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1. Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera

   https://doi.org/10.1029/2021GC009863  

   Canada, Andrew S.; Cassel, Elizabeth J.; Smith, M. Elliot (October 2021, Geochemistry, Geophysics, Geosystems)

   Abstract Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene “Lake Adobe” of limited aerial extent and a laterally extensive middle Eocene “Lake Elko,” are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ¹⁸O, δ¹³C and⁸⁷Sr/⁸⁶Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of¹²C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of⁸⁷Sr/⁸⁶Sr in the upper Elko Formation reflects an increasing aerial extent of low‐⁸⁷Sr/⁸⁶Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate. 

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2. Fluid Mixing and Spatial Geochemical Variability in the Lost City Hydrothermal Field Chimneys

   https://doi.org/10.1029/2023GC011011  

   Aquino, Karmina_A; Früh‐Green, Gretchen_L; Bernasconi, Stefano_M; Rickli, Jörg; Lang, Susan_Q; Lilley, Marvin_D (February 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (⁸⁷Sr/⁸⁶Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ¹³C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower⁸⁷Sr/⁸⁶Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate. 

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3. The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

   https://doi.org/10.1029/2021GL094301  

   Moynier, Frédéric; Jackson, Matthew G.; Zhang, Ke; Cai, Hongming; Halldórsson, Sæmundur Ari; Pik, Raphael; Day, James M. D.; Chen, Jiubin (September 2021, Geophysical Research Letters)

   Abstract The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ¹⁹⁹Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ¹⁹⁹Hg for terrestrial sub‐aerially‐derived materials and positive Δ¹⁹⁹Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐³He/⁴He lavas from Samoa and Iceland, providing an average of Δ¹⁹⁹Hg = 0.00 ± 0.10, Δ²⁰¹Hg = −0.02 ± 0.0.09,δ²⁰²Hg = −1.7 ± 1.2; 2SD,N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ¹⁹⁹Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ¹⁹⁹Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ¹⁹⁹Hg which correlate with⁸⁷Sr/⁸⁶Sr, consistent with variable proportions of continental (low Δ¹⁹⁹Hg and high⁸⁷Sr/⁸⁶Sr) and oceanic (high Δ¹⁹⁹Hg and low⁸⁷Sr/⁸⁶Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions. 

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4. Single Spot Rb‐Sr Isochron Dating of Biotite by LA‐MC‐ICP‐MS / MS

   https://doi.org/10.1111/ggr.12518  

   Cruz‐Uribe, Alicia M.; Craig, Grant; Garber, Joshua M.; Paul, Bence; Arkula, Cemil; Bouman, Claudia (July 2023, Geostandards and Geoanalytical Research)

   Laser ablation tandem mass spectrometry is a burgeoning field forin situRb‐Sr geochronology. Here, we determined simultaneous isotope ratios of⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC‐ICP‐MS/MS. Measurements were made on Faraday cups with Rb⁺at mass 87; Sr isotopes were reacted with SF₆gas and measured as SrF⁺at masses 103–107. Twenty‐two laser spots in biotite from a single sample yield a traditional Rb‐Sr isochron date of 289 ± 6 Ma. Time‐resolved signals reveal significant zoning in⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr within single spot analyses, which were used to construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb‐Sr isochrons that are decoupled from variations in Rb/Sr. Thirty‐five isochron dates were determined using this sub‐spot approach, with⁸⁷Sr/⁸⁶Sr intercepts that systematically vary with Rb‐Sr date; two‐point isochrons were calculated for individual integrations (n= 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb‐Sr system has the potential to record multiple heating, cooling or fluid‐alteration events spanning ~ 100 My within small domains in single biotite crystals. 

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5. Record of Neotectonics and Deep Crustal Fluid Circulation Along the Santa Fe Fault Zone in Travertine Deposits of the Lucero Uplift, New Mexico, USA

   https://doi.org/10.1029/2020GC009454  

   Garcia, V. H.; Ma, L.; Ricketts, J. W.; Dosseto, A. (April 2021, Geochemistry, Geophysics, Geosystems)

   Abstract Travertine deposits preserve an invaluable record of both ancient and modern fluid flow. The goal of this study is to reconstruct spatial and temporal patterns in travertine deposition associated with tectonic and climatic controls along the Lucero Uplift in New Mexico, USA. Uranium‐series ages of travertine deposits in the Lucero Uplift range from 0.94 ± 0.01 to 592 ± 110 ka, indicating that travertine formation has been episodically active since at least ∼600 ka. We find minimal evidence to attribute glacial and interglacial cycles to travertine formation in the Lucero Uplift. δ¹³C values in travertine deposits range from 2‰ to 9‰ (Vienna Pee Dee Belemnite), δ¹⁸O values range from 21‰ to 25‰ (Vienna Standard Mean Ocean Water). Positive correlation between travertine δ¹³C and δ¹⁸O values indicate travertine formation is closely associated with various degrees of CO₂degassing.⁸⁷Sr/⁸⁶Sr values in travertine deposits range from 0.714 to 0.717 and (²³⁴U/²³⁸U)_ivalues exhibit a remarkably wide range from 3.6 to 9.3, indicative of fluid‐rock interaction during deep crustal circulation in more radiogenic basement rocks. Reconstructed δ¹³C, δ¹⁸O, and (²³⁴U/²³⁸U)_ivalues in the inferred deep fluid sources showed systematic variations with travertine formation ages, while⁸⁷Sr/⁸⁶Sr values remain relatively constant. Based on dating of undeformed travertine deposits, which overlie tilted Santa Fe Group units, and high (²³⁴U/²³⁸U)_iwe infer that the Santa Fe fault has not produced a ground‐rupturing earthquake within the last 490 ± 52 to 592 ± 110 ka (2σ). Our study suggest that travertine formation is driven by fluid flow facilitated by tectonic and mantle structures. 

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