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10Be,26Al,53Mn, and light noble gases in the Antarctic shergottite EETA 79001 (A)
Earth and Planetary Science L~,tters, 75 (1985) 72-76 Elsevier Science Publishers B.V.. Amsterdam - Printed in ]'he Netherlands
72
Ill
~°Be, 26A1,53Mn, and light noble gases in the Antarctic shergottite EETA 79001 (A) R. Sarafin ~, U. H e r p e r s i, p. Signer 2 R. Wieler 2 G. Bonani ~ H.J. Hofmann 3 E. M o r e n z o n i 3, M. Nessi 3, M. Suter 3 a n d W. W61fli 3 J ,~thteilung Nukleam'henne der Unn'ers'itiJt zu Ki)ln, D-5000 Koln 1 (1-~R. G.) : Institut fitr Krtstallographie und Petrographie, E T t t Zi~rtch, ('11-8092 Ziirich (Swtt:erlandl ~ Institut fi~r Mttteh'nergtePhvsik, E T t l Himggerberg. C t t - 8 0 9 3 Zitrich (Swtt.:erland) Received February 18, 1985: revised version accepted June 18, 1985 The radionuclides I°Be, "~6AI, and "SMn as well as the noble gases He, Ne, and Ar were measured on aliquots of a sample of lithology A from the shergottite EETA 79001. The radionuclide data yield an exposure age of 0.78 + 0.14 Ma and a terrestrial residence time of 0.32 + 0.17 Ma. The exposure ages derived from ~He, -~lNe, and ~SAr all range between 0.4 and 0.6 Ma. These data rule out a common ejection of all known shcrgottites from their parent body as small objects in a single event about 2.5 Ma ago.
!. Introduction
During the last two decades planet Mars has been proposed as one of the parent bodies of meteorites. Based on orbital considerations W~nke [1] first suggested that some of the ordinary chondrites might come from Mars. Later, the chemical differentiation and the young crystallization ages of the SNC meteorites (shergottites, nakhlites, chassignites) became arguments for a Martian origin [2-4]. Recently, results of studies of noble gases and nitrogen in various lithologies of the Antarctic shergottite EETA 79001 [5-8] were again interpreted to argue that this object, and consequently the other SNC meteorites, come from the surface of Mars. ('osmic ray exposure ages can shed light on the time of ejection of meteorites from their parent bodies. The three nakhlites and Chassigny have exposure ages of about 10-11 Ma, while for the shergottites, with exception of EETA 79001, these ages are around 2.6 Ma [8], indicating ejection of the SNC meteorites in two events. 10 and 2.6 Ma ago, respectively. Puzzling, however, is the fact that the cosmic ray exposure age of the EETA 79001 shergottite is not the same as that of the other known shergotrites [8]. A crucial point in the determination of
0012-821X/85/$03.30
' 1985 Elsevier Science Publishers B.V.
exposure ages is the question of appropriate production rates of cosmogenic nuclides, given their dependence on chemical composition and preatmospheric shielding. Investigations of EETA 79001 exposure history along these lines have been performed by various authors [6-10]. According to all these studies, the noble gas exposure age is less than 1 Ma. The difference from the ages of other shergottites has been addressed by several scenarios for generating these meteorites, ranging from multiple impacts in the same source region on Mars a few million years apart to progressive break-up of a single meteoroid large enough to provide partial or complete shielding from cosmic radiation in its interior [6,8]. Diffusive losses of noble gases could lower the apparent exposure age of EETA 79001. This explanation would avoid the need to create a third group of exposure ages of SNC's. Conversely, the terrestrial age of EETA 79001 has to be considered. Investigations on Antarctic specimens revealed terrestrial ages of up to several hundred thousand years [ 11 -- 13]. An exceptionally long residence time of about 2 Ma for EETA 79001 would allow all shergottites to have been ejected in a single event [6]. By measurements of 26A1, t°Be, 5~Mn, and light noble gases in one single sample of lithology A, we
73
have attempted to clarify the question of the exposure and terrestrial ages of EETA 79001.
1. Experimental The sample investigated in this study was an aliquot of the combined fragments 23, 24, and 35 of lithology A. These fragments had a total mass of 15.2 g and had been well homogenized after grinding to a particle size < 160 /~m (E. Jarosewich, personal communication). The initial location of the chips is shown in Fig. 1. A 9.3 g split of the sample was subject to non-destructive 26A1 analysis by "/-3' coincidence counting. Details of the technique have been described earlier [14]. Another 1.0 g sample was split into aliquots for the determination of the other cosmogenic nuclides. V~Be was determined by AMS counting using the tandem accelerator facility at the ETH Ziarich. Sample preparation and reproducibility of analysis have been discussed elsewhere [15]. The determination of ~3Mn was performed by neutron activation [14,16]. Concentrations and isotopic compositions of the light noble gases were analyzed by static mass spectrometry [17]. 3. Results
Table 1 summarizes the results for the radionuclides. Note that the 1°Be concentration is about 30% lower than the value of 7.8 + 1.1 d p m / k g quoted by Pal et al. [18] although both measure-
EETA
79001
- SLAB
Llll"w)logyA Contoet H 1cm
Fig. 1. Sample location in EETA 79001.
TABLE 1 Results of radionuclide determinations in aliquot samples of EETA 79001 and comparison with calculated values. Production rates for 26AI and I°Be were derived from elemental production rates given by Fuse and Anders [19] and Pal et al. [20] using the chemical composition determined by Burghele et al. [21]. Activity calculations are based on T~p = 0.78 Ma. T~. = 0.32 Ma and the production rates quoted Nuclide
Measured activity
26A1 (dpm/kg) l"Be(dpm/kg)
32.9_+ 1.6 32.8 (1) 5.3_+ 0.4 5.3 (2) 5.3± 0.4 62 +_42 60.5
5~Mn(dpm/kgFe)
Calculated activity
Calculated production rate 84.6 21.2 478 [16]
ments were calibrated with the same standard, which was kindly provided to us by Dr. G.F. Herzog. In view of this discrepancy, we carried out an additional analysis on another split of the meteorite sample, As seen in Table 1, the agreement between our first and second analysis is excellent. When analyzing 53Mn by neutron irradiation, the activation product 54Mn is also produced by the reaction 55Mn(n,2n)~aMn. The resulting interference, which is easily correctable in most analyses, caused a large uncertainty of the 53Mn content in EETA 79001, because our sample had a high content of stable manganese whereas the 5~Mn concentration was very low due to the short exposure age. The noble gas data are given in Table 2. They are corrected for extraction blanks and the contribution of CO 2+ on the 22Ne peak. The latter correction amounted to about 20% and caused the relatively large error of the 22Ne/21Ne ratio. No correction for 4 ° A r 2 " o n 2°Ne was necessary. Errors given for gas concentrations also include the uncertainties in the absolute calibration of the mass spectrometers, which are estimated to be less than 3%. The :'He and 21Ne concentrations in Table 2 are higher by about 15% and 20%. respectively, than the corresponding average values for 3 samples of EETA 79001 (A) reported by Becker and Pepin [6] and by Becker [7]. However, these differences correspond to about three times the stated errors only. The calculated concentration of cosmogenic
74 TABLE 2
Light noble gases in EETA 79001. lithology A. Concentrations in 10 gcm 3 STP/g. exposure ages in Ma. Sample mass was 131 mg. A b u n d a n c e s of 3lie and 21Ne assumed to be purely cosmogenic. ( ~ A r ) .... and (22Ne/21Ne)¢,,~ calculated with the following assumptions: ( 2o Ne/21 Ne)~,,, = 0.91, ( 2o Ne/22 Ne) ....... ~.,,~= 9.8, (36Ar/3SAr),.,,, - 0.63, (3e'Ar/aSAr) .......~,,~ = 5.35. For production rates see text 3 He 1.00 +0.06
'~H e / 3 He 39.3 +0.2
31 l e / 2 1 N e 7.25_+0.7
2~ Ne 0.138 _+0.008
2o N e / " q Ne 1.21 .+ 0.06
2z N e / . , I Ne 1.27 _+0.07
( -': No/-~l No) .... 1.24 _+0.07
T( 21 Ne) 0.46-0.61
3XAr 0.076 +_0.004
a6Ar/~Ar 2.21 _+0.10
~a'Ar/~'Ar 623 _+40
(~8Ar) .... 0.050 + 0.(X14
T(3SAr) (/.38-0.54
3SAr is about 35% larger than the respective mean value in [6.7] but in view of the errors quoted it is uncertain whether this difference is significant. The 3He, 2~Ne, and 38Ar~,,~concentrations reported here are within the range of 3He, 2~Ne~,,~, and 38Ar~,,~ values, respectively, from several samples of EETA 79001 A measured by Bogard et al. [8]. 4. Discussion
T( 3 He) 0.470.61
(Tt/2 = 1.6 Ma) together with 26A1 (/'1/2=0.72 Ma) to calculate the exposure age and the terrestrial residence time. The observed S3Mn activity must then be compatible with the exposure history derived from the other radionuclides. We used an iterative calculation that, in the first step. assumed the terrestrial age T,~rr to be zero. A first estimate of exposure age T¢~p was then calculated from the measured roBe activity m Be,,, by solving the equation:
4.1. Radionuclide exposure age and terrestrial age
Pm
1
T.,p = ),,,~ • In Production rates of 26A1 and roBe were calculated from the contributions of the various target elements [19,20]. This procedure is valid if our sample of EETA 79001 was exposed to the cosmic radiation under shielding conditions comparable to those of the meteorites used for the determinations of the elemental production rates. Investigations by Bogard et al. [8] suggest this assumption to be reasonable. The chemical composition of the "Jarosewich powder" used here was taken from the work of Burghele et al. [21]. The resulting production rates for 26A1 and l°Be are given in Table 1 (last column). They are in good agreement with values calculated by Reedy [22]. "Also for the S3Mn production from FeNi we assume mean shielding conditions and use the average saturation activity of 91 S3Mn measurements in 61 ordinary chondrites [16]. With this set of production rates we derived the cosmic ray exposure age and terrestrial residence time of EETA 79001. S~Mn (Tt/2 = 3.8 Ma) should give the best estimate of the duration of cosmic ray bombardment. However, because of the large error in the 53Mn determination, wc use roBe
(1)
P I o - I ° B e m " eX"'r"+"
where X m and P,~ are decay constant and production rate of roBe, respectively. With this exposurc age a first value for the terrestrial age was deduced using the formula:
Tt~'r
1 In = --" X 26
i°26" (I
--
e a,,.t;.,, )
,~ - A1 m
(2)
with X,~,, P26 and 2~'Alm being the decay constant. production rate and measured activity of 26A1, respectively. Based on this terrestrial residence time an improved estimate for the exposure age was calculated via equation (1), and so on. This process resulted in T~p = (0.78 + 0.14) Ma and T ~ = (0.32 + 0.17) Ma. A comparison of the calculated and measured radionuclide activities is shown in Table 1. The 50% uncertainty in the terrestrial age results from the fact that 2~'A] and roBe in E E T A 79001 are far from saturation. Applying equatkm (2) the activity of 2~AI at the time of fall is calculated from the exposure age and so the uncertainty in this age also influences the determination of the
75 terrestrial residence time. Therefore, the error in the terrestrial age is much larger than the errors in the 26A1 and ~°Be measurements. 4.2 Noble gas exposure ages
Bogard et al. [8] argue that EETA 79001 was irradiated near the center of a relatively small object (50 2 5 0 kg). With this presumption the production rates for EETA 79001 (A) using Reedy's [9,10] calculations fall within the following ranges (in 10 -~ cm ~ S T P / g Ma): P(~He): 1.75-2.0; P(2~Ne): 0.24-0.28; P(~SAr): 0.10-0.12. For comparison, the elemental production rates given by Schultz and Freundel [23] yield a value for P(21Ne) of 0.26 x 10 -8 cm ~ S T P / g Ma. Using these ranges in production rates, we obtain the exposure ages given in Table 2. If EETA 79001 was indeed irradiated in a single exposure near the center of a rather small body, we conclude that its noble gas exposure age lies between 0.4 and 0.6 Ma. This is in agreement with the value of 0.5 Ma favoured by Bogard et al. [8] and is also compatible with the range of 0.4-0.9 Ma given by Becker and Pepin [6], whereby their ages above - 0.5 Ma apply for shallow exposure in a larger body. As noted by Bogard et al. [8], it is unlikely that the low age compared to other shergottites is the result of large diffusive noble gas losses, since these are expected to affect preferentially the light gases. No evidence for such losses is observed. The slight disagreement between the radionuclide exposure age and the noble gas exposure ages might indicate that the noble gas production rates applied are somewhat too high because the meteorite was irradiated in a larger body or at shallower depth than assumed. On the other hand the difference could be due to an underestimation of ~°Be prcucluction in EETA 79001 as well. Actually, recent investigations of elemental production rates of ~°Be [24] indicate a higher production rate for the meteorite than assumed here. It is important, however, that either of the exposure ages combined with the radionuclide data leads to a terrestrial residence time smaller than 0.5 Ma.
5. Conclusions The agreement between the radionuclide exposure age and the noble gas-derived exposure
ages rules out losses of cosmogenic gases as an explanation for the low exposure age of EETA 79001 compared to those of the other shergottites. Various scenarios have been proposed to account for the exposure age distribution among the SNC meteorites [6,8]. The most straightforward interpretation of our data is to suggest ejection of EETA 79001 by an impact on its parent body 0.6-1.4 Ma ago, taking into account our limits for exposure and terrestrial ages. This, however, would require three impacts on the parent body during about the last 10 Ma, each ejecting part of the SNC meteorites (of. [6,8]). At least the two events which produced the shergottites would have taken place presumably in a well confined region, unless the common 180 Ma Rb-Sr age of the 4 shergottites [3,25] does not reflect a shock event but a major igneous process on Mars [27]. On the other hand, uncertainties in experimental data as well as in production rates are too large to recognize a possible small contribution of a 1-2 Ma pre-irradiation under very heavy shielding within a shergottite fragment ejected some 2.5 Ma ago. This scenario, discussed by Becket and Pepin [6], would require ejection of only one body from the martian surface to account for the age distribution of the known shergottities. The diameter of this meteoroid, however, would need to be >/6 m [8]. According to Singer [26] this is about the largest possible size of a body that can be accelerated to Martian escape velocity in an impact. One other scenario [6,8], however, can be ruled out. It is not possible that EETA 79001 was ejected as a small object together w:ith the other shergottites in a single event 2.5 Ma ago. This would require that the Antarctic specimen was captured by the earth more than 1.5 Ma ago, after an interplanetary transit of some 0.4 0.9 Ma duration. Our results of radionuclide determinations in EETA 79001 yield a terrestrial age of less than 0.5 Ma and clearly contradict a capture of EETA 79001 by the earth 1.5 Ma ago.
Acknowledgements The authors wish to thank NASA for providing the meteorite sample. Thank are also due to the Jhlich reactor crew for performing the neutron irradiation. Constructive reviews bv R.O. Pepin, an anonymous reviewcr, and F. Begemann helped
76
to improve the manuscript, Financial support by the Bundesministerium liar Forschung und Technologie, Bonn, and the Swiss National Science Foundation (grants No. 2.263-0.81 and 2.443-0.82) is gratefully acknowledged.
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References
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1 1t. W~.nke, Der Mond als Mutterkt)rper der BronzitChondrite, Z. Naturforsch. 21a, 93-110, 1966. 2 H.Y. McSween, E.M. Stolper, L.A. Taylor, R.A. Muntean, G.D. O'Kelley, J.S. Eldridge, S. Biswas, H.T. Ngo and M.E. Lipschutz, Petrogenetic relationship between Allan Hills 77005 and other achondrites. Earth Planet. Sci. Lett. 45, 275- 284, 1979. 3 C.-Y. Shih, L.E. Nyquist, D.D. Bogard, G.A. McKay. J.L. W~×)den. B.M. Bansal and H. Wiesmann, Chronology and petrogenesis of young achondrites, Shergotty, Zagami and A L H A 77005: late magmatism on a geologically active planet, Geochim. C o s m o c h i m Acta 46, 2323-2344. 1982. 4 M.R. Smith, J.C. I,aul, M.S. Ma, T. Iluston, R.M. Verkouteren, M.E. Lipschutz and R.A. Schmitt, Petrogencsis of the SNC (shergottites. nakhlites, chassignites) meteorites: implications for their origin from a large dynamic planet, possibly mars. Proc. 4th l,unar Planet. Sci. Conf., in: J. Geophys. Res. 89, B612-B630, 1984. 5 D.D. Bogard and P. Johnson, Martian gases in an Antarctic meteorite?, Science 221. 651 .654, 1983. 6 R.II. Becker and R.O. Pepin, The case for a martian origin of the shergottitcs: nitrogen and noble gases in t'ETA 79001, Earth Planet. Sci. Lett. 69. 225-242. 1984. 7 R.tt. Becker, private communication. 8 I).D. Bogard, L.E. Nyquist and P. Johnson. Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites, Geochim. Cosmochim. Acta 48, 1723-1739, 1984. 9 R.C. Reedy, Calculated production rates of noble gases in the SNC meteorites (abstract). in: l,unar and Planetary ~'ience XV, pp. 677-678, Lunar and Planetary Institute. Houston, Texas, 1984. 10 R.C. Reedy, private communication. 11 J.C. Evans. J.l|. Reeves and L.A. Rancitclli, Aluminum -26: survey of Victoria Land meteorites, Smithsonian Contrib. Earth Sci. 24, 70-74. 1982. 12 K. Nishiizumi, M.T. Murrell, J.R. Arnold. 1-). Elmore, R.I). Fcrraro. H.E. Gore and R.('. Finkcl. Cosmic-ray-produced ~t'CI and 53Mn in Allan Hills-77 meteorites, Earth Planet. Sci. I,ctt. 52, 31 38, 1981. 13 R. Sarafin and U. Hewers, Spallogenic 26AI and 53Mn in Antarctic meteorites and determination of exposure and terrestrial ages. Meteoritics 18, 392, 1983. 14 U. Hewers, W. l-lerr and R. W/51fle, Determination of cosmic-racy-produced nuclides 53Mn. a~Sc and 'e'AI in
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meteorites by neutron activation and g a m m a coincidence spectroscopy, in: Radioactive Dating and Methods for l,ow-Level Counting, pp. 199-205. IAEA, Vienna, 1967. R. Sarafin, G. Bonani, U. Hewers. P. Signer, H.J. Hofmann, M. Nessi, E. Morenzoni, M. Surer, R. Wieler and W. W61fli, Spallogenic nuclides in meteorites by conventional and accelerator mass spectrometry, Nucl. lnstrum. Methods B5, 411-414, 1984. P. Englert and W. Herr, A study on exposure ages based on spallogenic S3Mn, Geochim. Cosmochim. Acta 42, 1635-1643. 1978. P. Signer, H. Baur, U. Derkscn, Ph. Etique, H. Funk, P. Horn and R. Wieler. Helium. neon, and argon records of lunar soil evolution. Proc. 8th Lunar Sci. Conf., pp. 3657. 3683, 1977. I).K. Pal, C. Tuniz, R.K. Moniot, W. Savin, S. Vajda, T. Kruse and G.F. Herzog, I~Be contents of SNC meteorites (abstract), in: Workshop on Cosmogenic Nuclides, pp. 33-34, Los Alamos National l,aboratory, Los Alamos, 1984. K. Fuse and E. Anders, Aluminium-26 in meteorites, VI. Achondrites, G e o c h i m C o s m t ~ h i m Acta 33, 653-670, 1969. D.K. Pal, C. Tuniz, R.K. Moniot, W. Savin. T.H. Kruse and G.F. Herzog. Composition dependence of the beryllium-10 production rate in stony meteorites (abstract), in: Lunar and Planetary ,Science XIV, pp. 588-589, Lunar and Planetary Institute. Houston, Texas, 1983. A. Burghelc, G. Dreibus, H. Palmc, W. Rammensee, B. Spettcl. G. Weckwerth and H. Wanke, Chemistry of shcrgottites and the Shergotty parent body (SPB): further evidence for the two component model of planet formation (abstract), in: I,unar and Planetary Science XIV, pp. 80 81, Lunar and Planetary Institute, Houston, Texas, 1983. R.C. Reedy, Cosmogenic radionuclides and exposure histories of SNC meteorites. Meteoritics 19, 297-298. 1984. I,. Schultz and M. Freundel, On the production rate of 21Ne in ordinary chondrites, in: Proc. Int. Conf. Isotope Ratios in the Solar System. Paris, 1984 (in press). C. Tuniz, ('.M. Smith, R.K. Moniot, T.H. Kruse, W. Savin, I).K. Pal. G.F. Iterzog and R.C. Reedy, Beryllium-10 contents of core samples from the St. Severin meteorite, Geochim. ('osmochim. Acta 48, 1867-1872, 1984. J. Wt~den, C.-Y. Shih, L. Nyquist, B. Bansal. H. Wiesmann and (i. McKay, Rb-Sr and Sm-Nd isotopic constraints on the origin of EETA 79001: a second Antarctic shergottite, (abstract), in: Lunar and Planetary Science XIII, pp. 879-880, l,unar and Planetary Institute, Houston. Texas. 1982. A.V. Singer. The effect of an impact-generated gas cloud on the acceleration of solid ejecta (abstract), in: Lunar and Planetary Science XIV, pp. 704-705, Lunar and Planetary Institute, Houston, Texas, 1983. J.tl. Jones, The youngest meteorit'es. !. A 180 m.y. igneous age for the shergottites .--the constraint of petrography (abstract), in: i,unar and Planetary Science XVI, pp. 406-407, l,unar and Planetary Institute, Houston. Texas, 1985.
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~°Be, 26A1,53Mn, and light noble gases in the Antarctic shergottite EETA 79001 (A) R. Sarafin ~, U. H e r p e r s i, p. Signer 2 R. Wieler 2 G. Bonani ~ H.J. Hofmann 3 E. M o r e n z o n i 3, M. Nessi 3, M. Suter 3 a n d W. W61fli 3 J ,~thteilung Nukleam'henne der Unn'ers'itiJt zu Ki)ln, D-5000 Koln 1 (1-~R. G.) : Institut fitr Krtstallographie und Petrographie, E T t t Zi~rtch, ('11-8092 Ziirich (Swtt:erlandl ~ Institut fi~r Mttteh'nergtePhvsik, E T t l Himggerberg. C t t - 8 0 9 3 Zitrich (Swtt.:erland) Received February 18, 1985: revised version accepted June 18, 1985 The radionuclides I°Be, "~6AI, and "SMn as well as the noble gases He, Ne, and Ar were measured on aliquots of a sample of lithology A from the shergottite EETA 79001. The radionuclide data yield an exposure age of 0.78 + 0.14 Ma and a terrestrial residence time of 0.32 + 0.17 Ma. The exposure ages derived from ~He, -~lNe, and ~SAr all range between 0.4 and 0.6 Ma. These data rule out a common ejection of all known shcrgottites from their parent body as small objects in a single event about 2.5 Ma ago.
!. Introduction
During the last two decades planet Mars has been proposed as one of the parent bodies of meteorites. Based on orbital considerations W~nke [1] first suggested that some of the ordinary chondrites might come from Mars. Later, the chemical differentiation and the young crystallization ages of the SNC meteorites (shergottites, nakhlites, chassignites) became arguments for a Martian origin [2-4]. Recently, results of studies of noble gases and nitrogen in various lithologies of the Antarctic shergottite EETA 79001 [5-8] were again interpreted to argue that this object, and consequently the other SNC meteorites, come from the surface of Mars. ('osmic ray exposure ages can shed light on the time of ejection of meteorites from their parent bodies. The three nakhlites and Chassigny have exposure ages of about 10-11 Ma, while for the shergottites, with exception of EETA 79001, these ages are around 2.6 Ma [8], indicating ejection of the SNC meteorites in two events. 10 and 2.6 Ma ago, respectively. Puzzling, however, is the fact that the cosmic ray exposure age of the EETA 79001 shergottite is not the same as that of the other known shergotrites [8]. A crucial point in the determination of
0012-821X/85/$03.30
' 1985 Elsevier Science Publishers B.V.
exposure ages is the question of appropriate production rates of cosmogenic nuclides, given their dependence on chemical composition and preatmospheric shielding. Investigations of EETA 79001 exposure history along these lines have been performed by various authors [6-10]. According to all these studies, the noble gas exposure age is less than 1 Ma. The difference from the ages of other shergottites has been addressed by several scenarios for generating these meteorites, ranging from multiple impacts in the same source region on Mars a few million years apart to progressive break-up of a single meteoroid large enough to provide partial or complete shielding from cosmic radiation in its interior [6,8]. Diffusive losses of noble gases could lower the apparent exposure age of EETA 79001. This explanation would avoid the need to create a third group of exposure ages of SNC's. Conversely, the terrestrial age of EETA 79001 has to be considered. Investigations on Antarctic specimens revealed terrestrial ages of up to several hundred thousand years [ 11 -- 13]. An exceptionally long residence time of about 2 Ma for EETA 79001 would allow all shergottites to have been ejected in a single event [6]. By measurements of 26A1, t°Be, 5~Mn, and light noble gases in one single sample of lithology A, we
73
have attempted to clarify the question of the exposure and terrestrial ages of EETA 79001.
1. Experimental The sample investigated in this study was an aliquot of the combined fragments 23, 24, and 35 of lithology A. These fragments had a total mass of 15.2 g and had been well homogenized after grinding to a particle size < 160 /~m (E. Jarosewich, personal communication). The initial location of the chips is shown in Fig. 1. A 9.3 g split of the sample was subject to non-destructive 26A1 analysis by "/-3' coincidence counting. Details of the technique have been described earlier [14]. Another 1.0 g sample was split into aliquots for the determination of the other cosmogenic nuclides. V~Be was determined by AMS counting using the tandem accelerator facility at the ETH Ziarich. Sample preparation and reproducibility of analysis have been discussed elsewhere [15]. The determination of ~3Mn was performed by neutron activation [14,16]. Concentrations and isotopic compositions of the light noble gases were analyzed by static mass spectrometry [17]. 3. Results
Table 1 summarizes the results for the radionuclides. Note that the 1°Be concentration is about 30% lower than the value of 7.8 + 1.1 d p m / k g quoted by Pal et al. [18] although both measure-
EETA
79001
- SLAB
Llll"w)logyA Contoet H 1cm
Fig. 1. Sample location in EETA 79001.
TABLE 1 Results of radionuclide determinations in aliquot samples of EETA 79001 and comparison with calculated values. Production rates for 26AI and I°Be were derived from elemental production rates given by Fuse and Anders [19] and Pal et al. [20] using the chemical composition determined by Burghele et al. [21]. Activity calculations are based on T~p = 0.78 Ma. T~. = 0.32 Ma and the production rates quoted Nuclide
Measured activity
26A1 (dpm/kg) l"Be(dpm/kg)
32.9_+ 1.6 32.8 (1) 5.3_+ 0.4 5.3 (2) 5.3± 0.4 62 +_42 60.5
5~Mn(dpm/kgFe)
Calculated activity
Calculated production rate 84.6 21.2 478 [16]
ments were calibrated with the same standard, which was kindly provided to us by Dr. G.F. Herzog. In view of this discrepancy, we carried out an additional analysis on another split of the meteorite sample, As seen in Table 1, the agreement between our first and second analysis is excellent. When analyzing 53Mn by neutron irradiation, the activation product 54Mn is also produced by the reaction 55Mn(n,2n)~aMn. The resulting interference, which is easily correctable in most analyses, caused a large uncertainty of the 53Mn content in EETA 79001, because our sample had a high content of stable manganese whereas the 5~Mn concentration was very low due to the short exposure age. The noble gas data are given in Table 2. They are corrected for extraction blanks and the contribution of CO 2+ on the 22Ne peak. The latter correction amounted to about 20% and caused the relatively large error of the 22Ne/21Ne ratio. No correction for 4 ° A r 2 " o n 2°Ne was necessary. Errors given for gas concentrations also include the uncertainties in the absolute calibration of the mass spectrometers, which are estimated to be less than 3%. The :'He and 21Ne concentrations in Table 2 are higher by about 15% and 20%. respectively, than the corresponding average values for 3 samples of EETA 79001 (A) reported by Becker and Pepin [6] and by Becker [7]. However, these differences correspond to about three times the stated errors only. The calculated concentration of cosmogenic
74 TABLE 2
Light noble gases in EETA 79001. lithology A. Concentrations in 10 gcm 3 STP/g. exposure ages in Ma. Sample mass was 131 mg. A b u n d a n c e s of 3lie and 21Ne assumed to be purely cosmogenic. ( ~ A r ) .... and (22Ne/21Ne)¢,,~ calculated with the following assumptions: ( 2o Ne/21 Ne)~,,, = 0.91, ( 2o Ne/22 Ne) ....... ~.,,~= 9.8, (36Ar/3SAr),.,,, - 0.63, (3e'Ar/aSAr) .......~,,~ = 5.35. For production rates see text 3 He 1.00 +0.06
'~H e / 3 He 39.3 +0.2
31 l e / 2 1 N e 7.25_+0.7
2~ Ne 0.138 _+0.008
2o N e / " q Ne 1.21 .+ 0.06
2z N e / . , I Ne 1.27 _+0.07
( -': No/-~l No) .... 1.24 _+0.07
T( 21 Ne) 0.46-0.61
3XAr 0.076 +_0.004
a6Ar/~Ar 2.21 _+0.10
~a'Ar/~'Ar 623 _+40
(~8Ar) .... 0.050 + 0.(X14
T(3SAr) (/.38-0.54
3SAr is about 35% larger than the respective mean value in [6.7] but in view of the errors quoted it is uncertain whether this difference is significant. The 3He, 2~Ne, and 38Ar~,,~concentrations reported here are within the range of 3He, 2~Ne~,,~, and 38Ar~,,~ values, respectively, from several samples of EETA 79001 A measured by Bogard et al. [8]. 4. Discussion
T( 3 He) 0.470.61
(Tt/2 = 1.6 Ma) together with 26A1 (/'1/2=0.72 Ma) to calculate the exposure age and the terrestrial residence time. The observed S3Mn activity must then be compatible with the exposure history derived from the other radionuclides. We used an iterative calculation that, in the first step. assumed the terrestrial age T,~rr to be zero. A first estimate of exposure age T¢~p was then calculated from the measured roBe activity m Be,,, by solving the equation:
4.1. Radionuclide exposure age and terrestrial age
Pm
1
T.,p = ),,,~ • In Production rates of 26A1 and roBe were calculated from the contributions of the various target elements [19,20]. This procedure is valid if our sample of EETA 79001 was exposed to the cosmic radiation under shielding conditions comparable to those of the meteorites used for the determinations of the elemental production rates. Investigations by Bogard et al. [8] suggest this assumption to be reasonable. The chemical composition of the "Jarosewich powder" used here was taken from the work of Burghele et al. [21]. The resulting production rates for 26A1 and l°Be are given in Table 1 (last column). They are in good agreement with values calculated by Reedy [22]. "Also for the S3Mn production from FeNi we assume mean shielding conditions and use the average saturation activity of 91 S3Mn measurements in 61 ordinary chondrites [16]. With this set of production rates we derived the cosmic ray exposure age and terrestrial residence time of EETA 79001. S~Mn (Tt/2 = 3.8 Ma) should give the best estimate of the duration of cosmic ray bombardment. However, because of the large error in the 53Mn determination, wc use roBe
(1)
P I o - I ° B e m " eX"'r"+"
where X m and P,~ are decay constant and production rate of roBe, respectively. With this exposurc age a first value for the terrestrial age was deduced using the formula:
Tt~'r
1 In = --" X 26
i°26" (I
--
e a,,.t;.,, )
,~ - A1 m
(2)
with X,~,, P26 and 2~'Alm being the decay constant. production rate and measured activity of 26A1, respectively. Based on this terrestrial residence time an improved estimate for the exposure age was calculated via equation (1), and so on. This process resulted in T~p = (0.78 + 0.14) Ma and T ~ = (0.32 + 0.17) Ma. A comparison of the calculated and measured radionuclide activities is shown in Table 1. The 50% uncertainty in the terrestrial age results from the fact that 2~'A] and roBe in E E T A 79001 are far from saturation. Applying equatkm (2) the activity of 2~AI at the time of fall is calculated from the exposure age and so the uncertainty in this age also influences the determination of the
75 terrestrial residence time. Therefore, the error in the terrestrial age is much larger than the errors in the 26A1 and ~°Be measurements. 4.2 Noble gas exposure ages
Bogard et al. [8] argue that EETA 79001 was irradiated near the center of a relatively small object (50 2 5 0 kg). With this presumption the production rates for EETA 79001 (A) using Reedy's [9,10] calculations fall within the following ranges (in 10 -~ cm ~ S T P / g Ma): P(~He): 1.75-2.0; P(2~Ne): 0.24-0.28; P(~SAr): 0.10-0.12. For comparison, the elemental production rates given by Schultz and Freundel [23] yield a value for P(21Ne) of 0.26 x 10 -8 cm ~ S T P / g Ma. Using these ranges in production rates, we obtain the exposure ages given in Table 2. If EETA 79001 was indeed irradiated in a single exposure near the center of a rather small body, we conclude that its noble gas exposure age lies between 0.4 and 0.6 Ma. This is in agreement with the value of 0.5 Ma favoured by Bogard et al. [8] and is also compatible with the range of 0.4-0.9 Ma given by Becker and Pepin [6], whereby their ages above - 0.5 Ma apply for shallow exposure in a larger body. As noted by Bogard et al. [8], it is unlikely that the low age compared to other shergottites is the result of large diffusive noble gas losses, since these are expected to affect preferentially the light gases. No evidence for such losses is observed. The slight disagreement between the radionuclide exposure age and the noble gas exposure ages might indicate that the noble gas production rates applied are somewhat too high because the meteorite was irradiated in a larger body or at shallower depth than assumed. On the other hand the difference could be due to an underestimation of ~°Be prcucluction in EETA 79001 as well. Actually, recent investigations of elemental production rates of ~°Be [24] indicate a higher production rate for the meteorite than assumed here. It is important, however, that either of the exposure ages combined with the radionuclide data leads to a terrestrial residence time smaller than 0.5 Ma.
5. Conclusions The agreement between the radionuclide exposure age and the noble gas-derived exposure
ages rules out losses of cosmogenic gases as an explanation for the low exposure age of EETA 79001 compared to those of the other shergottites. Various scenarios have been proposed to account for the exposure age distribution among the SNC meteorites [6,8]. The most straightforward interpretation of our data is to suggest ejection of EETA 79001 by an impact on its parent body 0.6-1.4 Ma ago, taking into account our limits for exposure and terrestrial ages. This, however, would require three impacts on the parent body during about the last 10 Ma, each ejecting part of the SNC meteorites (of. [6,8]). At least the two events which produced the shergottites would have taken place presumably in a well confined region, unless the common 180 Ma Rb-Sr age of the 4 shergottites [3,25] does not reflect a shock event but a major igneous process on Mars [27]. On the other hand, uncertainties in experimental data as well as in production rates are too large to recognize a possible small contribution of a 1-2 Ma pre-irradiation under very heavy shielding within a shergottite fragment ejected some 2.5 Ma ago. This scenario, discussed by Becket and Pepin [6], would require ejection of only one body from the martian surface to account for the age distribution of the known shergottities. The diameter of this meteoroid, however, would need to be >/6 m [8]. According to Singer [26] this is about the largest possible size of a body that can be accelerated to Martian escape velocity in an impact. One other scenario [6,8], however, can be ruled out. It is not possible that EETA 79001 was ejected as a small object together w:ith the other shergottites in a single event 2.5 Ma ago. This would require that the Antarctic specimen was captured by the earth more than 1.5 Ma ago, after an interplanetary transit of some 0.4 0.9 Ma duration. Our results of radionuclide determinations in EETA 79001 yield a terrestrial age of less than 0.5 Ma and clearly contradict a capture of EETA 79001 by the earth 1.5 Ma ago.
Acknowledgements The authors wish to thank NASA for providing the meteorite sample. Thank are also due to the Jhlich reactor crew for performing the neutron irradiation. Constructive reviews bv R.O. Pepin, an anonymous reviewcr, and F. Begemann helped
76
to improve the manuscript, Financial support by the Bundesministerium liar Forschung und Technologie, Bonn, and the Swiss National Science Foundation (grants No. 2.263-0.81 and 2.443-0.82) is gratefully acknowledged.
15
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16
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