Rates of melt lens replenishment at the East Pacific Rise, 9º50’N

Axial melt lenses (AMLs) are key features of fast and magmatically-robust spreading ridges. These sill-shaped bodies typically sit atop a lower crustal mush zone, and supply magma that gets intruded in the brittle axial lithosphere as dikes, or emplaced at the seafloor as lava flows. The replenishment rate of these shallow reservoirs is thus a critical control on the modes of crustal accretion, the timing of mid-ocean ridge eruptions, and the thermo-chemical output of hydrothermal convection, but remains scarcely documented.

Here we revisit estimates of magmatic inflation rates at the East Pacific Rise, 9º50’N based on measurements of vertical seafloor displacements carried out by Nooner et al. (2014). These measurements revealed seafloor uplift rates as fast as ~7 cm/yr above the AML, decaying over ~10 km in the cross-axis direction, between 2009 and 2011. We model this uplift profile as resulting from the inflation of a 1.5 km-deep, 1-km wide AML in a visco-elastic half-space that includes a viscous mush zone of uniform viscosity.

Our models reveal a tradeoff between the assumed viscosity of the mush zone and the sill inflation rate that is necessary to explain the observed seafloor uplift. Specifically, if we assume a strong mush (viscosity > 10¹⁸ Pa.s), the replenishment rate must be ~200 m³/yr per meter along axis. On the other hand, a weaker mush (viscosity < 10¹⁶ Pa.s) significantly damps the surface expression of sill inflation, requiring a replenishment rate of ~470 m³/yr/m to match the observations. Further constraints on AML replenishment rates can be obtained by assuming the associated heat flux sustains on-axis hydrothermal venting near 9º50’N (~100 MW). We also find that rapid AML deflation during an eruption can induce a characteristic deformation transient lasting up to a few years, which is akin to the post-seismic phase of the seismic cycle. Depending on the assumed viscosity of the mush zone, this post-eruption signal has the potential to bias estimates of steady AML replenishment rates.

Regardless of the assumed mush viscosity, our modeling yields replenishment rates comparable to the long-term crustal accretion rate (~600 m³/yr/m). This suggests that magmatic inflation is not an unusual event at a fast-spreading ridge like the East-Pacific Rise. By estimating the fraction of the ridge’s magma supply that transits through the AML, our results may also provide new constraints on the modes of accretion of the oceanic lower crust, i.e., help discriminate between the gabbro glacier and multiple-sills endmember models.