Alunite and natroalunite tell a story-the age and origin of Carlsbad Cavern, Lechuguilla Cave, and other sulfuric-acid type caves of the Guadalupe Mountains
Victor J. Polyak, W. C. McIntosh, Paula P. Provencio, and Necip Guven
Sulfate minerals alunite, natroalunite, hydrobasaluminite, and gypsum, produced from sulfuric-acid speleogenesis, are essential to the story of the origin of Carlsbad Cavern, Lechuguilla Cave, and many other major caves of the Guadalupe Mountains, southeastern New Mexico. Alunite is particularly important not only because it is a byproduct of sulfuric acid cave genesis, but because it contains telling isotopic signatures of sulfur, oxygen, hydrogen, potassium, and argon. Alunite has been reliably dated using the 40Ar/39Ar method yielding the timing of sulfuric-acid speleogenesis for Carlsbad Cavern, Lechuguilla Cave, and other sulfuric-acid caves as well as uplift and canyon incision rates for the Guadalupe Mountains. The repeatability of the 40Ar/39Ar results was demonstrated from analyses of alunite aliquots processed three different ways: 1) non-chemical alunite separation by simple gravity-method in water, 2) chemical separation using HF, 1 & 2 were not encapsulated for irradiation; and 3) encapsulation of HF-treated aliquots for irradiation. In addition to the chronology, negative sulfur isotope values for the alunite strongly point to a microbial-related process, and both alunite and natroalunite occur as micrometer-sized cubelike rhombs, an indicator of a low temperature origin of these large caverns. Larger a dimensions of the unit-cells of relatively well-formed alunite from the caves (determined by X-ray diffraction) are similar to those of synthetic alunites produced in the laboratory at <100ºC, and also support a low-temperature origin. Stable isotope values of the oxygen and hydrogen in the alunite have potential to generate more accurate values for the temperature and isotopic character of water in which the caves were formed. Timing of speleogenesis determined by 40Ar/39Ar dating of alunite shows this process was active as far back as ~12 Ma and slowly migrated eastward for at least 8-9 Ma up to 3-4 Ma ago. Rio Grande Rift faulting and tilting of the Guadalupe block during this period is probably the eastward-driving mechanism. If sulfuric-acid speleogenesis is taking place today, it is most likely happening east of the Guadalupe Mountains in the subsurface.
- Polyak, Victor J.; McIntosh, W. C.; Provencio, Paula P.; Guven, Necip, 2006, Alunite and natroalunite tell a story-the age and origin of Carlsbad Cavern, Lechuguilla Cave, and other sulfuric-acid type caves of the Guadalupe Mountains, in: Caves and karst of southeastern New Mexico, Land, Lewis; Lueth, Virgil W.; Raatz, William; Boston, Penny; Love, David L., New Mexico Geological Society, Guidebook, 57th Field Conference, pp. 203-209.