Fuzzy petrology in the origin of carbonatitic/pseudocarbonatitic Ca-rich ultrabasic magma at Polino (central Italy)

Abstract

The small upper Pleistocene diatreme of Polino (central Italy) is known in literature as one of the few monticellite alvikites (volcanic Ca-carbonatite) worldwide. This outcrop belongs to the Umbria-Latium Ultra-alkaline District (ULUD), an area characterized by scattered and small-volume strongly SiO 2 -undersaturated ultrabasic igneous rocks located in the axial sector of the Apennine Mts. in central Italy. Petrographic and mineralogical evidences indicate that Polino olivine and phlogopite are liquidus phases rather than mantle xenocrysts as instead reported in literature. The presence of monticellite as rim of olivine phenocrysts and as groundmass phase indicates its late appearance in magma chambers at shallow depths, as demonstrated by experimental studies too. The absence of plagioclase and clinopyroxene along with the extremely MgO-rich composition of olivine (Fo 92–94 ) and phlogopite (average Mg# ~93) suggest for Polino magmas an origin from a carbonated H 2 O-bearing mantle source at depths at least of 90–100 km, in the magnesite stability field. In contrast with what reported in literature, the ultimate strongly ultrabasic Ca-rich whole-rock composition (~15–25 wt% SiO 2 , ~31–40 wt% CaO) and the abundant modal groundmass calcite are not pristine features of Polino magma. We propose that the observed mineral assemblage and whole-rock compositions result mostly from the assimilation of limestones by an ultrabasic melt at a depth of ~5 km. A reaction involving liquidus olivine + limestone producing monticellite + CO 2 vapour + calcite is at the base of the origin of the Polino pseudocarbonatitic igneous rocks.

Complex Geological-Geophysical Studies of the Underwater Volcano 7.10 (Kuril Island Arc)

“…Существуют изоструктурные оливину редкие минералы группы монтичеллита -монтичелллит (CaMgSiO 4 ) и кирштейнит (CaFeSiO 4 ), составы которых изоморфны. Эти минералы образуются при пересыщении кальцием минералообразующих сред и отмечены преимущественно в метаморфитах, но встречены также в ультраосновных породах и вулканитах (Николаева, 2004;Плечов и др., 2017;Силаев и др., 2019;Lustrino et al, 2019). Монтичеллит, в частности, определен в районе вулкана Везувий (Principe, Marini, 2008).…”

New Inferences on Magma Dynamics in Melilitite‐Carbonatite Volcanoes: The Case Study of Mt. Vulture (Southern Italy)

“…Carbonatite magmatism is mainly associated with intraplate continental tectonic settings characterized by significant extension and even rifting, with a temporal distribution from Archean to the present (e.g., Jones et al., 2013; Woolley & Kjarsgaard, 2008; Yaxley et al., 2022), and currently, Oldoinyo Lengai (Tanzania) represents the only active carbonatite volcano, characterized by a natrocarbonatitic affinity (e.g., Berkesi et al., 2020). The growing number of carbonatite occurrences from unconventional tectonic settings, such as oceanic contexts (e.g., Carnevale et al., 2021; Day, 2022; Doucelance et al., 2010; Mata et al., 2010; Schmidt & Weidendorfer, 2018) or subduction zones (e.g., D’Orazio et al., 2007; Li et al., 2018; Lustrino et al., 2019, 2020), received considerable attention during last two decades, given their importance as source of rare elements such as La, Ce, Pr, and Nd (Anenburg et al., 2021; Verplanck et al., 2016), and, most importantly, because they provide meaningful information on the geochemical cycle of carbon and mantle metasomatism as well (e.g., Bouabdellah et al., 2010; Horton, 2021).…”

Petrogenesis of silica-rich carbonatites from continental rift settings: a missing link between carbonatites and carbonated silicate melts?

“…Carbonatitic melts span in age from Late Archean occurrences (Tupertalik, ~3.0 Ga; Bizzarro et al 2002) to active natrocarbonatite from Oldoinyo Lengai (Bell and Keller 1995). Their origin is usually attributed to either direct derivation from the upper mantle, fractional crystallization of parental silicate melts (Lee and Wyllie 1994;Veksler et al 1998), or separation from mantle-derived carbonated genetic indicators (e.g., Viladkar 2000Viladkar , 2017Reguir et al 2009Reguir et al , 2012Lustrino et al 2019). This leads to elevated whole-rock SiO 2 contents (silica-rich carbonatite containing > 5 wt.…”