Petrology and Trace Element Geochemistry of the Honolulu Volcanics, Oahu: Implications for the Oceanic Mantle below Hawaii

Abstract

The Honolulu Volcanics comprises small volume, late-stage (post-erosional) vents along rifts cutting the older massive Koolau tholeütic shield on Oahu, Hawaii. Most of these lavas and tuff of the Honolulu Volcanics have geochemical features expected of near-primary magmas derived from a peridotite source containing Fo87–89 olivine; e. g. 100 Mg/(Mg + Fe2+) >65, >250 p. p. m. Ni, and presence of ultramafic mantle xenoliths at 18 of the 37 vents. Consequently, the geochemistry of the alkali olivine basalt, basanite, nephelinite and nepheline melilitite lavas and tuff of the Honolulu Volcanics have been used to deduce the composition of their mantle source and the conditions under which they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmas were derived by partial melting of a garnet (<10 per cent) Iherzolite source, which we infer to have been carbon-bearing, from analogy with experimental results. This source was isotopically homogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980; Nd, Roden et al., 1981), and we infer that the source was compositionally uniform in all major-element oxides except TiO2, in compatible trace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatible trace elements (P, Th, La, Ce). However, the source appears to have been heterogeneous in TiO2, Zr, Hf, Nb, and Ta, elements that were not strongly incompatible during partial melting. Some nepheline melilitite samples may be derived from a source with distinct Sc and heavy-rare-earth-elements (REE) abundances, or which had a phase or phases controlling the distribution of these elements. The relatively limited abundance range for several elements, such as Ti, Zr, Nb, is partly a consequence of the low degrees of melting inferred for the series (2 per cent for nepheline melilitite, 11 per cent for alkali olivine basalt), which failed to exhaust the source in minor residual phases. We infer that these residual phases probably included phlogopite, amphibole, and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle composition and the mantle source of mid-oceanic-ridge basalt the garnet peridotite source of the Honolulu Volcanics was increasingly enriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, and Hf all <P <Nd <Sr ∼Ce <La <Nb ∼Ta. A multi-stage history for the source of the Honolulu Volcanics is required because this enrichment was superimposed on a mantle that had been previously depleted in incompatible elements, as indicated by the relatively low 87Sr/86Sr ratio, high 143Nd/144Nd ratio and low contents of K, Rb, Ba, and Th. The composition of the source of the Honolulu Volcanics differs from the source of the previously erupted tholeiitic shield. The modal mineralogy of the source of the Honolulu Volcanics is not represented in the upper-mantle xenoliths, e. g. the garnet pyroxenite and olivine-poor garnet Iherzolite included within the lavas and tuff of the unit.