西伯利亚雅库特金伯利岩中四颗古老锆石巨晶的微量元素和Hf同位素特征——对区分金伯利岩中锆石巨晶的启示

金伯利岩中的锆石按照颗粒大小可以分为细粒锆石（一般小于200μm）和巨晶锆石（一般大于500μm）。前人的研究结果显示在金伯利岩中粒径较大的巨晶锆石的U-Pb体系在高温的地幔中一直保持着开放状态,直到寄主金伯利岩浆的喷发才使地幔锆石的U-Pb体系封闭,因此这些巨晶锆石是确定金伯利岩年龄的重要矿物之一。然而,近年来的研究表明,金伯利岩中还存在一些时代远老于金伯利岩年龄的锆石,也具有较大的粒径（以下称古老锆石巨晶）。它们的存在无疑影响了利用锆石U-Pb方法确定金伯利岩年龄的准确性。本文以西伯利亚雅库特（Yakutia）金伯利岩省中的四颗古老锆石巨晶为研究对象,通过形态学、年代学、微量元素和Hf同位素组成,讨论古老锆石巨晶的来源。同时,我们统计和对比了全球多个金伯利岩中能够确定金伯利岩年龄的锆石和古老锆石巨晶的形态学、U、Th含量和微量元素组成、Hf-O同位素等特征。研究结果显示,金伯利岩中的古老锆石巨晶的t_DM年龄和O同位素组成与可以用来确定金伯利岩年龄的锆石巨晶具有明显的差别。这些手段在未来的研究中可以用来区分可确定金伯利岩年龄的锆石巨晶和古老的锆石巨晶。     

内蒙古中部毫义哈达岩体的年代学和地球化学特征

内蒙古中部毫义哈达岩体位于华北板块北缘陆缘增生带,通过对该岩体进行系统的年代学与地球化学研究表明,毫义哈达岩体是由辉石闪长岩类构成。在对两个样品中的锆石进行LA-ICP-MS U-Pb定年,结果(加权平均年龄)均为266±2 Ma,表明岩体的形成时代为晚二叠世。对14个样品进行主量元素和微量元素分析,表明岩体属钙—碱性系列;Eu有一定程度的负异常,富集大离子亲石元素Rb、Sr及轻稀土,亏损高场强元素Nb、Ta,富Cu、Cr等过渡元素。岩体在地球化学上还具有岛弧火山岩的特征,推测其岩浆源区曾遭受过古亚洲洋板片流体交代。依据区域构造演化,推测毫义哈达岩体侵位于造山后伸展环境,成岩物质来源于上地幔并且发生了结晶分离作用。地球化学的特征以及在该岩体发现的铜钨矿化,预示着该地区将是一个有利的成矿远景区。     

A comparison of procrystal and ab initio model representations of the electron-density distributions of minerals

 The procrystal calculation of the electron density is a very rapid procedure that offers a quick way to analyze various bonding properties of a crystal. This study explores the extent to which the positions, number, and properties of bond-critical points determined from the procrystal representations of the electron density for minerals are similar to those of first-principles ab initio model distributions. The purpose of the study is to determine the limits imposed upon interpretation of the procrystal electron density. Procrystal calculations of the electron density for more than 300 MO bonds in crystals were compared with those previously calculated using CRYSTAL98 and TOPOND software. For every bond-critical point found in the ab initio calculations, an equivalent one was also found in the procrystal model, with similar magnitudes of electron density, and at similar positions along the bonds. The curvatures of the electron densities obtained from the ab initio and the procrystal distributions are highly correlated. It is concluded that the procrystal distributions are capable of providing good estimates of the bonded radii of the atoms and the properties of the electron-density distributions at the bond-critical points. Because the procrystal model is so fast to compute, it is especially useful in addressing the question as to whether a pair of atoms is bonded or not. If the Bader criteria for bonding are accepted, then the successful generation of the bond-critical points by the procrystal model demonstrates that bonding is an atomic feature. The main difference between the critical-point properties of the procrystal and the ab initio model is that the curvature in the electron density perpendicular to the bond path of the ab initio model is sharper than for the procrystal model. This is interpreted as indicating that the electrons that migrate into a bond originate from its sides, and not from the regions closer to the nuclei. This observation also suggests that ab initio optimization routines could see an improvement in speed if the parameters relating to the angular components of atomic wave functions were to vary before the radial components. Received: 6 August 2001 / Accepted: 21 November 2001     

橄榄岩-熔体的相互作用:岩石圈地幔组成转变的重要方式

橄榄岩-熔体/岩浆的相互作用常被用来解释蛇绿岩套橄榄岩、造山带橄榄岩、超镁铁质侵入杂岩体、地幔橄榄岩捕虏体中某些具有不平衡结构和矿物组成的岩石的形成过程。橄榄岩-熔体的反应主要有两种方式,即消耗橄榄石(和单斜辉石)生成斜方辉石或消耗斜方辉石生成橄榄石(和单斜辉石)。反应的结果不仅造成矿物百分含量的变化,而且造成矿物组成的变化;后者更重要但未引起足够的重视。华北东部中生代玄武质岩石中具有环带状结构的橄榄石和辉石捕虏晶,特别是具有环带状结构的地幔橄榄岩捕虏体的发现,暗示这种橄榄岩-熔体的相互作用在华北东南部中生代岩石圈地幔中很可能普遍存在,为岩石圈地幔组成转变和快速富集的重要方式。这是全球首例由橄榄岩-熔体相互反应造成的岩石圈地幔大规模的组成变化。反应熔体来源途径主要有地壳来源和软流圈地幔来源。来源不同的熔体与橄榄岩的反应造成的组成变化完全不同。     

Iron Diffusion in Single-Crystal Diopside

 Iron tracer diffusion experiments in diopside have been performed using natural and synthetic single crystals of diopside, and stable iron tracers enriched in ⁵⁴Fe, at temperatures in the range 950–1100 °C, total pressure 1 atm, for times up to 29 days. Iron isotope diffusion profiles were determined with an ion microprobe. For experiments performed at log pO₂ = −13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe ∼ 1.8 at %) diopsides, the data obey a single Arrhenius relationship of the form D = 6.22_−5.9 ^+49.6×10⁻¹⁵ exp(−161.5 ± 35.0 kJ mol⁻¹/RT) m² s⁻¹. A single datum for iron diffusion in iron-free, single-crystal diopside at 1050 °C, is approximately 1 order of magnitude slower than in the natural crystals. The pO₂ dependence of iron diffusion in natural crystals at 1050 °C (power exponent = 0.229 ± 0.036) indicates a vacancy mechanism; this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotropy for iron diffusion in diopside. Received: 16 March 1999 / Accepted: 10 April 2000     

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C

Trace element partition coefficients (D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene D_REE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D₀) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D₀, and higher water content, which serves to decrease D₀, counteract each other to the extent that water has little effect on garnet–melt D₀ values. In contrast, the effect of water on clinopyroxene–melt D₀ overwhelms the effect of temperature, such that D₀ is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments.

Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes.

The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role.     

Performance of CRONUS-P – A pyroxene reference material for helium isotope analysis

Helium isotope analyses are central to modern earth science and measured by many noble gas laboratories around the globe (Burnard, 2013; Wieler et al., 2002), spanning a wide spectrum of fundamental research – from identifying primordial reservoirs in the Earth mantle to paleoclimate reconstructions. The CRONUS-Earth initiative included the manufacturing, distribution and analysis of a pyroxene reference material (CRONUS-P) that was designed to be useful for internal reliability control of ³He measurements within a few percent and potentially for ⁴He on a higher level of uncertainty.This short paper describes the CRONUS-P material and its performance as ³He and ⁴He reference sample for noble gas laboratories. The companion paper by Blard et al. 2015 describes in depth the inter-laboratory helium isotope experiment within CRONUS-Earth.We show normalized helium isotope data of CRONUS-P measured at three different noble gas laboratories. Data from all three laboratories show no relation between helium isotope concentrations and sample mass, implying that the material is homogeneous. The data show that CRONUS-P is useful as an internal standard for ³He within better 2% (1σ) and for ⁴He within better 10%.     

南太行山平顺闪长岩体斜长石捕虏晶特征及其对壳幔岩浆混合过程的记录

在南太行山平顺闪长岩体中偶见具有特殊环带结构的斜长石,其核部为钙含量高的基性斜长石（An>63）,幔部为富钠的斜长石,二者之间存在由绢云母和绿帘石组成的溶蚀带,且核、幔An值变化截然不同。根据斜长石的环带构造特征和成分分析,认为本区岩浆混合过程大致如下：起源于EMI型富集地幔的幔源岩浆底侵作用于下地壳,在高温下结晶出钙含量高的基性斜长石,且所携带的热使下地壳开始熔融形成壳源岩浆;壳、幔两种岩浆沿着太行山深大断裂快速上升侵位于地壳浅部,发生岩浆混合;与此同时,早期形成的基性斜长石由于压力突然减小发生溶蚀,形成形态不规则的溶蚀带;壳幔混合岩浆在基性斜长石基础上结晶生长富钠斜长石,形成具有特殊环带结构的斜长石。这为研究区内存在岩浆混合作用提供了最直接的证据。     

山东昌乐新生代碱性玄武岩中的巨晶单斜辉石成因研究

山东昌乐新生代碱性玄武岩中除斑晶单斜辉石和基质单斜辉石外,还发育大量巨晶单斜辉石和同源斜方辉石捕掳晶。巨晶单斜辉石与玄武岩间发育复杂和简单两种类型的反应边。复杂反应边往往发育在颗粒较大的巨晶单斜辉石外围。复杂反应边可分为4个带：内部带、过渡带、外部带和边缘带,其中,内部带、过渡带和外部带均由具不同结构特征的单斜辉石＋熔体＋金属氧化物构成,边缘带为不含熔体和金属氧化物的干净的单斜辉石;内部带的单斜辉石具梳状构造,过渡带的单斜辉石具细密筛孔构造,外部带为具似砂钟构造的筛孔状单斜辉石。反应边中单斜辉石、熔体及金属氧化物的成分显示,只有内部带受到单斜辉石巨晶的影响,其余带均受玄武岩浆的制约。简单反应边往往发育在颗粒较小的巨晶单斜辉石外围。简单反应边仅发育不含熔体和金属氧化物的干净的单斜辉石。巨晶单斜辉石的反应边是巨晶与玄武岩浆间温度差造成的。在与玄武岩浆反应的过程中,大颗粒的单斜辉石巨晶需要较长的时间与玄武岩浆达到温度平衡,从而有足够的时间发育包含4个带的复杂反应边;反之,由于缺乏足够的反应时间,小颗粒单斜辉石巨晶的外围只发育不含熔体和金属氧化物的简单反应边。同源斜方辉石捕掳晶的成分与地幔二辉橄榄岩包体中的斜方辉石类似,其边部发育类似于Bowen反应（1956）形成的单斜辉石环边。在单斜辉石Al^IV-Al^VI图解上,巨晶单斜辉石、二辉橄榄岩包体中的单斜辉石以及部分单斜辉石斑晶的核部,均位于较高压力的“麻粒岩和玄武岩中包体”区域,反应边中单斜辉石、基质单斜辉石以及绝大部分斑晶单斜辉石位于“火成岩”区域,说明部分斑晶单斜辉石核部来源于地幔二辉橄榄岩,属于捕掳晶。结合单斜辉石结晶压力的估算,笔者认为巨晶单斜辉石的成因模式为：来自软流圈的碱质基性熔体上侵到地幔岩石圈下部,结晶形成单斜辉石巨晶,后来该熔体携带巨晶单斜辉石与来自上地幔岩石圈、夹带大量二辉橄榄岩包体以及斜方辉石（和单斜辉石）捕掳晶的玄武岩浆混合,巨晶单斜辉石随混合后的碱性玄武岩浆上升、喷出地表。     

P-T-X-controlled element transport through granulite-facies ternary feldspar from Lofoten,Norway

Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions (∼600°C and 8–11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions.