Author

Mingzhe Zhai

Date of Award

5-1994

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

D.M. Shaw

Abstract

The efforts I made for the analysis of boron isotope composition in meteorites are first discussed. Alkali
fusion followed by boron-selective anion exchange resin
purification is not suitable for the analysis of silicate
rocks because of precipitation in the sample solution and
consequent loss of boron. An HF dissolution followed by
cation exchange resin and anion exchange resin purification
needs improvement to remove some interference elements.
Mass spectrometry of boron converted to potassium borate has
a standard deviation of 0.08% (2σ) which is acceptable for
boron isotope analyses.

Thirty six fragments of meteorite falls, never touched
by water or other possible sources of boron contamination
were analyzed for B by prompt gamma-ray neutron activation
analysis at McMaster University and at the U.S. National
Institute of Standards and Technology. Boron concentrations
are close to the sensitivity limit in both laboratories.
Results between the two laboratories agree well, but with
slight systematic differences attributable to blank and
background correction factors.

The mean B concentrations in different meteorites are similar, from 0.5 to 0.9 ppm, and the ranges in different
carbonaceous chondrite (CC), ordinary chondrite (OC) and
achondrite (ACH) classes overlap, mostly from 0.3 to 1.4
ppm. similar to previous measurements on falls. H, Land LL
ordinary chondrites overlap in B content with Antarctic
chondrites.

The solar system abundance, taken as the mean B content
of the matrix in all carbonaceous chondrites was calculated
from seven samples and is 0.69 ± 0.09 ppm. When normalized,
this abundance is 16.9 ± 2.2 (atoms/10^σSi). Normalized B
and S concentrations show a linear relationship in the CC;
the average OC lies on the sarne line, but individual OCs are
dispersed.

This solar system abundance supports a nucleosynthesis
model, in which boron was formed by continual bombardment of
interstellar medium (ISM) by the galactic cosmic rays
(GCRs), to which a very intense low energy is added.

The Si and CI meteorite normalized abundances of
moderately volatile and low-refractory elements in
carbonaceous chondrites show a linear correlation with their
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.

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