The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system?

Details The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system? The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system?

Dec 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..76..109b&link_type=abstract

Earth and Planetary Science Letters, Volume 76, Issue 1-2, p. 109-122.

Physics

Condensed Matter

Statistical Mechanics

50

Scientific paper

Many synthetic system thermobarometers have been proposed in recent years, for estimating the original equilibrium conditions of mantle and deep crustal rocks from quenched phase compositions. However, available natural system reequilibration data have never been used to test such thermobarometers, that is, to evaluate the best analogue of the natural system and to select an optimum thermodynamic model. Such test is used here together with a total inversion procedure applied successively to solution models and synthetic systems of increasing degrees of complexity. The a-posteriori non-significant terms are deleted after each inversion, which yields a simple though highly reliable equation for the solubility of the enstatite component (En) in coexisting natural phases (cpx, opx). The equilibrium constant for the En(cpx) <--> En(opx) transfer reaction is best written in terms of M2 site occupancies as (1 - XCaM2)cpx/(1 - XCaM2)opx, irrespective of the chemical systems considered. With such a convention, the mutual solubility of the two pyroxenes appears to be identical for the CMS and the CMAS systems unlike what has been often suggested and no correction for Al is needed. Theoretical corrections for Na for both pyroxenes according to the principles of statistical mechanics, and empirical ones for Fe in clinopyroxene according to CFMS data yield: [XCaM2]opx.CMS ⋍ Caopx* = [XCaM2/(1 - XNaM2)]opx and [XCaM2]cpx.CMS ⋍ Cacpx* = [XCaM2/(1 - XNaM2)]cpx + [-0.77 + 10-3T] . [Fe/(Fe + Mg)] where T is the absolute temperature (K). Our optimum CMS thermometer (2σCMS = 18°C at 1200°C J, GPa, K) is: TCMS = (36273 + 399P)/[19.31 - 8.314 ln(K*) - 12.15(Cacpx*)2] where K* = [1 - Cacpx*]/[1 - Caopx*]. This geothermometer is directly applicable to the natural system as illustrated by the remarkable agreement between the experimental conditions and the calculated temperatures for reequilibration experiments on natural systems. With corrections for Na, the CFMS system therefore is a very good analogue of the natural complex system represented by mantle peridotites for mutual solubility pyroxene thermometry. This model based on the total inversion of the synthetic system data and independent testing on natural system reequilibration data, implies the need for a considerable revision of the former temperature estimates based on natural assemblages.
Also affiliated to the University of Paris 7 (U.A. C.N.R.S. 1093).

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