GEOCHEMICAL VARIABILITY OF OCEAN RIDGE BASALTS AND CONSTRAINTS ON MANTLE AND CRUST MELT DISTRIBUTION: INGERSON LECTURE

A paradigm has emerged in recent years in which the hierarchy of tectonic segmentation of the mid-ocean ridge system is linked to a hierarchy of magmatic segmentation, in particular to differences in the depth in the mantle and crust where different geochemical signals originate. Examination of the geochemical variability of ocean ridge basalts shows that some axial discontinuities correspond to fundamental magmatic segment boundaries, while in other areas variations in composition grade continuously across significant tectonic offsets. Thus, the relationship between tectonic and magmatic segmentation remains enigmatic. A key piece in the puzzle is to understand how melt generation and crustal delivery processes effect the observed geochemical variations. Evidence from both geochemical and geophysical investigations can be used to constrain these processes at each level in the system from melt generation to eruption. A full evaluation includes consideration of source heterogeneity; the chemical effects of melting and melt focusing processes in the mantle melting regime (~100 km depth); crystallization of melt in the sub-crustal mush zone (~6 km); accumulation of melt in the over-riding melt lens (~2 km); vertical and lateral transport of magma from the melt lens through dikes; and the density driven filter on the eruption of magma on the surface. Focusing on the upper parts of this system, along-axis variations in the continuity of the melt lens are compared to changes in lava composition, suggesting in some areas a possible origin of larger scales of magmatic segmentation. In addition, evidence from the Hess Deep, a tectonic window into deep oceanic crustal structure and composition, and ophiolites suggests that along-axis transport of magma in dikes may account for some of the smaller scales of tectonic and magmatic segmentation.