Conditioned duality of the Earth system: Geochemical tracing of the supercontinent cycle through Earth history
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The balance between constructive versus destructive processes in the formation and recycling of continental crust over Earth history – or crustal growth – remains contentious; whereas some advocate continuous continental growth, others suggest episodic growth predominantly during periods of supercontinent assembly. In this paper, we review the geological record and present an analysis of time constrained hafnium and oxygen isotopes in dated zircon crystals, and of incompatible elements (Zr, Th) in dated magmatic rocks, to explore the operation of Earth's supercontinent cycle. This analysis reveals the importance of the supercontinent cycle to continental growth by demonstrating a link between periods of enhanced crustal recycling and elevated geochemical proxies of subduction flux. The temporal fluxes in subduction rate suggest a conditioned duality of the Earth system between alternating periods of hot, volatile-rich, and cold, volatile-depleted, mantle relative to an idealised power decrease curve. Hot, volatile-rich mantle periods accompany supercontinent dispersion events and are characterised by mantle superplumes and increased crustal recycling during rapid global subduction.Cool, volatile-depleted mantle periods that accompany aggregated supercontinents are interpreted to arise from a combination of the widespread subduction of cold oceanic lithosphere, volatile depletion arising from the preceding voluminous subduction-related magmatism, and core insulation by the slab graveyards that accompanied formation of the supercontinent: these periods are characterised by enhanced mantle influence on magmas but low rates of continental crust production. Pulses of rapid continental growth that accompanied supercontinent assembly led to crustal oversteps — which can be considered as periods when too much crust had formed relative to the thermal state of the mantle at that time. When combined with the anomalous mantle cooling that accompanied these pulses of rapid crust formation, we postulate that supercontinent assembly led to a stepwise increase in plate size via changes in tessellation during supercontinent dispersal.
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