Other
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56..319w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 1, pp.319-334
Other
3
Scientific paper
Lower Cretaceous euhedral dolomite crystals from the Gargano Peninsula, Italy, have been analyzed by scanning proton microprobe (SPM) for the trace elements Sr, Zn, Fe, Mn, Cu, Ni, Y, and Rb. Intricate, decoupled zoning patterns for Fe, Mn, and Zn are a ubiquitous feature of the outer 55 microns of the dolomite crystals: differences between peak and baseline concentrations are on the order of several hundred parts per million. In contrast, Sr concentrations are nearly constant at 110 ppm. The concentrations of the other cations are all low (Cu, 20 ppm; Y, 10 ppm; and Ni and Rb, 5 ppm). In order to investigate the origin of the Gargano dolomite, we have used the method of Tardy and Garrels to estimate equilibrium partition coefficients for divalent cations between aqueous fluids and the dolomite structure. 18 O measurements on Gargano dolomites show that they formed at a maximum temperature of 75°C (±7°C). Estimated equilibrium constants for substitution of the cation M into dolomite, calculated baseline molar cation ratios for fluids in equilibrium with Gargano dolomite, and measured cation ratios for seawater and marine sediment pore fluids at 75°C are as follows: Cation ratios of fluids in equilibrium with the baseline concentrations of Mn, Fe, and Zn in the Gargano dolomite samples are bracketed by the cation ratios measured in seawater and in marine sediment pore fluids. The factor often discrepancy for Sr in the predicted fluid compared with seawater is probably due to nonideal mixing of Sr on the Ca site. The decoupled fluctuations of the Mn and Fe concentrations in these samples can be explained by Eh fluctuations in the reactant fluid. Zn fluctuations may be a product of pH-dependent oxide surface reactions. To test this hypothesis of origin, unzoned Tertiary dolomites from Enewetak atoll and from the Bahama bank have also been analyzed. The homogeneous Sr concentration in the Gargano dolomite is similar to the Sr concentrations measured in the Enewetak atoll (175 ppm) and Bahama bank (150 ppm) dolomites, samples which in both cases almost certainly grew in equilibrium with seawater. Gargano baseline Mn (20 ppm) and Fe (80 ppm) concentrations are also much the same as the Bahama bank dolomite Mn (15 ppm) and Fe (140 ppm) concentrations. The lack of Sr zoning in the Gargano samples and the similarity of their Sr, Mn, and Fe content to dolomite that formed in equilibrium with seawater indicates that the dolomitizing fluid was most likely seawater or a seawater-type brine. Low Sr concentrations (12 ppm) in pore-filling calcite from the Gargano suggests that, in contrast to the dolomite, the calcite precipitated from meteoric fluids.
Feltham David J.
Fraser Donald G.
Whiteman Mark I.
Wogelius Roy A.
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