(Table 1) Isotopes, calcite, aragonite and sample descriptions at DSDP Hole 55-433C

Calcite in the cavities and veins of igneous rocks has long been recognized as an alteration by-product (Dana, 1892). Elementary mineralogy textbooks report that the most common occurrence of aragonite is in the cavities of basalts and andesites (e.g., Kerr, 1977). Therefore, it is not surprising to find both carbonate minerals in association with the moderately to extensively altered basalt flows recovered during deep sea drilling on Suiko Seamount in the Emperor Seamount chain (DSDP Leg 55, Hole 433C). The thickness and vesicularity of the flows, along with the presence of oxidized flow tops, indicate that the basalt erupted subaerially (Site 433 Report, 1980). The stable isotopic contents of the carbonate phases filling and lining the veins and vesicles denote the environment of alteration. An isotopic study was undertaken to secure supportive evidence for a subaerial period in the development of the seamount. Also, the subsequent alteration history after submergence may be interpreted from this isotopic record.

Stable oxygen isotopes of calcite veins in ODP Site 146-892

In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4'N, 125°07.1'W) along the Cascadia subduction margin. The average d18OPDB value of all samples is -9.9 per mil and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6 per mil. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are -14.4 per mil, -9.5 per mil, and -5.2 per mil, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher d18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2-3 km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin.

Isotope composition of dolomite, calcite and pore-waters of ODP Leg 210 samples

Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, d13C, d18O and 87Sr/86Sr values. The results are compared with the chemistry, and d13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre-dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the d13C values of the dolomite are not in equilibrium with the d13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (<30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate-methane interface. It has been hypothesized that microbial 'hot-spots', such as the sulphate-methane interface, may act as focused sites of dolomite precipitation. Varying d13C values from -15 per mil to +15 per mil for the dolomite are consistent with precipitation at a dynamic sulphate-methane interface, where d13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate-methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.