Abstract

The partitioning behavior of Cr into olivine in basaltic systems has been parameterized and can now be modeled over a wide range of redox conditions and liquid compositions. The Cr (super 2+) /Cr (super 3+) in spinel-saturated experimental systems can be estimated based on a simple model of Cr solubility in basalt. Fe (super 3+) appears to suppress the presence of Cr (super 2+) in basaltic systems. We predict that, in Fe-free systems, all Cr is trivalent at log f O2 = -3 (i.e., QFM+3 to QFM-4), whereas all Cr is trivalent at approximately Ni-NiO(QFM+1) in Fe-bearing systems. Cr (super 2+) predominates under redox conditions <IW-1 in both Fe-bearing and Fe-free systems. D (sub Cr2+) and D (sub Cr3+) (olivine/liquid) have been determined in various liquid compositions and temperatures. D (sub Cr3+) (i.e., f O2 > or =QFM, appropriate for most terrestrial or martian basalts) strongly covaries with the ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) (Mysen 1983) and can be estimated using the equation D (super (ol/liq)) (sub Cr3+) = -0.39.NBO/T+1.29. This relationship appears to be valid over the entire pressure range of olivine stability, from 1 atm to 15 GPa. D (sub Cr2+) (i.e., < or =IW-1, appropriate for lunar and some asteroidal basalts) is sensitive to liquid composition and temperature and can be estimated using either D (super (ol/liq)) (sub Cr2+) = 0.24.D (super (ol/liq)) Mg -0.07 or D (super (ol/liq)) (sub Cr2+) = 0.66.[10.000/T(K)]-4.48. The 1/T equation is probably only valid at 1 atm pressure, but the D Mg equation may be useful at higher pressures as well. The Cr content of spinel-saturated liquids is a function of temperature, composition, and f O2 . However, the Cr content of spinel-saturated liquids is buffered by spinel and is insensitive to the bulk Cr content of the system (e.g., Roeder and Reynolds 1991). Therefore, the Cr content of a crystallizing, spinel-saturated basalt cannot be modeled using Raleigh fractionation models.

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