超高压变质过程中的元素地球化学行为——CCSD主孔榴辉岩的矿物化学研究

中国大陆科学钻探主孔位于江苏东海县,苏鲁超高压变质带的南部.该钻孔的0～2050m深度获取了六种不同类型的榴辉岩和少量石榴石辉石岩岩心,它们是典型的基性超高压变质岩,为研究大陆深俯冲过程中的元素地球化学行为提供了非常好的样品.本文对各种超高压变质矿物的微量元素成分进行了系统的原位微区分析,结合全岩化学成分和矿物主量元素成分,深入地研究了超高压变质岩的微量元素赋存特征、分配规律、控制因素,及其对变质条件和流体.岩石相互作用的限定意义.结果表明,超高压榴辉岩中的LREE和Sr主要赋存在磷灰石、帘石和单斜辉石中,HREE赋存在石榴石中,Ba、Rb和Cs等LILE赋存在多硅白云母中,Ti、Nb和Ta等HFSE主要赋存在金红石、钛铁矿中,V、Sc、Co和Ni等元素大多赋存在石榴石和单斜辉石中.研究表明,全岩化学成分和矿物组成、及其含量的变化明显控制着超高压矿物的微量元素含量和分布形式.本研究也获得了如下重要的认识:超高压变质矿物之间的微量元素分配达到了化学平衡,并具有与地幔榴辉岩矿物之间类似的分配系数,表明榴辉岩的峰期变质温度很可能达到900℃～1000℃.部分高Ti和高Fe-Ti榴辉岩中的石榴石和绿辉石有明显的稀土元素成分环带,表明超高压变质岩经历了快速折返过程.金红石的Zr含量明显受到全岩成分和退变质作用影响,并不仅仅与形成温度有关,不是可靠的温度计.在超临界流体的作用下,榴辉岩中金红石的Nb、Ta发生了明显的分异,导致其Nb/Ta比值增大,由此推测俯冲到地幔深处的大量榴辉岩是地球内部高Nb/Ta比值的物质源区.在榴辉岩的不同程度退变质阶段,参与变质反应的流体具有不同的来源、成分和流体活动规模.     

山东海阳所麻粒岩向过渡榴辉岩转化的变质动力学过程及其构造意义

在山东半岛东南端的乳山海阳所,角闪岩和长英质片麻岩呈互层产出.在某些角闪岩层的中心残留有弱变形的麻粒岩和强变形的过渡榴辉岩(斜长石榴辉岩)团块.在弱变形的麻粒岩团块中发育有窄的(<1cm)剪切条带.沿着这些剪切条带和在强变形过渡榴辉岩团块中,原麻粒岩被转化为含斜长石的过渡榴辉岩.变形和流体在这一转化过程中加速了元素扩散和迁移的速度,在变形强和有流体加入的部位麻粒岩向榴辉岩转化的程度明显增强.麻粒岩相矿物组合(Cpx-Ⅰ+Opx+Pl-Ⅰ+ Hbl-Ⅰ + Grt-Ⅰ+ Ilm)及其花岗变晶结构被完好地保存于弱变形的麻粒岩中,其矿物成分显示出0.8～0.9 GPa和780℃～830℃的麻粒岩相变质条件.榴辉岩相叠加在麻粒岩中形成了一系列的环边状矿物和结构:环边状石榴石(Grt-Ⅱ)形成于麻粒岩相斜长石(Pl-Ⅰ)和暗色矿物(如Cpx-Ⅰ、Opx和Hbl-Ⅰ)的交界处,细粒石榴石(Grt-Ⅱ)、富钠单斜辉石(Cpx-Ⅱ)和石英形成于麻粒岩相角闪石(Hbl-Ⅰ)的外缘.同时榴辉岩相叠加造成麻粒岩相单斜辉石边缘钙契尔马克组份的降低和硬玉组份的升高.但在弱变形的麻粒岩中,麻粒岩相单斜辉石(Cpx-Ⅰ)的外缘并无榴辉岩相单斜辉石形成.沿着弱变形麻粒岩中的剪切条带和在过渡榴辉岩中,榴辉岩相矿物[Grt-Ⅱ+ Omp/Di(Cpx-Ⅱ)+ Ab(Pl-Ⅱ)+ Qtz + Hbl-Ⅱ+ Ky + Zo + Rut]广泛发育,它们的定向排列形成了明显的面理.其形成的温压条件为1.0～1.4 GPa和560℃～680℃,为过渡榴辉岩相变质.虽然其中仍然残留有被拉长的麻粒岩相单斜辉石,但它们含有很高的硬玉组份,且其外缘环绕有细粒的钠质透辉石或绿辉石(Cpx-Ⅱ).海阳所榴辉岩相变质的温压条件远远低于位于海阳所东南和东北的苏鲁超高压变质带中柯石英榴辉岩变质的温压条件.因此海阳所地区代表一条不同于超高压变质带的冷榴辉岩带,它可能与大别山南部的冷榴辉岩带相当.苏鲁超高压变质带的南部边界位于海阳所以北.     

隐藏在苏鲁地体斜长角闪岩锆石中超高压变质作用的信息

运用激光拉曼和阴极发光技术，配备电子探针测试，发现苏鲁地体地表露头和中国大陆科学钻探工程预先导孔CCSD-PP1和CCSD-PP2斜长角闪岩锆石中均保存以柯石英为代表的典型超高压矿物组合：柯石英 石榴石 绿辉石 金红石；柯石英 石榴石 绿辉石；柯石英 石榴石 绿辉石 多硅白云母 金红石 磷灰石；柯石英 绿辉石 金红石；柯石英 菱镁矿。该类矿物包体组合与苏鲁地体超高压榴辉岩的峰期矿物组合十分相似，表明斜长角闪岩可能是由超高压变质的榴辉岩在构造抬升过程中退变质而成。     

苏鲁造山带池庄超高压榴辉岩中变质脉:大陆俯冲带超临界流体活动的证据

高压-超高压变质岩中的变质脉能够反映俯冲带变质流体的组成和演化。为了探究大陆俯冲带超临界流体活动及伴随的元素迁移,本文系统地研究了苏鲁造山带南部江苏东海池庄地区的超高压榴辉岩及变质脉。变质脉主要是由石英、石榴石、绿辉石、多硅白云母、蓝晶石、黝帘石、金红石和锆石等矿物组成,与寄主榴辉岩的矿物组成类似。相比于榴辉岩,脉体中的石榴石更加富集重稀土元素(HREE);黝帘石强烈富集轻稀土元素(LREE)。变质脉和榴辉岩中各主要矿物的氧同位素组成在误差范围内一致(石英的δ18O分别为2.42‰和2.79‰;石榴石为-0.30‰和0.010‰;绿辉石为0.25‰和0.071‰),说明变质脉的形成与榴辉岩释放的内部流体有关。综合已有的研究,发现大别-苏鲁造山带不同地区的变质脉和榴辉岩具有极不均一的氧同位素组成,说明在陆壳深俯冲和折返过程中,流体活动有限。利用矿物温压计得到变质脉的峰期变质温压条件为692±65℃和3.6±0.3GPa,脉体中锆石U-Pb定年结果表明锆石的形成时代为218±2.4Ma,指示变质脉形成于深俯冲陆壳折返初期的超高压变质阶段。变质脉中矿物组合和矿物的主微量元素特征说明成脉流体富集Si、Al、Ca、K、LILE、REE和HFSE等元素,表明成脉流体可能是溶解能力极强的超临界流体。     

超高压变质岩的塑性流变：显微构造和变形机制

超高压变质岩是大陆深俯冲作用的产物。超高压变质岩在深俯冲和快速折返过程中,经历了长距离地构造搬运和构造力的作用。其构造变形主要集中在韧性剪切带中,并发生强烈地塑性流变。研究超高压变质构造岩的显微构造及其变形机制对于深入了解大陆壳岩石在深俯冲过程中的流变学行为有十分重要的意义,山东仰口的超高压韧性剪切带中榴辉岩质和花岗质糜棱岩记录了超高压变形的历史。在超高压条件下的稳定矿物绿辉石、多硅白云母、兰晶石和钾长石具有不规则波状消光、亚晶界、核幔构造和动态重结晶等显微构造特征,TEM 研究揭示了大量的位错构造,表明位错蠕变是其主要的变形机制。在花岗质糜棱岩中,金红石在刚性矿物的压力影中沉积,细粒的石榴石条带平行片理延伸,都说明超高压变形过程中有流体存在,流体助力的物质扩散迁移是又一个重要的变形机制。依据现有的流变学定律估算的流变应力应该在几十兆帕以上。     

韧性剪切带中片麻岩和超高压榴辉岩变形特征及其与地震波速各向异性的关系：来自中国大陆科学钻探（CCSD）680～1200米岩心的证据

中国大陆科学钻探(CCSD)680-1200米区段发育了多个韧性剪切带,带中主要岩石类型包括片麻岩和超高压榴辉岩。片麻岩中的变形石英、面理化榴辉岩中的拉长石榴石和绿辉石的应变轴比都表现为X>Y>Z,Flinn系数分别为0.11-0.27、0.22-0.23和0.23-0.24。随着糜棱岩化作用的增强,变形石英的C轴组构由Z轴极密逐渐向Y轴极密和叶理面上的大圆环带转变。在常温常压下测试了样品的波速,计算出片麻岩Vp和Vs的各向异性分别为30.17％-60.97％和11.52％-35.79％,榴辉岩Vp和Vs的各向异性分别为0.17％-11.19％和2.41％-6.70％。影响各向异性的主要因素有岩石的结构构造、矿物的晶格优选方位(LPO)、形态优选方位(SPO)和定向微裂隙。随着糜棱岩化作用的增强,岩石的P波各向异性逐 渐升高。变形岩石中的黑云母、石英、绿辉石的LPO和SPO是地震波各向异性的主要控制因素。饱水后的片麻岩样品的P波各向异性明显低于干燥片麻岩样品。在东海钻井中的强反射带主要是由于不同岩层之间的波阻抗差异而造成的,榴辉岩/强退变榴辉岩和黑云斜长片麻岩之间的接触界面会产生较强的地震深反射。此外,与LPO相关的地震波各向异性会增强地震波的反射,所以韧性剪切带中的糜棱岩化片麻岩可能是地震反射的良好载体。韧性剪切带中岩石弹性波速度的强各向     

中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体

中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。     

豫南——鄂北大别山北部高压角闪石榴辉岩的研究

一个高压角闪石榴辉岩带出现在豫南——鄂北大别山高压超高压变质单元的最北部。榴辉岩的矿物组合为石榴石+绿辉石+角闪石+绿帘石+多硅白云母+石英+金红石。采用Powell等(1994)的Thermocalc估计的温压条件:压力为1.8-2.4GPa,温度为490-592℃。这个带的榴辉岩在矿物组合,矿物成分和温压条件上明显不同于该高压变质单元中其它带的榴辉岩。     

CCSD主孔100-1100m榴辉岩中单矿物的原位微区微量元素地球化学研究

本文通过对CCSD主孔100～1100m范围内榴辉岩中单矿物的LA-ICP-MS分析，探讨了榴辉岩中单矿物之间的微量元素分配，发现超高压变质作用中石榴石和绿辉石之间Ti和C0的分配显著受Mg控制（如DCo^Grt/Omp=3．43DMg^Grt/Omp-0．34），而REE、Sr和Y的分配则受Ca分配所控制。绿辉石中REE、Pb和Th的含量则明显受超高压副矿物磷灰石的出现与否所控制。结合岩石学特征，对角闪石和绿辉石中微量元素的研究表明角闪石主要是绿辉石退变质的产物。但退变质矿物的微量元素组成不仅受原矿物控制，而且受退变质矿物组合类型影响。绿帘石的出现会显著降低共生角闪石中LREE和Sr的含量，而多硅白云母的分解则会增加角闪石中的Rb、Ba含量。另外，退变质过程中的流体活动也会影响退变质矿物中的LREE、Sr和Pb等。结合REE在榴辉岩各主要矿物间分配系数随温度、压力的变化，我们推测部分石榴石边部MREE的富集特征可能反映榴辉岩在折返过程中经历了短时增温作用，这可能是引起苏鲁地区榴辉岩相向麻粒岩相转变叠加现象以及超高压岩石经历部分熔融作用的重要原因。此外，榴辉岩中金红石Nb和Ta组成的高度不均一性为金红石形成于超高压变质阶段富Ti磁铁矿相变作用的成因机制提供了佐证。     

西藏松多榴辉岩矿物出溶体研究

西藏松多榴辉岩中的石榴石和单斜辉石中发育有大量的矿物出溶体。电子探针能谱分析结果表明,石榴石中的出溶体为金红石,绿辉石中的出溶体为石英和富钾质矿物(钾长石)。根据石榴石-绿辉石-多硅白云母矿物温压计计算出的松多榴辉岩变质温压范围接近石英-柯石英相转变线,结合这些出溶体的存在,指示了西藏松多榴辉岩曾经历过超高压变质作用,这为松多榴辉岩的形成条件提供了重要证据。     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

Geochemistry of eclogites from the Dabie–Sulu terrane, eastern China: New insights into protoliths and trace element behaviour during UHP metamorphism

The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.     

金红石-榍石转变过程中元素地球化学行为——以雅鲁藏布江缝合带角闪岩为例

金红石边缘形成榍石冠状边结构在变质中-基性岩中普遍存在,是金红石与退变质流体携带的SiO_2与CaO作用的结果,反应形成的榍石微量元素特征受到金红石和流体的共同影响。雅鲁藏布江缝合带中角闪岩LZ06-04在抬升过程经历近等温降压退变质作用,石榴子石分解导致同一样品中含石榴子石部分与不含石榴子石部分的退变质流体成分的差异。两种流体分别与金红石反应,对应形成的榍石具有相似的Nb、Ta含量和Nb/Ta比值特征,但截然不同的REE特征。榍石的Nb、Ta来源于金红石,残余金红石与含水流体再平衡Nb、Ta的分配系数增大,且D_(Nb)~(Rt/Fluid)≥D_(Ta)~(Rt/Fluid);虽然Nb和Ta在含水流体中都表现为不活动元素,但相对于Nb,Ta在含水流体中活动性较高。榍石的Zr-Hf体系特征受到锆石、石榴子石等矿物的综合影响,并且Zr-Hf在含水流体中表现出比Nb-Ta更高的活动性。榍石的REE特征受流体中REE特征、榍石与流体配分系数以及共生矿物的影响。在岩浆或变质体系,榍石形成过程中,REE富集矿物(如石榴子石、锆石、褐帘石、独居石、磷灰石等)形成或分解将影响榍石的REE分布特征或形成REE环带结构。含水流体中金红石退变质形成榍石反应的进行受流体中TiO_2、CaO和SiO_2活度的影响。因此榍石常见于钙碱性岩浆岩、富Ca基性变质岩和矽卡岩中。流体中CaO活度的变化影响榍石的形成,进而影响Ti、Nb、Ta在流体中的运移能力。俯冲板片产生流体在交代上覆富Ca地幔楔物质过程中形成榍石残留同样可以造成部分熔融体具有亏损HFSE特征。     

Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios < 0.09, lack Eu anomalies, and are only weakly enriched in HREE with respect to MREE. These features are consistent with a garnet-bearing, plagioclase-free, i.e., eclogite-facies paragenesis. Vein zircons yield an age of 45 ± 1 Ma, which is evidence for Eocene subduction-zone metamorphism of the Monviso ophiolite.In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values.Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.     

西藏松多榴辉岩的矿物原位微量元素地球化学研究

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。     

Rutile to Titanite Transformation in Eclogites and its Geochemical Consequences: An Example from the Sumdo Eclogite, Tibet

The formation of titanite coronae after rutile is common in retrograde high-to ultrahigh-pressure meta-mafic rocks, which provides a good opportunity to address the geochemical behavior of HFSE in crustal environments. In the Sumdo eclogite, titanite occurs either as a corona around rutile grains or as semi-continuous veins cross-cutting the major foliation, whereas rutile grains occur either as inclusions in garnet or omphacite or as a relict core surrounded by titanite. Textural relationships ...     

喜马拉雅造山带中新世岩浆型石榴子石的矿物化学特征:从高Sr/Y花岗岩到淡色花岗岩

石榴子石是演化花岗岩常见的重要副矿物之一,但石榴子石地球化学特征如何随岩浆演化而变化是有待探讨的问题之一。雅拉香波片麻岩穹隆发育年龄分别为20. 3±0. 5Ma和20. 1±0. 3Ma(锆石U-Pb年龄)的高Sr/Y比二云母花岗岩(TMG)和淡色花岗岩(Grt-LG)。虽然两类花岗岩都含石榴子石,且在形成时代和Sr-Nd同位素组成上相似,但在元素地球化学特征上具有明显的差异,淡色花岗岩和二云母花岗岩分别代表演化程度较高和较原始的岩浆。在同一件样品中,在石榴子石颗粒之间,存在一定程度的微量元素地球化学特征的不均一性,反映了局部熔体地球化学特征。在两类花岗岩中,岩浆型石榴子石具有以下相似的地球化学特征:(1)从核部到边部,Mn和HREE含量降低,表现出典型的生长环带特征;(2)富集HREE,亏损LREE;和(3)显著的Eu负异常。但在关键微量元素Zn、Sc和Y上,具有明显的差异性。在花岗质岩浆演化过程中,贫Fe、Mg和Mn矿物相的分离结晶作用,导致残留熔体的Ca和Sr含量降低,Eu负异常幅度增大,Sc、Zn、Y和HREE增高,是导致淡色花岗岩石榴子石相应元素含量增高的主要原因。上述观测表明:高Sr/Y花岗岩也可以结晶石榴子石,与通常的淡色花岗岩石榴子石相比,这些石榴子石的Sc、Zn和Y含量和Eu异常幅度明显较低。但随分异程度的升高,石榴子石的元素地球化学特征与源自变沉积岩的淡色花岗岩的类似。因此,花岗岩中的石榴子石矿物化学特征变化记录了花岗岩岩浆演化的重要信息。     

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.