UNIQUE MELT CLAST IN THE MARTIAN METEORITE BRECCIA NWA 7034

The first and currently only clastic rock sample from Mars is the meteorite NWA 7034 (paired with NWA 7533 and NWA 7475 that are believed to be part of the same meteorite fall). Its 2.1 Ga age is early Amazonian. Previous studies argue this sample represent a regolith breccia, suggesting an impact origin based on elevated Ni (~ 4-5% CI chondrite) and platinum-group elements. NWA 7034 exhibits a variety of clasts, all with different major element compositions suggesting it originated from distinct sources.

Here we present the interpretation of our new data on a large melt clast observed in NWA 7034. This 3x2.5 cm clast has a unique vitrophyric quenched texture that has not been observed in other martian meteorites. Additionally, this clast texture differs from the melt clasts and spherules described in previous studies. It displays elongate slightly zoned pyroxenes with core compositions of En₆₉Wo₃Fs₂₈ (average) and Fe-rich rims with compositions of En₆₀Wo₅Fs₃₅ (average) and olivine dendrites (Fo_55-63). The interstitial glass mainly exhibits a homogeneous basaltic composition. However, some areas show SiO₂, Al₂O₃, and alkali element enrichments compared to the rest of the glass.

The melt clast displays a finer-grained outer boundary, which is petrologically similar to the interior and does not contain unmelted clasts of material from the breccia. Despite the fact that this melt clast is rich in Fe²⁺ due to the presence of fayalite-rich olivine, the melt clast does not contain a substantial amount of P or Fe³⁺, which are found in abundance in phosphates and Fe-oxides in the host breccia. This along with the outer boundary texture imply that this clast did not form in situ. The quenched texture indicates that the melt droplet was mostly solidified before integration to the breccia. The bulk rock composition of the clast is basaltic (Mg# = 48) and is similar in composition to the host breccia and other basaltic samples studied from the martian surface, with the exception of Fe-enrichment. The FeO content (20 wt%) is comparable to the one found in shergottite and nakhlite meteorites. Therefore, preliminary findings indicate that this melt clast may be an impact melt clast and that the target rock may represent another sample of martian rock.