Compositional variability of terrestrial mantle apatites, thermodynamic modeling of apatite volatile contents, and the halogen and water budgets of planetary mantles

Abstract

Apatites in ultramafic xenoliths from Hawaii, Kamchatka, Nunivak Island, Lunar Crater volcanic field and the Navajo volcanic field have variable volatile (F, Cl, OH) compositions that correlate with their petrogenesis. Apatites in metasomatized lherzolite/harzburgite xenoliths are relatively Cl-rich whereas apatites in igneous pyroxenite and amphibolite xenoliths are relatively F-rich. Our results support the generalization of O'Reilly and Griffin (2000) that metasomatic apatites tend to be Cl-rich and magmatic apatites tend to be F-rich. Mantle metasomatism may largely be the consequence of devolatilization of Cl-rich fluids from subducted lithosphere. However, the occurrence of metasomatic xenoliths far removed from subduction zones (e.g., Bullenmerri) as well as the geochemistry of Cl in some plume basalts and diamonds indicates that not all metasomatism is subduction-triggered. Thermodynamic modeling of the influence on apatite compositions of variable F, Cl, H2O, and P abundances, as well as temperature, is consistent with the idea that terrestrial mantle fluids are H2O-rich, and that halogen enrichment in terrestrial apatites need not represent large absolute halogen abundances in the fluids. The models indicate that the occurrence of volatile-poor calcium phosphate (merrillite/whitlockite) in the Martian and terrestrial mantles is facilitated by halogen- and phosphorus-rich and water-poor environments — the rarity of this assemblage in the terrestrial mantle compared to Mars is consistent with the relatively wet and halogen- and phosphorus-poor nature of the terrestrial mantle compared to that of Mars.