ABSTRACT

Rare-earth element (REE) geochemistry on the igneous rocks from the Galapagos Islands has been discussed on the basis of precisely determined REE data for twenty rocks including alkali-olivine basalts, tholeiites, differentiated effusive rocks (ferrobasalt, icelandite and trachyte), plutonic rocks (eucrite and ferrobasalt) and inclusions of mafic and ultramafic compositions (gabbro and peridotite). It is disclosed that REE patterns of alkali olivine basalts from the Galapagos Islands have some features different from those observed in alkali olivine basalts from other regions. That is, alkali olivine basalts from the Galapagos Islands have essentially the same features in REE patterns as those seen in tholeiitic rocks from the same Islands and the ratios of normalized values between Ce and Yb are 2-4, which are small compared with the corresponding ratios of the typical alkali basalts. Most of REE patterns obtained in this study are similar to each other i.e., upward convex curve in the light and middle REE span with monoclinic rectilinear line in the heavy REE span, irrespective of a wide range in the major element compositions. These REE patterns appear to have an inflection point around the position of Gd, Th, Dy or Ho, mainly at Ho. These REE patterns most commonly observed in this study are considered to correspond to solid-type (light REE depleted) phase or to correspond to mixtures of conjugate solid-type and liquid-type (light REE enriched) phases as a consequence of incomplete separation of the two phases. It is suggested that the liquid-type phase was generated by the small-scale convection in the asthenosphere and that the liquid-type phase and the remnant solid-type phase were mixed at the top of the asthenosphere. We have considered that the essential features in these REE patterns were established during the processes in the asthenosphere and that fractionation process in the lithosphere gave rise to differentiation of only major elements and, during these processes in lithosphere, REE (except Eu) have been uniformly enriched or depleted with little change in the shape of REE patterns. It is probable that terrace-shaped REE partition pattern having the inflection point at Ho played a leading role in characterizing the shape of these REE patterns.