{"title":"原生铁同位素特征记录了浅海环境中沉积的 3.2 Ga 铁屑硅质沉积岩的氧化沉淀过程","authors":"Ryohei Suzumeji , Tsubasa Otake , Daizo Yamauchi , Yoko Ohtomo , Takeshi Kakegawa , Christoph Heubeck , Shin-ichi Yamasaki , Tsutomu Sato","doi":"10.1016/j.precamres.2024.107574","DOIUrl":null,"url":null,"abstract":"<div><div>Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for <em>ca</em>. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe<sup>2+</sup>-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ<sup>56</sup>Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ<sup>56</sup>Fe values (δ<sup>56</sup>Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> – δ<sup>56</sup>Fe and matrix ratio – δ<sup>56</sup>Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe<sup>2+</sup>-silicate precipitation from dissolved ferrous Fe (Fe<sup>2+</sup><sub>(aq)</sub>) was much smaller (Δ<sup>56</sup>Fe<sub>Fe2+-silicate–Fe2+(aq)</sub> < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe<sup>2+</sup><sub>(aq)</sub> supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ<sup>13</sup>C<sub>carb</sub> = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe<sup>3+</sup>-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> – δ<sup>56</sup>Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe<sup>2+</sup> in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment.</div></div>","PeriodicalId":49674,"journal":{"name":"Precambrian Research","volume":null,"pages":null},"PeriodicalIF":3.2000,"publicationDate":"2024-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Primary Fe isotope signatures record oxidative precipitation in 3.2 Ga ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean environment\",\"authors\":\"Ryohei Suzumeji , Tsubasa Otake , Daizo Yamauchi , Yoko Ohtomo , Takeshi Kakegawa , Christoph Heubeck , Shin-ichi Yamasaki , Tsutomu Sato\",\"doi\":\"10.1016/j.precamres.2024.107574\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for <em>ca</em>. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe<sup>2+</sup>-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ<sup>56</sup>Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ<sup>56</sup>Fe values (δ<sup>56</sup>Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> – δ<sup>56</sup>Fe and matrix ratio – δ<sup>56</sup>Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe<sup>2+</sup>-silicate precipitation from dissolved ferrous Fe (Fe<sup>2+</sup><sub>(aq)</sub>) was much smaller (Δ<sup>56</sup>Fe<sub>Fe2+-silicate–Fe2+(aq)</sub> < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe<sup>2+</sup><sub>(aq)</sub> supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ<sup>13</sup>C<sub>carb</sub> = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe<sup>3+</sup>-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe<sub>2</sub>O<sub>3(T)</sub>/Al<sub>2</sub>O<sub>3</sub> – δ<sup>56</sup>Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe<sup>2+</sup> in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment.</div></div>\",\"PeriodicalId\":49674,\"journal\":{\"name\":\"Precambrian Research\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":3.2000,\"publicationDate\":\"2024-10-01\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Precambrian Research\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0301926824002870\",\"RegionNum\":2,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"GEOSCIENCES, MULTIDISCIPLINARY\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Precambrian Research","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0301926824002870","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
Primary Fe isotope signatures record oxidative precipitation in 3.2 Ga ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean environment
Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for ca. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe2+-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ56Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ56Fe values (δ56Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe2O3(T)/Al2O3 ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe2O3(T)/Al2O3 – δ56Fe and matrix ratio – δ56Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe2+-silicate precipitation from dissolved ferrous Fe (Fe2+(aq)) was much smaller (Δ56FeFe2+-silicate–Fe2+(aq) < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe2+(aq) supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ13Ccarb = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe3+-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe2O3(T)/Al2O3 – δ56Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe2+ in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment.
期刊介绍:
Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as:
(1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology;
(2) Geochronology and isotope and elemental geochemistry;
(3) Precambrian mineral deposits;
(4) Geophysical aspects of the early Earth and Precambrian terrains;
(5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes.
In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes.
Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.