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

IODP多巡航任务的超快扩张地壳成功地将上洋壳的一整段钻入下面的辉长岩(1256D站；东赤道太平洋；15 Ma地壳形成于东太平洋隆起)。回收的岩石现在是通过快速扩张的洋壳的第一个参考剖面，揭示了岩浆和变质过程之间的复杂相互作用：原生结晶；低温和高温蚀变；接触变质作用；部分熔融/同化；岩浆混合。整个剖面的岩石学记录表明，所有过程都涉及Fe-Ti氧化物的形成或与其反应，因此可以作为监测快速扩张洋壳岩浆-变质演化不同阶段的合适指标。因此，我们计划对1256D地壳剖面中的Fe-Ti氧化物进行详细的微量分析研究。为了评价温度-氧逸度历史，应将电子探针数据应用于改进的Fe-Ti氧化物热氧压力计。铁同位素的原位分析将有助于我们更好地理解热液循环和原生岩浆作用之间的相互作用。为了限制岩浆室和轴向高温热液系统之间的导电边界层中普遍存在的条件，计划进行一项实验研究，使用富含氧化物(Fe-Ti)的辉长岩作为起始材料，以确定结晶过程中普遍存在的温度、氧化还原条件和挥发活动。这种演化的成分被解释为代表了岩浆和热液系统之间转换的关键岩性。

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.