“Primitive” basalts from the Papoose Canyon vent contain evolved and enriched “crystal cargo”.

The temporal compositional trends most likely reflect varying degrees of crustal assimilation.

A deflating sills model is proposed to explain the temporal trends.

Crustal signatures may be obscured within bulk lava but recorded in melt “crystal cargo”.

Evidence of pre-eruptive magma ponding may have implications for monitoring monogenetic volcanoes.