Geochimica et Cosmochimica Acta最新文献

{"title":"Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet","authors":"Getachew Agmuas Adnew ,&nbsp;Thomas Röckmann ,&nbsp;Thomas Blunier ,&nbsp;Christian Juncher Jørgensen ,&nbsp;Sarah Elise Sapper ,&nbsp;Carina van der Veen ,&nbsp;Malavika Sivan ,&nbsp;Maria Elena Popa ,&nbsp;Jesper Riis Christiansen","doi":"10.1016/j.gca.2024.11.009","DOIUrl":"10.1016/j.gca.2024.11.009","url":null,"abstract":"<div><div>Clumped isotopes of methane (CH₄), specifically <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH₄ provenance based on bulk isotopes. However, interpreting the origin of CH₄ using isotopes becomes challenging when the isotopic signature is altered by post-generation processes. In this study, we measured, for the first time, the bulk and clumped isotopic composition of sub-glacial CH₄ samples. These samples were collected from the air-filled headspace of the glacier portal (ice cave) at the edge of the Isunnguata Sermia glacier (ISG), located at the western margin of the Greenland ice sheet (GrIS). Our goal was to identify the processes underlying the sub-glacial production and potential processing of CH₄. The <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values are close to those of atmospheric CH₄. The elevated <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ values can be attributed to the alteration of the source’s isotope signal by aerobic oxidation. This conclusion is supported by previous studies at this site, which reported the presence of methanotrophic bacteria and dissolved oxygen close to saturation in the meltwater. Our results confirm that the correlation between <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>13</mn></mrow></msup></math></span>CH₃D and <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ is a useful tool for deciphering oxidation pathways. Our results support the inference that aerobic CH₄ oxidation can strongly modify the <span><math><msup><mrow><mi>Δ</mi></mrow><mrow><mn>12</mn></mrow></msup></math></span>CH₂D₂ isotope signal, which must be considered when determining the source signatures of environmental samples.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 249-264"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841590","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Iron isotope fractionation in highly evolved magmas results from ilmenite crystallization","authors":"Fengli Shao ,&nbsp;Yaoling Niu ,&nbsp;Haiquan Wei ,&nbsp;Yu Zhang ,&nbsp;Guodong Wang","doi":"10.1016/j.gca.2024.11.029","DOIUrl":"10.1016/j.gca.2024.11.029","url":null,"abstract":"<div><div>Globally, granites and rhyolites with SiO₂ &gt; 70 wt% show large Fe isotope variation (δ⁵⁶Fe = −0.05 to +0.65 ‰) relative to less silicic igneous rocks in δ⁵⁶Fe vs. SiO₂ space. The upper bound of the data tends to show δ⁵⁶Fe increase with increasing SiO₂. Granitic magma differentiation can be invoked to explain magma compositional variation, including Fe isotope variation, but clearly cannot explain the highly varied δ⁵⁶Fe values. The latter may result from magma differentiation of varying liquidus phases, magma mixing, assimilation and magma source compositional variation. To decipher how each of these and altogether explain the large δ⁵⁶Fe variation requires rigorous studies of varying well characterized sample suites. This paper is not to solve all these issues but demonstrates clearly using three sample suites with well-defined liquid lines of descent from alkaline basalts to peralkaline rhyolites to show that the δ⁵⁶Fe increases with continued magma differentiation (increasing SiO₂, SiO₂/MgO and decreasing MgO). The rapid δ⁵⁶Fe increase for samples with SiO₂ &gt; 70 wt% results from ilmenite (vs. magnetite) fractionation. Among all the major liquidus phases, ilmenite has a distinctive affinity with light-Fe isotope, whose crystallization elevates δ⁵⁶Fe in the residual melts. This result demonstrates the affinity of isotopically heavy Fe with Fe³⁺ and the correlation of isotopically light Fe with Fe²⁺ because δ⁵⁶Fe values of ilmenite (TiFe²⁺O₃) ≪ δ⁵⁶Fe values of magnetite (<span><math><mrow><msup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup><mi>O</mi><mo>∙</mo><msubsup><mrow><mi>F</mi><mi>e</mi></mrow><mrow><mn>2</mn></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msubsup><msub><mi>O</mi><mn>3</mn></msub></mrow></math></span>). We can conclude that ilmenite solid solution is likely the major oxide liquidus phase at the late-stage felsic melt evolution for relatively dry magmas with low <em>f</em>O₂ such as peralkaline rhyolites we study here and mid-ocean ridge basalts. We further predict that magnetite (vs. ilmenite) solid solution may be the important liquidus phase for wet magmas with high <em>f</em>O₂ such as volcanic arc magmas, where crystallization of magnetite with high δ⁵⁶Fe will deplete the heavy Fe isotopes in the residual melts. It is thus possible that the large Fe isotope variation of global igneous rocks with SiO₂ &gt; 70 wt% may result from varying parental magma compositions (varying water content and <em>f</em>O₂) plus bulk-rock modal mineralogy controls of granitic rocks. This work thus lays the foundation for testing this hypothesis through rigorous studies on ideal sample suites of global significance.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 30-42"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841591","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Molecular H2 in silicate melts","authors":"Dionysis I. Foustoukos","doi":"10.1016/j.gca.2024.10.020","DOIUrl":"10.1016/j.gca.2024.10.020","url":null,"abstract":"<div><div>A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H₂ between H₂O-saturated sodium aluminosilicate melts and H₂O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H₂ between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H₂ in silicate melts saturated with H₂O decreases as the temperature increases, with control exerted by the mole fraction of H₂O in the melt. The dissolution of H₂ in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:</div><div>ln <em>K</em>^m/f = 11.4 (±1.3) *1000/T (K) − 18.7 (±1.1)</div><div>where <em>K</em>^m/f is the equilibrium constant for the reaction H_2(g) = H_2(melt). This equation was derived by integrating data from the current and prior experimental studies that include silicate melts with varying H₂O saturation levels. It should be deemed applicable within the temperature range of 600–1450 °C and pressures ranging from 0.3 to 3 GPa. The implications are extended into developing an understanding of the H partitioning between H₂-rich atmospheres blanketing magma oceans in the early history of planetary bodies. For example, transferring H from primordial atmospheric envelopes to the interior of rocky exoplanets may be less efficient than previously believed, which should be considered in models of volatile retention. Experimental data also suggest that minimal amounts of solar nebula H₂ are likely to dissolve in the molten surface of primitive objects in the protoplanetary disk (∼10⁻⁵ to 10⁻⁹ mole faction of H₂ in the melt), contradicting the highly reducing conditions observed in chondrule mineral compositions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 125-135"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142718839","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The geochronology and cooling history of type 7 chondrites: Insights into the early impact events on chondritic parent body","authors":"Ye Li ,&nbsp;Yuting Wang ,&nbsp;Haoxuan Jiang ,&nbsp;Jia Liu ,&nbsp;Liping Qin ,&nbsp;Qiu-Li Li ,&nbsp;Yu Liu ,&nbsp;Zhenfei Wang ,&nbsp;Weibiao Hsu","doi":"10.1016/j.gca.2024.11.020","DOIUrl":"10.1016/j.gca.2024.11.020","url":null,"abstract":"<div><div>Type 7 chondrites, which record a higher degree of heating process than typical type 3 to type 6 chondrites, are characterized with textures and petrography of partial melting. Understanding the timing and cooling history of incipient melting event for type 7 chondrites could provide insights into the complex thermal process of the early solar system. Here, we studied two chondrites NWA 12272 and NWA 11021. Both samples display partial melting characteristics of LL chondrites, including interconnected plagioclase/high-Ca pyroxene network, zoned plagioclase and lack of chondrules, which concurs with their classification of LL7 chondrites in the Meteoritical Bulletin Database. The ⁵³Mn-⁵³Cr isotopic data of NWA 12272, determined by mineral separates and bulk samples, yielded an isochron with a ⁵³Mn/⁵⁵Mn ratio of (1.40 ± 0.59) × 10^-6 and a corresponding absolute age of 4558.8 ± 2.3 Ma (anchored to D’Orbigny angrite). Combined with the cooling rate estimated by the integration of REE-in-two-pyroxene thermometry and two-pyroxene thermometry, Mn-Cr isochron age of 4558.8 ± 2.3 Ma and Ca-phosphate Pb-Pb age of 4517 ± 6 Ma, we suggest that NWA 12272 experienced a two-stage cooling process after the incipient melting: it exposed to a relatively cold environment with a rapid cooling rate of ∼ 30-100°C/yr at 1150–1000 °C, and soon reburied with a slower cooling rate of ∼ 13 °C/Ma at 1000–475 °C. Although the Mn-Cr isotopic study was not conducted for NWA 11021, the average Ca-phosphate Pb-Pb age of 4509 ± 7 Ma and high-temperature cooling rate (∼1-30°C/yr) of NWA 11021 are indistinguishable from or slightly lower than those of NWA 12272. Assuming NWA 11021 cooled from the same incipient melting event as NWA 12272, it could have recorded a similar two-stage cooling process. We suggest that the studied LL7 chondrites were most likely formed in the early solar system when additional impact heat overlapped on the “heated” type 5–6 chondrites. Integrated with the previous cooling rates of LL6-7 chondrites, the prevailing two-stage cooling rates of LL chondrites provide compelling evidence for the fragmentation-re-accretion process in the early history of LL chondrite parent body. This early impact event also happened in other ordinary chondrite groups and some iron meteorites.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 280-293"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142763010","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Modern oceanic cycle of beryllium isotopes assessed using a data-constrained biogeochemical model","authors":"Kai Deng ,&nbsp;Gregory F. de Souza ,&nbsp;Jianghui Du","doi":"10.1016/j.gca.2024.10.025","DOIUrl":"10.1016/j.gca.2024.10.025","url":null,"abstract":"<div><div>Beryllium isotopes (stable ⁹Be and cosmogenic meteoric ¹⁰Be) enter the oceans through distinct pathways – i.e., from the continents and the atmosphere respectively – and display non-conservative behaviour in seawater. This isotope system has served as a powerful tool for quantifying a variety of processes, including geomagnetism, sedimentation, continental input, and ocean circulation. However, processes at land–ocean boundaries and within the ocean interior may either amplify or buffer the seawater isotope response to environmental changes. In the last decade, substantial effort has been invested in understanding external sources and internal cycling of Be isotopes, offering an excellent opportunity to revisit their modern oceanic cycle. Here, we investigate the controls on the modern oceanic cycling of Be isotopes using a three-dimensional ocean model, constrained by observational data on input fluxes and water-column distributions of ⁹Be and ¹⁰Be. In addition to modelling the previously known controls, we highlight the key role of marine benthic fluxes and scavenging onto particulate organic matter and opal in determining the mass balance and spatial distribution of Be isotopes. Inter-basin Be transport by the circulation is less important than external inputs at continent/atmosphere–ocean boundaries, except in the South Pacific. Therefore, the distribution of seawater ¹⁰Be/⁹Be ratios largely reflects that of the external inputs in most basins in the modern ocean. Finally, we apply our data-constrained mechanistic model to test the sensitivity of basin-wide ¹⁰Be/⁹Be ratios to changes of external sources and internal cycling. This analysis shows that seawater ¹⁰Be/⁹Be ratios are to some extent buffered against changes in continental denudation. For example, a 50 % decrease in denudation rates results in a 13–48 % increase in ocean-wide ¹⁰Be/⁹Be ratios. Moreover, the interplay between particle scavenging and ocean circulation can cause divergent responses in ¹⁰Be/⁹Be ratios in different basins. Weaker scavenging (e.g., 50 % decrease in intensity) would increase the homogenising effect of ocean circulation, making North Atlantic and North Pacific ¹⁰Be/⁹Be ratios converge (∼20 % change in isotope ratios). The mechanistic understanding developed from this Be cycling model provides important insights into the various applications of marine Be isotopes, and offers additional tools to assess the causes of spatio-temporal Be isotope variations. We also identify the key oceanic processes that require further constraints to achieve a complete understanding of Be cycling in the modern ocean and back through time.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 186-199"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673646","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Iron Isotopes reveal volcanogenic input during Oceanic Anoxic Event 2 (OAE 2 ∼ 94 Ma)","authors":"Lucien Nana Yobo ,&nbsp;Helen M. Williams ,&nbsp;Alan D. Brandon ,&nbsp;Chris Holmden ,&nbsp;Kimberly V. Lau ,&nbsp;Steven C. Bergman ,&nbsp;James S. Eldrett ,&nbsp;Daniel Minisini","doi":"10.1016/j.gca.2024.10.023","DOIUrl":"10.1016/j.gca.2024.10.023","url":null,"abstract":"<div><div>Anomalously high metal concentrations including iron enrichments are recorded in marine carbonates deposited during Ocean Anoxic Event 2 (OAE 2). These metal enrichments have been attributed to massive submarine eruptions during the formation of one or more large igneous provinces, the proposed trigger for OAE 2 (hydrothermal hypothesis), or to the release of metals from the reoxidation of formerly anoxic marine sediment during a period of temporary cooling during OAE 2 (sediment release hypothesis). Here we use iron stable isotopes to help distinguish between the two hypotheses for a trace metal enriched interval during OAE 2 in the Iona-1 core in the Western Interior Seaway, Texas. Our results show a two-step negative excursion during OAE 2 that is coincident with osmium isotope volcanic proxies measured in the same core, with peak negative values centered on a trace metal-enriched interval. After corrections for detrital and locally supplied iron to the study setting, the δ⁵⁶Fe value of the remotely supplied iron is –0.28 ± 0.05 ‰, falling in the range of iron δ⁵⁶Fe values observed in modern hydrothermal plumes (–0.1 to –0.5 ‰), thus supporting the hydrothermal hypothesis as the source of iron and other associated trace metals enriched in the study core during OAE 2. By contrast, the sediment release hypothesis predicts much lower δ⁵⁶Fe values, between –1.0 ‰ to –3.3 ‰ predicted for benthic supplies of iron from anoxic marine sediment overlying re-oxygenated bottom waters. This study shows that combining iron with other proxies for environmental change, particularly submarine volcanism, can distinguish hydrothermally supplied iron from dust, rivers, and shelf sediment supplies of iron despite iron’s reputation for complicated cycling.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 157-167"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673649","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Deciphering multiple episodes of partial melting, metasomatic and remelting processes in the Eastern Pyrenean orogenic mantle massif","authors":"Yujian Wang,&nbsp;Jingao Liu","doi":"10.1016/j.gca.2024.11.033","DOIUrl":"10.1016/j.gca.2024.11.033","url":null,"abstract":"<div><div>Secular variations in the composition of the subcontinental lithospheric mantle (SCLM) are intimately controlled by multiple geological processes, including diverse melting mechanisms and complex metasomatic processes. We present comprehensive analyses of whole rock and mineral chemistry, along with Re-Os isotopic system of the Eastern Pyrenean orogenic mantle massifs, supported by quantitative modeling through alphaMELTS thermodynamic software. The Eastern Pyrenean lherzolites display consistent negative correlations of TiO₂/Al₂O₃, TiO₂ versus MgO and relatively high and unfractionated heavy rare earth elements. These chemical features shed light on the diverse melting mechanisms responsible for these rocks, encompassing passive continuous melting in regions of lithospheric extension and decompression melting linked to the upwelling asthenospheric mantle. Some refractory harzburgites exhibit elevated TiO₂/Al₂O₃, TiO₂ contents and moderate light rare earth element enrichment. This suggests chromatographic metasomatism due to the reactive porous flow of evolved melts/fluids in the upper part of the lithosphere during their ascent to the surface. An isochron analogy between ¹⁸⁷Os/¹⁸⁸Os and Al₂O₃ yields an age of ∼ 1.5 Ga, constraining the timing of partial melting responsible for the formation of SCLM beneath the Eastern Pyrenean region. We then expand the scope of our study to encompass on– and off-cratonic SCLM on a global scale. This expanded analysis explores the variations in melting mechanisms across different tectonic settings and geological epochs and scrutinizes the role of diverse metasomatic processes in shaping the characteristics of the lithospheric mantle and its longevity on a broad scale. Silicate metasomatism typically produces fertile peridotites at the asthenosphere-lithosphere boundary, making them vulnerable to thermomechanical erosion, whereas cryptic metasomatism, commonly observed in refractory mantle rocks induced by evolved melts at decreasing melt-rock ratios (e.g., hydrocarbon-bearing silicate melt, carbonatite melt etc.), generally forms enrichment of highly incompatible elements but less significant influence on the mineral assemblages and major element geochemistry of the on-cratonic refractory SCLM, making them remain stable for extended periods.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 14-29"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841587","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Mineralogic control on the calcium and magnesium stable isotopic compositions of modern microbial carbonates","authors":"Elizabeth M. Griffith ,&nbsp;Matthew S. Fantle ,&nbsp;Brittan V. Wogsland ,&nbsp;Zijian Li ,&nbsp;Majie Fan ,&nbsp;David J. Wronkiewicz","doi":"10.1016/j.gca.2025.01.005","DOIUrl":"10.1016/j.gca.2025.01.005","url":null,"abstract":"<div><div>Modern microbialites grow in a variety of environments including the hypersaline, turbid, low alkalinity, high magnesium (Mg) and calcium (Ca) concentrations (and Mg/Ca ∼ 4.5 mol/mol), shallow (less than 2 m deep) Storr’s Lake on San Salvador Island, The Bahamas. Rather than growing via the trapping and binding of sediments, these largely micritic microbialites form via microbial processes creating crusts and mounds with laminated to clotted structures comprised of both high-Mg calcite and aragonite. The primary objective of this study was to determine whether the mineralogy of representative microbialite mounds dictates their pre-burial metal isotopic compositions. The Mg and Ca isotopic compositions (δ²⁶Mg, <em>δ</em>^44/40Ca) of two mounds collected from 0.6 to 1.1 m water depth range considerably, from −3.04 to −2.33 ‰ (relative to DSM3) and 0.38 to 0.94 ‰ (relative to SRM 915a), respectively. Along with Sr/Ca and Mg/Ca molar ratios, δ²⁶Mg and <em>δ</em>^44/40Ca are unambiguously related to mineralogy. This stands in contrast to the carbon and oxygen isotopic compositions of the microbial carbonates, which do not correlate solely with mineralogy. A simple bimineralic mixture of aragonite and high-Mg calcite can explain the observations; such a mixture could be a consequence of both phases forming independently or as one phase recrystallizes from another prior to burial. To evaluate the latter hypothesis, we used a time-dependent advection-recrystallization model. We found that recrystallization of high-Mg calcite to aragonite would require an unreasonable Sr partition coefficient, while recrystallization of aragonite to high-Mg calcite could explain the observed geochemical variations in the two mounds. No microbial isotopic effect is apparent for Mg or Ca, however a difference is seen in the carbon isotopic composition of inorganic carbon where or when aragonite and high-Mg calcite form suggesting they form in two different (micro)environments. Consequently, investigations of ancient microbialites (and carbonates in general) need to consider whether mixtures of primary (original) carbonate minerals and/or recrystallization and transformation pre-burial could impact δ²⁶Mg, <em>δ</em>^44/40Ca, δ¹³C and their interpretation using a combination of isotopic, elemental, petrographic, and modeling methods. This is especially important for Mg, whose elemental and isotopic composition can be dominated by even small amounts of calcite or dolomite.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 148-164"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143020255","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"One-pot synthesis of non-canonical ribonucleosides and their precursors from aldehydes and ammonia under prebiotic Earth conditions","authors":"Yuta Hirakawa ,&nbsp;Hidenori Okamura ,&nbsp;Fumi Nagatsugi ,&nbsp;Takeshi Kakegawa ,&nbsp;Yoshihiro Furukawa","doi":"10.1016/j.gca.2024.11.006","DOIUrl":"10.1016/j.gca.2024.11.006","url":null,"abstract":"<div><div>The formation of polymers that can hold gene information and work as catalysts is a crucial step for the origin of life. The discovery of catalytic RNA (i.e., ribozyme) supports the hypothesis that RNA might have served these functions at the early stage of life on the Earth. Given this, the spontaneous formation of RNA monomers (i.e., ribonucleotides) and their polymerization on Hadean Earth are essential steps for the origin of life. Previous experiments have investigated the chemical reactions that allow the formation of ribonucleotides and their components. These works have revealed the required molecules to form biological ribonucleotides (i.e., canonical ribonucleotides). Based on geochemical perspectives, abundantly available reactive molecules spontaneously react with each other to provide abundant products. Aldehydes and ammonia are reactive molecules assumed to have been present in considerable amounts on Hadean Earth. However, little is understood about whether or not nucleotides and their components were formed from these molecules under prebiotic conditions. We investigated the incubation products of alkaline aqueous solutions of aldehydes and ammonia. The product solution contained sugars (including ribose), various imidazole derivatives, and ribosyl imidazole (i.e., imidazole ribonucleoside). Ribosyl imidazole is formed via ribosyl amine, which reveals a new reaction pathway for prebiotic ribonucleoside synthesis. The imidazole ribonucleoside was then phosphorylated to imidazole ribonucleotide via a simple dry-down reaction with phosphate. Borate ion improved the reaction yields of these nucleosides and nucleotides. Because all the reactants were available on prebiotic Earth and the reactions progressed spontaneously, imidazole ribonucleotides could have accumulated in prebiotic environments. The experimental simplicity of the present reaction suggests that imidazoles were more abundant than canonical nucleobases on the prebiotic Earth. This further implies that prebiotic oligonucleotides contained imidazole bases in addition to the canonical nucleobases. The improvement of the reaction yields by borate indicates that borate-rich environments were conducive places for the formation and accumulation of non-canonical nucleosides and nucleotides. Such environments could have facilitated the formation of primordial ribonucleic acids on Hadean Earth.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 239-248"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673610","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Rhenium residency in molybdenite, compressional textures and relationship to polytypism","authors":"Mao Tan ,&nbsp;Yiping Yang ,&nbsp;Xiao-Wen Huang ,&nbsp;Jiaxin Xi ,&nbsp;Nuo Li ,&nbsp;Yu-Miao Meng ,&nbsp;Liang Qi","doi":"10.1016/j.gca.2024.12.012","DOIUrl":"10.1016/j.gca.2024.12.012","url":null,"abstract":"<div><div>Molybdenite has been established as a robust mineral for Re-Os isotope dating. However, higher-precision Re-Os molybdenite dating is necessary to accurately determine the absolute timing of metal mineralization and duration of ore-forming hydrothermal systems. To improve the precision and accuracy of molybdenite Re-Os dating, molybdenite from the Longmendian deposit—with its old age, heterogeneous distribution of Re, compression deformation, and complex polytypes—serves as a reference for enhancing the precision of in-situ Re-Os dating or grain-scale sampling during solution Re-Os isotope dating of molybdenite. High-resolution scanning transmission electron microscopy (TEM) analysis demonstrates that Re, Os, Pb, Bi, Cu, and Fe are incorporated into the molybdenite crystal through isomorphic substitution for Mo. Electron probe analysis shows that a single molybdenite grain exhibits heterogeneous textures consisting of Re-rich (∼0.29–0.82 wt%) and Re-poor (&lt;0.29 wt%) domains. Some molybdenite grains have undergone compression deformation. Rhenium can be enriched in either the deformed or the undeformed domains of molybdenite grains. Compression deformation in some grains induces delamination cracks in Re-rich domains, facilitating ore-forming fluid infiltration and leading to an inhomogeneous distribution of Re and other elements in molybdenite grains. Re-rich domain in molybdenite is enriched in other metals, including Fe, Co, Zn, Pb, and Bi, due to the overprint of the hydrothermal fluids with a lower temperature and a relatively high oxygen fugacity, leading to the formation of heterogeneous molybdenite. Micro-X-ray diffraction (μXRD) and TEM analyses have revealed that both the Re-rich and Re-poor domains belong to the 2<em>H</em>₁ polytype, indicating that Re concentration and distribution are not directly related to the polytype of molybdenite. The Re-poor and deformed domain of molybdenite shows the coexistence of 2<em>H</em>₁ and 3<em>R</em> polytypes (in a ratio of 9:1), suggesting that compression deformation led to polytype transformation. Therefore, the diverse characteristics of molybdenite in the Longmendian deposit present challenges for obtaining primary Re-Os age information. Nondestructive pre-characterization of molybdenite is essential before dating to ensure age homogeneity. Molybdenite samples with an undeformed and uniform distribution of elements (Re) within molybdenite grains are suitable candidates for analysis. Our findings collectively offer strategies to enhance precision while advancing the understanding of elemental and isotopic geochemical behavior in the contexts of metamorphism, deformation, and fluid flow environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 59-73"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841532","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}