The sulfur and carbon isotope composition of scapolite-rich granulites from southern Tanzania

Abstract

Sulfur and carbon isotope data are presented of 15 granulite samples from the Furua Complex, southern Tanzania, in which scapolite is a primary and major rock-forming constituent (up to 30 vol%). From these data, the isotopic composition is deduced of the sulfate and carbonate group in the scapolite structure. Subsequently, the composition and origin is discussed of the volatile species that are present in the deep crustal environment in which these scapolites formed.

The δ ³⁴S-values show a narrow range from 0.3 to 3.6‰, consistent with a deep-seated (mantle) origin of the sulfur; the mean value of 1.9‰ is slightly higher than usually found in rocks of assumed mantle provenance. The results of the carbon isotope analyses are more difficult to interpret; they suggest that the granulites contain two different carbon components with different isotopic compositions. Firstly, one component, liberated by phosphoric acid at room temperature, has δ ¹³Cvalues between −3.8 and −11.2‰ and a mean value of −6.7‰. This carbon component is assumed to occur as finely dispersed, submicroscopic carbonate inclusions. The second carbon fraction is liberated by phosphoric acid treatment at temperatures between 200 and 400° C and has considerably lower δ ¹³Cvalues with a mean value of −14.1‰ This seems to represent the carbon isotope composition in the scapolite structure. Such low δ ¹³C-values do not agree with the generally accepted value of −7‰ for “juvenile” carbon, but they are comparable to those found in early, primary carbonic inclusions from various granulite regions. It is argued that these low δ ¹³C-values are typical for granulite-facies metamorphism and that they may characterize an important fluid phase of the lower crust.