The ultimate source of magma erupting on Earth’s surface has always been mystifying, and it continues to be so. The Ferrar magmatic system necessitates a source that has a fairly uniform bulk composition over a large region, such that the magmas produced are broadly similar over thousands of kilometers, similar to the processes producing magma along ocean ridges and island arcs. And there must be a ready supply of massive quantities of primocrystic orthopyroxene that has an isotopic identity distinct from the basaltic magma but similar to the immediately overlying continental crust. Quartz eclogite is a possible primocryst source rock that has a large modal volume of pyroxene, and this may well exist just below the continental crust as a product of basaltic, even doleritic, materials having once been in the crust and over time sunken back into the uppermost mantle. An interaction between the continental crust and the underlying mantle has long been proposed by many scholars and here in the Ferrar is strong additional evidence for this process.