Fault-controlled hydrothermal fluid flow at the EPR

Investigation of an axial mid-ocean ridge fault to determine the character of focused hydrothermal fluid flow. Tectonic escarpments at Pito Deep expose ocean crust formed at the super-fast spreading portion of the southern EPR (ca. 3 Ma). Ocean crust from Pito Deep is cut by high angle axial faults striking NE-SW parallel to the sheeted dykes. We focus on trace element and Sr isotopic compositional variations across an approximately 35 m fault zone within the sheeted dyke complex, ca. 100m below the lava-dyke transition, to explore the relationship between fluid flow, alteration and deformation. Samples from across this axial fault can be divided into 1) wall-rock basalts, 2) fault zone basalts and 3) fault zone breccias, ranging from simple cataclasites, veined cataclasites to quartz cataclasites. Breccias and basalts occur closely juxtaposed within the fault zone suggesting heterogeneous, highly localized deformation and fluid flow. Wall-rock basalts have been sampled a few to 30 m away from the fault zone and show extents of alteration typical of dykes away from fault zones with dominant replacement of primary phases by amphibole. The alteration in the fault zone basalts is also amphibole dominated with wider veins than the wall-rocks of quartz, chlorite ± amphibole. The fault zone breccias range in alteration mineralogy from chlorite to quartz dominated assemblages. Wall rock and fault zone basalts have similar trace element compositions to fresh crust. Breccias show large deviations in trace element compositions in comparison to adjacent altered basalts, with depletion of highly mobile elements such as K, Sr and both enrichment and depletion of Cu and Zn. 87Sr/86Sr of the dykes away from the fault zones are slightly elevated over fresh oceanic crust (0.7025- 0.7029) and fault zone samples have similar to higher 87Sr/86Sr. Quartz-epidote veins precipitated from fluids at Pito Deep have 87Sr/86Sr of approximately 0.7040. These new data provide insight into the chemical evolution of axial hydrothermal fluids during focused fluid flow.