The fate of a travertine record: Impact of early diagenesis on the Y‐10 core (Mammoth Hot Springs, Yellowstone National Park, USA)

Abstract Spring systems are efficient and fast precipitating non‐marine carbonate factories, but they are also prone to (early) diagenesis. Evidence of diagenesis in travertines remains controversial as the resulting textures have been considered primary in other deposits and the impact on geochemical/palaeo‐environmental signals is often poorly addressed. This study revisits the Y‐10 core, taken in 1967 at one of the upper terraces of Mammoth Hot Springs in Yellowstone National Park (USA). The travertine depositional facies in the upper 50 m of the core show a general distal to proximal facies trend upwards. To unravel the nature, impact and timing of early diagenesis, fabrics and geochemical signatures are also compared to modern hot spring carbonates. Secondary ion mass spectrometry and detailed microscopy proved powerful in determining the stable isotope signature of different cement generations, thereby detangling the fluid history. Diagenesis starts during spring activity, but these warm waters not only precipitate CaCO3 at the top, they also seem to have circulated through the porous spring fabrics below. It resulted in coarsening and homogenization of the primary textures through neomorphism of aragonite and calcite cementation, often resulting in new intracrystalline microporosity within the calcite crystals. Subsequent circulation of meteoric fluids led to dissolution and cementation, particularly near the top of the core. Careful interpretation of U/Th dates suggests that travertine deposition started at the beginning of the Holocene/end Pleistocene. Calculated depositional (precipitation) rates vary between 2 and 20 mm/year, assuming a constant rate between samples at different depths. Aragonite deposits are completely transformed to calcite below depths of 5–10 m, corresponding to a time span of 4,000 years. Core and petrography observations, together with dating, suggest that travertine strata below marine sandstones in the lower part of the core may relate to fault displacement after the onset of travertine deposition.