Mantle source contributions to the diversity of primitive arc magmas

Primitive arc magmas compositions provide a valuable source of information to grasp the nature of mantle wedge and the role of heterogeneity in the diversity of arc magma suites. Surprisingly, we still know little about the nature of the mantle metasomatism and consequently the source lithology, the temperature at which melting occurs in the wedge, the depth at which melts segregate from their mantle wedge source, the style and degree of mantle melting and the pathway of the fluid/melt component from the slab to the surface. The aim of this study is to take some selected primitive melt compositions back to their mantle source conditions and identify the mineral assemblage that was in equilibrium with that magma using thermodynamic modelling (pMELTS) and experimental petrology (inverse approach). It is shown that pMELTS thermodynamic modelling provides an excellent mechanism to design an experiment strategy and make more efficient the search of multiple saturation points (MSPs). This study demonstrates that to find efficiently a robust enough MSP significant to a natural mantle wedge is crucial to constraint the water content as well the redox state appropriate to the context delimited by the chose natural samples and use pMELTS thermodynamic modelling as a reference tool. The chemical diversity of the Colima Volcanic Complex (CVC) suggests that the sub-arc mantle wedge from which they derive is chemically heterogeneous. The experiments of Comal Chico (COM-1) with 3.8 wt% H2O demonstrate that it is multiply-saturated at the liquidus around 1.9 - 2.4 GPa and 1300 oC with a phlogopite-bearing spinel pyroxenite assemblage (cpx+opx+phl+sp). This metasomatic pyroxenites require SiO2-rich fluxes from the slab that react to replace the original olivine in the peridotite. Examination of the olivine phenocryst chemistry in the magmas clearly shows a progression from a pyroxenite source (high-Ni; >5000 ppm, low-Ca; <1000 ppm) towards a more traditional olivine-rich peridotite source over a short distance. It has been shown experimentally and reinforced by other petrological tools (whole rock and olivine chemical analysis), that the geochemistry diversity in the primitive arc magmas is linked to the thermodynamic and lithological diversity of the mantle wedge. Moreover, the ubiquitous high-water content associated with arc magmatism controls and to some extent is buffered by the hydrous phase that are generated by the metasomatic reaction between the slab component and the peridotite mantle.