The importance of the melting process for quantifying mantle heterogeneity

Abstract:

A variety of data requires that the mantle source for basaltic magmatism is heterogeneous. Thanks to numerous experimental studies, parameterizations are available to model the melting behavior of peridotite and pyroxenite compositions that are thought to be present in the mantle (e.g., 1, 2). Based on these parameterizations, numerous studies have attempted to estimate the proportion of pyroxenites in magmatic sources. However, while almost all melting experiments correspond to a batch melting process, it is likely that oceanic basalts are formed by near fractional melting rather than batch melting (e.g., 3). Due to the limited extent of melting of peridotites under upper mantle conditions, their magmatic productivity and melt compositions are similar for batch and fractional melting (e.g., 4). In contrast, pyroxenites undergo much higher meting degrees during decompression of a heterogeneous, peridotite-rich mantle source. Using pMELTS, I investigated the effect of near-fractional melting of pyroxenite. Results suggest that the nature of the melting process for pyroxenites can significantly affect (1) the melt productivity of pyroxenites and thus their potential contribution in basalt genesis, (2) the major element composition of melts and thus their interaction with the surrounding peridodite, and (3) the concentration of minor elements such as Ni and consequently the estimation of pyroxenite proportion in magma-source (e.g., 5). In particular, calculations imply that the proportion of solid pyroxenite in the magma source is likely to be underestimated using “batch melting” rather than “fractional melting” parameterization. An increase in the pyroxenite proportion may affect the buoyancy of the mixture in the upper mantle and have important geodynamical implications. 1-Katz et al., 2003, GGG 4; 2-Lambart et al., 2013, Lithos 160-161; 3- Hirose & Kawamura, 1994, Geophy. Res. Let 21; 4-Johnson et al., 1990, J. Geophy. Res. 95; 5-Sobolev et al., 2007, Science 316