So "fertile" here means "can (partially) melt", while "depleted" means "cannot melt (because already did)". Corresponding mantle rocks are usually lherzolite and harzburgite, respectively. See for instance the peridotite classification article on Wikipedia:
- Harzburgite [...] is interpreted as depleted mantle rock, from which basaltic magma has been extracted.
- Lherzolite is thought to make up much of the upper mantle [...] and is the likely source rock for basaltic magma.