The Strontium Isotopic Composition and Origin of Carbonatites

The carbonatite problem can be stated as follows: Are carbonatites genetically related to sedimentary carbonate rocks, or are they derived from a silicate parent material? The isotopic composition of strontium in 21 carbonatites and in several related alkaline rocks has been determined. The mean Sr87/$r86 ratio for the 21 carbonatites is 0.7065 + 0.0003. The Sr87/sr86 ratios of sedimentary carbonate rooks in general vary from 0.709-0.713. The initial Sr87/Sr86 ratios of mafic igneous rocks are approximately 0.708. It is apparent that the Sr87/Sr86 ratios in carbonatites resemble those of igneous rocks rather than those of sedimentary carbonate rocks. It is concluded that the Sr87/Sr86 ratio can be used as a cri-terion for distinguishing carbonatites from carbonate rocks of metamorphic origin. Investigation of the sr87/sr86 ratio in a limestone xenolith at Mt. Royal in Montreal and in a skarn rock at Ice River, British Columbia indicates that carbonate xenoliths preserve the Sr87/Sr86 ratio of the limestones from which they were derived. The Sr87/Sr86 ratios of the carbonatites at Oka, Quebec and at Spitzkop, Transvaal, were found to be significantly different from those of the sedimentary carbonate rocks which have been suggested as the sources of these carbonatites. The Sr87/SrS6 ratios of the carbonatites at Oka, Quebec; Rocky Boy, Montana; Magnet Cove Arkansas; and Iron Hill Colorado, were found to be identical within the analytical precision to those of the associated alkaline rooks. These results strongly support the conclusion that carbonatites are not xenoliths of limestone but are comagmatic with the alkaline rocks.