Primary magma compositions for Kroenke-type basalts from the Ontong Java Plateau (OJP) have been estimated using a hybrid forward and universe model. For accumulated fractional melting of a fertile peridotite source, the primary magma had 16.8% MgO and lost 18% olivine by fractional crystallization to produce Kroenke-type basalts; the melt fraction was 0.27 and the potential temperature was 1500 degrees C. For equilibrium melting of a fertile peridotite source, the primary magma had 19.3% MgO and lost 25% olivine by fractional crystallization to produce Kroenke-type basalts; the melt fraction was 0.30 and the potential temperature was 1560 degrees C. The model peridotite source composition, melt fraction and potential temperature required to produce the primary OJP magmas are in excellent agreement with those that have been independently estimated from incompatible trace-element concentrations.