Reappraisal of Los Humeros Volcanic Complex by New U/Th Zircon and ⁴⁰Ar/³⁹Ar Dating: Implications for Greater Geothermal Potential

Abstract

Longevity and size of magmatic systems are fundamental factors for assessing the potential of a geothermal field. At Los Humeros volcanic complex (LHVC), the first caldera‐forming event was reported at 460 ± 40 ka. New zircon U/Th and plagioclase 40Ar/39Ar dates of pre‐, syn‐ and postcaldera volcanics allow a reappraisal of the evolution of the geothermally active LHVC. The age of the voluminous Xaltipan ignimbrite (115 km3 dense rock equivalent [DRE]) associated with the formation of the Los Humeros caldera is now constrained by two geochronometers (zircon U/Th and plagioclase 40Ar/39Ar dating) to 164 ± 4.2 ka, which postdates a long episode of precaldera volcanism (rhyolitic domes), the oldest age of which is 693.0 ± 1.9 ka (40Ar/39Ar). The inferred short residence time (around 5 ka) for the paroxysmal Xaltipan ignimbrite is indicative of rapid assembly of a large magma body and rejuvenation of the system due to recurrent recharge magmas, as it has been occurred in some other large magmatic systems. Younger ages than previously believed have been obtained also for the other voluminous explosive phases of the Faby fall tuff at ∼70 ka and the second caldera‐forming Zaragoza ignimbrite with 15 km3 DRE, which erupted immediately after. Thus, the time interval that separates the two caldera‐forming episodes at Los Humeros is only 94 kyr, which is a much shorter interval than suggested by previous K‐Ar dates (410 kyr). This temporal proximity allows us to propose a caldera stage encompassing the Xaltipan and the Zaragoza ignimbrites, followed by emplacement at 44.8 ± 1.7 ka of rhyolitic magmas interpreted to represent a postcaldera, resurgent stage. Rhyolitic eruptions have also occurred during the Holocene (<7.3 ± 0.1 ka) along with olivine‐rich basalts that suggest recharge of the system. The estimated large volume magmatic reservoir for Los Humeros (>∼1,200 km3) and these new ages indicating much younger caldera‐forming volcanism than previously believed are fundamental factors in the application of classical conductive models of heat resource, enhancing the heat production capacity and favor a higher geothermal potential.

The structural architecture of the Los Humeros volcanic complex and geothermal field

“…The first and largest caldera-forming eruption, producing the trap-door Los Humeros caldera, is associated with the emplacement of the rhyolitic Xaltipan ignimbrite, with an estimated volume of 115 km 3 DRE (Ferriz and Mahood, 1984) and a 40 Ar/ 39 Ar radiometric age of 164.0±4.2 ka (Carrasco-Nuñez et al, 2018). Following this catastrophic event, a sequence of explosive episodes occurred at 70±23 ka ( 40 Ar/ 39 Ar, Carrasco-Núñez et al, 2018). These eruptions emplaced thick rhyodacitic Plinian deposits grouped into the Faby Tuff unit (Ferriz and Mahood 1984;Willcox, 2011).…”

“…The post-caldera stage has been divided into two different intra-caldera eruptive phases ). The first one was a late Pleistocene phase characterized by the emplacement of rhyolitic and dacitic domes at about 50.7±4.4 -44.8±1.7 ka ( 40 Ar/ 39 Ar ages), followed by a sequence of explosive eruptions producing dacitic pumice fall units, volcaniclastic breccia and pyroclastic flows deposits with a maximum age of 28.3±1.1 ka (Rojas-Ortega, 2016;Carrasco-Núñez et al, 2018). The second one is characterized by alternated episodes of effusive and explosive volcanism with a wide range of compositions involving basalticandesitic and basaltic trachytic, trachyandesitic lava flows and dacitic, trachydacitic, andesitic and basaltic pumice and scoria fall deposits emitted by numerous monogenetic eruptive centers located in the LHVC (Ferriz and Mahood, 1984;Dávila-Harris and Carrasco-Núñez, 2014;Norini et al, 2015;Carrasco et al, 2017aCarrasco et al, , 2017b.…”

Outcrop analogue study to determine reservoir properties of the Los Humeros and Acoculco geothermal fields, Mexico

“…The younger Los Humeros caldera comprises Pleistocene to Holocene basaltic andesite-rhyolite volcanic rocks and has a 21 × 15 km irregular circular shape (Carrasco-Núñez et al, 2018). The emplacement of the Los Humeros caldera was associated with two main caldera-forming eruptions and multiple voluminous plinian eruptions as well as alternating episodes of dacitic and rhyodacitic dome-forming eruptions (Carrasco-Núñez et al, 2017a).…”

“…iutlán augite andesites (5-1.55 Ma; Norini et al, 2015 and references herein) form the geothermal reservoir in the subsurface of the Los Humeros geothermal system (Carrasco-Núñez et al, 2018). Ignimbrites (Xáltipan ignimbrite at ∼ 160 ka and Zaragoza ignimbrite at ∼ 69 ka; Carrasco-Núñez et al, 2017b), tuffs, ash fall deposits and diverse pyroclastic flows form the cap rock of the reservoir, which is overlain by Holocene to recent basalt lava flows.…”

Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)

“…The Pre-caldera volcanism extended between ca. 700 and 164 ka (based on U-Th and 39 Ar/ 40 Ar datings: Carrasco-Núñez et al, 2018), showing evidence for an extended building phase leading to the establishment of the large volume rhyolitic reservoir that fed the 115 km 3 caldera-forming, Xaltipan ignimbrite eruption. The Caldera stage started at ca.…”