The Rb–Sr decay system is one of the most widely used geochronometers for obtaining ages and cooling rates of terrestrial magmatic, metamorphic, and hydrothermal events. It has also been extensively applied to date extraterrestrial, early solar system events. The accuracy of Rb–Sr ages, however, strongly depends on the accuracy of the 87Rb decay constant (λ87Rb). We determined λ87Rb relative to the decay constants of 235U and 238U by comparing Rb–Sr ages of minerals with U–Pb ages obtained from the same intrusion. Comparison of U–Pb emplacement ages with high-precision Rb–Sr mineral ages from three rapidly cooled igneous rocks covering an age range of ca. 2.5 Ga yields an unweighted mean λ87Rb of 1.393 ± 0.004 × 10−11 yr−1 (i.e., ± 0.3%), corresponding to a half-life of 49.76 × 109 years. Because this decay constant is 2% lower than the presently recommended one, many previously published ages are 2% too young and the resulting geologic interpretations may need revision.
Discovering if hotspots observed on the Earth’s surface are explained by underlying plumes rising from the deep
mantle or by shallow plate-driven processes continues to be an essential goal in Earth Science. Key evidence
underpinning the mantle plume concept is the existence of age-progressive volcanic trails recording past plate
motion relative to surface hotspots and their causal plumes. Using the icebreaker RV Polarstern, we sampled
scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell, which projects down to one of the
Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province – caused by a massive thermo-chemical ‘pile’ on the core-mantle boundary.We showed recently using Ar/Ar isotopic ages – and crustal structure and seafloor ages – that these hotspot trails are age progressive and formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province (LLSVP) (O’Connor et al., 2012). We showed furthermore that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We concluded that if plume melts formed synchronous age progressive hotspot trails whenever they could penetrate the lithosphere, then hotspot trails in the South Atlantic are controlled by the interplay between deep plumes and the shallow motion and structure of the African plate. If the distribution of hotspot trails reflects where plume melts could or could not penetrate the continental or oceanic lithosphere then plumes could have been active for significantly longer than indicated by their volcanic chains. This provides a mechanism for extended late stage interplay between deep mantle processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of post-rift uplift on continental margins and nearby continents.
Combined in situ U–Pb and Hf–O isotope analyses for zircon are often used to date igneous events precisely and to gain insights into the origin of the magma from which the zircon crystallized. In conjunction with its resistance to weathering, zircon can therefore be considered a unique crystal toolbox and an ideal crustal archive. This concept, however, relies on the basic assumption that each zircon crystal is in Hf isotope equilibrium with its host magma. Here we test this hypothesis for zircon crystals from mafic–ultramafic layered intrusions and show that this assumption may not always be correct. We find Hf isotope disequilibrium between zircon crystals and their host-rocks in three Neoproterozoic mafic–ultramafic layered intrusions from the northwestern margin of the Yangtze Block, Central China, formed as part of convergent margin magmatism along the Hannan–Panxi subduction zone. Zircon crystals separated from diorite samples from these three intrusions confirm prolonged magmatism for over 90 Myr for the Beiba (869 ± 5 Ma), Wangjiangshan (822 ± 4 Ma) and Bijigou (785 ± 5 Ma) intrusions, with a chronologically progressive decrease in δ 18 O values from 7·4‰ to 6·3‰ and 6·0‰, respectively. We interpret the transition from an isotopically evolved (high δ 18 O) towards a progressively more primitive mantle source (lower δ 18 O) as the fading influence of subducted sediment-derived melts in a subduction zone, consistent with a reconstructed change in subducting plate motions from the northern to the western margin of the Yangtze Block. Unlike the coherent O isotopes, the εHf (t) values of zircon populations from each intrusion show a range of several εHf units (Beiba: –1·0 to+3·0; Wangjiangshan: +2·7 to +8·3; Bijigou: +2·3 to +7·8), outside analytical uncertainty and inconsistent with an origin from a single magma batch. Whole-rock Hf isotope analyses obtained by high-pressure dissolution indicate that the diorite samples from the Beiba, Wangjiangshan and Bijigou intrusions have εHf (t) of +8·2, +7·5 and +9·3, respectively. In contrast, table-top dissolutions for the same samples yield εHf (t) of +9·7, +10·0 and +11·7, respectively. The apparent systemic offset in εHf (t) values towards more crustal compositions in high-pressure dissolutions is interpreted here to reflect mixing of zircon-hosted Hf isotopes with less evolved Hf isotopes in associated mineral phases. The more crustal character of in situ Hf isotope determinations in zircons and their range of several εHf units are interpreted here to reflect progressive crustal contamination in magma chambers at the time of zircon saturation. This implies that Hf isotope compositions of zircon crystals can be biased towards crustal signatures, particularly in mafic–ultramafic intrusions that are more susceptible to crustal contamination. In such cases, source interpretation as well as Hf model ages calculated from these isotopic mixing pools are geologically meaningless. Inevitably, contaminated igneous suites, mafic–ultramafic complexes in particular, and detrital zircon populations derived therefrom may have a complex Hf isotopic history that cannot be resolved by fast in situ analyses of Hf isotopes alone. This history may be revealed only by an atypical range of Hf isotope compositions within single magmatic suites and, if unidentified, can lead to biased geological interpretations.
We undertook 87Sr/86Sr analyses for a range of carbonate bearing geological reference materials, and combined these with δ26Mg for a subset of samples. Following chemical purification in a series of chromatographic extractions, isotope ratios were measured by Multi-Collector-ICP-MS using a Plasma II (Nu instruments, Wrexham, UK). To validate efficient sample digestion procedures of carbonate fractions, total samples were treated with either 3 mol l−1 HNO3 and 0.5 mol l−1 HCl, respectively. Results of both leaching procedures are identical within reproducibility. Reference values for SRM 88A (formerly NBS 88A), SRM 1B (formerly NBS 1B), SARM 40, SARM 43, JDo-1, JLs-1, and San Carlos olivine range from 0.70292 to 0.73724 in 87Sr/86Sr and from -2.80 to -0.41 ‰ for δ26Mg, respectively. This set of geological reference materials can be used for sedimentary rock material with different carbonate mineral and matrix composition as quality control measurements of combined stable Mg and radiogenic Sr isotope analyses.•We present a protocol that facilitates the chemical separation of Mg and Sr in carbonate bearing geological reference materials including 87Sr/86Sr and δ26Mg of certified reference materials.
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