Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert,Chile

Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the Atacama Desert. ²³⁰Th–²³⁴U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting, was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate.     

Dissolved Uranium and 234U/238U in the Yamuna and the Chambal Rivers, India

Dissolved uranium concentration and ²³⁴U/²³⁸U activity ratio have been measured in two distinctly different Indian drainage systems: the Yamuna headwaters in the Himalaya and the Chambal river system in the plains to study the weathering and mobility of uranium in these watersheds. The dissolved uranium in the Chambal river system ranges from 0.2 to 1.74 μg L⁻¹ during September (tail end of monsoon), whereas in the Yamuna river system, its concentration varies from 0.1 to 3.18 μg L⁻¹ during October (post-monsoon) and from 0.09 to 3.61 μg L⁻¹ in June (summer). In the Yamuna main stream, uranium is highest at its source and decreases steadily along its course, from 3.18 μg L⁻¹ at Hanuman Chatti to 0.67 μg L⁻¹ at Batamandi, at the base of the Himalaya. This decrease results mainly from mixing of the Yamuna mainstream with its tributaries, which are lower in uranium. The high concentration of uranium at Hanuman Chatti is derived from weathering of the Higher Himalayan Crystalline series (HHC) and associated accessary minerals, which may include uranium-mineralised zones. The ²³⁴U/²³⁸U activity ratios in the samples from the Chambal watershed are in the range of 1.15±0.05 to 1.67±0.04; whereas in the Yamuna the ratios vary from 0.95±0.03 to 1.56±0.07, during post-monsoon and from 0.98±0.01 to 1.30±0.03, during summer. The relatively high ²³⁴U/²³⁸U activity ratios in the Yamuna system are in its tributaries from the lower reaches viz., the Amlawa, Aglar, Bata, Tons and the Giri. It is estimated that ~9×10³ and ~12 × 10³ kg of dissolved uranium are transported annually from the Yamuna at Batamandi and the Chambal at Udi, respectively. This corresponds to uranium weathering rates of 0.9 and 0.09 kg U km⁻² y⁻¹ in the basins of the Yamuna and the Chambal headwaters. This study confirms that uranium weathering rate in the Himalaya is far in excess (by about an order of magnitude) of the global average value of ~0.08 kg U km⁻² y⁻¹.     

Reaction history of garnet-sapphirine granulites and conditions of Archaean high-pressure granulite-facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H₂O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.     

Disequilibrium in co-existing clinopyroxene and garnet from blueschist-facies Fe-rich eclogites, Voltri Group, Italy

Abstract Clinopyroxenes and garnets from 11 blueschist-facies Fe-rich eclogite samples from the Voltri Group show a wide range of chemical compositions. Detailed analyses of single pyroxene and garnet grains show wide and scattered chemical inhomogeneity, the K_D(K_D= (Fe₂₊/Mg)^Gt/(Fe²⁺/Mg)^Cpx) ranges from 20 to 87 based on rim analyses only. The data obtained indicate that the mineral pairs never attained equilibrium under uniform P-T conditions and that the compositions of the metamorphic minerals were influenced mainly by the composition of the pre-metamorphic minerals and by topotactical reactions.     

Oxygen isotope ratios in magnetite core-and-mantle structures

In a polymetamorphic, felsic, biotite-bearing gneiss, biotite has reacted to form magnetite and microcline. The resulting structure is a magnetite core surrounded by a mantle of feldspar and quartz normally not exceeding 20mm in diameter. Measurements of oxygen isotope ratios disclose disequilibrium between mantle microcline and mantle quartz and also between mantle and matrix minerals of the same species. A clustering of temperature estimates from the oxygen isotope distribution between magnetite and quartz and between magnetite and microcline in the interval 550 to 600°C suggests an approach to oxygen isotope equilibrium. No signs of a re-equilibriation of the reacting biotite can be found.     

Uranium and thorium diffusion in diopside

This paper presents new experimental data on the tracer diffusion rates of U and Th in diopside at 1 atm and 1150–1300°C. Diffusion couples were prepared by depositing a thin layer of U–Th oxide onto the polished surface of a natural diopside single crystal, and diffusion profiles were measured by ion microprobe depth profiling. For diffusion parallel to [001] the following Arrhenius relations were obtained: log₁₀D_U=(−5.75±0.98)−(418±28 kJ/mol)/2.303RT log₁₀D_Th=(−7.77±0.92)−(356±26 kJ/mol)/2.303RT. The diffusion data are used to assess the extent to which equilibrium is obtained during near fractional melting of a high-Ca pyroxene bearing mantle peridotite. We find that the diffusion rates for both elements are slow and that disequilibrium between solid and melt will occur under certain melting conditions. For near-fractional adiabatic decompression melting at ascent rates >3 cm/yr, high-Ca pyroxene will exhibit disequilibrium effects. High-Ca pyroxene will become zoned in U and Th and the melts extracted will be depleted in these incompatible elements relative to melts produced by equilibrium fractional melting. U and Th diffusivities in high-Ca pyroxene are similar, and diffusive fractionation of these elements will be limited. Numerical solutions to a dynamic melting model with diffusion-controlled chemical equilibration indicate that the activity ratio [²³⁰Th/²³⁸U] in a partial melt of spinel peridotite will be slightly less than 1 for a broad range of melting parameters. This result reinforces the already widely accepted conclusion that melting of spinel peridotite cannot account for ²³⁰Th excesses in mid-ocean ridge and ocean island basalts, and that garnet must therefore be present over part of the melting column.     

分权化对旅游地空间不均衡发展的影响

分权化制度框架下的旅游政策、公共设施建设、旅游投资等发展要素的差异是旅游地空间不均衡发展的根本原因。本文对相关文献进行了系统分析,界定了中国旅游分权化的利益主体,探讨了旅游利益主体组成的旅游地权力网络的分权形式、不同旅游分权利益主体在旅游发展中的角色与功能,提出了不同层面的不均衡表现及未来应关注问题。结果表明,中国分权化框架下的财税制度、利益分割引起不同旅游发展主体间的利益冲突,利益权力网络的空间不对称导致旅游产业结构、旅游土地利用、公共基础设施与旅游服务设施和社区空间发展不均衡。未来研究应以各级政府、投资商和社区居民三方利益的均衡与权力重组为出发点,从区域旅游组织分权、社区参与制度保障及其技术支持、产业空间再结构化、乡村旅游发展的政治角色4个方面展开,以实现旅游地的可持续发展。     

A multi‐geochronological study of the Hakusan volcano,central Japan

The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko‐hakusan, and Shin‐Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. ²³⁸U(²³⁴U)–²³⁰Th disequilibrium and ²⁰⁶Pb/²³⁸U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko‐Hakusan, ca 50 ka for Shin‐Hakusan I, and <10 ka for Shin‐Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin‐Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin‐Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin‐Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.     

论铀镭平衡问题

铀镭平衡系数是表征铀矿样品铀镭平衡状态的物理参数,对准确估算矿区的铀资源量,评价研究区的铀成矿潜力具有重要意义。本文从铀镭平衡系数的定义和计算方法出发,讨论了铀镭平衡系数在铀矿勘查过程中的作用和影响,以及不同类型铀矿床的铀镭平衡系数特征,强调造成铀镭不平衡的本质是在铀系的1个平衡周期内（约1.9 Ma）,发生了铀或镭的再分配,并分析了可能造成样品偏铀或偏镭的原因。     

不平衡铀系定年的新技术突破——高精度、高灵敏度238U-234U-230Th同位素质谱测定和应用

80年代中期发展起来的高精度、高灵敏度不平衡铀系热电离质谱定年新技术是同位素定年技术的一次革命性突破，并已在古气候、古环境、古海洋、考古学以及近代火山—岩浆作用研究中取得了一系列重要成果，本文介绍了这方面的研究进展。这一新技术已引起越来越广泛的重视，显示出巨大的应用前景。尽快开展不平衡铀系热电离质谱定年技术的应用，对提高我国的高分辩率第四纪年代学分析技术和应用研究水平有重要意义。