APATITE AS A RECORDER OF HYDROTHERMAL AND METASOMATIC EVENTS ON MARS (Invited Presentation)

Apatite, Ca₅(PO₄)₃(F, Cl,OH), in extraterrestrial rocks is now a key recorder of volatiles in igneous systems and an important window into volatiles in the interiors of planetary bodies. A less appreciated use of apatite in these extraterrestrial rocks is its ability to record hydrothermal and metasomatic events. Specifically, in the case of Mars that had a wet past much like Earth, we recognized that apatite can record hydrothermal and metasomatic events, like those on Earth (Howarth et al., 2016; Liu et al., 2016).

In Martian regolith breccias NWA 7034 and 7533, a significant portion of apatite grains (≤10% of apatite grains) show signs of hydrothermal alteration. These apatite grains contain clusters of sub-μm monazite [(Ce, Nd, La, Y)PO₄] inclusions, and occasionally irregular, zoned merrillite inclusions (Na-merrillite in the interior to REE-merrillite toward the interface with apatite host). The apatite hosts are mainly chlorapatite, but apatite regions next to monazite and merrillite inclusions are F-rich, with compositions reaching pure fluroapatite. Cathodoluminescence (CL) images of monazite-bearing apatite grains display no igneous zonings, in contrast to zonings observed in monazite-free apatite in the same samples. The textural and compositional observations suggest that hydrothermal fluid leached REE from chlorapatite and merrillite, which then reprecipitated as monazite. Fluorine-rich apatite associated with monazite and merrillite inclusions suggest F-rich fluids were likely involved in the hydrothermal reactions. The monazite-bearing and merrillite-bearing chlorapatite, together with other evidence of hydrothermal alterations and metamorphic events in these meteorites, indicate multiple hydrothermal environments in crustal environments of Mars. Another evidence of post-crystallization alteration was observed in apatite grains in paired olivine-phyric shergottites (basaltic meteorites), LAR 06319 and 12011. Variations of Cl in apatite grains are best explained by interaction with a Cl-rich and possible water-rich fluid after crystallization (Howarth et al., 2016).

These recent observations emphasize non-trivial effects of secondary processes on apatite, and the large potential of using apatite to infer the nature of crustal fluids on Mars.