Авторы: Kopylova M., Matveev S., Raudsepp M.

2007 г.

Geochim.Cosmochim.Acta

    Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09–1.1 mm2 areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new ‘‘chemical point-counting’’ technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26– 29.5 wt% SiO2, 7 wt% of FeOT, 25.7–28.7 wt% MgO, 11.3–15 wt% CaO, 8.3–11.3 wt% CO2, and 7.6–9.4 wt% H2O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (g = 101–103 Pa s at 1100–1000 C) and depolymerized (NBO/T = 2.3–3.2), but entrains with 40–50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190–350 ppm of water suggesting crystallization from a low SiO2 magma (aSiO2 below the olivine-orthopyroxene equilibrium) at 30–50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO2-rich residual liquids. The melt crystallised phlogopite (6–10%), monticellite (replaced by serpentine, 10%), calcite rich in Sr, Mg and Fe (19–27%), serpentine (29–31%) and minor amounts of apatite, ulvo¨spinel-magnetite, picroilmenite and perovskite. The observed content of H2O can be fully dissolved in the primitive melt at pressures greater than 0.8–1.2 kbar, whereas the amount of primary CO2 in the kimberlite exceeds CO2 soluble in the primitive kimberlite melt. A mechanism for retaining CO2 in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.

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