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

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