Xirong Liang

Precise Sm–Nd and U–Pb isotopic dating of the supergiant Shizhuyuan polymetallic deposit and its host granite, SE China

The supergiant Shizhuyuan W-Sn-Bi-Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (∼ 10 km 2 ) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China. Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations. SHRIMP U-Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152 ± 2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm-Nd isochron age of 149 ± 2 Ma that is interpreted as the formation age of the Shizhuyuan deposit. Therefore, the mineralization of the supergiant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block: New constraints from Hf isotopes and Fe/Mn ratios

High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable εHf(T) values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3–1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.

The Bikou basalts in the northwestern Yangtze block, South China: Remnants of 820-810 Ma continental flood basalts?

National Natural Science foundation of China[40721063]; Chinese Academy of Sciences[KZCX3-SW-141]; Australian Research Council[DP0770228]

Geochemical and Nd isotopic variations in sediments of the South China Sea: a response to Cenozoic tectonism in SE Asia

Secular variations in geochemistry and Nd isotopic data have been documented in sediment samples at ODP Site 1148 in the South China Sea. Major and trace elements show significant changes at ca. 29.5 Ma and 26^23 Ma, whereas ONd values show a single change at ca. 26^23 Ma. Increases in Al/Ti, Al/K, Rb/Sr, and La/Lu ratios and a decrease in the Th/La ratio of the sediments beginning at 29.5 Ma are consistent with more intense chemical weathering in the source region. The abrupt change in Nd isotopes and geochemistry at ca. 26^23 Ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments, implying a drastic change in sedimentary provenance and environment at the drill site. Comparison of the Nd isotopes of sediments from major rivers flowing into the South China Sea suggests that pre-27 Ma sediments were dominantly derived from a southwestern provenance (Indochina^Sunda Shelf and possibly northwestern Borneo), whereas post-23 Ma sediments were derived from a northern provenance (South China). This change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the South China Sea, associated with a southwestward expansion of the ocean basin crust and a global rise in sea level. Thus, the geochemical and Nd isotopic changes in the sediments at ODP Site 1148 are interpreted as a response to a major plate reorganization in SE Asia at ca. 25 Ma. B 2003 Elsevier Science B.V. All rights reserved.

Precise 206Pb/238U age determination on zircons by laser ablation microprobe-inductively coupled plasma-mass spectrometry using continuous linear ablation

Abstract Laser ablation microprobe-inductively plasma-mass spectrometry (LAM-ICP-MS) with a frequency quadrupled Nd-YAG UV laser (266 nm) has been used as a fast and relatively low cost technique to successfully determine the 207Pb/206Pb age of single zircon grains with ages older than 1000 Ma; however, it has a problem with serious fractionation in Pb/U during the ablation process of in-situ depth profile analysis. Alternatively, a linear-scan laser ablation technique was carried out in this study with an ablation size of 15–20 μm in width, 100–120 μm in length and 15–20 μm in depth on single zircon crystals. Constant focusing of the laser beam on the polished zircon crystal is maintained, which successfully minimized the fractionation in Pb/U during the ablation process. Using this method, typical precision of the measurement for 206Pb/238U ratios determined on single zircons is improved to 0.8–5%. Thus, the LAM-ICP-MS technique is capable not only of achieving precise 207Pb/206Pb ages at about the 1% level for >1000 Ma zircons, but also of dating 206Pb/238U ages with precision of 2–5% for Mesozoic (156–126 Ma) zircons. It is likely that this simple and relatively low cost technique is able to achieve age results even comparable to the SHRIMP-type ion probe in the measurement of the relatively large (>100 μm) and homogeneous zircons. However, it still has the disadvantage of lower spatial resolution, which limits its application to precise dating of small and/or heterogeneous zircons of Phanerozoic ages.

Geochronology and geochemistry of single-grain zircons: Simultaneous in-situ analysis of U-Pb age and trace elements by LAM-ICP-MS

Simultaneous in-situ analysis of U-Pb age and 26 trace elements for single-grain zircon using the LAMICPMS technique was successfully achieved in this study. The precision of analysis is 1 to 3 % for 207 Pb/ 206 Pb isotopic ratios, 2 and 8 % for 206 Pb/ 238 U ratios and 5 to 15 % for most of the other trace elements. The concordia U-Pb and the mean 206 Pb/ 238 U ages for zircon grain of 1800 Ma to 150 Ma by LAM-ICP-MS spot analysis are in good agreement within analytical errors with the TIMS and SHRIMP results. All zircon grains are uniquely LREE-depleted in chondrite-normalized REE patterns. Zircon formed in mantle-derived magmas is significantly different from that of granitoids with regard to contents of many trace elements ( i.e. , U, Th, ΣREE, Y, Nb and P) and REE patterns ( i.e. , Nd/Yb ratio and Ce and Eu anomalies). This combined geochronological and geochemical investigation provides valuable information on timing and genesis of zircon.

Determination of rhenium content in molybdenite by ICP-MS after separation of the major matrix by solvent extraction with N-benzoyl-N-phenylhydroxalamine.

A simple and rapid analytical method for determining the concentration of rhenium in molybdenite for Re-Os dating was developed. The method used isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) after the removal of major matrix elements (e.g., Mo, Fe, and W) from Re by solvent extraction with N-benzoyl-N-phenylhydroxylamine (BPHA) in chloroform solution. The effect on extraction efficiency of parameters such as pH (HCl concentration), BPHA concentration, and extraction time were also assessed. Under the optimal experimental conditions, the validity of the separation method was accessed by measuring (187)Re/(185)Re values for a molybdenite reference material (JDC). The obtained values were in good agreement with previously measured values of the Re standard. The proposed method was applied to replicate Re-Os dating of JDC and seven samples of molybdenite from the Yuanzhuding large Cu-Mo porphyry deposit. The results demonstrate good precision and accuracy for the proposed method. The advantages of the method (i.e., simplicity, efficiency, short analysis time, and low cost) make it suitable for routine analysis.

Records of Toba eruptions in the South China Sea ——Chemical characteristics of the glass shards from ODP 1143A

Three layers of volcanic tephra, sampled from ODP 1143 Site in the South China Sea, were observed at the mcd depth of 5.55 m, 42.66 m, and 48.25 m, and named, in this paper, layers of A, B, and C, respectively. All of these tephra layers have an average thickness of ca. 2 cm. They were constrained in age of ca. 0.070 Ma, ca. 0.80 Ma, and ca. 1.00 Ma, respectively, by the microbiostratigraphy data. These tephra layers were predominated by volcanic glass shards with a median grain size of 70–75 μm in diameter. Major chemical compositions analyzed by EMPA and comparison with the previous data from other scatter areas suggest that these three layers of tephra can correspond to the three layers of Toba tephra, YTT, OTT, and HDT, respectively, erupting during the Quaternary. The occurrence of these tephra layers in the South China Sea implies that the Toba eruptions often occurred in the summer monsoon seasons of the South China Sea during the Quaternary, and that the strength of eruptions was probably stronger than that previously estimated.

Separation of magnesium from meteorites and terrestrial silicate rocks for high-precision isotopic analysis using multiple collector-inductively coupled plasma-mass spectrometry

We report a modified procedure for separating Mg and Al from meteorites and terrestrial igneous rocks for high-precision analysis of Mg isotopes with multiple collector-inductively coupled plasma-mass spectrometry. The separating procedure was carried out in a single ion-exchange column filled with AG50W-X12 resin, and Mg was eluted with 1 M HNO3, followed by Al eluted with 4M HNO3. The modified procedure efficiently eliminates most matrix elements (except for Ni, Co, and Cu) with a recovery yield of Mg > 99%. Measurements of Ni-, Co-, and Cu-bearing simulation solutions revealed no detectable matrix effects. However, test runs demonstrated significant mass-dependent fractionation during the chromatographic process; consequently, a high recovery yield of Mg (>99%) is required to limit the deviation to less than 0.05‰. Furthermore, analysis of Mg standard solution with a wide range of concentrations demonstrated negligible deviation for samples with concentrations of 0.3–2.5 μg ml−1Mg relative to Mg standard solution concentrations of 1 μg ml−1Mg. The total procedure bank is less than 2 ng. The long-term reproducibility of instrumental measurements of Mg isotopes is ±0.05‰ for δ25Mg, ±0.10‰ for δ26Mg, and ±0.06‰ for δ26Mg* (2 SD, n = 211), based on analyzing five pure Mg standard solutions. Analysis of three chondrites and seven igneous rock standards showed ranges of δ26Mg(DSM3) from 0.02‰ to 0.30‰ for the former and from 0.03‰ to 0.30‰ for the latter. This method is simple and actionable.