Fluoride and chloride melts included in phenocrysts of agpaitic acid volcanic rocks from Pantelleria Island

Abstract

978 Many deposits of rare elements (Be, Zr, Nb, REE, Y, and others) are genetically related to acid agpaitic (with the mole ratio (Na2O + K2O)/Al2O3 > 1) mag� mas [1, 2]. The concentration of these elements most likely results from crystallization differentiation lead� ing to their accumulation in the residual melts at the expense of low coefficients of minor element parti� tioning between silicate minerals and melts [3, 4]. Gradual accumulation of incoherent minor elements in melts may be violated by separation of saline melts from magma. According to the experimental data [5], minor elements are most effectively extracted by fluo� ride melts. Because of this, silicate–saline liquid immiscibility is an important factor of geochemical and metallogenic evolution of magmas. The formation of chloride melts during the evolu� tion of magmatic systems was established for some objects [6 and others]. The presence of fluoride melts in acid magmatic systems was described in individual cases [7, 8], and each new finding is of intense interest. In this paper we discuss the results of the study of melt inclusions in phenocrysts from magmatic rocks of Pantelleria Island (Central Mediterranean, Italy) [9], in which liquid immiscibility of alkaline acid magmas with the formation of saline melt was observed. Chlo� ride melts in inclusions and glasses from rocks of Pan� telleria Island were first discovered in 1987 [10] and hereafter were described in glasses of the rock ground� mass [11]. The melt and fluid inclusions were studied in phe� nocrysts from ignimbrite and pantellerite collected in different parts of Pantelleria Island and related to the precaldera and postcaldera stages of volcanism evolu� tion.