TY - JOUR
T1 - Influence of subsolidus processes on the chromium number in spinel in ultramafic rocks
AU - Voigt, Martin
AU - von der Handt, Anette
PY - 2011/10
Y1 - 2011/10
N2 - 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.
AB - 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.
KW - Chromium number
KW - Mid-ocean ridges
KW - Partial melting
KW - Spinel
KW - Ultramafic rocks
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U2 - 10.1007/s00410-011-0618-3
DO - 10.1007/s00410-011-0618-3
M3 - Article
AN - SCOPUS:80052346995
SN - 0010-7999
VL - 162
SP - 675
EP - 689
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 4
ER -