Magnesium isotopes in individual mineral grains of a Ca-Al rich inclusion from the Allende meteorite have been measured by secondary ion mass spectrometry. An electrostatic peak switching system was used to make a precise isotopic measurement in high mass resolution mode (M/&lrtri;M=&acd;4000). The inclusion shows excess of ^Mg correlated with the ^Al/^Mg ratio. The results suggest that live ^Al decayed in the inclusion which formed simultaneously in the solar nebula. The relative abundance of ^Al(^Al/^Al=3.12×10^) is close to the "canonical" value (^Al/^Al=&acd;5×10^) for coarse-grained CAIs

Hiroshi Nagasawa

Hisayoshi Yurimoto

Osamu Koike

Shigeho Sueno

P(論文)Magnesium isotopes in individual mineral grains of a Ca-Al rich inclusion from the Allende meteorite have been measured by secondary ion mass spectrometry. An electrostatic peak switching system was used to make a precise isotopic measurement in high mass resolution mode (M/&lrtri;M=&acd;4000). The inclusion shows excess of ^Mg correlated with the ^Al/^Mg ratio. The results suggest that live ^Al decayed in the inclusion which formed simultaneously in the solar nebula. The relative abundance of ^Al(^Al/^Al=3.12×10^) is close to the "canonical" value (^Al/^Al=&acd;5×10^) for coarse-grained CAIs.departmental bulletin pape

Koike, Osamu

Yurimoto, Hisayoshi

Nagasawa, Hiroshi

Sueno, Shigeho

National Institute of Polar Research Repository

Proc. NIPR Symp. Antarct. Meteorites, 6, 357-363, 1993 ION MICROPROBE MEASUREMENTS OF Mg ISOTOPES IN TYPE B l  CAI OF ALLENDE METEORITE Osamu KOIKE 1 , Hisayoshi YURIMOT0 1 , Hiroshi NAGASAWA2 , and Shigeho SuENo 1 1 Institute of Geoscience, University of Tsukuba, Tsukuba, lbaraki 305 2 Department of Chemistry, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171 Abstract: Magnesium isotopes in individual mineral grains of a Ca-Al rich inclusion from the Allende meteorite have been measured by secondary ion mass spectrometry. An electrostatic peak switching system was used to make a precise isotopic measurement in high mass resolution mode (M/JM = -4000). The inclusion shows excess of 26Mg correlated with the 2 7 Al/24Mg ratio. The results suggest that live 26 Al decayed in the inclusion which formed simultaneously in the solar nebula. The relative abundance of 26Al (26Al/2 7Al=3.I2 x 10- 5) is close to the "canonical" value (26 Al/2 7 Al= - 5 x 10- 5) for coarse-grained CAis. 1. Introduction Ca-Al rich inclusions (CAis) in the Allende meteorite have been studied by many researchers who have been very interested in the origin and evolution of the early solar system. The CAis are considered as the most primitive phases in primodal gas of the solar composition (LORD, 1965; LARIMER, 1967; MARVIN et al., 1970; GROSSMAN, 1972). Magnesium isotopic measurements of CAis have revealed the presence of excess 26Mg which is correlated with the 27 Al/24Mg ratio. The relationship indicates that live radioisotope 26 Al decayed in the CAis, and an upper limit of 26 Al/2 7 Al"' 5 x 1 o- 5 has been established for the early solar nebula (e.g., LEE et al., 1976; HuTCHEON, 1982). The excess is commonly cited in support of the view that CAis are the first solids to have formed in the solar nebula and that they formed more or less simultaneously, within an interval shorter than the half-life of 26 Al. However, the isochron of 26Mg excess, in some CAls, is disturbed by incomplete re-equilibration processes such as partial melts after all or a significant fraction of the initial 26 Al had decayed. The nature and setting for the disturbing event are still unknown (PooosEK et al., 1991 ). In this study, we have measured the excess of 26Mg in HN3- l Type B 1 CAI of the Allende meteorite (NAGAHARA et al., 1987; NAGAHARA and NAGASAWA, 1988) by secondary ion mass spectrometry (SIMS). As the first step for our isotopic analysis by SIMS, we report analytical procedures and the principal results of Mg isotopic measurement. 357 358 0. Kon<E, H. YURIMOTO, H. NAGASAWA ands. SUENO 2. Experimental 2.1. Sample preparation The sample used in this study was mineral grains separated from Allende HN3-l Type Bl CAI; some petrological and isotopic studies have been previously made (MAYEDA et al., 1986; NAGAHARA et al., 1987; NAGAHARA and NAGASAWA, 1988). Each mineral grain (spinel, fassaite, melilite, and anorthite) was mounted separately on epoxy disks. The size of each mineral grain was less than 20 µm. The disks were ground with SiC paper and then polished with alumina paste. The polished disks were coated with "'20 nm of gold film for SIMS analysis. 2.2. Standards Three different terrestrial single crystals (spinel, augite, and anorthite) were used as standards for Mg isotopic analysis (Table l ). The spinel, augite, and anorthite grains were from Russia, from Takashima, Japan, and from Miyakejima, Japan, respectively. The surfaces of the standards were coated by carbon film for electron microprobe analyses. Homogeneity of Mg and Al concentration on individual terrestrial standards was confirmed by electron microprobe analysis. The Al/Mg ratios for each crystal are shown in Table 1. After the electron microprobe analyses, the carbon film was removed, and then a gold film was evaporated for SIMS work. 2.3. Magnesium isotopic measurement Magnesium isotopic ratios and Al/Mg ratios were measured by the Tsukuba University CAMECA IMS-3F ion microprobe equipped with electrostatic peak switching system. All Mg isotopic measurements were made with a focused primary 10n beam of 160-. Primary beam currents ranged from 0.5 nA to 12 nA and were Table 1. Mg isotopic measurement of' terrestrial standards. Samples Spinel (Russia) Augite (Takashima) Anorthite (Miyakejima) ,12sMg1 (%o) ( ± 2amean) -11.3±0.7 -11.7±0.9 -5.3±0.6 -5.6±0.7 -3.0±0.8 -2.8± 1.2 (526Mg2(%0) 2 7 Al+ ;24Mg+ ( ±2amean) ( ± 2amean) -0.3 ±2.6 1.97 ±0.03 2.1±1.2 1.96±0.03 -0.5±1.5 0.51 ±0.02 -0.9±1.3 0.51 ±0.02 2.9 ± 1.5 309± I -0.9±3.0 317±3 Each line shows the measured values for a different spot of the given standard. 1 ,12sM = lOOO x g g MEAS 1 [(2sM +;24M +) J g (25Mg/24Mg)REF 2 (526Mg= 1000 x [(26M�_:;24Mg+2MEAS -l]-2 x ,12sMg (26Mg/24Mg)REF 27Al/24Mg3 2.58 0.655 423 (25Mg/24Mg)REF and (26Mg/24Mg)REF are assumed to 0.12663 and 0.13932, respectively (CATANZARO et al., 1966). 3 Determined by electron microprobe analysis. --E--,-i z E--z ,-i Mg Isotopes in Type B l  CAI of Allende Meteorite 106 24Mg+ 105 104 103 102 101 24.007 23.987 23.967 RUSSIA SPINEL 24MgJ-t 24.983 24.993 MASS (a.m.u.) 26Mg+ 25MgH+ 25.982 25.990 359 Fig. 1. High resolution mass spectra ( M/ '1M = "'4000) of positive secondary ions in the atomic mass 24, 25, and 26 regions. Jons were sputtered from a terrestrial spine/ single crystal by bombardment with 160- ions. The Mg isotope peaks are clearly resolved from all interferences. adjusted for each run to obtain a 24Mg + count rate of not more than -3.0 x 105 counts per second and typically -1. 5 x 105 counts per second. The beam size was 10-20 µm in diameter. The transfer optics of the instrument were adjusted to obtain an imaged field of 150 µm in diameter. The contrast aperture ( 150 µm) was used and the entrance and exit slits of the mass spectrometer were closed enough to achieve the desired mass resolution. The energy slit was set to select an energy window of 15 eV. A mass resolution power, M/L1M, of -4000 was sufficient to resolve all significant interferences from MgH + (Fig. 1) as well as other possible molecular and doubly-charged ions. Measurements were made by cycling through the mass sequences 24, 25, 26, and 27 in a peak jumping mode. The peak jumping was achieved by an automatic control of magnetic field strength. To minimize hysteresis effects, the magnet was repeatedly stepped through the set of field values corresponding to the selected masses before starting a measurement run. After the magnetic peak jumping, a precise peak centering of each mass was made by electrostatic peak switching scan. Principle of the electrostatic peak switching equipment was described by SLODZIAN et al. ( 1984). Small drifts in the position of a peak during the run were compensated by the electrostatic peak centerings every ten cycles through the masses to keep the ion beam centered in the exit slit of the mass spectrometer. Secondary ion signals were detected with an electron multiplier operated in a pulse counting mode. Signals measured in the electron multiplier were corrected for the counting system dead time. The dead time, which was determined by Ti isotopic measurements of a rutile standard, was 33.6 ± 0.2 ns. The correction was 0.5% when a 24Mg + count rate was - 1.5 x 105 counts per second. 360 Phase Spinel # 1 Spine! #2 Spine! #3 Fassaite # l Fassaite # 2 Melilite # 1 Melilite #2 Melilite # 3 Anorthite # I Anorthite # 2 Anorthite # 3 Anorthite # 4 Anorthite # 5 0. KOIKE, H. YURIMOTO, H. NAGASAWA ands. SUENO Table 2. Mg isotopic measurement of HN3-J Type Bl CAI. 27Al/24Mg L1 2s Mg (%0) ci 26Mg (%0) ( ± 2o-mean) ( ± 2o-mean) ( ±2o-mean) 2.45±0.02 3.61 ± l.99 0.8 ± 1.5 2.42±0.01 -1.05±0.84 3.5 ± 1.4 2.45±0.06 -1.30±0.38 4.9± 1.7 1.99±0.04 -2.28±0.67 3.92±0.39 2.45±0.06 0.48±2.70 -2.0±5.6 7.56±0.17 5.67±0.11 10.8 ±2.7 7.08±0.09 5.31 ±0.03 6.6±6.6 8.73 ± 1.00 6.54±0.75 2.2±4.9 718±66 5.4±2.4 163.6±4.0 451 ±8 1.0 ± 2.1 99.4± 7.4 429±6 6.5 ±3.2 87± 12 816±45 2.9 ± 3.4 198.0±9.4 484±31 4.7±3.0 117.6±7.8 The isotopic composition measured in the ion microprobe differs from true isotopic composition of the sample by the matrix dependent instrumental mass fractionation. The instrumental mass fractionation varies with the geometry of the sample measured, tuning condition of the instrument, and the position of the energy window, etc. In addition, the instrumental mass fractionation changes during the course of the measurement, inferred that this is the result of changes in the distribution of ion sputtered from a deepening hole (FAHEY et al., 1987). From these reasons, it is very difficult to determine precisely the intrinsic mass fractionation of the sample. In this study, no special efforts were made to determine precisely the mass-dependent Mg isotopic fractionation. Therefore, L1 25 Mg values reported in Table 2 (and in Table I) include the intrinsic and instrumental mass fractionational effects. The primary goal of the Mg isotopic measurements in this study is the determination of "non-linear" 26Mg excess. For the measurement of non-linear effects in 26Mg, a few independent runs were usually made on a given spot. The measured 26Mg/24Mg ratios were corrected for an assumed linear mass fractionation relationship on the basis of the measured 25Mg/24Mg ratios and the normal Mg isotopic ratios given by CATANZARO et al. (1966). As shown in Table 1, most <526Mg values on different spots for the terrestrial standards are zero within 2 am; although two b26Mg values are not. There are no significant discrepancy between variations for those original data; smaller errors of <526Mg values for each of spinel and anorthite standards arise occasionally when these data are statistically dealt in. Therefore, the linear fractionation correction and the chosen normal ratios agree with our SIMS data on the terrestrial standards (Table 1 ). A run consisted of 90 cycles through the mass sequence, 24, 25, 26, and 27. A measurement run was divided into "blocks" of 10 cycles each for the purpose of time interpolation and peak centering. In order to minimize instrumental instabilities, time interpolation was made for the mass intensities every ten cycles. The mass fractionation Mg Isotopes in Type BI CAI of Allende Meteorite 361 correction was applied to each block and the average of the individual c526Mg value was computed at the end of the run. This procedure minimized the c526Mg error, since it accounts for variations in the instrumental mass fractionation during a run. The c526Mg values reported in Table I and in Table 2 are averages over a series of a few runs on a single spot. The relative sensitivity factor of Mg + and Al + for each mineral under the analytical conditions used was determined with terrestrial standards of spinel, pyroxene, and anorthite. The 27 Al/24Mg ratios for CAI minerals were calculated from 27 Al + /24Mg+ intensity ratios multiplied by the relative sensitivity factors. Sensitivity factors determined from analyses of the respective standard minerals were 1.31, 1.26, and 1.33 for spinel, pyroxene, and anorthite, respectively. 3. Results and Discussion The results of the Mg-Al analysis are given in Table 2. Mg-Al evolution diagrams showing 26Mg/24Mg versus 21 Al/24Mg are shown in Fig. 2. All data points are plotted in Fig. 2. Individual anorthite grains are numbered in the figure. In this study, we assumed the same sensitivity factor for anorthite and for melilite because no terrestrial standard of melilite was prepared. Therefore, 27 Al/24Mg values for melilite in Table 1 may shift systematically. All data points for anorthite grains lie on a single correlation line. A least square fit to 13 data gives a line with slope 26Al/27Al=(3. l 2±0.I3)x 10- 5 and intercept c526Mg=3.37±0.75. The relative abundance of 26Al (26Al/27Al=3.12x 10- 5) is close to the "canonical" value (26Al/27Al= -5 x 10- 5) for coarse-grained CAis. The increases or decreases of Mg intensities during measurement have been observed in some spots of anorthite grains. The variation gives the larger error, especially in anorthite #1. The instability of Mg intensity corresponds to heterogeneous distribution 250 ALLENDE CAI HN3-1 200 - 26Al / 21\_t = I;;) 150 3.12 X 10·5 � -100 50 c..o 0 spinel+fassaite +melilite -50 0 200 400 600 800 1000 2�11 24Mg Fig. 2. 26 Al-26 Mg plot for Allende HN3-1 Type Bl CAI, showing a correlation of c:526 Mg with 2 7 Al/24 Mg ratio due to the in situ decay of 26 Al. A least square .fit to 13 data points gives a slope of 2 6 Al/27 Al= ( 3.12±0.13) X 10- 5• All errors are 2amean· 362 0. KOII(E, H. YURIMOTO, H. NAGASAWA and s. SUENO of Mg in anorthite, but not instrumental instability. Nevertheless, there are not any significant scatters from the isochron line. The results indicate that the internal isochron of anorthite is essentially equal to the mineral isochron in the CAI, and strongly suggest that live 26 Al decayed in the anorthite crystal and that CAis formed simultaneously in the solar nebula. Data points for one spinel, one fassaite pyroxene, and one melilite crystal fall slightly off the isochron line. We can not conclude that these scatters from isochron line are intrinsic effects or due to instrumental instabilities, although PooosEK et al. (1991) suggested that melilite crystals in Type Bl CAis migrated upward from the isochron probably by isotopic re-equilibration. Data points for the other melilite crys­tals (melilite n, :B) lie on this line within 2am. The '526Mg values for two melilites (melilite n, :B) and one fassaite (pyroxene n) have larger errors. We consider that these uncertainties are attributed to instrumental instabilities during Mg isotopic measurements. The large Al/Mg error for melilite :B is due to the chemical zoning of melilite solid solution. These results indicate that abundant data points obtained by much precise measurements are required to determine the intercept which represents the initial 26Mg/24Mg ratio at the CAI formation and that in situ magnesium isotope measurements combined with petrological studies are also necessary to discuss the CAI formation. As the next step, in situ magnesium, oxygen, and other isotopic measurements are required in a thin section sample of CAI, and the results should be combined with petrological studies. References CATANZARO, E. J. , MURPHY, T. J., GARNER, E. L. and SHIELDS, w. R. (1966): Absolute isotopic abundance ratios and atomic weight of magnesium. J. Res. Nat. Bur. Stand., 70a, 453-458. FAHEY, A. J., ZINNER, E., CROZAZ, G. and KORNACKI, A. S. (1987): Microdistributions of Mg isotopes and REE abundances in a Type A CAI from Efremovka. Geochim. Cosmochim. Acta, 51, 3215-3229. GROSSMAN, L. ( 1972): Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta, 36, 597-619. HuTCHEON, I. D. (1982): Ion probe magnesium isotopic measurements of Allende inclusions. Am. Chem. Soc. Symp. Ser., 176, 95-128. LARIMER, J. W. (1967): Chemical fractionation in meteorite: I. Condensation of the elements. Geochim. Cosmochim. Acta, 31, 1215-1238. LEE, T., PAPANASTASSIOU, D. A. and WASSERBURG, G. J. (1976): Demonstration of 26Mg excess in Allende and evidence for 26Al. Geophys. Res. Lett., 3, 109-112. LORD, H. C., I I I  (1965): Molecular equilibria and condensation in a solar nebula and cool stellar atmospheres. Icarus 4, 279--288. MARVIN, U. B., Wooo, J. A. and DICKEY, J. S., Jr. (1970): Ca-Al rich phases in the Allende meteorite. Earth Planet. Sci. Lett., 7, 346 350. MAYEDA, T. K., CLAYTON, R. N. and NAGASAWA, H. (1986): Oxygen isotopic variations within Allende refractory inclusions. Lunar and Planetary Science XVII. Houston, Lunar Planet. Inst, 526-527. NAGAHARA, H., NAGASAWA, H., NAKAMURA, N. and MATSUI, T. (1987): HN3-1 (Type B l  CAI) formed from isotopically and chemically heterogeneous interstellar and condensates of the solar system by incomplete melting. Lunar and Planetary Science XVIII.  Houston, Lunar Planet. Inst., 694-695. NAGAHARA, H. and NAGASAWA, H. (1988): Formation of Type B l  CAI. Papers Presented to the 13th NIPR Symposium on Antarctic Meteorites, June 7 9, 1988. Tokyo, Natl Inst. Polar Res., 29. PooosEK, F. A., ZINNER, E. K., MACPHERSON, G. J., LUNDBERG, L. L., BRANNON, J. C. and FAHEY, A. J. (1991 ): Correlated study of initial 87Sr/86Sr and Al-Mg isotopic systematics and petrologic Mg Isotopes in Type B 1 CAI of Allende Meteorite 363 properties in a suite of refractory inclusions from the Allende meteorite. Geochim. Cosmochim. Acta, 55, 1083-1 I IO. SLODZIAN, G., LORIN, J. C., DENNEBOUY, R. and HAVETTE, A. (1984): Isotopic Measurements at High Mass Resolution by Electrostatic Peak Switching. Secondary Ion Mass Spectrometry SIMS IV, ed. by A. BENNINGHOVEN et al. Berlin, Springer, 153-157. ( Received September 24, /992; Revised manuscript received January 14, 1993) 

Ion microprobe mesearments of Mg isotopes in Type B1 CAI of Allende meteorite

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Ion microprobe mesearments of Mg isotopes in Type B1 CAI of Allende meteorite

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