Ca-isotopes as a robust tracer of magmatic differentiation

The large mass difference (similar to 10%) between the two most abundant isotopes of calcium, Ca-40 and Ca-44, gives Ca great potential in tracking mass-dependent fractionation during magmatic processes. Resolvable Ca-isotope fractionation during fractional crystallization of magma, particularly by feldspar in evolved melts, has been theoretically inferred but not robustly tested in nature. To further explore the effects of magmatic differentiation on Ca-isotope systematics, we studied the late-Permian alkaline igneous suite of the empty setyangen Caldera, Oslo Rift, Norway, consisting of volcanic and intrusive units ranging from basanitic to rhyolitic compositions. Major and trace element variations and modeling demonstrate that the main series of samples (N = 21), including basanites, ring-dyke syenites, and central-dome syenites, likely documents a co-genetic and closed-system fractional crystallization sequence. Our data show minimal delta Ca-44/40 variation (< 0.05 parts per thousand) in the intermediate magma and a marked increase in delta Ca-44/40 in the felsic magma of the empty setyangen Caldera (from 0.62 +/- 0.02 parts per thousand to 1.15 +/- 0.03 parts per thousand relative to Ca standard, SRM915a). The systematic increase is best explained by equilibrium isotopic fractionation dominated by alkali feldspar in the fractionating mineral assemblage. This is further supported by strong correlations between delta Ca-44/40, CaO, and Eu/Eu* in the main-series samples. Implementing a Monte Carlo approach, isotopic modeling of the liquid line of descent using Rayleigh fractionation is highly consistent with the observed Ca-isotope evolution. For the first time, we confirm prominent Ca stable isotope fractionation in felsic-stage differentiation of alkaline magma and constrain the isotope fractionation factors of plagioclase and K-feldspar. Integrated with extant estimations on mineral fractionation factors from the literature, our results suggest increasing fractionation effects of rock-forming minerals with decreasing Ca content. The affirmation of significant Ca-isotope fractionation in alkaline magma by feldspar empowers the application of Ca as a versatile tracer of crustal evolution, allowing further tests in other magmatic conditions across various planetary objects. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

Significant calcium isotope fractionation occurs during magmatic processes, particularly in felsic magma due to the influence of feldspar. This finding supports the use of calcium isotopes as a tracer for crustal evolution.