Comparison of Volatile and Major Element Concentrations in Melt Inclusions from Juan de Fuca Ridge Seamounts and the Adjacent Ridge Axis
Abstract:Here we present volatile (CO2, H2O, F, S, Cl) and major element data from >200 naturally glassy, olivine-hosted melt inclusions and glasses erupted from five seamounts proximal to the Juan de Fuca Ridge. This includes 90 analyses of melt inclusions from the Vance Seamount chain and 126 analyses from two small, mafic (glass MgO > 9 wt%) near-axis cones (T461 and T881). We provide geochemical constraints on both the compositional variations and the depths of crystallization beneath the seamounts using vapor-saturation pressures derived from CO2-H2O concentrations. These data suggest crystallization occurs beneath the two near-axis cones from seafloor pressures to 6400 bars corresponding to depths up to 9 km below the seafloor. This range of crystallization is similar to that calculated from olivine-hosted melt inclusions from the adjacent Juan de Fuca Ridge axis. By contrast, crystallization pressures from Vance Seamounts are more limited with pressures ranging from 400 to 1300 bars or depths of 0.7 to 3.8 km below seafloor. The Vance Seamounts have a prominent peak in depths of crystallization at ~2-3 km below seafloor in histograms, perhaps suggesting that it is the preferred depth of melt storage and crystallization beneath seamounts chains. By contrast, crystallization peaks beneath the small, near-axis cones are less prominent and occur slightly deeper at 3–4 km below seafloor.
Overall, S and F concentrations in the seamount melt inclusions are similar to on-axis inclusions; however, Cl concentrations in the seamounts are remarkably low. On-axis inclusions have an average of 61 ppm Cl and range from 4–163 ppm. By contrast, Cl concentrations in the seamounts range from 3–82 ppm, but have an average of only 11 ppm. Excess Cl in mid-ocean ridge lavas is often attributed to contamination by seawater or brines associated with hydrothermal circulation. We suggest that the low Cl concentrations in the seamount inclusions may result from either a seamount mantle source that is depleted in Cl compared to the on-axis source (perhaps from a previous melting event on-axis) or it may indicate that the off-axis melt inclusions preserve the true upper mantle compositions, while all on-axis lavas are contaminated by interaction with the pervasive hydrothermal system or previously altered crust.