地球科学  2018, Vol. 43 Issue (4): 1011-1024.   PDF    
0
Fe-Mg同位素在蛇绿岩中铬铁矿床成因研究中的应用潜力
苏本勋1,2,3, 肖燕2,4, 陈晨1,2,3, 白洋1,2,3, 刘霞1,5, 梁子2,3,4, 彭青山6     
1. 中国科学院地质与地球物理研究所, 中国科学院矿产资源研究重点实验室, 北京 100029;
2. 中国科学院地球科学研究院, 北京 100029;
3. 中国科学院大学, 北京 100049;
4. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029;
5. 成都理工大学地球科学学院, 四川成都 610059;
6. 长春工程学院勘查与测绘工程学院, 吉林长春 130012
摘要:蛇绿岩中铬铁矿床成因一直存在较大争议,其主要原因可归结为:寄主蛇绿岩存在成因争议、产出状态不清、矿石及围岩矿物组合单一以及主要矿物成分简单但矿物包裹体复杂多样.针对这些研究瓶颈,率先对西藏普兰和罗布莎、土耳其Kızıldaǧ和Kop蛇绿岩中的地幔橄榄岩和铬铁岩进行了全岩和单矿物Fe-Mg同位素的探索性研究工作.结果表明:(1)蛇绿岩中的地幔橄榄岩具有较均一的Fe-Mg同位素组成,与世界上其他地区的地幔橄榄岩相似;(2)铬铁岩中铬铁矿和橄榄石之间存在明显的Fe-Mg同位素分馏,铬铁矿多具有比共存橄榄石轻的Fe同位素组成,与地幔橄榄岩中的尖晶石和橄榄石相反,Mg同位素变化较大;(3)铬铁矿和橄榄石的Fe-Mg同位素主要受控于结晶分异和Fe-Mg交换,且这两个过程造成的同位素变化趋势明显不同.因此,Fe-Mg同位素在揭示铬铁矿母岩浆来源、性质及成矿过程方面具有较大的应用潜力.
关键词蛇绿岩    铬铁矿    Fe同位素    Mg同位素    岩石学    矿物学    
Potential Applications of Fe and Mg Isotopes in Genesis of Chromite Deposits in Ophiolites
Su Benxun1,2,3 , Xiao Yan2,4 , Chen Chen1,2,3 , Bai Yang1,2,3 , Liu Xia1,5 , Liang Zi2,3,4 , Peng Qingshan6     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
5. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
6. Faculty of Prospecting and Survey Engineering, Changchun Institute of Technology, Changchun 130012, China
Abstract: Genesis of chromittie deposit in ophiolites has been debated for a long time. It is commonly related to host ophiolite, chromite occurrence, simple mineral assemblage and simple mineral chemistry, and unusual and complicated mineral inclusions. To constrain the genesis of chromite, a study on Fe and Mg isotopes of mantle peridotite and chromitite from Purang and Luobusa ophiolites in Tibet and Kızıldaǧ and Kop ophiolites in Turkey is conducted. The results reveal that (1) mantle peridotite of ophiolites has uniform Fe and Mg isotope compositions, falling in the same range of worldwide peridotite; (2) chromite and olivine in chromitite show large Fe and Mg isotope fractionations with overall lower δ56Fe and higher δ26Mg in chromite than olivine; (3) Fe and Mg isotope fractionations between chromite and olivine are mainly controlled by fractional crystallization and subsolidus Fe and Mg exchanges in two opposite processes. Therefore, Fe and Mg isotopes have great potentials to trace origin and nature of parental magmas of chromitite and crystallization and precipitation of chromite.
Key Words: ophiolite    chromite    Fe isotope    Mg isotope    petrology    mineralogy    

0 引言

蛇绿岩是大洋岩石圈的残片,是除大型层状岩体之外铬铁矿床的主要载体,且其产出的铬铁矿相对层状岩体更为富Cr.按照产出层位,蛇绿岩中的铬铁矿床分为两类:地幔橄榄岩中的豆荚状铬铁矿床和地壳超镁铁堆晶岩中的(似)层状铬铁矿床(图 1; Paktunc, 1990).豆荚状铬铁矿床主要产于显生宙以来各造山带的蛇绿岩中,少见于前寒武纪稳定地区,被认为是蛇绿岩的矿产特征之一;集中分布在莫霍面以下1 000~1 500 m方辉橄榄岩分布的位置,该带以下的方辉橄榄岩和二辉橄榄岩中几乎不含豆荚状铬铁矿床.豆荚状铬铁矿体常被纯橄岩薄壳包裹,但矿体大小和纯橄岩层的厚度并不具相关性.其特征与层状铬铁矿床迥然不同,矿体侧向延伸有限,形态也非常不规则,可呈透镜状、板状和宽窄不等的脉状,但以断续出露的豆荚状为最显著的特征,故以此得名(图 1; 杨经绥等, 2010).(似)层状铬铁矿床主要产于超镁铁质堆晶岩中,产出特征和成因均与大型层状岩体中的铬铁矿体相似,主要受控于岩浆结晶分异作用.

Download:
图 1 蛇绿岩剖面图中的铬铁矿产出状态 Fig. 1 Occurrence of chromite in ophiolite Paktunc(1990)修改

蛇绿岩中豆荚状铬铁矿床的研究已有140多年的历史,然而对于其成因说法不一,最初有学者认为是热液成因(von Groddeck, 1879; de Launay, 1913; Fisher, 1929),后来又有研究者认为是岩浆结晶分异(Fisher, 1929; Lago et al., 1982; 王恒升等, 1983)和部分熔融(Duke, 1982; Nicolas and Prinzhofer, 1983; 王希斌和鲍佩声, 1987; 鲍佩声, 2009),再后来多数研究者认为是熔-岩相互作用(Arai and Yurimoto, 1994; Zhou et al., 1994, 1996; Proenza et al., 1999).熔-岩相互作用模式强调不同性质的熔体与地幔橄榄岩反应形成不同类型的铬铁矿:俯冲带玻安质熔体与洋中脊玄武质熔体分别对应高Cr和高Al型铬铁矿.近年来,全球多个蛇绿岩中铬铁岩和橄榄岩中超高压矿物和超还原性矿物(八面体假象蛇纹石、金刚石、柯石英、碳硅石)的发现(Yang et al., 2007, 2014; 杨经绥等, 2008; Trumbull et al., 2009; Yamamoto et al., 2009; Robinson et al., 2015; Xu et al., 2015; Xiong et al., 2015),以及古老Re亏损年龄的报道(Shi et al., 2007),均表明形成铬铁岩的熔体中含有再循环地壳甚至古老大陆岩石圈地幔物质组分,进而又有学者提出了豆荚状铬铁矿的深部成因观点(杨经绥等, 2008; Yang et al., 2014; Arai and Miura, 2016).这些结果不仅使铬铁矿床成因研究变得更加复杂,也表明铬铁矿床成因研究对揭示地幔对流、壳幔相互作用及大洋岩石圈形成演化等全球尺度的地质过程具有重要意义.因此,对豆荚状铬铁矿开展新的研究,寻找新的方法和证据乃是当务之急.

1 铬铁矿床研究现状及存在的问题

铬铁岩具有非常简单的矿物组合,主要由铬铁矿和橄榄石两种矿物组成,且这两种矿物的化学组成较为简单,含微量元素种类较少且含量较低,同位素示踪方法也较为局限,这对研究其母岩浆的组成、性质和来源造成了较大的困难.目前,多数学者主要是通过铬铁岩中的铬铁矿与其他岩浆岩中铬铁矿的主量元素成分对比以及对铬铁矿中矿物包裹体进行研究,得出形成铬铁矿的母岩浆可能为富Mg、贫Si、富水氧化的镁铁质熔体.

1.1 铬铁矿成分对比及其局限性

不同构造背景以及不同性质熔体中产出的尖晶石(或铬铁矿)存在明显的成分差别,可以通过其Cr#值、Al2O3和TiO2含量等成分参数进行区分(Kamenetsky et al., 2001; Pagé and Barnes, 2009; Uysal et al., 2009).已有的成分对比结果表明,蛇绿岩中高Cr和高Al型铬铁矿在成分上分别与玻安岩和洋中脊玄武岩中的铬铁矿具有可比性,进而推断高Cr型铬铁矿主要产于俯冲带(SSZ: supra-subduction zone)型的弧前蛇绿岩,与玻安质熔体密切相关(图 2a);高Al型铬铁矿则产于洋中脊(MOR: mid-ocean ridge)型蛇绿岩,与MORB质熔体有关(Arai, 1992; Zhou et al., 1998; Arai and Miura, 2016).

Download:
图 2 罗布莎蛇绿岩中方辉橄榄岩、纯橄岩和铬铁岩与典型玻安岩的铬铁矿(a)和橄榄石(b)成分相关性 Fig. 2 Correlation of compositions of chromite (a) and olivine (b) in harzburgite, dunite and chromitite from the Luobusa ophiolite with comparisons of those in typical boninite 罗布莎数据引自Xiong et al., 2015; Su et al., 2016; Xiao et al., 2016; 玻安岩数据引自GEOROC(http://georoc.mpch-mainz.gwdg.de/georoc/Start.asp)

上述对比忽略了对铬铁矿的共生矿物—橄榄石的成分比较.实际上,不论高Cr还是高Al型铬铁岩,其中的橄榄石成分均与相应玻安岩和MORB中的橄榄石成分存在明显差别.例如:罗布莎高Cr型豆荚状铬铁矿石中橄榄石的Fo值为93~97(Xiong et al., 2015; Su et al., 2016),远高于玻安岩中橄榄石Fo值的变化范围87~90(Li et al., 2013)(图 2b).虽然橄榄石高Fo特征被认为是橄榄石和铬铁矿在亚固相下的元素交换造成的(Leblanc and Nicolas, 1992; Zhou et al., 1996; Melcher et al., 1997),但是最新的岩石学观察和模拟计算表明,浸染状和条带状铬铁矿石中的橄榄石受元素交换的影响较小或不明显,并且在元素交换过程中铬铁矿也会发生明显的不同程度的成分变化(Xiao et al., 2016; Bai et al., 2017).因此,铬铁矿的某些成分不能代表其最初组成,更不能以此来推断母岩浆的成分特征.

1.2 铬铁矿中的包裹体及其争议

许多蛇绿岩中的铬铁矿均含有富碱矿物包裹体(如角闪石、云母)和流体包裹体,这是推断形成铬铁矿的母岩浆具有富碱富水特征的主要依据(Johan et al., 1983; Schiano et al., 1997; Borisova et al., 2012; González-Jiménez et al., 2014; Zhou et al., 2014; Robinson et al., 2015; Arai and Miura, 2016; 刘霞等, 2018).然而,这些包裹体与寄主铬铁矿是否来自同一岩浆值得商榷,因为这些包裹体也可以形成于岩浆期后的热液活动过程或寄主铬铁矿经历的后期变形和重结晶过程(Augé, 1987; Lorand and Ceuleneer, 1989; Melcher et al., 1997; Huang et al., 2014).实验岩石学的结果揭示,在温度1 250±100 ℃和氧逸度f(O2)=±1 log units的条件下,铬铁矿可以从“干”岩浆体系中结晶(Irvine, 1977; Murck and Campbell, 1986; Roeder and Reynolds, 1991),但是富水体系是否更有利于铬铁矿的结晶,到目前为止仍然没有直接的证据.

水在铬铁矿结晶以及堆积过程中扮演的角色尚不清楚.Matveev and Ballhaus(2002)的实验结果表明,在玄武质岩浆和流体的不混溶体系中铬铁矿易于聚集在流体中,进而密度变轻而更易被上升岩浆携带运移.铬铁矿可以在振荡的岩浆中相互聚合,并以块状、豆状和浸染状铬铁岩的形式沉淀下来(Matveev and Ballhaus, 2002).这一模型可以很好地解释具有富水岩浆体系特征的SSZ蛇绿岩中铬铁矿床的形成,但无法解释相对贫水体系下MOR型蛇绿岩中的铬铁矿床成因(Arai and Miura, 2016).

1.3 铬铁岩围岩的成分解释及其质疑

豆荚状铬铁矿床的围岩方辉橄榄岩和薄壳纯橄岩的稀土元素配分呈U型(Zhou et al., 2005),与玻安岩的稀土元素配分模式相似,而不同于地幔橄榄岩通常由于部分熔融而亏损轻稀土的特征.这一特征被认为是俯冲带上的流体或熔体与残留橄榄岩反应的结果,同时这一反应又进一步促进了Cr的迁移并在冷却过程中发生Cr的富集(Dick and Bullen, 1984; Roberts and Neary, 1993; Matveev and Ballhaus, 2002).这一解释也得到了方辉橄榄岩的岩石地球化学(Kelemen et al., 1992)和西藏罗布莎铬铁岩及纯橄岩PGE配分特征的证实(Zhou et al., 1996, 1998, 2005).

然而,同时有学者质疑主量元素,尤其是成矿元素Cr的差异.例如,鲍佩声(2009)指出玻安岩的Cr含量总体较低且变化范围较大(250×10-6~2 350×10-6;通常小于1 000×10-6),与地幔橄榄岩中含Cr矿物的Cr2O3含量相比是微不足道的,如何能形成含Cr2O3达50%的块状铬铁矿呢?如此低Cr含量的熔体如何能够通过与方辉橄榄岩反应而聚集成几十万、几百万吨规模的铬铁矿呢?前人主要是通过全岩尺度上的微量元素对比推断出高Cr铬铁岩的母岩浆为玻安质熔体,但是由于缺少同位素以及俯冲组分的对比研究而无法对熔体的性质做进一步的制约和厘定.

1.4 铬铁岩母岩浆的氧化还原性质研究

豆荚状铬铁矿床中的铬铁矿普遍含有壳源包裹体,如斜长石、角闪石、磷灰石、锆石和金红石等(Robinson et al., 2004; Yang et al., 2007; Uysal et al., 2009; Zhou et al., 2014; 刘霞等, 2018),这被认为是结晶铬铁矿的母岩浆有地壳物质参与并具有高氧化特征的证据.而与此相矛盾的是,铬铁岩中还发现有高度还原性矿物(如碳化硅)和单质矿物包裹体(杨经绥等, 2013; Robinson et al., 2015; Xu et al., 2015).豆荚状铬铁矿石中的铬铁矿常具有比正常地幔橄榄岩高的Fe3+/∑Fe值,亦被认为是铬铁岩母岩浆具氧化特征的有力证据(Melcher et al., 1997; Lenaz et al., 2014).这一推论在两方面值得商榷:(1)测定的铬铁矿Fe3+值很难排除后期蚀变及其包裹体的影响(Rollinson and Adetunji, 2015),尤其值得一提的是已有研究发现铬铁岩中的锆石来自其围岩(Kapsiotis et al., 2016);(2)岩浆的Fe3+/∑Fe值与压力密切相关(Zhang et al., 2017a).因此,铬铁岩母岩浆的氧化还原性质尚需进一步认识和评估.

前人主要通过铬铁矿主量元素和包裹体研究铬铁矿成矿作用,并且主要集中在地幔橄榄岩中的豆荚状铬铁矿床上;对其母岩浆为玻安质岩浆的制约也主要是建立在铬铁矿主量元素成分对比上(Kamenetsky et al., 2001; Uysal et al., 2009; Zhou et al., 2014).地壳超镁铁质堆晶岩也是铬铁矿体的重要赋矿岩层,但对其研究甚少,尤其缺少地壳与地幔单元中成矿作用的对比.如果这些铬铁矿都是在俯冲起始阶段形成并与玻安质岩浆有关,那么各种不同的俯冲组分在此阶段的卷入会使其母岩浆发生怎样的变化?这种变化到底如何区分?非传统稳定同位素(Fe-Mg)在区分上述过程中可能有重要作用,例如Fe-Mg稳定同位素组成的差异可能记录了不同地质过程中的同位素分馏.Mg同位素体系是一个敏感的再循环物质的示踪体系,它可以很好地示踪俯冲带过程所造成的Mg同位素不均一性;Fe同位素的变化则多反映不同价态的Fe在部分熔融和分离结晶过程中的分馏,是研究成矿作用及来源的新工具.西藏罗布莎蛇绿岩中地幔橄榄岩和铬铁岩的Fe-Mg同位素的探索性研究表明,地幔橄榄岩和铬铁岩中不同矿物以及不同样品之间存在着Fe-Mg同位素的较大分馏,并且这些分馏很可能可以示踪铬铁矿的形成过程及其母岩浆的性质和来源(Chen et al., 2015; Su et al., 2015; Xiao et al., 2016; Zhang et al., 2017b).

2 为什么选择Fe和Mg同位素? 2.1 铬铁矿和共生橄榄石的组成特征

尖晶石族的化学式一般写为(Mg, Fe, Zn, Mn)(Al, Cr, Fe)2O4.其晶胞中阴离子呈现理想的六方最紧密堆积排列,阳离子充填入阴离子形成的四面体或八面体空隙.其中四面体空隙多被体积较大的二价阳离子占据而形成四次配位,如Mg2+和Fe2+;而八面体空隙多被体积较小的三价阳离子占据形成六次配位,如Al3+、Fe3+及Cr3+(图 3; Hagen and Mikkelsen, 2005).此外,一些微量元素例如Co2+、Zn2+及Mn2+可取代Mg2+和Fe2+,而V3+、Sc3+、Ga3+及Ti4+则可取代Al3+、Fe3+及Cr3+(Colás et al., 2014).

Download:
图 3 铬铁矿和橄榄石的结构 Fig. 3 Crystal structures of spinel (chromite) and olivine

橄榄石的化学式一般写为(Mg, Fe)2(SiO4),其阳离子Mg2+和Fe2+充填入络阴离子形成的八面体空隙,从而形成六次配位(图 3; Brown and Prewitt, 1973; Smyth and Hazen, 1973; Princivalle, 1990),该位置同样可充填入Ni2+、Mn2+等微量元素.而阳离子(如Li+、Na+)则会与Cr3+等三价离子发生电价平衡而占据两个八面体空隙位置,同时晶格发生畸变而部分形成八次配位(Papike et al., 2005).

蛇绿岩中的铬铁矿和橄榄石通常作为熔融残余或早期结晶相共生,铬铁矿一般为镁铬铁矿[(Mg, Fe)Cr2O4],橄榄石为贵橄榄石,其共同的主量元素为Fe、Mg和O. Fe和Mg在铬铁矿和橄榄石晶格中配位数的不同以及Fe3+在铬铁矿中的出现,直接决定了Fe和Mg元素及同位素在这两种矿物中的地球化学行为和组成差异.

2.2 Fe和Mg元素地球化学

Fe是基性矿物的主要组成元素之一,也是地球中含量最高的过渡族元素.同时,Fe是地球上最主要的金属变价(Fe2+、Fe3+)元素之一,在与熔体/流体有关的许多地质过程中,Fe元素皆可发生活化、迁移和再分配(Dauphas et al., 2017).Mg是地幔中含量仅次于O和Si的主要元素,也是地壳中最重要的元素之一.高温下,Fe和Mg的元素地球化学行为存在明显差异:部分熔融过程中,Mg为难熔元素富集于残留相,Fe为易熔元素在残留相中较Mg亏损;结晶分异过程中,Mg优先进入矿物相,Fe相对倾向于残留在熔体相中(Dauphas et al., 2017; Teng, 2017).

Fe较Mg在铬铁矿中更为相容,而在橄榄石中相反.因而,在部分熔融和结晶分异过程中,铬铁矿的Mg#和橄榄石的Fo值并无谐变趋势;在亚固相交换过程中,Fe由橄榄石扩散至铬铁矿,Mg则从铬铁矿扩散至橄榄石(Zhou et al., 1994; Melcher et al., 1997; Xiao et al., 2016; Bai et al., 2017).另外,Fe有变价,在部分熔融过程中,Fe3+表现为不相容性,优先进入熔体相,从而使得熔体相富集Fe3+,残留相富集Fe2+.由于橄榄石晶格中没有Fe3+的位置,因而在结晶分异过程中,Fe3+/Fe2+的比值即氧逸度的高低,直接影响到橄榄石及共结晶铬铁矿的成分.Mg在铬铁矿和橄榄石中均为+2价,但配位数不同,其含量变化很大程度上与Fe含量相关.

2.3 Fe和Mg同位素地球化学

近年来开展的Fe-Mg同位素研究表明:部分熔融过程中Fe同位素分馏主要发生在熔体相,残留相则几乎不受影响;结晶分异过程中亦存在Fe同位素分馏;Fe同位素分馏与价态相关,Fe3+相对富集重Fe同位素(Polyakov and Mineev, 2000; Weyer and Ionov, 2007; Teng et al., 2008; Dauphas et al., 2009; Polyakov and Mineev, 2000; 黄方, 2011; 何永胜等, 2015; 曹辉辉等, 2016)(图 4);地幔具有均一的Mg同位素组成,高温部分熔融和岩浆分异过程中均不发生Mg同位素分馏(Teng et al., 2010; Huang et al., 2011; 李曙光, 2015; 肖益林等, 2015)(图 4);地幔交代作用造成的Fe和Mg同位素分馏与熔体的组成和性质有关(Zhao et al., 2010, 2015; Xiao et al., 2013; Hu et al., 2016);固相与固相及固相与熔体之间化学扩散过程中轻Fe和轻Mg同位素显示相反的扩散方向,表现为负相关的δ56Fe-δ26Mg变化(Teng et al., 2011; Sio et al., 2013; Xiao et al., 2016).

Download:
图 4 洋中脊玄武岩、洋岛玄武岩、玻安岩、岛弧玄武岩、深海橄榄岩、地幔橄榄岩捕掳体和蛇绿岩的Fe-Mg同位素组成 Fig. 4 Fe and Mg isotope frequency distributions of boninites, island arc basalts, mid-ocean ridge basalts, oceanic island basalts, peridotite xenoliths, abyssal peridotites, and ophiolites 数据据Su et al.(2015)

Fe的变价性质是Fe同位素发生分馏的重要因素,硅酸盐矿物和岩浆中Fe3+富集重同位素(Polykov and Mineev, 2000).同位素分馏大小主要受体系氧逸度、Fe3+和Fe2+在熔体/矿物间的分配及部分熔融程度控制(Dauphas et al., 2009).一般情况下,体系中的Fe3+/∑Fe越高,熔融比例越低,产生的熔体δ56Fe相对于源区岩石越重(Dauphas et al., 2009).熔体中Fe3+/Fe2+的改变会使熔体的结构发生改变,从而影响到熔体的性质以及熔体与幔源岩石之间微量元素的分配系数(Dingwell and Virgo, 1988),同时,Fe3+/Fe2+比值也是反映氧逸度的重要参数.这些性质又会受到地幔交代程度、氧化还原状态、岩浆上升过程、结晶分异的矿物成分等影响,Fe同位素就会在熔体/流体与残留相间发生分馏.因而,Fe同位素已被广泛用来示踪各种地质过程.

深海橄榄岩Mg同位素的研究表明大洋岩石圈地幔具有与大陆岩石圈地幔相似的Mg同位素组成,蛇纹石化过程不会造成Mg同位素的分馏,但低温风化过程会造成Mg同位素变重(Su et al., 2015; Liu et al., 2017).另外,上地壳和水圈的Mg同位素组成很不均一,其中沉积碳酸盐岩具有最轻的Mg同位素组成(Young and Galy, 2004; Saenger and Wang, 2014).由于高温部分熔融过程中Mg同位素不发生显著分馏,因此在俯冲过程中由不同岩石部分熔融产生的熔体将具有不同的Mg同位素组成(李曙光, 2015; 肖益林等, 2015),其中俯冲硅酸盐沉积物熔融产生的熔体具有较重的Mg同位素组成;而俯冲碳酸盐岩熔融产生的熔体具有非常轻的Mg同位素组成.由于蛇绿岩中橄榄岩和铬铁岩的Mg同位素组成不会受到蛇纹石化的影响,Mg同位素可以反映其源区的特征,如果受到俯冲熔体的交代,Mg同位素组成可能会被改造,根据Mg同位素特征可以进一步探讨蛇绿岩的形成背景和经历的地质过程.

从理论上来说,Fe和Mg离子在铬铁矿和橄榄石中配位数的不同以及Fe3+在铬铁矿中的出现(图 3),导致平衡状态下铬铁矿相比橄榄石应显著地富集重Fe和重Mg同位素(Liu et al., 2011; Schauble, 2011).从上述的研究结果来看,Fe和Mg同位素相结合将可示踪/制约蛇绿岩中铬铁矿母岩浆的源区性质、铬铁矿的结晶分异过程及其形成后的成分改造.

3 蛇绿岩的Fe-Mg同位素组成

为了更好地了解Fe-Mg同位素在铬铁矿成矿作用研究中的应用潜力,笔者首次对蛇绿岩进行了全岩尺度的Fe-Mg同位素分析(Su et al., 2015).已有的岩石学、矿物学和地球化学工作揭示普兰蛇绿岩形成于大洋洋中脊,而后经历了俯冲阶段熔体改造,并且目前在该蛇绿岩中还没有发现有可开采规模的铬铁矿床(Miller, 2003; Liu et al., 2014, 2015; Li et al., 2015).笔者对西藏普兰蛇绿岩中二辉橄榄岩、方辉橄榄岩、纯橄岩和辉长岩的研究结果显示:(1)这些岩石的Fe-Mg同位素组成与烧失量没有相关性,表明蛇纹石化不会造成Fe-Mg同位素显著分馏,因而全岩Fe-Mg同位素可以用来探讨其形成和经历的地质过程;(2)Fe同位素比值与部分熔融的地球化学参数(如单斜辉石的Cr#、全岩的Sc含量等)显示很好的相关性,指示在大洋中脊环境下的部分熔融可以造成明显的Fe同位素分馏,即熔体相富集重Fe同位素,残留相Fe同位素变轻;(3)尽管Mg同位素与上述地球化学参数没有很好的相关性,但Mg和Fe同位素的耦合关系暗示二者在这些地质过程中具有密切相关的地球化学行为,即部分熔融和地幔交代过程均可以造成Fe和Mg同位素的分馏(图 5);(4)随氧逸度和交代程度的增加,Fe同位素显示系统的变化,辉长岩Fe同位素与MORB一致,残留相橄榄岩具弧岩浆交代趋势,揭示弧岩浆在对地幔橄榄岩的改造过程中可以改变其Fe同位素组成,从而解释高氧逸度的岛弧火山岩具有比洋中脊和洋岛玄武岩轻的Fe同位素特征以及Fe和Mg同位素的耦合关系(图 4);(5)纯橄岩中异常的Fe和Mg同位素组成可能与Fe-Mg交换有关,为制约铬铁矿矿床成因提供了潜在可能.因此,Fe和Mg同位素可以用来制约蛇绿岩的形成演化过程以及纯橄岩和铬铁矿床的成因.

Download:
图 5 普兰蛇绿岩岩石的Fe-Mg同位素组成的相关性 Fig. 5 Correlation of δ56Fe and δ26Mg of the rocks from the Purang ophiolite Su et al.(2015)
4 蛇绿岩中铬铁矿及共生矿物的Fe-Mg同位素组成

基于对普兰蛇绿岩研究,笔者率先对西藏罗布莎、土耳其Kızıldaǧ和Kop蛇绿岩中的铬铁岩及围岩进行了单矿物Fe和Mg同位素分析的探索性工作,其中罗布莎铬铁矿样品为地幔橄榄岩中的豆荚状铬铁矿,后两者为地壳超镁铁堆晶岩中的(似)层状铬铁矿.

4.1 地幔橄榄岩中矿物的Fe-Mg同位素组成

Kızıldaǧ和罗布莎蛇绿岩的方辉橄榄岩和纯橄岩中橄榄石和斜方辉石的Fe和Mg同位素均在平均地幔值范围内,而铬铁矿显示出较大的δ56Fe和δ26Mg变化范围(图 6; Chen et al., 2015; Xiao et al., 2016).同一样品的矿物作比较,Mg同位素在这两个蛇绿岩的样品中显示正常的序列即δ26MgChr>δ26MgOpx>δ26MgOl,与理论上的预测和地幔橄榄岩包体的研究结果(Schauble, 2011; Xiao et al., 2013; Hu et al., 2016)相一致;矿物之间的Fe同位素则表现出较大的差异,罗布莎样品从正常序列(δ56FeChr>δ56FeOpx>δ56FeOl)到逆序(δ56FeOl>δ26MgOpx>δ56FeChr)均有,Kızıldaǧ样品则完全为逆序.这一Fe同位素的异常特征是否与矿物之间的平衡状态有关,抑或与铬铁矿的成分差异有关还不得而知,但至少表明Fe同位素组成在不同蛇绿岩的地幔橄榄岩之间是不同的,有可能反映了蛇绿岩形成的温度压力、氧逸度条件或构造背景.

Download:
图 6 Kızıldaǧ和罗布莎蛇绿岩中方辉橄榄岩、纯橄岩和铬铁岩单矿物的Fe和Mg同位素组成 Fig. 6 Fe and Mg isotopic compositions of olivine, orthopyroxene and chromite in harzburgite, dunite and chromitite from Kızıldaǧ and Lubusa ophiolites 数据引自Chen et al.(2015),部分未发表;Xiao et al.(2016)
4.2 豆荚状铬铁矿床中矿物的Fe-Mg同位素组成

相对于橄榄岩,罗布莎豆荚状铬铁岩中铬铁矿和橄榄石的Fe和Mg同位素变化更大(δ56FeChr=-0.247‰~+0.043‰; δ56FeOl=-0.146‰~+0.215‰; δ26MgChr=-0.41‰~+0.14‰; δ26MgOl=-0.03‰~-0.29‰)(图 6).铬铁矿均具有比共存橄榄石还轻的Fe和Mg同位素组成,铬铁矿与橄榄石之间的Fe和Mg同位素分馏也没有落在理论计算的平衡线上,暗示着不平衡分馏(Xiao et al., 2016).另外,铬铁矿的δ56Fe和δ26Mg为负相关关系,而且铬铁矿的δ56Fe随着MgO和Fe3+/∑Fe的降低而降低.这些特征是部分熔融、结晶分异或弧岩浆交代作用所不能解释的,并且它们的Fe和Mg同位素组成应该不能代表其形成时的成分,而是两种矿物之间发生了Fe-Mg交换的结果.因而,笔者认为豆荚状铬铁矿中铬铁矿和橄榄石之间的不平衡分馏可能是铬铁矿结晶过程中与橄榄石发生Mg-Fe交换所造成的动力学分馏.

在铬铁矿和橄榄石之间发生Fe-Mg交换的过程中,轻Fe同位素(Fe2+)从橄榄石进入铬铁矿,轻Mg同位素从铬铁矿交换至橄榄石,铬铁矿的Fe同位素变轻、Mg#降低、Cr#不变、Fe3+/∑Fe值降低、Mg同位素变重、Fe-Mg同位素显示负相关关系;橄榄石Mg#升高(铬铁矿中的橄榄石包裹体会具有更高的Mg#)、Fe同位素变重、Mg同位素不变或稍变轻.模拟结果显示(图 7),Fe-Mg交换的程度与铬铁矿在矿石中的比例有关,这与笔者的观测结果相一致,即:不同类型铬铁矿石中铬铁矿的δ56Fe值的变化顺序为块状>豆状>条带>浸染,而δ26Mg相反.块状铬铁矿由于其中的橄榄石非常少,Mg-Fe交换对其影响很小从而保留了原有的Fe同位素特征(+0.043‰).铬铁矿在结晶过程中其Fe同位素重于熔体,说明熔体应该具有较轻的同位素组成,从而排除了具有较重Fe同位素组成的大洋玄武岩和洋岛玄武岩,可能来源于部分熔融程度很高的玻安岩(δ56Fe=0.028‰±0.008‰)(Xiao et al., 2016).

Download:
图 7 铬铁矿和橄榄石之间的Fe-Mg交换方式及模拟计算结果 Fig. 7 Modelling results of the effect by Fe-Mg exchange between chromite and olivine 修改自Xiao et al.(2016)
4.3 (似)层状铬铁矿矿物的Fe-Mg同位素组成

Kızıldaǧ蛇绿岩中(似)层状铬铁矿为岩浆结晶分异的产物,其铬铁矿的Fe同位素组成均比橄榄石轻,与罗布莎相似;而矿石类型之间铬铁矿δ56Fe的顺序则与罗布莎相反(图 6; Chen et al., 2015).铬铁矿δ56Fe与MgO的相关性图解(图 8)进一步表明结晶分异成因的铬铁矿与经历过Fe-Mg交换的铬铁矿的Fe同位素变化趋势明显不同,而Kop蛇绿岩中(似)层状铬铁矿可能是结晶分异和较低程度Fe-Mg交换的混合记录(Zhang et al., 2017b).

Download:
图 8 Kızıldaǧ、罗布莎和Kop蛇绿岩中铬铁岩的铬铁矿δ56Fe与MgO相关性 Fig. 8 Correlation of δ56Fe with MgO of chromite in chromitite from Kızıldaǧ, Luobusa and Kop ophiolites 数据引自Chen et al.(2015); Xiao et al.(2016); Zhang et al.(2017b)

Kızıldaǧ (似)层状铬铁矿中铬铁矿和橄榄石的Mg同位素组成亦与罗布莎豆荚状铬铁矿存在较大差别:(1)铬铁矿均具有较重的Mg同位素组成,与地幔橄榄岩捕掳体中的尖晶石结果相一致;(2)橄榄石均显示较轻的Mg同位素组成,低于平均地幔值;(3)同一样品中的铬铁矿均较橄榄石富集重Mg同位素,不同矿石类型之间的Mg同位素与Fe同位素变化趋势相反(图 6; Chen et al., unpublished).这反映出结晶分异和Fe-Mg交换过程中矿物间Mg同位素的分布不同.这种不同蛇绿岩、不同类型铬铁矿床之间的Fe-Mg同位素差异指示Fe-Mg同位素在揭示铬铁矿成因方面具有较大的应用潜力.

5 结论

针对蛇绿岩铬铁矿成因研究中存在的诸多问题,通过对蛇绿岩铬铁矿床的特点、Fe和Mg元素及同位素的地球化学行为分析,率先对西藏和土耳其蛇绿岩中的地幔橄榄岩和铬铁岩的Fe-Mg同位素进行了探索性研究.结果表明:(1)蛇绿岩形成与演化过程存在明显的Fe-Mg同位素分馏.(2)不同成因(不同环境下)的铬铁岩Fe-Mg同位素组成不同,受控因素不同:赋存在地幔橄榄岩中的铬铁岩同位素组成与矿物间的Fe-Mg交换有关;赋存在地壳(堆晶成因)的铬铁岩同位素组成与结晶分异有关.(3)Fe-Mg同位素在揭示铬铁矿床成因方面具有巨大的潜力.本研究为深入探讨蛇绿岩中橄榄岩和铬铁矿的成因提供了一种新的方法,对进一步认识铬铁矿的物质来源和形成机制具有重要的科学意义.

致谢 本工作开展过程中得到了周美夫、滕方振、张宏福、秦克章、林伟、何永胜等各位老师的大力支持,野外工作得到了史仁灯研究员和Ibrahim Uysal教授的协助,两位评审人对本文提出了建设性意见,在此一并表示感谢!

参考文献
Arai, S., 1992. Chemistry of Chromian Spinel in Volcanic Rocks as a Potential Guide to Magma Chemistry. Mineralogical Magazine, 56(383): 173-184. DOI:10.1180/minmag
Arai, S., Miura, M., 2016. Formation and Modification of Chromitites in the Mantle. Lithos, 264: 277-295. DOI:10.1016/j.lithos.2016.08.039
Arai, S., Yurimoto, H., 1994. Podiform Chromitites of the Tari-Misaka Ultramafic Complex, Southwestern Japan, as Mantle-Melt Interaction Products. Economic Geology, 89(6): 1279-1288. DOI:10.2113/gsecongeo.89.6.1279
Augé, T., 1987. Chromite Deposits in the Northern Oman Ophiolite:Mineralogical Constraints. Mineralium Deposita, 22(1): 1-10.
Bai, Y., Su, B.X., Chen, C., et al., 2017. Base Metal Mineral Segregation and Fe-Mg Exchange Inducing Extreme Compositions of Olivine and Chromite from the Xiadong Alaskan-Type Complex in the Southern Part of the Central Asian Orogenic Belt. Ore Geology Reviews, 90: 184-192. DOI:10.1016/j.oregeorev.2017.01.023
Bao, P.S., 2009. Further Discussion on the Genesis of Podiform Chromite Deposits in the Ophiolites-Questioning about the Rock/Melt Interaction Metallogeny. Geological Bulletin of China, 28(12): 1741-1761.
Borisova, A.Y., Ceuleneer, G., Kamenetsky, V.S., et al., 2012. A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-Hosted Inclusions. Journal of Petrology, 53(12): 2411-2440. DOI:10.1093/petrology/egs054
Brown, G.E., Prewitt, C.T., 1973. High Temperature Crystal Chemistry of Hortonolite. American Mineralogist, 58(7-8): 577-587.
Cao, H.H., Zhao, X.M., Zhang, H.F., 2016. Iron Isotope System and Its Applications in Mantle Geochemistry. Bulletin of Mineralogy, Petrology and Geochemistry, 35(5): 1053-1064.
Chen, C., Su, B.X., Uysal, I., et al., 2015. Iron Isotopic Constraints on the Origin of Peridotite and Chromitite in the Kızıldaǧ Ophiolite, Southern Turkey. Chemical Geology, 417: 115-124. DOI:10.1016/j.chemgeo.2015.10.001
Colás, V., González-Jiménez, J.M., Griffin, W.L., et al., 2014. Fingerprints of Metamorphism in Chromite:New Insights from Minor and Trace Elements. Chemical Geology, 389: 137-152. DOI:10.1016/j.chemgeo.2014.10.001
Dauphas, N., Craddock, P.R., Asimow, P.D., et al., 2009. Iron Isotopes May Reveal the Redox Conditions of Mantle Melting from Archean to Present. Earth and Planetary Science Letters, 288(1-2): 255-267. DOI:10.1016/j.epsl.2009.09.029
Dauphas, N., John, S.G., Rouxel, O., 2017. Iron Isotope Systematics. Reviews in Mineralogy and Geochemistry, 82(1): 415-510. DOI:10.2138/rmg.2017.82.11
de Launay, L.L., 1913. Traité de Métallogénie:Gîtes Minéraux et Métallifères. Béranger, Paris, 1: 257.
Dick, H.J.B., Bullen, T., 1984. Chromian Spinel as a Petrogenetic Indicator in Abyssal and Alpine-Type Peridotites and Spatially Associated Lavas. Contributions to Mineralogy and Petrology, 86(1): 54-76. DOI:10.1007/BF00373711
Dingwell, D.B., Virgo, D., 1988. Viscosities of Melts in the Na2O-FeO-Fe2O3-SiO2 System and Factors Controlling Relative Viscosities of Fully Polymerized Silicate Melts. Geochimica et Cosmochimica Acta, 52(2): 395-403. DOI:10.1016/0016-7037(88)90095-6
Duke, J.M., 1982. Ore Deposit Model 7-Magma Segregation Deposits of Chromite. Geochimica et Cosmochimica Acta, 39: 1061-1074.
Fisher, L.W., 1929. Origin of Chromite Deposits. Economic Geology, 24(7): 691-721. DOI:10.2113/gsecongeo.24.7.691
González-Jiménez, J.M., Griffin, W.L., Proenza, J.A., et al., 2014. Chromitites in Ophiolites:How, Where, When, Why? Part Ⅱ. The Crystallization of Chromitites.Lithos, 189(3): 140-158.
Hagen, A., Mikkelsen, L., 2005. XANES Study of the Oxidation State and Coordination Environment of Manganese, Chromium and Cobalt in Spinel Type Materials. Solid State Electrochemistry, Proceedings: 197-202.
He, Y.S., Hu, D.P., Zhu, C.W., 2015. Progress of Iron Isotope Geochemistry in Geoscience. Earth Science Frontiers, 22(5): 54-71.
Hu, Y., Teng, F.Z., Zhang, H.F., et al., 2016. Metasomatism-Induced Mantle Magnesium Isotopic Heterogeneity:Evidence from Pyroxenites. Geochimica et Cosmochimica Acta, 185: 88-111. DOI:10.1016/j.gca.2015.11.001
Huang, F., 2011. Non-Traditional Stable Isotope Fractionation at High Temperatures. Acta Petrologica Sinica, 27(2): 365-382.
Huang, F., Zhang, Z.F., Lundstrom, C.C., et al., 2011. Iron and Magnesium Isotopic Compositions of Peridotite Xenoliths from Eastern China. Geochimica et Cosmochimica Acta, 75(12): 3318-3334. DOI:10.1016/j.gca.2011.03.036
Huang, M.X., Yang, J.J., Powell, R., et al., 2014. High-Pressure Metamorphism of Serpentinized Chromitite at Luobusha (Southern Tibet). American Journal of Science, 314(1): 400-433. DOI:10.2475/01.2014.11
Irvine, T.N., 1977. Chromite Crystallization in the Join Mg2SiO4-CaMgSi2O6-CaAl2Si2O8-MgCr2O4-SiO2. Carnegie Institution of Washington, Yearbook, Washington D.C., 76: 465-472.
Johan, Z., Dunlop, H., LeBel, L., et al., 1983. Origin of Chromite Deposits in Ophiolitic Complexes-Evidence for a Volatile-Rich and Sodium-Rich Reducing Fluid Phase. Fortschritte der Mineralogie, 61: 105-107.
Kamenetsky, V.S., Crawford, A.J., Meffre, S., 2001. Factors Controlling Chemistry of Magmatic Spinel:An Empirical Study of Associated Olivine, Cr-Spinel and Melt Inclusions from Primitive Rocks. Journal of Petrology, 42(4): 655-671. DOI:10.1093/petrology/42.4.655
Kapsiotis, A., Rassios, A.E., Antonelou, A., et al., 2016. Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece:Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites. Minerals, 6(4): 124. DOI:10.3390/min6040124
Kelemen, P.B., Dick, H.J.B., Quick, J.E., 1992. Formation of Harzburgite by Pervasive Melt/Rock Reaction in the Upper Mantle. Nature, 358(6388): 635-641. DOI:10.1038/358635a0
Lago, B.L., Rabinowicz, M., Nicolas, A., 1982. Podiform Chromite Ore Bodies:A Genetic Model. Journal of Petrology, 23(1): 103-125. DOI:10.1093/petrology/23.1.103
Leblanc, M., Nicolas, A., 1992. Ophiolitic Chromitites. International Geology Review, 34(7): 653-686. DOI:10.1080/00206819209465629
Lenaz, D., Adetunji, J., Rollinson, H., 2014. Determination of Fe3+/∑Fe Ratios in Chrome Spinels Using a Combined Mössbauer and Single-Crystal X-Ray Approach:Application to Chromitites from the Mantle Section of the Oman Ophiolite. Contributions to Mineralogy and Petrology, 167(1): 958. DOI:10.1007/s00410-013-0958-2
Li, S.G., 2015. Tracing Deep Carbon Recycling by Mg Isotopes. Earth Science Frontiers, 22(5): 143-159.
Li, X.P., Chen, H.K., Wang, Z.L., et al., 2015. Spinel Peridotite, Olivine Websterite and the Textural Evolution of the Purang Ophiolite Complex, Western Tibet. Journal of Asian Earth Sciences, 110: 55-71. DOI:10.1016/j.jseaes.2014.06.023
Li, Y.B., Kimura, J.I., Machida, S., et al., 2013. High-Mg Adakite and Low-Ca Boninite from a Bonin Fore-Arc Seamount:Implications for the Reaction between Slab Melts and Depleted Mantle. Journal of Petrology, 54(6): 1149-1175. DOI:10.1093/petrology/egt008
Liu, C.Z., Zhang, C., Yang, L.Y., et al., 2014. Formation of Gabbronorites in the Purang Ophiolites (SW Tibet) through Melting of Hydrothermally Altered Mantle along a Detachment Fault. Lithos, 205(10): 127-141.
Liu, F., Yang, J.S., Dilek, Y., et al., 2015. Geochronology and Geochemistry of Basaltic Lavas in the Dongbo and Purang Ophiolites of the Yarlung-Zangbo Suture Zone:Plume-Influenced Continental Margin-Type Oceanic Lithosphere in Southern Tibet. Gondwana Research, 27(2): 701-718. DOI:10.1016/j.gr.2014.08.002
Liu, P.P., Teng, F.Z., Dick, H.J.B., et al., 2017. Magnesium Isotopic Composition of the Oceanic Mantle and Oceanic Mg Cycling. Geochimica et Cosmochimica Acta, 206: 151-165. DOI:10.1016/j.gca.2017.02.016
Liu, S.A., Teng, F.Z., Yang, W., et al., 2011. High-Temperature Inter-Mineral Magnesium Isotope Fractionation in Mantle Xenoliths from the North China Craton. Earth and Planetary Science Letters, 308(1-2): 131-140. DOI:10.1016/j.epsl.2011.05.047
Liu, X., Su, B.X., Bai, Y., et al., 2018. Ca-Enrichment Characteristics of Parental Magmas of Chromitite in Ophiolite:Inference from Mineral Inclusions. Earth Science, 43(4): 1038-1050.
Lorand, J.P., Ceuleneer, G., 1989. Silicate and Base-Metal Sulfide Inclusions in Chromites from the Maqsad Area (Oman Ophiolite, Gulf of Oman):A Model for Entrapment. Lithos, 22(3): 173-190. DOI:10.1016/0024-4937(89)90054-6
Matveev, S., Ballhaus, C., 2002. Role of Water in the Origin of Podiform Chromitite Deposits. Earth and Planetary Science Letters, 203(1): 235-243. DOI:10.1016/S0012-821X(02)00860-9
Melcher, F., Grum, W., Simon, G., et al., 1997. Petrogenesis of the Ophiolitic Giant Chromite Deposits of Kempirsai, Kazakhstan:A Study of Solid and Fluid Inclusions in Chromite. Journal of Petrology, 38(10): 1419-1458. DOI:10.1093/petroj/38.10.1419
Miller, C., 2003. Geochemistry and Tectonomagmatic Affinity of the Yungbwa Ophiolite, SW Tibet. Lithos, 66(3-4): 155-172. DOI:10.1016/S0024-4937(02)00217-7
Murck, B.W., Campbell, I.H., 1986. The Effects of Temperature, Oxygen Fugacity and Melt Composition on the Behaviour of Chromium in Basic and Ultrabasic Melts. Geochimica et Cosmochimica Acta, 50(9): 1871-1887. DOI:10.1016/0016-7037(86)90245-0
Nicolas, A., Prinzhofer, A., 1983. Cumulative or Residual Origin for the Transition Zone in Ophiolites:Structural Evidence. Journal of Petrology, 24(2): 188-206. DOI:10.1093/petrology/24.2.188
Pagé, P., Barnes, S.J., 2009. Using Trace Elements in Chromites to Constrain the Origin of Podiform Chromitites in the Thetford Mines Ophiolite, Québec, Canada. Economic Geology, 104(7): 997-1018. DOI:10.2113/econgeo.104.7.997
Paktunc, A.D., 1990. Origin of Podiform Chromite Deposits by Multistage Melting, Melt Segregation and Magma Mixing in the Upper Mantle. Ore Geology Reviews, 5(3): 211-222. DOI:10.1016/0169-1368(90)90011-B
Papike, J.J., Karner, J.M., Shearer, C.K., 2005. Comparative Planetary Mineralogy:Valence State Partitioning of Cr, Fe, Ti, and V among Crystallographic Sites in Olivine, Pyroxene, and Spinel from Planetary Basalts. American Mineralogist, 90(2-3): 277-290. DOI:10.2138/am.2005.1779
Polyakov, V.B., Mineev, S.D., 2000. The Use of Mössbauer Spectroscopy in Stable Isotope Geochemistry. Geochimica et Cosmochimica Acta, 64(5): 849-865. DOI:10.1016/S0016-7037(99)00329-4
Princivalle, F., 1990. Influence of Temperature and Composition on Mg-Fe2+ Intracrystalline Distribution in Olivines. Mineralogy and Petrology, 43(2): 121-129. DOI:10.1007/BF01164305
Proenza, J., Gervilla, F., Melgarejo, J.C., et al., 1999. Al-and Cr-Rich Chromitites from the Mayari-Baracoa Ophiolitic Belt (Eastern Cuba); Consequence of Interaction between Volatile-Rich Melts and Peridotites in Suprasubduction Mantle. Economic Geology, 94(4): 547-566. DOI:10.2113/gsecongeo.94.4.547
Roberts, S., Neary, C., 1993. Petrogenesis of Ophiolitic Chromitite. Geological Society, London, Special Publications, 76(1): 257-272. DOI:10.1144/GSL.SP.1993.076.01.12
Robinson, P.T., Bai, W.J., Malpas, J., et al., 2004. Ultra-High Pressure Minerals in the Luobusa Ophiolite, Tibet and Their Tectonic Implications. Geological Society, London, Special Publications, 226(1): 247-271. DOI:10.1144/GSL.SP.2004.226.01.14
Robinson, P.T., Trumbull, R.B., Schmitt, A., et al., 2015. The Origin and Significance of Crustal Minerals in Ophiolitic Chromitites and Peridotites. Gondwana Research, 27(2): 486-506. DOI:10.1016/j.gr.2014.06.003
Roeder, P.L., Reynolds, I., 1991. Crystallization of Chromite and Chromium Solubility in Basaltic Melts. Journal of Petrology, 32(5): 909-934. DOI:10.1093/petrology/32.5.909
Rollinson, H., Adetunji, J., 2015. The Geochemistry and Oxidation State of Podiform Chromitites from the Mantle Section of the Oman Ophiolite:A Review. Gondwana Research, 27(2): 543-554. DOI:10.1016/j.gr.2013.07.013
Saenger, C., Wang, Z.R., 2014. Magnesium Isotope Fractionation in Biogenic and Abiogenic Carbonates:Implications for Paleoenvironmental Proxies. Quaternary Science Reviews, 90: 1-21. DOI:10.1016/j.quascirev.2014.01.014
Schauble, E.A., 2011. First-Principles Estimates of Equilibrium Magnesium Isotope Fractionation in Silicate, Oxide, Carbonate and Hexaaquamagnesium (2+) Crystals. Geochimica et Cosmochimica Acta, 75(3): 844-869. DOI:10.1016/j.gca.2010.09.044
Schiano, P., Clocchiatti, R., Lorand, J.P., et al., 1997. Primitive Basaltic Melts Included in Podiform Chromites from the Oman Ophiolite. Earth and Planetary Science Letters, 146(3): 489-497.
Shi, R.D., Alard, O., Zhi, X.C., et al., 2007. Multiple Events in the Neo-Tethyan Oceanic Upper Mantle:Evidence from Ru-Os-Ir Alloys in the Luobusa and Dongqiao Ophiolitic Podiform Chromitites, Tibet. Earth and Planetary Science Letters, 261(1-2): 33-48. DOI:10.1016/j.epsl.2007.05.044
Sio, C.K.I., Dauphas, N., Teng, F.Z., et al., 2013. Discerning Crystal Growth from Diffusion Profiles in Zoned Olivine by In Situ Mg-Fe Isotopic Analyses. Geochimica et Cosmochimica Acta, 123(2): 302-321.
Smyth, J.R., Hazen, R.M., 1973. The Crystal Structure of Forsterite and Hortonolite at Several Temperatures up to 900℃. American Mineralogist, 58: 588-593.
Su, B.X., Teng, F.Z., Hu, Y., et al., 2015. Iron and Magnesium Isotope Fractionation in Oceanic Lithosphere and Sub-Arc Mantle:Perspectives from Ophiolites. Earth and Planetary Science Letters, 430: 523-532. DOI:10.1016/j.epsl.2015.08.020
Su, B.X., Zhou, M.F., Robinson, P.T., 2016. Extremely Large Fractionation of Li Isotopes in Chromitite-Bearing Mantle Sequence. Scientific Reports, 6: 22370. DOI:10.1038/srep22370
Teng, F.Z., 2017. Magnesium Isotope Geochemistry. Reviews in Mineralogy and Geochemistry, 82(1): 219-287. DOI:10.2138/rmg.2017.82.7
Teng, F.Z., Dauphas, N., Helz, R.T., 2008. Iron Isotope Fractionation during Magmatic Differentiation in Kilauea Iki Lava Lake. Science, 320(5883): 1620-1622. DOI:10.1126/science.1157166
Teng, F.Z., Dauphas, N., Helz, R.T., et al., 2011. Diffusion-Driven Magnesium and Iron Isotope Fractionation in Hawaiian Olivine. Earth and Planetary Science Letters, 308(3-4): 317-324. DOI:10.1016/j.epsl.2011.06.003
Teng, F.Z., Li, W.Y., Ke, S., et al., 2010. Magnesium Isotopic Composition of the Earth and Chondrites. Geochimica et Cosmochimica Acta, 74(14): 4150-4166. DOI:10.1016/j.gca.2010.04.019
Trumbull, R.B., Yang, J.S., Robinson, P.T., et al., 2009. The Carbon Isotope Composition of Natural SiC (Moissanite) from the Earth's Mantle:New Discoveries from Ophiolites. Lithos, 113(3-4): 612-620. DOI:10.1016/j.lithos.2009.06.033
Uysal, I., Tarkian, M., Sadiklar, M.B., et al., 2009. Petrology of Al-and Cr-Rich Ophiolitic Chromitites from the Mula, SW Turkey:Implications from Composition of Chromite, Solid Inclusions of Platinum-Group Mineral, Silicate, and Base-Metal Mineral, and Os-Isotope Geochemistry. Contributions to Mineralogy and Petrology, 158(5): 659-674. DOI:10.1007/s00410-009-0402-9
von Groddeck, A., 1879. Die Lehre von den Lagerstätten der Erze: Ein zweig der Geologie. Metzger and Wittig., Leipzig.
Wang, H.S., Bai, W.J., Wang, B.X., et al., 1983. Chromite Deposits in China and Their Genesis. Science Press, Beijing: 32-59.
Wang, X.B., Bao, P.S., 1987. The Genesis of Podiform Chromite Deposits-A Case Study of the Luobusa Ophiolitic Chromite Deposit. Acta Geologica Sinica, 22(2): 166-181+201-202.
Weyer, S., Ionov, D.A., 2007. Partial Melting and Melt Percolation in the Mantle:The Message from Fe Isotopes. Earth and Planetary Science Letters, 259(1): 119-133.
Xiao, Y., Teng, F.Z., Su, B.X., et al., 2016. Iron and Magnesium Isotopic Constraints on the Origin of Chemical Heterogeneity in Podiform Chromitite from the Luobusa Ophiolite, Tibet. Geochemistry, Geophysics, Geosystems, 17(3): 940-953. DOI:10.1002/2015GC006223
Xiao, Y., Teng, F.Z., Zhang, H.F., et al., 2013. Large Magnesium Isotope Fractionation in Peridotite Xenoliths from Eastern North China Craton:Product of Melt-Rock Interaction. Geochimica et Cosmochimica Acta, 115(5): 241-261.
Xiao, Y.L., Sun, H., Gu, H.O., et al., 2015. Fluid/Melt in Continental Deep Subduction Zones:Compositions and Related Geochemical Fractionations. Earth Sciences, Science China, 1063-11087.
Xiong, F.H., Yang, J.S., Robinson, P.T., et al., 2015. Origin of Podiform Chromitite, a New Model Based on the Luobusa Ophiolite, Tibet. Gondwana Research, 27(2): 525-542. DOI:10.1016/j.gr.2014.04.008
Xu, X.Z., Yang, J.S., Robinson, P.T., et al., 2015. Origin of Ultrahigh Pressure and Highly Reduced Minerals in Podiform Chromitites and Associated Mantle Peridotites of the Luobusa Ophiolite, Tibet. Gondwana Research, 27(2): 686-700. DOI:10.1016/j.gr.2014.05.010
Yamamoto, S., Komiya, T., Hirose, K., et al., 2009. Coesite and Clinopyroxene Exsolution Lamellae in Chromites:In-Situ Ultrahigh-Pressure Evidence from Podiform Chromitites in the Luobusa Ophiolite, Southern Tibet. Lithos, 109(3-4): 314-322. DOI:10.1016/j.lithos.2008.05.003
Yang, J.S., Ba, D.Z., Xu, X.Z., et al., 2010. A Restudy of Podiform Chromie Deposits and Their Ore-Prospecting Vista in China. Geology in China, 37(4): 1141-1150.
Yang, J.S., Bai, W.J., Fang, Q.S., et al., 2008. Ultrahigh-Pressure Minerals and New Minerals from the Luobusa Ophiolitic Chromitites in Tibet:A Review. Acta Geoscientica Sinica, 29(3): 263-274.
Yang, J.S., Dobrzhinetskaya, L., Bai, W.J., et al., 2007. Diamond-and Coesite-Bearing Chromitites from the Luobusa Ophiolite, Tibet. Geology, 35(10): 875-878. DOI:10.1130/G23766A.1
Yang, J.S., Robinson, P.T., Dilek, Y., 2014. Diamonds in Ophiolites. Elements, 10(2): 127-130. DOI:10.2113/gselements.10.2.127
Yang, J.S., Xu, X.Z., Rong, H., et al., 2013. Deep Minerals in Ophiolitic Mantle Peridotites:Discovery and Progress. Bulletin of Mineralogy, Petrology and Geochemistry, 32(2): 159-170.
Young, E.D., Galy, A., 2004. The Isotope Geochemistry and Cosmochemistry of Magnesium. Reviews in Mineralogy and Geochemistry, 55(1): 197-230. DOI:10.2138/gsrmg.55.1.197
Zhang, H.L., Hirschmann, M.M., Cottrell, E., et al., 2017a. Effect of Pressure on Fe3+/∑Fe Ratio in a Mafic Magma and Consequences for Magma Ocean Redox Gradients. Geochimica et Cosmochimica Acta, 204: 83-103. DOI:10.1016/j.gca.2017.01.023
Zhang, P.F., Zhou, M.F., Su, B.X., et al., 2017b. Iron Isotopic Fractionation and Origin of Chromitites in the Paleo-Moho Transition Zone of the Kop Ophiolite, NE Turkey. Lithos, 268-271: 65-75. DOI:10.1016/j.lithos.2016.10.019
Zhao, X.M., Zhang, H.F., Zhu, X.K., et al., 2010. Iron Isotope Variations in Spinel Peridotite Xenoliths from North China Craton:Implications for Mantle Metasomatism. Contributions to Mineralogy and Petrology, 160(1): 1-14. DOI:10.1007/s00410-009-0461-y
Zhao, X.M., Zhang, H.F., Zhu, X.K., et al., 2015. Effects of Melt Percolation on Iron Isotopic Variation in Peridotites from Yangyuan, North China Craton. Chemical Geology, 401: 96-110. DOI:10.1016/j.chemgeo.2015.02.031
Zhou, M.F., Robinson, P.T., Bai, W.J., 1994. Formation of Podiform Chromitites by Melt/Rock Interaction in the Upper Mantle. Mineralium Deposita, 29(1): 98-101. DOI:10.1007/BF03326400
Zhou, M.F., Robinson, P.T., Malpas, J., et al., 1996. Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet):Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle. Journal of Petrology, 37(1): 3-21. DOI:10.1093/petrology/37.1.3
Zhou, M.F., Robinson, P.T., Malpas, J., et al., 2005. REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet. Journal of Petrology, 46(3): 615-639.
Zhou, M.F., Robinson, P.T., Su, B.X., et al., 2014. Compositions of Chromite, Associated Minerals, and Parental Magmas of Podiform Chromite Deposits:The Role of Slab Contamination of Asthenospheric Melts in Suprasubduction Zone Environments. Gondwana Research, 26(1): 262-283. DOI:10.1016/j.gr.2013.12.011
Zhou, M.F., Sun, M., Keays, R.R., et al., 1998. Controls on Platinum-Group Elemental Distributions of Podiform Chromitites:A Case Study of High-Cr and High-Al Chromitites from Chinese Orogenic Belts. Geochimica et Cosmochimica Acta, 62(4): 677-688. DOI:10.1016/S0016-7037(97)00382-7
鲍佩声, 2009. 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. 地质通报, 28(12): 1741-1761. DOI:10.3969/j.issn.1671-2552.2009.12.008
曹辉辉, 赵新苗, 张宏福, 2016. Fe同位素体系及其在地幔地球化学中的应用. 矿物岩石地球化学通报, 35(5): 1053-1064.
何永胜, 胡东平, 朱传卫, 2015. 地球科学中铁同位素研究进展. 地学前缘, 22(5): 54-71.
黄方, 2011. 高温下非传统稳定同位素分馏. 岩石学报, 27(2): 365-382.
李曙光, 2015. 深部碳循环的Mg同位素示踪. 地学前缘, 22(5): 143-159.
刘霞, 苏本勋, 白洋, 等, 2018. 蛇绿岩中铬铁岩母岩浆的富Ca特征:矿物包裹体证据. 地球科学, 43(4): 1038-1050.
王恒升, 白文吉, 王炳熙, 等, 1983. 中国铬铁矿床及成因. 北京: 科学出版社, 32-59.
王希斌, 鲍佩声, 1987. 豆荚状铬铁矿床的成因—以西藏自治区罗布莎铬铁矿床为例. 地质科学, 22(2): 166-181+201-202.
肖益林, 孙贺, 顾海欧, 等, 2015. 大陆深俯冲过程中的熔/流体成分与地球化学分异. 中国科学: 地球科学, 1063-1087.
杨经绥, 巴登珠, 徐向珍, 等, 2010. 中国铬铁矿床的再研究及找矿前景. 中国地质, 37(4): 1141-1150.
杨经绥, 白文吉, 方青松, 等, 2008. 西藏罗布莎蛇绿岩铬铁矿中的超高压矿物和新矿物(综述). 地球学报, 29(3): 263-274.
杨经绥, 徐向珍, 戎合, 等, 2013. 蛇绿岩地幔橄榄岩中的深部矿物:发现与研究进展. 矿物岩石地球化学通报, 32(2): 159-170.