Controls on Explosive Eruptions along the Pacific-Antarctic Ridge

Sediment core OC170-26-159 was retrieved at 38.967°S, 111.35°W, a location that was 8-9km away from the Pacific-Antarctic Ridge (PAR) axis at the time of Glacial Termination II (T-II), 130ka, a period characterized by enhanced flux of hydrothermal metals to the near-ridge sediments on the East Pacific Rise (Lund et. al. 2016). An interval of enhanced Ti content in OC170-26-159 during T-II is rich in basaltic glass shards that we interpret to be the products of explosive submarine volcanic eruptions. Explosive eruptions of this scale are rare at mid-ocean ridges, so we studied the glass to evaluate whether sea level driven modulation in magmatic flux might be related to the frequency of such events though emplacement of distinct compositions or volatile contents. We report major element and volatile content data for the basaltic glasses and compare the results to literature data (PetDB) from on-axis sampling of the nearest ridge segment, to assess whether the glass was derived from the ridge axis and if it is unusual compared to the axial samples. Major element compositional data show that the glasses are a nearly homogenous population (MgO 5.8 to 6.5%). The heterogeneity is similar to that in single flows in Iceland (Maclennan et. al. 2003) and Hawaii (Garcia et. al. 2000), but the shards are dispersed across a gradient in δ18O, suggesting a closely spaced series of similar eruptions. The glasses are more evolved than any effusively erupted basalts on the PAR, yet are consistent with the same liquid line of descent, linking the explosive products to the axial magmatic system. The MELTS thermodynamic model allows us to calculate the changes in multiple variables along the liquid line of descent between the axial and explosive liquid compositions. Comparison of H2O and CO2 contents to those from axial flows will constrain whether variations in these components are related to eruption styles. These results will constrain the connection between sea level driven variations in magma supply rate, hydrothermal activity, thermal state of the axial magma chamber, volatile exsolution, and the potential for explosive submarine eruptions.