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The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm63Prp26Grs10)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm50Prp14Grs35)–jadeite (Jd80–97Di0–4Hd0–8Acm0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/PT. The inferred limiting a(H2O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6. 相似文献
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Activities of titanite (Ttn, CaTiSiO5) and/or rutile (Rt, TiO2) phase components were calculated for 45 well‐characterized natural titanite‐ or rutile‐undersaturated epidote–amphibolites by using the equilibria: (i) 3 anorthite + 2 zoisite/clinozoisite + rutile + quartz = 3 anorthite + titanite + water (referred to as TZARS) and (ii) anorthite + 2 titanite = grossular + 2 rutile + quartz (referred to as GRATiS). In titanite‐bearing and rutile‐absent samples aRt is 0.75 ± 0.26. In titanite‐absent, rutile‐bearing samples aTtn is 0.89 ± 0.16. Mean values derived for aRt/aTtn are 0.92 ± 0.12 for rutile + titanite‐bearing samples and 0.42 ± 0.27 for samples lacking both titanite and rutile. Use of these values with TZARS yields pressure estimates for epidote–amphibolites that differ on average by <0.5 kbar from those recorded by established mineral barometers, even where both titanite and rutile are lacking. Despite rather large uncertainties in the average values obtained for aRt, aTtn or aRt/aTtn, application of TZARS yields pressure estimates that agree with independent estimates to within ±0.5 kbar for titanite‐ and/or rutile‐saturated samples, and to within ±0.8 kbar for samples that contain neither Ti‐phase. The accuracy and precision of the TZARS barometer are comparable to that of many well‐calibrated barometers. TZARS offers a much‐needed barometer for mafic rocks metamorphosed at epidote‐bearing amphibolite and blueschist facies conditions. In addition, the results provide a basis for application of other thermobarometers, such as Ti‐in‐zircon, where rutile activity is required as input. 相似文献
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Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]–1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
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