Major and trace element compositions, stable H and 0 isotope
compositions and Fe 31 contents of amphibole megacrysts of
Pliocene-Pleistocene alkaline basalts have been investigated to obtain
information on the origin of mantle fluids beneath the
Carpathian-Pannonian region. The megacrysts have been regarded as
igneous cumulates formed in the mantle and brought to the surface by the
basaltic magma. The studied amphiboles have oxygen isotope compositions
(5.4 +/- 0.2 %., 1 sigma), supporting their primary mantle origin. Even
within the small 6180 variation observed, correlations with major and
trace elements are detected. The negative delta(18)O-MgO and the
positive delta(18)O-La/Sm(N) correlations are interpreted to have
resulted from varying degrees of partial melting. The halogen (F, Cl)
contents are very low (< 0.1 wt. %), however, a firm negative
(F+Cl)-MgO correlation (R(2) = 0.84) can be related to the Mg-Cl
avoidance in the amphibole structure. The relationships between water
contents, H isotope compositions and Fe 31 contents of the amphibole
megacrysts revealed degassing. Selected undegassed amphibole megacrysts
show a wide 813 range from -80 to -20 parts per thousand. The low delta
D value is characteristic of the normal mantle, whereas the high delta D
values may indicate the influence of fluids released from subducted
oceanic crust. The chemical and isotopic evidence collectively suggest
that formation of the amphibole megacrysts is related to fluid
metasomatism, whereas direct melt addition is insignificant.