ALBERT

Description: Hole 504B of the Ocean Drilling Program is dedicated to the study of crustal structure and hydrothermal processes in 5.9-m.y.-old oceanic basement. Continuing the work of previous legs, hole 504B was extended 212.3 m to a total depth of 1562.3 m below seafloor (bsf) during leg 111 in 1986. Quartz-sulfide veins occur at a depth of 1369–1388 m bsf in basalts of the sheeted dike complex. The ore minerals are predominantly pyrite, less chalcopyrite, rare Corich Cu-Fe-S phases, and a thiospinel (linnaeite/carrollite). Microprobe analyses yield a high Co content in zoned vein pyrites (〉8 wt %) as well as in the Cu-Fe-S phases (〉5 wt %). Up to 35.8 wt % Co was detected in the thiospinel. A Co/Ni ratio of 〉100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine (Co/Ni 〈5). The Co/Ni ratios correlate positively with Cu and negatively with As. Co-rich, nonstoichiometric Cu-Fe-S sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite. Fluid inclusions in quartz from the quartz-sulfide veins are two-phase and vary from liquid- to vapor-dominated. Their salinities range from 4.2 to 7.2 wt % equivalent NaCl and average 5.5 wt %. Pressure (360 bars) corrected average filling temperatures vary from 271° to 408°C with a maximum of 486°C. This is consistent with calculated quartz formation temperatures for a single quartz separate (+4.2‰ δ18O) using oxygen isotope thermometry. The δ18O value of the hydrothermal fluid was determined to be +1.7‰. The temperature data indicate fluid alteration of the sheeted dikes at about 350° to 500°C. The maximum homogenization temperatures intersect the liquid/vapor two-phase boundary above the critical point of seawater. Thus phase separation could have occurred before or during the formation of the mineralized veins and the alteration of the sheeted dike sequence.

Description: Al- and Ti-rich smectites associated with alkaline volcanic rocks were dredged during several cruises of the German research vessel “SONNE” as part of an extensive survey of manganese crusts in the Central Pacific seamount region. X-ray diffraction, chemical composition and infrared spectroscopy suggest that the smectites are constituents of the nontronite-beidellite solid solution series. X-ray fluorescence analyses yield considerable amounts of Al2O3 and Fe2O3 averaging 14.5 and 12.4 wt%, respectively. According to ASTM data the smectites are Al-rich nontronites rather than Fe-rich beidellites. A TiO2 content of up to 4.9 wt% is thought to be due to anatase impurities. δ18O-values in the range of +23.8 to +21.0‰ vs. SMOV indicate formation temperatures of 32–48°C. The rare-earth element patterns of the nontronitic smectites and associated alkaline volcanics are almost identical, showing a relative enrichment of light and a depletion of heavy rare-earth elements. It is concluded that the smectites formed as low-temperature alteration products during interaction of seawater with cooling basaltic/alkaline rocks rather than by mixing of hydrothermal fluids with seawater.

Description: Hole 841B was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The hole penetrated a roughly 500-m-thick series of Miocene volcanic sediments with a number of basaltic to andesitic units (sills?) varying in thickness between 7 cm and 17 m. The volcanics are slightly to moderately altered and contain analcite, chabazite, natrolite-thompsonite, heulandite (?), prehnite, and quartz as secondary phases. In addition, thaumasite [Ca3Si(OH)6 * 12H2O](SO4)(CO3) was identified in the altered sequence. Sulfur isotope data of two thaumasite separates (+23.5 per mil and +21.1 per mil d34S) indicate a seawater origin of the sulfate sulfur. It is suggested that thaumasite is a product of low-temperature (〈60 °C), seawater-derived CaCl2-rich fluids that were almost identical in composition to those presently circulating in the sub-seafloor.