Metric, kilometric and large-scale coherence of metamorphic conditions from graphitic phyllite in the Upper Lesser Himalaya of Nepal: Contribution to the estimation of carbon stored during Himalayan orogeny

Besides carbonates, carbonaceous material in meta-sediments represents an important carbon reservoir on Earth. In orogenic belts, carbon can be stored as graphite, a contribution to the global carbon cycle that remains insufficiently constrained. Here, we focus on the Himalayan belt where moderately crystalline graphite is found at the footwall of the Main Central Thrust. Using various analytical methods, we investigate metamorphic conditions of graphite at increasing spatial scales in the Upper Lesser Himalayan Sequence (upper-LHS) phyllites of Nepal. First, we develop and validate a new SEM-BSE technique to measure total organic carbon (TOC) content. Graphite shows mostly inter- and intra-granular flaky crystals associated with carbonates. Elemental carbon analyses yield large range of TOC content in phyllites with a mean of 3.7 +/- 0.4 vol.%. Inferred from micro-Raman spectroscopy, peak metamorphic temperature (peak-T) ranges from 519 +/- 9 to 590 +/- 8 degrees C. We observe a significant coherence of metamorphic conditions of graphitic phyllites at metric, decametric and kilometric scales using extensive data obtained in the Upper Trisuli valley, Central Nepal (n = 77), and, at larger spatial scales, using data from Far-Western to Eastern Nepal (n = 67). Mean carbon isotopic composition (delta 13C) of -26.3 +/- 0.1 parts per thousand indicates biogenic source. Less negative delta 13C values, also reported in other orogenic belts, suggest interaction with pre-existing sedimentary carbonates, probably related to metamorphic isotopic reequilibration. First-order extrapolations at the upper-LHS scale suggest meta-sediment organic carbon may represent up to 20 % of the total amount of carbon stored in orogens. Central Nepal segment, with higher TOC content and peak-T, is also characterized by the largest current metamorphic carbon dioxide (CO2) emissions of the chain. This hardly coincidental association, at the scale of the Himalayan belt, between organic carbon stored as graphite and mobile inorganic carbon released as CO2, represents an important clue to investigate further for global carbon budget estimations.