PLATINUM-GROUP ELEMENTS IN BASALTS FROM MAUI, HAWAI'I: LOW ABUNDANCES IN ALKALI BASALTS

Abstract

Platinum-group elements (PGE) in alkali basalts from Maui, Hawai’i are as much as eight times lower than in tholeiitic basalts from Kilauea. Correlations among different PGE and between abundances of PGE and base metals are entirely different in the alkaline and tholeiitic suites. The PGE-poor nature of the alkali basalts from the Haleakala and West Maui volcanoes is not readily explained by fractional crystallization of basic magma. There is very little evidence that the alkaline magmas were subject to scavenging of PGE by sulfide-rich liquids or that PGE were preferentially partitioned into ferromagnesian silicate or oxide phases such as chromite. Partial melting of mantle source-rocks would lead to slightly lower PGE contents in alkali melts by virtue of the lower degrees of partial melting associated with the generation of alkali melts. However, this effect is very small and not likely to account for the large differences in PGE abundances in alkaline and tholeiitic melts. As most models of melt generation in the Hawaiian plume require at least two lithological components of differing isotopic composition and abundances of incompatible trace elements, and as mixing of melts derived from these different components is required to generate the Maui alkaline suites, it is possible that the low-PGE alkaline melts result from mixing of larger proportions of melts from a PGE-poor source-rock such as MORB. Although acceptable as a low-PGE source, MORB is not a satisfactory plume component according to some recent models, which argues that recycled oceanic lithosphere is the only component required within the Hawaiian plume to account for the isotopic variations observed in Hawaiian lavas, and further that ophiolite sequences provide analogues of such lithosphere. A review of PGE abundances in ophiolites indicates that significant volumes of both low- and high-PGE lithologies are present in ophiolite sequences, which leads to the conclusion that from a PGE perspective, the concept that only recycled oceanic lithosphere is required in the Hawaiian plume is satisfactory. Hence, the lower PGE content of Maui alkaline lavas is considered to reflect a low degree of partial melting with a significant contribution from low-PGE lithologies in the source region.