The geochemistry of the stable isotopes of silicon

One hundred thirty two new measurements of the relative abundances of the stable isotopes of silicon in terrestrial materials are presented. The total variation of delta30Si found is 6.2 parts per thousand, centered on the mean of terrestrial mafic and ultramafic igneous rocks, delta30Si = -0.4 parts per thousand. Igneous rocks show limited variation; coexisting minerals exhibit small, systematic silicon isotopic fractionations that are roughly 1/3 the magnitude of concomitant oxygen isotopic fractionations at 11500C. In both igneous minerals and rocks, delta30Si shows a positive correlation with silicon content, as does delta18O. Opal from both sponge spicules and sinters is light, with delta30Si = -2.3 and -1.4 parts per thousand respectively. Large delta30Si values of both positive and negative sign are reported for the first time from clay minerals, opaline phytoliths, and authigenic quartz. All highly fractionated samples were precipitated from solution at low temperatures; however, aqueous silicon is not measurably fractionated relative to quartz at equilibrium. A kinetic isotope fractionation of approximately 3.5 parts per thousand is postulated to occur during the low temperature precipitation of opal and, possibly, poorly ordered phyllosilicates, with the silicate phase being enriched in 28Si. This fractionation, coupled with a Rayleigh precipitation model, is capable of explaining most non-magmatic delta30Si variations. (author)