Silicic melts produced by reaction between peridotite and infiltrating basaltic melts: ion probe data on glasses and minerals in veined xenoliths from La Palma, Canary Islands

Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Ca nary Islands include veined spinel harzburgites and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a grad ual transition in composition from broad veins to narrow veinlets. Broad ve ins contain alkali basaltic glass with semi-linear trace element patterns e nriched in strongly incompatible elements. As the veins become narrower, th e SiO2-contents in glass increase (46 ->, 67 wt% SiO2 in harzburgite, 43 -> 58 wt% in dunite) and the trace element patterns change gradually to conca ve patterns depleted in moderately incompatible elements (e.g. HREE, Zr, Ti ) relative to highly incompatible ones. The highest SiO2-contents (ca. 68% SiO2, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patt erns are exhibited by glass in unveined peridotite xenoliths. Clinopyroxene s shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-di opside in highly silicic glass. Estimates indicate that the most silicic gl asses represent melts in, or near, equilibrium with their host peridotites. The observed trace element changes are compatible with formation of the si licic melts by processes involving infiltration of basaltic melts into mant le peridotite followed by reactions and crystallization. The Fe-Mg interdif fusion profiles in olivine porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction proces ses have taken place in situ in the upper mantle. The CaO-TiO2-La/ Nd relat ionships of mantle rocks may be used to discriminate between metasomatism c aused by carbonatitic and silicic melts. Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with re latively constant, low-CaO contents which is compatible with metasomatism o f "normal" abyssal peridotite by silicic melts. Peridotite xenoliths from T enerife show somewhat higher CaO and TiO2 contents than those from the othe r islands and may have been affected by basaltic or carbonatitic melts. The observed trace element signatures of ultramafic xenoliths from La Palma an d other Canary Islands may be accounted for by addition of small amounts (1 -7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g. eastern Australia, show CaO-TiO2-La/N d relationships compatible with metasomatism by silicic melts. These result s suggest that silicic melts may represent important metasomatic agents.