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

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.