Mineralogenesis and geochemical signature of barite-celestine-series speleothems

• Applicant: Dr. Max Wisshak
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 468126028

Speleothems – mineral aggregates precipitating in caves – have come into focus in paleoclimatology as excellent geochemical archives for environmental change. Calcite stalagmites, for example, show growth increments that can be dated and analyzed for stable isotopes, yielding time series that portray mean annual temperatures in the cave and its surroundings. Apart from such classical carbonate dripstones, speleothems of many kinds are formed by a wide variety of cave minerals. Only few of these have so far been explored as geochemical archive. Two sulfate minerals are the subject of this project, barite (BaSO4) and celestine (SrSO4). They form a solid solution series and build up speleothems of various kinds, the largest variety of which has been recognized in Lechuguilla Cave, New Mexico, USA. These include, among others, greenish barite stalactites and bluish barite pool crystals that today grow in the vadose zone of this cave. They form despite the extremely low solubility particularly of barite, because of which there is a common perception that barite speleothems are a result of hydrothermal processes – a view that merits to be questioned. A preliminary investigation of the Lechuguilla barite speleothems has drawn a complex and yet unresolved picture of a temperate mineralogenesis.It is the aim of the proposed project to further elucidate the timing and the temperate formation of barite and celestine speleothems in Lechuguilla Cave, based on their mineralogical properties, their geochemical fingerprint, the chemistry of the host fluids, and the environmental conditions at the sites of active precipitation. For this purpose, two expeditions into the cave will document and sample the ‘fossil’ as well as active occurrences of barite-celestine-series speleothems and place data loggers to record a seasonal cycle of relevant environmental parameters at the sites of ongoing mineralogenesis. Analytical objectives performed on the recovered mineral samples include scanning electron microscopy and X-ray diffraction analyses for characterizing the mineralogical properties, radiometric dating for establishing a temporal framework, and wavelength-dispersive X-ray fluorescence as well as stable isotope analyses for assessing the character and diversity in their geochemical fingerprint. In addition, the ion load and stable isotope signature of the host fluids will be determined as a basis to model and interpret mineral precipitation.Together, these objectives support the overarching project goal of deciphering the diversity, timing, provenance and mineralogenesis of barite and celestine speleothems in the non-hydrothermal terrestrial setting of Lechuguilla Cave. Expected results will be of relevance beyond the chosen case study site and will provide a basis to further explore the value of these sulfate-speleothems as a new geochemical archive for environmental change.

• DFG Programme: Research Grants