Читать книгу Isotopic Constraints on Earth System Processes - Группа авторов - Страница 73

Most mantle‐derived rocks and peridotites studies report calcium isotope compositions similar to BSE (e.g., Amsellem et al., 2019; Ionov et al., 2019; Kang et al., 2017; Simon & DePaolo, 2010). There are reports of mantle rocks including peridotites that have distinctly heavy δ⁴⁴Ca isotopic compositions, e.g., Amini et al. (2009), Huang et al. (2010), Kang et al. (2017), and Lu et al. (2019), and a few that are distinctly light, e.g., Amsellem et al. (2019) and Zhao et al. (2017). In order to investigate this variability, Huang et al. (2010) measured mineral separates from mantle rocks and found that δ⁴⁴Ca in orthopyroxenes (opx) are significantly heavier than their associated clinopyroxenes (cpx) by 0.36–0.75‰. The magnitude and sign of the measured differences are generally consistent with first principles equilibrium intermineral isotope fractionation calculations that fundamentally depend on calcium concentration (i.e., Ca—O bond strength) and temperature (Antonelli et al., 2019a; Wang et al., 2017). In detail, calcium isotope fractionation between opx and cpx (Δ^44/40Ca_opx‐cpx) less than ~0.26‰ and greater than ~0.60‰ is likely related, at least in part, to disequilibrium calcium isotopes effects such as metasomatic metamorphism (Zhao et al., 2017). Disequilibrium effects have also been reported for volcanic settings during rapid crystal growth (Antonelli et al., 2019b) and between opx and cpx and with other minerals during granulite facies and ultrahigh‐temperature metamorphism (Antonelli et al., 2019a). In these cases, calcium concentration likely plays an important role, i.e., it is lower by ~1/32 in opx compared to cpx, and may be more easily affected by isotope fractionation governed by diffusive loss (or gain) of calcium. This would explain why cpx tends to have compositions closer to BSE but why opx compositions can vary wildly – opx as high as δ⁴⁴Ca ~6 has been found in mafic granulite samples (Antonelli et al 2019a). Interestingly, a recent investigation using calcium isotope signatures of carbonatite and silicate metasomatism and melt percolation found little evidence for calcium isotopic heterogeneity and concluded that metasomatism tends to decrease δ⁴⁴Ca values of metasomatized mantle materials, but that its effects are usually limited (≤0.3‰) (Ionov et al., 2019).