^1,2Christine E. Jilly-Rehak, ²Gary R. Huss, ²Kazu Nagashima, ³Devin L. Schrader
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.10.007]
¹Department of Geology & Geophysics, University of Hawai‘i at Mānoa, 1680 East-West Rd. POST 517A, Honolulu HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, 1680 East-West Rd. POST 602, Honolulu HI 96822, USA
³Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe AZ 85287, USA
Copyright Elsevier

The presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early Solar System; however, the process of aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) chondrites are particularly well-suited for the study of aqueous alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots of the alteration process through time. We studied oxygen isotopes in secondary-minerals from six CR chondrites of varying hydration states to determine how aqueous fluid conditions (including composition and temperature) evolved on the parent body. Secondary minerals analyzed included calcite, dolomite, and magnetite. The O-isotope composition of calcites ranged from δ18O ≈ 9 to 35 ‰, dolomites from δ18O ≈ 23 to 27 ‰, and magnetites from δ18O ≈ -18 to 5 ‰. Calcite in less-altered samples showed more evidence of fluid evolution compared to heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted on a single trend, with the exception of grains from the extensively hydrated chondrite MIL 090292. The MIL 090292 magnetite diverges from this trend, possibly indicating an anomalous origin for the meteorite. If magnetite and calcite formed in equilibrium, then the relative 18O fractionation between them can be used to extract the temperature of co-precipitation. Isotopic fractionation in Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (∼60°C). Assuming that the CR parent body experienced closed-system alteration, a similar exercise for parallel calcite and magnetite O-isotope arrays yields “global” alteration temperatures of ∼55 to 88 °C. These secondary mineral arrays indicate that the O-isotopic composition of the altering fluid evolved upon progressive alteration, beginning near the Al Rais water composition of Δ17O ∼ 1 ‰ and δ18O ∼ 10 ‰, and becoming increasingly 16O-enriched toward a final fluid composition of Δ17O ∼ -1.2 ‰ and δ18O ∼ -15 ‰.

^1,2Manuela A. Fehr, ¹Samantha J. Hammond, ^1,3Ian J. Parkinson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.10.010]
¹Department of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space & Astronomical Research, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
²Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
³Bristol Isotope Group, School of Earth Sciences, University of Bristol, Wills Memorial Building, BS8 1RJ, UK
Copyright Elsevier

New methodologies employing a 125Te-128Te double-spike were developed and applied to obtain high precision mass-dependent tellurium stable isotope data for chondritic meteorites and some terrestrial samples by multiple-collector inductively coupled plasma mass spectrometry. Analyses of standard solutions produce Te stable isotope data with a long-term reproducibility (2SD) of 0.064 ‰ for δ130/125Te. Carbonaceous and enstatite chondrites display a range in δ130/125Te of 0.9‰ (0.2‰ amu-1) in their Te stable isotope signature, whereas ordinary chondrites present larger Te stable isotope fractionation, in particular for unequilibrated ordinary chondrites, with an overall variation of 6.3‰ for δ130/125Te (1.3‰ amu-1). Tellurium stable isotope variations in ordinary chondrites display no correlation with Te contents or metamorphic grade. The large Te stable isotope fractionation in ordinary chondrites is likely caused by evaporation and condensation processes during metamorphism in the meteorite parent bodies, as has been suggested for other moderately and highly volatile elements displaying similar isotope fractionation. Alternatively, they might represent a nebular signature or could have been produced during chondrule formation.

Enstatite chondrites display slightly more negative δ130/125Te compared to carbonaceous chondrites and equilibrated ordinary chondrites. Small differences in the Te stable isotope composition are also present within carbonaceous chondrites and increase in the order CV-CO-CM-CI. These Te isotope variations within carbonaceous chondrites may be due to mixing of components that have distinct Te isotope signatures reflecting Te stable isotope fractionation in the early solar system or on the parent bodies and potentially small so-far unresolvable nucleosynthetic isotope anomalies of up to 0.27 ‰. The Te stable isotope data of carbonaceous and enstatite chondrites displays a general correlation with the oxidation state and hence might provide a record of the nebular formation environment.

The Te stable isotope fractionation of the carbonaceous chondrites CI and CM (and CO potentially) overlap within uncertainty with data for terrestrial Te standard solutions, sediments and ore samples. Assuming the silicate Earth displays similar Te isotope fractionation as the studied terrestrial samples, the data indicate that the late veneer might have been delivered by material similar to CI or CM (or possibly) CO carbonaceous chondrites in terms of Te isotope composition.

Nine terrestrial samples display resolvable Te stable isotope fractionation of 0.85 and 0.60‰ for δ130/125Te for sediment and USGS geochemical exploration reference samples, respectively. Tellurium isotopes therefore have the potential to become a new geochemical sedimentary proxy, as well as a proxy for ore-exploration.

¹T.S. Peretyazhko, ²P.B. Niles, ¹B. Sutter, ²R.V. Morris, ³D.G. Agresti, ¹L. Le, ²D.W. Ming
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.10.004]
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058
²NASA Johnson Space Center, Houston, TX 77058
³University of Alabama at Birmingham, Birmingham, AL 35294
Copyright Elsevier

The excess of orbital detection of smectite deposits compared to carbonate deposits on the martian surface presents an enigma because smectite and carbonate formations are both favored alteration products of basalt under neutral to alkaline conditions. We propose that Mars experienced acidic events caused by sulfuric acid (H2SO4) that permitted phyllosilicate, but inhibited carbonate, formation. To experimentally verify this hypothesis, we report the first synthesis of smectite from Mars-analogue glass-rich basalt simulant (66 wt% glass, 32 wt% olivine, 2 wt% chromite) in the presence of H2SO4 under hydrothermal conditions (∼200 °C). Smectites were analyzed by X-ray diffraction, Mössbauer spectroscopy, visible and near-infrared reflectance spectroscopy and electron microprobe to characterize mineralogy and chemical composition. Solution chemistry was determined by Inductively Coupled Plasma Mass Spectrometry. Basalt simulant suspensions in 11-42 mM H2SO4 were acidic with pH ≤ 2 at the beginning of incubation and varied from acidic (pH 1.8) to mildly alkaline (pH 8.4) at the end of incubation. Alteration of glass phase during reaction of the basalt simulant with H2SO4 led to formation of the dioctahedral smectite at final pH ∼3 and trioctahedral smectite saponite at final pH ∼4 and higher. Anhydrite and hematite formed in the final pH range from 1.8 to 8.4 while natroalunite was detected at pH 1.8. Hematite was precipitated as a result of oxidative dissolution of olivine present in Adirondack basalt simulant. Formation of secondary phases, including smectite, resulted in release of variable amounts of Si, Mg, Na and Ca while solubilization of Al and Fe was low. Comparison of mineralogical and solution chemistry data indicated that the type of smectite (i.e., dioctahedral vs trioctahedral) was likely controlled by Mg leaching from altering basalt and substantial Mg loss created favorable conditions for formation of dioctahedral smectite. We present a model for global-scale smectite formation on Mars via acid-sulfate conditions created by the volcanic outgassing of SO2 in the Noachian and early Hesperian.