Seafloor hydrothermal activity in sediment core MV0502-15JC

A large diameter piston core containing 8.35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote Southwest Pacific Basin (31°42.194'S, 143°30.331'W; 5082 m water depth), providing unique insight into hydrothermal activity and eolian sedimentation there since the early Oligocene. A combination of fish-teeth Sr-isotope stratigraphy and INAA geochemical data reveals an exponentially decreasing hydrothermal flux 31 Ma to the present. Although hydrothermal sedimentation related to seafloor spreading explains this trend, a complex history of late Eocene/early Oligocene ridge jumps, propagating rifts and plate tectonic reorganization of South Pacific seafloor could have also played a role. A possible hiatus in deposition, as recorded by changes in core composition just below 2 m depth, is beyond the resolution of the fish teeth Sr isotope dating method employed here; however, the timing of this interval may be coincident with extinction of the Pacific-Farallon Ridge at ~20 Ma. A low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth, consistent with dust-generating continental sources far to the west (Australia/New Zealand). This is the first long-term paleoceanographic record obtained from within the South Pacific "bare zone" (Rea et al., 2006), an anomalous region where Pacific seafloor has largely escaped sediment accumulation since the Late Cretaceous.

Chemical and isotopic compositions of rocks and minerals from the Ashadze and Logachev hydrothermal fields, Mid-Atlantic Ridge

This study was aimed at reconstructing a sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites from internal oceanic complexes of the Ashadze and Logachev hydrothermal vent fields. Collections of plutonic rocks from Cruises 22 and 26 of R/V "Professor Logachev", Cruise 41 of R/V "Akademik Mstislav Keldysh", and from the Serpentine Russian-French expedition aboard R/V "Pourquoi pas?" were objects of this study. Data reported here suggest that the internal oceanic complexes of the Ashadze and Logachev fields formed via the same scenario in these two regions of the Mid-Atlantic Ridge. On the other hand, an analysis of petrological and geochemical characteristics of the rocks indicated that the internal oceanic complexes of the MAR axial zone between 12°58'N and 14°45'N show pronounced petrological and geochemical heterogeneity manifested in variations in degree of depletion of mantle residues and in Nd isotopic compositions of rocks from the gabbro-peridotite association. Trondhjemites from the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under influence of hydrothermal fluids. It was supposed that presence of trondhjemites in internal oceanic complexes of MAR can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone, in which petrologically and geochemically contrasting internal oceanic complexes are spatially superimposed, serves as an area for development of large hydrothermal clusters with considerable ore-forming potential.

(Table 1) Calcite, siderite, and stable isotopes d13C and d18O at DSDP Hole 93-603B

Diagenesis of the fine-grained, feldspathic sandstones in the Lower Cretaceous submarine fan complex cored in DSDP Hole 603B can be considered to have occurred in three stages: (1) replacement of matrix and framework grains by pyrite, siderite, phillipsite (?), and particularly by ferroan calcite; (2) dissolution of ferroan calcite and feldspars to produce secondary macroporosity; and (3) development of sparse feldspar and quartz overgrowths, and authigenic modification of remnant matrix. Only ferroan calcite is a volumetrically important diagenetic mineral phase (up to 50 vol.%). Matrix in thin sandstone turbidite deposits has been extensively replaced by ferroan calcite. Carbon stable isotope data suggest that organic diagenesis had only a minor influence on calcite precipitation. Oxygen stable isotope data indicate that the minimum average calcite precipitation temperature was 40° C. Preliminary calculations show that steadystate diffusion of Ca+ + from the dissolution of nannoplankton skeletal material in the interbedded pelagic marls to the associated sandstones is a feasible transport mechanism. A thick sandstone unit from 1234-1263 m sub-bottom is extensively replaced by calcite near the upper and lower contacts. Farther into the sand body away from the contacts, the sandstone has good secondary porosity resulting from the dissolution of ferroan calcite that partially replaced matrix and framework grains. The central portion of the thick sand appears to be a channel with high-energy clean sand. We believe that the channel provided a conduit for focused flow of diagenetic compactional fluids responsible for dissolution. Focused flow may be the result of the earlier lithification of the pelagic limestones and thin-bedded sandstones which, then formed vertical permeability barriers. Calcite dissolution has occurred and may still be occurring at temperatures less than 65°C.