LI, SHUAI - Quantitative mapping of hydration in lunar pyroclastic deposits and implications for lunar volcanic processes

Abstract: 
Lunar pyroclastic deposits represent early volcanic processes on the Moon capable of informing us of volatiles and eruption processes. Mapping of lunar surface hydration (H2O/OH) using M3 spectral reflectance data, corrected for thermal emission effects, shows that pyroclastic deposits exhibit much higher hydration levels than surrounding terrains. We have examined eleven large pyroclastic deposits (> 1000 km2) between 30° N and 30° S to assess detailed variations in hydration level and possible links to morphology and eruptive processes. These eleven deposits can be classified into three groups based on their average hydration levels. Pyroclastic deposits at Aristarchus, Sulpicius Gallus, and Humorum exhibit areally extensive high abundances of hydration (e.g., < 500 ppm on average), whereas Rima Bode, Montes Harbinger, and Moscoviense exhibit moderately high abundances of hydration (< 200 ppm on average). Taurus Littrow, Montes Carpatus, Vaporum and Nectaris have lower hydration levels (< 100 ppm on average) and Aestuum is an outlier, showing no detectable hydration absorptions in M3 data. Locations with the highest hydration levels correspond to dark, smooth regions in LROC WAC and earth-based S band RADAR CPR data, consistent with previous observations of lunar pyroclastics. In addition, average hydration content in the pyroclastics is linearly correlated with the spatial extent (area) of each deposit. If the hydration signature in these deposits represents volatiles (water) from the lunar interior, as opposed to solar wind implantation, then these detections provide important information on the volatile content of magma sources and constraints on degassing during eruption events. The different levels of hydration that we observe might indicate heterogeneity in volatile content of magma sources, different cooling rates, or different degrees of degassing. The lack of detectable hydration at Aestuum may be due to water-poor magmas or significant loss of volatiles during eruption and emplacement, and this deposit is known to be associated with distinct (spinel-bearing) mineralogies.  The LROC NAC images at these large pyroclastic deposits show that the hydration abundances might be controlled by the thickness of glass-rich layers and/or the purity of the volcanic glass in the deposits. To understand the linear trend between the pyroclastic hydration level and areal extent we will also estimate the thickness of each pyroclastic deposit to provide an estimate on the volume of pyroclastic material. These volume estimates can be used as inputs for volcanic eruption models to calculate the required volatile content (H2O) to account for the observed size of each deposit, which in turn can be compared to our observed hydration levels from M3 data. Pyroclastic hydration can also be complicated by loss via diffusion after deposition, thus we also explore a post-emplacement diffusion model for lunar pyroclastics to understand the retention potential of water in volcanic glass at the lunar surface over geologic timescales.