tomtom5858 t1_jb6d12g wrote

>Why don't they just melt?

Pressure is too high to let them. That said, "melt" isn't a well defined term in conditions like this; at what point has ice cream melted?

>Are there fossils buried in the mantle?

Yep. If fossiliferous rocks are subducted, the fossils will be buried in the mantle until eventually, those fossils are somehow transformed beyond being recognizable as fossils (i.e. they're mixed enough, melted or not).