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Computer-assisted mass spectrometry and its application to rubidium-strontium geochronology Blenkinsop, John

Abstract

An on-line data acquisition system, centred around an Interdata Model 4 computer, has been designed for a mass spectrometer used primarily for rubidium-strontium geochron-ology. The digital system has been used for an investigation of the ages of certain gneisses within the southern Omineca Crystalline Belt of British Columbia. With the system, it has proved possible to achieve a precision of 0.02% (95% confidence limit) in the measurement of Sr⁸⁷/Sr⁸⁶ ratios, a precision which was necessary for the investigation described. The system employs a digital voltmeter at the mass spectrometer for analog-digital conversion of the ion beam. The "conversion complete" signal from the voltmeter is used to interrupt the computer. At each interrupt, the computer reads the mass spectrometer, filters the voltmeter reading, and adjusts the magnet scan speed as required. At the end of each mass spectrum, the data are processed and the isotopic ratios are calculated. They can be output to a digital display at the mass spectrometer as requested by the operator. Rock samples from the gneisses were obtained from areas near Revelstoke, Quesnel Lake, and Malton Range in order to test suggestions that they represent Precambrian crystalline basement. Samples from the first two locations exhibited a restricted range of Rb/Sr ratios, and required the precise determination of their Sr⁸⁷/Sr⁸⁶ ratios in order to be determined. All areas appear to have experienced an event about 700 m.yr. ago, and are, therefore, Precambrian in age. The event can possibly be correlated with the East Kootenay Orogeny. In addition, the gneiss at Revelstoke is clearly older than its surrounding metasediments, an observation which supports the thesis of J. V. Ross that it was tectonically emplaced. This latter finding has regional implications in that Precambrian crystalline basement has apparently been involved in the deformation of the region.

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