MAGNETITE-MELT HFSE PARTITIONING

Abstract

Results from doped, hydrous experiments on natural mafic-to intermediate-composition lavas at 2-5 kbar pressure were combined with existing 1 atm data to evaluate the effects of composition and temperature on the partitioning behavior of the high field strength elements (HFSE), Zr, Nb, Ta and Hf between magnetite and natural silicate melts. Magnetite composition was found to be the strongest controlling factor on partitioning behavior. The partition coefficients (D) for Zr, Nb, Hf, and Ta correlate with DTi, Ti and Al content of the magnetite, temperature and pressure. The partition coefficients for the HFSE are similar to one another for any given magnetite-melt pair, but range from <0.02 in Cr, Al-rich magnetites and chromites to >2 in titanomagnetite. In addition, the relationship between Ti and the HFSE changes as a function of pressure and temperature, with the HFSE becoming more incompatible relative to Ti at lower temperatures and/or higher pressures. This change in the relationship between DTi and DHFSE with temperature and pressure means that the expressions presented in Nielsen et al. (1994) [Nielsen, R.L., Forsythe, L.M., Gallaghan, W.E., Fisk, M.R., 1994. Major and trace element magnetite-melt partitioning. Chem. Geol. 117, 167-191.] are not valid for hydrous, aluminous systems. Expressions were derived to describe the relationship between DHFSE and temperature, pressure, Fe2+/Mg exchange, Ti/Al ratio of the magnetite, and DTi. These expressions reproduce the input data within 35-50% (1σ) over a range extending from highly incompatible to compatible (<0.02-3.0). This internal precision represents ~3-4% of the observed range of partition coefficients. These patterns of behavior are consistent with the observed miscibility gap between the Al, Cr spinel group end members and ulvospinel and magnetite.