Si and Mg isotope fractionations in melilite in coarse-grained CAIs measured by SIMS

Abstract

Si and Mg isotopic compositions of melilite in three coarse-grained CAIs were measured by SIMS. Isotopic compositions of these elements were fractionated (1.0 to 2.9 ‰/amu for Si and 4.6 to 9.6 ‰/amu for Mg) relative to terrestrial values. These fractionations are similar to those reported for bulk CAIs (Clayton et al., 1988) and suggest that coarsegrained CAIs were formed by evaporative loss of some Si and Mg from high-temperature condensates as proposed by Grossman et al. (2000). No significant zoning of isotopic fractionations was observed in these CAIs. This suggests that the evaporative loss of these elements from CAIs were slow compared with diffusive homogenization and also suggests that evaporative loss ceased early before crystallization of Mg-rich melilite.

Ion microprobe Al–Mg dating of single plagioclase grains in an Efremovka chondrule

“…In this session, the δ 25 Mg values of Miyake-jima plagioclase were from -2.4‰ to -5.4‰, whereas the δ 26 Mg values were from -5.8‰ to -9.4‰, suggesting again the slope of 1/2. The absolute fractionation values are significantly smaller than those of the multi-collector mode (see Table 1) and are consistent with -7‰/amu for Mg isotopes reported from the use of a modified ion microprobe (IMS-6f; Cameca) in a single collector mode (Sugiura et al, 2004). Sano et al (2008) reported that the instrumental mass fractionation of Sr isotopes in natural carbonate varied from -13.9‰ to -0.4‰ per amu within four days using a NanoSIMS, although the mass bias is negligibly small in Pb isotopes, +0.1 ± 2.6‰ for 206 Pb/ 204 Pb and -1.5 ± 2.6‰ for 207 Pb/ 204 Pb in a NIST SRM610 glass (Sano et al, 2006).…”

“…S1). The absolute fractionation value is much larger than -7‰/amu for reported Mg isotopes measured using an ion microprobe (IMS-6f; Cameca SAS) in a single collector mode (Sugiura et al, 2004). No explanation is sufficient for such a large Mg isotope variation in the terrestrial natural process.…”

Mg and Si isotopic fractionation patterns in types B1 and B2 CAIs: Implications for formation under different nebular conditions

“…This lack of correlation was also observed in another type B inclusion, “SH‐1,” by Sugiura et al. (; see also Mendybaev et al. ).…”