地幔的不均一性及地幔流体的形成机制

地幔的不均一性及地幔流体的形成机制朱永峰（北京大学地质系，北京１００８７５）关键词地幔不均一性，地幔流体，网络结构各种化学元素在地壳中的分布极不均一。因此，成矿带（群）、矿田（群）的分布在时代上和空间上也表现出极大的不均一性。原因是地球体系发展演化的...     

地幔地球化学研究进展:微量元素和同位素特征

系统介绍了近年来地幔同位素地球化学的研究进展,概述了Ｓｒ－Ｎｄ－Ｈｆ－Ｐｂ、Ｏｓ、Ｈｅ－Ａｒ和Ｎｅ同位素在的岩石示踪和成因鉴别上所取得的成果,指出同位素在地幔岩石研究的重要作用,简要总结了全球及区域地幔成分的主要研究手段和某些全球均一化比值的意义;并提出了分析技术的落后和基础理论的停滞是影响同位素和微量元素在地幔研究中应用和发展的主要因素。     

长白山火山1000年前火山泥流堆积及其灾害

长白山火山１０００年前大喷发,火山泥流堆积物沿松花江中上游分布。距火山较近的是火山泥流的岩屑流堆积,远离火山变为火山泥流的超高密度流堆积。探讨了火山泥流的成因,并指出长白山火山一旦再次爆发,火山泥流将是主要的火山灾害,沿松花江中上游可能造成巨大破坏,并可能危及鸭绿江、图们江中、上游。     

地幔不均一性与地幔循环的多维表征和原因思考

地幔早先经核- 幔- 壳分异形成,后受不同尺度对流和循环的影响,因而具有不均一性特征。近三十年来,地幔化学通过研究大洋玄武岩发现了多样地幔端元和非放射性同位素证据并证明了地幔不均一性,逐渐建全了地幔地球化学体系。然而,地幔不均一性如何对应于时空尺度的地幔循环,以及地球演化如何影响地幔不均一性等,仍不清楚。此外,地球物理研究显示,岩石圈厚度差异、中下地幔的波速异常体以及俯冲板片形态的观测为纵横向对流系统提供了空间不均一性证据支持。联合地球化学和地球物理手段对研究地幔不均一性至关重要,用好透视地幔成分与结构的“双目镜”已成为共识。本文从地幔不均一性结合地球化学场、地球物理的不同表现形式,以及现今及历史时期的洋陆格局的对比,多维度联系地幔循环和演化,思考了超大陆旋回与地幔不均一化的内在逻辑。强调了从全球演化角度看地幔不均一性的重要性和提出多手段联合建立地幔循环驱动模型的展望。     

长白山火山最新监测信息

长白山火山最新监测结果显示,自2010年5月份开始,聚龙温泉2个泉点水温同步上升了3℃;2010年长白山北坡垂直位移量打破了火山锥体每年升高4mm左右的规律:与2009年相比,火山锥体的位移在垂向上发生逆转,海拔最高的水准点在1年内突降12.72mm.本文针对上述2个最新的火山监测信息进行了报道,并对造成这种变化的可能原因进行了讨论.火山活动进入活跃时段的标志是大量火山地震、震群的发生,以及幔源岩浆气体He含量的大幅度上升.然而,现阶段长白山火山区并未出现这2种现象.因此,本文讨论的2个异常还不足以作为长白山火山已经进入新的活跃时段的证据,准确的预测应该密切关注火山地震和幔源气体含量的变化.最后通过N2-He-CO2三角图解初步研究了天池火山气体来源,结果显示:2002～2005年火山出现“初始扰动”,深部气体He含量升高,三角图解中气体向He角运移;2005年后火山转入平静状态,气体组份又开始向N2角运移.He浓度的变化特征反应了岩浆房内岩浆成分的变化过程.     

太古宙富铁苦橄岩与太古宙早期深部地幔不均一性

富铁苦橄岩是一类特殊的高镁地幔来源岩浆岩,具有高的FeO^T含量（> 14%）和MgO含量（> 12%）,并富集不相容元素和具有轻重稀土强烈分异的稀土元素配分模式。富铁苦橄岩通常与科马提岩产出于绿岩带中,或产出于大火成岩省中,因此富铁苦橄岩的形成与地幔柱活动有着密切的联系,与其他高镁地幔来源岩浆岩（科马提岩、苦橄岩）一道成为了探究地球深部地幔的最佳材料。根据太古宙科马提岩成分随时间的演化,以往的研究认为地球深部地幔中出现富集组分的时间约为2.7Ga。而最近在华北克拉通冀东地区发现的古太古代（3.45Ga）富铁苦橄岩表明地球深部地幔中铁和不相容元素的局部富集可能在3.45Ga前后便已出现。对太古宙富铁苦橄岩的进一步研究有助于我们理解地球早期各圈层相互作用的模式。

     

长白山火山灾害及其对大型工程建设的影响

长白山火山是世界著名的活火山,历史时期有过多次喷发,有再次爆发的危险.长白山火山最大的一次爆发发生在公元1199-1200年,这次大爆发的火山灰最远到达距其1 000km远的日本北部.依据这次大爆发由火山喷发空中降落堆积物、火山碎屑流和火山泥流造成的巨大火山灾害,预测了长白山火山未来爆发火山灾害的类型、强度和范围,并编制了长白山火山未来爆发火山喷发空中降落堆积物灾害预测图、火山碎屑流灾害预测图和火山泥流灾害预测图.该研究可预防和减轻火山灾害,指导核电站等大型工程选址.     

长白山火山历史上最大火山爆发火山碎屑物层序与分布

长白山火山历史时期规模最大的火山喷发发生在1199~1200年。这次大爆发分为两次普林尼（Plinian）式喷发:第一次（早期）喷发称赤峰期,第二次（晚期）喷发称园池期。赤峰期喷发模式为：普林尼式喷发柱（赤峰空落浮岩层）—火山碎屑流（长白火山碎屑流层）—火山泥流（二道白河火山泥流层）,主要由火山碎屑流诱发火山泥流;园池期火山喷发模式为：普林尼式喷发柱（园池空落浮岩火山灰层）—火山碎屑流（冰场火山碎屑流层）。两次普林尼式喷发空落火山碎屑物总量约120 km³,长白火山碎屑流层总量约8 km³,冰场火山碎屑流层总量约0.5 km³,火山泥流堆积总量约为2 km³。主要论述了这次大爆发的火山喷发碎屑堆积物的层序和分布。     

地球早期地幔化学不均一性及其成因机制

早期地球是地球科学前沿研究方向之一,涉及地-月形成、核幔分异、原始大气圈和水圈的形成等关键科学问题。地幔是硅酸盐地球的主要组成部分,也是地球上最大的化学储库,其早期演化为揭示早期地球增生、核-幔分异、壳-幔分异等重大地质事件提供重要制约。近年来,地球早期地幔不均一性逐渐被认知,本文在总结早期地幔不均一性的稀有气体同位素、钕同位素和钨同位素等证据基础上,探讨了早期地幔不均一性形成动力学机制,并指出发展高精度同位素分析技术,结合地球物理和实验岩石学,揭示核幔边界结构、核幔物质交换过程是深入研究早期地幔不均一的重要发展方向。

     

Erosion of lithospheric mantle beneath the East African Rift system: geochemical evidence from the Kivu volcanic province

This study presents new major and trace element and Sr–Nd isotopic results for a suite of Miocene–Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably <1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of ⁸⁷Sr/⁸⁶Sr, (La/Sm)_n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of ⁸⁷Sr/⁸⁶Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest ⁸⁷Sr/⁸⁶Sr (0.70314) and highest _Nd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr–Nd–Pb isotopic values among western rift lavas. Among Kivu lavas, variations in ¹⁴³Nd/¹⁴⁴Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.     

High velocity anomaly beneath the Deccan volcanic province: Evidence from seismic tomography

Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period.     

Mantle diversity beneath the Colombian Andes, Northern Volcanic Zone: Constraints from Sr and Nd isotopes

In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure–temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 °C) and MORB isotopic signature, ⁸⁷Sr/⁸⁶Sr ratio of 0.7030, and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29–35 kbar, 1250–1295 °C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature (⁸⁷Sr/⁸⁶Sr ratio of 0.7043 and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (>38 kbar, 1140–1175 °C) and OIB isotopic characteristics (⁸⁷Sr/⁸⁶Sr ratio=0.7041 and ¹⁴³Nd/¹⁴⁴Nd ratio=0.5135).     

CCSD主孔榴辉岩中金红石微量元素特征：LA-ICPMS分析及其意义

对中国大陆科学钻探（CCSD）主孔200~1005m范围内8件榴辉岩样品的金红石进行了LA-ICPMS原位微区微量元素分析,结合前人已发表的全岩和金红石分析数据,研究结果发现：在不同类型榴辉岩中,金红石的微量元素与其全岩成分具有不同的相关关系。金红石中的Nb和Ta元素含量不同程度地受控于全岩Nb和Ta含量。在高钛和低镁钛榴辉岩中,金红石的Cr与全岩Cr/TiO₂正相关;在富镁榴辉岩中,金红石的Cr含量受全岩MgO含量的控制;在高钛和富镁榴辉岩中,全岩成分明显影响着金红石的Zr含量,金红石Zr温度计可能不适用。低镁钛榴辉岩的金红石的平衡温度普遍低于榴辉岩峰期变质温度,可能是变质流体参与下的扩散作用和退变质作用所致;多数情况下,单个样品中大部分金红石颗粒的Zr含量是均匀的,金红石Zr温度计所给出的温度可能代表着退变质再平衡的温度;CCSD榴辉岩的全岩Nb/Ta比值普遍低于其中金红石的Nb/Ta比值,不支持金红石榴辉岩可能是地球上超球粒陨石Nb/Ta比值储库的观点。     

塔里木北部二叠纪长英质火山岩年代学及地球化学特征

大面积分布于塔里木盆地的二叠纪玄武岩构成了面积250000km²的大火成岩省(LIP),长英质火山岩的发现为塔里木二叠纪火山作用的研究打开了新的窗口。本文从塔北地区约5000m深的钻井中收集了4件二叠纪长英质火山岩的样品。通过对其进行锆石U-Pb同位素测试,得出其形成时代为274~282Ma,为塔里木大火成岩省晚期岩浆作用的产物。岩石具有高钾的特征K₂O+Na₂O=7.29%~8.34%,K₂O/Na₂O>1,大部分属于高钾钙碱性系列,且属于过铝质(A/CNK=1.32~1.53)。具有富集LREE和Zr、Hf、Y,亏损Sr、P、Ti、Nb、Ta等特征,微量元素分布曲线形态与地壳相近,具有右倾的稀土元素配分曲线,且显示出一定的负铕异常。通过Sr-Nd-Pb同位素的分析得出其源区有大量地壳物质,这与其具有较高的Th、U含量和与地壳平均值相似的Nb/La、Nb/U、Th/Ta相一致。综合年代学、地球化学特征及构造环境的判断,认为塔北地区二叠纪长英质火山岩形成于地幔柱活动背景下的地壳物质的部分熔融。     

微陆块属性及过程:我国东北地区岩石圈地幔性质差异之根本

我国东北地区位于中亚造山带的东段,由多个微陆块俯冲拼合所组成,是研究壳幔相互作用的理想地区。但该区岩石圈地幔性质,包括壳幔属性是否解耦、是否有古老地幔残留、导致地幔性质差异的机制是什么,都还不清楚。东北地区多地出露新生代玄武岩并有橄榄岩捕虏体,如松嫩-张广才岭地块的阿巴嘎、五大连池和蛟河,兴安地块的哈拉哈、诺敏和科洛,以及与华北拼合褶皱带处的双辽、伊通和汪清等。其中阿巴嘎、哈拉哈、诺敏、科洛和蛟河的橄榄岩含有Mg#≥91.5的橄榄石,最大Re亏损年龄是2.1~1.9Ga,富集Sr、Nd同位素组成且变化范围大,反映交代作用强烈,这些特征与大陆岩石圈地幔有明显的相似性;而拼合带处的双辽、伊通、汪清及松嫩-张广才岭地块中的五大连池地幔包体橄榄石,Re亏损年龄总体为中元古代,Sr、Nd同位素组成变化范围小,交代作用弱,与大洋岩石圈地幔亲缘性更明显。这些差别总体反映出地块内部与地块边缘相比,有岩石圈地幔形成时代相对较老、亏损程度较高、地幔交代作用较强的特点。但也有例外的情况,如五大连池与科洛相比,更远离贺根山-黑河断裂带,但地幔属性更饱满,反映地块内部深部作用过程可能更强烈。因此,我们认为东北岩石圈地幔性质差异的根本原因与微陆块初始属性和后来的俯冲拼合及软流圈-岩石圈相互作用等有关。     

Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P–T- composition lithospheric cross-sections in the region for the depth range of 35–80 km.Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of ^143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle.Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P–T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.     

粤北下庄铀矿田黄陂-张光营辉绿岩脉的成因：元素地球化学和Nd-Sr-Pb-O同位素证据

黄陂-张光营辉绿岩脉位于粤北贵东复式花岗岩岩体东部的下庄铀矿田内,形成于142．6±3．0Ma。岩石化学呈现拉斑玄武宕系列的特征,富集大离子亲石元素；在蛛网图上,Th表现出正异常,而Ba呈负异常,P轻度亏损,无Nb,Ta,Zr,Hf异常；轻、重稀土之间分馏不明显,基本无Eu异常。微量元素和稀土元素特征与板内玄武岩的相似；铅同位素具有典型的Dupal异常铅特征,ε_(Nd)(t)值为3．2～4．7,(~(87)Sr/~(86)Sr),为0．70446～0．70628；在Pb-Pb,Nd-Pd,Nd-Sr和Sr-Pb相关图解上,数据点都位于亏损地幔和富集地幔Ⅱ之间。地球化学特征表明,黄陂-张光营辉绿岩脉是在拉张的构造环境下由介于亏损地幔和富集地幔Ⅱ之间的、相对富集的地幔源区部分熔融形成的,而这种富集地幔可能是由俯冲带流体交代形成的。     

鲁西青山组火山岩形成的构造背景及其成因探讨:主元素和微量元素证据

研究区内火山岩从基性—中性—中酸性都有出露，包括橄榄玄武岩、安山岩和英安岩，且都属于非碱性系列。通过对主元素和微量元素的研究，认为本区火山岩为滞后型弧(陆缘弧)火山作用的产物。源区由于存在大量的因俯冲作用进入地幔的陆壳物质以及流体的交代作用，从而出现富含金云母和不相容元素的交代富集型地幔源，并具有壳源的元素组成特征。火山岩的形成是富集地幔部分熔融的结果，但在成岩过程中可能存在单斜辉石、斜长石、橄榄石和Ti—Fe氧化物等矿物的分离结晶作用，以及橄榄石的堆晶作用。     

Sr and O isotope constraints on source and crustal contamination in the high-K calc-alkaline and shoshonitic neogene volcanic rocks of SE Spain

The Neogene volcanic province of SE Spain (NVPS) is characterized by calc-alkaline (CA), high-K calc-alkaline (KCA), shoshonitic (SH), ultrapotassic (UP), and alkaline basaltic (AB) volcanic series. All these series, except the AB, have high LILE/LREE, LILE/HFSE and B/Be ratios and high but variable Sr, Pb and O isotope compositions. The KCA and SH lavas contain metapelitic xenoliths whose mineralogical and chemical composition are typical of anatectic restites. The geochemical characteristics of CA, KCA, SH and UP series suggest that they originated from the lithospheric mantle, previously contaminated by fluids derived from pelagic sediments. Additionally, the presence of restite xenoliths in the KCA and SH lavas indicates some sort of interaction between the mantle-derived magmas and the continental crust. Trace element and isotope modeling for the KCA and SH lavas and the restites, point towards the existence of two mixing stages. During the first stage, the lithospheric mantle was contaminated by 1–5% of fluids derived from pelagic sediments, which produced a fertile source heterogeneously enriched in incompatible elements (particularly LILE and LREE), as well as in ⁸⁷Sr/⁸⁶Sr, without significant modifications of the δ¹⁸O values. In the second stage, the primary melts derived from this metasomatized mantle, which inherited the enrichment in LILE, LREE and ⁸⁷Sr/⁸⁶Sr, interacted with crustal liquids from the Betic Paleozoic basement during their ascent towards the surface. This mixing process caused an increase in δ¹⁸O values and, to a lesser extent, in ⁸⁷Sr/⁸⁶Sr ratios. However, the incompatible trace elements abundances only change slightly, even for high mixing rates, due to their similar concentrations in both components. We suggest the following geodynamic scenario to account for the global evolution of this area: (1) a Late Cretaceous to Oligocene subduction scheme during which mantle metasomatism took place, shortly followed by Upper Oligocene to Lower Miocene continental collision, and (2) a Middle to Upper Miocene extensional event triggering partial melting of the previously metasomatized mantle and the extrusion of the CA and associated magmas.