Fractionation of rare earth elements in refractory inclusions from the Ningqiang meteorite: Origin of positive anomalies in Ce, Eu, and Yb
Fractionation of rare earth elements in refractory inclusions from the Ningqiang meteorite: Origin of positive anomalies in Ce, Eu, and Yb
复制标题
宁强陨石难熔包裹体中稀土元素的分馏：Ce、Eu 和 Yb 正异常的起源
DOI：
10.1016/j.gca.2011.03.029
复制
发表时间：
2011
期刊：
影响因子：
--
通讯作者：
M. Kimura
中科院分区：
文献类型：
--
作者：
H. Hiyagon;A. Yamakawa;T. Ushikubo;Y. Lin;M. Kimura
对宁强未分类碳质球粒陨石中不同类型的难熔包裹体（包括变形虫橄榄石聚集体（AOAs））进行了稀土元素（REE）、钡（Ba）和铪（Hf）离子探针分析，以寻找稀土元素丰度模式与包裹体化学成分之间的可能关系。识别出四种ci归一化稀土模式：(1)近平(未)模式有或没有欧盟(和Yb)异常(组我,第三,或V),(2)消逝ultrarefractory重里斯(三个)相对于光里斯(lree),与消逝的欧盟和Yb(第二组,但没有损耗的Yb在某些情况下),(3)消逝的ultrarefractory一定是积极的异常在Ce,(欧盟),和Yb(修改组II),和(4)近平模式与积极的异常Ce,(欧盟),Yb(修改组我)。包裹体化学组成与稀土元素模式无系统相关性。这表明，观察到的REE分馏发生在主要元素（如Mg和Si）凝聚之前，这些元素定义了包裹体的总体化学组成。值得注意的是，19个包裹体中有7个显示Ce、Yb正异常，个别还显示Eu正异常（修正I族和修正II族），表明这种异常在宁强包裹体中相当普遍，可能在其他原始陨石中也很常见。考虑了两种可能的机制来形成改良的第二类和改良的第一类图案。在模型1中，改性组II的形成过程与生成组II的过程类似，但超难熔粉尘的去除发生在稍低的温度下，不仅超难熔的hree，而且部分lree也被浓缩并从系统中去除。修改后的第一类组分可以通过在修改后的第二类组分中添加未分馏组分来解释，或者可以通过从系统中部分去除超耐火粉尘来解释。在模型2中，修饰的II族是由Ce、（Eu）和Yb添加到细粒粉尘或包裹体上形成的，这些尘埃或包裹体具有3 -贫稀土，类似II族的稀土模式。类似地，修改组I的解释是后来在几乎未分馏的REE模式上添加了Ce， （Eu）和Yb。观察到的稀土元素数据表明，改性组ⅱ的稀土元素耗竭程度（如Er耗竭）和稀土元素分馏程度（如Er/Gd比耗竭）与改性组ⅱ非常相似。模型1预测了系统中几乎完全去除超难分解的hree，导致修改组II的hree耗损高得多，这与目前的观测结果不一致。添加一种未分馏组分可以解释修饰组II中hree的适度耗竭，但它会减少hree之间的分馏，这与目前的观察结果不一致。相比之下，模型2预测Ce - (Eu) - yb富集与ree枯竭之间没有相关性，Ce - (Eu) - yb富集与hree之间的分馏之间也没有相关性，这与目前的观察结果一致。因此，模型2似乎更有可能出现。如果是这样的话，改性II族包裹体的形成至少需要两个具有不同稀土元素特征的不同区域：一个是高温区域，其中类II族（贫稀土）包裹体或其前体是在去除超难熔粉尘后由分馏气体冷凝形成的，另一个是气相中富集Ce、Eu和Yb的低温区域。大量正Ce - (Eu) - yb异常的出现表明固体物质从…
Ion microprobe analyses of rare earth elements (REEs), Ba, and Hf were performed for various types of refractory inclusions including amoeboid olivine aggregates (AOAs) from the Ningqiang ungrouped carbonaceous chondrite to search for possible relationships between REE abundance patterns and bulk chemical compositions of the inclusions. Four types of CI-normalized REE patterns were recognized: (1) nearly flat (unfractionated) pattern with or without Eu (and Yb) anomalies (Groups I, III, or V), (2) depletions of ultrarefractory heavy REEs (HREEs) relative to light REEs (LREEs), and depletions of Eu and Yb (Group II, but without depletion of Yb in some cases), (3) depletions of ultrarefractory HREEs with positive anomalies in Ce, (Eu), and Yb (Modified Group II), and (4) nearly flat pattern with positive anomalies in Ce, (Eu), and Yb (Modified Group I). No systematic correlation was found between bulk chemical compositions and REE patterns of the inclusions. This suggests that the observed REE fractionations occurred prior to condensation of major elements (e.g., Mg and Si) which defined bulk chemical compositions of the inclusions. It is remarkable that 7 out of 19 inclusions show positive anomalies in Ce, Yb, and in some cases, Eu as well (Modified Group I and Modified Group II), suggesting that such anomalies are rather common among inclusions in the Ningqiang and possibly in other primitive meteorites. Two possible mechanisms are considered for the formation of Modified Group II and Modified Group I patterns. InModel 1, Modified Group II is formed by a process similar to that produced Group II but removal of ultrarefractory dust occurred at slightly lower temperatures, where not only ultrarefractory HREEs but some fraction of LREEs had been condensed and removed from the system. Modified Group I may be explained by addition of an unfractionated component to the Modified Group II component, or alternatively, by partial removal of ultrarefractory dust from the system. InModel 2, Modified Group II is formed by later addition of Ce, (Eu), and Yb onto fine-grained dust or inclusions having HREE-depleted, Group II-like REE patterns. Similarly, Modified Group I is explained by later addition of Ce, (Eu), and Yb onto those with almost unfractionated REE patterns. The observed REE data show that both the degree of HREE-depletion (e.g., Er-depletion) and that of fractionation among HREEs (e.g., depletion in the Er/Gd ratio) for Modified Group II are very similar to those for Group II.Model 1predicts almost complete removal of ultrarefractory HREEs from the system, resulting in much higher HREE-depletion for Modified Group II, which is not consistent with the present observations. Addition of an unfractionated component may explain moderate depletion of HREEs in Modified Group II, but it will diminish fractionation among HREEs, which is not consistent with the present observations. In contrast,Model 2predicts no correlations between Ce–(Eu)–Yb-enrichment and HREE-depletion or between Ce–(Eu)–Yb-enrichment and fractionation among HREEs, consistent with the present observations. Hence,Model 2seems more likely. If this is the case, at least two distinct regions with different REE characteristics are required for the formation of Modified Group II inclusions: one is a high temperature region where Group II-like (HREE-depleted) inclusions or their precursors are formed by condensation from a fractionated gas after removal of ultrarefractory dust, and another is a low temperature region enriched in Ce, Eu, and Yb in the gas phase. Abundant occurrence of positive Ce–(Eu)–Yb anomalies suggests that migration of solid materials from …