A numerical model for the magmatic heat reservoir of the Las Tres Virgenes volcanic complex, Baja California Sur, Mexico

Highlights

• Conceptual model of a geothermal field in Mexico based on an Enhanced Seismic Tomography and detailed structural geology.

• Three-dimensional numerical simulations of the cooling of magmas of a currently exploited geothermal field.

• Petrologic processes of the volcanic complex in the light of simulated subsurface temperature field.

Abstract

Numerical models of geothermal systems have achieved considerable progress when dealing with fundamental transport processes. Site-specific geothermal models however, often face the lack of sufficient field data, which leads to under constrained models. Recent field studies in the Las Tres Virgenes volcanic complex (TVVC) provide us with a sound framework to build a numerical model for the magmatic heat reservoir of the geothermal area. Here we present a conceptual model of the system from 30 ka to present based on updated structural and geochronological studies. The heat reservoir is described from Enhanced Seismic Tomography of the TVVC. New field data also allow an estimate of the radiogenic heat generation in the batholith of the study area. Latent heat of crystallization is also considered in the cooling of magmatic intrusions. The transient heat transfer equation is solved with a finite volume numerical method and a conjugate gradient algorithm. The model is calibrated using the estimated heat flow in the area from the literature. The simulation results predict slow cooling of the heat reservoir, with the maximum temperature decreasing about 9 °C in 30 ky. The temperature field within the magmatic heat reservoir overall remains between about 790 °C (8 km bsl) and 300 °C (3 km bsl), in the order of the temperatures at which magmatic processes, such as magma mixing and replenishments, have been inferred from the eruptive history. Whereas the phase ratio (solid/melt) ranges from 1, at the top, to below 0.1 at the bottom of the heat reservoir. Convective transport and explicit thermal-chemical coupling in the heat reservoir is proposed as a future expansion of this work.

Introduction

The Tres Virgenes Volcanic Complex (TVVC) is a Quaternary structure that has been subject to intense research in recent years. This field currently hosts one of the four geothermal power plants operated in Mexico (10 MWe) by the Federal Commission of Electricity (CFE), and it represents a promising alternative for diversification of energy supply on the Baja California Peninsula, currently disconnected from the Electric National Network. New insights on the eruptive history and magma reservoirs of this volcanic complex provide a valuable opportunity to look into its recent thermal evolution. We present a numerical model of heat transport in order to elucidate the magmatic processes that govern the present state of the heat reservoir.

Most modeling of the cooling of magma reservoirs has focused on various fundamental magmatic processes. For instance, Usselman and Hodge (1978) analyzed the cooling and crystallizing of shallow basaltic magma chambers and explained commonly observed eruption rates of basaltic lavas as a consequence of periodic magma replenishment. Compositional layering in solidified basaltic intrusions was investigated by McBirney and Noyes (1979), who reported that layered mineral segregation is governed by chemical and thermal diffusion processes rather than with crystal settling by gravitation. Huppert and Sparks (1980) addressed heat transport in basic magma chambers after re-injection of ultrabasic magmas. Their model explained cyclic layering as a result of density differences, and also concluded that most of the temperature decline in the re-injected magma occurs within a few weeks to a few years as a consequence of vigorous convective transport. Marsh (1981) proposed a methodology to estimate the sequence of crystallization of lavas based on a probabilistic approach and also related crystallinity to the probability of eruption. Later, a series of authors investigated the critical role of magma convection on cooling, crystallization, and differentiation in early stages of emplacement (Marsh and Maxey, 1985; Brandeis and Jaupart, 1986; Martin et al., 1987; Marsh, 1989; Worster et al., 1990; Bagdassarov and Fradkov, 1993; Hort, 1997). Huppert and Sparks (1988) described the formation of granitic plutons as the result of heating of crustal rocks by underlaying basaltic sills. Pinkerton and Norton (1995) conducted experiments to evaluate rheological properties of basaltic magmas. Hort (1998) addressed the changes of liquidus temperature of magmas as functions of volatile concentration and magma mixing. A temporal analysis of cooling and chemical differentiation in a magma chamber was conducted by Kuritani et al. (2007) based on data of Rishiri volcano, Japan. Michaut and Jaupart (2011) studied the cooling of magma chambers in relation with their emplacement process; that is, magma is emplaced gradually or rapidly.

More recently, Eichelberger (2020) reviewed the few in situ studies of magma cooling, namely, Kilauea Iki lava lake, Hawaii, and Krafla magma-hydrothermal system, Iceland. He describes the Magma-hydrothermal boundary in these systems according with the concept of cracking front (Lister, 1974). In another recent investigation, Lamy-Chappuis et al. (2020) explored volatile outgassing in magma chambers and the role it plays in their cooling history.

There is also considerable progress in modeling transport and thermodynamics of hydrothermal fluids, particularly with a process-based approach (Ingebritsen et al., 2010). Site-specific models however, often face challenges such as poor field data availability, which leads to under constrained models. Structural complexity and heterogeneity can also be a major issue in the definition of numerical models. Nonetheless, continued efforts are required to build rigorously constrained models, taking advantage of novel methodologies for data acquisition and processing (e.g. Petrillo et al. (2013)), as well as increasingly efficient numerical and computational tools (e.g. Afanasyev et al. (2015)). In this context, we propose a model for the magmatic heat reservoir of the TVVC based on multidisciplinary field studies. The model is intended as a sound framework that can be progressively refined as more data are incorporated.