Isotope Geochemistry of Regionally Extensive Calcite Cement Zones and Marine Components in Mississippian Limestones, New Mexico

Carbonate cements from Mississippian skeletal limestones of southern New Mexico are dominated by echinoderm-syntaxid calcites that comprise four regionally extensive cotnpositiorlal zones. Previous petrography and cernent stratigraphy proves that the oldest [hree zones (zones 1, 2, 3) are pre-Pennsylvanian in age and were precipitated at temperatures around 25°C; they are interpreted as meteoric phreatic cements. The youngest cement, 7one 5, is interpreted to bc a pre-Pel-mian burial cement precipitated at burial depths less than I km and temperatures less than about 60°C. Originally defined on the basis of luminescent and staining characteristics, each one has a distinctive assemblage of 6l3C, 6180, Mg2+, Mn2' and Fe2' contents. The low MgO contents (less tlian 0.25 wt.%) in all zones indicates that seawater was an insignificant component of their prccipitational watera over most of the region. l'heir FeO and MnO contents are compatible with their subsurface interpretation. Mississippiall marine carbonate 6I3C(+4.0%c P D B ) and S180( 1.5% P D B ) are estimated fro111 analysis of micrite, synsedimentary marine cement, and crinoid skeletal carbonate. In addition to providing a base line with which to compare norlmarine cements, this estimate implies that the composition of Mississippian epeiric seawater was enriched in 13C and comparable in lSO to modem seawater. The pre-Pennsylvanian cements (zones 1, 2 and 3) are markedly distinct from one another and sIio\v a progressive decrease in 6'3C and 8'80 through time (81801.3, -2.8, 3 . 7 % PDB respectively; 6I3C= f 3.7, +2 .4 , -0.8% PDB respectively). We propose a model in which zones 1, 2 and 3 precipitated in meteoric phreatic groundwaters largely uncontaminated by seawater. Thc distinctive trend of pre-Pennsylvanian cements reflects progressive stages in the chemical evolution of the waterrock system. This involved a decrease in the degree of rock-water interaction. which in turn resulted from dcplction of metastable mineralogies through recrystallization or through isolation by cementation. Thus, the progressive decrease in 13C reflects equilibration of porewaters with Mississippian skeletal carbonates during carly cementation (zones 1 and 2) and increased contribution from soil-gas derived carbon during late cementation (zone 3). Similarly, the progressive decrease in 1 8 0 in zones 1 through 3 reflects decreasing contribution of IXO from Missisbippian niarine carbonates and progressive equilibration with meteoric porewaters. Hence, the isotopic cornposition of zone 3 cements reflects equilibrium precipitation with meteoric waters. Importantly, dccrcasing rock-water interaction during pre-Pennsylvanian cementation is supported by decreasing Mg2' contents among zones. High niagnesian calcite crinoids are the doniinant skeletal component in the Mirsisrippiali limcstoncs and likely were the major contributors of carbonate for cementation during diagenesis of all three zones. A progressive decrease in Mg2' content is compatible with a model of diminishing rock-water interaction. Zones 1 and 2 have the Ileaviest isotopic compositions and highest Mg2+ contents. The very low MgLcontent of zone 3 is predicted from very light 6132 co~npositions which predicates only minor contribution from crinoidal calcitc sources. Post-Mississippian zone 5 cement has a wide range of 6IXO values (mean= 7 . 4 % ~ P D B ) , all more depleted tlian any prePennsylvanian cement. This light 8180 probably reflecta elevated temperatures in the 40-60°C range. Zone 5 has intermediate 6°C values which reflect cornplex predominantly "rock" carbon sources, many of which were prohably extraformational as suggested by stratigraphic variations in Mnp and Fe2'