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Carbonate cements from Mississippian skeletal limestones of southern New Mexico are dominated by echinoderm-syntaxial calcites that comprise 4 regionally extensive compositional zones. Previous petrography and cement stratigraphy proves that the oldest three zones (zones 1, 2, and 3) are pre-Pennsylvanian
in age, and thus precipitated at temperatures around 25°C (77°F); they are interpreted as meteoric phreatic cements. The youngest cement, zone 5, is interpreted as a pre-Permian burial cement precipitated at temperatures less than about 80°C (176°F) and at burial depths less than about 1 km (3,300 ft).
Originally defined on the basis of luminescence and staining, each zone has a distinctive assemblage of C13, O18, Mg, Mn, and Fe contents. The low MgO contents (less than 0.25 wt. %) in all zones indicates that sea water was insignificant in their precipitational waters over most of the region. Their FeO and MnO contents are compatible with their subsurface interpretations.
The isotopically most positive values from bioherm muds and synsedimentary former high-Mg calcite cements (+4.5^pmil^dgrC13, -1.5^pmil^dgrO18 PDB) are interpreted as marine values, and offer a baseline with which to compare isotope values of non-marine cements.
The pre-Pennsylvanian cements, zones 1, 2, 3, are markedly different from one another and show a progressive decrease in ^dgrC13 and ^dgrO18 with decrease age (^dgrC13 = +3.7, +2.4, -0.8^pmil PDB, respectively; ^dgrO18 = -1.3, -2.8, -3.7^pmil PDB, respectively). This decrease in ^dgrC13 is interpreted to reflect increased contribution from soil or atmospheric CO2 carbon. The decrease in ^dgrO18 is interpreted to reflect a decrease in O18 content of precipitational waters rather than an increase in temperature. The ^dgrC13 of all three zones is less than the interpreted marine values, which reinforces their fresh-water interpretation. The ^dgrO18 of zone 1 is greater and the ^d rO18 of zones 2 and 3 is less than the interpreted marine values.
We propose a model in which zones 1, 2, and 3 precipitated in fresh phreatic groundwaters largely uncontaminated by sea water. Their chemistries reflect progressive stages in the chemical evolution of the water-rock system. This evolution resulted from either a progressive change from a rock-dominated to a water-dominated system, or may have involved a progressive climatic change from arid (zone 1) to wetter and more seasonal (zone 3). The carbon for these cements derived from Mississippian skeletal and lime mud components plus contributions of light organic carbon. Crinoids, the main skeletal component, could have been major sources only for zone 3 if they had C13 contents comparable to modern crinoids. More likely, they had C13 contents comparable to Mississipp an marine values and were major sources of carbon for all three zones via pressure solution and dissolution at the pre-Pennsylvanian unconformity.
The post-Mississippian zone 5 has a wide range of ^dgrO18 values (mean = -7.4^pmil PDB), all less than those of the pre-Pennsylvanian zones. This light ^dgrO18 reflects elevated temperatures in the 40 to 60°C (104 to 140°F) range. Zone 5 has intermediate ^dgrC13 values which reflect complex, predominantly rock carbon sources, many of which were extraformational.
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