GEO 5/6690 Geodynamics 01 Dec 2014

© A.R. Lowry 2014Read 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 VP, VS • slightly lower densityBuoyancy 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…

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…

Karlstrom et al GSA-Today Mar 2002

Shapiro et al. (Lithos, 1999) modeledsmall-scale convection for thick thermalboundary layer and found that (for laboratory values of flow-law parameters)buoyancy is not necessary for tectosphereto survive, but it helps resist drips… withmost likely compositional buoyancy ~-equal thermal (consistent with free air)

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

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

Kaban et al.,G3 in press

Inversionsfor thermal &compositionalmass imply~1-to-1correlation &nearly equal(opposite-signed)mass. The peak 2%densitychange = Tof 570°C or~67%hydration…

Buehler & Shearer, JGR 2010 Schutt et al., in preparation

Pn velocity variation Moho temperature fromPn & mineral physics

And the thermal variation may not be as large as many people think!

Wang et al. EPSL 2014

Most recent dynamicalmodeling suggestscompositional buoyancyhelps resist RT-instabilitybut does not preventdeformation…

And no compositionalbuoyancy is needed tokeep cratons stable ifthere is a 10x differencein compositionalviscosity!

No viscosity difference; yes chemical buoyancy

= 3x; yes chemical buoyancy

= 3x; no chemical buoyancy

= 10x; no chemical buoyancy