THE SYSTEMATICS OF CR3+ AND CR2+ PARTITIONING BETWEEN OLIVINE AND LIQUID IN THE PRESENCE OF SPINEL

The partitioning behavior of Cr into olivine in basaltic systems has b een parameterized and can now be modeled over a wide range of redox co nditions and liquid compositions. The Cr2+/Cr3+ in spinel-saturated ex perimental systems can be estimated based on a simple model of Cr solu bility in basalt. Fe3+ appears to suppress the presence of Cr2+ in bas altic systems. We predict that, in Fe-free systems, all Cr is trivalen t at log fO(2) = -3 (i.e., QFM+3 to QFM+4), whereas all Cr is trivalen t at approximately Ni NiO(QFM+1) in Fe-bearing systems. Cr2+ predomina tes under redox conditions <IW-1 in both Fe-bearing and Fe-free system s. DCr2+ and DCr3+, (olivine/liquid) have been determined in various l iquid compositions and temperatures. DCr3+ (i,e., f(O2) greater than o r equal to QFM, appropriate for most terrestrial or martian basalts) s trongly covaries with the ratio of non-bridging oxygens to tetrahedral ly coordinated cations (NBO/T) (Mysen 1983) and can be estimated using the equation D-Cr3+((ol/liq)) = -0.39.NBO/T + 1.29. This relationship appears to be valid over the entire pressure range of olivine stabili ty, from 1 atm to 15 GPa. DCr2+ (i.e., less than or equal to IW-1, app ropriate for lunar and some asteroidal basalts) is sensitive to liquid composition and temperature and can be estimated using either D-Cr2+( (ol/liq)) = 0.24.D-Mg((ol/liq)) - 0.07 or D-Cr2+((ol/liq))() = 0.66.[1 0,000/T(K)] - 4.48. The 1/T equation is probably only valid at 1 arm p ressure, but the D-Mg,, equation may be useful at higher pressures as well. The Cr content of spinel-saturated liquids is a function of temp erature, composition, and f(O2). However, the Cr content of spinel-sat urated liquids is buffered by spinel and is insensitive to the bulk Cr content of the system (e.g., Roeder and Reynolds 1991). Therefore, th e Cr content of a crystallizing, spinel-saturated basalt cannot be mod eled using Raleigh fractionation models.