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

We performed experiments between two different carbonated eclogite-derived melts and lherzolite at 1375 degrees 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 center dot 7 wt % dissolved CO2 (ABC), (2) basaltic andesite with 2 center dot 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 +/- olivine; 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 similar to 27-39 wt %, TiO2 similar to 2 center dot 8-6 center dot 3 wt %, Al2O3 similar to 4 center dot 1-9 center dot 1 wt %, FeO* similar to 11-16 wt %, MgO similar to 17-21 wt %, CaO similar to 13-21 wt %, Na2O similar to 4-7 wt %, CO2 similar to 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 similar to 42-53 wt %, TiO2 similar to 6 center dot 4-8 center dot 7 wt %, Al2O3 similar to 10 center dot 5-12 center dot 3 wt %, FeO* similar to 6 center dot 5-10 center dot 5 wt %, MgO similar to 7 center dot 9-15 center dot 4 wt %, CaO similar to 7 center dot 3-10 center dot 3 wt %, Na2O similar to 3 center dot 4-4 wt %, CO2 similar to 6 center dot 2-11 center dot 7 wt %) increase in alkalinity with decreasing melt-rock ratio. We demonstrate that owing to the presence of only 0 center dot 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, and < 20% 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 > 15 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 degrees C appear sufficient to produce high-MgO, primitive basanite-nephelinite if carbonated eclogite melt and peridotite interaction is taken into account.