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

Combined laser extraction and manometric-quadrupole-mass spectroscopic measurements permit the simultaneous analysis of CO2, He and Ar in se lected fluid inclusion generations trapped in a dunite nodule from Reu nion Island. Variations in Ar-40/Ar-36 in primary fluid inclusions (10 00-8500) implies that atmospheric Ar is present in all the trapped flu ids, possibly introduced by percolation of seawater into the magma fro m which the olivine crystallised. Later inclusion generations have low er Ar-40/Ar-36 (500-3500). CO2/He-3 ratios ((1-9.6) x 10(9)) are at th e lower range of reported values from high He-3/He-4 xenoliths ((1-20) x 10(9)) but show no systematic variation with inclusion generation. He-4/Ar-4 double dagger values are constant (2.4 +/- 0.1) for all incl usion generations and over wide ranges in CO2/He ((0.2-8.2) x 10(5)). This is not consistent with the relative solubilities of CO2, He and A r in basaltic melts but implies that f(CO2) is determined by some othe r 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 ca n generate the coupled high He-3/He-4-radiogenic Sr and Pb isotopic co mpositions of Reunion lavas. The range in C/He-3 likely for the primit ive He mantle ((1-20)x 10(9)) spans the estimated C/He-3 of chondrites (approximate to 2 x 10(9)). The primitive He component cannot represe nt undegassed primitive mantle as the C/Ar-36 values of Reunion (> 1 x 10(9)) are higher than those of chondrites (8 X 10(7)).