Variable Thermal Conductivity, Slab Mineralogy, and Subduction Rates

Models of subducting slabs have suggested that metastable olivine (MO) in the slabs' cold interiors can act like parachutes and reduce their density (ρ ), lowering the slabs' velocities by up to 30% [Marton et al., 1999; Schmeling et al., 1999] These models, however, use constant values of thermal conductivity (k)to solve the heat flow equation. Models that use ks that are functions of P, T, and mineralogy have wedges of MO that are 20-30% smaller in cross-sectional area and whose maximum extents are 30-50 km shallower [Hauck et al., 1999; Marton et al., 2004]. As a result of the decreased amount of MO, the parachute effect should also be decreased. Using the same thermo-kinetic model as Marton et al. [2004] I determine the mineralogy of three sets of subducting slabs and calculate the driving forces acting on them. Terminal velocities (v_t) are found via balances of the driving forces and the opposing viscous drag forces. For the same two sets of slabs examined using the previous constant k model [Marton et al., 1999] (thermal parameter ǎrphi = 3500-12000 km), this model shows reductions that are approximately half those of the former, with maximum reductions of 1-1.25 cm/yr or ∼15%. A third set, with older lithosphere (100 My) and ǎrphi = 5200-17000 km, show reductions of up to ∼2 cm/yr or 25%. In addition, there should be a negative feedback between the amount of MO and subduction velocity, narrowing the range of subduction rates [Marton et al., 1999; Tetzlaff and Schmeling, 1999]. This is tested by feeding v_ts back into the model, adjusting the durations of the iterations. The resultant v_ts and amounts of MO achieve a steady state within a few My after the slabs' tips exit the transition zone. In this case, the size of the parachute effect is dependent on lithospheric age. One set that has constant starting velocities and variable ages shows the same trend as without the feedback, but with v_t changes ∼1% smaller. The other two sets with constant ages each have v_t changes that are constant, +2.2% for 70 My old lithosphere, -1.3% for 100 My old lithosphere, that are in-line with the trends of the first group.