Major-Element Geochemistry and Fe3+/ΣFe of Metabasites

Abstract Metabasites (metamorphosed mafic rocks) are crucial for understanding metamorphic and tectonic processes. Their preservation in exhumed orogenic belts from throughout Earth’s history and the diverse mineral assemblages they form under different pressure–temperature conditions make them valuable for studying metamorphic processes. This work compiles a database of 6186 major-element whole-rock analyses of metabasites from different metamorphic facies (low-grade, greenschist, blueschist, amphibolite, granulite, and eclogite). These are used to explore the range and variability in their composition and assess geochemical differences among metamorphic facies. To mitigate the impact of outliers, median values and median absolute deviations (MAD) are used as measures of central tendency and dispersion. Metabasites show decreased volatile content with increasing metamorphic grade and generally consistent major-element contents across facies, with subtle differences interpreted to result from sampling bias. The median worldwide metabasite is as follows (anhydrous, normalised values in wt %, ±MAD): SiO2 = 51.36±3.40, TiO2 = 1.33±0.82, Al2O3 = 15.47±1.97, FeOtotal = 11.48±2.50, MnO 0.20±0.06, MgO = 6.83±2.25, CaO = 9.84±2.34, Na2O = 2.82±1.05, K2O = 0.50±0.61, and P2O5 = 0.18±0.16. The median XMg = MgO/(MgO+FeOtotal) is 0.51±0.09. The median Fe3+/ΣFe was measured by titration in 3153 samples and is 0.26±0.12, comparable to values in altered oceanic crust or arc basalts. Future research must carefully examine the distribution of Fe3+ amongst minerals in metabasites, allowing for a better evaluation of the median whole-rock Fe3+/ΣFe and its potential susceptibility to analytical interferences.