西南天山东德沟镁铁-超镁铁岩中尖晶石的矿物学特征及其地质意义

中国西南天山东德沟镁铁-超镁铁岩出露于天山伊犁-中天山板块和塔里木板块之间的碰撞造山带中,岩石类型主要为(方辉)橄榄岩,且普遍经历了含水流体(熔体)的渗透交代作用,含有形态和成分变化复杂的尖晶石,表现为从红褐色到黄褐色、再到深绿色和浅绿色,尖晶石的Cr2O3和FeO的含量依次降低,Al2O3和MgO的含量逐渐增高。研究认为红褐色和黄褐色的铬尖晶是原生矿物,而绿色的镁铝尖晶石为次生矿物。原生铬尖晶石的Cr#为0.41~0.87,Mg#为0.21~0.61,且成分变化大,显示其可能具多期熔融的复杂成因;次生绿色的尖晶石形态不一、成分相对有较大变化,表明其至少形成于两个期次的变质作用或流体交代过程。原生铬尖晶石具有SSZ环境橄榄岩的特征,指示其寄主岩石可能是俯冲上板片地幔楔岛弧环境的橄榄岩残片。     

Empirical geothermometers and geothermobarometers for spinel peridotite phase assemblages

Experimental synthesis of spinel peridotite phase assemblages for a range of compositions that mimic natural samples is used to derive a set of empirical geothermometers and geothermobarometers represented by multiple linear regression best-fit surfaces that link the variables of temperature, pressure, and composition. The calibrated geothermometers use reactions that govern the solubility of Al and Cr in both pyroxenes and the Mg–Fe exchange between silicates and spinel. Geothermobarometers map the Mg–Fe exchange between coexisting olivine and clinopyroxene and pyroxenes and Ca–Mg exchange between coexisting pyroxenes. Application of the geothermometers and geothermobarometers to suites of naturally occurring samples indicates that while reactions governing the Cr and Al solubility and solvus of orthopyroxene give useful estimates of ‘original’ mantle temperatures and pressures, respectively, comparable reactions for clinopyroxene yield estimates that are variably dependent on the transport phase of the sample suites. Temperature and pressure estimates from reactions governing Mg and Fe exchange between silicates and spinel and coexisting silicates are all sensitive to the later transport stage of the samples.     

东天山圪塔山口铜镍矿区镁铁-超镁铁质岩体橄榄石与尖晶石矿物学特征

新近发现的圪塔山口含铜镍矿化镁铁-超镁铁质岩体位于新疆东天山黄山-镜儿泉铜镍矿带东端,共有4个镁铁-超镁铁质岩体,其中Ⅰ、Ⅱ、Ⅲ号岩体均见铜镍硫化物矿化,研究表明其形成时代(282Ma)及岩浆来源与东天山地区其它铜镍矿化镁铁-超镁铁质岩体一致。本文对主要造岩矿物橄榄石及副矿物尖晶石进行了显微镜下观察及电子探针分析,结果表明橄榄石Fo值介于83.1~86.6之间,平均85.2,为贵橄榄石,其Ni含量变化于1273×10-6~2719×10-6,平均1918×10-6;尖晶石根据铝含量的不同可以分为高铝和低铝两种。圪塔山口岩浆为地幔源区发生15.8%~18.8%的部分熔融,并有过剩橄榄石加入的玄武质岩浆经结晶分异作用形成的派生岩浆。对橄榄石分离结晶和硫化物熔离的计算模拟表明橄榄石结晶前,岩浆已经达到S饱和,结晶过程始终伴随硫化物的熔离作用,虽然早期结晶的橄榄石与硫化物熔体间发生了Fe-Ni交换,但仍有很好的铜镍成矿潜力。     

与金刚石原生矿有关的铬尖晶石形态特征

铬尖晶石是寻找金刚石原生矿的重要指示矿物之一,重砂测量中往往发现大量的铬尖晶石。单纯利用矿物化学成分判别它们是否与金刚石原生矿有关,电子探针测试工作量大、耗资高。如果根据显微镜下矿物形态特征鉴定与选择性的电子探针分析相互验证,将是一种既经济又简单、而且更可靠的判别方法。本文主要介绍与金刚石原生矿有关的铬尖晶石在显微镜下的形态特征。     

Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic–ultramafic intrusions, Eastern Desert, Egypt

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr₂O₃ contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe³⁺-rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al₂O₃) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe³⁺-rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe³⁺, extending from the Fe³⁺–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations.     

雅鲁藏布江蛇绿岩中超高压矿物硅尖晶石的研究

从西藏雅鲁藏布江蛇绿岩带的罗布莎蛇绿岩的铬铁矿中，发现一个由70-80种奇异矿物组成的地幔矿物群，其中包括一种成分特殊的尖晶石类矿物。该种尖晶石呈包裹体分布在毒砂中，28粒该矿物的平均化学成分：Na2O 1.58％，MgO7．52％，Al2O3 36．59％，SiO2 44．45％，FeO 8．72％，并含少量CaO和TiO2。经激光拉曼谱仪测试，一部分颗粒具有Franclinite（锌铁尖晶石ZnFe2O4）拉曼谱。根据尖晶石结构和化学成分，可以得出两种分子式：（Mg0．52Na0．14Fe0.32Al0.74）1.72（Si2.00Al1.20）3.20O8和（Mg0.52Na0.14Fe0.32Si0.50）1.48相（Si1.50Al1.94）3．44O8。两种分子式都表明阳离子Si呈六配位占据尖晶石八面体晶格位置。Si离子呈六配位的硅酸盐。实验证明具有超高压性质，来自相当于过渡带400-670km的深部。表明西藏雅鲁藏布江蛇绿岩（古大洋岩石圈）的岩浆活动达到过渡带。可能是地幔柱活动将硅尖晶石类等超高压矿物搬运到上地幔浅部的。     

Petrological evolution of a suite of spinel granulites from Vizianagram, Eastern Ghats Belt, India, and genesis of sapphirine-bearing assemblages

A suite of spinel–cordierite granulites from Viziangram, Eastern Ghats Belt, India preserve mineral assemblages and reaction textures indicative of peak metamorphic conditions of >1000 °C, >8<10 kbar, followed successively by near isobaric cooling (down to 750–800 °C), near isothermal decompression (to 4–5 kbar), and late hydration. P–T conditions of each stage are evaluated through a combination of petrogenetic grid approach and thermobarometry. Sapphirine is developed in sillimanite‐bearing acid pegmatite veins that intruded the spinel–cordierite granulite close to peak metamorphic conditions, and also in the host rock in immediate contact with the pegmatite. Both sillimanite and sapphirine in the pegmatite are considered to be magmatic phases. Field observations and textural characteristics suggest that Al‐metasomatism of the spinel–cordierite granulite due to the intrusion of pegmatite was responsible for sapphirine formation in the spinel granulite.     

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO₂ content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.     

Petrology of spinel granulites from Araku, Eastern Ghats, India, and a petrogenetic grid for sapphirine-free rocks in the system FMAS

Abstract Spinel-quartz-cordierite and spinel-quartz are found as relic prograde assemblages in Fe-rich granulites from the Araku area, Eastern Ghats belt, India. Subsequent reactions produced orthopyroxene + sillimanite in the former association and garnet + sillimanite in the latter. The first reaction is univariant in the FMAS system, but is trivariant in the present case because of the presence of Zn and Fe³⁺ in spinel. The second reaction also has high variance because of Zn and Fe³⁺, but also because of the presence of Ca in garnet. Thermobarometry shows that the metamorphic conditions were approximately 950° C and 8.5 kbar and the fo ₂ was near the NNO buffer. In Fe-rich bulk compositions and low- P -high- T conditions of metamorphism, two of the univariant reactions around the invariant point [Sa], namely (Sa, Hy) and (Sa, Cd), change topology due to reverse partitioning of Fe-Mg between coexisting garnet and spinel. An alternative partial petrogenetic grid in the system FMAS is constructed for such conditions and is applied satisfactorily to several sapphirine-free spinel granulites. It is shown that bulk composition ( X _Fe and Zn) exerts greater control on the stability of spinel + quartz than fo ₂. The effect of the presence of Zn and Fe³⁺ in spinel on the proposed grid is evaluated. Reaction textures in the Araku spinel granulites can be explained from the petrogenetic grid as due to near-isobaric cooling.     

中国大陆科学深钻主孔岩屑中含钠和锌的尖晶石的发现

一种含钠和锌的尖晶石矿物产于中国大陆科学深钻主孔超镁铁岩中，以出溶的方式存在于钛铁矿和磁铁矿中，长5-60μm，宽2-3μm，化学组成（wB％）为MgO17．143％-22．337％，Al2O3 63．646％～66．492％，FeO9．784％～13．924％，ZnO 1．649％-6．386wt％．Na2O 0．029％～0．219wt％。Na2O与ZnO在尖晶石中的含量呈正相关关系。经同步辐射测得其为尖晶石单晶，空间群为Fm3m，晶胞参数为a=0．83781am。根据尖晶石与钛铁矿、石榴石（pyr=56-58mol％）共生和石榴石退变为顽火辉石和镁绿泥石的特点，表明该尖晶石来自超镁铁岩，它们一起经历了至少为角闪岩相的变质作用。广泛的混溶间断现象及其所造成的出溶结构出现在氧化物中，表现为Mt-Usp、Mt-Spl及Mt-Ilm之间的交生。根据前人有关Mt-Usp和Ilm-Gk-Hem固溶体系的实验。表明主孔岩石的氧逸度相对较低，形成温度应在720℃以上。这些早期在岩浆分离结晶过程中形成的铁钛氧化物出溶体，在后来的超高压变质作用下可能发生再次固溶和因降温减压造成的出溶。     

Cr and Al diffusion in chromite spinel: experimental determination and its implication for diffusion creep

Diffusion coefficients of Cr and Al in chromite spinel have been determined at pressures ranging from 3 to 7 GPa and temperatures ranging from 1,400 to 1,700°C by using the diffusion couple of natural single crystals of MgAl₂O₄ spinel and chromite. The interdiffusion coefficient of Cr–Al as a function of Cr# (=Cr/(Cr + Al)) was determined as D _Cr–Al = D ₀ exp {−(Q′ + PV*)/RT}, where D ₀ = exp{(10.3 ± 0.08) × Cr#^0.54±0.02} + (1170 ± 31.2) cm²/s, Q′ = 520 ± 81 kJ/mol at 3 GPa, and V* = 1.36 ± 0.25 cm³/mol at 1,600°C, which is applicable up to Cr# = 0.8. The estimation of the self-diffusion coefficients of Cr and Al from Cr–Al interdiffusion shows that the diffusivity of Cr is more than one order of magnitude smaller than that of Al. These results are in agreement with patterns of multipolar Cr–Al zoning observed in natural chromite spinel samples deformed by diffusion creep.     

Experimental Study on the Solubility of Cr^2 in Olivine,Orthopyroxene and Spinel Solid Solutions

Experiments have been performed on the system MgO-SiO2-Cr-O at 0-2.88 GPa and 1100-1450℃,focusing on the stability of Cr^2 in olivine(O1),orthopyroxene(Opx) and spinel(Sp) and its partitioning between these phases.Analytical reagent grade chemicals,MgO,SiO2,Cr2O3.and Cr were used to make starting mixtures.Excess Cr(50%) was then added in these mixtures to ensure that the resultant phases were in equilibrium with the metal Cr.Flux of BaO B2O3(%) was added for facilitating experimental equilibrium and crystal growth.Cr was used as capsule material.All phases in the product were identified by X-ray and analyzed by electron microprobe,The contents of CrO in the different phases(O1,Opx and Sp)were calculated according to stoichiometry.The obtained results of calculation indicate that Cr^3 in Ol and Opx is negligible.The experimental results show;(a) with increasing temperature and decreasing pressure,Cr^2 solubility in Ol,Opx and Sp increases;(b) with in creasing temperature,the partitioning coefficient of Mg and Cr^2 between Ol and Opx decreases,that between Opx and Sp increases,and that between Ol and Sp remains almost unchanged;(c) the effect of pressure on all partitioning coefficients is negligible.     

Evolution of spinel–pyroxene symplectite in spinel–lherzolites from the Horoman Complex,Japan

We examined the textural and geochemical characteristics of spinel-pyroxene symplectites in spinel-lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the symplectite minerals. Selective enrichment of Sr and LREE in the symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of symplectite.     

Magnetic properties of the Fe2SiO4-Fe3O4 spinel solid solutions

 Magnetic measurement of Fe_{3− x} Si _x O₄ spinel solid solutions indicates that their Curie temperatures decrease gradually, but not linearly, from 851 to 12 K with increasing content of nonmagnetic ions Si⁴⁺. Magnetic hysteresis becomes more noticeable in solid solutions having a larger content of Fe₂SiO₄. Saturation magnetizations of Fe_{3− x} Si _x O₄ samples increase up to x=0.357 and they are easily saturated in the field of H=0.1 T. However, magnetization of the sample of x=0.794 does not approach saturation even at high field of H=7.0 T and has a large coercive force. The Si⁴⁺ disordered distribution is confirmed to be ^tetr[Fe³⁺ _{1− x + x t} Si⁴⁺ _{x (1− t )}] ^octa[Fe²⁺ _{1+ x} Fe³⁺ _{1− x − x t} Si⁴⁺ _{x t} ] O₄ by the spin moment, which is consistent with site occupancy obtained from X-ray crystal structure refinement. Their molecular magnetizations would be expressed as M _B={4(1+x)+10xt}μ_B as functions of composition parameter x and Si⁴⁺ ordering parameter t of the solid solution. The sample of x=0.794 is antiferromagnetic below the Néel temperature, mainly due to the octahedral cation interaction M _O–M _O, while both M _T–M _O and M _O–M _O interactions induce a ferrimagnetic property. Concerning magnetic spin configuration, in the case of x>0.42, the lowest dɛ level becomes a singlet, resulting in no orbital angular momentum. Received: 20 April 2000 / Accepted: 11 September 2000     

新疆东准噶尔扎河坝橄榄岩单斜辉石、铬尖晶石地球化学特征及铬铁矿成矿作用

扎河坝蛇绿岩是东准噶尔地区一条重要的蛇绿岩带,主要由橄榄岩、层状辉长岩、玄武岩、斜长花岗岩、硅质岩等组成.其中橄榄岩主要由方辉辉橄岩(方辉橄榄岩)、二辉橄榄岩和少量纯橄岩组成.二辉橄榄岩中的单斜辉石Cr2O3平均1.11％,Al2O3平均4.77％,MgO平均16.99％,CaO平均21.84％,SiO2平均50.00...     

Cross-twinning in a natural spinel from Sri Lanka

A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in a previous paper. This particular type of twinning is derived from the combination of cyclic twinning with lamellar twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes: and They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped morphology of the face close to the twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal effect.

Petrography,chemical composition,and Raman spectra of chrome spinel: Constraints on the diamond potential of the No. 30 pipe kimberlite in Wafangdian,North China Craton

Conventional diamond exploration seldom searches directly for diamonds in rock and soil samples. Instead, it focuses on the search for indicator minerals like chrome spinel, which can be used to evaluate diamond potential. Chrome spinels are preserved as pristine minerals in the early Paleozoic (∼465 Ma), hydrothermally altered, Group I No. 30 pipe kimberlite that intruded the Neoproterozoic Qingbaikou strata in Wafangdian, North China Craton (NCC). The characteristics of the chrome spinels were investigated by petrographic observation (BSE imaging), quantitative chemical analysis (EPMA), and Raman spectral analysis. The results show that the chrome spinels are mostly sub-rounded with extremely few grains being subhedral, and these spinels are macrocrystic, more than 500 µm in size. The chrome spinels also have compositional zones: the cores are classified as magnesiochromite as they have distinctly chromium-rich (Cr₂O₃ up to 66.56 wt%) and titanium-poor (TiO₂ < 1 wt%) compositions; and the rims are classified as magnetite as they have chromium-poor and iron-rich composition. In the cores of chrome spinels, compositional variations are controlled by Al³⁺-Cr³⁺ isomorphism, which results in a strong Raman spectra peak (A_1g mode) varying from 690 cm⁻¹ to 702.9 cm⁻¹. In the rims of chrome spinel, compositional variations result in the A_1g peak varying from 660 cm⁻¹ to 672 cm⁻¹. The morphology and chemical compositions indicate that the chrome spinels are mantle xenocrysts. The cores of the spinel are remnants of primary mantle xenocrysts that have been resorbed, and the rims were formed during kimberlite magmatism. The compositions of the cores are used to evaluate the diamond potential of this kimberlite through comparison with the compositions of chrome spinels from the Changmazhuang and No. 50 pipe kimberlites in the NCC. In MgO, Al₂O₃ and TiO₂ versus Cr₂O₃ plots, the chrome spinels from the Changmazhuang and No. 50 pipe kimberlites are mostly located in the diamond stability field. However, only a small proportion of chrome spinels from No. 30 pipe kimberlite have same behavior, which indicates that the diamond potential of the former two kimberlites is greater than that of the No. 30 pipe kimberlite. This is also supported by compositional zones in the spinel grains: there is with an increase in Fe³⁺ in the rims, which suggests that the chrome spinels experienced highly oxidizing conditions. Oxidizing conditions may have been imparted by fluids/melts that have a great influence on diamond destruction. Here, we suggest that chrome spinel compositions can be a useful tool for identifying the target for diamond potential in the North China Craton.     

The near-solidus transition from garnet lherzolite to spinel lherzolite

The position of the transition from spinel lherzolite to garnet lherzolite in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) has been determined experimentally at near-solidus temperatures. In reversed experiments, the transition occurs between 18 and 20 kbar at 1200 °C and between 26 and 27 kbar at 1500 °C, corresponding to higher pressures than previously envisaged. A position for the transition deeper within the Earth further complicates the explanation of the so-called garnet signatures in the trace element and isotope patterns of mid-ocean ridge basalts. If melting during adiabatic upwelling beneath a mid-ocean ridge begins at the depth required for the stability of garnet in peridotitic compositions, simple melting models predict that the amount of melt produced should be much greater than the observed thickness of the oceanic crust. A partial solution to the apparent conflict might be that (1) the rather simplistic melting models are in error, (2) that melting begins in garnet pyroxenite veins that are believed to be stable at lower pressures than garnet lherzolite or (3) that melting does not involve garnet at all, but it is clinopyroxene causing the trace element patterns observed in basalts erupted at mid-ocean ridges. A second set of reversal experiments were conducted to investigate the solubility of alumina in both orthopyroxenes and clinopyroxenes at the high temperatures near the solidus in the system CMAS. The results are compatible with most previous studies, and may be used as a starting point to calibrate thermodynamic models for pyroxenes in chemical systems, approximating upper mantle chemistry. Received: 9 August 1999 / Accepted: 29 October 1999     

Rare minerals of the spinel group in ultrabasites from Bulgaria

Summary Chromite mineralization was found during a detailed geological mapping, carried out in 1962, in the region of the Seven Lakes, Northern Rila Mountain. Mineralogical studies showed that the ore minerals of this mineralization are of a great variety, and are members of the little known group of ferrospinels with varying chromium-, aluminium- and magnesium-content. Only three minerals are described in the present paper. They are well defined varieties, differing from one another by their chemical composition and physical properties.

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Seltene Vertreter der Spinell-Gruppe in Ultrabasiten von Bulgarien

Zusammenfassung Bei einer detaillierten geologischen Kartierung im Gebiet der Sieben Seen, nördliches Rila-Gebirge, wurden im Jahre 1962 Chromit-Mineralisationen gefunden. Die mineralogische Bearbeitung zeigte, daß die Erzminerale dieser Mineralisation stark variieren und daß sie zu der noch wenig bekannten Gruppe der Ferrospinelle mit wechselndem Chrom-, Aluminium- und Magnesiumgehalt gehören. In der vorliegenden Arbeit werden nur drei Glieder beschrieben. Es handelt sich um gut definierte Varietäten, die sich in der chemischen Zusammensetzung und in den physikalischen Eigenschaften unterscheiden.

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With 6 Figures     

雅鲁藏布江蛇绿岩再研究:从地质调查到矿物记录

蛇绿岩带出现于全球大多数碰撞构造带上,通常是板块缝合带的标志,西藏雅鲁藏布江缝合带蛇绿岩带对于此种概念模型是毫无争议的。根据近年来现代洋壳和蛇绿岩研究成果以及分析技术,在20世纪80年代中法合作项目研究成果基础上,笔者再次对西起日喀则拉孜、东至泽当金鲁,长约500km的雅鲁藏布江缝合带蛇绿岩进行了研究。在野外调查基础上,对雅鲁藏布江蛇绿岩中500多个尖晶石样品的地球化学特征进行了分析研究,以探讨蛇绿岩的成因及其动力学背景,提出雅鲁藏布江蛇绿岩带属于超俯冲作用(suprasubduction)的产物,与近年来其他研究成果的认识一致。