The calcium isotope systematics of Mars

- New Ca isotope data for >20 Martian meteorites (shergottites, nakhlites, ALH 84001).
- δ44/40Ca of 1.04‰ estimated for silicate Mars, identical to Earth.
- The lack of light Ca in mantle rocks precludes recycling of surface materials.
- Hint at light Ca during formation of low-temperature carbonates at Martian surface.

New Ca isotope data from a suite of Martian meteorites provide constraints on the Ca isotope composition of the Martian mantle and possible recycling of surface materials back into the mantle. A mean δ44/40Ca of 1.04±0.09‰ (2SD) is estimated for the Martian mantle which can also be taken as an approximation for Bulk Silicate Mars. This value is identical with the estimates for Bulk Silicate Earth, and the inner Solar System planets can therefore be considered homogeneous with respect to Ca isotopes. The Ca isotope composition of two Martian dunites varies by ∼0.3‰ despite strong chemical and mineralogical similarities and this difference can be caused by the presence of carbonate, probably of pre-terrestrial origin, implying a protracted period of the existence of CaCO3-rich fluids and sufficient amounts of CO2 on the surface of Mars. The variability of δ44/40Ca within the groups of shergottites and nakhlites (clinopyroxene cumulates) cannot be related to partial melting and fractional crystallization in any simple way. However, there is no necessity of incorporating surface lithologies with isotopically light Ca into the mantle sources of Martian meteorites. These inferences are consistent with the absence of large scale crust-mantle recycling and thus of plate tectonics on Mars.