Evaporative Fractionation of Volatile Stable Isotopes and their Bearing on the Origin of the Moon

Dr. James Day, Assistant Professor, Scripps Institution of Oceanography Geosciences Research Division

Abstract: The Moon is depleted in volatile elements relative to Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high-μ (238U/204Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile-loss through localised eruptive degassing are not consistent with the available S, Cl, Zn and K isotope and abundance data for the Moon. The most likely cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, the volatile-loss requires preferential delivery and retention of late accreted volatiles to Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately-volatile elements (e.g., Pb, Zn 5->10%) relative to highly siderophile elements (~0.5%) for Earth. Models of early magma ocean phases may be more effective in explaining the volatile-loss. Basaltic materials (e.g., eucrites, angrites) from highly-differentiated airless asteroids are volatile-depleted, like the Moon, whereas Earth and Mars have proportionally greater volatile contents. Parent body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.