Absolute isotopic abundances of Ti in meteorites

Abstract

The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using ^(46)Ti/^(48)Ti. Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. We provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components. The absolute Ti and Ca isotopic compositions still support the correlation of ^(50)Ti and ^(48)Ca effects in the FUN inclusions and imply contributions from neutron-rich equilibrium or quasi-equilibrium nucleosynthesis. The present identification of endemic effects at ^(46)Ti, for the absolute composition, implies a shortfall of an explosive-oxygen component or reflects significant isotope fractionation. Additional nucleosynthetic components are required by ^(47)Ti and ^(49)Ti effects. Components are also defined in which ^(48)Ti is enhanced. Bulk samples of carbonaceous meteorites (C2 and C3 types) show distinct excesses at ^(50)Ti but no nonlinear effects at the other Ti isotopes. Other chondrites, including Orgueil (Cl), show no nonlinear effects. Relative to terrestrial Ti, a small isotope fractionation is found for only an enstatite chondrite. The Ti absolute compositions in Ca-Al-rich inclusions show significant isotope fractionation effects corresponding to an enhancement in the heavier isotopes relative to the lighter isotopes as compared to Ti in a TiO_2 standard and in chondrites. The absence of a correlation of Ti isotope fractionation effects with those for Ca and Mg is indicative of multiple processes of condensation, volatilization and recondensation; however, the mechanisms causing the isotope fractionation are not well understood.

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