吉林陨石中铁镍合金的磁学性质

铁镍合金是陨石中重要的磁性物质,其中铁纹石、镍纹石和四方镍纹石是球粒陨石中的主要铁镍合金.然而,迄今针对陨石中铁镍合金的磁学性质研究仍非常缺乏.本文研究了吉林陨石中的铁纹石、四方镍纹石、以及陨硫铁的磁学特征.实验表明,镍含量为6%~7%的铁纹石是该陨石中最主要的铁镍合金物质,它具有低矫顽力和高的热稳定性,居里温度~750 ℃.镍含量为~48%的四方镍纹石具有高矫顽力和高的热稳定性,居里温度~565 ℃,它是剩磁的主要载体.陨硫铁在室温为反铁磁性,不具有载剩磁能力,在60 K左右存在一个低温转换,在氩气中加热较稳定而在空气中加热被氧化转化为磁铁矿.这些研究结果为鉴定球粒陨石中的磁性物质提供了依据.     

Mineralogy and petrology of Al-rich objects and amoeboid olivine aggregates in the CH carbonaceous chondrite North West Africa 739

The aluminum-rich (>10 wt% Al₂O₃) objects in the CH carbonaceous chondrite North West Africa (NWA) 739 include Ca,Al-rich inclusions (CAIs), Al-rich chondrules, and isolated mineral grains (spinel, plagioclase, glass). Based on the major mineralogy, 54 refractory inclusions found in about 1 cm² polished section of NWA 739 can be divided into hibonite-rich (16%), grossite-rich (26%), melilite-rich (28%), spinel-pyroxene-rich (16%) CAIs, and amoeboid olivine aggregates, (AOA's, 17%). Most CAIs are rounded, 25–185 μm (average=70 μm) in apparent diameter, contain abundant, tiny perovskite grains, and typically surrounded by a single- or double-layered rim composed of melilite and/or Al-diopside; occasionally, layers of spinel+hibonite and forsterite are observed. The AOAs are irregularly shaped, 100–250 μm (average=175 μm) in size, and consist of forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, and minor spinel. One AOA contains compact, rounded melilite-spinel-perovskite CAIs and low-Ca pyroxene replacing forsterite. The Al-rich (>10 wt% bulk Al₂O₃) chondrules are divided into Al-diopside-rich and plagioclase-rich. The Al-diopside-rich chondrules, 50–310 μm (average=165 μm) in apparent diameter, consist of Al-diopside, skeletal forsterite, spinel, ±Al-rich low-Ca pyroxene, and ±mesostasis. The plagioclase-rich chondrules, 120–455 μm (average=285 μm) in apparent diameter, are composed of low-Ca and high-Ca pyroxenes, forsterite, anorthitic plagioclase, Fe,Ni-metal nodules, and mesostasis. The isolated spinel occurs as coarse, 50–125 μm in size, subhedral grains, which are probably the fragments of Al-diopside chondrules. The isolated plagioclase grains are too coarse (60–120 μm) to have been produced by disintegration of chondrules or CAIs; they range in composition from nearly pure anorthite to nearly pure albite; their origin is unclear. The Al-rich objects show no evidence for Fe-alkali metasomatic or aqueous alteration; the only exception is an Al-rich chondrule fragment with anorthite replaced by nepheline. They are texturally and mineralogically similar to those in other CH chondrites studied (Acfer 182, ALH85085, PAT91467, NWA 770), but are distinct from the Al-rich objects in other chondrite groups (CM, CO, CR, CV). The CH CAIs are dominated by very refractory minerals, such as hibonite, grossite, perovskite and gehlenitic melilite, and appear to have experienced very low degrees of high-temperature alteration reactions. These include replacement of grossite by melilite, of melilite by anorthite, diopside, and spinel, and of forsterite by low-Ca pyroxene. Only a few CAIs show evidence for melting and multilayered Wark-Lovering rims. These observations may suggest that CH CAIs experienced rather simple formation history and escaped extensive recycling. In order to preserve the high-temperature mineral assemblages, they must have been efficiently isolated from the hot nebular region, like some chondrules and the zoned Fe,Ni-metal grains in CH chondrites.     

南极格罗夫山23个普通球粒陨石的化学群及岩石类型

对南极格罗夫山蓝冰地区收集的23个陨石进行了系统研究。经激光共聚焦显微镜观察,未查出碳质球粒陨石。经电子探针鉴定,也未发现顽火辉石球粒陨石。系统的研究确定这些陨石均为普通球粒陨石。运用X射线能谱仪进行全视域元素成分面分析方法,获得了23个陨石的化学成分。应用陨石中SiO_2、∑FeO、MgO、CaO的含量进行图解对比,划分出陨石的化学群,取得了初步的结果。又以电子探针测得的橄榄石、辉石矿物化学成分,采用Fa-Fs图解法进行化学群分类。这两种方法对23个陨石化学群的划分,取得了较为一致结果。陨石岩石类型的确定,主要根据陨石中球粒及基质的特征,参考其它标志,如橄榄石成分、辉石的结构、长石的特征等进行分类,效果较好。对于岩石类型为第3类的非平衡陨石,由于矿物成分变化较大,并可能有水等易挥发成分的存在,其化学群的划分不易确定。     

Petrography and Mineral Chemistry of 4 Carbonaceous Chondrites from the Grove Mountains, Antarctica

Petrography and mineral chemistry of four carbonaceous chondrites (GRV 020017, GRV 020025, GRV 021579, GRV 022459) collected from the Grove Mountains (GRV), Antarctica, were reported here. All four chondrites are unequilibrated, as indicated by well shaped chondrules and the chemical variations of olivine and low-Ca pyroxene. The modal abundance ratio of matrix/chondrule are 2(GRV 020017), 2.8(GRV 020025), 1.2(GRV 021579), 1(GRV 022459). GRV 022459 has the largest chondrules(0.6-2.0mm). A total of 30 Ca-Al-rich inclusions were found in the four meteorites. Most inclusions were highly altered, with abundant phyllosilicates in the inclusions of GRV 020017 and GRV 020025. On the base of petrography and mineral chemistry, these chondrites are classified as CM2 (GRV 020017 and 020025), CO3 (GRV 021579) and CV3 (GRV 022459).     

Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.     

100块南极格罗夫山陨石的分类及其成对陨石的初步判别

通过岩石矿物学特征的观察和矿物成分的电子探针分析,对采自南极格罗夫山的100块陨石进行了分类研究,确定了它们的化学群、岩石类型、冲击变质程度和风化程度,其中8块陨石划分为H4型,15块为H5型,3块为H6型,1块为L3型,2块为L4型,16块为L5型,47块为L6型,5块为LL3型,1块为LL4型,2块为LL5型。经过岩石矿物学特征和矿物成分的对比分析,初步判定成对陨石21组。     

南极陨石研究的启示Ⅶ:南极新的和独特的陨石类型

近年来在南极及澳大利亚、非洲和北美的沙漠地区发现和回收了大量的陨石，改变了以前的球粒陨石分类。球粒陨石群由９个增加到１３个（ＥＨ、ＥＬ、Ｈ、Ｌ、ＬＬ、Ｒ、ＣＩ、ＣＲ、ＣＨ、ＣＭ、ＣＯ、ＣＶ及ＣＫ），并提出了３个碳质球粒陨石小群。ＣＫ、ＣＲ、ＣＨ和Ｒ为１３个球粒陨石群中新的化学群。不同的球粒陨石群，其岩石学、矿物学、化学及氧同位素组成均有明显的差异。根据其结构、矿物、化学及氧同位素组成特征，将１３个球粒陨石群划分为碳质球粒陨石系（由ＣＩ－ＣＲ－ＣＨ、ＣＭ－ＣＯ及ＣＶ－ＣＫ族组成）和顽辉石－普通球粒陨石系（由ＥＨ－ＥＬ、Ｈ－Ｌ－ＬＬ及Ｒ族组成）。碳质球粒陨石系是在相对远离太阳的区域形成，它以高的氧化态为特征。顽辉石－普通球粒陨石系是在靠近太阳的内太阳系区域形成的     

Cosmic ray exposure ages of Rumuruti chondrites from North Africa

We analyzed noble gases and determined ³He, ²¹Ne, and ³⁸Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The ²¹Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their ²¹Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body.     

中国南极球粒陨石的高精度钾同位素研究

钾和其他中等挥发性元素亏损是类地行星普遍的全岩化学成分特征之一,能用来示踪不同的亏损过程.球粒陨石是组成行星的前体物质,研究球粒陨石中钾同位素的亏损和分异机制,对于太阳系物质或行星的起源、形成和演化具有十分重要的意义.本文利用近年来发展的高精度钾同位素分析技术,测试了 14 个中国南极陨石以及6 个目击型陨石(Murc...