Influence of subsolidus processes on the chromium number in spinel in ultramafic rocks

Martin Voigt, Anette von der Handt

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

The chromium number of spinel Cr#sp (atomic ratio of Cr/(Cr+Al)) is an important geochemical parameter for the estimation of the degree of partial melting, temperatures, and provenance in peridotites. In this study, a model has been developed in order to determine the effect of subsolidus reactions on the Cr#sp in ultramafic rocks. The final model includes temperature-dependent distribution coefficients of relevant reactions as well as solubility data and has been applied to lithologies common in mid-ocean ridge settings. Significant changes in the Cr#sp are predicted from the application of this model during cooling from 1300 to 800°C at mantle pressures. For spinel lherzolites and harzburgites, the Cr#sp is predicted to decrease proportional to the absolute values of the Cr#sp at (constantly) increasing spinel mass. Cpx-dunites show the same trend, although to a lower extent. Websterites show a different behavior with a slight increase in the Cr#sp due to their lack of olivine. Modal abundance of spinel correlates with the magnitude in Cr#sp change, too. Finally, these results were tested for possible effects on the calculated degree of partial melting as function of the Cr#sp. Application of the Cr#sp from a peridotite equilibrated down to 800°C would result in an underestimation of only 1. 5 % in the degree of melting, justifying the use of Cr#sp for estimations of this parameter.

Original languageEnglish (US)
Pages (from-to)675-689
Number of pages15
JournalContributions to Mineralogy and Petrology
Volume162
Issue number4
DOIs
StatePublished - Oct 1 2011

Keywords

  • Chromium number
  • Mid-ocean ridges
  • Partial melting
  • Spinel
  • Ultramafic rocks

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