LIGHT LITHOPHILE ELEMENT AND LI ISOTOPE IN SITU ANALYSES IN SHERGOTTITES: EVIDENCE FOR BOTH MARTIAN MAGMATIC WATER AND SUBSOLIDUS DIFFUSION

Investigations of the martian surficial mineralogy and geomorphology conducted by orbiters and rovers have shown that liquid water was present on the martian surface. The principal source for this surficial water was likely degassed magmatic water. Previous studies demonstrated that the parental magmas of shergottite meteorites, the most common type of martian meteorites, contained water, which was subsequently degassed and deposited at the surface. The presence of water in martian magmas was demonstrated by the occurrence of hydrous minerals (apatites, amphiboles in melt inclusions), O and H isotope systematics, and experimental studies.

Here we investigate water degassing in shergottite parental magmas by measuring in situ light lithophile elements (LLE), such as Li, B, and Be, in addition to Li isotopic compositions in shergottite pyroxene, olivine, and maskelynite. We analyzed four different shergottites (Shergotty, QUE 94201, LAR 06319, and Tissint), which have distinct incompatible-element enrichments (depleted versus enriched) and crystallization ages. Elemental Li and B are soluble in H₂O-rich fluids, whereas Be is insoluble. In addition, degassing would lead to fractionation of ⁶Li, which will escape preferentially with the water vapor, compared to ⁷Li. Thus, if degassing occurred in shergottite parental magma, we would expect both [Li] and [B] to decrease as well as δ⁷Li to increase from core to rim in pyroxene.

According to LLE and Li isotopic profiles as well as degassing models, only the degassing of the depleted QUE 94201 parental magma was confirmed, whereas pyroxene in three other shergottites was likely affected by subsolidus diffusion. Post-crystallization diffusion does not discount the fact that the parental magmas of Shergotty, LAR 06319, and Tissint also underwent water degassing. This study shows that water degassing of basaltic magmas on Mars likely occurred during the Amazonian and could have been the source of surficial water, although post-crystallization diffusion may have erased evidence of magmatic degassing.