Dy/Dy*: Variations Arising from Mantle Sources and Petrogenetic Processes

Abstract

Dy/Dy* is the measured value of Dy, a representative middle rare earth element (REE), compared with the value interpolated between La and Yb on a REE plot. It is essentially a measure of the ‘concavity’ of a REE pattern. The use of Dy/Dy* as a proxy for REE pattern shape allows us to compare large amounts of REE data, which can be difficult using standard REE patterns. When Dy/Dy* is combined with Dy/Yb (the slope of the middle-to-heavy REE pattern) REE patterns can effectively be classified by shape. We present a new set of high-quality REE (and other trace element) data for young volcanic rocks from six arcs. When plotted on the Dy/Dy*^Dy/Yb diagram they define a broad negative correlation from LREE-depleted (Tonga^Kermadec) to LREE-enriched (Philippines and Indonesia). This trend corresponds to decreasing Dy/Dy*, reflecting REE patterns varying from concave-down to concave-up respectively. When cogenetic suites from single volcanoes are plotted they define clear trends of decreasing Dy/Yb and Dy/Dy* with differentiation, roughly orthogonal to the general depletion^enrichment trend. The trends for single arc volcanoes are interpreted as most probably reflecting an amphibole control, consistent with the concomitant decreases in Ti/Ti* and increasing SiO2. Available distribution coefficients are, however, also permissive of cpx control on arc REE patterns. Estimated compositions of the continental crust fall along these same trends. In contrast, ocean island basalt (OIB) data all fall to high Dy/Yb, suggesting a significant control by garnet. A global consideration of the data suggests that (1) arc magmas are derived from variably depleted asthenospheric (mid-ocean ridge basalt) mantle sources, (2) arc magma (and continental crust) differentiation is controlled by a mineral phase (or phases) that preferentially partitions MREE and (3) OIB genesis appears to always involve garnet control. We propose that Dy/Dy* is potentially a powerful tool for representing the shapes of REE patterns, especially for large datasets. We also note the importance of using cogenetic rock suites to identify petrogenetic processes rather than regional suites.