Formation of Podiform Chromitite Deposits: Review and Prospects
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摘要: 豆荚状铬铁矿是蛇绿岩的特征性矿产，对其成因的认识还存在较大的分歧，包括：（1）早期岩浆熔离；（2）地幔熔融残余；（3）熔体-岩石反应.豆荚状铬铁矿及其围岩地幔橄榄岩中大量异常地幔矿物群的发现，引起了地质学家对其形成过程的重新思考.回顾了铬铁矿的研究，借助pMELTS热力学软件模拟浅部地幔过程，使用定量化的方法限定这些过程对豆荚状铬铁矿形成的贡献，通过一个新的角度讨论其形成.初步模拟结果显示，单独的地幔部分熔融、熔体分离结晶以及拉斑质熔体与亏损地幔的反应等过程形成的铬铁矿，无论在数量还是品位上都难以达到矿床水平，暗示豆荚状铬铁矿的形成可能为多种作用耦合的结果，或与深部地幔作用有关.Abstract: As the characteristic ore deposits in ophiolites, podiform chromitite deposits mainly formed in the ultramafic section of an ophiolite complex. However, the following issues on podiform chromitite remain controversial:(1) crystal-melt separation; (2) residual of mantle partial melting; (3) melt-rock interaction. It is of particular importance to study the formation processes of podiform chromitite when large amounts of unusual minerals are found in podiform chromitite and its host rock peridotite. Petrogeneses of chromitite are reviewed in this paper. The pMELTS thermodynamic software was used to calculate the contribution of shallow mantle processes to the formation of podiform chromitite. Preliminary simulation results show that it is impossible to explain the formation of large-scale podiform chromitite deposits by individual mantle partial melting, mantle melt separation or tholeiitic melt-depleted mantle interaction processes, which implies that the podiform chromitite deposits may have been formed by multi-stage or deep mantle processes. Further researches are needed to be conducted on the origin of podiform chromitite deposits.
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Key words:
- crystal-melt separation /
- partial melting /
- melt-rock reaction /
- podiform chromitite /
- ophiolite /
- petrology
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铬铁矿(FeCr2O4)属于尖晶石族矿物，广泛存在Cr2O3、Al2O3、Fe2O3、FeO和MgO等组分间的类质同象置换，化学式可变为(Mg, Fe2+)(Cr, Al, Fe3+)2O4，性质较稳定.最近实验岩石学指出，在1 000~1 600 ℃下，铬铁矿可稳定至14 GPa，该温度压力条件相当于地幔过渡带顶部~410 km(Ishii et al., 2014, 2015; Wu et al., 2016).铬铁矿的密度较大，为4.3~4.8 g/cm3，常与橄榄石共生于超基性岩中.铬铁矿是自然界中唯一可用于提炼铬的矿物原料.铬由于具有质硬、耐磨、耐高温、抗腐蚀等特性，被广泛运用于冶金工业、耐火材料以及化学工业中，是生产不锈钢不可替代的原料.我国的铬铁矿储量仅占世界的千分之一，每年消费量的90%以上均依靠进口(张建等, 2009).根据我国1952—2013年铬铁矿进口量统计图(图 1)可发现，随着我国工业技术的发展需求，我国对铬铁矿的需求量呈指数上升.铬铁矿作为我国的短缺矿种，勘探寻找新的矿床迫在眉睫.
图 1 1952—2013年中国铬铁矿进口量变化情况(百万t)1952—1995年数据来源于中国矿业网；1996—2013年数据来源于USGS Minerals Yearbook CHROMIUM各年册Fig. 1. Variety of chromitite imports of China from 1952 to 2013世界上原生的铬铁矿矿床主要包括两种类型：一是产于前寒武纪稳定克拉通内层状镁铁-超镁铁杂岩中的层状铬铁矿，此类铬铁矿位于莫霍面之上，由镁铁-超镁铁质岩浆结晶分异形成，在岩体的底部一般为铬铁矿，中间依次发育铜镍和铂族矿物矿床，上部为钒钛磁铁矿，为典型的高温岩浆矿床(姚凤良和孙丰月, 2006)，以南非Bushveld和津巴布韦大岩墙Great Dyke为代表，总储量占世界铬铁矿的70%以上(杨经绥等, 2010)；二是主要产于莫霍面之下，蛇绿岩的地幔橄榄岩中的蛇绿岩型铬铁矿，由于该种铬铁矿在形成后常受到强烈的构造作用，使得其在空间分布以及形态上常呈现不连续分布的透镜状或是特殊的豆荚状，故又得名豆荚状铬铁矿，该类矿床以俄罗斯极地乌拉尔(Polar Urals)和哈萨克斯坦肯皮尔赛(Kempirsai)的为代表.
我国镁铁-超镁铁岩分布十分广泛，类型多样，形成时期几乎贯穿元古宙到新生代所有大地构造发展阶段，蛇绿岩超基性岩体接近9 000个(鲍佩声, 1999)，然而目前发现的铬铁矿床却很少.西藏罗布莎铬铁矿床作为我国规模最大的豆荚状铬铁矿床，自1959年发现至今，许多代地质学家对其进行了考察和研究，积累了大量资料，也提出了许多豆荚状铬铁矿的成因模式(王希斌和鲍佩声，1987; Zhou and Robinson, 1994, 1997; Zhou et al., 1996, 2014；鲍佩声，2009; Arai, 2013; Yang et al., 2014, 2015; McGowan et al., 2015; Xiong et al., 2015; Griffin et al., 2016)，然而其形成过程至今仍存在争论，并且难以转换为找矿勘查模型.
特别是近年来在罗布莎铬铁矿中发现了大量的地幔异常矿物群，包括金刚石、碳硅石、自然元素及金属合金等(白文吉等, 2001, 2002, 2004, 2007; Robinson et al., 2004; Yang et al., 2007, 2014; Dobrzhinetskaya et al., 2009; Fang et al., 2009；徐向珍，2009; Xu et al., 2009, 2015; Yamamoto et al., 2009; Howell et al., 2015; Satsukawa et al., 2015; Zhang et al., 2016)，再度掀起了豆荚状铬铁矿的研究热潮.
豆荚状铬铁矿中高压-超高压还原矿物的发现引出了一系列新问题：(1)豆荚状铬铁矿形成于浅部地幔环境？还是深部地幔环境？或是两者都有可能？(2)高压-超高压还原矿物与铬铁矿的关系？这些矿物是如何进入铬铁矿晶体中的？(3)高压-超高压还原矿物是如何被保存下来并且没有发生退变？为了解决这些问题，有必要重新回顾国内外豆荚状铬铁矿甚至层状铬铁矿的研究现状.本文在综述前人研究的基础上，结合pMELTS热力学模拟岩浆系统相平衡软件(Ghiorso et al., 2002)，限定地幔过程中各种作用对形成铬铁矿的贡献，旨在更直观地厘清铬铁矿的形成过程，为建立成矿模型提供思路与基础.
1. 豆荚状铬铁矿床的基本特征
1.1 豆荚状铬铁矿的定义与分布
豆荚状铬铁矿，最先由Thayer(1964)定义：豆荚状铬铁矿一般产在阿尔卑斯型超镁铁质杂岩体中，呈扁平、铅笔或不规则状.透镜状的铬铁矿矿体经常被纯橄岩包围(纯橄岩壳)，并向外逐渐过渡为地幔橄榄岩(多为方辉橄榄岩)围岩.豆荚状铬铁矿在全球的分布不均一，主要存在于沿喜马拉雅-阿尔卑斯造山带的希腊、土耳其、阿尔巴尼亚、伊朗和巴基斯坦等，西太平洋的菲律宾、新喀里多尼亚，古巴、乌拉尔、哈萨克斯坦等显生宙蛇绿岩中(Mosier et al., 2012).此外，亦有研究在现代太平洋(Arai and Matsukage, 1998)、西南印度洋(Morishita et al., 2007; Payot et al., 2014)、大西洋洋脊(Abe, 2011)，芬兰(Vuollo et al., 1995)、中国辽西(李江海等，2002)、遵化(Huang et al., 2004; Kusky et al., 2004)、庙湾(Peng et al., 2012; Huang et al., 2017)等元古宙、太古宙蛇绿岩中发现豆荚状铬铁矿，但一般规模较小.
在中国，豆荚状铬铁矿床主要分布在古亚洲洋和喜马拉雅-特提斯成矿域中的造山带蛇绿岩内(图 2)，如中亚造山带(新疆萨尔托海、内蒙古索伦山、内蒙古贺根山等)、祁连-秦岭造山带(甘肃大道尔吉、青海玉石沟、陕西松树沟等)、班公湖-怒江缝合带(西藏东巧)和雅鲁藏布江缝合带(西藏罗布莎)(鲍佩声，1999).我国的铬铁矿探明储量较低，并且不同缝合带地幔橄榄岩中铬铁矿的规模、品位相差很大；虽然在各个重要的成矿时期均有分布，但是矿床的规模和品位远比不上全球同期的其他豆荚状铬铁矿.以我国最大的豆荚状铬铁矿床罗布莎为例，截至2015年，其已探明储量约为800余万t(崔成多吉, 2015)，而Kempirsai铬铁矿储量则大于300百万t(Melcher et al., 1999).
图 2 地球显生宙造山带及蛇绿岩分布简图据Dilek and Furnes(2011)；其中正方形显示国内外一些豆荚状铬铁矿床Fig. 2. Global distribution of major Phanerozoic orogenic belts and ophiolite1.2 豆荚状铬铁矿与蛇绿岩的关系
对豆荚状铬铁矿的成因探讨必然涉及到对蛇绿岩形成环境的认识.1972年Penrose会议定义了典型的蛇绿岩剖面自上而下由含放射虫硅质岩、枕状熔岩、席状岩墙、堆晶杂岩(层状辉长岩，堆晶辉石岩、橄榄岩)和地幔橄榄岩(不同比例的方辉橄榄岩、二辉橄榄岩以及纯橄岩)组成(Coleman, 1977).自蛇绿岩最初定义后，虽然随着研究者对现代大洋洋壳、全球构造背景认识的不断加深和地球化学方法的应用，蛇绿岩的形成环境得到了进一步细分(Pearce et al., 1984; Dilek and Furnes, 2011, 2014)，但蛇绿岩的剖面结构总体变化较少.铬铁矿常以层状或豆荚状出现在蛇绿岩的堆晶岩与地幔橄榄岩中(图 3).
图 3 世界显生宙典型蛇绿岩格铁矿分布柱状示意图图上厚度与实际不成比例，据Thayer(1964)、Boudier and Nicolas(1995)、Melcher et al.(1997)、Proenza et al.(1999)、Robertson(2002)、yumul(2004)、Rollinson(2008)、Hébert et al.(2012)、Johnson(2012)和Milushi(2015)Fig. 3. Idealized lithologic columns for typical Phanerozoic ophiolites in the world由图 3中典型的蛇绿岩铬铁矿剖面图可以看出，豆荚状铬铁矿的产出与纯橄岩密切相关，多以层状或似层状分布在纯橄岩中(如土耳其的Guleman岩体和阿尔巴尼亚的Bulqiza岩体)，或被纯橄岩薄壳包围呈透镜状产出在方辉橄榄岩中(如哈萨克斯坦的Kempirsai和中国西藏罗布莎).本文主要讨论产在地幔橄榄岩中的豆荚状铬铁矿的成因.
1.3 豆荚状铬铁矿的分类
蛇绿岩中铬铁矿的种类非常丰富，其产出、形态结构和成分等在不同的矿体、岩体中均存在差异，因而可从不同角度对其进行分类，已有的分类方案如下：
(1) 根据铬铁矿在蛇绿岩中的产出位置(围岩).具有高品位工业价值的蛇绿岩型铬铁矿矿体大多产于蛇绿岩莫霍面以下的地幔橄榄岩中，矿体多呈豆荚状，外常有纯橄岩薄壳赋存于方辉橄榄岩中，少量分布在二辉橄榄岩中，如菲律宾的Isabela蛇绿岩(Morishita et al., 2006)；而赋存于壳-幔过渡带厚层纯橄岩中的铬铁矿则多以浸染状、条带状矿石为特征，如罗布莎的香卡山(王希斌等, 1992)、土耳其的Klzllda (Chen et al., 2015).
(2) 根据豆荚状铬铁矿的构造形态.Cassard et al.(1981)基于对New Caledonia的铬铁矿进行研究，将产于地幔橄榄岩中的豆荚状铬铁矿划分为3类：不整合矿体、次整合矿体和整合矿体，明确地反映了豆荚状铬铁矿矿体的形成与大洋扩张、地幔变形组构-线理之间的关系.
(3) 根据豆荚状铬铁矿的矿石构造.蛇绿岩型铬铁矿与层状铬铁矿的矿石类型存在显著差异，层状铬铁矿矿石类型较为单一，以层状、浸染状为主；而蛇绿岩型铬铁矿则存在致密块状(粗粒和细粒)、豆状(或称瘤状)、反豆状、条带状、细脉状、浸染状等构造.并且在不同层位，其矿石特征有所不同.
(4) 根据豆荚状铬铁矿的化学成分.豆荚状铬铁矿的一个重要特征是铬铁矿石的Cr2O3与Al2O3含量具有较宽的变化范围，并且显示两者强烈的相互消长.按照矿石中造矿铬铁矿的成分可分为高铬型(Cr#为60~80或Cr2O3含量为45%~60%)与高铝型(Cr#为20~60或Al2O3＞25%)(Thayer, 1970).罗布莎铬铁矿属于高铬型，而萨尔托海铬铁矿属于高铝型(Zhou and Robinson, 1994)，亦有研究报道在同一蛇绿岩岩体中可同时产出高铬型与高铝型铬铁矿，如西藏普兰、古巴的Sagua de Tánamo(González-Jiméneznez et al., 2011; 熊发挥等, 2013).
2. 铬铁矿的研究现状
由于层状铬铁矿的品位高，吨位巨大，早在20世纪许多科学家已对其成因进行了研究，提出了许多模式，豆荚状铬铁矿的一些成因模式也继承于层状铬铁矿的研究，所以有必要先对层状铬铁矿的研究进行回顾.
2.1 层状铬铁矿的研究现状
层状侵入岩体是研究岩浆演化分异、与地壳物质同化混染过程的天然实验室，同时，其中还产有巨大的铬铁矿、铂族元素和贱金属元素矿床.铬铁矿常以层状产于这些侵入体之中，例如南非的Bushveld Complex、Great Dyke(津巴布韦)，美国的Stillwater Complex(Montana)和加拿大的Muskox intrusion(Northwest Territories)等(Irvine, 1975; Spandler et al., 2005; Mondal and Mathez, 2006).一般认为，层状铬铁矿矿床属早期岩浆矿床，主要由侵位于地壳的玄武质岩浆在岩浆房经结晶分异作用形成(姚凤良和孙丰月, 2006).
但由于铬元素为相容元素，主要赋存于地幔中，其在热液活动过程中相对稳定，并且基性母岩浆在封闭体系下的结晶过程仅能形成少量的铬尖晶石(Campbell and Murck, 1993)，所以，铬铁矿矿床的形成需要额外的岩浆过程(Spandler et al., 2005).前人通过对不同层状侵入体中铬铁矿的研究，提出了许多解释层状铬铁矿矿床形成过程的模式(Cameron and Desborough, 1969; Irvine, 1975; Cameron, 1977; Irvine, 1977; Lipin, 1993; Spandler et al., 2005).研究普遍认为(Irvine, 1975, 1977; Cameron and Desborough, 1969; Cameron, 1977; Lipin, 1993)，岩浆化学或物理性质的改变导致其中铬铁矿(铬尖晶石)发生饱和结晶并与液相分离，在岩浆房的底部形成了层状的铬铁矿.然而，对于何种性质的改变会导致岩浆饱和铬铁矿却存在许多观点，包括：岩浆房中压力的改变(Cameron, 1977; Lipin, 1993)、岩浆氧逸度f(O2)的改变(Cameron and Desborough, 1969)、岩浆在同化围岩(Spandler et al., 2005; Irvine, 1975)或混合过程(Irvine, 1977)中成分的改变等.其中，Irvine(1977)的观点比较普遍被接受，豆荚状铬铁矿的主要成因模式也由该模式发展而来(下文将详述).以下将简要介绍层状铬铁矿的主要成因模式.
(1) 硅铝质围岩同化混染机制(salic contamination mechanism).认为层状铬铁矿可能是由于基性的母岩浆在侵入过程中广泛地与岩浆房顶部的硅铝质围岩熔融形成的花岗质熔体发生混合(同化混染作用)而形成.基性熔体中二氧化硅及碱金属含量的增加会导致熔体趋向于聚合更少的八面体空隙，而Cr3+、Ni2+、Mg2+、Fe2+等离子稳定地存在于八面体晶体场中，所以熔体中的这些离子将优先被驱逐出来并结晶形成铬铁矿、橄榄石等矿物.K2O-MgO-Cr2O3-SiO2体系实验结果能很好地与Muskox Intrusion的Cr和Ni演化对比，并解释其结晶序列：橄榄岩-铬铁矿-斜方辉石岩.岩浆在同化混染过程中先结晶形成橄榄石与少量铬铁矿，随后铬铁矿结晶量不断增加，形成铬铁矿层，由于残余岩浆中的二氧化硅含量不断增加，斜方辉石开始代替橄榄石结晶.在Muskox intrusion铬铁矿和Stillwater Complex的铬铁矿中找到了许多富含碱金属(Irvine, 1975)及富钠的奥长花岗质熔体(Spandler et al., 2005)，这为其形成提供了重要的证据.综合前人研究，Spandler et al.(2005)细化了这一机制，并提出了层状铬铁矿的形成过程：高镁的基性母岩浆周期性地侵入岩浆房，由于母岩浆的温度非常高(＞1 400 ℃)，其可以上升至岩浆房顶部.岩浆房顶部的岩石(变沉积岩围岩或是早期结晶的基性岩石)受到岩浆侵入作用会发生部分熔融并形成高Na的奥长花岗质熔体.奥长花岗质熔体随后与岩浆房顶部的母岩浆发生混合，使得熔体急速冷却.玄武质或苦橄质熔体加入少量的二氧化硅和碱金属可能会抑制橄榄石的结晶，使得铬铁矿成为唯一的结晶相.大量的铬铁矿快速结晶会导致熔体中少量的流体相出溶并促进铬铁矿捕获混合的熔体以及流体成为包裹体，同时促进岩浆房的对流，使得大量的母岩浆在相对短的时间内与围岩发生反应，不断形成铬铁矿.
(2) 岩浆混合机制(mixing with more evolved magma mechanism).Irvine(1977)认为其之前提出的铬铁矿成因模式(Irvine, 1975)在解决类似于Bushveld Complex这种具有巨大岩体规模的矿床遇到了问题：虽然实验证明花岗质岩浆中的碱金属离子可以使橄榄石-斜方辉石的反应边界向更富含硅质的方向移动，然而像Bushveld Complex这种大规模的岩浆体需要大量的地壳物质混染才能引起岩浆结晶的改变，不太符合地质事实；并且Bushveld Complex的岩浆向富含Fe方向演化，一般而言，橄榄石的结晶会消耗高Mg/Fe2+比值的熔体，实验结果显示这种岩浆演化趋势只发生在贫碱金属离子的拉斑玄武质岩浆中，换而言之，是几乎没有发生混染的岩浆.基于以上问题，Irvine(1977)提出了新的层状铬铁矿成因模式，认为层状铬铁矿可能是形成于原始母岩浆与经过结晶分异的演化岩浆的混合作用.Irvine(1977)结合MgO-Cr2O3-SiO2实验体系(Keith, 1954)及Muskox intrusion层序的岩石矿物成分，建立铬铁矿的形成模型(图 4)：点f为侵入熔体演化后的成分(辉长岩的冷凝边)、原始熔体的成分为点a，原始的熔体成分沿着橄榄石-铬铁矿的共结线演化至点b，期间在岩浆房底部结晶形成了橄榄岩；但当岩浆房中注入新的岩浆，原始成分的岩浆会与演化的岩浆发生混合，混合的熔体成分落在点c，位于铬铁矿的首晶区，大量铬铁矿结晶，随着铬铁矿的不断结晶，熔体的成分会向着d点演化，进入斜方辉石的首晶区，形成斜方辉石岩.熔体的结晶顺序为橄榄岩-铬铁矿-斜方辉石岩，与Muskox intrusion、Stillwater Complex和Great Dyke的层序一致.
以上两种模式都能很好地解释Muskox Intrusion、Stillwater Complex和Great Dyke以橄榄岩-铬铁矿-斜方辉石岩或方辉橄榄岩作为循环单元的现象，然而，Bushveld Complex却与这些侵入体不同，绝大多数的Bushveld铬铁矿直接夹于斜方辉石岩之间.对于以上模式，部分学者认为Bushveld铬铁矿的含量远远超过目前岩体规模能解释的承载量(Eales, 2000; Mondal and Mathez, 2006).Eales(2000)曾作以下计算：基于对Bushveld Complex西北部剖面的研究考察，假设全部1 260 m厚的关键区域(critical zone)的Cr2O3的平均丰度约为1.40%，要形成这样规模的岩体需要Cr2O3含量为0.15%的岩浆覆盖该区域达15 km厚，或者需要近乎两倍的基性岩层厚度.人们为此提出了一些猜想，试图解决Bushveld Complex的超量铬铁矿的形成.Cawthorn and Walraven(1998)认为可能有大量的岩浆在注入岩浆房后又离开了；Eales(2000)注意到了这个缺失的证据，如火山岩的年龄及成分，他提出了Bushveld Complex的铬铁矿可能由一些含有悬浮铬铁矿的岩浆侵入Bushveld岩浆房，然后堆晶形成铬铁矿层，后Mondal and Mathez(2006)通过对UG2层铬铁矿的矿物学、岩石学及地球化学进行研究，也支持Eales(2000)的猜想.
对于层状铬铁矿的成因仍存在争议，不同侵入体可能经历了不同过程.其中Irvine(1975, 1977)借助含Cr2O3多元系统实验相图结合野外观察的研究方法及结果为豆荚状铬铁矿的研究奠定了理论基础.
2.2 豆荚状铬铁矿的研究现状
在层状铬铁矿还未被人类发现利用之前，豆荚状铬铁矿就已经用作提炼铬的主要矿床，然而由于豆荚状铬铁矿的种类丰富，在不同的缝合带和岩体中，其成矿性、成矿规模和品位差异非常大，直至今日，其成因相对于层状铬铁矿，仍存在许多分歧.下面将主要回顾前人对产于纯橄岩和方辉橄榄岩/二辉橄榄岩中铬铁矿的研究，因为具有工业价值的铬铁矿一般产于这两个层位中.
对于产于蛇绿岩中纯橄岩层位的铬铁矿，一般呈层状或浸染状，世界上该层位铬铁矿较成规模的有土耳其以及阿尔巴尼亚(Lian et al., 2017; Wu et al., 2017a, 2017b; Xiong et al., 2017)，我国大道吉尔(鲍佩声, 1999)和罗布莎香卡山(王希斌等, 1992; Zhou et al., 2005)也产有厚层状的纯橄岩，一般该层位的铬铁矿矿体规模较小，工业品位较低，对其成因的研究较少，甚至连其围岩纯橄岩(也称壳幔过渡带纯橄岩，transition-zone dunite)的成因也存在争议：
(1) 堆晶成因.认为蛇绿岩中地幔橄榄岩之上的纯橄岩属于地壳部分，为岩浆房中镁铁质岩浆通过结晶分异作用形成，其上逐渐过渡为辉长岩等(Coleman, 1977).
(2) 地幔熔融残余.Nicolas等学者对全球一些蛇绿岩进行考察后，认为其中过渡带纯橄岩与上部的层状辉长岩为截然接触，与下部的方辉橄榄岩为不规则接触，应为地幔部分熔融，辉石全部溶解进入熔体，熔融残余形成，同时他们还发现该层纯橄岩具有地幔特有的高温塑性变形以及组构，应属于地幔部分(Nicolas and Prinzhofer, 1983; Boudier and Nicolas, 1995).
(3) 熔体-岩石反应.Zhou et al.(2005)基于对罗布莎过渡带纯橄岩的地球化学研究，发现其地球化学特征与产于方辉橄榄岩中包围铬铁矿的纯橄岩薄壳成分相似，都具有U型稀土配分模式，与地幔中由玻安质熔体与MORB地幔橄榄岩反应形成的纯橄岩脉一致.
而对于产于莫霍面之下的铬铁矿，由于工业品位较高，经济价值高，故研究比较详尽，同时争议也较大：
(1) 早期岩浆熔离成因.20世纪60年代及以前，大多学者认为，豆荚状铬铁矿的成因与层状铬铁矿相似，由岩浆经过分离结晶作用形成(Thayer, 1964)，然而该观点无法解释为何豆荚状铬铁矿常产于蛇绿岩中的残留地幔橄榄岩中而非地壳岩浆房的堆晶岩中，后有学者发展该理论，认为铬铁矿在岩浆上升过程中已经开始发生结晶，并由于其重力作用留于Moho以下的方辉橄榄岩中(Dickey, 1975; Paktunc, 1990).
(2) 浅部地幔部分熔融残余成因.20世纪70~80年代，随着蛇绿岩研究的进展，蛇绿岩中的地幔橄榄岩被认为是抽取玄武质岩浆后的熔融残余(Moores and Vine, 1971)，随着熔融程度的增加，残余的地幔橄榄岩中的铬尖晶石的Cr#不断增加(Duke, 1982; 鲍佩声, 1999, 2009)，当熔融程度进一步增高时(熔融程度达50%)，呈分散状态的铬尖晶石逐渐融化并形成彼此隔开的熔滴状，借助地幔剪切作用汇聚成矿(王希斌和鲍佩声, 1987; 鲍佩声, 2009)；随后，地幔橄榄岩的部分熔融实验也证明了上地幔的部分熔融作用对于铬铁矿的预富集起着关键作用(金振民等, 1996).
(3) 熔体-岩石反应成因.20世纪80年代末至90年代，地球化学、实验岩石学等研究手段的发展成熟，特别是地幔中熔体-岩石反应的发现(Kelemen et al., 1990)，极大地促进了铬铁矿成因的研究：①以Arai(1994, 1997)为代表的学者认为，深部较高压地幔形成的原始熔体在上升过程中会与地幔浅部的地幔橄榄岩发生熔体-岩石反应，原始熔体会溶解地幔橄榄岩中的辉石，形成富硅的二次熔体(熔体Cr含量增加)以及橄榄石，当二次熔体遇到补给的原始熔体，两者会发生混合，使得混合熔体成分进入铬铁矿的首晶区，结晶形成铬铁矿并成矿(图 5)，围岩的亏损程度影响铬铁矿的成分以及规模大小.Arai and Miura(2015)补充，岩浆通道形成的应力环境(挤压或拉张)可能会影响单个豆荚状铬铁矿矿体的规模，因为在这两种环境下熔体通道的规模、熔体的量和铬铁矿在熔体中的饱和程度都会有所不同.②以Zhou and Robinson(1994, 1997)和Zhou et al.(1996)为代表的学者观点与Arai稍有不同，认为熔体-岩石反应过程中，辉石的不一致熔融形成的富SiO2熔体直接使得熔体的成分进入了铬铁矿结晶的稳定区域，并形成了纯橄岩薄壳(图 6).铬铁矿的成分主要由熔体成分决定，拉斑质熔体与地幔反应形成高Al型铬铁矿，而玻安质熔体由于具有较高的Cr含量，与地幔橄榄岩发生反应，形成高Cr型铬铁矿.③还有部分学者通过对铬铁矿中的Re-Os同位素研究，发现在铬铁矿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，提出了“熔体与古老大陆岩石圈地幔反应成矿”假说(Shi et al., 2007; 史仁灯等, 2012).上述几种观点都认为不同成分的豆荚状铬铁矿的成因与蛇绿岩的形成环境有关.
图 5 豆荚状铬铁矿熔体混合模型据Arai and Miura(2015)；Q.石英；Ol.橄榄石；Chr.铬铁矿Fig. 5. Petrologic model for the formation of podiform chromitite via melt mixing
图 6 橄榄石(Ol)-石英(Q)-铬铁矿(Chr)系统Fig. 6. Phase relations in the system olivine (Ol)-quartz (Q)-chromite (Chr)通过选矿矿物学的研究，首次在西藏罗布莎铬铁矿中发现金刚石(中国地质科学院地质研究所金刚石组, 1981; Bai et al., 1993)以及八面体假象蛇纹石(杨凤英等, 1981)等，这些矿物可能来自地幔过渡带.由于这些矿物都是由大量的铬铁矿矿石通过人工重砂的方式，经过破碎、重选、磁选等过程后再人工挑选出来，极容易受到混染，曾一度不为许多学者所认可(Taylor et al., 1995; 切切斯特钻石公司考察团, 1997).近年来，在罗布莎及乌拉尔铬铁矿中发现了大量的地幔异常矿物群，包括碳硅石、自然元素和金属合金等(白文吉等, 2001, 2002, 2004, 2007; Yang et al., 2007, 2014, 2015; 徐向珍等, 2008; Xu et al., 2009, 2015)，其中有不少新矿物(如罗布莎矿、藏布矿青松矿等)(Bai et al., 2006; Fang et al., 2009)，经过大量岩石光薄片的观察，终于在铬铁矿中找到了原位金刚石(Yang et al., 2014).Howell et al.(2015)对罗布莎铬铁矿中的金刚石进行了详细的矿物学研究，包括形态、大小、颜色、净度、内部生长结构、包裹体成分、微量元素、C和N同位素，认为这些金刚石与人造金刚石不同，是形成于自然环境的，并在地幔中具有较短的停留时间.除了罗布莎和乌拉尔，在雅鲁藏布江缝合带内的其他岩体(东波、普兰、当穷、日喀则、泽当)及国内(丁青、东巧、萨尔托海、贺根山等)甚至国外许多铬铁中(土耳其、阿尔巴尼亚等)或多或少都发现了这些矿物(杨经绥等, 2011; 郭国林等, 2015; Huang et al., 2015; Tian et al., 2015; 徐向珍等, 2015; Xiong et al., 2016, 2017; Lian et al., 2017; Wu et al., 2017a).
由于这些高压矿物大多呈包裹体产于铬铁矿中，其与铬铁矿的形成关系并不是十分清楚，部分学者转而研究罗布莎的铬铁矿.
Yamamoto et al.(2009)首次在罗布莎铬铁矿中发现出溶单斜辉石+柯石英的铬尖晶石，认为其至少在3 GPa(＞100 km)下形成，大量单斜辉石的出溶或可能为CF相铬尖晶石(CaFe2O4结构高压异质同象)在压力大于12.5 GPa(＞380 km)环境下的上升过程中减压形成.罗布莎铬铁矿的穆斯堡尔谱研究分析，发现致密块状铬铁矿的Fe3+/∑Fe值较高，Fe3+离子相对于其他价态更能稳定地存在于高压铬铁矿相中，这符合铬铁矿中其他高压矿物的发现这一现象(Ruskov et al., 2010).Satsukawa et al.(2015)对铬铁矿的电子背散射衍射(electro backscattered diffraction, EBSD)研究显示铬铁矿中橄榄石包裹体与铬铁矿都具有单一显著的优选方位(crystallographic preferred orientation, CPO)，指示橄榄石保留了高压同质异象体瓦兹利石(wadsleyite)的CPO，在细粒的铬铁矿颗粒中还发现地幔过渡带(mantle transition zone, MTZ)的高压相晶格位错蠕变.这些发现再度引起了国际研究豆荚状铬铁矿的热潮，人们不得不重新审视豆荚状铬铁矿的浅成观点.
(4) 深部地幔部分熔融成因认为早期俯冲的陆壳和洋壳物质在地幔过渡带会发生部分熔融并汇聚Cr，铬铁矿矿浆在地幔柱或地幔对流的驱动下携带超高压矿物上升，并伴随相变到达浅部地幔(Yang et al., 2014, 2015; Xiong et al., 2015).
(5) 铬铁矿重循环成因认为豆荚状铬铁矿形成于浅部地幔，但俯冲作用可以将大洋岩石圈中的铬铁矿带至深部地幔，经深部流体改造后带有深部地幔的特征，随后可通过地幔对流(Arai, 2010, 2013)或俯冲板片的回转(roll-back)作用形成的高速上升通道(high-velocity channelized upwelling)折返(McGowan et al., 2015; Griffin et al., 2016)，再次出现在浅部地幔，其原来的岩浆结构并未发生改变，却带有金刚石和大量还原矿物、铬铁矿出溶等超高压特征(Arai, 2013; Griffin et al., 2016).
(6) 板片断离，软流圈上涌.Zhou et al.(2014)认为，这些超高压矿物以及地壳物质的碎片为板片在俯冲过程中发生板片断离，上涌的软流圈将一些超高压矿物以及地壳物质带入铬铁矿中，同时，俯冲板片的地壳碎片及其脱出流体的加入，使得熔体发生混合作用，更易形成铬铁矿.Su et al.(2016)使用非传统同位素Li对罗布莎铬铁矿进行的示踪研究也支持该观点.文章系统测量方辉橄榄岩-纯橄岩-条带状铬铁矿中橄榄石的Li同位素含量，其结果显示纯橄岩中橄榄石的δ7Li值较高，与现代岛弧岩浆岩相似；而条带状铬铁矿中橄榄石的δ7Li值较低，与高度脱水的板片相似，认为可能有两种熔体与大洋岩石圈地幔发生反应形成纯橄岩和铬铁矿.
3. pMELTS模拟地幔过程的应用现状
MELTS是一款热力学模拟岩浆系统相平衡软件，主要由Mark Ghiorso开发(Ghiorso and Sack, 1995)，可用于计算矿物和硅酸岩熔体在500~2 000 ℃和0~2 GPa范围内的平衡相.pMELTS软件与MELTS软件类似，作为MELTS的升级版，更适用于接近固相(0~30%熔融程度)的地幔成分体系，设计温度和压力范围分别为1 000~2 500 ℃，1~3 GPa(Ghiorso et al., 2002).MELTS/pMELTS软件综合了常用的一些研究地幔部分熔融过程的工具，如分析地球化学、实验岩石学、物理化学理论(Ghiorso et al., 2002)，已经成功用于模拟大洋中脊之下的部分熔融和玄武岩的形成过程(Asimow, 2001).Ghiorso et al.(2002)使用pMELTS对MM3成分的地幔橄榄岩在1.0 GPa下部分熔融过程进行模拟，得到体系各个相的含量与实验岩石学得到的结果基本吻合，证明该软件在模拟地幔橄榄岩特别是尖晶石相地幔橄榄岩部分熔融过程的结果比较可靠.详细的MELTS软件计算算法、组成、热化学模型等可参考Ghiorso and Sack(1995)和Hirschmann et al.(1998).
由于MELTS/pMELTS软件包含了部分熔融、熔体结晶分异、同化混染等岩浆过程的模型，使用大量熔体-固体平衡实验的数据库进行校正，其模拟结果与岩石学实验结果可比较(Ghiorso et al., 2002)，近年来，其常被用于解决实际地质问题(Mazzucchelli et al., 2009; Kimura and Sano, 2012; Eason and Dunn, 2015).如Mazzucchelli et al.(2009)使用pMELTS热力学模型计算模拟地幔岩浆通道中岩浆与地幔橄榄岩反应，认为Balmuccia地幔岩体中高度亏损的纯橄岩形成于辉石岩熔体与方辉橄榄岩在尖晶石稳定域(1.5~0.7 GPa)的反应中.
4. 讨论
4.1 铬元素的来源和地球化学性质
铬(Cr)元素是铬铁矿中主要的有用元素，其以少量元素的形式存在于地幔中，并以微量元素的形式存在于大陆地壳中.Cr还可以替代一些硅等主要元素(如：Al、Fe3+)存在于硅酸盐矿物中，如角闪石(韭闪石，Cr2O3含量高达3.5%)、单斜辉石(铬透辉石，Cr2O3含量高达8%)、榴辉岩中的石榴子石和绿辉石(Cr2O3含量分别高至4.4%和5.9%)(Klein-BenDavid et al., 2011).而地幔中常见的含Cr矿物主要为尖晶石、单斜辉石和斜方辉石.
Cr元素的地球化学性质与其价态有着非常密切的联系，而Cr元素的价态又与氧化还原环境相关.Cr元素具有3个价态Cr2+、Cr3+和Cr6+.Cr2+一般存在于非常还原的环境(氧逸度低于IW缓冲剂，iron-wustite buffer)，但也有研究表明，在缺少Fe的体系中Cr2+也可以存在；Cr3+稳定存在于较还原的环境(氧逸度高于IW缓冲剂)；而Cr6+则存在于极度氧化的环境(氧逸度高于FMQ+4)(Papike et al., 2005; Mallmann and O'Neill, 2009; Klein-BenDavid et al., 2011).一般认为，地球岩石圈地幔相对还原，氧逸度在IW和FMQ(fayalite-magnetite-quartz)缓冲剂之间，并且大陆克拉通岩石圈地幔的石榴子石橄榄岩的氧逸度测量结果显示，氧逸度随着深度的增加而降低(Frost and McCammon, 2008).在地球浅部地幔中，Cr元素一般以Cr3+的价态存在，而Cr6+则可能仅稳定存在于一些流体相中，出现在地表环境中(Klein-BenDavid et al., 2011).Cr3+在地幔橄榄岩中表现为相容元素，在岩石圈地幔发生熔融过程中，Cr3+一般趋向于留在残余地幔矿物中，因此，随着熔融程度的增加，其在全岩中的含量一般也会不断增加(图 7).但是相对于Cr3+，Cr2+则相容性较低，当地幔岩石在还原环境时，Cr元素在矿物中的存在价态多为Cr2+，发生熔融时，Cr2+会趋向于进入熔体相，月球的氧逸度远远低于地球，氧逸度要低于IW缓冲剂，其形成的玄武岩一般Cr元素的含量要高于地球(Li et al., 1995; Papike et al., 2005).Cr6+相对于Cr3+则具有高度的迁移性，极易随热液流体运移(Li et al., 1995; Klein-BenDavid et al., 2011).
图 7 亏损地幔Cr2O3含量随部分熔融程度的变化使用pMELTS模拟计算亏损地幔(depleted MORB mantle, DMM, Workman and Hart, 2005)在尖晶石相(1.0 GPa)与石榴子石相(2.5 GPa)下分馏部分熔融成分变化Fig. 7. Cr2O3 contents vary with partial melting degrees of depleted mantle但也有研究表明，当熔体富含流体时，熔体会具有更多的八面体配位，因为水/流体的存在会降低熔体中硅氧骨架网络(silica network)的聚合程度，Cr3+具有高的八面体择位能，会提高Cr元素在熔体中的含量(Edwards et al., 2000).Matveev and Ballhaus(2002)对玄武岩-水不混溶体系(immiscible basalt-water system)在浅部地幔温度压力条件下的实验发现，铬铁矿趋向于进入流体相，而橄榄石(可能还包括其他硅酸盐矿物)则趋向于留在硅酸盐熔体之中，使得铬铁矿与橄榄石也显示类似不混溶的结构，如豆荚状的铬铁矿.
4.2 地幔部分熔融
前文讨论，岩石圈地幔发生部分熔融，由于Cr元素在地幔岩石中的相容性，其趋向于保存在残留相中，主要赋存在尖晶石相矿物中，所以一般随着地幔橄榄岩的熔融程度增加，尖晶石的Cr2O3含量会不断增加，而同为三价的Al元素的含量则会降低，所以尖晶石的Cr#是判断地幔橄榄岩亏损程度的极好指示(Dick and Bullen, 1984).随着地幔橄榄岩熔融程度的增加，尖晶石的Cr#不断增加，早在20世纪80年代，就有研究者提出豆荚状铬铁矿的熔融残余成因论点，认为铬铁矿和纯橄岩是地幔橄榄岩高度部分熔融的产物，并借助地幔物质的剪切流动聚集成矿(王希斌和鲍佩声, 1987).
通过pMELTS软件模拟亏损地幔在1.0 GPa和2.5 GPa下的熔融结果可见，随着熔融程度的增加，残余地幔的Cr2O3含量不断增加(图 7)，其中尖晶石的Cr#也不断增加(图 8).然而，从熔融程度与尖晶石Cr#的关系可见，即使亏损地幔遭受高达40%的熔融，残留尖晶石的Cr#仅勉强达到罗布莎纯橄岩中尖晶石的Cr#(~60)，要达到罗布莎铬铁矿的Cr#还需更高的熔融程度，显然这不太符合真实的地质情况.同时，熔融残留矿物相与熔融程度的关系也可见，随着地幔亏损程度的增加，尖晶石相矿物的含量变化不大(图 9)，这是因为随着熔融程度的增加，大量的Al、Fe等尖晶石族矿物组成元素会进入熔体，使残余地幔仍很难形成大量尖晶石相矿物.若由分散在地幔中的尖晶石富集形成，矿体的规模巨大，如Kempirsai中央成矿带的Voskhod矿体(600 m×170 m×360 m)，具有18 Mt的铬铁矿储量(Johnson, 2012)，这需要一个非常有效的富集机制才能解释.
图 8 亏损地幔中尖晶石Cr#随部分熔融程度的变化Cr#=100*Cr3+/(Cr3++Al3+).模拟条件同图 7Fig. 8. Cr# of spinel vary with partial melting degree of depleted mantle
图 9 DMM等压分馏部分熔融残余矿物质量分数与熔融程度关系a.模拟压力为2.5 GPa；b.模拟压力为1.0 GPaFig. 9. Calculation results for modal composition change with melting degree4.3 岩浆结晶分异作用
部分学者认为，豆荚状铬铁矿的成矿元素来源可能与层状铬铁矿相似，来自于岩浆(Thayer, 1964).4.1节讨论了Cr元素的地球化学性质，Cr元素在地幔熔融过程中一般显示为相容元素，其在熔体中的含量与Cr元素的配分系数、源区Cr元素含量以及源区矿物组成有关.如：原始的MORB岩浆的Cr含量在200×10-6~700×10-6(Roeder and Reynolds, 1991; Roeder et al., 2006)，而玻安质岩浆则含有较高的Cr含量，为1 000×10-6~1 600×10-6之间(Walker and Cameron, 1983; Taylor et al., 1994)，苦橄质岩浆的Cr含量则更高，为1 800×10-6~3 000×10-6(Liang and Elthon, 1990).
此处做一个简单的计算，假设岩浆中的Cr元素全部结晶形成尖晶石，要形成罗布莎规模的铬铁矿(储量约为800万t，Cr2O3含量约为50%)，即相当于274万t的Cr，将需要体积为铬铁矿体积的500~1 700倍的MORB岩浆，积将为铬铁矿体积的200~300倍的玻安质岩浆体，体积为铬铁矿体积的100~200倍的苦橄质岩浆.
若铬铁矿中Cr元素均来自于岩浆，还需仔细斟酌为何豆荚状铬铁矿多赋存于地幔橄榄岩中，而非Moho面之上的岩浆房中.
4.4 熔体-岩石反应
部分研究者认为熔体-岩石反应可以形成大规模铬铁矿，即铬铁矿的形成与熔体在上升运移过程中与周围的地幔橄榄岩发生反应时地幔橄榄岩的不一致熔融有关，形成的铬铁矿的成分与反应的熔体成分及围岩的亏损程度有着重要的关系(Arai and Yurimoto, 1994; Zhou and Robinson, 1994; Zhou et al., 1996; Arai and Miura, 2015).熔体-岩石反应能很好地解释为什么豆荚状铬铁矿经常存在一层纯橄岩薄壳，但是模型还停留于定性的描述，其许多条件还有待讨论.
为了了解熔体-岩石反应的产物与熔体和围岩成分的关系，本文尝试使用pMELTS软件进行模拟，使用不同亏损程度的DMM以及罗布莎地幔橄榄岩与不同成分的熔体以不同的岩石熔体反应比例在尖晶石稳定域(1.0 GPa)下进行反应，初步模拟结果见图 10.
图 10 模拟熔体-岩石反应产物a.熔体-岩石反应比例为1:2；b.熔体-岩石反应的比例为1:1；c.熔体-岩石反应的比例为2:1.其中行表示不同亏损程度的地幔橄榄岩：R1为DMM；R2为5%亏损的DMM；R3为10%亏损的DMM；R4为15%亏损的DMM；列表示不同成分的熔体，均为DMM在2.5 GPa下发生不同程度部分熔融的熔体：M1为3%；M2为7%；M3为11%；M4为15%；格子颜色表示反应产物的岩性；格子中的数字表示形成的岩石中尖晶石矿物的实际矿物含量Fig. 10. Calculated modal compositions for residual mantle and melt reaction由模拟结果可发现，熔体-岩石反应的产物与反应熔体、反应岩石以及两者反应比例有着重要的关系.一般而言，SiO2含量越高的熔体与亏损程度低的地幔橄榄岩反应，会趋向于形成单斜辉石含量较高的岩石.而熔体与岩石的反应比例则非常强烈地影响着反应产物的岩性，当熔体的量少于反应岩石的量时，地幔橄榄岩中的斜方辉石不能完全发生不一致熔融，使得产物一般还保留有斜方辉石；当熔体与地幔橄榄岩为等比例反应或熔体的量要高于参加反应的地幔橄榄岩时，地幔橄榄岩中的斜方辉石能刚好消耗完毕，形成纯橄岩；当熔体SiO2含量高时，将会生成单斜辉石，形成含单斜辉石的纯橄岩或是异剥橄榄岩(图 10).
而对于大家所关注的尖晶石，从模拟结果可以看到，产物中尖晶石的含量变化不大，并且与原始地幔橄榄岩的含量相似，没有发生尖晶石的富集，而且尖晶石的Cr#也较罗布莎铬铁矿和纯橄岩薄壳中的低(图 11).鉴于pMELTS软件受温压条件的限制，本次模拟并没有加入富含Cr元素的熔体端员(如玻安质熔体或是苦橄质熔体).那么，是否这种熔体与岩石的反应与熔体成分有关？近年来的研究都表明，玻安质熔体一般形成于浅部地幔(＜45 km，压力小于1.5 GPa)，由于流体的加入，亏损的地幔进一步熔融形成(Falloon and Danyushevsky, 2000; Umino et al., 2015; 彭松柏等, 2016a, 2016b).然而，这种浅部地幔形成的熔体一般与斜方辉石平衡，具有高的SiO2活度，难以与斜方辉石发生反应，形成纯橄岩，而能与斜方辉石反应的熔体应该是具有低的SiO2活度，形成于高压富集地幔(Shi et al., 2007).
图 11 熔体-岩石反应模型产物尖晶石Mg#-Cr#图解Fig. 11. Mg# vs. Cr# diagram of melt-rock reaction calculation results本文熔体-岩石反应的模拟没能得到大量的尖晶石，但是得到的结果却与深海橄榄岩中尖晶石成分的趋势极其相似(Payot et al., 2014).且模拟得到的产物(如纯橄岩、含单斜辉石纯橄岩和异剥橄榄岩)与Kelemen et al.(1990)计算的结果吻合，也符合在Oman等许多蛇绿岩中存在许多纯橄岩脉的地质观察(Kelemen et al., 1995; Sano and Kimura, 2006)，反映在洋中脊下的地幔可能经历过广泛的熔体-岩石反应作用，可形成大量的纯橄岩岩浆通道(Kelemen et al., 1995)，表明在洋中脊环境下形成的MORB熔体与地幔橄榄岩的反应难以形成大规模的铬铁矿床，熔体-岩石反应形成豆荚状铬铁矿的模型还有待进一步讨论或完善.
pMELTS模拟结果还显示，熔体-岩石反应在形成尖晶石的同时，也会形成大量的橄榄石，按此计算，规模越大的铬铁矿矿体，其纯橄岩壳应该越厚(Matsumoto and Arai, 2001).然而许多大的铬铁矿矿体通常仅被很薄的纯橄岩包围或直接与围岩接触(González-Jiméneznez et al., 2014)，罗布莎铬铁矿亦是如此.熊发挥等(2014)曾对罗布莎近436个钻孔岩心中的铬铁矿及围岩特征进行统计，发现罗布莎铬铁矿矿体(特别是致密块状的矿体)的围岩大多为方辉橄榄岩，其次为纯橄岩薄壳和破碎带等.这些观察显然与熔体-岩石模型(Zhou et al., 1996; Arai, 1997)不符.
因此，笔者认为蛇绿岩中的大量铬铁矿是由熔体和岩石反应形成这一结论值得商榷，豆荚状铬铁矿成因探讨应该另辟新的途径.
5. 结语
本文回顾了豆荚状铬铁矿的可能形成过程，特别是在其中发现大量地幔异常矿物群，极大地颠覆了人们以往认为豆荚状铬铁矿形成于浅部地幔的认识，然而要解决豆荚状铬铁矿成因这一问题还需要回答许多问题.本文借助了pMELTS热力学软件，模拟了浅部地幔一些作用过程对豆荚状铬铁矿形成的贡献，借助前人研究结果，尝试从一个定量化的角度重新看待豆荚状铬铁矿的形成.初步模拟结果显示，单独的地幔部分熔融过程、地幔熔体结晶分离作用以及拉斑质熔体与亏损地幔反应过程都不能形成大规模以及品位高的豆荚状铬铁矿床，或反映豆荚状铬铁矿的形成与多阶段的地幔作用有关，又或与深部作用有关.目前仍缺乏一个有效的机制解决豆荚状铬铁矿的富集问题，或许这些地幔异常矿物群的发现可以为豆荚状铬铁矿的成因指示新的思考方向.由于这些矿物群具有不同的稳定存在的物理化学条件，对这些矿物及其与铬铁矿之间的关系进行详细的研究可以限定深部地幔作用(Yang et al., 2014, 2015; Griffin et al., 2016; Zhang et al., 2016)以及复杂系统(罗照华等, 2014)对铬铁矿形成的贡献.豆荚状铬铁矿的形成还有待进一步研究.热力学模拟计算可能可以为辅助研究豆荚状铬铁矿的成因提供帮助.
致谢: 论文撰写过程中与中国地质科学院熊发挥、田亚洲博士等进行了有益的讨论.论文得到了郝梓国研究员的指教.罗照华教授、连东洋助理研究员及匿名审稿人审阅了本文，为本文提出了宝贵的修改建议.在此，一并致以诚挚的谢意！ -
图 1 1952—2013年中国铬铁矿进口量变化情况(百万t)
1952—1995年数据来源于中国矿业网；1996—2013年数据来源于USGS Minerals Yearbook CHROMIUM各年册
Fig. 1. Variety of chromitite imports of China from 1952 to 2013
图 2 地球显生宙造山带及蛇绿岩分布简图
据Dilek and Furnes(2011)；其中正方形显示国内外一些豆荚状铬铁矿床
Fig. 2. Global distribution of major Phanerozoic orogenic belts and ophiolite
图 3 世界显生宙典型蛇绿岩格铁矿分布柱状示意图
图上厚度与实际不成比例，据Thayer(1964)、Boudier and Nicolas(1995)、Melcher et al.(1997)、Proenza et al.(1999)、Robertson(2002)、yumul(2004)、Rollinson(2008)、Hébert et al.(2012)、Johnson(2012)和Milushi(2015)
Fig. 3. Idealized lithologic columns for typical Phanerozoic ophiolites in the world
图 5 豆荚状铬铁矿熔体混合模型
据Arai and Miura(2015)；Q.石英；Ol.橄榄石；Chr.铬铁矿
Fig. 5. Petrologic model for the formation of podiform chromitite via melt mixing
图 6 橄榄石(Ol)-石英(Q)-铬铁矿(Chr)系统
Fig. 6. Phase relations in the system olivine (Ol)-quartz (Q)-chromite (Chr)
图 7 亏损地幔Cr2O3含量随部分熔融程度的变化
使用pMELTS模拟计算亏损地幔(depleted MORB mantle, DMM, Workman and Hart, 2005)在尖晶石相(1.0 GPa)与石榴子石相(2.5 GPa)下分馏部分熔融成分变化
Fig. 7. Cr2O3 contents vary with partial melting degrees of depleted mantle
图 8 亏损地幔中尖晶石Cr#随部分熔融程度的变化
Cr#=100*Cr3+/(Cr3++Al3+).模拟条件同图 7
Fig. 8. Cr# of spinel vary with partial melting degree of depleted mantle
图 9 DMM等压分馏部分熔融残余矿物质量分数与熔融程度关系
a.模拟压力为2.5 GPa；b.模拟压力为1.0 GPa
Fig. 9. Calculation results for modal composition change with melting degree
图 10 模拟熔体-岩石反应产物
a.熔体-岩石反应比例为1:2；b.熔体-岩石反应的比例为1:1；c.熔体-岩石反应的比例为2:1.其中行表示不同亏损程度的地幔橄榄岩：R1为DMM；R2为5%亏损的DMM；R3为10%亏损的DMM；R4为15%亏损的DMM；列表示不同成分的熔体，均为DMM在2.5 GPa下发生不同程度部分熔融的熔体：M1为3%；M2为7%；M3为11%；M4为15%；格子颜色表示反应产物的岩性；格子中的数字表示形成的岩石中尖晶石矿物的实际矿物含量
Fig. 10. Calculated modal compositions for residual mantle and melt reaction
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