Fractionation effect of iron isotope during magmatism and its indication of submarine basalt formation process
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摘要: Fe是火成岩中丰度最高的变价元素,也是重要的成矿元素,主要以Fe2+或Fe3+价态赋存于固(矿物)、液(流体)相中,并全程参与岩浆作用过程和各种成矿作用。随着测试分析技术(如MC-ICPMS)的发展,Fe等非传统稳定同位素组成分析成为可能,并在最近十几年中被成功应用于岩浆物源追溯、结晶演化过程示踪和成矿作用分析等重要地质作用过程的研究。本文在分析了Fe同位素在岩浆作用过程中分馏效应的基础上,总结了Fe同位素组成在示踪海底玄武质岩浆(MORB、OIB、IAB和BABB等)作用过程研究的最新成果,并探讨了在应用Fe同位素组成示踪海底岩浆作用过程中所存在的主要问题。综合分析结果表明,火成岩中的Fe同位素分馏效应不仅受岩浆源物质部分熔融、岩浆扩散、流体出溶和结晶分异等作用过程的影响,而且还受到同化围岩物质、海底蚀变等作用的影响;由于Fe同位素分析技术(方法)至今仍待进一步完善,已有数据有限且需甄别去伪,因此在利用Fe同位素组成分析或恢复岩浆物源及作用过程时,仍需谨慎;于当前亟需建立完整可靠的Fe同位素示踪体系,这就需要在近期的工作中,尽可能多地选取代表不同构造环境和不同岩石类型的合适样品、获取(积累)更多原始(未经改造或蚀变)样品的精细分析数据,同时在利用Fe同位素示踪海底岩浆作用过程中还需注重多元数据的结合或相互佐证。Abstract: Fe is the most abundant variable-valence element in igneous rocks, and is also an important mineralizing element, mainly in the solid (mineral) and liquid (fluid) phases in Fe2+ or Fe3+ valence state, and participates in magmatic processes and various mineralization throughout. With the development of test analytical techniques (e.g. MC-ICPMS), the analysis of non-traditional stable isotope compositions such as Fe has become possible and has been successfully applied to the study of important geological processes such as magma source tracing, tracing of crystallization evolutionary processes and mineralization analysis in the last decade or so. Based on the analysis of the fractionation effect of Fe isotopes during magmatism, this paper summarized the latest results of Fe isotope composition studies in tracing the action of seafloor basaltic magmas (MORB, OIB, IAB and BABB, etc.) and discussed the main problems in the application of Fe isotope composition in tracing the action of seafloor magmas. The results of the comprehensive analysis show that the Fe isotope fractionation effect in igneous rocks is influenced not only by the processes of partial melting of magma source material, magma diffusion, fluid exsolution and crystallization differentiation, but also by the assimilation of surrounding rock material and seafloor alteration. Since Fe isotope analysis techniques (methods) have yet to be further refined, and the available data are limited and need to be screened for artifacts, caution is still needed when using Fe isotope compositions to analyze or recover magmatic sources and processes. It is urgent to establish a complete and reliable Fe isotope tracing system, which requires the recent work to select as many suitable samples as possible representing different tectonic environments and different rock types, to obtain (accumulate) more fine analytical data of original (unmodified or altered) samples, and to pay attention to the combination or mutual corroboration of multiple data in the process of using Fe isotope tracing for seafloor magmatism.
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Key words:
- iron isotope /
- fractionation effect /
- isotopic tracing /
- submarine magmatism
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1. 引言
近20年来,随着多接收电感耦合等离子质谱(MC-ICPMS)测试技术迅猛发展,非传统稳定同位素地球化学在分析方法、理论方法和实验方法3个方面已经渐近成熟[1-10]。特别是Fe同位素在天体地球化学[11-14]、高温地球化学[15-18]、低温地球化学[19-22]和生物地球化学[23]中的重要应用[24-26]。
大洋玄武岩样品在地球上覆盖范围广、形成过程简单、研究程度较高、受后期地质作用改造程度低。因此,大洋玄武岩样品是研究高温岩浆作用过程中Fe同位素分馏的极佳样品,也是研究更为复杂岩石和地质作用过程中Fe同位素分馏的基础。近年来,国内外学者对海底玄武岩的Fe同位素研究方兴未艾,极大丰富了Fe同位素分馏的理论体系[27-30]。
要将Fe同位素当作示踪岩浆演化的工具,首先需要厘清Fe同位素分馏的机制,而目前控制岩浆岩中Fe同位素分馏的机制还存在诸多争议,主要有部分熔融[31-33]、扩散[34-38]、流体出溶[39-41]和结晶分异[42-45] 4个方面。然而,海底火成岩Fe同位素分馏效应不仅受岩浆源物质部分熔融、岩浆扩散、流体出溶和结晶分异等作用过程的影响,而且还受到同化围岩物质[11]、海底蚀变[46]等作用的影响。
本文在分析了Fe同位素在岩浆作用过程中分馏效应的基础上,总结了Fe同位素组成在示踪海底玄武质岩浆(MORB、OIB、IAB和BABB等)作用过程研究的最新成果,探讨了在应用Fe同位素组成示踪海底岩浆作用过程中所存在的主要问题,并对Fe同位素组成在海洋地质学中的研究提出了几点工作展望。
2. Fe同位素组成及表达方式
在自然界中,Fe有4种稳定同位素,54Fe、56Fe、57Fe和58Fe分别占总量的5.845%、91.754%、2.119 1%和0.291 9%[47-48]。Fe的同位素组成特征通常表示为δ56Fe,这是56Fe/54Fe比值相对于IRMM-014参考标准的千分偏差。δ58Fe值鲜有报道,因为58Fe是一种罕见的同位素,并且与δ56Fe值所反映的质量分馏有关[49-50]。δ57Fe值也常被用来反映Fe的同位素分馏效应。在本文中,我们将重点讨论δ56Fe值的变化,从中亦可以得出δ57Fe和δ58Fe的近似值(δ57Fe=1.475×δ56Fe和δ58Fe≈2×δ56Fe)。
δ56Fe值被定义为
$$ \delta^{56}{\rm{Fe}}=\left[\frac{ (^{56}{\rm{Fe}}/^{54}{\rm{Fe}})_{样品}}{(^{56}{\rm{Fe}}/^{54}{\rm{Fe}})_{标样}}-1\right]\times 10^3 . $$ 其标准通常采用欧洲委员会参考物质及测量协会提供的IRMM-014[51]。尽管这是一个合成标准,但Craddock和Dauphas[52]认为,该标样与球粒陨石的Fe同位素组成类似,所有球粒陨石的Fe同位素组成都相对均一,与球粒陨石的岩石学类型关系不大。早期,Fe的同位素组成是相对于陆地火成岩的平均值来定义的[53],但后来的研究表明,这些岩石中Fe同位素组成变化较大,并不能代表地幔的Fe同位素组成[54-55]。目前,文献中发表的大多数Fe同位素数据都采用IRMM-014作为参考标准。
3. 岩浆作用过程中Fe同位素的分馏效应
3.1 部分熔融过程中Fe同位素的地球化学行为
已有研究[32, 56]表明,来自不同地区和不同构造环境的地幔橄榄岩中δ56Fe值和Mg#呈现良好的负相关关系,经历过越高程度熔体抽离的地幔橄榄岩(具有越高的Mg#)具有越轻的Fe同位素组成,这意味着在地幔部分熔融过程中Fe同位素发生了分馏,重Fe同位素优先进入熔体相中,而熔融残余物富集轻Fe同位素。对代表大洋中脊玄武岩(MORB)熔融残余的深海橄榄岩研究进一步证实了这一结论。深海橄榄岩具有类似于球粒陨石的Fe同位素组成(深海橄榄岩δ56Fe=0.01‰[57]),与MORB(δ56FeMORB=0.11‰[27])相比整体偏低。重Fe同位素优先赋存在熔体相中,说明重Fe同位素(例如56Fe)在部分熔融过程中比轻Fe同位素(例如54Fe)更不相容。如果从质量决定扩算速度的概念考虑,轻质量同位素应该比重质量同位素扩散更快而更不相容[58],但这明显和上述观察到的结论不符。然而,如果重Fe同位素(例如56Fe)在部分熔融过程中优先与Fe3+结合,即存在56Fe–Fe3+亲和性[16, 59-64],重Fe同位素更不相容的特点便很好理解,因为在地幔熔融过程中Fe3+比Fe2+更不相容[65]。在各种氧化还原条件下对橄榄石和玄武岩玻璃中铁键的力常数测量表明,在地幔温度下,Fe3+和Fe2+之间存在显著的Fe同位素分馏平衡,这使得能够对地幔熔融期间的Fe同位素分馏进行定量模拟[59, 66]。
另一方面,来自不同源岩(橄榄岩和石榴石辉石岩)的熔体相对于它们的源岩显示出不同程度的Fe同位素分馏[56-67]。具有相同初始Fe同位素组成的橄榄岩和石榴石辉石岩矿物的部分熔融模拟表明,在相同的部分熔融程度下,来自石榴石辉石岩的熔体比来自橄榄岩的熔体具有更重的Fe同位素组成[56],这归因于石榴石辉石岩部分熔融过程中单斜辉石对熔体的贡献较大,并且单斜辉石和石榴石之间存在较大的Fe同位素分馏(主要地幔岩矿物的δ56Fe值从小到大依次为:石榴石、橄榄石、斜方辉石、单斜辉石、尖晶石)[18, 31, 54, 68-71]。此外,天然辉石岩比橄榄岩具有更重的Fe同位素组成,这进一步增强了重Fe同位素在派生熔体中的富集作用。因此,地幔辉石岩成分通常被用来解释在世界范围内受地幔柱物质影响的洋岛玄武岩(OIB)和MORB观测到的重Fe同位素组成[30, 56, 67, 72-73]。
3.2 熔体演化过程中的Fe同位素分馏
火成岩的Fe同位素组成只有很小的变化[31, 45, 69, 74-75]。Weyer等[75]的研究发现地幔橄榄岩的δ56Fe值略低于玄武岩。MORB和板内玄武岩的δ56Fe值相对恒定,约为+0.1‰。正如Williams等[31]所认为的那样,Fe3+在熔融过程中的相对不相容性使得重Fe同位素进入熔体中。Poitrasson和Freydier[76]首先指出花岗岩具有比MORB更重的Fe同位素。随后的工作表明,这种重的Fe同位素富集不仅限于花岗岩,还包括流纹岩以及其他酸性岩类(如伟晶岩[39, 77])。火山岩的δ56Fe值与SiO2含量密切相关(图1)。在SiO2含量低于70 wt%时,火成岩的δ56Fe值基本是恒定的(+0.08‰~+0.14‰)。然而,当SiO2含量高于70 wt%时,δ56Fe陡然升高达+0.40‰上下(图1b)。
图 1 火山岩中Fe同位素分馏的控制(据文献[59])Fe2+的力常数在SiO2含量为(65~75)wt%之间出现突变(图1a),这种变化可以部分解释硅酸岩的δ56Fe值在SiO2含量高于70 wt%时快速增加的原因(图1b);b中灰色圆圈是由文献数据绘制的点,红色曲线是使用rhyolite-MELTS软件模拟安山岩熔体分离结晶计算得出,蓝色虚线显示了残余熔体的同位素演化Figure 1. Control of iron isotope fractionation in siliceous rocks (from reference [59])The force constant of Fe2+ shows an abrupt change between 65 wt% and 75 wt% of SiO2 content (Fig. 1a), which can partly explain the rapid increase of δ56Fe values of silicate rocks above 70 wt% SiO2 content (Fig. 1b); in b, gray circles are points drawn from literature data, red curves are calculated using rhyolite-MELTS simulation for the fractional crystallization of andesite melt, and blue dotted lines show the isotopic evolution of the residual melt3.2.1 流体出溶
随着压力的降低/岩浆结晶分异程度的增加,岩浆中挥发性组分可达到过饱和,发生流体出溶作用,出溶的流体可以氯化物或者其他络合物的形式带走一部分的Fe[78]。Poitrasson和Freydier[76]的研究认为,在岩浆结晶演化的后期,富含氯的水溶液可从岩浆中析出,并带走部分的Zn、Cu、Mo、Au和Fe等元素,移出流体中的Fe具有较低的δ56Fe值,使得残余岩浆中的Fe同位素组成具有更高的δ56Fe值。Heimann等[39]拓展了Poitrasson和Freydier[76]的工作,其研究结果表明,花岗岩和火山岩都具有重Fe同位素组成和比球粒陨石还低的Zr/Hf比值等特征,低的Zr/Hf比值是流体出溶的有效证据。
反对流体出溶模型的论据有两种:一是有的具有高δ56Fe值的花岗岩是由干的岩浆源物质部分熔融形成的A型无水花岗岩(非造山型),这些A型花岗岩不可能经历了含水流体的出溶作用[77, 79];二是来自Zn同位素组成和Zn/Fe比值的研究,Zn在氯化流体中具有很强的迁移性,大多数硅质岩浆岩中Zn/Fe比值变化很大,这很难用岩浆分异后期的流体出溶来解释[77]。Fe与Zn具有相似的地球化学性质,在Fe同位素发生分馏的过程中,Zn的同位素也必然会发生分馏。Telus等[77]测量了花岗岩中的Zn同位素,发现大多数具有Fe同位素分馏的样品并非具有Zn同位素的分馏特征。Schuessler等[80]测量了冰岛赫克拉火山岩样品的Li同位素组成,发现了与岩浆分异作用有关的Fe同位素分馏现象,但并没有检测到同样是流动元素的Li有同位素组成上的变化。这些研究表明,流体出溶对硅质岩中Fe的同位素分馏作用不大。然而,确实也有一些伟晶岩资料显示出Zn和Fe同位素的分馏特征,流体出溶可能在其中发挥了作用[77]。
3.2.2 扩散
扩散可分为热扩散和化学扩散,前者的扩散驱动力为热梯度,后者的扩散动力为化学梯度。已有研究发现,热梯度可以造成岩浆中同位素的分馏,热的端元趋于富集轻同位素,而冷的端元趋于富集重同位素[35-36, 81]。Lundstrom[82]和Zambardi等[83]提出热扩散可能是硅酸岩中Fe和其他同位素分馏的原因。Zambardi等[83]认为,美国Cedar Butte火山岩无论是结晶分异、地壳混染,还是晚期流体出溶,都不可能解释所观察到的Fe和Si的同位素变化,而将这些同位素分馏归因于热迁移过程,在此过程中,同位素变化主要受控于垂向温度梯度。然而,Telus等[77]测量了美国南达科他州黑山的花岗岩、混合岩和伟晶岩,以及澳大利亚拉克兰褶皱带的I型、S型和A型花岗岩的Fe、Zn、Mg和U的同位素组成,其中Mg、U、Fe的同位素组成之间没有明显的正相关关系,从而认为热扩散不是硅酸岩中Fe同位素分馏的主要因素。
化学扩散也可导致元素同位素的显著分馏[84]。Teng等 [36]通过对夏威夷小基拉韦厄熔岩湖中的橄榄石研究发现这些单矿物的Fe同位素与Mg同位素组成差异非常之大(δ56Fe= –1.1‰~0.49‰,δ26Mg= –0.43‰~0.03‰),并且δ56Fe与δ26Mg之间呈负相关关系,这是因为早先晶出的橄榄石与熔体之间发生了Fe同位素与Mg同位素的交换。除了矿物−熔体之间的互扩散之外,Fe同位素与Mg同位素的互扩散也可以发生在不同性质的熔体之间,Wu等 [37]发现在同时侵位的基性与酸性岩浆之间的接触剖面上(16 cm)同样也存在Fe同位素与Mg同位素的互扩散(δ56Fe=–0.07‰~0.25‰,δ26Mg=–0.63‰~–0.28‰),但远离接触界面则互扩散现象消失。
3.2.3 分离结晶
目前看来,最有可能适用于解释大多数岩浆中Fe同位素分馏的是分离结晶作用。当然,这并不排除热扩散或流体出溶在某些环境中所起的作用。
Schoenberg和von Blanckenburg[11]对瑞士Bergell侵入岩体的研究发现,样品的Fe同位素组成与SiO2含量呈正相关关系,辉长岩和石英闪长岩具有相对轻的Fe同位素组成(δ56Fe=0.03‰~0.09‰),而花岗闪长岩和富硅岩脉具有重的Fe同位素组成(δ56Fe=0.14‰~0.23‰),并将其归因于花岗质熔体的分离结晶作用。Teng等[55]对夏威夷小基拉韦厄熔岩湖玄武岩和堆晶岩的研究发现,这些岩石的δ56Fe变化范围很大(0.2‰),同时随着样品中MgO含量降低(Ol含量降低或结晶分异程度升高),熔体中Fe3+/∑Fe和δ56Fe值升高。这些样品的δ56Fe变化可以用熔体–堆晶间的Fe同位素分馏系数Δ熔体−堆晶=–0.1‰解释。Schuessler等[80]研究了冰岛赫克拉火山岩,发现富Si的英安岩和流纹岩中富集重Fe同位素,并将其归因于富轻Fe同位素的钛磁铁矿的分离结晶作用。Sossi等[79]在塔斯马尼亚Red Hill岩体中也观察到类似的现象。这是因为铁镁质矿物(如橄榄岩和单斜辉石)的分离结晶会优先从熔体中抽离Fe2+(轻Fe同位素),使残余体更富集Fe3+和重Fe同位素。
Dauphas等[59]和Foden等[85]利用rhyolite-MELTS软件对硅酸盐岩石的Fe同位素组成进行模拟,确定了岩浆在分离结晶过程中控制δ56Fe值变化的几个重要因素。Dauphas等[59]测量了矿物和玻璃(作为硅酸盐熔体的替代物)中Fe2+和Fe3+的β因子,并对岩浆的分离结晶进行模拟,以解释硅酸岩浆富重Fe同位素的趋势(图2)。橄榄石、辉石、尖晶石、磁铁矿等矿物的测定表明硅酸盐矿物和氧化物中Fe2+和Fe3+的β因子近似恒定,分别为0.57×106/T2和0.73×106/T2(T代表温度)[86]。玄武岩、安山岩、英安岩和流纹岩中Fe的力常数测量结果也表明岩浆中Fe3+的β因子近似恒定,为1.06×106/T2[59]。在力常数–Fe3+/Fe2+图中,玄武岩、安山岩和英安岩具有相同的趋势,对应于Fe2+的β因子为0.57×106/T2[59]。然而,流纹岩玻璃的力常数更高,对应于0.68×106/T2的β因子。流纹岩玻璃中Fe2+行为的这种差异也可以在X射线吸收近边结构(XANES)光谱中看到,该光谱可以探明Fe的配位环境。因此,核磁共振非弹性X射线散射(NRIXS)和XANES测试结果都表明,英安岩和流纹岩之间Fe的配位环境发生了变化,使亚铁与周围的原子形成了更强的键。Dauphas等[59]在假设缓冲氧逸度的条件下,利用rhyolite-MELTS软件模拟安山岩初始组分的分离结晶使用了这些分馏因子,重现了δ56Fe值与SiO2含量的变化趋势(图1)。
Foden等[85]研究了岩浆初始氧化还原状态以及开放体系(氧逸度缓冲,平衡结晶)和封闭体系(氧逸度非缓冲,分离结晶)对Fe同位素分馏的影响。为了模拟该过程,作者假设辉石–熔体的分馏系数为–0.17×106/T2,相当于在相关的岩浆温度下辉石/钛铁矿和熔体之间的分馏为–0.07‰和–0.11‰[79]。假设磁铁矿与熔体的分馏系数为+0.13×106/T2,对应于在相关岩浆温度为900~1 000℃时磁铁矿与熔体的分馏为+0.07‰~+0.09‰[79]。这些分馏系数是半经验的,以重现观察到的某些Fe同位素变化。在封闭体系(非缓冲)中,Fe3+在早期比Fe2+更不相容。岩浆在结晶过程中逐渐氧化,磁铁矿结晶较早,缓解了Fe3+在硅酸盐矿物中的不相容行为。由于封闭系统中Fe3+含量很少,当体系SiO2含量达到65 wt%~70 wt%时,熔体的Fe3+/Fetot比值较低,晚期结晶相以含Fe2+的硅酸盐为主,因此熔体–矿物分馏系数为正值。此时,熔体中也只剩下很少的Fe,矿物的结晶分离在很大程度上使Fe的重同位素在残余岩浆中富集。虽然该模型可以再现某些以重Fe同位素富集为特征的A型花岗岩的重Fe同位素组成,但矿物–熔体分馏系数具有一定的随机性,不会随着熔体中Fe3+/Fe2+比值的变化而演化。重同位素开始富集也与天然样品中的观察结果不太匹配,天然样品的δ56Fe值在SiO2含量为68 wt%出现转折,而Foden等[85]的模型预测了较低SiO2含量岩石中δ56Fe值的变化。
Dauphas等[59]和Foden等[85]的研究表明,分离结晶可以解释火成岩中的同位素变化,而无需引用流体出溶或热扩散等过程模型。
3.2.4 同化外界物质
Schoenberg和von Blanckenburg[11]测定Bergell岩石的δ56Fe值与SiO2含量呈正相关,而与Fe2O3总含量呈负相关,从而认为含铁相的分离结晶带走了轻Fe同位素,使岩浆房中残留熔体不断富集重Fe同位素。然而,放射性87Sr/86Sr比值和δ18O值也随着Bergell岩浆的不断分化而增加,正如大多数SiO2饱和体系的情况一样,这是由岩浆对地壳物质的同化作用和岩浆的分离结晶(AFC)同时发生所引起的。Bergell岩石的δ56Fe值也与δ18O值呈正相关,说明随着岩浆演化的进行,花岗岩的重Fe同位素组分增加,可能是持续受到Fe同位素重的寄主岩石或下地壳的混染,而不是分离结晶作用的结果。
3.2.5 海底蚀变对Fe同位素组成的影响
海水对洋壳的改变是控制全球元素循环及通量的最重要过程之一[87]。海水与大洋上部地壳的低温相互作用是海水中碱金属、Mg、18O、C和H2O重要的汇[88-90]。玄武岩洋壳最上部的400 m具有高渗透性的特征,该带中的玄武岩与含氧的深海海水反应形成铁的氢氧化物、蒙脱石和绿鳞石等次生矿物[91-92]。该过程蚀变了玻璃、橄榄石、硫化物等矿物,并在较小程度上蚀变了斜长石和单斜辉石等,蚀变产物填充了洋壳中的裂缝和空隙。
Rouxel等[46]揭示了海底蚀变对海洋玄武岩Fe同位素组成的显著影响,蚀变玄武岩显示出较高的δ56Fe值,这是由于蚀变过程中Fe2+和轻Fe同位素的优先析出所致。
4. 海底玄武岩中Fe的同位素组成
研究高温地质过程中Fe同位素的分馏机理,对于加深稳定同位素分馏理论的理解和利用Fe同位素作为岩石成因过程的示踪剂具有重要意义。例如,与球粒陨石相比[11, 33, 52],陆地和月球玄武岩具有重Fe同位素组成[32, 55, 80, 93],这最初被归因于形成月球的巨大撞击期间蒸发引起的动力学Fe同位素分馏,导致陆地和月球地幔的非球粒陨石Fe同位素组成[74]。然而,Polyakov [94]和Williams等[95]提出,这反映了在地核形成期间金属和硅酸盐之间的高压平衡Fe同位素分馏或Fe2+歧化成Fe0和Fe3+,产生了非球粒陨石地幔,后被带入地表形成陆地玄武岩。或者,玄武岩和球粒陨石之间的Fe同位素组成差异可能是地幔部分熔融过程中的Fe同位素分馏造成的[32-33]。
另外,岩浆分异过程也可以造成Fe同位素分馏[36, 55, 80, 96]。因此,玄武岩不能代表其地幔来源。事实上,全球地幔捕虏体和高程度部分熔融体的平均Fe同位素组成更类似于球粒陨石,显示出地球的球粒陨石Fe同位素组成[31-33, 54, 57, 68, 75, 97-100]。因此,形成月球的巨大撞击期间蒸发引起的Fe同位素动力学分馏和地核形成期间的Fe同位素平衡分馏或铁歧化反应可能并不能完全解释这一现象[95]。
为了进一步限制部分熔融和岩浆分异对火成岩Fe同位素组成的影响程度,Teng等[27]研究了覆盖主要大洋中脊段的43个MORB样品、来自夏威夷和法属波利尼西亚群岛的47个OIB样品以及来自北斐济盆地的3个弧后盆地玄武岩(BABB)样品的Fe同位素组成:MORB的δ56Fe值介于+0.07‰~+0.14‰之间,变化范围较小,平均δ56Fe为+0.105‰±0.006‰(2SE,n=43)(图3);3个BABB玄武岩样品的δ56Fe值在+0.09‰~+0.11‰之间,与MORB相似;OIB具有不均一的Fe同位素组成,夏威夷柯劳(Koolau)岛玄武岩的δ56Fe值为+0.05‰~+0.14‰,夏威夷罗希(Loihi)岛玄武岩的δ56Fe值为+0.05‰~+0.09‰,法属波利尼西亚社会(Society)群岛和库克–奥斯特拉尔(Cook-Austral)岛链的δ56Fe值为+0.09‰~+0.18‰。OIB中Fe同位素的不均一性主要反映了橄榄石和辉石的分离结晶,也有可能反映了岩浆源区地球化学性质上的差异。总体而言,MORB、OIB和BABB中的Fe同位素要比地幔橄榄岩重,这种差异应该反映了部分熔融过程中的Fe同位素的分馏效应。
虽然大多数MORB保留了近乎均一的Fe同位素组成[27],但岩浆演化或源不均一性的影响也需要考虑[13]。根据已公布的MORB数据中Fe同位素与MgO之间的弱相关性,Sossi等 [13]认为,在MORB演化过程中,分离结晶可能是导致Fe同位素分馏的重要原因。尽管这些推论是合乎逻辑的,但它们需要样品检验。为此,Chen等 [28]研究了东太平洋海隆(EPR)10°30′N轴上一组放射性同位素和不相容元素的成分均匀的MORB熔岩,其MgO((1.8~7.4)wt%)的成分范围很广,认为这些样品的Fe同位素组成系统性变化最好解释为以分离结晶为主的MORB熔体演化。
尽管地幔熔融[27, 31-33, 54, 59]和岩浆分异[13, 28, 55, 79-80]已被证明会引起可测量的Fe同位素分馏。另一方面,在岩石圈地幔岩石中观察到的显著Fe同位素变化被认为反映了地幔整体的不均一性[32, 56, 99, 101],MORB均一的Fe同位素组成解释为地幔中熔融橄榄岩反应过程中的均一化和地壳岩浆房中进一步的均一化[32, 69]。因此,对于MORB地幔源是均一还是非均一Fe同位素组成的根本问题仍未得到解答。为解决这一问题,Sun等[29]通过对东太平洋5°~15°N洋隆两侧火山玻璃样品的研究,发现MORB之下地幔的Fe同位素组成也是不均一的。这些样品显示出较大的Fe同位素变化范围(δ56Fe=+0.03‰~+0.36‰),超过了先前已知MORB的δ56Fe值变化范围(+0.05‰~+0.17‰)。这种高度变化的δ56Fe值难以用海底蚀变、分离结晶或部分熔融过程来解释,玄武岩中主量元素和微量元素特征与富集型地幔的低度熔融成因相一致,很可能指示Fe同位素组成明显不均匀的岩浆源地幔。这种富重Fe同位素的低度熔融物最有可能形成于诸如洋盆底下的岩石圈–软流圈边界的位置,其可通过结晶石榴石–辉石岩的岩脉/矿脉交代上覆的海底岩石圈而形成。
已有研究表明,洋壳的Fe同位素组成比较均一(图4,对文献中不合理的数据进行剔除并将δ57Fe全部换算为δ56Fe,δ57Fe=δ56Fe×1.475,SE=SD/n(n指样本总数)),全球MORB的δ56Fe值平均值为0.103‰±0.009‰(2SE,n=139);地幔柱成因的OIB的δ56Fe值平均值为0.099‰±0.010‰(2SE,n=150);而起源于地幔楔的IAB(岛弧玄武岩)的δ56Fe值平均值为0.066‰±0.020‰(2SE,n=63)。上述3种岩石的平均δ56Fe值为0.095‰±0.006‰(2SE,n=352),比地幔或者球粒陨石的δ56Fe值高了约0.1‰。
综上所述,海洋中不同构造单元玄武岩(MORB、OIB、IAB和BABB等)的δ56Fe值较为接近,由轻到重依次为:IAB<MORB≈OIB。MORB、IAB和BABB的δ56Fe变化范围略小于OIB。但是,也有少数MORB样品中δ56Fe值高达0.36‰左右(图4)。截至目前,已有文章(数据)显示海底玄武岩具有比球粒陨石富重Fe同位素组成,其主要解释为地幔部分熔融、矿物的分离结晶以及地幔源不均一性所造成。然而,这些解释是否具有全球普适性,还需更多不同构造环境的海底玄武岩进行验证,尤其是构造环境较复杂的弧后盆地,或许能提供新的解释。
5. Fe同位素在示踪岩浆过程中存在的主要问题
经过近20多年的发展,Fe同位素测试技术愈加精确、理论上更加完善,对不同地质储库中Fe同位素的组成以及可能导致Fe同位素发生分馏的机制已经有了一个较为全面的了解。因此,Fe作为一个变价元素,其同位素在示踪岩浆过程和成矿作用等方面潜力巨大。但是,要将Fe同位素当作示踪岩浆演化的工具,首先需要厘清Fe同位素分馏的机制,而目前控制岩浆岩中Fe同位素分馏的机制还存在诸多争议,主要有部分熔融、流体出溶、扩散和结晶分异4个方面。在实际地质过程中,是受单一机制控制分馏,还是受多机制分馏控制?哪一种机制才是控制岩浆岩中Fe同位素分馏的主要机制?如果是几种机制共同起作用,如何量化和评价每一种机制对同位素分馏的贡献?而且,熔离作用、岩浆混合作用、同化混染作用、多期次岩浆活动是否也影响Fe同位素的分馏,还有待于更进一步的系统性研究。
总之,Fe同位素的测试技术和理论方法已经较为成熟,为岩浆源区地幔物质的Fe同位素组成、高温岩浆作用、海底岩浆演化和成矿作用的研究提供了又一颇具潜力的示踪工具。
6. 研究工作展望
Fe同位素地球化学是非传统稳定同位素体系中发展最快的领域之一。测量Fe的化学键丰度与地球化学相似元素(如Zn、Mn等)的比值和氧化还原状态等指标有时不足以解开Fe复杂的地球化学过程。Fe同位素组成特征很可能成为消除这一过程中不确定性的独特工具,在海洋地质学研究中以下几个领域发挥重要的作用:
(1)地幔物质组成。地幔物质的组成(矿物组成、化学组成、元素同位素组成特征等)一直是地学界长期关注而至今又不十分清楚的科学问题。越来越多的证据表明,地幔物质无论是在横向上还是在纵向上都是不均一的,认识这种不均一性及其成因比了解先前认为的均一性更为重要。海底占了地球表面的70%以上,随着调查和分析资料的积累,Fe同位素组成很可能为研究大洋海底地幔物质组成及其发展演化提供新的视角或有效工具。
(2)海底岩浆岩岩石学研究。海底岩石圈主要是由不同性质的岩浆岩组成,其形成过程涉及地幔物质的熔融(产生岩浆)、上升及其过程中的岩浆混合和结晶分异、岩浆房过程中的结晶和重力分异、岩浆喷出(海底)及冷凝成岩、热液及海水的蚀变等一系列的复杂过程,许多非放射性元素同位素在这些高温过程中是基本不发生分馏的,而多价态的Fe同位素却对这些过程非常敏感。因此,Fe同位素组成特征很可能成为海底岩石(圈)形成过程的最好示踪剂(指标)。
(3)海底成矿作用研究。Fe不仅是重要的成矿元素,而且是海洋中众多矿产资源形成的载体或“中介”。例如,结核/结壳中贵重金属元素(如Pt、Co、Ni、REE等)的富集很可能是由于Fe–Mn胶体对海洋中稀有元素的吸附(清扫)作用所造成的;再如,在海底热液活动系统中,Fe是贯穿始终的常量元素,其含量及同位素组成可被用来指示成矿元素的来源、成矿作用过程(阶段)、热液柱的扩散过程及其影响范围等。
(4)同位素示踪地质作用过程的技术体系建立。利用Fe同位素组成分析或恢复岩浆物源及作用过程是近十几年才逐渐发展起来的新方法,但由于分析技术(方法)至今仍待完善,所获分析资料极为有限,且有限的资料大多由于源自不同的样品和测试方法,需要进一步验证和甄别,去伪存真。因此,建立完整可靠的Fe同位素示踪体系任重而道远。在近期的工作中,应注意尽可能地选取合适的样品、获取原始(未经改造或蚀变)样品的精细分析数据和多数据相互佐证等措施。
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图 1 火山岩中Fe同位素分馏的控制(据文献[59])
Fe2+的力常数在SiO2含量为(65~75)wt%之间出现突变(图1a),这种变化可以部分解释硅酸岩的δ56Fe值在SiO2含量高于70 wt%时快速增加的原因(图1b);b中灰色圆圈是由文献数据绘制的点,红色曲线是使用rhyolite-MELTS软件模拟安山岩熔体分离结晶计算得出,蓝色虚线显示了残余熔体的同位素演化
Fig. 1 Control of iron isotope fractionation in siliceous rocks (from reference [59])
The force constant of Fe2+ shows an abrupt change between 65 wt% and 75 wt% of SiO2 content (Fig. 1a), which can partly explain the rapid increase of δ56Fe values of silicate rocks above 70 wt% SiO2 content (Fig. 1b); in b, gray circles are points drawn from literature data, red curves are calculated using rhyolite-MELTS simulation for the fractional crystallization of andesite melt, and blue dotted lines show the isotopic evolution of the residual melt
图 2 玄武岩、安山岩、英安岩、流纹岩玻璃(a)和尖晶石(b)的力常数测量值
该力常数测量值是铁氧化还原状态的函数(数据取自文献[59, 86]);在高温平衡时,Fe同位素分馏与力常数成正比;1 000 ln β=2 904<F>/T2
Fig. 2 Force constant measurements of basalt, andesite, dacite, rhyolite glasses (a) and spinels (b)
The force constant measurements are function of the redox state of iron (data from references [59, 86]); at high temperature equilibrium, iron isotopic fractionation is directly proportional to the force constant; 1 000 ln β=2 904<F>/T2
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