徐州白露山含金刚石橄榄玄武岩地球化学特征及其岩浆演化特征

江苏徐州地区位于华北克拉通的东南部,其区内及周边广泛发育碱性超基性岩-基性岩,其中不乏含金刚石母岩,但是主要为金伯利岩。笔者在苏北寻找金刚石原生矿的过程中,在徐州白露山地区发现了含金刚石的橄榄玄武岩。本区橄榄玄武岩SiO₂含量为42.64%~51.13%,K₂O+Na₂O为2.57%~5.93%,Al₂O₃为10.48%~13.53%,MgO含量变化范围较大,为3.11%~19.34%。以主量元素做TAS图解可以看出本区基性岩的投影点基本都落于玄武岩范围,属于高钾钙碱性。微量元素特征显示白露山的样品靠近OIB和火山弧区域。富集端元明显偏高的Ba/Th比值暗示玄武岩中含有大陆下地壳物质。而白露山橄榄玄武岩元素地球化学特征及组成变化范围大并不是主要由地壳混染引起的,而是因为源区含有残留的再循环大陆下地壳。这些地球化学特征说明徐州白露山橄榄玄武岩与辽宁、山东地区金伯利岩具有一定的成因联系,为该区金刚石找矿提供了有利线索。     

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind,Pallasites and Trace Element Measurements

Electron probe microanalyser measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~ 100 µg g^?1 level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimise electron beam current density, a rastered 50 × 100 µm electron beam was adequate and minimised sensitivity to small Al‐rich contaminants. Reproducible analyses of eleven SIMS‐cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 µg g^?1. The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: Imilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination‐sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.     

High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine–anorthite mixtures

Amoeboid olivine aggregates (AOAs) are composed of forsteritic olivine, Fe,Ni-metal, and Ca,Al-rich nodules consisting mainly of Al-diopside, spinel, and anorthite. Although the textures, shapes, and mineralogy of AOAs are consistent with their being aggregates of nebular condensates, some textures are in conflict with equilibrium condensation calculations, indicating that AOAs were not produced by a simple one-stage condensation. To examine the origin of the constituent minerals within AOAs and their textural relationships, we performed heating experiments using mineral mixtures analogous to those in AOAs. Isothermal and cooling experiments on forsterite + anorthite mixtures reveal that a high-Ca pyroxene phase forms via the incipient melting of the two minerals. Comparative studies of heating experiments performed using the mineralogy of AOAs suggest that Al-diopside in AOAs can be produced from a small degree of melting of forsterite and anorthite. The formation of Al-diopside in this way is consistent with the annealing textures observed in AOAs, and it may account for the discrepancy between the observed mineralogy of AOAs and the results of equilibrium condensation calculations, the occurrence of two types of diopside (Al,Ti-rich diopside and Al-diopside), and the variable Al₂O₃ content of Al-diopside.     

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.     

Source and magmatic evolution of ocean island basalts from the Pohnpei Island,Northwest Pacific Ocean: Insights from olivine geochemistry

The compositional variability of ocean island basalts (OIBs) is thought to reflect partial melting of a lithologically-heterogeneous mantle source dominated by either pyroxenite or peridotite. The Pohnpei Island in Micronesia, which is associated with the Caroline hotspot, is suggested to have been generated from partial melting of a pyroxenite-rich mantle. To examine this hypothesis, we present new major- and trace-element compositions of olivine phenocrysts in basalts from the island. The olivines exhibit large systematic inter- and intra-crystalline compositional variability. In Sample DS1, olivines record compositional zonation, in which cores have relatively high Fo (77–85), Ni (550×10?6–2 392×10?6), and Fe/Mn ratios (66–82), whereas rims have lower Fo (71–78), Ni (526×10?6–1 537×10?6), and Fe/Mn ratios (51–62). By contrast, olivines within other samples preserve no clear compositional zonation, exhibiting similar or slightly lower Fo values (66–78), Ni contents (401×10?6–1 268×10?6), and Fe/Mn ratios (53–69) as the rims of zoned crystals. The distinct chemical contrast between the two different types of olivine suggests they formed in magma chambers at different depths. Analysis using forward petrological modeling and multi-element indicators (Fe/Mn, Zn/Fe, FC3MS (FeOT/CaO?(3×MgO/SiO₂)), Mn/Zn, and Ni/(Mg/Fe)) of whole-rock samples and high-Fo olivines is inconsistent with a pyroxenite-rich mantle source. We suggest these inconsistencies reflect an influence on the partition coefficients of Ni and Mn between olivine and liquid during melting at variable pressures and temperatures. In addition, magma recharge and mixing within the magmatic plumbing system can change the composition of olivine. We suggest that identification of the mantle source of OIBs in volcanic islands such as the Pohnpei Island using olivine geochemistry should be treated with caution.     

亚稳态橄榄石相变与深源地震研究进展

由于岩石圈俯冲板块内部温度低, 在400km左右的深度橄榄石不发生相变, 低压相的橄榄石便以亚稳态的形式进入更深的深度。 高温高压实验研究表明, 亚稳态橄榄石相变机制以相变形成的超塑性透镜状尖晶石反裂纹破裂为深源地震的触发机制, 能很好的解释深源地震的特征。 亚稳态橄榄石在俯冲带中存在的范围,是制约亚稳态相变机制的重要因素。 然而亚稳态橄榄石在俯冲带存在的范围尚存争议, 不同模型给出结果相差较大, 文中给出一些解决这个问题的建议。     

对一类相变体积分数计算公式的修正

考虑了相变过程中生成相晶核的真实形状,对1956年Cahn给出的颗粒边界成核情况下成核长大型相变的相变体积分数计算公式进行了修正;当初始实验数据是成核率、长大率时,两种相变体积分数计算公式的计算结果差别很大. 当初始实验数据是相变体积分数随时间变化的实验点时,两种相变体积分数计算公式所给出的计算结果的差别主要由拟合误差所致. 无论从哪种实验数据开始计算,两种不同的计算模型给出的生成相的颗粒大小都存在很大区别,这将影响对俯冲带流变结构的认识.     

Determination of cation distribution in (Co,Ni,Zn)2SiO4 olivine by synchrotron X-ray diffraction

 The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal diffraction method. The crystal data are (Co_0.377Ni_0.396Zn_0.227)₂SiO₄, M _r  = 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm³, Z = 4, D _x  = 4.864 g cm⁻³, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)_M1/(A/B)_M2] = −0.272 (IR _A -IR _B ) + 3.65 (EN _A −EN _B ), where IR (pm) and EN are ionic radius and electronegativity, respectively. Received: 8 April 1999 / Revised, accepted: 7 September 1999     

Subdivision of petrologic type of unequilibrated ordinary chondrites from Antarctica

We report results of petrologic and mineralogical studies of 25 unequilibrated ordinary chondrites (19 Chinese Antarctic meteorites) using electron microprobe, scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) techniques. With increasing degree of thermal metamorphism, chemical zoning of olivine was erased; plessite was transformed into intergrowths of coarser-grained kamacite and Ni-rich metal; Cr exsolved from ferroan olivine, produced needle-like crystals and coarsened into equant chromite grains; and feldspar crystallized during devitrification of glass and recrystallization of matrix. These features can help to identify different metamorphic grades. Based on the scheme by Sears et al (1982), we subdivided type 3 Chinese Antarctic ordinary chondrites into petrologic type 3.3 to 4 (3 type 3.3, 3 type 3.4, 1 type 3.5, 2 type 3.6, 2 type 3.7, 5 type 3.8, 3 type 4). This classification scheme is a quite effective way to subdivide Antarctic meteorites. Additionally, we propose to revise the chemical groups of GRV 020032 and GRV 020104 to L and H, respectively.     

江山-广丰地区早白垩世晚期玄武岩的岩石地球化学及其构造意义

江山-广丰地区所在的信江红色盆地与金衢红色盆地同属于NEE向赣杭断陷盆地带的组成部分.赣杭构造带自早白垩世晚期开始强烈拉张,红色盆地形成.红层下部夹有层状玄武岩,全岩K-Ar年龄为98～105 Ma,属早白垩世晚期.岩石地球化学特征表明研究区有两类玄武岩:江山陈塘坞一带的拉斑玄武岩,礼贤、广丰一带的碱性橄榄玄武岩.Nd、Sr同位素组成分析显示,拉斑玄武岩类(87Sr/86Sr)i值为0.705 4～0.707 0,εNd(t)值为1.69～0.70(接近于0),表明源区为基本未受壳源物质混染的原始地幔,在弧后陆缘环境下快速拉张而上升,呈熔岩被形式分布于信江盆地东部;碱性橄榄玄武岩(87Sr/86Sr)i值为0.707 6～0.708 0,而εNd(t)值偏低(-6.43～-5.18),表明来源于已受壳源物质混染的富集地幔,在拉张加剧、裂陷加深的构造背景下喷发,相对集中分布于盆地中部,范围小于拉斑玄武岩.二者的差别主要是地幔源区受到不均匀混染所致.研究表明,早白垩世晚期至晚白垩世期间,太平洋板块俯冲的角度由缓变陡以及向东南方向的后退作用,对弧后陆缘的快速拉张并形成赣杭带红色断陷盆地及其玄武岩起着关键性作用.