Crystal records of erupted vs. trapped magma in volcanic arcs

Arc andesites are hybrids generated by mixing of mafic and silicic magmas. The silicic endmember can contribute to crustal growth or trigger explosive eruptions, yet remains difficult to constrain due to limited representation in the erupted record. Here, we show that textural, elemental and isotopic records preserved in plagioclase can distinguish between trapped and erupted crystal mush, thus constraining conditions of magma storage vs. eruption in arc systems. We define the plagioclase eruptibility field, a narrow and recurrent anorthite range (An~61) that represents erupted melt across Central Andes volcanoes upon mafic recharge. The recycled plagioclase cargo is dominantly An~43, recording prolonged crystallisation and assimilation within silicic mushes. This pattern repeats in continental and composite arcs globally, indicating strong eruptive filtering of arc magmas and widespread occurrence of dormant felsic and mafic mushes that, in most cases, contribute to crustal growth; yet exceptionally, felsic mushes can rapidly remobilise to generate super-eruptions.