Determination of Boron, Lithium, and Chlorine in Meteorites

Abstract

The light elements Li, Be, and B are thought to be spallation products. In this case one would expect boron to be somewhat more abundant than lithium. To the contrary, determinations of these elements in meteorites yielded a Li/B ratio of about 5 [1]. Exposure to high neutron fluxes has been proposed to change the elemental and isotopic ratios of Li and B via the reaction ¹⁰B(n, α)⁷Li [2]. Recent measurements of the spallation cross-sections of Li, Be, and B by protons from C, N, O, Ne, and the constancy of the isotope ratio of lithium in meteoritic and terrestrial matter, show that neutron induced reactions cannot have had the attributed importance. The abundance of boron relative to lithium could then be explained by a chemical fractionation process, which caused a loss of boron due to the volatility of its compounds [3] in the early days of the planetary system.

For a systematic study, a highly sensitive fluorometric method has been developed to determine the boron content in iron and stone meteorites. The detection limit lies at the ppb-level. For the lithium determination we use a thermal-neutron activation technique.

In Canon Diablo and in Toluca iron we found 0.02 and 0.03 ppm boron, respectively. Roughly a factor of 10 lower than SHIMA’S [1] value and also somewhat lower than the upper limits given by WASSON [4]. In chondrites we found a noticeable correlation between the boron content and the petrological groups of VAN SCHMUS and WOOD [5]. Boron behaves like other volatile elements as carbon [6], indium [7] and the rare gases [8, 9], for which lower concentrations were found in the more recrystallized chondrites.

The lithium content seems to be independent of the petrological type. For carbonaceous chondrites we get a B/Li ratio of about 7. This supports the idea of a chemical fractionation as a cause of the depletion of boron. If boron was lost in an early evolution process, we may assume that some unfractionated material was added later in the history of the meteoritic matter in increasing amounts from higher to lower petrologic groups.