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 ± 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 ∼27–39 wt %, TiO2 ∼2·8–6·3 wt %, Al2O3 ∼4·1–9·1 wt %, FeO* ∼11–16 wt %, MgO ∼17–21 wt %, CaO ∼13–21 wt %, 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 ∼6·4–8·7 wt %, Al2O3 ∼10·5–12·3 wt %, FeO* ∼6·5–10·5 wt %, MgO ∼7·9–15·4 wt %, CaO ∼7·3–10·3 wt %, 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, 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°C appear sufficient to produce high-MgO, primitive basanite–nephelinite if carbonated eclogite melt and peridotite interaction is taken into account

始原的CRコンドライトに含まれる捕獲成分希ガスの起源への制約、及び太陽風希ガスに富む隕石の母天体位置における過去の太陽風フラックス

Tohoku University中嶋大輔課