Fluid evolution in slow-spreading environments

Abstract

Variably altered and deformed gabbroic and intercalated trondhjemitic to dioritic rocks, recovered from Ocean Drilling Program (ODP) Sites 921 to 923 near the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge (MARK), display a complex history of interaction with magmatic fluids, high-temperature hydration within localized plastic deformation zones, and brittle failure. Pegmatitic patches, oxide gabbros, and felsic veins, which occur throughout the core and that contain abundant apatite, zircon, oxide minerals, and coarse-grained amphibole, are thought to have crystallized under relatively high ∫O 2 from a hydrous magma. Analyses of primary fluid inclusions in magmatic apatite in these evolved rocks show that the earliest fluids to be exsolved from the melts involved CO 2 -rich vapors, which with progressive fractionation evolved to more H 2 O-rich compositions. These later fluids were most likely exsolved under immiscible conditions, resulting in the development of CO 2 -H 2 O-rich vapors and CO 2 -H 2 O-Nacl brines that were trapped during mineral growth, as well as during later high-temperature fracturing events. In trondhjemitic to dioritic veins that cut the plutonic sequence, subsequent cooling of the high-salinity, magmatic fluids during the final stages of crystallization, or direct exsolution of brines from the late-stage melts, resulted in formation of fluids with equivalent salinities of 38-59 wt% NaCl. With continued cooling, brittle fracturing at minimum temperatures of 275°-350°C allowed circulation of hydrothermal fluids with salinities that ranged from near seawater to six times that of seawater values (1-20 wt% NaCl equivalent). A striking result of this study is the discovery of fluids rich in CH4, H 2 O, ± H 2 along healed microfractures in the plutonic rocks that contain up to 11 mol% CH 4 , which were entrapped at minimum temperatures of 270°-365°C. The high concentration of CH 4 ± H 2 in these fluids most likely reflects high-temperature oxidation of iron within mafic minerals during seawater alteration of the plutonic rocks at low water-rock ratios, coupled with the release of H2 during magnetite formation, and attendant CO 2 reduction. The discovery of CH 4 -rich fluids in crustal Layer 3 rocks from both the MARK area and from ODP Hole 735B on the Southwest Indian Ridge suggests that oceanic crustal Layer 3 may represent a significant, presently unrecognized sink for carbon. Leaching during subsequent hydrothermal alteration may play an important role in the transfer of carbon from the lithosphere to the hydrosphere.