26Al homogeneity of the early solar system demonstrated by high precision Mg isotopic analyses of meteoritic chondrules with an ion microprobe (Invited)

Details 26Al homogeneity of the early solar system demonstrated by high precision Mg isotopic analyses of meteoritic chondrules with an ion microprobe (Invited) 26Al homogeneity of the early solar system demonstrated by high precision Mg isotopic analyses of meteoritic chondrules with an ion microprobe (Invited)

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v43j..01c&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #V43J-01

Mathematics

Logic

[1028] Geochemistry / Composition Of Meteorites

Scientific paper

Short-lived 26Al (T1/2 = 0.73 Myr) is potentially the most precise chronometer of events which took place during the first few Myr of the solar system. However, the resolution of the chronological information which could be obtained from variations of the 26Al/27Al ratios in the different components of chondrites (e.g. refractory inclusions -CAIs- or chondrules) has remained questionable because the level of homogeneity of 26Al in the accretion disk has never been assessed precisely from data on meteorites. Astrophysical hydrodynamic models of mixing in the solar nebula showed that 26Al injected onto the surface of the solar nebula can be spatially homogenized at a level of ±10% in a very short time of a few thousand years [1]. The level of homogeneity which could be reached for 26Al, if a significant fraction of it was produced close to the early Sun by high energy irradiation processes, has not been investigated in details. One way to quantify the level of homogeneity of 26Al in the accretion disk is to compare the composition (Mg and Al isotope compositions) of various components of meteorites (e.g. chondrules) with a theoretical growth curve of Mg isotopes for the solar nebula calculated under the assumption of homogeneous initial Mg and Al isotopic compositions. This has however never been possible because it implies that the slopes (26Al/27Al ratio at the time of crystallization) and the initial (26Mg/24Mg ratio at the time of crystallization) of mineral 26Al isochrons in chondrules are both measured with high precision. We developed high precision Mg isotopic measurements with the CRPG-CNRS (Nancy) Cameca ims 1270 multicollector ion microprobe. The measurements were made at a mass resolution of 2500 and using four Faraday cups to measure simultaneously the ion intensities of 24Mg+, 25Mg+, 26Mg+ and 27Al+. Special attention was paid to (i) the contribution of 24MgH+ on 25Mg, (ii) to the flatness of the Mg peaks and to (iii) the stability of the backgrounds of the Faraday cups. A two sigma of ±0.04‰ (and a two sigma error of ±0.005‰) was obtained for the measurement of Δ26Mg (i.e. the deviation of 26Mg/24Mg ratio from the terrestrial mass fractionation line) on our terrestrial olivine, pyroxene and silicate glass standards [2]. We determined mineral 26Al isochrons in 14 type II Fe-Mg chondrules and one Al-rich chondrule from the LL3.0 chondrite Semarkona. The chondrules Mg and Al isotope compositions plot within errors on the solar growth curve calculated with the solar (chondritic) Al/Mg ratio assuming initial 26Al/27Al and 26Mg/24Mg ratios taken to be that of bulk CAIs as determined by [3]. The fact that chondrules of various ages (various 26Al/27Al), bulk CAIs and the Earth plot on the same solar system growth curve of Mg isotopes implies that the solar system was homogeneous at better than ± 10% relative for 26Al/27Al and 26Mg/24Mg at the time of formation of CAIs [2]. The variable 26Al/27Al of chondrules imply that most of them crystallized between ≈ 1.5 Myr and ≈ 3 Myr after CAIs but that some of them formed as early as ≈ 0.9 Myr after CAIs [2]. [1] Boss A. P. (2007) Ap. J. 660, 1707-1714. [2] Villeneuve J., Chaussidon M. and Libourel G. (2009) Science 325, 985-988. [3] Jacobsen B. et al. (2008) Earth Planet Sci. Lett. 272, 353-364

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