1. GEO 5/6690 Geodynamics 01 Dec 2014 © A.R. Lowry 2014 Read for Wed 3 Dec: T&S 410-427 Last Times: Plate as Lithosphere; The Tectosphere Tectosphere is used to describe an upper layer that is depleted of melt relative to a “fertile” mantle lherzolite. Melting initially consumes garnet & aluminous pyroxene), & the melt “residuum” (olivine-rich peridotite ) has slightly higher V P, V S slightly lower density Buoyancy of the tectosphere would help to explain why “plate” material below ~100 km is not regularly removed by Rayleigh-Taylor instabilities… Mierdel et al: showed that aluminous enstatite can hold increasing amounts of water with increasing pressure (but decreasing with temperature). This is opposite the relation for olivine, leading to a hydration minimum ~ depths of seismic LAB…

2. Isopycnic hypothesis: Rudnick et al 1998; Lee and Rudnick 1999; Shapiro et al 1999; Forte and Perry 2000: Suggests that isostatic equilibrium is partially maintained by melt depletion offsetting the negative buoyancy associated with low temperature cratonic lithosphere…

3. Karlstrom et al GSA-Today Mar 2002 Shapiro et al. (Lithos, 1999) modeled small-scale convection for thick thermal boundary layer and found that (for laboratory values of flow-law parameters) buoyancy is not necessary for tectosphere to survive, but it helps resist drips… with most likely compositional buoyancy ~ -equal thermal (consistent with free air)

4. So low density of depleted mantle would help circumvent small-scale convection (there would be no density instability to drive it) and in such regions conductive (“half-space”) cooling can reach to greater depths. This might also enable other processes to kick in…

5. Problem though: The bulk composition that results from melting depends on pressure as well as degree of melting. Above 100 km depth (where most melting derives) the buoyancy effect is small; it peaks ~150 km… These are derived from mineral physics +melt experiments. Schutt & Lesher, JGR 2006

6. Kaban et al., G 3 in press Inversions for thermal & compositional mass imply ~1-to-1 correlation & nearly equal (opposite- signed) mass. The peak 2% density change =  T of 570°C or ~67% hydration…

7. Buehler & Shearer, JGR 2010Schutt et al., in preparation P n velocity variation Moho temperature from P n & mineral physics And the thermal variation may not be as large as many people think!

8. Wang et al. EPSL 2014 Most recent dynamical modeling suggests compositional buoyancy helps resist RT-instability but does not prevent deformation… And no compositional buoyancy is needed to keep cratons stable if there is a 10x difference in compositional viscosity! No viscosity difference; yes chemical buoyancy  = 3x; yes chemical buoyancy  = 3x; no chemical buoyancy  = 10x; no chemical buoyancy