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JournalISSN: 0009-2541

Chemical Geology 

About: Chemical Geology is an academic journal. The journal publishes majorly in the area(s): Carbonate & Mantle (geology). It has an ISSN identifier of 0009-2541. Over the lifetime, 10651 publication(s) have been published receiving 571678 citation(s). The journal is also known as: Chemical geology.
Topics: Carbonate, Mantle (geology), Basalt, Zircon, Calcite

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Journal ArticleDOI
Abstract: Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements. The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth. The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga). The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition. The Earth has a bulk Fe Al of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of Fe Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.

9,413 citations

Journal ArticleDOI
Abstract: The abundance and distribution of selected minor and trace elements (Ti, Zr, Y, Nb, Ce, Ga and Sc) in fresh volcanic rocks can be used to classify the differentiation products of subalkaline and alkaline magma series in a similar manner to methods using normative or major-element indices. A number of variation diagrams may be used to distinguish common volcanic rock types in terms of the above elements. As these elements are immobile during post-consolidation alteration and metamorphic processes, this method of rock-type classification may, when applied to metavolcanic rocks, prove more reliable than the commonly used methods that utilize major elements, some of which are known to be mobile.

4,126 citations

Journal ArticleDOI
Abstract: This paper reports new developments in in situ U–Pb zircon geochronology using 266 and 213 nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Standard spot ablation (spot diameters 40–80 μm) was employed, with no sampling strategies employed specifically to minimise elemental fractionation. Instead, He ablation gas and carefully replicated ablation conditions were employed to maintain constant ablation-related elemental fractionation of Pb and U between analyses. Combining these strategies with calibration on a new zircon standard (GJ-1) allows elemental fractionation and instrumental mass bias to be corrected efficiently, and accurate 206Pb/238U and 207Pb/235U ratios to be measured with short-term precision (2 r.s.d.) of 1.9% and 3.0%, respectively. Long-term precision (2 r.s.d.) of the technique (266 nm ablation), based on 355 analyses of the 91500 zircon (1065 Ma) standard over more than a year, was 3.8%, 4.0% and 1.4% for the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios, respectively. Long-term precision (2 r.s.d.) for the 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios of the Mud Tank zircon (732 Ma) was 3.9%, 4.1% and 1.7%, respectively (359 analyses). Selective integration of time-resolved signals was used to minimise the effect of Pb loss and common Pb enrichments on the measured ages. The precision and accuracy of our data compare very favourably with those obtained using more involved procedures to correct or minimise ablation- and ICP-MS-induced biases. 213 nm laser ablation produced comparable precision to 266 nm ablation using generally smaller spot sizes (40–50 vs. 60–80 μm), and offered significant advantages in terms of ablation duration and stability, particularly for small zircons (<60 μm). For the 91500 zircon, but not the Mud Tank zircon, 213 nm ablation also produced significantly older and more accurate Pb/U ages. This suggests that shorter wavelength ablation may have reduced a matrix-dependent elemental fractionation difference between sample and standard. The accuracy and precision of the technique for young zircons are demonstrated by analysis of three zircon populations ranging in age from 417 to 7 Ma. In each case, the zircons have yielded concordant ages or common Pb discordia which give concordia intercept ages that are in agreement with independently determined ages for the same samples. Application of Tera–Wasserburg diagrams [Earth Planet. Sci. Lett. 14 (1972) 281] was found to be the most useful approach to handling common Pb contributions that were not removed by selective integration of signals.

3,489 citations

Journal ArticleDOI
Tom Andersen1
Abstract: The presence of common lead contamination in zircons used for U–Pb geochronology is a potentially serious source of error. Traditionally, common lead is measured by analysis of 204Pb, and the isotopic composition of lead corrected accordingly. Some analytical methods (e.g. LAM-ICPMS) do not report 204Pb. Correction methods are available for such analyses, but these assume that the only source of discordance in a zircon is the presence of common lead. Using such a correction on a lead analysis that contains a discordance component caused by lead loss will invariably lead to overcorrection, and hence to a meaningless, young age. By assuming that the observed 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios of a discordant zircon can be accounted for by a combination of lead loss at a defined time, and the presence of common lead of known composition, a correction method can be designed that neither uses 204Pb nor assumes concordance. The method proposed here involves a numeric solution to a set of equations relating the content of radiogenic lead in a zircon or other U/Th-enriched mineral to its total lead content, the amount of common lead present, the age of initial crystallization, the age of lead loss and the amount of lead lost in that process. An estimate for the age of lead loss is needed, but in the absence of prior knowledge of this age, the recalculation procedure can be set up in such a way that the bias in initial age caused by a systematic error in the age of lead loss is minimized. Despite this limitation, the method will give less bias in the corrected ages than alternative correction methods.

3,072 citations

Journal ArticleDOI
Abstract: Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in inicrobeam U-Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plesovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U-Pb age with a weighted mean Pb-206/U-238 date of 337.13 +/- 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U-Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to similar to 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plesovice zircon (>0.9 wt.% Hf) is homogenous within and between the grains with a mean Hf-176/Hf-177 value of 0.282492 +/- 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plesovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.

2,880 citations

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