RENDICONTI DELLA SOCIETA ITAUANA Dl MINERALOGIA E PETROLOGIA, 1988, Vel. 43, pp_ 41·60

Dehydration melting of crustal rocks

ALAN BRUCE THOMPsONDepartement fUr Erdwissenshaften Eidgenossische Teehnische Hochschule Zurich CH-80n Zurich, Switzerland

Introduction

It has been long known from experimentalstudies (eg. TU'ITLE and BOWEN, 1958;WtNKLER, 1979) that excess H 20 greatlydepresses the solidus temperature of crustaltocks, and also that the amount of waterneeded to saturate such anatectic melts isconsiderable. LUTH (1976; Table 1), insummarising the data of previous workers,notes that at 10 bar PH20 the solidustemperature is as low as 620°C for the«granite mimmum» (in the systemNaAISi ° (Ab) + KAISi)OS (Or) + SiO(Qz) + H

20) and has a composition (wt%) at

(Ab) 46.15, (0,) 17.5, (Qz) 19, (HP) 17.This amount of H 20 is far in excess of thatavailable in common hydrated minerals(Muscovite (Mus) - 4.5 wt%; Biotite (Bio)- 4.3 wt%; Amphibole (Amp) - 2~1.5

wt%). Thus for water-saturated melting tooccur, either enormous volumes ofmetamorphic rocks need to dehydrateconveniently close to where feldspar + quartzrocks are waiting to be melted at depth in thecrust, or that the amounts of melt producedfrom the release of H20 from hydratedminerals is really quite small at temperaturesjust above the region of the H20-saturatedsolidus.

The amount of anatectic melt generatedfrom any rock composition (in addition to itscomposition and physical properties) willdetermine its ability to separate from itsrestite, segregate into melt pools and ascend,In addition to temperature and the amount

of water available at the melting site, thequantity of melt generated depends primarilyupon how closely the proportions of quartz,alkali-feldspar and plagioclase in the rockapproach the «granite minimum» compositionfor any particular pressure and aHzO. If freewater is not available then the amount of meltis proportional to the amount of hydrousminerals present. Thus most crustal rock types

TABLE 1

H20 contents of Ab + Or + Qz liquids doseto the H,O-saturated solidus, calculatedfrom the data summarised by LUTH (1976,Table 1) according to the method ofBURNHAM

(1979a, and b)..... i9M percu. DOle l"'reeot

Plbul ,', .. " ,. ',' ','...no ,,, .. .. " ,.. lO.6... '" l1. ~ H.' )6, l • • n.'

IUI '" J6. S 24.l 32 . 7 .. , U.t

H" .., l8 .S " lQ .l .. , S•. 8

4000 '" H.J lOo , " ••• U.l

sooo ... ".S It.6 H.t 11.0 U.O

10000 ... U.~ l' • ~ " l' . Q 71 ••

e using molecular weighl Ab .. 262,22';Or _ 278,337; Si40 s • 240,34; Hp. 18,01'_

will undergo vapour-absent melting as micaor amphibole dehydrate and simultaneouslymelt to produce H O-undersaturated«granitic» liquids. Mefting will thereforeproceed in a series of steps as the hydrousminerals undergo successive dehydration-

42 A.B, THOMPSON

melting. It is therefore appropriate to considerfirst the nature of dehydration-melting ofmuscovite, biotite and amphibole in simplesystems, then in model or real rocks, at variousdepths in the crust. In addition it is necessaryto consider the nature of the heat sourcesresponsibile for the endothermic meltingreactions and their duration, as well asconsidering the scale and duration of fluidmovement (especially H 0) in the lowercrust and in the anatectic fiquids themselves.

The approach used is deliberately simplifiedboth thermodynamically in calculations andgraphically in illustrations. As emphasised bythe philosophy of the meeting in Siena, oncpurpose is to provide a framework by whichimportant questions can be partly answeredby the available data and simplified theoretkalconcepts. I apologise to colleagues who rightlyexpect sometimes a more rigorous approachand I have attempted, where possible, toindicate where more thorough discussions areavailable, or need to be.

Some generalisations about the compositionsof crustal anatectic melts

The anatectk behaviour of most crustalrocks types is relatable to the experimentallyknown melting reactions of quartz-feldsparmixtures. Experimental studies (Fig. 1) in the«granite» system (Ab + Or + Qz + H20) haveshown that with increasing pressure at H20­saturation the minimum becomes eutectic(Fig. 1) and moves away from Qz towards Ab,whereas when dry, the minimum/eutecticmoves away from Qz towards Or (seesummary diagrams by WYLLlE, 1977; Fig. 1).The role of plagioclase melting through theincorporation of the CaAI Si 20 g (an)component is also reasonably wen unterstoodwith increasing pressure, especially at H 20­saturation (see summaries by THOMPSON andTRACY, 1979; ]OHANNES, 1985).

The most common rock types in the middleand lower crust are problably metabasalt (asamphibolite or mafk granulite/eclogite),quartzo·feldspathic rocks (referred to here as(preexisting) «granite» although including thewhole range of felsic plutonks as well asmetasediments) and metapelites. Carbonates

and ultramafics may be locally abundant butclearly their mehing is not controlled byquartz-feldspar reactions. Preexisting graniteshave the optimum proportions of quartz andtwo feldspars to generate large amounts of«minimum» melt composition but will onlydo so at the temperatures of the H20.saturated solidus given the influx of largeamounts of external water. The amounts ofhydrous minerals in «granites» is much lessthan in metapelites. So «granites» have theoptimum proportions of quartz and feldsparto procedure more melt than metapelites withan excess of H 20, but the latter containmore hydrous minerals for dehydration­melting.

Amphibolites rarely contain K-feldspar butoften contain small amounts of quartz incontrast to their basalt source rock, mainlybecause many metamorphic minerals inamphibolites (eg. chlorite and amphibole) arelow-silica minerals. So in amphibolites meltingis initially controlled by plagioclase + quartz,then if the small amount of quarz is removedinto the melt, plagioclase + amphibole meltingwould control subsequent anatexis. Aconsequence of quartz removal into earlymelts makes difficult many thoereticaltreatments of pelite or amphibolite meltingwhich, because of graphical constraints,assume quartz saturation well into the meltinginterval. However, CLEMENS ].0. (pers.comm. April 1987) has suggested that for mostquartzo-feldsparthic rocks their ratio incontrolling the amount of melt produced isprobably only important for H 20-saturatedmelting. Fluid-absent melting should not leadto near·solidus (early) exhaustion of eitherquartz or feldspar.

For most crustal rocks the meltingtemperature intervals are large, due mainly tothe considerable divergence from the quartz­feldspar proportions of the «graniteminimum» composition and the smallamounts of H 20 available from dehydrationmelting compared to that required to saturatethe melt (Figs. 2 and 3), Many minerals inmetapelites and amphibolites are quiterefractory and frequently persist to very hightemperatures after quartz and feldspardisappearance, even at low water contents.

DEHYDRATION MELTING OF CRUSTAL ROCKS

---Excess waler---Dry

43

Ab wt. .,.

Fig. I. - Effect of prC!;Sl.lre on liquidu5 field boundaries in the systems Ab -+- Or -+- Qz -+- HzO (solid lines) andAb -+- Or -+- Qz (dashed lines), as sumlIUlrised by WYLLIE (1977, Fig. I, p. 44). Crosses and circles mark thecompositions (wt%) of the minima/el.l1ectica at various PHzO (in kbar) and temperatures (OC). Abbreviations:qz (quartz), ab (albite), san (k-feldspar), jd Gadcite}, coes (coesite), fsp (Na·K feldspar).

a:6

4

2

700

Fig. 2. - Pressure (kbar) . temperal\lre ("C) diagram showing the anhydrous and HzO-saturated solidus andIiquidus curves for granite (heavy solid lines) as summarised by HARRts et al. (1970, Fig. I, p. 191). G .,granite,B - basalt, W - HzO-saturated, D _ anhydrous, S ~ solidus, L .liquidus. Light lines show displacement of theliquidii with 2 and 4 wt% Hp.

44 "B. THQMPSON

Fig. 3a. - Pressure-temperature projection of phase:rdations in the: sySlem NaAlSips(Ab) + H20 (V) fromBUJl.NlIAM 0979, FiS' 16.6), where the saturation(solubility) isopleths for the equilibrium (V + Ab. L)(Fig. 3b) have ~n extended to intersecl lhe: HLO.saturllc:d granile solidus (Ab + Or + Qz. + V • L). The:solid drde5 II the .. granite·minimum. (Fig. 3al sho....",·t% H 20 in the: melt and mol% (caku!IIc:d on dlebasis of Ab + Or + Si,O. + H 20, from the' dillsummarised by Ltml (1976). The: x: isople:ths,rep~nting the lmount of H~O dissolvc:d in H20.underutuTlnc:d ..granite .. mc:ltS, have been drawn usingan idc:al soIurion model. the H20 values al saturalion,and the: slopes compuled by HouowA't' J.R. (inJohannc:s. 198').

the H 20-deficient ~gion) can be treatedgraphically (or better numerically throughequations of state) in the same way. Ashas already been shown for several suchinvestigations, the comparison of experimentswith such simplified calculations providesrefinemeO[ of the acivity-compositionrelations in feldspar + quartz + H 20 system,also when the H?O is diluted with speciessuch as CO2, WhIch may variously interactwith melt components (especially An,CaAI2Si20" see BURNHAA1, 1979b, p. 77).

For the moment we will use the simplifiedversion of the BURNHAM model as outlinedabove to estimate the x: at any particular Pand T ~lative to the H

20-saturated and

dry-melting (no free H20 whatsoever) of therelevant quartz- feldspar systems. This means

Moreover, the «granitic» melts can dissolveincreasing amounts of oon-quartzo.feldspathiccomponents (especially AI20,. FeD. MgO,CaO) with increasing temperature, as isdiscussed further below.

p.T.XCHJO) relations in the granite system

From P-V-T (pressure-volume.temperature)measurements of partial molar volumes ofmelts in the system Ab + H20 (BURNI-lAM

and DAVIS, 1971, 1974) and additionaltheoretical considerations, BURNHA;-"'1 (1982)has developed a general model for P·T-aH 0(water activity related to mole fraction, 5C::;of water in the melt) for granite-like melts.The general applicability of the model can beseen in Fig. 3a where BURNHAM'S 0979; Fig.16.6) data for the solubility surface inAb + H20 (marked in the upper left insetdiagram, Fig. Jb, by the H

20-saturated

melting-reaction V + Ab", L) has beenextended to the granite system (Ab +Or + Qz + H20). To do this, the H 0­solubiliry data in such melts (summaris~b\Lun I, 1978, p. 351, see also B\JRl','UAM, 198:2.Table 9.5) have been renormalised on theb",i, (N.AlSi,o. + KA1Si,O. + S~P~ + H,OIto convert wt% H 20 to mol% H 2U (Table1). Both values (wt%, mol%, H

20 in the

(Ab + Or + Qz + H20) melts) are shown nextto the solid circles and the H20-saturatedsolidus for this system, where it can be seenthat the renormaIised values of Xm in(Ab + Or + Qz + H 20) correspond welt ;'iththe simple graphical extension of the H

20­

solubility saturation surface in Ab + H20.

From these intersection poinls, the P-T­X(H 20) surface for the H20-undersaturatedregion can be approxim:Helv contoured.Holloway (pers. comm. to JOH..... NNES, 1985,p. 41) has also calculated this surface usinga much more rigorous approach and showsthat dP/dT for Xm in (Ab + Or + Qz + H 0)is slightly difrerent from those iorAb + H 0. In general support of theextrapolation of BURNIIAM'S (1982) meltmodel for granitelike systems, it can be envisaged thatexperimentally measured solubiliry data forany feldspar + quartz + H20 system (also in

"

"

"

'" 1100 T. "c

DEHYDRATION MEUlNG OF CRUSTAL ROCKS 4'

Q6 Q.4 02 Ab·Or·Ollmol ./.

Fig. 3b. - A IOkbar T·X section foe Ab+ HP, parllyCOnStructed from BUJ.NUAM {l979a, Fig. 16.6) and therest is schematic (dashed lines).

Fig. k - A lOkbar pseudo-binary T·X section for(Ab. Or. Qrz). Hp. aKUtrtiCted pwJy from rig. 2and Fig. 3a (so1id lines) and the rest it schcma1ic.Superimposoed with lighter lines is the se:tion from Fig.3b. Schematic: de lines at 9QOOC show thr; relative Hpcontents of the phases. The dry liquidus from HUANCand WVWE (1981) is shown near 1240"C.

of H20 to enter the melt, for example by

the breakdown of muscovite. it is possibileto estimate the amount of melt foUowing themethods presented by BURNHAM (1967,1979a) and CLEMENS (1984).

Dehydration-melting of mi~a in simplifiedsystems

As examples of P-T-}\(H20) relations ofdehydration melting reactions, it is useful toconsider a(H20)-X(H20) relations in thereference system Or + Qz + H20, modifiedto account for the excess Al20) whenmuscovite, or excess MgO when phlogopite.undergoes dehydration melting. The P·Tschematics of muscovite + quartz dehydrationand melting reactions are shown in Fig. 4,with accompanying isobaric pseudo-binary T­X(rock.H

20) diagrams at three arbitrary

pressures. At pressures above the invariantpoint, muscovite rocks with excess waterwould melt at lower temperatures (throughH + V - L in the pseudo-binary so:tions)than for rocks without water, where meltingwould occur incongruently through (H ­L + A). The abbreviations H for hydrousminerals and A for anhydrous assemblage usedhere, permit wider extrapolation of theconcepts to biotit(' and amphibole systems.Important [0 note that at pressures below theinvariant lXlint (which occurs at about 'kbarancI7000C according to 1AMBERTet al., 1969,p. 622), that dehydration of Mus + Qtz occursat temperatures below the H 20.saturatedsolidus. In nature this dehydration waterwould need to remain at the production siteto be (conveniently) available for H 20.saturated melting during the progressiveheating period between the two reactions.Because the amount of H20 required tosaturate granitic melts at low pressures is muchless than at higher pressure (sec= Fig. 3a), eventhe amount of water from low-pressurebreakdown of the biotite available inmetapelities could produce sizeable meltproportions.

The dehydration melting of muscovite +quartz via:Mus + Qtz -+ San + Als +undersaturated melt

'. ,4'"_H'W

"" 0,1100 liQ

V.Ab·Or.Oll.

,,I1 /1

" "V" "L 1000I~---P/ 900

I' " «10q----+''-'7'''--0''"-'''''''::'::'''''''''_-1 '"l_ I. I

HlO-Ab 'r---"'------------! '"-at.·Or

1~()oAb IP. IlkllClrl T. 'c

b

"", , ''''yi ,l,

"", ,I>

Yol P L+Ab

'", ,'",~

, ,Hp'Ab

~---

'",,Y"Ab '"

H,O 08 06 '" OB Ab"'" ,.

that the reference melting reactions can alsochange during progressive melting if eitherplagioclase, alkali.feldspar or quartz is initiallyabsent from the subsolidus metamorphic rock,or is exhausted early by extraction into themelt (as discussed in section 2). By using X:COntours for the relevant feldspar( + quartz + H20) compositions, modifiedwhen no:essary to account for non·quartzo·feldspathic components in the melt (seebelow), it is possibile to calculate the amountof Hl..0 dissolved in any «granitic» melt atany 1~ and T in the H20-undersaturatedregion. Then by allowing a specific amount

46 A.a mOMPSON

O. Als T

k et---------- ------- ,'"'AI, L •

0'..

01, ,.,V

~

(Ms)I

,,., ,.,H10lV) , ,

••

,,"

,

,

,.,

,

c

,.,

-'--•

'"' c c"•

h'C.. ,,., ...,.,

T '

H10l't)

t1zO{V) H

IV}

••

(Or)

",OIz AlsV L

Ms L (Olz)

\l!:'--~y";;;----O;OrAI s V

T

IAls1",0,01'

V

",at,

V

III

p

Fig. 4. - Schematic P·T diagram of muscovite + quartz dehydration melting in the system KASH (from TUOMP'SONand Au;oR, 1977) togethc:r with SCHEMATIC ISOBARIC T.X(H20) sections at three preSSUTe$. 1be anhydrousasscmblage$ llre denoted by A and the hydrous mineral muscovite composition (H) is arbitrarily shown with avery high water content. 1be schematic isobaric T.X(H10) StttiOO$ show the evolution of pcritC'Ctic and eutecticmelting reactions with prC$5~.

has been studied experimentally by STORRE(1972) in the systemKAlOz+ AlzO, + SiOz+ H 20, and that ofphIogopite + Qtz via:Phi + Qtz - San + Ens + H 20.undersaturated melt (2)has been studied by BoHLEN et al. (1983) inthe systemKAlOz+ MgO + SiOz+ H20.Figu~ 5a shown the PoT locations of

mica + quartz dehydration-meltingexperiments superimposed on the P.T.aH20mehing relations for the systemsOr + Q,r + H Q( + I-CO) from BoHLEN etal. (983). The P-T.X(H20) relations werecalculated with reference to the dry solidus(San + Qtz -+ L) for which an average dP/dtof 77 bars KI and a d V of 0.54 ]bar l gave

an average dS of 42 JmoP KI, (using aone-site ideal-~tion model through theisobaric relationship dS.dT;; RTlnXarJ. Itwill be noticed in Fig. 5a that the X:surface calculated in this way whenextrapolated to the H 20-saturated solidus inOr -+ Qz + HzO corresponds poorly with thatevaluated Xli> at saturation (BmtNHAM, 1982,p. 211) with a discrepancy of about 10%.Both values of X: contours 010 be examinedin terms of their consequences for amount ofmelt produced by dehydration-melting ofmica.

The temperatures observed experimentallyfor the two mica + Qz dehydration meltingreactions can be used in conjunction with theX.. contours at any pressure to estimate thea~ount of H20 present in the melt through

DEHYDRATION MELTING OF CRUSTAL ROCKS 47

10 800Mus Qtz

GOD

1000

1200

L

15

20

, L________ '0 - ---

- =~ll~--v_giif>O-= :.':---

SonMus Qtz

o.a OS Q.4 0.2X~

Fig. 5b) Isobaric pseudo.binary T·X (San + Qtz.HJOlsections at 10, 15,20 kbar, showing the liquidus surtacelowering due to an ideal solution model for H20 in themelt, and the observed dehydration melting reactions.The amoum of H20 in the melt, X:;, can be obtainedfrom the horiwmalline representing the dehydration­melting reaction Phi + QtZ -. San + En(enstatite) + L,where the San .. Qrz + H.l0 liquidus has been loweredby about 1O-15°C 10 allow for excess MgO in thefeldspar + quartz liquid. These values can be comparedwith those at HIO.saturation defining the solidusSan .. Qtz + HIO ± (En) - L. The amount of meltcan be obtained by converting to wt% aoo applying thelever rule. Solid circles show the approximate amountof H20 dissolved in the eutectic liquid at the H20.saturated solidii.

line contours in Fig. 5a these correspond toX: values of about .68 and .52, respectively,and for the dashed line contours, X:; isabout .58 and .42. According to the data ofLUTH (1969), the composition of the

Fig. 5. - Phase relations for muscovite + quartzdehydration meltillll in KASH (from STORRE, 1972) andfor dehydration melting in KAlOz-MSH {from BoHLEN

et aL, 1983), and for the melting of KAISiP&(Sanidine) + SiOI (Qa) + V{HIO.C01).

Fig. 53) Pressure (kbar)· temperature (0C) P·T diagramshowing the experimental data, and X:, values for themelt calculated using an ideal solution model relative tothe anhydrous solidus (see lext).The solid .circles at the solidus show the values of X:;at saturatlon calculated by BURNHAM (1982, p. 211),which result in different x: values in the HIO.undersaturated region than those shown.

600

the dehydration melting reactions, and furtherto estimate the amount of melt formed for anygiven percentage of mica. Muscovite(KAI2(Si3AI)0lo(OH)2 and phlogopite(KMg/Si3AllO lO(OH)2 contain respectively(18.015 I 398.3111 4.52 'nd,(18.015/417.262) '" 4.32, wt% Hp. At forexample P '" 10 Kbar (Fig. 5a), the respectivedehydration-melting reactions (l) and (2) formica + quartz dehydration melting reactionsoccur at about 790 ± lOoe and 890 ± 100e.If no crystalline·solutions are involved thenthe dehydration-melting reactions shouldoccur abruplty (provided there are noretarding kinetic effects) and not over atemperature interval. According to the solid

48 A.B. THOMPSON

x·wt% H20wt% melt

PhI + Qtz "" San + Ens + L0.52 0.427.00 4.080.62 0.90

Mus+Qlz""Son+Sil+L0.68 0.58

12.08 8.70.35 0.5

x·wt% H 2wt% melt(pergram mica)

For quartz-feldspathic rocks it may bereasonable to assume that wt% "'" vol% melt,in view of the density ranges involved.However, as the melts dissolve othercomponents they will become denser.

These examples illustrate well the problemsinherent in estimating the amount of H 0·undersaturated melt formed when X: vafuesare poorly known. However, by determiningthe amount of melt fonned in an experimentalstudy of dehydration melting, the granite-meltmodel can obviously be refined.

aoo

T,OCL

kbor 20 1200

""Son

at,1000

PhI Qlz

-----!-- - ---20 IS 10-Son Qtz V(Ens) 'k~r

800

1000

1200

Fig. k) Isobaric pseudo-binary T-X (San ... Qa -+- H20)sections at 10,15 and 2Qkbat for Mus+Qtz .....San -+- Als(sillimanite) -+- L.

anhydrous Or+ Qz eutectic at lOKbar,1140°C is about 67 wt% Or and accordingto BURNHAM'S (1982, p. 211) interpretationsof the data of LAMBERT et al. (1969) theH 20.saturated eutectic at lOKbar, 7l0°C isat about 58mol% Or, which give respectivegram formula weights for the mixtures of 264and 262, on the basic of KAlSi30 S + Si40 S"

With these data, the values of X; may beconverted to wt% and for the above amountsof water available in the micas, the amountof melt can be calculated by simply assumingthat the amount of melt is directlyproportional to the amount of H 20 in theavailable dehydrating mica:

600

H,O oa 06 0.4Xm

w

Ph'

0.2 Sanat>

600Dehydration-melting of mica in peliticmetasediments

Because of crystalline solutions and therange in chemistries of anyone type of crustalrock, several dehydration.melting reactionscan occur in anyone rock and these will besmeared out over P·T space due to continuousreactions. Even with the little available datait is possibile to predict in some approximatefashion how the melting of say metapelites willoccur in terms of amount of melt formed forany given initial mineralogy over a range ofP-T conditions. For metapelites the situationis only slightly more complex than thebehaviour discussed in the previous sectionfor simple systems. Firsdy, dehydration·melting reactions are smeared-out in P·T spaceprincipally due to the exchanges KNa. 1 inmuscovite and AI2MK j Si.1' FedAI. l andFe' 2Mg I in both muscovite and biotite.

This also permits reaction coupling involvingplagioclase and AFM minerals (e.g. garnet andcordierite). Quite complex petrogenetic gridsfor dehydration-melting can be constructed(ABBOTr and CLARKE, 1979; THOMPSON,1982; GRANT, 1985a, b) which at the moment

DEHYDRATION MEUlNG OF CRUSTAl ROCKS 49

are poorly constrained experimentally.HUANG and WVlllE (1981), in their

ex~rimental investigation of dehydration(dry) mehing of muscovite granite, noted avery high liquidus temperature (11:;O°C at10kbar) defined by final disap~arance ofquartz but where corundum or kyanite~rsisted. The dry Iiquidus, H 20-saturatedsolidus and dry solidus (dehydration melting)curves are shown in Fig. 6. Also shown arethe results of PETO and THOMPSON (1974) andPETO (1976) for the reaction:Mus+Alb+Qtz - Ksp+A1s+L (3)in the system KNASH, which marks the lowertemperature limit of dehydration-melting withplagioclase (THOMPSON and TRACY, 1979).The dehydration-melting curve for (3) runsalmost parallel to the eslimated Xm contoursfrom Fig. 3 fot Ab + Ot + Qz, such that theamount of H

20 in the undetsaturated melt

does nO[ vary much with pressure.Also shown in Fig. 6 are the experimental

results of dehydration melting of:Bio+Plg+Als+Qtz - Ksp+Gar+L (4)where at IOkbar dehydration melting occurredbetween 7600C and 800°C for different pditecompositions (LEBRETON and THOMPSON,1988). VIELZEUF and HOLLOWAY (1988)reported extensive dehydration-melting(40·50%) just below biotite disappearance at860°C from 10kbar melting ex~riments ona natural pelite containing originally bothmuscovite and biotite. In synthesisexperiments on a synthetic pelite with 5wt%or 2wt% added HO. GREEN (1976)reported biotite stability to much highertemperature - 900°C and - 10000 C.respectively.

CLEMENS (1984. p. 281) has calculated theamounts of melts produced by micadehydration-melting at 5kbar. The datasummarised in Fig. 6 are used here to calcuJatethe amount of melt formed at IOkbar for hishypothetical medium-grade petite (44% Qz.23% Pig, 33% Mu" by weiglu) ,od high-gradepelite 05wt% Qz. 20% Pig. 5% Kfs.10% Sil. 25% Bio, 5% Gar). For muscovite·dehydration melting intervals at 10kbarbetween 740°C to 765°C. the correspondingX: values are .60 and . 57. and for a gramformula weight of the melt (sEw.. = 260)

leads to 9.4wt% and 8.1wt%. respectively.Thus 33wt% Mus ydds 03 x 4.5 = 1.485 gmH

20). which leads to 1.485/9.4 = 15.8 or

1.485/8.1 = 18.33wt% melt, respectivdy. Forbiotite-dehydration melting intervals of790°C to 840°C at 10kbar, the correspondingX'" values are .54 and .43 Oeading to 7..5an-d 4.96wt%. respectively). Thus 25wt%Bio yields (2.5 x 4.3 = 1.075gm H20). whichleads to 1.075 / 7.5 :0 14.33 and1.075/4.96,., 21.6wl% melt. respectively.

These values can be used 10 assess the amountof melt formed at any particular temperature(Fig. 7) and differ from those calculated byCLEMENS (1984) only because of the X:values used in Fig. 6. The mica dehydration'melting reactions have been shown overdistinct temperature intervals in Fig. 7 toillustrate the continous nature of the meltingreactions. In both cases the amount ofdehydration-melt produced is lower than thevalues calcuJated to disaggregate close-packedspheres (- 3.5 vol%. see summary diagramby WICKHAM, 1987, p. 282).

The rest of the melting path (a) is onlyconstrained by the temperature of themuscovite-granite .d.ry.. liquidus near 1240°C(HUANG and WYlllE, 1981) and will notnecessarily be continuous as shown whensuccessive minerals are dissolved in the highertemperature melts. For the schematic versionshown by the solid line (path a) significantfurther temperature increases are needed toproduce more melt (see also CLEMENS andVI.E.LZEUF. 1987). PIwINSKI (1975, p. 227 and}OHANNES, 1985, p. 73) have estimated theamount of glass at different te'mperatures frommelting of natural granitoids at H 20­saturation. and their data indicate a form asschematically represented by the dashed line(path b) in Fig, 7. The actual form of theliquidus surface depends upon the temperatureof specific cotectics and the degree ofsolubility of additional components in themelt. Clearly. path b would result ingeneration of higher melt fractions at lowertemperatures. As the amount of meltgenerated at particuJar temperatures is of greatimportance for melt extraction, furtherexperimental and theoretical studies aredearly needed.

50 A.B. THOMPSON

Dehydration melting of a two mica-bearingmerapelite would show two distinct meltingpulses in Fig. 7 but the total amount of meltis still proportional to the amount of mica,which is between .48 to .55 and .57 to.86wt% melt per gm of muscovite and biotiterespectively.

The amount of melt generated at anypressure depends upon how the dPjdT of thedehydration-melting reaction changes inrelation to the X"' contours, and as withH20-saturated meTting the amount of meltis still dependent both upon temperature andamount of H 20 available. Thus while micas

contain ahaunt 4.3 . 4-5wt% H 20 andundergo dehydration melting at reasonablylow temperatures, the amount of H 0 in themelt is high. Amphibolires contain 1ess H 20(1-2wt%) but because they dehydrate athigher temperatures where X~ is lower, theywill produce proportionally more melt, albeitat higher temperature. In any case muchhigher temperatures are needed for amphiboledehydration-melting than for biotite thanmuscovite, thus metapelites will generally bemore «fertile» (in the sense of amount of meltable to be generated) than amphiboJites at anytemperature (see also Cl.EJ\IIENS and VIELZEUF,

15

'0

5

500

.0~ J:'

~i

i '" "'

1100 T,·C

Fig_ 6. - poT diagram showing the wet solidus, dry solidus and dry liquidus for a muscovite-granite (HUANG andWVLLIE, 1980, together with the ?i: contours for Ab + Or + Qz + H20 (Fig. 3) and several mica dehydrationmelting reactions. The reaction (dashed line) Mus + Alb + Qtz -- Ksp + AIs + L (PEw and THOMPSON, 1974; PEw,1976) marks the lower T limit of plagiodase reactions (THOMPSON and TIlACY, 1979). The brackets at 10kbarshow the reaction (dotted line) Bio + PIg + Als + QtZ -- Gar + Ksp + L beginning at about 790°C (LE BRETON andTllOMPSON, 1988) and ending at about 860°C (V1ELZEUF and HOLLOWAY, 1986). The other brackets show thedisappearance of biotite at ,% and 2% added water to synthetic pelite (GRIillN, 1976). The calculated dehydrationmelting reaction (dot.<Jashed line) Bio + Pig + Qtz. -- Opx + Ksp + L is from CLEMENS and WALL (1981). Kyanite.sillimanite phase relations are from RtCHAR.OSON et al. (1969).

DEHYDRATION MELTING OF CRUSTAL ROCKS 51

1987). The compositional variation of liquidsproduced by dehydration melting of bothpelites and amphibolites will be discussedfurther below.

Dehydration-melting of amphibolites

Melting of amphibolites has been discussedby BURNHAM (1967, p. 64; 1979a, p. 85),where the reference solidii are usuallygoverned by plagiodase + I-quartz meltingreactions. The experimental results obtained

the basalts melted by HELz (1976) atPH10 '" 5kbar, liquid compositions between700 and 1000°C coexist initially withplagioclase and hornblende, then pyroxene.The enlargement of the compositional spacearound the feldspar composition (Fig. 8b)shows the peraluminous nature of «granitic»melts produced by amphibolite-melting. Thisresults in amphibole becoming less aluminouswith successive melting until clinopyroxeneappears (as shown in the model systemCMASH, ELus and THOMPSON, 1986). With

..E 80.~

~ 60

40

high-grade petite

700 800 900 1000 1100 1200T, DC

Fig. 7. - The amount of melt as a function of temperature at about lOkbar for the dehydration.melting of medium·and high-grade pelites discussed by ClEMENS (1984, p. 281), where the dehydration-melting temperature: intervalsand X: are obtained from Fig. 6, and the rest of the diagram, although highly schematic is based upon Fig. ,of HUANG and WYLlIE 0981, p. 1052) for path a, and upon PtWINSKI (1975, p. 227) and )OHANNES (1985, p.7J) for path b.

by HELZ (1976) at 5kbar for H20-saturated

melting of three basalts, have been replottedin Fig, 8 in ACF-deluxe coordinates (seeO'HARA, 1976, p. 110;}.8. THOMPSON, 1981,p. 175). The differences with conventionalACF diagrams can be seen in Figs. 10 and 11of ELUS and THOMPSON (1986). In ACF­deluxe, the molar gro!Jpings of A ( '" Al20J+ Na 0+ K20) anC( = CaO + 2NazO + 2KzO) result in allfeldspar components plotting at a singJe point,which at quartz and H 20 saturation definesthe entire «granite» system. Thus the diagram(Fig. 8a) shows divergence of evolvinganatectic melts as excess AI 0" FeO, MgOand CaO become progressive1y dissolved. For

increasing temperature (at around 1000°C,PHzO '" 5kbar; HELz, 1976), the meltsbecome metaaluminous then eventuallywollastonite (or diopside) normative. It is clearfrom HELz's (1976) results that even atIOOO°C at HzO-saturation, only about 12mol% of non-granitic components aredissolved in the melts.

Several reJevant questions arise from theseobservations, 1) does the melt contain smalleramount of non-granitic components whenHzO-undersaturated?; 2) how do the meJtcompositions vary when micas are involved?,and 3) how does the melting of mica oramphibole effect the Ab:Or:An:Qzcomposition of the meJt and hence the rock

52 A.B. TI-lOMPSON

classification (<<'membering that mica containslatent Or and amphibole latent An and Ab)?

BROWN and FYFE (1970) reported theresults of dehydration melting for the additionof 10 or 50wt% of either muscovite (M),biotitc (B) or hornblende (H). to naturalgranite (G) and diotile (0). Their recalculatedresults of melt compositions art: shown in theenlarged cqion of the ACF-deluxe diagramin Fig. Se. The divergence of their meltcompositions from the «J!ranire~system (Fig.Bc) are simiJar to those observed by HELZ(1976: shown in Fig. 8b). This could be takento indicate that both HzO-undersaruratedand H20-saturated melts contain similaramounts of non-quartzofe1dspathiecomponents, although presumably less in thelower amount of dehydration melt at anytemperature. However, data is insufficient toindicate whether or not, the excesscomponents CaD, FeD, MgO and AI

20}

dissolve equally in granitic melts at all pressureand aH O.

The giass compositions reported by BROWNand FYFE (1970), HEl.Z (1976), and NANEY

(1983) have bttn plotted in a mol% An-Ab­O, diagram (see Ku.mc, 1972) in Fig. 9.

The melt compositions for NANEY'S (1983,p. 1000) synthetic granite at PH20:: 8kbar,lie dose to the H..10 and quartz-saturatedeutectic in Ab + Ur + An + Qtz + H)O(WINKLER, 1979) and show with increasmgtemperature the influence of coexistingclinopyroxene, biotite then orthopyroxene.NANEY'S (1983, p. 10(0) melt compositionsfrom synthetic granodiorite show theinfluence of coexisting biotite, epidote thenhornblende with increasing temperature. Themelt compositions plot within O'CONNOR'S(1965, see KILINC, 1972) granodiorite field.

The melt compositions reported by BROWNand FYFE (1970) for granite and diorite showdearly the effects of addition of hornblende,biotite or muscovite, and plot in the vicinityof the H/O.saturated cotectic at theappropriate pressures.

The melt compositions reported by Haz(1976, Figs. 4a-<l) for H,p-sarurated meltingof various basalts (amphibolites) show well theeffects of progressive hornblende resorbtion

.~'Il'K-;<l

eo-,.hont. '" ~u OWlz.J916)~ncl c...oit.lO"lQ '" _(HI!I.Z.I!113)pr~ to, 110"lQ cMt..-tnl ....phiboliW

6~QK(PG·Plc.tur-tGorqo _ite.1l101-HuiliI.Io ~b1, biull • ...c<!'S!. HIOat $ kllir).T.....~rOI"'t "c

Fig. 8. - tt) ACF dcluxr: projr:ction £n:,m quart:l .nd HID showing glass composition and coexisting mineralsprOOuud by HP·§lIturaIM mdting of .mphiboliles 111 5kbar (HEU. 1976).

DEHYDRATION MELTING OF CRUSTAL ROCKS

on melt evolution. The hornblendecompositions have been plotted simply hvconsidering (2-CaO)/2, molar, as this is anapproximate representation of theCaAJNa.!Si.1 vector in amphibole. However,

the NaAlSi.\ vector in amphibole results indisplacement of the projected composition(from quartz) towards albite.

In all cases the melts are peraluminous,except at the highest temperatures for ~Iting

RU'lt.....,••illur~ il'. indOC:ilt..t ('ClA.8 il"'" P (P«>t>e-) .Her ladlf..."", r«.Ilo.Uiion~9""'" tJ1 HfIz l~.

""55C di

~I (om~Hetz.19?&1

~ by ......nv 1>0......: .od<..in .. I'fI'Wf'ClO e>l ••(ft.. HzO ill. 5_

• fIG - Pid Go<9l" tI""~.. 1IOl- 1 ........ l>Is.IollX 1'921 -lIiluoH oh·.... _it.

""'"

"Fig. 8b) Enlargcmem of dx area around the granite minimum showirc the penluminous and metaluminom natuTCof Imphibolilc melllr PHp .. :;kbu.

'"

30'SC ph

""

vd

GlCt50I (ClIT'Pl'lt_ can-.... FyIe.197O. p.lIS)

pnxM;" tJ1 cIfy rrdnq (11 ep.a<lz .ond

fl!oIdspAfl in tIw proportlonl III

G-grAlit" O' D-dlont.mi.e<:I "",Ih (10 Of SO"I.) hydrat"s

M-mllKO'ri~ Of B-bioIil.

20F45C Irdi

key: Brown .. Fyle)97D.t. ble 2

'T.u:ile "";'*.872 - 2 - GH 10 (216) 60f:C. ~kb. nhrnblC!<QOO'I.)

'0

@

Fig. Se) GlalS compositions prod~ by SIlOW,," and FVFE (1970) (rom dehydration.melting of muscovite, biotilcor hornblende with mixtures of quartz and feldspar at various pressures.

54 A.B. THOMPSON

of amphibolites (see ELUS and TnoMPsoN'S,1986, Figs. 10 and 11, representation ofHELZ'S, 1976 data) when wollasronite(diopside) normative melts are reached. Thedata plotted in Fig. 9 show well che influenceof the composition of the hydrated mineralundergoing dehydration melting, on the meltcompositional evolution.

Compositional evolution of liquids producedby dehydration melting of common crusta!rock types

With progressive melting not only do mostcrusta! rocks types produce pseudo-graniticliquids even through dehydration-melting,their compositions evolve progressively withincreasing temperature in ACF·deluxecoordinates (Fig. 8) and in Ab + Or + Anspace (Fig. 9). The compositional t;volutiondepends upon the amount of quartzo-

feldspathic componenlS in the original rockas well as the amounts and compositions ofmuscovite, biotite and amphibole, because thedehydration melting reactions involve bothfeldspathic and ACF-deluxe components.Melting of both muscovite and biotite releaserespectively Al 0 and (FeO + MgO)components, am] KAlSi}Og, which canresult in orthodase saturation in melts fromrocks in which alkali-feldspar was notpreviously present. Depending upon the actualamphibole compositions melting, the amountand composition of plagioclase is effected byNaSiCa.IAI. 1 exchange, as is the melt.

Possible evolution of a range of meltstowards or away from the «granite» minimumcomposition can be seen with aid of the ACF·deluxe projection in Fig. lOa, where thecompositional ranges of common minerals androck-types are also shown. The schematiccotectic surfaces are based upon case A of

An

• HELZ IPG THOLElltE• 1801 AI~. BASALT.. 192101. THOLEIITE

o NANEY {GRANODIORITE

• GRANITE

.. BAOWN{ GRANITE HORN8LENOEa. Or i-

D FYFE OIORITE 810TIlE<> MUSCOVITE

Hbl

I m"~ I

1

M",8io Or

7kbar H2O

,,

---

,--

'",,""I."",.'I ',.-/_ .>0,r~:;

" ,

.:~t~k'T/cr.,. GRANO-r," /j:?,,~IORITE ~%::Z-~. X I

TONALITE ~ '~.Q:lO

~\ --/.t. ,. .-%~"-

..,t /' '/TRQNOHJEMITE .... ------~

~".o~o8 GRANITE -.Ab

Fig. 9. _ Melt compositions obtained in various experimental SlUdies plotted in molar Ab-Or·An units. Thosefrom Haz (temperatures indicated in DC) and from Naney were obtained at H 20-saturation, those: from BROWNand FVFE without excess H.O. All melts are corundum normative with the ex~ption of those: produced byamphibolite-+H.O melting above alXlUt 10QOoC (see Er.us and TIIOMPSON'S, 1986; FIGS. 10 ANO ";REPReSENTATION OF Huz's, 1976; data). Also shown are coexisting amphiboles along the NaSiCa_1Al_ 1vector.Both muscovite and biotite plot near Or. Several experimental delerminations of the ternary cotectic inAb -+ Or -+ An -+ H20 are shown (WINKI..ER, 1979). The compositional fields of some siliceous plutonic rocks arealso presented (from KuNIC, 1972; after O'CONNOR, 1965).

DEHYDRATION MEUlNG OF CRUSTAl ROCKS "CAWIl-IORN and BROWN (1976. p. 471) but aredrawn to incorporate. as best as possibile, Ihedata available from dehydration-meltingstudies near lOkbar. Reactions involvingc1inopyroxene (Cpx) and amphibole (Hbl)

with fddspathic liquids are shown as peritecticand those among muscovite (Mus) and biotite(Bio) as eutectic. As nOled above the diagramstrictly applies at quartz and H 20­saturation, but nevertheless this «illegal»

I P-10kbor I

AI20)+N02°.f.K20

A"

®""

T.DO

PELITES

~~""'_'( ._. _. Crd

GRANITESGRAYWACkES

I'·W'-I @

001andesll.

.\ /'~1 /.!j.. / .... ",.

T. ·c

"'"

"'"

000

A

Fig. 10. _ 11) ACF.deluxe projection (from quaru andH 0) showins approximate cotectic surfaces fordehYdration.melting in the vicinity of the graniteminimum near IOkbar pressure (hased partly upon Fig.} of CAWTitOltN and BROWN, 1976; p. 471) andcompositional ranges of common minerals and rocktypes. The isothermal surf-ee5 ("Cl are highly schematicand are based upon the dat. of Figs. 6 and 8 here, Fig.12 of WYU.1E, 1977; p. 60, 2wt9& HP).b) Schematic pseudo.binary T-X seaion near p. IOkbarfor dehydruion.mdting reactions projecu:dapproximately into the pcriteclic valley of Fig. lOa, toillustrate the 5uccessi~ peritectic reactions andapproximate liquidus surface. The flat liquidus surfacewith pyroxene corresponds to the basall-andesite gap,and schematics of the fractionation sequence basalt­andesite.dacite-rhyolite (ELUS and THOMI'SON, 1986;

CFM Figs. 10 and III are indicated.

A.B. THOMPSON

projection can also illustrate some aspects ofH 20 undersaturated liquids that alsodissolve quartz or feldspar early (not alwaysthe case, see HUANG and \'QYLLIE, 1981). Theapproximate isothermal surface shows alsoHkely shape of valleys and ridges in thecomplex T-X space. Fig. lOb has ~en

constructed as an approximate pseudo-binaryT-X section along the peritectk valley of Fig.lOa. Although highly schematic in that thefeldspathic components are not shown, thediagram resembles in some respects T-Xdiagrams drawn for varying Si0

2content

(eg. WYLUE, 1977, Figs. 12 and 13). Theactual peritectic valley will be much morecomplex because of smearing due tocontinuous reactions. The intention is to showhow the dT/dX of the liquidus surface in avery complex composition space varies.Especially important are the very steep dT/dXvalues near the Mus + Bio + Fsp (pseudo)­eutectic which strongly constrains thecompositional evolution of non-quartzo­feldspathic components in the melts to a smallrange even up to about 1OOO°C. The shallowdT/dX of the liquidus surface towards mafkcompositions means that above about l100°C(for the conditions of Fig. lOb) rapidcompositional changes would occur for eitheranatectic liquids produced from amphiboHtesor for the crystallisation of basalt to andesite(ELUS and THOMPSON, 1986, Figs. 10, 11).

Dehydration-melting reactions at differentpressure and sequence of anatexis at variouscrustal depths

Earlier considerations of dehydration­melting reactions had suggested that they hadsteep positive dP/dT to at least 20kbar(BROWN and FYFE, 1970; BURNHAM, 1967,Fig. 2.9; 1979a, Fig. 3.4). Calculations byCLEMENS and WAJ.J.. (1981, Fig. 8) and byCLEMENS (1984, Fig. 2) indicated that somebiotite dehydration-melting reactions thatproduced garnet (curve c in Fig. 11) resultedin backbending of the curves at relatively low­pressure (eg. 3-4kbar). A similar proposal hasbeen put forward by PERCIVAL (1983, Fig. 5)for amphibolite melting producing garnet(curves e and/in Fig. 11). Backberxling occurs

because the reactions have small aVooIi<h andthe production of dense minerals, such asgarnet, and the high compressibility of H 20as vapour or dissolved in the melt, cause6 Vme'..... to go from positive to negati~e.Experimental data has shown that muscovitedehydration-melting has similar positivedP/dT until at least 20kbar (curves a·b in Fig.111.

If we consider the melting sequences atvarious pressures implied by curves (Mus (a)­Bio (c) and Hbl (e-j) then below about 8Rbarthe dehydration·melting sequence would firstinvolve Mus then Bio in pelitic rocks then HbIin amphibolites. Whereas at 15kbar, Biowould melt in metapelites then Hbl inapmibolites then Mus in peHtes. If true, thiswould provide another important diagnosticcharacteristic of high pressure magmatites, inaddition to individual mineral stabilities.However, the quantities of anatectites fromamphibolites for the curves (e-!J would alsobe restricted because of the lower X: atthese pressures.

Before looking too hard in the field for suchcharacteristic migmatites formed bydehydration-melting at high pressures, somelimited but important additional experimentalconstraints should be considered. Firstly,dehydration-melting of a natural biotiteassemblage according to reaction (4) did notapparently show backbending 10kbar (LEBRETON and THOMPSON, 1988) possiblybecause the Bio is stabilised to highertemperture in nature due to Ti substitution.Hence back~nding of the dehydration­melting curve would not occur until muchhigher pressure (e.g. curve d in Fig. 11). Evenhere biotite might undergo dehydration­melting before (or at the same time) asmuscovite in high-pressure migmatites.

Little experimental information is availablefor dehydration-melting of amphibolites. Inthe simple system CMASH, EillS andTHOMPSON (1986, p. 113) noted thatdehydration-melting occurred between900-950°C. With natural amphiboIitemineralogy, RUSUMER (1987) observeddehydration-melting to occur between 925and 950°C at 8kbar. Clearly, the effecrs ofTi in stabilising amphibole and Fe in

DEHYDRATION MELTING OF CRUSTAL ROCKS "stabilising garnet need further investigationto show in which pressure-range backbendingof amphibolite dehydralion-melting curveswill occur for the whole range of basalticcomposilions.

@

Control on crustal anatexis

The simple consideralions outlined abovedemonstrate how the proporlions of meltproduced by dehydration-melling of commoncrustal rock types can be crudely estimated

10

5

•,~

,,,,,,,,,,

1~'Amphibol. d.nydration-m.Uing

_ in CMASH

500 700 900 1100 T. CFig. 11. - poT diagram showing proposed backbcnding of dehydration-melting reactions for various mineralassemblages. The muscovite + quartz reactions (Fig. 6) have + dP/dT 10 at least 20kbar. The calculated backbendingfor biotite (CLEMENS and WAlL, 1981; CLEMENS, 1984; Fig. 2) and amphibole (PERClVAL, 1983; Fig. 5) dehydration.melting reactions, labelled as cod (stippled range) and e-f (dashed range), apparently show backbending due toformation of the dense assemblage with garnet (compositions given in X~ from P'EJtCtVAL, 1983). If the calculatedcurves are correct, then melting at about 5kbar would show successive dehydration.melting of muscovite thenbiotite then hornblende, whereas at high pressure (say 15kbar) amphibolites would melt before some pelites. Recentexperimental data however indicate that these dehydration melting curves apparently do not backbend withincrustal depths.

58 A.S. THOMPSON

BRADY (1983) has proposed a value of D ofabout 10·9m2s·1. For a characteristic diffusionlength defined by x 2 = 4Dt. distancespenetrat~as a function of time can be easilycalculated as order of magnitude estimates:

Mechanisms 0/melt segregation and separationinto intrusions

WICKHAM (1987) has discussed extensivelysome of the problems associated withsegregation and separation of anatectic melts.Not yet fully understood is the role of sheardeformation (ARZl. 1978 VAN DER. MOLENand PETERSON. 1979) on melt segregation.This becomes especially important for thesegregation of the low-melt fractions procluad

These simple calculations reveal that therate limiting step is not the diffusion of H20to the melt but through the melt (SHAW, 1965.1974; BURNHAM, 1967, p. 47). This can beviewed as a physical argument in favour ofdehydration-melting, where the H 20released from hydrous minerals only needs totravel short distance through melts atdispersed melting sites. Thereafter a movingmelt carries any dissolved H20 with it. untilthe solidus is crossed and bOiling occurs.

which, given the above time constraints onthe cooling of mafic plutons (10 km size inabout 10' years). means that H ° can onlydiffuse on the order of 100 m. )bis simplecalculation enables the evaluation of themaximum amount of H 20 that could beavailable for H 20-saturated melting of anyparticular high-grade metamorphic rock.

Diffusivities of H 20 through obsidianglasses and melts (see summary by HOFMANN,

1980, p. 401) reveal values of D about1O·IlUm2s·1 at lOOO°C and 1O. 12·'m2s· 1 at750°C. Using the characteristic diffusionequation above provides the following orderof magnitude estimates:

lol 10 1 100 (1101 10-ln..,

10S Ul tol 10-1

la' 10J lOl 101 U O

IOU IOU 10' u' u'

IOU 10' 10' 10' tal

,.. 1.,_tl,.... ,tlSOOel_. 1l0ooOel

a (.1 10lllla'

_tl,.uOI 10J

Heat sources for crustIJ! melting

Two commonly proposed causes foranatexis include high-temperature intrusionsand the intersection of the appropriate solidiiby metamorphic pressure-teMperature-time(PTt) paths.

Following the calculations by JAEGER (1957)of temperatures around cooling magma bodies,it appears that each volume of crustal meltrequires approximately the same volume ofmafic melt to generate it (BURNHMi, 1979,p. 97; ENGLAND and THOMPSON, 1986, p. 92)and that even a 10 km sized mafic pluton hassolidifi~ in about 10' years, consequentlyleaving only a short time for effectiveanatexis. If external H 20 sources areproposed (H 20-saturated rather thandehydration-melting) then this H20 mustdiffuse to the melting site also during the timewhen the heat source is active.

Some calrulated PTt paths following crustalthickening pass above the appropriatedehydration melting solidii for commoncrustal rock types (THOMPSON and ENGLAND,

1984; ENGLAND and Thompson, 1986) andprovide a feasible means of custal anatexiswithout necessarity requiring an augmentedheat suppy from mantle magma. Not yet wellunderstood are the effects of latent heat onPTt paths (but see RmLEY, 1986) and how thiswill effect melt fraction.

al anyp~ and temperaure. Several othermajor aspects should be considered before ageneral model for crusta! melting can bedeveloped, they include: - nature of heatsource for melting and the distribution oflaten[ heal with temperature, H

20­

migration to the melting site and diffusionof H 20 through the anatectic melt,mechanisms of melt segregation and thegeoclynamics of granite intrusion. As someaspects have bttn discussed elsewhere onlya short discussion will be included here.

H ,O-migration to the melting site and througha,iatectic melts

H 20-djffusivities through high-grademetamorphic rocks are poorly known, but

DEHYDRATION MEl.1'ING OF CRUSTAL ROCKS 59

by dehydration-melting, even though]UREWICZ and WATSONS'S (1985)measurements of dihedral anles of 44-600 indry granite systems implies that such meltswould form an interconnected granular filmcapable of being extracted. Because H 0diffusion is quite slow through granite mefis,on short anatectic time scales, viscosity anddensity of even thin melt layers will not behomogeneous. One simple conclusions mightbe in fact that migmatites are unsuccessful(aborted) granites. Clearly furtherconsiderations of the physical aspects ofmagma migration open a whole new andnecessary field of textural, mineralogical,chemical and isotopic investigations ofmigmatites and plutons.

Admowldgmrmts. - I am gntdulto Pro£. CA. Rkriand the organisers of the SIMP for the invitation to themeeting in Siena.

Discussions with or review by Nicole le Breton,]ohnCkmens, Ron Frost and Daniel Vielzeuf were ~thelpful. lam gratdulto U. Stidwill for typing and toB. Biihlmann for dlllfting. Financial support by lheS,hweiu-rische Nationalfonds and the ETH an::gratdully akoowledged.

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ARZI AA, (1978) . CriticQI phmommQ in IM rhtologyofpartially ~/ted rfXlt.s. Tcclonophysics, 44, 173-84.

BolllEN ST.R. et Al. (1983) - Stability ofph/Qgopi~-qUilmand sanidinNjuam: A modd for m~lling in lhe lowercroft. Contrib. Mineral. Petrol. 83: 270·277.

BRADY J.B. (1983) -lnleTgT'tlnulardiffusion in ~tIlmorphic

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Granilic Melts duriltg Ullrametamorphism. Contdh.Mineral. Petrol. 28: 310·318.

BURNllMt C WAYNE (1967) . Hydrothermal fluids allhemagmatic stllg~. In: Gcochemisuy of HydrothermalOre Ikposits, H.L. Barnes, ed., New York: Holt,Rinehart and Winston, p. }4·67.

BURNltMI C. W"YNE (1979a) • Magm4S and Hybotfxrm41Fluids. In: Gcochemistry of Hydrothermal OreIkpo$its, H.L. Bamcs, cd., New York: Holt, Rinehartand Winston, 21K1 edition, p. 71·136.

BURNI-IMl c. WAYNE (197%) - The importaf/Ct ofvolatiteconJtitllt71IS.lo: H.S. Yoder (Editod, The Evolutionof the Igna:Jus Rocks - Fiftieth AnniversaryPerspective$. Princeton, N.).. 439-482.

BUItNHAM C. W"YNE (l982) • Th~ nlItlJr~ of

multicompon~ntaluminosilical~melts. In: O. Rick.rdand F.E. Wickman (Editors), Chemistry andGcochemistry of solutions at high temperatures andpressures. Oxford, Pergamon Press 197·229.

BUilNIlMl C. WAYNE, OAVIS N.F. (1971) - The rote ofH]O in silic4/~ m~/ts: 1. p-V·T "/atio," in tM sysJcnsNaAlSi;O,-HP to IOkilobtm ana loooOC. Amcr.).Sd., 270, p. '4·79.

BURNIIAM C WAYNE, OAVIS N.F. (1974) . The rote ofHp in silicau mtlts; U. Tht-rmoJ,mrmic and~"/anons in tht syJte1I MlAlSi;O,-HP to 10 kiloba",7()()O to JJ()()0C. Amer. }. Sri., 274, p. 920-940.

CAwnlOllN R.G., BRO"'l:l1N P.A. U976}· A model for theformation and crystalli~tionof corundum·normativeC2k-alWne lJlIgIJW throush amphiboie fMJt:tionation.). GcoI., 84, 467·476.

CLEMEl'o'S J.O. (1984) • WakT conUtllS of silie4u toi1ft~NiaU magmas. Lithos, 17, 273·287.

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