Mixing processes between materials of different
compositions are known to occur frequently and at different scales in the Earth's mantle. In the SE Pacific mixing between material from the Easter plume and material from the adjacent East Rift and West Rift spreading axes of the Easter Microplate was described previously (Schilling et al., 1985; Hanan and Schilling, 1989) and is well constrained by radiogenic isotopes. Here we investigate the processes leading to the effective mixing using element and radiogenic isotope compositions.
The clearest isotopic plume signature is observed in alkaline basalts close to Easter Island. Incompatible element-enriched tholeiites from volcanic fields 200 km west of Easter Island and on the East Rift spreading segment 450 km west of the island both have similar isotopic compositions indicating less influence of the plume. The degree of melting is decreasing and the pressure of melting is increasing from the spreading axis to the intraplate region near Easter Island. From the central spreading segment strongly under plume influence the isotopic signatures of the plume decrease symmetrically to the north and to the south. Following major element models (Klein and Langmuir, 1987) there appears to be no variation in degree (Na8.0) and depth (Fe8.0) of melting along the East Rift spreading axis.
Because undiluted plume material is found near Easter Island the mixing must occur at relatively shallow levels during the flow of the plume material from Easter Island towards the spreading axis. 250 km east of the spreading axis the plume material becomes strongly diluted by MORB material giving rise to the isotopically homogeneous enriched tholeiites of the East Rift and the volcanic fields. Using fluid dynamic models (Christensen, 1989; Griffiths and Campbell, 1991) the thorough mixing over an area of 200 x 200 km2 suggests a relatively low viscosity of the materials and a weak plume underneath the region. West Rift-type MORB material cannot be the depleted endmember because it is too depleted in incompatible elements. The depleted endmember probably has the composition of depleted East Rift MORB material. Although mixing of magmas allows the most efficient mixing possibly not all magmas of the East Rift spreading axis and the intraplate region are generated by simple mixing of two endmember melts. Thus melting and mixing appears to occur over a wide range of depths giving rise to magmas reflecting variable degrees and average pressures of partial melting although isotopes suggest they come from similar mantle sources.
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