STEADY-STATE 226RA/230TH DISEQUILIBRIUM IN MANTLE MINERALS: IMPLICATIONS FOR MELT TRANSPORT RATES IN ISLAND ARCS

Abstract

Measurements of the concentrations of the 238U decay series isotopes (234U, 230Th, 226Ra) have been used to estimate the rates at which magma is generated and transported in the mantle. The usual assumption is that solid mantle minerals are in radioactive equilibrium prior to melting. However, if one or more of the nuclides in the chain is strongly concentrated by a minor mineral, and if the diffusivity of that nuclide is large enough, steady-state radioactive disequilibrium can result in the solid phase. It can be inferred from available data that radium is strongly concentrated in minor hydroxyl-bearing mantle minerals (phlogopite and amphibole) relative to Th, and Ra diffusion in clinopyroxene is fast relative to the typical grain diameter at ca. 1100°C. Consequently, we show with simple analytical models that a steady-state Ra deficiency in clinopyroxene (cpx), accompanied by a complementary steady-state Ra excess in neighboring phlogopite (phlog) or amphibole (amph), is likely to be the normal situation in hydrous mantle peridotite with average clinopyroxene grain radii of ca. 1 mm. The steady state (226Ra/230Th) (parentheses indicating activity ratio) in the hydrous mineral is limited roughly by the mass ratio with clinopyroxene (i.e. cpx/phlog or cpx/amph) and could be as high as 10-100. The exceptionally high (226Ra/230Th) of some island arc lavas could therefore be a result of preferential contribution of phlogopite or amphibole during partial melting of hydrous mantle. This effect may ease time constraints for source-to-surface melt migration at island arcs. Incipient melting of hydrous minerals from channel walls during melt transport and/or late-stage incorporation of phlogopite or amphibole into arc magmas may also contribute to generating high (226Ra/230Th). Steady-state (226Ra/230Th) disequilibrium due to diffusive loss of 226Ra from clinopyroxene is also important for melt/solid and fluid/solid partitioning, and must be incorporated into models relating isotopic disequilibrium to melt and fluid transport rates. Diffusive effects could be important for other U-series nuclides in some circumstances.