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

Susan A Carroll Webb, Bernard J. Wood*

*Corresponding author for this work

    Research output: Contribution to journalArticle

    193 Citations (Scopus)

    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: {Mathematical expression} 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: {Mathematical expression} where Xijrefers 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 WCr-AlSpof 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."

    Original languageEnglish
    Pages (from-to)471-480
    Number of pages10
    JournalContributions to Mineralogy and Petrology
    Volume92
    Issue number4
    DOIs
    Publication statusPublished - Apr 1986

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