Hydrothermal Alteration of Martian Zircons in NWA 7034/7533

The martian regolith breccia NWA 7034/7533 contains abundant zircon grains, which can provide insight into crustal processes through isotopic compositions. For example, insitu U-Pb measurements on these zircons suggest a crystallization age of ~4.4 Ga with resetting at 1.5-1.7 Ga [1-3]. On the basis of intragrain and intergrain oxygen isotope variations, Nemchin et al. [4] proposed that zircons crystallized from melts that had assimilated ^(17)O-enriched regolith, and that the metamict zircons were later altered by low-temperature fluids near the surface at ~1.7 Ga. Here, we describe the textural context of zircons in NWA 7034 and 7533, with an emphasis on those aspects touching on their hydrothermal alteration.