Combined noble gas and (sup 40)Ar-(sup 39)Ar study of Salt Lake Crater xenolith SL322 from Oahu, Hawaii

摘要

The microdistribution of noble gases in a garnet pyroxenite nodule from Salt Lake Crater (SLC), Oahu, Hawaii, was investigated by a detailed step- heating and -crushing analysis and a (sup 40)Ar-(sup 39)Ar-study. A noble gas component with MORB type argon, helium and neon resides in CO(sub 2)-rich fluid inclusions trapped in <30 km depth. This component was most probably derived from the nephelinitic SLC host magma and confirms the dominance of MORB type noble gases in the late post-erosional magmatic stages of Hawaiian volcanism, as suggested previsouly (Kurz et al., 1983; Valbracht et al., 1996). A second previously detected (Rocholl et al., 1996) low (sup 40)Ar/(sup 36)Ar ((proportional-to)5000) component turned out to be associated with two different reservoirs. The larger reservoir is most probably related to garnet, the other one is associated with low retentive sites containing few K and Cl and could not yet be adequately identified. The low (sup 40)Ar/(sup 36)Ar ((proportional-to)5000) component hosted by garnet can be interpreted as a mixture of MORB and plume type noble gas components with specific (sup 4) He/(sup 40)Ar ratios. The results demonstrate the complexity of the microdistribution of noble gases in ultramafic nodules and allow insight into plume induced metasomatism of the Hawaiian lithosphere. (orig.)