Multistage hydrothermal alteration of gabbroic rocks from the failed Mathematician Ridge

Abstract

Oxygen isotopic, mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed Mathematician Ridge. The Mathematician Ridge, though initially a fast-spreading ridge presently has a deep central rift valley more typical of slow-spreading rifts. The inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large-offset fracture zones. Undeformed Mathematician Ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post-crystallization hydrothermal alteration. Metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole, low-calcium plagioclase and ilmenite. Oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700°C, water-rock ratios of less than one, and produced modified seawater fluids with isotopic compositions of 3.5–6.0‰. The amphibolites are cross-cut by multiple generation quartz-epidote veins forming a network. Replacement of the amphibole-grade mineralogies by greenschist-grade mineralogies, fluid oxygen isotopic compositions of 2–4‰, and a two-phase separation in the fluid captured in conjugate vapor-fluid inclusions are all associated with the formation of this network. Fluid inclusion data for the quartz indicate initial temperatures of over 500°C. These samples may represent rocks from the 1–2 km deep plumbing system of black-smoker-type venting.