Geochemistry of the 2005-2006 East Pacific Rise Eruption: Chemical Changes and Possible Petrogenetic Relationships to the 1991-1992 Eruption

The first opportunity to study a well-documented temporal sequence of eruptions at one mid-ocean ridge (MOR) locality presented itself when a second eruption was discovered in 2006 on the East Pacific Rise (EPR) around 9 °50 'N, 15 years after an eruption was witnessed in 1991-1992. Major and trace element analyses of glass samples from the 2005-2006 eruption indicate that the flows are largely aphyric, relatively homogeneous N-type MORB but exhibit some geochemical variability outside of analytical uncertainties. Relative to the more than 1200 basalts that have been analyzed from this section of the Integrated Study Site (ISS), samples from the 2005-06 eruption are moderately evolved (7.5 to 8.3 weight percent MgO), and are more evolved than the basalts erupted in 1991-92 (8.3 to 8.9 weight percent MgO). However, both the 2005- 06 and 1991-92 eruptions appear to have originated from the same incompatible element-depleted mantle source. Modeling of the observed chemical variability suggests the 2005-06 flows could be related to the most primitive 1991-92 lava by up to ~10 percent low-pressure fractional crystallization. The compositions of the new flows fall along a liquid line of descent predicted by the crystallization of olivine and plagioclase in ratios that range, respectively, from approximately 6:1 to 1:2, and at crystallization temperatures that range from approximately 1200 °C to 1140 °C. Some gabbroic crystal clots enclosed in the young lavas support this model but others suggest complex crystal-melt interactions in the subaxial magma chamber. Additionally, chemical and spatial data from the 2005-06 eruption may indicate chemical differentiation of the lava along axis during the eruptive process. Although the data are consistent with small amounts of crystal fractionation of the 1991-92 parent over the past 15 years, it is possible the elemental trends were created by mixing between the parental melt and a more evolved magma from another part of the axial magma chamber.