Vapor-Absent Melting of Tonalite at 15–32 kbar

16 Pages·2005·0.46 MB·English

JOURNALOFPETROLOGY VOLUME46 NUMBER2 PAGES275–290 2005 doi:10.1093/petrology/egh071
Vapor-Absent Melting of Tonalite at
15–32 kbar
ALBERTO E. PATIN˜O DOUCE*
DEPARTMENTOFGEOLOGY,UNIVERSITYOFGEORGIA,ATHENS,GA30602,USA D
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RECEIVEDOCTOBER15,2003;ACCEPTEDAUGUST6,2004 de
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The behavior of igneous continental crust during subduction is considerableadvancesinourunderstandingofthemeta- s
modeledbymeansofvapor-absentpartialmeltingexperimentsona morphic reactions that affect crustal rocks at ultrahigh ://ac
a
tonalite, containing equal amounts of biotite and hornblende, at pressure (UHP) conditions (e.g. Massonne, 1995, 1999; d
e
pressures of 15–32kbar. The experiments produce leucogranitic Liou et al., 1998). Assemblages that are diagnostic of m
ic
melts coexisting with garnet þ omphacitic clinopyroxene þ UHP metamorphism (UHPM) have been identified and .o
u
K-feldspar þ kyanite þ quartz/coesite (cid:1) phengite (cid:1) zoisite. mineral equilibria that can be used to estimate UHPM p.c
Experimental constraints and geometrical analysis of phase equili- conditions quantitatively have been calibrated. Much of om
bria show that the hydrous phasesthat controldehydration-melting the interest has focused on identifying UHPM rocks at /p
e
of tonalites in deep thickened continental crust and in the upper the surface of the Earth. In contrast, there has been tro
lo
mantlearephengiteandzoisite.Thenegativelyslopingamphiboleþ relativelylessdiscussionofthefateofcrustalrocksduring g
y
quartzvapor-absentsoliduscharacteristicofamphibolitesislargely their mantle excursions. An important open question is /a
suppressed in tonalites, because amphibole is eliminated by water- whetherthereareinstancesinwhichcrustalrocksfailto rtic
le
conserving reactions that also consume K-feldspar and kyanite and returnfromthemantleandwhathappenstothem,andto -a
b
produce phengite and zoisite. The temperature at which melt first theenclosingmantle,inthosecases. stra
appears in the experiments varies from <900(cid:2)C at 15kbar, to ManyUHPMterranesappeartohaveundergonelittle c
1000(cid:2)C at 27kbar, to <925(cid:2)C at 32kbar. Moderate degrees of ornopartialmeltingatUHPconditions,althoughsome t/46
/2
partialmelting(20–30%)yieldresidualassemblageswithmantle- degreeofmelting hastaken place insomeoftheserocks /2
7
likedensitiesbutwhichcanstillcontainminoramountsofhydrous (e.g. in the Kokchetav massif: Shatsky et al., 1999; 5
/1
phases.Partialmeltingoftonaliticcrustduringcontinentalsubduc- Hermann et al., 2001; and in the Ezgebirge: St€oockhert 4
2
tioncanthusgenerateincompatibleelement-richresiduesthatwould et al., 2001). Dearth of partial melting reflects the low 27
4
be able to remain in the mantle indefinitely, acting as long-term geothermalgradients characteristicofultrahigh pressure 5
b
sourcesofmetasomaticfluids. metamorphism(5–20(cid:2)C/km,e.g.Chopin,2003).Equally y
g
importantly, it also demonstrates that free H O-rich u
2 e
s
fluids are not normally present during continental sub- t o
KEY WORDS: mantle; melting; metasomatism; tonalite; UHP duction. If free aqueousfluids were present, then partial n 0
metamorphism melting of quartzofeldspathic crustal lithologies would 9 A
teanktes(pel.agc.eCehvoepninu,n2d0e0r3t,hfeigs.e1d).eAprbessesnedcegoefopthaerrtimalamlgerlatidnig- pril 2
0
1
in UHP rocks has an important implication that stems 9
INTRODUCTION fromthefactthatiftherocksdonotmelt,thentheyare
Rocksoftheuppercontinentalcrustareburiedtodepths likelytopreserveH Oboundinhydrousminerals.Slices
2
in excess of 120km during continental collisions (e.g. ofsupracrustalrocksthataretrappedintheuppermantle
recentreviewsbyErnst,2001;Liouetal.,2002;Chopin, could thus become important reservoirs of H O and
2
2003). Ever since continental subduction was first con- incompatible trace elements. Such rocks may or may
firmedbytheoccurrenceofcoesitemineralinclusionsin not eventually melt while they reside in the mantle, and
eclogites (Chopin, 1984; Smith, 1984), there have been they may or may not be able to remain in the mantle
Journal of Petrology vol. 46 issue 2 # Oxford University Press 2004;
*E-mail:[email protected] allrightsreserved
JOURNALOFPETROLOGY VOLUME46 NUMBER2 FEBRUARY2005
Table 1: Composition of starting material (wt %)
SiO AlO TiO FeO* MgO MnO CaO NaO KO F Total
2 2 3 2 2 2
Plagioclase 61.40 24.11 n.d. 0.26 n.d. n.d. 6.01 7.32 0.57 n.d. 99.69
K-feldspar 64.29 19.08 n.d. 0.09 n.d. n.d. 0.20 1.86 13.56 n.d. 99.09
Biotite 38.26 15.55 2.68 14.60 14.42 0.17 0.06 0.41 9.12 1.52 96.79
Amphibole 47.48 7.12 1.02 15.65 12.31 0.61 11.93 1.02 0.64 0.08 97.84
Bulkrock 60.76 16.87 0.89 5.85 2.65 n.d. 5.33 3.83 2.54 n.d. 98.72
Wholecrust1 57.26 15.87 0.90 9.08 5.30 7.39 3.10 1.09 100.0
Uppercrust1 66.05 15.23 0.50 4.51 2.21 4.21 3.90 3.38 100.0 Do
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*TotalFe as FeO. loa
1Wholecrust andupper crust compositionsfrom Taylor (2001, table 12.5). de
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idnisdceufisnsiiotenlsy.ofTlhoensge-taerremqcuheasntigoenssinthmatanhtalveecoremlepvoasnitcioentso, rust WUphpoeler ccrruusstt ttps://a
mInaonrtdleermtoetaadsodmreasstitshmesaenqdueinsttiroancso,nittiinsennetcaelssmaraygmtoaktnisomw. erial/c 2 cadem
the conditions at which vapor-free crustal rocks even- at ic
m .o
tHu2aTOllhyeamngdoealitlnocaoftmtuhplitsartapihbailpgeeherleipsmreteosnsuetsxr.easm,itnheusthreelUeaHsiPngvatphoerir- Starting 0.15 up.com/p
absentmeltingbehaviorofaverageupper-crustaligneous e
rocks. A calc-alkaline tonalite was chosen as representa- SiO2 Al2O3 TiO2 FeO MgO CaO Na2O K2O trolo
g
tiveoftheaveragecompositionofuppercontinentalcrust y
Fig. 1. Comparison of bulk composition of the tonalite starting /a
of igneous origin. Some of the experimental results dis- material with whole continental crust and upper continental crust rtic
cussed here were previously reported in a preliminary [datafromTaylor(2001,table12.5)]. le
-a
fashionbyPatinn~oDouce&McCarthy(1998,CATstart- b
s
imngenmtsatwereirael,caotm15p,le2te1da,nadfte3r0ktbhaer)p.uAbdlidciatitoionnaloefxptehrai-t ahsosrunmblienngdeth2a(cid:4)t5bwiotti%te HcoOnt,aitnhse 3m(cid:4)5odwaltco%mpHos2iOtionanodf tract/4
2 6
paper, at 18, 27 and 32kbar. Full compositional data the tonalite results in 0(cid:4)8 wt% bulk H O-content. Iwill /2
for all phases in all melt-bearing experiments at 2 /2
useanintermediatevalueof1wt%asaroughestimateof 7
5
15–32kbararegivenhereforthefirsttime. theH O-contentofthetonalite. /1
2 4
2
2
7
4
Sample preparation and containment 5
EXPERIMENTAL AND ANALYTICAL b
Thestartingmaterialwasgroundfor1hinashatterbox y g
PROCEDURES with WC lining and balls, in order to attain a grain size ue
s
Starting material (cid:5)10 mm. Samples (approximately 3mg) were contained t o
n
The starting material for all of the experiments was a in welded Au capsules with 1(cid:4)0mm inner diameter 0
9
calc-alkalinetonalitefromtheSierraNevada(California, and 0(cid:4)2mm wall. The capsules were loaded and then A
UcoSmAp),osceodlleocftepdlagfrioocmlasLeeAenV3i0n(i4n5g%C),anqyuoanrt.zT(2h0e%r)o,cbkiois- sowtroedrreeerdctrooipmeepnleimdiniinnaasntiedoevatedhnseoraobtve1edn30ha(cid:2)unCmdifdoimirtyma.tedTleihaaetsetlyc2a4spehsau,lleiends pril 2019
tite(13%),hornblende(13%),K-feldspar(7%)andFe–Ti
byarcwelding.
oxides (2%). Bulk chemical composition and mineral
compositions of the starting material are given in
Table 1. The starting material is closer in composition Experimental apparatus
to average upper continental crust than to average bulk All experiments were performed in a solid-medium
continental crust [Table 1, Fig. 1; crustal data after piston-cylinder apparatus at the University of Georgia,
Taylor (2001)]. Difference of the bulk analysis from with 0(cid:4)5 inch diameter NaCl–graphite cell assemblies.
100 wt % (Table 1) suggests that the starting material Experiments were run at pressures of 15–32kbar and
may contain (cid:3)1(cid:4)3 wt % H O. On the other hand, temperaturesof900–1150(cid:2)C,withrundurationsvarying
2
276
PATIN˜ODOUCE VAPOR-ABSENTMELTINGOFTONALITE
Table 2: Run conditions, phase assemblages and modal compositions (wt %)
Run P(kbar) T((cid:2)C) Time(h) Qtz Kfs Plg Ky Grt Cpx Bt Phn Zo Rt Glass
number
TED53 15 900 198 X X X X X X X tr
APD651 15 950 175 11.4 13.3 27.6 18.2 9.2 2.2 X 18.1
APD-665 18 900 202 unmolten
TED51 21 940 189 13.2 31.4 6.0 17.6 21.6 X 10.2
TED50 21 960 187 11.2 29.5 4.5 20.8 21.6 X 12.4
TED47 21 975 175 9.4 27.8 3.5 22.0 21.4 X 15.9 Do
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TED44 21 1025 150 6.5 20.2 3.0 21.3 16.3 X 32.7 n
lo
TED52 21 1060 132 11.8 20.7 13.0 X 54.5 a
d
e
FCH-5 27 1000 118 unmolten d
APD-683 27 1025 149 18.9 14.6 1.5 16.6 30.3 4.5 5.3 8.3 fro
m
APD-682 27 1075 126 11.4 12.3 0.9 17.3 30.5 1.8 3.5 22.3 h
FCH-9 27 1100 118 9.2 12.6 1.4 18.0 29.5 2.8 26.4 ttp
s
APD-685 27 1125 78 6.7 8.4 1.5 19.9 24.3 39.2 ://a
c
APD-684 27 1150 53 9.9 9.9 1.0 22.7 11.8 44.7 a
d
TED48 30 975 194 11.5 10.4 1.4 14.4 41.1 5.8 6.7 X 8.6 em
TED49 30 1010 147 6.5 5.4 0.9 14.3 38.5 3.6 4.5 X 26.3 ic.o
u
TED45 30 1075 117 3.7 0.9 0.9 15.0 32.0 2.3 X 45.1 p
.c
APD-664 32 925 145 X X X tr om
APD-663 32 1000 173 21.4 1.0 16.8 34.4 17.3 3.9 X 5.2 /p
e
APD-686 32 1050 79 11.0 1.4 17.6 32.6 5.3 1.0 31.1 tro
APD-680 32 1100 29 5.9 0.9 16.4 27.1 49.7 log
y
/a
X,phaseispresentbutmodalabundancecouldnotbedetermined;tr,traceamounts;blankentry,phaseisabsent.Mineral rtic
abbreviations after Kretz (1983). le-a
b
s
tra
between 2 and 8 days (Table 2). Temperatures were Analytical procedures ct/4
6
measuredandcontrolledwithW74–Re26/W95–Re5ther- Capsules were retrieved from the high-pressure cells by /2
mocouples relative to electronic ice points (0(cid:2)C) feeding dissolvingtheNaClinwater.Theywereinspectedunder /27
5
Eurotherm 808 temperature controllers. Temperature the microscope for tears. Capsules with tears in them /1
stability during all runs was better than (cid:1)5(cid:2)C. The were discarded and the experiment was re-run. Phase 42
2
small size of the capsules that were used resulted in the compositionsweredeterminedbyelectronbeamanalysis, 74
5
entire sample volume being within 2mm ofthe thermo- usingtheJEOLJXA8600electronprobeattheUniver- b
y
couple.Sampleswerepressurizedatroomtemperatureto sityofGeorgia,withanacceleratingvoltageof15kVand g
u
about2kbarbelowtargetpressure,thenheatedtotarget sample current of 5nA. To minimize alkali migration, es
temperature and (owing to thermal expansion) the pres- andalsotoinsureaccuratetargeting,glassanalyseswere t o
n
sure was finally released to target pressure (hot, piston performed in scanning mode at 50 000(cid:7) magnification, 0
9
out).Thepressuresreportedarenominal(Heisegaugeoil with a focused beam rastered over an area of approx- A
p
pressures multiplied by ratioofram-to-piston areas) and imately 4 mm (cid:7) 4 mm. All other phases were also ana- ril 2
weremanuallymaintainedwithin(cid:1)0(cid:4)2kbarofthetarget lyzedinscanningmodebutwithmagnificationstypically 01
9
pressure. Oxygen fugacity was not buffered during the >100 000(cid:7). Na and K were counted first, and Na was
experiments, but the graphite-based cell assemblies that countedfor10s.Allotherelementswerecountedfor40s.
are used in the UGA piston-cylinders restrict log fO in Na count rates were corrected as described by Patinn~o
2
the samples to the interval QFM to QFM(cid:6)2 (where Douce&Harris(1998).Inthatandotherpreviousstudies
QFM is the quartz–fayalite–magnetite buffer) (Patinn~o (e.g.Patinn~oDouce&Beard,1995;Patinn~oDouce,1996),
Douce&Beard,1994,1995,1996;Patinn~oDouce,1996). decay of K count rates was not detected, so that no
The stabilities and compositions of ferromagnesian attempt was made to correct measured K-contents in
phases are not significantly affected by fO variations this study either. Water contents of the experimental
2
withinthisrange. glasseswereestimatedbymassbalance(seebelow).
277
JOURNALOFPETROLOGY VOLUME46 NUMBER2 FEBRUARY2005
D
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ic
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rtic
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t/4
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/2
Fig. 2. Back-scattered electron images of selected run products: (a) 27kbar, 1075(cid:2)C; (b) 27kbar, 1100(cid:2)C; (c) 32kbar, 1050(cid:2)C; /27
(d)32kbar,1100(cid:2)C. 5/1
4
2
2
Modal abundances in the experimental products DESCRIPTION OF RUN PRODUCTS 74
5
(Table 2) were calculated by a combination of mass The experimental work focused on the pressure range b
y
balance and partial estimates of modal abundances 21–32kbar, with a few additional experiments run at g
u
obtainedfromback-scatteredelectron(BSE)images[the 15 and 18kbar. Experimental conditions, phase assem- es
methodhasbeendescribedmorefullybyPatinn~oDouce& blages and modal compositions of the run products t o
n
Johnston (1991)]. The oxides used to carry out mass are given in Table 2 and BSE images of selected run 0
9
balance were: K O, Al O , SiO , MgO, CaO and products are presented in Fig. 2. Of the two hydrous A
2 2 3 2 p
FeO*. Estimates of modal abundances of melt and total minerals present in the starting material, only biotite ril 2
mafic phases, obtained by point-counting BSE images, persisted in two supersolidus experiments (at 15kbar). 01
9
were included as additional linear equations in the least Amphibolewasnotobservedinanysupersolidusexperi-
squares minimization routine, and were given the same ment. Garnet, clinopyroxene, kyanite, zoisite and phen-
weight as the chemical mass-balance equations. This gitecrystallizedentirelyduringtheexperiments,asnone
yieldsatotalofeightlinearequations,whichweresolved ofthesephaseswaspresentinthestartingmaterial.
for the abundances of all phases except rutile (which is Quartz(orcoesite)ispresentinalmostallexperiments,
always present in trace amounts). Analytical errors and as globular or interstitial crystals whose abundance,
the presence of zoned feldspars (see below) generate almost always lower than that in the starting material,
uncertaintiesinthecalculatedmodesofatleast(cid:1)2wt% decreases withrisingtemperature.It isonly absent from
absolute(Patinn~oDouce,1995). thehighesttemperatureexperimentat21kbar(1060(cid:2)C).
278
PATIN˜ODOUCE VAPOR-ABSENTMELTINGOFTONALITE
Table 3: Feldspar composition (wt %) thaninthestartingmaterial.Itissignificantlylessabun-
dant at 30kbar, where it may be largely a relic phase.
Alkali feldspar tends to become more Na- and Ca-rich
P(kbar) T((cid:2)C) SiO AlO FeO* CaO NaO KO Total
2 2 3 2 2 with rising temperature at constant pressure, and less
albitic with increasing pressure at constant temperature
Alkalifeldspar
(Table3).
15 950 66.39 18.98 0.19 0.17 3.17 11.36 100.25
Garnetispresentinallrunproducts(Tables2and4)as
21 940 65.97 19.34 0.15 0.47 4.05 9.88 99.84 euhedral crystals. No compositional zonation was
21 960 65.21 19.60 0.13 0.99 4.88 8.95 99.76 detected in garnet crystals (Fig. 2a, c and d), probably
21 975 66.13 19.45 0.16 0.65 4.29 8.78 99.45 reflectingthatfactthatthisphasegrewentirelyduringthe
21 1025 66.24 19.61 0.17 0.97 5.73 7.64 100.36 experiments. Almandine þ spessartine contents show
21 1060 64.02 21.45 0.14 2.94 6.18 4.74 99.45 little variability within the entire P–T range investigated Do
w
27 1025 64.67 19.44 0.24 0.24 1.58 13.10 99.27 (Table4,Fig.3).From15to27kbar,garnetalsochanges n
lo
27 1075 64.54 19.94 0.17 0.37 2.82 11.97 99.81 little in pyrope and grossular contents. At higher pres- a
d
27 1100 64.48 20.04 0.08 1.04 3.37 10.69 99.69 sures, the grossular content increases noticeably, at the ed
27 1125 65.17 19.91 0.05 1.67 6.10 7.07 99.97 expenseofpyrope. fro
m
30 975 64.23 19.18 0.16 0.15 1.15 14.50 99.37 Clinopyroxene is also present in all run products h
30 1010 64.56 19.17 0.14 0.09 1.16 14.13 99.25 (Tables 2 and 5), generally as subhedral to euhedral ttp
s
30 1075 64.68 19.25 0.12 0.11 1.73 13.54 99.43 unzonedcrystals(Fig.2a–d).Itscompositionvariesregu- ://a
larly with pressure and temperature. There is a clear ca
Plagioclase trend towards higher jadeite contents with increasing de
m
15 950 57.74 27.28 0.30 8.71 5.99 0.30 100.31 pressure but jadeite contents decrease with rising ic
27 1150 54.11 28.60 0.02 11.40 4.81 0.21 99.15 temperature at constant pressure (Figs 4 and 5a and b, .ou
p
Table 5). Jadeite contents incpx vary antithetically with .c
o
Values are averages of five to seven individual analyses of albite contents in alkali feldspar, reflecting the positive m
rimsofdifferentgrains.Relativeanalyticaluncertainties((cid:1)2 /p
dP/dT slope of the breakdown reaction of albite to e
S.D. of the average values) are: SiO2, 2%; Al2O3, 1%; tro
FeO*, 25%; CaO, 20% (Kfs) or 2% (Plg); Na2O, 6%; K2O, jadeiteþquartz.FollowingtheclassificationofMorimoto log
3%(Kfs)or 25%(Plg). (1988), experimentally produced clinopyroxene is augite y
*TotalFe as FeO. /a
at 15kbar and omphacite at all higher pressures, except rtic
at 27kbar, 1025(cid:2)C, where it is jadeite. In addition to le
-a
these major compositional trends, clinopyroxene shows b
s
The SiO2 phase in some of the 30 and 32kbar experi- an excess of [6]Al relative to [6]Al that is accounted trac
mentsislikelytobecoesite,butthiswasnotconfirmedas for by the jadeite (NaAlSi2O6) and Ca-Tschermak’s t/4
6
therunproductswerenotsubjectedtoX-raydiffraction (CaAlAlSiO6) components. The content of ‘excess’ /2
analyses. [6]Al is linearly correlated to the amount of vacancies in /27
5
Kyanite occurs in euhedral tabular and acicular crys- clinopyroxene (Fig. 5c), clustering close to a line with /1
tals (Fig. 2a–d) in all run products at 21–32kbar, except slope1.Thiscorrelationisconsistentwiththeincorpora- 42
2 2
at 21kbar, 1060(cid:2)C. Its modal abundance is greatest at tion of Ca-eskolaite component in clinopyroxene 74
5
21kbar(upto6wt%)anddecreasesathigherpressures (Ca0(cid:4)5&0(cid:4)5AlSiO6), which generally increases with by
((cid:3)1wt%),inconcertwiththeappearanceofzoisiteand increasing pressure (Fig. 5d). Clinopyroxene in the g
u
phengite(Table2). 27kbar, 1025(cid:2)C experiment is anomalously rich in Al, es
Feldsparsarepresentinallrunproductsatpressuresof Na and Ca-eskolaite, for unknown reasons. Ca-eskolaite t o
n
30kbar and lower, but not at 32kbar (Tables 2 and 3). contentsinallotherexperimentalproductsaregenerally 0
9
They occur both as euhedral homogeneous crystals similartothosereportedbyHermann(2002)atcompar- A
p
(Fig. 2a and b) and as zoned crystals with relic cores ableexperimentalconditions. ril 2
mantledbyeuhedralneoformed rims(Fig.2a). Withthe Micas are present at 15 and 27–32kbar (Tables 2 0
1
9
exception of alkali feldspar at 30kbar, euhedral crystals and 6). In supersolidus runs, biotite is present only at
andrimsarecompositionallydistinctfromfeldsparsinthe 15kbar.Phengitecrystallizedat27–32kbarandisparti-
starting material. Plagioclase is present only at 15kbar, cularlyabundantat32kbar(Fig.2aandc).Itcontainsup
950(cid:2)C, where it coexists with alkali feldspar, and at to3(cid:4)3Siatomsp.f.u,4–5wt%FeOþMgOand1–2wt%
27kbar, 1150(cid:2)C. Except in this last run, alkali feldspar TiO . Measured alkali contents (Table 6) may be some-
2
is always present at 15–30kbar, but absent at 32kbar. what low owing to the small size of phengite crystals in
Its modal abundance decreases with rising temperature someoftherunproducts.
and increasing pressure (Table 2). When present at Zoisitecrystallizedat27–32kbar(Fig.2aandb)andat
21–27kbar, alkali feldspar is always more abundant temperaturesofupto1100(cid:2)C(at27kbar).Compositions
279
JOURNALOFPETROLOGY VOLUME46 NUMBER2 FEBRUARY2005
Table 4: Garnet compositions (wt %)
P(kbar) T((cid:2)C) SiO AlO TiO FeO* MgO MnO CaO NaO Total
2 2 3 2 2
15 950 39.19 20.44 1.19 24.12 6.31 0.93 7.66 0.10 99.94
21 940 38.89 21.00 0.98 22.39 6.74 0.59 8.70 0.14 99.42
21 960 38.80 20.85 1.22 21.41 8.05 0.69 8.26 0.15 99.43
21 975 39.22 20.88 1.21 22.17 7.70 0.71 8.10 0.12 12.59
21 1025 38.78 20.62 1.27 22.51 7.43 0.59 7.64 0.22 99.05
21 1060 39.18 21.00 1.50 22.24 9.26 0.51 6.56 0.14 100.37
27 1025 37.91 21.61 0.67 26.75 5.61 0.65 6.71 n.d. 99.89 Do
w
27 1075 38.44 21.41 1.22 23.52 8.39 0.72 5.88 n.d. 99.57 n
lo
27 1100 37.93 21.56 1.64 23.57 9.23 0.31 5.66 n.d. 99.92 a
d
27 1125 39.09 21.35 1.44 21.53 9.83 0.52 5.97 n.d. 99.72 ed
27 1150 38.39 21.58 1.66 20.88 9.49 0.58 7.20 n.d. 99.77 fro
m
30 975 38.55 21.12 0.52 24.21 3.33 0.80 10.83 0.18 99.54 h
30 1010 38.08 21.00 0.98 24.65 3.50 0.63 10.16 0.26 99.25 ttp
s
30 1075 39.28 20.79 1.00 22.62 7.80 0.56 6.81 0.36 99.23 ://a
c
32 1000 38.25 20.53 0.57 24.33 2.19 0.74 12.79 0.16 99.56 a
d
32 1050 38.64 22.15 0.80 24.23 3.59 0.63 10.05 n.d. 100.09 em
32 1100 37.75 20.55 0.99 24.51 4.52 0.40 11.04 n.d. 99.76 ic.o
u
p
.c
Values are averages of four to six individual analyses of different grains. Relative analytical uncertainties ((cid:1)2 S.D. of the o
m
*aTveortaagleFevaalsueFse)Oa.re: SiO2, 2%;Al2O3, 2%;TiO2, 30%;FeO*, 3%;MgO,5%;MnO,30%; CaO,4%;Na2O, 35%. /pe
tro
lo
g
y
/a
Alm + Sps experimental products is zoisite or clinozoisite remains rtic
undetermined.It will be called zoisite in this paper, as a le
-a
genericname. b
s
Rutile is present in all experimental run products in tra
c
trace amounts, occurring as small crystals, sometimes t/4
6
included in garnet or clinopyroxene (Fig. 2a). No other /2
Fe–Tioxidephaseswerefound. /2
7
5
Glass (quenched melt) is present in all experiments /1
except at 18kbar, 900(cid:2)C and 27kbar, 1000(cid:2)C (Fig. 2). 42
2
Glass compositions and modal abundances (Tables 2 74
5
and 8) could be determined in all melt-bearing experi- b
y
Garnet compositions ments except at 15kbar, 900(cid:2)C and 32kbar, 925(cid:2)C, gu
where it is present in trace amounts. Glass compositions es
15 kbar are homogeneous throughout each experimental charge t o
21 kbar n
27 kbar in which it could be analyzed. The compositions in 09
30 kbar Table8arerecalculatedto100wt%H O-free.Original A
2 p
32 kbar microprobe totals and H2O-contents estimated by mass ril 2
Grs Prp balance(assuming2(cid:4)5wt%H2Oinzoisiteand3(cid:4)5wt% 01
9
H Oinphengite)arealsoincludedinTable8.Withafew
Fig.3.Molargarnetcompositions,recalculatedfromdatainTable4. 2
exceptions,thereisgoodagreementbetweentheseestim-
ated H O-contents and the difference between micro-
2
probe totals and 100%. The discrepancies arise from
are in every case near end-member zoisite, with negli- uncertaintiesintheglasscompositions,andtheresulting
gibleFe3þ-contents(Table7).Thismaybetosomeextent microprobetotals,compoundedwithuncertaintiesinthe
a consequence of the strongly reducing conditions (up calculationofmodalcompositions.Watercontentsestim-
to (cid:3)2 log units below QFM; see above) of the piston- ated by mass balance are probably a better approxima-
cylinder cell assembly. Whether the phase in the tiontomeltH O-contentsthandifferencesfrom100%.
2
280
PATIN˜ODOUCE VAPOR-ABSENTMELTINGOFTONALITE
Table 5: Clinopyroxene composition (wt %)
P(kbar) T((cid:2)C) SiO AlO TiO FeO* MgO MnO CaO NaO KO Total
2 2 3 2 2 2
15 950 53.21 4.74 0.48 11.72 12.84 0.23 15.40 1.11 0.10 99.82
21 940 52.81 11.06 0.75 6.00 9.56 0.15 14.38 4.06 0.42 99.19
21 960 52.87 9.45 0.70 6.06 11.53 0.18 15.65 3.43 0.11 99.98
21 975 53.98 9.99 0.71 5.92 11.21 0.11 15.01 3.68 0.10 100.71
21 1025 53.06 8.67 0.66 7.22 10.94 0.20 15.87 3.39 0.05 100.07
21 1060 52.22 10.72 0.90 3.89 11.38 0.19 18.04 3.00 0.08 100.41
27 1025 57.07 25.13 0.09 1.32 0.30 0.09 6.07 9.53 0.06 99.66 Do
w
27 1075 53.64 18.65 0.76 3.12 4.49 0.11 10.96 7.21 0.06 98.98 n
lo
27 1100 52.51 19.10 1.31 3.62 4.15 0.15 12.10 6.98 0.03 99.95 a
d
27 1125 52.15 18.42 1.16 3.96 4.91 0.13 12.97 5.91 0.12 99.73 ed
27 1150 51.61 17.06 1.11 4.71 6.43 0.15 13.51 5.22 0.05 99.85 fro
m
30 975 55.70 17.68 0.37 4.89 4.07 0.13 8.85 7.76 0.11 99.54 h
30 1010 55.65 19.21 0.40 4.13 3.62 0.05 9.03 7.64 0.16 99.88 ttp
s
30 1075 54.72 14.56 0.71 5.98 6.21 0.12 10.46 6.44 0.09 99.29 ://a
c
32 1000 55.88 21.01 0.23 3.58 2.88 0.00 7.94 8.45 0.19 100.16 a
d
32 1050 54.49 19.92 0.48 4.19 4.13 0.00 8.84 8.16 0.05 100.26 em
32 1100 53.63 19.60 0.61 4.93 3.05 0.22 10.50 7.65 0.09 100.28 ic.o
u
p
.c
Valuesareaveragesofseventotenindividualanalysesofdifferentgrains.Relativeanalyticaluncertainties((cid:1)2S.D.ofthe o
averagevalues)are:SiO ,2%;Al O ,4%;TiO ,30%;FeO*,5%;MgO,5%;MnO,30%;CaO,4%;Na O,5%;K O,50%. m
*TotalFe as FeO. 2 2 3 2 2 2 /pe
tro
lo
g
y
/a
Jd conditions generally approach equilibrium. Virtually all rtic
ofthephasesfoundinthemelt-bearingexperimentsdis- le
-a
Pyroxene compositions cussed here are neoformed(i.e. they werenot presentin b
s
15 kbar Jadeite the starting material) and reasonably euhedral (Fig. 2), tra
21 kbar c
27 kbar and they vary regularly in composition with changes in t/4
6
30 kbar PandT.Thecompositionsofmeltsandsolidphasesare /2
32 kbar constant throughout each experimental charge. These /2
7
5
observations are consistent with a good approach to /1
4
equilibrium in those experiments in which a melt phase 2
2
was present. Unmolten experiments, on the other hand, 74
5
clearly did not achieve equilibrium, as the minerals that b
Omphacite y
are present in them are unchanged from those in the g
u
starting material. There obviously are strong kinetic es
impediments to reaction in the absence of a fluid phase, t o
n
whichkeephydrousmineralsfromreactingoutuntilthe 0
9
vapor-absent solidus has been significantly overstepped. A
Augite p
Wo En + Fs In vapor-absent experiments such as these, performed ril 2
withstartingmaterialsequilibratedinnatureatpressures 0
1
Fig. 4. Molar clinopyroxene compositions, recalculated from data in far removed from the experimental conditions, there is 9
Table5.Jadeite,omphaciteandaugitefieldsafterMorimoto(1988).
thusanunavoidablegapbetweensubsolidusexperiments
that never attain equilibrium and experiments that
APPROACH TO EQUILIBRIUM equilibrate a finite distance above the solidus. This gap
Equilibrium cannot be strictly demonstrated in the makesitimpossibletodeterminetheexactlocationofthe
absenceofreversals.Patinn~oDouce(1995),Patinn~oDouce vapor-absentsolidus.Itisneverthelesspossibletoextract
& Beard (1995) and Patinn~o Douce & Harris (1998) used veryvaluablephaseequilibriuminformationfromsuper-
a limited number of reversals to show that melt- solidus experiments that have approached equilibrium,
present experiments performed under vapor-absent asdiscussedinthefollowingsection.
281
JOURNALOFPETROLOGY VOLUME46 NUMBER2 FEBRUARY2005
0.8 0.8
(a) (b)
0.6 0.6
u.
s p.f. p.f.u.
m 0.4 s 0.4
o m
at o
Na a at 15kbar
0.2 N 0.2 21kbar
27kbar
30kbar
32kbar D
o
w
0 0 n
15 20 25 30 950 1000 1050 1100 1150 lo
a
P (kbar) T (°C) de
d
0.3 0.15 from
p.f.u.0.25 (c) (d) https://ac
oms 0.2 p.f.u. 0.1 adem
6]Al at0.15 ncies ic.oup
[xcess 0.1 Vaca0.05 .com/p
E0.05 etro
lo
g
00 0.05 0.1 0.15 0 15 20 25 30 y/artic
le
Vacancies p.f.u. P (kbar) -a
b
s
Fig.5.Atomicclinopyroxenecompositions,recalculatedfromdatainTable5ona6oxygenbasis.(a)and(b)Na-contents(¼jadeite)asafunction tra
of pressure and temperature; (c) correlation between excess octahedral Al (i.e. octahedral Al not accounted for by jadeite nor c
Ca-Tschermak’s components) and vacancies. The line with a slope of 2 corresponds to the Ca-eskolaite component; (d) vacancy contents t/4
6
(proxyingforCa-eskolaite)asafunctionofpressure. /2
/2
7
5
Table 6: Mica compositions (wt %) /14
2
2
7
4
5
P(kbar) T((cid:2)C) SiO2 Al2O3 TiO2 FeO* MgO MnO CaO Na2O K2O F Total Tot(cid:6)Feq.1 b
y
g
u
e
Phengite s
27 1025 48.23 32.00 0.24 2.47 0.97 0.00 0.07 0.17 9.44 0.00 93.58 93.58 t on
27 1075 48.52 33.44 0.23 1.68 0.69 0.13 0.04 0.53 9.30 0.18 94.71 94.63 09
30 975 50.75 30.40 1.25 2.45 1.93 0.01 0.10 0.35 9.27 0.25 96.76 96.65 Ap
30 1010 48.83 29.87 2.24 2.09 2.04 0.06 0.05 0.30 10.50 0.23 96.21 96.11 ril 2
0
32 1000 50.68 29.81 1.77 2.23 2.08 0.02 0.16 0.33 10.42 0.22 97.72 97.63 1
9
32 1050 48.94 29.54 1.42 2.31 2.71 0.26 0.09 0.14 9.33 0.58 95.32 95.08
Biotite
15 950 40.14 15.96 6.24 11.02 14.52 0.08 0.07 0.51 9.08 0.82 98.41 98.07
Values are averages of six to ten individual analyses of different grains. Relative analytical uncertainties ((cid:1)2 S.D. of the
averagevalues)are:SiO2,2%;Al2O3,2%;TiO2,10%;FeO*,8%;MgO,10%;MnO,50%;CaO,30%;Na2O,10%;K2O,
5%;F,25%.
*TotalFe as FeO.
1Total minus fluorine oxygen-equivalent.
282
PATIN˜ODOUCE VAPOR-ABSENTMELTINGOFTONALITE
Table 7: Zoisite composition (wt %)
P(kbar) T((cid:2)C) SiO AlO TiO FeO* MgO MnO CaO NaO KO Total
2 2 3 2 2 2
27 1025 39.26 32.81 0.02 0.79 0.01 0.02 24.63 0.09 0.14 97.75
27 1075 39.95 32.10 0.03 0.71 0.00 0.22 23.30 0.08 0.11 96.50
27 1100 39.39 32.71 0.22 0.84 0.13 0.04 23.67 0.07 0.08 97.15
30 975 40.58 33.83 0.06 0.44 0.00 0.00 23.11 0.14 0.28 98.44
30 1010 40.43 32.74 0.06 0.60 0.01 0.06 23.21 0.15 0.29 97.55
30 1075 40.19 32.99 0.09 1.26 0.06 0.14 22.70 0.16 0.35 97.94
32 1000 39.69 33.50 0.01 0.29 0.01 0.00 23.71 0.09 0.14 97.44 Do
w
32 1050 39.58 33.24 0.01 0.37 0.02 0.00 23.62 0.12 0.15 97.11 n
lo
a
d
Values are averages of four to six individual analyses of different grains. Relative analytical uncertainties ((cid:1)2 S.D. of e
d
Kth2eOa,v3e0r%ag.e values) are: SiO2, 2%; Al2O3, 2%; TiO2, 50%; FeO*, 12%; MgO, 60%; MnO, 50%; CaO, 1%; Na2O, 30%; from
*TotalFe as FeO. h
ttp
s
://a
c
a
Table 8: Glass compositions (wt %) d
e
m
ic
.o
P(kbar) T((cid:2)C) SiO AlO TiO FeO* MgO MnO CaO NaO KO F HO1 Total2 u
2 2 3 2 2 2 2 p
.c
o
m
15 950 73.17 15.02 0.30 0.89 0.33 0.02 1.11 4.67 4.50 0.00 5.1 96.87 /p
21 940 73.00 15.74 0.21 0.50 0.27 0.03 0.94 5.51 3.81 0.09 9.8 96.61 etro
21 960 72.67 15.60 0.30 0.83 0.27 0.00 1.17 5.25 3.91 0.20 8.1 95.66 lo
g
21 975 72.63 15.85 0.21 0.70 0.23 0.10 1.12 5.20 3.95 0.07 6.3 95.70 y/a
21 1025 72.89 15.32 0.38 1.15 0.22 0.01 0.89 4.99 4.14 0.07 3.1 95.92 rtic
21 1060 71.25 16.14 0.49 0.69 0.39 0.06 1.52 4.05 5.41 0.16 1.8 95.54 le-a
b
27 1025 73.41 15.55 0.11 0.42 0.04 0.03 0.79 2.77 6.88 0.03 8.5 92.36 s
27 1075 74.86 16.44 0.10 0.42 0.03 0.02 0.97 3.02 4.14 0.10 3.8 92.10 trac
27 1100 73.80 16.57 0.29 0.78 0.11 0.03 1.09 3.04 4.30 0.16 3.5 95.14 t/46
27 1125 71.48 16.32 0.52 0.78 0.24 0.03 1.55 3.85 5.23 0.14 2.5 96.42 /2/2
27 1150 70.64 16.54 0.59 0.97 0.26 0.01 1.76 3.91 5.32 0.21 2.2 98.18 75
/1
30 975 75.48 16.62 0.17 0.52 0.10 0.02 0.98 2.10 4.00 0.19 7.3 93.43 4
2
30 1010 73.51 15.44 0.18 0.69 0.07 0.00 0.78 3.11 6.22 0.07 2.9 93.68 27
4
30 1075 72.16 16.05 0.37 0.65 0.12 0.03 0.88 3.60 6.13 0.27 2.1 94.35 5 b
32 1000 70.79 16.37 0.09 0.54 0.11 0.05 1.56 4.18 6.31 0.13 5.8 94.71 y g
32 1050 73.30 16.08 0.21 0.59 0.08 0.07 0.93 3.92 4.83 0.14 2.5 93.59 ue
s
32 1100 70.10 16.43 0.51 1.28 0.29 0.04 2.01 4.20 5.14 0.08 2.0 98.01 t o
n
0
9
Valuesareaveragesoffivetotenindividualglasspools.Relativeanalyticaluncertainties((cid:1)2S.D.oftheaveragevalues)are: A
*STiOot2a,l2F%e;aAslF2OeO3,.3%;TiO2, 15%;FeO*,10%; MgO,20%;MnO,50%; CaO,5%;Na2O, 10%;K2O, 5%;F, 40%. pril 2
12GOlraigsisnaHl2pOr-ocboentteontatsls.caGlclauslastceodmfrpoomsitmioondsaalrceogmivpeonsirteiocnaslc(uTlaatbelde t2o),1a0s0suwmtin%gw1awtetr%-frebeu.lkH2O-content (seetext). 019
PHASE RELATIONS and zoisite (Fig. 6, Table 2). These observations suggest
Thephaseassemblages produced inthetonalitemelting that when some amount of fluid becomes available in
experimentsaresummarizedinFig.6.Neitherbiotitenor the experimental charges (by breakdown of one or both
amphibole is present in experiments that contain even hydrous phases in the starting material, and possibly
traces of melt at P (cid:8) 21kbar. The only hydrous phases after overstepping of the vapor-absent solidus), other
present in such experiments, besides melt, are phengite hydrous phases that are stable at the experimental
283
JOURNALOFPETROLOGY VOLUME46 NUMBER2 FEBRUARY2005
ar) 30 UnmoHlteZn++Z CKopyxa+ ++C GPsK/rhQtnHt(zGZ)HZ+K KZ+Ho CHK Zp+yZx aK +H+f s GPZ +rh tQnKtHzZKZ1 CKpxZ ++ G CrK?ts + KyaP bar) 30 CpohQengusrpcatihtpzrmentxozegckltsrfstccspx vapor-absentsHoliZdzuoskgfrstqpthzn/mKcselHtZ(HGK)ZHKZHHzZocpZxzcogsgqr/ttrtqzmktzyaelmteltKKHZZ
ssure (kb 2205 K K CKpx + Grt + KyaK K+ K(Qfs)- (+AK )Qyatz ? essure (k 2205 amacpmpazopxmougtcppg,rphprtkxthnpfnqskhqty&zntaz zo in ampqtzmeltgrtcpx phKngrtczpkcfxospqgxtrKmztkelyqttakzfpsmhKnekltyqtazmelt K K(Q)(A)
Pre Pr btkyaqpthzncpx(kGya) capmkpbtgkrtykafsqtz(b) Dow
15 (GK)PB(A) + BKtPB(A) Cpx + Grt + Kfs + Plg + Qtz 15 vapor-abseacnmptspxobglitdrtkukysbfastpqtlgz(qat)z(bK)PB(A) KPB(A) bgctrptpxlkgkfsfqstmmzee(llatt)btampgbrtqttzckpyxakfqstzmaemlptqtzgrtcpxmelt nloaded fro
m
800 900 1000 1100 h
900 1000 1100 Temperature (oC) ttps
Temperature (oC) ://a
c
Fig.7.Phasediagramforvapor-absentmeltingoftonaliteinthedeep a
d
Fig. 6. Phase assemblages in experimental run products. All experi- crust and upper mantle. The diagram shows univariant equilibria in em
ments contain garnet and clinopyroxene. All also contain quartz (or the system K2O–Al2O3–MgO–CaO–SiO2–H2O, calibrated with ic
coesite)andaluminosilicate,exceptthoselabeled(Q)and(A),respec- experimental data summarizedin Fig. 6. In natural assemblages, the .ou
tfeivldelsyp.aIrn,Kad;dpitliaognio,celxapsee,riPm;eanntsdmbaioytictoe,ntBa.inSupbhseonlgiditues,Hex;pzeoriismiteen,Zts;aKre- elaqpu.ilTibhreiaswubilslohliadvueshPig–hTerinvtaerrivaanlcoevaenrdwwhiilclhdiaspmlapyhicboonlesidreeraacbtsleoouvtetro- p.co
m
KleaxbfpseelþreidmQ(eGnt)zt.s.FbTiuehltdeis1dnacosothntethdaeinlvisantpehoesre-aapsbasesremantebtslsaomgleideClut-spb(xeseaþerintGegxrttfrfþoormKdyiuscnþumsZsoiooltneþn). pPdhet>eanilg2iot0efkbþbiaozrtoitiaesriateen-ldboecaaamrtienpdghioabnsoslleyemdabeplhapygrodesxraiamtnioadntet-hlmyeev(lstaeinpegotrre-eaxabt)cs.teinIontnssseotalitdshpuorsewass-t /petro
sures in the neighborhood of 15kbar. The quartz–coesite phase lo
g
boundary is shown for reference; identity of the SiO2 phase in the y/a
conditions crystallize rapidly, even if the end-product of experimentalproductswasnotdetermined. rtic
this reaction contains relatively little hydrous melt (e.g. le
-a
Table2).Ihavecombinedthenear-solidusresultswitha by Hermann & Green (2001) and Hermann (2002). bs
geometrical analysis of phase relations in order to con- Note that there are some important differences between tra
c
struct a model for the UHP vapor-absent melting beha- thephaserelationsproposedinthosestudiesandtheones t/4
6
vior of tonalitic rocks. Because the experimental results presented here, especially with respect to the stability of /2
suggest that phengite and zoisite are the phases that are ferromagnesian phases. Thus, orthopyroxene and talc /27
5
likelytocontroldehydration-meltingoftonalitesatUHP were among the phases observed experimentally by /1
4
conditions, the model also seeks to explain subsolidus Hermann & Green (2001) and Hermann (2002), but 22
crystallization of these phases from breakdown of the neither of them was present in my experiments. This 74
5
low-pressure hydrous phase assemblage present in difference arises primarily from the strong effect of b
y
tonalites(biotiteþhornblende). Fe/Mg ratio on the stabilities of pyroxenes, talc and g
u
The resulting phase diagram is shown in Fig. 7. The garnet, and also reflects, in the case of talc, the higher es
pgehoamseebtroicuanldaanriaelsyssihsoowfnthinetshyisstedmiagKra2mO–aArel2bOa3s–eMd ogOn–a Hex2pOerimacetnivtistycoimnpaHreedrmwainthnm&yflGuirde-eanb’ssenHt2eOxp-seartimureantetsd. t on 09
CaO–SiO –H O, and of course translate into higher A
2 2 p
variance fields in natural systems. Their P–T locations Biotite and amphibole dehydration-melting ril 2
are constrained by results from the experiments pre- 0
inthedeepestcontinentalcrust(P (cid:3)15kbar) 19
sentedhere(Fig.6)aswellasbythoseofpreviouswork-
ers, as discussed below. I must emphasize that the Severalbiotiteandamphiboledehydration-meltingreac-
diagram in Fig. 7 is not intended to be an exhaustive tions converge at P (cid:3) 15kbar (Fig. 7, inset). In kyanite-
analysis of phase relations in the K O–Al O –MgO– free assemblages such as tonalites, vapor-absent melting
2 2 3
CaO–SiO –H O system. In particular, it is strictly at this pressure begins with the reaction [mineral
2 2
applicable only to tonalite bulk compositions and only abbreviationsafterKretz(1983)]:
in the absence of a free aqueous fluid phase. More gen-
BtþAmpþPlgþQtz ! CpxþGrtþKfsþMelt:
eral analyses of UHP phase relations in the system
ð1Þ
K O–Al O –MgO–CaO–SiO –H O have been given
2 2 3 2 2
284