This article presents Sm-Nd and geochemical data on fine-grained sediments of the northern margin from the Yangtze block, China, to understand the variations of Nd isotopic compositions and crustal evolution history i...This article presents Sm-Nd and geochemical data on fine-grained sediments of the northern margin from the Yangtze block, China, to understand the variations of Nd isotopic compositions and crustal evolution history in this area. The results are as follows： （1） Nd isotopic compositions for clastic sedimentary rocks of the Middle-Late Proterozoic have relatively positive Nd（t） values （＋ 2.72 to ＋ 0.69）, with Nd model ages from 1.38 Ga to 1.55 Ga, corresponding to the contemporaneous volcanic rocks from the Xixiang （西乡） Group. This indicates that the arc-related materials from Middle-Late Proterozoic dominate the provenances of the Middle-Late Proterozoic periods. （2） The gradual decrease in εNd（t） during the Cambrian-Carboniferous periods is likely to reflect the progressively increasing proportion of erosion materials from the Foping （佛坪） and Qinling （秦岭） complexes, corresponding to a gradually decreasing trend in the La/Th ratios. （3） A prominent increase in the εNd（t） value of the Late Permian strata probably reflects the significant incorporation of the mantle-derived materials. The trace element data are compared with data of the Emeishan （峨嵋山）flood basalts. These data indicate that the volcanic dust has been added to the Late Permian strata during the Late Permian, represented by periods of extremely high Emeishan flood basalt activity in the south-eastern margin of the Yangtze block.展开更多
The stable isotope systems of the transition metals potentially provide constraints on the current and past operation of the biological pump,and on the state of ocean redox in Earth history.Here we focus on two exempl...The stable isotope systems of the transition metals potentially provide constraints on the current and past operation of the biological pump,and on the state of ocean redox in Earth history.Here we focus on two exemplar metals,nickel(Ni)and zinc(Zn).The oceanic dissolved pool of both elements is isotopically heavier than the known inputs,implying an output with light isotope compositions.The modern oceanic cycle of both these elements is dominated by biological uptake into photosynthesised organic matter and output to sediment.It is increasingly clear,however,that such uptake is associated with only very minor isotope fractionation.We suggest that the isotopic balance is instead closed by the sequestration of light isotopes to sulphide in anoxic and organic-rich sediments,so that it is ocean chemistry that controls these isotope systems,and suggesting a different but equally interesting array of questions in Earth history that can be addressed with these systems.展开更多
Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spannin...Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ 98Mo values and εNd(t ) in these sedimets were observed to exhibit a large range of variation (?0.65‰―+1.87‰, ?1.46―?10.90, respec-tively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ 98Mo and εNd(t ) (+0.97‰, ?1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H2S and HS- into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO42? to be transferred into MoS24?, favoring the preferential deposition of the heavy Mo isotope as MoS24? and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ 98Mo value of +1.87‰, in association with the relatively negative εNd(t ) (?10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing pe-riod. The production of a huge quantity of HS? and H2S by the sulfate-reducing reaction led to the ex-istence of the elevated seawater δ 98Mo values during the Changxing period. The preliminary Phanero-zoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ 98Mo promotes our understanding on the evolution of the paleoenviron-mental event in the Earth history.展开更多
基金This paper is co-supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0441)the National Natural Science Foundation of China (Nos. 40173018, 40673020)
文摘This article presents Sm-Nd and geochemical data on fine-grained sediments of the northern margin from the Yangtze block, China, to understand the variations of Nd isotopic compositions and crustal evolution history in this area. The results are as follows： （1） Nd isotopic compositions for clastic sedimentary rocks of the Middle-Late Proterozoic have relatively positive Nd（t） values （＋ 2.72 to ＋ 0.69）, with Nd model ages from 1.38 Ga to 1.55 Ga, corresponding to the contemporaneous volcanic rocks from the Xixiang （西乡） Group. This indicates that the arc-related materials from Middle-Late Proterozoic dominate the provenances of the Middle-Late Proterozoic periods. （2） The gradual decrease in εNd（t） during the Cambrian-Carboniferous periods is likely to reflect the progressively increasing proportion of erosion materials from the Foping （佛坪） and Qinling （秦岭） complexes, corresponding to a gradually decreasing trend in the La/Th ratios. （3） A prominent increase in the εNd（t） value of the Late Permian strata probably reflects the significant incorporation of the mantle-derived materials. The trace element data are compared with data of the Emeishan （峨嵋山）flood basalts. These data indicate that the volcanic dust has been added to the Late Permian strata during the Late Permian, represented by periods of extremely high Emeishan flood basalt activity in the south-eastern margin of the Yangtze block.
文摘The stable isotope systems of the transition metals potentially provide constraints on the current and past operation of the biological pump,and on the state of ocean redox in Earth history.Here we focus on two exemplar metals,nickel(Ni)and zinc(Zn).The oceanic dissolved pool of both elements is isotopically heavier than the known inputs,implying an output with light isotope compositions.The modern oceanic cycle of both these elements is dominated by biological uptake into photosynthesised organic matter and output to sediment.It is increasingly clear,however,that such uptake is associated with only very minor isotope fractionation.We suggest that the isotopic balance is instead closed by the sequestration of light isotopes to sulphide in anoxic and organic-rich sediments,so that it is ocean chemistry that controls these isotope systems,and suggesting a different but equally interesting array of questions in Earth history that can be addressed with these systems.
基金Supported by the program for the Ministry of Education of ChinaNational Natural Science Foundation of China (Grant Nos. B07039, 40673020 and 40521001)the SinoPec project (Grant No. G0800-06-ZS-319)
文摘Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ 98Mo values and εNd(t ) in these sedimets were observed to exhibit a large range of variation (?0.65‰―+1.87‰, ?1.46―?10.90, respec-tively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ 98Mo and εNd(t ) (+0.97‰, ?1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H2S and HS- into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO42? to be transferred into MoS24?, favoring the preferential deposition of the heavy Mo isotope as MoS24? and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ 98Mo value of +1.87‰, in association with the relatively negative εNd(t ) (?10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing pe-riod. The production of a huge quantity of HS? and H2S by the sulfate-reducing reaction led to the ex-istence of the elevated seawater δ 98Mo values during the Changxing period. The preliminary Phanero-zoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ 98Mo promotes our understanding on the evolution of the paleoenviron-mental event in the Earth history.
