The thermal and therefore physical state of magma bodies within the crust controls the processes and time scales required to mobilize magmas before eruptions, which in turn are critical to hazard ...assessment. Crystal records can be used to reconstruct magma reservoir histories, and the resulting time and length scales are converging with those accessible through numerical modelling of magma system dynamics. The goal of this contribution is to summarize constraints derived from crystal chronometry (radiometric dating and modelling intracrystalline diffusion durations), in order to facilitate use of these data by researchers in other fields. Crystallization ages of volcanic minerals typically span a large range (10
-10
years), recording protracted activity in a given magma reservoir. However, diffusion durations are orders of magnitude shorter, indicating that the final mixing and assembly of erupted magma bodies is rapid. Combining both types of data in the same samples indicates that crystals are dominantly stored at near- or sub-solidus conditions, and are remobilized rapidly prior to eruptions. These observations are difficult to reconcile with some older numerical models of magma reservoir dynamics. However, combining the crystal-scale observations with models which explicitly incorporate grain-scale physics holds great potential for understanding dynamics within crustal magma reservoirs. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.
The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, ...result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage-that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.
Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such ...eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 103 to 105 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.
Extension within a continental back‐arc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental back‐arcs are ...poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental back‐arc basins—the Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatolia—to establish a geochemical framework for continental back‐arc magmatism. This analysis shows that continental back‐arcs yield geochemical signatures more similar to arc magmatism than intraoceanic back‐arcs do. We apply this framework to published data for Triassic‐Jurassic magmatic rocks from the Caucasus arc system, which includes a relict continental back‐arc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40Ar/39Ar and U‐Pb ages indicates Permian‐Triassic arc magmatism from ∼260 to 220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (∼220–210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from ∼180 to 140 Ma and back‐arc spreading in the Caucasus Basin from ∼180 to 160 Ma. Trace element and Sr‐Nd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back‐arcs and that the source to the Lesser Caucasus arc became more enriched at ∼160 Ma, likely from the cessation of back‐arc spreading.
Plain Language Summary
Ocean basins are consumed by the process of subduction, in which the crust and upper mantle of one tectonic plate are dragged beneath another plate. Although subduction happens when two plates are converging, about a third of subduction zones are characterized by areas where the overriding plate is stretched and thinned, sometimes forming a back‐arc basin: a small basin behind the volcanic arc associated with the subduction zone. The Caucasus Mountains within the Arabia‐Eurasia continental collision formed over the past 35 million years by closure of a poorly understood ancient back‐arc basin called the Caucasus Basin. This study compiles published data on the age and chemistry of igneous rocks within this basin, its associated volcanic arc, and comparable modern back‐arc basins to understand how extension began, which informs how the basin closed to form the Caucasus Mountains. These data are best explained by a model, previously proposed for modern back‐arc basins, in which the collision of Iran about 220–210 million years ago with Eurasia caused extension in the Caucasus region that resulted in magmatism about 180–160 million years ago. The main volcanic arc associated with the subduction zone migrated southward as a result.
Key Points
Modern continental back‐arc basins yield geochemical signatures more similar to modern arc magmatism than intraoceanic back‐arc basins do
Caucasus Basin magmatism is comparable to modern continental back‐arc basins, and back‐arc spreading likely occurred from ∼180 to 160 Ma
Caucasus Basin formation is explained by overriding‐plate extension due to along‐strike Late Triassic collision of Iran with Laurasia
We constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at Yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three ...rhyolites erupted during the (c. 170-70ka) Central Plateau Member eruptive episode-the most recent post-caldera magmatism at Yellowstone. We present super(238)U- super(230)Th crystallization ages and trace-element compositions of the interiors and surfaces (i.e. unpolished rims) of single zircon crystals from each rhyolite. We compare these zircon data with super(238)U- super(230)Th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals. Zircon age and trace-element data demonstrate that the magma reservoir that sourced the Central Plateau Member rhyolites was long-lived (150-250kyr) and genetically related to the preceding episode of magmatism, which occurred c. 256ka. The interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of Central Plateau Member magmatism or the preceding late Upper Basin Member magmatism (i.e. are antecrysts). Conversely, most zircon surfaces crystallized near the time of eruption from their host liquids (i.e. are autocrystic). The repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted Central Plateau Member rhyolites are genetically related. Sanidine separates from each rhyolite yield super(238)U- super(230)Th crystallization ages at or near the eruption age of their host magmas, coeval with the coexisting zircon surfaces, but are younger than the coexisting zircon interiors. Chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts, which considered along with their near-eruption crystallization ages suggests that nearly all Central Plateau Member sanidines are autocrystic. The paucity of antecrystic sanidine crystals relative to antecrystic zircons requires a model in which eruptible rhyolites are generated by extracting melt and zircons from a long-lived mush of immobile crystal-rich magma. In this process the larger sanidine crystals remain trapped in the locked crystal network. The extracted melts (plus antecrystic zircon) amalgamate into a liquid-dominated (i.e. eruptible) magma body that is maintained as a physically distinct entity relative to the bulk of the long-lived crystal mush. Zircon surfaces and sanidines in each rhyolite crystallize after melt extraction and amalgamation, and their ages constrain the residence time of eruptible magmas at Yellowstone. Residence times of the large-volume rhyolites (40-70km super(3)) are less than or equal to 1kyr (conservatively <6kyr), which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush. Because the lifespan of the crystal mush that sourced the Central Plateau Member rhyolites is two orders of magnitude longer than the residence time of eruptible magma bodies within the reservoir, it is apparent that the Yellowstone magma reservoir spends most of its time in a largely crystalline (i.e. uneruptible) state, similar to the present-day magma reservoir, and that eruptible magma bodies are ephemeral features.
Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat ...pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km
3
of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An
30
, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H
2
O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H
2
O and at an
f
O
2
of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and
f
O
2
s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.
Widespread mafic volcanism, elevated crustal temperatures, and plateau‐type topography in Central Anatolia, Turkey, could collectively be the result of lithospheric delamination, mantle upwelling, ...and tectonic escape. We use results from 40Ar/39Ar geochronology, basalt geochemistry, and a passive‐source broadband seismic experiment obtained in a collaborative international effort (Continental Dynamics‐Central Anatolia Tectonics) to investigate the upper mantle structure and evolution of melting conditions over an ∼2400 km2 area south and west of Hasan volcano. New 40Ar/39Ar dates for the basalts mostly cluster between 0.2 and 0.6 Ma, but some scoria cones are as old as 2.5 Ma. Basalts are dominantly Mg‐rich (Mg# = 62–71), moderately alkaline (normative Ne < 5 wt %), and, based on major and trace element signatures, derived from a peridotitic source. Covariations between radiogenic isotope and trace element signatures reveal contributions from a subduction‐related component and intraplate‐like mantle asthenosphere, as well as from ambient upper mantle. Central Anatolian basalts reflect maximum mantle potential temperatures of <1350°C and an average pressure of melt equilibration of 1.4 GPa, which are cooler and shallower than for basalts from Eastern and Western Anatolia. When considered in light of regionally slow upper mantle shear wave velocities, the mantle lithosphere may be thin and infiltrated by melts, or largely absent. An absence of secular changes in melting conditions suggests little to no lithospheric thinning over the past ∼1 Ma, despite evidence for lithospheric extension. Hasan basalts appear to be generated by decompression melting in response to the rollback of the Cyprean slab.
Key Points
The mantle lithosphere under Central Anatolia is relatively thin (<60 km) and infiltrated by melts, or largely absent
Mantle potential temperatures beneath Central Anatolia are lower than those of Eastern and Western Anatolia
Primitive basalts in Central Anatolia were generated by decompression melting of subduction‐modified and deeper intraplate‐like mantle
Abstract
The thermal histories of upper crustal magma reservoirs place important constraints on the formation, evolution, and remobilization of crustal magmas, yet quantifying thermal histories ...remains challenging. We report new in situ plagioclase trace-element data, Sr in plagioclase diffusion timescales, and plagioclase 238U-230Th-226Ra disequilibria data from Mount Saint Helens (MSH) 1980 cryptodome and 2004–2005 dacite domes to evaluate the thermal storage conditions and compositional diversity of recent MSH magmas. This approach allows us to more directly link the thermal (and therefore physical) conditions within the MSH magma reservoir to timescales of storage, thereby constraining the fraction of time 1980–2005 MSH magmas have spent in a mobile state. Plagioclase trace-element data and U-series characteristics reveal a compositionally heterogenous magmatic system beneath MSH and also require multi-stage plagioclase growth histories. The data also show that 2004–2005 dacites contain a different plagioclase population relative to the 1980 cryptodome dacite, comprised of either a compositionally (and possibly temporally) distinct plagioclase component or composed of the same plagioclase components but in significantly different proportions. Discordant plagioclase 238U-230Th and 230Th-226Ra apparent ages require a mixture of young (likely eruption related) and old (>20–40 ka) plagioclase crystals in both 1980 and 2004–2005 dacites. The low (230Th)/(232Th) in all measured 1980 and 2004–2005 plagioclase requires a significant fraction (>10–30%) of the plagioclase to be old (>10s kyr). Maximum modeled Sr diffusion timescales at 750°C range from decades to centuries for both eruptions, with a maximum of ~600 years found in 1980 cryptodome plagioclase. However, partially equilibrated Sr in the innermost exposed parts of some crystals found in both 1980 and 2004–2005 plagioclase indicate that a fraction (>20%) of plagioclase may have experienced a total of >10 kyrs at temperatures ≥750°C. Coupling Sr diffusion timescales with U-series measurements indicates that significant fraction (at least ~40%) of modeled MSH plagioclase spent <5% of their storage time at temperatures ≥750°C and thus in an easily mobilized rheological state. In contrast, the fraction of partially equilibrated plagioclase possibly spent >25–50% of their storage time at hotter conditions. Our data combined with data from other arc magmatic systems (e.g. Mt. Hood) imply that the process of remobilizing magma in arc systems toward successive eruptions requires thermally rejuvenating largely crystalline material, as opposed to sequential tapping of a persistent liquid-dominated magma body, even if successive eruptions are spaced closely in time (decades). In addition, rejuvenation events responsible for successive eruptions may sample spatially localized, but potentially overlapping, portions of the broader magma reservoir.
Using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology. Crystals utilized for
in situ dating hold great potential to link compositional ...and temporal information to better understand the evolution of a magmatic system. If micron-scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems, then new insights into long-term maintenance and storage of eruptible magma can be unlocked. This study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50,000
yrs of changing physical and chemical conditions within a magma reservoir. Trace elements (Hf and Y) in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized. Thus, different stages of growth can be associated with discrete time periods in the magmatic system.
Zircon from the two most recent rhyolite eruptions (Kaharoa and Whakatane) within the Okataina Volcanic Complex (OVC), New Zealand, have both isotopic (age) and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system. This discrete change is associated with caldera collapse and reflects a change from cold, wet source rhyolite to a relatively hotter and drier source rhyolite and back. The Kaharoa and Whakatane eruptions are separated by 4000
yrs and 15
km distance, yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse, suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 10
3 to 10
5
yrs that is periodically extracted during eruption.
► U–Th ages for zircon are connected with trace element variations at the micron-scale. ► Crystal chemistry is correlated to magmatic changes in the OVC over the past 60
kyr. ► Zircon composition and age distributions indicate different source rhyolite magmas. ► Rhyolite systems may host significant melt that is extracted before eruption. ► Combining chemical zonation and ages provide a more complete record of magmatism.
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