Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise
2013
Waters, Christopher L. | Sims, Kenneth W.W. | Klein, Emily M. | White, Scott M. | Reagan, Mark K. | Girard, Guillaume
No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of ²³⁸U–²³⁰Th–²²⁶Ra–²¹⁰Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report ²¹⁰Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of ²²²Rn from ²²⁶Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies.
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