The bimodal composition of carbonatites: Reality or misconception?     

J. Gittins  R.E. Harmer  D.S. Barker   《Lithos》2005,85(1-4):129-139

The concept of compositional bimodality in carbonatites has become widely accepted and has been used to impose restrictions on the composition of carbonatite magmas. We agree that mineralogical bimodality exists in carbonatites (most are either calcitic or dolomitic/ankeritic), but we argue that there is no compositional bimodality. The idea of bimodality is based on the interpretation of a variety of element distribution diagrams which were compiled only from chemical analyses in which SiO₂ is < 10 wt.%. All others were rejected. Even with such a restricted data set the case for compositional bimodality is extremely weak, but the inclusion of analyses with higher SiO₂ content destroys it completely. Yet these more siliceous compositions must be included, for many carbonatites contain substantial amounts of Fe–Mg silicates which are an essential part of the magmatic mineralogy of the rocks. They account for much of the Mg in carbonatites that are otherwise calcitic. Many such carbonatites contain well in excess of 10 wt.% SiO₂. Supporters of the bimodality concept argue that liquids having compositions between calcite and dolomite can precipitate neither calcite nor dolomite because the minimum on the solid solution loops in the system calcite–dolomite permits only a carbonate of intermediate composition. Therefore, it is argued, liquids of such intermediate composition cannot be parental to calcitic and dolomitic carbonatites; their parent magmas must be calcitic and dolomitic. This deduction is incorrect. It is well established that dolomitic liquids have calcite as the liquidus phase over substantial temperature intervals, and that this is followed by dolomite precipitation. Mixed calcite–dolomite carbonatites are explicable in this way. Therefore, dolomitic liquids can be parental to calcitic carbonatites. However, dolomitic carbonatites cannot crystallize from a calcitic liquid. We suggest that intermediate composition carbonatite magmas are probably common. Bimodality in carbonatites is solely mineralogical, not compositional.  相似文献   

Retention of cosmogenic 3He in calcite     

《Quaternary Geochronology》2015

Cosmogenic ³He can be used to date a wide range of mineral phases because it is produced from all target elements and can be readily measured above atmospheric contamination. Calcite is a particularly attractive target mineral due to its natural abundance, large crystal size (>1 mm), and low He closure temperature (<70 °C), which limit non-cosmogenic ³He components (Copeland et al., 2007). However, several recent studies have shown that some calcite may not be retentive to helium, even under surface temperatures (Cros et al., 2014; Copeland et al., 2007). This study thus explores ³He retention and production in natural calcite samples at four different sites. Samples from two high elevation sites appear retentive to ³He over 10 kyr timescales, whereas two additional sites clearly suffer from diffusive loss of ³He. Step-degassing experiments suggest that diffusion in calcite is controlled by multiple diffusion domains, with an apparent activation energy of 25–27 kcal mol⁻¹. Although minor ³He loss is expected from the smallest diffusion domains, the observed kinetics cannot explain the poor retention at all sites. We thus propose that opaque (non-transparent) calcite may be more retentive due to the presence of imperfections in the crystal lattice. We conclude that ³He dating of calcite shows promise in some settings. However, because retention depends on crystallographic variability it must be evaluated on a case-by-case basis until robust criteria for retention can be identified.  相似文献   

Diagenesis and origin of calcite cement in the Flemish Pass Basin sandstone reservoir (Upper Jurassic): Implications for porosity development     

《Marine and Petroleum Geology》2016

The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhrán Formation (Upper Jurassic) in the Flemish Pass Basin.The Ti-3 sandstones with average Q_86.0F_3.1R_10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300–500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature (T_h) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ¹⁸O and δ¹³C isotopic compositions of the intergranular calcite are −8.3 ± 1.2‰, VPDB and −3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ¹⁸O but similar δ¹³C values. The shale normalized rare earth element (REE_SN) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO₂/CH₄ and N₂/Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids.Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality.  相似文献   

细度和染色杂质对方解石白度的影响     

韩召峰  周清锋  邓涛 《江苏地质》2002,26(4):214-215

方解石经超细粉碎后通过水力分级分成若干粒级；考察细度对方解石白度的影响；运用高梯度强磁选考察弱磁性染色杂质对方解石白度的影响。  相似文献   

白云鄂博矿区周围火成碳酸岩岩墙地质特征   总被引：19，自引：0，他引：19       下载免费PDF全文

陶克捷  王文志 《地质科学》1998,33(1):73-83

首次填出白云鄂博矿区周围火成碳酸岩岩墙的分布图,深入系统地研究了岩墙的地质产状、主矿物类型、岩石结构、人工重砂矿物组成、稀土元素含量等特征。反映了白云鄂博矿区周围火成碳酸岩岩墙的岩浆演化分异过程存在差异。对于研究白云鄂博矿区铁与稀土的矿化提供了物质来源的证据。  相似文献   

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves     

Albert Coede  Russell S. Harmon  Tim C. Atkinson  Peter J. Rowe 《第四纪科学杂志》1990,5(1):29-38

Activity ratios of ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U, and ²³⁰Th/²³²Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U-series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of ²³⁰Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka. At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.  相似文献   

Ages of sediment-hosted Himalayan Pb–Zn–Cu–Ag polymetallic deposits in the Lanping basin,China: Re–Os geochronology of molybdenite and Sm–Nd dating of calcite     

《Journal of Asian Earth Sciences》2013

The Lanping basin is a significant Pb–Zn–Cu–Ag mineralization belt of the Sanjiang Tethyan metallogenic province in China. Over 100 thrust-controlled, sediment-hosted, Himalayan base metal deposits have been discovered in this basin, including the largest sandstone-hosted Pb–Zn deposit in the world (Jinding), and several Cu ± Ag ± Co deposits (Baiyangping, Baiyangchang and Jinman). These deposits, with total reserves of over 16.0 Mt Pb + Zn, 0.6 Mt Cu, and 7000 t Ag, are mainly hosted in Meso-Cenozoic mottled clastic rocks, and strictly controlled by two Cenozoic thrust systems developed in the western and eastern segments of the Lanping basin.To define the metallogenic history of the study area, we dated nine calcite samples associated with copper sulfides from the Jinman Cu deposit by the Sm–Nd method and five molybdenite samples from the Liancheng Cu–Mo deposit by the Re–Os method. The calcite Sm–Nd age for the Jinman deposit (58 ± 5 Ma) and the molybdenite Re–Os age for the Liancheng deposit (48 ± 2 Ma), together with previously published chronological data, demonstrate (1) the Cu–Ag mineralization in the western Lanping basin mainly occurred in three episodes (i.e., ∼56–54, 51–48, and 31–29 Ma), corresponding to the main- and late-collisional stages of the Indo–Asian orogeny; and (2) the Pb–Zn–Ag (±Cu) mineralization in the eastern Lanping basin lacked precise and direct dating, however, the apatite fission track ages of several representative deposits (21 ± 4 Ma to 32 ± 5 Ma) may offer some constraints on the mineralization age.  相似文献   

Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)     

《Journal of Structural Geology》2014

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.  相似文献   

Improved understanding of petroleum migration history in the Hongche fault zone,northwestern Junggar Basin (northwest China): Constrained by vein-calcite fluid inclusions and trace elements   总被引：1，自引：0，他引：1  

Jian Cao  Zhijun Jin  Wenxuan Hu  Yijie Zhang  Suping Yao  Xulong Wang  Yueqian Zhang  Yong Tang 《Marine and Petroleum Geology》2010

Calcite veins and cements occur widely in Carboniferous and Permian reservoirs of the Hongche fault zone, northwestern Junggar Basin in northwest China. The calcites were investigated by fluid inclusion and trace-element analyses, providing an improved understanding of the petroleum migration history. It is indicated that the Hongche fault behaved as a migration pathway before the Early Cretaceous, allowing two oil charges to migrate into the hanging-wall, fault-core and footwall reservoirs across the fault. Since the Late Cretaceous, the Hongche fault has been sealed. As a consequence, meteoric water flowed down only into the hanging-wall and fault-core reservoirs. The meteoric-water incursion is likely an important cause for degradation of reservoir oils. In contrast, the footwall reservoirs received gas charge (the third hydrocarbon event) following the Late Cretaceous. This helps explain the distribution of petroleum across the fault. This study provides an example of how a fault may evolve as pathway and seal over time, and how reservoir diagenetic minerals can provide clues to complex petroleum migration histories.  相似文献   

Geochronology and mineralogy of the Weishan carbonatite in Shandong province,eastern China     

《地学前缘(英文版)》2019,10(2):769-785

The Weishan REE deposit is located at the eastern part of North China Craton (NCC), western Shandong Province. The REE-bearing carbonatite occur as veins associated with aegirine syenite. LA-ICP-MS bastnaesite Th-Pb ages (129 Ma) of the Weishan carbonatite show that the carbonatite formed contemporary with the aegirine syenite. Based on the petrographic and geochemical characteristics of calcite, the REE-bearing carbonatite mainly consists of Generation-1 igneous calcite (G-1 calcite) with a small amount of Generation-2 hydrothermal calcite (G-2 calcite). Furthermore, the Weishan apatite is characterized by high Sr, LREE and low Y contents, and the carbonatite is rich in Sr, Ba and LREE contents. The δ¹³C_V-PDB (−6.5‰ to −7.9‰) and δ¹³O_V-SMOW (8.48‰–9.67‰) values are similar to those of primary, mantle-derived carbonatites. The above research supports that the carbonatite of the Weishan REE deposit is igneous carbonatite. Besides, the high Sr/Y, Th/U, Sr and Ba of the apatite indicate that the magma source of the Weishan REE deposit was enriched lithospheric mantle, which have suffered the fluid metasomatism. Taken together with the Mesozoic tectono-magmatic activities, the NW and NWW subduction of Izanagi plate along with lithosphere delamination and thinning of the North China plate support the formation of the Weishan REE deposit. Accordingly, the mineralization model of the Weishan REE deposit was concluded: The spatial-temporal relationships coupled with rare and trace element characteristics for both carbonatite and syenite suggest that the carbonatite melt was separated from the CO₂-rich silicate melt by liquid immiscibility. The G-1 calcites were crystallized from the carbonatite melt, which made the residual melt rich in rare earth elements. Due to the common origin of G-1 and G-2 calcites, the REE-rich magmatic hydrothermal was subsequently separated from the melt. After that, large numbers of rare earth minerals were produced from the magmatic hydrothermal stage.  相似文献