# Tourmalinisation in peraluminous granitic context : from experiment to thermodynamic modelling

Julien Fort, Stanislas Sizaret, Michel Pichavant, Arnault Lassin, Johann Tuduri, Olivier Blein

## ► To cite this version:

Julien Fort, Stanislas Sizaret, Michel Pichavant, Arnault Lassin, Johann Tuduri, et al., Tourmalinisation in peraluminous granitic context: from experiment to thermodynamic modelling. EGU General Assembly 2021 - VEGU21, Apr 2021, Virtual Event, Austria. 10.5194/egusphere-egu21-11559. insu-03322142

HAL Id: insu-03322142

https://insu.hal.science/insu-03322142

Submitted on 18 Aug 2021

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Distributed under a Creative Commons Attribution 4.0 International License
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## Tourmaline : «A unique [and stable] recorder of its geological past»

Van Hinsberg et al., 2011

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Tourmalines are rhombohedral cyclosilicates and the most widespread borosilicates. They are occurring in most rock types (from magmatic to sedimentary), are stable over a large P-T field and their crystal structure accommodate an exceptional range of different elements (van Hinsberg et al., 2011).

Thus, many studies tried to used them :

* to constrain the chemistry of hydrothermal fluids (see on the left of this section)

* as geothermobarometer (Henry & Guidotti, 1985)

Those two uses imply a good knowledge of the thermodynamic properties of those minerals. But, due to their complexity and the lack of precise knowledge over their stability conditions, the geochemical simulations are generally working without those classical alteration

## Experimental study of the stability field of the schorl-dravite solid solution, based on the metasomatic alteration of a (sim- plified) cordierite-bearing leucogranite by a boron-rich hydrothermal fluid.

$$3 \text{ Cordierite} + 2 \text{ Albite} + 6 \text{ H}_3\text{BO}_3 \text{ (aq)} \rightarrow 2 \text{ Dravite} + 8 \text{ Quartz} + \text{Andalusite} + 5 \text{ H}_2\text{O}$$

## Stability field and tourmaline textures

<figure><img src="image_4.png" /></figure>

* The boundaries of the stability field slightly change with temperature (1.): decreasing from 4 wt% at 600°C to 3 wt% at 400°C.

* First occurrence of tourmaline: **sheaf of fibrous crystal** (4.), growing from cordierite and/or albite nucleus.

At higher boron concentration: **fibro-radiated aggregates** (2.) and **homogeneous crystallisation** (3.).

* Presence of NaCl in the fluid (Weisbrod et al., 1986) seems to lower the boron concentration needed to precipitate tourmaline (1.).

## Experimental conditions

Metasomatic experiment using an internally heated gas apparatus.

* **Temperature** : 600°C (2 weeks), 500°C (1 month) and 400°C (2 months).

* **[B₂O₃]init** : from 0 to 10 wt%, with changes in the order of 1 wt%.

* **Pressure** : constant, at 200 MPa.

* **Oxygen fugacity** : included in the range $\Delta NNO$ + [1;2].

## Tourmaline chemistry

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## Comparison to natural and thermochemical data

* **Boron concentration in natural fluid inclusion generally doesn't exceed 1wt%**, even in systems known to present tourmaline as an hydrothermal alteration product (8.).

* **In geochemical models, tourmaline crystallise even with just 500 ppm of H₃BO₃ in the fluid and always as a Mg-foitite (9.).**

* **However, our experiments show a stability field requiring higher boron concentration to make tourmaline crystallise (1.).**

* **This difference with the fluid inclusion can be explained by the high salinities presented in those contexts.**

* **The actual thermodynamic properties seem to overestimate tourmaline stability field.**

<figure><img src="image_6.png" /><figcaption>Distribution of boron concentrations in natural fluid inclusions, according to the type of mineralization.</figcaption></figure>

$$Th = [100 : 450]^{\circ}\text{C}$$

$$\text{Salinity} = [1 : 40] \text{ wt\% eq. NaCl}$$

<figure><img src="image_7.png" /><figcaption>Alteration mineral phase diagram showing the condition of temperature and reaction ratio of a H₃BO₃+H₂O fluid with an assemblage Crd + Alb. Created using CHIM (Reed et al., 2016) and reproducing Weisbrod et al. (1986) experiment.</figcaption></figure>

## References

Beryman, E. J., Wunder, B., Rhode, D., Schetler, G., Franz, G., and Heinrich, W., 2016, P-T-X controls on Ca and Na distribution between Na-Mg-Al tourmaline and fluid: Contributions to Mineralogy and Petrology, v. 171, p. 31.

Outrow, B. L., and Henry, D. J., 2016, Fibrous Tourmaline: A Sensitive Probe of Fluid Compositions and Petrologic Environments: The Canadian Mineralogist, v. 54, p. 311-335.

Fritschi, M. H., and Plettenberg, E., 2018, Tetrabasic substitutions in tourmaline: a review, European Journal of Mineralogy, v. 30, p. 465-470.

Goerne, G. V., Franz, G., and Robert, J. L., 1999, Upper thermal stability of tourmaline + quartz in the system MgO-Al₂O₃-SiO₂-B₂O₃-H₂O and Na₂O-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O+HCl in hydrothermal solutions and siliceous melts: The Canadian Mineralogist, v. 37, p. 1025-1039.

Goerne, G. V., Franz, G., and Heinrich, W., 2001, Synthesis of tourmaline solid solutions in the system Na₂O-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O+HCl and the distribution of Na between tourmaline and fluid at 300 to 700 °C and 200 MPa: Contributions to Mineralogy and Petrology, v. 141, p. 160-173.

Goerne, G. V., Franz, G., and van Hinsberg, V. J., 2011, Experimental determination of Na-Ca distribution between tourmaline and fluid in the system Ca₂O-Na₂O-MgO-Al₂O₃-SiO₂-B₂O₃-H₂O: The Canadian Mineralogist, v. 49, p. 137-152.

Henry, D. J., and Guidotti, G. V., 1985, Tourmaline as a petrogenic indicator mineral: an example from the staurolite-grade metapetrites of NW Maine: American Mineralogist, v. 70, p. 1-15.

Hinsberg, V. J. V., and Schumacher, J. C., 2009, The geothermobarometric potential of tourmaline, based on experimental and natural data: American Mineralogist, v. 94, p. 761-770.

Hinsberg, V. J. M., Henry, D. J., and Outrow, B. L., 2011, Tourmaline as a Petrologic Forensic Mineral: A unique Recorder of Its Geologic Past: Elements, v. 7, p. 127-132.

Lussier, A., Aguirre, P. M., Michaelides, V. K., Kroeker, S., and Hawthorne, F. C., 2009, The occurrence of tetrahedrally coordinated Al and B in tourmaline: An 11B and 27Al MAS NMR study, American Mineralogist, v. 94, p. 785-792.

Morgan, G. B., and London, D., 1989, Experimental reactions of amorphous boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks: Contributions to Mineralogy and Petrology, v. 102, p. 281-297.

Reed, M. H., Spycher, N. F., and Plettenberg, E., 2016, Users Guide for CHIM-XPT A Program for Computing Reaction Processes in Aqueous-Mineral-Gas Systems and MINTAB Guide

Weisbrod, A., Polak, C., and Roy, D., 1986, Experimental study of tourmaline solubility in the system Na-Mg-Al-Si-B-O-H: Applications to the boron content of natural hydrothermal fluids and tourmalinization processes, in Experimental Mineralogy and Geochemistry - Nancy - France, p. 140-141.

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