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Fluid inclusions in thenardite from northern Mali: experimental stretching and microthermometric investigations

Published online by Cambridge University Press:  05 July 2018

A. Canals-Sabate ,
J. C. Touray  and
J. Fabre
A. Canals-Sabate
Affiliation:
Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009, Spain
J. C. Touray
Affiliation:
URA 1366 CNRS ESEM, Université d'Orléans, 45067 Orléans, France
J. Fabre
Affiliation:
URA 69 CNRS Université Joseph Fourier, Institut Dolomieu, 38031 Grenoble, France
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Abstract

Large thenardite crystals have been sampled at New Agorgott, in the Taoudenni area of northern Mali. They are still in equilibrium with a pressurized NaCl saturated brine capped by a halite layer. Clays located about 1 m above the thenardite occurrence have been dated at 6760 y.BP. The crystals contain numerous, large, brine and solid inclusions. Microcryscopic studies show that the fluids can be explained by the addition of MgCl2 to the Na2SO4-NaCl-H2O system (eutectic temperature: −31 to −35°C; possible bloedite Na2Mg(SO4)2.4H2O formed after freezing). The homogenization temperatures of primary fluid inclusions are in the range 28 to 50°C. In order to understand the significance of the highest Th values, overheating experiments under 1 bar pressure were performed at different heating rates up to 170°C. The results are as follows:

  • (i) When the temperature of stretching (TOh) is higher than about 10°C, overheating is recorded and fossilized (identical Th after some hours, several days or 8 months storage at 5°C).

  • (ii) The lowest Th values (28°C) are probably near the formation temperature of thenardite; the highest ones reflect stretching under present desert conditions.

  • (iii) With TOh lower than about 60°C, a fair correlation is observed between Th and TOh.

Finally, taking into account recent natural overheating, the fluid inclusion data are compatible with the formation of thenardite from underground brines later than the beginning of desert conditions in the Taoudenni area (i.e. about 3000 y.BP).

Keywords

fluid inclusionsthenarditemicrothermometrystretchingMali
Type
Near-surface and surficial environments
Information
Mineralogical Magazine , Volume 54 , Issue 375 , June 1990 , pp. 305 - 309
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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