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The intrinsic magnetic layered topological insulator MnBi 2 Te 4 with nontrivial topological properties and magnetic order has become a promising system for exploring exotic quantum phenomena such as quantum anomalous Hall effect. However, the layer-dependent magnetism of MnBi 2 Te 4 , which is fundamental and crucial for further exploration of quantum phenomena in this system, remains elusive. Here, we use polar reflective magnetic circular dichroism spectroscopy, combined with theoretical calculations, to obtain an in-depth understanding of the layer-dependent magnetic properties in MnBi 2 Te 4 . The magnetic behavior of MnBi 2 Te 4 exhibits evident odd-even layer-number effect, i.e. the oscillations of the coercivity of the hysteresis loop (at μ 0 H c ) and the spinflop transition (at μ 0 H 1 ), concerning the Zeeman energy and magnetic anisotropy energy. In the even-number septuple layers, an anomalous magnetic hysteresis loop is observed, which is attributed to the thickness-independent surface-related magnetization. Through the linear-chain model, we can clarify the odd-even effect of the spin-flop field and determine the evolution of magnetic states under the external magnetic field. The mean-field method also allows us to trace the experimentally observed magnetic phase diagrams to the magnetic fields, layer numbers and especially, temperature. Overall, by harnessing the unusual layerdependent magnetic properties, our work paves the way for further study of quantum properties of MnBi 2 Te 4 .

Mineral assemblages formed during hydrothermal alteration reflect the geochemical composition of ore-forming fluids. Gold is mainly transported in solution as AueCl and AueS complexes. The change of physicochemical conditions such as temperature, pressure, oxygen fugacity, and sulfur fugacity are effective mechanisms for gold precipitation. Gold tends to be concentrated in the vapor phase of fluids at high temperatures and pressures. AueAs and AueSb associations are common in gold deposit. Native antimony and/or arsenic e native gold assemblages may precipitate from hydrothermal fluids with low sulfur fugacity. Hydrothermal fluids forming epithermal gold deposits are Au-saturated in most cases, whereas fluids of Carlin-type are Au-undersaturated. Quasi-steady As-bearing pyrite extracts solid solution Au from hydrothermal fluids through absorption. The capability of As-bearing pyrite to absorb Au from under-saturated fluid is the key to the formation of large-scale Carlin-type deposits. With increasing new data, studies on the geochemistry of gold deposits can be used to trace the origin of ore-forming fluids, the source of gold, and the transporting form of Au and other ore-forming elements, such as Si, S, F, Cl, As and Ag. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.

Lower Permian volcanism was the first magmatic activity to occur after the collision events in the Mongolian orogenic zone, east China. The Permian volcanic rocks are therefore a key to understanding the dynamics of the unified continental lithosphere. The volcanic rocks consist of basic and intermediate rocks. The intermediate rocks with high initial 87 Sr/ 86 Sr ratios (0.7051 to 0.7052) and low ε Nd values (-0.73 to -3.57) generally overlie the basic rocks in the field. The basic rocks have relatively low initial 87 Sr/ 86 Sr ratios (0.7034 to 0.7051) and high ε Nd values (2.72 to -0.10). Two parallel Rb-Sr isochrons give almost the same age, about 270 Ma. One consists of the basic rocks giving an initial isochron 87 Sr/ 86 Sr ratio of 0.7035. The other consists of the intermediate rocks and one sample of basalt, which give an initial isochron 87 Sr/ 86 Sr value of 0.7051. The strong correlations between SiO 2 and other major elements suggest that fractional crystallization played an important role in the magmatic processes. However, fractional crystallization cannot explain the geochemistry of most incompatible trace elements and Sr-Nd isotope characteristics. The positive correlation between Th/Nb and (La/Sm) N ratios demonstrates the direct relation between the enrichment of the light rare earth elements and the contamination of continental sediments. The high contents of large ion lithosphere elements (LILE) in the Permian volcanic rocks may suggest an additional 'crust + fluid' component, especially in the intermediate rocks, which are highly enriched in Ba (> 400 ppm) relative to the basic rocks (< 200 ppm). We propose that the subduction slab dropped into depleted mantle and released fluid, which induced the mantle metasomatism and LILE enrichment. The metasomatized mantle partially melted and formed the 'primary' magma. This primary magma assimilated with the Proterozoic biotite-quartz schist during its rise, and finally formed the Permian volcanic rocks. Magma assimilated with the Proterozoic biotite-quartz schist in small amounts could have produced the basic rocks, while assimilation of larger amounts of magma (because of longer assimilation time) would generate intermediate rocks. §Author for correspondence: yongfeng@eyou.com

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