Magnetite-melt HFSE partitioning

Results from doped. hydrous experiments on natural mafic-to intermediate-co mposition 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 bet ween magnetite and natural silicate melts. Magnetite composition was found to be the strongest controlling factor on partitioning behavior. The partit ion coefficients (D) for Zr, Nb, Hf, and Ta correlate with D-Ti, Ti and Al content of the magnetite, temperature and pressure. The partition coefficie nts for the HFSE are similar to one another for any given magnetite-melt pa ir, 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 cha nges 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 D-Ti and D-HFSE with temperature a nd 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 descr ibe the relationship between D-HFSE and temperature, pressure, Fe2+/Mg exch ange, Ti/Al ratio of the magnetite, and D-Ti. These expressions reproduce t he input data within 35-50% (1 sigma) over a range extending from highly in compatible to compatible (< 0.02-3.0). This internal precision represents s imilar to 3-4% of the observed range of partition coefficients. These patte rns of behavior are consistent with the observed miscibility gap between th e A, Cr spinel group end members and ulvospinel and magnetite. (C) 2000 Els evier Science B.V. All rights reserved.