U-Th-Ra disequilibria and the extent of off-axis volcanism across the East Pacific Rise at 9°30′N, 10°30′N, and 11°20′N

Abstract

There is widespread interest in the distance that mid-ocean ridge magmatism extends beyond the neovolcanic zone. Off-axis magmas also provide a means to map out variations across the melting zone. We present 238U-230Th-226Ra data for 35 well-characterized samples that extend up to 50 km away from the ridge axis across the East Pacific Rise at 9°30′N, 10°30′N, and 11°20′N. The (230Th/238U) ratios range from 1.00 to 1.19, and the (226Ra/230Th) ratios range from 1 to 2.78. The samples have a bimodal (230Th/238U) distribution with approximately half overlying published axial data on the U-Th diagram and the remainder lying close to the equiline. The U series disequilibria in the majority of the samples can be explained by aging subsequent to eruption in a zone ∼8 km wide about the neovolcanic zone, consistent with visual evidence for sample age. Nevertheless, seven of the samples lie above calculated (230Th/238U) axial decay curves and/or have 226Ra excesses implying eruption tens of kilometers off axis. These are consistent with evidence from seamounts and seismic interpretations that magmatism can extend up to 20 km off axis. The implication is that magma is not as efficiently focused beneath the ridge axis as has generally been believed. There is a decrease in initial (230Th/238U) in both these and published samples inferred to have formed off axis, but there is no compelling evidence that this reflects source heterogeneity. Simple modeling suggests that this could be explained by a decrease in fertility and melt column length as the overlying lithosphere thickens with age and the solidus shallows.