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

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          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 2O-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.

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          Olivine-liquid equilibrium

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            An olivine-free mantle source of Hawaiian shield basalts.

            More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.
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              The compositions of primary carbonate melts and their evolution through wallrock reaction in the mantle

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                Author and article information

                Contributors
                michele.lustrino@uniroma1.it
                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group UK (London )
                2045-2322
                25 June 2019
                25 June 2019
                2019
                : 9
                : 9212
                Affiliations
                [1 ]GRID grid.7841.a, Dipartimento di Scienze della Terra, , Sapienza Università di Roma, ; P.le A. Moro, 5, 00185 Roma, Italy
                [2 ]GRID grid.7841.a, CNR – Istituto di Geologia Ambientale e Geoingegneria (IGAG) c/o Dipartimento di Scienze della Terra, , Sapienza Università di Roma, ; P.le A. Moro, 5, 00185 Roma, Italy
                [3 ]ISNI 0000 0004 1754 9227, GRID grid.12380.38, Faculty of Earth and Life Sciences, , Vrije Universiteit Amsterdam, ; de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
                Author information
                http://orcid.org/0000-0002-1212-7207
                Article
                45471
                10.1038/s41598-019-45471-x
                6592957
                31239468
                c8998971-c7fc-4a42-876a-18b5126b726a
                © The Author(s) 2019

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 26 September 2018
                : 4 June 2019
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                © The Author(s) 2019

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                geochemistry,petrology
                Uncategorized
                geochemistry, petrology

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