Phase relations and melting of carbonated peridotite between 10 and 20 GPa: a proxy for alkali- and CO2-rich silicate melts in the deep mantle

Abstract

We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite + 5.0 wt % CO2 and PERC: fertile peridotite + 2.5 wt % CO2) at 10–20 GPa and 1,500–2,100 °C and constrain isopleths of the CO2 contents in the silicate melts in the deep mantle. At 10–20 GPa, near-solidus (ACP: 1,400–1,630 °C) carbonatitic melts with < 10 wt % SiO2 and > 40 wt % CO2 gradually change to carbonated silicate melts with > 25 wt % SiO2 and < 25 wt % CO2 between 1,480 and 1,670 °C in the presence of residual majorite garnet, olivine/wadsleyite, and clinoenstatite/clinopyroxene. With increasing degrees of melting, the melt composition changes to an alkali- and CO2-rich silicate melt (Mg# = 83.7–91.6; ~ 26–36 wt % MgO; ~ 24–43 wt % SiO2; ~ 4–13 wt % CaO; ~ 0.6–3.1 wt % Na2O; and ~ 0.5–3.2 wt % K2O; ~ 6.4–38.4 wt % CO2). The temperature of the first appearance of CO2-rich silicate melt at 10–20 GPa is ~ 440–470 °C lower than the solidus of volatile-free peridotite. Garnet + wadsleyite + clinoenstatite + carbonatitic melt controls initial carbonated silicate melting at a pressure < 15 GPa, whereas garnet + wadsleyite/ringwoodite + carbonatitic melt dominates at pressure > 15 GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10–20 GPa. The liquidus is likely to be at ~ 2,050 °C or higher at pressures of the present study, which gives a melting interval of more than 670 °C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20 GPa at near mantle adiabat or even at plume temperature. These alkali- and CO2-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the zone of metal saturation, CO2 and H2O flux melting can take place and kimberlite parental magmas can be generated. These mechanisms might be important for mantle dynamics and are potentially effective metasomatic processes in the deep mantle.