Mantle metasomatism and carbonatites; An experimental study of a complex relationship

The solidus of peridotite in the presence of water and carbon dioxide displays two cusps. Consequently, water- and carbon dioxide-bearing magmas rising from depths in excess of 70 km will cross the peridotite solidus if they are at temperatures of less than 1,100°C. The melt may react with the peridotitic wallrock to produce a variety of mineral assemblages that will remain as veins in the mantle after the altered magma has passed on. The main effect of high-temperature interactions is shown here to be replacement of orthopyroxene in the wallrocks with clinopyroxenes. Greater degrees of reaction (at lower temperatures) produce more exotic mineral assemblages. At pressures above 17 kbar, the magma may react with the peridotite to completion, producing a carbonated and amphibolitized mantle. At lower pressures, at which carbonate is not stable, reaction of peridotite and magma may produce an exotic mineral assemblage (including feldspathoids and sanidine with or without amphibole and, possibly, apatite, sulfides, and oxides), while enriching the melt in alkalis and CO 2. The composition of the melt thus changes from ijolitic to carbonatitic, and may ultimately produce a CO 2fluid. Long-term storage of the alkaline magma in the mantle at 17 < P<22 kbar will result in the production of low Nd isotopic ratios, moderate Sr ratios, and high Pb ratios. Melting of this altered mantle at a later time will produce carbonated hyperalkaline magmas that also possess these isotopic characteristics. Several occurrences of such rocks with the suggested isotopic compositions are noted.