The Isheyevo meteorite: Mineralogy, petrology, bulk chemistry, oxygen, nitrogen, carbon isotopic compositions, and 40Ar‐39Ar ages
The Isheyevo meteorite: Mineralogy, petrology, bulk chemistry, oxygen, nitrogen, carbon isotopic compositions, and 40Ar‐39Ar ages
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伊谢耶沃陨石：矿物学、岩石学、本体化学、氧、氮、碳同位素组成和 40Ar-39Ar 年龄
DOI：
10.1111/j.1945-5100.2008.tb01090.x
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发表时间：
2008
影响因子：
2.2
通讯作者：
F. Brandstätter
中科院分区：
文献类型：
--
作者：
M. Ivanova;N. N. Kononkova;A. Krot;R. Greenwood;I. Franchi;A. Verchovsky;M. Trieloff;E. V. Korochantseva;F. Brandstätter
摘要- Isheyevo是一种富含金属的碳质球粒陨石，含有几种不同丰度的Fe，Ni金属（7-90体积%）的岩性。富含金属的岩性以50-60体积%的铁、镍金属为主。富金属和贫金属的岩性分别与CBb和CH碳质球粒陨石最相似，提供了这些球粒陨石群之间的潜在联系。所有岩石均经历了S4期的冲击变质作用。所有这些都由相似的成分组成-Fe，Ni金属，球粒，耐火夹杂物（富含Ca，Al的夹杂物[CAIs]和变形虫橄榄石聚集体[AOAs]），以及高度水化的岩屑碎屑-但在模态丰度，球粒尺寸以及斑状与非斑状球粒的比例方面存在差异。大量的化学和氧同位素组成的CH和CB方解石的范围内。整体氮同位素组成高度富集15 N（δ 15 N = 1122‰）。磁性部分在氮释放模式和同位素分布方面与散装样品非常相似;非磁性部分含有明显较少的重氮。在高温下释放的碳显示出相对较重的同位素特征。与CBb辉长岩类似，Isheyevo中约20%的Fe、Ni金属颗粒是化学分区的。类似于CH_2O_3，一些金属颗粒富含Ni（>20重量% Ni）。与CBb和CH相比较，大多数金属颗粒被热分解成富Ni相和贫Ni相。与CH陨石类似，球粒具有斑状和非斑状结构和铁镁（I型和II型），富含二氧化硅和富含铝的整体成分。一些层状铁镁球粒被铁橄榄石或层状硅酸盐边缘包围。球粒边缘的层状硅酸盐与水化岩屑碎屑中的层状硅酸盐在成分上是不同的。类似于CH石，CAIs主要是由富黑云母、富钙铝石和富黄长石类型; AOA非常罕见。我们推断，Isheyevo是由不同过程和不同物理化学条件下形成的材料的复杂混合物。两个族群的球粒和难熔内含物、金属颗粒和高度水合的碎屑在原行星盘的一个细粒尘埃耗尽的区域中聚集在一起进入伊舍耶沃母小行星。这种情况与伊舍耶沃的太阳风注入惰性气体的存在及其相对较老的K-Ar年龄相一致。我们不能排除K-Ar系统受到后来的碰撞事件的影响。宇宙成因的38 Ar测定的伊舍耶沃陨石的宇宙射线暴露年龄（CRE）为1034 Ma，与本库宾（CBa）陨石的年龄相似。
Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CBb and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15N (δ15N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CBb chondrites, ∼20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CBb and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic 38Ar is ∼34 Ma, similar to that of the Bencubbin (CBa) meteorite.