MORBCAL: a program for calculating the compositions of primary basaltic melts produced by decompression-induced melting below mid-ocean ridges

Abstract

A widely accepted model proposes that mid-ocean ridge basalts (MORB) are produced by decompression-induced partial melting of oceanic upper mantle which adiabatically upwells in response to plate separation (McKenzie, 1984; Klein and Langmuir, 1987; McKenzie and Bickle, 1988). In this model, rising mantle begins to melt as it intersects the solidus at a depth determined by its temperature and melting characteristics. Continued upwelling involves continuous melting, matrix compaction, and melt segregation. To quantitatively evaluate this type of melting, Niu and Batiza (1991) developed an empirical scheme based on the peridotite melting experiments of Jaques and Green (1980), Falloon and Green (1987, 1988), and Falloon and others (1988). We used the experimental data to calculate the bulk partition coefficients (Kd) for the major elements. In addition, we determined pressure and temperature (extent of partial melting) dependance of these Kds and used these to calculate the compositions of pooled polybaric melts created by incremental melting. Finally, we developed a method for calculating the melting parameters (initial and final melting pressures and temperatures, plus total extent of melting) for actual MORB melt compositions. This method assumes that primary polybaric pooled melts fractionate only at low pressure and employs the chemical back-tracking method of Klein and Langmuir (1987). The purpose of this Short Note is to present the MORBCAL program. This program is an easy-to-use tool for readers interested in mid-ocean ridge processes and igneous petrogenesis. For details of the melting model, readers are referred to Niu and Batiza (1991). In the rest of the note, we briefly discuss MORBCAL and its availability.