Boron Cosmochemistry

Abstract

<p>The efforts I made for the analysis of boron isotope composition in meteorites are first discussed. Alkali<br />fusion followed by boron-selective anion exchange resin<br />purification is not suitable for the analysis of silicate<br />rocks because of precipitation in the sample solution and<br />consequent loss of boron. An HF dissolution followed by<br />cation exchange resin and anion exchange resin purification<br />needs improvement to remove some interference elements.<br />Mass spectrometry of boron converted to potassium borate has<br />a standard deviation of 0.08% (2σ) which is acceptable for<br />boron isotope analyses.</p> <p>Thirty six fragments of meteorite falls, never touched<br />by water or other possible sources of boron contamination<br />were analyzed for B by prompt gamma-ray neutron activation<br />analysis at McMaster University and at the U.S. National<br />Institute of Standards and Technology. Boron concentrations<br />are close to the sensitivity limit in both laboratories.<br />Results between the two laboratories agree well, but with<br />slight systematic differences attributable to blank and<br />background correction factors.</p> <p>The mean B concentrations in different meteorites are similar, from 0.5 to 0.9 ppm, and the ranges in different<br />carbonaceous chondrite (CC), ordinary chondrite (OC) and<br />achondrite (ACH) classes overlap, mostly from 0.3 to 1.4<br />ppm. similar to previous measurements on falls. H, Land LL<br />ordinary chondrites overlap in B content with Antarctic<br />chondrites.</p> <p>The solar system abundance, taken as the mean B content<br />of the matrix in all carbonaceous chondrites was calculated<br />from seven samples and is 0.69 ± 0.09 ppm. When normalized,<br />this abundance is 16.9 ± 2.2 (atoms/10^σSi). Normalized B<br />and S concentrations show a linear relationship in the CC;<br />the average OC lies on the sarne line, but individual OCs are<br />dispersed.</p> <p>This solar system abundance supports a nucleosynthesis<br />model, in which boron was formed by continual bombardment of<br />interstellar medium (ISM) by the galactic cosmic rays<br />(GCRs), to which a very intense low energy is added.</p> <p>The Si and CI meteorite normalized abundances of<br />moderately volatile and low-refractory elements in<br />carbonaceous chondrites show a linear correlation with their<br />condensation temperatures. Compared with other elements, the normalized boron abundances in CM, CO, and CV meteorites indicate that the boron condensation temperature is about 910 ºK, similar to gallium.</p>