大麻坪地区二辉橄榄岩部分熔融实验研究

本文以采自大麻坪地区汉诺坝玄武岩中的二辉橄榄岩包体为初始实验物料,在压力1.0～3.0 GPa、温度1350～1550℃条件下进行了部分熔融实验,对实验产物进行了岩相学研究和电子探针成分分析。二辉橄榄岩在1350℃开始熔融,在实验的温度压力范围内,熔融程度为10%～30%。随熔融程度的升高,部分熔融后残余岩石倾向于向富镁、低铁、低钙、低硅的趋势演化,而部分熔融产生的熔体则倾向于富镁、富铁、低铝、低硅的趋势演化。在岩石化学图解上本次实验中二辉橄榄岩部分熔融产生的熔体化学组成与汉诺坝地区拉斑玄武岩的组成相近。随熔融程度升高,熔体具有从苦橄质→玄武质演化的趋势。     

岩浆熔融体性质及演化在成矿过程中的作用——以西藏玉龙斑岩铜(钼)矿床为例

玉龙斑岩铜(钼)矿床是亚洲最大的斑岩型铜矿床,包含多种矿化类型,成矿作用复杂。作为典型的斑岩型矿床,岩浆熔融体的性质在玉龙铜矿床的形成过程中起到了至关重要的作用。本文将分别从岩浆熔融体的物理和化学性质出发,解释玉龙矿床为何能成为玉龙成矿带中唯一一个超大型矿床的原因。在熔体演化方面,笔者主要通过对熔体密度、粘度的计算获得有关数据,以了解熔体运移、含矿流体分离的过程,以解释玉龙铜矿床为何能形成如此规模的矿床。     

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have beenwidely documented in island arc settings and may constitutea significant portion of the lowermost arc crust. Because ofthe low melting temperature of amphibole (1100°C), suchcumulates could melt during intrusion of primary mantle magmas.We have experimentally (piston-cylinder, 0·5–1·0GPa, 1200–1350°C, Pt–graphite capsules) investigatedthe melting behaviour of a model amphibole–olivine–clinopyroxenerock, to assess the possible role of such cumulates in islandarc magma genesis. Initial melts are controlled by pargasiticamphibole breakdown, are strongly nepheline-normative and areAl₂O₃-rich. With increasing melt fraction (T > 1190°Cat 1·0 GPa), the melts become ultra-calcic while remainingstrongly nepheline-normative, and are saturated with olivineand clinopyroxene. The experimental melts have strong compositionalsimilarities to natural nepheline-normative ultra-calcic meltinclusions and lavas exclusively found in arc settings. Theexperimentally derived phase relations show that such naturalmelt compositions originate by melting according to the reactionamphibole + clinopyroxene = melt + olivine in the arc crust.Pargasitic amphibole is the key phase in this process, as itlowers melting temperatures and imposes the nepheline-normativesignature. Ultra-calcic nepheline-normative melt inclusionsare tracers of magma–rock interaction (assimilative recycling)in the arc crust. KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite     

Compaction in a mantle with a very small melt concentration: Implications for the generation of carbonatitic and carbonate-bearing high alkaline mafic melt impregnations

Xenoliths entrained in alkaline basalts and kimberlites give strong evidence that mantle carbonatitic and carbonated high alkaline mafic silicate melts, which are initially produced at very low degrees of partial melting (?1%), percolate and accumulate to form impregnations with a melt concentration of up to 10%. At present no compaction model has explained such huge local amplification of melt concentration. Recently, Bercovici et al. [1] have shown that the commonly used equations of compaction are not sufficiently general to describe all melt percolation processes in the mantle. In particular, they show that, when the melt concentration in the mantle is very low, the pressure jump ΔP between the solid and liquid fractions of the mantle mush is very important and plays a driving role during compaction. 1-D compaction waves generated with two different systems of equations are computed. Three types of wave-trains are observed, i.e. (1) sinusoidal waves; (2) periodic waves with flat minima and very acute maxima (‘witch hat waves’); (3) periodic solitary waves with flat maxima and extremely narrow minima (‘bowler hat waves’). When the initial melt distribution in the mantle is quite homogeneous, the compaction waves have sinusoidal shapes and can locally amplify the melt concentration by a factor less than two. When there is a drastic obstruction at the top of the wetted domain, the pressure jump ΔP between solid and liquid controls the shape of the waves. If the computation assumes the equality of pressure between the two phases (ΔP=0), the compaction wave has a ‘bowler hat shape’, and locally amplifies the melt concentration by a factor less than 5. Alternatively, simulations taking into account the pressure jump between phases ΔP predict compaction waves with ‘witch hat shape’. These waves collect a large quantity of melt promoting the development of magmons with local melt concentration exceeding 100× the background melt concentration. It is inferred that in a mantle with very low concentrations of carbonatitic or high alkaline mafic silicate melt the magmons are about 1 km thick and reach, in less than 1 Ma, a melt concentration of about 10%. The magmons are likely generated below the lithosphere at some distance away from the center of hot spots. This can explain the development of mantle carbonatitic eruptions in the African rift and the carbonatite and high alkaline mafic silicate volcanic activity in oceanic islands.     

The putative impacts of the non‐native seaweed Sargassum muticum on native communities in tidepools of Southern California and investigation into the feasibility of local eradication

The ecological impacts of introduced seaweeds have been relatively understudied. Current research suggests that seaweed invasions often result in alterations of native marine communities and disruptions of normal ecosystem functioning, but the effects on native communities can vary among invasive seaweed species, among habitats and over small and large spatial scales. In this study, the impacts of Sargassum muticum, a non‐native brown alga introduced into southern California, USA, several decades ago, were examined by comparing community structure in rocky inter‐tidal pools with and without the seaweed. Sargassum muticum appeared to have little impact on the native community despite measures revealing changes in the abiotic conditions of pools, with S. muticum presence reducing light penetration and ameliorating pool temperature changes during low tides. In other regions and habitat types, S. muticum presence often, but not consistently, resulted in declines in macrophyte diversity and/or abundance and increases in faunal assemblages. The lack of effects of S. muticum in this study, combined with variable impacts by S. muticum and other invasive seaweeds worldwide, suggests that predicting the effects of introduced seaweeds is problematic and warrants further research. Regardless of the effects on native communities, there is often a desire to eradicate or control the spread of non‐native seaweeds. In this study, localized S. muticum eradication attempts, including manipulations of a native canopy and herbivorous urchins, proved unsuccessful as full recovery occurred in ~9 months. While eradication efforts conducted worldwide have resulted in mixed success, there is a trend that early detection and rapid response can increase success, highlighting a need for systematic monitoring and establishment of regional rapid response plans.     

Emplacement mechanisms of late-orogenic granites: structural and geochemical evidence from southern Finland

The country rock in southern Finland formed mainly during the Svecofennian orogeny ca. 1.9 Ga ago. The middle and lower crust was partially melted 1.83 Ga ago due to crustal thickening and subsequent extension. During this event, S-type migmatites and granites were formed along a 100×500 km zone. This Late Svecofennian Granite–Migmatite zone (LSGM zone) is a large crustal segment characterised by roughly E–W trending sub-horizontal migmatites and granites. Combined ductile E–W shear movements and NNW–SSE compressional movements defined a transpressional tectonic regime during the emplacement. Partial melts that moved through the crust pooled as granite sheets or froze as migmatites. Major transpressive shear zones border the LSGM zone, which forms a tectonic and metamorphic zone that crosscuts the earlier Svecofennian granitoids. Based on field observations and geochemical data from two sets of outcrops, we show that the great volumes of late-orogenic granites and migmatites in southern Finland were transported and emplaced as small chemically variable batches, possibly extracted from different protoliths. These melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust. In the Nagu area, the melt batches were trapped under a roof-layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and E–W trending fold axes. The sheets of microcline granite are, in places, strongly sheared; the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn-tectonic movements of the layers as well as a syn-tectonic mechanism for the late-magmatic fractionation. Depending on the degree of crystallisation of the individual melt batches during shearing at different intensities, the granites have slightly different appearances. Some sheared zones show a cumulate-like trace element geochemistry, indicating that melt fractions were expelled from the system, producing layers of deformation enhanced fractionated granites and cumulate layers. Our interpretation is that the Nagu area shows shear-assisted fractionation mechanisms in granitic melts, and that similar processes are responsible for the fractionation trends seen in the sub-horizontal sheeted granites in Hämeenlinna at higher levels in the crust.     

Theoretical models and experimental determination methods for equations of state of silicate melts: A review

Silicate melts are very active in the interior of the Earth and other terrestrial planets, and are important carriers for the transport of material and energy. The determination of the equation of state(EOS) for silicate melts and the acquisition of a precise quantitative relationship between molar volume(or density) and temperature, pressure, and composition is essential for simulating the generation, migration, and eruption processes of magmas and the evolution of the magma ocean stage during the early formation of the Earth and other terrestrial planets, for calculating and modeling the phase equilibria involving silicate melts, and for revealing the variation of the microstructure of silicate melts with pressure. However, it is experimentally challenging to determine the volumetric properties of silicate melts and the accumulated density data at high pressure are still very limited due to a series of problems such as: the high liquidus temperature of silicate rocks; proneness for silicate melts to react with sample capsules to change the melt composition; and proneness for melts to flow and leak during the high pressure and high temperature experiments. In recent years, there is rapid progress in the high pressure and high temperature experimental techniques, in terms of not only the extension of temperature and pressure ranges but also the improvement on the accuracy of measurements, and the emergence of new methods for in-situ measurements. Here, we review the widely-used theoretical models of ambient-pressure and high-pressure EOS for silicate melts, and illustrate some problems that need to be solved urgently:(1) the room pressure EOS for iron-and titanium-bearing silicate melts needs to be improved;(2) the partial molar properties of the H2 O and CO2 components in silicate melts containing volatile components may vary markedly with the melt composition, which need to be addressed in high-pressure EOS;(3) how the formulation and applicable range of EOS correspond to changes in melt structure and compression mechanism requires further study. We highlight the basic principle and applicable range of various methods for determining the EOS for silicate melts, and compare the advantages and disadvantages of doublebob Archimedes method, fusion curve analysis, shock compression experiments, sink-float method, X-ray absorption, X-ray diffraction and ultrasonic interferometry. Future trends in this field are to develop experimental techniques for in situ measurements on melt density or sound velocity at high temperature and high pressure and to accumulate more experimental data,and on the other hand, to improve the theoretical models of the EOS for silicate melts by a combination of research on the microstructure and compression mechanisms of silicate melts.     

锡在花岗质熔体和流体中的性质及分配行为研究进展

元素在流/熔体间的交换、分配过程是岩浆热液矿床形成的重要环节,作为与岩浆活动有密切成因联系的典型矿种之一,锡在花岗质熔体和流体中的存在形式、分配行为及其影响因素是认识其成矿机理的关键。锡在花岗质熔体和流体中的分配特征不仅受温度、压力、氧逸度等条件的制约,流体组成和熔体的NBO/T（非桥氧键/桥氧键）、碱含量、AlK/Al(总碱与铝含量比)也是制约锡分配行为的重要因素;挥发分F、Cl对锡在流体、熔体中的地球化学行为影响尤为明显。     

岩浆不混溶作用模拟

基于硅酸盐熔体不混溶相平衡实验资料,采用氧化物规则溶液的活度模型,建立了预测岩浆不混溶作用的热力学方法,研究了氧化物组分在不混溶两液相之间的分配系数与温度、压力和岩浆成分之间的关系．由此,可以预测天然岩浆不混溶作用,计算不混溶的起始温度、共轭两液相的成分及含量．计算的不混溶两液相中ＳｉＯ２,Ａｌ２Ｏ３,ＦｅＯ的摩尔分数平均残差为３．０％～４．０％,其他氧化物平均残差小于１．０％,不混溶两液相的摩尔分数平均残差约为１．０％．对阳原岩体的模拟计算表明,磁铁矿－磷灰石矿床的形成与球粒状黑云辉石正长岩岩浆在１１５０～１２５０℃下的不混溶作用有关;计算的共轭两液相的相对含量与岩相学证据吻合．     

祁连与伊豆-小笠原玻安岩的地球化学特征和成因模型对比

玻安岩为一类具有特殊地球化学性质的岩石,具有高SiO2（＞52%）、高MgO（＞8%）和低TiO2（＜0.5%）等特征。前人认为其形成主要是在俯冲起始阶段大洋板块所释放的流体导致亏损程度较高的难熔地幔楔发生熔融,因此其成因的研究对深入理解板块俯冲起始等地球动力学问题具有重要意义。虽然普遍认为俯冲物质对玻安岩岩浆源区具有重要贡献,但玻安岩中元素的不同富集程度反映了复杂的俯冲板片流体物理化学性质和对玻安岩形成的不同影响。通过对比分析伊豆–小笠原（Izu-Bonin）和北祁连造山带大岔大坂地区玻安岩样品,发现二者具有明显的地球化学差异：与伊豆–小笠原玻安岩相比,大岔大坂玻安岩中没有呈现“U”型稀土配分模式,不富集轻稀土元素或Zr、Hf等元素;而二者流体活动性/不相容元素比值（如Ba/La）变化较大,并具有较高的(87Sr/86Sr)i。这些特征反映了俯冲板片释放的流体和熔体分别对大岔大坂和伊豆–小笠原玻安岩岩浆地幔源区的贡献,从而表明大岔大坂玻安岩形成过程与伊豆–小笠原玻安岩所代表的俯冲初始形成模型不同,更可能形成于存在弧后扩张作用的成熟岛弧阶段。结合区域地质背景和前人研究,本文针对大岔大坂玻安岩成因提出了两种与俯冲初始阶段无关的可能形成机制：① 玻安岩产出于弧后扩张中心,弧后岩石圈的拉张环境和较热的地幔上隆区为玻安质岩浆的形成提供了温压条件,充分交代的水化地幔楔和蛇纹岩化地幔也参与了玻安质岩浆的形成;② 虽与弧后扩张中心相关,但玻安岩的产出位于前弧或弧。由于弧后地幔对弧下深度地幔楔进行侧向加热,导致地幔楔内部对流重新启动,弧后地区已经熔融出弧后玄武岩的残余橄榄岩进入前弧–弧下地幔楔,地幔楔底部和俯冲板片表面被重新加热而发生变质脱水,富水流体交代上部地幔楔使其部分熔融形成玻安质岩浆。