Excesses of seawater-derived 234U in volcanic glasses from Loihi Seamount due to crustal contamination

Abstract

The effects of crustal contamination on the chemistry of oceanic basalts are commonly assumed to be negligible due to the compositional similarity between the erupted basalt and the underlying oceanic crust or volcanic edifice. Here we evaluate this assumption with high-precision measurements of the 234U– 238U and 230Th– 238U disequilibria, Cl/K 2O ratios, and 87Sr/ 86Sr ratios of volcanic glasses from Loihi Seamount, a submarine Hawaiian volcano with an active hydrothermal system. The samples, including one from the volcano's 1996 eruption, have small to moderate amounts of excess 234U (~ 0.2–1.0%) and variable, elevated Cl/K 2O ratios. These excesses of 234U and enrichments in Cl are thought to result from contamination with seawater-derived U and Cl, but neither of these signatures can be explained by syn- or post-eruptive interaction between lava and seawater. Instead, mantle-derived magmas at Loihi appear to be variably contaminated with two distinct crustal materials: hydrothermal brines (which create enrichments in Cl) and U-enriched hydrothermally altered rocks (which create excesses of 234U). Both of these materials are expected to be found within the volcanic edifice as complementary parts of the volcano's hydrothermal system. The Loihi glasses display a wide measured range in the amount of excess 230Th from ~1 to 7% (due to the addition of seawater-derived U) that overlaps with lavas from Kilauea Volcano (~ 2% excess 230Th). We correct the 230Th– 238U disequilibria of the Loihi glasses back to their original pre-contamination values using their 234U– 238U disequilibria and a simple mass-balance calculation. This correction suggests that mantle-derived magmas at Loihi have a narrow range of ~ 6–9% excess 230Th, which is significantly larger than observed for lavas from the neighboring volcano, Kilauea. This difference is consistent with the idea that Loihi is tapping mantle that is upwelling slowly (~ 5–6 cm/yr) on the margin of the Hawaiian plume.