| Abstract |
In-hole temperature measurements can cause problems during geothermal drilling. Direct in-hole temperature measurements are relatively expensive, time-consuming and, in some cases, could be affected by errors due to thermal disequilibrium after drilling. In particular, the vapor-static nature of the Larderello geothermal system makes temperature estimations particularly difficult in this field, as in-hole thermal conditions can be strongly influenced by the pressure of the water introduced during drilling operations. Direct temperature measurements may also be a problem in deep wells characterized by hightemperatures (>375ÅãC), as in the case of some Larderello wells. Local temperature information can be obtained from fluid inclusions trapped in minerals if they contain hydrothermal fluids representative of the recent thermal conditions. In this work we compare present-day estimated temperatures with the homogenization temperatures of recent fluid inclusions in order to evaluate whether the latter can be used for temperature evaluations at Larderello. In the Larderello geothermal field, early fluid circulation was characterized by the presence of magmatic-derived and contact-metamorphic fluids trapped at high temperature (425- 690ÅãC) and under lithostatic pressure. At this stage, contactmetamorphic minerals (cordierite, biotite, corundum) and high-temperature hydrothermal minerals (tourmaline, biotite, plagioclase) crystallized. Early fluid circulation was replaced by a late hydrothermal activity characterized by the presence of meteoric fluids of lower temperature (usually <400ÅãC) under hydrostatic pressure. This hydrothermal activity was responsible for the precipitation of relatively low-temperature, late mineral assemblages (chlorite, epidote, carbonates, anhydrite, etc.). Finally, the Larderello system evolved to present-day vapor-static conditions. The relatively young maximum ages (20 and 270 ka) of late hydrothermal minerals suggest that the fluid inclusions trapped in these minerals may contain fluids representative of the recent thermal conditions at Larderello. Comparison of present-day temperature with average and minimum homogenization temperatures of late-stage inclusions found in 23 core samples indicates that these homogenization temperatures cannot precisely estimate the present-day temperature at the depth of core sampling. In general, both the average homogenization temperature and minimum homogenization temperatures approximate the present-day temperature with an error of Å}30ÅãC in most (. 65%) of the samples. Within this error, the average homogenization temperatures predict the present-day temperature in more cases than the minimum homogenization temperatures. Therefore, the average homogenization temperatures can be used for a broad evaluation of present-day temperature, when in-hole measurements are missing or dubious. |