Pb-isotopic evidence for an early, enriched crust on Mars

Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surface measured by orbiters and landers. NWA 7533 contains monzonitic clasts that have zircon with U-Pb ages of 4.428 Ga. The Pb isotopic compositions of plagioclase and alkali feldspars, as well as U-Pb isotopic compositions of chlorapatite in the monzonitic clasts of NWA 7533 have been measured by Secondary Ion Mass Spectrometry (SIMS). The U-Pb isotopic compositions measured from the chlorapatite in NWA 7533 yield an age of 1.357 +/- 81 Ga (2 sigma). The least radiogenic Pb isotopic compositions measured in plagioclase and K-feldspar lie within error of the 4.428 Ga Geochron. These data indicate that the monzonitic clasts in NWA 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4.428 Ga with a mu-value (U-238/Pb-204,) of at least 13.4 +/- 1.7 (2 sigma) and a K-value (Th-232/U-238) of similar to 4.3. This mu-value is more than three times higher than any other documented Martian reservoir. These results indicate either the Martian mantle is significantly more heterogeneous than previously thought (mu-value of 1-14 vs. 1-5) and/or the monzonitic clasts formed by the melting of Martian crust with a mu-value of at least 13.4. Therefore, NWA 7533 may contain the first isotopic evidence for an enriched, differentiated crust on Mars. (C) 2014 The Authors. Published by Elsevier B.V.