Chalcophile element constraints on magma differentiation of Quaternary volcanoes in Tengchong,SW China

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.     

Crustal structure in the Tengchong volcanic area and position of the magma chambers

In this paper, we extract 1500 P receiver functions in the Tengchong volcanic area from 211 teleseismic events recorded at nine digital seismic stations. A common conversion point stacking technique is used to improve the signal-to-noise ratio and to get the time delays of the Ps, PpPs, PsPs + PpSs phases within grids of 10 km × 10 km. Finally, the crustal thickness and Poisson’s ratio are calculated. The results show that the crustal thickness ranges from 28 to 40 km and the Poisson’s ratio ranges from 0.28 to 0.36. There exist two mantle-uplifting sites each with a horizontal scale of about 30 km × 30 km, one in Mazhan–Tengchong–Maanshan and the other in Wuhe–Longjiang–Tuantian. The high Poisson’s ratio is consistently located within these two sites. Recorded shocks with Ms > 2.0 reveal that most of the shocks are distributed around the two sites and few are located at the centers. The shocks, the geothermal distribution, and the crustal structure suggest that the magma is still active, and the two mantle-uplifting sites detected may be the positions of two magma chambers in the crust.     

Mix design and expansion behavior of sulpho-aluminate expansive cementitious composite

Sulpho-aluminate expansive cementitious composite is proved to be one of the most effective ways to prevent concrete cracking too soon． Mix design of sulpho-aluminate expansive cementitious composite complete- ly depends on experience and experiments at present． This method ignores the influence of expansion behavior which hinders the application of sulpho-aluminate expansive cementitious composite． The workability,free ex- pansion property,flexural strength and compressive strength of sulpho-aluminate expansive cementitious com- posite have been investigated by tests． The relationship between expansion ratio and mix design parameters such as curing age,water-cement ratio and cement abundance coefficient is deduced according to the experimental statistics． A new simplified approach to mix design of sulpho-aluminate expansive cementitious composite is pro- posed as a reference for construction application,which avoids experiments and experience．     

关于粉煤灰混凝土的研究

合理掺加扮煤灰可有效改善砼工作性能和节约费用。研究了粉煤灰在砼中的作用,并对不同掺量砼进行了对比,结合其它资料从理论上对粉煤灰砼的性能进行了讨论。     

Settling of metal droplets in a terrestrial magma ocean: On the correction of the Stokes velocity

The formation of magma oceans on at least the major terrestrial planets is widely assumed even if the full accretion history and early evolution of terrestrial planets is discussed rather controversial. Various processes occur within these magma oceans, among them the settling of small iron drops and their chemical equilibration with the silicate environment. Different models were proposed to explain the differentiation within a magma ocean. In order to model a magma ocean and give constraints about settling time of iron droplets, a constant velocity is used in most models, which is calculated using Stokes’ famous formula. According to the other model parameters this assumption is invalid, since Stokes’ assumption of a creeping flow is violated. However, former models investigated the behaviour of a large number of droplets, which makes it is impossible to solve the flow around each single iron drop. We introduce a model, in which the flow around iron drops in a molten silicate environment is computed using the appropriate hydrodynamic equations. We investigate the terminal velocity of a single drop descending through a magma ocean and additionally study the effects of the presence of other obstacles in the neighbourhood. We determine a new mean velocity, which may serve as an input parameter for the existent models of magma oceans. Although we used a full fluid-dynamical approach, the velocity computed with our model is not very different from the simple Stokes case and thus proves that the former assumptions were legitimate and can be used in future as well as the new velocity presented here.     

H<Subscript>2</Subscript>O diffusion in peralkaline to peraluminous rhyolitic melts

The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H₂O _t in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H₂O _t ) and water-rich (~2 wt% H₂O _t ) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H₂O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity () was derived from profiles of total water () using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between and Both methods consistently indicate that is proportional to in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al₂O₃ with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting:

Geochemistry and tectonic setting of mafic rocks from the Othris Ophiolite,Greece

We present new geochemical analyses of minerals and whole rocks for a suite of mafic rocks from the crustal section of the Othris Ophiolite in central Greece. The mafic rocks form three chemically distinct groups. Group 1 is characterized by N-MORB-type basalt and basaltic andesite with Na- and Ti-rich clinopyroxenes. These rocks show mild LREE depletion and no HFSE anomalies, consistent with moderate degrees (~15%) of anhydrous partial melting of depleted mantle followed by 30–50% crystal fractionation. Group 2 is represented by E-MORB-type basalt with clinopyroxenes with higher Ti contents than Group 1 basalts. Group 2 basalts also have higher concentrations of incompatible trace elements with slightly lower HREE contents than Group 1 basalts. These chemical features can be explained by ~10% partial melting of an enriched mantle source. Group 3 includes high MgO cumulates with Na- and Ti-poor clinopyroxene, forsteritic olivine, and Cr-rich spinel. The cumulates show strong depletion of HFSE, low HREE contents, and LREE enrichments. These rocks may have formed by olivine accumulation from boninitic magmas. The petrogenesis of the N-MORB-type basalts and basaltic andesites is in excellent agreement with the melting conditions inferred from the MOR-type peridotites in Othris. The occurrence of both N- and E-MORB-type lavas suggests that the mantle generating the lavas of the Othris Ophiolite must have been heterogeneous on a comparatively fine scale. Furthermore, the inferred parental magmas of the SSZ-type cumulates are broadly complementary to the SSZ-type peridotites found in Othris. These results suggest that the crustal section may be genetically related to the mantle section. In the Othris Ophiolite mafic rocks recording magmatic processes characteristic both of mid-ocean ridges and subduction zones occur within close spatial association. These observations are consistent with the formation of the Othris Ophiolite in the upper plate of a newly created intra-oceanic subduction zone. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

矿浆型铁矿的氧同位素判别标志:以宁芜玢岩铁矿为例

自然界是否存在矿浆型铁矿以及如何判别矿浆型铁矿是地质学家争论探索了几十年的问题。大量地质现象和实验研究证实,中酸性岩浆在高氧逸度、富磷等挥发分和助熔剂的条件下,硅酸盐熔体与铁氧化物熔体之间可以发生液态不混熔,熔离出富铁氧化物熔体或富铁岩浆。但有些学者认为实验无法直接一次性熔离出高品位铁矿浆,因此不存在矿浆型铁矿。实际上,高品位富铁矿浆可能是经过多次熔离富集形成的,而非一次简单熔离完成;磁铁矿中钛含量偏低,可能与矿浆型铁矿遭受后期热液改造、钛大量丢失有关,最典型的例子莫过于智利拉科铁矿。长江中下游玢岩铁矿是我国重要铁矿资源类型,其中是否发育矿浆型铁矿也一直存在激烈争论。为了避免不必要的争议,本文将铁矿浆限定为由岩浆熔离形成的铁氧化物浓度30%的富铁熔体,由铁矿浆演化形成的铁矿床称为矿浆型铁矿。根据宁芜成矿岩体中锆石的钛温度计确定了岩浆的温度,根据锆石的氧同位素组成及磁铁矿与锆石之间的氧同位素分馏方程,计算出岩浆温度下直接从熔体中熔离出来的磁铁矿的δ~(18)O_(Mt)值为4.2‰。据此建立了宁芜玢岩铁矿中矿浆型铁矿的氧同位素判别标志,如果矿体中磁铁矿的δ~(18)O_(Mt)≥4.0‰,则为矿浆型铁矿,否则为热液型或浆-液过渡型铁矿。判别结果表明,钟姑山矿田矿浆型和热液型矿体同时存在,梅山铁矿介于矿浆型-热液型之间,而凹山矿田铁矿则为热液型,与野外观测及前人研究结果一致。     

宁洱火山区壳内岩浆囊现今温度:来自CO_2-CH_4碳同位素地质温度计的估计

温度是岩浆囊的重要物理参数,获取温度参数并监测其变化对更好地理解岩浆囊的物理化学性质和行为、评价火山的活动性和喷发危险性具有重要的理论和现实意义。火山岩浆-水热系统丰富的含碳气体,这些含碳气体间碳同位素温度计为我们解决休眠火山区地下岩浆囊的温度问题提供了可能。我们对宁洱火山区地表地热异常强度最高区的4个温泉点CO_2、CH_4碳同位素组成进行了2年的连续观测和2次平行观测,利用观测数据对CO_2、CH_4气体的岩浆来源进行了甄别,对CO_2-CH_4间碳同位素分馏是否发到平衡进行了判断,然后利用Horita CO_2-CH_4碳同位素平衡分馏方程计算了宁洱-通关火山区逸出气体的源区温度。结果表明:宁洱火山区4个样点中,1个样点的CO_2和CH_4均为非岩浆成因,其他3个样点的主要属于岩浆来源。剔除了非岩浆来源的数据后的δ-Δ图解法判断CO_2-CH_4间碳同位素分馏达到了平衡。3个样点的CO_2、CH_4碳气体源区温度分别为425~475℃、941~995℃和1179~1578℃。结合最上部地壳温度场结果,我们认为,宁洱火山区现今存在2个壳内岩浆房,分别位于宁洱火山以南和通关火山以北,宁洱岩浆房的温度为1179~1578℃,通关岩浆房的温度为941~995℃。2个岩浆房的现今温度已达到流纹岩浆(600~900℃)、安山岩(800~1100℃)和玄武岩浆(1000~1250℃)的形成温度。思普地震带空间上密集的6级地震丛集活动可以用宁洱岩浆房的高活动性来解释。δ-Δ图解法判断CO_2-CH_4间碳同位素分馏平衡准则应修正为:在保持两拟合直线的斜率符号相反的条件下,δ~(13)CCO_2-ΔCO_2-CH_4拟合直线和δ~(13)CCH_4-ΔCO_2-CH_4拟合直线应相交于δ轴附近截距差Δb≤0.16处。     

花岗岩浆形成定位机制的思考与研究进展

花岗岩(广义)是陆壳的标志,也是地球岩石圈区别于其它行星岩石圈的标志。文章介绍了行星探测和大洋调查等方面的成果对花岗岩形成的地质约束:行星从岩浆表壳向岩石表壳转换过程以及现代地幔过程,均没有产生有规模意义的花岗岩;花岗岩及其所标志的陆壳,应是星球出现水圈和沉积岩之后的产物;花岗岩在地球岩石圈二维空间上的平均生长速率,大约为485×10~3km~2/Myr;岩浆主要来自地壳岩石的部分熔融(深熔)。在此基础上,文章介绍了深熔作用方面的研究进展,讨论了部分熔融岩石的流变行为与其内熔体比的关系,并比较了岩浆侵入模型与岩浆对流模型在解释花岗岩形成定位机制方面的异同。侵入模型的困难之一来自岩体与源区分离。由于源区位于岩体下方且远离岩体,因而是不可观察的,除非岩体及其与源区之间的岩石因风化或构造被剥蚀殆尽。文章最后介绍了"深熔-对流"模型的研究进展。该模型认为"源区"与"定位区间"是统一的,当"源区"岩石的熔体比例超过流变学的临界熔体比,岩石转变为"脏"岩浆;"脏"岩浆层内的重力分异诱发热对流,后者引起"顶蚀作用",导致重熔界面(MI)或固-液转换界面(SLT)不断向上移动和岩浆层的逐渐增厚。基本认识是:熔区内的热对流是深熔作用能够形成大规模花岗岩浆的必要条件;没有对流,陆壳岩石的部分熔融只能产生混合岩,不能产生岩基规模的花岗岩。