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1.
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   

2.
古老大陆岩石圈地幔再循环与蛇绿岩中铬铁矿床成因   总被引：2，自引：0，他引：2  
不同地区、不同时代蛇绿岩中不同类型铬铁矿岩的Re-Os同位素研究表明,在铬铁矿石或围岩中均存在极度亏损的具有大陆岩石圈地幔属性的物质。新疆达拉布特古生代蛇绿岩带中萨尔托海富Al铬铁矿岩的Os同位素组成为0.1109～0.1256,对应的模式年龄为3.5～0.6Ga;西藏班公湖—怒江中生代蛇绿岩带中东巧富Cr铬铁矿石及围岩Os同位素组成介于0.1175～0.1261,对应的模式年龄为1.5～0.1Ga;雅鲁藏布江中生代蛇绿岩带中罗布莎富Cr铬铁矿岩的Os同位素变化范围为0.1038～0.1266,对应的模式年龄为3.37～0.28Ga,而该带中不含矿的泽当二辉橄榄岩的Os同位素组成为0.1256～0.1261,没有古老大陆岩石圈地幔属性的物质存在,与新特提斯洋地幔Os组成较为接近。推测在蛇绿岩形成过程中,古老大陆岩石圈地幔参与循环有利于形成铬铁矿床,明确提出"熔体与古老大陆岩石圈地幔反应成矿"的假说,指出蛇绿岩带中存在的古老微陆块可能是找矿的指示标志。  相似文献   

3.
东南沿海地区古近纪大陆岩石圈地幔特征及成因   总被引：3，自引：0，他引：3  
东南沿海地区新生代玄武岩中的橄榄岩包体来自岩石圈地幔 ,上地幔橄榄岩包体的岩石学及地球化学特征都记录了地幔演化的历史。普宁橄榄岩包体斜方辉石含量与太古宙克拉通地幔类似 ,但在矿物学、REE、痕量元素和Sr Nd同位素上又与太古宙岩石圈地幔不同。橄榄岩包体的岩相学、矿物学、REE、痕量元素特征都提供了含H2 O富Si流体交代橄榄岩的证据 ,这种流体可能主要是洋壳物质局部熔融而成。流体交代使橄榄岩富Si,同时富Sr、Pb和强不相容元素等大洋岩石圈物质。这表明普宁大陆岩石圈地幔既保留太古宙岩石圈地幔的特征 ,又具有大洋俯冲地幔的特征 ,它是古老岩石圈地幔向大洋岩石圈地幔转换的一部分 ,这种转换可能是大洋岩石圈与大陆岩石圈地幔相互作用的结果。  相似文献   

4.
华北东部古生代岩石圈地幔经历过复杂的多旋回多阶段熔融岩浆、重结晶、变质和剪切变形历史,因而岩石组成、结构复杂.地幔橄榄岩的主元素、地幔矿物特别是金刚石中的包裹体的Mg#值及Cr#值等显示古生代岩石圈地幔高度难熔亏损玄武质的成分特征,而地幔橄榄岩的不相容元素、同位素显示高度富集的特点.这种高难熔和强富集两重性是克拉通岩石圈地幔的特征.Pb同位素反映华北东部古生代岩石圈地幔具冈瓦纳大陆地幔的特点.复县和蒙阴古生代岩石圈地幔尽管都表现为太古代克拉通地幔的特征,但存在着不均一性,前者受克拉通化后的深部作用影响强于后者.与新生代地幔Sr、Nd同位素对比表明富集的古生代岩石圈地幔向亏损新生代的地幔转变和减薄伴有新生地幔物质对古老岩石圈的置换和混合作用.  相似文献   

5.
苏皖地区发育的新生代玄武岩中富含地幔橄榄岩捕虏体，测定了25个橄榄岩全岩的锇同位素组成。结果表明大部分样品呈亏损特征,187Os/188Os=0.119～0.129.Os同位素比值与主量成分之间有显著的相关性.采用187Os/188Os-Al2O3代用等时线方法,由截距得到初始值(187Os/188Os)0=0.109,在对流地幔的187Os/188Os比值演化线上获得模式年龄t=2.5(±0.1)Ga,为晚太古-早元古代。用样品组中最低的锇同位素组成,即187Os/188Os=0.119,计算Re亏损模式年龄,t=1.2Ga,为中元古代。表明苏皖地区新生代玄武岩中的地幔橄榄岩捕虏体具有古老的形成年龄,它们是经过显生宙减薄作用后的残余地幔的碎片。大陆岩石圈地幔的古老形成年龄与上覆地壳克拉通的古老稳定年龄相耦合。  相似文献   

6.
位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。  相似文献   

7.
本文报道了汉诺坝新生代碱性玄武岩中地幔橄榄岩包体的主量、微量元素和Re-Os同位素。14个尖晶石橄榄岩Re、Os含量分别为0.022～0.193ng/g和1.237～4.304ng/g，^187Os/^188Os比值为0.1183～0.1244，与^187Os/^188Os比值相关性不好，但与熔体亏损指标如重稀土元素Yb的含量、全岩Al2O3的百分含量有很好的线性关系，可能反映了地幔熔融后的Re或/和Os的活动。全岩Al2O3、CaO、TiO2含量均与MgO有很好的负相关性，全岩原始地幔标准化REE丰度模式呈现了LREE亏损，表明该区橄榄岩包体是由软流圈地幔经过部分熔融，亏损了玄武质组分后形成的。由Os同位素代理等时线得到该区陆下岩石圈地幔的形成年龄为1.7～2.0Ga，表明尖晶石相橄榄岩所代表的岩石圈地幔是中元古代的陆下岩石圈地幔减薄后的残留体。  相似文献   

8.
大陆岩石圈地幔定年   总被引：4，自引：0，他引：4  
大陆岩石圈地幔是伴随地壳熔体抽取而形成的低密度地幔残留,是联系软流圈与地壳的重要纽带。虽然大陆岩石圈地幔一直是固体地球科学研究的重要内容,但对其形成时代的准确厘定则是当前研究的难点。传统方法是采用岩石圈地幔中橄榄岩主要元素的贫瘠程度来对岩石圈进行定年,即古老的岩石圈地幔Al2O3和CaO含量低,而MgO含量高(同时导致橄榄石高Fo值),而年轻岩石圈地幔的特征正好相反。显然,这种间接的方法不能给出确切的年龄信息。尽管Sr-Nd和锆石U-Pb等同位素方法也被用来进行地幔橄榄岩定年,但岩石圈地幔通常具有的较高温度使上述同位素体系不能保持封闭,因而给出的年龄大多与岩石圈地幔的形成时代无关。近几年发展起来的Re-Os同位素技术是目前进行岩石圈地幔定年的最理想工具,但也存在一系列需要研究的问题。文中对这一方法的基本原理、发展现状和存在的问题进行了全面的介绍。同时,根据目前获得的Os同位素资料,对中国东部岩石圈地幔的时代进行了讨论,并简要论述所获得的资料对岩石圈减薄研究的启示。  相似文献   

9.
橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素（如Mg和Fe）组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明：地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#（〉92）,而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#（〈91）。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明：该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。  相似文献   

10.
         二辉橄榄岩通常被认为是低程度部分熔融的残留,但熔体再富集作用为其成因提供了一种新的解释。熔体再富集作 用通常是指软流圈来源的玄武质熔体加入到难熔的方辉橄榄岩或纯橄岩形成更为饱满的二辉橄榄岩的过程。除了使主量元 素富集之外,熔体再富集作用还可以使微量元素与Sr-Nd 同位素从方辉橄榄岩中的富集特征转变为二辉橄榄岩所呈现的亏 损特征。对于熔体再富集作用是否改变橄榄岩的Os 同位素组成还存在较大的争议,它主要取决于加入熔体的比例,熔体中 硫的饱和程度以及熔体再富集作用发生的时间等因素。对于以低熔/岩比例为主的大陆岩石圈地幔来说,熔体再富集作用对 橄榄岩的Os 同位素组成的影响可能较为有限。除了化学成分上的影响之外,熔体的加入也会改变大陆岩石圈地幔的物理特 征。这一过程使得岩石圈地幔的渗透率增大和黏滞度降低,从而会破坏大陆岩石圈地幔的稳定性。虽然熔体再富集作用可 以影响和改变岩石圈地幔的性质,但它是否导致克拉通地幔的减薄以及克拉通破坏尚有疑问。  相似文献   

11.
Studies of mantle xenolith and xenocryst studies have indicated that the subcontinental lithospheric mantle (SCLM) at the Karelian Craton margin (Fennoscandian Shield) is stratified into at least three distinct layers cited A, B, and C. The origin and age of this layering has, however, remained unconstrained. In order to address this question, we have determined Re–Os isotope composition and a comprehensive set of major and trace elements, from xenoliths representing all these three layers. These are the first Re–Os data from the SCLM of the vast East European Craton.

Xenoliths derived from the middle layer B (at  110–180 km depth), which is the main source of harzburgitic garnets and peridotitic diamonds in these kimberlites, are characterised by unradiogenic Os isotopic composition. 187Os/188Os shows a good correlation with indices of partial melting implying an age of  3.3. Ga for melt extraction. This age corresponds with the oldest formation ages of the overlying crust, suggesting that layer B represents the unmodified SCLM stabilised during the Paleoarchean. Underlying layer C (at 180–250 km depths) is the main source of Ti-rich pyropes of megacrystic composition but is lacking harzburgitic pyropes. The osmium isotopic composition of layer C xenoliths is more radiogenic compared to layer B, yielding only Proterozoic TRD ages. Layer C is interpreted to represent a melt metasomatised equivalent to layer B. This metasomatism most likely occurred at ca. 2.0 Ga when the present craton margin formed following continental break-up. Shallow layer A (at  60–110 km depth) has knife-sharp lower contact against layer B indicative of shear zone and episodic construction of SCLM. Layer A peridotites have “ultradepleted” arc mantle-type compositions, and have been metasomatised by radiogenic 187Os/188Os, presumably from slab-derived fluids. Since layer A is absent in the core of the craton, its origin can be related to Proterozoic processes at the craton margin. We interpret it to represent the lithosphere of a Proterozoic arc complex (subduction wedge mantle) that became underthrusted beneath the craton margin crust during continental collision  1.9 Ga ago.  相似文献   


12.
A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re–Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03–1.34 ppb), while Os concentrations show a much more limited range (0.26–0.59 ppb). The three diamond-free eclogites have Re and Os concentrations that are at the extremes of the range of their diamond-bearing counterparts. 187Os/188Os ranges from 0.1579 to 1.4877, while 187Re/188Os varies from 0.54 to 26.2 in the diamond-bearing eclogites. The highly radiogenic Os in the diamond-bearing eclogites (γOs=23–1056) is consistent with their high 187Re/188Os and requires long-term isolation from the convecting mantle. Re–Os model ages for 9 out of 14 diamond-bearing samples lie between 3.08 and 4.54 Ga, in agreement with FTIR spectra of Newlands diamonds that show nitrogen aggregation states consistent with diamond formation in the Archean. Re–Os isochron systematics for the Newlands samples do not define a precise isochron relationship, but lines drawn between subsets of the data provide ages ranging from 2.9 to 4.1 Ga, all of which are suggestive of formation in the Archean. The Re–Os systematics combined with mineral chemistry and stable isotopic composition of the diamond-bearing eclogites are consistent with a protolith that has interacted with surficial environments. Therefore, the favored model for the origin of the Newlands diamond-bearing eclogites is via subduction. The most likely precursors for the Kaapvaal eclogites include komatiitic ocean ridge products or primitive portions of oceanic plateaus or ocean islands.  相似文献   

13.
Concentrations of Re and Os, and the isotopic composition of Os have been measured in the Japan Sea sediments to assess the response of the Japan Sea to glacial–interglacial climate change and associated weathering fluxes. The osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg/g, and 187Os/188Os from 0.935 to 1.042. Only 187Os/188Os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of Re and Os, and with ≥ 80% hydrogenous Os are explained in terms of seawater composition. Lower 187Os/188Os were observed for sediments deposited during the last glacial maximum (LGM) when planktonic foraminifera from the Japan Sea recorded lighter oxygen isotopic composition. Decrease in dissolved Os fluxes from continents and/or change in the composition of the dissolved load to the Japan Sea are suggested as the driving mechanisms for the observed lower LGM 187Os/188Os. The results of this study, coupled with lower 187Os/188Os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates, suggest that this trend is characteristic of the global oceans.

Data from this study show that the Japan Sea recorded higher 187Os/188Os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values. This is explained in terms of preferential release of 187Os during deglacial weathering and/or higher continental Os flux driven by warm and wet climate. This study demonstrates that Os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition. In addition, 187Os/188Os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi-enclosed basins such as the Japan Sea.  相似文献   


14.
Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γOs values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/188Os of a filtered suite yields a slope of −0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γOs value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory.Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.  相似文献   

15.
A double spike for osmium analysis of highly radiogenic samples   总被引：1，自引：0，他引：1  
Geologic samples containing highly radiogenic Os (molybdenites and low-level, highly radiogenic (LLHR) samples) have no internal means by which to correct for mass fractionation during isotopic measurement by mass spectrometry. We describe a double spike for use with highly radiogenic samples, created by combining isotopically enriched 188Os and 190Os. Spiking molybdenite and other highly radiogenic minerals with this tracer allows for a fractionation correction, as well as a more reliable determination of common Os relative to analysis using single spikes.

The precise isotopic composition of the double spike is determined by a calibration against natural Os, in which two separate measurements are necessary: one each for the pure double spike and the spike–standard mixture. An estimate of the true composition of the spike is obtained by least squares approximation, and the errors are obtained by Monte Carlo methods. Sample analyses are then much more straightforward than the calibration because isotopic compositions of all components are known a priori.

Results obtained with a mixed Re-double Os spike demonstrate an improved reproducibility over individual 185Re and 190Os spikes. For an Archean in-house molybdenite standard we now observe a reproducibility of 0.08%. The ability to make a fractionation correction is essential for Os measurements made by ion counting. With the double Os spike, young samples and those with low Re contents (i.e., LLHR) can now be accurately analyzed. The 188Os–190Os double spike also allows a determination of the common Os contents of highly radiogenic samples. Common Os is poorly determined for ancient samples with high concentrations of 187Os, which fortunately are not sensitive to estimates of common Os. Common Os can be reasonably well determined for younger samples and those with low Re contents. We report a common Os concentration of 0.4±0.1 ppb for an 11 Ma molybdenite. Consideration of common Os content is important for age determination of young samples and LLHR samples, and is not possible by other published means of Os analysis.  相似文献   


16.
Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in southern Washington represent fragments of mantle lithosphere from the back-arc side of the Cascade arc front. Previous studies have shown that metasomatism by either silica-rich fluids or hydrous melts crystallized phlogopite, imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and more radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re–Os, Sm–Nd, Sr, and U–Th–Pb isotopic data shed further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions (206Pb/204Pb=18.63–19.55, 207Pb/204Pb=15.56–15.63, 208Pb/204Pb=38.22–38.87). The most radiogenic Pb isotopic compositions extend beyond the most radiogenic Pb isotopic compositions for the Cascade arc lavas and display a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in 207Pb/204Pb and 208Pb/204Pb at the high 206Pb/204Pb end of this array. Therefore, models for rapid transfer of components from the slab to the mantle lithosphere are not viable in this case. Instead, a multi-stage model is preferred. In the first stage, the slab component is transferred via fluid or melt into, and reacts with the hanging wall mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting of the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures. In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compositions of the clinopyroxenes, and the presence of high alkali glasses in the xenoliths, are consistent with the metasomatic agent derived from the hybridized hanging wall being alkali-rich, and possibly similar to potassic-rich lavas found in arc and back-arc settings. These data therefore demonstrate the importance of the hybridized hanging wall mantle above slabs as a source for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at least tens of million years.  相似文献   

17.
High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr–Nd isotopic compositions (87Sr/86Sr = 0.7031–0.7048, 143Nd/144Nd = 0.5130–0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re–Os isotopic data for Cenozoic basalt-borne xenoliths show TRD ages of 3.0–1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2–0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6–0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re–Os data of sulfides in Hannuoba peridotites suggest that whole-rock Re–Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr–Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr–Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.  相似文献   

18.
The age of continental roots   总被引：39，自引：0，他引：39  
D. G. Pearson 《Lithos》1999,48(1-4):171-194
Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.  相似文献   

19.
Rhenium-Osmium (Re-Os) isotopic dating applied to the geochronology of hydrocarbon accumulation has been the forefront of international research in the last 10 years and has achieved significant achievements. However, our current researches are lacking in Re-Os isotopic chronology on the geochronology of hydrocarbon accumulation. The successful application of the Re-Os geochronology has enabled the determination of accurate and precise depositional ages for Organic-Rich Sedimentary rocks (ORS), the establishment of timing constraints of oil generation or migration, and the fingerprinting of the source of oil. On the basis of literatures study both at home and abroad, this article mainly dealt with the principle of Re-Os isotopic dating and analyzed the accumulation processes of Re and Os in a seawater-sediment system. Meanwhile, the test samples, sample preparation and analytical methodology of Re-Os isotopic dating were summarized. The results revealed that the initial 187Os/188Os ratio could constrain the age of oil generation or migration, and effectively evaluated the relationship between source rock and oil. The radiogenic 187Os/188Os ratio showed a positively correlated relationship with source rock age. This article proposed that hydrocarbon mature, biodegradation and water washing had minimal or no effect on data scatter about Re-Os isochron. However, the effects of postdepositional mobility of Re and Os, variations in the initial 187Os/188Os ratio, and thermochemical sulfate reduction could result in data scatter. The reproducibility and accuracy of Re-Os age was also closely related to the sample particle size and sample amount. Compared with other isotopic dating, the biggest advantage of Re-Os isotopic dating is to study geological body related to accumulation process, and to obtain direct evidence of hydrocarbon accumulation period. From what was discussed above, the article discussed the existing problems and put forward the future hot research and development direction of Re-Os isotope system, so as to promote the deep development of relative theory and technology of Re-Os isotopic dating.  相似文献   

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