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Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio

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Abstract

The partitioning of Cr and Al between coexisting spinel and clinopyroxene and the dependence of spinel-cpxgarnet equilibria on Cr/Al ratio have been investigated by a combination of phase equilibrium experiments, high temperature solution calorimetry and thermodynamic calculations.

The exchange equilibrium: \(\begin{gathered} {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {2 }}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${2 }$}}MgAl_2 O_4 + NaCrSi_2 O_6 = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {2 }}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${2 }$}}MgCr_2 O_4 + NaAlSi_2 O_6 \hfill \\ spinel clinopyroxene spinel clinopyroxene \hfill \\ \end{gathered} \) has a measured enthalpy change for pure phases of −2,100±500 cal at 970 K and 1 atm. Experimental reversals of Cr-Al partitioning between the spinel and clinopyroxene phases yield the following partitioning relationship: \(X\) where X ji refers to atomic fraction of i in the octahedral sites of phase j. The compositional dependence of partitioning implies that Al-Cr mixing in spinel is nonideal with, on the symmetrical model, a W SpCr-Al of 2,700±500 cal/gm. atom. In contrast, aluminum-chromium mixing in clinopyroxene is close to ideal.

The measured stability field of knorringite (Mg3Cr2Si2O12) and mixing properties of garnet have been used in conjunction with our experimental data to calculate the influence of Cr/Al ratio on the important reaction: orthopyroxene+clinopyroxene+spinel=olivine+garnet

The stability field of spinel lherzolite increases by about 2.8 Kb for every increase of 0.1 in Cr/(Cr+Al) ratio up to Cr/(Cr+Al) of 0.7. The calculated stabilization is in very good agreement with the experimental results of O'Neill (1981). The partitioning relationships are such that, at the low ratios of Cr/Al (∼0.07) of primitive lherzolite, clinopyroxene buffers spinel composition and sharpens the spinelgarnet reaction interval from 10 Kb (little or no clinopyroxene) down to about 2 Kb in pyroxene-rich “pyrolite.”

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  1. Department of Geological Sciences, Northwestern University, 60201, Evanston, IL, USA

    Susan A. Carroll Webb & Bernard J. Wood

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  1. Susan A. Carroll Webb
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  2. Bernard J. Wood
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Webb, S.A.C., Wood, B.J. Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio. Contrib Mineral and Petrol 92, 471–480 (1986). https://doi.org/10.1007/BF00374429

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  • DOI: https://doi.org/10.1007/BF00374429

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