Geology Club Seminar
The carbonate clumped (T(∆47)) and oxygen (d18O) stable isotope record provide temperatures of mineral formation and isotopic compositions of ancient waters used in reconstructions of tectonics, and ecological and environmental change. The robust use of these and other isotopic systems preserved in carbonate minerals requires that carbonate extracted from orogenic basins retains its primary isotopic composition through often complex post-depositional thermal histories. Here, I reconstruct the alteration history of Paleocene marine carbonates from the Tethys Sea using water-rock isotope exchange reactions and solid-state reordering models informed by a ZHe and AHe time-temperature history. Plausible alteration scenarios are considered within the framework of stable isotope data at different spatial resolutions (d18Oc [IRMS & SIMS], ∆47) and carbonate petrography. The proposed alteration history is: (1) cementation and microspar infilling of primary porosity and shells during early marine diagenesis; (2) high-T, rock-buffered recrystallization and solid-state reordering at >3km depth; and (3) low-T water-rock exchange on the exhumation pathway, and likely at the surface, to lock in intrasample ∆47 variability. Primary micritic fabrics and biogenic morphologies are retained through all diagenetic stages. The Jialazi Fm is ideal for studying the effects of deep to near-surface alteration because the original d18O and T(∆47) values are 'ground-truthed' by Paleocene shallow marine carbonate compositions (~-2 to +5‰VPDB) and temperatures (~20 to 30°C), but have experienced substantial burial and uplift throughout the India-Asia collision, and now interact with some of the lightest meteoric water on Earth at ~5km elevation. The marine Jialazi Fm now yields d18O values that are identical to high-altitude terrestrial carbonates formed on the Tibetan Plateau today (~-14 to -23‰) and T(∆47) ranging from ~10 to 120°C. Importantly, given the extent of high-T ∆47 alteration during burial, the low T(∆47) values must record recrystallization at the Earth surface post-exhumation, and suggests that carbonate can alter in outcrop. It is possible that texturally preservative, low-T carbonate alteration has impacted prior carbonate-based reconstructions of terrestrial environments.