Datensatz

Zusammenfassung

The rejuvenated Honolulu Volcanics (HV), Oahu, Hawaii, comprise small-volume magmas ranging from alkali basalt through basanite and nephelinite to melilite. Their origin is enigmatic and debated, because the magmas are highly enriched in incompatible trace elements but isotopically depleted and erupted some 1.3 Myrs after the Koolau shield building phase had ceased. We have analysed 26 HV basanitic to melilitic tuffs from Salt Lake Crater at Honolulu, Oahu (SLC). While trace element patterns of most samples overlap with those of magmas from other HV localities, four melilitites display 2 to 4-times higher concentrations for “carbonatite-affine” elements such like P, Mn, REE, Sr, Ba. These samples also show carbonatite-like supra-chondritic Nb/Ta (up to 40) and Zr/Hf (up to 60) ratios, suggesting the presence of a carbonatitic component, which is also supported both by modal mineralogy and mineral chemistry. For example, magnetite compositions differ significantly from those of mantle peridotites and basaltic magmas, varying between oceanic carbonatites and SLC Hawaiian plume xenoliths. In addition, many such xenoliths bear multiple evidence of carbonatitic (as well as silicate) metasomatism, e.g. aragonite, diamond, graphite (Wirth & Rocholl, 2003). The carbonatitic agents must therefore be derived from the Hawaiian plume itself, thus documenting auto-metasomatic processes within the rising plume. Because the formation of melilitite requires a carbonated peridotite mantle (Brey 1978), the SLC melts probably derived from such auto-metasomatised parts of the plume, with a possible derivation of the carbonatite agents from carbonated eclogite (Yaxley & Brey 2004). This model is consistent with a HV-source being intrinsic to the Hawaiian plume (Fekiacova et al., 2007), Sr-Nd-Hf systematics in SLC magmas and xenoliths (Bizimis et al. 2005) and very similar to a model proposed by Dixon et al. (2008) for the origin of Niihau rejuvenated lavas.