Magma diversity reflects recharge regime and thermal structure of the crust

Abstract

The chemistry of magmas erupted by volcanoes is a message from deep within the Earth's crust, which if decrypted, can provide essential information on magmatic processes occurring at inaccessible depths. While some volcanoes are prone to erupt magmas of a wide compositional variety, others sample rather monotonous chemistries through time. Whether such differences are a consequence of physical filtering or reflect intrinsic properties of different magmatic systems remains unclear. Here we show, using thermal and petrological modelling, that magma flux and the thermal structure of the crust modulate diversity and temporal evolution of magma chemistry in mid to deep crustal reservoirs. Our analysis shows that constant rates of magma input leads to extractable magma compositions that tend to evolve from felsic to more mafic in time. Low magma injection rates into hot or deep crust produces less chemical variability of extractable magma compared to the injection of large batches in colder or shallower crust. Our calculations predict a correlation between magma fluxes and compositional diversity that resembles trends observed in volcanic deposits. Our approach allows retrieval of quantitative information about magma input and the thermal architecture of magmatic systems from the chemical diversity and temporal evolution of volcanic products.

The Long-Term Life-Cycle of Nevado de Toluca Volcano (Mexico): Insights Into the Origin of Petrologic Modes

“…Increasing temperatures of the mafic recharge magma with time could therefore explain the occurrence of the early stage mafic amphibole and lack of opx ("U5 Raíces," "U14 Tepehuisco"), which are indicative of lower recharge magma T, as well as the decreasing abundances of mafic cpx through time. This scenario is consistent with thermal modeling of deep crustal magma reservoir assembly, which typically show trends of increasing temperature with time due to the thermal maturation of the crust (Annen and Sparks, 2002;Annen et al, 2006;Weber et al, 2020).…”

“…As An contents record similar ranges over time, while FeO decreases (Figure 11), either lower P H2O or temperature would be required in the early phase to compensate the effect of more mafic melt chemistry and thus keep An contents constant through time. As discussed before, a lower recharge magma temperature is consistent with the occurrence of mafic amphibole and cpx (Ulmer et al, 2018) in the early stages and thermal modeling of incremental hot-zone assembly (Annen et al, 2006;Weber et al, 2020). Interestingly, the plagioclase chemistry shows episodes with a stronger and weaker mafic magma component, indicated by higher or lower FeO contents in crystal rims.…”

“…The absence of mafic amphiboles in the later history of Nevado de Toluca may thus indicate either 1) complete digestion of mafic amphibole in the silicic magma of later eruptive stages, 2) a more evolved recharge magma composition in later stages of activity, in which mafic amphibole is not stable (e.g., Andesite >1,000°C; Blatter and Carmichael 2001) or 3) increasing temperature of the recharge magma with time beyond the stability field of mafic amphibole. While the second hypothesis is inconsistent with the presence of ultramafic pyroxene cores, the latter is consistent with thermal modeling of pulsed magma injection, which shows increasing temperatures with time during the build-up of reservoirs in mid to lower crustal hot zones due to progressive heat-up of the surrounding wall-rocks (Annen and Sparks, 2002;Annen et al, 2006;Weber et al, 2020). The temporal variation of En contents in orthopyroxenes shows that most eruptions record a large range with slight tendency toward lower variance in the early history of the volcano (Figure 13).…”

“…As we do not aim to model the particular phase equilibria of Nevado de Toluca in this study, but to investigate the general controls on compositional modes, we used a hypothetical linear melt fraction temperature relation in the presented model calculations. While the specifics of melt-fraction temperature relations impact on compositional distributions (Caricchi and Blundy, 2015a), it does not impact the direction of compositional change as a function of magma recharge or extraction rate (Weber et al, 2020) and is therefore considered of secondary importance here. A temperature dependent thermal conductivity (k) was implemented in the model based on the experiments of Whittington et al (2009) for average crust.…”

“…Predictions of existing numerical models, relating magmatic processes and the spectrum of chemical compositions erupted by volcanoes can be tested based on long-term eruptive records, but can also help to illuminate physical controls on geochemical distributions and temporal trends. Here we present a dataset of 52 new and 45 previously published (Weber et al, 2019, Weber et al, 2020 whole-rock analyses and 6247 new spot analyses on minerals of temporally resolved bulk-rock major and trace element analyses and mineral chemistry spanning the 1.5 Ma life cycle of the dacitic Nevado de Toluca volcano in Central Mexico. We use mineral zonation systematics and thermo-barometric FIGURE 3 | Compilation of stratigraphic relations for Nevado de Toluca volcano.…”