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1. Helium, inorganic and organic carbon isotopes of fluids and gases across the Costa Rica convergent margin

   https://doi.org/10.1038/s41597-019-0302-4  

   Barry, Peter H. ; Nakagawa, Mayuko ; Giovannelli, Donato ; Maarten de Moor, J. ; Schrenk, Matthew ; Seltzer, Alan M. ; Manini, Elena ; Fattorini, Daniele ; di Carlo, Marta ; Regoli, Francesco ; et al ( November 2019 , Scientific Data)

   In 2017, fluid and gas samples were collected across the Costa Rican Arc. He and Ne isotopes, C isotopes as well as total organic and inorganic carbon concentrations were measured. The samples (n = 24) from 2017 are accompanied by (n = 17) samples collected in 2008, 2010 and 2012. He-isotopes ranged from arc-like (6.8 R_A) to crustal (0.5 R_A). Measured dissolved inorganic carbon (DIC) δ¹³C_VPDBvalues varied from 3.55 to −21.57‰, with dissolved organic carbon (DOC) following the trends of DIC. Gas phase CO₂only occurs within ~20 km of the arc; δ¹³C_VPDBvalues varied from −0.84 to −5.23‰. Onsite, pH, conductivity, temperature and dissolved oxygen (DO) were measured; pH ranged from 0.9–10.0, conductivity from 200–91,900 μS/cm, temperatures from 23–89 °C and DO from 2–84%. Data were used to develop a model which suggests that ~91 ± 4.0% of carbon released from the slab/mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition with an additional 3.3 ± 1.3% incorporated into autotrophic biomass.

    

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2. Recycled Components in Mantle Plumes Deduced From Variations in Halogens (Cl, Br, and I), Trace Elements, and 3 He/ 4 He Along the Hawaiian‐Emperor Seamount Chain

   https://doi.org/10.1029/2018GC007959  

   Broadley, Michael W. ; Sumino, Hirochika ; Graham, David W. ; Burgess, Ray ; Ballentine, Chris J. ( January 2019 , Geochemistry, Geophysics, Geosystems)

   Halogens are primarily located within surface reservoirs of the Earth; as such they have proven to be effective tracers for the identification of subducted volatiles within the mantle. Subducting lithologies exhibit a wide variety of halogen compositions, yet the mantle maintains a fairly uniform signature, suggesting halogens may be homogenized during subduction to the mantle or during eruption. Here we present halogen (Cl, Br, and I), K, noble gas, and major and trace element data on olivines from three seamounts along the Hawaiian‐Emperor seamount chain to determine if the deep mantle source has retained evidence of halogen heterogeneities introduced through subduction. High Ni contents indicate that the Hawaiian‐Emperor mantle source contains a recycled oceanic crust component in the form of pyroxenite, which increases from the 46% in the oldest (Detroit) to 70% in the younger seamount (Koko). Detroit seamount retains mid‐ocean ridge basalts (MORB)‐like Br/Cl and I/Cl, while the Br/Cl and I/Cl of Suiko and Koko seamounts are higher than MORB and similar to altered oceanic crust and dehydrated serpentinite. Helium isotopes show a similar evolution, from MORB‐like values at Detroit seamount toward higher values at Suiko and Koko seamounts. The correlation between pyroxenite contributions, Br/Cl, I/Cl, and³He/⁴He indicates that subducted material has been incorporated into the primordial undegassed Hawaiian mantle plume source. The identification of recycled oceanic crustal signatures in both the trace elements and halogens indicates that subduction and dehydration of altered oceanic crust may exert control on the cycling of volatile elements to the deep mantle.

    

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3. Cycling of CO 2 and N 2 Along the Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, and Isotopic Approach

   https://doi.org/10.1029/2021GC009650  

   Epstein, Gabe S. ; Bebout, Gray E. ; Christenson, Bruce W. ; Sumino, Hirochika ; Wada, Ikuko ; Werner, Cynthia ; Hilton, David R. ( September 2021 , Geochemistry, Geophysics, Geosystems)

   We present a quantitative assessment of the input and output of CO₂and N₂along the Hikurangi margin based on the chemical and stable isotope composition of sediments and basalts (from IODP 375), previously accreted metasedimentary rocks, and volcanic/hydrothermal gases (together with noble gas data for the latter). We compare these results with 3‐D thermo‐petrologic models for four lithologic structures, representing different plateau inputs. The model results indicate that 59%–85% of initially subducted C and 5%–12% of N is lost from the slab during metamorphism, with both volatiles being dominantly sourced from altered oceanic crust with some contribution from subducted sediment at the forearc‐arc transition (75–90 km depth). The δ¹³C_VPDBand CO₂/³He values for the arc gases range from −8.3 to −1.4‰ and 2 × 10⁹to 2.7 × 10¹¹, indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ¹⁵N_airand N₂/³⁶Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 10⁴to 1.9 × 10⁵, indicating slab and mantle contributions of 74% and 26%. The δ¹³C signature of gases requires addition of organic C by tectonic erosion and/or shallow crustal assimilation. These calculations yield whole‐margin fluxes of 5.4–7.0 Tg/yr for CO₂and 0.0022–0.0057 Tg/yr for N_2,corresponding to ∼2.2% and 1%–30% of the global CO₂and N₂flux from subaerial volcanoes worldwide (assuming no loss during transit). This unique assessment of volatile cycling could prove useful in refining regional and global estimates of volatile recycling efficiency.

    

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4. The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

   https://doi.org/10.1029/2021GC010071  

   Chilson‐Parks, Benjamin H. ; Calabozo, Fernando M. ; Saal, Alberto E. ; Wang, Zhengrong ; Mallick, Soumen ; Petrinovic, Ivan A. ; Frey, Frederick A. ( January 2022 , Geochemistry, Geophysics, Geosystems)

   The Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ¹⁸O_olivinevalues from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ¹⁸O values, providing a different explanation for the low‐δ¹⁸O signatures observed in continental arc settings.

    

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5. Thallium Isotopes Reveal Brine Activity During Carbonatite Magmatism

   https://doi.org/10.1029/2020GC009472  

   Horton, Forrest ; Nielsen, Sune ; Shu, Yunchao ; Gagnon, Alan ; Blusztajn, Jerzy ( March 2021 , Geochemistry, Geophysics, Geosystems)

   Carbonatite volcanism remains poorly understood compared to silicic volcanism due to the scarcity of carbonatite volcanoes worldwide and because volcanic H₂O and CO₂—major components in carbonatite volcanic systems—are not well preserved in the rock record. To further our understanding of carbonatite genesis, we utilize the non‐traditional thallium (Tl) isotope system in Khanneshin carbonatites in Afghanistan. These carbonatites contain 250–30,000 ng/g Tl and have ε²⁰⁵Tl values (−4.6 to +4.6) that span much of the terrestrial igneous range. We observe that δ¹⁸O_VSMOW(+8.6‰ to +23.5‰) correlates positively with δ¹³C_VPDB(−4.6‰ to +3.5‰) and ε²⁰⁵Tl up to δ¹⁸O = 15‰. Rayleigh fractionation of calcite from an immiscible CO₂‐H₂O fluid with a mantle‐like starting composition can explain the δ¹⁸O and δ¹³C—but not ε²⁰⁵Tl—trends. Biotite fractionates Tl isotopes in other magmatic settings, so we hypothesize that a Tl‐rich hydrous brine caused potassic metasomatism (i.e., biotite fenitization) of wall rock that increased the ε²⁰⁵Tl of the residual magma‐fluid reservoir. Our results imply that, in carbonatitic volcanic systems, simultaneous igneous differentiation and potassic metasomatism increase ε²⁰⁵Tl, δ¹⁸O, δ¹³C, and light rare earth element concentrations in residual fluids. Our fractionation models suggest that the Tl isotopic compositions of the primary magmas were among the isotopically lightest (less than or equal to ε²⁰⁵Tl = −4.6) material derived from the mantle for which Tl isotopic constraints exist. If so, the ultimate source of Tl in Khanneshin lavas—and perhaps carbonatites elsewhere—may be recycled ocean crust.

    

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