Geochemical Patterns of Meteoric Diagenetic Systems and Their Application to Studies of Paleokarst


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The isotopic and cation chemistry of meteoric waters changes in response to the effects of rock—water interaction, uptake of organically derived CO2, and primary mineralogic differences among carbonate terranes. Moreover, variations in the dominance of these factors produce diverse chemical conditions within the meteoric systems which allow the sub- environments of vadose-phreatic, mixed-water, and spelean diagenesis to be distinguished. Therefore, geochemical patterns within the meteoric water system are examined to provide criteria for recognition of these subenvironments of meteoric diagenesis in ancient carbonate sequences. The δ18O composition of meteoric groundwater is largely constant at individual geographic sites. Variation in the amount of dissolved soil-gas CO2 and in the extent of rock—water interaction produces a distinct geochemical trend for diagenetic alteration and precipitation products. This pattern of invariant δ18O coupled with variable δ13C, termed here the meteoric calcite line, serves as the baseline relative to which chemical variations characteristic of vadose- phreatic, mixed-water, and spelean environments can be discriminated. Interaction of meteoric water with country rock produces spatial and temporal changes in the chemistry of the water, and likewise in diagenetic products. Areas proximal to exposure surfaces exhibit low rock—water exchange, and diagenetic phases possess compositions in equilibrium with surface-derived water. In contrast, at stratigraphic positions distal from a recharge surface, diagenetic phases become progressively enriched in Mg2+ and/or Sr2+ and isotopic values converge toward country rock values due to increasing rock-water interaction along fluid flow lines. In response to the diminishing availability of dissolving metastable carbonate phases with time as the rock system matures, sequentially precipitated phases will record a history of decreasing rock-water exchange. The δ18O and δ13C composition of water within the meteoric-marine mixing zone defines hyperbolic trends reflecting the relative proportions of intermixed marine and meteoric waters. Because calcite precipitation does not occur throughout the full range of mixing, and because the δ13C value of me-teoric waters vary through time, it is unlikely that discrete hyperbolic mixing trends can be deciphered in diagenetic products. Rather, the composition of replacive and precipitated phases formed in the mixed-water zone will define trends offset in δ18O which parallel the meteoric calcite line. Precipitation in spelean settings is induced by evaporation and/or CO2 degassing, both of which can modify the isotopic and cation chemistry of vadose seepage waters. While the isotopic compositions of spelean carbonates overlap the range of coevally precipitated meteoric vadose and phreatic calcite values, combined effects of degassing and evaporation produce covariant trends which deviate from the meteoric calcite line. Such patterns of variation serve to distinguish carbonate precipitated in spelean settings from the more typical vadose-phreatic en-vironments. However, similarities in mineralogy, fabric, and chemistry of spelean carbonates with primary cements precipitated in marine settings make it difficult to determine unambiguously a spelean origin for fibrous carbonates solely on the basis of geochemical or fabric critera.

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