Oxygen Isotopic Analyses of Water Extracted from Lunar Samples

Oxygen exists in lunar materials in distinct phases having unique sources and equilibration histories. The oxygen isotopic composition (δ¹⁷O, δ¹⁸O) of various components of lunar materials has been studied extensively, but analyses of water in these samples are relatively sparse [1-3]. Samples collected on the lunar surface reflect not only the composition of their source reservoirs but also contributions from asteroidal and cometary impacts, interactions with solar wind and cosmic radiation, among other surface processes. Isotopic characterization of oxygen in lunar water could help resolve the major source of water in the Earth-Moon system by revealing if lunar water is primordial, asteroidal, or cometary in origin [1].

Methods: A lunar rock/soil sample is pumped to high vacuum to remove physisorbed water before heating step-wise to 50, 150, and 1000°C to extract extraterrestrial water without terrestrial contamination. The temperature at which water is evolved is proportional to the strength with which the water is bound in the sample and the relative difficulty of exchanging oxygen atoms in that water. This allows for the isolated extraction of water bound in different phases, which could have different source reservoirs and/or histories, as evidenced by the mass (in)dependence of oxygen compositions. A low blank procedure was developed to accommodate the low water content of lunar material [4].

Results: Oxygen isotopic analyses of lunar water extracted by stepwise heating lunar basalts and breccias with a range of compositions, petrologic types, and surface exposure ages will be presented. The cosmic ray exposure age of these samples varies by two orders of magnitude, and we will consider this in discussing the effects of solar wind and cosmic radiation on the oxygen isotopic composition (Δ¹⁷O). I will examine the implications of our water analyses for the composition of the oxygen-bearing reservoir from which that water formed, the effects of surface processing on the isotopic composition of that water, and potential exogenous sources of water on the Moon.

References: [1] Greenwood, J. P. et al. (2011) Nat. Geosci., 4, 79-82. [2] Saal, A. E. et al. (2008) Nat., 454, 192-U38. [3] Hauri, E. H. et al. (2011) Sci., 333, 213-215. [4] Nunn, M. H., et al. (2013) 44^th LPSC, #2768.