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Authors: | Lustrino, Michele* Luciani, Natascia* Stagno, Vincenzo* Narzisi, Silvia* Masotta, Matteo* Scarlato, Piergiorgio* |
Title: | Experimental evidence on the origin of Ca-rich carbonated melts formed by interaction between sedimentary limestones and mantle-derived ultrabasic magmas | Journal: | Geology | Series/Report no.: | 4/50 (2022) | Publisher: | GSA | Issue Date: | 2022 | DOI: | 10.1130/G49621.1 | URL: | https://pubs.geoscienceworld.org/gsa/geology/article/50/4/476/610678/Experimental-evidence-on-the-origin-of-Ca-rich | Keywords: | carbonatite experimental petrology ultrabasic melts limestone assimilation |
Subject Classification: | 04.08. Volcanology | Abstract: | In this experimental study, we documented the formation of strongly ultrabasic and ultracalcic melts through the interaction of melilititic and basanitic melts with calcite. Three strongly to moderately SiO2-undersaturated volcanic rocks from the Bohemian Massif (central Europe) were mixed with 10, 30, and 50 wt% CaCO3 and melted at 1100, 1200, and 1300 °C at 2 kbar to evaluate the maximum amount of carbonate that can be assimilated by natural ultrabasic melts at shallow depths. Experiments revealed a surprisingly complete dissolution of the CaCO3, only rarely reaching carbonate saturation, with typical liquidus phases represented by olivine, spinel, melilite, and clinopyroxene. Only in the runs with the most SiO2-undersaturated compositions did abundant monticellite form instead of clinopyroxene. For all starting mixtures, strongly ultrabasic (SiO2 down to 15.6 wt%), lime-rich (CaO up to 43.6 wt%), ultracalcic (CaO/Al2O3 up to ~27) melt compositions were produced at 1200 and 1300 °C, with up to ~25 wt% dissolved CO2. When present, quenched olivine showed much higher forsterite content (Fo95–97) than olivine in the natural samples (Fo79–85). The two major results of this study are (1) silicate-carbonatite melt compositions do not necessarily imply the existence of carbonatitic components in the mantle, because they are also produced during limestone assimilation, and (2) Fo-rich olivines cannot be used to infer any primitive character of the melt nor high potential temperature (Tp). |
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