Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p33a1006y&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P33A-1006
Other
3662 Meteorites, 1040 Isotopic Composition/Chemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1094 Instruments And Techniques
Scientific paper
Variations in intrinsic Mg isotope compositions provide a potentially rich record of the physiochemical evolution of CAIs. Moreover, Mg excesses from the short-lived 26Al chronometer can be used to constrain when these processes occurred; e.g., during the nebular phase and/or during the development of planetisimals (< 4 Myr). We obtained in situ UV (213 nm) laser ablation MC-ICPMS measurements of Al and Mg isotope ratios within core-to-rim traverses of igneous CAIs to place temporal constraints on when features of CAIs formed. Results provide tests of models for the chemical and isotopic evolution of CAIs involving volatilization and recondensation of elements in the solar nebula. We studied five CV3 CAIs, including Allende 3576-1 "b", Allende M5, Leoville 144A, Leoville MRS3, and Efremovka E44. Our sample-standard comparison approach affords a precision <0.2 \permil per amu (2s) for intrinsic Mg isotope measurements and <0.3 \permil (2s) for measured 26Mg excesses. Intra-object variation in \delta25Mg exists with values ranging from as low as -2 \permil and as high as +8 \permil (compared to DSM3). The distinct Mg isotope patterns in the CAIs are difficult to explain by a single process or within a single nebular environment and likely require changing conditions or transfer of CAIs from one nebular environment to another. The ˜pristine Mg isotope profile of Leoville 144A is compared to results produced by implicit finite difference modeling. Model curves reflect isotopic fractionation at the moving surface of a shrinking molten sphere coupled with diffusion-limited transport within the sphere. We find that using mass-dependant diffusivities increases \delta25Mg with evaporation, but does not produce the tight curvature in the edgeward increases in \delta25Mg characteristic of Leoville 144A. Three CAIs that exhibit edgeward \delta25Mg decreases are well described by diffusion in a Mg-rich chondritic environment suggestive of nebular temperatures and timescales on order of 100 yrs at 1300 K (temperatures <900 K require heating times >2 Myr, and are improbable for parent body thermal histories). We concluded that: (1) CAIs exhibit enriched \delta25Mg interiors that require evaporation of molten spheres in low total pressures, and/or low Mg partial pressure environments and systematic edgeward mineral independent intrinsic Mg isotope variations (-2 to +8 \permil per amu) that require multiple evolutionary steps, (2) Isotopic profile measurements are accompanied by excess 26Mg and thus support a nebular origin for their development, (3) After initial isotopic enrichment CAIs undergo at least two divergent thermal histories as demonstrated by the two distinctive groups of Mg isotope profiles and their Al-Mg chronologies, and (4) Wark-Lovering rims are condensates from a nebular gas of chondritic or subchondritic Mg isotope composition that grew while 26Al was still extant.
Krot Alexander
Russell Stuart
Simon Justin I.
Tonui Eric
Young Edward D.
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