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1.
Zoned crystals can be important recorders of magmatic processes in space and time. However, in most situations, the temporal dimension is difficult to quantify. Here, we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non-polished crystal faces of zircon to obtain ~5 μm resolution U–Th disequilibrium ages (one pit) that can be correlated with trace element zoning at sub-μm resolution derived from a second pit. Data from 17 crystals representing each of the four rhyolite eruptions of Tarawera volcano, an intra-caldera edifice within the Okataina Volcanic Centre, reveal diverse zircon growth conditions over time. Most crystals display rimward depletions in Zr/Hf and Ti, broadly consistent with cooling and crystallization. However, a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization. Oscillatory zonation patterns in Y, Th, and U are superimposed on the Zr/Hf and Ti trends. Despite the limited number of crystals analyzed in this way, the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir. By combining data from all crystals, a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains <45 ka following the Rotoiti caldera-forming eruption. This contrasts with the more uniform conditions of zircon crystallization lasting >100 ka prior to caldera formation and is best explained by the post-caldera system consisting of small, isolated melt pockets that evolved independently. An important conclusion is that the zircon ‘cargo’ in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma.  相似文献   

2.
Sector zoned aegirine crystals occur in the interstices of peralkaline nepheline syenites in Ilímaussaq. The crystals have grass-green [001] sectors enriched in Ca and Fe2+ (as CaFeSi2O6), Mn and Zr; pale green {010} sectors enriched in Al (as NaAlSi2O6); blue-green {110} sectors enriched in Ti (as NaTi0.5Fe 0.5 2+ Si2O6); and light green {100} sectors enriched in Fe3+ (as NaFe3+ Si2O6).The crystals grew in the liquid with a rate that did not exceed the diffusion rate of most elements in the liquid. However. Fe3+ seems to have had diffusion rates lower than the crystal growth rate, and this probably caused the development of some sectors enriched in acmite and others enriched in the hedenbergite component. For Al, Ti and Zr a crystal structural control is envisaged although a recent structure-based model for sector zoning fails to explain the efficient separation of these elements into different sectors.Three more occurrences of sector zoned aegirine are noted, all from peralkaline nepheline syenites. The phenomenon is probably more widespread than hitherto realised.Contribution to the mineralogy of Ilímaussaq no. 62  相似文献   

3.
西藏罗布莎豆荚状铬铁矿中发现超高压矿物柯石英   总被引：18，自引：0，他引：18  
在西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床中发现典型的超高压矿物柯石英和蓝晶石, 二者呈针柱状交生, 产在一个以TiFe合金成分为主的颗粒(0.7mm× 0.5mm大小) 的最外部.该颗粒从内到外由4层矿物组成, 分别为TiFe合金主体、2 0~ 70 μm宽的自然钛、约10 μm宽的TiSi合金及30~ 5 0 μm宽的柯石英和蓝晶石为主的硅酸岩和氧化物层.主体矿物为高Ti低Fe的TiFe合金, 内部出现由细粒状低Ti高Fe的TiFe合金和自然钛组成的蠕英结构.最外层由柯石英和蓝晶石组成的格架中分布细粒的Si金红石和Ti-Mg -K -Na -Ca氧化物.初步认为TiFe合金从深部高温高压环境往浅部上升过程中, 内部发生局部熔融, 分解出自然Ti, 并在其边部与其他硅酸岩矿物或熔体发生反应, 形成柯石英和蓝晶石.这一过程可能发生在洋脊拉张环境, 由于地幔柱的上涌, 将深部的豆荚状铬铁矿带到浅部, 使得其中包裹的一些高温高压环境下稳定的矿物变得不稳定, 发生熔融和交代反应, 形成新的不平衡的矿物组合.罗布莎柯石英的这种不寻常产出特征说明是在减压过程中形成, 不同于造山带中常见的由板块俯冲增压过程中形成的柯石英   相似文献   

4.
Usually it is assumed that the partitioning of trace elements into titanite in metaluminous granitoid plutonic environments takes place under equilibrium conditions and that compositional zoning is due solely to progressive changes in melt chemistry and/or mineral/melt partition coefficients. Examination of titanites from a variety of Caledonian metaluminous granitoids and related rocks has revealed that sector zoning is present, indicating disequilibrium partitioning. The sector zoning in titanites is defined principally by the distribution of the rare earth elements (REE), Y, Nb, Al and Fe. The REE, Y and Nb preferentially occur within the minor (100) sectors relative to the morphologically important (111) sectors. The reverse is true of Al and Fe which preferentially occur within the (111) sectors relative to the (100) sectors. The patterns of sector zoning are complicated by the fact that the relative growth rates of the various crystal faces fluctuated during growth. Sector zoning indicates that crystal-interface kinetics are responsible for the observed patterns of element partitioning. It is concluded that differences in the lateral-layerspreading rates of crystal faces bring about the sector zoning. The results have implications for the use of trace element partition coefficients in the modelling of fractionation processes.  相似文献   

5.
Zoning of phosphorus in igneous olivine   总被引：2，自引：2，他引：0  
We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P2O5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.  相似文献   

6.
Reconciling the diverse records of magmatic events preserved by multiple crystals and minerals in the same sample is often challenging. In the case of basaltic–andesites from Volcán Llaima (Chile), Mg zoning in olivine is always simpler than Ca zoning in plagioclase. A model that explains a number of chemical patterns is that Llaima magmas stall in the upper crust, where they undergo decompression crystallization and form crystal-mush bodies. Frequent magma inputs from deeper reservoirs provide the potential for remobilization and eruption. The records of multiple recharge events in Llaima plagioclase versus an apparent maximum of one such event in coexisting olivine are addressed by using trace element zoning in olivine phenocrysts. We have integrated elements that (1) respond to changes in magma composition due to recharge or mixing (Mg, Fe, Ni, Mn, ±Ca), with (2) elements that are incorporated during rapid, disequilibrium crystal growth (P, Ti, Sc, V, Al). A more complex history is obtained when these elements are evaluated considering their partition coefficients, diffusivities, and crystal growth rates. The olivine archive can then be reconciled with the plagioclase archive of magma reservoir processes. Olivine (and plagioclase) phenocrysts may experience up to three or more recharge events between nucleation and eruption. Diffusion modeling of major and trace element zoning in two dimensions using a new lattice Boltzmann model suggests that recharge events occur on the order of months to a couple of years prior to eruption, whereas crystal residence times are more likely to be on the order of a few years to decades.  相似文献   

7.
Sub-micron scale distributions of trace elements in zircon   总被引：2，自引：1，他引：1  
Sub-micron scale zoning of Ti concentrations and correlations between concentrations of Ti and other trace elements (P, Ce, and Y) and cathodoluminescent (CL) banding is observed in natural zircons. Ion images were made using the Caltech Microanalysis Center’s CAMECA NanoSIMS 50L with an O− primary beam focused to ~300 nm on the sample surface. The high spatial resolution of this technique allows for interrogation of chemical variations at or below the scale of CL banding in natural zircons. Images produced in this manner display two types of correlations among Ti, P, Ce, and Y (which appears to be a proxy for CL intensity): strong (correlation coefficients >0.8) and subtle (correlation coefficients ~0.15–0.4). Strongly correlated images, which display Ti variations of ca. a factor of 3 between adjacent CL bands and overall elevated trace element concentrations in CL-dark bands, were found within an oscillatory-zoned, trace element enriched sector of a CL sector-zoned zircon. Three possible causes for such correlations include: temperature-dependent equilibrium partitioning, trace element partitioning limited by diffusion in the host melt and surface-controlled, non-equilibrium growth. Comparison of our data with the expected results of these processes suggests that: (1) Ti partitioning in zircon is dependent upon non-equilibrium effects in addition to temperature and/or (2) the incorporation of elements that co-vary with Ti in zircon (e.g., Y, P and Ce) is also temperature-dependent. Sub-micron scale, high-Ti regions are also found within Proterozoic Adirondack and >4 Ga Jack Hills zircons as well as trace element enrichments (including Ti) along cracks within Jack Hills zircons.  相似文献   

8.
Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved granites and aplites which are encountered mostly at the plutons roof and margins. Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization. The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the differentiates. The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation of mineral/melt partition coefficients (K d s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models constructed using the K d s thus obtained along with modal abundances from petrographic estimates. Lamont-Doherty Geological Observatory Contribution Number 3701  相似文献   

9.
Cathodoluminescence (CL) zoning in quartz crystals from rhyolitic pumices in two ignimbrite members of the ~340-ka Whakamaru super-eruption deposits, Taupo Volcanic Zone, New Zealand, is investigated in conjunction with the analysis of Ti concentration in quartz to reconstruct the history of changing magma chamber conditions and to elucidate the eruption-triggering processes. CL intensity images are taken as a proxy for Ti concentration and thus temperature and/or pressure and/or compositional variations during crystal growth history. Estimates of the maximum temperature changes (i.e., assuming other factors influencing Ti uptake remain constant) are made using the TitaniQ geothermometer based on the Ti concentration in quartz. These results are reviewed in comparison with Fe–Ti oxide, feldspar-melt and amphibole geothermometry. Core-to-rim quartz Ti profiles record a marked change in conditions (temperature increase and/or pressure decrease and/or change in melt composition) causing and then following a significant resorption horizon in the outer parts of the crystals. Two alternative models that could explain the quartz Ti zonation invoke a temperature increase caused by mafic recharge and/or a pressure decrease due to magma ponding and re-equilibration at shallow crustal levels. Concomitant changes in melt composition and Ti activity may, however, also have strongly influenced Ti uptake into the quartz. Some crystals also show other marked increases in CL brightness internally, but any accompanying magmatic changes did not result in eruption. Diffusion modelling indicates that this significant change in conditions occurred over ~10–85 years prior to caldera-forming eruption. This rapid thermal pulse or pressure change is interpreted as evidence for open-system processes, and appears to record a magma chamber recharge event that rejuvenated the Whakamaru magma system (melt-dominant magma plus crystal mush), and potentially acted as a trigger for processes that led to eruption.  相似文献   

10.
 Optical anomalies (deviations of the symmetry of optical properties from the ideal symmetry of the crystal) occur in many minerals and synthetic compounds and have been under investigation since the last century. An important feature of optically anomalous mixed crystals is a high degree of optical inhomogeneity, whereas the optical patterns of mixed crystals without anomalies are usually rather uniform. This work is devoted to the study of this phenomenon. As a model object we have chosen mixed alum crystals, which were known for their anomalous birefringence and which revealed the following types of optical inhomogeneities: (1) sector zoning; (2) concentric zoning; (3) subsector zoning; (4) stripes normal to growth front. The inhomogeneity of anomalous birefringence of mixed crystals of alums can be explained by superposition of several effects: mismatch strain, strain along dislocations and growth ordering of isomorphous components. Optical inhomogeneities due to the sector zoning of crystals and their dislocation structure arise even under stationary growth conditions and stationary micromorphology of the growing face. Both variable growth conditions and the relief of the growing face strongly intensify the optical inhomogeneity due to three interrelated factors: (1) a significantly inhomogeneous mismatch strain; (2) a variable degree of ordering of isomorphous components due to the compositional inhomogeneities; (3) different degrees of ordering of isomorphous atoms caused by different orientations, heights and velocities of growth steps. These effects lead to the formation of subsector zoning and zoning superimposed on the optical sector zoning. These optical structures are crossed by birefringent stripes arising from dislocations. Received: 29 March 2000 / Accepted: 11 March 2001  相似文献   

11.
独居石U-Pb定年在岩浆活动、变质作用和沉积作用等方面发挥着日益重要的作用,但是由于独居石成因复杂,因此从成因矿物学角度对不同成因独居石的特征进行总结将有助于解释独居石年代学数据.总结了不同成因独居石的矿物共生关系和组构特征、外部形貌-内部结构、化学元素特征,依托Th、U、Y、Ca、Pb、REE含量及其比值关系进行化学...  相似文献   

12.
The character of oscillatory zoning within a zircon crystal from the syenite Kipawa Complex, Quebec, varies with scale of observation. Analysis of an scanning electron microscopy (SEM) back-scatter gray-scale traverse at a resolution of one pixel = 2.43 μm revealed 145 zones over 5130 μm, whereas a detailed high-resolution (one pixel = 0.195 μm) section near the crystal rim revealed 225 zones over 795 μm. In order to mathematically characterize the zoning pattern, wavelet, Fourier, and nonlinear analysis techniques were used on profiles of the SEM gray-scale data, and a series constructed was from the zone widths. Results demonstrate that the zircon oscillatory zoning preserves nonlinear and periodic components. Secondary ion mass spectrometry, electron microprobe, and SEM analyses of trace elements show the SEM back-scatter bright zones are enriched in U, Th, and rare earth elements (REE) in comparison to the darker zones. REE patterns are sharply heavy REE enriched and have negative Eu anomalies and prominent positive Ce anomalies. We model the oscillatory zoning, including a measure of its chemical variation, by use of a periodically forced nonlinear system. Results of this data-driven model are quantitatively similar to the natural data. We envisage that the small-scale oscillatory zoning was the result of a nonlinear feedback process wherein the crystal growth modified the adjacent melt, which in turn affected the crystal composition. The large-scale harmonic zones likely reflect changes in the bulk geochemistry of the system from which the zircon grew.  相似文献   

13.
电子探针显微分析是常用的原位分析矿物中各种主量元素含量的技术方法,但是对于微量元素的分析,提高测试精确度和准确度长期以来面临着各种挑战。常用的提高精确度方法有改变加速电压、加大电流和增加计数时间,但是这些方法容易使样品遭到破坏,发生元素迁移从而造成测试结果的不准确,增加计数时间还会降低测试效率。文章提出了一种利用电子探针多道波谱仪同时测试同一个元素的方法来提高待测元素特征X射线的强度值。该方法利用电子探针不同波谱仪同时测试同一个待测元素,通过测试标准样品中待测元素的总净计数和待测样品中待测元素的总净计数,结合标准样品中待测元素的浓度计算待测样品中待测元素的含量。该方法在同等测试条件下可以有效的提高待测元素特征X射线的强度值,从而提高了测试的精确度,降低了测试的检测限。文章以石英中Al和Ti含量的测试为例,验证了多道波谱仪同时测试同一个元素的方法在微量元素测试中的应用。测试Al元素时使用一个TAP晶体和一个TAPL晶体,测试Ti元素时使用三个PETL晶体,结合多点背景测试法准确测试了石英中Al和Ti的含量,在20 kV电压、500 nA束流和20 μm束斑直径的条件下可将Al的检测限降低至2.6×10-6 (3σ),将Ti的检测限降低至2.1×10-6 (3σ),测试获得石英参考标样中Al和Ti的含量分别为163.8×10-6±5.8×10-6 (3σ)和56.5×10-6±2.0×10-6 (3σ),皆在参考值区间(Al：154×10-6±15×10-6;Ti：57×10-6±4×10-6)内,并且具有较好的长期稳定性。  相似文献   

14.
Kinetic effects on trace element partitioning have been measured for anorthite, forsterite, and diopside grown from synthetic compositions doped with REE. A seeding technique allowed determination of crystal growth rates and partitioning information was obtained from electron microprobe analyses. Compositional deviations from equilibrium values were sought in the crystals and as gradients in the quenched liquids adjacent to the crystals. The principal result is that large deviations in trace element distribution coefficients from equilibrium values do not occur because of a compensating effect. Rapid growth depletes the melt adjacent to the crystal in the elements of which the crystal is composed, leading to different values for apparent distribution coefficients. However, as the boundary layer melt becomes depleted in the components of the crystal, growth slows and the size of the compositional perturbations decreases. Crystals grown at very high rates (e.g., > 0.2 μm/sec for diopside) tended to be too small for accurate microprobe analyses, but are probably not compositionally extreme since the melts adjacent to the crystals did not acquire sizable compositional gradients. At moderately high growth rates (e.g., 0.02 μm/sec), crystals form in the presence of boundary layer compositions perturbed by as much as 10% from bulk melt values and, in diopside, attain concentrations for excluded trace elements about 70% higher than equilibrium values for crystals plus bulk melt. At the slower growth rates typical of igneous systems, kinetic effects on trace element partitioning are probably negligible.  相似文献   

15.
水热法合成黄铁矿微观形貌和结构的观测与表征   总被引：1，自引：0，他引：1       下载免费PDF全文
模拟热液型金矿床中黄铁矿生成的地质条件,采用硫酸亚铁(FeSO4)和硫代乙酰胺(CH3CSNH2)为铁源和硫源,在Fe/S比为1∶3、温度180~200 ℃、加热时间24 h条件下考察黄铁矿的结晶情况。用SEM扫描电镜观察在不同水热条件下合成黄铁矿的形态及成分;用多晶X射线衍射仪(XRD)确定了产物物相组成;用透射电子显微镜(TEM)观测表征了黄铁矿晶体的形态和结构。结果表明：在200 ℃时黄铁矿为粒状,粒度较均匀,粒径1 μm左右。在180 ℃时黄铁矿除有尺寸在1 μm左右粒状黄铁矿外,还有不规则带状黄铁矿晶体,带宽为200 nm左右。认为水热条件中,随着结晶温度的逐步升高,黄铁矿的形貌逐步由不规则形状向规则形状的转变。所以在实验温度较高阶段,形成规则的粒状形貌;在低温阶段,则会出现不规则带状形貌。  相似文献   

16.
Trace element zoning in garnets from two contact-metamorphosed rocks from the Kwoiek area, British Columbia (Hollister 1969a), was measured with an ion microprobe. Zoning profiles have three distinct parts with chemical breaks defined by co-variation of major and trace elements. Important features of the trace element zoning profiles are: (1) roughly bell-shaped zoning profiles for Y and the HREEs, (2) an abrupt increase in Ti at a point midway through each garnet with inflections in the zoning profiles of other elements (Li, Na, Cr, V, Y, Zr, and the HREE), and (3) irregular Cr and V profiles. Unlike Mn zoning, the zoning profiles of most other trace elements cannot be easily modeled using simple Rayleigh fractionation models. Ti activity in the two samples is buffered by phase relations with ilmenite. Garnets from a continuously heated contact metamorphic environment should display continuous Ti zoning profiles if equilibrium was maintained and provided the Ti buffering assemblage did not change during garnet growth. The irregular Ti profiles suggest disequilibrium behavior. Several elements (Cr, V) may indicate breakdown of a phase enriched in trace elements during metamorphism. The source for the mass excess of these elements is probably the refractory cores of ilmenite grains. Either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point. The preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth.  相似文献   

17.
滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶普遍出现正环带结构、反环带结构或韵律环带结构，少量为具绿色核部的单斜辉石(“绿核辉石”)。反环带斑晶和“绿核辉石”的幔部与正环带斑晶的核部具有相似并且相对较窄的成分范围，相对高Mg#（0.83 ~ 0.90）,低TiO2（0.13 % ~ 0.29 %）,Al2O3（0.73 % ~ 1.68 %）和Na2O （0.22 % ~ 0.42 %），为钾质岩浆平衡结晶的产物。反环带斑晶的核部相对低Mg#（0.77 ~ 0.84），但与反环带斑晶的幔部、正环带斑晶的核部均具有相似的Ti/Al比值（0.06 ~ 0.16）；韵律环带结构斑晶的成分变化均在正、反环带斑晶的成分范围之内。莴中高镁富钾火山岩中的这些环带结构单斜辉石斑晶应来源于相似的岩浆体系，反 环带结构表明在岩浆房存在较原始岩浆对较演化岩浆再补给的岩浆混合过程，而韵律环带结构特征揭示曾多次发生这种岩浆混合过程。“绿核辉石”的核部明显低Mg# （0.50 ~ 0.74）,相对富Al2O3（1.66 % ~ 3.63 %）和Na2O（0.87 % ~ 2.17 %），具有明显较低的Ti/Al比值（< 0.05）和较高的AlVI/AlIV比值（0.38 ~ 0.76），为下地壳捕虏晶 来源，证实了在滇西晚始新世富钾岩浆演化过程中存在少量地壳混染作用。  相似文献   

18.
Chemical and mineralogical characterizations of high Ti-bearing blast furnace slag in Panzhihua region were investigated and the technical approach "recycling Fe by magnetic separation-extracting Al by sulfation roasting and water leaching-extracting Ti by acidolysis" was proposed for recycling the valuable elements from the special mineral resources. The results show that the most valuable metal is Ti (20.46% TiO2, mass fraction). Other metals, such as Al, Mg and Fe can also be seen as valuable metal to be recovered. Minerals such as perovskite, diopside and spinel are disseminated in the slag. Metallic iron usually filling in the diopside, the Ti element in the slag is distributed widely in many types of mineral phases and Ti component enriches mainly in perovskite phase. It is highly euhedral and mostly disseminates in the diopside, usually presents an equiaxed or irregular granular shape, particles are commonly tiny, in the range of 10–20 μm.  相似文献   

19.
Two large pegmatitic crystals of sodic pyroxene (aegirine) and sodic amphibole (arfvedsonite) from the agpaitic igneous Ilímaussaq Complex, south Greenland were found to be suitable as reference materials for in situ Li isotope determinations. Lithium concentrations determined by SIMS and micro‐drilled material analysed by MC‐ICP‐MS generally agreed within analytical uncertainty. The arfvedsonite crystal was homogeneous with [Li] = 639 ± 51 μg g?1 (2s, n = 69, MC‐ICP‐MS and SIMS results). The aegirine crystal shows strongly developed sector zoning, which is a common feature of aegirines. Using qualitative element mapping techniques (EPMA), the homogeneous core of the crystal was easily distinguished from the outermost sectors of the crystals. The core had a mean [Li] of 47.6 ± 3.6 μg g?1 (2s, n = 33) as determined by SIMS. The seven micro‐drilled regions measured by solution MC‐ICP‐MS returned slightly lower concentrations (41–46 μg g?1), but still overlap with the SIMS data within uncertainty. Based on MC‐ICP‐MS and SIMS analyses, the variation in δ7Li was about 1‰ in each of the two crystals, which is smaller than that in widely used glass reference materials, making these two samples suitable to serve as reference materials. There was, however, a significant offset between the results of MC‐ICP‐MS and SIMS. The latter deviated from the MC‐ICP‐MS results by ?6.0 ± 1.9‰ (2s) for the amphibole and by ?3.9 ± 1.9‰ (2s) for the aegirine. This indicates the presence of a significant matrix effect in SIMS determinations of Li isotopes for amphibole and pyroxene relative to the basalt glasses used for calibration. Based on the MC‐ICP‐MS results, mean δ7Li values of +0.7 ± 1.2‰ (2s, n = 10) for the arfvedsonite crystal and of ?3.7 ± 1.2‰ (2s, n = 7) for the core of the aegirine crystal were calculated. Adopting these values, SIMS users can correct for the specific IMF (instrumental mass fractionation) of the ion probe used. We propose that these two crystals serve as reference materials for in situ Li isotope determinations by SIMS and pieces of these two crystals are available from the first author upon request.  相似文献   

20.
Rare strontium mineral tausonite and a peculiar Al- and F-rich titanite variety were found in the metamorphosed metalliferous sediments of the Triassic chert formation of the Sikhote Alin, which are distinguished by the abundance of native elements, intermetallic compounds, and metal solid solutions, as well as the presence of diverse Au, Ag, and PGE minerals. Tausonite was documented in the manganese (metamorphosed siliceous-rhodochrosite) rocks of the Ol’ga mining district and in the “brown cherts” (siliceous rocks with manganese garnet and spessartine) of the Dal’nerechensk district, Primorye. It forms rather numerous grains 2–10 μm across usually occurring as inclusions in quartz or rhodonite. According to the electron microprobe analysis, in addition to Sr, Ti, and O, the mineral contains only Fe3+ (up to 0.20 a.f.u.). Aluminum-fluorine titanite was found in the “brown cherts” of the Dal’nerechensk district of Primorye (upper reaches of the Gornaya River). Its crystals are up to 200 × 200 μm in size. The recalculation of the microprobe analyses to crystal chemical formulas indicated that up to half of the Ti sites in the structure of this mineral may be occupied by Al. The decrease of the total positive charge owing to the Al3+ substitution for tetravalent Ti4+ is compensated for by a decrease in the total negative charge owing to F? substitution for O2? via the scheme Al3+F? → Ti4+O2?. The occurrence of considerable amounts of F substituting for oxygen in the titanite structure and, as a consequence (owing to the crystal chemical features of the mineral), the high Al content were related to the reduced character of the metamorphism of the metalliferous deposits.  相似文献   

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