Lunar Surface Water in Agglutinates: Origin and Abundances

Introduction: Recent remote-sensing observations have demonstrated surface-correlated OH/H2O on the regolith of the Moon from highto mid-latitudes [1-3]. The estimated abundances of lunar surface water from the reflectance spectra ranged from 10-5000 ppm [1-3]. Three sources have been suggested: solarwind origin; volatile retention from meteoritic impact; and degassing of indigenous water [1-4]. The relative contributions from each of these sources are unclear. In addition, the absolute quantities of OH/H2O and where in the regolith it is stored remain unknown [5]. In order to investigate the origin and abundance of OH/H2O, we examined lunar agglutinates, which are aggregates of rock and mineral fragments cemented by melts (agglutinitic glasses) generated by micrometeorite impacts. These glasses are characterized by numerous vesicles, suggesting vapor presence in their formation. If there is pre-existing surface water, some of it will be volatilized, whereas the rest may be retained in these glasses. Samples: Lunar agglutinate grains from Apollo 11 soil 10084, Apollo 16 soil 64501, and Apollo 17 soil 70051 were studied. Soil 10084 is a mature soil, whereas soil 64501 is a sub-mature one. Sample 70051 was collected from the surface of the lunar rover at the conclusion of the Apollo 17 mission, and is considered to be immature [6]. The maturity index of lunar soils is defined as Is/FeO, where Is is the ferromagnetic resonance intensity from the nanophase metallic Fe and FeO is the total FeO content of the soil [7], which is roughly proportional to agglutinitic contents [8]. H2 and H2O and their hydrogen isotope values were obtained on bulk soils in 1970s with step-heating methods [e.g., 9-13]. These studies reported a total H2 content (H2+H2O+CH4) of 35-120 ppm in lunar soils. They also showed that D/H values of released H2 gas increased with step heating temperatures from near solar-wind values (210) to terrestrial values (~1.510), which was interpreted as a progressive exchange process between lunar H and terrestrial water [8]. However, Friedman et al. [12] cautioned that some of this H could have a meteoritic origin. Methods: Paired Fourier Transform InfraRed spectroscopy (FTIR) and Secondary Ion Mass Spectrometry (SIMS) were used to analyze polished impact glasses and agglutinates from the soils. The FTIR measurements were conducted at the University of Michigan. Back scattered electron (BSE) images and the composition of the glasses and minerals were acquired with a Cameca SX-100 EMP at the University of Tennessee. Analyses for H abundances and D/H values were conducted in two sessions with a Cameca ims-7f GEO ion probe at Caltech. For both sessions, the areas of interest were examined carefully using ion imaging to avoid C and H hotspots (cracks, vesicles). Spots chosen for SIMS analyses are near EMP points. Following a ~3 min sputtering, glassy regions were measured for 20 cycles through the mass sequence of C, OH , [O-], Si, [P], S, and Cl, where the masses in brackets were only measured in the second session. Terrestrial basaltic glasses with 150-260 ppm H2O [14] were used as standards for H abundances. The instrument H backgrounds of the two sessions were monitored with a dry olivine standard (GRR1017, <<1 ppm H2O, [15]). D/H measurements were conducted on the same spots where H abundances were obtained. The mass sequence of H, H, and O was measured for 15-20 cycles each with 1 s, 15 s and 1 s counting times. One terrestrial rhyolitic glass containing 0.69 wt% H2O with D = -69‰VSMOW (MC84-df, [16]) was used to evaluate the instrument mass fractionation (IMF) of D/ H. The matrix effect between basaltic and rhyolitic glasses is insignificant for the results presented in this study.