Low-volume melts from volatile-rich mantle domains, such as those
represented by the Tertiary melilitites from the Bad Urach-Hegau
volcanic district of southern Germany, are ideally suited for a study of
the fluids involved in intraplate volcanism. The delta(18)O values of
unaltered primitive melilitites (4.9 parts per thousand to 6.1 parts per
thousand) as well as those of hornblende (4.7 parts per thousand to 5.5
parts per thousand) and phlogopite (5.4 parts per thousand to 6.7 parts
per thousand) megacrysts from chemically evolved melilitite tuffs are
consistent with an origin from typical mantle, The delta D values of -91
parts per thousand td -98 parts per thousand of the unaltered primitive
melilitites support this inference, as they are similar to those of
other European Tertiary intraplate basalts. In contrast, the delta D
values for the hornblende and phlogopite megacrysts of -24 parts per
thousand to -42 parts per thousand indicate involvement of recycled
seawater probably during melt fractionation in the lithospheric mantle.
The combined H, O, and Sr isotope evidence is consistent with a
two-stage melt-evolution process in which melting was initiated in a
normal European-type mantle, followed in some cases by magma pending and
fractionation in the lithospheric mantle that had previously been
overprinted by recycled seawater.