The down-hole magmatic-metamorphic evolution in basalts and gabbros monitored by Fe-Ti oxides: A complete section of Superfast Spreading Crust at IODP Site 1256D

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The IODP multi-cruise mission Superfast Spreading Crust drilled successfully a complete section of the upper oceanic crust into the underlying gabbros (Site 1256D; eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). The recovered rocks, now representing the first reference profile through fast-spreading upper oceanic crust, reveal a complex interaction between magmatic and metamorphic processes: Primary crystallization; low- and high-temperature alteration; contact-metamorphism; partial melting/assimilation; magma mixing. The petrographic record of the whole section reveals that all processes involve the formation of, or the reaction with, Fe-Ti oxides, which can consequently be used as suitable proxies for monitoring the different stages in the magmatic-metamorphic evolution of fast-spreading oceanic crust. Therefore, we plan a detailed microanalytical study on the Fe-Ti oxides in 1256D crustal section. Electron microprobe data should be applied to an improved Fe-Ti oxide thermooxybarometer in order to evaluate the temperature-oxygen fugacity history. In-situ analysis of iron isotopes should improve our understanding of the interplay between hydrothermal circulation and primary magmatic processes. To constrain the conditions prevailing in the conductive boundary layer between the magma chambers and the axial high-temperature hydrothermal systems, an experimental study using very oxide-rich (Fe-Ti-rich) gabbros as starting material is planned to determine temperatures, redox conditions and volatile activities prevailing during crystallization. Such evolved compositions are interpreted as representing a key lithology at the transition between magmatic and hydrothermal systems.