Isotopic variations in the rock-forming elements in meteorites

Abstract

Variations in isotopic abundances of the major rock-forming elements can be used as tracers for chemical processes in the solar nebula, and may also provide links to the presolar cloud from which the solar nebula was derived. Emphasis in this paper is placed on the correlation of isotopic variations between pairs of elements, both for mass-dependent fractionation effects and for nucleosynthetic effects. Variations in oxygen isotope abundances, which are ubiquitous in all Solar System matter, are decoupled from those in other elements, probably because of the effect of a large oxygen reservoir in the nebular gas. Among the metallic rock-forming elements (Mg, Si, Ca, Ti, Cr, Fe, Ni) isotopic variations are small to immeasurable in ordinary chondrites and achondrites. Large variations are observed in refractory inclusions in carbonaceous chondrites in the elements Mg, Si, Ca and Ti. Fractionation effects result from evaporation and condensation at high temperatures. The dominant nucleosynthetic effects are seen as excesses and deficiencies of the neutron-rich isotopes in the region of the iron abundance peak: 48 Ca, 49 Ti, 50 Ti, 54 Cr, 64 Ni. These effects result from mixing in different proportions of the products of different regions of a supernova. The rock-forming elements also show isotopic variations due to extinct radioactivities: 26 Mg, 53 Cr and 41 K. A local source is likely, as is heterogeneous distribution within the solar nebula.