He and Ne isotopes in oceanic crust: implications for noble gas recycling in the mantle     

Manuel Moreira  Jerzy Blusztajn  Joshua Curtice  Stan Hart  Henry Dick  Mark D. Kurz 《Earth and Planetary Science Letters》2003,216(4):635-643

In an attempt to determine the helium and neon isotopic composition of the lower oceanic crust, we report new noble gas measurements on 11 million year old gabbros from Ocean Drilling Program site 735B in the Indian Ocean. The nine whole rock samples analyzed came from 20 to 500 m depth below the seafloor. Helium contents vary from 3.3×10⁻¹⁰ to 2.5×10⁻⁷ ccSTP/g by crushing and from 5.4×10⁻⁸ to 2.4×10⁻⁷ ccSTP/g by melting. ³He/⁴He ratios vary between 2.2 and 8.6 Ra by crushing and between 2.9 and 8.2 by melting. The highest R/Ra ratios are similar to the mean mid-ocean ridge basalt (MORB) ratio of 8±1. The lower values are attributed to radiogenic helium from in situ α-particle production during uranium and thorium decay. Neon isotopic ratios are similar to atmospheric ratios, reflecting a significant seawater circulation in the upper 500 m of exposed crust at this site. MORB-like neon, with elevated ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios, was found in some high temperature steps of heating experiments, but with very small anomalies compared to air. These first results from the lower oceanic crust indicate that subducted lower oceanic crust has an atmospheric ²⁰Ne/²²Ne ratio. Most of this neon must be removed during the subduction process, if the ocean crust is to be recirculated in the upper mantle, otherwise this atmospheric neon will overwhelm the upper mantle neon budget. Similarly, the high (U+Th)/³He ratio of these crustal gabbros will generate very radiogenic ⁴He/³He ratios on a 100 Ma time scale, so lower oceanic crust cannot be recycled into either MORB or oceanic island basalt without some form of processing.  相似文献   

Petrology and Geochemistry of the Volcanic Rocks Dredged from the Geophysicist Seamount in the Kurile Basin: Evidence for the Existence of Thinned Continental Crust     

Igor A. Tararin  Eugeniy P. Lelikov  Reinhard Werner 《Gondwana Research》2003,6(4):757-765

Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of ⁸⁷Sr/⁸⁶Sr ratios (0.70287-0.70652), varying ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin.  相似文献   

Pn arrivals and lateral variations of Moho geometry beneath the Kaapvaal craton     

M. T. O. G. Kwadiba  C. Wright  E. M. Kgaswane  R. E. Simon  T. K. Nguuri   《Lithos》2003,71(2-4):393-411

Pn arrivals from mining-induced earthquakes on the edge of the Witwatersrand basin show that the P wavespeeds in the uppermost mantle are almost constant throughout most of the Kaapvaal craton. The presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the Kaapvaal broad-band network using Pn times that has been compared with results from receiver functions. One thousand three hundred thirty-seven Pn arrivals were used to derive crustal thicknesses at 46 stations on the Kaapvaal craton. The average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well-constrained thicknesses were 50.52±0.88 km and 38.07±0.85 km, respectively. In contrast, the corresponding average thicknesses determined from receiver functions were 43.58±0.57 km and 37.58±0.70 km, respectively. The systematically lower values for receiver functions in the northern part of the Kaapvaal craton that was affected by the Bushveld magmatism at 2.05 Ga, suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust. The Pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8.40 km/s at azimuths of about 15° and 217° in the northern and southern regions of the craton, respectively, and minimum wavespeeds of about 8.25 km/s.  相似文献   

洋脊和年轻洋壳俯冲与埃达克岩生成的热模拟          下载免费PDF全文

张健  石耀霖  吴春明 《地震地质》2003,25(4):617-624

新生代以来 ,环太平洋周边分布的埃达克岩 (Adakite)主要与年轻洋壳俯冲时在 70～ 90km深处的部分熔融有关。利用数值方法 ,模拟了洋壳俯冲的热演化过程并讨论了脱水、熔融对埃达克岩浆活动的影响。结果表明 :仅在活动海岭俯冲前后约 10Ma内 ,年轻的、热的俯冲海洋板片在 75～85km深度范围内 ,温度升高至 82 5～ 10 0 0℃脱水 ,导致年轻洋壳中角闪岩部分熔融 ,形成埃达克岩(Adakite)。而一般洋壳俯冲在 10 0km以下深度才脱水 ,由于脱水区压力较高洋壳自身不能熔融 ,水进入上覆地幔楔状体导致部分熔融 ,形成安山岩 (Andesite  相似文献   

长白山天池火山区上地壳三维速度层析成像   总被引：6，自引：4，他引：6       下载免费PDF全文

赵金仁  张先康  杨卓欣  张成科  王夫运  张建狮  刘宝峰  段玉玲  王帅军 《地球物理学报》2003,46(6):796-802

利用三维地震透射观测系统所获得的莫霍界面反射波对长白山天池火山区下方上地壳的三维速度细结构进行了反演计算，得到了研究区下方地壳内不同深度的P、S波速度层析成像和VP/VS扰动分布图像，结果表明，天池火山区及其周边速度结构所呈现出的非均匀性变化与该区的地质构造和岩浆系统的存在有着密切的关系，异常变化比较明显的低P波速度异常体分布和高VP/VS扰动体意味这些介质体处在“软化高温”的状态，这与该地区广泛分布的地热异常和低Q值结构的研究结果是一致的.  相似文献   

Wide-angle seismic velocities in heterogeneous crust   总被引：5，自引：0，他引：5  

John Brittan  Mike Warner 《Geophysical Journal International》1997,129(2):269-280

Seismic velocities measured by wide-angle surveys are commonly used to constrain material composition in the deep crust. Therefore, it is important to understand how these velocities are affected by the presence of multiscale heterogeneities. The effects may be characterised by the scale of the heterogeneity relative to the dominant seismic wavelength (λ); what is clear is that heterogeneities of all scales and strengths bias wide-angle velocities to some degree. Waveform modelling was used to investigate the apparent wide-angle P -wave velocities of different heterogeneous lower crusts. A constant composition (50 per cent felsic and 50 per cent ultramafic) was formed into a variety of 1- and 2-D heterogeneous arrangements and the resulting wide-angle seismic velocity was estimated. Elastic, 1-D models produced the largest velocity shift relative to the true average velocity of the medium (which is the velocity of an isotropic mixture of the two components). Thick (width > λ) horizontal layers, as a result of Fermat's Principle, provided the largest increase in velocity; thin (width ≪λ) vertical layers produced the largest decrease in velocity. Acoustic 2-D algorithms were shown to be inadequate for modelling the kinematics of waves in bodies with multiscale heterogeneities. Elastic, 2-D modelling found velocity shifts (both positive and negative) that were of a smaller magnitude than those produced by 1-D models. The key to the magnitude of the velocity shift appears to be the connectivity of the fast (and/or slow) components. Thus, the models with the highest apparent levels of connectivity between the fast phases, the 1-D layers, produced the highest-magnitude velocity shifts. To understand the relationship between measured seismic velocities and petrology in the deep crust it is clear that high-resolution structural information (which describes such connectivity) must be included in any modelling.  相似文献   

Shear-Wave Velocity Models From Rayleigh-Wave Dispersion In the Broader Aegean Area   总被引：2，自引：0，他引：2  

Ioannis S. Kalogeras  Paul W. Burton 《Geophysical Journal International》1996,125(3):679-695

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Reinterpretation of the RRISP-77 Iceland shear-wave profiles   总被引：1，自引：0，他引：1  

William Menke  Bryndís Brandsdóttir  Páll Einarsson  Ingi Th. Bjarnason 《Geophysical Journal International》1996,126(1):166-172

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扬子克拉通北缘中、新元古代洋壳俯冲及壳幔再循环作用的同位素地球化学证据   总被引：9，自引：0，他引：9  

凌文黎  张本仁  张宏飞  骆庭川 《地球科学》1996,(3)

报道和总结了近期陕西汉中碑坝和西乡地区重要地质体系统同位素年代研究成果，在重新厘定的年代学基础上，就扬子克拉通北缘同位素特征及其地质意义进行剖析，依据该区岩浆作用的上地幔源区性质及演化规律，提出了中、新元古代扬子克拉通北缘存在与现代板块运动类似的洋壳俯冲和壳幔再循环作用的同位素地球化学证据  相似文献   

湘南地区稀土矿产资源开发经济意义探讨     

张传福 《湖南地质》1996,15(2):85-88

湘南地区风化壳型稀土资源丰富，对其开发的采、选工艺和流程简单，投入资金较少，产出颇丰、经济、社会效益明显，是湘南盆困山区脱贫致富的重要途径。本文就开发湘南稀土矿的成本、效益等进行了初步测算，可供参考。  相似文献