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Lunar and XXXII (2001) 1661.pdf

COSMOGENIC RADIONUCLIDES IN THE . R. G. Liberman1, J. O. Fernández Niello1,4, R. C. Reedy2, L. K. Fifield3 and M. L. di Tada3,4. 1Laboratorio TANDAR. Departamento de Física. Comisión Nacional de Energía Atómica. Av. del Libertador 8250 (1429) Buenos Aires, Argentina ([email protected]). 2Los Alamos National Laboratory, Mail Stop D436, Los Alamos, NM 87545 USA. 3Nuclear Physics Department, Research School of Physical Sciences and Engineering, The Australian National Uni- versity, Canberra ACT 0200, Australia. ). 4CONICET, Av. Rivadavia 1917 (1033), Buenos Aires, Argentina.

Long-lived cosmogenic radionuclides 10Be, 26Al, however, only upper limits were obtained. Table 1 36Cl, 41Ca and 59Ni have been measured in five samples shows the activities calculated from the measured con- from the Campo del Cielo meteorite using the accel- centrations for all the samples. erator mass spectrometry (AMS) technique. Campo del Cielo is an IA iron with TABLE 1. Activities for the five radioisotopes in many coarse silicate inclusions. It fell in an explosive Campo del Cielo. Experimental uncertainties of the way, creating more than twenty craters in the Chaco last digits are shown between parentheses. region of Argentina. The terrestrial age of this mete- orite was indirectly measured with radiocarbon in Sample Activities (dpm kg-1) charcoal from the bottom of one of the craters. It was 10Be 26Al 36Cl 41Ca 59Ni 14 established to be 3945 ± 85 C years BP [1,2]. It is TPB <0.0008 <0.0006 0.0018 <0.032 <3.8 possible to deduce a recovered mass of about 140 tons (2) by summing the masses remaining in the various cra- TPC <0.0023 <0.0010 0.0053 <0.040 <1.1 ters and those that are in museums around the world. (4) However, assuming that the terrestrial age for this me- ANU-I <0.0002 <0.0008 0.0006 <0.016 <2.8 teorite is not very large, we have estimated a much (1) larger preatmospheric size. ANU-B <0.0001 <0.0017 0.0009 <0.017 <3.1 The bulk composition of the Campo del Cielo me- (2) teorite is principally iron (92.7%), (6.15%), co- JFN1 <0.0134 <0.0036 - <0.038 <4.0 balt (0.42%), carbon (0.37%) and phosphorus (0.28%). (El Taco) Other elements like silicon, titanium, vanadium, gal- lium, copper, and sulfur are present at trace levels As the measured concentrations were very low, we [3,4]. assumed that Campo del Cielo parent meteoroid must Our samples were taken from two pieces of Campo have been very large. Hence, for 10Be, 26Al, 36Cl and del Cielo. Samples TPB, TPC, ANU-I and ANU-B 59Ni, we made an initial comparison between our ex- came from an unidentified fragment, which was clearly perimental results and theoretical radioisotope produc- metallic and with very little oxidation. This fragment tion rates for the Canyon Diablo [5,6], was approximately fist-sized and its mass was about 2 because both have very similar chemical kg. We divided each sample in two subsamples, one compositions. Those production rates were calculated for the Be-Al-Ca-Ni analyses and the other one for the using the LAHET Code System (LCS) considering a Cl analysis. The mass of these subsamples ranged sphere of 15 m-radius [5,6]. To obtain theoretical pro- between 1.4 and 2.8 g. While TPB and TPC samples duction rates for 41Ca, we used the 36Cl production rate came from near the crust of the fragment, ANU-I and profile for Canyon Diablo [5] and a ratio for 41Ca/36Cl. ANU-B samples were from the interior of it. This ratio was about (1.01 ± 0.08) [7] for low shield- JFN1 came from the El Taco fragment and was a ing, i.e., up to 100 g/cm2 depth, and was assumed to be very weathered sample with a lot of oxides and little 2 for higher shielding based on trends for this ratio in metal remaining before processing. We isolated 0.9 g [7]. For a better analysis of the experimental results, 10 26 41 59 of metallic material for the Be, Al, Ca and Ni new calculations of the production rates are being done samples. Unfortunately, the acid dissolution of the using LCS for with different radii ranging 36 subsample for Cl failed, and we could not measure between 1 and 6 m. that radioisotope in the JFN1 sample. By comparing our 36Cl experimental results with AMS measurements were performed with the theoretical production rates, we estimated each sam- 14UD tandem accelerator at the Department of Nuclear ple’s depth (Table 2). The ANU-I sample, which came Physics of the Australian National University (Can- from the interior of the fragment and was not influ- 36 berra). For Cl, the measured values were signifi- enced by the crust, gives the largest apparent depth: 10 26 41 59 cantly above the blank. For Be, Al, Ca and Ni, between 202 and 210 cm. The upper limits for the Lunar and Planetary Science XXXII (2001) 1661.pdf

COSMOGENIC RADIONUCLIDES IN CAMPO DEL CIELO: R. G. Liberman et al.

other radionuclides are consistent with this estimate. It is also interesting to compare the results of We conclude that the Campo del Cielo parent meteor- Campo del Cielo with data of other big bodies. Nagai oid must have been a >210 cm radius body with a mass et al. [9] have reported measurements in the of more than 300 tons. These results place Campo del , which they noted as having a slightly larger Cielo as one of the largest iron meteorites that has been shielding parameter than the El Taco fragment. That recovered. probably means that both meteorites had similar sizes. Honda and collaborators [10] have presented some TABLE 2. Preatmospheric depth (d, in cm) of dif- measurements of cosmogenic nuclides in that pallasite. ferent samples from the 36Cl measurements and upper They reported values ranging between (0.004 ± 1) and limits for the other radioisotopes, taking into account a (0.93 ± 3) dpm⋅kg-1 for metallic 10Be and between meteorite terrestrial age of about 4000 years. (0.0006 ± 2) and (0.80 ± 7) dpm⋅kg-1 for metallic 26Al. Nishiizumi and coworkers [11] reported some low Sample d(36Cl) d(26Al) d(10Be) d(41Ca) d(59Ni) activities of 10Be and 26Al in the Nantan iron meteorite: ± ± ⋅ -1 (cm) (cm) (cm) (cm) (cm) (0.00082 0.00004) and (0.00054 0.00003) dpm kg for beryllium and (0.00055 ± 0.00007) and (0.00029 ± TPB 182–187 > 155 > 140 > 142 > 173 0.00006) dpm⋅kg-1 for aluminum. They estimated that TPC 162–165 > 146 > 123 >138 > 204 these samples were from depths between 150 and 175 cm. As our experimental limits for these radioisotopes ANU-I 202–210 > 150 > 161 > 156 > 181 are lower than those values, we can say that the Campo ANU-B 194–200 > 136 > 180. > 154 > 178 del Cielo parent meteoroid was probably larger than Nantan or, at least, that our samples came from a larger JFN1 - > 123 > 96 > 139 > 172 preatmospheric depth than those that have been meas- (El Taco) ured in Nantan. Acknowledgments: We thank the Ing. Palacios (Dpto de Materiales, CNEA, Argentina) for providing Although only a few cosmogenic radionuclide four of our samples. This work was partially sup- studies in very large objects have been published, there ported by a grant of the FONCyT, Argentina. The are some previous measurements in Campo del Cielo work at Los Alamos was supported by NASA and was to compare with our results. In 1969, Chang and done under the auspices of the U.S. Department of -1 10 Wänke [8] obtained (0.31 ± 0.19) dpm⋅kg of Be in a Energy. sample from El Taco, which is probably an upper limit. References: [1] Romaña A. and Cassidy W. 10 36 They also measured Be and Cl in another sample (1972) Monografías, Universidad Nacional del from Campo del Cielo and obtained (0.21 ± 0.45) and Nordeste, Argentina. [2] Cassidy W. and Renard M -1 (0.59 ± 0.69) dpm⋅kg , respectively. (1996) & Planet. Sci., 31, 433-448. [3] Some years later, Nagai and coworkers [9] meas- Buchwald V. F. (1975) Iron Meteorites. Univ. Calif. 10 ured Be in different phases of the El Taco fragment Press, 373-379. [4] Talleres Metalúrgicos San Martín, 10 of Campo del Cielo. They calculated that Be in pure Chaco, Argentina (1989) Análisis Químico Nro. -1 silicate was 0.13 ± 0.03 dpm⋅kg and in pure carbon 00.108 de una muestra del meteorito de Campo del -1 was 0.77 ± 0.09 dpm⋅kg . Their metallic values Cielo. [5] Michlovich E. S. et al. (1994) JGR, 99, -1 ranged between 0.015 and 0.001 ± 0.001 dpm⋅kg , 23,187-23,194. [6] Schnabel C. et al. (1999) Science, with the higher values from samples that had some 285, 85-88. [7] Nishiizumi K. et al. (1998) Meteoritics silicates in them. In addition, metallic 26Al values of & Planet. Sci., 33, A117. [8] Chang C. and Wänke H. about 0.01 dpm⋅kg-1 were obtained. Our beryllium and (1969) Meteorite Research (P. M. Millman, ed.), 397- aluminum limits for JFN1 are comparable to their ac- 406. [9] Nagai H. et al. (1993) Geochim. Cosmochim. tivities, which is reasonable because both the Nagai et Acta, 57, 3705-3723. [10] Honda M. et al. (1996) al. samples and JFN1 come from the same fragment. Meteoritics, 31, A63-A64. [11] Nishiizumi K. et al. It can be seen, then, that the values from the litera- (1995) Meteoritics, 30, 556-557. ture were obtained from the El Taco fragment and are higher than our results for the TPB, TPC, ANU-I and ANU-B samples. Therefore, we can conclude that the fragment from which these four samples were taken was originally deeper in the than the El Taco fragment.