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

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 of 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.