The isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) derived from archaea are widely used in the reconstruction of past climate and environment through proxies such as the Methane Index (MI) and TEX86. A pre-requisite for the application of TEX86, which serves as a proxy for upper ocean temperature, is that the sedimentary GDGTs primarily originate from planktonic Marine Group I Thaumarchaeota. The MI is commonly used as a quality control measure for TEX86 reconstruction to identify samples affected by methanotrophic GDGTs. Recently, the MI has also been used for the reconstruction of past methane cycling. However, the spatial variability of GDGT-based proxies and the relationship between MI and TEX86 remain unclear. In this study, we generated a comprehensive suite of data, including gas, porewater, bulk sediment geochemistry, archaeal cell abundance, GDGTs and their derived proxies. We collected sediment cores from four study sites offshore Southwest Taiwan characterized by the absence or presence of anaerobic oxidation of methane (AOM) and differences in the depth of the sulfate-methane transition zone (SMTZ). The distance between the coring sites varied from ∼20 cm to ∼2 km. The geochemical and DNA data indicated the presence of SMTZs at depths ranging from 4 cm to 290 cm in sediments. AOM-related GDGTs were predominantly composed of GDGT-2, followed by GDGT-1, GDGT-0 and GDGT-3. Although MI values in the SMTZ could reach as high as 0.7, they did not strictly vary based on the SMTZ depth nor the methane consumption rate. This, coupled with the discrepancies in the downcore profiles of gas and porewater geochemistry compared to archaeal DNA and GDGTs, suggest that the duration of SMTZ presence could be a key factor influencing sedimentary MI values. We observed strong relationships between TEX86 and MI at AOM sites; however, the direction of these relationships varied across different locations. Despite this, TEX86-derived temperatures showed good agreement between sites (<1.5 °C between sites located ∼2 km apart), and with climatology data. Consequently, our findings suggest that even in samples with high MI values above the threshold of 0.3, the bias on TEX86-derived temperatures may not be as significant as generally assumed.
{"title":"Methane Index and TEX86 values in cold seep sediments: Implications for paleo-environmental reconstructions","authors":"Sze Ling Ho, Yu-Shih Lin, Pei-Ling Wang, Tzu-Ting Chen, Pei-Ting Lee, Hui-Hsin Wang, Tzu-Jung Cheng, Yun-Ju Wang, Chih-Chieh Su, Min-Te Chen","doi":"10.1016/j.gca.2024.12.033","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.033","url":null,"abstract":"The isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) derived from archaea are widely used in the reconstruction of past climate and environment through proxies such as the Methane Index (MI) and TEX<ce:inf loc=\"post\">86</ce:inf>. A pre-requisite for the application of TEX<ce:inf loc=\"post\">86</ce:inf>, which serves as a proxy for upper ocean temperature, is that the sedimentary GDGTs primarily originate from planktonic Marine Group I Thaumarchaeota. The MI is commonly used as a quality control measure for TEX<ce:inf loc=\"post\">86</ce:inf> reconstruction to identify samples affected by methanotrophic GDGTs. Recently, the MI has also been used for the reconstruction of past methane cycling. However, the spatial variability of GDGT-based proxies and the relationship between MI and TEX<ce:inf loc=\"post\">86</ce:inf> remain unclear. In this study, we generated a comprehensive suite of data, including gas, porewater, bulk sediment geochemistry, archaeal cell abundance, GDGTs and their derived proxies. We collected sediment cores from four study sites offshore Southwest Taiwan characterized by the absence or presence of anaerobic oxidation of methane (AOM) and differences in the depth of the sulfate-methane transition zone (SMTZ). The distance between the coring sites varied from ∼20 cm to ∼2 km. The geochemical and DNA data indicated the presence of SMTZs at depths ranging from 4 cm to 290 cm in sediments. AOM-related GDGTs were predominantly composed of GDGT-2, followed by GDGT-1, GDGT-0 and GDGT-3. Although MI values in the SMTZ could reach as high as 0.7, they did not strictly vary based on the SMTZ depth nor the methane consumption rate. This, coupled with the discrepancies in the downcore profiles of gas and porewater geochemistry compared to archaeal DNA and GDGTs, suggest that the duration of SMTZ presence could be a key factor influencing sedimentary MI values. We observed strong relationships between TEX<ce:inf loc=\"post\">86</ce:inf> and MI at AOM sites; however, the direction of these relationships varied across different locations. Despite this, TEX<ce:inf loc=\"post\">86</ce:inf>-derived temperatures showed good agreement between sites (<1.5 °C between sites located ∼2 km apart), and with climatology data. Consequently, our findings suggest that even in samples with high MI values above the threshold of 0.3, the bias on TEX<ce:inf loc=\"post\">86</ce:inf>-derived temperatures may not be as significant as generally assumed.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"32 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142985990","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}
Pub Date : 2024-12-28DOI: 10.1016/j.gca.2024.12.031
Jingye She, Danwen Qin, Andreas C. Scheinost, Mingliang Kang, Wujian Jin, Hanqin Weng, Hanyu Wu, Jianrong Zeng
Although the reduction of Se(VI) to Se0 and FeSe2 by pyrite is thermodynamically favorable, well-crystallized natural pyrite remains chemically inert to aqueous Se(VI) under conventional conditions. In this study, the interaction of aqueous Se(VI) with natural arsenic-rich and arsenic-free pyrites was studied under gamma-irradiation at pH ∼4.0 to ∼9.0, mimicking the conditions near a nuclear waste repository. The results showed that gamma-irradiation significantly boosts the reductive precipitation of Se(VI) by arsenic-rich pyrite at pH ∼4.0 to ∼9.0. For arsenic-free pyrite, this enhancement was observed within a narrower pH range of ∼5.0 to ∼6.0, and a distinct red precipitate, indicative of Se0, formed during the reaction at pH ∼6.0 under an absorbed dose of 100 kGy at a dose rate of 100 Gy/min. The reduction process involving arsenic-rich pyrite is more sensitive to the absorbed dose rate, with a decrease in Se(VI) removal efficiency at higher dose rates. Conversely, both the absorbed dose and the dose rate have significant effects on the reactions involving the arsenic-free pyrite. Transmission electron microscopy and X-ray absorption spectroscopy analyses verified the occurrence of Se0 nanoparticles on pyrite surfaces via the intermediate product of Se(IV). It is suggested that the ·OH radical generated by water radiolysis can be scavenged by pyrite and/or its non-oxidative dissolved solutes such as Fe2+ and H3AsO3/AsO33− impurities, allowing the concurrent eaq− and ·H, primarily eaq−, to reduce Se(VI), thereby altering the reaction pathway between pyrite and aqueous Se(VI) under gamma-irradiation. This work offers important insights into the geochemical behavior of 79Se in the near-field of a nuclear waste repository.
{"title":"Gamma-irradiation-induced reduction of aqueous Se(VI) by natural pyrite","authors":"Jingye She, Danwen Qin, Andreas C. Scheinost, Mingliang Kang, Wujian Jin, Hanqin Weng, Hanyu Wu, Jianrong Zeng","doi":"10.1016/j.gca.2024.12.031","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.031","url":null,"abstract":"Although the reduction of Se(VI) to Se<ce:sup loc=\"post\">0</ce:sup> and FeSe<ce:inf loc=\"post\">2</ce:inf> by pyrite is thermodynamically favorable, well-crystallized natural pyrite remains chemically inert to aqueous Se(VI) under conventional conditions. In this study, the interaction of aqueous Se(VI) with natural arsenic-rich and arsenic-free pyrites was studied under gamma-irradiation at pH ∼4.0 to ∼9.0, mimicking the conditions near a nuclear waste repository. The results showed that gamma-irradiation significantly boosts the reductive precipitation of Se(VI) by arsenic-rich pyrite at pH ∼4.0 to ∼9.0. For arsenic-free pyrite, this enhancement was observed within a narrower pH range of ∼5.0 to ∼6.0, and a distinct red precipitate, indicative of Se<ce:sup loc=\"post\">0</ce:sup>, formed during the reaction at pH ∼6.0 under an absorbed dose of 100 kGy at a dose rate of 100 Gy/min. The reduction process involving arsenic-rich pyrite is more sensitive to the absorbed dose rate, with a decrease in Se(VI) removal efficiency at higher dose rates. Conversely, both the absorbed dose and the dose rate have significant effects on the reactions involving the arsenic-free pyrite. Transmission electron microscopy and X-ray absorption spectroscopy analyses verified the occurrence of Se<ce:sup loc=\"post\">0</ce:sup> nanoparticles on pyrite surfaces via the intermediate product of Se(IV). It is suggested that the ·OH radical generated by water radiolysis can be scavenged by pyrite and/or its non-oxidative dissolved solutes such as Fe<ce:sup loc=\"post\">2+</ce:sup> and H<ce:inf loc=\"post\">3</ce:inf>AsO<ce:inf loc=\"post\">3</ce:inf>/AsO<ce:inf loc=\"post\">3</ce:inf><ce:sup loc=\"post\">3−</ce:sup> impurities, allowing the concurrent e<ce:inf loc=\"post\">aq</ce:inf><ce:sup loc=\"post\">−</ce:sup> and ·H, primarily e<ce:inf loc=\"post\">aq</ce:inf><ce:sup loc=\"post\">−</ce:sup>, to reduce Se(VI), thereby altering the reaction pathway between pyrite and aqueous Se(VI) under gamma-irradiation. This work offers important insights into the geochemical behavior of <ce:sup loc=\"post\">79</ce:sup>Se in the near-field of a nuclear waste repository.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"22 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936030","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}
Pub Date : 2024-12-27DOI: 10.1016/j.gca.2024.12.025
Samuel Ebert, Kazuhide Nagashima, Alexander N. Krot, Addi Bischoff
The nature of isotopic differences between ‘normal’ Ca,Al-rich inclusions (CAIs) characterized by the canonical initial 26Al/27Al ratio [(26Al/27Al)0] of ∼5 × 10−5 and the anomalous refractory inclusions characterized by the significantly lower (26Al/27Al)0, < ∼3 × 10−6, which include PLACs (platy hibonite crystals), PLAC-like inclusions, and some corundum-, hibonite-, and grossite-rich CAIs, remains controversial. The 26Al-poor inclusions may have formed earlier, prior to ‘normal’ CAIs, and recorded heterogeneous distribution of 26Al in the CAI-forming region, or they may have formed after nearly complete decay of 26Al, ∼ >4 Myr later than the canonical CAIs. Here we present the first high precision multi-isotopic (O, Mg, Ca, and Ti) study of refractory inclusions (RIs) in the EHa3 enstatite chondrite Sahara 97072 using in situ SIMS measurements. Our study revealed the presence of two isotopically distinct populations of CAIs in this meteorite: ‘normal’ CAIs and PLAC-like inclusions. The ‘normal’ CAIs composed of spinel, Al,Ti-diopside, ±hibonite, and secondary minerals, most likely replacing melilite, have solar-like Δ17O of ∼−23 ‰, ∼ the canonical (26Al/27Al)0, and no resolvable nucleosynthetic isotope anomalies in Ca and Ti. The PLAC-like inclusions composed of hibonite, corundum, and ± Al,Ti-pyroxene have Δ17O of ∼−19 ‰, no resolvable excess of radiogenic 26Mg, and large nucleosynthetic isotope anomalies in Ti and Ca: one inclusion has positive anomalies in 50Ti (835ε) and 48Ca (685ε), whereas another one has negative anomalies in 50Ti (−116ε), 46Ti (−112ε), 48Ca (−284ε).
{"title":"Two isotopically distinct populations of refractory inclusions in the EHa3 chondrite Sahara 97072 – Significance for understanding the evolution of the CAI-formation region","authors":"Samuel Ebert, Kazuhide Nagashima, Alexander N. Krot, Addi Bischoff","doi":"10.1016/j.gca.2024.12.025","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.025","url":null,"abstract":"The nature of isotopic differences between ‘normal’ Ca,Al-rich inclusions (CAIs) characterized by the canonical initial <ce:sup loc=\"post\">26</ce:sup>Al/<ce:sup loc=\"post\">27</ce:sup>Al ratio [(<ce:sup loc=\"post\">26</ce:sup>Al/<ce:sup loc=\"post\">27</ce:sup>Al)<ce:inf loc=\"post\">0</ce:inf>] of ∼5 × 10<ce:sup loc=\"post\">−5</ce:sup> and the anomalous refractory inclusions characterized by the significantly lower (<ce:sup loc=\"post\">26</ce:sup>Al/<ce:sup loc=\"post\">27</ce:sup>Al)<ce:inf loc=\"post\">0</ce:inf>, < ∼3 × 10<ce:sup loc=\"post\">−6</ce:sup>, which include PLACs (platy hibonite crystals), PLAC-like inclusions, and some corundum-, hibonite-, and grossite-rich CAIs, remains controversial. The <ce:sup loc=\"post\">26</ce:sup>Al-poor inclusions may have formed earlier, prior to ‘normal’ CAIs, and recorded heterogeneous distribution of <ce:sup loc=\"post\">26</ce:sup>Al in the CAI-forming region, or they may have formed after nearly complete decay of <ce:sup loc=\"post\">26</ce:sup>Al, ∼ >4 Myr later than the canonical CAIs. Here we present the first high precision multi-isotopic (O, Mg, Ca, and Ti) study of refractory inclusions (RIs) in the EH<ce:inf loc=\"post\">a</ce:inf>3 enstatite chondrite Sahara 97072 using <ce:italic>in situ</ce:italic> SIMS measurements. Our study revealed the presence of two isotopically distinct populations of CAIs in this meteorite: ‘normal’ CAIs and PLAC-like inclusions. The ‘normal’ CAIs composed of spinel, Al,Ti-diopside, ±hibonite, and secondary minerals, most likely replacing melilite, have solar-like Δ<ce:sup loc=\"post\">17</ce:sup>O of ∼−23 ‰, ∼ the canonical (<ce:sup loc=\"post\">26</ce:sup>Al/<ce:sup loc=\"post\">27</ce:sup>Al)<ce:inf loc=\"post\">0</ce:inf>, and no resolvable nucleosynthetic isotope anomalies in Ca and Ti. The PLAC-like inclusions composed of hibonite, corundum, and ± Al,Ti-pyroxene have Δ<ce:sup loc=\"post\">17</ce:sup>O of ∼−19 ‰, no resolvable excess of radiogenic <ce:sup loc=\"post\">26</ce:sup>Mg, and large nucleosynthetic isotope anomalies in Ti and Ca: one inclusion has positive anomalies in <ce:sup loc=\"post\">50</ce:sup>Ti (835ε) and <ce:sup loc=\"post\">48</ce:sup>Ca (685ε), whereas another one has negative anomalies in <ce:sup loc=\"post\">50</ce:sup>Ti (−116ε), <ce:sup loc=\"post\">46</ce:sup>Ti (−112ε), <ce:sup loc=\"post\">48</ce:sup>Ca (−284ε).","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"77 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142935996","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}
Pub Date : 2024-12-26DOI: 10.1016/j.gca.2024.12.029
Emily G. Wright, Xicheng He, Elaine D. Flynn, Daniel E. Giammar, Jeffrey G. Catalano
In surface and near-surface weathering environments, the mobilization and partial loss of palladium (Pd) under oxidizing and weakly acidic conditions has been attributed to aqueous chloride complexation. However, prior work has also observed that a portion of Pd is retained by iron (oxyhydr)oxides in the weathering zone. The effect chloride has on the relative amount of Pd mobilization versus retention by iron (oxyhydr)oxides is currently unclear. We studied the effect of chloride complexation on Pd(II) adsorption to two iron (oxyhydr)oxides, hematite and 2-line ferrihydrite, at pH 4. Increasing chloride concentration suppresses Pd adsorption for both hematite and ferrihydrite, which display similar binding affinities under the conditions studied. Thermodynamic modeling of aqueous Pd speciation indicates that greater suppression of binding to iron (oxyhydr)oxides should occur than is observed because of the strength of Pd-Cl complexation, implying that additional interactions at the mineral surface are counteracting this effect. While increasing dissolved chloride concentration does not measurably impact mineral surface charging, extended X-ray absorption fine structure (EXAFS) spectra indicate that ternary Pd-Cl surface complexes form on both hematite and ferrihydrite. The number of Cl ligands in the surface species increase at greater chloride concentration. A mixture of bidentate and monodentate surface species are indicated by the EXAFS spectra, although the fitting uncertainties precludes determining whether these vary in relative abundance with chloride concentration. In order to offset the effect of strong aqueous Pd-Cl complexation and align with our EXAFS results, a surface complexation model developed for Pd adsorption to hematite involves a mixture of three ternary surface complexes containing 1, 2, and 3 chloride ligands. Our results show that Pd is mobilized as a chloride complex in platinum-group element-rich weathering zones. Porewater chloride concentrations are thus a dominant control on Pd retention by iron (oxyhydr)oxides in these weakly acidic environments.
{"title":"Competitive and cooperative effects of chloride on palladium(II) adsorption to iron (oxyhydr)oxides: Implications for mobility during weathering","authors":"Emily G. Wright, Xicheng He, Elaine D. Flynn, Daniel E. Giammar, Jeffrey G. Catalano","doi":"10.1016/j.gca.2024.12.029","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.029","url":null,"abstract":"In surface and near-surface weathering environments, the mobilization and partial loss of palladium (Pd) under oxidizing and weakly acidic conditions has been attributed to aqueous chloride complexation. However, prior work has also observed that a portion of Pd is retained by iron (oxyhydr)oxides in the weathering zone. The effect chloride has on the relative amount of Pd mobilization versus retention by iron (oxyhydr)oxides is currently unclear. We studied the effect of chloride complexation on Pd(II) adsorption to two iron (oxyhydr)oxides, hematite and 2-line ferrihydrite, at pH 4. Increasing chloride concentration suppresses Pd adsorption for both hematite and ferrihydrite, which display similar binding affinities under the conditions studied. Thermodynamic modeling of aqueous Pd speciation indicates that greater suppression of binding to iron (oxyhydr)oxides should occur than is observed because of the strength of Pd-Cl complexation, implying that additional interactions at the mineral surface are counteracting this effect. While increasing dissolved chloride concentration does not measurably impact mineral surface charging, extended X-ray absorption fine structure (EXAFS) spectra indicate that ternary Pd-Cl surface complexes form on both hematite and ferrihydrite. The number of Cl ligands in the surface species increase at greater chloride concentration. A mixture of bidentate and monodentate surface species are indicated by the EXAFS spectra, although the fitting uncertainties precludes determining whether these vary in relative abundance with chloride concentration. In order to offset the effect of strong aqueous Pd-Cl complexation and align with our EXAFS results, a surface complexation model developed for Pd adsorption to hematite involves a mixture of three ternary surface complexes containing 1, 2, and 3 chloride ligands. Our results show that Pd is mobilized as a chloride complex in platinum-group element-rich weathering zones. Porewater chloride concentrations are thus a dominant control on Pd retention by iron (oxyhydr)oxides in these weakly acidic environments.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"94 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142985991","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}
Pub Date : 2024-12-25DOI: 10.1016/j.gca.2024.12.028
Joshua van Blerk, Geoffrey H. Howarth, Chris Harris, Philip E. Janney
Megacrysts in kimberlite are large (>1 cm) mineral grains that crystallized from sub-lithospheric proto-kimberlite melts in the lower sub-continental lithospheric mantle (SCLM), often after complex melt-SCLM interactions. Understanding the development of the proto-kimberlite melt system is important for two primary reasons: 1) a control on the melt diversification and range in Mg# observed in kimberlites that erupt at the surface and 2) it is associated with diamond-destruction in the deep lithosphere. We present δ18O values for a well-characterized suite of megacrysts from the Monastery kimberlite, South Africa, to evaluate variations in melt δ18O, the effect of melt-SCLM interactions on the primary δ18O of mantle-derived magmas, and further constrain the development of the proto-kimberlite melt system in the deep lithosphere.
{"title":"Oxygen isotope constraints on proto-kimberlite melt modification through assimilation of low δ18O recycled crust in the deep lithosphere","authors":"Joshua van Blerk, Geoffrey H. Howarth, Chris Harris, Philip E. Janney","doi":"10.1016/j.gca.2024.12.028","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.028","url":null,"abstract":"Megacrysts in kimberlite are large (>1 cm) mineral grains that crystallized from sub-lithospheric proto-kimberlite melts in the lower sub-continental lithospheric mantle (SCLM), often after complex melt-SCLM interactions. Understanding the development of the proto-kimberlite melt system is important for two primary reasons: 1) a control on the melt diversification and range in Mg# observed in kimberlites that erupt at the surface and 2) it is associated with diamond-destruction in the deep lithosphere. We present δ<ce:sup loc=\"post\">18</ce:sup>O values for a well-characterized suite of megacrysts from the Monastery kimberlite, South Africa, to evaluate variations in melt δ<ce:sup loc=\"post\">18</ce:sup>O, the effect of melt-SCLM interactions on the primary δ<ce:sup loc=\"post\">18</ce:sup>O of mantle-derived magmas, and further constrain the development of the proto-kimberlite melt system in the deep lithosphere.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"18 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936029","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}
Pub Date : 2024-12-24DOI: 10.1016/j.gca.2024.12.026
K.E. Miller, D.I. Foustoukos, G.D. Cody, C.M. O’D. Alexander
Titan’s abundant atmospheric N2 and CH4 gases are notable characteristics of the moon that may help constrain its origin and evolution. Previous work suggests that atmospheric CH4 is lost on geologically short timescales and may be replenished from an interior source. Isotopic and noble gas constraints indicate that N2 may derive from a mixture of NH3 ice and heating of organic matter. Here, we report experimental results from hydrothermal alteration of insoluble organic matter from the Murchison meteorite and analog insoluble organic matter at temperatures and pressures that are relevant to Titan’s interior. Our results indicate both CH4 and CO2 are formed, with the ratio between the two depending on a multitude of factors, particularly temperature and, to a lesser degree, the dielectric constant of water and carbonyl abundance in the starting material. Sufficient CH4 is produced to source Titan’s atmospheric reservoir if temperatures are greater than 250 °C. Nitrogen is volatilized, primarily in the form of NH3, in sufficient abundances to source at least 50 % of Titan’s atmospheric N2. The isotopic characteristics of volatilized material relative to the starting organics are consistent with current constraints for the nature of the accreted complex organics and Titan’s evolved atmosphere.
{"title":"Experimental heating of complex organic matter at Titan’s interior conditions supports contributions to atmospheric N2 and CH4","authors":"K.E. Miller, D.I. Foustoukos, G.D. Cody, C.M. O’D. Alexander","doi":"10.1016/j.gca.2024.12.026","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.026","url":null,"abstract":"Titan’s abundant atmospheric N<ce:inf loc=\"post\">2</ce:inf> and CH<ce:inf loc=\"post\">4</ce:inf> gases are notable characteristics of the moon that may help constrain its origin and evolution. Previous work suggests that atmospheric CH<ce:inf loc=\"post\">4</ce:inf> is lost on geologically short timescales and may be replenished from an interior source. Isotopic and noble gas constraints indicate that N<ce:inf loc=\"post\">2</ce:inf> may derive from a mixture of NH<ce:inf loc=\"post\">3</ce:inf> ice and heating of organic matter. Here, we report experimental results from hydrothermal alteration of insoluble organic matter from the Murchison meteorite and analog insoluble organic matter at temperatures and pressures that are relevant to Titan’s interior. Our results indicate both CH<ce:inf loc=\"post\">4</ce:inf> and CO<ce:inf loc=\"post\">2</ce:inf> are formed, with the ratio between the two depending on a multitude of factors, particularly temperature and, to a lesser degree, the dielectric constant of water and carbonyl abundance in the starting material. Sufficient CH<ce:inf loc=\"post\">4</ce:inf> is produced to source Titan’s atmospheric reservoir if temperatures are greater than 250 °C. Nitrogen is volatilized, primarily in the form of NH<ce:inf loc=\"post\">3</ce:inf>, in sufficient abundances to source at least 50 % of Titan’s atmospheric N<ce:inf loc=\"post\">2</ce:inf>. The isotopic characteristics of volatilized material relative to the starting organics are consistent with current constraints for the nature of the accreted complex organics and Titan’s evolved atmosphere.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"14 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142936032","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}
Pub Date : 2024-12-20DOI: 10.1016/j.gca.2024.12.021
Devin L. Schrader, Zachary A. Torrano, Dionysis I. Foustoukos, Conel M.O’D. Alexander, Jan Render, Gregory A. Brennecka
We report a coordinated bulk Cr-Ti-H-C-N isotopic and compositional study of six carbonaceous chondrites from Antarctica that are often considered to be related and termed Yamato-like carbonaceous (CY) chondrites. These meteorites are known to have undergone extensive aqueous alteration followed by different degrees of thermal alteration, to be similar to one another in regard to mineralogy, and share affinities with both the Ivuna-like carbonaceous (CI) and Mighei-like carbonaceous (CM) chondrites. While mineralogically similar, a key difference among these samples is that chondrules have been found in some of these samples, but not in others. The aim of this study is to evaluate the relationship of these meteorites to one another, and investigate how they relate to the CI and CM chondrite groups.
{"title":"Reassessing the proposed “CY chondrites”: Evidence for multiple meteorite types and parent bodies from Cr-Ti-H-C-N isotopes and bulk elemental compositions","authors":"Devin L. Schrader, Zachary A. Torrano, Dionysis I. Foustoukos, Conel M.O’D. Alexander, Jan Render, Gregory A. Brennecka","doi":"10.1016/j.gca.2024.12.021","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.021","url":null,"abstract":"We report a coordinated bulk Cr-Ti-H-C-N isotopic and compositional study of six carbonaceous chondrites from Antarctica that are often considered to be related and termed Yamato-like carbonaceous (CY) chondrites. These meteorites are known to have undergone extensive aqueous alteration followed by different degrees of thermal alteration, to be similar to one another in regard to mineralogy, and share affinities with both the Ivuna-like carbonaceous (CI) and Mighei-like carbonaceous (CM) chondrites. While mineralogically similar, a key difference among these samples is that chondrules have been found in some of these samples, but not in others. The aim of this study is to evaluate the relationship of these meteorites to one another, and investigate how they relate to the CI and CM chondrite groups.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"5 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884584","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}
Using a combination of first-principles density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the influence of Fe on the inter-site (i.e, between interstitial and substitutional sites) equilibrium isotopic fractionation of Li in olivine (M2SiO4; M: Mg, Fe). Our calculations indicate that among the substitutional sites, Li as well as Fe (Fe2+ and Fe3+) prefer to get incorporated in the smaller and more regular M1-octahedral site as opposed to the larger and more distorted M2-octahedral site in the olivine crystal structure. Calculation of the isotopic fractionation of Li between the interstitial and substitutional sites of olivine indicates that there is a Fe-concentration dependent enrichment of the interstitial site by the heavier Li isotope relative to the substitutional site. It is found that with the incorporation of 3.125 % Fe in the octahedral site of forsterite (Mg2SiO4), there is an enhancement of the ‘isotopic fractionation factor’ (103ln(αinterstitial-substitutional)) of Li by 41.751 ‰ at 300 K and 2.812 ‰ at 1200 K, which increases with further rise in Fe concentration. The correlation between the average Li-O bond length and the ‘reduced partition function ratios’ (103ln (β7-6)) of 7Li/6Li is found to break down in the presence of local charge imbalance. Our calculations offer further insights into the intricate nature of Li isotopic fractionation in natural olivine, which has long been used as a geochemical tracer to infer geological processes in the Earth.
{"title":"Lithium isotopic fractionation in olivine crystal in presence of iron at variable temperatures: A first-principles study","authors":"Rabindranath Mondal, Chirantan Pramanik, Swastika Chatterjee, Prosenjit Ghosh, Tanusri Saha-Dasgupta","doi":"10.1016/j.gca.2024.12.023","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.023","url":null,"abstract":"Using a combination of first-principles density functional theory (DFT) and density functional perturbation theory (DFPT), we investigate the influence of Fe on the inter-site (i.e, between interstitial and substitutional sites) equilibrium isotopic fractionation of Li in olivine (M<ce:inf loc=\"post\">2</ce:inf>SiO<ce:inf loc=\"post\">4</ce:inf>; M: Mg, Fe). Our calculations indicate that among the substitutional sites, Li as well as Fe (Fe<ce:sup loc=\"post\">2+</ce:sup> and Fe<ce:sup loc=\"post\">3+</ce:sup>) prefer to get incorporated in the smaller and more regular M1-octahedral site as opposed to the larger and more distorted M2-octahedral site in the olivine crystal structure. Calculation of the isotopic fractionation of Li between the interstitial and substitutional sites of olivine indicates that there is a Fe-concentration dependent enrichment of the interstitial site by the heavier Li isotope relative to the substitutional site. It is found that with the incorporation of 3.125 % Fe in the octahedral site of forsterite (Mg<ce:inf loc=\"post\">2</ce:inf>SiO<ce:inf loc=\"post\">4</ce:inf>), there is an enhancement of the ‘isotopic fractionation factor’ (10<ce:sup loc=\"post\">3</ce:sup>ln(α<ce:inf loc=\"post\">interstitial-substitutional</ce:inf>)) of Li by 41.751 ‰ at 300 K and 2.812 ‰ at 1200 K, which increases with further rise in Fe concentration. The correlation between the average Li-O bond length and the ‘reduced partition function ratios’ (10<ce:sup loc=\"post\">3</ce:sup>ln (β<ce:inf loc=\"post\">7-6</ce:inf>)) of <ce:sup loc=\"post\">7</ce:sup>Li/<ce:sup loc=\"post\">6</ce:sup>Li is found to break down in the presence of local charge imbalance. Our calculations offer further insights into the intricate nature of Li isotopic fractionation in natural olivine, which has long been used as a geochemical tracer to infer geological processes in the Earth.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"31 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142935995","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}
Pub Date : 2024-12-18DOI: 10.1016/j.gca.2024.12.020
Marina Martínez, Adrian J. Brearley
Phosphorus-bearing minerals in carbonaceous chondrites record early aqueous alteration effects in the parent asteroid and potentially provide clues on early solar nebular processes. Despite their importance, only a few studies exist dedicated to investigating P-bearing minerals in primitive carbonaceous chondrites and thus, their origins are not well constrained. Work on Ca phosphates around the edges of type IIA chondrules in primitive CR and CM chondrites has shown that Ca phosphates are generally associated with aqueous alteration in the parent body. The present study examines two different Ca phosphate occurrences in one of the least altered CR chondrites known, QUE 99177, by SEM, EPMA, and FIB-TEM techniques to better constrain their origins. The first type consists of elongate, submicron-sized rods of merrillite that occur in regions of mesostasis at the edge of type IIA chondrules adjacent to the surrounding matrix. The second type occurs as nanometer-sized grains around some type IA chondrules that are surrounded by smooth rims. These smooth rims are a type of rim that consists of an amorphous, Fe-rich, hydrous silicate phase that results from low-temperature aqueous alteration of silica in Silica-rich Igneous Rims (SIRs) at the earliest stages of parent body alteration. The Ca phosphates are located within discrete regions at the interface between smooth rims and adjacent matrix, ranging from whitlockite to apatite compositions. We argue that the first type of Ca phosphate has a solar nebular origin, formed by quenching of Ca- and P-bearing melts in chondrules at the final stages of crystallization, whereas the second type has a parent body origin, formed by oxidation of Fe,Ni metal grains in SIRs surrounding chondrules. Therefore, our new data and a reappraisal of previous data demonstrate, for the first time, that Ca phosphates formed by both primary (solar nebular) and secondary (parent body) processes. These results also provide additional insights into the formation conditions of type IIA chondrules in the protoplanetary disk and constrain the earliest stages of aqueous alteration in the CR chondrite parent body.
{"title":"Calcium phosphates associated with chondrules in the CR chondrite Queen Alexandra Range (QUE) 99177: Evidence for solar nebular and parent body processes","authors":"Marina Martínez, Adrian J. Brearley","doi":"10.1016/j.gca.2024.12.020","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.020","url":null,"abstract":"Phosphorus-bearing minerals in carbonaceous chondrites record early aqueous alteration effects in the parent asteroid and potentially provide clues on early solar nebular processes. Despite their importance, only a few studies exist dedicated to investigating P-bearing minerals in primitive carbonaceous chondrites and thus, their origins are not well constrained. Work on Ca phosphates around the edges of type IIA chondrules in primitive CR and CM chondrites has shown that Ca phosphates are generally associated with aqueous alteration in the parent body. The present study examines two different Ca phosphate occurrences in one of the least altered CR chondrites known, QUE 99177, by SEM, EPMA, and FIB-TEM techniques to better constrain their origins. The first type consists of elongate, submicron-sized rods of merrillite that occur in regions of mesostasis at the edge of type IIA chondrules adjacent to the surrounding matrix. The second type occurs as nanometer-sized grains around some type IA chondrules that are surrounded by smooth rims. These smooth rims are a type of rim that consists of an amorphous, Fe-rich, hydrous silicate phase that results from low-temperature aqueous alteration of silica in Silica-rich Igneous Rims (SIRs) at the earliest stages of parent body alteration. The Ca phosphates are located within discrete regions at the interface between smooth rims and adjacent matrix, ranging from whitlockite to apatite compositions. We argue that the first type of Ca phosphate has a solar nebular origin, formed by quenching of Ca- and P-bearing melts in chondrules at the final stages of crystallization, whereas the second type has a parent body origin, formed by oxidation of Fe,Ni metal grains in SIRs surrounding chondrules. Therefore, our new data and a reappraisal of previous data demonstrate, for the first time, that Ca phosphates formed by both primary (solar nebular) and secondary (parent body) processes. These results also provide additional insights into the formation conditions of type IIA chondrules in the protoplanetary disk and constrain the earliest stages of aqueous alteration in the CR chondrite parent body.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"26 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142911880","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}
Pub Date : 2024-12-18DOI: 10.1016/j.gca.2024.12.018
Dongmei Tang, Marina Lazarov, Dachuan Wang, Noreen J. Evans, Kezhang Qin, Shengchao Xue, Ingo Horn, Stefan Weyer
The initial silicate melt related to magmatic Ni-Cu deposits located in orogenic settings in China (e.g., East Tianshan orogenic belt, East Kunlun orogenic belt) had a high fO2 that progressively decreased with continued magmatic evolution. It is still unknown if the sulfide melt that separated from the silicate melt inherited this high fO2, or even whether oxygen fugacity in sulfide plays an important role in the mineralization processes. In this work we undertook new in situ Fe, Cu, and Ni isotopic analyses of base metal sulfides from the Xiarihamu magmatic sulfide deposit (East Kunlun orogenic belt), and combined this new data with previously published Fe and Cu isotopic results from orogenic and cratonic magmatic sulfide deposits to assess changes in fO2 in sulfide during sulfide melt evolution, and the role of these processes in metal enrichment.
{"title":"Response of Fe-Ni-Cu isotope systematics to sulfide liquid oxygen content: Implications for magmatic sulfide Co enrichment in orogenic settings","authors":"Dongmei Tang, Marina Lazarov, Dachuan Wang, Noreen J. Evans, Kezhang Qin, Shengchao Xue, Ingo Horn, Stefan Weyer","doi":"10.1016/j.gca.2024.12.018","DOIUrl":"https://doi.org/10.1016/j.gca.2024.12.018","url":null,"abstract":"The initial silicate melt related to magmatic Ni-Cu deposits located in orogenic settings in China (e.g., East Tianshan orogenic belt, East Kunlun orogenic belt) had a high <ce:italic>f</ce:italic>O<ce:inf loc=\"post\">2</ce:inf> that progressively decreased with continued magmatic evolution. It is still unknown if the sulfide melt that separated from the silicate melt inherited this high <ce:italic>f</ce:italic>O<ce:inf loc=\"post\">2</ce:inf>, or even whether oxygen fugacity in sulfide plays an important role in the mineralization processes. In this work we undertook new <ce:italic>in situ</ce:italic> Fe, Cu, and Ni isotopic analyses of base metal sulfides from the Xiarihamu magmatic sulfide deposit (East Kunlun orogenic belt), and combined this new data with previously published Fe and Cu isotopic results from orogenic and cratonic magmatic sulfide deposits to assess changes in <ce:italic>f</ce:italic>O<ce:inf loc=\"post\">2</ce:inf> in sulfide during sulfide melt evolution, and the role of these processes in metal enrichment.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"65 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2024-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142884629","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}
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