Correction for volatile fractionation in ascending magmas: Noble gas abundances in primary mantle melts

摘要

Accurate relative noble gas abundances of mantle-derived melts are required in order to further understand the distribution of noble gases in the mantle and fractionation of noble gases during the melting process. Noble gas relative abundances in the majority of oceanic basalts are highly fractionated, at least in part due to late stage, solubility controlled fractionation. Noble gas concentrations in the volatile phase (≡ noble gas:CO_2 ratio) will vary systematically during solubility controlled degassing of a magma. This contribution models the noble gas concentrations in the volatile phase during degassing at different pressures and vesicularities in order to develop a method for correcting fractionation resulting from magmatic degassing, and thereby estimate the "initial" (pre-degassing) noble gas compositions. Correcting for fractionation during magmatic degassing requires: a) a method for determining the volatile fractionation trajectory during degassing; and b) one well constrained mantle volatile composition with which to "fix" the extrapolation. The trajectory of volatile fractionation can be estimated by sequential crushing of basaltic glasses. Vesicles grow during ascent, therefore large vesicles trap early (less fractionated) volatiles while small vesicles trap late (fractionated) volatiles. Sequential crushing of basaltic glasses releases volatiles from progressively smaller vesicles, thereby allowing the fractionation trajectory resulting from degassing to be determined on individual samples. The production rate of both ~(21)Ne and ~4He in the mantle is a function of U concentration only, resulting in a constant ~(21)Ne/~4He production ratio in the mantle which can be used to "fix" the degassing fractionation trajectory determined by sequential crushing. This correction then allows fractionation of ~4He from ~(40)Ar prior to degassing to be assessed. This method is illustrated using multiple crushes of a single basaltic glass from the mid-Atlantic Ridge that shows that ~4He appears to have been fractionated from ~(40)Ar before degassing.