Constraints on the degree of dynamic partial melting and source composition using concentration ratios in magmas

Abstract

We present a dynamic melting inversion (DMI) method that permits calculation of the degree of partial melting for cogenetic primary magmas in the context of the dynamic melting model of Langmuir et al. (1977). The method uses variations of between-magma concentration ratios for two incompatible trace elements that have different bulk distribution coefficients, and does not require assumptions regarding source incompatible element concentrations or ratios. Source concentrations can be calculated after obtaining the partial melting degree. The method is applied to the nephelinitic Koloa suite of Kauai, the Honolulu volcanics of Oahu, and alkalic basalts from southeastern Australia. Results, which display remarkable intersuite and interelement consistency, suggest that melilitites and alkali olivine basalts are formed by 2–4% and 8–11% partial melting, respectively, of a LREE-enriched mantle source. This result for basalts that are characterized by positive ε Nd argues persuasively that the often-invoked “recent source enrichment event” is actually an integral part of the total melting process.