Title: A 5-km-thick reservoir with >380,000 km3 of magma within the ancient Earth's crust
Abstract: Abstract Several recent studies have argued that large, long-lived and molten magma chambers 1–10 may not occur in the shallow Earth’s crust 11–23 . Here we present, however, field-based observations from the Bushveld Complex 24 that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering was found to continuously drape across a ~4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within such a thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated to be >5-km-high in thickness and >380,000 km 3 in volume. This amount of magma is three orders of magnitude larger than any known super-eruptions in the Earth’s history 25 and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces 26 . This suggests that super-large, entirely molten and long-lived magma chambers, at least occasionally, occur in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’ 1–10 remains a viable research concept for some of Earth’s magmatic provinces.