CONSTRAINTS ON RARE-GAS PARTITION-COEFFICIENTS FROM ANALYSIS OF OLIVINE GLASS FROM A PICRITIC MIDOCEAN RIDGE BASALT

Knowledge of the rare gas distribution between crystal and silicate melt is a kev requisite when modelling the evolution of the Earth-atmosphere system. The concentrations and isotope compositions of He and Ar in glass and coexisting olivine in chemical equilibrium of a picritic MORB basalt from the Mid-Atlantic Ridge at 38-degrees-N have been measured. In an attempt to resolve components trapped in fluid inclusions from components dissolved in the glass or the olivine phases, gases were extracted from both phases by vacuum crushing and fusion. Since (1) olivine also contains glass inclusions and (2) the glass may be partially degassed, only upper limits of the partition coefficients between olivine and melt can be proposed. These upper limits (0.008 for He and 0.003 for Ar) indicate that rare gases behave as incompatible elements during magmatic processes. This validates assumptions about the efficiency of rare gas degassing from the solid Earth.