Major and trace element compositions, stable H and O isotope
compositions and Fe3+ 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 σ), supporting their primary mantle origin. Even within
the small δ18O variation observed, correlations with major and trace
elements are detected. The negative δ18O-MgO and the positive δ18O-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 (R2 =
0.84) can be related to the Mg-Cl avoidance in the amphibole structure.
The relationships between water contents, H isotope compositions and
Fe3+ contents of the amphibole megacrysts revealed degassing. Selected
undegassed amphibole megacrysts show a wide δD range from -80 to -20 ‰.
The low δD value is characteristic of the normal mantle, whereas the
high δD 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