The formation of saline mantle fluids by open-system crystallization of hydrous silicate-rich vein assemblages – Evidence from fluid inclusions and their host phases in MARID xenoliths from the central Kaapvaal Craton, South Africa

Abstract

The composition of texturally primary fluid inclusions and their host phases clinopyroxene, K-richterite, and zircon were investigated in two MARID-type (mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths sampled by the Kimberley cluster of Cretaceous kimberlites erupted in the central Kaapvaal Craton, South Africa. P–T conditions of crystallization for the MARID assemblages of 4.2GPa and 960°C were estimated based on Ca-in-opx thermometry and the assumption of a 40mW/m2 geotherm applied to two orthopyroxene-bearing MARIDs collected from the same locality. Cooling/heating stage measurements and Raman spectroscopy indicate a fluid system dominated by H2O–NaCl–MgCl2 with variable total salinities in the range ⩽6.4–32.4mass% and minor amounts of MgCl2, the latter inferred from the crystallization of MgCl2×12H2O during cooling of the inclusions. In addition to liquid and vapour, enstatite, baddeleyite, barite, calcite and a K–Ba–Fe–Cr-titanate were identified as solid phases in opened fluid inclusions, indicating high LIL-(HFS) element concentrations in the saline hydrous fluids prior to solid phase precipitation. The Cl contents of apatite (⩽0.35wt.%), phlogopite (⩽0.09wt.%) and K-richterite (⩽0.025wt.%) follow the enrichment pattern Clap≫Clphl>ClKr which is typical for upper mantle rocks. Fluid inclusion-bearing clinopyroxenes show very low H2O contents of ∼45μg/g which is consistent with a reduced aH2O of the fluids due to the presence of NaCl–MgCl2–SiO2–LILE combined with high fO2 and very low Al3+ contents of the clinopyoxenes. The zircons show a complex compositional zoning with variable and positively correlated Y (⩽1260μg/g), P (⩽1870μg/g) and Sc (⩽1373μg/g) contents, indicating a pretulite-type substitution Si4++Zr4+=P5++(Sc, Y, REE)3+. The Sc contents of the zircons are amongst the highest Sc concentrations reported so far for upper mantle silicates. Oxygen isotope analyses of selected zircons yields δ18O values as high as 6.7±0.2 (2σ)‰ and an overall average δ18O of 6.1±0.2‰ which is slightly higher than that of zircon megacryst δ18O of 5.3±0.3‰ reported from worldwide occurrences. The O-isotopic composition of the zircons is consistent with an open-system fractional crystallization of the MARID assemblage in an olivine-absent environment from very small batches of an alkali-rich hydrous saline high-density fluid or fluid-saturated melt. The hydrous saline fluids with LILE-(HFSE)-rich daughter crystals preserved in the two MARID xenoliths indicates that open-system fractional crystallization of MARID-type assemblages can generate small volumes of highly saline hydrous and incompatible trace element-enriched high-density fluids from larger volumes of alkali-rich hydrous melts of group-II kimberlite affinity. MARID crystallization is thought to be just one very specific example of a more general case of open-system crystallization in mantle veins of hydrous silicate-rich assemblages that can concentrate Cl in residual hydrous fluids to brine strength. Inclusions of K–Cl–P–LILE-rich fluids trapped in diamonds from many locations worldwide provide evidence for a widespread operation of these processes in the subcontinental lithospheric mantle.