Qandilite From Vesuvius Skarn Ejecta: Conditions of Formation and Miscibility Gap in the Ternary Spinel - Qandilite - Magnesioferrite

Abstract

Qandilite (Mg 2 TiO 4 ) and magnesioferrite occur in forsterite -spinel -calcite skarn ejecta from Mt. Vesuvius, Italy, with an exceptionally large compositional range that outlines the miscibility gap in the system spinel -qandilite -magnesioferrite (with a small amount of Fe 2+ ), at present solely determined on the spinel-qandilite binary at T > 1000°C. The analyzed spinelqandilite and spinel-magnesioferrite pairs are consistent with the solvus and tie-lines (except for a temperature offset) derived from the thermochemical model of spinel solid-solutions. Temperatures of formation in the range 650-700°C are inferred from the petrological study of the skarn-forming processes involved, which typically include two types of metasomatic reactions, i.e., formation of spinel -forsterite -calcite endoskarns by desilication of aluminosilicate bodies at the contact of dolostone wallrocks, and reaction of such pre-existing endoskarns with new influxes of magma ± fluid. Calculated phase-relations among qandilite, perovskite and geikielite show that qandilite with moderate magnesioferrite contents is the high-temperature Ti mineral stable in magnesian and extremely silica-deficient surroundings under oxidized conditions.

Magma Mingling in Kimberlites: Evidence from the Groundmass Cocrystallization of Two Spinel-Group Minerals

“…Similar compositions of qandilite-rich MUM were found in association with carbonates in the kimberlite groundmass at the Igwisi Hills and Benfontein [42,49]. This assemblage is consistent with the expected conditions of the crystallization of qandilite, favored by extremely high f (CO 2 ) and extremely low a (SiO 2 ) [50]. We believe that the mingling of an immiscible dolomite-rich melt (magma 2) might have led to the crystallization of dolomite and qandilite.…”

Magma evolution inside the 1631 Vesuvius magma chamber and eruption triggering

“…6c). A detailed mineralogical description of these skarns is found in [26,27]. Observation of composite igneous, skarn and marble contacts in the 1631 ejecta testifies the very limited thickness of the skarn shell.…”

“…Fresh sedimentary lithic clasts in the 1631 pyroclastic rocks are highly indicative of limited magma/crust interaction and thus potential assimilation. Skarn assemblages suggest temperatures of 650-700 ∘ C for the skarn forming processes, which typically include two types of metasomatic reactions, i.e., formation of spinelforsterite-calcite endoskarns by desilication of aluminosilicate bodies at the contact of dolostone wall rocks and reaction of pre-existing endoskarns with new influxes of magma and fluid as documented by [26,27]. According to these authors, these metasomatic processes are promoted by CO 2 -rich fluids and act as sinks of CO 2 , producing calcite as part of the skarn mineral assemblage.…”

Genetic implications, composition, and structure of trioctahedral micas in xenoliths related to Plinian eruptions from the Somma-Vesuvius volcano (Italy)

“…Skarns have calc-silicate compositions typically developing in contact-metamorphic aureoles in response to interaction between limestone or dolostone with a silicate magma. The formation of these rocks is known to progressively modify the composition of both the magma and the carbonate (Gilg et al, 2001;Pascal et al, 2011;Jolis et al, 2015 and references therein). Different varieties of spinels have been also described, i.e., spinel, magnetite, magnesioferrite and qandilite, the last two phases found in metasomatic forsterite-spinel-calcite skarns (Pascal et al, 2011).…”

“…The formation of these rocks is known to progressively modify the composition of both the magma and the carbonate (Gilg et al, 2001;Pascal et al, 2011;Jolis et al, 2015 and references therein). Different varieties of spinels have been also described, i.e., spinel, magnetite, magnesioferrite and qandilite, the last two phases found in metasomatic forsterite-spinel-calcite skarns (Pascal et al, 2011). Many studies characterized pressure and temperature of skarn formation, the fluid nature and the isotope evolution related to the limestone/dolostone-skarn transition, as reported by Jolis et al (2015 and references therein).…”