Tectonic setting and mantle source evolution reconstructed from deep time analysis of basalt geochemistry

Abstract The thermal state and volatile inventory of the Earth’s mantle changed through time as lithospheric architecture and crustal recycling also evolved. Here, we analyse a global geochemical database of continentally emplaced basalts to explore the reflection of these changes in normative mineralogy. The hypothetically crystallised basalt mineralogy is calculated from all 11 major oxides, affording a whole-rock secular petrological comparison. In the Phanerozoic, normative mineralogy distinguishes between oceanic and continental basalts. Because Precambrian oceanic crust is rarely preserved, the secular analysis was focused on continentally emplaced basalts. Results show that Archaean basalts were dominantly silica-oversaturated and closely resemble modern continental flood basalts. Their normative clinopyroxene-orthopyroxene relationships argue against a dominant hydrous flux-melting origin. Two lulls in Palaeoproterozoic basalt emplacement suggest reduced magma productivity. Alkali-rich, silica-undersaturated basalts were rare in the Archaean, emerged through the Proterozoic, and rose to prominence in the Phanerozoic, supporting a late onset of effective modern-style subduction.