The Origin of Hydrous Minerals in Peridotite Mylonites from an Oceanic Transform Fault

Abstract:

Previous studies of oceanic transform faults have assumed that fluid circulation ends when the transition from brittle failure to plastic flow occurs. However, we have identified significant amounts of hydrous minerals in peridotite mylonites from St. Paul’s Rocks, a small set of islets on the St. Paul Transform Fault on the Mid-Atlantic Ridge. These rocks, which are highly deformed, are interpreted as having originated within the brittle-ductile transition zone. Our analyses show that the peridotites contain syn-deformational amphibole (12% on average) and minor phlogopite. In addition, the mylonites contain cm-scale veins of gabbro that are semi-parallel to foliation, which have been altered to amphibole, sodalite, and scapolite. Microprobe analysis indicates that the amphibole is pargasite, which is relatively rich in Na and Cl and requires temperatures >~700°C to form. In addition, sodalite and scapolite contain Na and Cl as essential elements. Initial stable isotope analysis indicates that δD of the pargasite lies between mantle δD (~70‰) and seawater (0‰).

Based on our observations, we suggest that melt was introduced into the system either prior to or during deformation. In addition, we propose that either i) the melt was volatile-rich, providing the necessary water to form hydrous phases; or ii) seawater penetrated into the brittle-ductile transition zone by microfracturing, thus providing the necessary water, Na, and Cl to form the phases observed. While the 600°C isotherm is traditionally considered the limit of brittle deformation, this second hypothesis would suggest that seawater can penetrate to greater depths, in agreement with recent seismicity observations from an East Pacific Rise transform fault (McGuire et al., 2012). With additional stable isotope analysis and thermodynamic modeling, we plan to further constrain the source of melt/fluid at St. Paul’s Rocks and thus improve constraints on OTF processes. If seawater is the origin of syn-deformational hydrous phases in peridotite mylonites, our observations suggest that deep penetration of seawater on OTFs may be a pervasive process and possibly integral to deformation in the brittle-ductile transition zone.