Chemistry and Isotopic Composition

This latter statement is supported by carbon isotope data on limestone samples collected from the Pleistocene carbonate sequences of Barbados Island in 

Because the bulk of the limestone volume of Bermuda has spent most of its existence in the vadose zone, much of the limestone mass shows the imprint of vadose diagenesis, and progressive depletion in 13C with increasing age. However, limestones older than 125,000 years have spent more time in the phreatic meteoric zone. These limestones are more extensively altered by freshwater and freshwater-seawater mixtures. Their relatively light 813C values, however, imply alteration in a relatively open system in which soil carbon dioxide, depleted in 13C because of oxidation of organic matter, was an important source of carbon for replacement reactions. 

The chemistry of the groundwaters of Bermuda was investigated in detail by Plummer et al. (1976). To date this investigation still remains one of the few major integrative studies of phreatic meteoric water diagenesis in the literature (see also, e.g., Harris and Matthews, 1968, Barbados Back and Hanshaw, 1970, Florida and Yucatan Plummer, 1977, Florida Budd, 1984, 1988, Bahamas). A summary of the conclusions of this work is given here. 

The rainwater of Bermuda is in near equilibrium with atmospheric Pc02 = 10-3.5 atm., and contains small amounts of sea salt (0.07 wt. % seawater). The rainfall of 147 cm y1 is seasonally distributed. The rain enters the saturated zone by two main paths direct rainfall on marshes and ponds, and percolation downward from the vadose zone as vadose seepage and flow through rocks during times of soil water excess (Vacher, 1978). Total annual recharge of the saturated zone is about 40 cm y-1 (Vacher and Ayers, 1980). The residence time of the groundwater has been calculated as 6.5 years, and the average age of the sampled water as 4 years (Vacher et al., 1989). Such estimates are necessary for calculations of carbonate mineral stabilization rates, as shown in a later section. 

Vadose water moving by vadose flow through fractures has been sampled from the roofs of caves (Frantz, 1971) in the Town Hill and Walsingham formations. This water is lower in dissolved CO2 and contains less Ca2+ and HCO3  

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Kimura M., El Goresy A., Palme H., and Zinner E. (1993) Ca-, Al-rich inclusions in the unique chondrite ALH85085 petrology, chemistry, and isotopic compositions. Geochim. Cosmochim. Acta 57, 2329-2359. 

CHEMISTRY AND ISOTOPIC COMPOSITION OF GROUNDWATERS FROM CRYSTALLINE ENVIRONMENTS... 

Frape S. K. and Fritz P. (1982) The chemistry and isotopic composition of saline groundwaters from the Sudbury Basin, Ontario. Can. J. Earth Set 19, 645-661. 

Matsumoto, R. Matsuda, H. (1987) Occurrence, chemistry and isotopic composition of carbonate concretions in the Miocene to Pliocene siliceous sediments of Aomori, northeastern Japan. J. Fac. Sci., Univ. Tokyo, 21, 351-377. 

Pastorelli, S., Marini, L., Hunziker, J., 1999. Water chemistry and isotope composition of the Acquarossa thermal system, Ticino, Switzerland. Geothermics 28, 75-93. 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

Mozley PS, Carothers WW (1992) Elemental and isotopic compositions of siderite in the Kuparuk formation, Alaska effect of microbial activity and water/sediment interaction on early pore-water chemistry. J Sed Pet 62 681-692... 

Since we have no direct information about the chemistry of the Fountain fluid, we assume that its composition reflects reaction with minerals in the evaporite strata that lie beneath the Lyons. We take this fluid to be a three molal NaCl solution that has equilibrated with dolomite, anhydrite, magnesite (MgCC>3), and quartz. The choice of NaCl concentration reflects the upper correlation limit of the B-dot (modified Debye-Hiickel) equations (see Chapter 8). To set pH, we assume a CO2 fugacity of 50, which we will show leads to a reasonable interpretation of the isotopic composition of the dolomite cement. 

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