Experimental Mineralogy

The rates and mechanisms of reactions between aqueous fluids and rocks are being determined experimentally by me and colleagues at the Chemical Sciences Division of Oak Ridge National Laboratory. L. Anovitz, J. Blencoe, D. Cole, M. Fayek, and L. Riciputi are determining the thermodynamics of the system H2O–CO2–NaCl, the diffusivity of C and O in carbonate minerals, the kinetics of reactions in the system CaMg(CO3)2–H2O–CO2, and C and O isotopic exchange between calcite and H2O–CO2 fluid.

Isotope and cation exchange in alkali feldspar

Nanoscale isotope and chemical images of grains of Amelia albite that were reacted with 2 M 18O-enriched solution of KCl show a correspondence between O-isotope exchange and K–Na exchange. Experiments were conducted for 4–6 d at 600 °C and 200 MPa. After 6 d, the 150 μm diameter albite grains had 5–20 μm rims in which Na was nearly completely replaced by K and in which the O was strongly enriched in 18O. The boundary between the core albite and the K-feldspar replacement is sharp and decorated with numerous pores. The distribution of Na and K, determined by electron probe microanalysis, is uniform within the core and rim and has an abrupt discontinuity at the interface. There is no evidence for K–Na interdiffusion at the resolution of electron probe. The NanoSIMS shows that the interface is also sharp in the distribution of 18O and 16O. The NanoSIMS image data and the electron probe data were coregistered; principal components analysis of the merged data set shows that 86% of the total variance in the data result from a single principal component loaded by the replacement of Na by K and 18O. The combined electron probe and NanoSIMS analyses indicate that both cation and isotope exchange occurred during solution and reprecipitation of the feldspar.

Diffusion of C and O in carbonate minerals

The diffusivities of C and O in calcite were determined in a pure CO2 atmosphere at 100 MPa and temperatures ranging from 600 to 800 °C. The calcite crystals were preannealed and H2O was excluded from the system to determine the self-diffusion coefficients. The CO2 consisted of 99% 13C and 90% 18O. After heating for 7–147 d, diffusion profiles were measured with the use of secondary ion mass spectrometry. The results indicate that the diffusivity of C is DC = 7.77 × 10−9 exp(−166 ± 16 kJ/mol/RT) cm2/s and of O is DO = 7.5 × 10−3 exp (−242 ± 39 kJ/mol/RT) cm2/s. In comparison with other determinations of diffusivities in calcite, diffusion of O under the experimental conditions is consistent with vacancy migration, and diffusion of C seems to occur by diffusion of carbonate anions. Increased pressure appears to reduce the activation energy and the value of D0, and the presence of H2O greatly increases the diffusivity of O without appreciably changing the activation energy. Closure temperatures calculated for isotopic exchange by diffusion predict that C isotope compositions of calcite are preserved during cooling in most geologic environments, but that O isotope compositions in H2O-rich environments are preserved only in rapidly cooling environments, such as contact metamorphic aureoles.

Dolomite breakdown in mixed-volatile fluid

Metamorphism of carbonate rock is commonly the result of reaction between the minerals in the rock and an H2O-rich fluid. One example is the breakdown of dolomite, producing calcite + periclase. The reaction results in a reduction of the solid volume of the rock and a production of CO2. Both contribute to the increase in permeability during reaction, permitting continued ingress of H2O. We have attempted to determine some of the processes that occur during infiltration and reaction experimentally. We cut 4 mm diameter cores of dolomite rocks with a variety of textures ranging from fine-grained sedimentary dolostones to coarse-grained dolomitic marbles. Experiments are carried out using a conventional cold-seal hydrothermal apparatus. The dolomite rock cores were sealed in gold capsules with an aliquot of isotopically enriched water of composition HD18O0.516O0.5. The samples were held at a P = 100 MPa and T = 650–700 °C for durations ranging from a few days to a few months.

After experimentation, the cores were sectioned and examined by XRD, EMP, SIMS, and CL techniques. All experiments show some reaction, even in experiments lasting for only two days. In samples with a large water-rock ratio, complete reaction occurred within 30 d at 700 °C. Crystallization of new mineral phases within coarse samples is concentrated along fluid infiltration pathways, such as grain boundaries and fractures. Smaller grain size samples show more pervasive crystallization, with original dolomite nearly completely replaced by calcite. Although periclase is the stable phase for the conditions of these experiments, extensive brucite is observed and is believed to be the result of rapid hydration of periclase upon quench. SIMS ion imaging shows extensive enrichment of 18O along dolomite grain boundaries and in fractures. There is little evidence in the short-duration experiments for any exchange between fluid and the dolomite, but the newly formed products of reaction are strongly enriched in 18O. Under the conditions of the experiments with water–rock ratios > 1, grain-boundary transport readily occurs over the sample distances of 2–4 mm.

Some Recent References

DeAngelis, M.T., Rondinone, A.J., Pawel, M.D., Labotka, T.C., and Anovitz, L.A. (2012) Sol-gel synthesis of nanocrystalline fayalite (Fe2SiO4). American Mineralogist 97, in press.

Labotka, T.C., Cole, D.R., Fayek, M.J., and Chacko, T. (2011) An experimental study of the diffusion of C and O in calcite in mixed CO2–H2O fluid. American Mineralogist 96, 1262–1269.

DeAngelis, M. T., Labotka, T. C., Cole, D. R., Fayek, M., and Anovitz, L. M. (2007) Experimental investigation of the breakdown of dolomite in rock cores at 100 MPa, 650–750 °C. American Mineralogist 92, 510--517.

Labotka, T. C., Cole, D.R., Riciputi, L.R., and Fayek, M. (2004) Diffusion of C and O in calcite from 0.1 to 200 MPa. American Mineralogist 89, 799–806.

Anovitz, L.M., Labotka, T. C., Blencoe, J.G., and Horita, J. (2004) Experimental determination of the activity–composition relations and phase equilibria of H2O–CO2–NaCl fluids at 500 °C, 500 bars. Geochimica et Cosmochimica Acta 68, 3557–3567.

Labotka, T. C., Cole, D. R., Fayek, M., Riciputi, L.R., and Stadermann, F. J. (2004) Coupled cation and oxygen-isotope exchange between alkali feldspar and aqueous chloride solution. American Mineralogist 89, 1822–1825.

Student Theses

Michael DeAngelis, PhD 2011, Experimental investigations of fluid–mineral interactions in olivine and dolomite.

Cara Mulcahy, MS 2005, Experimental determination of the stability of the basic magnesium carbonate nesquehonite. 

Michael DeAngelis, MS 2004, Experimental investigation of fluid and isotope transport in dolomite rock.

© Theodore C. Labotka 2017