Tracking volatile elements in the lithospheric mantle through the orogenic cycle

The transport of volatile elements from the exosphere to the mantle via subduction, and their long-term storage in Earth’s interior - versus re-entry into the atmosphere viamagmatism and tectonic degassing - remain poorly constrained. The lithospheric mantle (LM), at the interface between these two major reservoirs, represents a potentially important sink and source of volatiles, and its role is only starting to be systematically addressed [1]. Here, we consider the effects of the orogenic cycle on the volatile element inventory of LM that evolved in a palaeo-convergent plate boundary affected by subduction, tectonic accretion, collapse and rifting. We take the European Variscan Orogen (EVO) and subsequent development of the European Cenozoic Rift System (ECRIS) through parts of the EVO as an example, keeping in mind the evolutionary diversity of this vast terrain.

 

LM is initially stabilised by decompression melting, where H₂O behaves like a highly incompatible element, and redox-melting will extract CO₂ initially stored in refractory graphite/diamond. Sulfur extraction efficiency depends on S solubility in the melt, S content and melt fraction, and refractory residues are predicted to be very S-poor. The effect of continental subduction (and preceding oceanic subduction) on the volatile element inventory of the mantle wedge is gauged by tectonically exhumed peridotite, which suggests a net addition of COHS via introduction of carbonate, hydrous and sulphide minerals accompanied by moderate oxidation [2]. The emplacement ages of syn-/late-/post-orogenic Mg-K-rich magmas (orogenic lamprophyres and lamproites) testify to the protracted (10s Ma) subsequent remobilization of earlier-formed hydrous mantle metasomes [3], the low solidi of which facilitate melting during heating and/or decompression [4]. The effects on C and S are unclear, but ƒO₂ remained mostly below sulphate stability and sulphides may have persisted in the residue. Tectonic reconstructions [5] and basalt-borne xenoliths with garnet break-down microstructures [6] concordantly point to crustal thinning and exhumation of LM by up to 30 km. This brought huge volumes of carbon-enriched garnet-facies LM to depths where decarbonation and associated mantle-CO₂degassing could occur if subsequently heating ± decompressed. We estimate this mantle volume at 7.2 Mio km³ in the French EVO alone, corresponding to a total of ~24 × 10⁶ Mt C for a C concentration ~1,000 mg/g ([2]).

 

During ECRIS development and minor associated LM thinning, magma affinities had shifted to OIB-like, reflecting (partial) consumption of earlier-formed volatile element-rich metasomes, leaving the LM largely below its solidus. Interaction with carbonated silica-undersaturated basalts and associated wehrlitisation of shallow LM suggests a CO₂flux of 1.7±1.1 Mt yr^-1 in the ECRIS [7]. Notwithstanding evidence in some EVO xenoliths for introduction of hydrous and sulphide minerals during rift-related carbonated melt metasomatism, stable isotope data are required to understand the inherited subduction- vs. rift-related mantle metasomatic origin of volatile elements in the LM.

 

[1] Gibson SA & McKenzie D 2023 EPSL; [2] Förster et al. 2024 EPSL; [3] Krmíček et al. 2020 JPet; [4] Prelević et al. 2024 ESR; [5] Vanderhaeghe et al. 2020 BSGF; [6] Puziewicz et al. 2025 Lithos; [7] Aulbach et al. 2020 GPL

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