HR: 1340h
AN: P33A-1006    [Abstracts]
TI: Unraveling the Environmental Record of the Early Solar System: High Precision Laser Ablation Al-Mg Isotopes of Igneous CAIs
AU: Young, E D
EM: eyoung@ess.ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Young, E D
EM: eyoung@ess.ucla.edu
AF: IGPP, UCLA, Los Angeles, CA 90095 United States
AU: * Simon, J I
EM: jisimon@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Russell, S S
AF: Department of Mineralogy, Natural History Museum, London, SW7 7BD United Kingdom
AU: Tonui, E
AF: Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095 United States
AU: Krot, A
AF: HIGP/SOEST, UHM, Manoa, HI 96822 United States
AB: Variations in intrinsic Mg isotope compositions provide a potentially rich record of the physiochemical evolution of CAIs. Moreover, Mg excesses from the short-lived $^{26}$Al 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 $\it in situ$ 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 $<$0.2 \permil per amu (2$s$) for intrinsic Mg isotope measurements and $<$0.3 \permil (2$s$) for measured $^{26}$Mg excesses. Intra-object variation in \delta$^{25}$Mg 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 $\sim$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 \delta$^{25}$Mg with evaporation, but does not produce the tight curvature in the edgeward increases in \delta$^{25}$Mg characteristic of Leoville 144A. Three CAIs that exhibit edgeward \delta$^{25}$Mg 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 $>$2 Myr, and are improbable for parent body thermal histories). We concluded that: (1) CAIs exhibit enriched \delta$^{25}$Mg 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 $^{26}$Mg 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 $^{26}$Al was still extant.
DE: 3662 Meteorites
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1094 Instruments and techniques
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting