The transition between spinel lherzolite and garnet lherzolite,and its use as a Geobarometer |
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| Authors: | Hugh St C O'Neill |
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| Institution: | 1. Research School of Earth Sciences, Australian National University, 2600, Canberra, ACT, Australia
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| Abstract: | The equilibrium between spinel lherzolite and garnet lherzolite has been experimentally determined in the CaO-MgO-Al2O3-SiO2 system between 800° and 1,100° C. In confirmation of earlier work and predictions from thermodynamic data, it was found that theP-T slope of the reaction was close to zero, the equilibrium ranging from 16.1 kb at 800° C to 18.7 kb at 1,100° C (±0.3 kb). The addition of Cr2O3 to the system raised the stability field of spinel to higher pressures. It was found that the pressure at which both garnet and spinel could exist with olivine+orthopyroxene+clinopyroxene in the system CMAS ?Cr2O3 could best be described by the empirical relationship: $$P = P^{\text{O}} + \alpha X_{{\text{Cr}}}^{s{\text{p}}} $$ whereP 0 is the equilibrium pressure for the univariant reaction in the Cr2O3-free system,α is a constant apparently independent of temperature with a value of 27.9 kilobars, andX Cr sp is the mole fraction of chromium in spinel. Use was made of the extensive literature on Mg-Fe2+ solid solutions to quantitatively derive the effect of Fe2+ on the equilibrium. The effect of other components (Fe3+, Na) was also considered. The equilibrium can be used as a sensitive geobarometer for rocks containing the five phases ol+opx+cpx+gt+sp, and thus provides the only independent check presently available for the more widely applicable geobarometer which uses the alumina content of orthopyroxene in equilibrium with garnet. |
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