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Post-Hydration Thermal Metamorphism of Carbonaceous Chondrites*

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Post-Hydration Thermal Metamorphism of Carbonaceous Chondrites* 260 Journal of Mineralogical and PetrologicalT. Nakamura Sciences, Volume 100, page 260─ 272, 2005 Post－hydration thermal metamorphism of carbonaceous chondrites 261 REVIEW Post-hydration thermal metamorphism of carbonaceous chondrites* Tomoki NAKAMURA Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812－8581, Japan The presence of abundant phyllosilicates in many carbonaceous chondrites indicates a prevailing activity of low－temperature aqueous alteration in primitive asteroids. However, among the hydrous carbonaceous chon- drites known, more than 20 samples show evidence of having been heated at elevated temperatures with corre- sponding phyllosilicate dehydration. The mineralogical features of dehydration suggest that the heating oc- curred in situ in meteorites, which demonstrates that there are some hydrated asteroids that have been heated at a certain period after aqueous alteration. Recent studies have uncovered details of heating and dehydration processes in hydrous carbonaceous chondrites: step－by－step changes in mineralogy, trace element chemistry, carbonaceous materials, and reflectance spectra have been clarified. Based on data from synchrotron X－ray dif- fraction analysis of the matrix, heated hydrous carbonaceous chondrites have been classified as Stages I－IV, with the temperature of heating increasing from I to IV. In spite of recent progress, heat sources are poorly defined, mainly due to a lack of chronological information on the timing of the heating, and therefore more data are needed to fully clarify the thermal metamorphism of hydrous carbonaceous chondrites. Keywords: Thermal metamorphism, Hydrous carbonaceous chondrites, Serpentine, Dehydration INTRODUCTION sky, 1984; Kozerenko et al., 1996; Browning et al., 1996). The radiogenic 53Cr concentration in some hydrous carbo- Carbonaceous chondrites are the most pristine solar－sys- naceous chondrites indicates that alteration occurred very tem material in terms of elemental abundance, and there- early until 20 Ma after the formation of Ca－ and Al－rich fore they are an important source of information for the inclusions (CAIs), which are the oldest materials in the processes occurring in the early solar system. Many car- solar system (e.g., Hutcheon et al. 1999). bonaceous chondrites are regarded as having been essen- In the early 1980s, three unusual Antarctic carbona- tially unaffected by post－accretionary, parent－body pro- ceous chondrites were found: Yamato (Y−) 82162, Y− cesses, except for extensive hydration at low temperatures, 86720, and Belgica (B−) 7904. They show mineralogies i.e., aqueous alteration. These alteration reactions replaced similar to both CM− and/or CI−type carbonaceous chon- anhydrous phases with newly formed phyllosilicates, and drites, but all three meteorites had been heated and dehy- the degree of the replacement varies between meteorites drated. A research consortium was organized, and many (e.g., Zolensky and McSween, 1988; Brearley and Jones, groups in the fields of mineralogy, petrology, inorganic 1998). Phyllosilicates are the dominant mineral in CI－ and organic chemistry, and noble gases carried out inten- and CM－type carbonaceous chondrites (e.g., Barber, sive work for five years, and the results are summarized 1981). The presence of a unique hydrous sulfide, tochili- in Ikeda (1992). In addition to the three meteorites, nite, (e.g., Tomeoka and Buseck, 1983, 1985; MacKinnon another thermally metamorphosed CM−type chondrite, Y− and Zolensky, 1984) in many CM chondrites suggests that 793321, has been found (Akai, 1988). The mineralogy aqueous alteration took place in extremely reduced condi- and the petrology of these four meteorites indicate that tions (Eh < 0 V), at low temperatures (< 120 °C), and in they had been heated in their parent bodies after aqueous variable pH (> 7) and pS2− (< −3) conditions (e.g., Zolen- alteration (Akai, 1988; Tomeoka et al., 1989a, 1989b; Tomeoka et al. 1990a, 1990b; Akai 1990; Bischoff and * Invited review based on the lecture at the 2004 Symposium of the Mineralogical Society of Japan “The Frontiers of Science for Metzler, 1991; Ikeda, 1991). These findings changed the Primitive Solar System Materials: The Role of Mineral Science”. view that hydrous carbonaceous chondrites had not expe- T. Nakamura, [email protected]－u.ac.jp Corresponding author rienced extensive thermal metamorphism. This work pres- 260 T. Nakamura Post－hydration thermal metamorphism of carbonaceous chondrites 261 ents a summary of the progress in studies on heated heated), Stage III (moderately heated), and Stage IV hydrous carbonaceous chondrites, with the main focus (strongly heated). This classification was mainly based on being on recent findings occurring after work of the above the characteristics of the X－ray diffraction patterns of the consortium over the period 1987−1992. matrix, and therefore, classification of the samples that had no diffraction data, such as Y－82162, was less cer- SAMPLES AND EXPERIMENTS tain. Among the heated hydrous carbonaceous chondrites, the CM－type was the most dominant, and this was pres- For this review, some CM carbonaceous chondrites, ent in all the heating stages from Stages I to IV, whereas which had already been characterized as heated speci- other groups, such as CI and CR, were rare. Thus, the CM mens in previous studies, were investigated using syn- matrix mineralogy was used as a reference to define the chrotron X－ray diffraction and scanning electron micros- scheme of the heating stages. copy. Textural details were characterized using a scanning electron microscope equipped with an energy dispersive Stage I spectrometer (SEM/EDS JEOL 5800), operating at an electron－accelerating voltage of 20 keV and an electron－ Samples classified as Stage I were those that showed no beam current of 0.5 nA. Based on the textural and compo- heat−induced mineralogical changes in their X－ray diffrac- sitional characterization obtained from optical and elec- tion patterns, because they were heated at very low tem- tron microscopy, several small particles, approximately peratures. The effect of heating was only identified using 100 μm in size, were separated from the matrix of each other characterization methods, such as reflectance spec- CM chondrite sample, using an edged tool, for X－ray dif- troscopy and transmission electron microscopy. Figure 1 fraction measurements. The individual small particles shows synchrotron X－ray diffraction patterns of unheated were mounted on a thin 5 μm diameter glass fiber, and (Fig. 1a, b), and very weakly heated (Fig. 1c, d) matrices exposed to synchrotron X－rays in a Gandolfi camera that of CM carbonaceous chondrites. The latter two samples allowed powder diffraction patterns to be obtained from a were classified as Stage I (Table 1). The matrices of all single particle. Synchrotron radiation X－rays from Beam four samples in Figure 1 contained serpentine as the dom- line 3A of the Photon Factory at the Institute of Materials inant phase, which was verified by the presence of a Science, High Energy Accelerator Research Organization strong basal 001 reflection at 7.1－7.2 Å (2θ = 18° in Fig. (Tsukuba, Japan) were utilized in this study. The X－rays 1), an 002 reflection at 3.6 Å (2θ = 35° in Fig. 1), and were monochromated to 2.161 ± 0.001 Å. The ultrahigh prism reflections occurring above θ2 = 27°. Prism reflec- intensity and well－monochromated synchrotron X－rays tions are peculiar to phyllosilicates with heavy stacking allowed clear X－ray powder diffraction patterns from disorder along their c−axis. The major polytype of the ser- each small particle to be collected, with a short exposure pentine in all the matrices was probably lizardite, which time of approximately 30 min (Nakamura et al., 2001, was identified by the presence of strong 11l reflections: 2003). 111 at 2.51 Å (2θ = 51° in Fig. 1), 112 at 2.15 Å (2θ = 60° in Fig. 1), 113 at 1.79 Å, and 114 at 1.50 Å (Wicks CLASSIFICATION OF HEATING OF STAGES I－IV and O’Hanley, 1991). The small 001 spacing of lizardite BASED ON MATRIX MINERALOGY (< 7.2 Å) suggested that some Si4+ in the tetrahedral sites had been substituted by Fe3+ to form a lizardite−cronst- Table 1 lists 21 chondrite samples that were identified as edtite solid solution (Nakamura and Nakamuta, 1996). being heated hydrous carbonaceous chondrites from their Lizardite and cronstedtite have similar structures, and thus constituent minerals, volatile element concentrations, and form a solid solution, where Fe3+ occupies both the tetra- reflectance spectra. The intensity of heating varied be- hedral and octahedral sites in cronstedtite (Wicks and O’ tween the samples: some meteorites had been heated Hanley, 1991). strongly and showed complete dehydration, while others Another hydrous phase present was tochilinite, had been heated weakly and only showed traces of devol- which was detected in Y－791198 (Fig. 1a), Murchison atilization. Mineralogically, phase changes due to heating (Fig. 1b), and A－881458 (Fig. 1d). Tochilinite consists of are most easily recognized in the matrix, because the a 1:1 stacking of brucite (Mg,Fe)(OH)2 and mackinawite matrix is an assemblage of heat－sensitive hydrous phases, FeS layers. The strong 002 reflection at 5.4 Å relative to and thus provides critical evidence of any temperature the 001 reflection at 10.8 Å indicated a dominant occu- rises. Based on the matrix mineralogy (Table 1), the pancy of Fe2+ in the brucite layer (Nakamura and Naka- heated hydrous carbonaceous chondrites were classified muta, 1996). A mixed layer mineral of tochilinite (inter- into: Stage I (very weakly heated), Stage II (weakly layer spacing = 10.8 Å) and serpentine (interlayer spacing 262 T. Nakamura Post－hydration thermal metamorphism of carbonaceous chondrites 263 Table 1. List of heated hydrous * Heating stage represents the degree of heating in the order: I < II < III < IV. Stages of samples whose X－ray diffraction is not avail- able are estimated from other properties. $ ○ in columns from reflectance spectra to microtextures means that analysis data are available. # Abbreviation of synchrotron X－ray diffraction.
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