Understanding sources and timescales of melt generation in continental rear arcs using high-precision measurements of uranium and thorium isotopes in lavas from Reventador volcano, Ecuador.

摘要

Although analysis of U-series isotopes in arc magmas have revolutionized our understanding of magmatic processes and their timescales, no U-series studies have been undertaken in continental rear arc volcanoes. I present high-precision measurements of 238U, 234U, 230Th and 232Th, measurements of 226Ra, and major and trace element data measured in lavas collected in the Reventador volcanic center, Ecuador. Of the 16 samples analyzed for U-series data, I find that 10 samples have measured values of (230Th/ 238U) > 1, 3 samples are within measured error of (230Th/ 238U) ∼ 1, and 3 samples have measured values of (230Th/ 238U) < 1. Reventador lavas also show enrichments in (226 Ra/230Th) values in 9 of the 16 samples, while the remaining 7 samples are within error of equilibrium. The thorium excesses dominant in this rear-arc volcano are likely the product of partial melting of variably slab fluid metasomatized mantle leaving a garnet residual. This suggests that melting occurs at great depths in the Ecuadorian rear arc system. Radium enrichments may have been added through slab fluid addition and metasomatization of mantle melts or during ascent of magmas. These values may have been depressed by shallow magmatic fractionation, placing the timescales of magma/fluid ascent 75 km depth, as required by the mantle garnet signature. Measurements of (230Th/238U) > 1 in samples of the volcanic edifice that the active cone resides suggests that this structure is younger than previously thought (<127 ka) This implies that process of volcanic cone construction and destruction occurred twice in the last 127 ka.