Abstract
Abstract. Understanding the anatomy of magma plumbing systems of active volcanoes is essential not only for unraveling magma dynamics and eruptive behaviors but also to define the geometry, depth, and temperature of the heat sources for geothermal exploration. The Pleistocene–Holocene Los Humeros volcanic complex is part of the eastern Trans-Mexican Volcanic Belt (central Mexico), and it constitutes one of the most important exploited geothermal fields in Mexico with ca. 90 MW of produced electricity. With the aim to decipher the anatomy (geometry and structure) of the magmatic plumbing system feeding the geothermal field at Los Humeros, we carried out a field-based petrological and thermobarometric study of the exposed Holocene lavas. Textural analysis, whole-rock major-element data, and mineral chemistry are integrated with a suite of mineral-liquid thermobarometric models. Our results support a scenario characterized by a heterogeneous multilayered system, comprising a deep (depth of ca. 30 km) basaltic reservoir feeding progressively shallower and smaller discrete magma stagnation layers and batches, up to shallow-crust conditions (depth of ca. 3 km). The evolution of melts in the feeding system is mainly controlled by differentiation processes through fractional crystallization (plagioclase + clinopyroxene + olivine + spinel). We demonstrate the inadequacy of the existing conceptual models, where a single voluminous melt-controlled magma chamber (or “Standard Model”) at shallow depths was proposed for the magmatic plumbing system at Los Humeros. We instead propose a magmatic plumbing system made of multiple, more or less interconnected, magma transport and storage layers within the crust, feeding small (ephemeral) magma chambers at shallow-crustal conditions. This revised scenario provides a new configuration of the heat source feeding the geothermal reservoir at Los Humeros, and it should be taken into account to drive future exploration and exploitation strategies.
Highlights
Recent views on the structure of volcanic plumbing systems have moved from the “Standard Model” of a single bowl-shaped magma chamber where all petrological processes of differentiation and assimilation occur (e.g., Hildreth, 1979, 1981; Hildreth and Wilson, 2007) to more complex arrays of stratified and variably interconnected transient magma accumulation zones, set in largely crystallized and vertically extensive mush zones (e.g., Bachman and Bergantz, 2004, 2008; Cashman and Giordano, 2014; Cashman et al, 2017)
We demonstrate the inadequacy of the existing conceptual models, where a single voluminous melt-controlled magma chamber at shallow depths was proposed for the magmatic plumbing system at Los Humeros
In this paper we present a geothermobarometric study of the post-caldera Pleistocene–Holocene products of the Los Humeros volcanic complex (LHVC), located at the eastern termination of the Neogene–Quaternary Trans-Mexican Volcanic Belt (TMVB) (Fig. 1), with the goal of reconstructing the present-day geometry and structure of the magmatic plumbing system
Summary
Recent views on the structure of volcanic plumbing systems have moved from the “Standard Model” (sensu Gualda and Ghiorso, 2013) of a single bowl-shaped magma chamber where all petrological processes of differentiation and assimilation occur (e.g., Hildreth, 1979, 1981; Hildreth and Wilson, 2007) to more complex arrays of stratified and variably interconnected transient magma accumulation zones, set in largely crystallized and vertically extensive mush zones (e.g., Bachman and Bergantz, 2004, 2008; Cashman and Giordano, 2014; Cashman et al, 2017). A key factor in determining the internal architecture of the magmatic systems is the magma intrusion rate. It controls whether successive pulses of magma will coalesce to form progressively larger chambers, as well as the formation of ductile shells surrounding the magma chamber that prevent country rock failure, favoring the inflation of the reservoir (Jellinek and DePaolo, 2003; Annen, 2009). Numerical simulations suggest that caldera systems smaller than 100 km are fed by plumbing systems encapsulated by country rock that remains sufficiently brittle, while larger systems are more ductile, which favors an increase in size (Gregg et al, 2012)
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