Nabil Z. Boctor

Abiotic nitrogen reduction on the early Earth

The production of organic precursors to life depends critically onthe form of the reactants. In particular, an environment dominated by N2 is far less efficient in synthesizing nitrogen-bearing organics than a reducing environment rich in ammonia (refs 1, 2). Relatively reducing lithospheric conditions on the early Earth have been presumed to favour the generation of an ammonia-rich atmosphere, but this hypothesis has not been studied experimentally. Here we demonstrate mineral-catalysed reduction of N2, NO2− and NO3− to ammonia at temperatures between 300 and 800 °C and pressures of 0.1–0.4 GPa — conditions typical of crustal and oceanic hydrothermal systems. We also show that only N2 is stable above 800 °C, thus precluding significant atmospheric ammonia formation during hot accretion. We conclude that mineral-catalysed N2 reduction might have provided a significant source of ammonia to the Hadean ocean. These results also suggest that, whereas nitrogen in the Earths early atmosphere was present predominantly as N2, exchange with oceanic, hydrothermally derived ammonia could have provided a significant amount of the atmospheric ammonia necessary to resolve the early-faint-Sun paradox.

A Reduced Organic Carbon Component in Martian Basalts

Abiotic Martian Organics Understanding the sources and the formation mechanisms of organic carbon compounds on Mars has implications for our understanding of the martian carbon cycle. Steele et al. (p. 212, published online 24 May) present measurements of organic material in 11 martian meteorites, including the Tissint meteorite, which fell in the Moroccan desert in July 2011. Ten of the meteorites contain complex hydrocarbons encased within igneous minerals. The results imply that the organics formed as the magma melt crystallized and are thus of abiotic origin. Analysis of 11 martian meteorites reveals complex hydrocarbons associated with magmatic minerals in 10 of them. The source and nature of carbon on Mars have been a subject of intense speculation. We report the results of confocal Raman imaging spectroscopy on 11 martian meteorites, spanning about 4.2 billion years of martian history. Ten of the meteorites contain abiotic macromolecular carbon (MMC) phases detected in association with small oxide grains included within high-temperature minerals. Polycyclic aromatic hydrocarbons were detected along with MMC phases in Dar al Gani 476. The association of organic carbon within magmatic minerals indicates that martian magmas favored precipitation of reduced carbon species during crystallization. The ubiquitous distribution of abiotic organic carbon in martian igneous rocks is important for understanding the martian carbon cycle and has implications for future missions to detect possible past martian life.

Geochemical roots of autotrophic carbon fixation: Hydrothermal experiments in the system citric acid, H2O-(′FeS)-(′NiS)

Recent theories have proposed that life arose from primitive hydrothermal environments employ- ing chemical reactions analogous to the reductive citrate cycle (RCC) as the primary pathway for carbon fixation. This chemistry is presumed to have developed as a natural consequence of the intrinsic geochemistry of the young, prebiotic, Earth. There has been no experimental evidence, however, demonstrating that there exists a natural pathway into such a cycle. Toward this end, the results of hydrothermal experiments involving citric acid are used as a method of deducing such a pathway. Homocatalytic reactions observed in the citric acid-H2O experiments encompass many of the reactions found in modern metabolic systems, i.e., hydration- dehydration, retro-Aldol, decarboxylation, hydrogenation, and isomerization reactions. Three principal de- composition pathways operate to degrade citric acid under thermal and aquathermal conditions. It is concluded that the acid catalyzed decarboxylation pathway, leading ultimately to propene and CO2, may provide the most promise for reaction network reversal under natural hydrothermal conditions. Increased pressure is shown to accelerate the principal decarboxylation reactions under strictly hydrothermal conditions. The effect of forcing the pH via the addition of NaOH reveals that the decarboxylation pathway operates even up to intermediate pH levels. The potential for network reversal (the conversion of propene and CO2 up to a tricarboxylic acid) is demonstrated via the Koch (hydrocarboxylation) reaction promoted heterocatalytically with NiS in the presence of a source of CO. Specifically, an olefin (1-nonene) is converted to a monocar- boxylic acid; methacrylic acid is converted to the dicarboxylic acid, methylsuccinic acid; and the dicarboxylic acid, itaconic acid, is converted into the tricarboxylic acid, hydroaconitic acid. A number of interesting sulfur-containing products are also formed that may provide for additional reaction. The intrinsic catalytic qualities of FeS and NiS are also explored in the absence of CO. It was shown that the addition of NiS has a minimal effect in the product distribution, whereas the addition of FeS leads to the formation of hydrogenated and sulfur-containing products (thioethers). These results point to a simple hydrothermal redox pathway for citric acid synthesis that may have provided a geochemical ignition point for the reductive citrate cycle. Copyright

RADIOMETRIC AGES OF BASALTIC ACHONDRITES AND THEIR RELATION TO THE EARLY HISTORY OF THE SOLAR SYSTEM

PbPb mineral isochrons were determined for three cumulate eucrites (Moama, Moore County, and Serra de Mage) and three noncumulate eucrites (Nuevo Laredo, Bouvante, and Stannern). Two noncumulate eucrites (Bereba and Cachari) show disturbed Pb isotope patterns. The mineral isochron ages of cumulate eucrites range from 4.40 Ga to 4.48 Ga. The latter is the age of Moore County (4.484 ± 0.019 Ga), which was also dated by the SmNd method at 4.456 ± 0.025 Ga. Nuevo Laredo and Bouvante give the same Pb-Pb age: 4.514 ± 0.015 Ga and 4.510 ± 0.004 Ga, respectively. The Pb isochron of Stannern corresponds to 4.128 ± 0.016 Ga, which is a reequilibration age. Both Cachari and Bereba show evidence of isotope resetting at about 4 Ga. A Pb-Pb multicotrelation approach is introduced and applied to the data. The younger ages of the cumulate eucrites preclude their formation by fractional crystallization from the melts that produced the noncumulate eucrites. The μ1 of the source of the cumulate eucrites is ∼ 14, whereas that of the source of the noncumulate eucrites is -150. This large difference in μ1 values suggests that eucrites were derived from separate planetesimals or from a single heterogeneous (layered?) parent body.

Mineral-solution equilibria—IV. Solubilities and the thermodynamic properties of FeCl20 in the system Fe2O3-H2-H2O-HCl

Abstract The solubility of hematite in chloride-bearing hydrothermal fluids was determined in the temperature range 400–600°C and at 1000 and 2000 bars using double-capsule, rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method ( Frantz and Popp , 1979). The changes in the molalities of associated hydrogen chloride ( m HC l 0 ) as a function of the molality of total iron in the fluid at constant temperature and pressure were used to identify the predominant species of iron in the hydrothermal fluid. The molality of associated HCl varied from 0.01 to 0.15. Associated FeCl20 was found to be the most abundant species in equilibrium with hematite. Determination of Cl/Fe in the fluid in equilibrium with hematite yields values approximately equal to 2.0 suggesting that ferrous iron is the dominant oxidation state. The equilibrium constant for the reaction Fe2O3 + 4HCl0 + H2 = 2FeCl20 + 3H2O was calculated and used to estimate the difference in Gibbs free energy between FeCl20 and HCl0 in the temperature range 400–600°C at 1000 and 2000 bars pressure.

Mineral-solution equilibria—V. Solubilities of rock-forming minerals in supercritical fluids

The solubility constants of sixty-nine rock-forming minerals have been computed for temperatures between 400 and 600°C at 1000 and 2000 bar pressure using the free-energy data for aqueous solutes presented in Parts I through IV of this series combined with the thermodynamic properties of minerals from Helgesonet al. (1978). An example describing solution compositions in equilibrium with a spilite is discussed. A computer program for calculating solution compositions in equilibrium with mineral assemblages is included as an appendix.

Characterization of a high-pressure phase of silica from the Martian meteorite Shergotty

Abstract Recently, there has been substantial interest in post-stishovite high-pressure polymorphs of SiO2, discovered as extraterrestrial minerals, or synthesized in the laboratory. Previous investigators reported the presence of “α-PbO2-like” and “baddeleyite-like” SiO2 in the Martian meteorite Shergotty, and also the synthesis of an a-PbO2-like phase at pressures of 60-80 GPa in a laser-heated diamond anvil cell. To provide definitive information about the nature of the natural “a-PbO2 phase,” we recovered a small sample from the Shergotty meteorite, obtained powder X-ray diffraction patterns, and performed a Rietveld refinement of the structure. The resulting cell parameters and space group are a = 4.097(1) Å, b = 5.0462(9) Å, c = 4.4946(8) Å, and Pbcn. The structure refinement confirms that this sample does have the α-PbO2 structure

Seifertite, a dense orthorhombic polymorph of silica from the Martian meteorites Shergotty and Zagami

Seifertite is a dense orthorhombic polymorph of silica with the scrutinyite (α-PbO2) type structure that was found as lamellae occurring together with dense silica glass lamellae in composite silica grains in the heavily shocked Martian meteorite Shergotty. The mineral is also intergrown in some grains with minor stishovite and a new unnamed monoclinic dense silica polymorph with a ZrO2-type structure. Seifertite has also been found in the Martian shergottite Zagami and is a minor constituent in other Martian shergottites. Chemical analyses of seifertite in Shergotty indicate major SiO2 with minor concentrations of Al2O3 and Na2O. Selected-area electron diffraction (SAED) and X-ray diffraction can be interpreted in terms of an orthorhombic pattern from a scrutinyite (α-PbO2) structure. The cell parameters are a = 4.097(1) A, b = 5.0462(9) A, c = 4.4946(8) A, V = 92.92 A3, Z = 4, and the space group is Pbcn or Pb 2 n . Density is (calc.) = 4.294 g/cm3 (with pure SiO2), 4.309 g/cm3 (with empirical formula). It is inferred that seifertite was formed by shock-induced solid-state transformation of either tridymite or cristobalite on Mars at an estimated minimum equilibrium shock pressure in excess of 35 GPa. The new mineral is named after Friedrich A. Seifert (b. 1941), founding Director of the Bayerisches Geoinstitut, Universitat Bayreuth, Germany, for his seminal contributions to high-pressure geoscience.

Lafayette meteorite - Petrology and opaque mineralogy

The Lafayette meteorite is a calcium-rich achondrite composed predominantly of cumulus clinopyroxene (Wo39En39Fs22) with minor iron-rich olivine (Fa66) and rare interstitial feldspar (Or4Ab62An34-Or22Ab65An13). The opaque oxide minerals are magnetite-ilmenite intergrowths, an apparently homogenous Cr-rich (Cr2O3 ∼ 7 wt.%) titaniferous magnetite, and discrete ilmenite. Pyrite, the major sulfide mineral, occasionally displays lamellar intergrowths of marcasite that appear to be of primary origin. Troilite occurs as inclusions in ilmenite or interstitial to the silicate minerals. Bulk chemical composition of the glass in the fusion crust is comparable with that of the Nakhla meteorite. The glass contains abundant skeletal crystals of magnetite, as well as very rare, zoned magnetite grains with rims enriched in Mg and depleted in Ti relative to the cores. Although no brecciation is visible in the Lafayette meteorite, the presence of deformed twin lamellae in the clinopyroxene indicates some deformation has occurred.

Rhondonite solubility and thermodynamic properties of aqueous MnCl2 in the system MnOSiO2HClH2O

Abstract The solubility of rhodonite, represented by the reaction MnSiO 3 ( rhodonite ) + 2 HCl 0 = MnCl 2 0 + SiO 2 ( quartz ) + H 2 O , was investigated experimentally in the temperature range 400°–700°C at 1 and 2 kbar by rapid-quench hydrothermal techniques and the Ag-AgCl buffer methods. Variations in the molalities of associated hydrogen chloride ( m HCl 0 ) as a function of the molalities of total Mn indicate that Mn in the fluid in equilibrium with the assemblage rhodonite + quartz is predominantly associated as MnCl 2 0 . The Mn:Cl in the fluid ≅2, indicating that Mn +2 is the dominant oxidation state. The solubility data were used to calculate the equilibrium constant of the above reaction as a function of temperature, pressure, and the difference in Gibbs free energy of formation between MnCl 2 0 and HCl 0 . The equilibrium constants of solubility for Mn minerals for which thermochemical data are available were also calculated. Calculated mineral solubilities were used in conjunction with the data of Frantz et al . (1981) to calculate the composition of supercritical fluids in equilibrium with Mn-bearing phases and assemblages. At 400°C and 1000 bars, supercritical fluids in equilibrium with olivines of compositions similar to those present in MORB tend to be enriched in Mn, despite the low mole fraction of tephroite in the olivine. Supercritical fluids in equilibrium with the assemblage quartz-hematite-rhodonite at 500° and 400°C and 1000 bars show high concentrations of Mn relative to Fe. Manganese concentrations in the fluids increase with decrease in the mole fraction of H, whereas Fe concentrations decrease. The data indicate that H fugacity plays a significant role in the separation of Mn from Fe in chloride-bearing hydrothermal fluids at supercritical temperatures.