^aNicole X. Nie, ^aNicolas Dauphas, ^bRichard C. Greenwood
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.035]

^aOrigins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, United States
^bPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Copyright Elsevier

Banded iron formations (BIFs) contain appreciable amounts of ferric iron (Fe³⁺). The mechanism by which ferrous iron (Fe²⁺) was oxidized into Fe³⁺ in an atmosphere that was globally anoxic is highly debated. Of the three scenarios that have been proposed to explain BIF formation, photo-oxidation by UV photons is the only one that does not involve life (the other two are oxidation by O₂ produced by photosynthesis, and anoxygenic photosynthesis whereby Fe²⁺ is directly used as electron donor in place of water). We experimentally investigated iron and oxygen isotope fractionation imparted by iron photo-oxidation at a pH of 7.3. The iron isotope fractionation between precipitated Fe³⁺-bearing lepidocrocite and dissolved Fe²⁺ follows a Rayleigh distillation with an instantaneous ⁵⁶Fe/⁵⁴Fe fractionation factor of +1.2‰. Such enrichment in the heavy isotopes of iron is consistent with the values measured in BIFs. We also investigated the nature of the mass-fractionation law that governs iron isotope fractionation in the photo-oxidation experiments (i.e., the slope of the δ⁵⁶Fe–δ⁵⁷Fe relationship). The experimental run products follow a mass-dependent law corresponding to the high-T equilibrium limit. The fact that a ∼3.8 Gyr old BIF sample (IF-G) from Isua (Greenland) falls on the same fractionation line confirms that iron photo-oxidation in the surface layers of the oceans was a viable pathway to BIF formation in the Archean, when the atmosphere was largely transparent to UV photons.

Our experiments allow us to estimate the quantum yield of the photo-oxidation process (∼0.07 iron atom oxidized per photon absorbed). This yield is used to model iron oxidation on early Mars. As the photo-oxidation proceeds, the aqueous medium becomes more acidic, which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing UV-photons. Iron photo-oxidation in centimeter to meter-deep water ponds would take months to years to complete. Oxidation by O₂ in acidic conditions would be slower. Iron photo-oxidation is thus likely responsible for the formation of jarosite–hematite deposits on Mars, provided that shallow standing water bodies could persist for extended periods of time.

The oxygen isotopic composition of lepidocrocite precipitated from the photo-oxidation experiment was measured and it is related to the composition of water by mass-dependent fractionation. The precipitate-fluid ¹⁸O/¹⁶O isotope fractionation of ∼+6‰ is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water.

^aJ.J. Bellucci, ^aM.J. Whitehouse, ^bT. John, ^a,cA.A. Nemchin, ^aJ.F. Snape, ^cP.A. Bland, ^cG.K. Benedix
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.028]

^aDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^bInstitut für Geologische Wissenschaften, Freie Universität Berlin, Malteser Str. 74-100, 12249 Berlin, Germany
^cDepartment of Applied Geology, Curtin University, Perth, WA 6845, Australia
Copyright Elsevier

The Cl isotopic compositions and halogen (Cl, F, Br, and I) abundances in phosphates from eight Martian meteorites, spanning most rock types and ages currently available, have been measured in situ   by Secondary Ion Mass Spectrometry (SIMS). Likewise, the distribution of halogens has been documented by x-ray mapping. Halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in Martian samples or on the Martian surface, and the inter-element ratios are highly variable. Similarly, Cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks. Phosphates in ancient (>4 Ga) meteorites (orthopyroxenite ALH 84001 and breccia NWA 7533) have positive δ³⁷Cl anomalies (+1.1 to +2.5‰). These samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity. In contrast, the phosphates in the younger basaltic Shergottite meteorites (<600 Ma) have negative δ³⁷Cl anomalies (−0.2 to −5.6‰). Phosphates with the largest negative δ³⁷Cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative δ³⁷Cl anomalies suggestive of interaction with the surface of Mars during the latest stages of basalt crystallization. The phosphates with no textural, major element, or halogen enrichment evidence for mixing with this surface reservoir have an average δ³⁷Cl of −0.6‰, supporting a similar initial Cl isotope composition for Mars, the Earth, and the Moon. Oxidation and reduction of chlorine are the only processes known to strongly fractionate Cl isotopes, both positively and negatively, and perchlorate has been detected in weight percent concentrations on the Martian surface. The age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines, which have been documented on the Martian surface, has been active throughout Martian history.

^a,bJacob A. Richardson, ^aJames A. Wilson, ^aCharles B. Connor, ^bJacob E. Bleacher, Koji Kiyosugi^c
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.09.040]

^aSchool of Geosciences, University of South Florida, Tampa, FL, USA
^bPlanetary Geology, Geophysics, and Geochemistry Laboratory, Code 698, NASA Goddard Space Flight Center, Greenbelt, MD, USA
^cOrganization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
Copyright Elsevier

Magmatism and volcanism have evolved the Martian lithosphere, surface, and climate throughout the history of Mars. Constraining the rates of magma generation and timing of volcanism on the surface clarifies the ways in which magma and volcanic activity have shaped these Martian systems. The ages of lava flows on other planets are often estimated using impact crater counts, assuming that the number and size-distribution of impact craters per unit area reflect the time the lava flow has been on the surface and exposed to potential impacts. Here we show that impact crater age model uncertainty is reduced by adding stratigraphic information observed at locations where neighboring lavas abut each other, and demonstrate the significance of this reduction in age uncertainty for understanding the history of a volcanic field comprising 29 vents in the 110-km-diameter caldera of Arsia Mons, Mars. Each vent within this caldera produced lava flows several to tens of kilometers in length; these vents are likely among the youngest on Mars, since no impact craters in their lava flows are larger than 1 km in diameter. First, we modeled the age of each vent with impact crater counts performed on their corresponding lava flows and found very large age uncertainties for the ages of individual vents, often spanning the estimated age for the entire volcanic field. The age model derived from impact crater counts alone is broad and unimodal, with estimated peak activity in the field around 130 Ma. Next we applied our volcano event age model (VEAM), which uses a directed graph of stratigraphic relationships and random sampling of the impact crater age determinations to create alternative age models. Monte Carlo simulation was used to create 10,000 possible vent age sets. The recurrence rate of volcanism is calculated for each possible age set, and these rates are combined to calculate the median recurrence rate of all simulations. Applying this approach to the 29 volcanic vents, volcanism likely began around 200–300 Ma then first peaked around 150 Ma, with an average production rate of 0.4 vents per Myr. The recurrence rate estimated including stratigraphic data is distinctly bimodal, with a second, lower peak in activity around 100 Ma. Volcanism then waned until the final vents were produced 10–90 Ma. Based on this model, volume flux is also bimodal, reached a peak rate of 1–8 km³ Myr⁻¹by 150 Ma and remained above half this rate until about 90 Ma, after which the volume flux diminished greatly. The onset of effusive volcanism from 200–150 Ma might be due to a transition of volcanic style away from explosive volcanism that emplaced tephra on the western flank of Arsia Mons, while the waning of volcanism after the 150 Ma peak might represent a larger-scale diminishing of volcanic activity at Arsia Mons related to the emplacement of flank apron lavas.

^aQueenie H. S. Chan, ^aMichael E. Zolensky, ^bRobert.J. Bodnar, ^bCharles Farley, ^cJacob C. H. Cheung
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.10.048]

^aNASA Johnson Space Center, Houston, Texas 77058, USA
^bDepartment of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA
^cMet Office, Exeter, UK
Copyright Elsevier

Carbonates record information regarding the timing, nature and conditions of the fluids circulating through asteroid parent bodies during aqueous alteration events. Determining carbonate abundances and their relationships with organic matter improves our understanding of the genesis of major carbonaceous components in chondritic materials. In this study, five CM2 carbonaceous chondrites (CM2.2 Nogoya, CM2.3 Jbilet Winselwan, CM2.5 Murchison, CM2 Santa Cruz, and CM2TII Wisconsin Range 91600) were studied with Raman spectroscopy. Carbonates were identified in these meteorite samples by the distinctive Raman band in the ∼1100 cm^-1 region, representing the symmetric stretching vibration mode (v₁) of the (CO₃)^2- anion. Carbonates identified in the meteorite samples are all calcite, with the exception of a single dolomite grain in Nogoya. The v₁ positions of the CM calcites are 2−3 cm^-1 higher than in pure calcite, which suggests that they contain significant impurity cations. Typical graphitic first-order D and G bands were identified in the meteorite matrix as well as in ∼25% of the analyzed carbonate grains. From the Raman results, we postulate that the carbonates might not have formed under equilibrium conditions from a single fluid. The first generation of carbonate is interpreted to have formed from highly oxidized fluids that led to the oxidation of organic matter (OM) and produced carbonates that are OM-barren. The second generation of carbonate was formed from a more evolved aqueous fluid with the presence of OM. The Raman parameters of the organics in carbonates clearly deviate from the matrix OM which suggests that the carbonate organics contain very different carbonaceous components that are distinct from the typical amorphous OM of the CM matrix. The occurrence of different generations of carbonate in close proximity may be partly responsible for the wide range in estimated ages of carbonates in carbonaceous chondrites reported in previous studies.