Whitlockite solubility in silicate melts: Some insights into lunar and planetary evolution

The solubility of both beta-whitlockite and alpha-whitlockite has been expe rimentally determined between 1200 and 1400 degrees C at 1 atm using a wide range of natural rocks and synthetic mixtures as starting materials. The s olubility of both phases depends strongly on melt composition, decreasing s ystematically with increasing silica content and aluminosity. Experiments a lso show that alpha-whitlockite contains much more Na (0.5-6.3 wt.% Na2O) t han beta-whitlockite (<0.5 wt.% Na2O). Lunar low- and high-Ti mare basalts are far below the saturation limit of whitlockite and need 90-99% fractiona tion of olivine, pyroxene, plagioclase, and ilmenite to precipitate whitloc kite, whereas KREEP basalts need less but at least 80-95% fractionation. It is shown here that both lunar mafic and felsic immiscible melts, the forme r enriched in Fe, REE, P, U, and Th, and the latter in Si and K, are unders aturated in whitlockite, and further fractionation of fayalite, ilmenite, p lagioclase, and K-feldspar is required to reach the saturation limit. Thus, lunar whitlockite must have crystallised from highly fractionated residual melts. Lunar whitlockite, which is low in Na (0.09-0.49 wt.% Na2O), crysta llised originally as beta-whitlockite from low-temperature residual melts. In contrast, meteoritic whitlockite contains more Na (0.5-3.3 wt.% Na2O and therefore, had formed initially as alpha-whitlockite at higher temperature s and transformed into beta-whitlockite upon cooling. It is proposed that t he interior of the Martian mantle and crust was enriched in volatiles in it s early history (4.6-1.3 Ga), but has become essentially dry and very deple ted in water and halogens at least since the last 180 Ma. Calculations show that the Earth's crust and mantle as a whole contains only 5% of the total P of the Earth, and the remaining 95% is stored in the core. In contrast, the crust and mantle of Mars are much more enriched in P and contain as muc h as 43% of the Martian total P budget, with the remaining 57% being distri buted in the relatively smaller Martian core. This difference in the distri bution of P among planetary shells must have resulted from a more oxidising environment during the accretion and early evolution of Mars compared to t he more reducing conditions under which Earth formed. Copyright (C) 2000 El sevier Science Ltd.