CONTROLS ON HYDROTHERMAL ALTERATION IN A SECTION OF FAST-SPREADING OCEANIC-CRUST

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 la va sequence, sheeted dike complex, and uppermost plutonic sequence. Al teration assemblages record the initiation and subsequent cooling of a n axial hydrothermal system(s) that was periodically perturbed by the injection of sheeted dikes. Mineralogical acid geochemical data indica te that temperatures increased across the lava-dike transition (less t han or equal to 50- > 180 degrees C) and that the grade of metamorphis m was heterogeneous within the sheeted dike complex in that temperatur es varied laterally from 100-300 degrees C to 600-800 degrees C within the upper dikes and were 400-450 degrees C within the lower sheeted d ikes. During vigorous hydrothermal circulation at the EPR, the base of the hydrothermal system was located within or at the top of a sequenc e of gabbros that is interpreted to be the root-zone of the sheeted di ke complex. The base of the hydrothermal cell did not penetrate into t he subjacent plutonic sequence until the section of crust had migrated off-axis and the magma lens had solidified. The vigor of the hydrothe rmal system would have waned by this time and hydrothermal fluids migr ated into the lower crust along broadly distributed microfracture netw orks at temperatures between 450 and 650 degrees C, although locally t emperatures may have reached 800 degrees C. The highest temperature al teration assemblages within the sheeted dikes formed in response to th e injection of a dike or swarm of dikes that initiated a short-lived h ydrothermal system. The sheeted dikes and plutonic rocks experienced v ery little retrograde alteration and there is no evidence for a discre te, 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 d ifferences in the distribution and mineralogical expression of incipie nt alteration in plutonic sequences. The evolution of alteration as a section of crust cools and moves off-axis reflects its particular magm atic-tectonic history and, thus, may vary significantly in ridges with all spreading rates.