1999 Spring AGU Abstract (Kirby)

Bina, C. R., and S. H. Kirby, Model estimates of the petrological buoyancy cost of descent into the transition zone and lower mantle of slabs containing metastable olivine: Results for the Tonga subduction zone, Eos, Transactions of the American Geophysical Union, 80, Spring Supplement, S31B-09, 1999.

S31B-09

Published tomographic images and the presence of numerous deep earthquakes outboard of the bottom of the Tonga deep slab suggest some form of increase in resistance to slab descent into the transition zone and lower mantle. Recent work by Okal and Kirby [PEPI 109, 25-63, 1998] suggested that the petrological buoyancy of metastable olivine may be an important source of such resistance for the very rapidly descending and hence very cold Tonga slab. The presence of such hypothetical metastable olivine may facilitate deep earthquake occurrence by, for example, transformational faulting, dehydration embrittlement, or thermal instability.

We model the deep Tonga slab by computing the thermal structure for 120-Ma lithosphere subducting at 45° at 24 cm/yr. Employing a bulk composition of Fo₉₀ olivine and published thermodynamic parameters for the olivine polymorphs, we calculate the resulting mineralogy in such a slab for both the equilibrium case and the case in which alpha olivine persists metastably at temperatures below 1000 K. We then calculate the net thermal and petrological buoyancy of the slab, thereby estimating the resistance to slab descent into the transition zone and lower mantle.

We find that the petrological buoyancy of low-density metastable olivine increases stepwise near 410 and 550 km and again at 660 km, where it is more than twice that expected for thermal depression of the equilibrium gamma -> pv + mw transformation. We suggest that the intense deformation of the Tonga slab near 660 km and the resulting slab deflection may be linked with the petrological buoyant resistance to the descent of metastable olivine below 660 km. This large buoyancy cost of the deep descent of metastable olivine also offers a simple explanation of why no deep earthquakes have been detected in the lower mantle.