Alkali differentiation in LL-chondrites
Alkali differentiation in LL-chondrites
复制标题
LL-球粒陨石的碱分化
DOI：
10.1016/0016-7037(83)90108-4
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发表时间：
1983
影响因子：
5
通讯作者：
A. Noonan
中科院分区：
文献类型：
--
作者：
F. Wlotzka;H. Palme;B. Spettel;H. Wänke;K. Fredriksson;A. Noonan
LL-族的Krähenberg（Krbg）和Bhola是异质的聚集体，含有各种岩屑和球粒以及晶体碎片。大多数橄榄石颗粒的Fe Fe+ Mg含量是均匀的（Fa 28），尽管发现了少数Fe含量明显较低的（Fa 19）。这两颗陨石都含有大的厘米级碎片，相对于LL-陨石而言，其K（~ 12×）、Rb（~ 45×）和Cs（~ 70×）的富集程度较高，而稀土元素浓度正常（除了负Eu异常）; Na和Sr贫化（~ 0.5×），Na K重量比为0.33，而寄主为11。但是，Na+ K原子的总和没有差异。此外，主元素Si、Al、Mg、Ca和Fe在碎片中与在主体材料中几乎相同。富钾火成岩岩屑在含~ 4%K2O的部分失透玻璃中具有自形到骨架橄榄石的微斑状结构。Krbg和Bhola的主盘都含有以斑状球粒和岩屑形式存在的中稳态玻璃，其K含量（0.1- 8.6%K2O）和Na K比（0.2-100）各不相同。结晶斜长石贫钾，平均Na K比为22，即高于普通钙钛矿斜长石的Na K比8.4。富含钾的大陨石碎片和陨石中的橄榄石具有相同的铁含量（Fa 28），表明两者都是在相同的氧逸度下形成的，可能是在同一母体上形成的。从正常LL球粒陨石母质中形成富钾岩石的可能机制为：1、岩浆分异;通过气相进行Na-K交换; 3.硅酸盐液体不溶性; 4.在撞击事件中的挥发和凝结。过程2似乎是最可行的，形成一个岩石只富集在K和较重的碱和亏损的钠，而不会显着改变其他元素，包括稀土元素。
The LL-group chondrites Krähenberg (Krbg) and Bhola are heterogeneous agglomerates containing a variety of lithic fragments and chondrules as well as crystal fragments. The Fe Fe+ Mg content of most olivine grains is uniform (Fa 28), although a few with distinctly lower Fe contents were found (Fa 19). Both meteorites contain large, cm-sized, fragments with high enrichments of K (~ 12×), Rb (~ 45×) and Cs (~ 70×) relative to LL-chondrites, while the REE concentrations are normal (except for a negative Eu anomaly); Na and Sr are depleted (~ 0.5×) and the Na K weight ratio is 0.33 compared to 11 in the host. However, there is no difference in the sum of Na+ K atoms. Also, the major elements, Si, Al, Mg, Ca and Fe, are nearly the same in fragments as in the host material. The K-rich igneous lithic fragments have a microporphyritic texture of euhedral to skeletal olivines in a partly devitrified glass with~ 4% K 2 O. The main pans of both Krbg and Bhola contain mesostasis glasses in porphyritic chondrules and lithic fragments with varying K content (0.1–8.6% K 2 O) and Na K ratios (0.2–100). Crystalline plagioclase is depleted in K with an average Na K ratio of 22, ie higher than that for ordinary chondritic plagioclase, 8.4. Olivines in the large, K-rich fragments and in the host meteorites have the same iron content (Fa 28), indicating that both formed under the same oxygen fugacity and probably on the same parent body. Conceivable mechanisms for the formation of the K-rich rocks from normal LL-chondrite parent material are: 1, magmatic differentiation: 2. Na-K exchange via a vapor phase; 3. silicate liquid immiscibility; 4. volatilization and condensation in impact events. Process 2 appears most feasible for forming a rock enriched only in K and heavier alkalies and depleted in Na without noticeably changing other elements including the REE.