448 Earth and Planetary Science Letters, 107 (1991) 448-462 Elsevier Science Publishers B.V., Amsterdam [CL] The Tasmantid Seamounts: shallow melting and contamination of an EM1 mantle plume S.M. Eggins 1, D.H. Green and T.J. Falloon Department of Geology, Unit,ersity of Tasmania, P.O. Box 252c, Hobart, Tasmania 7001, Australia Received May 18, 1990; revision accepted September 17, 1991 ABSTRACT Primitive basalts (Mg# ~ 61-70; Ni = 232-322 ppm) dredged from the Tasmantid Seamounts range between incompati- ble element-rich alkali-olivine basalt and incompatible element-poor tholeiite compositions. They are primary and near- primary magma candidates, consistent with a decrease in depth of magma segregation from ~ 2.5 GPa (for alkali-olivine basalts) to ~ 1 GPa (for tholeiites). Variation in geochemical and isotopic data in these basalts indicates mixing involving two distinct mantle sources: (1) an EM1 mantle source dominating the tholeiitic basalts (Ba/Nb > 15, ~7Sr/86Sr > 0.7050, ENd <--3.5) and (2) a "long-term depleted" upper mantle source dominating the alkali-olivine basalts (Ba/Nb <8, 87Sr/S6Sr < 0.7035, end > +3). This mixing can be explained by the interaction of a deeply derived EM1 mantle plume (or diapirs) with surrounding upper mantle (asthenosphere or oceanic lithosphere). The hot plume core penetrates to shallow levels to generate tholeiitic magmas, whereas the plume margins interact with surrounding upper mantle, resulting in cooler temperatures, smaller degrees of melting, and the formation of alkaline magmas. The fractionated REE patterns (i.e. Dy/Yb > chondrites) of the tholeiitic and transitional basalts imply melting in the presence of residual garnet but are inconsistent with the low-pressure segregation of these magmas. This may be resolved if small melt fractions, generated in the presence of garnet, have been added to the tholeiitic magma source volume. This requires a melt segregation process where small melt fractions migrate from relatively deep levels within the plume and its margins to enrich the plume core at shallow levels. Alternatively, the plume could incorporate incompatible-element enriched oceanic lithosphere, formed through addition of small melt fractions derived from underlying garnet-bearing asthenosphere prior to Tasmantid magmatism. 1. Introduction Magma generation and melt segregation pro- cesses at hotspots remain poorly understood, de- spite the continuing growth of geochemical, iso- topic and geophysical data. Several recent synthe- ses have developed petrogenetic models con- strained principally by data from the Hawaiian Islands [1-3]. Each has adopted a mantle plume framework in which melting is the consequence of buoyant ascent of anomalously hot mantle. These models are also notable for their endeav- ours to be consistent with trace element evidence Present address: Research School of Earth Sciences, Aus- tralian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia for melting of garnet peridotite sources, yet they are unable to reconcile this interpretation with phase equilibria-based evidence for relatively shallow generation of Hawaiian tholeiitic magmas [4]. It has been proposed that dynamic melt seg- regation processes may resolve this conflict through the addition of small melt fractions gen- erated at depth, in the presence of garnet, to major melt fractions formed at shallower levels where garnet is absent [5]. In this study an integrated approach combin- ing experimental, trace element and isotopic data is used to evaluate the petrogenesis of the Tas- mantid Seamounts. Primary melt composition es- timates are employed to constrain depths of magma generation and the nature of the mantle source residue. This approach highlights the ex- 0012-821X/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved