Origin and abundances of volatiles on Mars from the zinc isotopic composition of Martian meteorites

Volatile elements are key to understanding the accretion and differentiation processes that formed the terrestrial planets. Relative to the solar photosphere or carbonaceous Ivuna-type (CI) chondrites, the terrestrial planets are depleted in moderately volatile elements (MVEs), with the Earth being more depleted in MVEs than Mars. Mass-dependent and mass-independent stable isotope variations of MVEs, such as Zn, can be used to determine the origin of volatile depletion on Mars. Here we present Zn isotopic compositions of ten Martian meteorites, spanning the major petrologic groups, and show that the Zn isotope composition of the Martian mantle, representative of the Bulk Silicate Mars (BSM), has a δ66Zn (per mille deviation of the 66Zn/64Zn from the JMC-Lyon standard) of +0.50 ± 0.18. This is distinctly heavier than that of the Earth (+0.16 ± 0.06‰). The BSM Zn isotope composition could partly reflect fractionation during volatile loss by evaporation consistent with the isotopic composition of other volatile elements (e.g., K). The mass-independent Zn isotopic composition of Mars, ε66Zn (per 10,000 deviation of the 66Zn/64Zn from the terrestrial standard JMC-Lyon, normalized to 68Zn/64Zn) is intermediate between carbonaceous chondrites (CC) and non-carbonaceous chondrites (NC), but with stronger affinity to the NC compared to that of the Earth. Therefore, planets further from the Sun are not necessarily made from a higher fraction of CC-like material.