Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults.

Albers, Elmar; Schroeder, Timothy; Bach, Wolfgang

The footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide. Plain Language Summary: At slow spreading mid‐ocean ridges, the formation of oceanic crust is commonly accommodated by geologic faulting, whereby tectonic plates are displaced relative to one another. Some of these faults, termed detachments, are active over millions of years and displacement can sum up to several tens of kilometers. It is not well understood why detachment faults form and remain active over such long periods of time. We present evidence from drill core from the Mid‐Atlantic Ridge that fault evolution and longevity are favored by a process that is related to the presence of a particular mixture of rocks: where magma has intruded and crystallized in the prevalent mantle rocks, chemical reactions with seawater that commonly penetrates deep into the seafloor cause the formation of mechanically weak minerals. Extensive tectonic strain is subsequently localized into these weak spots where it leads to deformation and faulting. This faulting enables further inflow of seawater that results in further weakening—a positive feedback loop is initiated. Comparison to other detachment faults implies that such rock mixtures are present at many slow spreading ridges, and we suggest that the proposed process significantly contributes to the formation of new seafloor around the globe. Key Points: Impregnation of mantle peridotite by mafic or more evolved melts is common in oceanic detachment faultsSeawater interaction at >500 °C turns impregnated peridotite into weak secondary assemblages in which strain localizesStrain localization initiates deformation and faulting and facilitates the concentration of extensional tectonics into long‐lived detachments

PERIDOTITE; MASS transfer; FAULT zones; METAMORPHIC rocks; CHLORITE minerals

Geochemistry, Geophysics, Geosystems: G3, 2019, Vol 20, Issue 1, p84

1525-2027

Academic Journal

10.1029/2018GC007783