ISOTOPIC MASS FRACTIONATION LAWS AND TEH INITIAL SOLAR SYSTEM 26AL/27AL RATIO.

Abstract

Excess 26Mg from the decay of 26Al was discovered in 1976 by thermal ionization mass spectrometry of mineral separates from a CAI 1. Since that time, the 26Al- 26Mg system has been of great interest because of its potential for precise early solar system chronology. Until recently, most of the data generated was by ion microprobe using a single collector and data that most precisely defined the early solar system 26Al/27Al ratio was based on ion probe analyses of minerals with high Al/Mg ratios. In the last few years, multi- collector ICP and multicollector ion probe mass spectrometry have made possible high precision magnesium isotopic analy- ses that provide chronologically useful data on low-Al/Mg phases. Some of these studies have suggested that the initial solar system 26Al/27Al ratio may have been higher than the canonical value of 4.5105 2,3. Magnesium has three isotopes, 24, 25 and 26, and isotope ratios vary in solar system objects by mass dependent frac- tionation and by the effects of 26Al decay. In order to infer the effects of the latter, the exact behavior of mass dependent fractionation must be known. CAIs, where most 26Al-26Mg studies are done, typically have mass fractionation effects of a few /amu and so-called FUN inclusions have larger mass fractionation signatures. The algorithm or law used to cor- rect CAI data for natural mass fractionation effects can sig- nificantly affect the inferred amount of radiogenic 26Mg, espe- cially in cases where the degree of mass fractionation is large and/or the Al/Mg in the phase analyzed is small. A variety of fractionation laws have been used to correct isotopic data. We review these laws, describe the results of experiments in which the mass fractionation law has been determined for vacuum evaporation of CAI compositions and recalculate a number of published supercanonical 26Al/27Al ratios. Mass

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