Influence of subsolidus processes on the chromium number in spinel in ultramafic rocks |
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Authors: | Martin Voigt Anette von der Handt |
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Institution: | 1.Institut für Geowissenschaften,Mineralogie - Geochemie Albert-Ludwigs-Universit?t,Freiburg,Germany |
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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. |
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