Abstract

Concentration ratio An assemblage of MORB analyses (n=792 samples), including a suite of new, high-precision LA-ICP-MS measurements (n=79), has been critically compiled in order to provide a window into the chemical composition of these mantle-derived materials and their respective source region(s), commonly referred to as the depleted MORB mantle (DMM). This comprehensive MORB data set, which includes both “normal-type ” (N-MORB, defined by (La/Sm)Nb1.00) and “enriched-type ” samples (E-MORB, (La/Sm)N≥1.00), defines a global MORB composition that is more enriched in incompatible elements than previous models. A statistical evaluation of the true constancy of “canonical ” trace element ratios using this data set reveals that during MORB genesis Ti/Eu, Y/Ho and Ce/Pb remain constant at the 95 % confidence-level; thus, the ratios recorded in MORB (Ti/Eu=7060±1270, 2σ; Y/Ho=28.4±3.6, 2σ; Ce/Pb=22.2±9.7, 2σ) may reflect the composition of the DMM, presuming the degree of source heterogeneity, component mixing and conditions of melting/crystallization of the DMM are adequately recorded by global MORB. Conversely, Ba/Th, Nb/U, Zr/Hf, Nb/Ta, Sr/Nd, and Th/U are shown to fractionate as a function of MORB genesis, and thus these ratios do not faithfully record the composition of the DMM. Compared to samples from the Pacific and Indian Oceans, MORB derived from Atlantic ridge segments are