What's the current thinking regarding the importance of metastability for subduction zone processes?

I'm pretty convinced blueschist-to-eclogite metastability is rare at large scale, unsure about antigorite metastability, and totally unsure for the transition zone.

The blueschist-eclogite one is, so far as I know, traceble to Simon Peacock's work on flat slabs and how to keep them buoyant.

Our more recent observations seem to show there are eclogitized flat slabs, and normal dipping slabs seem to eclogitize readily.

Antigorite metastability has been invoked sometimes to explain why seismicity continues to >50 km depth in slabs, idea being that its dehydration causes a localized change in rheology promoting brittle fracture.

Petrology people seem to be pushing back on this lately?

Lab and theoretical studies for high pressure, high temperature changes in olivine are pretty consistent and having large delay in its shift to denser phases at the bottom of the transition zone.

This gets translated into an idea that slabs don't readily enter the lower mantle.

Which is true, in some places. But more recent observations from South America tend to show both a tendency to push into the deeper mantle and to deform at the boundary in different locations for similarly old slab.

So, not really a clear fit for that idea....

(This thread brought to you by my stumbling across a kinda shouty paper entitled "Neither antigorite nor its dehydration is 'metastable' ". No mystery about its conclusions! )