Jorge Andaverde

Three-dimensional temperature field simulation of a cooling of a magma chamber, La Primavera caldera, Jalisco, Mexico

The La Primavera caldera lies close to the triple junction of the Tepic-Zacoalco, Colima, and Chapala rifts in the western part of the Mexican Volcanic Belt. It is a promising geothermal field with 13 deep wells already drilled. We calculated solute geothermometric temperatures (Na–K, Na–Li, and SiO2) from the chemistry of geothermal water samples; determined values are generally between 99°C and 202°C for springs and between 131°C and 298°C for wells. Thermal modelling is an important geophysical tool as documented in the study of this and other Mexican geothermal areas. Using the computer program TCHEMSYS, we report new simulation results of three-dimensional (3-D) thermal modelling of the magma chamber underlying this caldera through its entire eruptive history. Equations (quadratic fit) describing the simulated temperatures as a function of the age, volume and depth of the magma chamber are first presented; these indicate that both the depth and the age of the magma chamber are more important parameters than its volume. A comparison of 3-D modelling of the La Primavera and Los Humeros calderas also shows that the depth of the magma chamber is more important than its volume. The best model for the La Primavera caldera has 0.15 million years as the emplacement age of the magma chamber, its top at a depth of 4 km, and its volume as 600 km3. Fresh magma recharge events within the middle part of the magma chamber were also considered at 0.095, 0.075, and 0.040 Ma. The simulation results were evaluated in the light of actually measured and solute geothermometric temperatures in five geothermal wells. Future work should involve a smaller mesh size of 0.050 or 0.10 km on each side (instead of 0.25 km currently used) and take into account the topography of the area and all petrogenetic processes of fractional crystallization, assimilation, and magma mixing as well as heat generation from natural radioactive elements.

Thermal sensitivity analysis of emplacement of the magma chamber in Los Humeros caldera, Puebla, Mexico

We present results of our simulation study of the effects of the depth (top of the magma chamber at 5–10 km) and volume (1000–1400 km3) of the primary heat source beneath the Los Humeros caldera. The thermal gradient in the vicinity of the magma chamber calculated from the temperature excess (difference between the simulated and the initial temperatures prior to emplacement of the magma) is more sensitive to its depth of intrusion than to its volume. This relationship was quantified from multiple linear regression equations. The temperature excess at 2–3 km depth due to the emplacement of magma and its conductive cooling is also more dependent on the chamber depth than on its volume. Therefore, in the study of calderas, volcanoes, and geothermal fields, constraining the chamber depth is more important than its volume. Similarly, comparison of the thermal regime inferred along vertical and horizontal profiles shows the importance of solving the thermal transport equations in three dimensions instead of one or two dimensions.