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The Effects of Fluorine on the Vapor-Absent Melting of Phlogopite

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The Effects of Fluorine on the Vapor-Absent Melting of Phlogopite American Mineralogist, Volume 76, pages 470-476, l99l The effects of fluorine on the vapor-absentmelting of phlogopite * quartz: Implications for deep-crustalprocesses J. W. PrrpnsoN* Department of the Geophysical Sciences,University of Chicago,Chicago, Illinois 60637, U.S.A. T. CH.lcro Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada S. M. KuoHNBn Department of Geological Sciences,University of Washington, Seattle,Washington 98195, U.S.A. Ansrru.cr The vapor-absentpartial melting of biotite-bearing quartzofeldspathicassemblages may play an important role in the petrogenesisof high-temperaturegranitoid rocks. F-bearing biotite has been reported in high-grade terranes; therefore, melting equilibria involving fluorhydroxy micas may be better models for lower-crustal anatexis than those involving the hydroxy end-member. In order to make an initial assessmentof the effect of F on vapor-absent melting, the reaction fluorhydroxy phlogopite + qu"rtz: enstatite + liquid has been investigated experimentally near 8 and 15 kbar, using a synthetic phlogopite of F/(F + OH) : 0.58 (t0.12). Near l5 kbar, melting occursbetween 1020 and 1050"C; at 8 kbar, melting occursbetween 950 and 1000'C. The equilibrium was reversedat l5 kbar by a two-stageexperiment between I100 and 1000 "C, and it was reversedat 8 kbar by a two-stage experiment between 1050 and 950 "C. Reversal criteria included textural evi- dencefor mica regrowth from melt. Results of this study indicate that -60 molo/osubsti- tution of F for OH in phologopite stabilizes the phlogopile + qtrarlz assemblageby as much as 175 "C relative to enstatite and liquid. This significant stabilizing effect indicates that vapor-absentpartial melting of rocks containing high fluorine micas may be restricted to temperaturesin excessof 950 oC,leaving a residual unmelted micaceousassemblage of higher F content. The stabilizing effectofF in this systemis broadly consistentwith current models for the petrogenesisof A-type granites. Quantitative analysesof quenched liquid generatedfrom the vapor-absentfluorhydroxy phlogopite + quartz assemblage,however, indicate melts are lamproitic in composition, not granitic. Ir.mnonucrroN (Guidotti, 1984),which indicates that the KMASH-F sys- Initial melting of most quartzofeldspathiccrustal rocks tem may be a closer analogueto rocks. It has been dem- occurs at the solidi of constituent mica-bearing assem- onstrated experimentally that the F/(F + OH) ratio of phlogopite blages. Becausethe stability of biotite exceedsthat of increaseswith increasing temperature upon (Munoz muscovite in quartzofeldspathicrocks (Brown and Fyfe, equilibration with a fluid of constant {*. and Lu- presumed 1970), biotite is the mica applicable to the modeling of dington, 1974, 1977). Therefore, it is that F lower-crustal anatexis. To a first approximation, the va- substitution for OH increasesthe decomposition temper- phlogopite pressures. quantitative por-absent melting of biotite * qoartz representsa limit ature of at low No data partial to the beginning of melting in crustal rocks and is con- exist, however, for the vapor-absent melting of phlogopite quartz pressures sequentlyan important constraint on the processofgran- fluorhydroxy + at applicable itoid genesis. to the middle and lower crust. To obtain this informa- The vapor-absentmelting temperatureof the biotite + tion, we have conductedan experimental investigation of phlogopite quartz assemblagehas been determined for an end-mem- the vapor-absent melting of fluorhydroxy + qvartz ber hydroxy phlogopite in the system K,O-MgO-AlrO.- in the system KrO-MgO-AlrO3-SiOr-HrO-F. The principal SiO,-HrO (KMASH) (Peterson and Newton, 1989a); objective of the study was to obtain reversed however, many biotite samplesfrom high-grade terrains experiemntal P-Z determinations of the solidus of the phlogopite quartz: contain substantial fluorine (FJ in place of hydroxyl (OHJ reaction fluorhydroxy * enstatite + liquid. This information provides an initial quantifica- tion of the stabilizing effect of F in biotite, and thereby * Present address: Amoco Production Company, P.O. Box placesfurther constraints on the partial melting processes 3092,Houston, Texas 77253, U.S.A. responsiblefor the genesisof magmas in the lower crust. 0003-004x/9I /0304-0470$02.00 470 PETERSON ET AL.: MELTING OF PHLOGOPITE + QUARTZ 47r of the componentswas loaded into an Pt capsulewith an ExprnrvrnlvrAl. PRoCEDUR-ES outside diameter of 8 mm and reacted at 1.5 kbar and Apparatus 750'C for 5 d. Optical examination, as well as the ab- Experimentswere performedat -8 and -15-16 kbar senceof nonmica reflectionson X-ray diffractograms,in- in a solid-medium piston-cylinder apparatus.All exper- dicated a successfulsynthesis. Natural quartz from Lis- iments were conducted at the University of Chicago, ex- bon, Maryland, was used as the SiO, phase in the cept the 8-kbar reversalexperiment (Fl8), which was per- experiments. formed at the University of Alberta. Two types of salt Starting materials of fluorhydroxy phlogopite + quartz (NaCl) pressuremedia were used in the experiments. In (2:3 by weight) were ground in an agate mortar under one type, the sample was surrounded by NaCl cylinders distilled HrO to a fine-grained homogeneous mixture. inside a cylindrical graphite furnace. The furnace assem- Sampleswere dried under a heat lamp at -80 "C for I h, bly wasjacketed by a 1.905cm (o.d.)NaCl outer sleeve. then loaded in 4-7 mg amounts into Pt capsuleswith an In a secondtype, the samplewas placedbetween two soft- outside diameter of 1.5 mm. The encapsulatedsamples glasscylinders (Ca-Na glass,Corning no. 47057), inside werethen dried for a minimum of 1.5h at I l0'C before a graphite furnace that was separatedfrom the NaCl outer sealing of the capsules. sleeveby a l-mm thick soft-glasssheath. The assembly containing soft-glassinternal parts was necessaryfor high- Analytical methods temperature experiments in which the melting tempera- Starting fluorhydroxy phlogopite was characterizedby ture of NaCl was exceeded.Pressure was applied to the powder X-ray diffraction and electron microprobe tech- all-NaCl assembly 3-4 kbar below the desired pressure niques. X-ray diffraction unit-cell refinements,using Ni- at room temperature.A precalibratedamount of thermal filtered CuKa radiation, were obtained by scanning at a expansion allowed final pressuresto be attained within rate of t/q"2-0/min with an internal standard of synthetic 200 bars ofthe desired value upon heating. For the soft- corundum. Refinements were facilitated by the least glassassembly, a cold pressureof 4-5 kbar was initially squares refinement program of Appleman and Evans applied to the assembly.The temperaturewas then raised (1973), for lM (C2/m) mica polytype. The F content of to approximately 520'C, at which point the soft glass the phlogopite was determined by the refinement equa- relaxed enoughto allow the desiredpressure to be applied tion for F-OH substitution in phlogopite of Noda and to the assembly without shattering it. The temperature Ushio (1964). Unit-cell parametersand an F/(F + OH) was then raised to the appropriate conditions. No pres- value for starting mica are shown in Table l. According sure correction was applied to either of the NaCl pressure to the X-ray chancteiaalion, F/(F + OH) : 0.58 (+9.12; media. A third type of assemblywas used for the reversal for the synthetic phlogopite. experiment at 8 kbar. The assembly consisted of talc, Quantitative chemical analysesof starting mica were - pyrex, and alumina. A l0o/ofriction correction was ap- obtained from polished sections on a JEOL 733 Super- plied to this assembly,based on delineation of the reac- probe with Tracor Northern software at the University tion fayalite + quartz: ferrosilite (R.W. Luth, personal of Washington in Seattle.Two different electron micro- communication). Nominal pressureswere measuredwith probe techniques were employed. In the first technique a Heise bourdon tube gauge.Measured pressuresfor all (method l, Table l), major element oxides were analyzed assembliesare considered accurate to within 500 bars, by EDS using 15 keV acceleratingvoltage and l0 nA and precise to within 100 bars. beam current, and resultsare consideredaccurate to with- Temperatures were measured with W-WRe thermo- in 2o/oof the amount present. Quantitative F analyses couples situated directly above the encapsulatedsample. were obtained through WDS methods using a TAP crys- The cold junction of the thermocouple was laboratory tal. A calibration curve of counts vs. concentration was temperature. Temperature was controlled to within a de- constructedusing a natural fluorite (48.7 wto/oF) and syn- greewith an electronic, solid-state controller. No correc- thetic flurophlogopite (9.02 wto/oF) as standards. Con- tion was made for pressure-inducedemf. Temperature centration of F in the starting mica was determined from gradients were measured for the graphite furnaces used the calibration curve by collecting counts over 60 s time in this study. Reported temperaturesare considered ac- intervals. Representativeanalyses are shown in Table I curate to within l0 "C. and indicatean F/(F + OH) :0.52 (a0.03) for the start- Starting material was synthesizedin an interndlly heat- ing mica. In the second technique (method 2, Table l), ed Ar pressurevessel (design after Holloway, 197l). Pres- all elements were analyzed by WDS, including F with a sure was measured with an Astra bourdon tube gauge, synthetic LDE I crystal. Analytical data were collectedat
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