1,2,3,1,- 1The University of Manchester, Earth and Environmental Sciences, Manchester, M13 9PL, United Kingdom of Great Britain – England, Scotland, Wales
- 2Indian Institute of Technology Kharagpur Geology and Geophysics, Kharagpur, 721302, India
- 3The University of Edinburgh, NERC Ion Microprobe Facility, Edinburgh, EH9 3FE, United Kingdom of Great Britain – England, Scotland, Wales
H2O and F are among the most soluble volatile species in basaltic melts, exerting strong control over volcanic eruptive style and influencing climate at regional and global scales. In this study, we aim to quantify the pre-eruptive magmatic H2O and F content of the voluminous Ambenali Formation (Wai Subgroup; ~65.9-65.8 Ma [1]) lavas in the Deccan Traps large igneous province, for which magma volatile contents have not previously been reported. Owing to the scarcity of analysable glassy melt inclusions, the pre-eruptive H₂O and F contents of Ambenali Formation are estimated using clinopyroxene-melt equilibrium calculations. The trace H2O and F contents in clinopyroxene grains were measured using Secondary Ion Mass Spectrometry (SIMS) along core-rim transects. Equilibrium melt H2O and F contents were calculated using appropriate clinopyroxene-melt partitioning (KDcpx/melt) values, which are determined based on clinopyroxene crystallisation temperature and pressure conditions, major element compositions, and degree of melt polymerisation (NBO/T). The calculated KDcpx/melt ranges from 0.012 to 0.016 for H2O, and 0.128 to 0.135 for F. Using these values, the equilibrium melt H2O and F contents are 625 ± 416 ppm and 550 ± 260 ppm, respectively. The calculated melt F contents are broadly consistent with reported melt F contents (634 ± 411 ppm) from the Poladpur and Mahabaleswar Formations of the Wai Subgroup[2]. However, the calculated melt H2O contents are substantially lower than the reported high melt inclusion H2O contents (14571 ± 2621 ppm) from the Poladpur and Mahabaleswar Formations[3]. We propose that clinopyroxene H2O contents diffusively re-equilibrated with the surrounding degassed lava during stagnation and cooling. The absence of H2O concentration variation across core-rim transects suggests ~100% re-equilibration with the degassed lava. We applied a simple one-dimensional, isothermal diffusion model to estimate the minimum timescale for the near-complete diffusive re-equilibration of H2O in our clinopyroxene crystals. Assuming an initial homogeneous clinopyroxene H2O content of 204 ppm[3] and a crystal diameter of 1.3 mm, our diffusion model indicates complete H+ re-equilibration can be achieved on timescales of ~0.01 to 1 year. Therefore, analyses of rare melt inclusions remain critical if we are to accurately estimate pre-eruptive H2O contents of Deccan magmas.
References
[1] B. Schoene, et al., U-Pb Constraints on Pulsed Eruption of the Deccan Traps across the End-Cretaceous Mass Extinction. Science (2019), 363 (6429), 862–866.
[2] S. Callegaro, et al., Recurring Volcanic Winters during the Latest Cretaceous: Sulfur and Fluorine Budgets of Deccan Traps Lavas. Sci. Adv. (2023), 9 (40), eadg8284.
[3] B. Choudhury, et al., Melt Inclusion Evidence for Mantle Heterogeneity and Magma Degassing in the Deccan Large Igneous Province, India. Lithos (2019), 346–347, 105135.
How to cite: Sen, R., Hartley, M., De Hoog, C.-J., Polacci, M., and Gupta, S.: Quantifying H2O and F Contents of Younger Deccan Traps Eruptions using Clinopyroxene-Melt Equilibrium Calculations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-606, https://doi.org/10.5194/egusphere-egu26-606, 2026.
