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1.
地幔中铂族元素丰度是基于地球形成理论所得计算值的两三百倍,无论是地幔中还是球粒陨石中,铂族元素之间的比值是相近的;对于这一现象有很多解释,其中被普遍接受的是:地核形成后(核幔分异完成后),大约占现在地球质量0.5%-1%的球粒陨石物质加入地幔,即可造成现在地幔中的铂族元素在丰度与比值方面的特征,这就是后增薄层模型(Late-veneer model)。但是,这一模型受到很多地幔样品铂族元素具非球粒陨石比值和高压下Co、Ni亲铁性测定结果的挑战。最近的地幔橄榄岩各相铂族元素精确测定、高压下Pt、Pd金属相与硅酸盐分配系数的测定,以及Re-Os同位素的研究都支持后增薄层模型。  相似文献   

2.
亲铜(亲铁)元素在行星增生演化、核幔分异、地幔岩浆过程、壳幔相互作用以及金属矿床成因等领域具有举足轻重的作用。本文从亲铜元素的地球化学性质出发,介绍了"高维度思维"的亲铜元素含量比值及其在地球科学领域的初步应用:①获得同一份样品中不同亲铜元素含量可在一定程度上降低样品的不均一性(块金效应)对含量比值的影响;②Cu/Ag值可以约束不同高温岩浆过程中硫化物固液状态和亲铜元素地球化学性质,进而认识地幔、洋壳和大陆地壳间的联系;③具有不同分配系数的亲铜元素含量的比值可以鉴别岩浆硫化物饱和史,比如通过亲铜元素的分异约束火星陨石母岩浆的硫化物不饱和演化历史;④依据In-Cd-Zn在硅酸盐地球的含量以及它们的相对亲铜亲铁性质,地球主体增生物质已经消失,不能由陨石代表。  相似文献   

3.
榆树沟变质基性-超基性岩带出露于塔里木板块与哈萨克斯坦板块之间的南天山北缘,主要由变质橄榄岩和变质基性岩组成。变质橄榄岩富相容元素Cr、Co和Ni,贫不相容元素,太离子亲石元素Ba、Rb和Sr含量较低,与世界典型蛇绿岩相似,代表了地幔残留物特征。REE分布模式为LREE亏损型,REE含量小于或等于2.5倍球粒陨石,类似于阿尔卑斯型变质橄榄岩,显示榆树沟的变质橄榄岩是原始地幔岩部分熔融萃取出玄武岩后的残留物。变质基性岩绝大部分为LREE亏损型,类似于N-MORB。所有样品均以富集Nb和Ta、高场强元素不分异,以及微量元素含量低为特征,批示岩浆源区总体上类似于MORB,Nb、Ta富集可能与OIB型源区有关,Nd、Sr同位素特征也显示其具有OIB型源区特征。综合分析认为,榆对沟变质基性岩石的岩浆可能经历了两个阶段的演化过程,即上地幔底部或下地幔顶部的OIB型原始岩浆形成阶段和软流圈地幔亏损阶段。  相似文献   

4.
科学地按不同类型考察地幔岩的稀土元素丰度是深部地球化学领域重要的基础研究。由于稀土元素作为微量示踪元素包含着夭体演化和地球演化的大量信息,所以地幔岩稀土地球化学在揭示核-幔分离和壳-幔分异规律方面具有重要的意义。普遍估计,原始地幔的稀土元素丰度应是普通球粒陨石简单的倍数关系。因此,实际考察各种类型地幔岩的稀土元素含量和相对丰度,可能加深我们对不同构造区域地幔地球化学特征及其演化的认识。例如地幔微  相似文献   

5.
广东普宁麒麟幔源包体及其寄主玄武岩稀土元素的研究   总被引:1,自引:0,他引:1  
球粒陨石标准化的稀土元素分布型式表明二辉橄榄岩和方辉橄榄岩具有平坦的分布型式,二辉橄榄岩分布在世界二辉橄榄分布区域内,方辉橄榄岩HREE含量较低;辉石岩的分布型式与印度洋中脊玄武岩公布型式相近,玄武岩的分布型式明显亏损重稀土元素,它与夏威夷碱性橄榄玄武岩分布型式相近以.稀土元素分配系数的计算表明,二辉椰榄岩和玄武岩岩浆之间、方辉橄榄岩阳辉石岩岩浆之间处于平衡.辉石岩岩浆和玄武岩岩浆是起源于上地慢的原始岩浆.这与辉石岩和玄武岩主要元素的化学成分一致.稀土元素的研究暗示了该区土地馒曾发生过稀土元素的分异.在辉石岩浆形成之前,在上地幔的较高部位即玄武岩岩浆的源区轻稀土元素微具富集,而在上地幔的梢下部位即辉石岩岩浆形成的源区轻度亏损了轻稀土元素  相似文献   

6.
陈仁旭  郑永飞 《地球科学》2019,44(12):4095-4101
俯冲带是地壳与地幔之间物质交换的主要场所.前人对大洋俯冲带壳幔相互作用进行了大量研究,但是对俯冲带壳幔相互作用的物理化学过程和机理仍缺乏明确认识.在大陆俯冲带出露有造山带橄榄岩,它们来自俯冲板片之上的地幔楔,是解决这个问题的理想样品.通过对大别-苏鲁和柴北缘造山带橄榄岩进行系统的岩石学和地球化学研究,发现地幔楔橄榄岩由于俯冲地壳的交代作用而含有新生锆石和残留锆石,它们能为地壳交代作用时间、交代介质来源、性质和组成提供制约.地幔楔橄榄岩在大陆碰撞过程的不同阶段受到了俯冲大陆地壳衍生的多期不同性质流体的交代作用.地幔楔橄榄岩还受到了陆壳俯冲之前古俯冲洋壳衍生流体的交代作用.深俯冲陆壳衍生熔体与橄榄岩反应形成的石榴辉石岩具有高的水含量,能提供高水含量的地幔源区.   相似文献   

7.
陈意  苏斌  郭顺 《地球科学》2019,44(12):4086-4094
俯冲地壳衍生流体交代地幔楔,是产生俯冲带岩浆作用的重要机制.但是,目前人们对俯冲大陆物质改造地幔楔的岩石学过程和机理仍缺乏深入认识,造山带橄榄岩是解析这一问题的直接样品.通过对大别-苏鲁造山带橄榄岩进行系统的矿物学、岩石学和地球化学研究,发现橄榄石Ni/Co比值可有效区分幔源和壳源造山带橄榄岩,揭示幔源造山带橄榄岩起源于华北岩石圈地幔.苏鲁李家屯纯橄岩在进入俯冲带之前就已在地幔内部经历了碳酸盐熔体交代.大别毛屋和苏鲁蒋庄橄榄岩及其交代脉体记录了约170~200 km深度的俯冲带壳幔相互作用过程.深俯冲陆壳释放的富Si-Al质熔体可不同程度地改造地幔楔底部,形成富石榴石和富辉石的交代岩,并引发强烈的Os同位素分馏效应.该过程不仅改变地幔楔岩性和化学组成,还能够改变交代介质成分,为俯冲带各类深部地幔岩浆提供源区物质.因此,大陆深俯冲是导致上地幔不均一的重要途径.   相似文献   

8.
支霞臣  秦协 《地球科学》2006,31(1):23-30
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.  相似文献   

9.
佘宇伟  朱祥坤  何源  万洪清 《中国地质》2019,46(6):1372-1383
日喀则蛇绿岩位于雅鲁藏布构造带中段,其成因和构造环境仍存在较大争议。日喀则蛇绿岩下部为蛇纹石化地幔橄榄岩,壳幔过渡带缺失超镁铁质堆晶岩。少量辉长岩脉呈块状或韵律结构并侵入到地幔橄榄岩和辉绿岩中。辉绿岩呈席状岩床侵入到地幔橄榄岩之上,且少量辉绿岩脉侵入到下覆的地幔橄榄岩中。通过野外关系和地球化学研究,日喀则辉长岩可能并不是洋壳中岩浆房原位结晶堆积而成,而是深部位置岩浆囊经过不同程度分异演化形成富晶粥岩浆并向上侵入的结果。而席状辉绿岩床则是基性岩浆沿着构造薄弱面顺层侵入的结果。拆离断层可能导致了岩石圈地幔抬升和剥露,进而引起下覆软流圈地幔减压熔融和岩浆上侵。日喀则辉长-辉绿岩形成于慢速扩张脊较小规模的岩浆供应和不连续的岩浆侵入。  相似文献   

10.
橄榄岩-熔体的相互作用:岩石圈地幔组成转变的重要方式   总被引:31,自引:2,他引:31  
张宏福 《地学前缘》2006,13(2):65-75
橄榄岩-熔体/岩浆的相互作用常被用来解释蛇绿岩套橄榄岩、造山带橄榄岩、超镁铁质侵入杂岩体、地幔橄榄岩捕虏体中某些具有不平衡结构和矿物组成的岩石的形成过程。橄榄岩-熔体的反应主要有两种方式,即消耗橄榄石(和单斜辉石)生成斜方辉石或消耗斜方辉石生成橄榄石(和单斜辉石)。反应的结果不仅造成矿物百分含量的变化,而且造成矿物组成的变化;后者更重要但未引起足够的重视。华北东部中生代玄武质岩石中具有环带状结构的橄榄石和辉石捕虏晶,特别是具有环带状结构的地幔橄榄岩捕虏体的发现,暗示这种橄榄岩-熔体的相互作用在华北东南部中生代岩石圈地幔中很可能普遍存在,为岩石圈地幔组成转变和快速富集的重要方式。这是全球首例由橄榄岩-熔体相互反应造成的岩石圈地幔大规模的组成变化。反应熔体来源途径主要有地壳来源和软流圈地幔来源。来源不同的熔体与橄榄岩的反应造成的组成变化完全不同。  相似文献   

11.
铂族元素地球化学研究评述   总被引:17,自引:2,他引:15  
随着分析技术的发展和数据的积累,人们逐渐认识到PGE在地球化学研究方面具有潜在的应用价值。但地幔中PGE的存在形式目前尚不清楚,在许多方面还有争议。文中通过大量的实例综述了近年来PGE的分异机制和其在上地幔分布不均一性方面取得的进展以及存在的问题,结果表明:除Au外,蚀变作用并不影响PGE的分异;PGE主要以硫化物或合金的形式赋存于地幔岩石中,其分布不均匀,单一地依靠PGE与MgO,Cr,Ni的相关性来探讨部分熔融、分离结晶过程中橄榄石、尖晶石、铬铁矿对PGE分异的影响是不全面的,必须考虑硫化物的作用;地幔岩石具有包裹体和粒间两种不同PGE分配模式的硫化物。地幔源区或板内携带PGE流体交代以粒间硫化物为主的地幔岩石。使其PGE发生分异;不管是核幔分离后外核物质的返回,还是单一硫化物的作用都不能完全否定陨石撞击的地球增生假说。在大的区域,上地幔PGE的分布是均一的,但在一定范围内由于不同的大地构造背景,其PGE的分布显示不均一性。  相似文献   

12.
PLATINUM-GROUP ELEMENTS MINERALIZATION IN THE OPHIOLITES OF INDUS SUTURE ZONE, EASTERN LADAKH,THE HIMALAYA  相似文献   

13.
贵金属配分模式及地质应用   总被引:2,自引:0,他引:2  
张生  李统锦 《世界地质》1996,15(2):39-45
通过与REE模式对比,介绍了贵金属配分模式的构筑方法和组成参数;并对包括地幔岩石、科马提岩和玄武岩在内的各种基性、超基性岩石的特性PGE+Au配分模式和贵金属分馏机制进行了综述。  相似文献   

14.
Osmium, Ru, Ir, Pt, Pd and Re abundances and 187Os/188Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al2O3. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al2O3 contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the 187Os/188Os ratio (a proxy for Re/Os) show positive correlations with Al2O3, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high 187Os/188Os, and scatter off the correlations defined by incompatible HSE and Al2O3. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/IrPUM of 1.12 ± 0.09, Pt/IrPUM = 2.21 ± 0.21, Re/OsPUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/IrPUM = 2.03 ± 0.12, Pd/IrPUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.  相似文献   

15.
An attempt has been made to estimate the chemical composition of the earth's primitive mantle by a critical evaluation of data derived from ultramafic mantle samples and partial melting model calculations for mafic and ultramafic magmas of various ages.Compatible (Al, Ca, Si, Mg, Fe) and moderately incompatible (Ti, Zr, heavy and middle rare earth) elements in basaltic magma sources have not changed significantly since the early Archaean (~3.5 Byr). Estimated abundances for refractory lithophile elements (such as Al, Ca, Ti, Zr, Y, Se, REE etc.) in the primitive mantle are about 2.0 times ordinary chondrites (~ 1.1 times Cl chondrites relative to Mg). Highly incompatible volatile elements (K, Rb, Cs, Tl, Pb etc.) are depleted in the mantle throughout geological time. Abundances of Fe, Ni and Co are obtained on the basis of values for ultramafic nodules and model calculations using komatiites of various ages. The results show little (? 20%?) dispersion and there is no obvious secular variation since 3.5 Byr. Noble metals show similar effects. These data permit constraints to be placed on the timing of core formation.The estimated elemental abundances for the primitive mantle are normalized to Cl chondrites relative to Mg and plotted against the solar condensation temperature at 10?4 atm. Above 700 K there are two parallel trends which are defined by lithophile elements (Al, Ca, REE, Ti, Mg, Si, Cr, Mn, Na, K, Rb, F, Zn etc.) and siderophile elements (W, Ni, Co, P, As, Ag, Sb and Ge) respectively. The depletion factor for the siderophile trend relative to the lithophile trend is about 0.085. Within each trend there is a continuous depletion towards lower temperature. A third trend is defined by noble metals (Ir, Os, Re, Pd, Pt and Au) with a depletion factor of about 0.003 relative to Cl chondrites. These trends are interpreted in terms of core-mantle differentiation and volatility-controlled processes operating before and during earth accretion.  相似文献   

16.
Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO2 of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the 53Mn-53Cr and 26Al-26Mg systems.  相似文献   

17.
西藏达巴—休古嘎布绿岩带铂族元素的分布特征   总被引:1,自引:0,他引:1       下载免费PDF全文
区内各岩体的地幔橄榄岩中PGE含量及分布型式有所差异.相对于原始地幔,拉昂错和东坡岩体的PGE含量没有显著异常,标准化分布曲线大致平坦,PPGE与IPGE之间分馏不明显,总体特征与中国日喀则地区以及阿尔卑斯和东地中海地区的地幔橄榄岩相似.而当穷岩体的PPGE含量变化范围较大,出现IPGE与PPGE强分馏.单斜辉石岩脉的PGE含量及分布型式与地幔橄榄岩不同:Os、Ir、Ru、Rh亏损而Pt、Pd富集,模式曲线向左倾斜,可能反映其较高程度部分熔融作用和熔体分离作用的产物.铬铁矿石的PGE含量较高,而且变化幅度大(19.7×10-9~5526×10-9),各样品∑PGE平均值(729.33×10-9)是原始地幔的25.6倍.与原始地幔相比,Os、Ir、Ru、Rh总体上表现为较强富集,Pt、Pd富集与亏损并存.其标准化模式曲线较分散,曲线向右倾斜,IPGE与PPGE分馏较强,多数样品呈现Pt负异常.这些特点与中国罗布莎、希腊、南斯拉夫以及塞浦路斯等世界各地蛇绿岩中的铬铁矿床类似.铬铁矿石的PGE含量与相应样品的BMS含量无相关性,PGE不是主要赋存于BMs内.推测PGE主要以PGM的形式存在,具体矿物名称有待进一步发现与研究.  相似文献   

18.
Platinum-group element (PGE) abundances in mantle rocks are generally considered to result from a late meteorite addition to the early Earth, post-dating the core separation event. As such, PGEs are key tracers for the Earth accretion history. For decades, the PGEs systematics of undepleted mantle peridotites has been used to constrain the composition of meteorite impactors involved in the late veneer material. Despite multiple evidence of considerable modifications by partial melting, harzburgites from the Sumail ophiolite (Oman) display a mean PGE composition very akin to recently refined estimates for the Primitive Upper Mantle (PUM) of the Earth. These rocks document a resetting of the PUM signature by percolating basaltic melts, which precipitated Pd-enriched Cu–Ni sulphides within a strongly Pd-depleted residual harzburgitic protolith. Such a resetting casts doubt on both the reliability of any PUM estimates and relevance of the PUM concept itself, at least for PGEs.  相似文献   

19.
High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes.The observed variations of the 62Ni/58Ni, 61Ni/58Ni, and 60Ni/58Ni ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no 60Ni anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45‰ per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation.  相似文献   

20.
Devonian, spessartite dykes, known as the Weekend dykes, onthe Eastern Shore of Nova Scotia contain panidiomorphic texturesand mineral (amphibole, clinopyroxene, and biotite) compositionstypical of shoshonitic lamprophyres. The major element and traceelement geochemistry of the Weekend dykes is also representativeof shoshonitic lamprophyres with high large ion lithophile elementconcentrations (LILE, e.g., Rb, K, and Ba) relative to the lightrare earth elements (LREE, e.g., La) and very low Nb and Ti.Only Ta concentrations are uncharacteristically high relativeto Nb and Ti. The dykes are variably evolved but many samplesshow primitive compositions with high mgnumber [>0.70, wheremg-number=Mg/(Mg+09total Fe) atomic] high MgO and Ni concentrations(>10 wt.% and 150 ppm, respectively), and low heavy REE concentrations.All noble metal concentrations (Au, Pd, Pt, Rh, Ru, and Ir)tend to be lower in evolved samples than in primitive rocks,suggesting that evolved magmas were efficiently scavenged byimmiscible sulphide globules and that shoshonitic lamprophyreplutons may hold economic promise for the platinum-group elements(PGE). Noble metal abundances in even the most primitive rocksare low compared with many basaltic rocks excepting mid-oceanridge basalt (MORB). This makes doubtful the idea that Megumagroup mesothermal gold deposits, which are spatially and temporallyassociated with the dykes, derived their gold from the lamprophyres.Models that explain PGE concentrations and Pd/Ir ratios in awide variety of mafic rocks suggest that the low noble metalabundances probably reflect metal retention in mantle sulphides,olivine, and PGE alloys at low percentages of melting. However,noble metal abundances are higher than expected if the oceanicslab was involved in the melting process or if the source regionresembled that for most MORB, which appears to have low PGE.Mantle metasomatism represents an unlikely explanation for thePGE abundances, which are equivalent to those expected in alkalinehot-spot magmas. The high PGE abundances of hot-spot magmas(compared with MORB) may be related to the incorporation ofcore materials in deep mantle plumes. Presumably the hot-spotlikemantle became part of the subcontinental lithospheric mantlebefore the subduction and transpressional event that producedthe shoshonitic magmas.  相似文献   

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