Vapor Element Transport in the Lunar Crust and Implications for Lunar Ore Deposits

Introduction: In planetary environments with stable H2O in the target (Earth, Mars), large impacts will generate extensive hydrothermal systems associated with the ejecta blankets [i.e. 1,2 and references within]. Although the Moon’s crust is considered to be dry by most standards (with the possible exception of the polar deposits), there are textures preserved in the lunar sample collection that imply the mobility of elements in this type of environment. As an example, Norman [3] and Lindstrom and Salpas [4] reported the existence of troilite fracture-fillings, veins and textures suggesting the replacement of olivine and pyroxene by troilite. Haskin and Warren [5] speculated that C-O-S compounds of various forms might serve as circulating fluids in ejecta blankets, causing fractional distillation, sublimation, or extraction of individual vapormobilized elements. A detailed theoretical analysis of these lunar replacement textures by Colson [6] indicated that they may be described by the reaction: Olivine + S2(g) ⇔ Enstatite + 2FeS + O2 (g) with the oxygen fugacity (fO2) being internally buffered. In contrast to Haskin and Warren [5], Colson [6] concluded that such textures only required the mobility of S and not other chalcophile elements. This contrast in models for the mobility of chalcophile elements (and other elements such as H and C) in such an environment has significant implications for the presence or absence of “ore deposits” on the Moon. The goal of this study is to differentiate between the transport models proposed by Haskin and Warren [5] and Colson [6] and thereby gain a clearer understanding of volatile element transport in the relatively dry lunar crust. Analytical Methods: During the initial stage of this study, thin section 67016,294 was documented using backscattered electron imaging (BSE). BSE maps were used to identify individual phases for follow-on analyses and to document textural relationships between sulfides and silicates. Following phase identification, quantitative electron microprobe (EMP) point analyses were conducted on the phases of interest, using an accelerating voltage of 15 kV, a beam current of 20 nA and a ~1 μm spot size. Analyses were standardized using Taylor mineral and metal standards. A trace element analysis of sulfides via EMP was accomplished using trace element sulfide standards and analytical methodology documented by Donnelly and Brearley [7]. The sulfides were further analyzed for a suite of trace elements using a Cameca 4f ims ion microprobe (UNM). The precision of most of the trace elements was approximately 10%. Sulfur isotope analysis of the sulfides was accomplished with a Cameca nanoSIMS 50L at the California Institute of Technology. The precision on δS measurements is approximately 0.8 per mil for 2-5 μm sulfide grains.