Light rare-earth enrichment of supra subduction-zone mantle: Evidence from the Troodos ophiolite, Cyprus

Geochemical data (including rare-earth elements [REE]) are presented for the primitive (high-MgO) glassy lavas from the upper sections of the Troodos ophiolite. Al2O3/TiO2 ratios are used as an index of degrees of mantle melting and/or successive depletion events. The sources of the lavas have undergone both a ubiquitous introduction of low field-strength elements and a selective addition of light REE, suggesting a two-stage mantle contamination model. Correlations between Al2O3/TiO2 ratios (= depletion index) and the amount of light-REE enrichment present in the lavas suggest a component induced melting model. In this model, progressive addition of a light-REE component induced higher degree batch melts, or enabled melting of highly refractory source material. The light-REE-enrichment event is shown to be of the same character across the ophiolite, but it is limited to the older ocean crust and a transform-influenced regime.