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.     

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).     

Shock metamorphism of ordinary chondrites from Grove Mountains, Antarctica

Shock effects of 93 Grove Mountains (GRV) ordinary chondrites were studied in this work, including fracture, various types of extinction, and recrystallization of silicates observed under optical microscopy. Shock-induced veins and pockets show various microtextures, decomposition and phase transformation of minerals. The confirmed high-pressure polymorphs of silicates are ringwoodite, majorite, pyroxene glass and maskelynite. Based on the shock effects and assemblages of high-pressure minerals, shock stages of all of 93 GRV chondrites were classified. In comparison with literature, the Grove Mountains meteorites have a higher fraction (23 out of 93) of heavily shocked samples (S4-S5). Most of the heavily shocked meteorites are L group (22 out of 23), except for one H chondrite. The distinct shock metamorphism between H and L groups may indicate different surface properties of their parent bodies. In addition, there is relationship between petrologic types and shock stages, with most heavily shocked samples observed in equilibrated ordinary chondrites (especially Type 5 and 6).     

NWA 13943(CK5型)碳质球粒陨石的矿物岩石学和稳定同位素组成研究

CK型陨石是一类高度氧化的碳质球粒陨石, 金属/磁铁矿的比值接近零. 与其它类型的碳质球粒陨石(岩石类型: 1-3)不同, 大多数CK型陨石在母体上经历了强烈的热变质过程(550--1270K), 以4-6型为主. 多项证据表明, CK和CV3型陨石具有成因联系. 但是, 两者在岩相结构和化学组成方面仍存在微小差异. 因此, 精细地区分和比较两者的地球化学特征对于验证CK-CV单一母体假说非常重要. Northwest Africa (NWA) 13943是一块新发现的陨石, 经历过较强烈的热变质作用. 利用扫描电子显微镜和电子探针, 确定了NWA 13943的岩石类型. 并运用质谱分析技术, 重点测定了NWA 13943陨石的全岩氧同位素和铬同位素组成. 综合岩石结构、矿物化学成分、氧同位素异常(△¹⁷O,△代表同位素分馏值)和铬同位素异常(ε⁵⁴Cr, ε表示样品中的同位素比值与标样中的同位素比值的相对偏差的10⁴倍),CK和CV型陨石的母体可能形成于原行星盘中两个相似但不同的化学源区.     

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(Murchison、Allende、Ningqiang、Tagish Lake、Xinyang和Banma)的全岩钾同位素组成。结果显示,21个碳质球粒陨石全岩δ⁴¹K值分布范围为-0.62‰±0.05‰至0.37‰±0.08‰,平均值为-0.32‰±0.24‰(2SD),比全岩硅酸盐地球(BSE)稍重。18个普通球粒陨石全岩数据(如果异常值GRV 021603除外)的δ⁴¹K值分布范围为-1.02‰±0.05‰到-0.61‰±0.02‰,平均δ⁴¹K值为-0.81‰±0.15‰(2SD),比全岩硅酸盐地球稍轻。2个目击型陨石Murchison(CM2型)和Allende(CV3型),呈现较大的内部钾同位素差异(分别为0...     

Weathering, shock metamorphism and type distribution patterns of 98 ordinary chondrites from the Grove Mountains, Antarctica

Petrography and mineral chemistry of ninety-eight ordinary chondrites from Grove Mountains (GRV), Antarctica, were presented and their.weathering effect, shock metamorphism and type distribution patterns were discussed in this study. Among them, six are unequilibrated ordinary chondrites, including 3 H3 and 3 L3; and 92 meteorites are equilibrated ordinary chondrites, including 24 H-group (13 H4, 10 H5, 1 H6), 64 L-group (2 L4, 44 L5, 18 L6) and 4 LL-group (3 LL4, 1 LL5). Most GRV chondrites (>90%) displayed minor weathering effect (W1 and W2). About half of the meteorites experienced severe shock metamorphism. They commonly contain shock-induced melt veins and pockets. These heavily shocked meteorites provide us with natural samples for study of high-pressure polymorphs of minerals. In addition, the Grove Mountains collection seems to have more abundant unequilibrated and L group ordinary chondrites compared to the US Antarctic meteorite collection which were mainly found along the Transantarctic Mountains.     

Lack of relationship between Al ages of chondrules and their mineralogical and chemical compositions

Previous studies have suggested the existence of a correlation between ²⁶Al relative crystallisation ages and mineralogical and bulk chemical compositions of ferromagnesian chondrules from the Bishunpur and Semarkona unequilibrated ordinary (0120 and 0150). However, because the precision in ²⁶Al ages was moderate, these correlations are questionable. Here, we report mineralogical and chemical compositions of 14 ferromagnesian chondrules from Semarkona for which precise ²⁶Al ages were previously obtained (Villeneuve et al., 2009). We find global correlation of ²⁶Al ages neither with bulk chemical composition of chondrules, nor with the different types of ferromagnesian chondrules, i.e. PO, POP and PP. This indicates that if some kind of correlations between chemical compositions of chondrules and their ages of formation exists, they do not exist at timescales that can be measured with the ²⁶Al-²⁶Mg systematics but presumably at much shorter timescales.     

南极格罗夫山碳质球粒陨石的研究与展望

本文报导了3块南极格罗夫山碳质球粒陨石(GRV 020025, 021579和022459)的矿物岩石学特征。3块碳质球粒陨石具有明显的非平衡陨石的特征,基质和球粒的模式比值分别为2.8 (GRV 020025)、1.2 (GRV 021579)和1 (GRV 022459)。GRV 022459具有明显较其他两块陨石大的球粒直径(0.6-2.0 mm)。在3块碳质球粒陨石中,一共发现了27个富Ca, Al包体,除了在GRV 020025和021579中各发现了一个富尖晶石球粒状包体,其余25个包体均属于A型(似A型)和富尖晶石-辉石型包体。大部分包体发生了强烈的蚀变,GRV 020025中的包体出现了大量层状硅酸盐,GRV 022459中的尖晶石具有富FeO的特征。最后,对南极碳质球粒陨石的下一步研究工作进行了展望。     

INTERPLANETARY DUST AND CARBONACEOUS METEORITES: CONSTRAINTS ON POROSITY, MINERALOGY AND CHEMISTRY OF METEORS FROM RUBBLE-PILE PLANETESIMALS

This paper discusses measured textures, porosity, chemical compositions and the minerals of interplanetary dust and meteorites from primitive planetesimals and how this information can be used to explain some of the observed physical and chemical properties of meteor phenomena.