Formation temperature and chemistry of brine inclusions in euhedral quartz crystals from Permian salt in the Danish Trough

Abstract

Classic microthermometry has been carried out on fluid inclusions in euhedral quartz from the Batum saltdome in the dome area of N.Jutland, Denmark. The salt studied is of Zechstein 1 and 2 age. The fluid inclusions are either isolated regular inclusions or populations of irregular thin inclusions on an internal crystal face. The inclusions are either fluid-gas inclusions or inclusions also containing a solid phase of NaCl. The salinity and the CaCl2:MgCl2 ratio have been measured using phase diagrams of the system (CaCl2 + MgCl2)-NaCl-H2O, the Ca:Mg ratios 3:1, 1:1 and 1:3. The salinity determined is from c.35 weight % to more than 45 weight % and the Ca:Mg ratio from 3:1 to 1:4. The homogenization temperatures of 231 fluid-gas inclusions give a mean Th=98.1 °C indicating a formation of the quartz crystals late in Zechstein 3. The formation temperatures (115 °C-180 °C) and pressures (15 MPa-90 MPa) have been measured/calculated on 14 quartz crystals indicating an individual crystallization in the period Early Triassic-Early Jurassic times. The calculated Rayleigh numbers indicate conditions of thermal convection in Late Triassic-Early Jurassic times. Both the Zechstein 1 salt and the Zechstein 2 salt were sedimented under uniform conditions.