Xuan-Ce Wang

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]

Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume

Mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the Neoproterozoic supercontinent Rodinia, with supportive evidence including radiating dike swarms, globally synchronous anorogenic igneous activity, large-scale lithospheric doming and unroofing, and geochemical signatures similar to recent plume-related magmatism. However, high-temperature magmas such as picrite or komatiite, a requisite product of mantle plume activities, have not previously been identified for those events. We present here geochronological and geochemical data from a suite of pillow lavas in central South China. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 ± 6 Ma. All but a few highly evolved, crust-contaminated basaltic rocks are characteristically high in MgO (10.2%–17.5%), Ni (183–661 ppm), and Cr (677–1672 ppm), but low in TiO2 (0.5%–0.7%), Al2O3 (10.6%–12.7%), and FeOT (total Fe as FeO) (7.4%–10.5%), typical of komatiitic basalts. Our geochemical modeling, which removes the effect of olivine crystallization, suggests that their primary magma has typical komatiitic compositions with MgO ≈ 20%, FeOT ≈ 11%, SiO2 ≈ 47%, TiO2 ≈ 0.48%, Al2O3 ≈ 10%, Ni ≈ 860 ppm, and Cr ≈ 1780 ppm. Such a high MgO content in the primary melts implies a melt temperature of >1500 °C, suggesting that the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature ( T p) 260 ± 50 °C higher than the ambient mid-oceanic-ridge basalt (MORB)–source mantle, similar to that of modern mantle plumes. The Yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca. 825 Ma mantle plume that played a key role in the breakup of the supercontinent Rodinia.

Continental flood basalts derived from the hydrous mantle transition zone

It has previously been postulated that the Earths hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts.

Eocene Neo-Tethyan slab breakoff constrained by 45 Ma oceanic island basalt–type magmatism in southern Tibet

Slab breakoff is one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoff of the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in-situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)–type oceanic island basalt and have depleted Sr-Nd isotopes [87Sr/86Sr(t) = 0.70312-0.70317; eNd(t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoff model may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.

Neoproterozoic S-type granites in the Alxa block, Westernmost north China and tectonic implications: in situ zircon U-Pb-Hf-O isotopic and geochemical constraints

The Alxa Block in northern China has been traditionally considered as the westernmost part of the Archean North China Craton (NCC). However, recent studies revealed that there are few Archean rocks exposed in the Alxa Block, and the Paleoproterozoic geology of this block is different from that of the western part of the NCC. Thus, the tectonic affinity of the Alxa Block to the NCC and/or other Precambrian blocks needs further investigations. In this study, we carry out integrated analyses of in situ zircon U–Pb age and Hf–O isotopes as well as whole-rock geochemistry and Nd isotopes for the Neoproterozoic Dabusushan and Naimumaodao granites from central Alxa Block. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating results indicate that the Naimumaodao and Dabusushan granite plutons were formed at ca. 930 Ma and ca. 910 Ma, respectively. These granites are peraluminous (A/CNK value >1.0), and contain peraluminous minerals such as muscovite and tourmaline, similar to those of S-type granites. They are characterized by high zircon δ18O values of ca. 8.2 to 12.1 permil, corresponding to a calculated magmatic δ18O value of ca. 10.5 to 14.3 permil, variable zircon εHf(t) values of −6.2 to +3.8 (corresponding to Hf model ages of 2.2 to 1.6 Ga) and whole-rock εNd(t) values of −10.1 to −4.5 (corresponding to Nd model ages of 2.4-1.9 Ga). The petrological and Nd–Hf–O isotopic study indicated that these granites were most probably generated by remelting of dominant (meta)sedimentary rocks in an orogenesis-related compressional environment. There is a clear contrast in the Precambrian geological evolution, including basement rock age data, Precambrian magmatism and detrital zircon age patterns, between the Alxa Block and the NCC. Furthermore, the new in-situ detrital zircon ages on Neoproterozoic (meta)sedimentary rock suggest that Alxa Block is likely related to the Cathaysia Block of South China during the Neoproterozoic, and amalgamated with the NCC since the Early Paleozoic. Thus, our new data suggest that the Alxa Block is most likely a separated Precambrian terrane from the Western Block of the NCC.

Nonglacial origin for low-δ18O Neoproterozoic magmas in the South China Block: Evidence from new in-situ oxygen isotope analyses using SIMS

Low-δ 18 O signatures in supracrustal rocks have been used as geochemical proxies for cold paleoclimates, e.g., glaciations. Unusual low-δ 18 O values found in Neoproterozoic igneous rocks in parts of the South China Block have thus been genetically linked to Neoproterozoic glaciation events. However, we report here new oxygen isotope compositions from Neoproterozoic magmatic zircons in central southern China using in-situ techniques that argue against such an interpretation. Our results show that (1) low-δ 18 O magmatic zircons started to appear in the South China Block from ca. 870 Ma, coinciding with the tectonic switching from Sibao orogenesis to postorogenic extension, which occurred more than 150 m.y. prior to the first glaciation event. The most abundant low-δ 18 O magmatic zircons have ages of 800–700 Ma. (2) The 830–700 Ma magmatic zircons are characterized by their bimodal nature of oxygen isotope compositions, i.e., mantle-like δ 18 O values (+4.4‰ to +5.8‰) and high-δ 18 O values (+9.3‰ to +10.8‰). (3) A sharp temporal change in maximum zircon δ 18 O values in the South China Block coincided with the onset of continental rifting and the possible arrival of a plume head. (4) No negative δ 18 O zircons have been identified in this study, contrary to previous studies. These features strongly argue against a glaciation origin for low to negative δ 18 O values in Neoproterozoic magmatic zircons from southern China. We propose that two stages of high-temperature water-magma interaction during plume-driven magmatism and continental rifting best explain the low-δ 18 O magmas. The most important implication of this study is that formation of such low-δ 18 O magmatic zircons was not necessarily related to glacial events and should not be used as a geochemical proxy for a cold paleoclimate.

Oriented nanotwins induced by electric current pulses in Cu-Zn alloy

After applying an electrical current pulse (ECP) to samples of Cu–Zn alloy, {111} oriented nanotwins parallel to the ECP direction in α phase grains have been observed at ambient temperature. It seems that (a) these samples have heated up to a temperature much higher than the α to β phase transformation temperature and (b) new β nuclei on the {110} planes have formed in the original α phase. As a result, with the samples being rapidly cooled, these oriented nanotwins will be formed with the β to α martensitic transformation.

Rapid separation scheme of Sr, Nd, Pb, and Hf from a single rock digest using a tandem chromatography column prior to isotope ratio measurements by mass spectrometry

A straightforward tandem column separation procedure is presented for the separation of Sr, Nd, Pb, and Hf from silicate materials. It allows rapid purification, without any intervening evaporation, of these four elements of great interest in Earth science and cosmochemistry. After sample loading, the upper Sr Spec column adsorbs Sr and Pb, while the lower TODGA Spec column adsorbs Hf and Nd. Strontium-lead and hafnium–neodymium elements are then back-extracted from the Sr Spec and TODGA Spec columns, respectively. The whole separation procedure, including column setup, cleaning, and pre-conditioning, takes approximately eight hours for separating a batch of 25 samples. The proposed procedure offers significant improvement in separation efficiency of these often-used four elements, compared with conventional four column methods. Fractions of Sr, Nd and Pb are then measured by TIMS and the Hf fraction is determined by MC-ICP-MS. The stability of this procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios of eight international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained in this study agree well with published data. The external reproducibility (2RSD, n = 8) of standard BCR-2 was ±0.0026% for 87Sr/86Sr, ±0.0020% for 143Nd/144Nd, ±0.0049% for 176Hf/177Hf, and ±0.026–0.034% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios.

Single-step separation scheme and high-precision isotopic ratios analysis of Sr–Nd–Hf in silicate materials

Thermal ionization mass spectrometry and multiple-collector inductively coupled plasma mass spectrometry are considered to be “gold standards” for the determination of the isotope ratios of Sr–Nd and Hf in geological samples because of the extremely high precision and accuracy of these methods. However, the sample throughputs are hindered by time-consuming and tedious chemical procedures. Three-step ion exchange resin separation is traditionally employed to purify Sr–Nd–Hf from matrix elements. In this study, a one-step Sr–Nd–Hf separation scheme was developed to process geological samples. The separation scheme is based on the combined use of conventional AG50W-X12 cation-exchange resin and LN Spec extraction chromatographic material without any intervening evaporation step. The protocol not only prevents cross-contamination during operation using multiple-stage ion exchange resins but also significantly improves the efficiency of sample preparation. The stability of our chemical procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios in six international reference materials of silicate rocks. The analytical results obtained for these standard rocks compare well with the published data. The external reproducibility (2 SD, n = 10) of a BCR-2 standard sample was ±0.000018 for 87Sr/86Sr, ±0.000010 for 143Nd/144Nd, and ±0.000014 for 176Hf/177Hf.

Ce–Nd separation by solid-phase micro-extraction and its application to high-precision 142Nd/144Nd measurements using TIMS in geological materials

In view of the low initial abundance of 146Sm, 142Nd anomalies are expected to be extremely small (less than 40 ppm), and their detection requires ultra-precise 142Nd/144Nd measurements. A rapid solid-phase micro-extraction (SPME) technique, using HEHEHP resin as sorbent, is established to completely separate Ce from rare earth element (REE) mixtures. This technique is applied to ultra-high-precision 142Nd/144Nd measurements in geological materials. In contrast to the traditional liquid–liquid micro-extraction (LLME) technique, the benefits of the SPME tandem column are high Nd recovery, low residual Ce (Ce/Nd 3.0. Thus, 142Ce interferences on 142Nd never exceed 1.3 ppm. Ultra-high-precision thermal ionization mass spectrometry analyses of silicate standards show that the internal precision of all runs are better than 4 ppm (2 RSE) for 142Nd/144Nd values. 142Nd/144Nd values for JNdi-1, JR-3, and BCR-2 have external precisions of ±4.8, ±4.4, and ±3.9 ppm (2 RSD), respectively. The external reproducibility is sufficient to distinguish and resolve 5 ppm anomalies in 142Nd/144Nd values.