HR: 14:40h
AN: V43F-05    [Abstracts]
TI: Calcium Isotope Heterogeneity Among Planetary Objects
AU: * Simon, J I
EM: simon@eps.berkeley.edu
AF: Center for Isotope Geochemistry, University of California, Berkeley, Berkeley, 94720,
AU: * Simon, J I
EM: simon@eps.berkeley.edu
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, 94709,
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Center for Isotope Geochemistry, University of California, Berkeley, Berkeley, 94720,
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., MS 90-111, Berkeley, 94720,
AB: A critical issue for understanding the origin of the Earth's geochemical reservoirs is whether chondritic meteorites are an adequate model for the bulk composition of the Earth, even for refractory lithophile elements. One way to address this issue is to document the extent of isotopic homogeneity in the early solar system by studying in detail the isotopic composition of different meteorite classes, and comparing them to Earth, Mars, and the Moon. We present new Ca isotope measurements of bulk chondritic meteorites that show non-mass dependent differences among ordinary chondrites and Earth for the more abundant and lighter Ca isotopes–masses 40, 42, 43 and 44. The measurements are made using TIMS, and isotope ratios are normalized to a constant ⁴²Ca/⁴⁴Ca. The fractionation-corrected ⁴⁰Ca/⁴⁴Ca ratios in meteorites vary from -3 to +2 epsilon units (0.01%) relative to the terrestrial ratio. The typical analytical uncertainty is ±0.015%. The observed variations could be caused by differences in either ⁴⁰Ca or ⁴²Ca abundance. The ⁴³Ca/⁴⁴Ca ratio shows minimal variation. The observed effects are near the limits of analytical resolution, due to incompletely understood inconsistencies in Ca isotope analysis using multi-collector TIMS, but must be nucleosynthetic in origin. Hence the data suggest that the solar system was heterogeneous with respect to Ca isotopes, and some fraction of this heterogeneity was preserved through the accretion process. The Ca data add to previously documented variation in isotopes of Cr, Mo, Ba, Sm, and Nd in chondrites. Carbonaceous chondrites Allende (CV3) and Murchison (CM2) yield the highest ⁴⁰Ca enrichments (+2.0 ±1.2 and +1.1 ±0.8 in units of 0.01%), whereas ordinary chondrites Saint Severin (LL6) and Bruderheim (L6) exhibit the lowest (-3.3 ±0.7 and -3.6 ±1.8). The ⁴⁰Ca isotopic effects in chondrites show correlations with reported effects in \triangle ¹⁷O, ⁵⁴Cr, and ⁵³Cr. There is a weak correlation between high ⁴⁰Ca and low ¹⁴²Nd, but meteorites with both high and low ⁴⁰Ca have ¹⁴²Nd lower than terrestrial. The observed nuclear effects in Ca isotopes can be accounted for by differences in the relative contributions of Ca made during supernova explosions (explosive O-Si burning) versus Ca made in dominantly s-process nucleosynthesis such as occurs in AGB stars. There is still no single model that can account for all observed isotopic data, but evidence is mounting that incomplete mixing of a heterogeneous solar nebula must be accounted for in the assessment of the formation age and evolution of geochemical reservoirs in the Earth's interior.
DE: 1027 Composition of the planets
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 6240 Meteorites and tektites (1028, 3662)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting