Excess 36Ar in the Efremovka Meteorite: Evidence for Live 36Cl in the Early Solar System

The recent discovery of the presence of ^41Ca (t = 0.15 Ma) in the early solar system at the time of formation of the Efremovka CAIs [1] has prompted us to look for the possible presence of ^36Cl (t = 0.43 Ma) in this meteorite. Since ^36Cl decays to ^36Ar, an excess ^36Ar will in principle be a signature of the presence of extinct ^36Cl. However in situ (n,g) reaction on ^35Cl can also produce ^36Cl and generate excess ^36Ar. A study of all noble gases is helpful in decoupling the various ^36Ar components. Here we report the noble gas results on a bulk sample of Efremovka (a single chip adjacent to CAI E60). Work on two CAIs E40 (coarse-grained) and E42 (fine-grained) is currently in progress. All the noble gases have been analysed on VG 1200 mass spectrometer by standard procedure. After 400 degrees C combustion in 2 torr O2, mainly intended to get rid of surfical contaminants, the subsequent extractions at 900, 1200 and 1500 degrees C are carried out by pyrolysis. The Ne and Ar data for the total sample given in Table-1 have been corrected for blanks, mass discrimination and interferences as detailed in [2]. From the Ne data, we derive ^21Ne(sub)c = 4.1x10^-8ccSTP/g and (^22Ne/^21Ne)(sub)c = 1.043 +/-0.011. Using the ^21Ne production rate of Eugster [3] an exposure age of 9.0 Ma is obtained. The maximum ^36Ar release (about 86%) occurs at 1200 degrees C wherein the minimum ^38Ar/^36Ar (0.1795) is also observed. Even the ^38Ar/^36Ar value of 0.1801 for the total sample is less than the value of 0.1880 for the planetary component[4], indicating the presence of additional Ar-components in Efremovka and in particular, Ar produced by neutron capture on chlorine. If we assume that the measured ^38Ar/^36Ar value in Efremovka is a mixture of three components (trapped = 0.1880; spallogenic = 1.45-1.60 and neutron produced from Cl = 3x10^-3 [5,6]), the abundance of each of these components can be obtained, with the aid of Ne data, to be (in units of 10^-8ccSTP/g) ^36Ar(sub)t= 164.4; ^36Ar(sub)c = 0.35 and ^36Ar(sub)cl = 9.8. This value of ^36Ar(sub)cl for Efremovka is about 20 times larger than that found in the Allende meteorite belonging to the same CV3 group[5,6]. Eventhough the exposure age of Efremovka is twice that of Allende, its preatmospheric size is expected to be smaller (recovered mass about 21 kg compared to >2 tons for Allende) and hence the neutron fluence in it cannot be higher than in Allende by such large factor. Further, if we attribute the excess ^128Xe (over trapped) found in the sample to (n,g) reaction on 127I and use Cl/I = 1350 for CV3 meteorites [7], we estimate ^36Arcl due to in-situ (n,g) production in Efremovka to be about 0.3x10^-8cc STP/g only. Hence a majority of ^36Ar(sub)cl component has to be accounted for by an alternative source and the most plausible one is the in situ decay of ^36Cl present in the sample at the time of its formation. Although further studies will be needed to substantiate this conclusion, our data suggest a value of 10-6 for 36Cl/35Cl, if we assume a Cl abundance of 228 ppm for Efremovka (based on Br content of 0.76 ppm (G. Dreibus, Private communication) and Cl/Br = 300 for CV3 [7]). An earlier experiment on Allende has yielded an upper limit of about ^36Cl/^35Cl about 3x10^-9 [8]. The higher value of ^36Cl/^35Cl in Efremovka suggests the presence of ^36Cl in the early solar system and is in principle consistant with the observation of excess ^41K from the decay of ^41Ca in Efremovka CAIs [1]. References: [1] Srinivasan G. et al. (1994) Astrophys. J. Lett., 431, L67-L70. [2] Murty S. V. S. and Goswami J. N. (1992) Proc. LPS, Vol. 22, 225-237. [3] Eugster O. (1988) GCA, 52, 1649-1662. [4] Swindle T. (1988) in Meteorites and the Early Solar System, pp. 535-564. [5] Smith S. P. et al (1977) GCA, 41, 627-647. [6] Gobel R. et al. (1982) GCA, 46, 1777-1792. [7] Dreibus G. et al. (1979) in Origin and Distribution of the Elements, pp. 33-38. [8] Jordan J. and Permika E. (1981) Meteoritics, 16, 332-333.