IODP Expedition 345: Characterizing Hydrothermal Alteration of Fast-Spreading EPR Lower Crust using O, Sr and Nd isotopics

Abstract:

The Integrated Ocean Drilling Program (IODP) Expedition 345 sampled lower crustal primitive gabbroic rocks that formed at the fast-spreading East Pacific Rise (EPR) and are exposed at the Hess Deep Rift. The metamorphic mineral assemblages in the rocks recovered at Site U1415 record the cooling of primitive gabbroic lithologies from magmatic (>1000°C) to zeolite facies conditions (<200°C) associated with EPR spreading, Cocos-Nazca rifting and exposure onto the seafloor. The dominant alteration assemblage is characterized by lower grade greenschist (<400°C) and subgreenschist facies (<200°C) alteration of olivine to talc, serpentine, or clay minerals, and is commonly accompanied by prehnite microveins in plagioclase. The intensity of alteration varies with igneous lithology, in particular, the modal abundance of olivine, as well as proximity to zones of brittle fracturing and cataclasis. We have attempted to characterize the nature and extent of isotopic exchange associated alteration and cooling and present a record of variations in O, Sr, and Nd isotopic compositions in altered rocks from the lower plutonic crust at Hess Deep. The Rb-Sr and ¹⁸O/¹⁶O systems exhibit sensitivity to hydrothermal interactions with seawater; whereas the Sm-Nd system appears essentially undisturbed by the minimal alteration experienced by the rocks drilled at Site U1415. The ⁸⁷Sr/⁸⁶Sr isotopic compositions of olivine gabbros (Mg# 0.81-0.89) range from 0.702536-0.703950 (±0.000008). Higher ⁸⁷Sr/⁸⁶Sr ratios are strongly correlated with percentage of hydrous minerals, and are higher in samples with a greater modal abundance of olivine. These rocks have somewhat higher ⁸⁷Sr/⁸⁶Sr ratios than upper plutonic rocks from the Northern Escarpment at Hess Deep (Kirchner and Gillis, 2012), although their percentage of hydrous phases is apparently similar. The d18O in these rocks ranges from 0.23‰ to 4.65‰ (±0.67); troctolites have systematically lower d18O than the gabbro and gabbronorites. Preliminary Nd isotopic data appears to be consistent with previous measurements from the Site 894 gabbros, implying derivation from depleted MORB (e.g. Pedersen et al., 1996).