A noble gas profile across a Hawaiian mantle xenolith: Coexisting accidental and cognate noble gases derived from the lithospheric and asthenospheric mantle beneath Oahu

Abstract

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 absolute and relative noble gas concentrations are large and comparable to those observed between individual Salt Lake Crater pyroxenite xenoliths. 3He/4He varies from 7.7 to 9.4 times the atmospheric value (Ra) and correlates inversely with 40Ar/36Ar, which ranges between 4100 and 9700. Neon krypton and xenon isotopes are uniform and indistinguishable from air, with the exception of excess 129Xe. Overall, the observed noble gas compositions reflect a derivation from depleted MORB-type mantle sources.The spatial distribution of noble gas signatures within the xenolith and the observed correlation between helium and argon isotopes suggest the presence of two different noble gas components which are trapped in different phases and are unevenly distributed within the xenolith. 40Ar/36Ar and 1/36Ar correlate inversely, indicating that atmospheric contamination is insignificant. Hence, the observed isotopic variations reflect mixing of two mantle-derived noble gas components. Correlations between HeAr isotopes and CO2/H2O in different pyroxenites from Salt Lake Crater, including our sample, reveal that the first component is characterized by highly radiogenic helium and argon isotopes and related to abundant secondary CO2-rich fluid inclusions. 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. This interpretation is supported by the low entrapment depths of fluid inclusions in Salt Lake Crater pyroxenites (<30 km; Murck et al., 1978). This implies that the fluids are genetically related to the host magma 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 proposed that this component resides inside the mineral lattices and was trapped during magmatic crystallization.Our results indicate that the depleted lithospheric mantle source of the post-erosional host magma (Honolulu Volcanic Series) is characterized by 40Ar/36Ar ≥ 10000, R/Ra ≤ 7.5, and excesses in129Xe. 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 MORB-type mantle, but became slightly modified by noble gases and other incompatible trace elements derived from the Hawaiian mantle plume prior to partial melting.This study provides evidence that noble gas isotopes and solid radiogenic isotopes are coupled in the mantle sources of basalts (e.g., Allègre et al., 1983), but decoupled in xenoliths (e.g., Vance et al., 1989).