Carbonatite melt-CO2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands:: a story of trapping, immiscibility and fluid-rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO2 (Type A); carbonate-rich CO2-SO2 mixtures (Type B); and polyphase inclusions dominated by silicate glass +/- fluid +/- sp +/- silicate +/- sulfide +/- carbonate (Type C). Type A inclusions commonly exhibit a coating (a few microns thick) consisting of an aggregate of a platy, hydrous Mg-Fe-Si phase, most likely tale, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the coating, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid-wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO2-H2O-NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO2-SO2 fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 degreesC, the mixed CO2-H2O-NaCl fluid inside the Type A inclusions were separated into CO2-rich fluid and H2O-NaCl brine. At T<650 degreesC, the residual silicate melt reacted with the host olivine, forming a reaction rim or coating along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO2 fluid. The homogenization temperatures of +2 to +25 degreesC obtained from the Type A CO2 inclusions reflect the densities of the residual CO2 after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000-1200 degreesC, and the spinel therzolite phase assemblage of the xenolith, which is 0.7-1.7 GPa. (C) 2002 Elsevier Science B.V. All rights reserved.