[Tagbert]

Heat and pressure like that would convert that ash into a metamorphic rock and destroy any fossils. Surface rocks don’t get down to the Mantle except through subduction where they are melted and mixed with other melted rocks.

If those fossils were buried in ash or mud, the normal rock-forming processes would have fossilized the bone and converted the ash and much to sedimentary rock. You see the same thing in other fossils bearing rocks like limestone and sandstone.