Mineralogical and Geochemical Evolution of a Basalt-Hosted Fossil Soil (Late Triassic, Ischigualasto Formation, Northwest Argentina): Potential for Paleoenvironmental Reconstruction * Neil J. Tabor 1, Isabel P. Montañez 2, Robert Zierenberg 2, Brian S. Currie 3 1 Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275-0395 (ntabor@mail.smu.edu) 2 Department of Geology, University of California, Davis, CA 95616 3 Department of Geology, Miami University, Oxford, OH
Mineralogy Whole-rock samples were ground in an agate mortar and pestle, dispersed in deionized water and shaken overnight. Mineralogical analysis by X-ray diffraction of rock powders was carried out on randomly oriented samples (in an aluminum mount) between 2-60 2 A separate aliquot of the bulk paleosol matrix was used to isolate the <2µm-size fraction by centrifugation. The octahedral coordination of the phyllosilicates was determined from the d(060)-spacing of powdered <2 m-size fraction samples (Moore and Reynolds, 1997). All other X-ray diffraction mineralogical analyses of the <2µm fraction clays were carried out on samples that were exchange-saturated with K or Mg on filter membranes and transferred to glass slides as oriented aggregates. Duplicate Mg-saturated clays were also prepared with glycerol. Oriented aggregates of all Mgtreated samples were analyzed at 25 C; the K-saturated clays were analyzed, after 2 hours of heating at each temperature, at 25 C, 300 C, and 500. Step-scan analyses were performed on a Diano 8500 X-ray diffractometer using CuK radiation between 2 and 30 2 with a step size of 0.04 2 and count time of 1s. Mineral composition of the samples was determined following the methods of Moore and Reynolds (1997). Mineralogy of a bulk Fe-oxide sample was determined by X-ray diffraction (XRD) analysis using Cu-K radiation on a Diano 8500 X-ray. Powders were backmounted into an aluminum holder and step-scanned from 2-70 2 with 0.01 steps, a dwell time of 12s, 40KV and 20ma, 0.5/1.0mm for the primary slits and 0.2/0.3 mm for the receiving slits. The amount of Al 3+ substituted for Fe in goethite was determined by the XRD method of Schulze (1984), with an analytical uncertainty of ±3 mole %. 1
Mineralogic characterization of the host-basalt and calcite nodules was made by petrographic analysis of polished thin sections under reflected and transmitted-light. Chemical Pretreatment and Analysis The <2µm-size phyllosilicate fraction was pretreated to remove non-phyllosilicate constituents that can complicate interpretation of measured 18 O and D values. Chemical treatments follow in the order of application: (1) 0.5M NaAOc to remove carbonate (Savin and Epstein, 1970), (2) successive aliquots of 30% H 2 O 2 solution (room temperature) to remove occluded organic matter, and (3) Sodium citrate-bicarbonatedithionite solution (25 C) to remove admixed iron oxy-hydroxides (Jackson, 1979). One aliquot of each bulk and chemically treated <2µm-size fraction sample was fused into glass at high temperature (~1200 C) on molybdenum strips in an Ar atmosphere. Samples were analyzed for major oxide compositions with a Cameca SX 50 electron microprobe. Microprobe data are reported as the average value of six analyses for every sample. Phyllosilicate samples were stored in 2M NaCl solution and subsequently washed to remove excess NaCl before chemical analysis, so most of the exchange sites in these phyllosilicates should have been occupied by Na + prior to fusion. Chemical characterization of each component in the host basalt was estimated from microprobe analysis of major and minor elemental compositions of individual analyses. Chemical pretreatment of the iron-oxide sample follows the methods of Yapp (1998). Samples were ground in a corrundum mortar and pestle under reagent-grade acetone and sized by passage through a 63 m brass sieve. Only powders from the <63 m particle size fraction were used in this study. Samples were treated with ~40mL of 0.5N 2
HCl solution overnight to remove carbonates and then rinsed with deionized H 2 O. The sample was subsequently treated over a period of 28 days with successive 40mL aliquots of 30% H 2 O 2 at room temperature. Samples undergoing this treatment are designated as bulk in the subsequent figures and tables. An aliquot of the bulk sample was combined with lithium tetraborate to produce a 2:1 mixture on a mass basis prior to chemical analysis (c.f. Poage et al., 2001). Chemical analyses of dilute HNO 3 + sample solutions were performed with ICP-OES. The relative analytical error of these analyses is ±2% of the reported value for the oxide component. In order to assess the isotopic composition of non-iron oxide constituents within the bulk Fe-oxide rich sample, ~200mg of bulk sample were treated with Citrate- Bicarbonate-Dithionite (CBD) digestion solution to remove Fe-oxide (Jackson, 1979). The remaining non-iron oxide residue was then washed with deionized H 2 O, subsequently treated with four successive 40 ml aliquots of 30% H 2 O 2, and then dried at 23 C in a vacuum desiccator. These non-iron oxide constituents are designated residue sample. Isotopic Analysis The <2µm-size fraction phyllosilicates, bulk Fe-oxide and residue samples were reacted with BrF 5 at ~560 C to release oxygen, following the methods of Clayton and Mayeda (1963). Mineral-bound hydrogen for D analysis was extracted as H 2 O by heating the samples to ~850 C under vacuum in closed-system conditions. The liberated structural water was then converted to H 2 by passage over hot (~750 C) U-metal (Biegelsen et al., 1952). Both oxygen (as CO 2 ) and hydrogen gas samples were analyzed on a Finnigan 3
MAT 252 in the Department of Geological Sciences at the Southern Methodist University. 18 O and D values are reported relative to the V-SMOW standard (Gonfiantini, 1984) with an analytical uncertainty, based on replicate analyses of samples and quartz standard NBS 28, of 0.2 for 18 O and 4 for D. Finally, the wt. % H 2 O of bulk and residue samples were measured manometrically after complete dehydration and used in conjunction with chemical analyses to calculate the mole fraction of H 2 O in each sample. Representative carbonate samples from each paleosol horizon were drilled directly from thin-sections or matching billets using a hand-held dental drill equipped with faceted 100 m diamond bits. Approximately 50 g of carbonate powder were roasted at 375 C in vacuum for three hours to remove organics. Stable isotope analysis of calcites were carried out on a Fisons-Optima IR gas source mass spectrometer in the Department of Geology at UC Davis. Carbonate 18 O values are given relative to the standard V-SMOW (Gonfiantini 1984). Replicate analysis of NBS-19 (n=39) yielded 18 O values of 2.07 0.15 over the period of analysis. 4
Biegelsen, J., Perlman, M. L., and Prosser, H. C., 1952, Conversion of hydrogenic materials to hydrogen for isotopic analysis: Analytical Chemistry, v. 24, p. 1356-1357. Clayton, R. N., and Mayeda, T. K., 1963, The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis: Geochimica et Cosmochimica Acta, v. 27, p. 43-52. Gonfiantini, R., 1984, Advisory group meeting on stable isotope reference samples for geochemical and hydrological investigations: Rep Director General IAEA Vienna. Jackson, M. L., 1979, Soil Chemical Analysis Advanced Course: Published by the author, Madison, Wisconsin. Moore, D. M., and Reynolds, R. C., 1997, X-ray diffraction and the identification and analysis of clay minerals: Oxford University Press, New York. Poage, M.A., Sjostrom, D.J., Goldberg, J., Chamberlain, C.P., and Furniss, G., 2001, Isotopic evidence for Holocene climate change in the northern Rockies from a goethite-rich ferricrete chronosequence: Chemical Geology, v. 166, p. 327-340. Savin, S. M., and Epstein, S., 1970, The oxygen and hydrogen isotope geochemistry of clay minerals: Geochimica et Cosmochimica Acta, v. 34, p. 25-42. Schulze, D.G., 1984, The influence of aluminum on iron oxides. VIII. Unit cell dimensions of Al-substituted goethites and estimation of aluminum from them: Clays and Clay Minerals, v. 32, p. 36-44. Yapp, C.J., 1998, Paleoenvironmental interpretations of oxygen isotope ratios in oolitic ironstones: Geochimica et Cosmochimica Acta, v. 62, p. 2409-2420. 5