Scapolite as a major fractionating mineral phase in deep-seated arc magmas

Scapolite is a relatively common constituent of lower crustal rocks, rendering them a significant reservoir in the global volatile cycle. The mineral in these rocks can be of either metamorphic, metasomatic, or igneous origin. Unequivocal examples of igneous scapolite are mostly related to alkaline magmas and pegmatites. Here we demonstrate by means of high-pressure experiments that scapolite can be a major (>1−2 vol%) mineral phase in ordinary arc magmas that fractionate in the lower crust. Slow-cooling and isothermal crystallization experiments performed on a natural arc basalt doped with geologically realistic amounts of H2O, CO2, SO3, and Cl show that at 1.0 GPa confining pressure, scapolite is stable at 800−1000 °C, constituting a major fraction of the minerals precipitating within this temperature interval. At 1.3 GPa, scapolite is stable at 750−950 °C, whereas at pressures <0.7 GPa, scapolite did not form in our bulk composition. These observations suggest that the relatively common occurrence of scapolite in lower crustal arc rocks could be partly a consequence of igneous differentiation at high pressure. The presence of 2−5 vol% scapolite in garnet-free, hornblende-rich lower crustal cumulates of the exhumed New Zealand arc support this hypothesis.