ABSTRACTS OF RECENT PAPERS


1. Vennemann, T.W. and O'Neil, J.R. (1996). Hydrogen isotope fractionation between hydrous minerals and molecular hydrogen. Geochim. Cosmochim. Acta, 60, 2437-2451.

Abstract: Molecular hydrogen was used as an exchange medium to make indirect determinations of hydrogen isotope fractionation factors between hydrous minerals (epidote, kaolinite, muscovite, biotite, and hornblende) and water at temperatures between 150 and 400^oC. Hydrogen isotope exchange between hydrous minerals and H₂ is found to be unusually rapid at these temperatures and in the absence of reducible iron, may represent true diffusive exchange. Two methods were used for the exchange: (1) the molecular hydrogen provided an ''infinite'' reservoir of H₂ and (2) the mineral provided an ''infinite'' reservoir of H₂. Results of these methods are in good agreement for epidote and kaolinite, suggesting that ''surface effects'' are not important for these experiments. Fractionation factors in the epidote- H₂ system, expressed as 1000ln(alpha), increase linearly with increasing 1/T² (or decreasing temperature) between temperatures of 150 and 400^oC. This behavior is also true for epidote-water fractionation factors calculated from published H₂O(vapor)-H₂ and H₂O(vapor)-H₂O(liquid) fractionation factors. Values of 1000ln(alpha) for kaolinite- H₂ and muscovite-H₂ fractionation factors are similar and also decrease linearly with increasing 1/T² between 200 and 275^oC, and 200 and 400^oC, respectively. There is a small departure from such linearity at temperatures of less than 200^oC or the kaolinite-H₂ system. The trends of the calculated kaolinite-H₂O fractionation factors with temperature are similar to those previously published, with a maximum at about 200^oC. Muscovite - H₂O fractionation factors also decrease with decreasing temperatures and become increasingly positive at temperatures lower than about 225^oC. Problems of reduction of iron and a concomitant increase in the amount of water were encountered in exchange experiments involving biotite, hornblende, and, to a lesser extent, muscovite. The consequences of these reactions were the production of apparent relatively small mineral-H₂ and large mineral-H₂O equilibrium fractionation factors and support the hypothesis that hydroxyl groups are added to sites linked to iron and that there is indeed a compositional effect on hydrogen isotope fractionations in mineral-H₂O systems. The new equilibrium fractionation factors determined in this study do not unambiguously resolve serious conflicts in data published by previous workers, but do provide some important constraints on the direction and magnitude of the D/H fractionation factors between water and the common rock-forming minerals.

2. Fricke, H. C. and O'Neil, J.R. (1996) Inter- and intra-tooth variation in the oxygen isotope composition of mammalian tooth enamel: some implications for paleoclimatological and paleobiological research. Palaeo., Palaeo., Palaeo., 126, 91-100.

Abstract: Significant differences in the d¹⁸O value between teeth, and even within a single tooth were observed in a detailed study of the oxygen isotope composition of tooth enamel phosphate (d¹⁸O_p) of hypsodont teeth from bison and sheep jaws. The permanent molars and premolars of a fossil adult bison from eastern Wyoming (similar to 500 yr B.P.) and a modern sheep from California were analyzed. The bison is assumed to have been free-ranging with a variety of possible water sources, whereas the sheep was raised on a ranch. Inter-tooth variability in d¹⁸O_p for the bison compared to the sheep (5.6 per mil and 3.5 per mil, respectively) may be a result of behavioral differences. Analyses of multiple samples from the m3 of both the bison and sheep vary to a similar degree (3.5 per mil) in a similar cyclic pattern down the length of the tooth, a pattern which is interpreted to be seasonal. When present, inter- and intra-tooth variations in d¹⁸O_p are controlled by the water and food ingested by the mammals during the period of enamel formation. In these localities, well waters, surface waters, and mother's milk have different isotopic compositions at different times of the year.

The data underscore the role of biology and behavior in determining d¹⁸O_p values, and the need to understand how they vary for a population of interest. If these variations are taken into account, the d¹⁸O_p values of single samples from small, late-forming teeth (e.g. premolars) can be used as a proxy for the d¹⁸O value of local meteoric water for long-term climate studies. Multiple samples from a single third molar may provide information on the duration and timing of enamel growth, seasonality, as well as long-term climate change.

3. Kim, S.-T. and O'Neil, J.R. (1997) Equilibrium and non-equilibrium oxygen isotope effects in synthetic carbonates. Geochim. Cosmochim. Acta, 61, 3461-3475

Abstract: A suite of divalent metal (Ca, Cd, Ba) carbonates was synthesized over the temperature range 10-40^oC by the classical method of slowly bubbling N₂ through a bicarbonate solution. It was discovered that carbonates could be precipitated reproducibly in or out of isotopic equilibrium with the environmental solution by varying the concentrations of bicarbonate and cation, Precipitation rate had little or no influence on the isotopic composition of the product. Relatively high initial concentrations of up to 25 mM in both bicarbonate and cation were prepared by adding solid metal chlorides to solutions of NaHCO₃. On the basis of results of equilibrium experiments and a new determination of the acid fractionation factor, a new expression is proposed for the oxygen isotope fractionation between calcite and water at low temperatures:

10001n alpha (calcite-H₂O) = 18.03(10³T^-1) -32.42

where alpha is the fractionation factor, and T is in kelvins. Combining new data for low-temperature precipitations and the high-temperature equilibrium fractionations published by O'Neil et al. (1969) results in a revised expression for the oxygen isotope fractionation between octavite (CdCO₃) and water from 0 to 500^oC:

10001n alpha (CdCO₃-H₂O) = 2.76(10⁶T^-2) -3.96

The ability to produce nonequilibrium carbonates allowed assessment to be made, for the first time, of the temperature dependence of nonequilibrium stable isotope fractionations in mineral systems. The temperature coefficients of cu(carbonate-water) for nonequilibrium divalent metal carbonates are greater than those for equilibrium carbonates, a finding that may bear on the interpretation of analyses of biogenic carbonates forming out of isotopic equilibrium in nature.

New determinations of acid fractionation factors (10001n alpha) at 25^oC for calcite (10.44 ± 0.10), aragonite (11.01 ± 0.01), and witherite (10.57 ± 0.16) are mildly to strongly different from those published by Sharma and Clayton (1965) and point to a control on this fractionation by some physical property of the mineral. Reproducible values for octavite (CdCO₃) varied from 11.18 to 13.60 depending on the conditions of preparation of the carbonate. These new values need to be considered in determinations of absolute ¹⁸O/¹⁶O ratios of international reference standards and in relating analyses of carbonates to those of waters, silicates, and oxides.

4. Blake, R.E., O'Neil, J.R. and Garcia, G. A. (1997) Oxygen isotope systematics of biologically-mediated reactions of phosphate. I. Microbial degradation of organophosphorous compounds. Geochim. Cosmochim. Acta, 61, 4411-4422.

Abstract: Microbial activity has been invoked to explain anomalous oxygen isotope compositions of phosphate mineral deposits as well as fossil biogenic apatite. Results of laboratory experiments on enzyme-mediated reactions of phosphate and microbially-mediated degradation of organic matter, an important mechanism for the regeneration of dissolved phosphate in modern porewaters, demonstrate that significant exchange of oxygen isotopes between phosphate and water accompanies the hydrolytic cleavage of organically-bound phosphate as well as the metabolism of inorganic orthophosphate. Evaluation of the oxygen isotope systematics of microbially-mediated reactions of phosphate demonstrates that oxygen isotope exchange between phosphate and water mediated by bacteria is governed by equilibrium rather than kinetic factors. Under certain conditions, the microbially-mediated exchange appears to result in complete re-equilibration of oxygen isotopes between phosphate and water and in other instances equilibrium exchange may be masked by inheritance of phosphate-oxygen from the organic substrate. Analogous bacterial processes in natural sediments may be important in the release of dissolved phosphate to pore fluids, precipitation of authigenic apatite, and in the diagenetic alteration of phosphorite deposits and biogenic apatite. These results have important implications for paleoclimatological and paleoenvironmental studies in which oxygen isotope ratios of biogenic phosphate are used as paleotemperature indicators, as well as studies employing phosphate oxygen isotopes as a tracer of P transport and cycling in the environment.