What fraction of subducted mass crosses the 660 km boundary?
In plain words
Some sinking plates pass straight into the deep mantle while others flatten and stall near 660 km or 1000 km depth. How much material crosses, and for how long stalled plates stay, controls how well the mantle mixes.
Precise statement
Given that subducted slabs penetrate the 660 km discontinuity while some stagnate near 660 to 1000 km, determine the time-averaged fraction of subducted mass flux that enters the lower mantle and the residence time of stagnant slabs. An answer is a mass-exchange rate with uncertainty consistent with global tomography, plate-reconstruction slab budgets and convection models that fit the geoid.
What would settle it
Comparison of subduction histories from plate reconstructions with slab volumes imaged by tomography above and below 660 km, and convection models matching the geoid.