Highly siderophile element and 187Os isotope systematics of Hawaiian picrites: Implications for parental melt composition and source heterogeneity

Abstract

Absolute and relative abundances of the highly siderophile elements (HSE) are reported for 52 Hawaiian picrites (MgO > 13 wt.%) and 7 related tholeiitic basalts (~ 7–12 wt.% MgO) from nine volcanic centers (Mauna Kea, Mauna Loa, Hualalai, Loihi, Koolau, Kilauea, Kohala, Lanai and Molokai). The parental melts for all the volcanic centers are estimated to contain ~ 16 wt.% MgO. Samples with higher MgO contents contain accumulated olivine. Samples with lower MgO contents have lost olivine. Osmium, Ir and Ru abundances correlate positively with MgO. These elements are evidently sited in olivine and associated phases (i.e. chromite and PGE-rich trace phases) and behaved compatibly during crystal-liquid fractionation of picritic magmas. Platinum, Pd and Re show poor negative correlations with MgO. These elements behaved modestly incompatibly to modestly compatibly during crystal-liquid fractionation. Effects of crustal contamination and volatile losses on parental melt compositions were likely minor for most HSE, with the exception of Re for subaerially erupted lavas. The HSE abundances for the parental melts of each volcanic center are estimated by consideration of the intersections of HSE-MgO trends with 16 wt.% MgO. The abundances in the parental melts are generally similar for most volcanic centers: 0.5 ± 0.2 ppb Os, 0.45 ± 0.05 ppb Ir, 1.2 ± 0.2 ppb Ru, 2.3 ± 0.2 ppb Pt, and 0.35 ± 0.05 ppb Re. Samples from Loihi contain higher abundances of Pt, Pd and Re which may be a result of slightly lower degrees of partial melting. Hualalai parental melts have double the Os concentration of the other centers, the only discernable HSE concentration heterogeneity among these widely distributed volcanic centers. Two types of HSE patterns are observed among the various picrites. Type-2 patterns are characterized by greater fractionation between IPGE (Os, Ir, Ru) and PPGE (Pt, Pd), with higher Pt/Ir and Pd/Ir ratios than Type-1 patterns. Both pattern types are present in most of the volcanic centers and do not correlate with MgO content. The differences between the two patterns are attributed to the inter-relationship between partial melting and crystal-liquid fractionation processes, and likely reflect both differences in residual sulfides and the loss of chromite-associated IPGE alloys or Mss during magma ascent. The ranges in 187Os/ 188Os ratios obtained for rocks from each of these volcanic centers are in good agreement with previously published data. Variations in 187Os/ 188Os isotope ratios between volcanic centers must reflect ancient source heterogeneities. The variations in Os isotopic compositions, however, do not correlate with absolute or relative abundances of the HSE in the picrites. Minor source heterogeneities have evidently been masked by partial melting and crystal-liquid fractionation processes. There is no evidence of derivation of any picritic lavas from sources with highly heterogeneous HSE, as has been implied by recent studies purporting to explain 186Os isotopic heterogeneities.