A new chlorite geothermometer for diagenetic to low-grade metamorphic conditions

Franck Bourdelle (1) , Teddy Parra (2) , Christian Chopin (3) , Olivier Beyssac (1)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 IFPEN - IFP Energies nouvelles
3 LGENS - Laboratoire de géologie de l'ENS

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior Fe3+ knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350 °C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of ‘Al-free chlorite S’, sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-Fe2+ thermometer performs at least as well as the recent models, which require an estimate of Fe3+ content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets—provided it is used in the calibration range (T < 350 °C, P < 4 kbar).

Mots clés: Chlorite; Geothermometry; Diagenesis; Low-grade metamorphism; Solid-solution.

Domaines: Minéralogie

Franck Bourdelle, Teddy Parra, Christian Chopin, Olivier Beyssac. A new chlorite geothermometer for diagenetic to low-grade metamorphic conditions. Contributions to Mineralogy and Petrology, 2013, 165 (4), pp.723-735. ⟨10.1007/s00410-012-0832-7⟩. ⟨hal-02270198⟩