Abstract

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 δ18O values of unaltered primitive melilitites (4.9‰ to 6.1‰) as well as those of hornblende (4.7‰ to 5.5‰) and phlogopite (5.4‰ to 6.7‰) megacrysts from chemically evolved melilitite tuffs are consistent with an origin from typical mantle. The δ D values of −91‰ to −98‰ of the unaltered primitive melilitites support this inference, as they are similar to those of other European Tertiary intraplate basalts. In contrast, the δD values for the hornblende and phlogopite megacrysts of −24‰ to −42‰ 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 ponding and fractionation in the lithospheric mantle that had previously been overprinted by recycled seawater.

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