President Jackson Seamounts, northern Gorda Ridge: Tectonomagmatic relationship between on‐ and off‐axis volcanism

Abstract

Basaltic lavas and hyaloclastites dredged from the President Jackson Seamounts, a 65‐km‐long, linear volcanic chain starting near latitude 42°20′N west of the northern Gorda Ridge, are normal mid‐ocean ridge basalt (MORB), with compositions ranging from highly depleted (0.07% K2O, 0.54 La/SmN) to moderately enriched in incompatible elements (0.24% K2O, 0.86 La/SmN). The seamount lavas are similar in many respects to those erupted at the adjacent ridge but have some important geochemical differences. Seamount lavas are concentrated toward primitive compositions with higher MgO contents (to 9.3%, Mg number 69.5) than most of the nearby ridge lavas. They also have systematically lower TiO2 and FeO and higher CaO, Na2O, and Sr at comparable MgO content than adjacent ridge basalts. Seamount basalts contain phenocrysts in equilibrium with the melt and lack compositionally diverse glass inclusions and compositional zoning common in phenociysts of most ridge basalt. Minor and trace element variability at a given MgO content indicates complex petrogenetic processes. Radiogenic isotopic ratios indicate somewhat less depleted sources than for basalt from the adjacent ridge axis. The higher Pb isotopic ratios of some lavas trend toward enriched MORB. The seamounts typically have multiple, nested calderas or pit craters, stepping downward toward the ridge axis, indicating formation in the active, near‐ridge, extensional environment. The predominantly primitive nature of the lavas suggests that they pass through crustal magma reservoirs, presumably underlying the calderas, very rapidly. The lack of evidence for magma mixing suggests that batches of magma are delivered to the seamounts episodically and either solidify or are drained into ridge‐parallel faults before the next batch arrives. In contrast, lavas from the ridge axis show evidence for magma mixing involving more fractionated melts that show evidence for clinopyroxene fractionation. Despite a lack of seismic evidence for magma chambers under slow spreading centers, continuous melt zones must be present under the Gorda Ridge axis to give the ubiquitous imprint of magma mixing.