Reappraisal of chlorite composition along a subduction metamorphic gradient (Schistes Lustrés Complex, Western Alps)

   This study aims at reappraising the evolution of the crystal chemistry of metamorphic chlorite in metasediments along a well-constrained geothermal gradient, in order to challenge current chlorite semi-empirical thermometers and thermodynamic models, currently proposing contradictory views upon the chemical evolution of chlorite with temperature and its phase relations. For that purpose, we studied metasediments originating from the Schistes Lustrés complex (Western Alps). Metamorphic peak pressure and temperature were determined by Si-content in white mica and Raman Scattering on Carbonaceous Matter (RSCM) respectively, for the different units of the Liguro-Piemont domain¹. Peak conditions increase from west to east, from blueschist- to eclogite-facies (1-2GPa, 350-550°C).
   Chlorite and white mica crystallized in the successive deformation structures that record the peak and retrograde path of the rock. However, the link between structure and chlorite composition is not straightforward. Even chlorite in textural equilibrium with peak pressure mica (high celadonite content) displays significant local composition variation down to the micrometre scale, emphasising small scale re-equilibration of chlorite upon retrogression. After selection of chlorite on microtextural bases, the evolution of composition has been determined through exhaustive chemical analysis (major elements, minor elements, Fe³⁺/Fe_Tot ratio and oxygen analysis).
   Results highlight the importance of at least the homovalent Fe-Mg, Al-Fe, Tschermak (^IVSi⁴⁺ + ^VIMg²⁺ ↔ ^IVAl³⁺ + ^VIAl³⁺) and di-trioctahedral (2Al³⁺ + vacancy ↔ 3Mg²⁺) substitution. The clearest signal is the increase of X_Mg (Mg/[Mg + Fe] atom per formula unit) with temperature, contrary to the suggestion of Bourdelle et al. (2013)². The presence of an oxychlorite³ component is not observed. Variations in Fe³⁺ content are in the range Fe³⁺/Fe_Tot = 10% to 25%. The sudoite content (chlorite with vacant octahedral sites) is significant, although it is neglected in the thermodynamic model of White et al 2014⁴. The sudoite content appears increasing with decreasing temperature in agreement with the semi-empirical model of Bourdelle et al. (2013)² but opposite to the thermodynamic model of Vidal et al. (2006)⁵ and Lanari et al. (2014)⁶.
   Associated with further analysis, these data will allow to set new ground for the comprehension of metasediment chlorite composition evolution along a subduction metamorphic gradient, and to select the most suited formalism for thermodynamic modelling.

 

References

1. Herviou C. et al., Tectonophysics, 827, 0040-1951 (2022).
2. Bourdelle F. et al., Contribution to Mineral Petrology, 166:423–434 (2013).
3. Masci L. et al., American Mineralogist, 104, 403–417 (2019).
4. White R. W. et al., Journal of metamorphic Geology, 32, 261–286 (2014).
5. Vidal O. et al., Journal of metamorphic Geology, 24, 669–683 (2006).
6. Lanari P. et al., Contributions to Mineralogy and Petrology, 167, 968 (2014).