The behaviour of scandium during crustal anatexis: Implications for the petrogenesis of Sc-enriched granitic magma

Abstract

Scandium (Sc) is a compatible element in mafic silicate minerals, in particular amphibole, garnet and clinopyroxene, and is enriched in ultramafic rocks. Nevertheless, its concentration is also sufficiently high in some evolved granites and granite pegmatites to form minerals with essential Sc. In some granitic occurrences, Sc-bearing minerals occur in miarolitic cavities, indicating the importance of late- to post-magmatic fluids. In contrast, some granite pegmatites have thortveitite (Sc₂Si₂O₇) and other Sc-rich minerals, including Sc-enriched garnet as part of evolved magmatic mineral assemblages. The maximum Sc concentration in garnet in thortveitite-bearing, Mesoproterozoic granite pegmatites of probable anatectic origin in South Norway is ca. 2000 ppm, corresponding to ca. 100 ppm in a coexisting silicate melt. As for any trace element, the behaviour of Sc during crustal anatexis is controlled by the amount of melt formed and the mineralogy of the solid residue. The melting process can be modelled from thermodynamical data on solids and melts for given protolith compositions, temperature and pressure. Simulations in a range of mafic/ultramafic to felsic systems show that mafic protoliths will form Sc-depleted anatectic melts, and correspondingly Sc-enriched solid residues under relevant pressure and temperature conditions (2-10 kbar, 700-800 °C). Limited enrichment in melt relative to protolith is only seen in granodioritic-tonalitic bulk compositions, reaching maximum concentrations of 30–60 ppm. The effect of fluorine during melting and subsequent fractionation is to lower the solidus temperature, depolymerise the silicate melt, and lower partition coefficients for Sc between mafic silicate minerals and felsic melt. Contamination with mafic material has only limited effect, as it will eventually sequester Sc into hybrid mafic silicate mineral assemblages and lead to reduction of the Sc concentration of the remaining melt fraction. Further increase of the Sc concentration requires fractional crystallisation of minerals with low K_D, i.e. mainly feldspar minerals and quartz, which may be facilitated by selective, local contamination by quartz and feldspar from granitic country rocks.