The origin of low delta(18)O values for ocean island basalts (OIBs) is
debated as such low values have been taken as evidence for melt
contamination by altered oceanic crust with low delta(18)O values or for
the involvement of altered and subducted oceanic crust during melting at
mantle depths. For the submarine-subaerial Transitional Volcanic Group
of Fuerteventura (Canary Islands, Spain) low delta(18)O values (4.8 +/-
0.3 parts per thousand) measured for phlogopite and amphibole megacrysts
with close-to-HIMU Sr-Nd-Pb isotope characteristics are interpreted to
be primary and, on the basis of hydrogen isotope compositions, exclude
the influence of crustal assimilation. The model that best describes the
chemical and H-O-Sr-Nd-Pb isotope compositions of these minerals favours
melting of subducted ocean crust in the OIB source as a cause of low
delta(18)O values. Low deltaD values of -115 +/- 4 parts per thousand
have been measured for amphibole megacrysts and are attributed to
preceding phlogopite crystallization from a fluid-rich magma. However,
phlogopites with close-to-HIM Sr-Nd-Pb isotope ratios are also depleted
in deuterium relative to upper mantle compositions with deltaD values of
-96 +/- 1 parts per thousand, which may be regarded as the pristine H
isotope composition characteristic for plume material. (C) 2004 Elsevier
B.V. All rights reserved.