Evolution of rare-earth mineralization in the Bear Lodge carbonatite,Wyoming: Mineralogical and isotopic evidence

The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~ 120–183 m, and grades into unaltered carbonatites at depths greater than ~ 183–190 m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd)cn and (La/Ce)_cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd)_cn, and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single sample of primary calcite shows mantle-like δ¹⁸O_V-SMOW and δ¹³C_V-PDB values, whereas most other samples are somewhat depleted in ¹³C (δ¹³C_V-PDB ≈ − 8 to − 10‰) and show a small positive shift in δ¹⁸O_V-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more pronounced in the transitional and oxidized zones; large shifts in δ¹⁸O_V-SMOW (to ~ 18‰) reflect the input of meteoric water during pervasive hydrothermal reworking and supergene oxidation. The textural relations, mineral chemistry and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in ²⁰⁸Pb/²⁰⁴Pb value (~ 39.2), the Bear Lodge carbonatites are remarkably uniform in their Nd, Sr and Pb isotopic composition: ¹⁴³Nd/¹⁴⁴Nd_t = 0.512591–0.512608; εNd_t = 0.2–0.6; ⁸⁷Sr/⁸⁶Sr_t = 0.704555–0.704639; εSr_t = − 1.5–2.7; ²⁰⁶Pb/²⁰⁴Pb_t = 18.071–18.320; ²⁰⁷Pb/²⁰⁴Pb_t = 15.543–15.593; and ²⁰⁸Pb/²⁰⁴Pb_t = 38.045–39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust.     

三叠纪西秦岭西北部洋俯冲的记录:来自镁安山岩/高镁安山岩的证据

三叠纪以来,西秦岭地区构造-岩浆活动极为强烈,中酸性侵入岩呈北西向弧状分布,前人做了大量卓有成效的工作,但对出露较少的火山岩鲜有研究。本次工作选取青海省同仁县麦秀和甘肃省夏河县甘加、德乌鲁3个地区的安山岩,从岩石学、岩石地球化学、年代学等进行分析。结果显示安山岩均属于钙碱性系列,具有富钾火山岩的特征。其中,麦秀安山岩中既有高镁安山岩,也有镁安山岩,而甘加与德乌鲁安山岩属于镁安山岩,为洋壳俯冲作用形成的两种典型火山岩类型。西秦岭地区安山岩形成时间东早西晚,由西向东安山岩基性程度降低,K_2O、Na_2O、∑REE等含量增高,MF、FL、DI、La/Sm等值变大,SI、Mg~#等值降低,并且呈明显的线性特征,构成同仁—合作镁安山岩(镁闪长岩)岩浆弧组合,和北部隆务峡蛇绿岩带一起,组成岩性分布极性,从而判断出西秦岭西北部在三叠纪存在隆务峡蛇绿岩代表的洋壳板块向南俯冲作用。     

Mineralogical and isotopic properties of inorganic nanocrystalline magnetites

Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF₅ lines. At low total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite. At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10 nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope fractionation factor between magnetite crystals and water was of 0-1‰, similar to the fractionation factor associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH.     

Carbon isotopic disequilibrium in the shell of the freshwater mussel Elliptio complanata

Isotopic analyses of shell material from the freshwater mussel Elliptio complanata produced mean C concentrations that were depleted in ¹⁴C and ¹³C relative to the lake water DIC. Depleted C isotope values were attributed to the incorporation of ¹²C-enriched metabolic C into the shell. Microsampling of longitudinal sections of shell revealed a seasonal pattern in the δ¹³C values. The seasonal pattern matched predictions that DIC δ¹³C and metabolic rate determine the final shell δ¹³C concentration. The greatest depletion of δ¹³C occurred in the summer. Similar δ¹³C patterns in marine and freshwater mollusks probably record not only temperature fluctuations, upwelling events, and phytoplankton blooms as reported, but also the changes in metabolic activity brought on by such events.     

云南来利山锡矿矿物学研究及氢氧同位素特征

在详细的野外调研及岩相学研究基础上,通过X射线粉晶衍射和红外光谱等测试手段,研究云南来利山锡矿不同成矿阶段石英和锡石的晶胞参数及红外光谱特征,解译石英和锡石的成因信息。研究表明,来利山锡矿热液成矿期可分为4个成矿阶段,即云母-黄铁矿-黄玉-粒状锡石阶段(Ⅰ)、云母-石英-黄铁矿-柱状锡石阶段(Ⅱ)、石英-黄铁矿-放射状锡石阶段(Ⅲ)和萤石-石英-黄铁矿-球粒状锡石阶段(Ⅳ)。从第Ⅰ阶段到第Ⅳ阶段,石英晶胞参数中a0、c0、V0和c0/a0均有减小的趋势,石英轴变化率比值为0.773 07~3.496 88;石英红外光谱各吸收峰的吸光度有增大的趋势。推测第Ⅰ、Ⅱ阶段石英所含杂质以Al3+、Fe3+等置换杂质为主,第Ⅲ阶段较为复杂,可能存在置换杂质和填隙杂质两种类型,第Ⅳ阶段所含杂质以Na+、K+等填隙杂质为主。锡石晶胞参数中a0、c0和V0随着成矿阶段的演化先减小后增大,其中第Ⅱ阶段a0、c0和V0值最小。这与不同成矿阶段锡石中杂质元素总量的变化规律一致,反映了锡石的晶胞参数变化主要受混入晶格元素的影响,而温度对锡石晶胞参数的影响较小。锡石红外光谱特征在不同成矿阶段的基本相似,属于变形谱,反映了锡石-硫化物热液矿床的谱形特征。此外,石英氢氧同位素组成显示成矿流体为岩浆水与大气降水混合物,推测初始成矿热液主要来自岩浆,随着热液演化,大气水沿构造裂隙混入到热液中,使热液的氢氧同位素组成向大气水一侧偏移。     

Uranium-series disequilibrium in tuffs from Yucca Mountain,Nevada, as evidence of pore-fluid flow over the last million years

Samples of tuff from boreholes drilled into fault zones in the Exploratory Studies Facility (ESF) and relatively unfractured rock of the Cross Drift tunnels, at Yucca Mountain, Nevada, have been analysed by U-series methods. This work is part of a project to verify the finding of fast flow-paths through the tuff to ESF level, indicated by the presence of ‘bomb’ ³⁶Cl in pore fluids. Secular radioactive equilibrium in the U decay series, (i.e. when the radioactivity ratios ²³⁴U/²³⁸U, ²³⁰Th /²³⁴U and ²²⁶Ra/²³⁰Th all equal 1.00) might be expected if the tuff samples have not experienced radionuclide loss due to rock-water interaction occurring within the last million years. However, most fractured and unfractured samples were found to have a small deficiency of ²³⁴U (weighted mean ²³⁴U/²³⁸U=0.95±0.01) and a small excess of ²³⁰Th (weighted mean ²³⁰Th/²³⁴U 1.10±0.02). The ²²⁶Ra/²³⁰Th ratios are close to secular equilibrium (weighted mean=0.94±0.07). These data indicate that ²³⁴U has been removed from the rock samples in the last ∼350 ka, probably by pore fluids. Within the precision of the measurement, it would appear that ²²⁶Ra has not been mobilized and removed from the tuff, although there may be some localised ²²⁶Ra redistribution as suggested by a few ratio values that are significantly different from 1.0. Because both fractured and unfractured tuffs show approximately the same deficiency of ²³⁴U, this indicates that pore fluids are moving equally through fractured and unfractured rock. More importantly, fractured rock appears not to be a dominant pathway for groundwater flow (otherwise the ratio would be more strongly affected and the Th and Ra isotopic ratios would likely also show disequilibrium). Application of a simple mass-balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at ESF level is about 400 a. Processes of U sorption, precipitation and re-solution are believed to be occurring and would account for these anomalous results but have not been included in the model. Despite the difficulties, the U-series data suggest that fractured rock, specifically the Sundance and Drill Hole Wash faults, are not preferred flow paths for groundwater flowing through the Topopah Spring tuff and, by implication, rapid-flow, within 50 a, from the surface to the level of the ESF is improbable.     

Mineralogical evidence for multiple dust sources in an early Triassic loessite

Loessite present in a borehole into the Smith Bank Formation (early Triassic age, Central North Sea) differentiates five coeval source terranes for aerosol dust, three long-distance sources and two local sources. All were active immediately following the end Permian mass extinction. Long-distance sources are sedimentary, basic magmatic and acid–intermediate volcanic. Although predominantly silt-sized and dominated by quartz with subordinate feldspars, muscovite and illite, evidence of basic and acid–intermediate magmatic/volcanic sources are pervasive. Baddeleyite is diagnostic of basic magmatism, an origin supported by enrichment of plagioclase relative to potassium feldspar. Deduction of acid–intermediate volcanism comes from the collective occurrence of irregular geometry quartz, volcanic shards, Ti-mineralization, euhedral biotite, sanidine, the co-occurrence of apatite and zircon, and the common occurrence of a tosuditic clay mineral. The tosuditic phase occurs as an unusual diagenetic dioctahedral chlorite/smectite formed at low temperature (<45°C), during very shallow burial by the decomposition of unstable rhyo-dacitic and andesitic grains in alkaline pore water from an adjacent lake that yielded pore fluids with a high Al : Si ratio. The Siberian Traps large igneous province is the likely source terrane for the magmatic and volcanic silt. Locally sourced clay pellets and kaolinite booklets formed from aeolian erosion of an adjacent, periodically desiccated lake-floor and a kaolinitic regolith, respectively. Inference of a prolonged harsh, arid climate leaves no evidence of any periods of sustained humidity or climatic fluctuation, such as pedogenesis. The association between the end Permian mass extinction, emplacement and aeolian erosion of the Siberian Traps large igneous province, and location of the Smith Bank Formation in a large lacustrine endorheic basin, combine to preserve a record of prolonged harsh climate in the early Triassic.     

Theory of alpha recoil effects on radon release and isotopic disequilibrium

Using the present understanding of the mechanisms of release from rocks and soils of recoiling products of alpha emission, physical predictions can be made of the fraction of these nuclei that are directly freed and the fraction that can subsequently be freed by the action of water. The fractions depend on the size and nature of the grains in which the recoiling nuclei originate, the size and shape of the pore spaces, the nature of the surrounding grains, the decay lifetime of the recoiling nuclei, and the time sequence of the presence of water. For simplified assumptions quantitative predictions are made of Rn emanation rates and of isotopic disequilibrium.     

Possible role of amphibole in the origin of andesite: some experimental and natural evidence

Experiments in the system high-A1 basalt (HAB)-water have been conducted in the melting range at pressures between 1 atm. and 10 kbar, defining the amphibole stability field and the composition of liquids which coexist with this amphibole. Plagioclase is the anhydrous liquidus phase between 1 atm. and 10 kbar but in the hydrous runs this role is taken by olivine at <7 kbar and then by clinopyroxene at higher pressures. Because amphibole is never on the high-A1 basalt liquidus it is not likely that andesite is derived from primary basalt by pure fractional crystallisation, although as we discuss, other mechanisms including equilibrium crystallisation might implicate amphibole. If primary basaltic magma undergoes closed-system equilibrium crystallisation, then the amphibole field will be intersected at between 50 and 100°C below the liquidus. The compositions of melts coexisting with amphibole alone do not match those of any of the natural andesite or dacitic lavas associated with the particular high-A1 basalt investigated. Like natural andesites, they become rapidly silica enriched, but they also become far more depleted in TiO₂ and MgO. However, the compositions of liquids lying directly on the divariant amphibole-out reaction zone, where amphibole +liquid coexist with clinopyroxene or olivine (±plagioclase), do resemble those of naturally occurring low-silica andesites. With increasing temperature pargasitic amphibole breaks down via incongruent melting reactions over a narrow temperature range to form a large volume of relatively low-silica basaltic andesite liquid and a crystalline assemblage dominated by either clinopyroxene or olivine. Our important conclusion is that basaltic andesite liquid will be the product of reaction between cooling, hydrous mafic liquid and anhydrous ferromagnesian phases. The solid reactants could represent earlier cumulates from the same or different magma batches, or they could be peridotite wall-rock material. Because the amphibole-out boundary coexisting with liquid is one of reaction, it will not be traversed so long as the phases on the high temperature side remain. Thus, the assemblage amphibole+clinopyroxene±olivine±plagioclase+liquid is one in which the liquid is buffered (within limits), and results reported here indicate that this buffering generates melts of low-silica andesite composition. When tapped to lower pressures these liquids will rise, eventually to fractionate plagioclase-rich assemblages yielding silicarich andesite and dacite melts. Conversely, the partial melting of hornblende pyroxenite, hornblende peridotite or hornblende gabbro can also yield basaltic andesite liquids. The phase relationships suggested by these experiments are discussed in the light of naturally occurring phenocryst and xenolith assemblages from the east Sunda Arc. Primary magmatic additions to the lithosphere of volcanic arcs are basaltic and voluminous upper crustal andesite in these terranes, complemented by mafic and ultramafic crystalline deposits emplaced in the lower crust or close to the Moho. Together these components constitute total arc growth with a basaltic composition and represent the net accreted contribution to continental growth.     

大别山区石英质玉宝石矿物学特征研究

大别山区石英质玉属新近发现的一种石英质玉（简称＂大别山玉＂）,具色彩绚丽、质地细腻温润等特点。利用偏光显微镜、扫描电子显微镜、X射线粉末衍射、化学分析、红外光谱分析等方法,重点对＂大别山玉＂的矿物组成、化学成分、物理光学性质、微结构、红外吸收光谱等特征进行研究。结果表明,＂大别山玉＂具较典型的微粒-细粒结构,主要矿物为石英,含少量的绢云母、绿泥石、萤石、黄铁矿及其它粘土矿物等次要矿物,实属石英质玉;其化学成分相对较纯,主成分为SiO2,含少量的Al2O3,CaO,MgO,Fe2O3,FeO,K2O等;其红外反射光谱以Si—O非对称伸缩振动致特征的1177,1104 cm-1谱带、Si—O—Si对称伸缩振动致800,781cm-1分裂谱带以及由Si—O弯曲振动致492 cm-1较强谱带和542 cm-1弱谱带为特征。同时,还对＂大别山玉＂的结晶度及水的赋存状态一并给予了探讨。     

Lithium isotopic disequilibrium of minerals in the spinel lherzolite xenoliths from Boeun,Korea

Spinel lherzolite xenoliths found in Boeun, Korea, have protogranular to porphyroclastic textures and are enclosed in a Miocene alkali basalt. The lithium concentration and isotopic compositions of olivine, clinopyroxene, and orthopyroxene separates from the spinel lherzolite, and whole rocks of the spinel lherzolites and alkali basalt were determined by inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The lithium concentrations of the olivines and orthopyroxenes range from 2.2 to 5.0 ppm and from 2.1 to 6.4 ppm, respectively. In contrast, the clinopyroxenes have larger lithium concentrations, from 2.0 to 8.4 ppm, which reflect their preferential lithium enrichment. The lithium isotopic compositions (δ⁷Li) of olivines (-5.4 to + 3.5‰), orthopyroxenes (-11.4 to -0.1‰), and clinopyroxenes (-14.4 to -4.7‰) range far beyond the average mantle composition of + 4 ± 2‰. The lithium isotopic composition of the host rock, alkali basalt (3.4‰), is within the range of the intraplate and oceanic island basalts. The spinel lherzolites from Boeun exhibits strong elemental and isotopic disequilibria due to the different lithium and lithium isotope diffusion velocities in the olivine, orthopyroxene, and clinopyroxene minerals after eruption and magma cooling.     

Carbon isotopic evidence for the catalytic origin of light hydrocarbons

The molecular proportionality between C₆ and C₇ isomers reported recently (F. D. Mango, Geochim. Cosmochim. Acta, 2000, 64, 1265; ref. 1) is probably the strongest evidence for catalysis yet published. It implicates two cyclopropane-like precursors, [S₆] and [S₇] (where S denotes any substrate), of similar structures, each yielding three isomers along similar kinetic pathways:     

Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sr_i rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sr_i. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial ⁸⁷Sr/⁸⁶Sr (Sr_i) and a less radiogenic modeled ²³⁸U/²⁰⁴Pb composition (μ₂) than coexisting orthopyroxene. The range of calculated Sr_i for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sr_i between mineral pairs was 0.00095. The range of calculated μ₂ for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured ²⁰⁸Pb/²⁰⁶Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sr_i = 0.70654 (36) with calculated μ₂ of 10.32 for larger grains as compared with Sr_i = 0.70290 (32) and calculated μ₂ of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system.     

高庙子膨润土中蒙脱石碱性溶蚀的矿物学证据

高放废物地下处置库运营过程中,衬砌混凝土在地下水作用下产生的强碱性溶液,会扩散进入膨润土缓冲屏障,长期将使屏障性能发生退化。采用不同 pH 值的 KOH 溶液模拟衬砌混凝土溶出的碱性溶液,在室内进行为期一年的常温接触扩散试验,开展X射线荧光光谱（X-ray fluorescence spectroscopy,简称XRF）、X射线衍射（X-ray diffraction,简称 XRD）、扫描电镜（scanning electron microscope,简称 SEM）和能谱（energy dispersive spectrometry,简称 EDS）试验,研究强碱性溶液缓慢扩散对高庙子膨润土矿物的影响。XRF试验结果表明,当 KOH 溶液的 pH>12.6时,膨润土中 Si 元素的含量开始减少,即含Si 矿物蒙脱石、石英、方石英等发生了溶解;同时,K元素含量增多,表明膨润土与碱溶液发生离子交换反应,溶液中大量的K⁺进入蒙脱石层间。XRD 试验结果表明,在 pH=12.6时,蒙脱石矿物的 001峰开始右移、变宽,峰值强度大大减弱;在pH>13时,衍射角θ整体向右偏,晶面间距由原始膨润土样的1.385 3 nm（13.853 Å）减小为1.221 0 nm（12.210 Å）,表明蒙脱石矿物晶层被压缩。随着 KOH 溶液 pH 值增大,膨润土中蒙脱石、石英等矿物的含量明显减少,长石类矿物的含量逐渐增加,伊利石和斜发沸石的含量也呈现略微增多的趋势。SEM 试验结果表明,随着溶液 pH 的增大,蒙脱石部分晶层会发生重叠,进而产生部分裂隙和孔洞,加速蒙脱石的溶蚀进程。在为期一年的接触扩散试验中,pH=13.8的 KOH 溶液扩散深度超过7.5 mm,并且在碱溶液与膨润土接触面上观察到新生成的伊利石微晶,证实强碱性溶液会导致蒙脱石溶解及伊利石化。     

Nd isotopic evidence for juvenile crust in the Carolina terrane,southern Appalachians

 Nd isotopic analyses of whole-rock samples from the older portion of the Carolina terrane, one of the largest terranes in the Appalachian orogen, demonstrate that part of this terrane is composed of juvenile, mantle-derived crust. These data suggest that the terrane may not have originally been built upon old, evolved basement material but rather may have been built upon oceanic crust. A recent study by other workers demonstrates a more crustally evolved Nd isotopic signature for younger components of the Carolina terrane. These data may indicate that the terrane interacted with evolved crust at a later time, possibly by amalgamation with a more evolved crustal fragment before final accretion to Laurentia, rather than indicating a primary old basement. A juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting — a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane. It is herein suggested that Carolina is a chemically evolved but isotopically juvenile crustal fragment, because it remained in an oceanic setting for an unusually long time. In this regard the Carolina terrane is similar to some of the large accreted terranes in the Canadian Cordillera, such as Wrangellia and Alexander. The presence of juvenile crust in the Carolina terrane documents that at least part of the southern Appalachian orogen is not composed solely of reactivated pre-existing continental crust. The importance of this part of the orogen in terms of the volume of juvenile Phanerozoic crustal material in North America may be larger than previously thought. However, until additional major Appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown. The isotopic make-up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions, while even widespread pieces of a single terrane should have very similar isotopic characteristics. The Nd isotopic data for the Carolina terrane form the beginning of an isotope database for terranes in the southern Appalachians. Received: 15 June 1994/Accepted: 31 January 1995     

Lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions

The Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex were determined by laser ablation multi-collector ICPMS (LA MC-ICPMS). The samples are of the upper Critical Zone in the northeast corner of the Complex and were collected from drill core and underground mine exposures. All the rocks are fresh and exhibit no evidence for alteration, weathering, or disruption of the Pb isotope systematics subsequent to the initial cooling of the intrusion. Furthermore, individual plagioclase and sulfide crystals do not contain enough U to warrant correction for radiogenic in-growth. For these reasons, the measured Pb isotope ratios approximate the initial ones. For plagioclase, ²⁰⁷Pb/²⁰⁶Pb ranges from 0.98 to 1.02 and ²⁰⁸Pb/²⁰⁶Pb from 2.26 to 2.35. Low ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios characterize grain boundaries and partially annealed microcracks, some of which contain minute fragments of sulfide and other phases, and this accounts for most, if not all, the heterogeneity exhibited by individual samples. Real compositional differences exist, however, in plagioclase from different lithologic layers. For example, plagioclase ²⁰⁷Pb/²⁰⁶Pb values vary from 1.004 in norite beneath the Merensky pyroxenite to 1.009 in the mineralized pyroxenite, and 0.997 in overlying norite. In most samples in which sulfide and plagioclase coexist, the sulfide ²⁰⁷Pb/²⁰⁶Pb ratio is lower and ²⁰⁸Pb/²⁰⁶Pb ratio higher than the corresponding ones in plagioclase. For example, in a mineralized Merensky reef sample, average sulfide ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios are 0.993 and 2.313, respectively, while those in plagioclase are 1.000 and 2.292. In one sample, the sulfide is extremely heterogeneous, with ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios as low as 0.84 and 2.12. In this particular sample, the compositions must represent an isolated occurrence of addition of a young Pb component.The array of sulfide and plagioclase compositions requires multiple sources of Pb at the time of crystallization or soon thereafter. The disequilibrium between plagioclase and sulfide implies that some of the Pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified. Thus, Bushveld sulfide, and to some extent plagioclase, do not reliably record the initial Pb isotopic composition(s) of the parent magma(s).     

Late Ordovician high-Mg adakitic andesite in the western South China block: evidence of oceanic subduction

The Early Palaeozoic was an important period in the geologic evolution of the South China block (SCB), marking the intracontinental orogen in the Wuyi-Yunkai region. One salient feature of the western SCB is the absence of Early Palaeozoic oceanic subduction-related magmatism. Here, we report the first known occurrence of Late Ordovician andesites from the Shimian area, western SCB. Zircon SHRIMP U–Pb dating reveals that the andesites formed at ca. 451 Ma. They have geochemical features of high-Mg adakitic andesite (HMAA) and are characterized by low K₂O (1.09–2.24 wt.%) and Th (2.50–5.65 ppm) and high MgO (4.02–6.91 wt.%) and Mg# (56–71). Furthermore, their zircon grains display positive ε_Hf(t) (+11.4 to +19.6) and low δ¹⁸O (4.72–6.20‰) values. The andesites are interpreted to have been derived from partial melting of a peridotitic mantle wedge in an oceanic subduction setting and subsequent fractional crystallization. Integrating previous studies of the Qinling-Dabie orogenic belt with the data presented in this contribution, we suggest that the SCB was probably involved in the Early Palaeozoic Andean-type orogeny along the Gondwanan proto-Tethyan margin.