The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8–10°N and 12–14°N): Evidence from volatiles (H 2O, CO 2, S) and halogens (F, Cl)

Abstract

New volatile (H 2O, CO 2, S), halogen (F, Cl) and trace-element data for selected fresh MORB glasses are reported from two geologically and geophysically well-studied regions on the East Pacific Rise (8–10°N and 12–14°N) with distinct differences in spreading rate and magma supply. Sample locations include on-axis and young off-axis eruptions, as well as off-axis fissures, abyssal hills and pillow mounds. H 2O, F, S and trace-element concentrations increase with decreasing MgO content, displaying over-enriched liquid lines of descent consistent with combined fractional and in-situ crystallization. A negative correlation between CO 2/Nb and MgO indicates simultaneous degassing and magma crystallization, while broadening of this correlation to lower CO 2/Nb at constant MgO indicates shallow degassing and CO 2 loss during magma transport to the seafloor. Excess Cl concentrations and associated high Cl/Nb and Cl/K ratios of some northern EPR MORB result from variable pre-eruption contamination by high-salinity brines derived from supercritical phase separation of seawater within deeply-rooted hydrothermal circulation systems. In the faster-spreading 9–10°N and the Siqueiros Transform region magma-brine contamination is limited to off-axis samples, while on-axis samples contain no excess Cl. In the slower-spreading 12–14°N region, on-axis and off-axis samples exhibit similar extents of Cl addition and magma-brine contamination. This contamination appears to affect only Cl, and does not appear to impact concentrations of H 2O or highly incompatible trace elements such as K and Ba. Dissolved H 2O and CO 2 concentrations provide pressure estimates of eruption initiation, and indicate the majority of on-axis samples in the 9–10°N region (including Siqueiros Transform) and 12–14°N region were extracted from crustal depths equivalent to the top of the local axial magma chamber without significant degassing. Samples recovered from off-axis locations in both regions were in magma-vapour equilibrium at eruption depths or at best extracted undegassed from within layer 2B. These results indicate vertical transport of magma leading to eruption is generally faster on-axis than off-axis. Pressure estimates show that Cl addition mainly occurs within or below layer 2B, and rarely at the top of the axial magma chamber. Off-axis Cl contamination occurred similarly in both regions, independent of spreading rate. Uncontaminated samples from both regions yield similar estimates for H 2O and halogen abundances in the northern EPR mantle. Combined estimates (128 ± 5 ppm H 2O, 178 ± 1 ppm F, 1298 ± 6 ppm S, and 5.38 ± 1.1 ppm Cl) are similar to previous estimates based on volumetrically and geographically more-restricted host glass and melt inclusions from Siqueiros Transform samples.