Pyroxene chemistry and crystallization history of basaltic units in the Mare Humorum on the nearside of the Moon: Implications for the volcanic history of the region

Abstract

Mare basalts are volcanic features on the lunar surface and are vital for understanding the Moon’s volcanic and thermal history. The present study investigates the mineralogy and chemical variations of the mare basaltic units in the Mare Humorum on the Moon’s nearside using orbital remote sensing data from recent lunar exploration missions. The previous model ages available for the Humorum basaltic units have been utilized in the present study to understand the chemical evolution of these basaltic units with time. Basalts of Imbrian-Eratosthenian periods occur in the Mare Humorum. Hyperspectral data from Moon Mineralogy Mapper onboard Chandrayaan-1 mission showed pigeonites and augites as the major compositions in these basaltic units. The compositional trend between pigeonites and augites point towards the differentiation of the basaltic magma while cooling. The older units crystallized from a Fe-enriched fractionated magma while the younger unit formed from an Mg and Ca-rich magma. The distinct chemical trends in the pyroxene quadrilateral revealed that multiple volcanic eruptions have occurred in the Mare Humorum. The longward and shortward shifts in the Band I and Band II centres in the younger and older basalts respectively, also indicate that the older basaltic units crystallized more Fe2+-rich, Ca-depleted pyroxenes while the younger basaltic unit was formed from a rather Ca-rich magma. Hence, the basaltic magmas erupted in the Mare Humorum during Imbrian and Eratosthenian periods were derived from heterogeneous source regions.