第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日（木） 国立国語研究所 2階講

Fujiya W.

Hiyagon H.

Sano Y.

Sasaki Shogo

Takahata N.

National Institute of Polar Research Repository

Hibonite-bearing inclusions from Murchison (CM2) meteorite: A Mg isotopic study us-ing a NanoSIMS. Shogo Sasaki1, H. Hiyagon1, W. Fujiya1, N. Takahata2 and Y. Sano2,  1Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2Atmosphere and Ocean Research Institute, The University of Tokyo.   Introduction Hibonite (CaMgxTixAl12-2xO19) is one of the most refractory minerals [e.g., 1] and may record high temperature process(es) in the early solar sys-tem. Hibonite-bearing inclusions in CM chondrites are morphologically divided into several types, such as SHIBs (Spinel-HIBonite inclusions), PLACs (PLAty Crystals) and BAGs (Blue AGgregates) [2]. It is known that these morphological types are strongly correlated with their isotopic characteristics [2, 3]. For example, SHIBs tend to show resolvable excesses in 26Mg with the inferred initial 26Al/27Al ratios of ~4.5 x 10-5 (~canonical value [4]), while PLACs and BAGs generally lack resolvable excess 26Mg or even show small apparent deficits in 26Mg by 3-4 ‰ [5]. Also SHIBs tend to show no anomalies in Ca and Ti isotopes, while PLACs and BAGs tend to show isotopic anomalies (positive or negative) in 48Ca and 50Ti.  Lack of excess 26Mg in PLACs and BAGs suggests either they formed after 26Al decayed completely or they formed before 26Al was injected into the solar system from (a) nearby stellar source(s). The presence of Ca and Ti anomalies in PLACs and BAGs seems to support the latter interpretation [5]. If this is the case, various types of hibonite inclusions may represent different stages of early solar system evolution. Our goal is to understand the process(es) of isotopic homogenization in the early solar system.  Samples and analytical conditions First, about 10 grams of Murchison meteorite was disaggregated using the freeze-thaw method, then a size separation, a magnetic separation and a density separation (using methylene iodide: ~3.3 g/cm3) were applied. Candidates of hibonite-bearing inclusions (usually having light blue to blue colors) were hand-picked under an optical microscope from non-magnetic, dense fractions of the separated grains. After preliminary examinations of these grains with SEM-EDS, they were fixed on a glass slide with epoxy and were examined using an optical micro-scope. Finally the glass slide was polished so that surfaces of most of the grains were exposed together. About 30 hibonite-bearing inclusions were recovered in this study. So far, we analyzed two PLACs (MC026 and MC028), two SHIBs (MC042 and MC043), one “blue spinel” (MC003, spinel with some hibonite grains embedded in it [6]) and two grains with Fe-rich silicates (MC037 and MC040, composed of hibonite, spinel and Fe-silicates). Backscattered electron images of the selected grains are shown in Fig. 1. The Al-Mg isotopic analyses were performed using NanoSIMS in Atmosphere and Ocean Research Institute, The University of Tokyo. A primary beam of 16O‾ with a diameter of 3-5µm and an intensity of 30-200pA was used for the analyses. Positive ions of Mg isotopes, 24Mg+, 25Mg+ and 26Mg+, were detected using an electron multiplier (EM) (Tr4) by a peak jumping mode, and 27Al++ was detected simulta-neously with 24Mg+ using another EM (Tr2). In order to detect Al ions using an EM for high Al/Mg sam-ples, 27Al++ instead of 27Al+ was used in this study. Madagascar hibonite, placed and polished together with the Murchison hibonite grains on the same slide glass, was used as a terrestrial standard for Mg iso-topes and Al/Mg ratios. Terrestrial hibonite standard was analyzed repeatedly during the analysis period and all the hibonite inclusion data were normalized to the terrestrial standard data to correct for instru-mental mass fractionation of Mg isotopes and rela-tive sensitivity factor of Al++/Mg+ for hibonite. In order to precisely estimate excess 26Mg, a correction for mass-dependent fractionation is essen-tial, especially for extremely mass-fractionated sam-ples. For this purpose, we adopted the formula rec-ommended by Davis et al. [7], that is,                                                      and similarly for      ,  and                                Their evaporation experiments on CAI-like melt show that Mg isotope data line along a straight line with a slope of 0.514 on a       vs       plot. This fractionation law may also be applied to our hibonite samples.  Results Preliminary data for Mg isotopes so far obtained are shown in a Δ26Mg vs 27Al/24Mg plot (Fig. 2) and a       vs       plot (Fig. 3). Figure 2 clearly shows that there are two distinct groups in the hibo-nite-bearing inclusions. One group is SHIBs (MC042 and MC043), which show resolvable excess 26Mg and the inferred 26Al/27Al ratio is consistent with the canonical ratio (~4.5 x 10-5). The other group in-cludes PLACs (MC026 and MC028), blue spinel (MC003) and two inclusions containing Fe-rich sili-cates (MC037 and MC040), which show Δ26Mg val-ues of almost zero or even negative (up to -3 to -4 ‰). These results are very consistent with the pre-vious studies (see e.g., [5]), but in our case, blue spi-nel and other two inclusions (MC003, MC037 and MC040) also belong to the latter group. It should be noted that the two inclusions containing Fe-rich sili-cates (MC037 and MC040) show extremely large mass fractionation in Mg isotopes even exceeding ~50 ‰/amu (Fig. 3). Blue spinel (MC003) also shows significant mass fractionation (~18 ‰/amu). Such large mass fractionation of Mg isotopes may be produced by significant evaporation of precursor melts and suggests some relations to so-called FUN inclusions [4].  An interesting observation is that PLACs may have negative Δ26Mg of -3 to -4 ‰. This is consistent with the results of [5]. This can be interpreted by the presence of deficit in 26Mg and/or excess in 25Mg in PLACs. However, because of the limited number of analyses so far conducted and because of relatively large uncertainties including large corrections for mass-fractionation, the results are not conclusive at present. Further studies are required to confirm these results. It is also essential to study other isotope sys-tems, such as O, Ca, Ti, Be, etc., and trace element studies as well, for better understanding the forma-tion processes of hibonite-bearing inclusions, and hopefully, isotopic homogenization processes in the early solar system.   References [1] Davis A. M. and Richter F. M. (2007) In Treatise on Geochemistry (2nd ed.; eds. H. D. Hol-land and K. K. Trekian), Elsevier, Oxford. [2] Ireland T. R., (1988) Geochim. Cosmochim. Acta 52, 2827-2837. [3] Ireland T. R. (1990) Geochim. Cos-mochim. Acta 54, 3219-3237. [4] MacPherson et al. (1995) Meteoritics 30, 365-386. [5] Liu M.-C. et al. (2009) Geochim. Cosmochim. Acta 73, 5051-5079. [6] Ireland et al. (1986) Geochim. Cosmochim. Acta 50, 1413-1421. [7] Davis A. M. et al. (2005) LPS XXXVI, Abstract #2334.     Fig. 1. Backscattered electron images of selected hibonite-bearing inclusions from Murchison: MC043 (SHIB), MC028 (PLAC) and MC003 (blue spinel).    Fig. 2. Excess 26Mg (Δ26Mg) vs 27Al/24Mg diagram for hibonite-bearing inclusions. SHIBs show resolv-able excesses in 26Mg, while PLACs, blue spinel and two inclusions with Fe-rich silicates show almost no, or even slightly negative, Δ26Mg.         Fig. 3. A 25Mg vs 26Mg plot. PLACs show ex-tremely large mass fractionation of up to ~50 ‰/amu, and blue spinel also shows relatively large mass frac-tionation (~18 ‰/amu).  50μmMC003hibonitespinel-10-50510150 50 100 150MC026MC042MC037MC028MC003MC043MC040SHIB27Al/24Mg-100102030405060-20 0 20 40 60 80 100 120MC026MC042MC037MC028MC003MC043MC040stdMg*=026Mg50μmMC028hibonite50μmMC043hibonitespinel

Hibonite-bearing inclusions from Murchison (CM2) meteorite: A Mg isotopic study using a NanoSIMS.

https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=13379&item_no=1&attribute_id=16&file_no=1

Hibonite-bearing inclusions from Murchison (CM2) meteorite: A Mg isotopic study using a NanoSIMS.

Abstract

Similar works

Full text

Available Versions

National Institute of Polar Research Repository