Titanite-bearing calc-silicate rocks constrain timing, duration and magnitude of metamorphic CO2 degassing in the Himalayan belt

The pressure, temperature, and timing (PTt) conditions at which CO2 was produced during the Himalayan prograde metamorphism have been constrained, focusing on the most abundant calcsilicate rock type in the Himalaya. A detailed petrological modeling of a clinopyroxene, scapolite, Kfeldspar, plagioclase, quartz, calcite calcsilicate rock allowed the identification and full characterization, for the first time, of different metamorphic reactions leading to the simultaneous growth of titanite and CO2 production. The results of thermometric determinations (Zr in Ttn thermometry) and UPb geochronological analyses suggest that, in the studied lithology, most titanite grains grew during two nearly consecutive episodes of titanite formation: a nearpeak event at 730 740 {\deg}C, 10 kbar, 30 26 Ma, and a peak event at 740 765 {\deg}C, 10.5 kbar, 25 20 Ma. Both episodes of titanite growth are correlated with specific CO2producing reactions and constrain the timing, duration and PT conditions of the main CO2producing events, as well as the amounts of CO2 produced (1.4 1.8 wt percent of CO2). A firstorder extrapolation of such CO2 amounts to the orogen scale provides metamorphic CO2 fluxes ranging between 1.4 and 19.4 Mt yr; these values are of the same order of magnitude as the presentday CO2 fluxes degassed from spring waters located along the Main Central Thrust. We suggest that these metamorphic CO2 fluxes should be considered in any future attempts of estimating the global budget of non volcanic carbon fluxes from the lithosphere.

💡 Research Summary

The authors investigate the timing, duration, and magnitude of metamorphic CO₂ release associated with the growth of titanite in the Himalayan orogen, focusing on the most abundant calc‑silicate rock type (clinopyroxene + scapolite + K‑feldspar + plagioclase ± calcite). Using a combination of whole‑rock micro‑XRF mapping, SEM‑EDS mineral chemistry, and in‑situ LA‑ICP‑MS analyses of titanite, they apply the Zr‑in‑titanite thermometer (Hayden et al., 2008) together with U‑Pb dating to obtain simultaneous temperature‑time (T‑t) constraints for individual titanite grains.

Two distinct titanite growth episodes are identified. The first, termed a “near‑peak” event, records temperatures of 730–740 °C at ~10 kbar and ages of 30–26 Ma. The second, a “peak” event, records 740–765 °C at ~10.5 kbar and ages of 25–20 Ma. Both episodes coincide with specific decarbonation reactions that produce CO₂ while simultaneously crystallising titanite, such as clinopyroxene + scapolite + K‑feldspar → titanite + CO₂.

Quantitative phase‑modal analysis and thermodynamic modelling indicate that these reactions liberated 1.4–1.8 wt % CO₂ from the host rock. Scaling this proportion to the estimated volume of calc‑silicate rocks throughout the Greater Himalayan Sequence yields an annual metamorphic CO₂ flux of 1.4–19.4 Mt yr⁻¹. This range is comparable to present‑day CO₂ emissions measured from spring waters along the Main Central Thrust, suggesting that metamorphic degassing is a significant, ongoing carbon source in the Himalaya.

The study emphasizes that titanite growth occurs at isobaric invariant points, implying that CO₂ release was concentrated in relatively brief intervals (likely a few hundred thousand years) rather than being spread uniformly over the entire prograde metamorphic history. This temporal constraint refines previous estimates of the duration of metamorphic degassing in collisional orogens.

Methodologically, the work showcases the power of combined trace‑element thermometry and U‑Pb geochronology in a single mineral phase to directly link temperature and age, overcoming the limitations of separate temperature proxies and age determinations. The authors also demonstrate that internal Zr diffusion in titanite is sufficiently slow to preserve high‑temperature signatures up to ~775 °C, making titanite a reliable recorder of peak metamorphic conditions.

In conclusion, the paper provides the first quantitative, petrologically constrained assessment of when, under what pressure‑temperature conditions, and how much CO₂ was released during Himalayan metamorphism. The inferred fluxes are large enough to warrant inclusion of metamorphic CO₂ in global carbon budget models, especially when evaluating past greenhouse warming events linked to orogenic activity. The approach sets a benchmark for similar investigations in other “large‑hot” collisional belts, where metamorphic degassing may represent an underappreciated component of the long‑term carbon cycle.