The Search for Water in Igneous Lunar Materials: a Re-assessment of the Lunar Magmatic Volatile Budget

Introduction: Magmatic volatiles such as H 2 O, F, Cl, CO 2 , and S play a critical role in physiochemical processes that control thermal stabilities of minerals and melts, magma eruptive processes, and transport of economically important metals. Of these volatiles, water is the dominant magmatic volatile in terrestrial magmas both because of its abundance and its strong effect on physical properties. From samples collected thus far, it has become widely accepted that lunar magmas were depleted in magmatic volatiles, particularly dissolved water. There are two primary lines of reasoning behind the general acceptance of lunar mag-mas as nominally anhydrous. One is based upon micro-FTIR analyses of lunar volcanic glasses. These yielded H 2 O and OH-species concentrations below detection limits [1]. The other is linked to the currently accepted model for lunar formation, which is that of a giant impact [2]. The high-temperatures associated with this event are theorized to cause depletion in volatile elements and compounds (including H 2 O) [3, 4]. This is consistent with analysis of lunar materials for relatively volatile elements such as Na and K, which are both depleted [3, 4]. However, neither the direct evidence for dry glasses nor giant-impact volatile depletion can completely rule out the possibility that lunar magmas did contain dissolved water. Lunar Volcanic Glasses: Low abundances of measured OH-and H 2 O were obtained for lunar fire-fountain glasses. This, however, does not necessarily constitute proof that the magmas from which these glasses quenched did not contain magmatic water at any stage of their evolutionary history. Volatile contents of quenched glasses only reflect magmatic volatile budgets if quenching is rapid enough to minimize degassing effects. However, most models of lunar fire-fountain eruptions predict a fumarolic gas cloud that allows sufficient time for degassing and volatile diffusion [1, 5]. It has been suggested by [5] that the fu-marolic gas cloud associated with lunar fire-fountain eruptions contained at least some water, possibly in the same abundance as Cl-, in order to facilitate precipitation of the observed Zn-sulfides on the glass surfaces. However, degassing likely begins upon ascent through the lunar crust at more elevated pressure. By the time that the very low pressure of the surface is reached, most magmas should be nearly anhydrous because water is nearly insoluble in silicate liquids at lunar surface pressures [6], and any direct information on the water contents at the higher pressure stages would …