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1.
Three fresh kimberlites are shown to have87/Sr86Sr ratios of 0.704; altered kimberlites yield87/Sr86Sr ratios of 0.707 and 0.708. All kimberlites previously analysed for Sr isotopes may in fact have been altered. 相似文献
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Ichiro Kaneoka Nobuo Takaoka Ken-Ichiro Aoki 《Earth and Planetary Science Letters》1977,36(1):181-186
Rare gas isotopes in a phlogopite nodule and a phlogopite-bearing peridotite nodule in South African kimberlites were studied to examine the state of rare gases in the deep interior of the kimberlite region.Within the experimental error of 1 ~ 2%, rare gas isotopic compositions are atmospheric except for radiogenic4He and40Ar. No excess129Xe was observed.In phlogopite, Ne is more depleted, whereas the heavier rare gases are more enriched than the atmospheric rare gases relative to36Ar.Together with other data these results suggest that the state of rare gases in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by a process such as mixing of crustal materials. 相似文献
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The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions 总被引:1,自引:0,他引:1
Richard A. Brooker R. Stephen J. Sparks Janine L. Kavanagh Matthew Field 《Bulletin of Volcanology》2011,73(8):959-981
Kimberlites are volatile rich magmas that ascend from deep in the mantle at high velocities, then as they reach a ‘root zone’
at 1–3 km in depth they either discharge explosively through to the surface or stall to form dykes and sills. Understanding
this eruptive behaviour is difficult due to a lack of data on volatile solubility, particularly at conditions where the magmas
enter the ‘root zone’ (∼30–80 MPa). In this study, we perform experiments on some putative primary kimberlite magma compositions
to assess the amount of CO2 and H2O retained if these compositions represent magma as it enters the root zone. At the conditions investigated (100–200 MPa and
1,275–1,100°C) the results suggest that none of these particular kimberlite compositions reproduce a magma that can retain
the observed high volatile content when intruded at these pressures (∼4–8 km). In our experiments, the low volatile retention
is due to a combination of factors including a high proportion of solid phases, none of which are volatile-bearing, and inadequate
volatile solubilities in the subordinate amounts of melt present. Modelled solubilities also suggest that the dissolved volatile
contents remain too low even at super-liquidus temperatures (i.e. 100% melt). For water, the higher values observed in natural
rocks can be explained by the addition of H2O associated with ubiquitous post-emplacement serpentinization. The high CO2 contents in hypabyssal rocks are unlikely to be related to alteration. We suggest that most kimberlites originally had lower
SiO2 contents and as such may have been ‘transitional’ between silicate and carbonate melts. This results in both higher CO2 solubilities and lower liquidus temperatures. For such compositions, it is possible that both CO2 and water solubility may first decrease and then increase as magmas decompress and crystallize. Such unusual behaviour can
help explain why kimberlite magmas can be very explosive or form shallow hypabyssal intrusions. 相似文献
4.
Maksimochkin V. I. Trukhin V. I. Minina Yu. A. Garanin V. K. Bovkun A. V. Anashkin S. M. 《Izvestiya Physics of the Solid Earth》2013,49(2):289-305
Izvestiya, Physics of the Solid Earth - The magnetic properties and mineralogy of kimberlites from four pipes (BK53, BK56, AK08, and AK06) in Botswana are studied. It is shown that magnetic... 相似文献
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Diamond bearing kimberlite pipes are exposed across the north-central part of the Siberian platform. Three main time intervals are considered to be the age of emplacement: the Devonian–Early Carboniferous, Triassic, and Cretaceous. However, isotopic age data from of the pipes are scattered and provide a very broad age interval for the magmatic activity. New paleomagnetic poles from four kimberlite pipes (Eastern Udachnaya, Western Udachnaya, International and Obnazhennaya) are obtained to estimate their paleomagnetic age. The mean primary magnetization directions for the pipes are as follows: D = 4.3°, I = − 44.5° (k = 29.4, α95 = 7.4°, N = 14); D = 340.5°, I = − 65.6° (k = 12.9, α95 = 19.4°, N = 6); D = 291.1°, I = − 78.1° (k = 27.5, α95 = 14.9°, N = 5); and D = 306.7°, I = − 82.6° (k = 38.4, α95 = 5.8°, N = 17), respectively. On the basis of a comparison with the Siberian apparent polar wander path (APWP) we estimate the age of kimberlite magmatism, assuming primary magnetizations in these rocks. The paleomagnetic ages are as follows: 428 ± 13 Ma for Eastern Udachnaya; 251 ± 30 Ma for International pipe; and 168 ± 11 Ma for Obnazhennaya pipe. The Western Udachnaya pipe was remagnetized and no clear paleomagnetic age could be determined. The ages of magmatic activity span the Early Silurian to Middle Late Jurassic. Early Silurian magmatism could be associated with the formation of the Viluy rift. Middle to Late Jurassic magmatic activity is most likely related to subduction related to the accretion of surrounding terranes to Siberia. 相似文献
6.
40Ar/39Ar analyses have been made on phlogopite-bearing peridotite nodules from Bultfontein and phlogopite nodules from Du Toitspan, Kimberley area, South Africa. Neither definite plateau nor isochron age could be obtained due to the occurrence of an excess40Ar in phlogopite. However, the extrusion age of a phlogopite nodule from Du Toitspan has been estimated to be about 86 m.y. from the combination of the youngest40Ar/39Ar age in the intermediate temperature fraction with Rb/Sr age data reported for this area.Excess40Ar correlates with K-derived39Ar in some phlogopites suggesting that it is trapped in K- or K-similar sites and has been incorporated during phlogopite formation.The occurrence of large amounts of excess40Ar in phlogopite suggests that it was not formed at a shallow depth. 相似文献
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Houjun Mei Chunjing Tang Sunrong Li Yongming Li Xingchun Zhang Dengrong Lu Lianchang Zhang 《中国科学D辑(英文版)》1998,41(1):54-92
It is known that the lamproites occur in the southeastern Guizhou Province and in the Dahong Mountains area, Hubei Province; and many para-lamproite occurrences spread in the west half part of the South China landmass. The para-larnproite diatremes in Ningxiang County, Hunan Province, contain a few of fine grains of diamond. Parts of the kimberlite pipes and dykes in Mengyin County, Shandong Province, consist locally of basic kimberlite; and the kimberlite dykes in the Maping kimberlite area, Zhenyuan County, Guizhou Province, consist of basic kimberlite principally. Although the diamondiferous kimberlites and lamproites occur always in the cratons within continental plate, both the potassium-rich ultramafic rocks display the geochemical features of the magmatism of post-collision in orogenic belt. Both the kimberlite and lamproite magmas may originate from the local parts of the mantle transition zone, where the mantle contains the matter of ancestor slab of ancient subduction zone. And, both the K-rich ultramafic magmas generated in an active mantle plume, which came from the boundary between the core and the lower mantle. The basic kimberlite magma may be more capable of preserving the crystals of the diamond type 11. 相似文献
10.
赣南区域地质构造特征与地震分布关系研究 总被引:2,自引:0,他引:2
利用赣南地区的基础地质资料,分析了该区域在大受力背景下的应力调整方式。赣南新构造运动特征明显。NNE向,NW向及EW向构造体系受近东西向应力场作用在局部形成应力集中,并在多组构造交接复合处释放;新构造在对老构造的改造复合过程中,构造带上岩石常处于强烈破碎状态。积聚不了大的能量。因而不太可能发生较大(Ms≥6.0级)地震。 相似文献
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Victor A. Piedrahita Roberto S. Molina-Garza Gloria M. Sierra José F. Duque-Trujillo 《Studia Geophysica et Geodaetica》2017,61(4):772-800
Mio-Pliocene hypabyssal rocks of the Combia event in the Amagá basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α95 = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α95 = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amagá basin may be tentatively explained by the obduction of the Cañas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system. 相似文献
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This paper considers the focal distribution of subcrustal earthquakes and the dynamic features of seismic wave propagation under some volcanic areas of Kamchatka. Under active volcanoes there are aseismic areas of anomalous attenuation of seismic waves. These features suggest the presence of low viscosity zones under volcanoes, in which there are no concentration of sufficient stresses to generate earthquakes. These zones extend down to the focal layer. 相似文献
15.
We report two examples from the south of the French Alps, showing that radon emanation monitored by alpha-sensitive film may be used to locate certain discrete structural features revealed in data collected by remote sensing from a satellite. The variations observed in our data, over a period of several months, are in accordance with atmospheric changes and might correlate with local seismic activity when the detectors are located directly above structural fractures and the magnitude of the seismic event is greater than 2. 相似文献
16.
AbstractKnowledge of the relationship between rainfall intensity and kinetic energy and its variations in time and space is important for the prediction of erosion hazard. Kinetic energy and erosivity are also strongly controlled by raindrop size. However, studies on raindrop measurement and different practical techniques have been rarely documented. The current study therefore aimed to apply existing raindrop-size measurement techniques—the photographic, flour-pellet and stain methods, as well as an innovative flour-stain method—and to evaluate their applicability at several intensities in Mazandaran Province, Iran. The distribution of raindrop size obtained by the different methods was recorded and compared with those obtained through applying a high-speed imaging technique. All the analyses were made with the help of a SPSS software package. The results showed that the raindrop diameters ranged from 0.2 to 5.16 mm at different rainfall intensities. Statistical comparison of the methods using the Duncan test showed that the flour-pellet method presented similar results to the photographic technique; it was concluded that this can be used as a practical and inexpensive method to estimate a wide range of raindrop sizes.Editor Z.W. KundzewiczCitation Sadeghi, S.H., Abdollahi, Z., and Khaledi Darvishan, A., 2013. Experimental comparison of some techniques for estimating natural raindrop size distribution on the south coast of the Caspian Sea, Iran. Hydrological Sciences Journal, 58 (6), 1374–1382. 相似文献
17.
The stress release model and results from modelling features of some seismic regions in China 总被引:1,自引:0,他引:1
ThestresreleasemodelandresultsfrommodelingfeaturesofsomeseismicregionsinChinaJIANCANGZHUANG(庄建仓)LIMA(马丽)CentreforAnalysisan... 相似文献
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Are some of the deep crustal conductive features found in SW Iberia caused by graphite? 总被引:1,自引:0,他引:1
Fernando A. Monteiro Santos António Mateus Jaume Pous 《Earth and Planetary Science Letters》2002,201(2):353-367
Recent results obtained from two-dimensional inversion of magnetotelluric (MT) data acquired in SW Iberia reveal high-conductive features at the middle-lower levels of the crust. The top of these anomalous structures correlates very well with the depth (10-13 km) of an important seismic interface that has been interpreted as a regional detachment horizon. Very shallow and relatively narrow conductors in the Ossa Morena Zone appear to correspond to small-scale fluid-deposited graphite systems in the preorogenic metasedimentary sequences. Some of the midcrustal conductors can be ascribed to graphite-bearing thrust zones, the formation of graphite with variable crystallinity being a consequence of Variscan shearing processes. Deep-seated conductors are tentatively interpreted as a result of relatively continuous, highly crystalline grain-boundary graphite films presumably preserved in basement, granulite(?) rocks. Assuming that graphite occurs as interconnected films, calculations indicate that a fraction of 0.006-0.02% of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of MT models. The role of graphite on the thermal behaviour of the crust is also discussed. The results show that low contents of graphite do not significantly change the thermal behaviour of earth materials. 相似文献
20.
An explosive eruption of Young Shiveluch Volcano occurred on September 22, 2005, discharging a pyroclastic flow about 20 km long in the Baidarnaya River valley and an ashfall in the area of the Northern cluster of volcanoes. 相似文献