Showing papers by "Gerhard P. Brey published in 1999"
TL;DR: In this paper, in-situ single crystal X-ray diffraction was used to identify a new inclusion paragenesis of Ca-silicates in the Kankan district of Guinea.
103 citations
TL;DR: In this paper, the authors investigated the influence of chromium on the spinel peridotite transition in the FMASCr system at pressures of 30-50 kbar and temperatures of 1200-1500 degrees C. The experiments were carried out using mixtures of synthetic minerals as starting materials with different initial compositions of phases.
Abstract: Experiments in the FMASCr system at pressures of 30-50 kbar and temperatures of 1200-1500 degrees C model mineral equilibria in depleted mantle harzburgite. Special emphasis was on the partitioning of chromium between garnet, spinel and orthopyroxene and the influence of chromium on the garnet to spinel peridotite transition. The experiments were carried out using mixtures of synthetic minerals as starting materials with different initial compositions of phases, approaching the conditions of reversed experiments. The equilibrium between minerals was attained rapidly with respect to Fe-Mg exchange, whereas a considerable scatter in Cr/(Cr-+Al) values was observed in all phases studied. The extreme compositions of phases in reversed experiments give insight on the equilibrium phase composition. Cr/(Cr+Al) ratio of garnet coexisting with orthopyroxene, olivine and spinel increases with pressure and temperature, exceeding 0.4 at 50 kbar and 1500 degrees C. At high pressure spinel becomes also more chromian, while orthopyroxene composition approaches the enstatite-ferrosilite join. After quench, the minerals were studied by X-ray diffraction in order to obtain unit-cell parameters of solid solutions and refine their volume properties. Spinels in the system (Mg, Fe)(Cr, Al) 2 O 4 show negative deviation from ideal mixing volume at high Cr contents and positive deviation in Al-rich composition (Margules excess mixing volume parameters: W CrAl = 0.017 and W AlCr = -0.007 J/bar). Small negative excess volume of mixing is related to Fe-Mg mixing in spinel. Cr-Al mixing in garnet is characterised by a small excess mixing volume, which may be approximated by a symmetrical model (W V AlCr = W V CrAl = 0.018 J/bar). Our results are compatible with zero excess mixing volume of orthopyroxene solid solution. All the excess volumes of mixing are small and their influence on equilibria is substantial only at very high pressures (>50 kbar).
55 citations
TL;DR: In this article, the effects of non-ideal mixing in solid solutions are approximated by a subregular Margules formulation, independent for each site of spinel, garnet, and orthopyroxene solid solutions.
Abstract: Experimental results for the four-phase assemblage in the MASCr (Doroshev et al., 1997) and FMASCr (Brey et al., 1999) systems are evaluated with respect to exchange (Fe-Mg and Cr-Al) and displaced equilibrium reactions (Al solubility in orthopyroxene and spinel-orthopyroxene to garnet-olivine transition). These equilibria are considered on the basis of the thermodynamic data of Berman (1988). The effects of non-ideal mixing in solid solutions is approximated by a subregular Margules formulation, independent for each site of spinel, garnet, and orthopyroxene solid solutions. Mixing parameters were calculated by linear programming considering each experimental point as an independent half-bracket for the concentration term in the respective equilibrium. Cr-Al mixing in all phases is not ideal, but the magnitudes of excess mixing energies are not large. Spinel solid solution shows asymmetric Cr-Al and Mg-Fe excess mixing energy. Cr-Al mixing in garnet is close to an ideal model with a symmetric Margules parameter 0 to 6 kJ/mol. Standard state properties of knorringite, Mg 3 Cr 2 Si 3 O 12 and orthopyroxene end-member MgCrAlSiO 6 are estimated. The major factor determining concentration dependencies of equilibrium constants is the Gibbs free energy changes of reciprocal reactions FeCr+MgAl = FeAl+MgCr in spinel and garnet. The derived thermodynamic parameters are used to calculate phase relationships in the four-mineral region of the FMASCr system, which models depleted mantle harzburgite. The Cr/(Cr+Al) ratio of garnet increases with both pressure and temperature, while that of spinel depends mainly on pressure. The distribution of Cr and Al between spinel and garnet in equilibrium with orthopyroxene and olivine may be used as a thermobarometer for deep mantle xenoliths and inclusions in diamonds.
42 citations