Geochemistry of DSDP Hole 83-504B minerals

Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.

Physical properties and minerals of Leg 65 Holes basalts

Velocities and densities of submarine basalts obtained during Leg 65 are distinguished by their relatively high values (about 6.3 km/s and 2.9 g/cm/**3, respectively). This is consistent with their low degree of alteration. The range of velocities covered by these samples correlates well with porosity, and a comparison with in situ velocities from refraction data suggests maximum porosities ih the upper 100 meters of the crust of about 10%.

Chemical composition of hydrothermal sulfide minerals from the Rainbow, Logachev-1, and Logachev-2 hydrothermal fields

Composition of ore minerals in MAR sulflde occurrences related to ultramaflc rocks was studied using methods of mineragraphy, electron microscopy, microprobe analysis, and X-ray analysis. Objects are located at various levels of maturity of sulflde mounds owing to differences in age, duration and degree of activity of the following hydrothermal systems: generally inactive Logatchev-1 field (up to 66.5 ka old), inactive Logatchev-2 field (3.9 ka), and generally active Rainbow field (up to 23 ka). Relative to MAR submarine ore occurrences in the basalt substrate, mineralization in the hydrothermal fields mentioned above is characterized by high contents of Au, Cd, Co, and Ni, along with presence of accessory minerals of Co and Ni. The studied mounds differ in quantitative ratios of major minerals and structural-textural features of ores that suggest their transformation. Ores in the Logatchev-1 field are characterized by the highest Cu content and development of a wide range of multistage contrast exsolution structures of isocubanite and bornite. In the Logatchev-2 field, sphalerite-chalcopyrite and gold-arsenic exsolution structures are present, but isocubanite exsolution structures are less diverse and contrast. The Rainbow field is marked by presence of homogenous isocubanite and the subordinate development of exsolution structures. The authors have identified four new phases in the Cu-Fe-S system. Phases X and Y (close to chalcopyrite and isocubanite, respectively) make up lamellae among isocubanite exsolution products in the Logatchev-1 and Logatchev-2 fields. Phase Y includes homogenous zones in zonal chimneys of the Rainbow field. Phases A and B formed in the orange bornite domain at low-temperature alteration of chalcopyrite in the Logatchev-1 field. Mineral assemblages of the Cu-S system are most abundant and diverse in the Logatchev-1 field, but their development is minimal in the Logatchev-2 field where mainly Cu-poor sulfides of the geerite-covellite series have been identified. Specific features of mineral assemblages mentioned above reflect the maturity grade of sulfide mounds and can serve as indicators of maturity.