¹T.S. Peretyazhko, ²A. Fox, ¹B. Sutter, ³P.B. Niles, ⁴M. Adams, ³R.V. Morris, ³D.W. Ming
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058
^{2/sup>Indiana University, Bloomington, IN 47406
3NASA Johnson Space Center, Houston, TX 77058
4University of Hawaii at Hilo, Hilo, HI 96720}

Akaganeite, a Cl-bearing Fe(III) (hydr)oxide, has been recently discovered in Yellowknife Bay in Gale crater on Mars by the Mars Science Laboratory (MSL) Curiosity Rover. Akaganeite was associated with sulfate and sulfide minerals at Yellowknife Bay indicating that sulfate ions could be present in solution during akaganeite formation. The mechanism and conditions of akaganeite formation in the Yellowknife Bay mudstone are unknown. We investigated formation of akaganeite through hydrolysis of ferric chloride solution in the presence of 0, 0.01, 0.05, 0.1 and 0.2 M sulfate and at initial pH of 1.5, 2 and 4 at 90 °C. Mineralogy of the precipitated Fe(III) phases was characterized by X-ray diffraction and infrared spectroscopy. The precipitates were also acid digested to determine total sulfate and chloride contents. Akaganeite and natrojarosite formed at initial solution pH of 1.5; akaganeite, goethite and natrojarosite precipitated in initial pH 2 solutions and goethite, hematite and 2-line ferrihydrite precipitated at initial solution pH of 4. Sulfate addition did not inhibit akaganeite formation. Increasing initial solution sulfate concentrations resulted in increasing sulfate to chloride ratio in the precipitated akaganeite. Infrared spectroscopy revealed akaganeite bands at ∼2 μm (H2O combination band) and at ∼2.46 μm (OH combination band). The H2O combination band position linearly correlated with total chloride content in akaganeite. Overall, laboratory studies demonstrated formation of akaganeite at initial sulfate concentration ⩽ 0.2 M (sulfate to chloride molar ratio ⩽0.3) and pH ⩽ 2, implying that those conditions might prevail (perhaps as micro-environments) during akaganeite formation in Yellowknife Bay mudstone. The occurrence of Fe(II) sulfides (pyrite and pyrrhotite) in Yellowknife Bay mudstone is a potential acidity source. Dissolution of sulfide minerals might occur under localized oxidizing water-limiting Cl-rich conditions creating favorable environments for akaganeite formation.

Reference
Peretyazhko TS, Fox A, Sutter B, Niles PB, Adams M, Morris RV, Ming DW (2016)
Synthesis of akaganeite in the presence of sulfate:Implications for akaganeite formation in Yellowknife Bay, Gale Crater, Mars. Geochmica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.06.002]
Copyright Elsevier

¹Toni Schulz, ²Ambre Luguet, ¹Wencke Wegner, ²David van Acken,^1,3Christian Koeberl
¹Department of Lithospheric Research, University Vienna, Vienna, Austria
²Steinmann Institut of Geology, Mineralogy and Palaeontology, University of Bonn, Bonn, Germany
³Natural History Museum, Vienna, Austria

The Lonar crater is a ~0.57-Myr-old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best-preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr-Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact-related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re-Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re-Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower 187Re/188Os and 187Os/188Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near-chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.

Reference
Schulz T, Luguet A, Wegner W, van Acken D, Koeberl C (2016) Target rocks, impact glasses, and melt rocks from the Lonar crater, India: Highly siderophile element systematics and Sr-Nd-Os isotopic signatures. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12665]
Published by arrangement with John Wiley & Sons