Observation of Correlated 41Ca and 26Al in CV3 Hibonites
Abstract
The demonstration of the presence of ^41Ca in the early solar system, based on the observation of excess ^41K in Efremovka CAIs [1,2] has led to several attempts that sought to identify plausible stellar sites and processes that could have produced and subsequently injected a host of short-lived radionuclides(e.g. ^41Ca, ^26Al, ^60Fe, ^53Mn and ^107Pd) into the proto-solar cloud [3,4,5]. Radiogenic excess of ^26Mg (from ^26Al decay) was also found in the Efremovka CAIs where excess ^41K was present [6,7]. This association suggests possible co-production and subsequent injection of these two short-lived nuclides to the solar nebula. We have measured K and Mg isotopic composition in a single mineral phase (hibonite) having contrasting ^26Mg excesses to further substantiate this hypothesis. We have chosen hibonites from two CV3 meteorites, Efremovka and Allende for the present study. The Efremovka hibonite is from a CAI (E50) having a well-behaved Mg-Al isotopic systematics with initial ^26Al/^27Al of (4.87 +/-0.87)x10^-5 [6]. The Allende sample (HAL hibonite) on the other hand has a very low initial ^26Al/^27Al of (5.2 +/-1.7)x10^-8 [8]. We have carried out K and Mg isotopic studies of E50 hibonite and K isotopic study of HAL hibonite using procedures described previously [1,2,6]. Because of the small size of E50 hibonites, K isotopic composition could be measured in only a few hibonite grains, where the contribution from neighbouring K-rich phase(melilite) towards the measured K signal was negligible. In the case of HAL hibonite there is a large variation in K content, and the signal at ^39K is often close to the system background, while the signal at mass 41 is dominated by the [^40Ca^42Ca]6++ interference. The magnitude of this interference was estimated by measuring the [^40Ca^43Ca]^++/^43Ca^+ ratio in terrestrial (Madagascar) hibonite as well as in the HAL hibonite. We obtained a value of (7.3 +/-0.6)x10^-6 which is about four times lower than the corresponding value for pyroxene [1,2]. Definite excess of ^41K has been found in E50 hibonites with Ca/K ratio exceeding 5x10^5. A forced linear fit of the data through the solar system ^41K/^39K ratio (0.072) yields an initial ^41Ca/^40Ca of about 1.4x10^-8, which is close to the value reported for Efremovka pyroxene [1]. The Al-Mg isotopic data for the E50 hibonites reveal ^26Mg excess in them, consistent with the results reported earlier [6]. In contrast, no excess 41K was found (within the limits of our experimental uncertainties) in HAL hibonite even though its Ca/K ratio is much higher than in Efremovka hibonite. Our data provide an upper limit of 3x10^-9 for initial ^41Ca/^40Ca in HAL hibonite. These results and data obtained from studies of other Efremovka CAIs [1,2,6,7] show that : (i) ^41Ca and ^26Al are present at the same level (^41Ca/^40Ca about 1.4x10^-8 and ^26Al/^27Al about 5x10^-5) in several Efremovka CAIs and (ii) ^41Ca is very low or absent when ^26Al is at very low level (HAL hibonite). It therefore appears that ^41Ca and ^26Al are coupled either in their original stellar source(s) or were well mixed in some parcels of interstellar material before they were injected to the solar nebula. The lower initials in HAL hibonite may reflect either a heterogeneity of ^26Al and ^41Ca in the solar nebula (due to variable mixing of the source material with nebular material) or processing of material and/or redistribution of Mg and K isotopes at a later time. Although our data cannot decide between these alternatives, extensive studies of HAL and other refractory objects indicate that many CAIs evolved through multiple events in the early solar system spreading over a significant time interval [e.g. 9-12]. References: [1] Srinivasan G. et al. (1994) Astrophys. J. Lett., 431, L67-L70. [2] Srinivasan G. et al. (1995) GCA, submitted. [3] Wasserburg G. J. et al. (1995) Astrophys. J. Lett., 440, L101-L104. [4] Cameron A. G. W. et al. (1995) Astrophys. J. Lett., in press. [5] Ramaty R et al. (1995) Astrophys. J., submitted. [6] Goswami J. N. and Srinivasan G. (1994) Proc. Indian Acad. Sci. (EPS), 103, 57-82. [7] Goswami J. N. et al. (1994) GCA, 58, 431-447. [8] Fahey A. et al. (1987) GCA, 51, 329-350. [9] Lee T. et al. (1980) GRL, 7, 493-496. [10] Brigham C. A. (1990) Ph.D. thesis, Caltech. [11] Sheng Y. J. et al. (1991) GCA, 55, 581-599. [12] Ireland T. R. et al. (1992) GCA, 56, 2503-2520.
- Publication:
-
Meteoritics
- Pub Date:
- September 1995
- Bibcode:
- 1995Metic..30..570S
- Keywords:
-
- HIBONITE;
- INCLUSIONS;
- REFRACTORY;
- ION PROBE;
- METEORITES;
- ALLENDE;
- EFREMOVKA;
- NUCLIDES;
- EXTINCT