Chemical Geology最新文献

{"title":"Analysis of nano-mineral chemistry with single particle ICP-Time-of-Flight-MS; a novel approach to discriminate between geological environments","authors":"Aaron J. Goodman ,&nbsp;Hark Karkee ,&nbsp;Shiqiang Huang ,&nbsp;Katharina Pfaff ,&nbsp;Yvette D. Kuiper ,&nbsp;Zhaoshan Chang ,&nbsp;Alexander Gundlach-Graham ,&nbsp;James F. Ranville","doi":"10.1016/j.chemgeo.2024.122498","DOIUrl":"10.1016/j.chemgeo.2024.122498","url":null,"abstract":"<div><div>Studies of indicator minerals and mineral chemistry are widely used in geochemistry and are particularly useful in mineral exploration. Due to the low abundance of indicator mineral grains, large field samples and extensive laboratory processing are required for these studies. However, nano- and submicron-scale mineral particles (NPs, diameter &lt; 1 μm) are highly abundant in geochemical sample media, containing millions to billions of particles per gram of soil or sediment. In this study, we analyze mineral NPs using single particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS), a recently developed technique for high throughput elemental characterization of NPs. We investigate the limitations of the technique that arise from the working range of the ICP-TOFMS instrument and the analytical uncertainty of measured element masses in single particles. Despite these limitations, spICP-TOFMS can be used to determine accurate element mass ratios in NPs, which we validated through analysis of mineral specimens. Elemental mass ratios obtained from spICP-TOFMS in the mineral specimens were supported by SEM-EDS. We analyzed the mineral chemistry of two pairs of elements, Zr<img>Hf and Nb<img>Ta in geochemical samples (sediments, soils, and mine waste) adjacent to a carbonatite and a lithium‑cesium‑tantalum-type pegmatite. Hundreds to thousands of NPs were detected in only 30–80 min of spICP-TOFMS analysis, indicating that these particle types are highly abundant. Pegmatite-associated samples contained Hf and Ta-rich NPs, compared to carbonatite-associated samples that displayed the chondritic or crustal abundance mass ratios in single particles. Zr:Hf mass ratios measured in NPs by spICP-TOFMS were supported by LA-ICP-MS analysis of zircons from selected samples. Diagnostic nano-mineral compositions including Nb-Ta-Bi-Sb (large grains of which are rarely found) were abundant in pegmatite-associated samples, but virtually absent in carbonatite-associated samples. In this study, we demonstrate that the chemistry of mineral nanoparticles can be used to discriminate between geological environments, and for the first time, we show that spICP-TOFMS is an effective tool for this type of analysis.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122498"},"PeriodicalIF":3.6,"publicationDate":"2024-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652763","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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":"Differences in Sm/Nd ratios between early magmatic and late sulfides: The role of fluids and Nd mobility","authors":"Pavel A. Serov","doi":"10.1016/j.chemgeo.2024.122497","DOIUrl":"10.1016/j.chemgeo.2024.122497","url":null,"abstract":"<div><div>This paper presents the results of a Sm<img>Nd study of sulfide minerals and whole-rock samples from Cu-Ni-PGE layered complexes of the Fennoscandian Shield. Syngenetic (early) and epigenetic (late) sulfides were analyzed in each complex using the Sm<img>Nd method. Late sulfide minerals with low Sm/Nd values (the ¹⁴⁷Sm/¹⁴⁴Nd ratio is often in the range of 0.02–0.07) are associated with an increased mobility of Nd relative to Sm, resulting in a relative excess of Nd compared to Sm in these sulfides. Simultaneously, early sulfides, which are deposited during the magmatic stage of ore formation, typically exhibit higher Sm/Nd values (the ¹⁴⁷Sm/¹⁴⁴Nd ratio is frequently above 0.07). Additionally, Sm<img>Nd isotope data for sulfide minerals were used to date ore-forming processes in two Cu-Ni-PGE complexes—Nyud-II (Monchegorsk area, Russia) and Ahmavaara (Finland). The Sm<img>Nd ages of syngenetic and metamorphic ore from these complexes were determined. Syngenetic ores formed at 2496 ± 36 Ma (Nyud-II) and 2441 ± 93 Ma (Ahmavaara), while metamorphic ores formed at 1940 ± 32 Ma (Nyud-II) and 1904 ± 24 Ma (Ahmavaara). Thus, Sm<img>Nd isochrons yield the timing of sulfide mineralization and its relationship with the ages of the rocks containing it, while Sm/Nd ratios in sulfides help understand the processes of ore formation. A comprehensive analysis of the full isotopic dataset (this study and other published data) showed the potential of using Sm<img>Nd isotope data to trace the sequence of sulfide mineralization, which has been confirmed for some hydrothermal deposits. However, this sequence has not been confirmed for magmatic sulfides; this opens up the possibility for further research.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122497"},"PeriodicalIF":3.6,"publicationDate":"2024-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652762","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geochemical behaviour of biotite during interaction with aqueous and brine fluids: Constraints from hydrothermal batch experiments","authors":"M.M. Repczyńska ,&nbsp;J.F. Molina ,&nbsp;A. Cambeses ,&nbsp;P. Montero ,&nbsp;F. Bea ,&nbsp;K. Itano ,&nbsp;N. Pujol-Solà ,&nbsp;I. Novo-Fernández ,&nbsp;L. Barcos ,&nbsp;A. Garcia-Casco","doi":"10.1016/j.chemgeo.2024.122486","DOIUrl":"10.1016/j.chemgeo.2024.122486","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Biotite plays an important role in the geochemical cycle of Li, Rb, Cs, and Ba in the upper continental crust, as it is a significant carrier of Li and large-ion lithophile elements in felsic igneous rocks and high-grade detrital metasedimentary rocks. During its interaction with meteoric and hydrothermal fluids, biotite can be transformed into various types of clay minerals (mostly, interlayer-deficient biotite, vermiculites and smectites). These transformations can cause fractionation of the alkaline trace-element ratios Rb/Li, Cs/Li and Rb/Cs between biotite and its replacement products. This study examines the mineral transformations that occur when biotite interacts with aqueous and saline fluids and the poorly understood geochemical behaviour of the resulting phyllosilicates. For this purpose, we performed batch hydrothermal experiments of the interaction of biotite + quartz ± graphite with ultrapure H&lt;sub&gt;2&lt;/sub&gt;O, and 2 M NaCl, 2 M CaCl&lt;sub&gt;2&lt;/sub&gt; and 1 M NaF brine fluids at 170 °C and 10 bar using Teflon bombs, and at 550 °C and 800 to 1400 bar using autoclave apparatus. At &lt;em&gt;lower-T conditions&lt;/em&gt;, biotite was replaced by 2:1 trioctahedral clay minerals (interlayer-deficient biotite, smectite, vermiculite, and other phyllosilicate species with higher interlayer charge) and Fe oxy-hydroxide minerals by coupled dissolution-precipitation mechanisms. At &lt;em&gt;higher-T conditions&lt;/em&gt;, these mechanisms caused the transformation of biotite into the mineral assemblages (quartz ± graphite): diopside + anorthite + titanite (CaCl&lt;sub&gt;2&lt;/sub&gt; brine experiments), albite + ilmenite + clay minerals (NaCl brine experiments), and cryolite + alkali feldspar with albite rimmed by K-feldspar + Fe-oxides (NaF brine experiments). Therefore, a significant reduction of the clay mineral stability in the presence of NaF and CaCl&lt;sub&gt;2&lt;/sub&gt; brine fluids is inferred. The biotite replacements by phyllosilicates were mostly controlled by the ion exchange of K&lt;sup&gt;+&lt;/sup&gt; by H&lt;sup&gt;+&lt;/sup&gt; (or its hydrate state H&lt;sub&gt;3&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt;), hydrated Na&lt;sup&gt;+&lt;/sup&gt; and Ca&lt;sup&gt;2+&lt;/sup&gt;, and NaF in the interlayer site. Conservation of the total mass and the Si, Al and Mg abundances occurred in most experimental phyllosilicates. However, in the products of the low-T NaF brine ± graphite experiments, the total mass may have a gain of 5.3–11 % assuming Mg conservation. Sc, V, Nb and Ta abundances were also conserved, but a significant fractionation of the Rb/Li, Cs/Li, and Ba/Li ratios occurred in the experimental phyllosilicates. The experiments predict the generation of highly fractionated Rb/Li and Cs/Li phyllosilicates by replacement of biotite during interaction with aqueous fluids and, mostly, NaCl and NaF brine fluids at high-T and low-T conditions, respectively. This demonstrates a key role of biotite in the fractionation of Rb/Li, Cs/Li and Rb/Cs during the hydrothermal alteration of felsic igneous rocks. Conversely, a reversal in the mobility of Li wi","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"672 ","pages":"Article 122486"},"PeriodicalIF":3.6,"publicationDate":"2024-11-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142678959","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Using tourmaline to trace Li mineralization in the Mufushan granitic batholith, South China","authors":"Leguang Li ,&nbsp;Lianxun Wang ,&nbsp;Rolf L. Romer ,&nbsp;Changqian Ma ,&nbsp;Liang Cao ,&nbsp;Yang Tian","doi":"10.1016/j.chemgeo.2024.122485","DOIUrl":"10.1016/j.chemgeo.2024.122485","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Granites and rare metal pegmatites of the Mufushan granitic batholith form a continuous magmatic sequence linked by fractional crystallization. Tourmaline is present in muscovite leucogranites and all types of pegmatites, including highly evolved Li-rich pegmatites. We utilized major element, trace element and in-situ B isotope analyses of tourmaline to investigate the effects of magmatic fractional crystallization and magmatic volatile phase (MVP) exsolution on Li migration and exceptional Li enrichment. Eight types of tourmaline are identified across three rock units: (i) Isolated (Tur Ia) and nodular (Tur Ib) tourmaline within muscovite leucogranites; (ii) black tourmaline in veins and/or clusters (Tur IIa), as isolated crystals (Tur IIb) and in tourmaline-quartz segregations (Tur IIc) within Li-poor pegmatites; and (iii) tourmaline as isolated pink crystals with zoning patterns (Tur IIIa), as isolated pink crystals and/or radiating clusters (Tur IIIb), and as isolated crystals enclosed in quartz block (Tur IIIc) within Li-rich pegmatites. Tourmaline in Mufushan muscovite leucogranites and Li-poor pegmatites belongs to the alkali-group and schorl series with Mg/(Mg + Fe) ratios of 0.10–0.31 and 0.12–0.48, respectively, containing almost no Li* and F (apfu, based on X + Y + Z = 15). In contrast, tourmaline in Li-rich pegmatites exhibits schorl-elbaite and elbaite-rossmanite compositions with low Mg/(Mg + Fe) ratio (avg. = 0.01), and evolved Li* (0.01–0.90 apfu, avg. = 0.41 apfu) and F (0.00–0.91 apfu, avg. = 0.36 apfu) contents. A pronounced increase in &lt;sup&gt;Y&lt;/sup&gt;Al, (Li* + Mn) contents, and &lt;sup&gt;Y&lt;/sup&gt;[Al/(Al + Fe)] ratio is observed across the transition from Li-poor to Li-rich pegmatites, consistent with the anticipated pattern of fractional crystallization. The concentration of Li exhibits a sharp increase in Li-rich pegmatites (avg. Li = 6786 ppm) compared to Li-poor pegmatites (avg. Li = 114 ppm) and leucogranites (avg. Li = 469 ppm). Lithium contents increase and reach a peak during the crystallization of Tur IIIb (6686–11,667 ppm), and have lower peak contents during the precipitation of Tur IIIc (8261–9160 ppm), indicating that the incorporation of Li is influenced by MVP accumulation and exsolution. MVP exsolution significantly reduces the solubility of Nb, Ta, and Be in the residual melt, promoting the precipitation of beryl and columbite group minerals and facilitating the migration of fluid-mobile elements such as Li, Rb, Cs, and Ga to form lepidolite. The B isotope compositions of tourmaline range from −14.8 ‰ ∼ −12.6 ‰ in Li-poor pegmatites to −17.1 ‰ ∼ −14.0 ‰ in Li-rich pegmatites. Rayleigh fractionation modeling reveals that MVP saturation occurs after approximately 60 % B was removed from the pegmatite melt. The compositional variation of tourmaline demonstrates that Li enrichment is not only governed by continuous fractional crystallization, but also by MVP-related accumulation and exsolution mechanism.&lt;/div&gt;&lt;/d","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122485"},"PeriodicalIF":3.6,"publicationDate":"2024-11-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142678960","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"17O-excess of grass leaf phytoliths across the North American prairies records late-growing season daytime relative humidity","authors":"Minger Guo ,&nbsp;Anne Alexandre ,&nbsp;Fred J. Longstaffe ,&nbsp;Christophe Peugeot ,&nbsp;Elizabeth A. Webb","doi":"10.1016/j.chemgeo.2024.122484","DOIUrl":"10.1016/j.chemgeo.2024.122484","url":null,"abstract":"<div><div>Because it is an important climate parameter, an accurate proxy is required for the reconstruction of past Relative Humidity (RH). Triple oxygen isotopes of grass-leaf phytoliths have a high potential for this purpose. A strong correlation between RH and ¹⁷O-excess of grass leaf phytoliths has been previously demonstrated from calibration experiments in growth chambers and at natural Sub-Saharan African and Mediterranean sites. Here we focus on the potential of ¹⁷O-excess of grass leaf phytoliths as a proxy of RH across the North American Great Plains. We tested the relationship between the ¹⁷O-excess of naturally grown <em>Calamovilfa longifolia</em> phytoliths and RH for different periods of the grass growing season. The results show that ¹⁷O-excess of leaf phytoliths has a strong correlation with late-growing season (July–August) daytime RH, which varies from 29 to 72 % across North America. The correlation can be expressed as: ¹⁷O-excess_{leaf phytoliths} = 4.14(±0.47) × RH (%) – 497(±26) (R² = 0.85, <em>p</em>-value&lt;0.0001). To use the ¹⁷O-excess of bulk phytoliths buried in soils and sediments as a quantitative proxy of past RH, future calibration studies on soil phytoliths are needed to evaluate the contribution of non-transpiring phytoliths. The ¹⁷O-excess of stem water, which was determined by ¹⁷O-excess of stem phytoliths, shows similar values to the ¹⁷O-excess of precipitation in North America without evaporation, which therefore does not affect the ¹⁷O-excess of leaf phytoliths in North America. These results demonstrate that the ¹⁷O-excess of phytoliths is a promising proxy for regional growing season RH.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122484"},"PeriodicalIF":3.6,"publicationDate":"2024-11-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142697402","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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":"Contrasting molecular structures and photooxidation behaviors between dissolved organic sulfur released from rice straw-biochar and aerobically decomposed rice straw","authors":"Chenhui Wei ,&nbsp;Heyun Fu ,&nbsp;Xiaolei Qu ,&nbsp;Shu Tao ,&nbsp;Patrick G. Hatcher ,&nbsp;Dongqiang Zhu","doi":"10.1016/j.chemgeo.2024.122494","DOIUrl":"10.1016/j.chemgeo.2024.122494","url":null,"abstract":"<div><div>Photooxidation of dissolved organic sulfur (DOS) in soils and natural waters plays an important role in the sulfur biogeochemical cycle. However, the structural-dependent photoliabilities of DOS from different sources remain unclear. Here, the molecular structures and photooxidation behaviors of DOS in pyrogenic dissolved black matter (PyDOM) derived from rice straw-pyrolyzed biochar (referred to as PyDOM-S and considered to be representative of black carbon from prairie fires) were thoroughly characterized and compared with those of DOS in leached dissolved organic matter (LDOM) derived from aerobically decomposed rice straw (referred to as LDOM-S and considered to be generally representative of organic-rich horizons in soils and peats). The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed that both PyDOM-S and LDOM-S were, respectively, dominated by aliphatic (60.4 % and 41.1 %) and lignin-like compounds (35.1 % and 40.1 %), followed by minimal aromatic and polyaromatic compounds (2.8 % and 11.9 % in total). As demonstrated by the sulfur K-edge X-ray absorption near-edge structure (S-XANES) analysis, PyDOM-S consisted mainly of organosulfate (80.4 %) contrasting to the diversified and mingled reduced sulfurs (62.7 %) and oxidized sulfurs (37.3 %) of LDOM-S. Under simulated sunlight irradiation, 74 % of sulfur in PyDOM-S was photomineralized to sulfate within 24 h and totaling 89 % after 168 h, but only 9 % and 42 % for LDOM-S given the same periods of time, confirming the much faster photomineralization of PyDOM-S. After 168-h irradiation, almost all molecules in PyDOM-S disappeared, whereas a large proportion (44.2 %) of LDOM-S molecules (mainly aliphatic and lignin-like compounds) were photo-resistant. Furthermore, the photomineralization of PyDOM-S was mainly contributed by the final and complete oxidation of organosulfate to sulfate; however, the photooxidation of LDOM-S was dominated by the sequential oxidation of exocyclic sulfur and heterocyclic sulfur to organosulfate prior to releasing sulfate. These results highlight that pyrogenic-sourced PyDOM-S and diagenesis-derived LDOM-S exhibit contrasting photooxidation behaviors due to the associated distinct molecular structures.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122494"},"PeriodicalIF":3.6,"publicationDate":"2024-11-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652395","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Magmatic maturation of Archean continental crust via a three-step crustal reworking, western Singhbhum Craton","authors":"Yousef Zoleikhaei ,&nbsp;Trisrota Chaudhuri ,&nbsp;Peter A. Cawood ,&nbsp;Rajat Mazumder ,&nbsp;Oliver Nebel ,&nbsp;Shuvabrata De","doi":"10.1016/j.chemgeo.2024.122483","DOIUrl":"10.1016/j.chemgeo.2024.122483","url":null,"abstract":"<div><div>The western part of the Singhbhum Craton preserves Paleo-Mesoarchean mafic greenstone lava flows, felsic tonalite-trondhjemite-granodiorite (TTG)-granite associations, and high-K granite and volcanic suites, similar to other Archean cratonic blocks. These successions are crucial components of early continental crust, and unravelling their respective petrogenetic relations is important for understanding the evolution from mafic to felsic crust. This study presents detailed investigations of zircon U<img>Pb age and Hf isotope data from the Bonai TTG/gneiss-granite Suite, and the overlying Tamperkola high-K granite and rhyolite Suite. Our results indicate concurrent crystallization of the Bonai TTG gneiss (3316 ± 9 Ma), associated porphyritic high-K granite (3299 ± 9 Ma), and their amphibolite enclaves (3325 ± 9 Ma) with older, inherited zircon grains intercepting at 3586 ± 25 Ma. The entire Bonai Suite yields an overall juvenile Hf isotope composition (ɛHf_(t) = −1.7 to +4.6, 95 % ɛHf_(t) &gt; 0). Combined with the mantle-like Hf isotope signatures of the inherited zircons grains (ɛHf_(t) = +1.7 to +6.2), this indicates a Hf isotope evolution array with a mafic crustal ¹⁷⁶Lu/¹⁷⁷Hf ≈ 0.022. Considering that these grains represent the source of the TTGs, this implies lower crustal residence of ca. 300 Myr of the mafic precursor rocks. The Tamperkola high-K magmatic suite yields a crystallization age of 2810 ± 8 Ma with subchondritic Hf isotope composition (ɛHf_(t) = −3.2 to −0.6). This Tamperkola Suite plots on the Hf isotope evolutionary array defined by the Bonai Suite and its mafic precursor, suggesting remelting of the Bonai (transitional) TTGs to produce these high-K granitoids in an internal reworking process. Our new and published data yield a threefold crustal evolution with (i) initial formation of the mafic crust at ca. 3586 Ma, (ii) subsequent residence for ca. 300 Myr and crustal reworking at ca. 3316–3299 Ma to form TTGs and (iii) their melting at ca. 2810 Ma to form high-K magmas. This succession of re-melting of igneous rocks drove the transition from mafic to felsic continental crust in the Singhbhum Craton. Given the consistent lithological sequence of predominantly mafic greenstone rocks, TTG-granite suites, and high-K granites observed across global cratons, this Paleo-Mesoarchean process likely reflects the order of crustal maturation in the Archean continental crust.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122483"},"PeriodicalIF":3.6,"publicationDate":"2024-11-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652394","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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":"The chemical index of alteration (CIA) and interpretation of ACNK diagrams","authors":"Thomas J. Algeo ,&nbsp;Hanlie Hong ,&nbsp;Chaowen Wang","doi":"10.1016/j.chemgeo.2024.122474","DOIUrl":"10.1016/j.chemgeo.2024.122474","url":null,"abstract":"<div><div>The chemical index of alteration (CIA) and ACNK ternary diagrams (i.e., with apices of Al₂O₃, CaO + Na₂O, and K₂O) are among the most widely used tools for evaluation of chemical weathering patterns in soils and sediments. However, incorrect use of these tools has become prevalent in recent literature, necessitating a “corrective” discussion of best practices for the CIA proxy and ACNK diagrams, their implications for the chemical weathering intensity of transported sediments, and the recognition of metasomatic effects in deep-time formations. As shown by the diversity of weathering trends in modern soil profiles, the assumption of an “ideal weathering trend” (IWT) for paleosols that runs parallel to the A-CN axis is generally unjustified. Use of an IWT to infer the presence of excess potassium (K) in a paleosol can lead to incorrect inferences of K-metasomatism and to unwarranted “corrections” of CIA values. Protolith reconstruction from a weathering path requires that the latter form a well-defined linear array in ACNK space that can be accurately projected backward to the feldspar join; projections based on poorly defined data arrays are meaningless. In transported sediments, the interpretation of ACNK data as a weathering proxy is further complicated by factors such as hydraulic sorting, two-component mixing systems, and sediment recycling. In metasedimentary formations, metasomatic alteration commonly disrupts weathering paths in ACNK space accompanied by pronounced shifts toward more K- (or Ca-Na-)rich compositions; however, it is doubtful whether this process can operate on a linear sample array representing a well-defined weathering path and shift all samples in a coordinated manner to produce a new array with an equal degree of alignment but an entirely different slope. Although K-enrichment of Paleoproterozoic paleosols has been widely attributed to K-metasomatism, the potential role of supergene weathering linked to rising atmospheric oxygen levels in promoting pedogenic K retention through early illitization of clay minerals requires consideration. For maximum robustness, studies of weathering in paleosols and paleoformations must combine information for the mobile bases (i.e., Ca-Na-K, in the form of the CIA proxy and ACNK relationships) with compositional data for Fe-Mg, SiO₂, and resistate elements as well as petrographic data documenting mineral phase relationships and sample paragenetic histories. In summary, more nuanced use and interpretation of the CIA proxy and ACNK diagrams are necessary.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122474"},"PeriodicalIF":3.6,"publicationDate":"2024-11-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652764","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Trace metal evolution of the Late Cretaceous Ocean","authors":"Mingzhao Sun ,&nbsp;Corey Archer ,&nbsp;Florian Scholz ,&nbsp;Tim Sweere ,&nbsp;Derek Vance","doi":"10.1016/j.chemgeo.2024.122477","DOIUrl":"10.1016/j.chemgeo.2024.122477","url":null,"abstract":"<div><div>The Cenomanian-Turonian boundary (Late Cretaceous) witnessed the last spectacular manifestation of Mesozoic Anoxic Events (OAE 2, <span><math><mo>∼</mo></math></span>94 Ma), marked by a prominent carbon isotope excursion (CIE) and burial of organic-matter-rich sediments under high atmospheric CO₂ concentrations. But the Late Cretaceous generally was a time of profound environmental change. OAE 2 was preceded by other CIEs, including the Mid-Cenomanian Event (MCE), and was punctuated by a short re‑oxygenation and cooling event (the Plenus Cold Event, PCE). Extensive previous studies, including many trace metal studies, have focused on OAE 2, but there is still debate concerning the degree of drawdown of oceanic trace metal reservoirs during OAE 2, whether this drawdown is global or local, its causes and consequences for ocean ecology. Here, we present records of eight trace metals, over about 5 Myr of the Late Cretaceous, from the Tarfaya Basin in the proto-North Atlantic. The long records from a core preserving a continuous sedimentary succession allow us to set changes occurring across OAE 2 in the broader context of Late Cretaceous, including the lead up to OAE 2. Moreover, the multiple trace metal dataset allows us to broadly investigate the oceanographic setting in the context of recent studies of multiple trace metals in modern organic-rich sediments aimed at refining the proxies.</div><div>Trace metal enrichments in these organic-rich sediments are discussed on three different timescales. Firstly, comparison of these Late Cretaceous sediments with modern organic-rich sediments are consistent with deposition in an open ocean upwelling margin in the Late Cretaceous, very like the modern Peru or Namibian Margin, although the deep proto-North Atlantic was probably partially restricted. Secondly, in common with previous studies, metal/TOC ratios often show sharp drops in the early part of OAE 2. Thirdly, however, this sharp drop occurs within a framework of pseudo-cyclical variations in metal/TOC, with a period of about 143 <span><math><mo>±</mo></math></span>19 kyr (1 SD), that is a feature of these long records well before OAE 2, including across the MCE. Different metals respond differently to the perturbation in the early part of OAE 2 itself. Simple mass balance considerations suggest that trace metal drawdown with organic carbon must be at least partially compensated by changes in the rate of chemical weathering on the continents, as previously inferred from Li and Ca isotopes. Moreover, changes in the patterns of variation between different metals, as well as covariation of metal/TOC ratios and Os isotopes, hint at changes in the pattern of chemical weathering, most prominently in the contribution of mafic rocks to the chemical weathering flux.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122477"},"PeriodicalIF":3.6,"publicationDate":"2024-11-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652390","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 effects of carbonate contamination on Ni-Cu-PGE deposit genesis in the Platreef, northern Bushveld Complex: A case study using Niggli numbers","authors":"Erin S. Thompson ,&nbsp;David A. Holwell ,&nbsp;Iain McDonald ,&nbsp;Marc Reichow ,&nbsp;Thomas G. Blenkinsop ,&nbsp;Hannah S.R. Hughes ,&nbsp;Katie McFall ,&nbsp;Kate R. Canham ,&nbsp;Matthew A. Loader ,&nbsp;Lara Du Preez ,&nbsp;Kofi Acheampong ,&nbsp;Andy Lloyd","doi":"10.1016/j.chemgeo.2024.122481","DOIUrl":"10.1016/j.chemgeo.2024.122481","url":null,"abstract":"<div><div>Crustal contamination in Ni-Cu-PGE deposit genesis is generally regarded as an essential, or at least highly beneficial, process in triggering sulfide saturation, either through the addition of external sulfur or as a mechanism for increasing oxygen fugacity through the incorporation of volatile-rich lithologies. The Platreef, northern limb of the Bushveld Complex, South Africa, forms one of the world's largest resources of platinum-group elements (PGE), with additional Ni-Cu-Co mineralisation, and represents a unique deposit, intersecting numerous footwall lithologies from the underlying Transvaal Supergroup. In this study, Niggli Numbers, a geochemical tool used to classify rocks on the molecular proportions of their major element geochemistry, are used to examine the degree and styles of contamination in the Platreef at three locations: Tweefontein, Overysel, and a newly drilled deeper section at Sandsloot. The potential impact of these varying contamination styles on PGE-Ni-Cu mineralisation is then discussed. At Tweefontein and Overysel, the highest PGE-Ni-Cu grades are found in largely uncontaminated pyroxenites, exemplified by Niggli <em>c</em> values &lt;20. Where carbonate contamination is strong, and Niggli <em>c</em> exceeds 30, it is not associated with elevated grades. In contrast, at Sandsloot elevated PGE and Ni grades are strongly associated with carbonate contamination, with Niggli <em>c</em> commonly exceeding 20, and Niggli <em>mg</em> exceeding 0.8, indicating dolomitic contamination, in the highest-grade horizons. Although other pre-emplacement models may yet explain the elevated grades observed at Sandsloot, there remains a clear correlation between interactions with the Malmani dolomite and elevated Ni-Cu-PGE contents which warrants further investigation.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"671 ","pages":"Article 122481"},"PeriodicalIF":3.6,"publicationDate":"2024-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142652389","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}