Lithium Isotopic Composition of Chondritic Meteorites
Lithium Isotopic Composition of Chondritic Meteorites
复制标题
球粒陨石的锂同位素组成
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
R. Rudnick
中科院分区：
文献类型：
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作者：
W. McDonough;F. Teng;P. Tomascak;R. Ash;J. Grossman;R. Rudnick
前言：考虑到锂的潜在溶解性，锂同位素研究可以提供对涉及水相流体的低温蚀变过程的洞察。在这方面，陨石的锂同位素研究可能有助于阐明陨石母体上的流体作用。然而，太阳系物质的Li同位素组成仍然知之甚少，只有少数球粒陨石和CaI、SNC陨石的精选相和一块球粒陨石(Orgueil)的数据[2-4]。这些数据显示δLI值范围很大，没有明显的系统变化。球粒陨石的LI值变化高达100Sigma(以2‰水平表示)，而δ的LI值变化高达200Sigma。此外，Ci平均比球粒(分别为-20‰和+10‰)轻[2]。Chaussidon等人。[2]认为材料的δLI值较低可能反映了自生Li的加入。地球物质显示出约60‰的δLi总变化，其中很大一部分变化可归因于低温下的水-岩石反应。根据对地球岩石和流体的研究，只有低温(<；700K)过程被认为对锂同位素组成的分馏有显著影响[1，5]。Chan等人。[1]证实，随着低温蚀变和粘土形成的增加，新鲜的地幔玄武岩的δLi值从+4增加到+14‰。最近，扎克等人。[6]论证了在稍高的温度下变质脱水将使岩石的Li同位素组成向轻值转变，低至-12‰。鉴于这些观察，我们仔细选择了一套反映了广泛的岩石学类型和水蚀变程度的球粒陨石，以表征它们的Li同位素变化，并从岩石学的角度来理解这种变化。我们只选择了坠落的样本，而不是发现的样本，以避免任何可能伴随着地球上的水处理而来的不确定性。通过对球粒陨石的锂同位素组成的研究，我们发现球粒陨石和球粒陨石中的Li/Li值有很大的差异。此外，这些数据将对(1)陨石母体上的流体-岩石相互作用，以及(2)脱水作用和热变质作用对球粒陨石母体的相对影响提供深入的认识。
Introduction: Li isotope studies can provide insights into low temperature alteration processes involving aqueous fluids, given the potential solubility of lithium [1]. In this respect, Li isotope studies of meteorites may help to elucidate fluid processes on meteorite parent bodies. The Li isotopic composition of solar system materials, however, remains poorly understood, with data available for only a few chondrules and CAI, selected phases from SNC meteorites, and one bulk chondrite (Orgueil) [2-4]. These data show a wide range in δLi values with no obvious systematic variation. Chondrules possess up to 100‰ (expressed at the 2 sigma level) variation in δLi values, whereas CAI have up to 200‰ variation. In addition, CAI are lighter, on average, than chondrules (–20‰ and +10‰, respectively [2]). Chaussidon et al. [2] suggested that the lower δLi values of CAI materials may reflect the addition of spallogenic Li. Earth materials show ~60‰ total variation in δLi, with much of this variation attributed to water-rock reactions at low temperatures. Based on studies of rocks and fluids from the Earth only low temperature (<700 K) processes are recognized as having any significant effect in fractionating the Li isotopic compositions [1,5]. Chan et al. [1] established that the δLi of fresh, mantle-derived basalts increases from values of about +4 to values as high as +14 ‰, as the amount of low temperature alteration and clay formation increases. Recently, Zack et al. [6] demonstrated that metamorphic dehydration at slightly higher temperatures will shift the Li isotopic compositions of rocks to light values, as low as –12‰. Given these observations, we have carefully selected a suite of chondritic meteorites that reflect a broad range of petrological types and degrees of aqueous alteration in order to characterize their Li isotopic variation and to understand this variation in a petrological context. We selected only samples that are falls, not finds, to avoid any uncertainties that might accompany Earth-based aqueous processing. By characterizing the Li isotopic composition of chondritic meteorites we place into context the wide variation of Li/Li values observed in chondrules and CAI. In addition, these data will provide insights into (1) fluid-rock interactions on meteorite parent bodies, and (2) the relative effects of dehydration versus thermal metamorphism on chondritic parent bodies.