106 lm. After first sieving, all material >990 lm was

making the pumice ‘aphyric’.

further crushed in small proportions until all pumice material passed through 990lm mesh. Material for glass separates was handpicked before heavy liquid separation to insure uncontaminated Br values. Using bromoform to gray and are sparsely phyric. Phases observed in all and tetrabromoethane, sequential heavy liquid mineral phyric pumices are alkali feldspar, Fe-rich clinopyroxene, separations were done for each size fraction. First glass titanomagnetite, quartz and accessory zircon and apatite. was floated, keeping the density of the liquid close to Additional minerals that occur in some pumices are that of the glass to insure that minerals with attached fayalite, biotite, pyrrhotite, and chevkinite (Table 2). glass also sank. The separated glass was weighed. Next, A rough estimate of the volumetric proportions of the each size fraction was either separated into individual different rhyolite groups can be made by using the phases or into a feldspar–quartz and ‘mafic’ fraction distribution of gray vs white shards in the matrix, which through handpicking, magnetic or heavy liquid sep- roughly groups composition A and B and composition aration. The proportion of fsp:qtz and of timt:cpx was D and E. This way, a 1:1 proportion is derived which sometimes visually estimated and recalculated to weight seems to be a maximum value for the ‘gray magma’ as per cent. The total mode of the pumice (Table 2) rep- white shards dominate distally. resents the sum of the weights of different species in all size fractions. The agreement between weights of total mineral yields before and after separation into individual phases deviated between 1 and 10%, with lower yields in the separated minerals partly attributable to dissolution ANALYSIS AND MINERAL

of glass rinds during washing with dilute hydrofluoric PREPARATION acid after first determination of total mineral yields.

Extrapolation of mineral size distribution indicates that Mainly unbanded, glassy pumices were selected for ana-

loss of minerals in the <106lm fraction is minor.

lysis. Other analyzed glassy samples included three macro-

Mineral separates for INAA analysis were prepared by scopically banded pumices and one bulk tuff and its

different shard populations (Table 1). Sampling was done hand-picking from the 300–500lm fraction for feldspars,

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106–300lm for titanomagnetites, and mainly 106–180 systematically with silica. Elements that increase with lm fraction for pyroxenes. Separates were not powdered. silica, that is, those that are enriched compared with the All selected feldspars were clear and inclusion free. Titan- least evolved compositions (Group E), are Cs, Rb, U, omagnetite separates were superficially 100% clean. In Th, Ta, Nb, Pb, Y, HREE (heavy rare earth elements), the case of the pyroxenes, the selected size fraction was Sb, and probably Ni (Fig. 4). Depleted elements are Fe, small enough so that most pyroxenes were translucent, Ti, Mg, Ca, Ba, Sr, Eu, Zr, Hf, LREE (light rare making screening for inclusions feasible. All pyroxene earth elements), and Zn. Ga, As, V, Cr, Co, and Mn separates were visually clean at >99%. Mineral separates concentrations are nearly constant; precise Mn data were multiply washed in mild acids, distilled water, and (INAA) exist only for pumice glass separates and suggest acetone. Br values of <6 ppm for all mineral separates a minimum for Group B pumices. Similarly, a minimum verify the almost complete removal of heavy liquid res- is suggested by Sc and Al and a maximum by Tb.

idues from mineral surfaces (see Table 4, below). Glass Na and K concentrations become more variable from separates were picked from the 300–500lm fraction and Group A to Group E rhyolites (Fig. 3), whereas total separates were completely clean of crystals, except sample alkalis (Na2O+K2O) are nearly constant, with an average RT165A where very tiny (±10lm) Fe-oxide(?) crystals of 9·1 wt %. Early post-emplacement ion exchange is were sparsely but evenly dispersed. likely to have caused most of this scattering, by increasing Mineral and glass separates were analysed by INAA K and reducing Na contents (Fisher & Schmincke, 1984, at the Oregon State University Radiation Center using p. 328). The samples richest in sodium are presumably a 1 MW Triga reactor. Weights of analyzed samples closest to the magmatic concentration. With this as- ranged from 10 to 70 mg. Short activation was performed sumption, Na2O slightly decreases by ~0·6 wt % and at a power level of 50 kW for 5 min and long activation K2O increases by the same amount from Group E to at 1 MW for 12 h. Counting was done sequentially, three Group A. The peralkalinity index [AI=molar (Na+K)/

times after short activation and five times after long Al] of the most sodic sample of each group ranges from activation using intrinsic germanium and low-energy 0·98 to 1·03, suggesting almost constant alkali–aluminum photon (LEP) detectors. balance throughout the compositional range. Alkali mo- One non-welded, glassy bulk-tuff sample was used bility did not affect other ‘mobile’ elements (Zielinski, for separation of different shard populations. Magnetic 1982) because neither Na nor K correlate with Cs, Rb, procedures ( Frantz magnetic divider) separated loosened and U (Fig. 3).

glass shards of the 180–500lm fraction (weighing ~100 The observed enrichment and depletions are nearly g) into a white shard and a mixed-gray shard fraction. identical with trends in the Lava Creek and Huckleberry Using heavy liquids, the white shard fraction was split Ridge Tuffs of the Yellowstone caldera complex (Hildreth into a lighter and heavier fraction, yielding materials for et al., 1984) and similar to those of the metaluminous the first two shard samples analyzed by INAA. Similarly, Bishop Tuff (Hildreth, 1979). For enrichment trends of splitting the mixed-gray fraction into successively denser 36 elements, the differences between the Rattlesnake and fractions yielded seven shard fractions from which four Bishop Tuffs are mainly enrichment of Na, Mn, Sc, and were selected for analysis. Therefore, the different shard Sm, and constant Hf and Zn compared with depletions populations observed in the matrix of the tuffare thought of these elements in the Rattlesnake Tuff (Fig. 4). Cl to be only approximately represented by the six analyzed decreases from Group E to A, probably reflecting de- bulk shard samples because of imperfect separation. gassing during eruption. Comparison of enrichment Shard separates used for analysis were free of crystals trends from Group B to A with the Bishop Tuffreduces and lithic fragments. INAA was performed after long the degree of discrepancy between the two. The slightly activation at 1 MW for 6 h with corresponding counting peralkaline Tala Tuff( Mahood, 1981) with constant Si, procedures (see above). Fe, Mg, and Eu, and enriched Zr, Hf, Zn, Sm, and Tb

Microprobe analyses on minerals were done using a differs strongly from the Rattlesnake Tuff.

fully automated Cameca SX-50 electron microprobe at Oregon State University. For most minerals, beam cur- rent was 30–50 nA, accelerating voltage 15 kV and beam diameter 1–5lm.

COMPOSITIONAL CLUSTERS

High-silica rhyolite compositions of the Rattlesnake Tuff bulk pumices cluster in several Groups (A–E). The clusters COMPOSITIONAL VARIATION are mainly established by La(LREE), Eu, Ba, Ta, Nb, Zr, Hf, Rb, Cs, Th, U, Ti, and Fe. Pumices within a The Rattlesnake Tuffhigh-silica rhyolite pumices range

cluster are macroscopically the same. The relative po- in composition from 74·5 to 77·8 wt % SiO2 (Table 1,

Fig. 3). Despite this narrow range, many elements vary sition among groups and of individual pumices within

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Fig. 3. Major and trace element variation diagrams for Rattlesnake Tuff pumices and shards. Cs–K2O and U–Na2O diagrams show that probably limited ion-exchange of Na with K did not affect significantly even mobile elements because clustering of pumices in terms of Cs and

U contents is mainly intact; sh, shard separate.

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Fig. 3.