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Geothermobarometry in Pelitic Schists

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Geothermobarometry in Pelitic Schists American Mineralogist.Volume 78, pages68 I-693, 1993 Geothermobarometryin pelitic schists:A rapidly eyolyingfieldx M. J. Hor,o,q.wav, Brsw.l;rr MuxrroploHyAy Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A. Ansrnrcr The study of pelitic rocks for the purpose of deciphering pressure-temperature(P-Z) information is an important area for collaboration betweenmineralogists and petrologists. Accurate geothermobarometryis especiallyimportant for studiesof pressure-temperature- time paths of terranesduring tectonism and for studies of the movement of metamorphic fluids. During the past decade, new developments have included thermodynamic data bases,sophisticated crystal structure determinations and associatedsite assignments,and analysesfor Fe3+ and light elements (especially H and Li) for many minerals within a petrogeneticcontext. It is now necessaryto apply thesenew findings to geothermobarome- try in several important ways: (l) The stoichiometric basis for hydrous minerals should be revised in light of highly variable H and variable Fe3*, which can now be estimated with considerableaccuracy as a function of grade or mineral assemblage.(2) Mole fraction (activity) models should be based on the best crystal chemical considerations.For some minerals, H may be omitted from the model if all the substitutionsinvolving H are coupled substitutions. (3) Thermodynamic data should be basedon careful analysisof all available experiments and secondarycomparison with natural assemblages.(4) The possibility of nonideal solid solution should be considered,as ideality is merely a specialcase of nonide- ality. It is better to estimate binary interaction parameters than to assumethat they are zero. However, it is difficult to determine ternary interaction parameters.In such cases, little error is likely to result from assumingthat strictly ternary interaction parametersare zero, while evaluating all the binary terms. Our understanding of garnet and biotite solu- tions has improved, and the garnet-biotite geothermometerhas undergonenumerous re- finements,but additional information on theseminerals and improvements in the garnet- biotite thermometer are still necessary. As the P and ?"determinations become more precise,we can better evaluate Xnro, X"o, and X.ro in the fluid phase. In all probability, patterns will be discovered in the compo- sitions of pelitic metamorphic fluids, especially when graphite is present. The hematite- ilmenite solid solution system is potentially useful for determining .fo2,oree the solvus is better understood. In graphitic rocks at known P and T, knowledge of /o, from coexisting hemoilmenite and rutile or hemoilmenite and magnetite enables one to determine the mole fraction of all fluid species. Careful application of the present state of knowledge,combined with modest improve- ments, should allow accuracyapproaching +250 bars and +25 "C in rocks formed at low to moderate pressures.It is very important to continue to revise thermochemical data, activity models, Margules parameters,and stoichiometric information, and it is equally important for petrologistsand tectonicists to make use of these revisions as they become available. INrnooucrroN if we are not continually watching for them and correcting possible, get preci- In recent years, we have become concernedabout the them wherever we will not the best simplified view taken by some petrologists and tectoni- sion, especiallyin the P-T ratge outside of the range of cists regardingquantitative estimation of the pressureand calibration. This is an ideal area for collaboration be- temperature of formation of metamorphic rocks. There tween mineralogists and petrologists. Mineralogists pro- are many possiblesources of error in this procedure,and, vide valuable information on mineral structure and site assignments,and petrologistsapply the results to activity models. Both mineralogists and petrologists are making t This report is a revised, expanded version of the Mineral- ogical Society of America Presidential Address given by M.J. important contributions to careful, complete chemical Holdaway at the Geological Societyof America Meeting in Cin- analysis of common metamorphic minerals. cinnati, Ohio, on October 27, 1992. Interesting work is being done on pressure-tempera- 0003-004x/93l0708-o68I $02.00 681 682 HOLDAWAY AND MUKHOPADHYAY: GEOTHERMOBAROMETRY IN PELITES P difference between these two estimates is 450/oof the lower value. Here we review the processof P, T, and X"ro estima- tion, focusing on five important areas:(l) determination of mineral stoichiometry, (2) mineral mole fraction (ac- o tivity) models, using the mixing-on-sites model, (3) non- E (4) and use o ideality in mineral solid solutions, derivation U) -6 of thermodynamic data, and (5) fluid composition and o petrologists carefully consider c its use to test results. If E l what is known in eachofthese areasand continue to heed z developmentsas they becameavailable, we will converge on a consistent P-T-Xtro grid for all metamorphic ter- ranes and grades.In our analysis, we will focus on me- dium-grade pelitic metamorphism, but some of what we have to say is equally applicable to other gradesor bulk compositions. The major silicate minerals we will be dis- Wt.%Water cussing are muscovite, biotite, garnet, staurolite, cordi- Fig. 1. Histogramof H.O contentof all muscoviteand bio- erite, aluminum silicates, and chlorite; the Fe-Ti oxides tite samplesanalyzed by M.D. Dyar.After Dyaret al. (1993), and rutile, as well as reprintedwith the kind permissionof PergamonPress. ilmenite, hematite, magnetite, graphite, will also come up for discussion.We will con- sider mainly the dominant mineral end-membersand will ture-time paths (e.9.,Spear and Selverstone,1983) and not be consideringsulfide equilibria. The effectsofsulflde on fluid composition and movement in metamorphic ter- on Xrro in medium-grade reduced pelites can be ignored ranes(e.9., Ferry and Dipple, l99l). There are also sig- ifthe sulfideis pyrrohotite and not pyrite (Shi, 1992). nificant controversies over how much P-Z information MrNsnAl, SToICHIoMETRY one can learn from zoned minerals (e.g.,Frost and Tracy, 1991) and how much fluid actually passedthrough the Extensiveand, commonly, exclusiveuse of the electron rocks (e.g.,Gray et al., l99l). The quality of the results microprobe for mineral analysis has left us short on im- of many of these studies may well be only as good as the portant data for many minerals, especiallyhydrous ones. quality of the geothermobarometryused. Only when sig- In many studies, mineral chemistry is based on micro- nificant improvementsare made to the accuracyof P-T probe data alone. The resultant shortcomings include a determination will we possibly be able to answer these lack of data for Fe3+/(Fe2++ Fe'*) and a lack of light- questions. That alone is an important justification for elementanalyses, such as H, Li, B, and Be, on minerals continuing our efforts to improve geothermobarometry. as diverse as micas, chlorite, staurolite, garnet, and tour- To illustrate the seriousnessof the problem, we com- maline. In this list all may contain significant amounts of pare two recent studiesinvolving geothermobarometryin Fe3+,all but garnethave variable H, biotite and staurolite pelitic rocks. In Acadian M3 in Maine and in the Dela- may have significant Li, and tourmaline has variable B merian of South Australia. near Adelaide. isobaric ther- (Dyar et a1.,1992,1993; Holdaway et al., 1986a,1986b; mal metamorphism produces assemblagescontaining Dutrow et al., 1986). The assumptionof ideal or zero staurolite * andalusite followed by sillimanite at higher content of light elementsand zero Fe3* may have pro- grade. In both areasthe appearanceof sillimanite corre- ducedinaccurate stoichiometry, and it has precludedthe sponds approximately to the disappearanceof both an- possibility of several important substitutions that are dalusite and staurolite. In both areas,cordierite is locally coupledwith thoseof major elements.Depending on the present,probably becauseofan earlier, lower-P subevent, mineral, the H content of hydrous minerals may be a a local decreasein P, or a differencein bulk composition. function of metamorphic grade,of mineral assemblage, All petrologic evidenceindicates that thesetwo localities or both. With light-elementand Fe3* analyticaldata for have enjoyed about the same P-T conditions. Average representativeminerals, we can make appropriatecorrec- metamorphic conditions for the staurolite-bearingrocks tions to the stoichiometricbasis. Here we summarizewhat p":+1 deducedby Holdaway et al. (1988) for Maine M3 are P we know about Fe3*/(Fe2+* and light elements, : 3.1 kbar, T : 531 'C; conditionsdeduced by Dymoke how they affect stoichiometry,and possiblesite assign- and Sandiford (1992\ for the Delameriannear Adelaide mentsfor Fe3*. are P : 4.5 kbar, T : 594 oC, corresponding to an ap- parent P difference of 1.4 kbar or a depth difference of Muscovite nearly 5 km. Whereaspetrologists who work in rocks at Muscovite suffers from two compositional problems. higher P and I might not consider these to be serious (l) Preliminary Mdssbauerstudy (M. D. Dyar, unpub- differences, we believe it should be possible to obtain lished data) shows that half to two-thirds of the Fe in much greater accuracyin low- to moderate-P rocks than muscovite is Fe3+.Except for highly oxidized, Iow-grade, is implied by this comparison. It is worth noting that the or high-P rocks, this efect can be ignored becauseit has HOLDAWAY AND MUKHOPADHYAY: GEOTHERMOBAROMETRY IN PELITES 683 TABLE1. HrO content of Maine peliticmicas as a functionof Trale 2. Stoichiometryof micas,specimen 143' grade- Muscovite.* Biotitel Muscovite No H, Fe3* No H, Fe3* Average Average correction corrected correction corrected (%) ('/") No. No. rrlSi 3 034 3 076 2.666 2.696 Garnet 8 3.54 0 86 16 4.52 0.30 rrlAl 0.966 0.924 1.334 1.201 Staurolite 10 3.78 0.39 10 4.16 0.47 rrlFe3r 0.000 0.000 0.000 0 103 Sillimanite 26 4.06 0.35 29 3.83 0 35 rMrAl 1.866 1.947 0 390 0.542 Ksp-Sil-Mus 4 3.84 020 5 3.21 0.30 rMtTi 0.038 0.039 0 173 0.175 Atl 48 3.90 0.49 60 4.02 0.51 rMtFe2+ 0.060 0.031 1.277 1.136 rMtFes+ 0.000 0.030 0.000 0.052 .
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