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1. Lithogeochemical Vectors and Mineral Paragenesis of Hydrothermal REE-Bearing Fluorite Veins and Breccia Deposits in the Gallinas Mountains, New Mexico

   Owen, E. ; Gysi, A.P. ; McLemore, V. ( September 2021 , SEG 100)

   The Gallinas Mountains district located in Lincoln and Torrance Counties, New Mexico, is host to hydrothermal REE-bearing fluorite veins and breccia deposits. The rare earth elements (REE) are found in bastnäsite-(Ce) ([La,Ce]CO3F) which is also the primary ore mineral mined in several important carbonatite deposits (e.g. Mountain Pass in California; Bayan Obo in China). Minor production of REE, fluorite, Cu, Pb, Zn, Ag, and Fe has been recorded in the Gallinas Mountains district between the early 1900s and the 1950s. The REE-bearing fluorite veins and breccias are hosted in Permian sedimentary rocks as well as genetically related trachyte/syenite sills and dikes emplaced between 28-30 Ma. Previous studies have described the REE occurrences in the Gallinas Mountains but the controls of hydrothermal processes on the transport and deposition of REE in the district remain unclear. In this study, we combine microtextural observations with mineral and bulk rock chemistry of hydrothermal REE-bearing fluorite veins and breccias to determine the vein types, alteration styles and establish a detailed mineral paragenesis. The goal of this study is to determine lithogeochemical vectors towards REE enriched zones in the district by linking thin section and deposit scale observations with mineral and bulk rock geochemistry. This district is an exceptional natural laboratory for studying the role of hydrothermal processes for transport/deposition of REE in an alkaline F-rich magmatic- hydrothermal system because very few deposits worldwide have such well-preserved and exposed geology. Hand samples of hydrothermal veins and breccias containing fluorite ± calcite ± barite ± bastnäsite-(Ce) were collected from outcrops, prospect pits, and mine dumps. Optical microscopy was used to identify minerals and determine the textural features and crosscutting relationships of the different fluorite veins. The veins were classified into: i) hematite-fluorite veins; ii) barite- bearing bastnäsite-fluorite veins; iii) barite-bearing (fluorite)-calcite veins. Nearly all of the barite crystals in the fluorite veins display dissolution textures (skeletonized crystals) with infilling of mostly fluorite and minor calcite, suggesting that barite is part of an earlier paragenetic mineral assemblage. Bastnäsite-(Ce) is commonly found in veins containing barite and occurs either as disseminated crystals in the fluorite veins or together with fluorite infills around large barite crystals. A few of the barren fluorite-calcite veins display an intergrowth with euhedral barite crystals indicating that these could be part of an earlier barite paragenesis. These textural observations suggest a key control of REE mineralization in the Gallinas Mountains district by a coupled dissolution of barite-bearing (fluorite)-calcite veins and precipitation of later bastnäsite- fluorite veins. Geochemical bulk rock data collected from the New Mexico Bureau of Geology and Mineral Resources database were analyzed using the IMDEX ioGASTM program to definegeochemical signatures of rock types, alteration styles, and vein types. Preliminary data analysis indicates a positive correlation between Ba, F, and total rare earth oxides (TREO). These trends corroborate with the observed vein microtextures, suggesting that the interaction of a hydrothermal fluids with the barite-bearing (fluorite)-calcite veins represents a key process for defining geochemical vectors in the district. 

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2. CO 2 reduction driven by a pH gradient

   https://doi.org/10.1073/pnas.2002659117  

   Hudson, Reuben ; de Graaf, Ruvan ; Strandoo Rodin, Mari ; Ohno, Aya ; Lane, Nick ; McGlynn, Shawn E. ; Yamada, Yoichi M. ; Nakamura, Ryuhei ; Barge, Laura M. ; Braun, Dieter ; et al ( September 2020 , Proceedings of the National Academy of Sciences)

   null (Ed.)

   All life on Earth is built of organic molecules, so the primordial sources of reduced carbon remain a major open question in studies of the origin of life. A variant of the alkaline-hydrothermal-vent theory for life’s emergence suggests that organics could have been produced by the reduction of CO 2 via H 2 oxidation, facilitated by geologically sustained pH gradients. The process would be an abiotic analog—and proposed evolutionary predecessor—of the Wood–Ljungdahl acetyl-CoA pathway of modern archaea and bacteria. The first energetic bottleneck of the pathway involves the endergonic reduction of CO 2 with H 2 to formate (HCOO – ), which has proven elusive in mild abiotic settings. Here we show the reduction of CO 2 with H 2 at room temperature under moderate pressures (1.5 bar), driven by microfluidic pH gradients across inorganic Fe(Ni)S precipitates. Isotopic labeling with 13 C confirmed formate production. Separately, deuterium ( 2 H) labeling indicated that electron transfer to CO 2 does not occur via direct hydrogenation with H 2 but instead, freshly deposited Fe(Ni)S precipitates appear to facilitate electron transfer in an electrochemical-cell mechanism with two distinct half-reactions. Decreasing the pH gradient significantly, removing H 2 , or eliminating the precipitate yielded no detectable product. Our work demonstrates the feasibility of spatially separated yet electrically coupled geochemical reactions as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alkaline hydrothermal systems to couple carbon reduction to hydrogen oxidation through biologically relevant mechanisms, these results may also be of significance for industrial and environmental applications, where other redox reactions could be facilitated using similarly mild approaches. 

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3. Formation of Complex Organic Molecules in Hot Molecular Cores through Nondiffusive Grain-surface and Ice-mantle Chemistry

   https://doi.org/10.3847/1538-4365/ac3131  

   Garrod, Robin T. ; Jin, Mihwa ; Matis, Kayla A. ; Jones, Dylan ; Willis, Eric R. ; Herbst, Eric ( February 2022 , The Astrophysical Journal Supplement Series)

   A new, more comprehensive model of gas–grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g., methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. The observed ratios of structural isomers methyl formate, glycolaldehyde, and acetic acid are well reproduced by the models. The main temperature regimes in which various complex organic molecules (COMs) are formed are identified. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH₂are found to contribute substantially to the formation of certain COMs.

    

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4. Prebiotic Origin of Pre‐RNA Building Blocks in a Urea “Warm Little Pond” Scenario

   https://doi.org/10.1002/cbic.202000510  

   Menor Salván, C. ; Bouza, Marcos ; Fialho, David M. ; Burcar, Bradley T. ; Fernández, Facundo M. ; Hud, Nicholas V. ( September 2020 , ChemBioChem)

   Urea appears to be a key intermediate of important prebiotic synthetic pathways. Concentrated pools of urea likely existed on the surface of the early Earth, as urea is synthesized in significant quantities from hydrogen cyanide or cyanamide (widely accepted prebiotic molecules), it has extremely high water solubility, and it can concentrate to form eutectics from aqueous solutions. We propose a model for the origin of a variety of canonical and non‐canonical nucleobases, including some known to form supramolecular assemblies that contain Watson‐Crick‐like base pairs.The dual nucleophilic‐electrophilic character of urea makes it an ideal precursor for the formation of nitrogenous heterocycles. We propose a model for the origin of a variety of canonical and noncanonical nucleobases, including some known to form supramolecular assemblies that contain Watson‐Crick‐like base pairs. These reactions involve urea condensation with other prebiotic molecules (e. g., malonic acid) that could be driven by environmental cycles (e. g., freezing/thawing, drying/wetting). The resulting heterocycle assemblies are compatible with the formation of nucleosides and, possibly, the chemical evolution of molecular precursors to RNA. We show that urea eutectics at moderate temperature represent a robust prebiotic source of nitrogenous heterocycles. The simplicity of these pathways, and their independence from specific or rare geological events, support the idea of urea being of fundamental importance to the prebiotic chemistry that gave rise to life on Earth.

    

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5. Expedition 376 Preliminary Report: Brothers Arc Flux

   https://doi.org/10.14379/iodp.pr.376.2018  

   de Ronde, C.E.J. ; Humphris, S.E. ; Höfig, T.W. ( June 2019 , Preliminary report)

   null (Ed.)

   Volcanic arcs are the surface expression of magmatic systems that result from the subduction of mostly oceanic lithosphere at convergent plate boundaries. Arcs with a submarine component include intraoceanic arcs and island arcs that span almost 22,000 km on Earth’s surface, the vast majority of which are located in the Pacific region. Hydrothermal systems hosted by submarine arc volcanoes commonly contain a large component of magmatic fluid. This magmatic-hydrothermal signature, coupled with the shallow water depths of arc volcanoes and their high volatile contents, strongly influences the chemistry of the fluids and resulting mineralization and likely has important consequences for the biota associated with these systems. The high metal contents and very acidic fluids in these hydrothermal systems are thought to be important analogs to numerous porphyry copper and epithermal gold deposits mined today on land. During International Ocean Discovery Program (IODP) Expedition 376 (5 May–5 July 2018), a series of five sites was drilled on Brothers volcano in the Kermadec arc. The expedition was designed to provide the missing link (i.e., the third dimension) in our understanding of hydrothermal activity and mineral deposit formation at submarine arc volcanoes and the relationship between the discharge of magmatic fluids and the deep biosphere. Brothers volcano hosts two active and distinct hydrothermal systems: one seawater-influenced and the other affected by magmatic fluids (largely gases). A total of 222.4 m of volcaniclastics and lavas was recovered from the five sites drilled, which include Sites U1527 and U1530 in the Northwest (NW) Caldera seawater-influenced hydrothermal field; Sites U1528 and U1531 in the magmatic fluid-influenced hydrothermal fields of the Upper and Lower Cones, respectively; and Site U1529, located in a magnetic low that marks the West (W) Caldera upflow zone on the caldera floor. Downhole logging and borehole fluid sampling were completed at two sites, and two tests of a prototype turbine-driven coring system (designed by the Center for Deep Earth Exploration [CDEX] at Japan Agency for Marine-Earth Science and Technology [JAMSTEC]) for drilling and coring hard rocks were conducted. Core recovered from all five sites consists of dacitic volcaniclastics and lava flows with only limited chemical variability relative to the overall range in composition of dacites in the Kermadec arc. Pervasive alteration with complex and variable mineral assemblages attest to a highly dynamic hydrothermal system. The upper parts of several drill holes at the NW Caldera hydrothermal field are characterized by secondary mineral assemblages of goethite + opal-A + zeolites that result from low-temperature (<150°C) reaction of rock with seawater. At depth, NW Caldera Site U1527 exhibits a higher temperature (~250°C) secondary mineral assemblage dominated by chlorite + quartz + illite + pyrite. An older mineral assemblage dominated by diaspore + quartz + pyrophyllite + rutile at the bottom of Hole U1530A is indicative of acidic fluids with temperatures of ~230°–320°C. By contrast, the alteration assemblage at Site U1528 on the Upper Cone is dominated by illite + natroalunite + pyrophyllite + quartz + opal-CT + pyrite, which attests to high-temperature reaction of rocks with acid-sulfate fluids derived from the disproportionation of magmatic SO2. These intensely altered rocks exhibit extreme depletion of major cation oxides, such as MgO, K2O, CaO, MnO, and Na2O. Furthermore, very acidic (as low as pH 1.8), relatively hot (≤247°C) fluids collected at depths of 160, 279, and 313 meters below seafloor (mbsf) in Hole U1528D have chemical compositions indicative of magmatic gas input. In addition, preliminary fluid inclusion data provide evidence for involvement of two distinct fluids: phase-separated (modified) seawater and an ~360°C hypersaline brine, altering the volcanic rock and potentially transporting metals in the system. The material and data recovered during Expedition 376 provide new stratigraphic, lithologic, and geochemical constraints on the development and evolution of Brothers volcano and its hydrothermal systems. Insights into the consequences of the different types of fluid-rock reactions for the microbiological ecosystem elucidated by drilling at Brothers await shore-based studies. 

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