Boron cosmochemistry. Part II: Boron nucleosynthesis and condensation temperature

Abstract

The new B solar-system abundance calculated by Zhai and Shaw (1994), 16.9 atoms/106 Si (or 606 atoms/10'2 H) is used to reevaluate the different possibilities of LiBeB (except 'Li) nucleosynthesis. The revised abundances support two models: (1) Light elements were formed by continual bombardment of interstellar medium (ISM) by galactic cosmic rays (GCRs), but these galactic cosmic rays should contain a very intense low-energy component, in the form of E-' , which cannot be observed near the Earth due to solar modulation effects; (2) Light elements are a mixture of two sources. In the first source, light elements were synthesized by continual bombardment of interstellar medium by galactic cosmic rays. In the second source, they were made by the interactions of C and 0 nuclei ejected from supernovae with the H and He in the surrounding gas. The first source constitutes -46% of total B.The Si-normalized and CI-meteorite-normalized abundances of common and volatile elements in carbonaceous chondrites show a linear correlation with their condensation temperatures. Using this relationship and the normalized B abundances in CM, CO, and CV meteorites, we can estimate the B condensation temperature to be -910 K, which is similar to Ga. Author Observed solar-system abundance* Conclusion for LiBeB Major source Li Be B Nucleosynthesis Meneguzzi eta[. (1971) Weller et al. (1977) Walker et al. (1985) Present work Solar photosphere CI meteorites Stellar photosphere CI meteorites lo** 53,197-214. COMPTEL observations of the orion complex: Evidence for cosmic-ray induced gamma-ray lines. Astron. Astrophys. 281, L5-L8. BOESGAARD A. M. (1985) Big Bang nucleosynthesis: Theories and observations. Ann. Rev. Astrophys. 23,319-378.BOESGAARD A. M. AND HEACOX W. D. (1978) The abundance of boron in Band A-type stars. Astrophys. J. 226,

Distribution of lithium, beryllium, and boron in meteoritic chondrules

“…Volatilization of Li from molten chondrules during their formation is plausible by analogy with what is observed for Na: Na is clearly depleted (1) in chondrules relative to whole rocks and (2) in reduced type I relative to oxidized type II chondrules (e.g., Hewins, 1991). This latter difference between type I and type II can be attributed to a higher volatility of Na from silicate melts under reducing conditions Zhai, 1995;Lauretta and Lodders, 1996) than Li-bearing minerals (e.g., 50% condensation temperatures of Li is -1225 K; Wasson, 1985).…”

Reply to Comment by M. Chaussidon and F. Robert: Are the boron isotopic ratio and abundance homogeneous or heterogeneous in meteorites?

“…Therefore, it is possible that the lowenergy galactic cosmic rays (GCRs) are more intensive and the lowenergy B is more abundant in places that are far away from the Sun. The hypothesis involving the B nucleosynthesis by a low-energy flux of GCRs is not in conflict with the B abundance and isotopic compositions in meteorites (Zhai, 1995) and is not in conflict with the recent measurement of the B-isotope ratio in the interstellar periods (after Shapiro, 1991). Lowenergy GCRs are less affected by solar modulations and have higher intensity at solarquiet period than at solar-active period.…”

Comment on “Boron cosmochemistry II: Boron nucleosynthesis and condensation temperature” by M. Zhai.

“…It is now possible to evaluate the contribution of SN boron (I1B/I0B between 4.2 and 5.0) mixed with GCR boron (IIB/lOB = 2.5) in the bulk meteoritic B. This gives a range of a relative contribution of GCR boron between 1 and 34%, which is quite different from the 46% calculated by Zhai (1995).…”

“…In a recent paper, Zhai (1995) addresses the problem of the nucleosynthesis of B using the B solar-system abundance calculated by Zhai and Shaw (1994) (B/Si = 16.9 x 10"). Although it is now widely accepted that spallogenic reactions between cosmic rays and interstellar matter are the only feasible process to account for the cosmic abundance of B, a central difficulty of these models is the 'lB/loB isotope ratio.…”