Mass Spectrometric Determination of the Reiative Isotope Abundances of Caiciujm and the Determination of Geologic Age

Title: Mass Spectrometric Determination of the Relative Isotope Abundances of Calcium and the Determination of Geologic Age. Author: Milo M. Backus. Submitted to the Department of Geology and Geophysics on September 14, 1955, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Techniques of mass spectrometric determination of the relative isotope abundances of calcium were investigated, in order that the K4 0+ Ca4O decay scheme might be utilized as a check on other methods of geologic age determination, and for the purpose of investigating natural variations in the isotopic composition of calcium. Using Ca(NO3)2 on a platinum-plated tantalum filament as a primary ion source, and measuring ion currents with a vibrating reed electrometer, complete isotopic assays (except Ca4 6 ) were made with a precision of t 1% and an absolute accuracy known to within * 3% or better, on samples as small as one microgram. Isotope abundance results obtained for "'common" calcium (Devonian limestone) were, in atom percent: 4o 96.88 t 0.05 43 0.138±0.002 42 0.655±0.006 44 2.12 ± 0.04 48 0.200t0.006 Calcium extracted from the Homestead meteorite was fsotopically identical to "common" calcium to within t 1%. and the Ca46 /Ca ratio was measured as 0.000033 * 3%. Original calcium in lepidolites is f rctionated relative to "common" calcium by no more than t 2% in the Ca4/Ca4 ratio. Techniques for the separation and purification of microgram quantities of calcium from lepidolites without excessive contamination were developed. The method of isotope dilution was applied to the determination of total potassium, total calciump and radiogenic calcium in lepidolites. The K40-Ca40 "ages" for five different lepidolites were determined as: Bikita Quarry, Southern Rhodesia 1980 t 100 m.y. Bob Ingersoll Mine, Black Hills, South Dakota 1180 ± 90 m.y. Skuleboda, Sweden 1190 t 150 m-y. Brown Derby Mine, Colorado 1) 1370 * 150 m.y. 2) 1570 t130 m.y. These "ages"' are lower than "'ages" obtained on the same samples, or closely associated samples, by the Rb-Sr method and the Pb-U method. The extent of disagreement is variable, indicating that post-formational geochemical alteration of some of these lepidolites may have occurred. A determination of the radiogenic argon content of a specimen of the Bob Ingersoll lepidolite by L. Strickland, when compared to the radiogenic calcium determination for the same sample in this research, gave a branching ratio for K40 decay, M / , * 0.15 t 0.02. This determination is not consistent with the recent published determinations of /. suggested by comparisons of K40/A4O ages and Pb/U ages. Certain aspects of thermal ion production were studied and the literature on this subject reviewed. It is suggested that the mechanism of positive ion production in the system mentioned above is extremely complex, and is more intimately connected with the properties of the salt than of the filament. The measured calcium isotope ratios were observed to vary by as much as 17% during evaporation of a single sample, and the form of this fractionation as a function of percent sample emitted was not entirely reproducible. Abnormal emission of Ca*, Mg*, and Al* ions was observed at temperatures as low as 900-10000 K. Isotopic assay of alkalis is not an entirely straight forward procedure, and incorrect results may be easily obtained due to isotopic fractionation during analysis. By consistently observing only the first portion of emission, results with a reproduciblity of better than t 0.5% may be obtained.