Ion microprobe Al Mg dating of single plagioclase grains in an Efremovka chondrule

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1 Geochemical Journal, Vol. 48, pp. 133 to 144, 2014 doi: /geochemj Ion microprobe Al Mg dating of single agioclase grains in an Efremovka chondrule YUJI SANO, 1 * MIO TAKADA, 1,2 NAOTO TAKAHATA, 1 WATARU FUJIYA 2 ** and NAOJI SUGIURA 2 1 Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba , Japan 2 Department of Earth and Planetary Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan (Received May 16, 2013; Accepted November 7, 2013) We report here technical details of 26 Al 26 Mg dating of single agioclase grains located in a chondrule of the Efremovka CV3 chondrite using a high spatial resolution secondary ion mass spectrometer (NanoSIMS). A ca. 500 pa mass filtered 16 O primary beam was used to sputter 5 7 µm diameter craters. Secondary positive beams were extracted for mass analysis. We detected 27 Al ++ (at mass 13.5), 24 Mg +, 25 Mg +, and 26 Mg + simultaneously under a static magnetic field for the first session. To obtain accurate Mg isotopic ratios but at the expense of a longer analytical time, we have also measured Mg isotopes ( 24 Mg, 25 Mg, 26 Mg) using a single detector with a magnet scanning mode. Apparent 26 Mg excesses were observed in small agioclase grains (#1 and #4) of the chondrule. A positive correlation between the Al/Mg ratio and the excesses 26 Mg engender a 26 Al 26 Mg isochron with an initial 26 Al/ 27 Al ratio of (3.3 ± 1.3) 10 6 for #1 and (1.7 ± 1.4) 10 6 for #4. In contrast, large agioclase grains (#2 and #3) show no resolvable excesses 26 Mg. The initial 26 Al/ 27 Al ratios of (0.13 ± 0.71) 10 6 (#2) and (0.12 ± 1.26) 10 6 (#3) are consistent with zero. A substantial difference exists among the initial 26 Al/ 27 Al ratios of individual grains in the same Efremovka chondrule, which may be attributable to parent body alteration of the meteorite. Careful treatment is necessary to calculate the formation age combining the data from different grains, even in a single chondrule. Keywords: Al Mg dating, chondrule, ion microprobe, Efremovka INTRODUCTION Formation interval between refractory inclusions (CAIs) and chondrules in primitive chondrites is an important issue to study the duration of high-temperature processes in the solar nebula. Efremovka (CV3) is a lowalteration CV chondrite (Kimura and Ikeda, 1997). Several chronological studies were made for its CAIs. Based on Pb Pb isochrons for the TIMS Pb isotopic analyses of acid-washed fractions from the Efremovka CAIs called E49 and E60, Amelin et al. (2002) reported one of the oldest ages identified in the solar system: ± 0.6 Ma. They also presented the initial 26 Al/ 27 Al ratio of (4.63 ± 0.44) 10 5, as deduced from excess 26 Mg in E60-CAI measured using a modified ion microprobe (IMS-3f; Cameca SAS). This value is consistent with the canonical value of representative of CAIs in carbonaceous chondrites (MacPherson et al., 1995; McKeegan and Davis, 2007). However, Goswami et al. *Corresponding author ( ysano@aori.u-tokyo.ac.jp) **Present address: Max Planck Institute for Chemistry, Hahn-Meitner- Weg 1, Mainz, Germany. Copyright 2014 by The Geochemical Society of Japan. (1994) found a much smaller initial 26 Al/ 27 Al ratio of (4.60 ± 1.10) 10 6 based on the anorthite grains in E44-CAI using a Cameca IMS-4f. Sugiura et al. (2001) measured Al Mg systematics of the Efremovka CAIs (E38, E44, E48, and E69) using a Cameca IMS-6f ion microprobe. No substantial correlation exists between Al/Mg ratios and Mg isotopic compositions in inclusions of E38, E44 and E69, suggesting a disturbed Al Mg system. In addition, no excess 26 Mg was detected in the E48 inclusion. Recently Wadhwa et al. (2009) reported a small 26 Al/ 27 Al ratio of (3.20 ± 0.35) 10 5 in diopside melilite agioclase grains in the E60-CAI, which is consistent with the value of (3.11 ± 0.33) 10 5 of the same CAI (Goswami et al., 1994). Consequently, the Al Mg systematics of anorthite and melilite in the Efremovka CAIs is comicated probably because of the partial reequilibration and disturbance to various degrees (Sugiura et al., 2001). In contrast to the 26 Al chronology of CAIs, that of chondrules in CV chondrites is not well documented in the literature (McKeegan and Davis, 2007; Sheng et al., 1991; Srinivasan et al., 1999). Until recently, there was a conference abstract reporting an Al Mg age of the Efremovka chondrule (Srinivasan et al., 2000). The initial 26 Al/ 27 Al ratio was Hutcheon et al. (2009) 133

2 (a) (b) Chondrule Grain#4 Grain#1 Grain#2 Grain#3 1 mm (c) Grain#4 Grain#1 altered vein Grain#3 Grain#2 Fig. 1. (a) Photograph of the thin section of the Efremovka CV3 chondrite. (b) Backscattered Electron Image (BEI) of the chondrule examined in this study. Locations of analyzed grains are shown as open squares. (c) Enlarged image of the analyzed area with altered vein. (d) (g) Analyzed pits on agioclase grains in the chondrule measured using the NanoSIMS. Spot numbers correspond to those in Table 1. Pl, Plagioclase;, high-ca Pyroxene; fa, Fayalite. reported four initial ratios ranging from to , which are comparable to the value of Srinivasan et al. (2000) within the experimental error. However all data are derived from one or two pairs of agioclase and pyroxene in the chondrules (see figures 6 and 7 of Hutcheon et al., 2009). Therefore more work on the Al Mg dating of the Efremovka chondrules should be done. 134 Y. Sano et al. To study the formation age of Efremovka chondrules, we have conducted Al Mg dating of a few agioclase grains with a high spatial resolution secondary ion mass spectrometer (NanoSIMS 50; Cameca SAS) installed at the Atmosphere and Ocean Research Institute, The University of Tokyo. The NanoSIMS has higher sensitivity at a smaller analytical spot than a conventional SIMS (e.g.,

3 (d) Grain# (e) Grain# Grain# (g) fa m Grain# fa m 10 m (f) fa m Fig. 1. (continued). Slodzian et al., 1991). It can measure several spots, even on a single grain of about 30 µm. Experimental details are given because this report is the first of such a study using a NanoSIMS. EXPERIMENTAL A thin section of Efremovka (Fig. 1a) was first examined using scanning electron microscope-energy dispersive spectroscopy (SEM-EDS; JSM-5310; JEOL Ltd.) after apying a carbon coating to dissipate the charge during analysis. We found agioclase grains in a chondrule and selected four grains for Al Mg isotope analyses. Chemical compositions of these grains were determined using EDS where we measured them at several positions in the individual grains (Supementary Table S1). Parts of some grains showed altered signature, which is discussed latter. The Mg concentrations are close to the detection limit (about 500 ppm) and are not well measured by EDS. For that reason, they are not shown in Table S1. We also found low Ca pyroxene grains for Mg isotope measurements in the same chondrule. The Efremovka section was set in a same holder together with standard agioclase (Miyake-jima) and olivine (San Carlos) grains. The sames were evacuated in the airlock system of the NanoSIMS to reduce water adsorbed onto the mount. The first session of analysis was conducted in January Using a critical illumination mode, a ca. 500 pa mass-filtered 16O primary beam was used in the case of agioclase grains to sputter a 5 7 µm diameter crater (see Fig. 1c). Then secondary positive beams were extracted for mass analysis using the Mattauch Herzog geometry of the NanoSIMS. Before the actual analysis, the same surface was sputtered with a rastered primary beam over 7 10 µm for 180 s to reduce the surface contaminant elements. We detected 27Al ++ (using a secondary electron multiier detector called EM#1) at mass 13.5, 24Mg+ (EM#2) Al Mg dating of Efremovka chondrule 135

4 Table 1. Al Mg data of Miyakejima standard and Efremovka chondrule for session one Spot 27 Al ++ / 24 Mg + 25 Mg + / 24 Mg + ±2σ 26 Mg + / 24 Mg + ±2σ Corrected Corrected δ 25 Mg ±2σ δ 26 Mg ±2σ 27 Al/ 24 Mg 26 Mgex ±2σ 25 Mg/ 24 Mg 26 Mg/ 24 Mg Miyake-jima standard Eflemovka chondrule: agioclase grain # at 24, 25 Mg + (EM#3) at 25 and 26 Mg + (EM#4) at 26 simultaneously under a static magnetic field for 10 s. This measurement was repeated 100 times so that the total counting time was 1000 s. Mass resolution of 3500 at 10% of the peak height was attained for separating 24 Mg + from 48 Ca ++ with adequate flat-topped peaks. Difference of sensitivity between Mg and Al was corrected using a standard agioclase (Miyake-jima) whose Al/Mg ratio of 356 ± 7 was determined from repeated measurements conducted with an electron probe microanalyzer (EPMA). For olivine and low-ca pyroxene sames, we first used a 1 pa primary and detected Mg isotopes using the multicollector mode. However we observed undesirable isotope fractionations (Supementary Materials). To avoid the effect, we used a strong primary ion beam of 10 na with a 15 µm spot diameter and detected Mg isotopes using a single Faraday cup with a magnetic scanning mode where the secondary 24 Mg ion beam was 40 pa. Total analysis times for 24 Mg, 25 Mg and 26 Mg were 50 s, 100 s, and 100 s, respectively. Abundance of 27 Al in olivine and low Ca pyroxene sames was less than the detection limit of the Faraday cup. The second and third sessions of analyses were conducted in November 2009 and November 2011, respectively. To obtain accurate Mg isotopic ratios at that time but at the expense of a deeper and larger pit by a critical focusing mode with a radial gradient of primary ion density, we measured Mg isotopes ( 24 Mg, 25 Mg, 26 Mg) using an EM#2 detector and a magnet scanning mode together with the multi-collector mode for 27 Al ++, 24 Mg +, 25 Mg +, and 26 Mg +. The total analysis time was 2500 s. The expected pit might be 2.5 times deeper than that of the multicollector mode assuming a constant digging speed. After SIMS analysis, the same section was examined again using SEM-EDS. RESULTS The chondrule studied here is an Al-rich chondrule with the size of 1200 µm 1800 µm (Kita and Ushikubo, 2012) with an oval outline (see Fig. 1b) and a layered structure. The inner region consists of euhedral agioclase and high-ca pyroxene (En 59 Wo 40 ) grains, whereas the outer region shows a porphyritic pyroxene (PP) structure (En 98 Wo 1 ). The lath agioclase grains are at most 30 µm 100 µm. According to the EBSD analysis, we confirmed that the agioclase grains were not transformed into maskelynite during possible shock events in the CV parent body. Iron sulfide (FeS) grains are included in the outer PP region. Figures 1c f present backscattered electron imaging (BEI) depictions of agioclase grains analyzed using SEM-EDS. Finegrained mesostasis and/or glass together with coarse grain high-ca pyroxene surround the target agioclase grains. 136 Y. Sano et al.

5 Table 2. Mg isotope data of San Carlos and Efremovka olivine for session one Spot 25 Mg + / 24 Mg + ±2σ 26 Mg + / 24 Mg + ±2σ Corrected Corrected δ 25 Mg ±2σ δ 26 Mg ±2σ 26 Mg ex ±2σ 25 Mg/ 24 Mg 26 Mg/ 24 Mg San Carlos olivine: Multi-collector San Carlos olivine: single-faraday Eflemovka low Ca pyroxene: single-faraday To calculate the initial 26 Al/ 27 Al ratio using the Al Mg isochron method, a precise measurement of Al/Mg ratio is required. For agioclase analysis, the monovalent aluminum ion ( 27 Al + ) beam intensity is too strong to be measured using an EM detector. Instead we measured the divalent ion ( 27 Al ++ ). There is no large difference between energy distributions of 27 Al + and 27 Al ++ ions. Production rate of 27 Al + / 27 Al ++ ratio was 3120 calculated using the first measurements of the day using a magnet scanning mode. The rate usually remains constant during the day. The 27 Al/ 24 Mg ratio was obtained using the following equation. 27 Al/ 24 Mg = A ( 27 Al ++ / 24 Mg + ) measured. Therein, ( 27 Al ++ / 24 Mg + ) measured and A respectively denote the 27 Al ++ / 24 Mg + ratio obtained by SIMS and a constant determined by experimentation. Differences of counting efficiency of each detector for Mg isotopes (EM#2 #4) were also measured at the first day of the session by scanning the 24 Mg signal sequentially over the EMs. The values of EM#3/EM#2 and EM#4/EM#2 were determined, respectively, as ± and ± Table 1 presents the Al Mg data of the Miyake-jima standard and Efremovka chondrule for the first session. The constant A is 4570 ± 380 (2σ), calculated using the average of 27 Al ++ / 24 Mg + ratios of the standard and the estimated 27 Al/ 24 Mg ratio of 451. Considering the 27 Al + / 27 Al ++ ratio of 3120, one might calculate the relative secondary ion yield of 1.46 for the Al/Mg ratio, which is larger than 1.2 for a synthetic glass standard (Kunihiro et al., 2004) and ± for anorthite glass standard (Kita et al., 2012). The 25 Mg/ 24 Mg and 26 Mg/ 24 Mg ratios corrected for the difference of the counting efficiency in the multicollector mode are also shown in Table 1. The standard error of Al/Mg measurement is estimated about 3% at 2σ by repeated analyses. Error assigned to the Mg isotopic ratio is calculated using the reproducibility of the ratio of 100 cycles, which is consistent with the statistical error estimated by total ion counts of about in minor isotopes ( 25 Mg and 26 Mg). The δ 25 Mg values of Miyakejima agioclase ( 25 Mg/ 24 Mg ratios in delta notation; δ 25 Mg = {( 25 Mg/ 24 Mg) meas / } 1000) were 24.9 to 29.1, whereas the δ 26 Mg values ( 26 Mg/ 24 Mg ratios in delta notation) were 50.0 to 58.4, suggesting a typical mass-dependent fractionation with a slope of 1/2 (Supementary Fig. S1). The absolute fractionation value is much larger than 7 /amu for reported Mg isotopes measured using an ion microprobe (IMS-6f; Cameca SAS) in a single collector mode (Sugiura et al., 2004). No exanation is sufficient for such a large Mg isotope variation in the terrestrial natural process. These isotope fractionations might be attributable to the instrumental mass bias of the NanoSIMS using the multi-collector mode. Dead time correction of the individual ion counters was set to 44 ns following the detector characteristics, confirmed by independent experimentation using Ti isotopes. Because the count rate of the major isotope ( 24 Mg) is as low as cps, insufficient dead time correction is unlikely to exain the iso- Al Mg dating of Efremovka chondrule 137

6 Table 3. Al Mg data of Miyake-jima standard and Efremovka chondrule for the second session Spot 27 Al ++ / 24 Mg + 25 Mg + / 24 Mg + ±2σ 26 Mg + / 24 Mg + ±2σ δ 25 Mg ±2σ δ 26 Mg ±2σ 27 Al/ 24 Mg 26 Mg ex ±2σ Miyake-jima standard Eflemovka chondrule: agioclase grain # Eflemovka chondrule: agioclase grain # topic fractionation. Such a large instrumental mass discrimination may be due to the trajectory effect (Supementary Materials). Five spots in a single agioclase grain (#1) in the Efremovka chondrule (Table 1) showed that the δ 25 Mg and δ 26 Mg values respectively change markedly from 38.9 to 43.3, and from 70.2 to Mass fractionations of δ 25 Mg values are larger than those of standard agioclase, which might be again problem of the trajectory effect (Supementary Materials). It is possible to correct the observed δ 26 Mg value based on the δ 25 Mg value as follows if we assume that the discrimination can work the same effect on the Mg isotope fractionation as the standard. 26 Mg ex = δ 26 Mg 2 δ 25 Mg. In that equation, 26 Mg ex denotes the excess 26 Mg. All 26 Mg ex values of the terrestrial standard are consistent with zero within experimental error margin. However, there is apparently excess 26 Mg in a few spots of agioclase grain #1 in the Efremovka chondrule (Fig. S1), which might be attributable to the extinct 26 Al. To check the absence of excess 26 Mg in Mg-rich and Al-poor sames as well as mass discrimination of the instrument, we measured Mg isotopes of terrestrial olivine based on the multi-collector mode in the first session. Table 2 presents the Mg isotope data of San Carlos olivine. Because Mg is the major chemical component, we used a weak primary current of 1 pa oxygen beam, with spot diameter of less than 1 µm. We have a sufficient counting rate of cps for 24 Mg + ions. The δ 25 Mg values vary from 23.3 to 26.5, consistent with those of Miyake-jima agioclase. The mass discrimination might result from the trajectory effect (Supementary Materials). The δ 26 Mg values change from 52.7 to 56.5 and positively correlate with the δ 25 Mg values. Precisely speaking, data are not located on the sime mass fractionation line (Supementary Fig. S2). Apparent negative 26 Mg ex values are observed in the San Carlos same. No exanation can be found to produce a negative anomaly during natural processes. These data might be attributable to the experimental artifact resulting from the use of some SIMS instruments, which is called QSA effect (Slodzian et al., 2004; Nishizawa et al., 2010; Supementary Materials). Even though the QSA effect might exain the negative anomaly, the possible non-mass independent fractionation is not desirable. Therefore, we have measured Mg isotopes by a single Faraday cup with a stronger primary beam of 10 na and a magnetic scanning mode. Data of San Carlos olivine and low Ca pyroxene in the Efremovka chondrule are presented in Table 2. Most 25 Mg/ 24 Mg and 26 Mg/ 24 Mg ratios of San Carlos olivine are consistent with terrestrial values. There is a noteworthy lack of anomalous 26 Mg ex values, which suggests that the instrumental mass fractionation is probably derived from the trajectory effect of the multi-em detector system (Supementary Materials). However, measured 25 Mg/ 24 Mg and 26 Mg/ 24 Mg ratios are higher than those of olivine standard. These data show a typical mass dependent fractionation with a slope of 1/2. The observed absolute fractionation value of 4 7 /amu might be attributable to the instrumental mass discrimination. Thirlwall (1991) 138 Y. Sano et al.

7 Table 4. Al Mg data of Miyakejima standard and Efremovka chondrule for the third session Spot 27 Al ++ / 24 Mg + 25 Mg + / 24 Mg + ±2σ 26 Mg + / 24 Mg + ±2σ δ 25 Mg ±2σ δ 26 Mg ±2σ 27 Al/ 24 Mg 26 Mg ex ±2σ Miyake-jima standard Eflemovka chondrule: agiclase grain # Eflemovka chondrule: agiclase grain # Eflemovka chondrule: agiclase grain # Eflemovka chondrule: agiclase grain # reported that non-normalized 86 Sr/ 88 Sr ratios of the SRM 987 standard varied markedly from 6.7 to using a TIMS with multi-faraday cup system over a period of several months. Similar instrumental fractionation might occur in a NanoSIMS with a Faraday cup, although we cannot provide a ausible reason. Whatever is the case, the intrinsic mass dependent isotope fractionation of chondrule minerals is not the main goal of this work. No anomaly of 26 Mg ex values is observable in the Efremovka low Ca pyroxene. The second set of experiments was conducted in November 2009 when the magnet scanning method was apied to Mg isotopes as a single collector mode using an EM#2 detector. We measured only agioclase sames at the time; olivine and low Ca pyroxene sames were not analyzed. Table 3 presents the Al Mg data of Miyakejima standard and agioclase (grain #2 and #3) in the Efremovka chondrule for the second session. In this session, the δ 25 Mg values of Miyake-jima agioclase were from 2.4 to 5.4, whereas the δ 26 Mg values were from 5.8 to 9.4, suggesting again the slope of 1/2. The absolute fractionation values are significantly smaller than those of the multi-collector mode (see Table 1) and are consistent with 7 /amu for Mg isotopes reported from the use of a modified ion microprobe (IMS-6f; Cameca) in a single collector mode (Sugiura et al., 2004). Sano et al. (2008) reported that the instrumental mass fractionation of Sr isotopes in natural carbonate varied from 13.9 to 0.4 per amu within four days using a NanoSIMS, although the mass bias is negligibly small in Pb isotopes, +0.1 ± 2.6 for 206 Pb/ 204 Pb and 1.5 ± 2.6 for 207 Pb/ 204 Pb in a NIST SRM610 glass (Sano et al., 2006). The observed mass fractionation of Mg isotopes is within an acceptable level of the instrument specification. Large fractionation of δ 25 Mg observed in spot 3.1 and 3.2 (grain #3) might result from the charge-up effect of the same surface, although the fractionations of δ 25 Mg and δ 26 Mg are mass-dependent (see Table 3). In order to check the reproducibility between the multi- EM collector and single collector-magnetic scanning mode, the third set of experiments was conducted in November 2011 after polishing the same surface slightly. Spot 3.4 disappeared by polishing. At the time, we made new grain analysis (#4) as well as additional spot analy- Al Mg dating of Efremovka chondrule 139

8 a b c d Fig. 2. Al Mg isochron diagram for a grain #1 (a), grain #2 (b), grain #3 (c), and grain #4 (d) in a chondrule of Efremovka CV3 where 26 Mg ex denotes excess 26 Mg after correction of the mass-dependent fractionation. Errors are two sigma values. Correlation line fitted by weighted least-squares method is shown by a dotted line, where the error of Al/Mg ratio is assumed 3.0% at 2σ. sis of #1 #3 together with the Miyake-jima standard. These data are presented in Table 4. In this session, the absolute fractionation of Mg isotopes of the standard is similar to that of session two. The trend is consistent with the slope of 1/2, suggesting identical experimental quality of both sessions. In addition the 26 Mg ex data of session three are consistent with those of session one. Figure 2 presents a correlation diagram between Al/ Mg ratios and 26 Mg ex of a small agioclase grain (#1). Five spot data with the multi-collector mode were obtained in the first session (Table 1); three with the magnetic scanning mode were obtained in the third session (Table 4). The former are not distinguishable from the latter within experimental error in the diagram, supporting the validity of the multi-collector mode. These data are located along a line with a positive inclination together with a low Ca pyroxene. Because Al contents are almost uniform within grain #1 (except #5 value in Table S1), the variation in Al/Mg ratios is probably derived from the heterogeneity of Mg abundances. An inferred 26 Al/ 27 Al ratio becomes (3.3 ± 1.3) 10 6 (2σ error, r coeff = 0.917; MSWD = where r coeff and MSWD are correlation coefficient and Mean Squared Weighted Deviates, respectively) based on the slope if the excess 26 Mg was attributable to the extinct 26 Al, where we do not take into account of the Al/Mg error, which is significantly small compared with the excess 26 Mg. When the pyroxene data are omitted, the slope is calculated as (3.2 ± 1.9) Y. Sano et al.

9 (r coeff = 0.876; MSWD = 0.150), identical to the calculation with the pyroxene presented above. At the second and third sessions, we measured two relatively larger grains (#2 and #3). Figure 2b shows the Al Mg diagram of seven spots in the agioclase grain #2. No resolvable excess 26 Mg in the same exists where the initial 26 Al/ 27 Al ratio is calculated as (1.3 ± 7.1) 10 7 or < (MSWD = 0.358), which is not distinguishable from zero within two sigma error. Figure 2c shows the Al Mg diagram of six spots in the agioclase grain #3. Again no large excess 26 Mg in the same and the initial ratio is (1.2 ± 12.6) 10 7 or < (MSWD = 0.963). At the third session, we measured a relatively small grain (#4). Figure 2d shows the Al Mg diagram of seven spots in the agioclase grain #4. Apparent excess 26 Mg has been observed in a spot with the 27 Al/ 24 Mg ratio of about 300. A least-squares fitting provides a non-zero slope, leading to the initial 26 Al/ 27 Al ratio of (1.7 ± 1.4) 10 6 (2σ, r coeff = 0.678; MSWD = 0.751). In summary, the initial 26 Al/ 27 Al ratios are consistent with zero in relatively large agioclase grains of #2 and #3 in the Efremovka chondrule, whereas smaller grains (#1 and #4) show significant excess 26 Mg derived from 26 Al. DISCUSSION Variation of initial 26 Al/ 27 Al ratios in the Efremovka chondrules Table 5 presents a comparison of initial 26 Al/ 27 Al ratios of the Efremovka chondrules observed in this work together with those reported in the literature (Hutcheon et al., 2009; Srinivasan et al., 2000). The ratios range considerably from to Our data show values that are lower than those given in the literature but comparable to them within experimental error. To discuss the formation interval between CAIs and chondrules of Efremovka, it is necessary to study the possible causes of the large variation. Hutcheon et al. (2009) argued that several individual chondrules within Efremovka should have different formation (crystallization) ages. However, there is clear evidence that some CAIs in Efremovka are disturbed for Al Mg system, and therefore, some chondrules also could be disturbed. One Efremovka CAI shows a canonical value (Amelin et al., 2002), although others indicate disturbed (Sugiura et al., 2001) or younger (Wadhwa et al., 2009) ages. It is unlikely that the variation in CAI ages is due to prolonged duration of the CAI formation, because unaltered whole rock sames of Allende CAIs show a well-defined isochron consistent with internal mineral isochrons from these CAIs (Jacobsen et al., 2008). Instead, the variation is more likely to be attributable to the partial re-equilibration (the disturbance of Al Mg system) of the chondrite by parent body altera- Table 5. Comparison of intial 26 Al/ 27 Al ratios of Efremovka chondrules Same 26 Al/ 27 Al ( 10 6 ) Reference Plag-rich chondrule 25 ± 8 Srinivasan et al. (2000) 1-1 chondrule 6.4 ± 5.0 Hutcheon et al. (2009) 2-1 chondrule 12.6 ± 8.4 Hutcheon et al. (2009) 2-2 chondrule 6.9 ± 6.7 Hutcheon et al. (2009) 2-12 chondrule 14.7 ± 9.4 Hutcheon et al. (2009) grain #1 3.3 ± 1.3 This work grain #2 <0.6 This work grain #3 <1.1 This work grain #4 1.7 ± 1.4 This work Error assigned to the ratio is 2 sigma. tion. It has been often assumed that many agioclase grains in a single chondrule show the same origin and history, which is the basis upon which one might draw a 26 Al 26 Mg isochron from the spot analysis of different grains in the same chondrule measured using a SIMS. However such is not the case in the present same. Initial 26 Al/ 27 Al ratios are variable among individual agioclase grains in the single Efremovka chondrule. This may derive from parent body alteration and metamorphism. Grain size of agioclase and possible Fe alkali metasomatic alteration To discuss more details related to each grain, we have considered chemical compositions of the agioclase grains (Table S1). These four grains have higher Na contents and much higher Al/Mg ratios (except for three spots of grain #4) than those reported in the literature (Hutcheon et al., 2009; Srinivasan et al., 2000). These signatures are inconsistent with crystallization from typical ferromagnesian chondrule melts (Phinney and Morrison, 1990; Simon et al., 1994) and suggest that these grains gained Na and lost Mg during post-crystallization metamorphism. Typical agioclase grains reported by Hutcheon et al. (2009) have Al/Mg ratios lower than 100, which is consistent with primary crystallization from a Mg-rich melt. On the other hand Al-rich chondrule shows the Al/Mg ratio of (Kita and Ushikubo, 2012). This might exain the discrepancy of the initial 26 Al/ 27 Al ratios in Table 5. Krot et al. (1998) reported that progressive alteration resulted from fluid-rock interaction in the CV3 asteroid that accreted as a heterogeneous mixture of ice and anhydrous materials. Subsequently, it was heated during parent body thermal metamorphism. This metamorphism might assist the mobilization of Fe, Mg, and Na and reacement of primary phases in chondrules. The Fe alkali metasomatic alteration might have occurred and/ or more proceeded in the large grains (grain #2 and #3) Al Mg dating of Efremovka chondrule 141

10 (a) (b) M 2.1 ne ne M ne Grain #1 Fig. 3. (a) Composite electron image of the area close to the spot 1.6 of grain #1 obtained by the SEM and (b) that close to the spot 2.1 of grain #2. The white and black lines are attributable, respectively, to nepheline and FeS. Pl, Plagioclase; M, FeS; ne, Nepheline. than in small ones (#1 and #4). This is contrary to the general idea that a large grain is more resistant and robust to the thermal metamorphism than a small one when the molecular diffusion is the dominant process. It is necessary to exain the cause of such a difference in the grain size. Figure 6 presents detailed images of (a) grain #1 and (b) grain #2 close to the spot 1.6 and 2.1, respectively, measured using the SEM. Many irregular cracks and thin black lines are apparent in grain #2, but fewer in grain #1, even though we do not have three-dimensional information. The black and white lines are probably attributable, respectively, to nepheline and FeS, even though measuring pure chemical compositions of these lines is difficult because the excitation volume by the electron beam was expanded to about 2 3 µm even though the M Grain #2 electron beam diameter was confined to less than 0.1 µm. Similar black lines were found in a chondrule of a CO3 chondrite by Tomeoka and Itoh (2004). They suggested that the lines were generated by reacement of agioclase by nepheline through the sodiummetasomatism because of parent body thermal metamorphism. Kita et al. (2004) reported that the absence of 26 Mg excess is related to the presence of nepheline in Al-rich chondrules (or POI) from the Ningqiang ungrouped carbonaceous chondrite. Finding apparent cracks and black lines in the small grain #1 is difficult (Fig. 6a). Instead there are several small white lines (probably because of FeS) observed passing through the spot #4.2, #4.5, #4.6 and #4.7 in grain #4 (Fig. 1f). These four spots have no excess 26 Mg (see Table 5). The metamorphism might have proceeded more in the large grains #2 and #3, which is supported by higher average contents of FeO and Na 2 O in these grains than in grains #1 and #4 (see Table S1). Anyway they are not pure agioclase, but probably altered by the metamorphism that changed their chemistry. The FeO abundance values are highly variable in #2 and #3. The effective diffusion distance of Mg is probably reduced to about 0.5 µm (average distance between black lines in Fig. 6b) in the large grains because of metamorphism. However, the distance in the smaller grains might be kept longer than those of the large grains (Fig. 3). The mechanism to produce such a difference is not well understood and will be clarified in a future work. Closure temperature of agioclase grains The heat source of the Fe alkali metasomatic alteration might be attributable to thermal and/or shock metamorphism (Krot et al., 1998; Scott et al., 1992). To estimate the maximum temperature of the metamorphism, we calculated the closure temperature (T c ) of chronological system based on the equation presented by Dodson (1973). E/RTc = ln(art c 2 (D 0 /a 2 )/ET ). Therein, E, R, A, D 0, a, and T respectively denote the activation energy, the gas constant, a constant depending on geometry, the diffusion constant, the diffusion size, and the cooling rate. Considering the experimental diffusion data of Mg reported by LaTourrette and Wasserburg (1998), one might use E = 278 kj/mol, D 0 = m 2 /s in the present case. It is possible to estimate the closure temperature based on the diffusion size (a = 5 µm for grain #1) and the cooling rate (T ) if a cylinder shape is assumed (A = 27) and the variation of the MgO content is primary. For metamorphism, Harrison and Grimm (2010) have reported the early thermal history of the H- chondrite parent body. The estimated closure temperature is 579 K when one assumes T = 30 K/Ma based on data in figure 3d of Harrison and Grimm (2010). Because 142 Y. Sano et al.

11 the initial ratio is (4.63 ± 0.44) 10 5 in the Efremovka CAI with the 207 Pb/ 206 Pb age of ± 0.6 Ma (Amelin et al., 2002), the formation interval between the CAI and the small grain (#1) is calculated as Ma), which suggests that the small agioclase grain (#1) in the Efremovka chondrule has never experienced slow cooling through 579 K after Ma of the chondrule formation, even though this is a rough estimate. Similar calculations suggest that grain #4 underwent slow cooling through 708 K after Ma. CONCLUSIONS Our study has revealed the possibility of conducting Al Mg dating on a single grain of 30 µm using a NanoSIMS instrument. Two agioclase grains located in a chondrule of Efremovka CV3 chondrite show apparent excess 26 Mg with an initial 26 Al/ 27 Al ratio of (3.3 ± 1.3) 10 6 and (1.7 ± 1.41) 10 6, although two others in the same chondrule do not show marked excesses. Textural and chemical differences exist between these grains. 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