Calcium isotopic fractionation during magma differentiation: Constraints from volcanic glasses from the eastern Manus Basin. (15th July 2021)

• Record Type: Journal Article
• Authors: Zhu, Hongli; Liao, Renqiang; Liu, He; Du, Long; Li, He; Li, Congying; Zhang, Zhaofeng; Sun, Weidong
• Is Part Of: Geochimica et cosmochimica acta. Volume 305(2021)
• Page Start: 228
• Page End: 242
• Publication Date: 2021-07-15
• Subjects: Calcium isotopes -- Isotopic fractionation -- Magma differentiation -- Felsic -- The eastern Manus Basin; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.05.032
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: To better constrain Ca isotopic fractionation during volcanic magma differentiation, we report Ca isotope data for a series of volcanic glasses from the eastern Manus Basin that represent a continuum of compositions comprising basaltic andesite, andesite, dacite, and rhyolite. Our results show that these volcanic glasses exhibit ~0.25‰ variation in Ca isotopic compositions, with δ 44/40 Ca ranging from 0.81 ± 0.04 to 1.05 ± 0.04‰. The similar trace element patterns and Sr-Nd isotopic compositions exclude source heterogeneity as the cause of the varied Ca isotopic compositions. Due to their low K/Ca ratios and young age, the Ca isotopic variations cannot be attributed to 40 K decay. Instead, the observed Ca isotopic variations are primarily controlled by their lithology. Specifically, δ 44/40 Ca of basaltic andesite (0.81 ± 0.04) and andesite (0.82 ± 0.01) are similar to the averages of reported mid-ocean ridge basalts (MORBs; 0.84 ± 0.10; 2SD, N = 26) and back-arc basin basalts (BABBs; 0.80 ± 0.08; 2SD, N = 21) in literature, but do not correlate with SiO2 or MgO contents, indicating the Ca isotopic fractionation is insignificant during intermediate-mafic magma evolution. As magma differentiation continues, δ 44/40 Ca of dacite and rhyolite evolves from 0.85 ± 0.01 to 1.05 ± 0.04‰. Moreover, δ 44/40 Ca of felsic glasses are negatively correlated with Eu/Eu*, which suggests that the increasing δ 44/40 Ca of felsic glasses is most likely related to plagioclase (Pl) fractional crystallization. Therefore, we conclude that Ca isotopes can behave differently at different stages of volcanic magma differentiation. Modeling of fractional crystallization using rhyolite-MELTS shows that the relative Ca fractions in crystallized minerals (e.g., Pl vs. Cpx) and the Ca isotopic fractionation factor between the residual melt and crystallized Pl (αmelt-Pl, which is likely related to the An number of Pl, magma temperature and kinetic effects during crystallization), provide a plausible explanation for the behavior of Ca isotopes during volcanic magma differentiation.