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Mineralogical and Geochemical Study of Zhaoping, Xifu and Hami Iron Meteorites

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Mineralogical and Geochemical Study of Zhaoping, Xifu and Hami Iron Meteorites CHINESE ASTRONOMY AND ASTROPHYSICS ELSEVIER Chinese Astronomy and Astrophysics 32 (2008) 293–305 Mineralogical and Geochemical Study of Zhaoping, Xifu and Hami Iron Meteorites LIN Su1,2 HSU Wei-biao1 1Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2Graduate School of Chinese Academy of Sciences, Beijing 100039 Abstract The mineralogy and bulk chemical compositions of three iron mete- orites (Zhaoping, Xifu and Hami) recently found in China are reported here and are classified on the basis of their bulk chemical compositions. Zhaoping contains 93.4 mg/g Ni, 85.9μg/g Ga, 418 μg/g Ge, 5.24 mg/g Co, 1.94 μg/g Ir, 0.774 μg/g W, and 1.62 μg/g Au and belongs to the low-Ni, low-Au subgroup of IAB. It is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite and cohenite. The cohenite has entirely decomposed to graphite and low-Ni kamacite in our samples. Zhaoping contains some inclusions of Mn-free sarcopside which were rarely reported in IAB iron meteorites. Xifu has 74.1 mg/g Ni, 58.8gμg/g Ga, 150μg/g Ge, and 0.913μg/g W. Xifu is a member of group IIICD iron me- teorite. Like most of IIICD irons, Xifu is a coarsest octahedrite with kamacite bandwidth larger than 3mm, and contains kamacite, taenite and schreibersite. Carbides and graphite are not found in the sample because of its being het- erogeneous. Hami has 106 mg/g Ni, 5.36 mg/g Co and 0.922μg/g Ir. We did not obtain the Ga and Ge contents in Hami because of their low concentra- tions and the limited precision of the INAA technique. Hami is an unclassified iron meteorite on the basis of the contents of other trace elements, structure and mineralogy. On mineralogy and structure, Hami resembles Rafruti, another unclassified iron meteorite. Key words: iron meterites—physical data and processes—astrochemistry † Supported by National Natural Science Foundation for Distinguished Young Scholars, “One-Hundred- Talent Program” of Chinese Academy of Sciences, and Minor Planet Foundation of China Received 2006–07–04; revised version 2006–10–12 A translation of Acta Astron. Sin. Vol. 48, No. 3, pp. 328–342, 2007 [email protected] 0275-1062/08/$-see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2008.07.003 294 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 1. INTRODUCTION Iron meteorites consist mainly of high-density Fe-Ni metal (Ni > 4%) alloys (kamacite and taenite) with minor amounts of sulfides, phosphides, carbides, silicate and phosphate inclu- sions. The study of iron meteorites not only helps us to understand the composition and structure of the earth’s interior, but also increases our knowledge of melting and differentia- tion processes on planetary bodies. In the early 19th century, scientists began to analyze the chemical composition of iron meteorites [1], and found that some meteorites display similar structures and cooling rates and that their trace elements show special fractional crystalliza- tion trends and so on. In order to study iron meteorites systematically and comparatively, scientists made classification. There are three classification schemes for iron meteorites: structural, genetic and chemical classification[2]. The chemical classification is the most widely used one, which groups most iron meteorites into IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA and IVB group[3−11]. It is believed that iron meteorites of a given chemical group formed on a common parent body. Iron meteorites were thought to have formed by two mechanisms. 11 groups of iron are designated as magmatic groups, which formed by fractional crystallization of molten cores in asteroids and are possibly sim- ilar to the core of the earth. Another three groups (IAB, IIICD and IIE) are considered to be nonmagmatic. They originated in small impact melt pools of asteroids[10,12−14].Some iron meteorites structurally and chemically do not fit into any of the above groups, and are defined as ungrouped iron meteorites. Zhaoping, Xifu and Hami iron meteorites are objects recently found in China. In this paper we systematically analyze the petrography, mineralogy and chemical compositions of the three iron meteorites, and make chemical classification. 2. SAMPLES AND ANALYTICAL METHODS Zhaoping, Xifu and Hami iron meteorites have experienced prolonged terrestrial weathering processes. Their fusion crusts were weathered in various degrees. The most significantly corroded iron meteorite is Xifu which was found in Pine Village of Xifu town, Qingdao Municipality (36◦ 18 N, 120◦ 29 E). On the morning of May 18, 2004, Xifu was dug out of the ground from 3 meters below during some construction work. The surface is covered with irregular air vents. Its fusion crust and regmaglypts are not recognized and in some areas the surface had been heavily weathered . It is roughly cone-shaped (130 cm high) and weighs about 3 tons. The perimeter of the largest section is 260 cm. Xifu local government holds the main mass. Zhaoping iron mete- orite was discovered in Zhaoping County, Guangdong province (24◦ 14 N, 111◦ 11 E). The egg-shaped mass weighs about 2 tons and is about 120 cm-long, with a maximum diameter of 65 cm. The main mass still resides in Huangyao Village of Zhaoping County. Hami iron meteorite is preserved relatively fresh. It was found in the East-Station of Thirteen-Houses of Hami, Xinjiang province (43◦ 45 N, 92◦ 55 E). The surface of Hami displays well-preserved fusion crust and “thumb prints” . It weighs about 160 kg. Its height is about 30cm and its underside has a maximum length of 42 cm. A private collector holds the main mass. We prepared 5 thick sections (two sections of Zhaoping, two of Hami and one of Xifu) at the Laboratory for Astrochemistry and Planetary Sciences at Purple Mountain Observa- LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 295 Fig. 1 The Xifu iron meteorite. Its fusion crust and regmaglypts are not recognized. tory, Chinese Academy Sciences. These sections were first etched with a 10% nital solution, and then were observed under reflected light with a Nikon E400 POL optical microscope. Backscattered-electron imaging and quantitative analyses of some small mineral grains were performed with LEO 1530VP field emission scanning electron microscope at Nanjing Insti- tute of Geology and Paleontology. The chemical compositions of most of the minerals were measured with a JEOL JXA-8800M electron microprobe at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The accelerating voltage was 15 keV and the beam current was 15 nA. Both synthetic (NBS) and natural mineral standards were used, and matrix collections were made based on ZAF procedures. The Raman spectra of the minerals were measured with a Renishaw RM2000 micro- Raman spectrometer equipped with a CCD detector in the Department of Earth Sciences of Nanjing University. A microscope was used to focus the excitation laser beam (514.5 nm lines of a Princeton Instruments Ar+ laser). The scanning time was 30 seconds. Twelve elements were determined by instrumental neutron-activation analysis (INAA). Our neutron-activation procedure is similar to that used by Tian et al. [15].Samples and standards were all sawn into roughly 1×6×6 mm pieces and two pieces of each were prepared, one for short and middle irradiations, the other for long irradiation. Table1 gives the bulk chemical composition of Zagora and Caddo County. All the irradiations were conducted at the heavy water research reactor (HWRR) in China Institute of Atomic Energy. Thermal neutron fluxes were 1×1013n.cm−2.s−1 for the short and middle irradiations and 5×1013n.cm−2.s−1 for the long irradiation. The time of the short irradiation was 20 s; the samples were counted two times, at about 7 and 30 minutes after the irradiation; elements of Au, Ca, Cu and Ni were determined in this process. The duration of the middle irradiation was 1000s and that of the long irradiation was 30 minutes; the samples were counted at two times, at about 1 and 3 days after the middle irradiation (As, Au, Co, Cr, Cu, Fe, Ga, Ge, Ir, Re and W) and at 4 and 15 days after the long irradiation (As, Co, Fe and Ni). Table 1 The bulk chemical compositions of standard samples Cr Co Ni Cu Ga Ge As W Re Ir Au μg/g mg/g mg/g μg/g μg/g μg/g μg/g μg/g ng/g μg/g μg/g Zagora Measured data < 8.3 4.84 nd 264 67.9 218 14.8 0.732 273 2.76 1.72 Literature data 9—146 4.84 253 67.7 239 15.1 0.910 264 2.77 1.61 Caddo County Measured data 44.1 4.95 93.2 nd 97.6 nd 14.2 0.956 nd 2.13 1.61 Literature data 29—343 4.93 93.2 68.7 14.9 1.11 2.55 1.608 296 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 Fig. 2 The Hami iron meteorite Literature data are from [23], and are mean values (underlined value is a range), nd means “not determined”. In our analysis, Zagora was used as the standard for the short and middle irradiations and Caddo County, for the long irradiation. All counts were carried out with an HPGe gamma-ray spectrometer (the resolution was 18Kev at 45% detection). Zr foil was used as the neutron flux ratio monitor and high - purity Fe wire, as comparator for K0-NAA.
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