Geology and petrogenesis of lavas from an overlapping spreading center: 9°N east Pacific rise.

摘要

In contrast to relatively homogeneous mid-ocean ridge basalt (MORB) compositions typically erupted on fast-spreading oceanic ridges, a wide range of rock types from basalts to dacites have been recovered at overlapping spreading centers (OSC). This study focuses on the petrogenesis of lavas erupted at the 9°N OSC on the East Pacific Rise in order to better understand the complex magmatic plumbing system beneath a ridge discontinuity. Lavas that span the entire compositional range observed on the global mid-ocean ridge (MOR) system, including basalts, ferrobasalts, FeTi basalts, basaltic andesites, andesites and dacites have erupted along the eastern, propagating limb of the OSC. Major and trace element analyses, radiogenic (Pb, Sr, Nd) and oxygen isotopic ratios, volatile contents (Cl, H2O, CO2) and geochemical modeling are used to determine the petrogenesis of MORB and genetically related high-silica magmas.;The formation of high-silica dacites on MOR remains a petrologic enigma despite eruption on several different ridges. They are characterized by elevated U, Th, Zr, and Hf; relatively low Nb and Ta; and Al2O3 and K2O concentrations that are higher than expected from fractional crystallization. Additionally, high Cl and H2O concentrations and relatively low delta18O values in dacitic glasses require contamination from a seawater-altered component. Extensive petrologic modeling of MOR dacites suggests that fractional crystallization of a MORB parent combined with partial melting and assimilation of altered ocean crust can generate magmas with geochemical signatures consistent with MOR dacites. This suggests that crustal assimilation is a much more important process on ridges than previously thought and may be significant in the generation of evolved MORB in general.;Petrologic models indicate that ferrobasalts and FeTi basalts erupting at the OSC can be explained by low-pressure fractional crystallization of a primitive MORB parent; however, both fractional crystallization and magma mixing produce intermediate compositions. Geochemical analyses suggest that there are two distinct populations of andesites erupted at the OSC. Andesites with high-P2O5 are the most evolved MOR compositions produced through fractional crystallization. In contrast, low-P2O 5 andesites and basaltic andesites appear to have formed primarily through mixing of ferrobasaltic and dacitic magmas.