(Table 1) List of samples with carbonate mineralogy, stable isotope, and accessory-mineral information

Petrographic and stable-isotope (d13C, d18O) patterns of carbonates from the Logatchev Hydrothermal Field (LHF), the Gakkel Ridge (GR), and a Late Devonian outcrop from the Frankenwald (Germany) were compared in an attempt to understand the genesis of carbonate minerals in marine volcanic rocks. Specifically, were the carbonate samples from modern sea floor settings and the Devonian analog of hydrothermal origin, low-temperature abiogenic origin (as inferred for aragonite in serpentinites from elsewhere on the Mid-Atlantic Ridge), or biogenic origin? Aragonite is the most abundant carbonate mineral in serpentinites from the two modern spreading ridges and occurs within massive sulfides of the LHF. The precipitation and preservation of aragonite suggests high Mg2+ and sulfate concentrations in fluids. Values of d18OPDB as high as +5.3 per mill for serpentinite-hosted aragonite and as high as +4.2 per mill for sulfide-hosted aragonite are consistent with precipitation from cold seawater. Most of the corresponding d13C values indicate a marine carbon source, whereas d13C values for sulfide-hosted aragonite as high as +3.6 per mill may reflect residual carbon dioxide in the zone of methanogenesis. Calcite veins from the LHF, by contrast, have low d18OPDB (-20.0 per mill to -16.1 per mill) and d13C values (-5.8 per mill to -4.5 per mill), indicative of precipitation from hydrothermal solutions (~129°-186°C) dominated by magmatic CO2. Calcite formation was probably favored by fluid rock interactions at elevated temperatures, which tend to remove solutes that inhibit calcite precipitation in seawater (Mg2+ and sulfate). Devonian Frankenwald calcites show low d18O values, reflecting diagenetic and metamorphic overprinting. Values of d13C around 0 per mill for basalt-hosted calcite indicate seawater-derived inorganic carbon, whereas d13C values for serpentinite-hosted calcite agree with mantle-derived CO2 (for values as low as -6 per mill) with a contribution of amagmatic carbon (for values as low as -8.6 per mill), presumably methane. Secondary mineral phases from the LHF for which a biogenic origin appears feasible include dolomite dumbbells, clotted carbonate, and a network of iron- and silica-rich filaments.

Some physical and chemical proceddes in fluids

The ability to reproduce reduced gravity conditions for long periods is one of the reasons why the orbiting laboratory is so attractive. In this paper several fluid dynamics problem areas are reviewed in which zero-gravity conditions are of great importance. Although emphasis is placed on space processing, there are some older problems also in which gravity masks the phenomcna, impeding a reasonably simple approach to the solution. Three problems are considered: Thermal convection under reduced gravity. The dumping effect ofsurface gravity waves at the outset of convection induced by surface tractions is discussed in particular. The existence of convection is of concern for some satellite thermal control techniques presently used, and for most of the proposed manufacturing processes. Whereas convection should be normally avoided, problems related to the containerless stirring ofa melt constitute an exception. Secondly, gravity and chemical reactions. Although chemical reactions are independent of gravity because of the small mass of the molecules and atoms involved, in many cases the reaction rate dcpends on the arrival of the species to the reaction zone. When the arrival process is buoyancy-controlled, the net specd of the reaction will be affected by the gravity. Thirdly, two-phase flows under reduced gravity provkle interesting problems from boiling heat transfer to degasslng of melts. This part of the paper deals only with the measurement of sound veiocity in a liquid containing bubbles. It is suggested that such measurements should be mude under reduced gravity to provide reliable residís.