Experimental evidence on the origin of Ca-rich carbonated melts formed by interaction between sedimentary limestones and mantle-derived ultrabasic magmas

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).