Tracking the Evolution of Magmas from Heterogeneous Mantle Sources to Eruption

This chapter reviews the effects of source heterogeneities, melt-rock reactions, and intracrustal differentiation on magma chemistry across mid-ocean ridges, intraplate settings, and subduction zones using experimental studies and natural data. We compare melting behaviors of pyroxenites and peridotites and their relative contributions to magmas as functions of composition, mantle potential temperature, and lithospheric thickness. We also discuss the fate of chemically distinct melts derived from heterogeneities as they travel through a peridotitic mantle. Using nearly 60, 000 natural major element compositions of volcanic rocks, melt inclusions, and crystalline cumulates, we assess broad petrogenetic trends in as large of a global dataset as possible. Consistent with previous studies, major element chemistry of mid-ocean ridge basalts(MORBs), and their cumulates favor a first-order control of intracrustal crystal-liquid segregation, while trace-element studies emphasize the role of melt-rock reactions, highlighting the decoupling between the two. Ocean islandbasalts (OIB) showa largercompositional variability thanMORB, partly attributed to large variations of pyroxenite proportions in the mantle source. However, the estimated proportions vary considerably with heterogeneity composition, melting model and thermal structure of the mantle. For arcs, we highlight current views on the role of the downgoing slab into the source of primary arc magmas, and the role of the overriding lithosphere as a magmatic chemical filter and as the repository of voluminous arc cumulates. Our approach of simultaneously looking at a large database of volcanic + deep crustal rocks across diverse tectonic settings underscores the challenge of deciphering the source signal versus intracrustal/lithospheric processes.