Constraints on mid-ocean ridge processes from uranium series disequilibria.

摘要

Uranium series disequilibria represents a powerful tool for examining the processes of melting, melt extraction, and solid upwelling beneath mid-ocean ridges. To reach its full potential, an integration of experiments, numerical modeling and geochemical analysis is required.; The partitioning of U and Th between clinopyroxene and melt is compositionally controlled by the Al content of clinopyroxene. Given the higher Al content in mantle clinopyroxenes, the relevant {dollar}rm sp{lcub}cpx/l{rcub}{dollar}D for Th is estimated to be 0.015. Under the oxygen fugacities relevant to MORB petrogenesis, clinopyroxene does not fractionate U from Th.; U-series disequilibria in MORB produce systematic trends for three different parent-daughter pairs, best exemplified in young samples from the Juan de Fuca ridge. These trends may be explained by mixing of melts derived from heterogeneous sources that have migrated by an equilibrium porous flow process. They are not consistent with dynamic or near-fractional melting models. The conclusion that equilibrium porous flow describes the sub-ridge mantle requires that melt porosities be less than 0.2%.; The slopes of the mixing trends on a ({dollar}sp{lcub}238{rcub}{dollar}U)/({dollar}sp{lcub}232{rcub}{dollar}Th) versus ({dollar}sp{lcub}230{rcub}{dollar}Th)/({dollar}sp{lcub}232{rcub}{dollar}Th) (equiline) diagram are shown to be a direct function of the half spreading rate of a ridge. This relation can be explained by an equilibrium porous flow model using solid upwelling rates similar to the half spreading rates. Such a one to one correlation is consistent with passive flow of the upwelling mantle. The major and trace element chemistry of MORB are well explained by an equilibrium porous flow process acting on a peridotite mantle that is veined with pyroxenite.; The mid-Atlantic ridge at 33{dollar}spcirc{dollar}S produces the largest gravity anomaly yet observed along the global ridge system. U-series data demonstrate that this segment has anomalous melting behavior producing the lowest zero age ({dollar}sp{lcub}231{rcub}{dollar}Pa)/({dollar}sp{lcub}235{rcub}{dollar}U) observed on any ridge. The shallow trends on an equiline diagram of all three segments suggest that upwelling beneath this area is passive and that the low {dollar}sp{lcub}231{rcub}{dollar}Pa excesses reflect variations in the porosity structure beneath the anomalous segment.