| Popis: |
The efforts I made for the analysis of boron isotope composition in meteorites are first discussed. Alkalifusion followed by boron-selective anion exchange resinpurification is not suitable for the analysis of silicaterocks because of precipitation in the sample solution andconsequent loss of boron. An HF dissolution followed bycation exchange resin and anion exchange resin purificationneeds improvement to remove some interference elements.Mass spectrometry of boron converted to potassium borate hasa standard deviation of 0.08% (2σ) which is acceptable forboron isotope analyses. Thirty six fragments of meteorite falls, never touchedby water or other possible sources of boron contaminationwere analyzed for B by prompt gamma-ray neutron activationanalysis at McMaster University and at the U.S. NationalInstitute of Standards and Technology. Boron concentrationsare close to the sensitivity limit in both laboratories.Results between the two laboratories agree well, but withslight systematic differences attributable to blank andbackground correction factors. The mean B concentrations in different meteorites are similar, from 0.5 to 0.9 ppm, and the ranges in differentcarbonaceous chondrite (CC), ordinary chondrite (OC) andachondrite (ACH) classes overlap, mostly from 0.3 to 1.4ppm. similar to previous measurements on falls. H, Land LLordinary chondrites overlap in B content with Antarcticchondrites. The solar system abundance, taken as the mean B contentof the matrix in all carbonaceous chondrites was calculatedfrom seven samples and is 0.69 ± 0.09 ppm. When normalized,this abundance is 16.9 ± 2.2 (atoms/10^σSi). Normalized Band S concentrations show a linear relationship in the CC;the average OC lies on the sarne line, but individual OCs aredispersed. This solar system abundance supports a nucleosynthesismodel, in which boron was formed by continual bombardment ofinterstellar medium (ISM) by the galactic cosmic rays(GCRs), to which a very intense low energy is added. The Si and CI meteorite normalized abundances ofmoderately volatile and low-refractory elements incarbonaceous chondrites show a linear correlation with theircondensation 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. Doctor of Philosophy (PhD) |
