Radial Distribution of Spallogenic K, Ca, Ti, V and Mn Isotopes in Iron Meteorites Mineo Imamura *, Masako Shima **, and Masatake Honda The Institute for Solid State Physics, The University of Tokyo Z. Naturforsch. 35 a, 267-279 (1980); received January 7, 1980 Dedicated to Professor H. Hintenberger on the occasion of his 70tli birthday Cosmic-ray-produced stable nuclides of Ca (mass number: 42, 43, 44 and 46), Ti (46, 47, 49 and 50), V (50), Cr (50, 53 and 54) and the long-lived nuclides, 40K and 53Mn were determined along the radial axes of the iron meteorites Grant and Treysa. Grant was extensively examined and the results compared with rare gas data. Although Treysa does not include enough samples to allow detailed analysis, the depth profile shows typical features for a small meteorite. The results were compared with calculated profiles of 40K, 49Ti and 53Mn using thick bombard- ment data. The approximate pre-atmospheric radii of Grant and Treysa were determined to be 30 cm and 14 cm, respectively. The effect of space erosion was also estimated by comparing the data of 49Ti and radioactive 53Mn in Grant and Treysa with the calculated patterns. It is suggested that space erosion of both meteorites is small (<0.8Ä/y) during the cosmic-ray exposure of several hundred million years. 1. Introduction Among the spallogenic nuclides in iron meteorites, the production rate of rare-gas isotopes decreases The interaction of primary cosmic radiation with with the depth of the sample, that is, production cor- meteorites produces many secondary particles of relates almost linearly with the attenuation of high various energies, causing very complex nuclear energy particles. Compared with rare gas isotopes, reactions and a variety of nuclear species to be nuclides of mass number closer to target nuclides, produced. The distribution of these products in the such as 53Mn, 54Cr, 50V, 50Ti, etc., are produced in meteorite is a function of the shielding depth, viz. it a large proportion by moderate energy secondaries. depends on the shape and size of the meteoroid as These nuclides give a maximum in the distribution well as the depth of the sample in the body. The in- patterns along the depth of samples [20, 21]. As is vestigation of these depth dependences of cosmogenic well known, the maximum positions in the distribu- nuclides is important to the understanding of histo- tion patterns of these spallogenic nuclides are almost ries of cosmic-rays, meteorites and lunar surface inversely related to the mass differences between materials. target and produced nuclides. Therefore, it would Since the first determination of 3He in iron mete- be ideal to study spallogenic nuclides in a wide range orites by Paneth et al. [1], a large number of spal- of mass differences from the main target nuclides. logenic rare gas data as well as their radial distribu- Further, it is also desirable to measure spallogenic tion has been published [2 — 12]. On the other hand stable and radioactive nuclides in the same sample the data of cosmic-ray produced spallogenic stable because comparisons of stable nuclides and radio- nuclides other than rare gases are limited to certain active nuclides with various half-lives make it possible nuclides [13 — 19]. There have been many studies to estimate the erosion rate in space and ablation in on spallogenic radioactive nuclides, but only a little air, as well as the constancy of cosmic rays. work [18] has been reported on depth profiles in the iron meteorites. In this paper, we report the distribution of cos- mic-ray-produced isotopes of Mn, Cr, V, Ca and K 53 6 * Present Address: Institute for Nuclear Study, The including Mn (7^/2 = 3.7 x 10 y) [22] and University of Tokyo, Midori-cho, Tanashi, Tokyo, 188, 40K (7^2 = 1.28 Xl09y) [23,24] in two iron mete- Japan. orites, Grant and Treysa. Since, all the spallogenic ** Present Address: National Science Museum, Ueno- Park, Taito-ku, Tokyo, 110, Japan. nuclides in iron meteorites are predominantly pro- Reprint requests to Dr. Masako Shima, National Science duced from Fe and Ni, the analysis of the data is Museum, Ueno-Park, Taito-ku, 110, Japan. much less complex than in the case of stone mete- 0340-4811 / 80 / 0200-283 $ 01.00/0. - Please order a reprint rather than making your own copy. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 268 M. Imamura et al. • Radial Distribution of Spallogenic Isotopes orites. Grant, a medium-sized iron meteorite with I recovered weight 530 kg, was chosen because a con- siderable number of detailed rare gas data are avail- able [4, 9] and comparison with other data is pos- sible. Treysa has not been investigated in detail, but it is an interesting object due to its smaller size with recovered weight 63.3 kg. The experimental data are compared with model calculations and discussed in connection with the pre-atmospheric size of mete- orites, space erosion and the constancy of the cosmic- rays intensity. 2. Experimental 2.1. Samples Sample locations in the plane cut out from the meteorites are shown in Figures 1 and 2. The planes are supposed to pass near the center of the mete- orites based on rare gas distributions [4, 9]. The samples from Grant are from the plane investigated -150mm -100 -50 0 +50 +100 by Hoffman and Nier [4], Fireman [7], and Signer and Nier [9] for depth profile measurements of rare Fig. 2. Sample location of Treysa iron meteorite specimens gases. used in this work. Location is shown as (x, y) coordinates. About lg chunks of each sample location were cut from Grant and Treysa with a steel fretsaw or a circular rubber saw. The surfaces of the specimens 2.2. Chemical Procedure were etched and washed successively with 6 N HCl, The sample was dissolved in a minimum amount 5 N HN03, water and ethanol or acetone. The of aqua regia. For the isotope dilution analyses, ap- absence of inclusions such as troilite and schreiber- propiate amounts of mixed spike solution containing site was checked in the course of these treatments. :J9K-, 42Ca-, and 46Ti-enriched isotopes and V and Mn of terrestrial isotopic composition were added to the solution at the beginning of the dissolution process. Because of the complex chemical behaviour of Cr in the solution, another chunk was taken for Cr extraction and a 50Cr-enriched spike was added to the sample. All acids, ammonia and organic solvents used were purified by isopiestic- [25] or distillation methods under normal or reduced pressures. Chem- ical treatments were performed in a dust-free chamber positively pressurized by clean N.>. In most cases, teflon or quartz vessels were used. The de- tailed chemical procedures have already been de- i . i - • . • L1NE I , i i i -400 -300 -200 -100 0 +100 +200 scribed in our previous papers [14 — 16], and are mm schematically summarized in Figure 3. Fig. 1. Location of Grant samples used in this work. For example, "A-350" indicates that the specimen was taken from Bar A and at — 350 mm (at the center of the specimen) 2.3. Mass Spectrometry from the reference line, Contour lines shown in the figure are taken from the study of 3He distribution by Hoffman The isotopic ratios of K, Ca, Ti, V and Cr were and Nier [7]. The unit of the 3He content is 10~6 ccSTP/g. measured by a surface ionization source mass 269 M. Imamura et al. • Radial Distribution of Spallogenic Isotopes Iron Meteorite (Ig) spike soln? (Mg)-* | weigh into teflon beaker Xn carrier soln. ('vlg)-* pure** conc HCl -» KX03 Cvl.5g)-» dissolve heat. - evaporate -*• dry pure** SN' KCl C-7g} - dissolve isopropyl ether ("*10mI/3; shake -<• extract Fe"1 Aqueous Phase Organic Phase evaporate (Fe+++) H-0 Cv-3.-r.D- , pure** 6N NH46h (M-inl)-* j precipitate ce.-.trifugc soln. ppt. war." 1.2N HCl (^2-1) - H2S in pure**NH/OH('v2örops)- centrifuge cupferron ("vQ.Ic agitate CHC1, (5ml/3)*3-* i shake extract Ti s. V soln. ppt- 3 i evaporate Org. Phase NiS, COS ?ha3c KNO, (1 drops) vaporat conc HXOi (3 drops) - evaporate evaporate use conc HMO, (1 drop)- Ka2C03 (^O.Ig) melt conc KC10, (1 drop)- heat decompose cupferon X, Ca evaporate Xass spectrometry H20 (a few ml)- conc HC1 (2 dro s) dissolve ethcir.ol (1 drop)- heat - aissolve ? filter |0.5ml anion exchange resin - Cl - form soln. ppt. 12N HCl (1ml) I EN* HCl (1ml) j 5N H.SO, neutralize dil. HCl- dissolve V Ti 5N H,S04 (Imlj heat •* evaporate -»-dry j evaporate j evaporate K2O -<• total volume lC.nl 0.5N' HCl- dissolve 50% diphenyl carbazide- [ 3-t,1 cation exchange resin - H .Mass spectrometry acetona soln. (lml)->• form 1 sat. NaCl (15ml)+ 0.5N' HCl (12ml) ! 2.0:: HCl (10ml) i-amyl-OH (lCml/3)x - shake •> extract „ ' ?, ir I heat - evaporate dry 1 5 5% -PrOK-4N KCl*I dissolve Aq. Phase Org. Phase 3ml anion exchange resin - i-prOil-jN' HCl 1 10M NH4NO3 (1Cml/7)*3 - wash 'til no Cl" KNO3 (1 crop) evaporate 55% i-?rOH--;.\' HCI(2Cml) 1 7" HCl(Cml) K2S04 (1 drop)}- H202 (I crop) heat decompose diphenyl carbazide ignite evaporate Cr r.-irradiation y.ass spectrometry Fig.