Phase transformations in a harzburgite composition to 26 GPa: implications for dynamical behaviour of the subducting slab

摘要: Abstract The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. pyroxene-garnet component (harzburgite minus 82 wt.% olivine) transforms to majorite garnet 18–19 GPa, and further disproportionates assemblage + stishovite Mg 2 SiO 4 spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) then perovskite MgSiO 3 (24–26 are found coexist with garnet. Garnet disappears 26 almost complete transition is achieved this pressure. mineral proportions density profiles in subducting oceanic lithosphere, modelled combination 80% 20% primitive MORB compositions calculated as function depth under conditions isothermal surrounding pyrolite mantle, also for temperature distribution which slab substantially cooler than mantle below 700 km. Under conditions, similar 600 However, between km, up 0.08 g/cm denser mantle. This caused primarily alumina content compared harzburgite, causes occur pressures harzburgite. presence smears out garnet-perovskite 50 whereas transformation much sharper composition. Calculations based on observed phase equilibria show that subducted cool remains (by 0.1–0.3 ) km but possesses depth. These results have important implications dynamical behaviour slabs possessing different thermal regimes when they encounter 670 discontinuity nature discontinuity.