DEPTH OF ORIGIN OF BASALTS INFERRED FROM TI/V RATIOS AND A COMPARISON WITH THE K

Abstract

The location of tectonic fields on the Ti/V discrimination diagram suggests that this ratio in least fractionated basalts may be a function of the depth to magma source. The hypothesis is tested using chronostratigraphic data for selected intraplate volcanics, and the idea is then applied to island-arc settings. The temporal trends suggest that the initial magma is mid-ocean ridge basalt (MORB)-like and may be subducted oceanic crust alone. This develops into a deeper source with the involvement of subducted oceanic crust and depleted upper-mantle materials. In the mature arc there is a dominance of mantle-wedge-derived materials.Such depth trends suggest that the subducted oceanic crust is not always directly involved in magma genesis, implying that the inference of paleosubduction zones by the K2O—depth relationships may be invalid for much of the life of an island arc. The consideration of mixing models suggests that the early and mature arc volcanics obtain their K from the continental crust, whereas the arc of intermediate maturity obtains its K at least partly from K-bearing subcreted sediments included in the mixing of depleted upper mantle and subducted oceanic crust.