Silicate melt inclusions in the cumulate minerals of gabbroic nodules from Stromboli Volcano (Aeolian Islands, Italy): main components of the fluid phase and crystallization temperatures

摘要

The studied gabbroic nodules occurring in the Petrazza pyroclastic rocks consist mainly of plagioclase (An_(95-87)), olivine (Fo_(83-73)) and clinopyroxene (Mg# 90-77), with subordinate opaques (Ti-magnetite) and amphibole (Mg-hastingsite), which constitute the cumulate minerals. Interstitial material has a relatively high, but variable, degree of vesicularity and consists of variable amounts of glass and quenched crystals of plagioclase (An_(71-55)), amphibole, clinopyroxene and rare biotite, olivine and opaques. Silicate melt inclusions are abundant in the cumulate minerals, but complete homogenization to melt has been observed only in the inclusions occurring in clinopyroxene, where the temperatures of homogenization vary from 1134 to 1190 deg C. Microthermometric investigations of fluid inclusions and of the shrinkage bubble of the melt inclusions suggest that the magma contained CO_2. The apparent scarcity of H_2O indicates that this component was strongly partitioned into the magma at the time of crystallization of the investigated minerals; this is further supported by the occurrence of (1) daughter biotite- and amphibole-bearing inclusions which show that the H_2O activity in the magma was sufficiently high to allow their crystallization, and (2) calcic plagioclase (An_(95-87)) which can be crystallized from a high-alumina basaltic magma at pressure <= 2 kbar, temperatures in the range 1050-1100 deg C and in the presence of 3-4 wt. percent of water (MELTS software simulations). The composition of the melt inclusions suggests that the hosting plagioclase, olivine and clinopyroxene crystallized from slightly different batches of magma. The S content in the melt inclusions of clinopyroxene and olivine is high (up to 0.41 wt. percent). The presence of Fe-Cu(-Ni)-rich blebs of sulphide in plagioclase, olivine, amphibole, and locally in the melt inclusions too, further supports the important role of sulphur in the primitive magmas of the investigated gabbros. Small differences in redox conditions or in the Fe content of the melts favoured S mobilization as sulphide.