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Manganese‐chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites
Author(s) -
NYQUIST L.,
LINDSTROM D.,
MITTLEFEHLDT D.,
SHIH CY.,
WIESMANN H.,
WENTWORTH S.,
MARTINEZ R.
Publication year - 2001
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2001.tb01930.x
Subject(s) - chondrule , chondrite , meteorite , carbonaceous chondrite , allende meteorite , olivine , formation and evolution of the solar system , nebula , volatiles , manganese , geology , analytical chemistry (journal) , mineralogy , chemistry , materials science , astrobiology , astrophysics , physics , metallurgy , stars , chromatography
— 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‐chondrite‐normalized Mn/Cr, Mn/Fe, and Sc/Fe served to identify chondrules with unusually high or low Mn/Cr ratios, and to correlate the abundances of other elements to Sc, the most refractory element measured. A subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x‐ray spectrometer prior to high‐precision Cr‐isotopic analyses. 53 Cr/ 52 Cr correlates with 55 Mn/ 52 Cr to give initial ( 53 Mn/ 55 Mn) I = (9.4 ± 1.7) × 10 −6 for Chainpur chondrules and ( 53 Mn/ 55 Mn) I = (9.5 ± 3.1) × 10 −6 for Bishunpur chondrules. The corresponding chondrule formation intervals are, respectively, Δ t LEW = −10 ± 1 Ma for Chainpur and −10 ± 2 Ma for Bishunpur relative to the time of igneous crystallization of the Lewis Cliff (LEW) 86010 angrite. Because Mn/Sc correlates positively with Mn/Cr for both the Chainpur and Bishunpur chondrules, indicating dependence of the Mn/Cr ratio on the relative volatility of the elements, we identify the event dated by the isochrons as volatility‐driven elemental fractionation for chondrule precursors in the 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 ( 53 Mn/ 55 Mn) I = (2.8 ± 0.3) × 10 −5 for calcium‐aluminum‐rich inclusions (CAIs), 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 chondrites. Mn‐Cr formation intervals for meteorites are presented relative to average I(Mn) = ( 53 Mn/ 55 Mn) Ch = 9.46 × 10 −6 for chondrules. Mn/Cr ratios for radiogenic growth of 53 Cr in the solar nebula and later reservoirs are calculated relative to average (I(Mn), ∍( 53 Cr) I ) = ((9.46 ± 0.08) × 10 −6 , −0.23 ± 0.08) for chondrules. Inferred values of Mn/Cr lie within expected ranges. Thus, it appears that evolution of the Cr‐isotopic composition can be traced from condensation of CAIs via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids. Measured values of ∍( 53 Cr) for individual chondrules exhibit the entire range of values that has been observed as initial ∍( 53 Cr) values for samples from various planetary objects, and which has been attributed to radial heterogeneity in initial 53 Mn/ 55 Mn in the early solar system. Estimated 55 Mn/ 52 Cr = 0.42 ± 0.05 for the bulk Earth, combined with ∍( 53 Cr) = 0 for the Earth, plots very close to the chondrule isochrons, so that the Earth appears to have the Mn‐Cr systematics of a refractory chondrule. Thus, the Earth apparently formed from material that had been depleted in Mn relative to Cr contemporaneously with condensation of chondrule precursors. If, as seems likely, the Earth's core formed after complete decay of 53 Mn, there must have been little differential partitioning of Mn and Cr at that time.