Mineralogical and fluid inclusion analyses of gabbroic rocks recovered from Site 894 on the intrarift ridge at Hess Deep indicate that the gabbroic sequence is a product of a complex history involving multiple magmatic and hydrothermal events. Pegmatitic patches throughout the core that contain high concentrations of apatite, zircon, oxide minerals, and coarse-grained amphibole are believed to have crystallized under relatively high fO2 from a hydrous magma. The patches of micropegmatite represent the most evolved compositions sampled at Site 894 and may reflect reequilibration with evolved, volatile-rich fluids that percolated through the gabbroic sequence late in the magmatic history. Analyses of fluid inclusions entrapped within these zones, indicate that 30-40 wt% NaCl ± Fe ± CO2-rich brines formed either under immiscible conditions in association with the evolved patches, or were exsolved in the absence of a vapor phase directly from the compositionally evolved melts. Migration of the high-salinity fluids may have resulted in the crystallization of apatites that contain up to ~6 wt% chlorine.
The transition from magmatic to hydrothermal seawater-dominated conditions in the plutonic sequence is marked by penetration of fluids that exhibit equivalent fluid salinities that range from ~0.1% to 200% of seawater values (3.2 wt% NaCl) and exhibit uncorrected homogenization temperatures that cluster at ~250°C. The extremely low-salinities were most likely formed during supercritical phase separation of seawater at temperatures >407°C. Their preservation requires that the vapor phase remained isolated from the associated brine and that subsequent mixing with seawater did not occur. The compositions of the plutonic-hosted fluids are similar to those of fluids exiting hydrothermal vents, indicating that the fracture networks may represent the feeder systems for sustained hydrothermal flow. Migration of these fluids along microfracture and vein networks resulted in the heterogeneous replacement of the gabbroic rocks by greenschist facies mineral assemblages. This alteration is most intense adjacent to vein networks and in cataclastically deformed zones, in which brittle failure facilitated enhanced fluid flow. Sealing of the microfracture and fracture networks with zeolite facies mineral assemblages marks the cessation of fluid circulation in the plutonic sequence. The plutonic sequence may have undergone as much as 2 km of uplift attendant with crustal thinning and propagation of the Cocos-Nazca spreading center.
Date of initial receipt: 2 August 1994
Date of acceptance: 24 February 1995
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