olivine and chromian spinel in primittve calc-alkaline and tholeiitic lavas from the southernmost

20 Pages·2006·1.77 MB·English

453
Thz Ca wdian Miner alo gist
Vol. 35, pp.4 53472 (1997)
OLIVINEA ND CHROMIANS PINELIN PRIMITTVCEA LC-ALKALINAEN D THOLEIITIC
LAVASF ROMT HES OUTHERNMOSCTA SCADER ANGEG, ALIFORNIA:
A REFLECTIOONF RELATIVEF ERTILITOY FT HES OURCE
MICHAELA. CLYNNE1
U.S. GeologicalS umey,V obano HazardsT eam"3 45 MidllefrzW Road
Mail Stop9 10,M enloP arh Califurnin94025U, .S.A.
LARS E. BORGX
Departmento f GeologicalS cieru:esU, niversityo f Texasa t Awtin
Austin Texas7 8712.U .SA.
ABSTRAcT
'.9
Chromians pinela ndc oexis olivine phenocrystsfr om a geochemicallyd iverses uiteo f primitive tholeiitic andc alc-alkaline
basaltsa ndm aguesiana ndesitesfr om the Lassenr egion,i n the southernmosCt ascadeR ange,i n California,s howt hat the sub-arc
mantle is zoned.D epletedc alc-alkalineb asaltsa ad magnesiana ndesitese rupt in the forearc region, and calc-alkalineb asalts
containi ncreasinga bundanceos f incompatiblee lementst oward the backmc.H igh-aluminao livine tholeiites erupt from the arc
andb ackarca reas.O livine from all thesel avasd isplaysa limited compositionarl ange,f rom Fo86t o Fo9l, andc rystallizeda t high
temperatureg, enerally1 225-Ln5"C. Chromians pinelt rappedi n the olivine phenocrystsd isplaysa large raageo f composition:
Cr# valuess pant he range9 J6. ExcessA l in the spinel relative to that in l-atm spinel suggeststh at it crystallizeda t elevated
pressureT. he phenocrystsin thesel avasa rei n equilibrium with their hostl iquids. The fulI rangeo f Cr# of the spinelc ompositions
cannotb e explainedb y differentiationo r variablep ressurev, ariationsi n.t(Oz), subsoliduse quilibrationo r variationsi n degreeo f
partial melting of a single peridotitic source.R ather, the systematicc ompositionald ifferencesa mong phenocrystsin these
primitive lavas result from bulk chemical variability in tlefu mantle sources.C orrelationsb etween spinel and host-rock
compositionss upportt he assertiont hat the geochemicadl iversity of Lassenb asaltsr eflects tle relative fertility of their mantle
sources.
Keywords: chromians pinel,o livine, arc basalt,m antlef ertility, Lassen,C ascadea rc, Califomia.
Sotrfiraens
Les ph6nocristauxd e spinelle chromifbre et d'olivine coexistantep rovenantd 'une suite g6ochimiquemendt iversifi6e de
basalt€s thol6iitiquese t calco-alcalinse t d'anddsitesm agn6siennepsr ovenantd e la r6gion de Lassen,d ansl e secteurs ud de la
chalned es Cascadesd, ansl e nord de la Califomie, montrent que le manteaus ousc et arc est zon6.l rs basaltesc alco-alcalins
st6rilese t les and6sitesm agn6siennesse trouventd ansI 'avant-arc,e t les basaltesc alco-alcalinsd avantagee nrichis en 6ldments
incompatibless e trouventp lutdt vers l'arridre-arc.L es thol6iites d olivine riches en Al affleurent aussib ien da::sl a zonea xiale
de I'arc que dans I'arribre-arc. L'olivine de toutes ces vari6tds de laves basiquesf ait preuve d'un intervalle restreint de
composition,e nte Fq6 et Fo91,e t a cristallisd I une temp6raturer elativement6 1evdee, nte 1225 et lns"C. Ir spinelle
chromifdre pi6g6 dans les ph6nocristauxd 'olivine, en revanche,m ontre un intervalle important de composition,c orlme en
t6noigne I'intervalle desv aleursd e Cr#, entre9 et 76. Un exc6ds6 d'slrrminium dansl e spinellep ar rapport aux compositions
dquilibrdesd un atmosphdrefa it penserq ue sa cristallisationa eu lieu i une pression6 lev6e.L es ph6nocristauxd ansc es laves
semblent6 quilibr6sa vecl a compositionprobabled u liquide coexistaatL. 'intervalle complete n Cr#du spinelled esl avesn e peut
rdsultern i d'une diff6rentiationo u d'une pressionv ariable de cristallisation,n i de variationse n fugacit6 d'oxygbneo u de taux
de fusion partielle d'un seul socle pdridotitique, ni de r6-6quilibrage subsolidus.N ous croyons plutOt que les variations
systfmatiquese n compositioni mpliquant les phdnocristauxd esl avesp rimitives illustrent une variabilit6 dansl a compositiond e
leurs sourcesd ansl e manteau.D 'aprbs les corr6lationse ntre la compositiond u spinelle et cel1ed es rochesh 6tes,l a diversitd
g6ochimiquep armi les basaltesd e la suite de Lassen d6pendraits urtout de diff6rencesd ans la fertilit6 de leurs sources
manteuiques.
(Traduit par la R6daction)
Mots-cl6s: spinellec hronifbre, olivine, basalted 'arc, ferdlit6 du manteau,L assen,a rc desC ascadesC, alifomie.
I E-mail address:m [email protected]
* Presenta ddressN: ational Aeronauticsa nd SpaceA dministrationJ ohnsonS paceC enter,S N4, Houston,T exas7 7058,U .S.A,
454 THE CANADIAN MINERALOGIST
INTR.oDUcnoN hundreds of coalescingv olcanoesh as built a broad
mafic pladorm in the Lassena rea.I ndividual volcanoes
The large rangeo f solid solution in chromians pinel range from monogeneticc inder cones of basall and
and its common occurrencei n basaltsa nd peridotites basaltic andesitet o larger lava cones and shields of
have promoted its use as a pefrogenetici ndicafor of basaltict o andesiticc omposition( Clynne 1990).L inear
environment of crystallization. The composition of alignmentso f vents demonstrateth at the locations are
chromians pinel in igneousr ocks exhibits a significant related to north- to north-northwest-striking normal
lange of variation( Nlan et al. 1988),a nd correlates faults that reflect extensionr esulting from overlap of
with the major-element composition of coexisting Basin andR anget ectonicso n the volcanic arc (Guffanti
primitive liquids (rvine 1967, Eales 1979, Dick & et al. L990).A lthough primitive lavas are sparse,t hey
Bullen 1984).T his observationc. ombinedw ith the fact are widely distributedi n the region, and as a result of
thatcbromians pineli s the liquidus phasein manymelts crustale xtension,a rem ore commont han in otherp arts
ofbasalticc ompositions, uggesath ati t potentiallyr ecords of theC ascadRe ange@ acone t aL L997).T\em ajority
information about the most primitive aspectso f melt of primitive basalts discussedb y Clynne (1993) and
composition.O n this basis, spinel compositionsh ave Borg et al. (1997) are lesst han 0.5 Ma in age,a nd thus
beenu sedt o consfraind ifferentiationp rocessess ucha s areb roadly contemporaneousS.a mplel ocationsc an be
fractional crystallization,r nagmam ixing, and magma- found in Clynne (1993) and Borg et al. (1997).
chamberrecharge(e .9.,N atlande t al. 1983, AJlaneta l.
1988). The strong correlation between the Cr# GsocHnvtrsrRYA NDP ETRocENETIc
[100Cr(Cr + Al)] of spinel and the modal mineralogy Moosls ron TIfi OruCNO r LESSENM AGMAS
of the host peridotiteh as led severala uthorst o suggest
that the compositiono f spinel in lavas may be useda s
This paper focuses on tle constraintst hat can be
an indicator of the degreeo f partial melting (Dick &
imposed on models of the petrogenesiso f primitive
Bullen 1984, Dick 1989) and source fertility (Fujii
lavas from the Lassenr egion by the compositionso f
1989).B ut the compositiono f cbromians pinel also is
their phenocrystsW. e use the termp rimitive to denote
dependento n intensive variabless uch as pressurea nd
/(Or) (Sigurdsson& Schilling 1976, Fisk & Bence magnesianl avas with simple assemblageso f pheno-
crysts and low contents of phenocrysts (generally
1980,R oeder& Reynolds 1991),a nd on tle extent of
melt differentiation (Arai & Takahashi 1987, Ami 4Vo), that [6v6 high contentso f compatiblee lements
(e.g.,N i > 100p pm, Cr > 200 ppm), and FeO*/IvIgO
1994a)T. hereforeb, eforec bromians pinelc anb e applied
values and compositionso f ferromagnesianm inerals
as a pefogenetic indicator, compositional variation
in equilibrium with mantle assemblage(sM g# > 86).
resulting from thesef actorsm ust be assessed.
Generally,t hesel avash ave2 8 wtVoMgO and4 3 wt%o
himitive arc lavasf rom the Lassenr egiono f northem
California containc bromians pinel that exhibits a large SiO2, although some sampleso f magnesiana ndesite
compositionalr ange. Here, rr\ieu se the compositions also meet thesec riteria. Lavas containingp henocrysts
with reversez oning or anomalousa bundanceosf one or
of coexisting cbromian spinel and olivine to infer the
relative depletioni n the peridotite sourceso f the suite more compatiblee lementso, r lavasc ontainingm ultiple
ofprimitive Lassenla vas.T hesel avasp rovide an excel- populationso f phenocrysts,o r other evidences ugges-
lent test of the usefulnesso f chromians pinela s a petro- tive of a complexh istory, are commoni n the area but
genetic indicator becauseb oth the degree of partial are not consideredh ere.
melting and relative fertility of the source peridotite Lavasf rom the Lassena rqr can be divided into two
haveb eene stimai€d on the basiso f major,t racee lement, types;t he mostm agnesiano f eacht ype meett he criteria
and isotope geochemistry( Clynne 1993, Borg et al. above.T he fust goup is similar to circumpacificc alc-
1997). We review the compositional dependenceo f alkalineb asaltf ound worldwide, and is called high-
chromians pinel on the intensivev ariablesp ressurea nd aluminab asaltb y many investigators( e.9.,K uno 1960,
as well as on extent of fractional crystallization Waterc1 962.E wart & LeMaitre 1980"W alker 1981,
"f(Oz),
andp artial melting. Subsequentlyc, orrelationsb etween Kay et al. 1982). Calc-alkalinem agnesiana ndesites
the compositionso f coexistings pinela ndo f olivine and with primitive mineralogyt haf arec hemicallys imilar to
geochemicailn dices of differentiation,p artial melting, boninites and rocks found in Japan (Iatsumi 1981,
and source composition in the Lassenl ava suite are 1982)a ndt he Mt. Shastaa rea@ akere t al. 1994)a lso
discussed. are presenti n the Lassen area. The secondg roup is
similar to low-K, high-Al olivine tholeiite from the
GEoLoGIcALS grrwc Medicine Lake Highland @onnelly-Nolane t al. L99L,
Baker et al. 1991)a ndt he Devils Gardena reao f north-
The subduction-related magmatic regime of the easternC alifornia McKee et aI. 1983), and is found
southernmostC ascadea rc was describedb y Guffanti tbroughout the northwestemG reat Basin (Hart 1985,
et al. (1990),a nds ummariesc anb e found in Bcrg et al. Haft et aL 1984).T hesel avasw erec alledh igb-Al olivine
(1997) andBacor et al. (1997).E ruption of lavasf rom tholeiite or HAOT by the above investigators and
OLIVINE AND CIIROMIAN SPINEL IN PRIMITT\TE LAVAS 4f)
Bacon et al. (L997). HAOT vents are found from the lavas.P rimitive CA lavasa re found from the forearct o
arc axis to the backarc,b ut are absentf rom the forearc. the backarc. Although exceptionse xist, low-(Sr/Fh,
basaltsc haracterizeth e arc axis and backarc,w hereas
Calc- a llcalinel ava s ( CA) high-(Sr/P[' basaltsa nd -agnesian andesitesc harac-
terize the forearc. Using incompatible trace-element
Calc-alkaline( CA) lavas display a considerable abundancesB, org et al. (1997) modeledt he composi-
continuumo f compositionald iversity Grg. 1). At one tional continuuma s the result of the productiono f arc
end of the continuuma re magmnsw ith low contentso f magrnast hrough the interaction of mantle peridotite
incompatiblee lementsh, igh ratios of concentrationso f with a decreasingp roportion of a slab-derivedf luid
large-ion lithophile element to high field-strength compone,nfrto m the forearct o the backarcT. heir models
element$( ULEIHFSE), and Sr and Nd isotopic ratios suggestt hat the degreeo f melting of the mantler anges
approachingth oseo f mid-ocean-ridgeb asalt.T he high from a maximumo f about 107of or high-(Sr/P),'b asalts
abundanceo f Sr and bt$t STILREE( light rare-earth and magnesiana ndesitese ruptedi n the forearct o a few
element) and Sr/P values relative to primitive mantle pocent for the low-(Sr/P)rbya saltse ruptedin theb ackarc.
led Borg ar aJ.Q gn) to chtract€rize this endm embera s
having a high (Sr/P);yv alue. The other end of the con- Ittw-K, high-Al olivine tholeiite (HAOT)
tinuum is characterizedb y magmasw ith higher abun-
danceso f incompatible elements,l ower LILEIHFSE, The most stiking featureo f HAOT from the Lassen
and Sr, Nd, and Pb isotopic ratios that are like thoseo f region (and in general)i s the limited range of major-
ocean-islandb asalts.R elativee nrichmento f ZREEo ver and frace-elemenvt ariability displayedb y thesel avas.
Sr and low Sr/P led Borg et al. (1997) to chuacteizn, Representativer esults of major- and trace-element
this end membera s having a low (Sr/P)yvalue.C lynne analyseso f HAOT areg iven in Bacone t al. (7997)n d
(1993)c lassifiedm afic lavaso f the Lassena reab y their Clynne (1993).D istinctive featureso f primitive HAOT
K2Oc ontent,u sing an arbitraryd ivision betweenlo wer- arel ow SiO2( 48 wtVo),FeO*lMgOo f 0.9,h igh dlrQ,
and higher-IlO lavas.S ince( Sr/P),,c orrelatesin versely (LSwtVo), and low alkali content, especially K2O
with the concentrationo f.t he ULE and particularly K, (around0 .2 wtVoo r less).I n comparisonto calc-alkaline
classificationso f CA lavasb asedo n K2O and Sr/P are lavas, HAOT have lower concenffationso f. ULE,
essentiallye quivalent.L ow-(Sr/P)r,l,a vasa ree quivalent higher concentrationso f I/RE4 lower ULEIHFSE, and
to higher-K basaltic lavas, and high-(Sr/P);,l.a vas are Towq IREEIHREE( Fie. 1). IIAOT displayw eake mich-
equivalen!to lower-K basalticl avas.T he highe.st-(Sr/Plvmentso f Ba" Sr, and Pb relative to ULE or /,REE Sr
lavasa rem ostly primitive magnesiana ndesitesB. elowo andN d isotopic systematicso f HAOT are distinct from
we adoptt he (Sr/P)at erminology,a ndu se (Sr/P)" = 3.3 those of CA lavas, primarily in having higher
to distinguishlo w- andhigh-(Sr/PlylavasT. he magnasian 144Ndrf143aNtd e quivalent 875r/865@r ullen & Cllnne
andesitesfo nn a subgroupo f high-(Sr/P)1lsa vas. 1989,B org et al. 1997,Bacone t al. L997).
Clynne (1993) and Borg et al. (1997) discussed Bartels et aL (L99I) have experimentally equili-
across-arcv ariation in the geochemistryo f primitive brated a magnesianT IAOT compositionw ith a spinel
100
o I HAOT
(C6 x (Sr/P)p< 3.3
o n (Sr/P)x> 3.3
E 10
0L-
o
E.
E
(U
U)
CsBa U Ta LaPbPSmHfTiTbYTmLu
Rb Th NbK Ce Sr Nd Zr Eu Gd Dy Er Yb
Ftc. 1. Spider diagram showing the differencesb etween the types of Lassenb asalts.
Averaget race-elemenct oncentrationsfo r the groupso flavas, takenf rom datareported
by Clynne (1993), Borg et aL (1997), and Bacon et al. (L997), are normalizedt o the
primitive mantlev alueso f Sun & McDonough( 1989).
456 THE CANADIAN MINERALOGIST
lherzolitea ssemblagea t a temperatureo f about I290'C U.S. Geological Survey in Menlo Park. We used the
andp ressureo f -l 1 kbar. Clynne (1993) suggestedth at maffix-correctionp rocedureso fBence & Albee (1968)
HAOT were derived by approximately ll%o parttal and the modificationso f Albee & Ray (1970). Correc-
melting of depleteds ubcontinentaml antle subjectedt o tions were madef or interferenceso f CrKp on MnKot,
Fe and Al enrichmentb y addition ofpyroxene through and Ti(p on V/(cr for spinel analyses.S ynthetic and
metasomatismB. aker et al. (1994) concluded that naturalm ineralsw ereu seda ss tandardsT. he proportion
HAOT at Mt. Shastar epresentsa nearly anhydrous of Fd* in spinel was calculatedb y chargeb alancea nd
6-l0vo pwltal melt of subcontinentaml antle that last stoichiometry( fixing 8 positive chargesa nd 3 cations
equilibratedn eart heb aseo f the crusLB acr,ne t al. (1997) per formula unit). The Tiebaghi chromite (USBM
attributed their weak subduction-relatedg eochemical 11 7075.J arosewiche t al. 1980)w asu seda s an internal
signature( elevatedS r, Ba, Pb, and Sr/P) to passageo f standardfo r analyseso f chromians pinel.P recisiona nd
ancient( probablyM esozoic)a rc magmast hrought heir accuracya ssociatedw ith analyseso fthe Tiebaghic hro-
mantles ource-regionC lynne( 1993)p roposeda ndBacon mite arer eportedi n Clynne (1993),a nd comparef avor-
et al. (1997) agreedt hat the mantle source-regionso f ably with thoser eportedb y Ballhaus et al. (1991) and
IIAOT and CA lavas are compositionallvd istinct. Forsythe& Fisk (1994).
ANarvncer TBCHMeTJFS Mtr{ERALocY
Minerals were analyzed with an automatedn ine- CA lavas in the Lassenr egion, like arc basaltsa nd
spectrometerA RL SEMQ electron microprobe at the andesitesw orldwide, tend to be porphyritic, but we
TABLE 1. REPRESENTATWE COMPOSITIONS OF OLTVINE IN IIAOT AND CALC-ALKALINE LAVAS
569 1046 l3n 951 1398 910 ',135 514 347 671 l3t4 1333 831 2443 671 905
SiO, Oelo) 39.94 40.?2 39.91 39.U 40.52 39.91 39.88 40.34 39.74 40.00 39,37 39.59 40.00 40.38 39.99 39.80
MgO 4'7.49 47.12 47.20 47,12 46.U 47.25 46.40 46.96 44.82 M.45 46,42 46.21 46.24 47.93 46.44 46.97
FeO 12.18 11.72 1t,67 11.71 t2.t0 12.34 12;73 t3.67 15.21 13.08 t2.93 13.72 t3.43 1t.62 t2.54 12.20
MlO O.20 O.2l 0,2t 0.19 020 0.22 o,22 020 0.25 0.23 0.18 0.23 023 0.16 0.19 0.16
Nio 0.30 0.26 o2'7 0.34 029 0.30 0,28 0.32 0.n 0.30 o.31 0.24 0.37 0.32 0.37 0.31
CaO 0.21 0.26 028 0.2t 0 029 o24 0.4. o.t1 0.18 0.13 0.t7 0.15 0.18 0.16 0.18
Total 100.31 W.79 9.55 99.41 100.19 100.31 99.74 100.71 100.4t 100.24 99.41 100.16 100.41 100.59 99.69 9.62
si 0.989 0.994 0.983 0.980 1.003 0.990 0.990 0.991 0.996 0.995 0,975 0.989 0.995 0.995 0.990 0.984
Mg 1.153 1.735 1.734 t;128 1.728 1.746 1.7t6 1.729 t.675 t,722 1.7t3 1;721 t.714 1.760 t,7t5 t.731
Fe 0.252 0242 0.241 0241 0.250 0.256 0264 0.261 0.319 02J2 0.268 0287 0.279 0.239 0260 0.252
Mn 0.004 0.004 0.004 0.004 0.004 0.005 0.005 0.004 0.005 0.005 0.0M 0.005 0.005 0.003 0.0M 0.003
Ni 0.006 0.00s 0.005 0.007 0.006 0.006 0.006 0.006 0.005 0.006 0.007 0.005 0.007 0.006 0.007 0.006
Ca 0.005 0.007 0.007 0.006 0.006 0.@8 0.006 0.006 0.005 0.005 0.004 0.005 0.004 0.005 0.0M 0.005
Fo 87.4 87.8 87.8 87.8 87.4 87.2 86.7 86.9 84.0 86.4 86.5 85.7 86.0 E8.0 86.8 87.2
Ni(ppn) 2318 2035 2122 2255 2381 2200 2483 1760 2389 2939 1847 2892 2't5.358'7 2892 2420
(Sr/P\. HAOT HAOT HAOT HAOT HAOT HAOT HAOT IIAOT 125 1.45 1.52 1.52 l.J3 t.6'1 t.69
834 1006 1,108 l37l 1305 1005 829 2116 2697 1303 1056 l3l2 255 l3l0 1308 1009
SiO, (wtolo) 40.81 40,73 4.69 40.01 39.49 40.13 40.38 40.99 41.16 40.72 402t 41.51 40.59 41.09 40.72 40.51
Mgo 48,69 50.58 49.83 4'7.48 43.10 45.24 49.t6 49.54 50.16 49.63 41.42 48.80 46.45 48.93 47.9t 48.94
FeO 10.89 8.49 923 12.80 16.68 14.16 10.31 1032 9.65 10.07 11.36 9.91 11.57 9.t4 10.67 9.54
MnO o.t4 0.14 0.17 0.21 0.24 0.23 0.t7 0.14 0.16 0.16 0.19 0.19 0.16 0.15 0.19 0.13
Nio 0.32 0.38 0.38 0.19 0.31 0.23 0.33 0.34 0.32 0.25 0.10 0.15 0.41 0.35 0.27 0.73
CaO 0.14 0.13 0.14 0.21 0.1I 0.t2 0.t7 0.16 0.19 0.13 022 021 0.12 0.ll 0.20 0.08
Total 100.99 100.44 100.46 100.89 99.94 100.10 100.53 101.49 101.52 100.96 99.50 100.78 99.3t 99.77 99.96 99.93
si 0.994 0.991 0.993 0.988 1.000 1.003 0.990 0.993 0.992 0.9v2 0.987 1.009 0.994 1.001 1.002 0.994
Mg lJ18 1.834 1.813 1.749 t.626 1.685 1.797 1.789 1.807 1.802 t.735 t.768 1.696 t.77'1 1.758 t.190
Fe 0223 0.173 0.188 026/. 0.353 0,296 0,2t1 0,2@ 0.195 0205 0.233 0201 0237 0.186 0220 0.196
Mn 0.003 0.003 0.004 0.004 0.005 0.005 0.004 0.003 0.003 0.003 0.004 0.004 0.003 0.003 0.004 0.003
Ni 0.006 0.008 0.008 0.004 0.006 0.005 0.007 0.007 0.006 0.005 0.002 0.003 0.008 0.007 0.005 0.014
Ca 0.004 0.004 0.004 0.005 0.003 0.003 0.005 0.004 0.00s 0.004 0.006 0.005 0.003 0.003 0.005 0.002
Fo 88.9 91.4 90.6 86.9 4.2 85.1 89.5 89.5 90.3 89.8 88.2 89.8 8'7.7 90.5 88.9 90.1
Ni (ppn) 2s38 3002 3010 t477 2467 1776 2593 2672 2538 t94l 802 t202 3245 n58 2153 4420
($/P\N l.9l 2.42 2.50 3.M 3.89 3.91 4.66 4.79 5.10 5.35 5.47 5.88 5.96 6.59
CompositioNd qived frm miqoprcb€ dd S€ Table2 for bostr uk SiOzm d MgO; t calealkaline lavs showv alue fo! (Sr/P\
OLTIIINE AND CHROMIAN SPINEL IN PRIMITTVE I.AVAS 457
emphasizeth at the majority of lavass electedfo r detailed Olivine
studya rep rimitive basaltsa ndm agnesiana ndesitesth at
have simple assemblageos f minerals (many of them Olivine is a ubiquitousp henocrystp hasei n both CA
containo livine ast he only phenocrystp hase)a nd a low and HAOT lavas. Most of the primitive lavas in this
content of phenocrysts( <57o). Although titaniferous
study containl ess than 5Voo tline, usually as small,
magnetitei s the stableo xide phasei n the groundmass
<l mm, unzonedt o weakly zoned, euhedralc rystals.
of CA lavas,t he most magrresianla vasu sually contain
Primitive CA lavash avec ore compositionsin the range
chromians pinel preserveda s inclusionsi n olivine phe- Foro-srw, itl 2500-3500p pm Ni (Table1 , Fig. 2A). In
nocrysts.C rystalsi n primitive CA lavasa re small, usu- general,t he most magnesianfo rsterite @o3e-e1o)c curs
ally less than L mm in maximum dimension" and in CA lavas with high (Sr/PL'. The Ca content of
unzoned,e xceptf or a thin, normally zonedq uench-
forsterite from CA lavasr anEesfr om 0.10 to 0.22 wt%o
induced rim. The groundmassr anges from being
Cao @ig.2B).
holocrystallinet o holohyaline,a nd is composedo f
plagioclase,c linopyroxene,a nd titaniferousm agnetite,
although olivine also is presenti n some of the more
magnesianC A lavas.
HAOT lavas are distinctive rocks, typically aphyric
or with sparseo livine or plagioclase (or both) in a
holocrystalline, usually diktytaxitic groundmass( as
E><E
defined by Williams et al. 1,954)c omposedo f plagio-
clase,c linopyroxeneo r olivine (or both), and titanifer- 5***f u )N(
a
ospuisn eml aags ninectliutes.iM onoss itn H oAliOvinTe lapvhaesn occornytsatisn,a cnhdr oam feiawn zEoo.. x)qBExl T** *E
also contain chromian spinel as microphenocrysts. o
Slightly evolvedH AOT lavasm ay containg lomero- il*]ao.t **
porphyritic clots composedo f olivine and plagioclase,
nsgqmFaniedra rely by clinopyroxene.
Abont 3OVoo f the primitive CA lavas reported bv
Clynne( 1993)a nd Boig et at. (1997)c ont,in clinopy-
roxene phenocrysts.C linopyroxenec ompositionsf all
in a small range aroundW oa5En5sFsc5o,n tain high Cr
contents (generally 0.5-1.0 wtVo Cr2O3),a nd corre- OlMnec ore (Fo)
spond to chromian diopside or chromian Mg-rich
augiie. They haveM g# [100Mg(Mg + Fd*)] valuesi n
the range 88-92 and are in equilibrium with the host-
rock compositioni f one assumesa Kn of 0.23 (Grove
et al. 1982).T he compositionso f the clinopyroxenei n
CA lavas 61eg imi.lari n all respectst o those found in
primitivel avasi n otler arcs( e.g.,B VSP 1981,T atsumi
& Ishizakal 982,Kay & I(ay 1985,L uhr & Carmichael rE!tr
1985, Bailey et aI. L989), and are similar in major c
* t<
componentsto thosef ound in many lherzolites( Dick &
Fisher 1984,W ilshire et al. 1988). i )a< x r(
xx)<
Plagioclasep henocrystsa res parselyp resenti n about (u XX
B)€r(
25Vo of. the primitive CA lavas and nearly 5OVoo f E
HAOT lavasd escribedin Clynne( 1993) atd,bory et al.
xxa
(L997).P lagioclasep henocrystsin CA lavasa re gener- aEl
ally small, up to a few mm in size and unzoned,b ut
occasionally display weak normal or reverse zoning
toward their margin.T he compositiono f plagioclasein
CA lavasr angesf rom An6st o Ane6.T he compositiono f
plagioclasep henocrystsi n HAOT is in the range OlMnec ore (Fo)
Aryzsti the phenocrystsa re unzonedo r display weak
normal zoning of a few mo7.VAo n. The An contento f
Ftc. 2. Plot of abundanceos f (A) NiO and (B) CaO versusF o
plagioclasefr omHAOT ando f low- to high-(Sr/P)rvCA of olivine phenocryss from HAOT and CA basalt and
lavasp lot asd istinct parallell inear arraysv er,ra.wr hole- magnesiana ndesite( MA) lavas from the Lassenr egion.
rock CaO/I.{a2Oa, nd have Sr and K2O contentsa ppro- Olivine in HAOT lavas has consistentlyh ighsl l6ysh ef
priate for equilibrium with their hosts( Clynne 1993). C4 andg enerallylo werl evelso fNi, thano livinei n CA lavas.
458
0.
0.6
o
o
x
o
E
o 0.4
o
o)
o 0.3
o)
o
(U
o
0.
0.1
o.4 0.5 0.6 o.7 0.8 0.9 1.0 1.1 1.2 1.3
CaOlMgOm elt (molar)
Ftc. 3. CaOq/MgO (X100) in olivine versresm olar CaO/1VIgOin coexistingw hole rock
(melt), where CaOq is the Ca contento f olivine normalizectl o Foeao livine (Jurewicz
& Watson 1988). Temperaturec ontours from Jurewicz & Watson (1988) have an
uncertaintvo f t25'C.
Olivine phenocrystsin sparselyp hyric HAOT lavas effect of iron on the equilibriumC a contente f magmatic
are euhedrala nd generally unzoned or display weak olivine. The measuredp artition-coefftc ient [normalized
normal 26ning of a few mol.VoF o. The range of core to Foeeu sing the procedureo utlined by Jurewicz &
compositionsis quite restricted,g enerallyF o6646w, ith Watson( 1988), and assumingth at bulk-rock Ca equals
Ni contenti n therange?,000-2400p pm, andC a content the amounto f Ca in the meltl for olivine in the Lassen
in therange0.224.28v'ivoC aO( Iable 1,F igs.24,B). samplesis 0.021+ 0.008,i denticalt o that of Jurewicz
Compared to the sparsely phyric lavas, the olivine & Watson (1988). Crystallization temperaturesb ased
phenocrystsi n porphyritic HAOT lavas shows a on amounto f Ca in olivine (Jurewicz& Watson 1988)
slightly wider and more evolved compositionalr ange rangef rom about I2i75 n about1 225"C for them ajority
(Fos2-sew), ith generallyl ower Ni content,i n the range of HAOT and CA lavas @ig. 3). Temperaturesc alcu-
1500-2000 ppm, and A22 vrt%oC aO. Olivine pheno- lated for HAOT using an empfuicafl unction basedo n
crysts in HAOT contain a grc*;tora mount of Ca than whole-rockS iO2a ndM gO content( AlbarbdeL 992)a rc
phenocrystsi n CA lavas having a similar Fo content mostly in the range from 1250 to 1315'C, whereas
@rg.2B). temperaturesc alculatedf or CA lavas arem ostly lower,
but in the range1 20G-1300"C.
There is considerabled isagreemenot n the factors
that influencet he partitioningo f Ca betweenm agnesian It is important to demonsfrateth at in terms of trace
olivine and melt; temperaturep, ressure,c oncentrations elements, the phenocrysts are in equilibrium with
of Ca and Fe in the melt have all beenc ited as confol- whole-rock( wr) compositionsA. t equivalenFt o, olivine
ling factors by various investigators.A synopsisa nd phenocrystsin most primitive CA lavas containh igher
critical evaluationo f the role of eacho f rhesef actorsa re Ni than olivine phenocrystsin HAOT, despites imilar
given by Jurewicz( 1986),a ndt hesef acto$ werei nves- to lower Ni contenti n the host lava @ig. 2A). Measured
tigated experimentallyb y Jurewicz & Watson (1988). Dxi(oUwr) arep lotted againstw hole-rockM gO content
Their results indicate that the Ca content of olivine in Figure 4. Most fall close to the DNi(oywr) reported
dependso n the concenhationo f Ca in the melt and the by Hart & Davis (1978). The exceptionsa re mostly
relative activity of iron, as indicatedb y the Fe/IvIg of samples6 f high-(Sr/P)1my agnesiana ndesite,a nd their
the olivine; pressurea nd/(O) have no direct influence apparentlyh igherD pi is probablyd uet o the increaseo f
at conditions relevant to crystallization of olivine in DM of olivine v/ith increasingS iO2o f the melt andw ith
Lassenm agmas.J urewicz& Watson( 1988)r eporteda decreasingte mperatureo f crystallization( Kinzler et aL
partition coefficient for Ca in olivine of 0.021 t 0.015, 1990). Olivine is less abundanti n the magnesian
and presenteda n empirical equation to evaluate the andesitesin which DNi is high; instead,t heser ocls also
OLMNE AND CTIROMIAN SPINEL IN PRIMTTVE LAVAS 459
El
25 a6
E
a I
; 20 I\D \"-_ffi_- " sI
\HFsx 6
2 W.-
o 15 g3
n
I
OlMne core (Fo)
12
whoter ockM gO( wtZ o) FIc.5. Plot of whole-rockFeo*/1VlgOve rsarc ompositiono f tle
core of forsteritep henocrystsS. olid lines display contours
Ftc.4 . Distributionc oefEcienfot r Ni in olivinev ersaMs gOi n of olivine-liquid Kr. The majority of HAOT and CA lavas
wholer ock Solidl ine denotetsh e equilibriumc urveo f plot betweenK 2 valueso f 0.29 and 0.26, and thus are in
Hart& Davis( 1978f)o r olivinei n equilibriunw ith liquid. equilibrium with whole-rock FeO*/1VIgOS. ee text for
Errorb ara pproximatethse e rrori n them easuremeonf tN i discussiono f the exceptions.
in wholer ock ando livine( Clynne1 993)H. axt& Davis
(1978)d id not reporta n uncertaintfyo r the distribution
coefficient.
containc linopyroxenep henocrystsC. onsequentlyt,h ey Chrominns pinel
may have undergone some combination of olivine
accumulationo r fractionation, which may well have The majority of primitive lavas of the Lassena rea
modified Dm(ol/wr). Howeverot he presenceo f contain chromian spinel tapped as inclusions in for-
chromians pinel and high-Ni olivine at low whole-rock steriiep henocrystsS. pinel crystalst rappeds equentially
FeO*/NIgO values indicates that crystal fractionation in growing phenocrystsm ay record changesin compo-
has not significantly modified their bulk compositions,
sition due to crystal fractionation, decompressiono r
whereasre lativelyl ow whole-rockN i contenta ndo livine destabilizationo f chromians pinel.T he langeso f com-
- \ilhole rock equilibrium suggestt hat olivine accumu- position of cbromian spinel in many Lassenl avas are
lation is likewise not an important factor in the evolu- similart o thoser eportedb y Lubr & Carmichae(l1 985),
tion of the magnesiana ndesites.C onsequently,t he Arai &Takahashi( 1987),a tdUmino et a/. (1991).T he
higher SiO2o f the high-(Sr/P)i,mr agnesiana ndesitesis compositionsr eportedh ere are from the core of euhe-
probablyt he mostimportantfactor in the origin of their dral grains located in the core of olivine phenocrysts,
high DNi.
and arer epresentativeo f thosew ith lowest Cr#, highest
Olivine compositionso f HAOT and CA lavas are Mgf, andl owestp roportion of the titaniferousm agnet-
plotted as a function of whole-rock total iron as FeO ite component taken from the .uray of analyzed
versusM gO in Figure 5. The majority of IIAOT and chromian spinel crystals in each rock examined
CA lavas form a coherent tend lying between the (Clynne 1993). Thus the effects of fractional crystal-
Ko= 0.26 and 0.28 contours,s lightly lower than the lization and reaction with host liquid (seeb elow) are
Ko= 0.29 detenninedfo r MedicineI akeV olcanob asalt minimized.F urtherd iscussionis limited to this specially
at the quartz - fayalite - magnetite( QFM) buffer by selectedg roup of cbromians pinel compositionsw, hich
Grove et al. (1982). Thus the data suggestt hat HAOT arer eportedi n Table2, alongw ith the forsteritec ontent
andC A lavase xhibit equilibriumb etweeno livine phe,no- of the host olivine phenocrystm onitoredd irectly adja-
crystsa ndh ost lava in termso f FeO*/IvIgO,p robablya t cent to the cbromians pinel.
a/(O) higher than QFM. A few sampleso f magnesian
andesitec ontain olivine with lower Fo content than Chromians pineli s found includedi n olivine pheno-
expectedf or their FeO*/TVIgOy,e t are sparselyp hyric crysts, but does not occur in the groundmasso f CA
and containh igh lsysfu ofNi. Thesel avas also contain lavas.I n the CA lavas,t he grainsa res maller( 1G-50p m
chromian spinel with high psz+/Ir.r+,a nd may have in diameter,2 0 pm.i s typical), but more abundan(t there
crystallizsd under more reducing conditions than the are generally ten to one hundred crystals per cross
majority of CA lavas.C onversely,a few sampless how sectiono f an olivine phenocryst)t han in IIAOT lavas.
evidencef or a small amounto f olivine accumulation. The crystalsr angei n color from goldenb rown through
460 THE CANADIAN MINERALOGIST
TABLE 2. REPRESENTATTVE COMPOSITIONS OF CIIROMIAN SPINEL IN IIAOT AND CALC.ALKALINE LAVAS
569 1M6 t3t1 951 1398 970 735 514 347 67',7 l3l4 1333 831 2443 855 67t
SiO,( rtolo) 0.00 0.00 0.00 0.00 0.00 0.11 0.06 0.05 0.00 0.00 0.00 0.00 0.00 0.08 0.00 0.00
Tio, 0.32 0.15 0.18 0.33 0.26 0.18 0.18 023 0.98 0.66 0.96 0.87 0.98 0.49 0.62 0.68
Al,O, 41.88 58.16 57.11 45.93 47.72 56.52 52,44 49.63 36.87 39.83 25.15 36.39 30.,!0 22.89 3E.02 28.30
Cr2O3 21.88 8.50 10.01 18.58 18.79 9.61 12.01 14.89 22.46 23.10 33.53 24.31 28.72 39.91 21.55 31.20
FeO 12.56 10.44 10.85 11.41 11,17 11.80 10.03 11.83 16.61 13.96 16.87 14.65 16.17 14.42 16.45 1520
rqq 6.48 3.31 3.35 6.21 3.83 2.62 5.20 5.64 8.40 5.93 1026 8.23 9.M 7,48 9.77 10.06
MnO 0.15 0.10 0.ll 0.14 0.14 O.12 0.13 0.16 020 0.13 0.20 0.13 0.20 023 0.19 O.l7
MeO 17.23 20.31 20.16 18.53 18.53 19.22 19.89 18.62 14.41 16.29 12.97 15.74 13.91 13.94 14.45 14.25
Nio 025 029 0.27 0.28 0.22 0.21 023 0.30 0.1',7 0,24 0.26 0.22 0.10 0.08 0.18 0.28
cao 0.00 o.o2 0.00 0.01 0.01 0.03 0.00 0.00 0.01 0'01 0.02 0.00 0.00 0.01 0.00
vo, 0.09 0.08 0.ll 0.09 0.13 0.10 0.12 0.11 0.18 0.14 0.14 0.17 0.18 0.16 0.14 0.17
Toral 100.84 101.37 102.15 101.50 101.39 t00.58 to0.29 t0t.4't 100.36 100:9 100.36 100.78 99.90 99.68 101.38 100.31
si 0.000 0.000 0.000 0.000 0.000 0.003 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000
Ti o.oo7 0.003 0.003 0.007 0.005 0.003 0.004 0.005 0.021 0.014 0.022 0.019 0.022 0.011 0.013 0.015
AJ 1.366 1.153 1.7t9 1.458 1.505 1.732 1.628 1.554 t.250 1.321 0.901 1223 1.064 0.826 1275 0.994
Cr 0.481 0.173 0203 0.39'7 0.399 0.198 0251 0.314 0.513 0.516 0.809 0.552 0.617 0.970 0.487 0.738
Fd' 0.29t 0.223 0232 025'1 0.263 0.256 0221 0263 0.400 0.328 0.429 0.349 0.492 0.369 0.391 0.379
Fe* 0.135 0.064 0.064 0,126 0.007 0.051 0.103 0.113 0.182 0.126 0235 0.176 0.202 0.172 0209 0226
Mn O.OO3 0.002 0.002 0.003 0.003 0.003 0.003 0.003 0.005 0.003 0.005 0.003 0.005 0.006 0.005 0.004
Mg O.7ll 0.774 0.768 0.744 0.139 0.745 0.781 0.738 0.621 0.683 0.588 0.669 0.616 0.631 0.613 0.633
Ni 0.006 0.006 0.006 0.006 0.005 0.006 0.005 0.006 0.004 0.006 0.006 0.005 0.007 0.002 0.004 0.007
Ca 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000
v 0.002 0.002 0.002 0.002 0.003 0.002 0.003 0.002 0.004 0.003 0.003 0.004 0.004 0.004 0.003 0.004
Mgs 1l.O 77.6 76.8 74.3 73.7 74.4 78.0 73.7 60.8 67.6 57.8 65.7 60,6 63.3 61.0 62'6
c# 26.0 9.0 10.6 21.4 21.O 10.3 13.4 16.8 29.1 28.1 M.3 3l.l 38.9 54.0 27.6 42.6
Fe# 6.8 32 32 6.3 3.9 2.6 52 5.7 19.3 6.4 12.1 9.0 10.4 8.7 10.6 11.5
ol Fot 87.4 8?.8 8?.8 8?.8 87.4 87.3 86.7 87.0 84.0 86.4 86.5 85.7 86.0 88.0 U.2 86.8
hoslS iO, 49.58 48.12 48.37 48.E4 49.50 48.33 48.61 48.58 51.23 51.17 50.&2 50.71 52.E1 51.60 49.94 51.35
hoslMgo 8.71 10.08 9.41 9.56 10.30 9.78 8.76 9.90 6.83 7.53 8.43 7.74 7.09 9.34 826 8.05
(Sr/P)a.*i IIAOT IIAOT IIAOT I{AOT I{AOT IIAOT HAOT HAOT l.X 1.45 1.52 r.52 1.53 1.57 1.66 1.67
905 834 1408 1371 1305 1005 829 2116 2697 1056 l3l2 255 743 1310 1308 1009
Sior(*xolo) 0.04 0.13 0.09 0.09 0.00 0.00 0.09 0.ll 0.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Tio, 0.51 0.31 0.30 0.43 0.23 0.48 0,34 0.34 0.35 0.42 0.29 0.91 0.1'l 0.27 0,27 0.50
Al,o! 35.60 45.33 25.35 32.31 t7.U 23.33 29.32 28.91 19.46 32.95 31.49 14.15 18.30 10.70 28.21 13.61
Cr,O, 26.85 18.14 39.50 29.36 43.74 37.22 34.32 35.39 4528 31.72 35.05 39.14 41.35 51.q2 36.1'1 49.32
FeO 13.59 9.95 11.45 14;77 22.32 19.00 12.36 11.82 1s.14 13.49 12.31 19.89 17.52 1726 13.63 t6.23
'1.40
FqO. 7.31 6.57 6.90 7.1't 1.21 8.85 7.87 6.95 5.93 5.30 3.42 15.30 10.45 7.20 6.55
Mro 0.17 0.13 0.17 0.22 0.30 023 0.13 020 0.29 0.18 0.13 0.20 0.17 023 0.21 0.21
MgO 16.05 19.15 16.11 14.74 8.19 11.3'7 16.08 16.35 l3.M \5.94 16.20 9.U 11.88 10.50 15.15 l2,M
Nio 0.?3 021 0.21 0.02 0.ll 0.05 022 0.07 0.10 0.06 0.13 0.22 0.13 0.ll 0.16 0.13
Cao 0.00 0.00 0.05 0.01 0.03 0.02 0.01 0.03 0.03 0.01
V,O, 0.14 0.ll 0.12 0.18 0.10 020 0.15 0.12 0.10 0.22 0.18 0.16 0.15 0.13 0.17 0,25
Total 100.49 100.03 10010 99.89 100.03 100.79 100.88 100.26 9.19 t00.29 9922 100.43 100.66 98.36 100.55 99.70
si 0.001 0.004 0.003 0.003 0.000 0.000 0.003 0.003 0.003 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ti 0.011 0.006 0.007 0.010 0.005 0.011 0.007 0.008 0.008 0.009 0.006 0.022 0.017 0.007 0.006 0.012
At 1200 1.452 0.890 l.ll9 0.681 0.847 1.009 0.999 0.715 1.123 1.086 0.543 0.677 0.421 0.982 0.517
Cr 0.609 0.391 0.934 0.683 1.124 0.910 0.795 0.823 l.l2l 0,128 0.814 1.026 1.031 1.374 0.848 1.261
Fea 0325 0226 0.285 0,362 0.604 0.489 0.302 0290 0.395 0.326 0.301 0.541 0.460 0.481 0.337 0.437
Fe$ 0.157 0.134 0.154 0.1?0 0.176 0205 0,173 0.153 0.139 0.115 0.075 0.374 0247 0.181 0.146 0.179
Mn 0.004 0.003 0.004 0.005 0.008 0.006 0.003 0.005 0.008 0.004 0.003 0.006 0.005 0.006 0.005 0.006
Me 0.684 0.776 0.715 0.644 0.396 0.523 0.700 0.715 0.606 0.687 0.706 0.478 0.556 O52 0.6'1 0.578
Ni 0.005 0.005 0.005 0.000 0.003 0.001 0.005 0.002 0.003 0.001 0.003 0.006 0.003 0.003 0.004 0.003
Ca 0.000 0.000 0.002 0.000 0.001 0.001 0.000 0'00i 0.001 0.000
v 0.003 0.002 0.003 0.004 0.003 0.005 0.003 0.003 0.003 0.005 0.0M 0.004 0.004 0.004 0.004 0.006
Mg# 67,8 77.5 71.5 &.0 36.9 51.6 69.9 71,2 60.6 6',1.8 70.1 46.9 54.7 52.0 66.5 57.0
Cr# 33.7 2t2 51.2 37,9 62.3 51.6 M.l 452 61.0 39.3 42.8 65.4 60.3 76.6 46,3 '70.9
Fe# 8.0 6.8 7.8 8.6 8.9 10.4 8.1 7.8 7.0 5.9 3.8 19.3 12.6 9.1 7.4 9.2
ol Fo 87.3 88.9 90.6 86.9 80.6 65.1 89.5 89.5 90.3 88.0 89.8 86.0 86.7 91.0 88.9 89.0
host SiO2 50.36 49.38 54.08 51.99 53.68 51.13 51.51 51.17 51.54 50.57 50.83 54.41 54.53 56.14 51.47 58.1E
hostM gO 8.43 t0.46 8.89 8.42 10.86 11.06 920 10.t5 1321 8.24 9.98 '.1.73 7.55 10.20 9.O9 7.57
(Sr/P)N.3* 1.69 1.91 2.50 3,U 3,29 3.79 3.89 3.91 4.66 5.10 5.35 5.41 5.83 5.88 5.96 6.59
[e-mail]imediatelyadj@nttoualyzedspinel;MgF(t00Mgl(Mg+Fet),Cr$=
(l00Crl(C$Al), FetE (l00Fd'y(Fd'+c*Al), ** (SIPX ofhost r@k for cslealkalhe larc
OLIVINEAND CHROMIAN SPINEL IN PRJMITIVELAVAS 461
shadeso f reddishb rown that becomei ncreasinglyd ark Grainso f chromians pineli n HAOT containh igh lgvelg
with increasingC r andF e contentC hromians pinelw ith a of A1 andM g, and low levels of Cr andF e, haveC r# in
significanpt roportiono f thet itaniferousm agnetitec ompo- the range 9-26, andMg# n the range 8G-65 (Fig. 6),
nent (moret han L-2'xt{o TiO, is abundanitn slightly to anda res imilar 1ea luminquss pinelf ound in MORB and
moderatelye volvedl avas,a nd can be readily identified abyssal peridotite @ick & Bullen 1984). Although
optically becauseit is opaque( Sigurdsson1 977). therei s considerableo verlap,g rainso f cbromians pinel
Primitive CA lavas from the Lassenr egion contain in HAOT generallyc ontain lower amountso f Fe3",T i,
chromians pinel with a wide rangeo f Cr# at high Mg# and V, and higher amountso f Ni than chromians pinel
(Fig. 6); this range is similar to that reportedf rom all in CA lavas.
other arcs combined( Arat 1992).T he compositiono f Spinel compositionsp lotted againsth ost FeO* and
chromians pineli n CA lavasc orrelatesw ith (Sr/P)pa nd Al2O3o f primitive lavas from the Lassena rear eveal
A1 contento f the host lava. Low-(Sr/P)ryb asaltsc ontain systematicc orrelationsT. he Cr#of spinels howsa good
a chromian spinel with a Cr# in the range 20-50, negativec orrelationw ith FeO* and A12O3o f the host
slightly to significantly lessa luminoust hatthe spineli n lava for all Lassens ampleso f basalt@ igs.7 A, B). The
HAOT lavas. High-(Sr/?)1yb asalts and magnesian entirer angeo f spinelC r# is presenta t constanht ost-lava
andesitesc ontain a chromians pinel with a large range CrlN2O3a ndC r in Lassenla vas@ gs. 7C, D).
of Cr#, from 40 to 77, but most arei n the range5 0-70.
There is a good negativec orrelationb etweenC r# and FAcroRsT uer Arrscr
Mg# ia the collection of chromians pinel compositions TTn Col/PosmoNoF SPINELINV orcamrc Rocrs
in al1 Lassenb asaltsa nd magnesiana ndesites,a nd a
good positive correlationb etweenC r# artd( Sr/P),' for The compositiono f the chromians pinel crystallizing
CA lavas (Frg. 6). ast he liquidus phasef rom aprimitive magmas houldbe
Chromian spinel is occasionally found in HAOT closet o that of the residuals pinel in the sourcer egion.
lavas as euhedralp henocrystsu p to 1 mm in size, but Differences between residual and phenocrysts pinels
usually occur$ as sparse( a few crystals per olivine reflect differencesb etweent he physical conditions at
the sites of melt formation and crystallization. The
phenocryst),1 0- to 100-p:ne uhedralc rystalsi n olivine
analysiso f chromians pinel crystallizedi n natural and
phenocrysts;4 0- to 60-pm.d iametersa re typical, and
experimentasl ystemsd emonstratetsh at its composition
they areg reenishb rown to light yellow-brown in color.
is affectedb y extent of fractional crystallization,p res-
sure,/(O), and degreeo f partial melting. However,t he
only significant factor affecting the composition of
cbromians pinel crystallizing at liquidus temperatureis
changein melt composition(N Ian et al. 1988).B elow,
we review the factors that affect the composition of
spinel in volcanic rocks and evaluatet heir relevancein
connectionw ith the origin of the compositionaal nay of
cbromians pinel found in the Lassenr ocks.
8ts Effec t of frac ti on al cr ys t a lli zatio n
on spinel cornposition
x
x -[
#*'ry Chromian spinel is typically the liquidus phasei n
Bx x mafic magmas,b ut crystallizes over only a na:row
sxx temperature-interv(aHl ill & Roeder1 974,F isk & Bence
xx 1980,L ubr & Carmichae1l 985,A rai & Takahash1i 987),
,s& so that its occurrenceis typically restrictedt o minimally
fractionatedl avas (Sigurdsson& Schilling 1976).T\e
xrl
composition of chromian spinel closely reflects the
I
compositiono f its host melt, andi s therefored ependent
on differentiation processes(I rvine 1965, 1976, lJlan
et al. 1988, Sack & Ghiorso L99la). Chromian spinel
reactsw ith its host liquid either during fractional crys-
100 80 60 40 20 0 tallization,o r oncei t ceasesto be a stablep hase( Ridley
Mg# 1977. Tatsumi & Ishizaka 1982. Luhr & Carmichael
1985). heservation of chromian spinel with a near-
Ftc. 6. Cr# and Mg# of cbromians pinelf ound in HAOT lavas liquidusc ompositionre quirest hat the magmab e rapidly
and the array of low- to high-(Sr/P)rvC A lavas from the decompresse4e rupted,a nd quenchd or that olivine
Lassenr esion. entert he crystallizinga ssemblagseim ultaneouslwy ith or
462
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