Controls on hydrothermal alteration in a section of fast-spreading oceanic crust

Abstract

Young oceanic crustal sections that formed at the fast-spreading East Pacific Rise (EPR) were mapped by submersible along steep fault scarps that bound the Hess Deep rift valley. Excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence, sheeted dike complex, and uppermost plutonic sequence. Alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system(s) that was periodically perturbed by the injection of sheeted dikes. Mineralogical and geochemical data indicate that temperatures increased across the lava-dike transition (≤ 50→ 180°C) and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100–300°C to 600–800°C within the upper dikes and were 400–450°C within the lower sheeted dikes. During vigorous hydrothermal circulation at the EPR, the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root-zone of the sheeted dike complex. The base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off-axis and the magma lens had solidified. The vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650°C, although locally temperatures may have reached 800°C. The highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short-lived hydrothermal system. The sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete, off-axis hydrothermal event(s). Incipient alteration conditions in crustal sections that formed at mid-ocean ridges with slower rates of spreading than the EPR are similar to Hess Deep, although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences. The evolution of alteration as a section of crust cools and moves off-axis reflects its particular magmatic-tectonic history and, thus, may vary significantly in ridges with all spreading rates.