NanoSIMS analyses of apatite and melt inclusions in the GRV 020090 Martian meteorite: Hydrogen isotope evidence for recent past underground hydrothermal activity on Mars

Abstract

We measured the hydrogen isotopes and water contents of melt inclusions and apatite that locate far from shock-induced melt veins in the Martian meteorite GRV 020090, using nano-scale secondary ion mass spectrometry (nanoSIMS). The melt inclusions in olivine show hydration profiles, with both the water contents and the δD values increasing from the cores (2σ) (170±7ppm and 3386±126‰) to the rims (5337±200ppm and 5519±65‰). The extremely high δD values of the melt inclusions relative to a maximum of 4239±81‰ of apatite that crystallized from the last residual melt convincingly argue that the observed hydration postdated emplacement of the parent magma of GRV 020090. This is also a robust line of evidence for past-presence of liquid water on Mars. Diffusion simulation of the hydration profiles of both water contents and δD values constrains the duration of liquid water up to 130,000–250,000years at 0°C or 700–1,500years at 40°C. All analyses of the melt inclusions, including the hydration profiles, show a logarithmic correlation between the water contents and the δD values, plotting within a two-endmember mixing area. The extremely D-enriched endmember represents Martian underground water, with a δD value of 6034±72‰ (2σ). This is the highest δD value reported in Martian meteorites and it is consistent with the recent analyses of Martian soils by the Curiosity rover (Leshin et al., 2013), suggestive of more water escaped from Mars than previous estimates.The analyses of apatite show a distinct positive correlation between the water contents (0.10–0.58wt%) and the δD values (737–4239‰), which can be explained by assimilation of D-rich Martian crustal materials and enhancement of water via fractional crystallization. The observed correlation suggests that the water contents of Martian mantle reservoirs might have been overestimated from D-enriched apatite in previous studies. Our estimation based on the least contaminated apatite grains from GRV 020090 turned out a low water content of the primordial parent magma (380–750ppm), which was likely derived from a relatively dry Martian mantle reservoir of GRV 020090 (∼38–75ppm H2O).