Melting depths and mantle heterogeneity beneath Hawaii and the East Pacific Rise

14 Pages·1999·1.05 MB·English

JOURNALOFGEOPHYSICALRESEARCH,VOL.104,NO.B2,PAGES2817–2829,FEBRUARY10,1999
Melting depths and mantle heterogeneity beneath Hawaii
and the East Pacific Rise: Constraints from Na/Ti
and rare earth element ratios
Keith Putirka
LawrenceLivermoreNationalLaboratory,Livermore,California.
Abstract. Mantle melting calculations are presented that place constraints on the
mineralogy of the basalt source region and partial melting depths for oceanic basalts.
Melting depths are obtained from pressure-sensitive mineral-melt partition coefficients for
Na, Ti, Hf, and the rare earth elements (REE). Melting depths are estimated by
comparing model aggregate melt compositions to natural basalts from Hawaii and the
East Pacific Rise (EPR). Variations in melting depths in a peridotite mantle are sufficient
to yield observed differences in Na/Ti, Lu/Hf, and Sm/Yb between Hawaii and the EPR.
Initial melting depths of 95–120 km are calculated for EPR basalts, while melting depths
8
of 200–400 km are calculated for Hawaii, indicating a mantle that is 300 C hotter at
Hawaii. Some isotope ratios at Hawaii are correlated with Na/Ti, indicating vertical
stratification to isotopic heterogeneity in the mantle; similar comparisons involving EPR
lavas support a layered mantle model. Abundances of Na, Ti, and REE indicate that
garnet pyroxenite and eclogite are unlikely source components at Hawaii and may be
unnecessary at the EPR. The result that some geochemical features of oceanic lavas
appear to require only minor variations in mantle mineral proportions (2% or less) may
have important implications regarding the efficiency of mantle mixing. Heterogeneity
required by isotopic studies might be accompanied by only subtle differences in bulk
composition, and material that is recycled at subduction zones might not persist as
mineralogically distinct mantle components.
1. Introduction sensitive to both P and mineralogy. Partition coefficients for
theseelementshavealsorecentlybeencalibratedtohighPand
Fundamental questions in the Earth sciences concern the T [Putirka, 1998a, b; Salters, 1996], making these elements
mineralogy of the basalt source region and the temperatures particularly useful for examining mineralogic heterogeneity
and depths at which basalts form. It has been proposed that andpotentialP-dependentgeochemicalvariations.
isotopic and other compositional differences between basalts Thenatureanddegreeofheterogeneityimpactuponissues
reflectmineralogicalheterogeneity[Langmuiretal.,1977;Hof- of mantle composition and the efficiency of mantle mixing
mannandWhite,1982;Zindleretal.,1984;AllegreandTurcotte, [Hofmannetal.,1986;Kellogg,1992;Farberetal.,1994],aswell
1986](andmorerecently,HirschmannandStolper[1996]and as the thermal structure of the mantle. It is thus essential to
Hauri [1996]). However, isotopic differences need not be ac- differentiate between heterogeneity with respect to mineral-
companied by substantial differences in mineralogy (compare ogy, isotope ratios, or differences in major or minor element
wholerocksandmineralseparatesfromOkanoandTatsumoto composition.Toexamineissuesofheterogeneityandthecon-
[1996]andXuetal.[1998]).Moreover,variationsinthepres- ditions of partial melting, alkalic and tholeiitic basalts from
sures (P) and temperatures (T) of partial melting have a Hawaii and tholeiites from the EPR are compared. Hawaii is
significantimpactonbasaltcomposition[KleinandLangmuir, the type example for a mantle plume, while EPR basalts are
1987;KinzlerandGrove,1992b;Kinzler,1997].Thenatureand representative of mid-ocean ridges [Langmuir, 1988]. As Ha-
degree of mantle heterogeneity cannot be fully ascertained waiirepresentsamantlehotspot[Wilson,1963;Morgan,1972;
until such effects are evaluated. To investigate the potential Sleepetal.,1988],meltingdepthsshouldbedeepcomparedto
effects of P and T on melt composition, melting models are the EPR, where melting occurs in response to passive up-
welling [Oxburgh and Turcotte, 1968; Cawthorn, 1975; Ahern
developed here, and the results are compared to tholeiites
and Turcotte, 1979; Spiegelman, 1996]. The Hawaiian island
from the East Pacific Rise (EPR) and tholeiitic and alkalic
chainisalsounderlainbythicklithosphere(70–100km[Bock,
lavasfromHawaii.Themodelstestwhethersomeofthegeo-
1991; Woods and Okal, 1996]). Faced with such geologic dif-
chemicalvariationsattheselocalitiescanbeexplainedbydif-
ferences,wemightask:Candifferencesinmantletemperature
ferences in the P-T conditions of melting. These calculations
or lithosphere thickness explain some of the geochemical dif-
alsoconstrainminimumdegreesofsourceregionheterogene-
ferences between Hawaii and the EPR? This question is ad-
ity.ThemodelsuseNa,Ti,Hf,andrareearthelement(REE)
dressed by comparing natural basalts to calculated melt com-
abundance’ssincetheirmineral/meltpartitioncoefficientsare
positions derived from a homogenous depleted peridotite
Copyright1999bytheAmericanGeophysicalUnion. source (see the appendix and Table 1). While the mantle is
Papernumber1998JB900048. knowntobeheterogeneous,suchcalculationsconstrainheter-
0148-0227/99/1998JB900048$09.00 ogeneity.Geochemicalvariationsthatareunexplainedbysuch
2817
2818 PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY
Table 1. ModelCompositions tholeiitic rocks are examined (Figure 1). The melting models
test whether P-T conditions during melting can explain such
Major
variations.
Oxides Value MineralModes Value
SiO 46.316 olivine 0.60
2 2. Geochemical Data and Corrections
TiO 0.174 orthopyroxene 0.09
2
Al O 3.935 clinopyroxene 0.17
2 3 Toexaminegeochemicalvariationduetomantlemelting,it
FeO 7.653 garnet 0.14
is essential to minimize the effects of shallow level fraction-
MgO 38.44
CaO 3.192 ation. As a test for shallow level fractionation, Na/Ti and
Na O 0.281 Sm/Yb ratios are compared to MgO (Figure 2). For basaltic
2
rocks,MgOisausefulindicatoroffractionation,sinceolivine
is normally crystallized at shallow depths. While some scatter
exists, Na/Ti and Sm/Yb ratios are independent of MgO for
models provide minimum estimates on source region differ-
ences. To test for mineralogic heterogeneity, melting models
usingeclogiteandvariousgarnetpyroxenitestartingcomposi-
tionsarealsodeveloped.
BasaltsfromHawaiiandtheEPRexhibitnotablegeochemi-
cal differences. Both tholeiites and mildly alkalic Hawaiian
basaltshavelowerNa/Tiratios,duetohigherTiandlowerNa
(Figure 1). Lower melt fractions at Hawaii could account for
increased Ti but not for Na if Na is also incompatible. This
conundrummightbeexplainedifmineral-meltpartitioncoef-
ficients for Na increase with increased P [Frey et al., 1994;
Kinzler, 1997]. Hawaiian basalts also exhibit high Sm/Yb and
lowLu/HfratioscomparedtoEPRlavas[Salters,1996;Salters
and Hart, 1991, 1989], and greater melting depths at Hawaii
have been invoked to explain such differences [Salters, 1996].
Finally,thereexistsignificantintershieldvariationsinNa/Tiat
Hawaii; these intershield differences hold whether alkalic or
Figure 2. (a) Na/Ti cation fraction ratios are compared to
MgO (weight percent) for EPR and Hawaiian basalts. Note
Figure 1. WeightpercentNa OandTiO arecomparedfor that alkalic and tholeiitic lavas from Loihi and Mauna Kea
2 2
tholeiites from the East Pacific Rise (EPR) and from several feature similar Na/Ti ratios and are distinct compared to
Hawaiian volcanoes. The Mauna Kea and Loihi groups also Koolau,MaunaLoaorEPRtholeiites.AtHawaii,Na/Tiratios
includealkaliclavas.MostHawaiianbasaltshavelowerNa/Ti areinsensitivetoMgOcontent.Na/TiratiosatHawaiiarethus
duetobothlowerNaandhigherTicontents.Basaltsfromthe unlikelyaffectedbylow-pressurefractionationandshouldre-
Koolau shield have intermediate Na/Ti ratios. Data are for flect source composition and melting conditions. In contrast,
LoihifromGarciaetal.[1993,1995];MaunaKeafromFreyet EPR basalts are affected by low-pressure fractionation. Only
al. [1991], Yang et al. [1996], Rhodes [1996], and Rhodes and EPRbasaltswith.8wt%MgOwereusedtoconstrainEPR
Hart[1995];MaunaLoafromRhodes[1988],Yangetal.[1996], melting depths; Na/Ti ratios for such EPR basalts are higher
andRhodes[1996];KilaueafromGarciaetal.[1992]andChen than at Hawaii. (b) The concentration ratio Sm/Yb is com-
et al. [1996]; and Koolau from Frey et al. [1994]. East Pacific pared to wt % MgO for the same samples as in Figure 2a.
Rise basalt compositions are from Langmuir [1988] (see also Sm/YbratiosareindependentofMgOcontentandshouldthus
Langmuiretal.[1986]). reflectmeltingconditionsandmantlecomposition.
PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY 2819
Hawaiian tholeiites and alkalic lavas and thus unaffected by Crough,1978;LiuandChase,1989].However,heatflow[Von-
shallow level processing (Figure 2). Since accumulation or Herzenetal.,1989]andseismicstudies[WoodsandOkal,1996]
fractionationofolivinedoesnotaffectNa/TiorSm/Ybratios, areinconsistentwithsuchthinning,andanaveragelithosphere
Hawaiian basalts with 7–13 wt % MgO are examined. An thickness of 100 km is indicated [Woods and Okal, 1996].
interesting interisland difference is apparent (Figure 2a): ba- Lithospherethickness,though,isunlikelytobeconstantalong
saltsfromMaunaKea,Loihi,andKilaueahaveNa/Ti52–2.5, the Hawaiian chain and may influence magma production
whilebasaltsfromMaunaLoaaresomewhathigherat2.5–3.0, [Morganetal.,1995;Wessel,1993].Sinceestimatesof75–80km
andKoolaubasaltsarehigherstillwithNa/Ti,rangingfrom3 lithospherehavebeenobtainedatOahu[Bock,1991],a75–120
to4. kmrangewasexplored.
In contrast to Hawaiian lavas, EPR basalts display Na/Ti When thick lithosphere is present, Na/Ti and Sm/Yb ratios
ratiosthatdecreasesharplywithdecreasingMgO(Figure2a); decreaseasinitialmeltingdepth(orT ) increases(Figure3).
p
this probably reflects plagioclase fractionation. Ratios of Na/Tiratiosarehigherbeneaththinlithosphereanddecrease
Sm/YbdonotvarywithMgO(Figure2b)andarethusappar- as initial melting depth increases (Figure 3a). Na/Ti ratios
ently unaffected by such fractionation. Experimental studies decrease with increased melting depth because the Kcpx/melt
Na
indicatethatplagioclasesaturationformid-oceanridgebasalts (Napartitioncoefficientforclinopyroxene/melt,5Ccpx/Cmelt,
Na Na
(MORB) occurs near 8–10 wt % MgO [Tormey et al., 1987; where Ci is the concentration of Na in the superscripted
Na
Grove and Bryan, 1983]. The highest Na/Ti ratios (Figure 2a) phase) increases with increased P, while clinopyroxene and
maythusapproachprimitivevalues.Tominimizetheeffectsof garnetKmin/meltremainconstantordecrease[Langmuiretal.,
Ti
fractionationonNa/Ti,onlybasaltswith.8wt%MgOwillbe 1992; Blundy et al., 1995; Kinzler, 1997; Walter, 1998; Putirka,
compared to model values for constraining partial melting 1998a,b].Mostimportantly,withouttheP-inducedchangein
depths. Na/Ti ratios for such EPR basalts (4–5) are signifi- Kcpx/melt,TiandNaarenotsignificantlyfractionatedfromone
Na
cantly higher, and Sm/Yb ratios are lower, compared to Ha- another,andaggregatemeltNa/Tiratiosarenearlyinvariant.
waiiantholeiitesandalkaliclavas. LowNa/Tiratiosthusindicateacontributionfrommeltspro-
ducedatsubstantialdepth.
The decrease in Sm/Yb beneath thick lithosphere is due to
3. Results
the progressive removal of garnet from the residue during
The melting model of this study follows the work of Lang- partialmelting.Whenmeltingbegins,theamountofgarnetis
muiretal.[1992](seetheappendix)andexaminesthecompo- highest,andsoisDsol/melt,(sinceKgarnet/meltishigh).Asmelt-
Yb Yb
sitions of aggregate melts, i.e., melts that are pooled from a ing progresses, though, garnet is removed from the residue,
rangeofdepthswithinameltingcolumn.Thuseachchoicefor and Dsol/melt decreases. By increasing initial melting depths
Yb
mantle potential temperature (T ) corresponds to a unique (i.e.,increasingmeanandmaximumF) theamountofgarnet
p
initial depth of partial melting, and the choice of lithosphere in the total melting column is decreased, as is the average
thicknesscorrespondstoafinaldepthofmelting.Inaddition, Dsol/meltinthemeltingcolumn.Aggregatemeltsatthesurface
Yb
for a particular starting composition, when lithosphere thick- reflect this average Dsol/melt. When lithosphere thickness ex-
Yb
nessandT arespecified,thereexistsauniquemaximumand ceeds 75 km, all melting occurs in the garnet stability field
p
meanextentofpartialmelting,orF(meltfraction).Sinceonly [Takahashi et al., 1993; Longhi, 1995; Kinzler, 1997; Walter,
twovariables(T , initialmeltingdepth,lithospherethickness, 1998], and the progressive consumption of garnet dominates
p
andmeanF)needbespecifiedforagivensourcecomposition, the Sm/Yb signal. For both Na/Ti and Sm/Yb the aggregate
thefollowingdiscussionwillrefertodifferencesininitialmelt- melt curves flatten with depth because melt productivity is
ing depths and lithosphere thickness; differences in mean F reduced at increased P [Putirka, 1997]. The Na/Ti curves ex-
andT areimplied. hibit less flattening with depth, compared to Sm/Yb, because
p
Initialmeltingdepthsandlithospherethicknessbothsignif- Kcpx/meltincreasessteadilywithdepththroughoutthemelting
Na
icantlyaffectNa/TiandSm/Ybratiosinaggregatemelts.This column.Incontrast,Dsol/melt,whichdependsuponthemodal
Yb
is true for both peridotite and eclogite or garnet pyroxenite abundance of garnet, does not vary greatly due to the small
mantle sources. In general, partial melting of a garnet pyrox- amountofpartialmeltthatisproduced(andhencethesmall
enite leads to aggregate melts that have higher Na/Ti and amountofgarnetthatisconsumed)athighP.
Sm/Yb ratios, compared to aggregate melts from a peridotite Inthecaseofthinlithosphere,changesinSm/Ybratiosare
source.HighNa/Tiforgarnetpyroxenitepartialmeltsresult,in lessdramatic(Figure3b)becausemeltsproduceddeepinthe
part, from higher Na/Ti in the source (5, compared to 4 for column (in the presence of garnet) are overwhelmed by the
peridotite).Moresignificant,though,isthegreateramountof comparatively large amounts of melt produced at shallow
garnet in the garnet pyroxenite (see the appendix), which in- depths (,75 km). With 10 km lithosphere, Sm/Yb first de-
creases the garnet pyroxenite bulk mineral/melt distribution creasesthenexhibitsarapidincreaseoncethegarnetstability
coefficient for Ti (5Dsol/melt 5 Csol/Cmelt, where Csol is the field is entered (75 km). The initial decrease in Sm/Yb, be-
Ti Ti Ti Ti
concentrationofTiinthetotalsolidandCmeltistheconcen- tween initial melting depths of 50 and 75 km, is due to varia-
Ti
tration of Ti in the melt). High Sm/Yb ratios similarly reflect tions in mean F. At these depths Dsol/melt . Dsol/melt; in the
Yb Sm
the high modal proportion of garnet, which results in an in- absence of garnet (,75 km), though, these D are very
REE
creased Dsol/melt. For both peridotite and pyroxenite starting small,andSmandYbareonlyfractionatedfromoneanother
Yb
compositions,Na/TiratiosdecreaseandSm/Ybratiosincrease whenF isverylow.Asmeltingdepthsincreasefrom75to50
inaggregatemelts,aslithospherethicknessincreases. km, both maximum and mean F increase, and Sm/Yb ratios
Mid-oceanridgebasaltsweremodeledusinga10-kmlitho- approach initial (unfractionated) values. Sm/Yb ratios for ag-
sphere; at Hawaii, lithosphere thickness’ are less certain. For gregate melts increase sharply when initial melting depths
example,convectivethinningispresumedtoreduceaninitially reach the garnet stability field (75 km), since Kgar/liq is very
Yb
90–100 km thick lithosphere to 50 km or less [Detrick and high.Sm/Ybratiosarealsosensitivetomeltingstoichiometry
2820 PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY
(Figure3c).Asanillustration,peridotite1(meltingcoefficient
forgarnetis0.2)iscomparedtoperidotite2(coefficientis1.0).
These values range beyond observed values (0.7 [Walter,
1998]), but Figure 3c shows the magnitude by which melting
stoichiometrymightinfluenceSm/Ybsystematicsatdepth.
4. Discussion
Variations in the P-T conditions of melting alone might
account for substantial geochemical variations in natural ba-
salts(Figures3–5).Suchvariationsencompassthelarge-scale
differences between the EPR and Hawaii, as well as some
intershieldvariationattheHawaiianislands.Inparticular,the
presentmodelsupportstheFreyetal.[1994]andKinzler[1997]
hypothesesregardingtheimportanceofmeltingdepthforrec-
oncilingvariationsinNa OandTiO inoceanicbasalts.These
2 2
results do not imply that the mantle is homogenous; hetero-
geneity appears to be required at both Hawaii and the EPR
[LangmuirandHanson,1980;Rodenetal.,1994;Prinzhoferet
al.,1989;Hekinianetal.,1989;Reynoldsetal.,1992].However,
such heterogeneity might not encompass all major oxides, let
alone involve major differences in source region mineralogy.
SincetheP-Tconditionsofmeltingcanaccountforsignificant
major and minor element systematics, source region differ-
encesmaybelimited.
4.1. EstimatesofMeltingDepthsandTemperatures
AttheEPR,initialmeltingdepthsof120615kmreproduce
theapproximateconcentrationsofNa,Ti,Sm,andYb(Figure
4).ThesedepthsoverlapwithdepthestimatesimpliedbySalt-
ers [1996, Figure 2] and are consistent with depths of low-
velocity and electromagnetic anomalies at mid-ocean ridges
[MELT Seismic Team, 1998; Zhang and Tanimoto, 1993;
Andersonetal.,1992].Mantletemperatureestimatesaresen-
sitive to the temperature of the mantle solidus and are much
lessrobustthanmeltingdepthestimates(seetheappendix).If
the mantle has a composition similar to KLB-1, a melting
depth of 110 km suggests a T of 1831 K (15588C). While
p
mantletemperaturesarenotwellconstrainedfromgeophysical
studies [Parsons and Sclater, 1977; Stein and Stein, 1992], T
p
inferredusingaKLB-1compositionoverlapswithmantletem-
peratures of 1723 6 250 K (at 95 km depth), obtained from
global studies of heat flow and ocean bathymetry [Stein and
Stein, 1992]. It cannot be overemphasized, though, that tem-
perature estimates are only as reliable as our knowledge of
mantle composition and solidus temperatures. Experimental
studies show a range of melting temperatures for different
peridotitecompositions[HiroseandKushiro,1993];actualtem-
peratures could be lower by as much as 50 K, or more, if the
mantleismorefertilethanKLB-1.
MuchgreaterinitialmeltingdepthsareimpliedforHawai-
Figure 3. (a) Na/Ti ratios (cation fraction) of aggregate iantholeiiteandalkaliclavas.AbundancesofNa,Ti,Hf,and
meltsproducedfromperidotiteandgarnetpyroxenitesources REE can be produced using the same depleted mantle com-
are compared at different P-T conditions. Lithosphere thick- position as that for the EPR but with increased lithosphere
nessindicatesdepthtothetopofthemeltingcolumn;“initial thicknessandinitialmeltingdepthsof200–400km.Anunde-
melting depth” represents depth to the base of the melting pleted mantle source would yield higher initial Na/Ti ratios
columnandhencealsoreflectsmantlepotentialtemperature.
(Figure 5); to achieve the low Na/Ti ratios of Hawaii for an
(b) Sm/Yb ratios for aggregate melt compositions are com- undepleted source, increased initial melting depths (by ;50–
pared,and(c)aclose-upviewofthe10-kmlithospherecalcu-
100km)wouldberequired.
lations is given. In Figure 3c the “peridotite 1” model uses a
Estimates of melting depths at Hawaii exceed those ob-
garnetmeltingcoefficientof0.2;the“peridotite2”modeluses
agarnetmeltingcoefficientof1.Foragivenlithospherethick- tained by Watson and McKenzie [1991]. There are numerous
nessagarnetpyroxenitesourceyieldshigherNa/TiandSm/Yb reasonsforthesedifferences.First,itshouldbenotedthatthe
ratioscomparedtoaperidotitesource. Watson and McKenzie [1991] model does not successfully re-
PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY 2821
inameltproductivitythatistoohigh.Highmeltproductivity
results in (1) elevated estimates of mean F and (2) lower
incompatible element concentrations for a given initial depth
of melting. The present model, with greater initial melting
depthsandamorerealisticDS (;420J/kg),successfullyyields
f
Na,Ti,andREEconcentrationsforHawaiianandEPRlavas
(Figure4).
Sincegarnetisgraduallyconsumedduringmelting,agarnet-
free residue is produced at the top of the melting column at
Hawaii. If the melting coefficient for clinopyroxene is .0.5,
then clinopyroxene is also consumed by depths of 100 km,
leavingaharzburgiteresidueatshallowdepths.Thispredicted
residue might resolve the apparent conflict that high-MgO
Kilaueatholeiitesareharzburgitesaturatedatpressures,3.5
GPa[Eggins,1992],yetshowagarnettrace-elementsignature
[BudahnandSchmitt,1985;seealsoFreyetal.,1994].Usingthe
solidus of KLB-1, a T of 2130 K (18578C), is implied to
p
achieve melting at 300 km. Such temperatures represent a
mantlethatisapproximately300KhotteratHawaiicompared
to the EPR, which is consistent with temperature differences
inferredfromtomographic[Romanowicz,1994;Andersonetal.,
1992] and geodynamic studies [Sleep, 1990; Ribe and Chris-
tensen, 1994]. Calculated melting depths are also consistent
with depths for low seismic velocities and high seismic wave
attenuation [Bhattacharyya et al., 1996; Romanowicz, 1995;
Zhang and Tanimoto, 1993]. The temperature difference be-
tweenHawaiiandtheEPRisprobablymorerobustcompared
toabsolutetemperatureestimatesatHawaii,whicharesubject
tothesameuncertaintiesasnotedabovefortheEPR.
4.2. MantleHeterogeneity
Thedegreeandnatureofheterogeneityinthebasaltsource
region can be constrained when the potential P-T effects on
partial melt compositions are evaluated. Geochemical varia-
tionsthatareunexplainedbyahomogenousperidotitemantle
model provide minimum estimates on source region differ-
Figure 4. (a) Natural and model Sm concentrations (ppm)
ences. In some cases, such as at Hawaii, differences in P-T
andSm/Ybratiosarecompared.(b)Calculatedandobserved
concentrationsforNaandTiarecompared.Hawaiianbasalts conditionsareprobablynotsufficienttoexplainallgeochemi-
weremodeledusinga100-km-thicklithosphereandaninitial cal variations, and minimum differences in modal mineralogy
melting depth of 300 km; EPR basalts were modeled using a for source regions are calculated. In addition to peridotite,
10-km lithosphere and an initial melting depth of 115 km; eclogite and various garnet pyroxenite starting compositions
thesemodelcalculationsareindicatedwiththelettersH(Ha- arealsoexaminedtoresolvewhethersuchsourcesareconsis-
waii)andE(EPR)(circled).Observedabundance’sofNa,Ti, tentwithobservedgeochemicalsignals.
andsomeREEcanbereproducedbyvaryingtheP-T melting 4.2.1. Heterogeneity beneath the EPR. Hirschmann and
conditionsofamantlethathasadepletedpyrolitebulkcom-
Stolper [1996] have proposed that garnet pyroxenite exists in
positionandaperidotitemineralogy.
the MORB source region. Their argument was in part based
upon the apparent discrepancy between the deep levels of
melting required to produce the garnet signature in MORB
produce Na O and TiO contents at Hawaii. Their predicted andtherelativelythincrustthatisformedatoceanicspreading
2 2
Na/Ti ratios [Watson and McKenzie, 1991, Table 2] are much centers.AsnotedbyPutirka[1997],though,fractionalmelting
too high, indicating that their melting depth estimates (parti- processeslimitmeltproductivityandmaybeinvokedtosatisfy
tion coefficients for Na) are too low. Watson and McKenzie bothobservedcrustalthickness[Whiteetal.,1992]andinferred
[1991]alsouseafinalmeltingdepthof82kmandassumethat melting depths of .100 km [Salters, 1996]. Regarding geo-
spinel peridotite is stable to depths apparently exceeding 100 chemical variation, Na/Ti and Sm/Yb ratios at the EPR are
km.ThelithospherebeneathsomepartsofHawaiiisconceiv- consistent with partial melting of a peridotite source. Some
ably as thin as proposed by Watson and McKenzie [1991], but mixing with a garnet pyroxenite source is allowed, since the
spinelperidotiteisprobablynotstable,evenatdepthsasshal- EPRsamplesplotbetweentheperidotiteandgarnetpyroxen-
low as 82 km [Takahashi et al., 1993; Longhi, 1995; Kinzler, ite curves (Figure 5b). However, only the major oxides carry
1997; Walter, 1998]. Using a more realistic garnet lherzolite any significant leverage on mineral abundances. It is possible
mineralogy, such shallow melting depths yield Sm/Yb ratios thattheobservedgeochemicaldifferences,atleastwithrespect
that are too high compared to observed values (Figure 5). to Na and Ti, might be supported by minor mineralogic vari-
Finally, the entropy of fusion (DS ) used by Watson and ations within a source that has an overall peridotite mineral-
f
McKenzie[1991]istoolow,perhapsbyafactorof2,resulting ogy. For example, a variation of 60.2 wt % Na in the EPR
2822 PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY
sourceregioncouldexplainobservedrangesofNa/Tiratiosin
the high MgO EPR lavas; such differences can be accommo-
dated by very small (,2%; see below) changes in peridotite
mineralabundances.
As recognized by Hirschmann and Stolper [1996], partial
melts from neither garnet pyroxenite nor peridotite sources
exhibitsufficientvariationtoexplainthefullrangeofMORB
Lu/Hf and Sm/Nd ratios. If the lithosphere thickness is in-
creased to 30–50 km, peridotite partial melts can reproduce
the low Lu/Hf and Sm/Nd of the MORB array (Figure 5c).
Thickerlithospheremaynotbesurprisingatsuchlocalitiesas
theAtlantic-AntarcticDiscordance(AAD),sincecoolerman-
tletemperaturesareindicatedfromcomparisonsofmid-ocean
ridgebathymetryandgeochemistry[KleinandLangmuir,1987;
Langmuiretal.,1992;Salters,1996].Neitherthegarnetpyrox-
enitenortheperidotitemodels,though,arecapableofrepro-
ducing the high Lu/Hf and Sm/Nd ratios observed at Kol-
beinsy. In contrast, Salters’ [1996] peridotite melting model
appearstoaccountforallbuttheKolbeinsydatabytakinginto
accountvariationsinthetimingofsourceregiondepletion.
4.2.2. HeterogeneitybeneathHawaii. AtHawaii,eclogite
hasbeenhypothesizedtoexistinthesourceregion,witheclo-
gite partial melts contributing most greatly (up to 20%) to
some Koolau basalts [Hauri, 1996]. Hauri’s [1996] hypothesis
stems, in part, from the observation that Hawaiian lavas are
enriched in SiO compared to partial melts obtained from
2
experimental studies of lherzolites. However, several isotope
and trace element studies suggest that a recycled source at
Koolau might be minimal [Roden et al., 1994; Bennett et al.,
1996].Furthermore,Freyetal.[1994]interpretinterislandSiO
2
systematics as indicating depth of melting differences, rather
than source region heterogeneity. Wagner and Grove [1998]
have also shown that basalts can obtain elevated SiO if or-
2
thopyroxene is assimilated at shallow depths. (Note: orthopy-
roxene assimilation does not affect Na/Ti or Sm/Yb. Using
data from Wagner and Grove [1998], assimilation of 20% or-
Figure 5. (opposite) (a) Aggregate melt compositions are
fromadepletedperidotitesource.HighNa/Ti(cationfraction)
at Koolau may be explained by the presence of thinner litho-
sphereandcoolermantletemperatures.Intravolcanovariation
probably reflects heterogeneity. Peridotite models use a de-
pleted mantle composition (see the appendix and Table 1);
“bulk composition uncertainty” reflects the increase in Na/Ti
andSm/Ybratiosthatareobtainedwhenanundepletedman-
tlesourceisinputintothemeltingmodels(seetheappendix).
(b) Calculated aggregate melts are shown for eclogite and
garnet pyroxenite sources (“HS” is the average garnet pyrox-
enite from Hirschmann and Stolper [1996]; “massif” refers to
garnetpyroxenitesfrommassifperidotites;seetheappendix;K
istheKoolauandMListheMaunaLoa1MaunaKea1Loihi
fields, as in Figure 5a). Na/Ti and Sm/Yb ratios at Hawaii
overlap only with some massif pyroxenites, but the predicted
Na, Ti and REE abundances are too high compared to lava
compositions (see text). (c) Lu/Hf and Sm/Nd from MORB
[Salters,1996]arecomparedtoHawaiiancompositions(AAD,
Atlantic-AntarcticDiscordance).Initialmeltingdepthsforthe
10-km lithosphere peridotite and garnet pyroxenite models
range from 50 to 400 km, moving downward in the figure.
Hawaiian data are adequately described with a 100-km litho-
sphere and initial melting depths of 200–400 km (increasing
upward in the figure). Lu/Hf and Sm/Nd for other MORBs
span a range of values that cannot be covered by varying the
initialmeltingdepthsforeclogiteorperidotitepartialmelting.
PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY 2823
thopyroxenedecreasesNa/TiandSm/Ybratiosby.015and.05 compared to Hawaiian tholeiites and alkalic lavas (Figure 5).
respectively). Finally, Okano and Tatsumoto [1996] observe Partial melts from some massif garnet pyroxenites exhibit
that Hawaiian spinel lherzolite xenoliths exhibit a wide range Na/TiandSm/YbratiosthatoverlapwithsomeHawaiiantho-
ofisotoperatios,withsomeisotopesapproachingKoolau-like leiiteandalkalicbasalts.However,Na,Ti,Hf,andREEabun-
values. This observation shows that a wide range of isotope dances from such sources are much higher than observed at
ratios can be supported by a lherzolite mineralogy and that Hawaii. Partial melts from pyroxenite sources yield Na abun-
isotope variations at Hawaii might not require an eclogite dances between 0.08–0.20 cation fraction, and Sm abun-
source. It is thus unclear that eclogite is a necessary source dance’sof6.2–12.5ppm(comparetoFigure4)andarehighest
componentatHawaii. for massif pyroxenites. Minor element abundances might be
Most of the interisland variation in Na/Ti and Sm/Yb at dilutedifmixedwithmeltsfromaperidotitesource.However,
Hawaii can be accounted for by plausible ranges in the P-T toaccountforNa/TiratiosatKoolau,massifpyroxenitepartial
conditionsofmeltingforaperidotitemantle(Figure5a).For melts would appear to dominate the Koolau aggregate melts
example, Na/Ti-Sm/Yb partial melting arrays overlap the (up to 100%; Figure 5b). In addition, massif pyroxenites ex-
Koolaufieldaslithospherethicknessdecreasesto75km,and hibitawiderangeofgarnet/pyroxeneratios;onlywhengarnet
initialmeltingdepthsdecreaseto170km.Owingtodifferences is,20%areSm/Ybratiosinthecorrectrange.Finally,thethin
ininitialmeltingdepth(andT ), differentvaluesformeanF lithosphere and cooler melting temperatures required in the
p
arealsoobtainedfortheHawaiiantholeiites(Figure5a).For peridotite model (above) for Koolau are not avoided by as-
comparison,estimatesofF atLoihiaresimilarto,butslightly sumingapyroxenitesource.Thus,incontrasttotheperidotite
higher than, those obtained by Garcia et al. [1995] and are model,pyroxeniteandeclogitesourcesprovidepoorfitstoNa,
within the very wide range of values noted by Feigenson et al. Ti,andREEabundancesatHawaii.Itwouldthereforeappear
[1996]forMaunaKea.Asnotedabove,though,differencesin thatsuchsourcesatHawaiiarenotonlyunnecessarybutper-
meltingdepth(intheabsenceofheterogeneity)arerequiredto hapsalsounlikely.
fractionateNafromTi;initialmeltingdepth,ratherthanF, is AsisevidentfromFigure5,variationsinlithospherethick-
thecriticalvariableforexplainingtherangeofNa/Tiratiosat ness may be invoked to explain interisland variations at Ha-
Hawaii. waii.Ontheotherhand,ifthehypothesizedlithospherevari-
Undoubtedly,someintraislandvariationisduetoheteroge- ations are incorrect, only small changes in the source region
neity,especiallysincesomeHawaiianlavasplotoutsideofthe mineralogyarerequiredbasedonNaandTiabundances.Mass
partial melting grid (Figure 5). Limited variations in melting balance calculations using a mantle peridotite and high-
depthsandlithospherethicknessatHawaii,though,areatleast pressuremineralcompositions[Walter,1998;Putirka,1998a,b]
plausible. Melting depths might vary depending upon the po- underscorethisconclusion.Forexample,ifthedifferencebe-
sitionoftheshieldvolcanoandtheplumeaxis[Farnetaniand tween the lowest and highest Na O values at Koolau and
2
Richards,1995],especiallyifshieldpositionsareinfluencedby Kilauea(about1wt%;Figure1)aredueonlytoheterogeneity
thestateofstressinthelithosphere[JacksonandShaw,1975]. inthesourceregion,massbalanceimpliesthatthisdifference
RelativemaximummeltingdepthsforKoolau,Kilauea,Mauna canbeaccommodatedbyasourceregiondifferenceinclinopy-
Loa,andLoihiarequalitativelyconsistentwithmeltingdepths roxene 1 garnet of ,2.0% (assuming mean F is 0.15, mean
inferred from major oxide systematics [Frey et al., 1994]; the Dmin/melt is 0.06, and minerals have constant composition).
Na
modelisfurthermoreconsistentwithtraceelementandisoto- Such source differences are reduced if any Na O variation
2
pic observations [Bennett et al., 1996; Roden et al., 1994] that resultsfromtheP-T conditionsofmelting,assimilationinthe
indicate a peridotite source for Hawaiian lavas. Moreover, shallow mantle, adjustments in mineral composition in the
KoolauliesnorthoftheMolokaifracturezone[Mammerickx, source region, or shallow level fractional crystallization. It
1989; Atwater, 1989], and seismic work [Bock, 1991] at Oahu shouldbenotedthatwhilethesecalculationsarebasedonlyon
indicatesa75–80kmlithosphereinthisregion. Na O, this oxide carries more leverage on calculated mineral
2
If the Koolau lithosphere approaches 100 km, the mantle proportions compared to other major oxides. This is because
source at Koolau must differ from that beneath Loihi and Naoccursonlyinclinopyroxenetoanygreatextent,andeven
MaunaKea.Additionally,sincelithospherethicknessprobably at high pressure it is never the dominant component in this
does not vary greatly at any single volcano, intraisland heter- phase.LargechangesinsourceNa Oconcentrationstherefore
2
ogeneityisalsoimpliedbytherangeofNa/Tiratiosdisplayed translate into large changes in clinopyroxene modal abun-
ateachoftheHawaiianshields(Figure5a).ThelowestNa/Ti dances.Na Oisthusanidealoxidefordeterminingwhethera
2
ratios at Mauna Kea are also not explained by the present mantlesourceiscomprisedofperidotiteorpyroxenite.While
model.TheselowNa/Tiratioscanonlybeexplainedif(1)the an analysis of other elements, especially Ca or Al, might also
topofthemeltingcolumnexceeds.150kmdepthor(2)the beuseful(givensuitablehigh-pressurecalibrationsofK ),itis
d
mantle source has higher clinopyroxene (by 2%) and lower tentativelyconcludedthatsmallmodalvariationsinaperido-
garnet(by2%)comparedtothemineralogyofTable1.Option tite assemblage of minerals can probably accommodate ob-
2 seems more likely, but lower garnet results in lower Sm/Yb servedgeochemicalvariationsatHawaii.
ratios,perhapsindicatingREEheterogeneity. Mantleheterogeneityisaflexiblehypothesisandcannotbe
Whileperidotitepartialmeltingmayexplaininterislandvari- excluded. If eclogite is not required to describe geochemical
ationatHawaii,cangarnetpyroxeniteoreclogitesourcesalso variationsinoceaniclavas,though,thereareimportantimpli-
yield observed Na, Ti, and REE abundances at Koolau? To cationsformixinginEarth’smantle.Itseemslikelythatsmall
illustrate the potential range of such sources, several starting amounts of recycled material contribute to the trace element
compositions were explored, including eclogite, and garnet and isotopic signature of some oceanic lavas [Hofmann and
pyroxenites from various sources (see the appendix). As is White, 1982; Zindler et al., 1984; Hauri et al., 1996]. Such ma-
evident from Figure 5b, most pyroxenites and eclogites are terial, though, might not survive the recycling process as a
unsuitableassourcecomponents,sinceSm/Ybratiosarehigh distinctmineralogicalcomponent.Mantlemixingmightbeef-
2824 PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY
Figure 6. Hawaiian shield averages for isotopes and major oxides [Hauri, 1996] are compared to test for
depth-dependent isotopic heterogeneity at Hawaii. Na/Ti ratios are correlated with (a) 87Sr/86Sr, (b) 206Pb/
204Pb,and(c)187Os/188Os.(NotethatHauri[1996]givestwoaveragesforLoihi:oneforalkalicsandonefor
tholeiites; both are plotted. R2 values are for Hauri’s [1996] shield averages.) The Loa trend volcanoes are
consistent with two-component mixing between an upper isotopically enriched mantle component and a
deeper more isotopically depleted/less enriched source. Kea trend volcanoes are not consistent with two-
componentmixingandmayindicateathirdcomponent[Chen,1987].Interestingly,Loihilavas[Garciaetal.,
1995;Lanphere,1983;FreyandClague,1983]andMaunaLoalavas[RhodesandHart,1995]showlocalinternal
correlationsfor87Sr/86Sr-Na/TithatareoppositetotheintershieldtrendbutdirectedtowardMORB(Figure7).
ficient at erasing such heterogeneity, or perhaps the melting (Figure7)indicatealarge-scaleverticalcomponenttoisotopic
processisitselfahomogenizingagent. heterogeneity in the Pacific upper mantle. The Hawaii-EPR
comparison supports the conventional layered mantle model,
4.3. SpatialVariabilityofIsotopeSources or perhaps a “marble cake” mantle with a systematic depth
To test for depth distributions of isotopic heterogeneity in distributionofisotopicallydistinctblobs.
theHawaiianmantle,compositionalaveragesfortheHawaiian
shields [Hauri, 1996] were examined. Such shield averages
5. Summary
yieldgoodcorrelation’sbetweenNa/Tiandsomeisotoperatios
(Figure6).Since,asnotedabove,Nacanonlybefractionated A mantle melting model was developed to estimate partial
fromTiatdepth,itappearsthatmeltsproducedatdepth(low meltingdepthsforoceaniclavasandtoconstrainmantlehet-
Na/Ti ratios) tap mantle sources that are isotopically distinct erogeneity,usingNa/TiandREEratios.Na/Tiratiosarepar-
from melts produced at more shallow levels of the mantle ticularlyusefulsinceNaandTiareonlystronglyfractionated
(higherNa/Ti).TheorderingoftheLoatrendvolcanoesisthe fromoneanotherathighP. HafniumandtheREE’sarealso
sameineachplotinFigure6andisthusconsistentwithmixing usefulsincetheirmineral/meltpartitioncoefficientsaresensi-
between two, vertically stratified, mantle sources. This order- tive to pressure and mineralogy [Salters, 1996; Kinzler, 1997;
ing for the Loa trend holds, even for the more poorly corre- Walter, 1998; Putirka, 1998a, b]. Several important results are
lated Sr isotope ratios (Figure 6), and supports prior argu- obtainedfromthemodels.First,theP-Tconditionsofmelting
ments for two-component mixing at Hawaii [Bennett et al., aloneappeartobecapableofexplainingbothlocalvariationat
1996; Chen, 1987; Chen and Frey, 1985]. It is also possible, Hawaii, as well as some large-scale geochemical differences
though,thatisotoperatiosvarycontinuouslybutsystematically betweenHawaii(anoceanisland)andtheEPR(amid-ocean
with depth in the mantle. So far as Na/Ti ratios represent ridge spreading center). This implies that bulk compositional
meltingdepth,Koolaulavasreflectpartialmeltingofashallow differencesbetweenHawaiianandEPRsourceregionsmight
and variably enriched/depleted [see Richardson et al., 1982] beminimal;thisisconsistentwithnear-uniform,butnonprimi-
mantle component (Figure 7). A two-component mixing tive,minorelementratiosforoceanicbasalts[Hofmannetal.,
scheme is not consistent with Kea trend lava compositions, 1986] and high-pressure diffusion experiments which indicate
whichmayindicatealessverticallystratifiedsource[Staudigel efficienthomogenizationinpartsoftheuppermantle[Farber
etal.,1984;Stilleetal.,1983;Chen,1987]. etal.,1994].Mantleheterogeneityisalsoconstrained;inferred
Interestingly, Sr isotopes for individual lavas at Loihi and sourceregiondifferencescanbeaccommodatedbysmall(2%)
MaunaLoashowalocal,orinternal,correlationthatisoppo- variations in peridotite phase proportions. Na O, TiO , and
2 2
site to the intershield trend (Figure 6) but directed toward REEabundancesatHawaiialsoappeartobeinconsistentwith
MORB.Theintershieldtrendwouldappeartoindicatealat- avarietyofpotentialeclogiteorgarnetpyroxenitesources.The
eral(along-strike)componentaswellasaverticalcomponent mantleisundoubtedlymineralogicallyheterogeneous,butthis
to local heterogeneity at the Hawaiian islands. This might be and other work [Sims and Depaolo, 1997] reveal the need to
an enriched shallow layer, perhaps at or near the base of the test models of heterogeneity against possible P-T dependent
Hawaiianlithosphere,thatismoststronglysampledatKoolau. variationsinmeltcomposition.
Incontrast,boththelocaltrendatHawaiiandthedifferences AbundancesofNa,Ti,Hf,andtheREEforEPRlavasare
between isotope and Na/Ti ratios at Hawaii and the EPR bestreproducedwhenmeltingbeginsat120615km,consis-
PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY 2825
Figure 7. IsotopesandNa/TiratiosforHawaiianlavasarecomparedtolavasfromtheEPR.Differencesin
Na/Ti between EPR and Hawaii indicate that the shallow upper mantle has experienced greater extents of
long-term depletion of (a) Rb relative to Sr and (b) Nd relative to Sm. Isotopic field for EPR lavas (and
neighboringseamounts)arefromZindleretal.[1984].NotethatindividualLoihiandMaunaLoalavasappear
totrendtowardMORB,perpendiculartothemeaninterislandtrend.SymbolsanddataareasinFigure6.
tentwithrecentseismicwork[MELTSeismicTeam,1998]and decrease with increasing depth in the mantle [Kinzler, 1997;
prior depth estimates [Salters, 1996]. In contrast, Hawaiian Walter, 1998]. This appears not to be an intrinsic pressure
tholeiitescanbemodeledusinga100-km-thicklithosphereand effect but instead results from the low Al content of near
initial melting depths of 200–400 km. This implies a temper- solidusliquidsathighpressure[Putirka,1998b].Adecreasein
aturedifferencebetweenHawaiiandtheEPRof3008C,close the partition coefficient for Ti amplifies the pressure depen-
to estimates inferred from geodynamic studies [Sleep, 1990; dence of Na/Ti and decreases estimates of initial melting
Schilling, 1991] and mantle tomography [Romanowicz, 1994]. depths.Sincetheclinopyroxene-meltpartitioncoefficientsfor
Thecompositionaldependenciesofperidotitesolidustemper- Ti observed by Kinzler [1997] and Walter [1998] are probably
atures, though, need to be explored so that these depth esti- most appropriate for modeling mantle melting, they were
mates may be used to better resolve the thermal structure of adoptedforthisstudy.
Earth’s upper mantle. The present model also indicates that
A1. MantleComposition
garnet is consumed during melting, while aggregate melts re-
tain a garnet signature. This result may help to resolve the One set of mantle mineral proportions used for peridotite
apparent conflict between trace element abundances, which calculations (Table 1) are from Ita and Stixrude [1992, Figure
indicate garnet in the source region [Budahn and Schmitt, 4]. The Ita and Stixrude [1992] estimates were obtained as a
1985;Freyetal.,1994],andexperimentalstudieswhichindicate best fit to seismic data for mineral elastic properties. Such
thatsomeprimitiveKilaueabasaltsarenotgarnetsaturatedat mineral modes are consistent with other similar studies of
P , 3.5 GPa [Eggins, 1992]. uppermantlemineralogy[WeidnerandIto,1987]andprovide
areasonablestartingpointformodelinganinitialmantlemin-
eralogy.Initialelementconcentrationsforaperidotitemantle
Appendix: Mantle Melting Model
arefromKinzlerandGrove[1992b]andHirschmannandStol-
Oceanic basalt compositions can be successfully explained per [1996] for a depleted mantle and McDonough and Sun
bypolybaricpartialmeltingmodels[KleinandLangmuir,1987; [1995] for an undepleted source. Minerals compositions from
KinzlerandGrove,1992b;Langmuiretal.,1992;Kinzler,1997]. high-pressure experimental studies [Walter, 1998; Putirka,
Aggregate melt compositions were thus calculated for batch 1998a] were used to compare the major oxide [Kinzler and
andfractionalmeltingprocessesusingtheequationsofLang- Grove, 1992b] and mineral modes [Ita and Stixrude, 1992] of
muir et al. [1992] and Plank and Langmuir [1992] and the Table 1. Mass balance calculations indicate overlap between
mineral-melt partitioning relationships of Putirka [1998a, b] the major oxide composition and mineral proportions, but
and Salters [1996]. The calculations employ a step-wise ap- theseestimatesareverysensitivetothemineralcompositions
proach[Putirka,1997].Inthismethod,degreesofpartialmelt- selectedformassbalance.UsingtheKinzlerandGrove[1992b]
ing (F) and aggregate melt compositions are calculated at major element abundances, it is possible to obtain mineral
depthincrementsof2km.Theresidualmineralogyisadjusted modesthathavelowergarnet(by2%)andhigherclinopyrox-
ateachstepusingtheestimatedvalueforF andthestoichio- ene (up to 6%) balanced by lower olivine; these mineralogys
metriccoefficientsofthepertinentmeltingequilibria(seebelow). were also explored for modeling purposes. A model mantle
Recentexperimentalworkindicatesthatatnear-soliduscon- withslightlyhigherclinopyroxeneandlowergarnetandolivine
ditionstheclinopyroxene-meltpartitioncoefficientforTimay abundances results in lower depth estimates for Hawaiian la-
2826 PUTIRKA:MELTINGDEPTHSANDMANTLEHETEROGENEITY
vas.Suchamineralogycanalsomoreeasilyexplainthelowest peridotite solidi have not been measured with the same tem-
Na/Ti ratios observed at Mauna Kea, although lower-than- perature precision (Walter [1998] and Zhang and Herzberg
observed Sm/Yb ratios are also obtained. The mineralogy of [1994]accountforthermalgradientsintheirexperiments;see
Table 1 appears to be the most successful mineralogy for si- Zhang and Herzberg [1994] for comparisons), and composi-
multaneously explaining EPR and Hawaiian lava composi- tional effects on the mantle solidus cannot yet be quantified.
tions.Modelresultsarelesssensitivetosmallchangesininitial The data presented by Hirose and Kushiro [1993], though,
Na/Tiratio,andarangeofdepletedmantlestartingcomposi- indicate that Tsolidus differences of up to 50 K are possible,
tions are successful at explaining major element variations in dependingonmantlecomposition.
oceanicbasalts.
The peridotite bulk composition uncertainty (Figure 5) re- A3. MeltingStoichiometry
flectsthedifferencebetweendepletedandundepletedsources Meltingstoichiometryaffectstheestimationofmineralpro-
when these sources are input into the melting models. The portions throughout the melting column and thus impacts
average garnet pyroxenite composition is from Hirschmann upon Dsol/melt. Stoichiometric coefficients to depths of 0–75
i
and Stolper [1996], and their modal mineralogy is used. Eclo- km are from J. Longhi [see Salters, 1996], Longhi [1995], and
gitecompositionsarefromGriffinandBrueckner[1985],Grif- Kinzler and Grove [1992a]. These coefficients (experimentally
fithsandCornichet[1985],Milleretal.[1988],TaylorandNeal determinedatP , 3.5 GPa)mightnotextrapolatetohighP.
[1989],andBeardetal.[1992];modelcalculationsuseagarnet With increased pressure, clinopyroxene and garnet survive to
fraction of 0.25. Eclogite garnet contents vary widely (0.25– greater extents of melting [Wei et al., 1990; Takahashi et al.,
0.50), and higher amounts of garnet yield partial melts with 1993].InworkbyTakahashietal[1993],clinopyroxenedisap-
higherSm/YbandNa/Tiratios(Figure5).Garnetpyroxenites pearsat38%meltingat3GPa,whileat7GPaclinopyroxene
from massifs are from Loubet et al. [1982], Bodineir et al. survives to .70% melting. Less clinopyroxene must be con-
[1987],andStoschandLugmair[1990];agarnetfractionof0.2 sumed for each increment of melt produced at 7 GPa, and
wasusedformodelcalculations.Aswitheclogites,pyroxenites meltingcoefficientsshouldbereduced.Atdepths.75kmthe
feature a wide range of garnet fractions (0–50%), and in- models(Figures3–5)useWalter’s[1998]coefficients,although
creased garnet yields increased Sm/Yb and Na/Ti ratios for lowercoefficientswereexplored.
partialmelts(Figure5). The amount of orthopyroxene on the mantle solidus is ex-
pected to change with depth since solid solution between or-
A2. TheMantleLiquidusandSolidusandMantle
thopyroxene and clinopyroxene increases with increased tem-
Adiabat
perature [Lindsley, 1983; Longhi and Bertka, 1996]. While
The mantle melting model uses a mantle adiabat of 1.2 orthopyroxene appears in the products side of the melting
K/GPa and liquidus and solidus curves from Zhang and Her- equilibriumat3.0–3.5GPa[Longhi,1995],orthopyroxenemay
zberg[1994].Liquidusandsoliduspressureswereconvertedto disappearfromthemeltingequilibriaathighpressure[Zhang
depthusingthepreliminaryreferenceEarthmodel[Anderson, andHerzberg,1994;Takahashietal.,1993].However,thepre-
1989]. cisePatwhichorthopyroxenedisappearsisunclear[seeCanil,
The spinel-garnet transition was placed at 2.3 GPa [Gud- 1992;Walter,1998;BertkaandHolloway,1993].Orthopyroxene
finnssonandPresnall,1996;Kinzler,1997];acusponthesolidus was treated as being absent from the melting interval at P .
atthespinel-garnetphasetransitionwasnotmodeled.Whilea 6.5 GPa. Since orthopyroxene exerts little leverage on Na/Ti
changeinslopemustoccuratthespinel-garnettransition,no or REE ratios, small errors regarding the placement of or-
clearconstraintsonthemagnitudeofthischangeexist.Acusp thopyroxeneinthemeltingintervaldonotaffectmodelcalcu-
is not clearly detectable in the data of Takahashi et al. [1993] lations.
forKLB-1,norintheCaO-MgO-Al O -SiO (CMAS)system
2 3 2
[GudfinnssonandPresnall,1996],whichmayindicatethatthe
transitionisnotsharp.Anyerrorintroducedbyignoringsmall Acknowledgments. IthankCharlieLangmuirformanyhelpfuldis-
cussionsconcerningthisprojectandforintroducingtomesomeofthe
cusps is also insignificant compared to error on the solidus
problemsconcerningNaandTiabundancesinoceanicbasalts.Ialso
temperature. greatlyappreciatethereviewsofMichaelGarcia,VincentSalters,and
Langmuiretal.[1992]showedthatcompositionaldifferences KennethCameron,whichresultedinamuchimprovedversionofthis
between batch and fractional melting may not be very great manuscript. Finally, I thank David Walker, Rosamond Kinzler, and
JohnLonghiforhelpfulcommentsandreviewsofearlierversionsof
when polybaric aggregate melts are compared. The present
thismanuscriptandRickRyersonforhissupport.UCRL-JC-131482
calculations support this notion. The results of fractional and
batchmeltingmodelsaresimilar,thoughbatchmeltingresults
comparesomewhatbettertothegeochemicaldata(fractional References
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