High-pressure apatite solubility in carbonate-rich liquids: Implications for mantle metasomatism

Abstract

At pressures higher than ∼22 kbar, near-solidus melts of peridotite + CO_2 + H_2O are carbonatitic and may be able to separate from their source regions at liquid fractions as low as 0.01%. At such low degrees of partial melting, trace-element-rich accessory phases may remain in the residue and have a dramatic effect on abundances and ratios of incompatible elements. We have determined the solubility of apatite in carbonate-rich melts at 30 kbar and over a temperature range of 1120–1430°C. The P_2O_5 content of an apatite-saturated liquid is given by the expression In (P_2O_5 wt%) = 4.834 − 3,809/7+ ∑ d_iC_i, where T is the absolute temperature in Kelvin and the di represent the constants 0.0113 and −0.00574, which modify wt% concentrations of CaO and CO_2 in the liquid. The quenched liquids have silica contents between ∼1.5 and 10 wt% and are also characterized by low solubilities of garnet, rutile, and ilmenite. Our experiments yield a partition coefficient for P between clinopyroxene and carbonate melt of 0.0067 ± 0.0016. Based on mineral/clinopyroxene P ratios from the literature, high-pressure olivine, orthopyroxene- and garnet-carbonatitic liquid partition coefficients for P are therefore approximately 0.003, 0.001, and 0.01, respectively. For mantle P_2O_5 contents between 0.03 and 0.05 wt%, <1 wt% apatite coexists with low-degree melts, and apatite is rapidly consumed near the solidus. At concentrations < 0.02 wt%, P probably resides completely in the silicate phases. Nodule data suggest that relatively large volumes of the mantle have P contents ≤ 0.05 wt%. Because near-solidus carbonatitic liquids probably represent very low degrees of partial melting, even minor amounts of residual apatite can substantially affect LREE contents in the melt. Our experimental data also indicate that near-solidus carbonatitic melts can have P_2O_5-TiO_2 ratios much higher than those observed in primitive nephelinitic lavas or kimberlites. Data on cryptically metasomatized spinel harzburgites suggest that the metasomatizing agents also had high P-Ti ratios.