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金刚石(又称钻石)主要由碳元素组成,是目前已知的天然存在的最硬的物质.金刚石是天然存在的3种碳的同素异形体之一,另外两种碳的同素异形体分别为石墨和六方碳(Lonsdaleite)(Frondel and Marvin, 1967; Shirey et al., 2013).由于金刚石特殊的物理和化学性质,其自身具有重要的经济和科研价值.
自然界中的金刚石主要产出于浅成侵入岩/火山岩(如金伯利岩和钾镁煌斑岩) (郑建平等, 1994; Tappert et al., 2005; Gurney et al., 2010; Shirey et al., 2013; Stachel and Luth, 2015),此外在超高压变质岩(董振信, 1991; 徐树桐和季寿元, 1991; Dobrzhinetskaya et al., 2001; 杨经绥等, 2002; Yang et al., 2003; Ogasawara, 2005)、陨石以及陨石冲击坑中也报道有金刚石的产出(Lewis et al., 1987; Huss and Lewis, 1995; Huss, 2005).然而,近二十年来,金刚石相继被报道产出于世界范围内不同地区的蛇绿岩铬铁矿(白文吉等, 2001; Yang et al., 2007; 杨经绥等, 2007; Xu et al., 2009; Howell et al., 2015a; Tian et al., 2015; Lian et al., 2017; Wu et al., 2017; Xiong et al., 2017)和地幔橄榄岩(Xu et al., 2015, 2017)中,被认为是一种新产出类型的金刚石.
地球的地壳和地幔之间的碳和氮的循环,以及地球的深部物质组成是地球科学研究中的热点.但是来自地幔的样品通常具有较低的氮含量,容易受到大气圈氮的污染,因此制约了对地球深部的氮含量及其同位素组成的研究(Cartigny et al., 2001).同时,由于目前科学技术条件的限制,所能获得的来自地球深部的样品极为有限,对于地球深部的物质组成的研究存在较大障碍.金刚石形成于地球深部大于150 km的深度或者压力大于5 GPa的条件下,因此金刚石及其内部的包裹体为探索深部地球的物理和化学性质提供了重要的研究对象(Stachel et al., 2004; Harlow and Davies, 2005; Tappert et al., 2005).在过去的四十多年里,研究人员对世界不同地区的金刚石进行了大量详细的研究,为地球内部物质组成和物理化学条件提供了大量的信息(Cartigny, 2005; Stachel and Harris, 2009; Shirey et al., 2013).
我国的金伯利岩主要分布于华北地台和扬子地台区内,如辽宁、山东、贵州、湖北、河南等地,而其中具有重要经济意义的岩体则主要分布在华北地台的辽宁和山东两地(董振信, 1991; 郑建平等, 1994; 刘观亮等, 1995).我国的含矿钾镁煌斑岩主要产于黔东南、鄂中以及湘中地区(梅厚钧等, 1998).与国外学者对于金刚石的研究程度相比,我国的金刚石研究较为薄弱,尤其是对金刚石的碳和氮同位素的组成特征以及内部包裹体的特征缺乏系统的研究(郑建平等, 1994; 刘观亮等, 1995; 陈华等, 2013).
本文针对国内外金刚石的研究现状,梳理总结了全球金刚石的不同产出类型、碳同位素组成、氮同位素组成以及内部的包裹体组成特征,以期为我国不同类型的金刚石的研究工作以及金刚石矿床的找矿工作提供借鉴和参考.
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根据金刚石的来源和成因的不同,可以将金刚石划分为幔源型金刚石、超高压变质型金刚石和陨石相关型金刚石(Harlow and Davies, 2005; Huss, 2005; Ogasawara, 2005; Tappert et al., 2005; Shirey et al., 2013; 杨经绥等, 2013),不同类型的金刚石在全球的分布具有不均一性(图 1).杨经绥等(2013)通过前期大量的研究工作,提出了一种新的类型的金刚石,即蛇绿岩型金刚石(或罗布莎型金刚石),并将其与上述3种类型的金刚石进行了较为详细的对比和介绍(Bai et al., 1993; Robinson et al., 2004; 杨经绥等, 2011, 2013).蛇绿岩型金刚石产出于蛇绿岩中的地幔层位,也应归属于幔源型金刚石,但是由于其特殊性,本文将其单独归为一类.
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幔源型金刚石主要形成于深度大于140 km的岩石圈地幔深部(图 2),这些金刚石主要被3种类型的岩浆携带运移至地球的浅部(Shirey et al., 2013).这3种类型的岩浆岩分别是金伯利岩、钾镁煌斑岩以及煌斑岩,其中金伯利岩最为重要,钾镁煌斑岩的经济价值仅次于金伯利岩,但二者均是宝石级金刚石的主要来源.在全世界范围内已经发现的几千个金伯利岩岩体和几百个钾镁煌斑岩岩体中,约有30%的岩体含有金刚石(Shirey et al., 2013).澳大利亚的Argyle钾镁煌斑岩岩体,是目前世界上最大的金刚石矿体(Jaques et al., 1990; Luguet et al., 2009).不同于金伯利岩和钾镁煌斑岩,仅有少数的煌斑岩被报道含有金刚石,其经济价值较弱.加拿大的Wawa和Abitibi省的煌斑岩是目前世界上已知的最古老的金刚石矿,具有重要的研究价值(Lefebvre et al., 2005; Wyman et al., 2006).
金伯利岩的原始岩浆可能来源于200~650 km的地幔,一般呈岩管、岩墙或岩脉形式产出,是一种富含挥发分和钾质的偏碱性超基性超浅成侵入岩/火山岩(Harte, 2010; 杨经绥等, 2013; 杨志军等, 2016).钾镁煌斑岩主要呈岩管,部分呈脉状形式产出,是一种来源于岩石圈地幔的超钾质和超镁质基性或超基性浅成侵入岩或火山岩(叶德隆, 1993; 梅厚钧等, 1998; 曾璇, 2016).此外在与这些含金刚石的岩体相关的冲积物中,也被广泛报道有金刚石产出(池际尚等, 1996; Tappert, 2006; Afanas’ev et al., 2009; Shatsky et al., 2014).根据大量金刚石中的硅酸盐包裹体的温压计算的结果,幔源型金刚石主要来源于地球140~200 km的深度范围(Boyd and Gurney, 1986; Meyer, 1987; Stachel et al., 2005).然而,近30年来,大量包裹体的研究证实了金刚石可以来源于更深的地幔过渡带,甚至下地幔的深度,这种类型的金刚石在全世界金刚石总量中所占的比例 < 1% (图 3)(Moore and Gurney, 1985; Stachel, 2001; Tappert et al., 2005; Kaminsky, 2012; Anzolini et al., 2016).这些来源岩石圈下部的金刚石(sublithospheric diamond)主要在世界范围内的4个地区有较多产出,分别为南非的Monastery (Moore and Gurney, 1985; Moore et al., 1991)和Jagersfontein (Deines et al., 1991; Tappert et al., 2005),巴西的Juina (Harte and Harris, 1994; Harte and Cayzer, 2007; Bulanova et al., 2010)以及几内亚的Kankan地区(Stachel et al., 2000).对于地球深部结构和物质组成的研究是地球科学领域的热点,然而由于目前技术手段的限制,尚无法直接对地球的深部地幔进行直接取样,因此造山带中的地幔橄榄岩以及岩浆中的地幔捕掳体为研究地球地幔的物质组成以及地球动力学过程提供了重要的素材(郑建平等, 2004; Luguet et al., 2009; Yang et al., 2015; Griffin et al., 2016).然而这些地幔样品往往来源于深度小于200 km的地幔,而来自更深部的岩石样品则极为稀少(Sautter et al., 1991; MacDougall and Haggerty, 1999),因此大大制约了对深部地幔物质组成的认识.大家对岩石圈下部地幔的认识,主要间接地来源于高温高压实验和地震数据资料,而来源于岩石圈下部地幔的金刚石及其包裹体为认识地球的深部地幔提供了更为直接和丰富的研究对象(Kaminsky, 2012; Stachel and Luth, 2015).
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超高压变质型金刚石产出于经历了超高压变质作用(> 4 GPa)的壳源岩石之中,对于理解和研究板片的深俯冲(> 150 km)以及快速折返至地球浅部的动力学机制具有重要的价值(Rozen et al., 1972; Sobolev and Shatsky, 1990; Ogasawara, 2005; Dobrzhinetskaya, 2012; 杨经绥等, 2013).Rozen et al.(1972)首次报道了哈萨克斯坦的Kokchetav岩体中的微粒金刚石的产出,但是其变质成因在近20年之后才被学者所接受(Sobolev and Shatsky, 1990).Kokchetav变质岩中的金刚石粒径在12 μm左右,主要呈包裹体形式产于锆石之中(图 3),此外在变质岩中的石榴石中还发现了自形的石墨颗粒,Sobolev and Shatsky(1990)认为这些金刚石可能形成于一种新的环境,代表一种新类型的金刚石(Sobolev and Shatsky, 1990).徐树桐和季寿元(1991)在大别山的榴辉岩、石榴石榴辉岩以及硬玉岩中发现有微粒金刚石.大别山变质岩中的金刚石呈包裹体形式产出于石榴石中,粒径主要分布在10~60 μm的粒度范围内,但也有少数金刚石的粒径可达240 μm,部分金刚石中可见包裹体(徐树桐和季寿元, 1991; Xu et al., 1992).Dobrzhinetskaya et al.(1995)从挪威的Western Geiss地区的高级片麻岩中分选出了3颗微粒金刚石,呈黄绿色,粒径在10~45 μm,但是并未发现原位的金刚石(Dobrzhinetskaya et al., 1995). van Roermund et al.(2002)对挪威西部的Fjortoft岛的石榴石二辉辉石岩进行了研究,在石榴石中的一颗铬尖晶石中发现了原位的金刚石,证实了Dobrzhinetskaya et al. (1995)在高级片麻岩中所发现的金刚石(van Roermund et al., 2002).在挪威西部石榴石二辉辉石岩中发现的金刚石粒径约为5 μm,同时与金云母、独居石、尖晶石、方镁石和白云石等矿物共生(van Roermund et al., 2002).Massonne (1999)报道了德国Erzgebirge的石榴石白云母片麻岩中发现有微粒金刚石的产出,粒径可达20 μm (图 3a).德国Erzgebirge片麻岩中的金刚石主要呈包裹体产出于石榴石、蓝晶石以及锆石中(Massonne, 1999, 2001; Nasdala and Massonne, 2000; St ckhert et al., 2001).在希腊的Rhodope地体的变质沉积岩中也报道有微粒金刚石的产出,这些金刚石的粒径在10~20 μm,呈包裹体产于石榴石和黑云母中;这些金刚石的发现使得Rhodope变质省(Rhodope Metamorphic Province)成为世界上又一条重要的变质岩带(Mposkos and Kostoponlos, 2001).在世界不同地区相继发现金刚石之后,我国的超高压变质带研究也取得了重要进展.Yang et al. (2003)在北秦岭地区的榴辉岩和片麻岩中发现了微粒金刚石以及大量的石墨,该发现具有重要意义,它将中国西部的北柴达木超高压变质带与东部的北大别超高压变质带连接在了一起,在中国境内构成了一条超过4 000 km加里东期超高压变质带(Xu et al., 1992; 杨经绥等, 2002; 许志琴等, 2003; Yang et al., 2003).在捷克共和国境内的Bohemian岩体中的Moldanubian带中的尖晶石-石榴石地幔橄榄岩以及长英质麻粒岩中也报道有金刚石的产出,其中地幔橄榄岩中的金刚石(一粒)来源于重矿物分选(Naemura et al., 2011),而麻粒岩中的金刚石(两粒)为锆石中的原位金刚石包裹体(Perraki and Faryad, 2014).Janák et al. (2015)在奥地利的东阿尔卑斯的Pohorje地区的变质沉积岩的石榴石中发现有原位的金刚石包裹体,粒径仅为若干微米,呈粉色、黄色至棕色,与碳硅石伴生产出(Janák et al., 2015).
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陨石相关型金刚石可以进一步划分为陨石型金刚石(Lewis et al., 1987; Daulton et al., 1996; Huss, 2005)以及陨石冲击型金刚石(Hough et al., 1995; Koeberl et al., 1997; Goresy et al., 2001).陨石型金刚石是指产出于陨石中的金刚石,这种类型的金刚石由于含具有异常同位素组成的稀有气体元素,而在1987年被首次发现(Lewis et al., 1975, 1987).Lewis et al. (1987)报道了Allende陨石含有400×10-6的极细粒的碳质物质.由于这些碳质物质中的稀有气体元素氙具有异常的同位素组成,而被认为是太阳系之外的物质(presolar material) (Lewis et al., 1987).这些碳质物质的粒径仅为2 nm,通过对这些碳质物质进行X射线衍射分析(X⁃ray diffrection),最终确定这些物质为纳米级的金刚石(Lewis et al., 1987).Lewis et al. (1987)对莫奇森陨石(Murchison meteorite) (Bernatowicz et al., 1996)和Indarch陨石(McCoy et al., 1999)进行了同样的处理,最后在残余的物质中发现纳米级的金刚石.Daulton et al. (1996)将陨石中的纳米金刚石与人工合成的高压冲击型金刚石和低压化学蒸气沉积金刚石进行对比,认为陨石中的纳米金刚石可能同样形成于一种化学蒸汽沉积过程(Huss,2005),与Lewis et al. (1987)的观点一致.陨石冲击型金刚石通常产出于因陨石撞击形成的陨石坑中的岩石(Hough et al., 1995; Abbott et al., 1996; Koeberl et al., 1997; Langenhorst et al., 1998; Goresy et al., 2001),此外在白垩纪/第三纪(K/T)界限中也发现有金刚石(Carlisle and Braman, 1991; Gilmour et al., 1992).在白垩纪/第三纪界限中发现的金刚石为纳米级(3~5 nm),粒度稍大于陨石中的纳米金刚石(Lewis et al., 1987; Gilmour et al., 1992).不同的是,K/T边界中的纳米级金刚石与陨石中的纳米级金刚石具有不同的碳(C)和氮(N)同位素,因此可能具有不同的物质来源(Gilmour et al., 1992).Hough et al.(1995)对Ries陨石坑中的冲击成因的凝灰角砾岩(suevite)进行酸处理,在残余物中发现了粒径从纳米级到微米级的金刚石,同时伴随有SiC的发现,这些金刚石可能是在陨石冲击过程中通过化学蒸汽沉积形成的(Hough et al., 1995).Abbott et al.(1996)再次报道了Ries陨石坑的爆炸玻璃中存在有粒径达300 μm的板状金刚石.除了化学蒸气沉积成因,陨石冲击型金刚石也可能由石墨直接发生固相转变而来(Hough et al., 1995; Koeberl et al., 1997; Goresy et al., 2001).Koeberl et al., (1997)发现俄罗斯Popgai陨石坑中的金刚石与该地区的变质片麻岩中的石墨具有相似的矿物学、结晶学和地球化学特征,指示了金刚石可能是由石墨在陨石冲击下直接发生固相转变而来.然而由于化学处理破坏了含有金刚石的岩石结构,隐藏了陨石冲击型金刚石的成因信息,因此原位的(in situ)金刚石的发现对于解决其成因问题至关重要.Goresy et al.(2001)报道了在德国Ries陨石坑中的片麻岩中发现的原位的石墨-金刚石矿物组合,为陨石冲击型金刚石可以由石墨直接转变而来提供了依据.
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蛇绿岩型金刚石(或罗布莎型金刚石)是指产出于蛇绿岩地幔橄榄岩以及相关的豆荚状铬铁矿中的一种新的类型的金刚石,这种类型的金刚石先后在全世界多个地区的蛇绿岩和铬铁矿中被发现(图 4) (杨经绥等, 2013; Yang et al., 2014, 2015; Lian and Yang, 2019).中国地质科学院地质研究所金刚石组于1981年首次在西藏雅鲁藏布江缝合带东段的罗布莎铬铁矿中发现了金刚石.Bai et al.(1993)报道了在西藏的罗布莎和东巧蛇绿岩中的地幔橄榄岩和铬铁矿中发现有金刚石、SiC、石墨、自然Cr以及Ni⁃Fe合金等矿物,这些金刚石最初被认为是在板片的深俯冲过程中形成的.在此之后,研究人员对罗布莎铬铁矿和地幔橄榄岩进行了大量的选矿工作,再次发现了金刚石、碳化硅等超高压/超还原矿物(Xu et al., 2009).蛇绿岩和豆荚状铬铁矿通常被认为是形成于地球浅部 < 30 km的深度范围(Boudier and Nicolas, 1985; Zhou and Robinson, 1997; Dilek and Furnes, 2011; 程晨等, 2018; 刘飞等, 2018),而金刚石则形成于 > 120 km的深度范围(Cartigny, 2005).此外,由于这些金刚石是通过矿物分选而来,并非原位发现,因此其天然成因受到了质疑(切切斯特钻石公司考察团,1997).杨经绥等(2004)在从罗布莎铬铁矿中分选出的钛铁(TiFe)合金的外部发现了原位的柯石英和蓝晶石的矿物组合,指示了高压的形成条件(> 2.6 GPa),一定程度上支持了铬铁矿中超高压矿物金刚石的发现.随后,Yang et al.(2007)再次在罗布莎铬铁矿中分选出来的锇铱合金中发现了一颗原位的微米级金刚石(1~2 μm).这颗原位金刚石的发现为蛇绿岩型金刚石的天然成因而非人工混染提供了强有力的证据.由于原位的金刚石的发现对于蛇绿岩型金刚石分类的建立具有重大的意义,因此需要发现更多的原位金刚石来提供更多的支持.通过大量的镜下观察工作,Yang et al.(2014, 2015)在中国西藏罗布莎铬铁矿以及俄罗斯的Ray⁃Iz铬铁矿的铬尖晶石矿物中再次发现6颗原位的金刚石.蛇绿岩型地幔橄榄岩和铬铁矿中金刚石的发现,引起了国外学者的广泛关注(Trumbull et al., 2009; Howell et al., 2015a; McGowan et al., 2015; Griffin et al., 2016; Rollinson, 2016).澳大利亚麦考瑞大学的研究团队对西藏罗布莎铬铁矿进行了独立的取样和矿物分选工作,并从手标本大小的铬铁矿中分选出了若干粒金刚石(Howell et al., 2015a).通过对这些金刚石进行微量元素、氮含量以及碳、氮同位素组成的研究,Howell et al.(2015a)认为罗布莎铬铁矿中的金刚石与人造金刚石存在有较大的差异,排除了其人为混染的可能.Trumbull et al.(2009)通过对阿曼的Semail铬铁矿进行独立选矿工作,在其中发现了碳硅石(又称碳化硅,SiC)矿物.Das et al.(2017)首次报道了在Nidar蛇绿岩地幔橄榄岩中发现原位的金刚石(图 5d).Nidar蛇绿岩中的金刚石粒径约为40 μm,典型八面体外形,呈包裹体形式产于地幔橄榄岩的斜方辉石中,同时与其他原位超高压矿物伴生产出(Das et al., 2015, 2017).对金刚石开展的傅立叶变换红外光谱(FTIR)和同位素组成研究工作(Howell et al., 2015a; Xu et al., 2017)均证实了这些金刚石并非人工混染,而是天然成因.Butler and Beaumont (2017)的数值模拟工作也为蛇绿岩型金刚石的成因提供了参考,这些不同研究团队的研究成果以及原位金刚石的发现为蛇绿岩型金刚石概念的建立提供了有力的支撑.然而,关于蛇绿岩型金刚石的另一个重要科学问题是,这种类型的金刚石是否在不同的蛇绿岩中普遍存在?杨经绥及其研究团队针对该问题,对全球多个地区的蛇绿岩进行了采样工作.Yang et al.(2015)报道了在俄罗斯Ray⁃Iz铬铁矿中发现有金刚石、自然元素和金属合金等矿物.Tian et al. (2015)和Huang et al. (2015)分别在新疆的萨尔托海和内蒙古的贺根山高铝型铬铁矿中分选出金刚石等矿物.此外,在中国西藏的当穷岩体(Xiong et al., 2016),阿尔巴尼亚Skenderbeu(Wu et al., 2017)和Bulqiza岩体(Xiong et al., 2017)以及土耳其的Pozantı⁃Karsantı岩体(Lian et al., 2017)中均被报道有蛇绿岩型金刚石的产出(图 4).近期,在缅甸的Myitkyina蛇绿岩中也发现了金刚石等其他异常矿物(Chen et al., 2018).这些新的研究成果指示了蛇绿岩型金刚石可能普遍存在于蛇绿岩地幔橄榄岩和铬铁矿中(Yang et al., 2015).
不同地区的蛇绿岩型地幔橄榄岩和铬铁矿中金刚石的含量存在有较大差异.Xu et al. (2009)从罗布莎蛇绿岩1 100 kg铬铁矿样品中发现了超过1 000粒的金刚石,约1粒/kg;内蒙古贺根山蛇绿岩中2 000 kg的样品中被报道含有130多粒金刚石,约为0.06粒/kg (Huang et al., 2015);新疆萨尔托海铬铁矿的900 kg样品中仅发现有20多粒金刚石(Tian et al., 2015);土耳其Pozantı⁃Karsantı蛇绿岩约500 kg铬铁矿样品中含有100多粒金刚石(Lian et al., 2017);阿尔巴尼亚Mirdita蛇绿岩的Skenderbeu岩体600 kg样品中含有20多粒金刚石(Wu et al., 2017);缅甸Myitkyina蛇绿岩1 540 kg样品中仅发现有6粒金刚石(Chen et al., 2018).不同地区蛇绿岩中金刚石含量的差异会在一定程度上受到人为因素的影响,但是更重要的是,这种差异性在一定程度上可能反映了地幔物理化学条件和物质组成的不均一性.
蛇绿岩型金刚石在晶体形态、颜色、粒度以及内部结构方面具有相似性.这些金刚石通常为单晶,呈自形至半自形,无色至浅黄色,粒径为0.2~0.5 mm,少数可达0.7 mm (图 5).Yang et al.(2007)报道的一粒锇铱合金中的原位金刚石粒度较小,仅为2 μm左右(图 5b).在中国西藏的罗布莎铬铁矿以及俄罗斯的Ray⁃Iz铬铁矿中发现的原位金刚石与通过重砂矿物分选所获得的金刚石在颜色、粒径、晶型等方面具有一致的特征(图 5c, 5d) (Yang et al., 2014, 2015).通过对铬铁矿中原位金刚石进行元素面成分扫描分析,可以发现这些原位金刚石的周围发育有无定形碳,指示了金刚石可能结晶于碳饱和的流体(Yang et al., 2015).金刚石的晶体形态较为多样,包含有八面体、立方八面体以及十二面体晶型等(图 6a,6b).在罗布莎蛇绿岩中发现的部分金刚石还呈现有多晶或者骸晶的形态(图 6c, 6d).蛇绿岩型金刚石的阴极发光图像揭示了该类型金刚石与其他类型金刚石不一样的内部结构特征(图 6e, 6f).罗布莎铬铁矿中的金刚石在阴极发光图像下显示清晰的明暗分区的特征(Howell et al., 2015a),不同于金伯利岩型金刚石普遍发育的振荡环带的特征(陈华等, 2013).Howell et al. (2015a)指出罗布莎蛇绿岩型金刚石在阴极发光图像下的明暗分区分别代表着金刚石的八面体生长区({111}区,亮区)和立方体生长区({100}区,暗区).这种明暗分区的现象同样存在于土耳其的Pozantı⁃Karsantı蛇绿岩(Lian et al., 2018)和阿尔巴尼亚的Skenderbeu蛇绿岩的金刚石中(Wu et al., 2017).
图 6 蛇绿岩型金刚石单晶(a,b),多晶(c)以及骸晶(d)二次电子图像;蛇绿岩型金刚石不同生长区阴极发光图像(e,f)
Figure 6. Secondary electron images for single-crystal(a, b), polycrystal(c) and skeletal-crystal (d) of ophiolitic diamond; cathodoluminescence images showing different growth sectors of ophiolitic diamond(e, f)
蛇绿岩中金刚石的产出通常伴随有一系列的其他矿物,这些矿物既包含来自地球深部的超高压/超还原矿物,也包含有来自浅部的壳源矿物,详见附表 1.除去主要造岩矿物,如橄榄石、单斜辉石、斜方辉石和铬尖晶石,这些伴生矿物主要包含有碳硅石(SiC)、金红石、锆石、八面体硅酸盐矿物、金属合金、单质元素以及硫化物等.在这些矿物中碳硅石是与金刚石伴生的最重要的矿物之一.碳硅石,又称为碳化硅,主要成分是硅和碳.该矿物首次被发现于Canyon Diablo陨石中(Moissan, 1904).在该矿物发现之初,碳化硅被认为是来源于人造碳化硅的混染.但是,金刚石中碳化硅包裹体的发现(Leung et al., 1990)以及原位碳化硅的产出证实了碳化硅的天然成因(di Pierro et al., 2003; Xu et al., 2008).蛇绿岩铬铁矿中的碳化硅通常呈无色、浅绿色、浅蓝色或者深蓝色,粒径通常在50~200 μm,具有6H、4H、3C和15R等多种类型.实验岩石学和理论计算证实了碳化硅形成于极其还原的环境,其形成的氧逸度条件较铁-方铁矿氧逸度缓冲线低5~7个数量级(Mathez et al., 1995; Golubkova et al., 2016),指示了含金刚石和碳化硅的蛇绿岩地幔橄榄岩和铬铁矿经历了超还原的环境,然而这种超还原的环境是代表了地球深部的下地幔环境(Trumbull et al., 2009; Yang et al., 2015),还是地球上地幔的一种局部的显微超还原环境(Schmidt et al., 2014; Golubkova et al., 2016),仍然存在有较大的争议.
与金刚石伴生的另一个重要矿物为锆石,该矿物可以为金刚石的寄主岩石提供年龄方面的限制,然而仅有罗布莎蛇绿岩中的锆石开展了U⁃Pb定年工作(Yamamoto et al., 2013; McGowan et al., 2015; Robinson et al., 2015).罗布莎蛇绿岩被认为初始形成于177±31 Ma的大洋中脊环境,随后在120±10 Ma经历了俯冲带作用的改造(Zhou et al., 2002;Malpas et al., 2003).罗布莎铬铁矿中的锆石呈无色或浅褐色,粒度整体在30~200 μm之间,自形柱状至他形浑圆状.Yamamoto et al. (2013)利用激光剥蚀电感耦合等离子质谱(LA⁃ICP⁃MS)对罗布莎铬铁矿中的锆石进行了U⁃Pb定年,所获得的锆石年龄值具有较大的变化范围,介于100~2 700 Ma.Robinson et al.(2015)利用二次离子质谱仪对罗布莎铬铁矿的锆石进行了精确的定年,获得锆石年龄为90~2 500 Ma,与Yamamoto et al.(2013)获得的年龄范围接近.罗布莎铬铁矿中的锆石年龄整体老于蛇绿岩中基性岩的年龄.结合铬铁矿中锆石的包裹体组成,这些锆石被认为是地壳物质再循环进入地幔、并最终被铬尖晶石包裹返回地表的(Yamamoto et al., 2013; Robinson et al., 2015).McGowan et al. (2015)在罗布莎铬铁矿中发现了自形的锆石,该锆石具有较好的振荡环带,其U⁃Pb年龄为376±7 Ma,并认为该锆石可能代表了铬铁矿从玻安质熔体结晶的年龄.然而,这种观点与罗布莎蛇绿岩的构造演化明显不符,与蛇绿岩中大量岩浆岩的年龄是矛盾的(McDermid et al., 2002; Zhou et al., 2002; Guilmette et al., 2009; Xu et al., 2011).为了更好地理解蛇绿岩铬铁矿及地幔橄榄岩中锆石的来源,需要对其他地区的蛇绿岩中的锆石开展更多精细的锆石定年工作.
在已经发现的异常矿物组合中,有一类特殊的八面体硅酸盐矿物,该矿物具有良好的八面体晶形,被认为可能是林伍德石假象(杨凤英等, 1981).林伍德石是一种超高压的矿物,是橄榄石的高压多形(Ringwood and Major, 1970; Pearson et al., 2014).在地球410~600 km的深度范围内,橄榄石将转变为瓦兹利石或林伍德石(Kerschhofer et al., 1996; Pearson et al., 2014).铬铁矿中的这类特殊的八面体硅酸盐矿物首次被发现于中国西藏的罗布莎铬铁矿,电子探针和X射线粉晶衍射分析指示该矿物为利蛇纹石(杨凤英等, 1981).易隶文(1987)在西藏安多超镁铁岩再次发现有八面体硅酸盐矿物的产出,这些八面体硅酸盐矿物包含两种类型,一种类型矿物与罗布莎铬铁矿中的八面体硅酸盐矿物一致,均是由蛇纹石组成;另一种类型的八面体硅酸盐矿物则主要由斜绿泥石组成.晶体结构和化学分析显示,西藏罗布莎和安多岩体中的八面体硅酸盐矿物均已经发生完全蚀变,仅保留有原生矿物的假象.Robinson et al. (2004)从罗布莎铬铁矿中分选出了几百颗八面体硅酸盐矿物,其中有若干粒八面体硅酸盐矿物仍然保有立方体的尖晶石结构,化学式为(Mg1.8Fe0.2)SiO4,与林伍德石的晶体结构和化学组成一致,指示了超高压的成因.随着选矿工作的开展,尽管多个地区的豆荚状铬铁矿中被报道含有金刚石和碳化硅等矿物,然而八面体硅酸盐矿物的产出则鲜有报道.Lian et al. (2017)通过土耳其卡桑特铬铁矿进行矿物分选工作,除发现有金刚石、碳化硅、金红石和锆石等矿物产出外,还在该铬铁矿中发现有八面体硅酸盐矿物.然而,Lian et al.(2017)仅对铬铁矿中的八面体硅酸盐矿物进行了简要的描述,对于该矿物的化学组成以及晶体结构特征并未进行深入研究,矿物来源及成因仍不清楚.
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金刚石在形成过程中通常会包裹有周围环境中的物质,这些包裹体保存有金刚石的形成环境、形成时代和形成机制等重要信息(Stachel and Luth 2015; Smit et al., 2016).金刚石中的包裹体主要为硅酸盐矿物(如橄榄石、单斜辉石和石榴石)(图 7) (Stachel and Haris, 2008),此外还包括氧化物(如铬尖晶石和柯石英)(Moore et al., 1991; Hunt et al., 2012; Miller et al., 2014)、单质元素(如自然铁和自然镍)(Kaminsky et al., 2001; Davies et al., 2004);硫化物(Pearson et al., 1999; Smit et al., 2016)以及碳化物(如碳硅石,SiC)(Leung, 1990)和碳化铁(Fe3C、Fe2C和Fe23C6) (Kaminsky and Wirth, 2011) (图 7).地球的不同深度地幔的矿物组成存在差异,研究显示,地球的地幔可以划分为3层:(1)上地幔,主要是由橄榄石以及低钙辉石组成;(2)地幔过渡带(410~660 km),在该层位,橄榄石首先转变为具有似尖晶石结构的β相瓦兹利石,随后转变为尖晶石结构的γ相林伍德石,同时辉石发生溶解并进入石榴子石中,形成超硅石榴石;(3)下地幔,主要以镁硅钙钛矿,钙硅钙钛矿以及铁方镁石为主要组成矿物(图 8) (Stachel, 2001).因此根据金刚石的矿物包裹体,可以推断金刚石来源深度的信息(图 8).
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人类所采集到的金刚石主要来源于大陆下部的岩石圈地幔,然而,根据大量包裹体的研究结果,还有少量的金刚石来源于岩石圈下部的软流圈、过渡带、下地幔甚至核幔边界的深度(Stachel and Harris, 2008; Harte, 2010;Stachel and Luth, 2015). Scott Smith et al. (1984)首次提出一部分金刚石可能起源于下地幔的深度.Moore and Gurney(1985, 1989)首次在Monastery矿床的金刚石中发现有超硅石榴石包裹体,指示金刚石可能形成于软流圈或地幔过渡带.Stachel and Harris(2008)对3 145粒含有硅酸盐和氧化物矿物包裹体的金刚石进行了统计分析,约有90.4%的金刚石来源于克拉通岩石圈地幔;3.6%的金刚石含有超硅石榴石(majorite)包裹体,其中2.8%为深部榴辉岩型,0.8%为深部地幔橄榄岩型;此外还有,6%的金刚石含有铁方镁石(ferropericlase)包裹体,可能来自于地球下地幔(图 9).根据前人的研究成果,铁方镁石和钙钛矿等矿物形成于地幔过渡带下部以及下地幔的压力条件下(Hayman et al., 2005; Stixrude and Lithgowbertelloni, 2007),因此,含有此类包裹体的金刚石被认为来自于大于410 km的深部地幔(Tappert et al., 2005; Kaminsky et al., 2009).
图 9 不同类型的幔源型金刚石的相对比例
Figure 9. The relative abundance of different types of mantle-derived diamonds according to the mineral assemblages
根据包裹体的组合以及成分特征,可以将来自于克拉通岩石圈地幔的金刚石划分为3种类型,分别是:(1)地幔橄榄岩型(或超基性岩型),(2)榴辉岩型;(3)二辉辉石岩型(图 9)(Stachel and Harris, 2008).这3种不同类型的金刚石中,地幔橄榄型金刚石所占的比例约为65%,榴辉岩型金刚石所占比例约为33%,其中二辉辉石岩型金刚石所占比例最小,仅为2%(Meyer, 1987; Stachel and Harris, 2009).通过对大量不同类型的包裹体矿物组成进行统计,Stachel and Harris (2009)获得了不同类型金刚石的包裹体矿物的组成比例.地幔橄榄岩型金刚石的包裹体组成中,石榴石所占比例为29%,橄榄石比例为27%,铬尖晶石比例为27%,斜方辉石为12%,单斜辉石为4%;榴辉岩型金刚石的包裹体组合含有56%的石榴石,39%的单斜辉石,3%的金红石,以及2%的柯石英;二辉辉石岩型金刚石的包裹体组合中石榴石占比为38%,单斜辉石占比为37%,斜方辉石占比为21%,柯石英占比为3%,还有1%的橄榄石.需要指出的是,这些包裹体中不同矿物的比例并不是单颗金刚石中不同矿物的比例,而是统计了大量金刚石包裹体所得到的结果,并且,该比例并非矿物的体积百分比,而是包裹体矿物的数量百分比.这就导致了金刚石包裹体所指示的围岩的矿物组成与地幔岩石实际的矿物组成存在差异,相对于典型的克拉通地幔的石榴石地幔橄榄岩,由统计所得的地幔橄榄岩型金刚石的包裹体组成中,橄榄石的含量偏低而石榴石和铬尖晶石的含量偏高(McDonough and Rudnick, 1998).
地幔橄榄岩型金刚石可以进一步划分为二辉橄榄岩型、方辉橄榄岩型以及异剥橄榄岩型(图 9)(Stachel and Harris, 2009).地幔橄榄岩型金刚石中,方辉橄榄岩型所占比例为86%,二辉橄榄岩型占比13%,异剥橄榄岩型金刚石所占比例为1%(图 9)(Stachel and Harris, 2008, 2009).
如前所述,由于单颗金刚石中所包含的矿物包裹体的矿物类型有限,因此不能通过矿物包裹体的相对比例来确定金刚石的类型.不同类型的金刚石的划分主要是通过包裹体中关键矿物的成分来确定的,例如地幔橄榄岩型金刚石和榴辉岩型金刚石的主要区别在于其中的石榴子石包裹体的成分的差异(Schulze, 2003; Grütter et al., 2004).榴辉岩型金刚石中的石榴子石包裹体的Cr2O3的含量通常小于1%,而地幔橄榄岩型金刚石的石榴子石包裹体的Cr2O3的含量通常大于1%(Grütter et al., 2004).此外,不同类型的金刚石中单斜辉石包裹体的成分也存在差异,榴辉岩型金刚石中单斜辉石的包裹体的Cr#值(Cr/(Cr+Al)×100)通常小于10,而二辉橄榄岩型金刚石的单斜辉石包裹体通常具有较高的Cr#值(图 10)(Stachel and Harris, 2008).
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橄榄石是地球上地幔的主要组成矿物之一,在410 km的深度,橄榄石会发生相变转变为瓦兹利石;在520 km的深度,瓦兹利石会转变为林伍德石;当深度大于660 km,林伍德石会分解为钙钛矿(perovskite,又称布里基曼石,bridgmanite)和铁方镁石(ferropericlase)(图 8)(Perrillat et al., 2006; Stixrude and Lithgowbertelloni, 2007; Harte, 2010).不同类型的幔源型金刚石的橄榄石包裹体的成分往往存在差异(Stachel and Harris, 2008).根据Stachel and Harris (2008)的统计结果,二辉辉石岩型金刚石的橄榄石包裹体通常具有较低的镁值(Mg#=Mg/(Mg+Fe2+)×100),通常小于88;二辉橄榄岩型金刚石中的橄榄石的镁值介于90.1~93.6,平均值为92.0;方辉橄榄岩型金刚石的橄榄石包裹体具有较高的镁值,介于90.2~95.4,平均值为93.2.金刚石中橄榄石包裹体镁值的增高,反映了形成金刚石的围岩的亏损程度的增加,这种从二辉橄榄岩到方辉橄榄岩逐渐亏损的趋势,与现代大洋地幔橄榄岩和蛇绿岩型地幔橄榄岩的亏损特征是一致的.如果金刚石中的橄榄石与单斜辉石共生,可以根据这两种矿物中的氧化钙的含量进行压力计算,提供金刚石生长条件的信息(Khler and Brey, 1990),但是由于天然橄榄石中的Ca含量较低,使得该压力计的使用受到了限制.
金刚石中的斜方辉石包裹体通常为无色、黄色或者绿色.不同类型的金刚石中斜方辉石包裹体的化学成分存在差异,其中二辉辉石岩型金刚石的斜方辉石包裹体具有较低的Mg#,通常小于86;方辉橄榄岩型和二辉橄榄岩型金刚石中的斜方辉石包裹体成分具有一定的重叠,但是整体上方辉橄榄岩型金刚石中的斜方辉石具有较高的Mg#值和较低的CaO和Na2O含量,显示了更为亏损的特征,与橄榄石包裹体的成分具有一致的特征规律(Stachel and Harris, 2008).金刚石中典型的地幔橄榄岩型单斜辉石包裹体通常为翠绿色,含有较高的Cr2O3含量(0.6%~2.4%),同时具有较高的Mg#值(92.5~93.5);榴辉岩型金刚石中单斜辉石包裹体通常为浅绿色,具有较低的Mg#值(< 85)(Stachel and Harris, 2008).利用单斜辉石的Mg#值和Cr#值(Cr3+/(Cr3++Al3+)×100)作图,可以对榴辉岩型单斜辉石和地幔橄榄岩型单斜辉石进行有效的区分,其中地幔橄榄岩型单斜辉石相对榴辉岩型单斜辉石具有更高的Cr#(图 10)(Stachel and Harris, 2008).实验研究显示,石榴子石地幔橄榄岩中的单斜辉石的Cr含量与压力具有相关性,其顽火辉石端元的含量与温度具有相关性,因此单斜辉石可以作为温压计来计算平衡温度和压力(Nimis and Taylor, 2000).利用金刚石中的单斜辉石包裹体的Cr和顽火辉石端元的含量,可以计算金刚石形成的温度和压力.
金刚石中的尖晶石族矿物包裹体以铬尖晶石为主,不透明,通常为深红色,具有较高的Cr/Al比值.金刚石中的铬尖晶石包裹体通常独立产出,少数情况下可见与石榴子石共同产出(Stachel and Harris, 2008).铬尖晶石的Cr#值可以为岩石形成的构造环境提供重要信息(Dick and Bullen, 1984; Kaminsky et al., 2001).根据实验研究的结果,可以预测在金刚石稳定区域内,克拉通型二辉橄榄岩和方辉橄榄岩中铬尖晶石的Cr#值至少大于80(Doroshev et al., 1997).根据Stachel and Harris(2008)的统计结果,98%的金刚石中的铬尖晶石包裹体的Cr#值大于80,普遍高于蛇绿岩型地幔橄榄岩和现代大洋地幔橄榄岩中铬尖晶石的Cr#值(Lian et al., 2016).此外,尖晶石中Zn的含量可以用于计算金刚石的形成温度(Ryan et al., 1996).
方辉橄榄岩型石榴子石和二辉橄榄岩型石榴子石在化学成分上的差异较小.相对于榴辉岩型石榴子石,地幔橄榄岩型石榴子石具有更高的Cr2O3含量(> 1%),而98%已发现的榴辉岩型金刚石中石榴子石包裹体的Cr2O3含量低于0.4%,因此,如前所述,利用石榴子石的Cr2O3含量,可以对金刚石类型进行有效的划分(图 10) (Stachel and Harris, 2008).地幔捕掳体中共生的橄榄石和铬镁铝榴石的Ni含量主要是受温度控制(Griffin et al., 1989;Griffin and Ryan, 1995),因此,金刚石中共生的橄榄石和镁铝榴石组合可以提供金刚石形成的温度信息.Griffin and Ryan(1995)指出,其实验室所测得的石榴子石地幔橄榄岩中橄榄石的Ni含量,基本保持在2 900×10-6±360×10-6的数值范围,而石榴子石的Ni含量则随温度有明显变化,因此石榴子石的Ni含量可以作为一个独立的地质温度计.Ryan et al. (1996)对石榴子石的Ni温度计进行了改进,使得该温度计的准确度达到±50℃.根据Cr和Al元素在共生的石榴子石与尖晶石之间的交换,Grütter et al.(2006)开发出了石榴子石的铬压力计.若石榴子石没有共生的铬尖晶石,这种情况下利用石榴子石的铬压力计计算出来的压力则代表金刚石形成压力的下限.
硫化物是金刚石中除硅酸盐矿物和氧化物矿物之外的一种较为常见的矿物包裹体(Deines and Haris, 1995; Richardson et al., 2001).金刚石中的硫化物包裹体通常被认为代表了最原始的地幔硫化物的样品,可以为岩石圈深部的亲铜元素的分布和丰度提供重要信息(Bulanova et al., 1996).金刚石中的硫化物包裹体中通常可见强烈的出溶现象,转变为一系列低温的矿物相(Stachel and Harris, 2008).因此,在金刚石生长过程中,被包裹进金刚石的原始硫化物包裹体组合可能是高温的固溶体,随着温度的降低,固溶体发生出溶现象,形成一系列的黄铁矿、磁黄铁矿、镍黄铁矿、黄铜矿、方黄铜矿以及硫镍矿(Craig and Kullerud, 1969).硫化物包裹体的Ni含量对于金刚石的类型具有一定的指示意义.Gurney(1989)对西伯利亚地区的金刚石中硫化物进行综述研究发现,榴辉岩型金刚石中的硫化物包裹体具有较低的Ni含量(0.5%~8.2%,平均值为2.9%),而地幔橄榄岩型金刚石中的硫化物包裹体则具有较高的Ni含量(15.6%~29.8%,平均值为22.8%).Deines and Harris(1995)对分别产于南非和博茨瓦纳的5个金伯利岩体中的金刚石的硫化物包裹体进行了分析,发现这些硫化物包裹体的Ni含量介于3%~27%,小于西伯利亚金刚石中硫化物包裹体的Ni含量变化范围.由于缺乏对应的硅酸盐矿物的限定,这些产于南非和博茨瓦纳的金刚石可能同时包含榴辉岩型和地幔橄榄岩型(Deines and Harris, 1995).Deines and Harris(1995)发现硫化物的Ni含量与硫化物的Cu含量以及寄主的金刚石的碳同位素之间没有相关性.Re和Os是强亲铁元素,187Re经过β衰变会形成187Os,因此可以利用Re⁃Os同位素体系来进行定年和物质来源示踪(Pearson et al., 1998, 1999; Richardson et al., 2001;史仁灯等, 2006; 杨红梅和凌文黎, 2006; Marchesi et al., 2011).在岩浆作用的过程中,Re为中等不相容元素,Os为强相容元素,地幔中的Re和Os主要存在铂族矿物,合金和金属硫化物中(Birck and Allègre, 1994; Shirey and Walker, 1998).地幔岩石中的硫化物通常会经历复杂的重结晶、与硅酸盐矿物再平衡以及交代等作用,这些作用会对硫化物的成分产生改造(Lorand, 1989, 1990);而金刚石中的硫化物由于金刚石的保护作用,则可以保留其最原始的成分信息.Pearson et al.(1998)利用负离子热表面电离质谱(N⁃TIMS)对南非的Koffiefontein金伯利岩中的地幔橄榄岩型和榴辉岩型金刚石的硫化物包裹体进行了Re⁃Os同位素体系分析,其中单粒地幔橄榄岩型金刚石中的两粒硫化物形成的Re⁃Os等时线年龄为60±30 Ma,与金伯利岩喷发的年龄较为一致;而榴辉岩型金刚石中硫化物产生较老的Re⁃Os模式年龄,范围为1.1~2.9 Ga(Pearson et al., 1998).单粒金刚石内部不同生长区内的硫化物也可能存在有较明显的化学成分组成差异(Rudnick et al., 1993; Pearson et al., 1999).Rudnick et al.(1993)利用二次离子探针质谱(SIMS)对俄罗斯Yakutia金伯利岩中单粒金刚石不同生长区内的硫化物包裹体进行了Pb同位素组成分析,所获得的Pb⁃Pb同位素模式年龄的差异可达2 Ga;然而Pearson et al.(1999)指出该Pb⁃Pb同位素模式年龄可能存在有较大的误差.Pearson et al.(1999)再次利用N⁃TIMS对该金刚石中的不同硫化物包裹体进行Re⁃Os同位素组成分析,所获得的Re⁃Os模式年龄具有较小的变化范围,介于3.1±0.3 Ma和3.5±0.3 Ma,体现Re⁃Os同位素体系在限定金刚石形成年龄的优越性.
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根据金刚石中包裹体的研究,金刚石主要形成于地球内部150~200 km深度范围内的岩石圈地幔(Boyd and Gurney, 1986; Meyer,1987);但还有少量金刚石可能来自于岩石圈下部的软流圈、地幔过渡带以及下地幔的环境(Moore and Gurney, 1985; Harte and Harris, 1994; Tappert et al., 2005; Kaminsky, 2012; Stachel and Luth, 2015; Nestola et al., 2018).如上所述,形成于岩石圈地幔金刚石中的包裹体主要为橄榄石、辉石、石榴石、尖晶石以及硫化物等矿物,而来源于岩石圈下部地幔的金刚石,其包裹体矿物的组合、成分以及晶体结构与岩石圈地幔中金刚石的包裹体矿物存在较大差异.Tappert et al.(2005)指出来自软流圈地幔和地幔过渡带的金刚石中的包裹体几乎全部是榴辉岩型,而来自于下地幔的金刚石的包裹体则主要为地幔橄榄岩型,指示了在地球200~410 km的深度范围内可能存在有一个榴辉岩成分的物质层(Gasparik, 2002).
高温高压实验研究显示,在压力为24 GPa的条件下,橄榄石的高压相矿物瓦兹利石或林伍德石会发生相变,转变为铁方镁石+镁硅钙钛矿的矿物组合,即(Mg, Fe)2SiO4→(Mg, Fe)O+(Mg, Fe)SiO3(Ringwood and Irifune, 1988).该相变具有重要的意义,标志着过渡带地幔向下地幔条件的转变,因此铁方镁石+镁硅钙钛矿的矿物组合成为鉴定金刚石是否为下地幔来源的重要指示标志(Kaminsky, 2012).需要指出的是,由于铁方镁石(Mg, Fe)O稳定于地球地幔的整个压力区间内,因此单一的方镁石矿物的出现并不能代表下地幔的条件;只有当方镁石与镁硅钙钛矿构成矿物组合时,才能有效地指示下地幔的环境(Kaminsky, 2012; Stachel and Luth, 2015).金刚石中常见的下地幔包裹体还包含有斯石英、四方铁铝榴石-镁铝榴石矿物相(TAPP, Tetragonal Almandine⁃Pyrope Phase)和钙硅钙钛矿等.含有下地幔矿物包裹体的金刚石主要被发现于南非、澳大利亚、巴西、几内亚以及加拿大的金伯利岩矿床或冲积型金刚石矿床中(Harte and Harris, 1994; Stachel et al., 2000; Kaminsky et al., 2001; Davies et al., 2004; Tappert et al., 2009).Kaminsky(2012)对这些来自于下地幔金刚石中的矿物包裹体进行了归纳和总结,根据不同矿物之间的共生关系,并划分出了3种类型的下地幔矿物组合,分别是:(1)超基性矿物组合,主要包含有铁方镁石(Ferropericase)、镁硅钙钛矿(MgSi⁃perovskite)、钙硅钙钛矿(CaSi⁃perovskite)、钙钛钙钛矿(CaTi⁃perovskite)、斯石英、四方铁铝榴石-镁铝榴石矿物相、尖晶石、锰钛铁矿(Manganoilmenite)、榍石、自然镍、自然铁、磁铁矿、超硅石榴石以及碳化硅等;(2)似榴辉岩型矿物组合,主要包含有AlSiO3OH相矿物(Wirth et al., 2007)、斯石英以及其他矿物相;(3)碳酸岩型矿物组合,主要包含有方解石和白云石(Bulanova et al., 2010)、菱钠钙石(nyerereite,Na2Ca(CO3)2)、苏打石(nahcolite,NaHCO3)以及卤化物等矿物.
根据下地幔矿物独特的化学成分,可以有效地将来源于下地幔的金刚石与其他深度范围内的金刚石区分开来;而相对于成分差异较小的软流圈和地幔过渡带,超硅石榴石(majorite)则成为鉴别来自于该源区金刚石的主要指示矿物(Stachel, 2001; Tappert et al., 2005).高温高压实验研究显示,超硅石榴石的形成是由于在高压条件下石榴子石中辉石的溶解度逐渐增高导致的(Ringwood, 1967; Ringwood and Major, 1970; Akaogi and Akimoto, 1977),其中辉石在石榴子石中的初始溶解发生在大约250 km的深度范围,终止于大约450 km的范围(Irifune, 1987).在超过550~600 km的深度,超硅石榴石开始出溶钙硅钙钛矿(Irifune, 1987; Wood, 2000).尽管超硅石榴石包裹体较为稀有,但是已经在世界范围的多个金刚石矿床的金刚石中发现超硅石榴石包裹体,例如南非的Jagersfontein矿床(Tappert et al., 2005)、南非的Monastery矿床(Moore and Gurney, 1989)、巴西的Juina⁃Sao Luiz地区(Kaminsky et al., 2001)以及几内亚的Kankan地区(Stachel et al., 2000).
除了上述软流圈、地幔过渡带以及下地幔矿物组合之外,还有两组已发现的较为特殊的矿物包裹体组合,分别为:(1)方铁矿(wüstite)+方镁石(periclase)、(2)铁的碳化物(iron carbide)+自然铁矿物组合,这两种矿物组合可能来自于核幔边界D”层(Kaminsky, 2012).方铁矿+方镁石的矿物“组合”被首次报道发现于南非的Monastery金伯利岩岩筒的金刚石中,但是这两种矿物并非真正以组合形式产出,而是产于不同的金刚石颗粒中(Moore et al., 1986).在委内瑞拉境内的Guaniamo地区的金刚石中也报道有单粒的方铁矿产出,其镁指数(Mg2+/(Mg2+Fe2+))仅为0.002 (Kaminsky et al., 2000; Kaminsky, 2012).Kaminsky et al.(2009)在巴西的Juina地区的金刚石中发现了相互接触产出的方铁矿和铁方镁石矿物组合,其中方铁矿的镁指数为0.02~0.03.Dubrovinsky et al.(2001)通过高温高压实验研究,在压力超过80 GPa的条件下,将镁方铁矿(Mg0.5Fe0.5)O和(Mg0.8Fe0.2)O加热到1 000 K以上的高温,获得了方铁矿和方镁石的矿物组合.该实验结果可能指示了金刚石中的方铁矿和方镁石矿物组合可能来源于核幔过渡带的深度(Dubrovinsky et al., 2001; Kaminsky et al., 2009).Kaminsky and Wirth(2011)在巴西Juina地区的金刚石中还发现了铁的碳化物、自然铁、磁铁矿和石墨的矿物包裹体组合.铁的碳化物包含有Fe3C(cohenite,陨碳铁)、Fe2C(chalypite,碳铁陨矿)和Fe23C6(haxonite,碳镍铁矿),其中Fe2C和Fe23C6是首次在地球样品中被发现,且Fe2C具有较高的N含量.结合前人对金刚石中包裹体的研究成果以及高温高压实验的结果(Lord et al., 2009),Kaminsky and Wirth (2011)认为碳铁陨矿(Fe2C)形成于压力在50~130 GPa范围内的富集氮元素的Fe⁃C熔体,这套金刚石+碳铁陨矿+石墨的矿物组合可能形成于地核的外层或者核幔边界的位置.
相对于地幔橄榄岩型和榴辉岩型金刚石,蛇绿岩型金刚石具有较为特殊的包裹体组合.Xu et al.(2009)在罗布莎铬铁矿的金刚石中发现了Ni⁃Mn⁃Co合金、Ta⁃W合金以及Ta合金包裹体,其中Ni⁃Mn⁃Co合金粒径约为20 μm,Ta⁃W合金和Ta合金的粒径在8 μm左右.Howell et al.(2015a)同样在罗布莎铬铁矿中的金刚石发现了数粒Ni⁃Mn⁃Co合金包裹体,这些包裹体呈他形球根状,可能指示了这些包裹体是以熔体的形式进入金刚石的(图 11a).Yang et al. (2015)不仅在Ray⁃Iz铬铁矿中发现了微米级的合金包裹体,还利用聚焦离子束结合透射电镜技术在金刚石中发现了纳米级的合金、石墨和柯石英包裹体(图 11b).内蒙古贺根山铬铁矿中的金刚石具有更为丰富的包裹体组合,Huang et al. (2015)在七粒金刚石中发现了Ni⁃Mn⁃Fe,Fe⁃Ni⁃Al和Ni⁃Mn⁃Co合金,以及单质锰、单质钙、石墨、KCl和FeO包裹体.利用聚焦离子束结合透射电镜技术,在土耳其Pozantı⁃Karsantı蛇绿岩金刚石中发现了Ni⁃Mn⁃Co合金、(Ca, Mn)SiO3以及流体包裹体(图 11c) (Lian et al., 2018).由此可见,Ni⁃Mn⁃Co合金包裹体是蛇绿岩型金刚石较为常见的包裹体之一,可能指示了Ni⁃Mn⁃Co对于蛇绿岩型金刚石的形成发挥着重要作用.Moe et al. (2017)利用傅立叶变换红外光谱分析,在罗布莎和Ray⁃Iz蛇绿岩金刚石中发现了一系列的流体包裹体,如水、碳酸盐、碳氢化合物以及硅酸盐等,同时利用激光拉曼分析,发现了铬尖晶石、磁铁矿、斜长石、碳化硅和赤铁矿等矿物.
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金刚石是由碳原子组成的晶体,其中的每个碳原子与周围的4个碳原子以共价键相连,构成正四面体结构.金刚石的组成元素除了碳元素之外,往往还含有一些其他的杂质元素,如氮(N)、硼(B)、氢(H)、硅(Si)和氧(O)等(Mainwood, 1979; Briddon and Jones, 1993; Burgess et al., 1998; Kaminsky et al., 2001; Breeding and Shigley, 2009).在这些杂质元素中,N和B元素含量最高且最为重要,是金刚石划分的重要依据(Kaiser and Bond, 1959; Smith et al., 2000; Breeding and Shigley, 2009).
利用红外吸收光谱仪对金刚石中N的含量进行分析,金刚石可以被划分为Ⅰ型金刚石和Ⅱ型金刚石(Robertson et al., 1934; Breeding and Shigley, 2009).Ⅰ型金刚石中杂质氮的含量通常高于20×10-6,足以被红外吸收光谱仪检测到;而Ⅱ型金刚石中杂质氮的含量通常低于20×10-6,不足以被红外吸收光谱仪所检测到(Kaminsky et al., 2001; Breeding and Shigley, 2009).在红外光谱中,Ⅰ型金刚石具有很强的杂质氮的吸收峰,主要表现为1 370 cm-1、1 430 cm-1、1 282 cm-1、1 175 cm-1、1 100 cm-1、1 010 cm-1等吸收峰;而Ⅱ型金刚石则表现为无或很弱的杂质N吸收峰(何雪梅, 2000).
Ⅰ型金刚石根据其内部的杂质氮原子在晶格中的聚集状态,可以被划分为Ⅰa和Ⅰb型金刚石(图 12).Ⅰa型金刚石中的氮原子表现出较为复杂的聚集状态,在晶格中的氮原子通常与其他的氮原子相邻(Breeding and Shigley, 2009).Ⅰa型金刚石可以进一步划分为ⅠaA型、ⅠaB型和ⅠaAB型(ⅠaA和ⅠaB型之间的过渡类型)(Davies, 1976; Kaminsky et al., 2001).ⅠaA型金刚石以偶氮(N2,又称作A心)为主,表现为在晶格中有两个氮原子相邻,替换一对碳原子.ⅠaA型金刚石中两个氮原子组成的A心与晶格中的其他氮原子相互独立,互不接触.ⅠaB型金刚石的晶体结构中则包含由4个氮原子围绕着一个空缺,形成B心(N4).Ⅰa型金刚石中的氮原子还有其他类型的聚集状态,如3个氮原子围绕一个空缺(N3),两个氮原子被两个空缺所分隔(H4)等,但是这些类型的氮原子的聚集状态并不参与金刚石的分类(Collins, 1982, 2001; Breeding and Shigley, 2009).Ⅰb型金刚石的氮原子的聚集状态较为简单,仅仅由一个氮原子代替一个碳原子,不同的氮原子之间相互独立、互不相邻,这种类型的杂质氮被称作孤立氮,单替代氮或者C心.金刚石中氮的聚集状态是受金刚石结晶的地幔源区的温度以及在地幔中的滞留时间所确定的(Evans and Qi, 1982).Ⅰa型金刚石可能在上地幔1 000~1 400 ℃的温度滞留了200~2 000 Ma,而Ⅰb型金刚石则可能在上地幔800℃的温度滞留了与Ⅰa型金刚石相似的时间,或者在与Ⅰa型金刚石相似的温度范围内滞留了很短的时间(Evans and Qi, 1982).
图 12 不同类型的金刚石的内部原子的排布特征
Figure 12. Diamonds of different types showing different arrangements of carbon and impurity atoms
Ⅱ型金刚石同样可以被进一步划分为Ⅱa型和Ⅱb型(Breeding and Shigley, 2009).Ⅱa型金刚石不含或含有极微量的杂质氮或硼,且杂质氮或硼的含量不足以被红外吸收光谱仪所检测到,而Ⅱb型金刚石不含杂质氮,但是含有杂质硼原子.Ⅱb型金刚石中的杂质硼原子以单原子的形式替代金刚石中的碳原子(图 12).
由于金刚石的颜色受控于晶体内部的杂质元素以及晶格缺陷,金刚石的类型往往与颜色密切相关,例如天然的Ⅰa型金刚石通常为无色、棕色、粉色或者紫罗兰色;天然的Ⅰb型金刚石通常为棕色、黄色或者橙色;天然的Ⅱb型金刚石通常为天蓝色(Breeding and Shigley, 2009; Smith et al., 2018).
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如前所述,金刚石主要由碳元素组成,此外还包含有一些杂质元素,如氮、硼、氢和氧等.金刚石同位素组成的研究主要围绕金刚石的碳和氮元素进行,为研究形成金刚石的物质来源提供了重要的信息.早期金刚石碳同位素组成的分析主要运用燃烧/氧化法,将金刚石中的碳转变为二氧化碳,随后利用质谱仪对碳的同位素组成进行分析(Swart et al., 1983; 韩友科和安娜, 1986).这种测试方法会将金刚石完全消耗,同时具有较低的空间分辨率.二次离子探针质谱仪(SIMS)的发展实现了金刚石碳和氮同位素的原位分析,大大提高了分析测试的空间分辨率和分析精度,降低了分析检出限(Bulanova et al., 2002; 张健等, 2012; Lian et al., 2018).前人对全球金刚石的碳和氮同位素组成开展了大量的研究分析工作,产出了大量的研究数据(Harte et al., 1999; Cartigny et al., 2001;Bulanova et al., 2002;Cartigny,2010).Cartigny (2005)和Shirey et al. (2013)对世界范围内不同地区的金刚石同位素组成进行了系统的综述,归纳整理了不同类型的金刚石以及不同储库的同位素组成.
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自然界中碳存在3种同位素,分别是12C、13C和14C,其中12C和13C为稳定同位素,在自然界中的相对比例分别为98.90%和1.10%(Rosman and Taylor, 1998).金刚石的碳同位素组成通常用δ13C = (13C/12C样品/13C/12C参考-1)×1 000表示,其中13C/12C样品代表未知样品的13C/12C比值;13C/12C参考代表标准样品的参考值,通常以美国南卡罗莱纳州白垩纪皮狄组的拟箭石化石PDB (Peedee Belemnite)为国际标准,其“绝对”碳同位素比值为13C/12C=0.011 237 2±0.000 002 9(Craig, 1957).
Cartigny(2005)统计了全球范围内超过4 400个金刚石的碳同位素组成的分析数据(图 10).统计结果发现,有72%的金刚石δ13C值介于-8‰~-2‰.值得注意的是,这些金刚石在δ13C=-5‰的位置出现峰值,且在该峰值两侧金刚石的频数逐渐降低(Cartigny, 2005).由于地幔储库的δ13C=-5‰±1‰,全球金刚石在此出现峰值,指示了形成金刚石的碳可能主要来源于地幔.
根据Cartigny(2005)的统计结果,地幔橄榄岩型和榴辉岩型金刚石的碳同位素组成分布特征存在差异(图 13).地幔橄榄岩型金刚石的δ13C值介于-26.4‰~+0.2‰,其中仅有2%的金刚石的δ13C小于-10 ‰.而榴辉岩型金刚石的δ13C值具有较大的范围,介于-38.5‰和+2.7‰,且有34%的金刚石的δ13C小于-10‰ (Cartigny, 2005; Shirey et al., 2013).这种碳同位素组成的差异指示了部分榴辉岩型金刚石可能具有与地幔橄榄岩型金刚石不同的碳的来源.来源于下地幔的金刚石的碳同位素组成具有较窄的变化范围,其δ13C介于-8.5‰~-0.5‰,但主要分布在δ13C=-8‰~-2‰的地幔范围内.纤维型/复合型金刚石(Fibrous/coated diamonds)δ13C值介于-8‰~-5‰,位于正常地幔碳组成范围(Javoy et al., 1984; Boyd et al., 1992).根据碳同位素分布直方图可以发现,尽管不同类型的幔源型金刚石的碳同位素组成存在有差异,但是以上不同类型的金刚石的δ13C值均集中在-8‰~-2‰的地幔范围内,且δ13C峰值出现在-5‰±1‰的位置.该特征指示了来自于地幔的碳是形成以上这些幔源型的金刚石的主要物质来源.
图 13 不同类型金刚石和沉积物碳同位素、氮同位素以及氮含量频率分布直方图
Figure 13. Comparative frequency histograms of δ13C, δ15N, and nitrogen contents of diamonds, recycled carbon and metasediment
产出于超高压变质岩石中的变质型金刚石的δ13C值介于-30‰~-3‰,其碳同位素组成频率分布直方图与幔源型金刚石具有较明显的差异.超高压变质型金刚石的δ13C值主要集中在-16‰~-5‰的范围内,整体偏移地幔的碳同位素组成范围(Ogasawara, 2005; Shirey et al., 2013).Russell et al.(1996)分析了不同类型的陨石中的金刚石同位素组成.不同类型的陨石中的金刚石具有13C非常亏损的特征,δ13C值变化于-38 ‰和-31 ‰之间,显著低于地球上的金刚石的值.
尽管越来越多的蛇绿岩铬铁矿被报道有金刚石的产出(Yang et al., 2007; 杨经绥等, 2007, 2011; Xu et al., 2009; Huang et al., 2015; Tian et al., 2015; Xiong et al., 2016, 2017; Lian et al., 2017; Wu et al., 2017),但是对这些蛇绿岩型金刚石的同位素组成的研究却相对滞后,目前仅有我国西藏罗布莎铬铁矿,俄罗斯的Ray⁃Iz铬铁矿以及土耳其的Pozantı⁃Karsantı铬铁矿中金刚石同位素组成的报道(Yang et al., 2014, 2015; Howell et al., 2015a; Lian et al., 2018).Yang et al.(2014, 2015)报道了蛇绿岩型金刚石的碳同位素组成介于-28‰~-18‰,δ13C值分布范围较窄.Howell et al.(2015a)利用二次离子探针质谱仪(SIMS)对罗布莎铬铁矿中的金刚石的碳和氮同位素进行了分析,结果显示罗布莎铬铁矿中的金刚石具有较窄的变化范围,δ13C=-24.0‰~-28.3‰,平均值为-25.8‰.Lian et al.(2018)报道了土耳其Pozantı⁃Karsantı铬铁矿中的金刚石与上述两个岩体中的金刚石具有相似的同位素组成.因此,蛇绿岩型金刚石总体具有13C较为亏损的碳同位素组成特征,与地幔储库的碳同位素组成差异较大(图 13),同时不同于大多数幔源型金刚石、超高压变质型金刚石以及陨石相关的金刚石的碳同位素组成.蛇绿岩型金刚石这种较轻的碳同位素组成的成因目前仍不清楚.
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氮是金刚石中最主要且最重要的杂质元素,金刚石中氮的含量和氮同位素的组成同样可以为金刚石的物质来源和成因提供重要的信息.自然界中的氮存在有两种稳定同位素,分别是14N和15N,其相对丰度分别为99.634 %和0.366 % (Rosman and Taylor, 1998).金刚石的氮同位素组成通常用δ15N=(15N/14N样品/15N/14N参考-1)×1 000表示,其中氮同位素的参考值选用大气中N2的同位素组成(15N/14Nair=0.003 676 5±0.000 008 1).
Cartigny(2005)统计了全球不同类型的金刚石以及沉积物的氮含量和氮同位素组成(图 13).2 242个地幔橄榄岩型金刚石的氮含量数据显示,该类型金刚石的氮含量介于0~2 100×10-6,氮含量的峰值出现在0~300×10-6的含量范围内(图 13).榴辉岩型金刚石具有与地幔橄榄岩型金刚石相似的氮含量组成,其频率分布直方图同样呈现右倾递减的趋势.相对于地幔橄榄岩型和榴辉岩型金刚石,纤维型/复合型(fibrous/coated)金刚石具有较高的氮含量,主要分布在600×10-6~1 800×10-6的范围内,其峰值出现在900×10-6~ 1 200×10-6的位置.超高压变质岩型的金刚石的氮含量变化范围较大,最高可达9 600×10-6,与上述3种类型金刚石的氮含量相比,存在较大差异.
地球地幔的氮同位素组成主要是通过对大洋中脊玄武岩、地幔橄榄岩型金刚石以及纤维型/复合型金刚石的研究来推算的(Javoy et al., 1986; Marty and Humbert, 1997; Cartigny, 2005).Exley et al.(1987)对不同大洋中脊的玄武岩玻璃进行了氮同位素分析,结果显示大洋中脊玄武岩的氮同位素组成具有较大的变化范围,δ15N介于-4.5‰~+15.5‰.但是由于不能有效地对浅部过程的混染作用(如地壳物质混染、大气或有机物质的污染)以及去气作用进行约束,因此这些大洋中脊玄武岩的氮同位素的组成并不能真实地反映地幔的氮同位素组成(Exley et al., 1987; Pineau and Javoy, 1994; Marty and Humbert, 1997).Javoy and Pineau(1991)采集了大西洋中脊的富集气体的玄武岩(孔隙度,17%),利用质谱仪对其中的孔隙气体进行了分析.分析结果显示, 这些玄武岩中的气体具有典型的大洋中脊玄武岩的稀有气体同位素组成(3He/4He=8Ra,40Ar/36Ar最高可达28 000),指示了该岩石几乎没有遭受浅部海水或大气物质的混染,此外,这些气体的δ15N变化于-2.6‰~-5.9‰,可能代表了原始的同位素组成(Javoy and Pineau, 1991).Marty and Humbert(1997)再次对大洋玄武岩中的氮和氩同位素组成进行了分析,显示这些玄武岩的δ15N介于-1.4‰~+5.4‰,然而,通过氩的同位素组成分析,显示了地表成分端元与地幔端元的混合作用.因此,利用大洋玄武岩对地幔的氮同位素组成进行约束,具有一定的局限性,使得地幔的氮同位素组成未能得到很好的限定.Jia and Kerrich (2015)利用碳质(carbonacous)球粒陨石和顽火(enstatite)陨石混合模型,对原始地幔的氮同位素组成进行计算,认为原始地幔的δ15N值为-7‰±3‰.
金刚石来源于地幔,因此为研究地幔的氮同位素组成提供了更为直接的研究对象(Boyd et al., 1987; Cartigny and Marty, 2013).纤维型/复合型(fibrous/coated)金刚石的δ15N值变化于-12‰~0(Cartigny, 2005),而地幔橄榄岩型和榴辉岩型金刚石的δ15N值则具有较大的变化范围,分别介于-24‰~+12‰和-12‰~+18‰.变质型金刚石具有截然不同的氮同位素组成,有限的数据显示其δ15N均为正值,与变质沉积物中的氮具有相似的同位素组成,指示了其成因之间的联系(Cartigny and Marty, 2013; Shirey et al., 2013).变质沉积物中15N的富集,是由于随着变质程度的增加,沉积物会发生脱挥发分作用(devolatilization process),导致沉积物中氮含量的降低,同位素发生动力学分馏作用,重的15N残留在沉积物中,导致δ15N的升高(图 14) (Haendel et al., 1986).
图 14 不同变质等级的区域变质岩中氮含量与氮同位素δ15N值的相关性图解
Figure 14. Correlation between nitrogen content and δ15N values in regional metamorphism
蛇绿岩型金刚石的氮含量以及同位素组成的分析数据较为有限,目前仅有我国西藏罗布莎和土耳其Pozantı⁃Karsantı蛇绿岩中金刚石的氮含量和同位素组成的报道.罗布莎蛇绿岩型金刚石的氮含量介于9×10-6~430×10-6,但是蛇绿岩型金刚石的立方体生长区(cubic growth sector)和八面体生长区(octahedral growth sector)的氮含量具有较明显的差别(Howell et al., 2015a).罗布莎蛇绿岩型金刚石的立方体生长区的氮含量的平均值为46×10-6,而八面体生长区的氮含量平均值为214×10-6明显高于立方体生长区,因此蛇绿岩型金刚石的不同生长区之间明显存在氮含量的差异.罗布莎金刚石的立方体生长区的δ15N介于-7.6‰~+8.6‰,但是由于该生长区的氮含量较低,因此误差值较大(2σ≈±3‰).八面体生长区的δ15N值具有较大的变化范围,变化于-5.6‰~+28.6‰之间.土耳其Pozantı⁃Karsantı铬铁矿中的金刚石氮含量介于7×10-6~541×10-6,δ15N介于-19.1‰~+16.6‰ (Lian et al., 2018).土耳其金刚石不同生长区氮含量和氮同位素的组成具有与罗布莎金刚石相似的特征.形成蛇绿岩型金刚石的不同生长区之间的氮含量和氮同位素差异的原因目前仍不清楚(Howell et al., 2015a),可能是由于某种结晶效应导致了不同生长区对15N具有不同的捕获能力(Boyd et al., 1988; Reutsky et al., 2008; Lian et al., 2018).将蛇绿岩型金刚石不同生长区的氮含量及氮同位素组成作为整体与其他类型的金刚石进行对比,笔者发现,蛇绿岩型金刚石与地幔橄榄岩型和榴辉岩型金刚石具有相似的分布特征,但区别于变质型金刚石(δ15N > 0)(图 13),可能指示了蛇绿岩型金刚石与其他幔源型金刚石相似的氮物质来源.
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Boyd and Pillinger(1994)系统地分析了南非、澳大利亚以及北美的43颗金刚石的同位素组成,结果显示这些金刚石可以分为两组,分别是高δ13C组和低δ13C组.其中高δ13C金刚石的δ13C=-6.4‰~-2.9‰,δ15N=-12.3‰~+5.9‰;而低δ13C金刚石相对亏损13C、富集15N,其δ13C=-19.4‰~-9.5‰,δ15N=-3.0‰~+16.4‰.Boyd and Pillinger(1994)认为这种不同金刚石的碳同位素和氮同位素的差异可能主要是由俯冲再循环物质的改造作用引起的,但是金刚石形成过程中同位素的分馏作用也不能被完全排除.尽管Boyd and Pillinger(1994)识别出了高δ13C组和低δ13C组的金刚石(图 15a),但是由于这些金刚石不含包裹体,不能对这些金刚石的类型进行鉴别(地幔橄榄岩型还是榴辉岩型?),因此无法探讨金刚石的碳同位素组成与金刚石类型之间的联系.
图 15 幔源型金刚石δ13C-δ15N (a)以及δ13C-N (b)含量相关性图解
Figure 15. Correlation diagrams of δ15N values (a), N (b) against δ13C values of mantle-derived diamond
Cartigny et al.(1998)对来自世界范围内不同地区的150颗已知类型的金刚石的碳和氮同位素进行了耦合研究,发现地幔橄榄岩型金刚石的δ13C值变化较小,而δ15N则具有较大的变化范围.不同地区的榴辉岩型金刚石的δ13C值主体上与地幔橄榄型金刚石的δ13C值具有一致的变化范围,仅有个别的榴辉岩型金刚石具有较低的δ13C值(< -10.0‰),且具有较低δ13C值的榴辉岩型金刚石具有负的δ15N值(Cartigny et al., 1998).由于俯冲再循环的有机物通常具有较低的δ13C值(< -15‰)和正的δ15N值(Cartigny, 2005),如果这些榴辉岩型金刚石形成于俯冲的有机物,那么这些榴辉岩型金刚石将具有较低的δ13C值和正的δ15N值,然而这与Cartigny et al.(1998)观察的实际情况是不符的.因此,Cartigny et al.(1998)认为地幔橄榄型和榴辉岩型金刚石并非是由俯冲的有机碳直接形成的,这两种类型的碳同位素组成的变化反映的是高温条件下的同位素的分馏过程或者是原始地幔(primordial mantle)组成的不均一性.然而,值得注意的是,Cartigny et al.(1998)认为金刚石中氮同位素的较大变化是由于俯冲再循环的氮的改造作用的引起的.如果俯冲的物质对地幔的氮同位素进行了改造作用,那么俯冲在循环物质中的碳也应当对地幔的碳同位素组成进行有效的改造.因此,仅用同位素分馏效应或者原始地幔的不均一性不能有效地解释金刚石的同位素组成的差异.
Cartigny et al.(2001)统计了1 200颗幔源型金刚石的碳同位素和氮含量的数据,发现在金刚石的N⁃δ13C相关性图解中,幔源型金刚石普遍分布在一个有限的区域(limit sector)内,且该有限的区域边界曲线可能指示了地幔熔体的演化趋势(图 15b).金刚石中的氮含量可能并非直接受其生长基质中的氮含量控制,其氮含量的高低可能还受控于一种动力学过程.当金刚石具有较高的生长速率时,金刚石与生长基质处于不平衡的状态,此时碳和氮原子进入晶格的比例可能与生长基质中两种原子的比值接近.因此,在金刚石的N⁃δ13C相关性图解中,当δ13C值一定时,对应的最高的氮含量可能反映了形成金刚石的熔体/流体的氮含量.因此,N⁃δ13C相关性图解中幔源型金刚石分布的上边界则定义了形成金刚石的熔体/流体的氮含量演化.幔源型金刚石的氮含量随着δ13C值的减小呈现降低的趋势,反映了地幔熔体/流体在分异过程中的一种演化趋势(Cartigny et al., 2001).
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与其他产出类型的金刚石相比,蛇绿岩型金刚石具有非常低的δ13C值,且δ13C变化范围较窄(~10‰).这种具有低δ13C值(< -10‰)的金刚石在榴辉岩型金刚石和变质岩型金刚石中也占有一定的比例(Kaminsky et al., 2000; Tappert et al., 2005; de Stefano et al., 2009; Cartigny, 2010).例如,Kaminsky et al.(2000)报道了委内瑞拉的Guianamo金伯利岩中的榴辉岩型金刚石的δ13C值介于-17‰~-24‰;Tappert et al.(2005)报道了南非的Jagersfontein金伯利岩中金刚石的δ13C值介于-24‰~-30‰;de Stefano et al.(2009)报道了加拿大北部的Jericho金伯利岩中的金刚石δ13C值介于-5‰~-41‰,其中榴辉岩型金刚石的δ13C值介于-24‰~-41‰.Cartigny (2010)报道了在法属圭亚那的Dachine科马体岩中金刚石的δ13C值介于-32.6 ‰~+0.15‰,峰值为-27‰.前人对金刚石的低δ13C值的成因,主要提出了3种模型.
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该模型认为金刚石中较轻的碳同位素组成代表了地球最原始地幔的碳同位素组成(Deines et al., 1993; Haggerty, 1999).陨石被认为是与地球原始物最为类似的地外物质,不同类型的陨石按一定的比例混合,可以用来模拟原始地球的化学组成(侯渭等, 1996).陨石中碳的δ13C值介于-31‰~+5‰,与地球上的金刚石的同位素组成的变化范围一致,为金刚石代表地球原始成分这一模型提供了支持.然而,铁陨石通常被认为是代表了行星地核的物质组成,因此在将陨石的成分与地球中金刚石的碳同位素组成进行对比之前,需要对陨石中不同的碳进行区分.Kirkley et al.(1991)指出,铁质陨石中铁镍合金(taenite)和陨碳铁(cohenite)中的碳(δ13C=-18‰~-22‰)代表地核中的碳组成,而非地幔的碳组成;铁质陨石中的石墨球体以及硅酸盐矿物中的碳(δ13C=-3‰~-14‰)才真正代表地球地幔的碳组成.由于金刚石形成于不同深度的地幔,如果形成金刚石的碳来源于原始的地幔碳,那么这些金刚石的碳同位素组成应当主要分布在δ13C=-3‰~-14‰之间,显然这与金刚石尤其是榴辉岩型金刚石的碳同位素组成是不一致的.蛇绿岩型金刚石具有更小的碳同位素变化范围以及更轻的碳同位素组成,显著区别于铁质陨石中的石墨和硅酸盐矿物中的碳.
Cartigny (2005)指出其他类型同位素的研究并不支持地幔中保存有原始碳这一模型.在地球形成之后,地球的地幔可能经历了强烈的对流作用,这种作用会导致原始地幔中的碳同位素组成发生均一化,消除原始碳同位素组成的不均一性(Kirkley et al., 1991; Walter et al., 2011).地幔橄榄岩型金刚石的碳同位素组成具有较窄的变化范围(δ13C=-10‰~-1‰),进一步证明地幔碳同位素组成的均一性(Gurney, 1989).因此,原始不均一性模型并不能有效地解释金刚石中的较低的δ13C值.
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该模型认为金刚石中较轻的碳同位素是金刚石形成过程中由正常的地幔碳通过同位素分馏形成的,由于在地幔的高温条件,金刚石与共存的液态或固态的含碳相物质之间同位素分馏值(ΔC=~-3.5‰~+1‰)较低(Bottinga 1968; Deines, 1980),与具有正常地幔碳同位素(δ13C=-5‰±3‰)组成的含碳物质之间的热力学平衡分馏远不能形成金刚石如此轻的碳同位素组成.
然而,瑞利分馏效应可能会引起金刚石同位素的较大分馏.瑞利分馏是指在开放体系中,反应生成的产物一旦形成后马上从系统中分离开,从而实现同位素分馏效应的过程(Taylor, 1980).Deines(1980)利用瑞利分馏模型,分别模拟了金刚石从CO2和CH4蒸汽中结晶金刚石的过程,其中CO2和CH4蒸汽的初始碳同位素组成与正常的地幔一致.瑞利分馏模型显示,从CO2蒸汽中结晶的金刚石的δ13C值介于-10‰~0,峰值为-9‰;而从CH4蒸汽中结晶的金刚石的δ13C介于-8‰~-2‰,峰值为-4‰(Deines, 1980).因此,模拟结果均与榴辉岩型和蛇绿岩型金刚石的结果不一致.
虽然,以正常的地幔碳同位素组成作为形成金刚石的物质的初始碳同位素组成来进行同位素分馏模拟,均不能模拟出榴辉岩型和蛇绿岩型金刚石的碳同位素组成,但是这并不意味着在金刚石形成的过程中没有发生同位素的分馏作用.
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该模型认为金刚石中较轻的碳同位素来源于俯冲的有机碳俯冲板片中的有机碳,通常具有较轻的碳同位素组成,其δ13C值介于-40‰~-10‰,平均值约为-25‰ (Peters et al., 1978; Cartigny, 2005; Li and Bebout, 2005).陆源有机碳和海洋的有机碳的同位素组成存在一定的差异,其中陆源有机物质的δ13C值主要介于-28‰~-24‰,平均值为-27‰;而海洋的有机物质的δ13C值主要介于-22‰~-20‰,平均值为-21‰ (Meyers and Eadie, 1993; Minoura et al., 1997).因此,伴随俯冲板片进入地幔的有机物质,可能是具有较轻碳同位素组成的蛇绿岩型、变质岩型和榴辉岩型金刚石的物质来源(Ogasawara, 2005; Walter et al., 2011; Schulze et al., 2013; Yang et al., 2015; Nestola et al., 2018).
该模型得到了金刚石中包裹体的化学组成和氧同位素组成研究的支持(Walter et al., 2011; Schulze et al., 2013; Burnham et al., 2015).Schulze et al.(2013)和Burnham et al.(2015)利用离子探针对金刚石中的包裹体矿物的氧同位素分析显示,这些包裹体具有高于正常地幔的δ18O值.由于氧同位素只有在较低温度下才能发生显著的分馏效应,因此偏离正常地幔值的氧同位素组成是地壳来源的有效指示剂.此外,Walter et al.(2011)发现巴西Juina金伯利岩中金刚石的包裹体的全岩化学成分组成与玄武岩在下地幔条件下结晶的矿物相的化学组成一致,指示了这些金刚石的包裹体来源于俯冲到下地幔的洋壳物质.这些具有俯冲洋壳来源包裹体的金刚石通常具有较轻的碳同位素组成.由于蛇绿岩型金刚石中包裹体非常稀有,且类型独特,无法利用这些包裹体对蛇绿岩型金刚石的物质来源进行有效的探讨.
据图 13,俯冲再循环的物质的δ15N均为正值,由于板片深俯冲形成的变质型金刚石的δ15N均为正值,与俯冲再循环的物质一致.而蛇绿型和榴辉岩型金刚石的δ15N既有正值也有负值,与正常地幔氮同位素组成相似.Cartigny et al.(1998)以及Cartigny(2005)认为具有较轻碳同位素组成的金刚石的碳和氮同位素之间的解耦现象并不支持俯冲再循环物质的模型.然而,具有低δ13C值金刚石的碳和氮同位素的解耦现象可能指示了金刚石并非直接从俯冲的有机碳转变而来,而是从俯冲在循环物质形成的含碳流体/熔体中结晶出来.在这些含碳流体形成的过程中,由于碳和氮分馏程度的差异,可能导致了碳和氮同位素的解耦(Zedgenizov et al., 2017).
因此,尽管俯冲物质模型存在不足,笔者认为俯冲再循环的有机物质是具有较轻碳同位素组成的金刚石的物质来源,但还需进一步的研究工作去揭示金刚石形成过程中碳和氮的解耦现象.
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在阴极发光图像中,蛇绿岩型金刚石通常显示明暗不同的分区,其中亮区被认为是代表{111}的八面体生长区(octahedral growth sector),而暗区被认为是代表{100}的立方体生长区(cuoid growth sector) (Howell et al., 2015a).这种具有不同生长习性的金刚石被称为混合生长型金刚石(Frank, 1967; Lang, 1974).根据抛光的金刚石表面的蚀刻图案(Harrison and Tolansky, 1964; Seal, 1965),Frank (1967)首次认识到了金刚石内部存在不同生长形式.随后,阴极发光手段和X射线形貌学技术的应用进一步揭示了金刚石内部结构的差异(Lang et al., 2007).这种混合生长型金刚石在金伯利岩型金刚石中也占有一定的比例.例如,Welbourn et al.(1989)对博兹瓦纳的Jwaneng矿床的金刚石进行形态学、光学以及X射线形貌学的研究显示,约有8%的金刚石具有混合的生长习性.金刚石的生长习性主要受温度、压力以及流体中碳的饱和度等因素控制(Boyd et al., 1987; Sunagawa, 1990).大量的研究显示,这些混合生长型金刚石的不同生长分区不仅存在内部结构的差异,同样存在有化学组成的差异.
Lang(1974)利用X射线形貌学对混合型金刚石进行研究,结果显示金刚石立方体的生长区的片状体含量低于八面体生长区,指示了较低的氮含量.金刚石中八面体生长区相对富集氮的特征,同样被红外吸收光谱和二次离子探针质谱的研究所证实(Welbourn et al., 1989; Reutsky et al., 2008; Howell et al., 2013; Xu et al., 2017).Boyd et al.(1988)分析了人造混合生长型金刚石内部的元素组成,发现金刚石的立方体生长区相对八面体生长区更富集15N,不同生长区的δ15N的差值可达45‰.Reutsky et al.(2008)对两粒人造混合型金刚石的研究,同样发现了金刚石的不同生长区的氮含量和氮同位素组成存在有较大差异.尽管人造金刚石的不同生长区的氮的组成存在较大不同,但是不同生长区的碳同位素组成则较为一致(Boyd et al. 1988; Burns et al., 1990; Reutsky et al., 2008).前人对天然混合生长型金刚石的化学组成也开展了研究工作.与人造金刚石一致的是,天然的混合生长型金刚石的不同生长区之间碳同位素同样不存在差异或差异很小,且八面体生长区相对立方体生长区具有更高的氮含量(Bulanova et al., 2002; Cartigny et al., 2003; Zedgenizov and Harte, 2004).然而,Bulanova et al. (2002)指出俄罗斯Mir金伯利岩中的一粒混合生长型金刚石不同生长区的氮同位素组成不存在差异.Cartigny et al.(2003)研究发现澳大利亚新南威尔士的一粒混合生长型金刚石显示了相似的结果,即氮同位素组成不存在差异.Howell et al.(2015b)指出Cartigny et al.(2013)所研究的金刚石并非真正的混合生长型金刚石.Howell et al.(2015b)发现天然混合生长型金刚石不同生长区的δ15N值存在0.4‰~1‰的差异,其中八面体生长区具有更高的15N含量.西藏罗布莎铬铁矿中发现的蛇绿岩型金刚石为混合生长型金刚石,碳和氮的组成分析结果显示不同生长区的碳同位素组成差异较小,但是氮含量及同位素组成差异较大.罗布莎金刚石的八面体生长区相对于立方体生长区具有更高的氮含量和δ15N值,与金伯利岩混合生长型金刚石特征一致,但是罗布莎金刚石不同生长区的δ15N差值可达18.5‰,远高于金伯利岩型金刚石(Howell et al., 2015b).Lian et al.(2018)对土耳其Pozantı⁃Karsantı铬铁矿中的金刚石的研究,取得了相似的结果.
可以看出,人造的和天然的混合生长型金刚石不同分区的碳同位素往往不存在差异,但是八面体生长区相对于立方体生长区普遍具有更高的氮含量.不同的是,人造混合型金刚石的立方体生长区更富集15N,而天然金刚石的八面体生长区往往更富集15N.然而,导致金刚石不同生长分区的元素组成差异的机制目前仍不清楚.
单颗晶体内部同期生长的不同生长区之间,同位素组成的差异在其他矿物(如石英、方解石)中同样存在(Dickson, 1991; Onasch and Vennemann, 1995).这种元素组成的差异被认为是由于不同生长区不同的表面结构或生长机制引起的(Onasch and Vennemann, 1995).Reutsky et al.(2008)认为金刚石不同生长区的氮含量和氮同位素组成的差异是由于不同生长区的生长面对氮原子的吸附能力的差异引起的.然而,为何天然金刚石和人造金刚石的不同生长区的氮同位素行为会存在差异,仍然不清楚,有待进一步的研究工作.
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蛇绿岩型金刚石的发现是近年来蛇绿岩、铬铁矿以及金刚石研究领域的重要进展,金刚石与蛇绿岩中发现的其他异常矿物被认为是“来自地幔过渡带的惊喜”(Liou et al., 2014; Rollinson, 2016; Dilek and Yang, 2018).全世界不同地区多个蛇绿岩体中金刚石的发现,指示了大洋地幔橄榄岩和铬铁矿中金刚石的存在并非个例,金刚石可能在全球蛇绿岩中具有普遍的产出(Yang et al., 2015).尽管蛇绿岩型金刚石因粒径小、品位低而不具有经济开采价值,然而其对于研究蛇绿岩铬铁矿的形成、理解板块构造过程以及认识地壳物质的再循环具有重要的意义.
针对蛇绿岩型金刚石的发现,国内外学者曾提出了不同的成因模式:
Arai(2013)将豆荚状铬铁矿分为低压型和超高压型铬铁矿,并认为超高压铬铁矿是由低压铬铁矿俯冲至地幔过渡带甚至下地幔的深度形成的,铬铁矿中的金刚石是在深俯冲的过程中形成的(图 16a).但是,Arai (2013)的成因模型不能解释低压铬铁矿俯冲到下地幔的深度、随后又返回地球浅部的动力学机制,此外,在罗布莎蛇绿岩中的洋壳单元(如辉长岩,玄武岩等)中并未发现有超高压的证据.
图 16 蛇绿岩型金刚石及相关铬铁矿和地幔橄榄岩的不同成因模型
Figure 16. Different models for the origin of ophiolitic diamond and the hosting peridotite and podiform chromitite
Zhou et al.(2014)认为豆荚状铬铁矿中的金刚石来源于深俯冲(> 140 km)板片中的变质型金刚石,形成铬铁矿的母岩浆在通过俯冲板片断离形成的板片窗时,混染了俯冲板片的物质,如金刚石、锆石等(图 16b).豆荚状铬铁矿中壳源矿物的发现,较好地支持了该成因模型(Zhou et al., 2014; Robinson et al., 2015).然而,将蛇绿岩型金刚石与超高压变质带中的变质型金刚石进行简单的形态学方面的对比,发现蛇绿岩型金刚石与变质型金刚石存在明显的差异,例如蛇绿岩型金刚石通常为单晶,粒径在50~200 μm (Yang et al., 2015; Das et al., 2017; Lian et al., 2018),而超高压变质型金刚石通常粒径小于10 μm,以多晶为主(Ogasawara, 2005).此外,蛇绿岩型金刚石的同位素组成与超高压变质岩型金刚石也存在有较大差异.蛇绿岩型金刚石的δ13C值介于-18‰~-28‰,不同于变质型金刚石的碳同位素组成(δ13C=-30‰~-3‰),此外二者的氮同位素组成以及氮含量也存在有较大不同.变质型金刚石的δ15N值均为正值,而蛇绿岩型金刚石的氮同位素组成具有较大的变化范围,与地幔橄榄岩型和榴辉岩型金刚石较为一致.变质型金刚石可以具有很高的氮元素含量,最高可达9 000×10-6,而蛇绿岩型金刚石的氮元素含量通常小于600×10-6.这些形态与成分上的差异,指示了蛇绿岩中的金刚石并非来源于深俯冲的板片物质.
近期,Ballhaus et al.(2017)以玄武岩为研究对象,进行电子放电实验,该实验生成的矿物组合与蛇绿岩地幔橄榄岩和铬铁矿中所发现的异常矿物组合相似.因此,这些金刚石等异常矿物可能是地幔橄榄岩在野外遭受雷击形成的(Ballhaus et al., 2017, 2018).然而一系列的证据并不支持闪电成因模型(Griffin et al., 2018; Yang et al., 2018).这些证据主要包含有以下几点:(1)在这些已经发现含有异常矿物的蛇绿岩中,仅位于西藏的蛇绿岩体具有较高的海拔,而其他岩体(如土耳其Pozantı⁃Karsantı岩体、阿尔巴尼亚的Mirdita岩体、中国内蒙古的萨尔托海岩体等)海拔较低,遭受雷击的可能性较小;(2)蛇绿岩中发现的超高压矿物,如具有斯石英假象的柯石英(Yang et al., 2007)、铬尖晶石中的单斜辉石和柯石英出熔体(Yamamoto et al., 2009)、高压的八面体硅酸盐矿物(Griffin et al., 2016; Lian et al., 2017),都没有在Ballhaus et al.(2017)的电子放电实验中生成;(3)蛇绿岩型金刚石中存在有大量的包裹体,这种包裹体未在电子放电实验中观察到;(4)蛇绿岩中这种数百微米的金刚石不可能通过仅仅持续几秒钟的雷击形成(Griffin et al., 2018).
大量地球物理的研究资料显示,俯冲的大洋岩石圈板片可以到达地球410~660 km的地幔过渡带的深度(Fukao, 1992; Grand, 1994; Zhao, 2004).板片的深俯冲将壳源物质带入地球的深部,为蛇绿岩型金刚石的形成提供了物质来源.Robinson et al.(2015)以及Yang et al. (2015)认为蛇绿岩型金刚石形成于地幔过渡带的深度,这些金刚石以及其他的超高压矿物被深部流体携带,并向上运移,在300 km左右的深度铬尖晶石发生结晶,并包裹了金刚石等超高压矿物(图 16c).Howell et al.(2015a)以及Xu et al.(2017)通过对罗布莎蛇绿岩中的金刚石的研究指出,这些蛇绿岩型金刚石为Ⅰb型金刚石,缺少塑性变形以及再吸收的特征,指示了较短的地幔滞留时间(几个百万年),但是金刚石的形成深度和形成机制仍然不清楚.因此,现有的蛇绿岩型金刚石的成因仍然存在较大争议.
目前,我们对蛇绿岩型金刚石的物质来源、形成深度、运移和保存机制等关键问题的认识仍然不清楚.伴随俯冲板片进入地幔的有机物质被普遍认为是蛇绿岩型金刚石的物质来源,然而,俯冲板片中再循环的物质的碳同位素组成具有较大的变化范围(图 13),为何已知的罗布莎、Ray⁃Iz、Pozantı⁃Karsantı和Mirdita蛇绿岩中的金刚石均具有一致的较轻的碳同位素组成?具有较重碳同位素组成的再循环物质为何没有作为碳的来源,而形成具有高δ13C值的金刚石?
金刚石的形成深度目前无法进行有效的限制.产于金伯利岩中的金刚石的形成的温压条件,可以通过其内部含有的硅酸盐包裹体进行限制;而蛇绿岩型金刚石中最常见的包裹体为合金包裹体.尽管Moe et al.(2017)在蛇绿岩型金刚石中发现了多种类型的流体和固体包裹体,但是这些包裹体类型无法为金刚石的形成深度提供准确信息.
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金刚石矿床资源在全球的分布极为不平衡,仅集中分布在澳大利亚、加拿大、俄罗斯、南非、博茨瓦纳、刚果(金)、巴西等少数国家(彭艳菊等, 2013).俄罗斯、博茨瓦纳、刚果(金)、加拿大、南非、澳大利亚和安哥拉这7个国家的金刚石的总产量已经占据了全球金刚石总产量的91%,金刚石的总产值占全球金刚石总产值的87%(Janse, 2007; 袁姝等, 2011).相比之下,中国金刚石资源极为匮乏,产量仅占全球金刚石产量的0.1%,且近年来,我国的金刚石资源濒临枯竭,金刚石主要采矿区基本停产(彭艳菊等, 2013).
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中国的金刚石矿床包含有原生金刚石矿床和金刚石砂矿,主要分布在我国的华北地台和杨子地台,其中有经济价值的矿床包含山东蒙阴和辽宁复县的金伯利岩矿床,以及湖南沅水流域的金刚石砂矿(池际尚等, 1996; 彭艳菊等, 2013).
中国具有经济价值的山东蒙阴和辽宁复县金伯利岩矿床均位于华北地台.华北地台目前共发现有9个金伯利岩岩体群和1个钾镁煌斑岩岩体群,分别是蒙阴、复县、铁岭、恒仁、鹤壁、涉县、柳林、应县、阳高金伯利岩岩体群和阳高饮牛沟钾镁煌斑岩岩体群.不同于华北地台、扬子地台内部发育的与金刚石相关的岩体为钾镁煌斑岩.池际尚等(1996)将这些岩体划分为4个岩区,分别是:(1)贵州镇远及麻江钾镁煌斑岩-钾质煌斑岩区;(2)湖南宁乡钾镁煌斑岩-钾质煌斑岩区;(3)湖北大洪山金伯利岩-钾镁煌斑岩-钾质煌斑岩区;(4)扬子地台西缘超钾质煌斑岩区.前人通过岩体重砂选矿以及河流重砂选矿工作,仅在贵州镇远及湖南宁乡的钾镁煌斑岩-钾质煌斑岩区发现有金刚石,但是两个岩区不具有工业开采价值(梅厚钧等, 1998; 黄远成等, 2016).
我国塔里木地台的河流砂矿中也曾发现有7颗微粒金刚石,塔里木地台也被认为是我国的金刚石找矿的靶区之一.但是自1950年以来,开展的金刚石找矿工作一致未取得进展,仅发现有大量的金刚石指示矿物,而未获得金刚石(梁日暄和方青松, 1992).
除了上述具有经济价值的幔源型金刚石,我国还分布有超高压变质型(徐树桐和季寿元, 1991; Xu et al., 1992; 杨经绥等, 2002; Yang et al., 2003)和蛇绿岩型金刚石(杨经绥等, 2008, 2011).Xu et al.(1992)在秦岭造山带东段的大别山地区的榴辉岩中发现有微粒金刚石,这些金刚石主要呈包裹体的形式产出于石榴子石中.大别山榴辉岩中的原位金刚石粒径主要在10~60 μm,少数金刚石粒径可达240 μm;而通过重砂分选获得的金刚石粒度稍大,一般在150 μm左右,最高可达700 μm (Xu et al., 1992).杨经绥等(2002)在中国中部的北秦岭造山带变质岩中的锆石中发现了10余粒金刚石,这些锆石样品分别来自于榴辉岩和片麻岩(图 3c).秦岭造山带中的超高压变质型金刚石粒度远小于大别山榴辉岩中的金刚石,粒度仅为1~2 μm,个别颗粒甚至小于1 μm (杨经绥等, 2002; Yang et al., 2003).由于超高压变质型金刚石微米级的粒度限制以及有限的分布,使其不能成为具有经济价值的金刚石矿床.
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由于早期技术条件的限制,我国的金刚石的研究局限于对金刚石晶体粒度、颜色、形态及包裹体的描述,对于关系金刚石物质来源和成因的同位素组成的研究较少(朱和宝等, 1982; 郭九皋等, 1986; 刘观亮等, 1995).随着离子探针质谱仪方法的应用,金刚石的同位素分析由破坏性真空燃烧法(刘观亮等, 1995),到激光烧熔质谱法(张宏福等, 2009),转变为精确的原位微区且近于无损的分析测试,使得金刚石内部更精细的同位素组成的变化得以被识别(张健等2012;陈华等2013).
刘观亮等(1995)利用真空燃烧法结合气体稳定同位素MAT⁃251质谱仪对来自山东蒙阴、辽宁复县、贵州镇远以及湖南沅水的230多颗金刚石进行了碳同位素分析.这4个地区的金刚石的δ13C值具有较大的变化范围,总体介于-26.1‰~+1.5‰,集中分布在-9‰~-2‰之间,与世界范围内金刚石的碳同位素组成较为一致.不同地区的金刚石的碳同位素组成具有一定的差异,其中:(1)山东蒙阴金刚石δ13C值介于-11.8‰~-2.8‰,集中分布在-8‰~-3‰;(2)辽宁复县金刚石δ13C值介于-14.7‰~-0.3‰,集中分布在-9‰~-2‰;(3)贵州镇远金刚石δ13C值介于-22.2‰~-2.5‰,集中分布在-11‰~-5‰;(4)湖南沅水金刚石δ13C值介于-26.1‰~+1.5‰,集中分布在-9‰~-2‰.根据刘观亮等(1995)分析结果,可以看到华北地块的蒙阴和复县与扬子地块的镇远和沅水地区的金刚石碳同位素组成存在有一定的差异,镇远和沅水的部分金刚石具有较轻的碳同位素组成,这种同位素组成的差异可能反映了形成金刚石的地幔源区和物质来源的差异.张宏福等(2009)同样利用燃烧法对山东蒙阴和辽宁复县的14颗金刚石进行分析,获得山东蒙阴金刚石δ13C值变化范围为-6.37‰~-0.42‰,辽宁复县金刚石δ13C值为-5.03‰~-1.37‰,介于刘观亮等(1995)取得的区间范围内.同时,张宏福等(2009)指出,对同一颗金刚石进行3次激光烧熔质谱分析的结果明显不同、且大于测试误差,指示了金刚石内部具有明显的同位素成分分带现象.因此,传统的金刚石分析方法无法有效地揭示金刚石内部成分的变化,进而限制了对金刚石形成环境和物质来源变化的解读.
二次离子质谱SIMS (secondary ion mass spectroscopy)仪器的应用,有效地解决了这一问题,使得金刚石的研究向精细化、原位分析发展.张健等(2012)和和陈华等(2013)先后利用二次离子质谱仪对中国不同地区的金刚石进行了碳同位素组成分析,识别出了金刚石内部的成分变化.这些被分析的金刚石粒径为2~5 mm,晶体形态包含八面体与十二面体聚形和八面体,颜色为近无色或浅褐色.金刚石的阴极发光图像显示出了清晰的生长环带,同时部分金刚石显示核幔结构,其核部区域可能代表了金刚石结晶早期的晶核(图 17).分析结果显示,山东蒙阴和辽宁复县的金刚石的碳同位素组成较为一致,而扬子地台的金刚石具有相对较轻的碳同位素组成,与前人的研究结果一致(刘观亮等, 1995; 张健2012;陈华2013).此外,这些金刚石的核部还显示出了与幔部层状生长区不一样的同位素组成.
图 17 中国山东蒙阴代表性金刚石形态特征以及阴极发光图像
Figure 17. Representative images of diamond from the Mengyin kimberlite in Shandong, China
上述对华北地台和扬子地台的金刚石的同位素研究工作仅仅涉及了碳同位素,而对于金刚石中的重要杂质元素氮的研究尚未开展,限制了对两个地台早期地幔深部氮组成以及金刚石物质来源的认识.
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国内金刚石中包裹体的研究主要集中在山东蒙阴和辽宁复县的金伯利岩岩体中的金刚石中(陈丰和陈积昌, 1992; 陈丰等, 1992;郑建平等, 1994; 亓利剑等, 1999; 刘惠芳, 2002; 殷莉等, 2008; 陆琦等, 2011, 2012),而对扬子地台中金刚石包裹体的研究则较为有限(陈丰和陈积昌, 1992; 池际尚等, 1996).
山东蒙阴和辽宁复县的金刚石具有相似的矿物包裹体组合,主要包含橄榄石、铬尖晶石、透辉石、石榴石、石盐、自然铁和硫化物等矿物.殷莉等(2008)收集并整理了前人已发表的金刚石中包裹体的主量元素和微量元素化学组成的数据,通过分析,发现山东蒙阴和辽宁复县金刚石中的包裹体类型主要为地幔橄榄岩型,仅有少数矿物显示榴辉岩型矿物包裹体的化学特征(Wang et al., 1998; 殷莉等, 2008).根据山东蒙阴和辽宁复县的金刚石的包裹体的组合特征,同时结合包裹体成分稳压计算的结果(1 050~1 250℃,5~7 GPa),华北地台的金刚石主要形成于200 km以上的岩石圈地幔(Wang et al., 1998; 尹作为等, 2005).然而,除了这些岩石圈类型的包裹体矿物组合之外,陆琦等(2011, 2012)先后在辽宁复县的金刚石中发现有碳化钛(TiC),超硅石榴石和钙钛矿等高压-超高压矿物.金刚石中的TiC包裹体呈板状,颗粒大小约为50 μm×35 μm×8 μm,X射线衍射分析结果显示矿物具有立方结构,同时可能保存有超高压的菱面体结构的残余,可能指示18 GPa的压力(Dubrovinskaia et al., 1999; 陆琦等,2011).辽宁复县金刚石中超硅石榴石的发现进一步证实了一部分金刚石可能来源于华北克拉通的岩石圈下部的软流圈甚至地幔过渡带(陆琦等, 2012).与辽宁复县金刚石中超硅石榴石相伴生的矿物包裹体还有钙钛矿、二氧化硅、刚玉、氧化铁(方铁矿?) (陆琦等,2012),对这些伴生矿物开展晶体结构的研究工作,可以进一步限定金刚石的来源深度.
扬子地台的贵州镇远和湖南宁乡钾镁煌斑岩中金刚石包裹体的研究工作较少,仅池际尚等(1996)及及陈丰等(1992)对这些金刚石中包裹体进行了简单描述.池际尚等(1996)提到贵州镇远金刚石中含有石墨、赤铁矿、碳化钨等包裹体,而湖南宁乡的金刚石中含有大量细小的深色包裹体以及个别透明包裹体,包裹体成分不详.因此对扬子地块的金刚石亟需开展进一步的研究工作,以详细确定金刚石中包裹体的组合特征,为判断金刚石的来源提供依据.
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中国的金刚石类型较为丰富,既有来自金伯利岩和钾镁煌斑岩中的典型的幔源型金刚石,产于超高压变质带的变质型金刚石,也有产出于蛇绿岩地幔橄榄岩和铬铁矿中的蛇绿岩型金刚石,为金刚石的研究提供了丰富的素材.金刚石的研究对于认识深部物质循环,区域构造演化以及我国金刚石矿产的找矿工作发挥着重要作用.
蛇绿岩型金刚石主要通过重矿物分选而来,仅在罗布莎和Ray⁃Iz的铬铁矿中发现有限的原位金刚石产出,人们对该类型的的金刚石仍然存在质疑,因此在不同地区的蛇绿岩地幔橄榄岩和铬铁矿中寻找原位的金刚石及其他超高压矿物具有重要的意义.原位金刚石可以提供地幔橄榄岩和铬铁矿中金刚石的产出状态,为揭示形成金刚石的流体/熔体的组成提供线索.相对于国外对幔源型金刚石的研究,我国在这方面研究仍然存在明显的不足.对于我国不同地区的金刚石的晶体结构类型、矿物包裹体类型以及碳氮同位素组成,报道很少,缺乏系统详细的研究,制约了对我国金刚石的成因认识,因此亟需加强这些方面的研究工作.随着测试技术的不断发展,应当利用先进的分析仪器(如聚焦离子束-透射电子显微镜和大型二次离子探针质谱仪),对金刚石开展一系列原位的微米级乃至纳米级的结构和成分的研究工作,高温高压实验和数值模拟计算的手段也应当被应用于金刚石的研究中.利用这些手段可以为金刚石的形成过程中元素的分馏、不同类型的金刚石形成的温压条件、形成金刚石的不同熔体/流体的性质提供直观的定量的限定.
金刚石的研究不能脱离对于金刚石的宿主岩石的研究,应当同时开展金伯利岩、钾镁煌斑岩、地幔橄榄岩和铬铁矿等岩石的精细的岩相学、矿物学和地球化学的研究工作,为金刚石的成因研究限定背景条件.
金刚石分类、组成特征以及我国金刚石研究展望
Diamond Classification, Compositional Characteristics, and Research Progress: A Review
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摘要: 金刚石由于其独特的物理化学性质,在经济生产与科学研究中均具有重要价值.金刚石形成于地球大于150 km的深度范围内,是人类可以获得的来自地球深部地幔乃至核幔边界的最直接的样品,因此可以为研究地球深部物质组成和物理化学条件提供重要的素材.金刚石由碳元素组成,还含有微量的杂质元素(如氮、硼、氢、氧等),其中氮和硼元素对于划分金刚石的晶体结构类型发挥着重要的作用.根据金刚石的产出类型,金刚石可以划分为幔源型、超高压变质型、陨石相关型以及蛇绿岩型金刚石.全球约百分之一的幔源型金刚石含有包裹体,对这些包裹体的研究显示,金刚石主要来源于地球150~200 km深度的岩石圈地幔.这些含有包裹体的金刚石中,仅有1%的金刚石来自于地球深部的软流圈、地幔过渡带、下地幔、甚至核幔边界.我国的金刚石产出类型多样,但是,目前仅山东蒙阴、辽宁复县的金伯利岩矿床以及湖南沅水的砂矿具有经济价值.蛇绿岩型金刚石是近年来金刚石研究领域取得的重要进展,该类型金刚石分布在全球多个造山带不同时代、不同构造属性的蛇绿岩地幔橄榄岩和铬铁矿中,被认为是一种新的金刚石的产出类型.相对于其他国家和地区的金刚石的研究,我国的金刚石领域的研究程度相对较低,缺乏对金刚石结构、化学组成以及包裹体组成的系统研究,制约了对我国金刚石成因的认识,限制了我国的金刚石的找矿工作.因此,亟需结合先进的分析手段对我国的金刚石及其围岩做进一步的研究,以期揭示金刚石的形成过程,为金刚石的找矿提供理论基础.Abstract: Due to the unique physicochemical characteristics, diamonds are of great significance in economic production and scientific research. Diamonds record valuable information about the physicochemical conditions and compositions of Earth's deep mantle and even the core-mantle boundary. Diamond mainly consists of carbon, with minor impurities such as nitrogen, boron and hydrogen. Of these impurities, nitrogen and boron play important roles in the classification of diamonds. Carbon and nitrogen are vital elements for life, the cycles of which are closely related to the environment that humans live in. Diamonds can provide clues for the deep carbon and nitrogen cycle on the Earth. Studies on the mineral inclusions show that diamonds mainly form between a depth window of 150-200 km, with 1% of the diamond population forming in the asthenosphere (>200 km), the mantle transition zone, the lower mantle or even the core-mantle boundary. China hosts different types of diamonds, however, diamonds of economic significance only exist in the Shandong Province and Liaoning Province. The discovery of ophiolite-hosted diamonds (also called ophiolitic diamonds) is an important progress in the study of diamonds. Ophiolite-hosted diamonds in China have been discovered in ophiolitic peridotites and chromitites in different orogenic belts, and have been accepted as a new occurrence for diamonds. Diamond research in China is very limited, which has hampered the understanding of diamond genesis and diamond exploration. New analytical methods need to be applied on diamond and country rock research to provide new constraints on diamond formation and theoretical basis for diamond explorations.
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Key words:
- diamond /
- carbon isotope /
- nitrogen isotope /
- inclusion /
- ophiolite-hosted diamond /
- petrology
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