Hydrothermal Fluid Flow in Oceanic Gabbros, IODP Site 1309, Mid-Atlantic Ridge: Strontium and Oxygen Isotopic Composition

IODP Hole U1309D was drilled to 1400 mbsf in the footwall of the Atlantis Massif detachment fault at the Mid- Atlantic Ridge 30°N. The core is composed of gabbroic rocks interlayered with olivine rich troctolites and ultramafic rocks, with several basalt/diabase sills in the top 130 m. The dominant alteration occurred in the greenschist facies, at depths at least 1 km below seafloor, and decreases in intensity downhole. Whole rock oxygen isotope values range from +5.5 ‰ to +1.5 ‰, indicating variable degrees of interaction with seawater at temperatures generally > 250°C. Gabbroic rocks and diabases exhibit a range of Sr isotope ratios from MORB values (0.70261) to intermediate ratios (0.70429), while four serpentinite samples from < 400 mbsf show much higher values (0.70687 to 0.70904) close to seawater composition (0.70916). A key question is how fluid with a seawater isotopic signature passed through partially altered gabbros to reach thin ultramafic horizons. Samples of individual minerals and alteration zones have been extracted from thin sections using a microscope-mounted drill. In gabbroic samples, igneous plagioclase and pyroxene have MORB-like values. Actinolite replacing pyroxene is generally very low in strontium but can have 87Sr/86Sr ratios significantly higher than the whole rock. Secondary plagioclase is isotopically close to the whole rock. Values close to seawater have been found in vug-filling actinolite veins, although bleached feldspar in vein margins is much less radiogenic. These data suggest that fluid percolating through the gabbros was significantly more radiogenic in strontium than the whole rock analyses might suggest. Where fluid flowed through high flux pathways with little exchange with wall rocks, minerals isotopically similar to seawater were precipitated. Serpentinite layers were altered either by fluid passing along such high flux pathways, or at significantly lower temperatures (200-250°C) than the gabbros. Models for fluid flow around detachment faults must allow for deep penetration of seawater-like fluids.