Mn‐Cr chronology of five IIIAB iron meteorites

Abstract

Apparently, old Mn-Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn-Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model-dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.

Solar System Time Scales from Long-Lived Radioisotopes in Meteorites and Planetary Materials

“…Several Re-Os studies (e.g., Chen et al, 2002;Horan et al, 1998;Smoliar et al, 1996) have assumed a more precise value for the 187 Re decay constant under the assumption that the Re-Os system attained closure in the IIIAB irons almost contemporaneously with the formation of the angrites ADOR and LEW at 4557.8 AE 0.5 Ma (Lugmair and Galer, 1992). Indeed, the Mn-Cr model ages for the IIIAB irons (Hutcheon and Olsen, 1991;Hutcheon et al, 1992;Sugiura and Hoshino, 2003) and these angrites (Lugmair and Shukolyukov, 1998;Nyquist et al, 1994) are approximately coincident within $AE5 Ma and, therefore, this assumption may be valid at this level of uncertainty. Assuming then that the IIIAB magmatic irons attained closure of the Re-Os system at 4558 Ma, most iron meteorite groups give relatively old Re-Os ages (i.e., older than $4.5 Ga) (Cook et al, 2004;Horan et al, 1998;Shen et al, 1996;Smoliar et al, 1996).…”

Diffusion kinetics of Cr in olivine and 53Mn–53Cr thermochronology of early solar system objects

“…Excess 53 Cr relative to the terrestrial values has been detected in olivines, orthopyroxenes, spinels, and plagioclase from different types of meteorites (Hutcheon et al, 1998;Lugmair and Shukolyukov, 1998;Sugiura, 2002), and has led to the calculation of Mn-Cr mineral ages. The interpretation of these mineral ages requires quantitative understanding of the closure temperatures (T c ) below which there is no significant diffusive loss of the daughter products from a mineral.…”

“…The 53 Mn-53 Cr decay system, with a half life of 3.7 Myr, has recently emerged as an important chronometer for the study of time scales of processes in the early history of the solar system when 53 Mn was still extant (e.g., Hutcheon et al, 1998;Lugmair and Shukolyukov, 1998;Nyquist et al, 2001;Sugiura, 2002). Excess 53 Cr relative to the terrestrial values has been detected in olivines, orthopyroxenes, spinels, and plagioclase from different types of meteorites (Hutcheon et al, 1998;Lugmair and Shukolyukov, 1998;Sugiura, 2002), and has led to the calculation of Mn-Cr mineral ages.…”

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

“…In the case of typical IIIAB iron meteorites with an exposure age of ∼650 Ma, the cosmogenic effect was estimated to be δ 53 Cr ∼ (1.7 × 10 −4 ‰) × ( 56 Fe/ 52 Cr) (Sugiura and Hoshino, 2003. Note that the formula given there is incorrect.).…”

“…(Sugiura and Hoshino, 2003), 52 Cr/ 50 Cr was used for estimating the instrumental isotopic fractionation. In the case of olivine, however, 50 Cr cannot be measured accurately because of severe 50 Ti interference and hence 52 Cr/ 50 Cr cannot be used for estimating instrumental effects (Hsu, 2005).…”