Abstract
Hydrothermal circulation at slow-ultraslow spreading ridges represents the primary means by which seawater can penetrate the oceanic crust. This process redistributes volatile elements that can be sequestered in hydrous alteration minerals and later remobilised during plate subduction. Accordingly, the hydration state of the oceanic crust fundamentally influences a variety of fluid-mediated processes across subduction zones. Numerous drill cores penetrate the upper oceanic crust and provide relatively robust constraints on the volatile content of sedimentary and volcanic crustal components. However, very few drill cores sample the lower gabbroic and lithospheric mantle crustal domains, which are rich in hydrous minerals (serpentine, brucite, amphibole, chlorite, talc, zeolites, etc.). This has led to considerable uncertainty regarding the volatile makeup of the crust entering subduction zones, and the behaviour of fluid tracers such as boron and its isotopes. Lower crustal units are exposed by domal detachment faulting at the Atlantis Massif (30°N; Mid-Atlantic Ridge). We will present preliminary data from IODP Expedition 399, which recovered 1268m of serpentinized mantle peridotite and subordinate gabbroic rocks from the southern wall of the massif, proximal to the famous Lost City Hydrothermal Field. This is by far the longest core ever drilled in situ in serpentinized oceanic peridotite. We will report whole-rock and mineral (serpentine, amphibole, chlorite, prehnite, talc, Cr-spinel, oxide) chemistry in order to investigate the down-hole style of alteration. This will include whole-rock 11/10 B and 87/86 Sr isotope ratios to assess the role of seawater vs. possible ongoing metamorphic alteration at depth. These data represent an important step towards quantifying the fluid mobile element budgets and specifically the boron and 11/10 B content of the lower oceanic crust.
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