Degassing Mechanisms of Kimberlite Magma at Its Initial Ascent: Experimental Data at 5.5 and 3.0 GPa

— The mechanisms of kimberlite magma effervescence were experimentally studied for the first time with the use of kimberlite-like melts, which are formed by the dissolution of garnet lherzolite in model CO 2 - and H 2 O-rich carbonate melts at 3.0 and 5.5 GPa, in the temperature range of 1200–1350°C. In the studied systems at 3.0 GPa and 1200°C, the melting degrees vary from 30 to 68 wt %. In the systems with the addition of 9 wt % CO 2 and 2.5 wt % H 2 O in the form of oxalic acid, the Ol–Opx–Cpx–Phl (?) –Mgs−L assemblages are stable (in the Lc(lerzolite)−B10 system (asthenospheric carbonate melt)–CO 2 –H 2 O) or Ol–Opx–Phl (?) –Mgs−L assemblages (in the Lc−GS (subduction carbonate melt)–CO 2 –H 2 O system). Phlogopite in the assemblages may be a quenching phase. In the systems with the addition of 21 wt % CO 2 and 6 wt % H 2 O, the assemblages Opx−Cpx−Mgs−L or Opx−Mgs−L are stable in the samples. At 3.0 GPa and 1350°C, garnet lherzolite almost completely dissolves, and the only stable phases are Opx–L or melt ( L ) alone. The estimated solubility of molecular CO 2 in carbonate–silicate melts at 3.0 GPa decreases from 14–23 to <10 wt % as the temperature decreases from 1350 to 1200°C. Experiments conducted following a specially developed two-stage scenario to reproduce the rapid magma ascent from levels of 5.5 to 3.0 GPa with a temperature drop from 1350 to 1270°C made it possible to achieve effervescence or “boiling” of the samples. The main process controlling the release of CO 2 fluid is the reaction of magnesite with the melt without formation of olivine: Mgs + L 1 = L 2 + Fl , where L 2 is enriched in MgO and depleted in SiO 2 compared to L 1 . The orthopyroxene remains stable. Thus, the capture and/or formation of magnesite in xenogenic material transported by magma can create prerequisites for its effervescence at magma ascent from 5.5 to 3.0 GPa and a temperature decrease from 1350 to 1270°C.