C-HE-AR VARIATIONS WITHIN A DUNITE NODULE AS A FUNCTION OF FLUID INCLUSION MORPHOLOGY

Abstract

Combined laser extraction and manometric-quadrupole-mass spectroscopic measurements permit the simultaneous analysis of CO2, He and Ar in selected fluid inclusion generations trapped in a dunite nodule from Reunion Island. Variations in Ar-40/Ar-36 in primary fluid inclusions (1000-8500) implies that atmospheric Ar is present in all the trapped fluids, possibly introduced by percolation of seawater into the magma from which the olivine crystallised. Later inclusion generations have lower Ar-40/Ar-36 (500-3500). CO2/He-3 ratios ((1-9.6) x 109 are at the lower range of reported values from high He-3/He-4 xenoliths ((1-20) x 109 but show no systematic variation with inclusion generation. He-4/Ar-40* values are constant (2.4 +/- 0.1) for all inclusion generations and over wide ranges in CO2/He ((0.2 - 8.2) x 105. This is not consistent with the relative solubilities of CO2, He and Ar in basaltic melts but implies that fCO2 is determined by some other process, possibly the partitioning of CO2 into an immiscible phase. Mixing between high He-3/He-4, high He abundance mantle (primitive He mantle) with a degassed and incompatible element enriched component can generate the coupled high He-3/He-4 -- radiogenic Sr and Pb isotopic compositions of Reunion lavas. The range in C/He-3 likely for the primitive He mantle ((1 - 20) x 109 spans the estimated C/He-3 of chondrites (approx. = 2 x 109. The primitive He component cannot represent undegassed primitive mantle as the C/Ar-36 values of Reunion (greater than 1 x 109) are higher than those of chondrites (8 x 107).