Isotopic Composition of Strontium in Carbonatites

Abstract

IN view of Dr. Bailey's remarks it must be admitted that my phrase “carbonatites show little evidence of assimilation on this scale” was rather ill-chosen. Near the margins of carbonatite plugs inclusions of gneiss transformed into feldspathic or phlogopite rocks are common, and local enrichment of the carbonatite in ferromagnesian silicates is very suggestive of assimilation. The significant point, however, and the one I had intended to convey, is that this assimilation has surprisingly little effect on the carbonatite as a whole, apart from causing a modest increase in the silicate content. So far as strontium is concerned, the negligible effect of assimilation is witnessed by the very limited range of 87Sr/86Sr ratios in the 30 carbonatite specimens examined by Powell and Hamilton, as these could scarcely have failed to include some with sialic contamination. This may be largely due to the great abundance of strontium in carbonatites, which ranges from about 1,000 to more than 10,000 p.p.m., and may average perhaps 4,000 or 5,000 p.p.m., a value so high as to be little affected by assimilation of granite with probably less than 400 p.p.m. strontium, despite its higher 87Sr/86Sr ratio.