Meteoric calcite line

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Figure 11.8 Photomicrographs of meteoric cements. (A) Low-magnesium calcite cement with meniscus fabrics in cayrock. Constituent grains are Halimeda fragments. Harry Jones Point, Turneffe Islands, Belize. Diameter of picture is 1 mm. (B) Blocky and bladed low-magnesium calcite line a dissolution cavity in cayrock. Cay Bokel, Turneffe Islands, Belize. Diameter of picture is 1.2 mm. (C) Close-up of same sample. Diameter of picture is 750 ptm.

Figure 5. Plot of and 5 C values for the Tierra Blanca Limestone and hydrothermal calcites. Symbols represent distance from the skam/marble contact. Curves A-B and A-C show depletion resulting from Batch and Rayleigh decarbonation, respectively. Curves A-D, A-E represent progressive depletion in limestone resulting from isotope exchange with meteoric water in eqnilibrinm with the Hanover-Fierro pluton at 315°C and water-rock ratios of 0.01-0.001. Cnrve A-E (dotted line) is calculated at 400°C using the fractionation factor of Chiba et al. (1989), assuming X(C02) = 0.02. Curve A-F (dashed line) defines progressive depletion resulting from water/rock interaction at 400°C using the fractionation factor of O Neil et al. (1969) (from Turner and Bowman 1993).

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