Composition and age of ores and bottom sediments accumulated within and near the Rainbow hydrothermal field

Results of direct geological and geochemical observations of the modern Rainbow hydrothermal field (Mid-Atlantic Ridge, 36°14'N; 33°54'W) carried out from the deep-sea manned Mir submersibles during Cruises 41 and 42 of the R/V Akademik Mstislav Keldysh in 1998-1999 and data of laboratory studies of collected samples are under consideration in the paper. The field lacks neovolcanic rocks and the axial part of the rift is filled in with a serpentinite protrusion. In this field there occur metalliferous sediments, as well as active and relict sulfide edifices composed of sulfide minerals; pyrrhotite, chalcopyrite, isocubanite, sphalerite, marcasite, pyrite, bornite, chalcosine, digenite, magnetite, anhydrite, rare troilite, wurtzite, millerite, and pentlandite have been determined. Sulfide ores are characterized by concentric-zoned textures. During in situ measurements during 35 minutes temperature of hydrothermal fluids was varying within a range from 250 to 350°C. Calculated chemical and isotopic composition of hydrothermal fluid shows elevated concentrations of Cl, Ni, Co, CH4, and H2. Values of d34S of H2S range from +2.4 to +3.1 per mil, of d13C of CH4 from -15.2 to -11.2 per mil, and d13C of CO2 from +1.0 to -4.0 per mil. Fluid inclusions are homogenized at temperatures from 140 to 360°C, whereas salinity of the fluid varies from 4.2 to 8.5 wt %. d34S values of sulfides range from +1.3 to +12.5 per mil. 3He/4He ratio in mineral-forming fluid contained in the fluid inclusions from sulfides of the Rainbow field varies from 0.00000374 to 0.0000101. It is shown that hydrothermal activity in the area continues approximately during 100 ka. It is assumed that the fluid and sulfide edifices contain components from the upper mantle. A hypothesis of phase separation of a supercritical fluid that results in formation of brines is proposed. Hydrothermal activity is related to the tectonic, not volcanic, phase of the Mid-Atlantic Ridge evolution.

Composition of sediments, hydrothermal manifestations, interstitial waters, and aeolian material from the TAG Hydrothermal Field and Central Atlantic

The book is devoted to regularities of spatial distribution, mineralogy and geochemistry of hydrothermal and hydrothermal-sedimentary manifestations of the Mid-Atlantic Ridge rift zone.

(Table T1) Geochemical composition of xenoliths and host rocks from ODP Hole 193-1188A and dredge haul MD-138, PACMANUS field

A number of intensely altered, dark xenoliths with palimpsest quench textures were recorded within altered dacitic host rocks at Site 1189 (Roman Ruins, PACMANUS) during Ocean Drilling Program (ODP) Leg 193. Several of these displayed puzzling marginal fringes, apparently of altered plagioclase with variolitic texture, protruding into adjacent host rocks. Despite their alteration, the xenoliths were interpreted as fragments of rapidly chilled, possibly olivine-bearing basalts incorporated into the dacitic magmas either within the crustal plumbing system or during eruption at the seafloor (figures F15, F16, F17, F42, and F43 in Shipboard Scientific Party, 2002, doi:10.2973/odp.proc.ir.193.104.2002). An additional example of formerly spinifex-textured xenolith, the first from Site 1188 (Snowcap) and the first from the upper cristobalite-bearing zone of alteration, has been revealed by postcruise studies. Furthermore, a pristine sample of the parent lithology has been found within a dredge haul (MD-138, Binatang-2000 Cruise of Franklin; 3°43.60'S, 151°40.35'E, 1688 meters below sea level) from the Satanic Mills hydrothermal field at PACMANUS, near ODP Site 1191. The purpose of this report is to document these discoveries and thereby to confirm and build on shipboard interpretations. To my knowledge, similar xenoliths have never before been found in modern island arc or backarc volcanic sequences. Spinifex textures are most common in Archean komatiites, some of which are bimodally associated with calc-alkaline felsic volcanic rocks.

Description, geochemistry, mineralogy and isotopes of sulfides from ODP Site 193-1189, PACMANUS field

Chemical and isotopic data for rare massive and semimassive sulfide samples cored at Site 1189 (Roman Ruins, PACMANUS) suggest their genetic relationship with sulfide chimneys at the seafloor. Sand collected from the hammer drill after commencement of Hole 1189B indicates that at least the lower section of the cased interval was occupied by material similar to the stockwork zone cored from 31 to ~100 meters below seafloor (mbsf) in this hole, but with increased content of barite, sphalerite, and lead-bearing minerals. Fractional crystallization of ascending hydrothermal fluid involving early precipitation of pyrite may explain vertical mineralogical and chemical zoning within the stockwork conduit and the high base and precious metal contents of Roman Ruins chimneys. A mineralized volcaniclastic unit cored deep in Hole 1189A possibly represents the lateral fringe of the conduit system. Lead isotope ratios in the sulfides differ slightly but significantly from those of fresh lavas from Pual Ridge, implying that at least some of the Pb within the Roman Ruins hydrothermal system derived from a deeper, more radiogenic source than the enclosing altered volcanic rocks.

(Table 1) Rock magnetic parameters of core samples obtained from Morne Rouge, Ile de Possession

The magnetic high field susceptibility chi(hifi) indicates the presence of mafic paramagnetic minerals in the scoriae and points to an absence of clay minerals, and in turn of pedogenic processes, in the peat samples. Due to their volcanic origin, scoriae contain much more ferrimagnetic minerals than peat with is indicated by their enhanced saturation remanence Mrs.