Earth’s mantle: probing a hidden heavy-weight

Earth’s mantle comprises the vast region between the crust and core, some 84% by volume and 67% by mass – a real heavyweight. The superposition of key events – all ultimately rooted in the mantle – conditioned the evolution of the whole Earth system that produced our hospitable and resource-rich, yet fragile habitat. These include magma ocean crystallisation during Earth’s tumultuous beginnings, the formation of first atmospheres and oceans, the emplacement and emergence of weatherable crust, the transition to plate tectonics, and volcanism at and away from plate margins that continues today.

The mantle is poorly exposed on the ocean floor and as tectonically exhumed rocks, and sampled by sparsely distributed mantle-derived magmas carrying small fragments plucked from their wallrocks. Much of what we know relies on applying the petrologist’s ever-growing toolbox to these direct and indirect mantle samples. This is complemented by high-pressure experiments and computational models, which establish the phase relations and dynamic behaviour of mantle rocks, and the physical properties of its minerals.

Many mantle mysteries remain, reflecting the dearth of samples from deep Earth and deep time. Among these are the origin, size, locus and persistence of different mantle reservoirs recognised in mantle magmas of various ages, as reflections of mantle structure and convective vigour. Other key questions concern the mantle’s temperature and redox evolution, affecting its dynamic behaviour and melting relations through time, and the origin and cycling of its volatile element inventory, the partial degassing of which contributes to the air we breathe today.