OCCURRENCE OF BORON-RICH POTASSIC MELTS IN THE VULSINI VOLCANIC DISTRICT, ITALY - EVIDENCE FROM MELT INCLUSIONS

Boron was analyzed by nuclear microanalysis technique, using the B-11( p, alpha)Be-8 reaction, in potassic to highly potassic melts (Mg# [Mg2 +/(Mg2+ + Fe2+)] > 0.70) trapped as melt inclusions in olivine (Fo(91. 7-90.5)) phenocrysts from the Vulsini Volcanic District (northwestern Roman Province, Italy). These data evidence unusual B concentrations f rom 60 to 100 ppm, up to five times higher than any documented values in primitive subduction-related magmas. The melt inclusions display va riable concentrations in B, K, Sr, and F, with constant ratios between elements, for a single volcanic event, that strongly suggest variatio ns of the extents of batch partial melting of a mantle source enriched in B, K, Sr, and F. Conversely B/K and B/Cl ratios in potassic magmas (Mg# > 0.70), as exemplified by melt inclusions in olivine (Fo > 90), are variable on the scale of the whole Roman Province. These variatio ns cannot be explained by either fractional crystallization or differi ng extents of the mantle source melting alone. This is also supported by the range in Th/Ta ratios (52-10) of the whole rock samples. Variab ility of the B/K or B/Cl ratios cannot be related to a single-stage me tasomatism event. The data indicate the possible occurrence of B-rich contaminant and/or high inputs of subducted-derived components in the mantle wedge beneath the Vulsini area, within the peculiar geodynamic setting of the northern part of the Apenninic are. In addition, by det ermining boron concentrations using nuclear microanalysis involving th e B-11 (p, alpha)Be-8 nuclear reaction, we have illustrated the sensit ivity of the nuclear reactions in the analysis of the light elements. Copyright (C) 1998 Elsevier Science Ltd.