A sequential sample treatment system is described that permits use of an isotope-ratio mass spectrometer for automated Rittenberg analyses of ammonium salts. This system is based on the design of McInteer and Montoya [Recent Developments in Mass Spectrometry in Biochemistry, Medicine and Environmental Research, edited by A. Frigerio (Elsevier, Amsterdam, 1981), Vol. 7, p. 343]. An x-y table and a set of valves under computer control are used to automatically perform hypobromite oxidations, introduce the N2 thus liberated to the mass spectrometer for isotope-ratio analysis, and effect spent sample pumpaway. Analyses can be performed on samples containing 20 μg of N. When operated with a Nuclide Model 3-60-RMS double-collector mass spectrometer, the standard deviation for measurements at the natural abundance level (12 samples, 50 or 100 μg of N per sample) ranged from 0.0002 to 0.000 06 at. % 15N. Throughput capacity is 100–250 samples per day, and unattended operation can continue for at least three days.
Eighteen mica specimens from pegmatites of the Blue Ridge and Piedmont provinces yield Rb/Sr apparent ages of 250 to 510 million years (m.y.). Muscovites from the Piedmont generally appear to be younger (avg. about 285 m.y.) than those from the Blue Ridge (avg. about 315 m.y.). Highly discordant 'ages' were found for coexisting biotite and muscovite from two Blue Ridge pegmatites; the biotite 'ages' are about 275 m.y. while the muscovites both yield 'ages' of 510 m.y. Loss of radiogenic Sr 87 from the biotite seems the most likely explanation of the differences; some of this may have been gained by the muscovite, thereby raising its 'age.' Some possible causes of discordance seem excluded by this study; these include (a) different crystallization times for biotite and muscovite, (b) preferential loss of rubidium from the muscovite, (c) original Sr 87 abundances in the pegmatitic fluids markedly higher than 7.02 per cent, and (d) assimilation by the crystallizing pegmatites of country rock with an anomalously high Sr •* abundance. In general, the study supports the previous suggestion by others that widespread metamorphism and pegmatite emplacement took place in the Blue Ridge in two periods, about 500 and 350 m.y. ago; however, the 'ages' found for the eight Piedmont samples range from 255 to 330 m.y. and do not support the idea of a single 250-m.y. event in this region.
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