Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise

Abstract

Volcanic accretion at the fast-spreading East Pacific Rise (EPR) occurs over a ~2-4 km wide neo-volcanic zone on either side of the axial summit trough (AST). Eruption ages are critical for understanding the distribution and timing of volcanic and magmatic activity. Uranium series nuclides are susceptible to fractionation by magmatic processes that occur beneath mid-ocean ridges, and the half-lives of 22 6Ra (1.6 kyrs) and 23Th (75 kyrs) make them ideally suited for determining eruption ages and placing constraints on eruption frequency and temporal changes in magma chemistry. Accordingly, major and trace element, and long-lived radiogenic and 8 U-2 Th-226Ra isotope compositions were measured in basalts from 9'-10 N EPR to determine eruption ages and to place temporal constraints on volcanic and magmatic processes. At 9030'N EPR, 238U-2 Th-2 26Ra compositions indicate that trace elementally and isotopically enriched mid-ocean ridge basalt (MORB) collected off-axis erupted >8 ka and that E-MORB magmatism is interspersed with normal, depleted MORB magmatism. Lava ages are consistent with eruption from the AST and flow down the ridge flanks, which is in contrast to previous studies that suggested E-MORB erupted from off-axis vents. At 9'50'N EPR, discrete eruptive units are distinguished by high precision 238U, 32 Th, and 226Ra sample concentrations, but because the resolution of the 2 Th-226Ra model age dating technique is ~±1 kyrs, the surprisingly young ages of these lavas prohibit the construction of an explicit, time-constrained lava stratigraphy. Nonetheless, seven different flows identified within 0.8-2.0 km west of the AST imply greater frequency of flows to these distances than previously recognized. Model age dating of ferrobasalts, basaltic andesites, andesites, and dacites sampled from the east limb of the overlapping spreading center at 9'03'N EPR is difficult due to uncertainties in magma residence times. However, (22 6Ra/23 Th) disequilibria indicate recent basaltic volcanism (<<8 ka) up to -4 km off-axis. The axial graben at the rise crest sources the most recent volcanic activity and is the dominant location for eruption of high-silica magmas. Major element, trace element, 87Sr/86Sr, and (3 4U/238U) isotope compositions are consistent with the formation of dacite magmas by extensive crystallization, and 238U-2Fh-226 Ra systematics imply crustal residence times of -8 kyrs.