Al-Solubility in Orthopyroxene in Equilibrium with Garnet: A Reinterpretation of Existing Experimental Data and the Petrogenetic Implications in Garnet Peridotite Xenolith

Abstract

Finnerty and Boyd (1984)discovered that the application ot available correction schemes -for solution o-f other components (concerning MAS ) in pyroxenes and garnet decreased both accuracy and precision of P,T estimates. The discrepancies are likely to be a result of imprecision in activity models for silicates. The proposed orthopyroxene -garnet geobarometer is recalibrated incorporating the determination of the high temperature equilibrium fractionation of elements between different sites in orthopyroxene structure,based on the statistical thermodynamic principles We took into account the experiments in MAS,CMAS,FMAS,CFMAS systems and obtained a close agreement with thermochemical properties of minerals. Only three empirical constants are used (aS^ and-dH'^of the reaction Mg Al^Si 0^= Mg^ Al^ Sij Function of Mg-Tschermak component). The results of isopleth calculations and of estimated stability fields in P,T space of sdme garnet peridotite xenolith in CFMAS (and partly Cr) system are shown on Fig. 1,2.The remarkable feature of the new P,T determinations is that the equilibrium field of diamond -bearing peridotite and inclusions in diamond (vertical bars on Fig.2 ) are drawnd along the line of conductive geotherm for surface heat flow 40 mW/m^calculated by Pollack et al (1977) (fat solid line on Fig.2).The T estimates were made with two-pyroxene (Wells,1977 and Lindsley,1983) and garnet-olivine (0>Nei11, Wood,1979) geothermometers. The field of diamond-bearing peridotite covers a temperature range T>1000 C which satisfies the diamond-graphite constraint.Temperature of other garnet peridotite xeno.liths (dotty area on Fig.2) are higher in relation to "cool" shield geotherm.The estimated boundary between this field and the field of P,T distributions for suits of garnet Iherzolite from southeast Austral i a, Sol omon Islands (Mai ai te) , Canada (He Bizard ) as well as from alkali basalts ,1amprophi res and some carbonatites (dashed area on Fig.2)--lies ne^ar the ledge on the solidus for per i dot i t e-CO^-H^ 0 after Wylli,1979 (broken fat line on Fig.2).