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1.
Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait) 总被引：3，自引：2，他引：1
Sonja Aulbach Roberta L. Rudnick William F. McDonough 《Contributions to Mineralogy and Petrology》2008,155(1):79-92
Lithium concentrations and isotopic compositions of olivine and 87Sr/86Sr and 143Nd/144Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence
of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both
δ7Li and Li concentrations. Olivines in iron-rich peridotites (Fo85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ7Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively
light δ7Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas.
A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has
a 87Sr/86Sr of 0.7029 and 143Nd/144Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling
mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared
to previous estimates of the plume that were based on rift basalts. Although elevated δ7Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics
in Sr, Nd and Li isotope composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
2.
Neoproterozoic chemostratigraphy 总被引：3，自引：0，他引：3
Chemostratigraphy has diverse applications to investigating the rock record, such as reconstructing paleoenvironments, determining the tectonic setting of sedimentary basins, indirect dating, and establishing regional or global correlations. Chemostratigraphy is thus an integral component of many investigations of the ancient sedimentary record. In this contribution, we review the principle inorganic geochemical methods that have been applied to the Neoproterozoic sedimentary record. Analysis of the traditional stable and radiogenic isotope systems, such as δ13C, δ18O, δ34S, and 87Sr/86Sr, is routine, particularly in successions rich in carbonate. These mainstay applications have yielded invaluable data and information bearing on the chronology and evolution of this eventful era in Earth history. Alongside the growing database of traditional data, a series of novel geochemical techniques have given rise to important new models and constraints on Neoproterozoic biogeochemical change. In particular, a range of proxies for water column redox, mainly obtained from black shales, have shed light on the pace and tempo of Neoproterozoic oxygenation and its link to the appearance of early animal evolution. Increased integration of diverse geochemical, sedimentological, and paleontological datasets, and the gradual radiometric calibration of the stratigraphic record promise to bring the details of the evolution of the Neoproterozoic Earth system into ever greater focus. 相似文献
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4.
Concentrations of rare earth elements (REE) and Nd isotopic ratios were analyzed for seawater, filtered suspension and sediment trap samples collected in the tropical Atlantic Ocean (EUMELI program, EUtrophic, MEsotrophic and oLIgotrophic sites, 20°N, 18°–21°W). This is the first REE/Nd dataset on solution and different-sized particles collected at the same site. We present direct evidence of the Nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer. This exchange is probably one of the main factors that simultaneously constrains the Nd concentration and isotopic ratio budget. We propose a new approach to estimate the residence time of Nd in the ocean (τNd) based on isotopic exchange: 200 yr < τNd< 1000 yr. The exchange requires a partial dissolution of lithogenic Nd. We estimate that the fraction of soluble Nd proportion in atmospheric dust is of the order of 20% based on the isotopic ratios. We suggest that the partial dissolution of atmospheric fallout is probably one of the main REE sources of the ocean. 相似文献
5.
江西修水地区震旦纪前后沉积岩的Nd同位素组成和物质来源 总被引：14，自引：1，他引：14
本文报导了江西修水地区前震旦系与震旦—寒武系沉积岩的Nd同位素组成。前震旦系浅变质沉积岩:~(143)Nd/~(144)Nd=0.512000～0.512214,ε_(Nd)(t)=-2.57～3.91,t_(DM)=1563～1859Ma;震旦—寒武系沉积岩:~(143)Nd/~(144)Nd=0.511710～0.511902,ε_(Nd)(t)=-8.04～-9.99,t_(DM)=1833～2426Ma。两者之间的显著差异反映了震旦纪前后沉积岩物质来源的不同及沉积构造环境的改变,前震旦纪地壳活动性强,沉积岩中含有较多的幔源物质;震旦纪以后地壳活动性减弱,沉积岩以陆壳再循环碎屑为主。 相似文献
6.
S. H.?Withers R. E.?PealeEmail author A. F.?Schulte G.?Braunstein K. M.?Beck W. P.?Hess R. J.?Reeder 《Physics and Chemistry of Minerals》2003,30(7):440-448
Calcite crystals were grown from solution with single-crystal dimensions up to 3 mm and doped up to 0.1 at% with Nd3+ ions. Phase purity was verified by powder X-ray diffraction. The concentration of Nd3+ was measured by energy-dispersive spectrometry and Rutherford backscattering spectrometry. Micro X-ray fluorescence mapping of the calcite grains indicates uniform Nd distribution in as-grown crystal grains. X-ray absorption fine structure suggests that Nd3+ is substituted for Ca2+ with local lattice distortion. Temperature-dependent near-infrared spectroscopy of Nd3+ impurities in calcite reveals large inhomogeneous linewidths and smooth line profiles that are characteristic of glassy hosts, though the samples are well crystallized. 相似文献
7.
Relict soils provide insights into Quaternary soil formation and erosion on the Edwards Plateau of central Texas and into soil-forming processes in karst terranes. Late Quaternary climate-driven soil erosion produced a mosaic of thick and thin soils on the Edwards Plateau landscape. Thick soils on uplands of the Edwards Plateau are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late Pleistocene to middle Holocene. The relict, thick soils are silicate-rich and most commonly overlie the relatively silicate-poor Cretaceous Edwards Limestone, which supports the idea that the thick soils did not form from weathering of the underlying limestone. Other potential sources of silicates for the relict soils include dust, alluvial sediments, and the Del Rio Clay, a stratigraphically higher but locally eroded clay-rich unit. Here we investigate the geographic distribution, texture, clay-sized mineralogy, rare earth element geochemistry, and neodymium isotope composition of the relict soils. We have found that the relict, thick soils are deeply weathered soils that occur dominantly over the Edwards Limestone and have a high clay content and a neodymium isotope composition that is similar to that of the Del Rio Clay. Thus, we propose that in situ weathering of the Del Rio Clay, along with partial weathering of the upper portion of the underlying Edwards Limestone produced thick chert- and clay-rich soils over resistant limestone. In areas like the Edwards Plateau, where pure limestones are interbedded with clay-rich strata, the overlying clay-rich strata must be considered as a possible silicate source to soils on pure limestone bedrock. 相似文献
8.
Nd and Hf isotope systematics of oceanic basaltic rocks are often assumed to be largely immune to the effects of hydrothermal alteration. We have tested this assumption by comparing Nd and Hf isotope data for acid-leached Cretaceous oceanic basalts from Gorgona and DSDP Leg 15 with unleached data on the same rocks. Hf isotope values and Lu/Hf ratios are relatively unaffected by leaching, but 143Nd/144Nd values of leached samples are significantly higher than those of unleached fractions of the same sample in most cases. Furthermore, the Sm/Nd ratios of the majority of leached samples are 10–40% greater than those of unleached samples. X-ray diffraction studies indicate that selective removal of secondary minerals, such as smectite, during the acid leaching process is responsible for the fractionation of Sm/Nd ratios. These results have implications for interpretation of the Hf–Nd isotope systematics of ancient submarine rocks (older than ~ 50 Ma), as (1) the age-corrected 143Nd/144Nd ratio may not be representative of the primary magmatic signature and (2) the uncertainty of the age-corrected εNd value may exceed the assumed analytical precision. 相似文献
9.
Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive εNd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative εNd(t) values, whereas Al-rich BIF samples possess consistently positive εNd(t) features. Significantly, the associated supracrustal rocks in the study area have positive εNd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (εNd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (εNd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (εNd(t) > 0). 相似文献
10.
The current use of untreated river water for drinking purposes by the population of French Guiana has important impacts on public health. Consequently, groundwater is of major importance as a possible alternative drinking water supply to reduce these impacts. Since French Guiana belongs to the Guyana Shield, sustainable water management can be expected to depend increasingly on water from fissured aquifers in hard rocks. Groundwater samples were collected from shallow drill holes in the densely populated coastal area, and deeper wells in the basement (around Cayenne and along the Maroni and Oyapock rivers). This study reports on major and trace elements for which Na+ and Ca2+ excess with regard to Cl reflect the role of water-rock interaction, as well as Sr and Nd isotopes that reflect the role of the different lithologies. δ18O and δD in waters give constraints on the water cycle (recharge and evaporation processes). 相似文献