The Behavior of Chalcophile and Siderophile Elements during Magmatic Differentiation as Observed in Kilauea Iki Lava Lake, Hawaii
Friday, 18 December 2015: 10:35
300 (Moscone South)
In 1959, Kilauea Iki Lava Lake formed as a single pulse of picritic lava ponded in a preexisting crater. The lava cooled and differentiated over the following decades, providing an excellent natural laboratory to study basaltic differentiation. Major element, trace element, and data for numerous isotope systems of both eruption and drill core samples have been previously published. In this study, twelve chalcophile and siderophile elements (V, Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, W, Tl, and Bi) were measured in sixteen whole rock samples using standard addition solution ICP-MS, which provides high precision data for elements that were previously undetectable. Samples ranging from 26.9 to 2.4 wt% MgO show that all of these elements display incompatible behavior throughout the lake as they increase exponentially with decreasing MgO wt%. Olivine and chromite are the only phases varying significantly in abundance in samples from 27 to 7 wt% MgO. Ferro-diabasic segregation veins and other internal differentiates (5.8 to 2.4 wt% MgO) consist of augite, plagioclase, Fe-Ti oxides, and an immiscible Cu-Fe sulfide phase. These veins are significantly more enriched in the listed elements than are the olivine basalts. Several elements (Mo, Ag, Cd, Sn, Sb, Tl) are well correlated with Cu (R2>0.84), which is assumed to be chalcophile in this system. Germanium appears to follow Ti while Ga, In, W, and Bi display incompatible behavior but don’t directly correlate with other elements. Vanadium shows overall incompatible behavior but is depleted in extremely differentiated samples, suggesting it is sequestered in a late stage fractionating phase. Molybdenum, Sb, Tl, and Sn are also very well correlated (R2>0.95) with several incompatible lithophile elements (REE, Ba, Hf, Nb, Ta, Th). This suggests their overall behavior in Kilauea Iki Lava Lake isn’t controlled by any fractionating phase, including sulfides, and they may behave in a more lithophile manner.