Petrology and shock history of the first depleted‐like poikilitic shergottite Asuka 12325

Abstract

Asuka (A) 12325 is the first poikilitic shergottite having a depleted pattern in light rare earth elements (REE). Compared with known poikilitic shergottites, A 12325 has smaller but more abundant pyroxene oikocrysts with remarkable Fe‐rich pigeonite rims, indicating that A 12325 cooled relatively faster at a shallower part of the crust. The redox condition (logfO2 = IW + 0.6‐IW + 1.7) and Fe‐rich chemical compositions of each mineral in A 12325 are close to enriched shergottites. The intermediate shergottites could not form by a simple mixing between parent magmas of A 12325 and enriched shergottites. Although A 12325 contains various high‐pressure minerals such as majorite and ringwoodite, plagioclase is only partly maskelynitized. Therefore, the maximum shock pressure may be within 17–22 GPa. Thermal conduction and ringwoodite growth calculation around a shock vein revealed that the shock dwell time of A 12325 is at least 40 ms. The weaker shock pressure and longer shock dwell time in A 12325 may be attained by an impact event similar to those of nakhlites and Northwest Africa (NWA) 8159. Such a weak shock ejection event may be as common on Mars as a severe shock event recorded in shergottites. Alteration of sulfide observed in A 12325 may imply the presence of magmatic fluid in its reservoir on Mars. A 12325 expands a chemical variety of Martian rocks and has a unique shock history among poikilitic shergottites while A 12325 also implies that poikilitic shergottites are common rocks on Mars regardless of their sources.

Petrogenesis of the Dar al Gani (DaG) 1.1 Ma ejection‐paired olivine‐phyric shergottites and implications for ~470 Ma Martian volcanism

“…The more Mg-rich pigeonite may have initially crystallized along with these phases at depth but predominantly crystallized along with plagioclase and augite in the nearsurface environment (Figure 12). Such a polybaric model is similar to that proposed for poikilitic shergottites (e.g., Combs et al, 2019;Howarth et al, 2014;Rahib et al, 2019), and is consistent with rare poikilitic textures observed in the DaG shergottites as well as other 1.1 Ma ejection-paired shergottites (e.g., NWA 2046 and NWA 4925;Peel et al, 2023), and more recently the first description of a depleted poikilitic shergottite (i.e., Asuka (A)-12325; Takenouchi et al, 2023).…”