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 |
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Authors: | E Wulff-Pedersen E -R Neumann R Vannucci P Bottazzi L Ottolini |
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Institution: | (1) Mineralogical-Geological Museum, University of Oslo, Sarsgt. 1, N-0562 Oslo, Norway, NO;(2) Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy, IT;(3) CNR-Centro di Studio per la Cristallochimica e la Cristallografia, via Ferrata 1, I-27100 Pavia, Italy, IT |
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Abstract: | Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites
and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition
from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched
in strongly incompatible elements. As the veins become narrower, the SiO2-contents in glass increase (46 → 67 wt% SiO2 in harzburgite, 43 → 58 wt% in dunite) and the trace element patterns change gradually to concave 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 patterns are exhibited by glass in unveined peridotite
xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic
glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites.
The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration
of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine
porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes
have taken place in situ in the upper mantle. The CaO-TiO2-La/Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts.
Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant,
low-CaO contents which is compatible with metasomatism of “normal” abyssal peridotite by silicic melts. Peridotite xenoliths
from Tenerife show somewhat higher CaO and TiO2 contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace
element signatures of ultramafic xenoliths from La Palma and 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/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent
important metasomatic agents.
Received: 15 November 1998 / Accepted: 17 May 1999 |
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