Perspectives on Ocean Ridge Basalts from the Segment to the Global Scale.

摘要

This study addresses the influences on ridge basalt chemistry, through analysis of their major and trace element and isotopic composition at scales ranging from individual ridge segments to the entire length of the ridge system. Local-scale studies of basalts along the Mid-Atlantic Ridge shed light on crustal accretion at slow-spreading ridges, and on the nature of plume-ridge interaction in this region. We show that segments must have multiple supplies of magma delivered along their length, but with preferential delivery of magma to segment centers. Plume-ridge interaction near the Azores is not simple two-component mixing between "plume" mantle and "depleted" mantle as previously argued. The elevated highly incompatible trace element ratios possessed by basalts well south of the plume are the definitive sign of a low-degree melt, which can fractionate highly incompatible element ratios. We show that a low-degree melt of plume mantle acts to metasomatize ambient depleted mantle, creating a mixed source that melts to produce the enriched basalts south of the Azores. This metasomatized source is the enriched mixing component that produces the observed geochemical gradient, rather than bulk plume mantle.;The latter half of this study is global in scope, involving a carefully compiled ridge basalt geochemical database. This database is unparalleled in size and coverage -- including data from portions of the Gakkel and Southwest Indian Ridges and Lau basin that were unavailable in prior data compilations. It includes a catalog of 771 global ridge segments, enabling the calculation of mean MORB by averaging the "segment means", including weighting on segment length and spreading rate and a quantitative treatment of errors. We show that the mean composition of ocean ridge basalts is more enriched than previously suggested, and argue for a re-definition of "normal MORB". Segment basalt compositions are individually corrected for crystal fractionation, arriving at parental magma compositions that can be interpreted in terms of mantle processes. The fractionation-corrected mean segment compositions correlate with ridge depth, and with each other, in a manner that is consistent with control by mantle temperature variations. Mantle compositional heterogeneity is also seen, but appears to be a second-order effect.