Controls on the chemical composition of flood volcanics
Previously Published Material: The broad ideas have been published in several publications, but new ideas are introduced in this talk
Abstract ID#: 34158
High-Ti magmas have high contents of incompatible trace elements and strongly fractionated REE patterns. They result from low-degree melting of a possibly enriched portion of the mantle source – a mantle plume in most cases. Volatile components like H2O and CO2 act like incompatible elements and they are strongly concentrated in the melt. Their presence decreases melt density which drives the magma rapidly into the crust and the ascent accelerates once CO2 exsolves near the base of the crust. These magmas interact minimally with wall rocks and partly for this reason do not contain Ni-sulfide deposits.
Low-Ti magmas have lower contents of incompatible trace elements accompanied by the chemical signature of crustal contamination (negative Nb-Ta anomalies, high 87Sr/86Sr, negative epsilon Nd). They result from high degrees of melting of the hotter parts of the mantle source. Their low volatile content and more primitive composition imparts high densities that cause the magma to be trapped at density discontinuities at the base of, and within, the crust. In magma chambers, the magmas differentiate to less dense evolved melts that continue towards the surface and assimilate wall rocks, a process instrumental to the formation of Ni-sulfide ore deposits.
This type of model explains the geochemical characteristics of magmas in most continental LIPs, including Siberia, Emeishan, Deccan, Karoo and Ethiopia. The Bushveld Complex consists of low-Ti magma that assimilated massive amounts of crustal rock. Interaction with the continental lithospheric mantle had little impact on the major and trace element contents of flood basalts, but might have influenced the concentrations and isotopic compositions of platinum-group metals.