The geochemistry of the stable isotopes of silicon

Details The geochemistry of the stable isotopes of silicon The geochemistry of the stable isotopes of silicon

Aug 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982gecoa..46.1449d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 46, Issue 8, pp.1449-1458

Physics

30

Scientific paper

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 30 Si found is 6.2%., centered on the mean of terrestrial mafic and ultramafic igneous rocks, 30 Si = -0.4%.. Igneous rocks show limited (1.1%.) variation; coexisting minerals exhibit small, systematic silicon isotopic fractionations that are roughly 1/3 the magnitude of concomitant oxygen isotopic fractionations at 1150°C. In both igneous minerals and rocks, 30 Si shows a positive correlation with silicon content, as does 18 O. Opal from both sponge spicules and sinters is light, with and -1.4%., respectively. Large 30 Si values of both positive and negative sign are reported for the first time from clay minerals (-2.3 to +1.8%.), opaline phytoliths (-1.4 to +2.8%.), and authigenic quartz (+ 1.4%.). 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 3.5%. is postulated to occur during the low temperature precipitation of opal and, possibly, poorly ordered phyllosilicates, with the silicate phase being enriched in 28 Si. This fractionation, coupled with a Rayleigh precipitation model, is capable of explaining most non-magmatic 30 Si variations. Chert 30 Si values are largely inherited, but the primary opal 30 Si values can be modified by isotopic equilibration of silicate silicon and dissolved silicon during the transformation of opal into quartz.

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