Characteristics And Implications For Magmatism Of The Morbs In The East Pacific Rise 13°N

The East Pacific Rise (EPR) is located to the east of pacific and is not far from the western coast of America. It is a typical fast spreading ridge, which has produced the major component of the upper part of the Pacific ocean crust. The study of petrology and geochemistry of MORBs from the EPR is critical for understanding the magmatic process and the dynamics for generating new oceanic crust. In this study, we analyzed the mineralogy, whole-rock major and trace elements, U-series isotopic compositions and melt inclusions hosted in olivine- and plagioclase-phenocrysts in MORBs from the EPR 13°N. This study aims to investigate the melting behavior of upper mantle and magma evolution process in fast spreading ridge, and to reveal the control of melting dynamics on chemical and isotopic compositions of basaltic magma.The mineralogical characters and major element compositions show that MORB magmas in this study area have experienced extensive low-pressure crystallization of olivine plus plagioclase, whereas clinopyroxene has not taken part in crystallization. The calculation using COMAGMAT resulted in that these magmas had fractionated clinopyroxene under pressure of 4-8 kbar. The results of analyses for sixty-five initial melt inclusions show that their host magmas have experienced high pressure fractionation of clinopyroxene. The MORB magmas have experienced fractionation not only in the ocean crust but also beneath the ocean crust, and the MORB magmatism is not an ideal adiabatic process. The variations of Ni in these samples can be addressed by multiple magma mixing processes in the magma chamber. High Mg# olivine hosts early-stage melt– melt inclusion, and the study based on these melt inclusions shows that magmas beneath the EPR have experienced mixing with magmas from various melting depths and degrees.The comparison of U-series data between MORBs and OIBs show that the excess 230Th are dominated by mantle melting conditions. We have proposed a density-control model to address the characteristics of U-series isotopic compositions. In this model, the deep-sourced melt has higher (230Th/238U), Fe8, La/Sm and density than that from shallow depth. The high density of the deep-sourced melt leads to its weak buoyancy, thus would tend to reside in the magma chamber for long time; whereas the shallow-sourced melt has a low density and intensive buoyancy, and would have short residence time.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. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from ~80 mm/yr in the north (16°N) to ~160 mm/yr in the south (19°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.