Hydrothermal Alteration of Hyaloclastites Adjacent to Sill-Like Intrusives in the HSDP 3-km Core Hole.

Hyaloclastites at present depths below1880 mbsl on the submarine flanks of Mauna Kea volcano have been intruded by numerous, < 10 m-thick, sill-like bodies. The contact metamorphism of the hyaloclastites has resulted in up to 1 m-thick bleached zones, characterized by the presence of Na-and Mg-enriched alteration rinds on sideromelane clasts as well as the precipitation of drusy hydrothermal clinopyroxene (calcic augite to hedenbergite) and analcime within void spaces. The intrusive activity associated with contact metamorphism appears to have occurred early in the diagenetic history of the hylaoclastites, when they possessed porosities of 40-50%, because (1) early induration and pore-filling by hydrothermal minerals apparently strengthened them, preventing significant grain compaction during subsequent burial, (2) hydrothermal minerals have been coated or overgrown by smectite, zeolites, and palagonite during subsequent diagenesis and microbial innoculation, and (3) 87Sr/86Sr ratios of hydrothermal rinds on glass shards, averaging .7069 ± .0006, imply extensive interaction with seawater, whereas 87Sr/86Sr ratios of adjacent palagonitized glass, averaging .7042 ± .0002, imply interaction with comparatively less fluid, presumably after diagenetic pore-filling. Thermal modeling, which assumes (1) convective cooling, (2) that hydrothermal clinopyroxenes formed at minimum temperatures of 350°C, and (3) that hyaloclastite porosities approached 50% at the time of intrusion, implies that the observed contact aureoles must have been produced by mafic intrusions that maintained temperatures above the solidus rather than being rapidly cooled and frozen. This may have occurred because magma continued to flow in the intrusion conduit, consistent with the suggestion that these intrusions fed overlying pillow flows (Garcia et al., 2007). If this intrusive activity occurred at shallow depths within the edifice of Mauna Kea (Seaman et al. 2004), then hydrothermal clinopyroxene must have precipitated from superheated steam, as hydrostatic pressures would have been too low to allow precipitation from a liquid aqueous phase.