Massive formation of lawsonite in subducted sediments from the Schistes Lustrés (W. Alps): Implications for mass transfer and decarbonation in cold subduction zones

Abstract

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Missing lawsonite and aragonite found: P–T and fluid composition in meta-marls from the Combin Zone (Western Alps)

“…The hourglass textures also correspond to a chemical sector distribution, as reported both in fresh (e.g. Ueno 1999;Tsujimori and Ernst 2014;Vitale Brovarone et al 2014a;Fornash et al 2019;Lefeuvre et al 2020) and pseudomorphed lawsonite (Philippon et al 2013;López-Carmona et al 2014). Specifically, Ti is preferentially concentrated in sectors opposite to those Fig.…”

“…Lawsonite (or pseudomorphs after lawsonite) is widespread in metabasic rocks and calcschists of the oceanic units of both the MUM (Fry and Fyfe 1971;Bearth 1973;Dal Piaz 1974;Angiboust et al 2009) and S type (Bearth 1962;Caron 1974;Pognante and Kienast 1987;Lefeuvre et al 2020). In the S-type units, lawsonite tends to be largely preserved in the western part of the belt (Fig.…”

“…However, given (i) the consistency of their orthorhombic shapes (which excludes garnet), (ii) the nature of the breakdown products, and their similarity to those found in partial pseudomorphs observed in similar rocks from the Western Alps (e.g. Caron 1974;Lefeuvre et al 2020), and (iii) their occurrence in Jurassic or Cretaceous (hence monocyclic) meta-sediments, which exclude some potential precursors like andalusite or diopside/omphacite, the aggregates are confidently interpreted as pseudomorphs after lawsonite.…”

“…However, great care should be taken in identifying breakdown products, because some of the phases actually occurring in the pseudomorphs may be former inclusions in the lawsonite porphyroblasts, known to contain a large amount of inclusions (e.g. Caron 1974;Lefeuvre et al 2020). At least three phases can be safely considered as being co-stable with lawsonite rather than a breakdown product.…”

“…The western, lower T zone (Fraitève, Triplex in Fig. 11a) is characterized by abundant preserved lawsonite and carpholite (Agard et al 2001;Lefeuvre et al, 2020), whereas the eastern, higher T zone (Assietta-Albergian in Fig. 11a) displays pseudomorphs after lawsonite (frequently containing zoisite) and chloritoid instead of carpholite (Agard et al 2001).…”