Knowing that gypsum is not an ideal host mineral for fluid inclusion studies due to the ease of deformation and perfect cleavage along {010}, the aim of our study (Krüger et al., 2013) was to test the applicability of fluid inclusion liquid-vapor homogenization temperatures (Th) for an accurate determination of gypsum growth temperatures. To do so, we used fluid inclusions in synthetic gypsum crystals that were grown under well-defined pressure-temperature conditions. Because most of the fluid inclusions were in a metastable liquid state at room temperature, we used ultra-short laser pulses to generate the vapor bubble for the subsequent Th measurements. We then applied the findings from the synthetic gypsum crystals to determine the growth temperatures of natural gypsum crystals using the example of Naica (Mexico). Two crystals, one from the Cave of Swords (120 m below the surface) and the other from the Cave of Crystals (290 m below the surface), were analyzed and we found a difference in crystal growth temperatures of ∼7 °C based on the interpretation of the measured Th data.
In their Comment, Garofalo et al. (2013) criticize that we did not refer to their studies published on the formation of gypsum in Naica. In our paper, however, we do not provide a review of the current state of research in Naica simply because the genetic models proposed for the precipitation of the giant gypsum crystals were not subject of this study. Naica was chosen only as an example to demonstrate the applicability of Th measurements to accurately determine the growth temperatures of natural gypsum crystals.
The second statement of Garofalo et al. (2013) concerns the pressure correction that needs to be applied if the fluid pressure is significantly higher than 1 atm. In this case, the gypsum growth temperature is no longer equal to the homogenization temperature, yet it needs an additional correction given by the slope of the fluid isochore. For the Cave of Swords, we assumed crystal growth only a few meters below the water table, close to atmospheric pressure conditions and, therefore, we did not apply a pressure correction. There might be reasons to assume a higher water pressure in the Cave of Swords and to apply a pressure correction. In an interconnected fluid system, however, this would also imply a higher fluid pressure and, accordingly, a larger pressure correction than we assumed in our paper for the crystals grown in the Cave of Crystals. Thus, the question of whether or not we need to apply a pressure correction for the Cave of Swords only shifts the two growth temperatures, but it has no effect on the temperature difference between the two crystals.
In this context, Garofalo et al. (2013) argue that a 7° temperature difference between the Cave of Swords and the Cave of Crystals implies an unreasonably high geothermal gradient of 41 °C/km. However, their argument seems to be rather speculative. In fact, the accuracy of our growth temperatures (±1.5 °C for the Cave of Swords and ±2 °C for the Cave of Crystals) does not allow for one to draw a serious conclusion on the geothermal gradient at the time of gypsum precipitation. In addition, we have no evidence to indicate whether or not the two crystals grew simultaneously. In any case, our results clearly demonstrate that the two crystals grew at significantly different temperatures.
Finally, Garofalo et al. (2013) refer to their own study (Garofalo et al., 2010), in which they observed rather high salinities in the fluid inclusions. Based on measurements of the ice melting temperature, they determined salt concentrations of up to 7.7 wt% NaCl-equivalent. In our samples, in contrast, we measured ice melting temperatures between –0.2 and –1.4 °C, corresponding to 0.35–2.4 wt% NaCl-equivalent (salinity data were not shown in the paper). In all the measurements, the ice melting proceeded under equilibrium conditions (i.e., in the presence of the vapor bubble) and we can exclude a systematic temperature shift due to ice melting under metastable conditions. Therefore, we are confident that our measurements are correct and we can only note that the salinities reported by Garofalo et al. (2010) are obviously higher than those measured in our two samples. The only motivation to determine the salinity of the inclusions in our study was to check the applicability of the thermodynamic model of Marti et al. (2012) to calculate the effect of surface tension on liquid-vapor homogenization: the model we use actually refers to a pure water system, but it still provides a reasonable approximation for low-saline fluid inclusions.
In conclusion, we notice that the comments of Garofalo et al. (2013) do not challenge the results of our paper, with which they apparently agree. In regard to our study of natural gypsum crystals from Naica, we hope that our Reply has clarified the points that were subject to criticism.
