氏 名 カオ
カ オドン
ド ンヴ
ヴ学 位 の 種 類 博士（理学）
学 位 記 番 号 理工博第
220号 学位授与の日付 平成
29年
3月
2日
課程・論文の別 学位規則第４条第
2項該当
学 位 論 文 題 名
Cosmochemical study of carbonaceous chondrites炭素質コンドライトの宇宙化学的研究（英文）
論 文 審 査 委 員 主査 教 授 海老原 充 委員 教 授 竹川 暢之 委員 准教授 大浦 泰嗣
委員
准教授 山口亮
(総合研究大学院大学
)【論文の内容の要旨】
Chemical compositions of meteorites vary depending on their formation history. These variations result from condensation processes, and secondary processes such as aqueous alteration and thermal metamorphism on their parent bodies etc. Many of these processes are considered to take place in accordance to the region of the protosolar nebula in which meteorite parent bodies were formed [1].
Carbonaceous chondrites are rare types of meteorites that comprise less than 5% of the meteorite falls. These meteorites display a wide range of petrographical, chemical and O-isotopic features, and are classified in eight main groups (CI, CM, CO, CV, CK, CR, CH and CB), and C-ungrouped derived from very diverse asteroids. The CI chondrite group is small.
Chemical composition of CI chondrite is similar to the Sun for most elements. CI chondrites are considered as the most primitive materials available for solar system. Therefore, they are used as reference materials in cosmochemistry. CI chondrite comprises only five falls and possibly a few finds from Antarctica [2]. It was reported that Antarctic CI chondrites are petrologically different from CI chondrites falls such as Ivuna, Orgueil and Alais [3]. These Antarctic CI chondrites were metamorphosed. Therefore, Antarctic CI chondrites could have been derived from a parent body different from that of CI chondrite falls. Even with the importance of CI chondrites, chemical compositions of Antarctic CI chondrites have not been reported.
The CM chondrites were considered to have pristine chemical compositions. This point of view was changed in the 1980’s because of the finding of the some unusual CM chondrites (e.g., B-7904). These unusual CM chondrites were metamorphosed on the their parent body after aqueous alteration, and they have petrological and mineralogical characteristics similar to those of CI and CM chondrites. Such unusual meteorites have been studied in the view of petrology and mineralogy [4-6] while their chemical characteristics are less known. The understanding of metamorphism on the CM parent body based on chemical compositions is limited.
Within the scope of this study, the chemical compositions, especially rare earth elements (REE), Th and U, of Antarctica carbonaceous chondrites were determined in order to unravel their formation history. On the other hand, a number of major and minor elements were also analyzed to support for further discussions. For these objectives, the abundances of fifteen elements (Mg, Al, P, Ca, Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sr and Ba) by inductively coupled atomic emission spectrometry (ICP-AES), and sixteen elements (REE, Th and U) by inductively coupled plasma mass spectrometry (ICP-MS) for fifteen Antarctica carbonaceous chondrites are determined.
Experimental
A total of 54 carbonaceous chondrite samples were analyzed. In addition to these samples, an ordinary chondrite, Saratov, was analyzed as a control sample. Each sample weighing around 15 mg was placed into a Teflon beaker with 149Sm-enriched spike, and digested by heating on a hot plate with mixture of 0.150 ml of HF, 0.075 ml of HNO3 and 0.075 ml of HClO4. A procedural blank sample was also prepared by using the same method as those of samples. In ICP-MS, dilution factors of 2,000 were applied to determine REE, Th and U, and In, Tl and Bi were used as internal standards. Samarium abundances were determined by both isotope dilution method using an enriched isotope of 149Sm and external calibration method, whereas the other REE, Th and U were obtained by external calibration method. Chemical recoveries were calculated from Sm concentrations obtained from isotope dilution and external calibration methods. These values were applied to all elements (except Sm) for the correction of the sample loss during the preparation of sample solutions assayed to ICP-AES and ICP-MS.
Fifteen major and minor elements were determined by ICP-AES with different dilution factors.
Magnesium, Al, Ca, Ti, Mn, Fe and Ni were measured with dilution factors of 6,000 and 60,000, while P, Sc, V, Co, Cu, Zn, Sr and Ba were measured with dilution factor of 450 and matrix matching were applied. Lutetium and Be were used as internal standards in ICP-AES. The
Allende meteorite was used as the main control material for all experiments, whereas Saratov and Murchison meteorites were also analyzed for internal quality control.
Results and discussion
The accuracy of our data for most elements are estimated to be less than 5% except for Cu (7%) and Ba (6%) by repeating analyses of Allende. The relative standard deviations (RSD) for 5 individual experiments are less than 5% except for U (6.4%). Thirty-one elements in four Antarctica meteorites could be determined by ICP-AES and ICP-MS with high accuracy and precision for further discussions.
Kallemeyn and Wasson [7] suggested that the ratios of lithophile and/or siderophile to moderately volatile and/or volatile elements such as Al/Mg, Ca/Mg, Fe/Mn, Fe/Mg and Zn/Mn ratios are able to classify carbonaceous chondrites into some subgroups. As Y 980115 has similar these elemental ratios to those of CI falls, this meteorite is chemically classified into CI chondrite. Our chemical observations are consistent with those from mineralogical and petrological studies [e.g., 3]. In comparison of chemical compositions of Y-980115 and other CI fall (Alais, Ivuna, Orgueil [7-9]), the chemical composition of Y-980115 for most elements lied in the range of other CI chondrites within errors with some minor differences. Although meteorites collected from Antarctica suffered from terrestrial weathering [10], chemical compositions of Y-980115 for major and minor elements are less affected by terrestrial weathering. In general, Y-980115 has similar REE, Th and U abundances to those for other CI chondrites. Nevertheless, it is noticed that there are slight differences between Y-980115 and other CI chondrites. Y-980115 is slightly depleted in light REE (LREE) relative to heavy REE (HREE). Y-980115 has low ratios of (La/Lu)CI (0.947) and (Lu/Yb)CI (0.945), whereas (La/Lu)CI
and (Lu/Yb)CI ratios for other CI chondrites are in range of 0.981-1.07 and 0.968-1.06, respectively. Y-980115 has no Ce and Tm anomalies ((Ce/Ce*)CI = 1.00 and (Tm/Tm*)CI = 0.987), while all other CI chondrites have negative Ce anomaly ((Ce/Ce*)CI < 1) and positive Tm anomaly ((Tm/Tm*)CI > 1). It is well known that negative Ce and positive Tm anomalies are characteristics of REE abundances pattern of type II CAI (Mason and Taylor, [11]). Probably, Y-980115 contains less amount of early condensates than those in other CI chondrites. It is concluded that Y-980115 and other CI chondrites have different formation history and/or parent body.
Chemical compositions of B-7904, Y-793321 and Y-86720 are compared with those for Murchison. Based on mineralogical study, Nakamura [6] showed the degree of heating for several carbonaceous chondrites including B-7904, Y-793321 and Y-86720. Y-793321 has experienced the thermal metamorphism at the stage II (300oC – 500oC), while B-7904 and Y-86720 have undergone the thermal metamorphism at the stage IV (> 750oC). Even though
Y-793321 was metamorphosed, there is no any noticeable difference between Y-793321 and Murchison. Although B-7904 and Y-86720 have similar chemical compositions to those for other CM chondrites, their Zn abundances are strongly depleted. Considering that these meteorites were strongly metamorphosed, these depletions of Zn may be due to thermal metamorphism on its parent body. REE abundances for B-7904 are enriched compared with those for other CM chondrites. These enrichments will be discussed below. It is recognized that B-7904 has a high (~1.4 × CM) and fractionated REE, Th and U abundances pattern and comparable to those of Allende. Negative Ce and Eu anomalies, and a positive Tm anomaly along with enrichment in LREE and depletion in HREE are the main features of REE abundances pattern in Allende, implying that B-7904 contains amount of early condensed material such as type II CAI. Although having undergone the different thermal metamorphism histories [6], Y-793321 and Y-86720 have the similar REE, Th and U abundances patterns to those for other CM chondrites, suggesting that REE, Th and U abundances are not affected by thermal metamorphism.
Summary
Chemical compositions of Y-980115 for major and minor elements are consistent with those for other CI-chondrites, implying that Y-980115 is classified into CI-chondrite and terrestrial weathering on Antarctica was not significant for this meteorite. Although REE abundances of Y-980115 are within the range of other CI-chondrites, LREE is slightly depleted relative to HREE. No Ce and Tm anomalies were detected in Y-980115, while other CI-chondrites have negative Ce and positive Tm anomalies. Therefore, amount of early condensed materials in Y-980115 is less than those in other CI-chondrites. It is suggested that Y-980115 and other CI-chondrite falls have different formation history and/or parent body.
The three Antarctica CM meteorites, B-7904, Y-793321 and Y-86720, are classified into CM chondrite based on Fe/Mn and Fe/Mg ratios and have similar chemical compositions to those for CM such as Murchison. However, B-7904 and Y-86720, which were strongly heated on their parent body [5,6], have lower Zn abundances than those for other CM chondrites. Such a difference is due to metamorphism on their parent body. Y-793321 and Y-86720 have similar REE abundances patterns to those for other CM chondrites. REE, Th and U are not affected by metamorphism. In contrast, B-7904 has about 1.4 times higher REE abundances than those for other CM chondrites, and similar REE abundances pattern to those for Allende, implying that this meteorite contains early condensed material such as type II CAI.
References: [1] McSween and Huss (2010) Cambridge University press, 543; [2] Barrat et al.
(2012) GCA, 83, 79-82; [3] King et al. (2015) GCA, 165, 148-160; [4] Ivanova et al. (2010)
MAPS, 45, 1108-1123; [5] Nakato et al. (2008) EPS, 60, 855-864; [6] Nakamura (2005) MAPS, 100, 260-272; [7] Kallemeyn and Wasson (1981) GCA, 45, 1217-1230; [8] Nakamura (1974) GCA, 38, 757-775; [9] Barrat et al. (2012) GCA, 83, 79-92; [10] Huber et al. (2006) GCA, 70, 4019-4037; [11] Mason and Taylor (1982) Smithsonian Contributions to the Earth Sciences, 25.
