Stability and migration of slab-derived carbonate-rich melts above the transition zone

We present a theoretical model of the stability and migration of carbonate-rich melts to test whether they can explain seismic low-velocity layers (LVLs) observed above stalled slabs in several convergent tectonic settings. The
LVLs, located atop the mantle transition zone, contain small (∼1vol%)
amounts of partial melt, possibly derived from melting of subducted carbonate-
bearing oceanic crust. Petrological and geochemical evidence from inclusions
in superdeep diamonds supports the existence of slab-derived carbonate melt,
which may potentially explain the origin of the observed melt in the LVL.
However, the presumptive reducing nature of the ambient mantle can be an
impediment to the stability of carbonated melt. To reconcile this appar-
ent contradiction, we test the stability and migration rates of carbonate-rich
melts atop a stalled slab as a function of melt percolation, redox freezing,
amount of carbon supplied by subduction, and the metallic Fe concentration in the mantle. Our results demonstrate that carbonate-rich melts in the LVL
can potentially survive redox freezing over long geological time scales. We
also show that the amount of subducted carbon exerts a stronger influence on
the stability of carbonate melt than does the mantle redox condition. Con-
centration dependent melt density leads to rapid melt propagation through
channels while a constant melt density causes melt to migrate as a planar
front. Our calculations suggest that the LVLs can sequester significant frac-
tions of carbon transported to the mantle by subduction.