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The Reaction Talc * Forsterite = Enstatite + Hro: New Experimentalresults and Petrologicalimplications Ar,Rsour

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The Reaction Talc * Forsterite = Enstatite + Hro: New Experimentalresults and Petrologicalimplications Ar,Rsour AmericanMineralogist, Volume 83, pages 5l-57, 1998 The reaction talc * forsterite = enstatite + HrO: New experimentalresults and petrologicalimplications Ar,rsouR. PlwrBv Departmentof Earth Sciences,University of Manchester,Oxford Road, ManchesterM13 gPL, U.K. Ansrnacr The reaction talc * forsterite = enstatite + HrO has been investigatedbetween 6 and 20 kbar. Previous high-pressureexperimental studies suggestedvarious reaction positions, mostly with positive P-7 slopes. The new results show that the reaction has a negative slope in the pressurerange studied. It was bracketedbetween 640 and 680'C at l0 kbar, between 620 and 640 "C at 15 kbar, and between 580 and 600 'C at 20 kbar. The growth of antigorite at high pressuresprevented the reaction from being determinedabove 20 kbar. The reaction position is consistentwith the previous low-pressuredata of Chernosky et al. (1985), is in very good agreementwith the position calculated using the thermodynamic databaseof Berman (1988), and is in reasonableagreement with the calculation using the thermodynamic databaseof Holland and Powell (1990). The high reaction temperatures suggestedby previous high-pressureexperiments may reflect metastableor quench growth of talc in those experiments.Talc in equilibrium with forsterite is stable over a wide P-7 range in hydrothermally altered and metamorphosedulffamafic rocks and may be important for carrying HrO into subduction zones. However, talc dehydration occurs at too shallow a depth for the HrO to contribute directly to subduction zone volcanism. INrnonucrroN and releasesits HrO through a series of dehydration re- Talc, Mg.SioO,o(OH)r,is a common mineral in hydro- actions. These reactions may play a major role in sub- thermally altered and metamorphosedultramafic rocks. In duction zone processesby providing the HrO necessary these rocks it is commonly associatedwith olivine and for volcanism or by triggering earthquakes.In addition, other hydrous minerals such as serpentineand amphibole. HrO releasedby these actions may become incorporated The maximum stability of end-membertalc, as given by in new high-pressurehydrous phases,e.g., densehydrous the reaction talc : enstatite + quartz + H,O, occurs at magnesiumsilicates or in nominally anhydrousminerals. -800 "C at 10-30 kbar (Chernoskyer al. 1985;Jenkins These phases may transport the HrO to much greater et al. 1991). However, in typical altered ultramafic rock depth than the stability of common metamorphicminerals in which talc coexistswith olivine, it breaksdown at low- such as talc. er temperaturesthrough reaction with the olivine. The Several experimental investigationsof reaction t have end-member reaction in the system MgO-SiOr-HrO been published. For example, as part of a comprehensive (MSH) is: study of phaseequilibria in the systemMSH at low pres- sures,Chernosky et al. (1985)determined its positionup talc + forsterite: 5enstatite+ H,O (1) to 6 kbar. Their results on this reaction and others in- Ta, MgisiaOr0(OH)2 Fo, Mg2SiOl En, MgSrOl volving talc formed the basis for the determinationof the This reaction is significant becauseit determinesnot only enthalpy of talc in the internally consistentthermodynam- the maximum stability of talc in equilibrium with for- ic databasesof Holland and Powell (1990) and Berman sterite but also the highest temperatureat which HrO can (1988).The experimentaldata of Chernoskyet al. (1985) be stored in typical altered ultramafic rock at moderateto and the position of reaction I calculated using the data- high pressures.The amphibole-bearingMSH assemblage basesare shown in Figure 1, togetherwith other previous anthophyllite + forsterite is stableto higher temperatures data on reaction1. than talc + forsterite but is not stable above -8 kbar The results of Chernoskyet al. (1985) on talc reactions (Chernosky et al. 1985). Serpentine is more abundant are generally self-consistentand so the calculatedreaction than talc at low pressuresand temperatures,but below I passesthrough their low-pressure brackets. However -15 kbar it breaks down at a temperature lower than this consistency does not extend to the previous high- reaction 1. Reaction I could therefore play an important pressureexperimental studies, which have produced re- role in affecting the conditions of HrO storage in the sults inconsistent with each other and with the calcula- Earth's mantle, parlicularly in subduction zones. Here, tions. The most-cited high-pressurereversal experiments hydrated oceanic lithosphereis subductedinto the mantle are those of Kitahara et al. (1966), who obtained phase- 0003-004x/98/0102-0051$05.00 sl 52 PAWLEY: TALC-FORSTERITE REACTIONS H a2O Chedoslyeral i1985) H Kilahanerdl (1966) Fo- o 15 H Yamamoroand Atimoto (1977) L i3'---O Ulrer and Tronmdod ( I 9951 700 Temperature('C) FrcurB 1. Resultsof previousstudies on thereaction Ta f Temperature("C) Fo - En + HrO,and calculation of thereaction using the com- puterprograms THERMOCALC (Powell and Holland 1988) and TWQ (Berman1991). Program versions are THERMOCALC v2.5 (thermodynamicdata generated February 1997) and TWQ v1.0.Reaction temperatures were also calculated using TWQ v2.01(March 1997) and up to 30 kbarare within 2 'C of those calculatedusing vl 0 Thecalculation using v2.01 is notshown, however,as this versionof the databasedoes not includeanti- gorite,as requiredfor the other reactionsshown in Figure2. Fo En SiO2 : grew, : Closedsymbols Ta t Fo opensymbols En grew. Frcuns2. Equilibriain thesystem MSH around the invariant pointinvolving Th, Atg, Fo, En, and H.O, calculated using TWQ. Solidlines are stable equilibria, dashed lines metastable. Com- equilibrium brackets between 10 and 30 kbar. As shown patibility diagramsare shownfor stableassemblages in each in Figure 1, their bracket at 10 kbar occurs at a much field.The numberscorrespond to thereactions given in thetext. point higher temperaturethan the 6 kbar bracket of Chernosky Notethat stability levels relate to thisinvariant only. et al. (1985). Given that the data of Chernosky et al. (1985) suggestthat the maximum thermal stability of Ta The stabilities of other hydrous MSH phasesare also + Fo is at -5-6 kbar, with the reaction bending back to relevant to this study. For example, the experiments of lower temperaturesat higher pressures,the two sets of Ulmer and Trommsdorff (1995) showed that the serpen- resultscannot be reconciled.Chernosky et al. (1985)sug- tine mineral antigorite [Atg, Mgo.Si.oO*.(OH)ur]is stable gestedthat the apparentlyhigher-temperature brackets of at low temperaturesand high pressures.At low pressures Kitahara et al. (1966) are becauseof inaccurate calibra- and high temperatures,antigorite dehydratesthrough the tion of their piston-cylinder apparatus,whereas Berman reaction (1988) suggestedthat the error lies in the use of poorly antigorite : talc * forsterite + HrO (2) characterizedstarting materials synthesizedin very short experiments.This substantialdiscrepancy between high- which has been reversedbetween 2 and 15 kbar by Evans and low-pressure data indicates the need for a redeter- et al. (1976).Their bracketswere usedby Berman(1988) mination of reaction 1 at pressuresabove 6 kbar. in deriving the enthalpy data for antigorite in his data- Reaction I has been investigatedin other studiesof the base.The position of reaction 2 and all others around the MHS system. For example, in synthesis experiments at invariant point involving Ta, Atg, Fo, En, and H,O, as =29 kbar, Yamamoto and Akimoto (1977) obtained a re- calculated using the program TWQ, are shown in Figure action position -80 'C higher than that suggestedby Ki- 2. (Becauseantigorite is not presentin THERMOCALC, tahara et al. (1966), which is itself much higher than the a similar calculation cannot be performed using that calculatedcurve (Fig. 1). Yamamoto and Akimoto (1977) database.) accepted the possibility of disequilibrium in their short Anthophyllite is anothermineral potentially relevant to synthesis experiments and cited the need for reversal the presentstudy, becauseat low pressuresand high tem- expenments. peraturesTa + Fo react to anthophyllite rather than en- The assemblageTa + Fo was produced in the inves- statite. However the reaction is not directly relevant to tigation of serpentinestability by Ulmer and Tiommsdorff this study, becausebelow 700'C anthophylliteis only (1995) and again appearedto be stable to higher temper- stable below 8 kbar, breaking down at higher pressures atures than calculated. Those experiments, although not to Ta + En (Chernoskyet al. 1985). designed specifically to investigate reaction l, reinforce This paper reports the reinvestigation of reaction I be- the need for a redeterminationof this reaction by reversal tween 6 and 20 kbar. The results are in very good agree- expenments. ment with the position of the reaction calculated using PAWLEY: TALC-FORSTERITE REACTIONS 53 TWQ and are close to the position calculated using TeeLe 1. Exoerimentalresults THERMOCALC. The reasonsfor the discrepancies with EXOer- previous experimentsare examined. ment Starting Duration number material P (kbar) f ("C) (hours) ExpBmlrBNrAL PRocEDURES The reaction talc + forsterite = enstatite + H2Ousing natural talc Starting materials for the experiments on reaction I 23 TFE 6 0 670 280 No reaction were 1:1:5mixtures of naturalor synthetictalc, synthetic 21 TFE 6 0 679 312 No reaction 19 Expt 4 8.0 660 353 Ta + Fo'f. forsterite, and synthetic enstatite. The natural talc has 8 TFE 10 O 620 792 Ta + Fo'l been used in previous studies(Pawley et al. 1995;
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