Mantle Potential Temperature through time: Constraints from Greenstone Basalts and Komatiites

English Abstract:

Basalts can be divided into three groups using incompatible element distributions: undepleted/enriched (UE), depleted (D), and hydrated (H). We have calculated mantle potential temperatures (Tp) for each of these groups and for komatiites (K) based on major element contents filtered for alteration and extreme fractionation. Median Tp for the H and D groups decreases with time from 1500°C in the Archean to 1380°C today, whereas the UE and K groups maintain high and relatively constant Tp values through time (UE,1500°C; K, 1650°C). Decreasing Tp in ambient mantle may be expected with time, but the rather constant Tp in UE and K mantle sources requires an explanation. One possibility is that before 2.5 Ga, most of the mantle was well stirred and low-viscosity and only near the core-mantle thermal boundary layer (CMB) was the thermal gradient large enough to give rise to mantle plumes. Although plumes may have existed throughout Earth history, only some of them (those that gave rise to komatiites) made it to the base of the lithosphere before 2.5 Ga due to smaller plume sizes resulting in rapid diffusive heat loss. Archean UE basalts may thus reflect partial melting of compositional inhomogeneities embedded in the D-H upper mantle rather than thermal plumes. Widespread plume production in the late Archean may be related to the onset of plate tectonics, where the accumulation of cool slabs at the CMB increased the thermal gradient, thus enhancing plume generation. Plume heads may have been more robust in this cooler mantle and survived to the base of the lithosphere, producing most of the post-2.5 Ga UE basalts. Supporting this interpretation are increases in Ti and Nb in UE basalts after 2.5 Ga. After 2.5 Ga, a maintained high Tp for UE and K mantle plume sources may reflect sustained high CMB temperatures, perhaps due to growth of the inner core.