Adélie Delacour

Detachment fault control on hydrothermal circulation systems: interpreting the subsurface beneath the TAG hydrothermal field using the isotopic and geological evolution of oceanic core complexes in the Atlantic

The geology and alteration history of two well-studied and very similar oceanic core complexes (OCCs) along the Mid-Atlantic Ridge are compared: the Atlantis Massif at 30°N (Integrated Ocean Drilling Program Site U1309) and a dome-like massif at 15°45′N (Ocean Drilling Program Site 1275). Both massifs are charac-terized by (1) a fault surface formed by talc-tremolite-chlorite schists; (2) little deformed gabbroic bodies a few kilometers in size, intruded into serpentinized peridotite and affected by mainly greenschist facies alteration; and (3) syntectonic basaltic intrusions within and below the detachment fault. Sr and O isotope data show that seawater-derived fluids were responsible for alteration in the gabbro, but fluid fluxes were moderate to low. Deformation in these “low-temperature” OCCs contrasts with the Atlantis Bank (Southwest Indian Ridge), where deforma-tion dominantly occurred at temperatures >800°C. The trans-Atlantic geotraverse (TAG) hydrothermal field, located ~4 km east of the Atlantic neovolcanic axis, is underlain by a convex-upward zone of seismicity reaching 7 km below seafloor, interpreted as a detachment fault. This suggests cooling to temperatures <700°C by hydrothermal circulation down to this depth. The geology and thermal evolution of the TAG detachment fault and its footwall are predicted on the basis of observa-tions on OCCs in the Atlantic. We suggest that the hydrothermal system is driven by gabbros emplaced at depth (7 km) and rapidly cooled during exhumation due to hydrothermal discharge along the fault at 350–400°C. The difference between low- and high-temperature OCCs is that gabbros in the former are intruded into the roots of an active hydrothermal system.

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid‐Atlantic Ridge 36°14′N)

Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones

Subduction zones modulate the chemical evolution of the Earths mantle. Water and volatile elements in the slab are released as fluids into the mantle wedge and this process is widely considered to result in the oxidation of the sub-arc mantle. However, the chemical composition and speciation of these fluids, which is critical for the mobility of economically important elements, remain poorly constrained. Sulfur has the potential to act both as oxidizing agent and transport medium. Here we use zinc stable isotopes (δ66Zn) in subducted Alpine serpentinites to decipher the chemical properties of slab-derived fluids. We show that the progressive decrease in δ66Zn with metamorphic grade is correlated with a decrease in sulfur content. As existing theoretical work predicts that Zn-SO42− complexes preferentially incorporate heavy δ66Zn, our results provide strong evidence for the release of oxidized, sulfate-rich, slab serpentinite-derived fluids to the mantle wedge.