Reactive Infiltration of MORB-Eclogite-Derived Carbonated Silicate Melt into Fertile Peridotite at 3GPa and Genesis of Alkalic Magmas

We performed experiments between two different carbonated eclogite-derived melts and lherzolite at 1375°C and 3 GPa by varying the reacting melt fraction from 8 to 50 wt%.The two starting melt compositions were (1) alkalic basalt with 11.7 wt % dissolved CO₂ (ABC), (2) basaltic andesite with 2.6 wt % dissolved CO₂ (BAC).The starting melts were mixed homogeneously with peridotite to simulate porous reactive infiltration of melt in the Earth's mantle. All the experiments produced an assemblage of melt+orthopyroxene+clinopyroxene+garnet ólivine; olivine was absent for a reacting melt fraction of 50 wt % for ABC and 40 wt % for BAC. Basanitic ABC evolved to melilitites (on a CO₂-free basis, SiO₂ ~27-39 wt %,TiO₂ ~2.8-6.3wt %, Al₂O₃ ~4.1-9.1wt %, FeO^* ~11-16 wt %, MgO ~17-21wt %, CaO ~13-21wt %, Na2O ~4-7 wt%, CO₂ ~10-25 wt%) upon melt-rock reaction and the degree of alkalinity of the reacted melts is positively correlated with melt-rock ratio. On the other hand, reacted melts derived from BAC (on a CO₂-free basis SiO₂ ~42-53 wt %, TiO₂ ~6.4-8.7wt %, Al₂O₃ ~10.5-12.3wt %, FeO^* ~6.5-10.5wt %, MgO ~7.9-15.4wt %, CaO ~7.3-10.3wt %, Na2O ~3.4-4 wt%, CO₂ ~6.2-11.7 wt%) increase in alkalinity with decreasing melt-rock ratio.We demonstrate that owing to the presence of only 0.65 wt%of CO₂ in the bulk melt-rock mixture (corresponding to 25 wt%BAC +lherzolite mixture), nephelinitic-basanite melts can be generated by partial reactive crystallization of basaltic andesite as opposed to basanites produced in volatile-free conditions. Post 20% olivine fractionation, the reacted melts derived from ABC at low to intermediate melt-rock ratios match with 20-40%of the population of natural nephelinites and melilitites in terms of SiO₂ and CaO/Al₂O₃, 60-80% in terms of TiO₂, Al₂O₃ and FeO, and520% in terms of CaO and Na²O. The reacted melts from BAC, at intermediate melt-rock ratios, are excellent matches for some of the Mg-rich (MgO 415 wt %) natural nephelinites in terms of SiO₂, Al₂O₃, FeO^*, CaO, Na₂O and CaO/Al₂O₃. Not only can these reacted melts erupt by themselves, they can also act as metasomatizing agents in the Earth's mantle. Our study suggests that a combination of subducted, silica-saturated crust-peridotite interaction and the presence of CO₂ in the mantle source region are sufficient to produce a large range of primitive alkalic basalts. Also, mantle potential temperatures of 1330-1350°C appear sufficient to produce high-MgO, primitive basanite-nephelinite if carbonated eclogite melt and peridotite interaction is taken into account.