Ancient geologic events on Mars revealed by zircons and apatites from the Martian regolith breccia NWA 7034

Abstract

Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U-Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U-Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two-endmember mixing trend. The D (δD up to 2459‰) and 37 Cl heavy core (3.8‰) of a large apatite grain suggest a D-, 37 Cl-rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P-rich. The D-, 37 Cl-poor H 2 O-rich rim (<313‰) suggests the degassing of water from the Martian Cl-poor interior at a later time. This D-, 37 Cl-poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.