Ultrapotassic lava flows from Colli Albani Volcanic District shed light on the origin of calcite-bearing magmas

This work reports a detailed petrological, geochemical and experimental study of lava flows belonging to the whole eruptive activity of the Colli Albani Volcanic District (central Italy). Colli Albani lava flows are particularly intriguing due to the occurrence of calcite crystals in the groundmass of some of these products. By exploring still unclear aspects of the origin of the investigated lava flows, the study aims at shedding light on the origin of calcite-bearing magmas at Colli Albani, thus providing inferences on carbonate-bearing magmas from other magmatic systems worldwide. The detailed microtextural investigation of lava flows has shown that calcite mainly occurs i) interstitially, associated with clinopyroxene, nepheline and phlogopite, ii) in spherical ocelli, associated with nepheline, fluorite and tangentially arranged clinopyroxene, iii) in corona-like reaction zones around K-feldspar xenocrysts. Rarely calcite is enclosed in clinopyroxene xenocrysts and, only in one deposit (the Vallerano lava flow), occurs in amigdalae associated with zeolite. The observed microtextural features indicate that in all investigated lavas but one (the Vallerano lava flow) calcite is not a hydrothermal phase but crystallized from a carbonate melt when the groundmass was still partially molten (i.e. at temperature above the solidus of the hosting lava flow). The high activity of fluorine in the Colli Albani magmas, recorded by the occurrence of F-rich mineral phases such as amphibole, mica and fluorite, had a key role on the stabilization of calcite at atmospheric pressure and magmatic temperature. The calcite stable isotope values account for a crustal, rather than mantle, origin of the carbonate melt and the calcite trace element patterns are consistent with the melting of limestone fragments ripped from the substrate and entrapped at shallow level into the rising magma. The phase equilibria experiments performed at variable pressure and temperature, mimicking different possible carbonate contaminants for the magmas, support the crustal, rather than mantle, origin of the contaminating carbonate. In particular, experiments performed using a natural olivine-leucitite (Palazzolo lava flow, representative of Colli Albani parental composition) doped with natrocarbonatite (as mantle contaminant component) produced melilite as liquidus phase and alkali-rich, silica-poor residual glasses. These results, inconsistent with the composition of the Colli Albani natural products, allow ruling out the mantle origin of the contaminant. Additionally, to better constrain the mantle source of Colli Albani primitive magmas, new Sr and Nd isotope data have been produced. On one hand, the assimilation and fractional crystallization modeling applied to the studied composition shows that the observed Sr and Nd isotope values have been not influenced by limestone assimilation. It would have required, indeed, a too high amount of assimilation to reproduce the observed trend. On the other hand, the progressive decrement of 87Sr/86Sr with the age of the products is consistent with a decrease of the contribution of phlogopite during melting of the metasomatized mantle source. The absence of an appreciable correlation between 143Nd/144Nd and the age of the products can be due to the absence of a Sm-rich phase in the mantle source.