Andesitic Magma Research Articles

Micro-Textural Controls on Magma Rheology and Vulcanian Explosion Cyclicity

Understanding the relationship between degassing, crystallization processes and eruption style is a central goal in volcanology, in particular how these processes modulate the magnitude and timing of cyclical Vulcanian explosions in intermediate magmas. To investigate the influence of variations in crystal micro-textures on magma rheology and eruption dynamics, we conducted high-temperature (940°C) uniaxial compression experiments at conditions simulating a shallow volcanic conduit setting on eight samples of high-crystallinity andesite with variable plagioclase microlite populations from the 2004 to 2010 Vulcanian explosions of Galeras volcano, Colombia. Experiments were conducted at different strain rates to measure the rate-dependence of apparent viscosities and assess the dominant deformation processes associated with shear. Variations in plagioclase micro-textures are associated with apparent viscosities spanning over one order of magnitude for a given strain rate. Samples with low numbers of large prismatic microlites behaved consistently with published rheological laws for crystalline dome samples, and displayed extensive micro-cracking. Samples with high numbers of small tabular microlites showed a lower apparent viscosity and were less shear-thinning. The data suggest a spectrum of rheological behavior controlled by concurrent variations in microlite number, size and shape. We use previously published micro-textural data for time-constrained samples to model the apparent viscosity of magma erupted during the 2004–2010 sequence of Vulcanian explosions and compare these results with observed SO2 fluxes. We propose that variations in magma decompression rate, which are known to produce systematic textural differences in the plagioclase microlite cargo, govern differences in magma rheology in the shallow conduit. These rheological differences are likely to affect the rate at which magma densifies as a result of outgassing, leading to magmatic plugs with a range of porosities and permeabilities. The existence of magmatic plugs with variable physical properties has important implications for the development of critical overpressure driving Vulcanian explosions, and thus for hazard assessment during volcanic crises. We suggest a new conceptual model to explain eruption style at andesitic volcanoes based on micro-textural and rheological differences between “plug-forming” and “dome-forming” magma. We advance that existing rheological laws describing the behavior of andesitic magma based on experiments on dome rocks are inappropriate for modeling large Vulcanian explosions (∼106 m3), as the magma involved in these eruptions lacks the characteristics required to form exogenous lava domes.

Hekla Revisited: Fractionation of a Magma Body at Historical Timescales

AbstractHekla is an elongate volcano that lies at the intersection of the South Iceland Seismic Zone and the Eastern Volcanic Zone. We report major and trace element, oxygen isotopic, and H2O analyses on rocks, glass, melt inclusions, and minerals from almost all of the historical lavas and tephra deposits. This new dataset confirms the remarkable observation that not only are many eruptions compositionally zoned from felsic to mafic, but the extent of zoning relates directly to the length of repose since the previous eruption. Compositional data are consistent with the origin of the basaltic andesites and andesites by fractional crystallization, with no measurable crustal interaction once basaltic andesite has been produced. Although the 1104 CE Plinian rhyolite and 1158 CE dacite are also created by fractional crystallization, uranium–thorium isotopic disequilibria measured by others require that they evolved in a separate body, where magma is stored in a molten state for >104 years. Consistent trace element trends and ratios, as well as oxygenisotopic data, preclude significant crustal input into the evolving magma. The phenocryst assemblages are dominated by crystals that formed from their host melt; an exception is the 1158 CE dacite, which contains abundant crystals that formed from the 1104 CE rhyolite melt. A suite of thermobarometers indicates that most crystals formed in the lower crust at temperatures ranging from ∼1010 to 850 °C. Hekla’s unique and systematic petrological time series and geophysical activity are attributed to the unusual geometry of the magma body, which we propose to be a tabular, vertically elongate macrodike, extending from the lower to the upper crust. The vertical body is recharged with basaltic andesite magma at the end of each eruption, which then undergoes cooling and crystallization until the subsequent eruption. The entire system is supplied by a lower-crustal body of basaltic andesite, which is produced by fractional crystallization of basaltic magma in a reservoir that is thermochemically buffered to ∼1010 °C. Cooling and crystallization of recharged basaltic andesite magma in a background geothermal gradient from the lower to the shallow crust accounts for the systematic relationship between repose and composition.

Petrogenesis of volcanic rocks of the Devonian – Carboniferous Dahalajunshan Formation, Western Tianshan: Implications for crustal growth in an accretionary orogen

Magmatism associated with plate subduction and collision plays an important role in crustal growth during the evolution of the Earth, but the detailed mechanisms regarding how magmatism contributes to the vertical growth of the crust remain unclear, in part due to controversies on the petrogenesis of intermediate-felsic rocks. In the Central Asian Orogenic Belt (CAOB), although it has been recognized that both pre-existing crustal component and juvenile mantle-derived melt played a role in the crustal evolution of the orogens, the mechanics of how they contributed to vertical crustal growth remain uncertain. This study presents new geochronological, major and trace element geochemical, and Nd–Hf isotopic data for the Late Paleozoic intermediate-felsic volcanic rocks of the northern Yili terrane, Western Tianshan Orogen, with the aims to constrain the petrogenesis of the andesitic magma and to offer new insights into the deep interaction processes between crustal- and mantle-derived magmas. According to zircon U–Pb dating, trace element and isotopic compositional data, these volcanic rocks can be divided into two groups. Group 1 rocks (365–375 Ma) have relatively low whole-rock εNd(t) values (−4.2 – −1.6) and display high contents of Th (11.8–20.4 ppm), U (2.7–5.1 ppm), Nb (14.1–36.7 ppm) and Ta (1.1–3.1 ppm) that suggest they were derived from the partial melting of pre-existing crust. Group 2 rocks (346–349 Ma) show REE and trace elemental patterns typical of subduction-related calc-alkaline volcanic rocks with relatively high whole-rock εNd(t) values (−1.89 – +2.4), and with variable zircon Hf isotopic values (εHf(t) = +1.9 – +13.5) that can be attributed a juvenile crust with variable input of mantle material. Hf–Nd isotopic decoupling of the group 2 rocks suggests a replenishment by more mafic magmas. Samples from the entire Yili terrane show an increasing trend in εNd(t) values with younger ages that suggest an increasing involvement of melted juvenile crustal rocks. Overall, these results suggest a crustal evolutionary scenario in which: 1) pre-existing crustal components were melted and ascended to the upper crust (group 1) during the southward subduction of the Junggar-North Tianshan ocean in the early stage; and, 2) a later stage in which newly input mantle material solidified to form new arc crust that was also melted by basaltic underplating and that was probably associated with the rollback of the Junggar oceanic slab. This two-stage crustal growth model seems to be an efficient process for vertical crustal growth during the formation of an accretionary orogen.

Petrology, geochemistry, stratigraphy, zircon U–Pb geochronology and Hf isotopic compositions of subsurface lithologies in northwestern Mesa Central, Durango, Mexico: Implications for the tectonomagmatic evolution of northwestern Mexico

We present whole-rock geochemistry, LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions of subsurface lithologies in the northwestern corner of Mesa Central, Mexico. Zircon U–Pb ages identify five magmatic events: (1) ca. 162 Ma Nazas volcano-sedimentary province; (2) 88–61 Ma early stage Laramide age magmatism; (3) 57–48 Ma late stage Laramide age rhyodacitic/dacitic magmatism; (4) 46–41 Ma transition period andesitic magmatism; and (5) 32–28 Ma silicic volcanism of the Sierra Madre Occidental silicic large igneous province. A ca. 162 Ma zircon U–Pb crystallization age obtained for a dacitic flow interbedded within the Guanacevi conglomerate yielded generally negative εHf (t) composition varying from −3.3 to +0.8 suggesting magmas derived from crustal source(s). The new geochronological data, lithological features, and stratigraphic position indicate deposition of the conglomerate was synchronous with intermittent volcanism of the Nazas volcano-sedimentary province. Late stage Laramide age (ca. 51 Ma) rhyodacites/dacites yielded largely positive and heterogeneous εHf (t) compositions ranging from −1.6 to +6.1 indicating the dominant role of mantle-derived sources. These ca. 51 Ma rhyodacites/dacites have radiogenic Hf isotopic compositions comparable to those of Late Cretaceous – Early Tertiary Laramide plutons of the southern granite domain suggesting that they share similar subcontinental mantle sources. The 46–41 Ma andesite samples yielded dominantly negative and heterogeneous εHf (t) compositions varying from −4.8 to +4.0. Zircon U–Pb ages, and geochemical and Hf isotopic compositions indicate a rapid shift (~ 2 Ma) from predominantly mantle-derived magmas during the 57–48 Ma late stage Laramide age magmatism to more crustal dominated sources during the 46–41 Ma transition period magmatism. Therefore, we propose the development of a tear in the subducting slab underneath central Durango. The proposed tear in the subducting slab would have facilitated asthenospheric upwelling, providing the necessary heat to induce crustal melting during the 46–41 Ma transition period magmatism.

Geochemical and geochronological constraints on the origin and emplacement of the Shimo‐ondori diorites in Shikoku, Southwest Japan

AbstractIn this study, the LA‐ICP‐MS zircon U–Pb dating of the Shimo‐ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo‐ondori diorites are suggesstive that they could be influential for the estimate of the early‐Cretaceous tectonic evolution for the eastern Asian margin. Their whole‐rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO2 and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (εHf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge‐mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo‐ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo‐ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.

Mechanism of the historical and the ongoing Vulcanian eruptions of Ebeko volcano, Northern Kuriles.

Ebeko is one of the most active volcanoes of the Kurile island arc, producing frequent mild Vulcanian explosions with eruption clouds up to 5 km high. The volcano poses a serious threat to the Severo-Kurilsk town with a population of around 2500 inhabitants, located at a distance of only 7 km on a fan of the volcano’s laharic deposits. Here, we report an overview of the activity of the volcano in the 20th–21st centuries and the results of our geological and petrological investigations of the ongoing Vulcanian eruption that started in 2016. We have found that eruptions of Ebeko span a range of mechanisms from purely magmatic to phreatic/hydrothermal. Three of its historical eruptions (the 1934–1935, 1987–1991, and the 2016–ongoing) involved fresh magma, while during the others (1967–1971, 2009–2011) fresh magma was not erupted. Juvenile material of the ongoing eruption represents highly crystalline and highly viscous (more than 108 pa s) low-silica (56–58 wt% SiO2) andesite. Historical data and our observations of the ongoing eruption allowed us to suggest a functional model of the volcano where Vulcanian explosions are caused by shallow intrusions of small diapir-like batches of strongly crystallized and highly viscous andesitic magma ascending into water-saturated, hydrothermally altered rocks composing the volcano summit. We suggest that the diapir’s ascent is governed by their positive buoyancy. Some of the diapirs reach and breach the ground surface producing magmatic eruptions of Ebeko, while the others are stuck at the shallow subsurface level and feed intensive hydrothermal activity as well as phreatic eruptions of the volcano. Positive buoyancy of the diapirs is too weak to allow them to extrude high above the ground surface to form lava domes.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00445-020-01426-z.

Orogenic andesites and their link to the continental rock cycle

Continents are created and destroyed at convergent margins but understanding the interconnectedness of this system—a complex interplay of subaerial and tectonic erosion, volatile exchange and metamorphism, partial fusion and melt evolution—is far from complete. Here we use geochemical data from the Mexican convergent margin to reveal a strong connectivity between subducted materials and typical orogenic andesites. We find that the along-strike isotopic variations of andesitic stratovolcanoes in Mexico are mirrored by those of riverine sediments sampled along the continental margin, which presumably represent the compositions of tectonically ablated forearc crust entering the subduction zone. Isotopic modelling further indicates that the Mexican andesites represent partial melts of subduction mélanges, constituted by mixtures of altered subducted ocean crust, forearc debris and mantle peridotite in similar proportions. Remarkably, the concentrations and ratios of incompatible trace elements in the volcanoes also correlate with the major element compositions of their corresponding forearc sediments. This suggests that the modal contents of key mineral phases in the partially molten mélanges control the trace element abundances of the andesitic magmas. These findings demonstrate that a significant proportion of tectonically eroded subducted crust is not lost to the deep mantle, but effectively reincorporated into arc magmas in a closely interlaced continental rock cycle.

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge: Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins. For this reason, andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries. With respect to the origin of andesites themselves, however, there is still a hot debate on how they have acquired their geochemcial compositions. The debate is mainly centralized on the relative contributions of crustal contamination, magma differentiation and source mixing, which reaches an impasse in the past decades. The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites, leading to insufficient discrimination among them. Nevertheless, the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes. If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites, there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis. This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data. The modeling has run with four steps from early to late: (1) dehydration of the subducting oceanic crust at forearc depths; (2) partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts; (3) the generation of basaltic metasomatites (e.g., Si-excess pyroxenite) in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts; (4) the production of andesitic melts by partial melting of the basaltic metasomatites. The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites. The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources, followed by different degree of partial melting under different pressures and temperatures. Therefore, the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites. In this regard, it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

Understanding the storage conditions and fluctuating eruption style of a young monogenetic volcano: Blue Lake crater (

Blue Lake crater (<3 ka) is monogenetic volcano that produced one of the youngest eruptions in the central Oregon Cascades. Understanding monogenetic volcano behavior – from storage through eruption – is imperative in planning for future eruptions. Here we combine physical volcanology and geochemistry to determine the pre-eruptive storage conditions, ascent rate, eruption style, and deposit distribution of this young eruption. We find that the eruption of Blue Lake was initially phreatomagmatic, producing lithic-rich fall deposits and thin surge deposits and excavating the maar crater, before transitioning rapidly to a final voluminous magmatic-volatile driven explosive eruption. The mapped fall deposit has an estimated volume of 3.9 × 107 m3 (2.2 × 107 m3 DRE) which suggests a VEI of 3. Although similar in magnitude (as measured by fall deposit volume) to many other recent cinder cone eruptions in the Cascades, the Blue Lake crater eruption lacks an effusive phase. The absence of lava flows may reflect the lack of evidence for syn-eruptive magma storage at shallow levels. Indeed, corrected volatile contents of olivine-hosted melt inclusions (2.9–4.2 wt% H2O, 910–1330 ppm CO2) are strikingly uniform and indicate storage and crystallization at a restricted pressure range (average ~ 235 MPa), equating to a depth of ~8.6 km. Melt inclusion geochemistry indicates that the basaltic andesite magma cooled and crystallized ~25% during storage at this pressure. Crystals in the Blue Lake magma show evidence of mixing with, or entrainment in, a more evolved magma. Feldspar crystals have large An-rich cores (An80–85) and abrupt An-poor rims (An60–70); olivine crystals have large, broad cores (~Fo82–84) and thin rims with lower Fo and NiO contents. Diffusion modeling of olivine zoning suggests that an intrusion event occurred ~10–60 days prior to eruption. Diffusive loss of H+ from melt inclusions was minimal (<1.3 wt% H2O) during magma ascent, from which we calculate minimum ascent times from 235 MPa of <1 day. Many inclusions indicate ascent times of <3 h, corresponding to ascent rates of ~1 to >13 m/s. This study illustrates the pre-eruptive and eruptive complexities of monogenetic volcanoes and highlights the minimal warning that may precede future eruptions.

High-Mg# Adakitic Rocks Formed by Lower-crustal Magma Differentiation: Mineralogical and Geochemical Evidence from Garnet-bearing Diorite Porphyries in Central Tibet

AbstractModern arc adakites with high Mg# values (molar 100 × Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg# adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg# adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg# (Mg# = 45–56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg# &amp;gt; 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg# adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at ∼1 GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([87Sr/86Sr]i &amp;gt; 0·709, ɛNd[t] &amp;lt; −3·4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O &amp;gt;5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg# adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge.

Disequilibrium phenocrystic assemblage within dacites reveals magma mixing and stratified chamber after crustal assimilation at El Hoyazo volcano, SE Spain

In volcanic areas where evolved and primitive magmas coexist, it is common to observe evidence of magma mixing. At the Neogene silicic volcano of El Hoyazo in SE Spain, the detailed petrological and geochemical characteristics of the phenocrysts (plagioclase, biotite, amphibole and pyroxene) as well as their host dacitic lavas allow the reconstruction of the structure of the magma plumbing system. Microtextures, modal abundances, and geochemistry of zoned phenocrysts reveal a stratified magma composed of (i) high-SiO2 dacites with An-poor (<50%) plagioclase and Mg-poor biotite; (ii) high-SiO2 andesites with intermediate-An (50–70%) plagioclase and intermediate-Mg biotite; and (iii) an andesitic magma with An-rich (>80%) plagioclase, Mg-rich pyroxene, and tschermakitic hornblende, whose zoning records different crystallization depths during magma ascent. Evidence from the hornblende varieties and from mafic enclaves or inclusions indicates that the stratified chamber was repeatedly injected by hotter magmas from greater depth.

Correlation of Andesite Complexes in the Southern Framing of the Eastern Part of the Mongol–Okhotsk Orogenic Belt according to Geochronological, Geochemical, and Isotope-Geochemical Data

Abstract —The evolution of the eastern part of the Mongol–Okhotsk Orogenic Belt marks the late Mesozoic accretion–collision processes involving the Siberian and Sino–Korean plates. Tectonic restructuring in the region was accompanied by the formation of igneous complexes, which replaced each other almost without a temporal break. Andesitic magmatism was widely manifested in the southern framing of the eastern part of the belt. Its study is complicated by the isolation of volcanic fields and their confinement to the Amur–Zeya depression. Study of the geochemical and geochronological characteristics of igneous complexes there made it possible to recognize and systematize the temporal stages and substantiate the geodynamic formation of these rocks. However, there are controversial geologic objects, such as the Isikan volcanic field. We present the first data on the isotopic age and chemical and isotope-chemical compositions of the rocks of this field. 40Ar/39Ar dating showed that the integrated age of the dacitic matrix is 113.0 ± 2.6 Ma and the plateau age is 121.0 ± 1.6 Ma. In chemical composition the studied volcanics are similar to rocks of the Poyarkov volcanic complex developed within the Bureya– Jiamusi superterrane, which formed under subduction in the period 120–105 Ma. In geochemical and isotope-geochemical characteristics the volcanics are similar to rocks of igneous belts of Andean-type active continental margins.

Extraction of high-silica granites from an upper crustal magma reservoir: Insights from the Narusongduo magmatic system, Gangdese arc

AbstractThe genesis of high-silica igneous rocks is important for understanding the behavior of shallow magmatic systems. However, although many such studies have focused on the eruption of crystal-poor high-SiO2 rhyolites, the origin of high-silica granites (HSGs) has received comparatively little attention. Here, we present a detailed study of HSGs from the Narusongduo volcanic complex, Gangdese arc. Combining zircon U-Pb geochronology with stratigraphic investigations, we show that the Narusongduo magmatic system was constructed over a period of ≥3.7 Myr with or without lulls. On the basis of zircon textures and ages, diverse zircon populations, including antecrysts and autocrysts, are recognized within the HSGs and volcanic rocks. All of the igneous rocks within the Narusongduo volcanic complex have highly radiogenic Sr–Nd isotopic compositions. Our results indicate the presence of an andesitic magma reservoir in the upper crust at a paleodepth of ~8 km. Ubiquitous zircon antecrysts in the HSGs, combined with compositional similarities between the HSGs and evolved melts of the andesitic magma reservoir, indicate that the Narusongduo HSGs represent melts extracted from the shallow magma reservoir. In addition, our results suggest that magma recharge promoted the escape of high-silica melts to form the Narusongduo HSGs. This work presents an excellent case that kilometer-scale high-silica granites are the differentiated products from an upper crustal magma reservoir. It would make a contribution to contemporary debates concerning the efficiency of crystal–melt separation in upper crustal magmatic systems.

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

The petrologic diversity of volcanic rocks reflects the dynamics of magma reservoirs and the temporal evolution of magma chemistry can provide valuable information for hazard assessment. While some stratovolcanoes monotonously produce intermediate magmas (55–68 wt% SiO2), dominantly erupted magma types (e.g., basaltic andesite, andesite or dacite) frequently differ even between neighboring volcanoes. If such differences arise due to thermal maturation processes over time or are predetermined by other properties of magmatic systems remains poorly understood. This study helps to elucidate the underlying factors modulating the chemistry of the magma preferentially erupted by Nevado de Toluca volcano in Central Mexico. We present a new dataset of bulk-rock and mineral chemistry spanning the entire 1.5 Million years of the volcanos’ eruptive history. The results reveal that Nevado de Toluca dacites and minor andesite originate in a stable configuration of pre-eruptive processes and plumbing system architecture by hybridization between an upper crustal silicic mush and deeper sourced basaltic andesite magmas. Yet, a subtle trend toward increasing silica content with time (2 wt% in 1.5 Ma) and episodicity in magma hybridization conditions are observed. We use thermal simulations of pulsed magma injection to probe the controlling variables on the temporal variation and compositional mode of magma geochemistry. The results show that the subtle temporal trend toward increasing bulk-rock SiO2 content is plausibly explained by slightly dropping recharge rates and continued upper crustal reservoir growth. Our modeling also shows that the dominant composition of eruptible magmas (“petrologic mode”) can shift as a function of magma flux, extrusive:intrusive ratio and temperature of the recharge magma. A comparison of SiO2 whole rock distributions for monotonous Mexican stratovolcanoes and their peripheral cones shows that their petrologic modes vary in concert, indicating that the recharge magma chemistry or temperature is a major control on the preferentially erupted magma composition for these volcanoes.

Element and Sr isotope zoning in plagioclase in the dacites from the southwestern Okinawa Trough: Insights into magma mixing processes and time scales

The processes and time scales of magma mixing in shallow silicic magma chambers prior to eruptions in the Okinawa Trough (OT) are still poorly understood. Here, we employed in situ major-trace elements and Sr isotopic analyses, coupled with textural investigations, on plagioclase phenocrysts in the mafic magmatic enclaves (MMEs) and their host dacites from the southwestern OT to address this issue. The plagioclase phenocrysts in the MMEs have high An% contents (82–88) and Sr/Ba ratios (7.2–17.2) and relatively unradiogenic 87Sr/86Sr ratios (0.70536–0.70595), suggesting that they crystallized in the chilled basaltic andesitic magma. In contrast, plagioclase phenocrysts in the host dacites show a wide composition range, with An% contents of An47 to An93, Sr/Ba ratios of 3.4–18.0, and 87Sr/86Sr ratios of 0.70535–0.70791. The plagioclase phenocrysts are zoned, with high-An% cores characterized by resorption textures and mantled by euhedral low-An% rims. The transition between cores and rims is very abrupt, with a decrease of 30 to 40 mol% An. This shift coincides with large and abrupt changes in Sr/Ba ratios. The plagioclase phenocryst rims have low Sr/Ba values (3.4–5.1) and 87Sr/86Sr ratios (0.70632–0.70675) that are similar to those of the whole-rock dacites, suggesting that the plagioclase rims grew from the host dacitic magma. In contrast, Sr isotopes in the plagioclase cores are either radiogenic (0.70724–0.70791) or unradiogenic (0.70535–0.70595) compared with rims, combined with glomerocryst textures, consistent with their derivation from distinct deep mafic crystal mush zones, and were then entrained by ascending melts and transported rapidly to the shallow silicic magma chamber and underwent decompression during ascent, leading to plagioclase core resorption. Mg diffusion calculations indicate that incorporation of the calcic cores into the shallow silicic magma chamber experienced a long pre-eruptive storage (~600 years). The distinct Sr isotopes of the plagioclase, particularly the different compositions of titanomagnetite in the MMEs and host dacites, reflect the mafic magma injection into the silicic magma chamber and formation of the MMEs must have occurred over very short time scales before eruption to inhibit complete re-equilibration, which likely triggered the dacitic magma to finally erupt. Hence, the silicic magma in the southwestern OT contains a crystal cargo with a complex, open-system crystallization history and that magma mixing/mingling is an important process for controlling the chemical and textural diversity of these silicic magmas as well as a likely eruption trigger.

Slow Ascent of Unusually Hot Intermediate Magmas Triggering Strombolian to Sub-Plinian Eruptions

AbstractTo assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years bp to ad 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (&amp;gt;30 µm) display complex growth zoning, and only the smallest crystals (&amp;lt;30 µm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16·5 km) from hot andesitic magmas (1010–1130 °C) with low H2O content (0–1·5 wt%). Size distributions of a total of &amp;gt;60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of &amp;lt;5 cm s–1 prior to H2O exsolution, which occurs only at &amp;lt;33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 and 12 m s–1; that is, at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s–1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke changing to cylindrical conduit).

The Influence of Magma Mixing on the Composition of Andesite Magmas and Silicic Eruption Style

AbstractAlthough inhibited on established fluid mechanical grounds, extensive magma mixing can play a critical role in creating andesite magmas at volcanic arcs and triggering effusive or explosive volcanism. We use analog experiments, scaling theory, and thermodynamic modeling of natural volcanic systems to show that mechanical mixing is enhanced if an intruding magma crystallizes to acquire a yield strength. Mafic magmas intruding highly silicic reservoirs will fragment as crystal‐sized blobs/enclaves to produce textural homogeneity at the outcrop scale and heterogeneity at the scale of individual crystals. Rapid degassing from these enclaves can favor the production of permeable magmatic foams that facilitate gas loss from the reservoir and effusive volcanism. Crystallizing magmas intruding similar composition reservoirs of comparable or smaller effective viscosity will fragment at scales approaching the dike width. Sluggish degassing and consequent volatile retention in the reservoir can enhance volatile exsolution in an erupting conduit and tendency for explosive volcanism.

Transition from passive to pre-extrusion degassing of ascending andesitic magma before the lava dome-building eruption as derived from the seismic signals and tilt changes: Volcán de Colima, México, August–September 2004

Volcanoes emit volcanic gas during various types of activity. In the case of andesitic and dacitic volcanoes the high viscosity of silicic magma inhibits the release of volatiles. This increases the possibility that gas escape will occur explosively. The explosive emission of volatile constituents, presented in magmas, generates the seismic signals which may give information about the process of degassing at different stages of eruption. Transition between the eruptive episodes of lava dome-building at andesitic volcanoes represents the transition from passive degassing of the final stage of one episode to pre-extrusion degassing of the next episode. In this article we use the seismic parameters of volcanic explosions, volcanic tremor and micro-explosions, recorded at Volcán de Colima during August–September 2004, to describe the process of transition from passive to pre-extrusion degassing from episode 2 (May 2001–September 2004) to episode 3 (30 September 2004–January 2007). Variations in the tilt changes allowed us to identify three periods in development of this transition: period of passive degassing at the final stage of episode 2 (until 6 September) and two periods of pre-extrusive degassing at the initial stage of episode 3 defined by processes of inflation and deflation of volcanic structure from 6 to 30 September. Intensity of degassing of magma, expressed in the energy of volcanic explosions, decreased during transition from passive to pre-extrusive stages. During pre-extrusion periods of inflation and deflation of volcanic structure the depths of explosive events became shallower. Period of deflation was characterized also by a dominance of degassing processes occurring within the permeable cap of ascending magma and generating volcanic tremors and swarms of micro-earthquakes. Comparison of degassing of magma during pre-extrusive period and co-extrusive and post-extrusive periods showed the similarity in intensity of degassing of magma during the pre-extrusive and co-extrusive periods but a sharp increase in degassing intensity during the post-extrusive period of lava dome destruction with large explosions. It was shown also minor intensity of degassing processes just preceding the beginning of the lava dome building comparing with intensity of degassing processes just preceding the beginning of the lava dome destruction with great explosions.

Magmatic evolution of the mineralized Štiavnica volcano (Central Slovakia): Evidence from thermobarometry, melt inclusions, and sulfide inclusions

The factors that control the fertility (e.g. capacity to form an ore deposits) of a magmatic system are still a matter of debate, requiring more studies on the evolution of ore-related magmas. This study presents a detailed reconstruction of the magmatic system forming the Štiavnica volcano, the biggest mineralized center of the Western Carpathians (Central Slovakia). The Štiavnica volcano represents an interesting study object because the magmatic system was sulfide-saturated during its entire evolution, and porphyry systems with contrasting Cu/Au ratio are present in the same volcano. This situation allows to study the impact of magmatic sulfide saturation on the Cu/Au ratio of porphyry deposits. Pressure, temperature, and the compositional evolution of pre-, syn-, and post-mineralization magmas were reconstructed based on the chemical composition of phenocrysts and hosted silicate melt inclusions and sulfide inclusions. Dominantly andesitic magmas forming the volcanic and intrusive rocks of the volcano were sourced from an upper crustal (~1 to ~3 kbar) reservoir in which most phenocrysts grew from a sulfide-saturated dacitic to rhyolitic melt. This upper crustal magmatic reservoir was active during >3 My, and most ore-deposits, including Cu-Au porphyries, formed during cooling periods where the residual melt reached fluid saturation. In contrast, the Au-only porphyry deposit formed after shallow emplacement (<1 km) of a hot magma that was produced by mixing between a magma derived from a lower crustal (>8 kbar) reservoir and one from the upper crustal reservoir. Inclusions of monosulfide solid solution were found in phenocrysts crystallizing over a large range of temperature and pressure in ore-related and barren magmas suggesting that sulfide saturation doesn't play a critical role on the fertility of the magmatic system. Finally, this study reveals that composition of the magmatic sulfides does not play a direct role in the Cu/Au ratio of the formed porphyry deposits.

GEOLOGY, PETROGEOCHEMISTRY AND CRYSTALLIZATION CONDITIONS OF PLASTOVSKY GRANITOIDS MASSIVE (EASTERN URAL TROUGH)

The article describes the geological structure of the Plastovsky granitoid massif located in the northern part of the East Ural trough. It is composed of granitoids, the formation of which took place in two stages: in the first of them granodiorites, quartz diorites and biotite granites were formed, and in the second - plagiogranites. The massif lies among the metamorphic rocks of the yeremkinsky, oseysky strata and volcanic rocks of the silurian-lower carboniferous age. Chemical composition of the granitoids corresponds to normal alkaline granodiorites and plagiogranites with a predominantly sodium type of alkalinity. According to the content of Rb and Sr, they are derivatives of orogenic andesite magma, clearly fit into the field of tonalite-granodiorite formation and fall into the field of continental trondjemites and quartz diorites. The distribution of rare earth elements in granodiorites and muscovite-biotite granites of the plastovsky complex corresponds to the trondjemites of the continental margins, and in plagiogranites - to the interior of the continents. It was established that the crystallization of the Plastovsky granitoid massif occurred at a temperature of 850 - 1030 °С. The presence of melt inclusions in quartz indicates their primary magmatic nature. The water pressure in the melt was 1460 - 3380 bar, which corresponds to depths of 5 - 11 km (abyssal zone). High content of water (1.0 - 3.1 wt. %), chlorine (0.09 - 0.16 wt. %) and salts (6.2 - 10.2 wt. %) in melt inclusions in quartz granitoids, providing the extraction and transfer of many metals, they make it possible to attribute them to ore-bearing granites of the final plutonic series.