Home – Home – Jornals – Russian Geology and Geophysics 2015 number 1-2

Melting and multiple saturation experiments with systems simulating primary kimberlite magma compositions at 5.5-6.5 GPa provide constraints on magma generation conditions. The liquidus of model kimberlitic systems exceeds the hottest temperatures of lithospheric mantle (1400 ºC) but is 150-200 ºC lower in systems with lower CO ₂/(CO2 + H2O) ratios. The high melting points require additional heat sources for the generation of kimberlite magmas. Multiple saturation of experimental melts and the stability of individual near-liquidus phases depend on both major-element contents and X CO2 (as the CO2/(CO2 + H2O) molar ratio) in the starting composition. Generally, olivine-bearing assemblages are stable at X CO2 < 0.5, while an increase in MgO/CaO from 1.8 to > 4.0 leads to progressive changes in the equilibrium assemblages: Ol + Grt + Cpx → Ol + Grt + + Opx + Cpx → Ol + Grt + Opx. The results of geochemical reconstructions and multiple saturation experiments indicate partial or complete wehrlitization of the kimberlitic source regions. Most of primary magmas with X CO2 < 0.5 may have been derived from carbonated garnet lherzolite. Some highly calcic (MgO/CaO < 2) magmas with X CO2 < 0.5 likely originated from carbonated garnet wehrlite. A probable scenario is that melts and/or fluids repeatedly metasomatized and oxidized the protolith (caused its carbonation and phlogopitization) and thus provided conditions for buffering CO2 and H2O fugacities in the forming kimberlitic magma, at least early in the melting history. During later magma generation, water was, likely, extracted from nominally anhydrous minerals having hydrated (OH) defects in the structure.