A NOBLE-GAS PROFILE ACROSS A HAWAIIAN MANTLE XENOLITH - COEXISTING ACCIDENTAL AND COGNATE NOBLE-GASES DERIVED FROM THE LITHOSPHERIC AND ASTHENOSPHERIC MANTLE BENEATH OAHU

A noble gas profile across a garnet pyroxenite xenolith from Salt Lake Crater, Oahu, Hawaii, provides information about the scale and origin of noble gas heterogeneities within such rocks. Variations in both ab solute and relative noble gas concentrations are large and comparable to those observed between individual Salt Lake Crater pyroxenite xenol iths. He-3/He-4 varies from 7.7 to 9.4 times the atmospheric Value (Ra ) and correlates inversely with Ar-40/Ar-36, which ranges between 4100 and 9700. Neon krypton and xenon isotopes are uniform and indistingui shable from air, with the exception of excess Xe-129. Overall, the obs erved noble gas compositions reflect a derivation from depleted MORB-t ype mantle sources. The spatial distribution of noble gas signatures w ithin the xenolith and the observed correlation between helium and arg on isotopes suggest the presence of two different noble gas components which are trapped in different phases and are unevenly distributed wi thin the xenolith. Ar-40/Ar-36 and 1/Ar-36 correlate inversely, indica ting that atmospheric contamination is insignificant. Hence, the obser ved isotopic variations reflect mixing of two mantle-derived noble gas components. Correlations between He-Ar isotopes and CO2/H2O in differ ent pyroxenites from Salt Lake Crater, including our sample, reveal th at the first component is characterized by highly radiogenic helium an d argon isotopes and related to abundant secondary CO2-rich fluid incl usions. Given the high diffusivity of He at mantle temperatures (Hart, 1984), the observed helium isotope heterogeneities on a sub-mm scale require that the fluids were introduced concurrently with eruption. Th is interpretation is supported by the low entrapment depths of fluid i nclusions in Salt Lake Crater pyroxenites (<30 km; Murck et al., 1978) . This implies that the fluids are genetically related to the host mag ma itself and reflect its composition. The second noble gas component is interpreted as being magmatic, i.e., cognate to the basaltic magma from which the pyroxenite precipitated within the mantle. It is propos ed that this component resides inside the mineral lattices and was tra pped during magmatic crystallization. Our results indicate that the de pleted lithospheric mantle source of the post-erosional host magma (Ho nolulu Volcanic Series) is characterized by Ar-40/Ar-36 greater than o r equal to 10000, R/Ra less than or equal to 7.5, and excesses in Xe-1 29. Noble gas signatures of the asthenospheric mantle source parental to the pyroxenite-producing magma are less radiogenic with respect to these isotopes. Combining noble gas and strontium, neodymium, and lead isotope evidence, we propose that the asthenosphere beneath Oahu was originally similar to depleted MORE-type mantle, but became slightly m odified by noble gases and other incompatible trace elements derived f rom the Hawaiian mantle plume prior to partial melting. This study pro vides evidence that noble gas isotopes and solid radiogenic isotopes a re coupled in the mantle sources of basalts (e.g., Allegre et al., 198 3), but decoupled in xenoliths (e.g., Vance et al., 1989).