Spreading dynamics of an intermediate ridge: Insights from U-series disequilibria, endeavour segment, juan de fuca ridge

Abstract

We present new U-series disequilibria results for 37 basalts collected from the flanks and axial valley of the Endeavour segment of the Juan de Fuca Ridge, making Endeavour one of the most wellcharacterized spreading ridges along the global ridge system. Detailed studies of Endeavour basalts provide a geological framework for the tectonic, magmatic, and hydrothermal evolution of the ridge, and the geochemical diversity of basalts that result from short spatial- and temporal-scale variations in mantle sources and melting processes. Because the geologic and geochemical context of these basalts is well known, new uranium series disequilibria and model ages place additional constraints on the recent magmatic evolution of the ridge. While Endeavour basalts vary widely and include depleted, normal, transitional, and enriched compositions, relatively little variation is observed in Th/ U ratios or 238U-230Th disequilibria in near zero-age basalts. The largest variation in both exists in lavas erupted most recently in the axial valley (the 'Graben Trend') with overall higher 230Th excesses. Some basalts with lower 230Th excesses collected from pillow mounds on the flanks of the ridge (the 'Inflated Trend') have aged significantly since eruption and document the episodic nature of volcanic and tectonic regimes at Endeavour. Uranium-series melting models using two lithologies (peridotite and eclogite) can account for U-Th systematics in near zero-age basalts. These models indicate that mixing of diverse, deeply-generated, enriched melts and homogeneous, shallowlygenerated, depleted melts can account for the geochemistry of Endeavour basalts. In addition, decreasing porosity and melting rate of the eclogite component is primarily responsible for chemical and physical changes occurring at the Endeavour segment through time.