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

Abstract

Abstract-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 MdCr 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. 53CrPCr correlates with 55Md52Cr to give initial (53Mn/55Mn)1= (9.4 2 1.7) x 10-6 for Chainpur chondrules and (53Mn/55Mn)1= (9.5 ? 3.1) x 10-6 for Bishunpur chondrules. The corresponding chondrule formation intervals are, respectively, AtLEW = -1 0 ? 1 Ma for Chainpur and -10 k 2 Ma for Bishunpur relative to the time of igneous crystallization of the Lewis Cliff (LEW) 86010 angrite. Because MdSc correlates positively with MdCr for both the Chainpur and Bishunpur chondrules, indicating dependence of the MnlCr 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 (53Mn/55Mn)1= (2.8 ? 0.3) x 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) = (53MdssMn)ch = 9.46 x 10-6 for chondrules. MdCr ratios for radiogenic growth of 53Cr in the solar nebula and later reservoirs are calculated relative to average (I(Mn), c(53Cr)1) = ((9.46 5 0.08) x 10-6, -0.23 k 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 ~(53Cr) for individual chondrules exhibit the entire range of values that has been observed as initial c(53Cr) values for samples from various planetary objects, and which has been attributed to radial heterogeneity in initial 53MnPMn in the early solar system. Estimated 55Md52Cr = 0.42 rt 0.05 for the bulk Earth, combined with c(53Cr) = 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 53Mn, there must have been little differential partition...

Dating the First Stage of Planet Formation

“…A complete digestion of fluoride complexes and Cr-rich refractory phases, such as spinel, is ensured by a subsequent dissolution in aqua regia (concentrated HCl and HNO 3 mixture in 3 : 1 ratio). The Cr purification technique is adapted from Birck & Allegre (1985), Lugmair & Shukolyukov (1998), Glavin et al (2004), Nyquist et al (2001), Ueda et al (2006), and Trinquier et al (2007), and has already been described in Yin et al (2007). Cr was separated from the major elements by elution through cationic-exchange resin in HCl medium.…”

“…We can see that the value of the bulk CI is also in this zone. 53 m However, Yin et al (2007) recently revised Nyquist et al (2001) ages of chondrules (Kita et al 2005), and resolved a major discrepancy in the chronology of the early solar system.…”

Mn‐Cr chronology of five IIIAB iron meteorites

“…The 53 Cr systematics in various solar system objects show that 53 Mn was alive in the early solar system and therefore, the 53 Cr system can be used as a chronometer among objects that had a constant 53 Mn/ 55 Mn initial ratio Shukolyukov 1998, 2001;Carlson and Lugmair 2000;Nyquist et al 2001). In the case of IIIAB iron meteorites, studies by Davis and Olsen (1990), Hutcheon and Olsen (1991), and Hutcheon et al (1992) showed that 53 Mn was alive in these meteorites.…”

Mn−Cr ages of Fe-rich olivine in two Rumuruti (R) chondrites

“…The exposure age of NWA 753 is 25 Ma (Ebisawa et al, 2003) and hence the cosmogenic effect is expected to be slightly smaller than that in Sahara 99531. The insignif- icance of the cosmogenic effect in ordinary chondrites was shown based on the production rate of 53 Mn by cosmic-ray spallation reactions (Nyquist et al, 2001). Hsu (2005) also reached a similar conclusion that the cosmogenic effect is negligible in the case of pallasite olivine.…”

“…The inferred initial 53 Mn/ 55 Mn ratio is (1.84 ± 0.42(2σ )) × 10 −6 . This translates to an age of 2.1 Ma before the angrite LEW 86010 (Lugmair and Shukolyukov, 1998) and 8.7 Ma after chondrules in ordinary chondrites (Nyquist et al, 2001 …”