Fig. 1. Sequential room-temperature photomicrographs of inclusion # 25 from sample ODP193-1188A-16R-2 (Piece 15, 109–111 cm), illustrating the changes that took place during freezing and heating runs. (A) Inclusion # 25 prior to any microthermometry. (B) After a freezing run (during which numerous other inclusions were studied), inclusion # 25 is observed to contain newly formed gypsum crystals. (C) On heating, the gypsum crystals disappeared and T_h was measured at 284 °C. After cooling, T_m(ice) was measured at − 1.4 °C. (D) After more of the freezing run, new gypsum crystals formed, and T_m(ice) was measured at − 1.7 °C. (E) One day later T_m(ice) was re-measured at − 1.7 °C. (F) After heating to 130 °C the gypsum disappeared, and a new measurement of T_m(ice) gave the original − 1.4 °C. Note how the inclusion shape has been modified significantly relative to the initial shape.

Fig. 2. (A) Inclusion # 36 from sample ODP193-1188A-16R-2 (Piece 15, 109–111 cm) photographed at room temperature after a series of freezing runs. The inclusion bears the telltale graininess that hints at the presence of newly formed gypsum. T_m(ice) was measured at − 1.8 °C. (B) After heating to 145 °C the same inclusion appears clear and gypsum-free at room temperature. A new measurement of T_m(ice) gave − 1.5 °C, which is considered to be the temperature yielding the “true” fluid salinity.

Fig. 3. The relative stability of anhydrite and gypsum as a function of temperature and water activity; the activities of Ca^+ 2 and SO₄^− 2 are fixed at the values shown. Decreasing temperature favors the hydration of anhydrite to gypsum. However, at very low values of water activity (because of very high salinity, for example) both anhydrite and gypsum may be undersaturated. The diagram was constructed using Geochemist Workbench 4.0 (Bethke, 1996) and the thermo.dat database. Pressure is 1 bar. Note that the effect of pressure is to increase in the anhydrite–gypsum transition temperature by 1 °C per 80 bars (Blount and Dickson, 1973).

Fig. 4. Values of T_m(ice) and T_h determined and then redetermined for four inclusions over the course of about 120 h, sample ODP193-1189B-14R-1 (Piece 17, 123–129 cm). Two cycles of gypsum growth as a result of freezing and gypsum decomposition as a result of heating were carried out. The inclusions were heated to destroy the gypsum at about 72 and 115 h. The original T_m(ice) values are reliably determined even after multiple cycles of gypsum growth. Similarly, three out of four inclusions gave the same T_h after multiple cycles of gypsum growth.

Fig. 5. (A) Inclusion # 3 from sample ODP193-1188F-26Z-1 (Piece 4, 62–69 cm) photographed at room temperature prior to microthermometry. (B) The same inclusion after freezing runs, and photographed several days later at room temperature. Gypsum growth on the inclusion walls and shape modification are apparent. (C) The same inclusion photographed at room temperature after it was heated to greater than 130 °C, which caused the gypsum to disappear. Note that the inclusion shape was strongly modified.

Fig. 6. Sequential photomicrographs taken at room temperature illustrating the stretching of inclusions that were heated as little as a few tens of degrees above their homogenization temperature. Sample ODP193-1188A-7R-1 (Piece 12, 66–68 cm), inclusion # 1 through 5. (A) and (B) show five inclusions prior to heating. The inclusions were then heated, and they are annotated with T_h values, determined in sequence as T increased. (C) and (D) show the inclusions after they cooled to room temperature. The highest T reached was 280 °C; the inclusions are annotated with the degree to which they were overheated. In each inclusion except the one that was not overheated (ΔT = 0 °C), the vapor bubble has expanded, indicating that the inclusion has stretched. One inclusion (ΔT = 113 °C) decrepitated.

Fig. 7. The relative salinity increase [100 * (new salinity − initial salinity) / initial salinity] calculated within a spherical fluid inclusion owing to the development of a homogenous hydration front of gypsum.

Examples of observed changes in ice melting temperature and inclusion salinity as a result of the growth of gypsum during freezing runs

Note: T in °C; salinities in wt.% NaCl equivalent, determined using the equation of Bodnar (1993).