MAGNESIUM ISOTOPE FRACTIONATION IN LUNAR SOILS

Oxygen and silicon extracted by partial fluorination from lunar soils show large, percent-level, mass-dependent enrichments in heavy isotopes. Similar isotope fractionations occur in sulphur in grain size separates and possibly in potassium in bulk soils. In contrast calcium, extracted using similar methods, exhibits only marginal isotope effects. We have measured magnesium in water and acid leaches and in surface layers of individual soil grains. We have not been able to find any large isotope fractionation in magnesium. There is a small 0.9-1.7 parts per thousand per mass unit effect in one soil sample. The magnesium data are in agreement with the calcium data within errors and in sharp contrast to the large effects in O, Si, S, and K. The results for calcium and magnesium place severe constraints on any theory attempting to account for the large mass fractionation effects in O, Si, S, and K in lunar soils. We propose that the large effects in oxygen and silicon are probably artifacts of analytical procedures.