Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts: Constraints on Magma Supply and Thermal Structure Beneath Mid-ocean Ridge

Major elements of 2202 basalts from the East Pacific Rise (EPR) and 888 basalts from near-EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%<8.0wt % shows that magma crystallization pressures beneath near-EPR seamounts are positively and negatively correlated with Na8 and Fe8, respectively. However, these correlations are indistinct in axial lavas, which can be explained by chemical homogenization induced by extensive mixing processes. In each segment divided by major transforms and over-lapping spreading centers (OSCs), near-EPR seamount lavas have higher magma crystallization pressures, higher Fe8 and lower Na8 than the EPR lavas, which indicate cooler lithosphere, lower degrees and shallower melting depths beneath near-EPR seamounts than the EPR. The correlations between magma crystallization pressures and melting conditions beneath near-EPR seamounts imply that the source thermal state controls the melting degree and melt flux, and then melting process controls the shallow lithosphere temperature and magma crystallization depth (pressure). The cooler mantle sources beneath near-EPR seamounts produce a lower degree of melting and a less robust magma supply, which results in a deep thermal equilibrium level and high magma crystallization pressure. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from similar to 80 mm/year in the north (16 degrees N) to similar to 160 mm/year in the south (19 degrees S), while this trend is unobvious in near-EPR seamounts. This suggests that the magma supply controlled by spreading rate dominates the ridge crust temperature and magma crystallization depth, while the near-EPR seamount magma supply is not dominated by the axial spreading rate. Because most seamounts form and gain most of their volume within a narrow zone of 515 km from ridge axis, they provide good constraint on magma supply and thermal structure beneath the EPR. High magma crystallization pressures in seamounts indicate dramatic temperature decrease from the EPR. The crystallization pressures of seamount lavas are well correlated with mantle melting parameters but in a blurry relationship with axial spreading rate. Despite the adjacency of the EPR and nearby seamounts, the thermal structure beneath the near-EPR seamounts are controlled by their own magma supply and conductive cooling, chemically and thermally unaffected by magmatism beneath the ridge axis.