Geochemistry and crystal morphology of aragonite cements of mixing-zone origin, Barbados, West Indies

Isopachous aragonite ray cement (ARC) lines large rugs (> 5 cm) through a 4 m interval recovered in core samples from a borehole drilled along the southeast coast of Barbados. The vuggy/cement interval is 12-16 m below core top, and most of the interval is below the modern water table (13 m). The top of the interval corresponds to the top of the 194 ka coral reef terrace, which is unconformably overlain by the 122 ka terrace. ARC was precipitated after a period of meteoric diagenesis during which vuggy porosity was produced. ARC shows variable ultrastructures, including (1) loosely packed, randomly oriented rays (< mm long), (2) coalescing splays of elongote fibers (up to 5 mm long) radiating from the substrate, and most commonly, (3) closely packed isopachous rays (4 mm long) that are oriented perpendicular to large vug walls. Generally, ARC is pristine, but evidence of minor dissolution and incipient replacement by LMC microspar is present in some samples from below the modern water table. The 100% ARC has uncharacteristically low delta ¹³ C values relative to predicted and reported values for marine aragonite cement. Likewise, the delta ¹⁸ O values are lower than both reported and most predicted O ¹⁸ O values for marine aragonite. Stable-isotope compositions of this cement fall along a mixing line between aragonite equilibrium compositions calculated for meteoric and marine end-member fluids. Strontium concentrations are greater than in most known marine aragonite; this suggests precipitation from fluids with an elevated Sr/Ca ratio. Such fluids are found in modern Barbados mixed waters whose elevated Sr/Ca ratios result from updip meteoric dissolution of aragonite. Magnesium concentrations are slightly higher and more variable than in reported Pleistocene marine aragonite. Sodium concentrations are lower than expected marine values and covary with changes in delta ¹³ C and delta ¹⁸ O values. Isotope and minor-element geochemistry of these cements suggests precipitation from mixed meteoric-marine waters. Fluid mixing models for modern Barbados waters suggest precipitation from mixed fluids ranging from 50% to 75% seawater. Precipitation of aragonite from mixed waters may be facilitated by degassing of CO ₂ -charged mixed waters within large vugs and by Mg/Ca ratios sufficiently elevated to inhibit precipitation of calcite.