蛇绿岩型金刚石的特征

蛇绿岩型金刚石产在蛇绿岩地幔橄榄岩和铬铁矿中,不同于产在大陆克拉通的金伯利岩型金刚石,也不同于产在板块俯冲带中的超高压变质型金刚石和陨石撞击成因的金刚石。蛇绿岩型金刚石的主要特征是粒度普遍较小,多数在200~500μm之间,C同位素显示极低的δ13CVPDB值(-28‰~-18‰),金刚石中包裹体以含富Mn的矿物为特征,金刚石产在蛇绿岩大洋地幔橄榄岩中,其构造背景为板块缝合带。金伯利岩型金刚石粒度可达厘米级,是大颗粒宝石级金刚石的主要来源,其C同位素显示轻微低的δ13CVPDB值(-10‰~-5‰),包裹体矿物多为富Mg的矿物组合,金刚石产在克拉通和大陆岩石圈构造背景。超高压变质带中的金刚石颗粒十分细小,由数微米至100μm,C同位素为中等低的δ13CVPDB值(-15‰~-7‰),金刚石通常与碳酸岩和地壳成因矿物伴生,含有金刚石的超高压变质岩石形成的构造背景为板块深俯冲边界。形成于陨石撞击的金刚石产出和研究均较少,金刚石通常也是微米级,产出在陨石撞击形成的变质岩中,伴生矿物的成分与与撞击变质的原岩有关。以上四类金刚石无论野外和室内区分显著,由此,可以将蛇绿岩型金刚石归为地球上一种新的金刚石产出类型。     

蛇绿岩中金刚石和异常矿物的全球记录

蛇绿岩型金刚石产在地幔橄榄岩和铬铁矿中,是新建立的金刚石产出类型,不同于产在金伯利岩和超高压变质岩中的金刚石。全球已在21个蛇绿岩中发现了该类金刚石,含金刚石的蛇绿岩主要分布在特提斯造山带、乌拉尔-中亚造山带、日高变质岩带和北美克拉马斯-阿卡特兰造山带。本文梳理了含金刚石蛇绿岩的全球分布和地质背景以及蛇绿岩中超高压-强还原矿物与其它壳幔矿物组合的特征,讨论了已有的含金刚石铬铁矿和地幔橄榄岩的四种成因机制。金刚石和伴生的超高压-强还原矿物组合产在不同时代的造山带蛇绿岩中,不仅仅揭示了金刚石在蛇绿岩中普遍存在,需要重新思考蛇绿岩和铬铁矿的成因以及它们形成的地质构造背景,还证实了蛇绿岩地幔橄榄岩和铬铁矿是地球深部矿物重要的储存库,为认识地球深部的物质组成和物理化学环境,以及深部物质运移的轨迹和动力学过程等提供了天然样品。     

内蒙古贺根山蛇绿岩的铬铁矿中发现金刚石等深部地幔矿物

前人报道在西藏中生代和俄罗斯极地乌拉尔早古生代蛇绿岩地幔橄榄岩铬铁矿中发现了金刚石等深部地幔矿物,认为需重新考虑铬铁矿浅部成因的传统认识。为了查明不同造山带蛇绿岩的铬铁矿中金刚石等深部矿物的分布规律和豆荚状铬铁矿的成因,笔者开展了内蒙古贺根山晚古生代蛇绿岩中的铬铁矿床的人工重砂矿物学研究,本研究获得约2000 kg的内蒙古贺根山蛇绿岩铬铁矿石样品,对所采样品开展人工重砂选矿,表明该铬铁矿矿石样品中至少有金刚石、碳硅石及其他自然元素类、金属互化物类、氧化物类、硫化物类、硅酸盐类等30余种矿物。内蒙—大兴安岭造山带晚古生代的内蒙古贺根山蛇绿岩带铬铁矿石中,发现金刚石等深部地幔矿物表明,贺根山铬铁矿可能为深部成因。     

蛇绿岩型铬铁矿中金刚石的浅源成因

蛇绿混杂岩中的橄榄岩和铬铁矿中发现有大量的显微金刚石(microdiamond),其通常伴有超高压矿物组合(柯石英、斯石英等),这对于认识地球深部物质循环及动力学过程具有重要的指示意义。然而,实验表明,金刚石可以形成在低温低压环境(石墨稳定区域)中,不过至今在天然环境中还没有发现相关的证据。     

再论蛇绿岩中豆荚状铬铁矿的成因——质疑岩石/熔体反应成矿说

着重论述了蛇绿岩地幔橄榄岩中豆荚状铬铁矿的成因,并对现今盛行的岩石/熔体反应成矿说提出了质疑。世界含铬铁矿的地幔橄榄岩均显示上部偏基性、下部偏酸性的垂直熔融分带,与蛇绿岩堆晶岩中上部偏酸性、下部偏基性的岩浆分异垂直层序恰恰相反。豆荚状铬铁矿与熔融剖面上部的纯橄岩或纯橄岩-方辉辉橄岩杂岩带紧密伴生。豆荚状铬铁矿是原始地幔岩高度熔融再造的产物,高铬型铬铁矿与PPG型蛇绿岩伴生,形成于岛弧或弧前盆地环境;高铝型铬铁矿与PTG型蛇绿岩伴生,形成于扩张脊（MOR）或弧后盆地环境。玻安岩（boninite）与高铬型豆荚状铬铁矿无成因关系,铬铁矿（或富铬矿浆）的形成反而为boninite提供了其形成所需的残余地幔;高铝型铬铁矿不是地幔橄榄岩/拉斑玄武质熔体反应形成的,而是富铬矿浆与基性熔体发生再平衡的产物。豆荚状铬铁矿中超高压矿物包体的出现为其地幔深部成因提供了佐证,而boninite形成于浅部较低压的条件;豆荚状铬铁矿中富集强相容元素IPGE（Os、Ir、Ru）合金,boninite富集不相容元素PPGE (Pt、Pd)硫（砷）化物, 而亏损IPGE,显示其形成较晚。因此,boninite与铬铁矿无生因关系,两者均受岛弧（或弧前盆地）环境的制约而在空间上相伴产出。     

极地乌拉尔豆荚状铬铁矿中发现金刚石和一个异常矿物群

1前言自1980报道在西藏罗布莎蛇绿岩铬铁矿的人工重砂样品发现金刚石[1]和八面体假象蛇纹石[2]以来,已经发现了一个由70～80种矿物组成的地幔矿物群[3]。近些年的重要新进展有:(1)铬铁矿中发现超高压矿物柯石英和蓝晶石,EBSD研究     

铬在橄榄石中的赋存状态：西藏罗布莎地幔橄榄岩和铬铁矿中的富铬橄榄石及对深部地幔成因的启示

西藏罗布莎蛇绿岩的地幔橄榄岩和铬铁矿中含有目前世界上已知最富铬的包裹体橄榄石(Cr2O3含量最高达1.49%)和富含铬铁矿出溶体的变形残晶橄榄石。通过对富铬橄榄石产出特征和其中Cr与其他元素相关关系的分析,结合前人的研究,指出铬是以还原态的Cr2+进入橄榄石晶格的,Cr2+可能以占据空位和部分替代Fe2+的方式稳定于富铬橄榄石初始相的晶格中。鉴于前人在罗布莎地幔橄榄岩和铬铁矿中大量超高压强还原相矿物的发现,认为这些富铬橄榄石的初始相可能为形成于地幔过渡带或下地幔的瓦兹利石或林伍德石,富铬橄榄石的产出也反过来证明了罗布莎地幔橄榄岩和铬铁矿中部分物质的深部地幔来源。     

豆荚状铬铁矿床的研究现状及进展

豆荚状铬铁矿床是工业上冶金级铬铁矿石的最主要来源,对于其成因研究依然是各国地质学家最为关注的热点之一。文章着重概述了近年来国内外地质学者对豆荚状铬铁矿床成因研究的现状和最新进展。最新研究表明,显生宙以来的豆荚状铬铁矿床具有一定的成矿专属性,主要赋存于蛇绿岩套底部(壳-幔边界,即岩石莫霍面)地幔橄榄岩中的一定层位中。世界上含矿的地幔橄榄岩普遍具有垂直熔融分带特征,即上部偏基性,下部偏酸性。豆荚状铬铁矿床与纯橄岩-方辉橄榄岩相密接相关,却很少见有豆荚状铬铁矿床产于二辉橄榄岩中。豆荚状铬铁矿的成矿作用经历了由洋中脊(MOR)扩张环境向岛弧体系俯冲环境的转变过程,而岛弧环境(岛弧、弧后盆地、弧前盆地等)是形成冶金级豆荚状铬铁矿的最为有利的构造环境。富铬铬铁矿与纯橄岩、玻安岩(Boninite)均为亏损地幔橄榄岩再次高度熔融的最终产物,而玻安岩普遍产于岛弧环境。虽然玻安岩不是铬的有效载体,但玻安岩的熔离促使铬铁矿达到进一步的富集。铬铁矿中的铬来自原始地幔,主要来自于地幔橄榄岩中两种辉石的不一致熔融及其对副矿物铬尖晶石的改造。随着部分熔融程度的增高,地幔橄榄岩逐渐向富镁方向演变,而对应的造矿铬尖晶石也逐渐向富镁、富铬方向演变。     

西藏雅鲁藏布江蛇绿岩带罗布莎地幔橄榄岩的成因

广泛分布在雅鲁藏布江蛇绿岩带的地幔橄榄岩是没有蛇纹石化的新鲜岩石,因而通过显微镜下观察和全岩化学分布探索其成因和地幔活动,是十分有利和简便的。最近几年来,利用岩石化学资料揭示远洋橄榄岩,认为它不是原始地幔岩经分或分离熔化作用萃取出玄武岩熔体后的直接残余物。罗布落蛇绿岩橄的镜下观察和岩石化学资料也证明,它不是原始地幔岩经局部熔化后的残余物,而且由来自更深层的地幔过渡带的硅酸盐超高压矿物底辟上升经熔取     

大洋地幔橄榄岩-铬铁矿中的金刚石和深地幔再循环

全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成（δ13C-18‰~-28‰）,并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。     

蛇绿岩地幔岩中自由SiO2的发现及其地质意义

自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。     

西藏泽当地幔橄榄岩中的异常矿物及其指示意义

雅鲁藏布江蛇绿岩带内多个地幔橄榄岩体产有金刚石、碳硅石等异常矿物组合,为了进一步探讨这些异常矿物形成的物理化学条件,在前人已有的研究基础上,对泽当地幔岩体中526 kg的方辉橄榄岩样品开展人工重砂矿物学研究工作。研究表明,同雅鲁藏布江蛇绿岩带内的其他岩体相似,泽当地幔橄榄岩也选出了包含金刚石、碳硅石、锆石等30余种矿物。异常矿物组合指示泽当地幔橄榄岩中存在局部的超高压、极还原环境,可能经历了复杂的演化过程:即古老地壳物质通过深俯冲或者折沉作用,进入地幔甚至是地幔过渡带(410~660 km),随后经历了超高压、极还原环境的改造,在后续的地幔柱或地幔对流作用中从洋中脊上升至浅部环境并返回到地壳中。该过程中地幔橄榄岩中的异常矿物组合记录了岩石的演化信息,因此开展地幔橄榄岩中异常矿物组合的精细矿物学研究,对认识壳-幔物质交换以及深部地幔动力学过程都有重要的研究意义。     

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

In recent years diamonds and other unusual minerals(carbides,nitrides,metal alloys and native elements) have been recovered from mantle peridotites and chromitites(both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings.Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite,west Albania.So far,more than 20 grains of microdiamonds and 30 grains of moissanites(SiC) have been separated from the podiform chromitite.The diamonds are mostly light yellow,transparent,euhedral crystals,200~300 μm across,with a range of morphologies;some are octahedral and cuboctahedron and others are elongate and irregular.Secondary electron images show that some grains have well-developed striatums.All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ~1325 cm~(-1).The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue,but some are yellow to light yeUow.All the analyzed grains have typical Raman spectra with shifts at 766 cm~(-1),787 cm~(-1),and 967 cm~(-1).The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon.This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys.Our new findings suggest that diamonds and moissanites are present,and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.     

Petrochemical Investigation of the Antique Ophiolite (Philippines): Implications on Volcanogenic Massive Sulfide and Podiform Chromitite Deposits

Abstract: The Antique ophiolite, located in Panay island (west‐central Philippines), corresponds to several tectonic slices within the suture zone between the Philippine Mobile Belt (PMB) and the North Palawan Block (NPB). It includes dismembered fragments of a basaltic sequence, dominantly pillow‐lavas with minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, subordinate layered mafic and ultramafic rock sequences and serpentinites. Most of the ophiolite units commonly occur as clasts and blocks within the serpentinites, which intrude the whole ophiolitic body, as well as, the basal conglomerate of the overlying Middle Miocene sedimentary formation. The volcanic rock sequence is characterized by chemical compositions ranging from transitional (T)‐MORB, normal (N)‐MORB and to chemistry intermediate between those of MORB and island arc basalt (IAB). The residual upper mantle sequence is harzburgitic and generally more depleted than the upper mantle underlying modern mid‐oceanic ridges. Calculations using whole‐rock and mineral compositions show that they can represent the residue of a fertile mantle source, which have undergone degrees of partial melting ranging from 9‐22.5 %. Some of the mantle samples display chondrite‐nor‐malized REE and extended multi‐element patterns suggesting enrichments in LREE, Rb, Sr and Zr, which are comparable to those found in fore‐arc peridotites from the Izu‐Bonin‐Mariana (IBM) arc system. The Antique ultramafic rocks also record relatively oxidizing mantle conditions (Δlog f_O2 (FMQ)=0.9‐3.5). As a whole, the ophiolite probably represents an agglomeration of oceanic ridge and fore‐arc crust fragments, which were juxtaposed during the Miocene collision of the PMB and the NPB. The intrusion of the serpentinites might be either coeval or subsequent to the accretion of the oceanic crust onto the fore‐arc. Volcanogenic massive sulfide (VMS) deposits occur either in or near the contact between the pillow basalts and the overlying sediments or interbedded with the sediments. The morphology of the deposits, type of metals, ore texture and the nature of the host rocks suggest that the formation of the VMS bodies was similar to the accumulation of metals around and in the subsurface of hydrothermal vents observed in modern mid‐oceanic ridge and back‐arc basin rift settings. The podiform chromitites occur as pods and subordinate layers within totally serpentinized dunite in the residual upper mantle sequence. No large coherent chromitite deposit was found since the host dunitic rocks often occur as blocks within the serpentinites. It is difficult to evaluate the original geodynamic setting of the mineralized bodies since the chemistry of the host rocks were considerably modified by alteration during their tectonic emplacement. A preliminary conclusion for Antique is that the VMS is apparently associated with a primitive tholeiitic intermediate MORB‐IAB volcanic suite, the chemistry of which is close to the calculated composition of the liquid that coexisted with the podiform chromitites.     

In-situ Moissanite in Dunite: Deep Mantle Origin of Mantle Peridotite in Luobusa Ophiolite, Tibet

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite, Tibet. The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 μm× 10 μm and 20 μm× 7 μm, respectively. The moissanite is green with parallel extinction. The absorption peaks in its Raman spectra are at 967-971 cm-1, 787-788 cm-1, and 766 cm-1. The absorption peaks in the infrared spectra are at 696 cm-1, 767 cm-1, 1450 cm-1, and 1551 cm-1, which are distinctly different from the peaks for synthetic silicon carbide. Moissanites have been documented to form in ultra-high pressure, high temperature, and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin. Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite, the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite. Further, we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment. Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.     

西藏班公湖-怒江缝合带中段切里湖豆荚状铬铁矿地球化学特征及构造意义

切里湖蛇绿岩出露于班公湖-怒江缝合带中段,蕴含较为丰富的铬铁矿资源。蛇绿岩主要由蚀变地幔橄榄岩、辉长岩和辉绿岩组成,铬铁矿矿体呈透镜状分布在地幔橄榄岩内部。切里湖地幔橄榄岩橄榄石Fo(90.06～90.74)和铬尖晶石Cr^#值(67.45～85.42)较高,全岩富集MgO、LREE和大离子亲石元素(LILEs,如Rb、Ba等),亏损Al₂O₃、CaO、高场强元素(HFSE,如Th、Ta、Ce、Nb)和PPGE(Rh、Pt、Pd),这些特征与缝合带其他地幔橄榄岩相似,指示它们具有深海地幔橄榄岩及弧前地幔橄榄岩特征,经历了早期高程度熔体抽取和晚期熔体交代过程。切里湖铬铁矿矿石具有致密块状和浸染状构造,包含磁铁矿、黄铜矿、方铅矿等多种包裹体。铬铁矿Cr^#值为65.5～75.8,Mg^#值为64.89～76.04,平衡熔体显示出玻安质熔体的亲缘性。与地幔橄榄岩相比,切里湖铬铁矿的IPGE与PPGE分馏更加明显,IPGE更加富集。地幔橄榄岩和铬铁矿Δlg f_O2