LIGHT LITHOPHILE ELEMENT AND LI ISOTOPE IN SITU ANALYSES IN MARTIAN METEORITES: POTENTIAL EVIDENCE FOR MARTIAN MAGMATIC WATER?

According to past and recent geomorphological and mineralogical observations in meteorites and by rovers and orbiters, water has flowed on the ancient martian surface. In addition, previous analyses of shergottites, the most common type of martian meteorites, indicate the presence of water in their parental magmas, which likely subsequently degassed and formed surficial water. However the role of martian magmatic water still remains controversial.

Here we investigate in situ light lithophile elements (LLE) such as Li, B, and Be as well as Li isotopic compositions in shergottite pyroxenes, olivines, and maskelynite in order to better understand water degassing in martian magmas. We measured LLE and Li isotopic profiles in pyroxenes of four different shergottites (Shergotty, QUE 94201, LAR 06319, and Tissint). These meteorites are martian basalts that have distinct ages and are distinguished by their different enrichment of incompatible elements (enriched versus depleted). During degassing, elemental Li ([Li]), [B], which are both soluble in H₂O-rich fluids, and ⁶Li compared to ⁷Li are predicted to escape with water vapor. Water degassing might have occurred between the formation of shergottite pyroxene cores and rims; Therefore, we expect [Li] and [B] to decrease as well as δ⁷Li to increase from cores to rims of pyroxenes, if their parental magmas experienced water degassing.

Our results show that the depleted shergottites might have undergone degassing according to their LLE elemental profiles and Li isotopic profiles. Previous studies have shown that all depleted shergottites originate from the same volcanic complex. Because the two investigated depleted shergottites in this study span the whole range of crystallization ages of depleted shergottites (575-325 Ma), water degassing may have occurred at the martian surface during the half-billion years. Nevertheless, Li isotopic profiles in enriched shergottites were likely affected by sub-solidus diffusion and may indicate an overlay to a degassing pattern.