HR: 1330h
AN: V42C-0371    [PDF]
TI: Platinum-Group Element Variations in Hawaiian Lavas: Constraints on the Role of Sulfides during Melt Generation and Fractional Crystallization
AU: * Lassiter, J C
EM: lassiter@mpch-mainz.mpg.de
AF: Max-Planck-Institut f. Chemie, Postfach 3060, Mainz, 55020 Germany
AB: Platinum-group elements (PGE) are highly compatible in mantle and magmatic sulfides, with sulfide melt/silicate melt partition coefficients typically on the order of 10$^{4}$ or higher. PGE abundances in basaltic melts are therefore very sensitive to the presence or absence of residual sulfides during melt generation and the fractionation of magmatic sulfides during crystallization. PGE abundances (Ir, Os, Ru, Pt, Pd) were measured in lavas from Mauna Kea and Koolau volcanoes, Hawaiian Islands to constrain the abundance of residual sulfide in the Hawaiian plume during melt generation as well as the role of sulfide fractionation during melt evolution. Iridium, Os, and Ru are positively correlated with MgO content in lavas ranging from $\sim$6-28 wt.% MgO. Bulk partition coefficients during fractional crystallization range from $\sim$4 (Ir) to $\sim$7 (Os). The compatible behavior of Ir, Os and Ru in Hawaiian melts likely reflects the high compatibility of these elements in Cr-spinel, which coprecipitates with olivine in most Hawaiian lavas. In contrast, no significant trend is observed in Pt or Pd abundances with MgO content, indicating bulk partition coefficients for these elements of $\sim$1. Pt and Pd are predicted to be incompatible in Cr-spinels, but are highly compatible in magmatic sulfides (D$_{sulfide/silicate}$ = 4.5x10$^{4}$) . The low bulk partition coefficients for Pt and Pd in the Koolau and Mauna Kea lavas indicate that sulfide segregation was insignificant during fractional crystallization, even in lavas that have experienced up to 25% olivine fractionation. Lack of sulfide saturation/segregation could reflect sulfur degassing in shallow magma chambers. However, deep submarine lavas from the HSDP-2 Mauna Kea drillcore display similar PGE trends. Therefore, it is likely that primary Hawaiian magmas (with $\sim$15-16 wt.% MgO) are at least $\sim$20-25% sulfur undersaturated when they reach crustal levels. If the source of Hawaiian lavas contains residual sulfide, primary Hawaiian melts should be sulfur-saturated at their depth of origin. However, because sulfur solubility increases with decreasing pressure, sulfur-saturated melts generated at depth become undersaturated as they rise, provided they do not reequilibrate with sulfide-bearing mantle during ascent. Sulfur undersaturation in primary Hawaiian lavas thus precludes significant interaction with the oceanic lithosphere during magma ascent. Bulk partition coefficients for the PGEs during melt generation are constrained from the PGE abundances in primitive Hawaiian lavas assuming primitive mantle abundances in the Hawaiian plume. Bulk partition coefficients during melt generation are higher than those for fractional crystallization, ranging from $\sim$3.5 (Pt) to 14 (Ir). Significantly, MgO-Ir, -Ru, and -Os trends are identical in the Koolau and Mauna Kea lavas, suggesting similar PGE and sulfide abundances in the sources of these two suites. Based on available sulfide/basalt PGE partition data, the bulk partition coefficients for melt generation are consistent with no more than $\sim$100-150 ppm residual sulfide ($\sim$30-50 ppm sulfur) in the Hawaiian plume. If primary Hawaiian melts are generated by $\sim$5% partial melting and contain $\sim$1000 ppm sulfur, this suggests a sulfur content in the plume prior to melting of $\sim$100 ppm, significantly less than the sulfur content of primitive mantle. Low sulfur abundances in the plume may reflect the presence of significant quantities of recycled oceanic crust and lithospheric mantle that have had volatile species removed during subduction-induced dehydration.
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting