The Changing Nature of Mantle Hotspots through Time: Implications for the Chemical Evolution of the Mantle

The melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time. The highest-temperature melts (komatiites and picrites) derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates. A survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the Archean-Proterozoic boundary. Archean komatiites have depleted or neutral geochemistry, whereas most Protero-zoic and younger picrites have originated from enriched (OIB-type) mantle, although rare depleted plumes persist to the Tertiary (e.g., Gorgona). If plumes originate from a thermal boundary layer near the core-mantle interface, the composition of this layer must have changed from a mixture of depleted and neutral mantle during the Archean to mainly enriched mantle in the Proterozoic. This change coincides with a drop in the maximum MgO content of the magmas from >30% in the Archean to <22% in younger magmas, indicating a decrease in plume source temperature. We suggest that the change in the temperature and composition of the hotspot sources reflects a fundamental change in the dominant form of the component of the earth's convection, driven by surface heat loss: from cold plumes or drips breaking away from beneath a stable lithosphere during the pre-Archean to lithospheric subduction in the Archean and post-Archean mantles. The absence of enriched or OIB-type plumes in the Archean is interpreted to reflect the length of time for oceanic crust to become unstable, sink to the core-mantle boundary, heat up, and return to the upper mantle in plumes. The fall in the maximum MgO content of plume-related magmas between the Archean and Proterozoic may be due to the accumulation of a compositional boundary layer above the core that lowers the temperature of the plume source region by partially insulating it from the core, or to the material in OIB-type plumes being slightly compositionally lighter than the rest of the mantle. Continental flood basalts and oceanic plateau basalts have less extreme geochemical characteristics than those of the associated komatiites or picrites. If these magmas form by melting in the head of a starting plume that is a mixture of mantle from the plume source and entrained lower mantle, the modern lower mantle must have geochemical and isotopie characteristics closer to chondritic than the depleted or enriched mantle reservoirs.