Chemical variations and regional diversity observed in MORB

Abstract

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) N < 1.00) 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 characterized by statistically significant (≥ 95% confidence-level) enrichments in both highly incompatible elements (e.g., light REE, TITAN group elements, Sr, Ba, Pb, Th, and U) as well as less incompatible elements (e.g., heavy REE), indicating: i) a prominent recycled source component; ii) variable proportions of pyroxenite in the Atlantic source region; and/or, most likely iii) smaller degrees of melting and/or greater extents of fractional crystallization due to slower ridge spreading rates. Conversely, Pacific MORB has the most depleted regional signatures with regard to highly incompatible elements (e.g., Ba, Pb, Th, and U), likely due to faster ridge spreading rates. Indian Ocean MORB exhibit limited variation in incompatible element enrichments/depletions but are generally the most depleted in more compatible elements (e.g., Ti, Cr, Sc, and heavy REE), potentially due to distinct source characteristics or deep source melting in the garnet field. Atlantic, Pacific and Indian MORB can also be distinguished by trace element ratios, particularly Ce/Pb and Th/U, which is distinct at the > 99% confidence-level. Global MORB, and by inference the DMM, are characterized by enrichments in Y/Ho and depletions in Th/U relative to the chondritic ratios, and are complementary to the continental crust. However, the median of global MORB and the bulk continental crust both have sub-chondritic Ti/Eu and Nb/Ta ratios, suggesting an under-represented Ti- and Nb-rich reservoir in the Earth, potentially refractory, rutile-bearing eclogite at depth in the mantle.