Knowledge Bank

The East Pacific Rise (EPR) is a fast-spreading mid-ocean ridge characterized by several discontinuities including large transform faults and overlapping spreading centers. Current models for mid-ocean ridges correlate spreading rate with depth of partial crystallization of magmas; faster spreading ridges are coupled with shallower magma chambers. Anomalously high pressures and hence depths of partial crystallization have been calculated for some magmas erupted along the Northern EPR. One possible explanation is that discontinuities are associated with cooler mantle and crust, promoting thermal arrest and crystallization of magmas at high pressures. An alternative view is that high rates of magma input into crustal reservoirs promotes melting and assimilation of crustal lithologies. Assimilation of crust can lead to anomalous compositions, and calculated pressures for such compositions do not represent the actual pressure of magma evolution. Geochemical analyses of lavas erupted along the EPR between ~6 and 12°N were compiled from Gale et al. (2013) and examined to identify evidence for assimilation of oceanic crust. This ridge segment was chosen because some samples yield anomalously high pressures and hence depths of partial crystallization. The analyses were normalized to the composition of average Normal Mid-Ocean Ridge basalt (NMORB) to aid interpretation. Lavas erupted along the southern part of the ridge segment are depleted in highly incompatible elements, and are interpreted to have formed by melting of a depleted mantle source followed by crystallization en-route to the surface. Lavas erupted along the northern part of the ridge are not depleted in highly incompatible elements and formed by melting of a "normal" mantle source. However, some of these lavas have geochemical characteristics (enrichment in incompatible elements, high SiO2) consistent with assimilation of crustal lithologies. Work in progress will determine whether there is a correlation between these geochemical characteristics and high calculated pressures of partial crystallization.