Multiple Sills Tapped by a Series of Explosive Eruptions from the East Pacific Rise

Incorporation of volatiles into mid-ocean ridge basalts has a significant effect on the compositional evolution and eruption style of these melts, which can result in the dispersion of glass by explosive eruptions. To investigate these relationships, we have analyzed a series of basaltic glass shards distributed within a 20 cm interval of sediment core Y71-07-53, sampled from 35 km west of the axis of the East Pacific Rise (EPR) at 11°S. At the time of eruption (348-343 ka, slightly preceding glacial termination IV), the core location was 8.7-7.6 km from the axis of the EPR. Therefore, we posit that a series of strombolian eruptions ejected the glass shards to such a distance. The drop in sea level associated with the preceding glacial maximum likely enabled this atypical eruption style. We report major element, trace element, and volatile concentrations of the glass shards and compare the results to published compositions of lavas from the nearest EPR ridge axis segment (PetDB) to examine the factors contributing to eruption explosivity.

The range of MgO contents in the glass shards (5-7 wt%) overlaps with but also extends to significantly more evolved values than the range of axial compositions (6-8.3 wt% MgO). The glass shards define three compositional groups: 1) a high-MgO, low-Al group, 2) a high-MgO, high-Al group, and 3) a low MgO group. Using the MELTS thermodynamic model, we show that these groups represent two distinct fractional crystallization liquid lines of descent that differ in the H₂O content of their primitive melts. Because the two parental melts have very similar anhydrous compositions, we propose that they both originated from a typical primary mid-ocean ridge basalt. However, the higher-H₂O melt assimilated more hydrothermally altered crust prior to its later stages of fractionation. To explain their coexistence in the erupted suite, we propose that these melts remain in isolated, stratified sills during storage and fractionation in the axial magmatic system, such that a given sill might be tapped by a given dike propagation event and be the source of a particular eruption. Falling sea level results in increased magma and heat flux to the ridge axis, which may reactivate fractionated, high-viscosity, volatile-bearing magma stored in the crust, thus contributing to the unusual eruption style of these lavas.