Ancient impactor components preserved and reworked in martian regolith breccia Northwest Africa 7034

作者：Goderis Steven; Brandon Alan D; Mayer Bernhard; Humayun Munir

来源：Geochimica et Cosmochimica Acta, 2016, 191: 203-215.

DOI：10.1016/j.gca.2016.07.024

• Osmium isotope systematics
• Highly siderophile elements
• Meteoritic contribution
• Martian regolith breccia
• NWA 7034 and paired meteorites

Northwest Africa (NWA) 7034 and paired stones represent unique samples of martian polymict regolith breccia. Multiple breccia subsamples characterized in this work confirm highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) contents that are consistently elevated (e.g., Os similar to 9.3-18.4 ppb) above indigenous martian igneous rocks (mostly <5 ppb Os), equivalent to similar to 3 wt% of admixed CI-type carbonaceous chondritic material, and occur in broadly chondrite-relative proportions. However, a protracted history of impactor component (metal and sulfide) breakdown and redistribution of the associated HSE has masked the original nature of the admixed meteorite signatures. The present-day Os-187/Os-188 ratios of 0.119-0.136 record a wider variation than observed for all major chondrite types. Combined with the measured Re-187/Os-188 ratios of 0.154-0.994, the range in Os isotope ratios indicates redistribution of Re and Os from originally chondritic components early in the history of the regolith commencing at similar to 4.4 Ga. Superimposed recent Re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces. Elevated Os concentrations (38.0 and 92.6 ppb Os), superchondritic Os/HSE ratios, and Os-187/Os-188 of 0.1171 and 0.1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at similar to 1.5-1.9 Ga, possibly overlapping with a (partial) resetting event at similar to 1.4 Ga recorded by U-Pb isotope systematics in the breccia. Martian alteration of the originally chondritic HSE host phases, to form Os-Ir-rich nuggets and Ni-rich pyrite, implies the influence of potentially impact-driven hydrothermal systems. Multiple generations of impactor component admixture, redistribution, and alteration mark the formation and evolution of the martian regolith clasts and matrix of NWA 7034 and paired meteorites, from the pre-Noachian until impact ejection to Earth.

• 出版日期2016-10