Manganese-chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites

Whole-chondrule Mn-Cr isochrons are presented for chondrules separated from the Chainpur (LL3.4) and Bishunpur (LL3.1) meteorites. The chondrules were initially surveyed by instrumental neutron activation analysis. LL-chondri te-normalized Mn/Cr, Mn/Fe, and Sc/Fe served to identify chondrules with un usually high or low Mn/Cr ratios, and to correlate the abundances of other elements to Sc, the most refractory element measured. A subset of chondrule s from each chondrite was chosen for analysis by a scanning electron micros cope equipped with an energy dispersive x-ray spectrometer prior to high-pr ecision Cr-isotopic analyses. Cr-53/Cr-52 correlates with Mn-55/Cr-52 to gi ve initial (Mn-53/Mn-55)(I) = (9.4 +/- 1.7) x 10(-6) for Chainpur chondrule s and (Mn-53/Mn-55)(I) = (9.5 +/- 3.1) x 10(-6) for Bishunpur chondrules. T he corresponding chondrule formation intervals are, respectively, Deltat(LE W) = -10 +/- 1 Ma for Chainpur and -10 +/- 2 Ma for Bishunpur relative to t he time of igneous crystallization of the Lewis Cliff (LEW) 86010 angrite. Because Mn/Sc correlates positively with Mn/Cr for both the Chainpur and Bi shunpur chondrules, indicating dependence of the Mn/Cr ratio on the relativ e volatility of the elements, we identify the event dated by the isochrons as volatility-driven elemental fractionation for chondrule precursors in th e solar nebula. Thus, our data suggest that the precursors to LL chondrules condensed from the nebula 5.8 +/- 2.7 Ma after the time when initial (53Mn /55Mn)(I) = (2.8 +/- 0.3) x 10(-5) for calcium-aluminum-rich inclusions (CA Is), our preferred value, determined from data for (a) mineral separates of type B Allende CAI BR1, (b) spinels from Efremovka CAI E38, and (c) bulk c hondrites. Mn-Cr formation intervals for meteorites are presented relative to average I(Mn) = (53Mn/55Mn)Ch 9.46 x 10(-6) for chondrules. Mn/Cr ratios for radiog enic growth of Cr-53 in the solar nebula and later reservoirs are calculate d relative to average (I(Mn), epsilon(Cr-53)(I)) = ((9.46 +/- 0.08) x 10(-6 ), -0.23 +/- 0.08) for chondrules. Inferred values of Mn/Cr lie within expe cted ranges. Thus, it appears that evolution of the Cr-isotopic composition can be traced from condensation of CAls via condensation of the ferromagne sian precursors of chondrules to basalt generation on differentiated astero ids. Measured values of epsilon(Cr-53) for individual chondrules exhibit th e entire range of values that has been observed as initial 6(53Cr) values f or samples from various planetary objects, and which has been attributed to radial heterogeneity in initial 53Mn/55Mn in the early solar system. Estim ated Mn-55/Cr-52 = 0.42 +/- 0.05 for the bulk Earth, combined with epsilon( Cr-53) = 0 for the Earth, plots very close to the chondrule isochrons, so t hat the Earth appears to have the Mn-Cr systematics of a refractory chondru le. Thus, the Earth apparently formed from material that had been depleted in Mn relative to Cr contemporaneously with condensation of chondrule precu rsors. If, as seems likely, the Earth's core formed after complete decay of Mn-53, there must have been little differential partitioning of Mn and Cr at that time.