Geochimica et Cosmochimica Acta最新文献

{"title":"Mechanism of mineral adsorption enhancing the reduction of hexavalent chromium by natural organic matter","authors":"Peng Zhang ,&nbsp;Yiran Liu ,&nbsp;Yingxiao Tian ,&nbsp;Chenglong Yu ,&nbsp;Songhu Yuan","doi":"10.1016/j.gca.2025.03.011","DOIUrl":"10.1016/j.gca.2025.03.011","url":null,"abstract":"<div><div>Natural organic matter (NOM) serves a crucial electron reservoir for the reduction of hexavalent chromium (Cr(VI)) in subsurface environments. However, the influence of mineral adsorption on Cr(VI) reduction by NOM remains poorly understood, despite the widespread interaction among NOM, mineral matrices and Cr(VI) in natural environments. In this study, aluminum oxides (Al₂O₃) and Fe oxyhydroxides (such as ferrihydrite) were chosen as representative minerals to investigate how mineral adsorption influences on the reduction of Cr(VI) by both native and reduced NOM across a pH range of 5–9. Results of this study showed that the extent of Cr(VI) reduction (10 μM) was 1.5–11.1 % in the native NOM (12 mg/L) system, while it increased to 10.6–19.4 % in the native NOM-Al₂O₃ systems and to 19.4–25.4 % in the native NOM-ferrihydrite systems. Similarly, the extent of Cr(VI) reduction was 16.7–23.1 % in the reduced NOM system, while it increased to 17.5–38.3 % in the reduced NOM-Al₂O₃ systems and to 30–54.9 % in the reduced NOM-ferrihydrite systems. This enhancement effect increased with higher amounts of Al₂O₃ and ferrihydrite but diminished as the solution pH increased from 5 to 9. The enhancement of Cr(VI) reduction by Fe oxyhydroxides at the same dosage followed by the sequence: ferrihydrite &gt; lepidocrocite &gt; goethite &gt; hematite. In NOM system alone, phenolic moieties in NOM mainly contributed to Cr(VI) reduction. In NOM-Al₂O₃/Fe oxyhydroxide systems, mineral adsorption shifted the reaction site from solution phase to mineral surface, subsequently inducing oxidative polymerization of polyphenolic compounds in NOM, which generated more phenolic moieties that further facilitated the reduction of Cr(VI). Additionally, Fe oxyhydroxides served as electron shuttles, facilitating electron transfer from NOM to Cr(VI). This study highlights a previously unrecognized catalytic role of mineral adsorption in enhancing Cr(VI) reduction by NOM in subsurface environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 32-43"},"PeriodicalIF":4.5,"publicationDate":"2025-03-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143643488","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":"Metallogeny of subcontinental lithospheric mantle driven by sulfide-saturated pyroxenite-forming melts: evidence from the Balmuccia peridotite massif","authors":"Bartosz Pieterek ,&nbsp;Magdalena Matusiak-Małek ,&nbsp;Riccardo Tribuzio ,&nbsp;Marina Lazarov ,&nbsp;Magdalena Pańczyk ,&nbsp;Harald Strauss ,&nbsp;Thomas Kuhn ,&nbsp;Zbigniew Czupyt ,&nbsp;Jakub Ciazela ,&nbsp;Stefan Weyer","doi":"10.1016/j.gca.2025.03.007","DOIUrl":"10.1016/j.gca.2025.03.007","url":null,"abstract":"<div><div>Focused melt flow is a common phenomenon in the subcontinental lithospheric mantle. Although it exerts significant control on the magmatic differentiation of the upper mantle, its role in metal transport remains poorly constrained. To improve our understanding of the subcontinental mantle metallogeny, we investigated the Balmuccia massif of the Ivrea-Verbano Zone (Italian Alps), which consists of fresh mantle peridotites that experienced a prolonged period of multistage melt intrusions. As a result, this massif hosts two suites of pyroxenite dykes, known as Cr-diopside and Al-augite pyroxenites, which enable us to provide undisturbed insights into mantle metallogeny. Here, through scrutiny of the pyroxenite dykes and their contacts with mantle peridotites, we provide insights into the sulfide and associated chalcophile metals (e.g., Cu and Ag) distributions. We demonstrate that the Balmuccia mantle pyroxenites are enriched in magmatic sulfides and sulfide-loving elements compared to the Balmuccia mantle peridotites. In particular, the pyroxenites contain up to 8 times more Cu (on average 227 ± 58 ppm; 1SD; n = 8) than the mantle peridotites (29 ± 20 ppm Cu; n = 20). Additionally, we found that each sulfide phase has distinct S-Fe isotopic signatures among sulfides. Such differentiation indicates that the S-Fe isotopic fractionation is most likely controlled by the mass-dependent fractionation that follows the (re)crystallization under high-T subsolidus magmatic conditions.</div><div>The increased amount of sulfides and associated chalcophile metals (Cu and Ag) within the studied pyroxenites evidence the heterogeneous distribution of sulfides and metals in the subcontinental lithospheric mantle, similarly to observations from other pyroxenite dykes within mantle rocks and metasomatized mantle xenoliths. Specifically, we estimate that from 12% to 42% of the Cu and from 11% to 40% of the Ag of the upper mantle inventory could be accumulated within mantle pyroxenites. Our results indicate that mantle pyroxenites constitute a critical metal reservoir for subcontinental lithospheric metallogeny.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 248-266"},"PeriodicalIF":4.5,"publicationDate":"2025-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143724028","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":"Barium uptake and isotope fractionation by a marine diatom: Implications for oceanic barium cycle","authors":"Yu-Te Hsieh ,&nbsp;Po-Kai Yang ,&nbsp;Tung-Yuan Ho","doi":"10.1016/j.gca.2025.03.006","DOIUrl":"10.1016/j.gca.2025.03.006","url":null,"abstract":"<div><div>Barium (Ba) is a nutrient-type element in the ocean and is commonly used as a tracer for reconstructing marine productivity. Despite recent developments in Ba stable isotope analysis and growing research interest, the controls on biological Ba uptake and isotope fractionation remain largely unknown. This study presents a series of culture experiments using the model marine diatom <em>Thalassiosira weissflogii</em> to explore biological Ba uptake contributing to pelagic barite (BaSO₄) formation and its associated isotope fractionation for the first time. The results show that Ba cell quotas (Ba/P) are positively correlated with Ba concentrations in the culture medium, with slopes influenced by diatom-specific growth rates under high and low light levels. Similar trends in Ba, Ca, and Sr uptake suggest that Ba is likely taken up passively through Ca transporters, as a leakage of seawater Ba into the cells. This study also investigates Ba/C ratios in <em>Thalassiosira weissflogii</em> for the first time, revealing significantly lower ratios (down to 43500-fold) compared to those observed in marine field particles. This finding suggests that additional Ba sources are required to sustain particulate Ba flux associated with export production in marine water columns. The Ba isotope compositions of the cultured species indicate preferential uptake of isotopically lighter Ba from seawater, with isotope fractionation Δ¹³⁸Ba_bio-sw values ranging from −0.47 ‰ to −0.14 ‰ as Ba concentrations in the medium increase from 90 to 200 nmol/kg. The fractionation pattern is independent of the growth rates. The Ba isotope results from cultured diatoms provide the first evidence explaining the mismatch between Ba isotope fractionation factors in pelagic and laboratory-precipitated barite, suggesting that initial isotope fractionation from seawater through biological uptake can lead to a more negative fractionation factor in pelagic barite than that observed in laboratory-precipitated barite. Considering the Ba/C ratios between cultured diatoms and sediment traps, biological uptake is unlikely to be the sole or primary source of Ba for pelagic barite formation. This study provides the first constraint on marine diatom Ba cell quotas and their isotope fractionation factors, emphasizing the need to investigate the contribution of Ba from other sources (e.g., microbial processes) and the associated isotope fractionation during pelagic barite formation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 238-247"},"PeriodicalIF":4.5,"publicationDate":"2025-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143724017","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":"Exogenous iron mitigates photo-facilitation of soil organic matter","authors":"Yongli Wen ,&nbsp;Jiali Wu ,&nbsp;Xueting Wen ,&nbsp;Zhuoyue Zhang ,&nbsp;Jian Wang ,&nbsp;Guanghui Yu ,&nbsp;Xinhua He ,&nbsp;Maohong Xu ,&nbsp;Man Cheng ,&nbsp;Wenjuan Liu ,&nbsp;Jian Xiao","doi":"10.1016/j.gca.2025.03.010","DOIUrl":"10.1016/j.gca.2025.03.010","url":null,"abstract":"<div><div>Subalpine meadow soil carbon (C) is susceptible to ultraviolet B (UV-B) radiation, yet the mechanisms of UVB-induced soil organic C (SOC) photodegradation and the influence of iron (Fe) on this susceptibility remain largely unknown. In this study, soils from the southeastern (SE) and northwestern (NW) slopes of a subalpine meadow were exposed to three UVB treatments, elevated (ele-UVB, ∼120 μW·cm⁻²), ambient (amb-UVB, ∼60 μW·cm⁻²), or attenuated (no-UVB, 0 μW·cm⁻²), to assess the effects of UV radiation and Fe addition on SOC mineralization. A two-phase lignin incubation experiment was then conducted to elucidate the mechanism by which Fe influences the abiotic and biotic processes of lignin photodegradation. Results showed that ele-UVB increased SOC degradation by 137 % and 34 % in the SE and NW soils, respectively. Lignin phenols underwent significant photochemical degradation, which was mitigated by Fe addition. Furthermore, our findings revealed that photo-facilitation (i.e., microbial decomposition) significantly contributed to lignin photodegradation, releasing over 5 times more CO₂ than abiotic degradation did. This occurred mainly due to the depolymerization of lignin macromolecules, which increased the substrate availability for microbes, rather than shifts in microbial community composition. Fe impacted photo-facilitation by binding with lignin derivatives, reducing microbial accessibility and limiting their decomposition. These findings highlight the intricate interactions among UV-B radiation, Fe, and microbial processes in SOC turnover, offering critical insights for soil C management under global environmental change scenarios.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 1-11"},"PeriodicalIF":4.5,"publicationDate":"2025-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143629384","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":"Loss and accretion of moderately volatile elements K and Na in Australasian microtektites from Antarctica","authors":"M. Del Rio ,&nbsp;L. Folco ,&nbsp;E. Mugnaioli ,&nbsp;S. Goderis ,&nbsp;M. Masotta","doi":"10.1016/j.gca.2025.03.005","DOIUrl":"10.1016/j.gca.2025.03.005","url":null,"abstract":"<div><div>Recent studies on alkali metals, Ar-, Fe- and K-isotope distribution in Australasian microtektites have revealed the complex interplay of multiple fractionation processes in establishing their moderately volatile elements record, particularly in those deposited in Antarctica, most distal from the hypothetical source crater. To provide a better understanding of moderately volatile elements fractionation during microtektite formation, we studied the distribution of K, Na, Rb and Cs in twenty-seven Australasian microtektites from Antarctica ranging in size from 180 to 680 µm. Compositional profiles were determined using electron probe microanalyses (major elements) and laser ablation-inductively coupled plasma-mass spectrometry (trace elements), following a petrographic study at the nanoscopic scale by means of scanning and transmission electron microscopy. The Australasian microtektites from Antarctica contain nanometer-sized, partly digested lechatelierite inclusions and rare vesicles, and record significant moderately volatile elements depletion (Na₂O = 0.30 ± 0.07 (1σ) wt%; K₂O = 0.94 ± 0.25 (1σ) wt%) relative to: i) upper continental crust (Na₂O = 3.46 wt%; K₂O = 3.45 wt%), ii) microtektites from deep sea sediments (Na₂O = 1.15 ± 0.43 (1σ) wt%; K₂O = 2.47 ± 0.82 (1σ) wt%), and iii) Australasian tektites (Na₂O = 1.20 ± 0.19 (1σ) wt%; K₂O = 2.43 ± 0.24 (1σ) wt%). They are also characterized by moderately volatile elements enrichments at their rims (up to ∼2x for K₂O; ∼1.6x for Na₂O), and the enrichment factor typically decreases with increasing diameter. Lastly, there is an inverse correlation between bulk Na₂O content (but not K₂O) and diameter. We propose that the most distal Antarctic microtektites originated as impact melt droplets and not as vapor condensate spherules. Their moderately volatile elements geochemical budget was established through three subsequent stages of fractionation in the context of a hypervelocity impact. 1) Gross Na and K and other moderately volatile elements loss which occurred during the melting and vaporization of the target precursor materials. 2) Re-accretion of Na, K and other moderately volatile elements from the condensation of a hot gas envelope of vaporized target materials onto volatile depleted droplets cores. 3) Size-controlled partial evaporation of (mainly) Na, caused by aerodynamic drag heating, during deceleration from high ejection velocities either during the decoupling from the hot gas envelope in ambient air, or during atmospheric re-entry, as suggested by alkalis and Fe-isotope data in the literature. The late accretion of K vapor also provides plausible explanations for the contamination by extraneous Ar and K-isotopic systematics reported in the literature.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 212-228"},"PeriodicalIF":4.5,"publicationDate":"2025-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143724018","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":"Abiotic and biotic transformation of petroleum hydrocarbons coupled with redox cycling of structural iron in clay mineral","authors":"Yuan Liu ,&nbsp;Hongyu Chen ,&nbsp;Yizhi Sheng ,&nbsp;Weiguo Hou ,&nbsp;Wenhui Zhang ,&nbsp;Wenhui Hu ,&nbsp;Hailiang Dong","doi":"10.1016/j.gca.2025.02.038","DOIUrl":"10.1016/j.gca.2025.02.038","url":null,"abstract":"<div><div>Geothermal systems are hot spots for interaction among minerals, microorganisms, and hydrocarbons. Coupled hydrocarbon transformation and redox cycling of iron in minerals is important to ecosystem functions but remains poorly understood. This work studied abiotic transformation of petroleum hydrocarbons by reactive oxygen species produced upon oxygenation of a reduced clay mineral (nontronite NAu-2). Subsequently, the impact of such abiotic petroleum-clay interactions on coupled reduction of structural Fe(III) in clay mineral and petroleum transformation was studied under anaerobic condition. In the abiotic phase, hydrocarbons in a crude oil sample were oxidized by hydroxyl radicals (^<img>OH) generated upon oxygenation of reduced NAu-2, forming partially oxygenated compounds with hydroxyl, carbonyl, and carboxyl groups. In the subsequent bio-reduction experiments, these more bioavailable compounds significantly enhanced Fe(III) bio-reduction by a thermophilic microbial community enriched from a terrestrial hot spring. In particular, <em>Sphingomonas</em> and <em>Phyllobacterium</em> were enriched, both of which possessed genes for anaerobic hydrocarbon activation and Fe(III) reduction. Notably, the thermophilic community possessed more genes for breaking down C, H, and O-containing compounds. In contrast, when the same thermophilic community was exposed to the original (unoxidized) petroleum hydrocarbons, they possessed more genes for degrading C and H compounds. These findings enhance our understanding of the important role of minerals in regulating hydrocarbon transformation and in shaping subsurface microbial community.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 44-63"},"PeriodicalIF":4.5,"publicationDate":"2025-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666482","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":"Planktic foraminifera record the succession of anaerobic metabolisms in particle microenvironments across a pelagic oxygen gradient","authors":"Shannon C. Doherty ,&nbsp;Catherine V. Davis ,&nbsp;Jennifer S. Fehrenbacher","doi":"10.1016/j.gca.2025.03.008","DOIUrl":"10.1016/j.gca.2025.03.008","url":null,"abstract":"<div><div>Microenvironments inside marine organic particles can host anaerobic microbial respiration outside of ocean anoxic zones, but these environments are challenging to directly observe. We present evidence that the planktic foraminifer <em>Globorotaloides hexagonus</em> inhabits a particle microenvironment and suggest that their shell chemistry records anaerobic microbial metabolisms inside particles. We propose a novel interpretation of intrashell trends in Ba/Ca, Mn/Ca, and Zn/Ca ratios as signals of denitrification, Mn respiration, and sulfate reduction. We measure these trace elements in <em>G. hexagonus</em> collected from discrete depth horizons across a pelagic oxygen gradient in the Eastern Tropical North Pacific. Using this intrashell trace element framework, we find that denitrification may have occurred inside particles throughout the water column, and that Mn respiration and sulfate reduction may have occurred inside particles throughout the oxygen minimum zone. Our results have implications for budgets of nitrogen, sulfur, manganese, and other trace elements in regions with expanding oxygen minimum zones and suggest a new method of interpreting intrashell trends in trace element-to-calcium ratios in planktic foraminifera.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 267-276"},"PeriodicalIF":4.5,"publicationDate":"2025-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666529","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":"Clay minerals and the stability of organic carbon in suspension along coastal to offshore transects","authors":"Michael Fettweis ,&nbsp;Saumya Silori ,&nbsp;Rieko Adriaens ,&nbsp;Xavier Desmit","doi":"10.1016/j.gca.2025.03.003","DOIUrl":"10.1016/j.gca.2025.03.003","url":null,"abstract":"<div><div>Particulate (POC) and dissolved organic carbon (DOC) concentration, clay mineral content and composition and the suspended particulate matter (SPM) concentration have been analyzed in water samples taken along transects from the high turbid nearshore to the low turbid offshore on the North Sea shelf. The suspended POC has been classified into a mineral-associated (POC_mineral), a slowly degrading (POC_slow) and a fresh fraction (POC_fresh). The POC_mineral has been estimated based on the clay mineral composition and on literature data of the mineral specific surface area per g and the OC content per specific surface area. It consists of organic molecules adsorbed onto mineral surfaces and is thereby the most refractory fraction. The POC_fresh content (% of POC_fresh in SPM) has been calculated using the semi-empirical POC-SPM model of Fettweis et al. (2022) and is intrinsically labile. The POC_slow content is refractory with variable rates of degradation. The total POC content of the SPM was between 2 and 11%, from which about 0.3–6.6% (0.1–2.1%) was POC_fresh in spring (resp., winter). The POC_mineral content was between 0.4% and 1.1% and decreased towards the offshore, meaning that the POC offshore is less refractory than nearshore. The organic molecules adsorbed onto clay minerals, are dynamically exchanging with the DOC, and thus influencing its fate and concentration. However this process is not sufficient to explain the increasing POC/DOC ratio with increasing SPM concentration, which is further explained by primary production, advection and diffusion, density gradients and seabed erosion. Our results highlight the difficulty and the necessity of estimating the respective sample-POC and DOC concentrations, fluxes and fates along SPM concentration gradients in coastal zones. This is needed as organo-mineral interactions influence the vertical dynamics and horizontal transport of SPM and have an impact on particles and organic carbon fluxes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 229-237"},"PeriodicalIF":4.5,"publicationDate":"2025-03-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666341","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":"Helium isotopes in geothermal fluids reveal off-rift plume degassing and localized seismicity-induced processes in North Iceland","authors":"Carolina Dantas Cardoso ,&nbsp;Raphaël Pik ,&nbsp;Antonio Caracausi ,&nbsp;Sæmundur Ari Halldórsson ,&nbsp;Andri Stefánsson ,&nbsp;Laurent Zimmermann ,&nbsp;Guillaume Paris ,&nbsp;Andrea Ricci ,&nbsp;Hreinn Hjartarson","doi":"10.1016/j.gca.2025.03.004","DOIUrl":"10.1016/j.gca.2025.03.004","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Iceland is a location of geological interest due to the combination of upwelling mantle plume and divergent plate boundary, which resulted in the formation of its extensive surface area (&gt;100,000 km&lt;sup&gt;2&lt;/sup&gt;) that rises above sea-level. This unique setting facilitates assessing the role of the underlying mantle plume and tectonic activity on crust-forming processes. Helium isotopes provide a useful tool in this regard, as they can identify physical processes and resolve deep and shallow fluid sources in the crust. In Iceland, the highest &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He for geothermal fluids are found in Vestfirðir with values up to 29 R&lt;sub&gt;a&lt;/sub&gt; (where R&lt;sub&gt;a&lt;/sub&gt; is the &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He of air), more than 110 km away from current active rift zones. Such locations are key to understand the extent of mantle degassing processes associated with the high buoyant Icelandic mantle plume. Other off-rift regions, such as most of North Iceland, have not been extensively investigated, despite the widespread presence of geothermal activity. Although North Iceland has been volcanically inactive for the past 0.8 Ma, severe earthquake hazards associated with mature and partially on-land transform zones have occurred, rendering the monitoring of the full tectonic-hydrogeochemical system of societal importance. Our study in North Iceland aimed to (i) assess temporal variations in helium isotopic signatures in low-T geothermal water and their relationship with regional earthquakes, (ii) diminish the helium isotope data gap in geothermal fluids of this region, and (iii) elucidate both local and regional processes controlling the He isotope systematics in this region as a case study for other off-rift contexts on Earth. In order to achieve these goals, we report helium isotope time series data collected from June 2020 to October 2022 from a borehole in Hafralækur, Aðaldalur valley (95 samples collected on a near-weekly basis), along with an isotope survey (δ&lt;sup&gt;2&lt;/sup&gt;H-&lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He-δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;TDIC&lt;/sub&gt;-δ&lt;sup&gt;18&lt;/sup&gt;O-δ&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt;) of North Iceland geothermal fluids (T &lt; 130 °C, n = 36 samples). The results indicate a large regional variability in helium isotope ratios (4 to 27 R&lt;sub&gt;a&lt;/sub&gt;) that is comparable to the entire range evident in geothermal fluids across Iceland (∼1 to 29 R&lt;sub&gt;a&lt;/sub&gt;) where the maximum &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He signature is among the highest measured in geothermal fluids from oceanic and continental hotspots globally. Several processes, both on regional and local scales, are needed to account for this large range: (i) influence of a deeply-derived mantle flux evidenced by a high &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He mantle component, degassing via fault systems, (ii) release of local radiogenic helium components, potentially associated with seismic events along the Dalvík Lineament, and (iii) local groundwater mixing, for example evident at the Hafralækur sit","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 12-31"},"PeriodicalIF":4.5,"publicationDate":"2025-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143629448","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":"Mineral-melt calcium isotope fractionation factors constrained using ab initio molecular dynamics simulations and their implications to calcium isotope effects during partial melting in the upper mantle","authors":"Yonghui Li ,&nbsp;Justin Hardin ,&nbsp;Wenzhong Wang ,&nbsp;Zhongqing Wu ,&nbsp;Shichun Huang","doi":"10.1016/j.gca.2025.02.032","DOIUrl":"10.1016/j.gca.2025.02.032","url":null,"abstract":"<div><div>To better constrain the Ca isotope effect during partial melting of Earth’s mantle, we used <em>ab initio</em> molecular dynamic simulations to calculate the equilibrium mineral-silicate melt Ca isotope fractionation factors for the major Ca-bearing minerals of the upper mantle (orthopyroxene, clinopyroxene, olivine, and garnet), as well as plagioclase. We found that mineral-melt Ca isotope fractionation factors are dependent on pressure, temperature, and mineral major element compositions, but not the silicate melt composition. Specifically, our calculations show that under equilibrium, clinopyroxene has a slightly heavier Ca isotope composition compared to silicate melt, consistent with the inference of published research that studied the Ca isotope effects during basaltic magma evolution.</div><div>We then utilized the calculated mineral-melt Ca isotope fractionation factors to model the Ca isotope effects on both silicate melts and residues during partial melting of spinel peridotite at 1–2 GPa, garnet peridotite at 3–7 GPa, and garnet pyroxenite/eclogite at 2–5 GPa. Our model predicts that silicate melts only have <em>δ</em>^44/40Ca up to 0.12 lower than their source value, consistent with previous estimates. Importantly, partial melts of spinel peridotite, garnet peridotite, and garnet pyroxenite/eclogite exhibit overlapping <em>δ</em>^44/40Ca values, if their mantle sources have the same <em>δ</em>^44/40Ca. Partial melting alone cannot explain the full range of <em>δ</em>^44/40Ca values observed in natural basalts, and at least part of this variation must reflect <em>δ</em>^44/40Ca variation of their mantle sources and/or other processes such as crystal fractionation.</div><div>Our calculations show that melting residues always have <em>δ</em>^44/40Ca higher than their mantle source, with the highest <em>δ</em>^44/40Ca at 0.40 higher than their source value. This range is much smaller than that observed in natural ultramafic rocks that might represent melting residues. In addition, the range and direction of inter-mineral Ca isotope fractionation factors predicted in our modeled residues for the mineral pairs orthopyroxene-clinopyroxene and garnet-clinopyroxene are much more restricted than those observed in natural ultramafic rocks, including peridotites and pyroxenites/eclogites. Therefore, most natural ultramafic rocks have likely experienced more complicated petrogeneses than partial melting.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"396 ","pages":"Pages 51-70"},"PeriodicalIF":4.5,"publicationDate":"2025-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666342","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}