Browsing by Author "Mollo, Silvio"

Crystal-rich lithic clasts occurring in volcanic deposits are key tools to understand processes of storage, cooling, and fractionation of magmas in pre-eruptive volcanic systems. These clasts represent snapshots of the magma-chamber-host-rock interface before eruption and provide information on crystallization, differentiation, and degrees of interaction between magma and wall-rock. In this study we have focused on the petrology of clasts of cumulate and skarn rocks from the Colli Albani Volcanic District with the aim of shedding light on magma-carbonate interaction and CO2 emission in volcanic areas. By means of phase relations, bulk-rock chemistry, mineral compositions and isotope data we have identified different types of cumulates and skarns. Cumulates containing either clinopyroxene +/- olivine associated with Cr-bearing spinel or glass + phlogopite have been classed as primitive and differentiated, respectively. Cumulates originate at the interface between either a primitive or differentiated magma and carbonate-bearing wall-rock characterized by the occurrence of CaO-rich melt. Skarns have been classed as exoskarns, characterized by xenomorphic textures and abundant calcite, and endoskarns, characterized by a hypidiomorphic texture, Ca-Tschermak-rich mineral phases, and interstitial glass. Exoskarns formed by means of solid-state reactions in a dolomite-bearing protolith whereas endoskarns crystallized from a silicate melt that experienced exoskarn assimilation. Our study indicates that magma-carbonate interaction is a multi-step process that proceeds beyond the formation of skarn shells. Magma and carbonate rocks, when in contact, continuously interact leading to the formation of exoskarns, endoskarns, cumulates (primitive and differentiated types), and differentiated melts. The geochemical characteristics of the studied endoskarn and cumulate rocks indicate that crustal contamination of the Colli Albani magmas occurs through the simultaneous assimilation of both solid crustal material (dolomite and/or exoskarns) and partially molten crustal material (CaO-rich melt). The oxygen and carbon isotope compositions of calcite in equilibrium with the skarns suggest that dolostone-limestone assimilation and decarbonation are able to provide the massive CO2 release observed in carbonate-hosted magmatic systems, such as Colli Albani.

We have investigated lava flows representative of the whole eruptive history of the Colli Albani ultrapotassic volcanic district (Central Italy). One of the most intriguing features concerning some of these lava flows is the occurrence of primary, magmatic calcite in the groundmass. The primary, magmatic nature of calcite has been inferred by microtextural investigations showing that it typically occurs i) interstitially, associated with clinopyroxene, nepheline and phlogopite, ii) in spherical ocelli, associated with nepheline, fluorite and tangentially arranged clinopyroxene, and iii) in corona-like reaction zones around K-feldspar xenocrysts. These microtextural features distinctly indicate that calcite crystallized from a carbonate melt in a partially molten groundmass, implying that the temperature of the system was above the solidus of the hosted lava flow (>850 degrees C). Geochemical features of calcite crystals (i.e., stable isotope values and trace element patterns) corroborate their primary nature and give insights into the origin of the parental carbonate melt. The trace element patterns testify to a high-temperature crystallization process (not hydrothermal) involving a carbonate melt coexisting with a silicate melt. The high delta C-18 (around 27 parts per thousand SMOW) and wide delta C-13 (-18 to + 5 parts per thousand PDB) values measured in the calcites preclude a mantle origin, but are consistent with an origin in the crust. In this framework, the crystallization of calcite can be linked to the interaction between magmas and carbonate-bearing wall rocks and, in particular, to the entrapment of solid and/or molten carbonate in the silicate magma. The stability of carbonate melt at low pressure and the consequent crystallization of calcite in the lava flow groundmass are ensured by the documented, high activity of fluorine in the studied system and by the limited ability of silicate and carbonate melts to mix at syn-eruptive time scales. (C) 2013 Elsevier B.V. All rights reserved.