地球深部に沈み込む炭酸塩鉱物の分解反応

抄録

Considerable amounts of carbonates are introduced deep into the Earth's interior by the subduction of oceanic plates. Here I report on high-pressure and high-temperature experiments involving carbonates and silicates up to 100 GPa and 3200 K, corresponding to depths within the Earth of approximately up to 2200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO₃ (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO₃ (magnesite) via reactions with Mg-bearing silicates under conditions that are 200-300 K colder than the mantle geotherm. With increasing temperature, the magnesite is decomposed into an assemblage of CO₂-V + perovskite via reactions with SiO₂. Diffraction experiments for CO₂-V show that the phase is consistently interpreted in terms of a β-cristobalite structure, which has been indicated by theoretical studies. Furthermore, CO₂-V itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.