STABLE ISOTOPE VALUES OF DEEPLY FORMED CAVE SPAR AND OTHER RELATED PHREATIC SPELEOTHEMS FROM THE GUADALUPE MOUNTAINS, NEW MEXICO AND TEXAS – CONTRIBUTIONS TOWARDS LANDSCAPE EVOLUTION STUDIES

Using cave spar (large euhedral phreatic calcite crystals) and other phreatic speleothems for landscape evolution studies requires an understanding of depth of formation and the precipitating environment of these speleothems. While we have determined the depth of formation to be 100s of meters below the water table and land surface, the stable isotope geochemistry of spar calcite contributes important constraints on the tectonic evolution and uplift of the region of interest. In this study we compare the δ¹⁸O and δ¹³C of spar collected in the Guadalupe mountains with regional data collected by Hill (1996) in order to place the samples within known spar categories. δ¹⁸O and δ¹³C of cave spar from the Guadalupe Mountains (two groups: δ¹⁸O of -12 to 16‰; δ¹³C of -4 to +3‰ and δ¹⁸O of -7 to 10‰; δ¹³C of -7 to +0‰), a mammillary (δ¹⁸O of -7 to -9‰; δ¹³C of -7‰), and a fibrous fault spar (δ¹⁸O of -8‰; δ¹³C of -22 to -26‰) show distinct differences, where the two cave spar groups are consistent with formation at temperatures between 30 and 90° C (Hill, 1996; Spotl et al., 2009). These spar samples fall within two broadly defined "thermal spar" fields, one at 50-90° C, and the other at <50° C. At least two spar crystals and the fibrous fault spar are petroliferous and are consistent with values reported by Hill (1996) for oil spar. Stable isotope values for the mammillary calcite are characteristic of meteoric groundwater, and likely formed at a temperature <30° C. The stable isotope values of cave spar reinforce data showing that these same spar crystals formed from hydrothermal waters which were most likely generated by two well-known volcanic/magmatic events near the region of study at 40 to 34 Ma and 30 to 28 Ma. The stable isotope values of the cave spar calcite support deep phreatic origin, and thereby add to evidence that indicates a long-term stable water table and little uplift during the early to middle Cenozoic.

Hill, C. A., 1996, Permian Basin Seciton - SEPM, 480 p.

Spotl, C., Dublyansky, Y., Meyer, M., and Mangini, A., 2009, Int. Journal of Earth Sci., v. 98, p. 665-676.