Dunites of the Inagli massif (Central Aldan), cumulates of lamproitic magma

We consider a hypothesis for the origin of PGE-bearing ultramafic rocks of the Inagli massif (Central Aldan) through fractional crystallization from ultrabasic high-potassium magma. We studied dunites and wehrlites of the Inagli massif and olivine lamproites of the Ryabinovy massif, which is also included into the Central Aldan high-potassium magmatic area. The research is focused on the chemistry of Cr-spinels and the phase composition of Cr-spinel-hosted crystallized melt inclusions and their daughter phases. Mainly two methods were used: SEM-EDS (Tescan Mira-3), to establish different phases and their relationships, and EPMA, to obtain precise chemical data on small (2-100 μm) phases. The obtained results show similarity in chromite composition and its evolutionary trends for the Inagli massif ultramafites and Ryabinovy massif lamproites. The same has been established for phlogopite and diopside from crystallized melt inclusions from the rocks of both objects. Based on the results of the study, the conclusion is drawn that the ultramafic core of the Inagli massif resulted from fractional crystallization of high-potassium melt with corresponding in composition to low-titanium lamproite. This conclusion is consistent with the previous hypotheses suggesting an ultrabasic high-potassium composition of primary melt for the Inagli ultramafites.     

新疆东昆仑阿其克库勒湖西南缘蛇绿岩--铬铁矿证据及其构造环境探讨

新疆东昆仑阿其克库勒湖西南缘蛇绿岩沿近东西走向的昆中断裂出露,以构造岩片的形式夹持于泥盆纪和志留纪地层中,由变质橄榄岩、堆晶岩和火山岩三部分组成.对变质橄榄岩中的铬铁矿成分分析证实了它们属蛇绿岩的成员,并且铬铁矿的Cr#=Cr/(Cr Al)为0.59～0.65,指示蛇绿岩经历了从洋中脊到俯冲带环境的演化,形成机制为岩石-熔体反应机制.     

上地幔Os同位素组成的演化

Re-Os同位素体系为研究地幔的成分-结构-演化提供了新的地球化学示踪和定年的工具.上地幔Os同位素组成演化的球粒陨石模型是Re-Os体系用于地幔物质定年的基础, 尤其在采用Re亏损模式年龄和Os同位素代理等时线年龄时.综合了铁陨石和各类球粒陨石、地幔橄榄岩包体和蛇绿岩豆荚状铬铁矿的Re-Os同位素体系研究的近期成果, 为认识对流上地幔Os同位素组成的演化提供了制约.对河北遵化蛇绿岩豆荚状铬铁矿岩的研究, 获得新太古代(2.5 Ga)时形成豆荚状铬铁矿的对流上地幔的187Os/188Os=0.110 2, 与球粒陨石型模式的一致.文献中常用的球粒陨石模式的参数如下: 地球形成时(4.558 Ga)初始值187Os/186Os为0.095 31, 现代值分别采用碳质球粒陨石的187Os/186Os比值为0.127 0和原始上地幔(PUM)的187Os/186Os比值为0.129 6, PUM与普通球粒陨石和顽火球粒陨石的187Os/186Os比值接近.      

A New Method for Low‐Temperature Decomposition of Chromites and Dichromium Trioxide using Bromic Acid Evaluated by Chromium Isotope Measurements

In recent years, routine application of the stable isotope determination of chromium (Cr) in environmental and health protection research has led to the search for simpler chromite decomposition techniques. As the range of Cr isotope abundance ratios in nature is very narrow, conventional chromite decomposition techniques are no longer suitable, due to the relatively high risk of contamination during laboratory procedures. We have developed a protocol for the decomposition of chromites based on oxidation by bromic acid at room temperature. The procedure takes 15 d and requires two doses of bromic acid during the reaction period (day 1 and 8), due to the limited stability of the reagent. Chromium extracted by alkaline oxidative fusion and by bromic acid decomposition yielded statistically indistinguishable δ⁵³Cr values, measured by multi‐collector inductively coupled plasma‐mass spectrometry following addition of a ⁵⁰Cr‐⁵⁴Cr double‐spike.     

Metasomatic alteration of chromite from parts of the late Archaean Sittampundi Layered Magmatic Complex (SLC), Tamil Nadu,India

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section.     

Mineralogical,geochemical, and age characteristics of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and Cr-diopside mineralization (Aldan Shield)

We consider the mineralogical and geochemical features of the rocks of the Inagli dunite-clinopyroxenite-shonkinite massif with platinum-chromite and unique jewelry Cr-diopside mineralization, which is a reference object of concentric zonal complexes. The massif rocks, from dunites to pulaskites, including peridotites, clinopyroxenites, shonkinites, and melanocratic alkali syenites, form a single continuous comagmatic series. This is confirmed by a clear dependence of the compositions of olivine, pyroxene, phlogopites, and Cr-spinels on the MgO content of the rocks and on the behavior of trace elements in them. The similar compositions of pyroxenes and trace-element patterns of clinopyroxenite rocks and Cr-diopsidite veins indicate a genetic similarity of these rocks. The age and mineralogical and geochemical compositions of the rocks and the geologic and morphological features of the intrusion prove that the Inagli massif formed from high-K picritoid melts, which underwent gradual decompression solidification during the ascent and formed a cylindrical diapir-like body at the subsurface level in the Early Cretaceous. The new portions of differentiates supplied from the lower horizons of the magma column determined the complex composition of the massif: It has a concentric zonal structure cut by numerous radial-circular vein bodies of pegmatites and pure anchimonomineral rocks (Cr-diopsidites), in places, of jewelry quality.     

西藏铬铁矿找矿方向和找矿方法问题探讨——兼论铬矿物探效果

铬铁矿一直是我国的急缺矿产,西藏是我国铬铁矿最大的产出区和远景区,目前也面临后备资源不足的困境,主要原因是投入不足。为了国家的经济安全,建议在已知矿体外围和深部找新矿,同时加速新岩体铬铁矿的普查工作．要特别重视物探方法的运用。     

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...