Discussion on Fractionation Mechanism of Lithium Isotopes
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摘要: 作为一种新兴的稳定同位素示踪工具, 锂同位素地球化学的研究近年来受到了国际地学界日益广泛的关注.其应用领域涵盖了从地表到地幔的流体与矿物之间的相互作用.在地表风化作用过程中, 轻锂同位素(6Li) 优先进入固体相, 而7Li则进入流体相, 因而地表风化作用淋滤出了岩石中的重锂, 致使河水具有重的锂同位素组成, 河水又将重锂同位素组分补给海洋, 洋壳的低温蚀变作用使得海水的锂同位素组成进一步变重.在俯冲带, 由于俯冲板片释放的流体具有重锂同位素组成的特征, 它们上升并交代上覆的地幔楔和相邻的地幔, 使得地幔楔的锂同位素组成变重.同时, 深俯冲的板片由于脱水而具有较轻的锂同位素组成, 它们在地幔中可能形成一个局部轻锂的地幔储源.影响地幔橄榄岩锂同位素分馏的因素主要有3个方面: 温度、扩散机制以及外来熔体的反应.由于高温下地幔矿物之间的锂同位素分馏很小, 而单纯的扩散分馏机制不能够很好的解释我国华北汉诺坝地区地幔橄榄岩中矿物之间的锂同位素分馏.因此, 具有轻锂同位素组成的熔体与橄榄岩之间的反应是上述现象的一个合理解释.需要指出的是, 在橄榄岩-熔体反应的过程中, 锂同位素的扩散作用也对地幔矿物之间的同位素分馏有一定的贡献.Abstract: As a burgeoning tracer of stable isotopes, lithium geochemistry has attracted increasing attention from the international geological community recently.Its application field covers the interactions between fluids and minerals from the surface to the mantle.During weathering, the lighter isotope 6Li is preferentially retained in the solid phase while 7Li goes into solution.Thus river waters have heavy Li isotopic compositions compared to the original bedrock.The heavy Li isotopic compositions of river water feed the oceans.Low-temperature alteration of oceanic crust then makes seawater heavier than river water.In a subduction zone, heavy Li of fluids escaping the slab at low temperature likely enrich the forearc mantle wedge and hydrate the adjacent mantle and make them isotopically heavier than before.Meanwhile, an isotopically light component of slab dehydrated is subducted deeply and may form a locally light-Li mantle reservoir.There are three main factors that can result Li isotopic fractionation in mantle peridotites.They are temperature, diffusion-driven fractionation and mixing with exotic melts.At high temperatures of magmatic processes, Li isotopes do not show per mil-level mass fractionation, and simple diffusive process can not account for the Li isotopic fractionation between the minerals in peridotitic xenoliths from the Hannuoba area in the North China Craton, thus the reaction between peridotites and melts with light Li isotopic compositions maybe a reasonable explanation.It should point out that diffusion of Li isotopes during peridotite-melt reaction makes some contributions to the isotopic fractionation between the mantle minerals.
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Key words:
- Li isotopes /
- fractionation mechanism /
- geochemical tracer
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图 1 不同储源的锂同位素组成
数据来源: 海水(You and Chan, 1996; Chan and Edmond, 1988; Moriguti and Nakamura, 1998b; Tomascak et al., 1999b; James and Palmer, 2000); 河水(Huh et al., 1998); 高温孔隙流体(Foustoukos et al., 2004; Kisakürek et al., 2004); 岛弧熔岩(Moriguti and Nakamura, 1998a; Tomascak et al., 2000, 2002; Chan et al., 2002b); 洋岛玄武岩(Tomascak et al., 1999b; Chan and Frey, 2003); 新鲜玄武岩(Chan et al., 1992, 2002b; Moriguti and Nakamura, 1998a; Tomascak and Langmuir, 1999); 蚀变玄武岩(Chan et al., 1992, 2002a); 海洋沉积物(Chan et al., 1994a; Zhang et al., 1998; James et al., 1999; Chan and Kastner, 2000); 黄土、页岩和大陆上地壳(Teng et al., 2004); 榴辉岩(Zack et al., 2003; Marschall et al., 2007); 橄榄岩和辉石岩捕虏体(Tang et al., 2007b); 其他数据来源(Tomascak, 2004)
Fig. 1. Li isotopic composition of different reservoirs
图 2 锂同位素的地球化学行为示意图(据Tang et al., 2007b修改)
地表风化作用使得河水具有高的7Li, 河水将高7Li组分补给海洋, 低温洋底蚀变作用使海水的锂同位素组成进一步变重.来自俯冲板片的流体及蛇纹岩底辟块(均高δ7Li) 的重锂同位素特征说明在蚀变洋壳脱水过程中发生了锂同位素分馏.由于俯冲板片释放的流体具有高δ7Li的特征, 它们上升并交代上覆地幔楔, 从而使地幔楔具有重的锂同位素组成.因此, 一些岛弧熔岩具有高δ7Li的特征
Fig. 2. Schematic illustration of Li isotope geochemical behaviors
图 3 橄榄石、单斜辉石和斜方辉石中δ7Li的变化(a), Δ7LiCpx-Ol和Δ7LiOpx-Ol随δ7LiCpx的变化(b), δ7LiCpx和Δ7LiCpx-Ol随单斜辉石的(La/Yb) N的变化(c), δ7LiOpx-Ol和Δ7LiCpx-Opx随Δ7LiCpx-Ol的变化(d)
图(a) 和数据来自Tang et al. (2007a)
Fig. 3. Plots showing the variations in δ7Li between olivine, Cpx, and Opx (a), Δ7LiCpx-Ol and Δ7LiOpx-Ol versus δ7LiCpx (b), δ7LiCpx and Δ7LiCpx-Ol versus (La/Yb) N ratios in Cpx (c), and δ7LiOpx-Ol and Δ7LiCpx-Opx versus Δ7LiCpx-Ol (d)
图 4 扩散模型计算结果(model曲线) 与汉诺坝橄榄岩中矿物Li-δ7Li的对比(a), 橄榄石、斜方辉石和单斜辉石的δ7Li随扩散距离的变化(b), 模拟计算的Δ7LiCpx-Ol和Δ7LiOpx-Ol随扩散距离的变化(c), 计算结果与实测的Δ7LiCpx-Ol和Δ7LiOpx-Ol的对比(d)
Model 1和model 2分别代表两种不同组成的熔体, 它们代表MORB的变化范围(Moriguti and Nakamura, 1998a).计算参数见表 1
Fig. 4. Diffusion model calculations (model curves) compared to data for Li andδ7Li of minerals in Hannuoba peridotites (a), δ7Li of Ol, Opx and Cpx versus distance from the contact for melt diffusion model (b), Δ7LiCpx-Ol and Δ7LiOpx-Ol versus distance from the contact (c), and Δ7LiCpx-Ol versus Δ7LiOpx-Ol in the diffusion calculations compared to data measured (c)
图 5 不同的β值对扩散分馏模型的影响
a.橄榄石的Li-δ7Li变化; b.橄榄石的δ7Li随扩散距离的变化; c.单斜辉石的Li-δ7Li变化; d.单斜辉石的δ7Li随扩散距离的变化; e.单斜辉石与橄榄石之间的锂同位素分馏Δ7Licpx-ol随扩散距离的变化; f.Δ7Licpx-ol随β值的变化, 并与汉诺坝橄榄岩数据对比
Fig. 5. Effects of different β values on diffusion fractionation mode
图 6 汉诺坝橄榄岩中矿物的锂含量和同位素组成以及瑞利蒸馏和同位素二端元混合模型
图中的百分数表示蚀变洋壳脱水的百分比.二端元混合模型参数: 端元1 (熔体) 锂含量为11.5×10-6, δ7Li=-15‰, 端元2 (矿物) 与图 4相同
Fig. 6. Diagram of Li versus δ7Li for minerals in Hannuoba peridotites, and Rayleigh distillation model and two end-member mixing model
表 1 扩散模型计算所采用的参数
Table 1. Parameters used in diffusion model
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