Assessing environmental controls for candidate microbial ichnofossils in hydrothermally altered basaltic tuffs of Upsal Hogback, NV

Abstract

Putative biogenic alteration has previously been identified in basaltic glass of the ocean floor, in subglacial hyaloclasts, in meteorite impact glass, and more recently in tuff erupted in a lacustrine environment. Documentation of putative biogenic alteration and characterization of the environments in which they have formed improves understanding of the thermophilic chemotrophic euendoliths to which the ichnofossils are attributed, which may enhance humankind’s understanding of the origins of life on Earth and potential evidence of life elsewhere. Upsal Hogback is a series of monogenetic tuff rings in the topographic low of the Carson Sink in the Lahontan Basin in Western Nevada. These tuff rings erupted into pluvial Lake Lahontan between 19 ka and 33.7 ka. Tufas emplaced on the tuff rings are especially abundant at two elevation intervals, which can be correlated to stillstands of Lake Lahontan, providing the minimum age constraint for the emplacement of Upsal Hogback. Chemical analysis of basalts from each ring indicates a common magmatic source that was likely affected by fractional crystallization of olivine and plagioclase as the rings erupted from north to south. In basaltic tuff of Upsal Hogback, petrographic analysis has revealed the presence of putatively biogenic alteration textures in association with palagonite, calcite, and zeolites. Tuff samples with the most abundant putative biogenic alteration are those from the interior of the tufas. Biogenic origin of the textures is indicated by their size, the log-normal distribution of their sizes, and the complex morphology which cannot readily be explained by known abiogenic processes. Measurement of palagonitization and consideration of zeolite assemblage indicates a hydrothermal fluid of sustained temperature between 40°C and 60°C. Upsal Hogback’s autochthonous carbonates, basaltic tuff, palagonitization, and the emplacement of the tuff rings in an alkaline lake may make it a good analogue for the Mars 2020 landing site in Jezero crater, Mars. The geologic characteristics of Upsal Hogback, including the hydrothermal regime, together with the early stage of putative biogenic alteration identified, provide an instructive example for future work investigating terrestrial and extraterrestrial environments for evidence of life.