VEIN GROWTH RELATED TO EXTENSION AND HYDRATION OF THE OCEANIC LITHOSPHERE, ATLANTIS MASSIF, MID-ATLANTIC RIDGE

This study analyzed several thin sections using petrography and Raman spectroscopy of serpentinized harzburgite from the Atlantis Massif oceanic core complex sampled during IODP Expedition 399 to study vein formation related to hydration of the oceanic lithosphere. Expedition 399 drilled a 1.2 km deep section of oceanic lithosphere exhumed ~1.5 Ma along the Mid-Atlantic Ridge by a detachment shear zone/fault causing embrittlement and fracturing. Identified vein minerals include aragonite, dolomite, calcite, diopside, serpentine (chrysotile, lizardite, antigorite), talc, iowaite, brucite, and sulfides. Chrysotile and lizardite are typically fibrous to platy whereas antigorite is fine-grained and granular. Aragonite is only found shallower than ~60 mbsf and forms aligned crystals. Calcite veins also contain sulfides in variable amounts. Diopside and antigorite are found between ~630 and 750 mbsf. We speculate that a higher proportion of gabbro deeper in the hole affected the composition of hydrothermal fluids and replacement reactions were recorded in distinct secondary mineral assemblages. Fluids that interacted with gabbro and subsequently reacted with peridotite were likely involved in the formation of talc in serpentinite. Antigorite can either form during metamorphism or via the addition of Si. Hence, antigorite may reflect interactions with Si-rich fluids that previously interacted with gabbro. Secondary diopside may be related to rodingitization of gabbroic dikes in serpentinizing peridotite; alternatively, diopside may have formed during moderate temperature serpentinization when silica activities were low. In most cases, these veins are not sheared which likely indicates they formed in a static environment or strain was accommodated on the edges of this interval. Carbonate ranges from aligned to granular suggesting these veins either formed at different times or under changing stress fields. The presence of sulfides with carbonate likely indicates a slightly oxidized fluid. The presence of iowaite at shallow intervals (~30 mbsf) likely indicates seawater derived fluids overprinted the serpentinites. This interval represents a zone of differential fluid compositions, silica and oxygen activities across a range of temperatures during and after serpentinization and exhumation.