Into the deep and beyond: Carbon and nitrogen subduction recycling in secondary peridotites

• Published in: Earth and planetary science letters 2020-08, Vol.543, p.116328, Article 116328
• Main Authors: Cannaò, E., Tiepolo, M., Bebout, G.E., Scambelluri, M.
• Format: Article
• Language: English
• Subjects: blue boron diamonds; carbon; carbon isotope; nitrogen; serpentinites; subduction zone
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2020.116328

•Secondary peridotites carry significant carbon and nitrogen into subduction zones.•Isotopically fractionated C are recycled into the deep mantle via metaperidotites.•Deep sinking of metaperidotites feeds Earth's mantle heterogeneity.•C isotope composition of metaperidotites overlaps those of peridotitic diamonds.•Light δ13C and B-rich metaperidotites may contribute to B-bearing diamonds genesis. Understanding the volatile cycles at convergent margins is fundamental to unravel the Earth's evolution from primordial time to present. The assessment of fluid-mobile and incompatible element uptake in serpentinites via interaction with seawater and subduction-zone fluids is central to evaluate the global cycling of the above elements in the Earth's mantle. Here, we focus on the carbon (C), nitrogen (N) and C isotope compositions of chlorite harzburgites and garnet peridotites deriving from subduction-zone dehydration of former oceanic dehydration of serpentinite – i.e., metaperidotites (Cima di Gagnone, Swiss Central Alps) with the aim of evaluating the contribution of these rocks to the global C-N cycling. These ultramafic rocks, enclosed as lenses in a metasedimentary mélange, represent the destabilization of antigorite and chlorite at high-pressure/temperature (P/T) along a slab-mantle interface. Chlorite- and garnet-bearing rocks have similar ranges in C concentration ([C] = 210 – 2465 ppm and 304 – 659 ppm, respectively), with one magnesite-bearing chlorite harzburgite hosting 11000 ppm C. The average N concentrations ([N]) of the garnet peridotites (54 ± 15 ppm, one standard deviation indicated) are higher than those of the chlorite harzburgites (29 ± 6 ppm). The δ13C of total C (TC) and total organic C (TOC) values of the Gagnone metaperidotites range from -12.2 to -17.8‰ and from -27.8 to -26.8‰, respectively, excluding the magnesite-bearing chlorite harzburgites with higher values of -7.2‰ (TC) and -21.2‰ (TOC). The [C] of these rocks are comparable to those of serpentinites form modern and ancient oceanic environments and with [C] of high-P serpentinites. However, the lack of preserved serpentinite precursors makes it difficult to determine whether release of H2O during high-P breakdown of antigorite and chlorite is coupled with significant C release to fluids. The δ13C values appear to reflect mixing between seawater-derived carbonate and a reduced C source and a contribution from the host metasedimentary rocks ([C] = 301 ppm; [N] = 33 ppm; TC δ13C =