Magma Chamber Dynamics Research Articles

Magma Chamber Response to Ice Unloading: Applications to Volcanism in the West Antarctic Rift System.

Volcanic activity has been shown to affect Earth's climate in a myriad of ways. One such example is that eruptions proximate to surface ice will promote ice melting. In turn, the crustal unloading associated with melting an ice sheet affects the internal dynamics of the underlying magma plumbing system. Geochronologic data from the Andes over the last two glacial cycles suggest that glaciation and volcanism may interact via a positive feedback loop. At present, accurate sea-level predictions hinge on our ability to forecast the stability of the West Antarctic Ice Sheet, and thus require consideration of two-way subglacial volcano-deglaciation processes. The West Antarctic Ice Sheet is particularly vulnerable to collapse, yet its position atop an active volcanic rift is seldom considered. Ice unloading deepens the zone of melting and alters the crustal stress field, impacting conditions for dike initiation, propagation, and arrest. However, the consequences for internal magma chamber dynamics and long-term eruption behavior remain elusive. Given that unloading-triggered volcanism in West Antarctica may contribute to the uncertainty of ice loss projections, we adapt a previously published thermomechanical magma chamber model and simulate a shrinking ice load through a prescribed lithostatic pressure decrease. We investigate the impacts of varying unloading scenarios on magma volatile partitioning and eruptive trajectory. Considering the removal of km-thick ice sheets, we demonstrate that the rate of unloading influences the cumulative mass erupted and consequently the heat released into the ice. These findings provide fundamental insights into the complex volcano-ice interactions in West Antarctica and other subglacial volcanic settings.

Ground surface displacements and stress localization driven by dual magma chamber dynamics: analytical and numerical model estimates

Ground surface displacements and stress localization driven by dual magma chamber dynamics: analytical and numerical model estimates

A multi-methodological approach to record dynamics and timescales of the plumbing system of Zaro (Ischia Island, Italy)

Determining the time spans of processes related to the assembly of eruptible magma at active volcanoes is fundamental to understand magma chamber dynamics and assess volcanic hazard. This information can be recorded in the chemical zoning of crystals. Nevertheless, this kind of study is still poorly employed for the active volcanoes of the Neapolitan area (Southern Italy), in particular, for Ischia island where the risk is extremely high and this information can provide the basis for probabilistic volcanic hazard assessment. For these reasons, we acquired chemical composition on clinopyroxene crystals erupted at Ischia during the Zaro eruption (6.6 ± 2.2 ka) and performed numerical simulations of the input of mafic magma into a trachytic reservoir, in order to investigate various aspects of pre-eruptive dynamics occurring at different timescales. This event emplaced a ~ 0.1 km3 lava complex, in which the main trachytic lava flows host abundant mafic to felsic enclaves. Previous petrological investigation suggested that mafic magma(s) mixed/mingled with a trachytic one, before the eruption. In this work, the clinopyroxene zoning patterns depict the growth of crystals in different magmatic environments, recording sequential changes occurred in the plumbing system before the eruption. The evolution of the plumbing system involved a hierarchy of timescales: a few hours for magma mingling caused by mafic recharge(s) and likely occurred multiple times over a decade during which a dominant magmatic environment was sustained before the eruption. Such timescales must be considered in volcanic hazard assessment at Ischia and similar active volcanoes in densely populated areas.

Degassed versus pristine: Evaluating melt inclusions with a new ATR-FPA-FTIR calibration and water imaging method in rhyolitic melts

The concentration of pre-eruptive volatiles is a key parameter that controls the eruptive behaviour of volcanoes. Among these, water is the most abundant volatile and has a major influence on magma chamber dynamics and a primary role in driving eruptions. While accurately quantifying the pre-eruptive dissolved water concentration of magmas is a crucial step in understanding eruptive styles, we still encounter problems in this regard. One of the main challenges is in discriminating pristine melt inclusions from those that lost water due to syneruptive and post-emplacement processes. To this end, we have calibrated a Fourier Transform Infrared (FTIR) setup with a Germanium crystal objective for Attenuated Total Reflectance analyses (ATR) coupled to a Focal-Plane Array detector (FPA), which allows the measurement of single-sided polished rhyolitic melt inclusions. The samples do not require any other special preparation routine, making it a versatile method (i.e. it can be applied to samples prepared for microprobe analysis). The setup generates high-resolution (4 μm) water distribution images that allow the quantification of dissolved water with an uncertainty of ±0.35 wt%. Water diffusion inside the melt inclusions can be identified on the spectral images, allowing for a qualitative analysis that, in most cases, can easily distinguish between pristine and degassed inclusions. We apply this setup to eight units (four explosive and four effusive) from the Nisyros-Yali volcanic center, in the South Aegean Sea, and compare the results with previously published hygrometry data. The results show that even in explosive units, which are generally considered unaffected by gas loss, up to 75% of the melt inclusions lost water. Because of this, the measured water contents show a spread over a range of ~2 wt%. Dissolved water contents in pristine melt inclusions match those from previous hygrometry calculations, indicating ~4 to 5 wt% H2O. In effusive units, all the melt inclusions are degassed, yielding water contents that vary from 0 to 3.6 wt%, as compared to ~5.6 wt% H2O given by mineral-melt hygrometry. The new setup and calibration provide a fast and cheap way for measuring water contents in calc-alkaline rhyolitic melt inclusions. Furthermore, our results indicate the importance of determining the pristine versus degassed nature of inclusions before any interpretations are made.

Garnet petrochronology reveals the lifetime and dynamics of phonolitic magma chambers at Somma-Vesuvius.

Somma-Vesuvius is one of the most iconic active volcanoes with historic and archeological records of numerous hazardous eruptions. Petrologic studies of eruptive products provide insights into the evolution of the magma reservoir before eruption. Here, we quantify the duration of shallow crustal storage and document the evolution of phonolitic magmas before major eruptions of Somma-Vesuvius. Garnet uranium-thorium petrochronology suggests progressively shorter pre-eruption residence times throughout the lifetime of the volcano. Residence times mirror the repose intervals between eruptions, implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution, thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longer-lived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that long-lived deep-seated reservoirs and transient upper crustal magma chambers are common features of subvolcanic plumbing systems.

Anisotropy of magnetic susceptibility, gravimetry and geochronology of the Cerne granite, Ribeira Belt, southern Brazil

The Cerne granite is an elongated pluton in the NE-SW direction, parallel with the Ribeira Belt trend. This Neoproterozoic massif crosscuts supracrustal metamorphic rocks and has been considered late to post orogenic in the Ribeira belt evolution (Apiaí domain). The Lancinha and Morro Agudo shear zones are the major tectonic structures that frame the studied area. The southern border of the pluton is deformed by the dextral Cerne brittle-plastic shear zone, which is considered a Lancinha shear zone synthetic component. The intrusion consists mainly of fine-to medium-grained alkali feldspar granites and quartz alkali feldspar syenites. Conversely to its elongated shape, striking N20E, the rocks display isotropic structure. Analyzing field data, and integrating with petrography, gravimetry profiles, anisotropy of magnetic susceptibility (AMS) and geochronology techniques, we sought to contribute to understand the architecture of the Cerne granite and its emplacement mechanisms. The rocks display isotropic structure and very locally an incipient preferred orientation of biotite crystals. Quartz grains do not show preferred orientation but undulose extinction, and bulging-related recrystallization features are more commonly observed on low-temperature mylonites located close to the southern border of the pluton. The Cerne granite gravimetric data show two roots of two inverted teardrop-shaped plutons. ASM fabric is mostly controlled by coarse-grained magnetite, with a mean susceptibility of 7.15 mSI. The magnetic foliation (K3) shows a concentric arrangement shallowly dipping and the magnetic lineation (K1) displays subhorizontal plunging. AMS ellipsoids varies from neutral to oblate (T ≥ 0) in 75% of the sites. Concentric magnetic foliations and lineations seem to follow the plutons boundaries and reflect mostly magma chamber dynamics. The set of data suggests that the Cerne magma emplacement occurred in a place of local distension stresses associated with sinistral strike-slip system. Deep-seated tension gash probably controlled the Cerne granite ascent. SHRIMP U–Pb zircon dating provided an age of 614 ± 8 Ma attributed to the emplacement and crystallization of the granite.

Role of dyke geometry in understanding dyke-emplacement mechanisms and magma-chamber dynamics: A critical appraisal from the Chotanagpur Gneissic Complex, India

We present a detailed account of the structural aspects of dykes from the Chotanagpur Gneissic Complex (CGC) of eastern India, in order to understand dyke-emplacement mechanisms and magma-chamber dynamics. The study area comprises two temporally distinct parts of the CGC, namely the Proterozoic CGC and the Gondwana CGC. Dykes of the Gondwana CGC are divided into two types based on their intrusion into either sandstones or basalts. The size distributions of dyke lengths and of thicknesses respectively follow a power-law and an exponential distribution. Power law explains the nonlinear functional relationship between two quantities in which one quantity varies as the power of the other. The essential properties of power-law are that it is scale-invariant and represents the universality of a problem. An exponential distribution explains that a quantity sharply decreases with respect to other quantities over a period of time. We calculate the overpressure conditions for 60 selected dykes using their aspect ratios. Calculated overpressure is utilized to estimate the depth of magma origin, which in turn is used to estimate the magnitude of maximum and minimum principal stresses. Based on this collective information, we propose a conceptual model of dyke emplacement. Consequently, the results have critical implications in understanding magma chamber dynamics.

Stress fields around magma chambers influenced by elastic thermo-mechanical deformation: implications for forecasting chamber failure

Defining the conditions that lead to the rupture of a magma chamber is essential to forecast eruptions. So far, models simulating magma chamber dynamics have neglected the effects of elastic thermal expansion in the host rocks surrounding a new injection of magma, focusing instead primarily on elastic-plastic deformation and more recently, on visco-elastic deformation. Here we fill this gap by building a suite of elastic thermo-mechanical models to determine the stress field around a variably heated crustal magma chamber. We first consider linear elastic mechanical models with only the effect of magma pressure. We then present purely thermal models simulating heat distribution around a heated chamber. Finally, we present coupled linear elastic thermo-mechanical models that highlight the influence of temperature on the distribution of crustal stresses. Results show that thermal expansion–induced stresses generate two competing consequences: (1) they increase the level of shear stress around the magma chamber and (2) they partially suppress the level of tensile stress generated by the magmatic pressure. These competing effects influence the short-timescale conditions required for the failure of immature magmatic systems and hence the nucleation of dikes which may ultimately feed eruptions during unrest. Therefore, soon after a new magmatic recharge event, the contribution of temperature increase in the host rocks, following the new influx of magma into a crustal magma chamber, should be considered.

Magmatic karst reveals dynamics of crystallization and differentiation in basaltic magma chambers

An understanding of magma chamber dynamics relies on answering three important yet highly controversial questions: where, why, and how magma chambers crystallize and differentiate. Here we report on a new natural phenomenon—the undercut-embayed chamber floor in the Bushveld Complex—which allows us to address these questions. The undercut-embayed floor is produced by magmatic karstification (i.e. erosion by dissolution) of the underlying cumulates by replenishing magmas that form basal flows on the chamber floor. This results in a few metres thick three-dimensional framework of spatially interconnected erosional remnants that separate the floor cumulates from the overlying resident melt. The basal flow in this environment is effectively cooled through the floor, inducing heterogeneous nucleation and in situ growth against much of its three-dimensional framework. The solidification front thus propagates in multiple directions from the surfaces of erosional remnants. Fractional crystallization may occur within this environment by convective removal of a compositional boundary layer from in situ growing crystals and is remarkably efficient even in very confined spaces. We propose that the way magma crystallizes and differentiates in the undercut-embayed chamber floor is likely common for the evolution of many basaltic magma chambers.

Volcano dynamics vs tectonics on Mars: evidence from Pavonis Mons

Volcanic activity is widespread within the inner Solar system and it can be commonly observed on rocky planets. In this work, we analyse the structures of Pavonis Mons in the Tharsis volcanic province of Mars by performing structural mapping, azimuth, and topographic distribution of linear features on the flanks of Pavonis, such as grabens and pit chains. We tested whether their formation is to be ascribed to the volcano dynamics and magmatic activity or the tectonics. Through the length size distribution and fractal clustering analyses of the structural features, we found that large grabens are vertically confined in the upper mechanical layers of the brittle crust whereas pit chains penetrate the whole crust up to the magmatic source, indicating that they can be considered the main feeders of Pavonis Mons. We inverted the topography with dykes and faults models to test whether grabens at the surface are the expression of intrusions at depth and we suggest that thin dykes inducing normal faulting are the most likely mechanism. Furthermore, two azimuthal distribution of the grabens are identified: concentric grabens occur on the volcano summit while linear grabens at its base show NE-SW trend as the Tharsis Mons volcanos alignment. The occurrence of linear grabens suggests that Pavonis likely experienced a phase of active rifting with the formation of such structures, followed by a phase of volcano growth and concentric magma intrusions when volcano and magma chamber dynamics prevailed.

Magma buoyancy and volatile ascent driving autocyclic eruptivity at Hekla Volcano (Iceland)

AbstractVolcanic eruptions are typically accompanied by ground deflation due to the withdrawal of magma from depth and its effusion at the surface. Here, based on continuous high‐resolution borehole strain data, we show that ground deformation was absent during the major effusion phases of the 1991 and 2000 eruptions of Hekla Volcano, Iceland. This lack of surface deformation challenges the classic model of magma intrusion/withdrawal as source for volcanic ground uplift/subsidence. We incorporate geodetic and geochemical observables into theoretical models of magma chamber dynamics in order to constrain quantitatively alternative co‐ and intereruptive physical mechanisms that govern magma propagation and system pressurization. We find the lack of surface deformation during lava effusion to be linked to chamber replenishment from below whilst magma migrates as a buoyancy‐driven flow from the magma chamber towards the surface. We further demonstrate that intereruptive pressure build‐up is likely to be generated by volatile ascent within the chamber rather than magma injection. Our model explains the persistent periodic eruptivity at Hekla throughout historic times with self‐initiating cycles and is conceptually relevant to other volcanic systems.

Geochemical and radiogenic isotope probes of Ischia volcano, Southern Italy: Constraints on magma chamber dynamics and residence time

The active volcano of Ischia, an island off-shore the city of Naples, Southern Italy, has a discontinuous volcanic activity characterized by caldera-forming paroxysmal eruptions, lava flows, and lava domes, and thus offers the opportunity to study the complexity of magma storage, differentiation, and extraction mechanisms in a long-lived magma reservoir. The overall geochemical composition of erupted magmas varies from shoshonite to latite and trachyte/trachyphonolite. Their Sr and Nd, isotope composition variation is typical of subduction-related magmas, akin to other potassic magmas of the Neapolitan District, and there is a complete overlap of radiogenic isotope composition among shoshonite, latite, and trachyte/trachyphonolite. The lack of systematic radiogenic isotope covariation during differentiation suggests that the radiogenic isotope variability could be a signature of each magma pulse that subsequently evolved in a closed-system environment. Erupted magmas record a recurrent evolutionary process consisting of two-step fractional crystallization along similar liquid lines of descent for each magma pulse, suggesting near steady-state magma chamber conditions with balanced alternating periods of replenishment, differentiation, and eruption. The dominant role of fractionating feldspars determines a significant depletion of Sr ( 200) in the residual trachyte magma. Several more-evolved trachytes have anomalous radiogenic 87 Sr/ 86 Sr i (>0.707) coupled with high 87 Rb/ 86 Sr (>50), all other geochemical and isotopic characteristics being similar to normal 87 Sr/ 86 Sr i trachytes at the same degree of evolution. This radiogenic Sr isotope signature is not consistent with assimilation of crustal material and demands for a time-related in-growth of 87 Sr during storage within the magma chamber. Rb-Sr isochrons on separated mineral-groundmass pairs provide robust constraints on a prolonged pre-eruptive history ranging from a few tens to hundreds of thousands of years at relatively low temperature (~750 °C). Remarkably, also normal trachytes with high 87 Rb/ 86 Sr (>200) yield a magma residence time from some 4 to 27 kyr, implying that the long-lived history of Ischia magmas is not limited to the anomalous 87 Sr/ 86 Sr i trachytes. This long-lived history could be a characteristic feature of the magma chamber reservoir of this active volcano, which other volcanic products (i.e., shoshonite and latite) cannot disclose due to their lower Rb/Sr (i.e., low 87 Sr in-growth rate) and higher magma storage temperature (>900 °C) (i.e., rapid Sr isotope homogenization via diffusion). The magma chamber dynamics of the active volcano of Ischia, probed on the basis of geochemical and radiogenic isotope tools, is consistent with recent models of complex magma chamber reservoirs made up of multiple discrete melt pockets, isolated by largely crystalline mush portions, maintained in a steady-state thermal flux regime with no mass exchange, and with reactivation shortly before eruption.

How temperature-dependent elasticity alters host rock/magmatic reservoir models: A case study on the effects of ice-cap unloading on shallow volcanic systems

In geodynamic numerical models of volcanic systems, the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir. We assess this assumption by deriving an empirical relation between elastic parameters and temperature for Icelandic basalts by conducting a set of triaxial compression experiments between 200 °C and 1000 °C. Results show a significant decrease of Young's modulus from ∼38 GPa to less than 4.7 GPa at around 1000 °C. Based on these laboratory data, we develop a 2D axisymmetric finite-element model including temperature-dependent elastic properties of the volcanic basement.As a case study, we use the Snæfellsjökull volcanic system, Western Iceland to evaluate pressure differences in the volcanic edifice and basement due to glacial unloading of the volcano. First, we calculate the temperature field throughout the model and assign elastic properties accordingly. Then we assess unloading-driven pressure differences in the magma chamber at various depths in models with and without temperature-dependent elastic parameters. With constant elastic parameters and a sharp transition between basement and magma chamber we obtain results comparable to other studies. However, pressure changes due to surface unloading become smaller when using more realistic temperature-dependent elastic properties. We ascribe this subdued effect to a transition zone around the magma chamber, which is still solid rock but with relatively low Young's modulus due to high temperatures. We discuss our findings in the light of volcanic processes in proximity to the magma chamber, such as roof collapse, dyke injection, or deep hydrothermal circulation. Our results aim at quantifying the effects of glacial unloading on magma chamber dynamics and volcanic activity.

Pre-eruptive magmatic processes re-timed using a non-isothermal approach to magma chamber dynamics.

Constraining the timescales of pre-eruptive magmatic processes in active volcanic systems is paramount to understand magma chamber dynamics and the triggers for volcanic eruptions. Temporal information of magmatic processes is locked within the chemical zoning profiles of crystals but can be accessed by means of elemental diffusion chronometry. Mineral compositional zoning testifies to the occurrence of substantial temperature differences within magma chambers, which often bias the estimated timescales in the case of multi-stage zoned minerals. Here we propose a new Non-Isothermal Diffusion Incremental Step model to take into account the non-isothermal nature of pre-eruptive processes, deconstructing the main core-rim diffusion profiles of multi-zoned crystals into different isothermal steps. The Non-Isothermal Diffusion Incremental Step model represents a significant improvement in the reconstruction of crystal lifetime histories. Unravelling stepwise timescales at contrasting temperatures provides a novel approach to constraining pre-eruptive magmatic processes and greatly increases our understanding of magma chamber dynamics.

A multi-data stream assimilation framework for the assessment of volcanic unrest

Active volcanoes pose a constant risk to populations living in their vicinity. Significant effort has been spent to increase monitoring and data collection campaigns to mitigate potential volcano disasters. To utilize these datasets to their fullest extent, a new generation of model-data fusion techniques is required that combine multiple, disparate observations of volcanic activity with cutting-edge modeling techniques to provide efficient assessment of volcanic unrest. The purpose of this paper is to develop a data assimilation framework for volcano applications. Specifically, the Ensemble Kalman Filter (EnKF) is adapted to assimilate GPS and InSAR data into viscoelastic, time-forward, finite element models of an evolving magma system to provide model forecasts and error estimations. Since the goal of this investigation is to provide a methodological framework, our efforts are focused on theoretical development and synthetic tests to illustrate the effectiveness of the EnKF and its applicability in physical volcanology. The synthetic tests provide two critical results: (1) a proof of concept for using the EnKF for multi dataset assimilation in investigations of volcanic activity; and (2) the comparison of spatially limited, but temporally dense, GPS data with temporally limited InSAR observations for evaluating magma chamber dynamics during periods of volcanic unrest. Results indicate that the temporally dense information provided by GPS observations results in faster convergence and more accurate model predictions. However, most importantly, the synthetic tests illustrate that the EnKF is able to swiftly respond to data updates by changing the model forecast trajectory to match incoming observations. The synthetic results demonstrate a great potential for utilizing the EnKF model-data fusion method to assess volcanic unrest and provide model forecasts. The development of these new techniques provides: (1) a framework for future applications of rapid data assimilation and model development during volcanic crises; (2) a method for hind-casting to investigate previous volcanic eruptions, including potential eruption triggering mechanisms and precursors; and (3) an approach for optimizing survey designs for future data collection campaigns at active volcanic systems.

Volcano–tectonic interactions as triggers of volcanic eruptions

Surface displacements and edifice deformations at active volcanoes can occur when magma reservoirs begin to inflate as new magma enters them. Volcanoes are also subjected to a variety of external lithospheric stresses that are thought to be responsible for triggering volcanic unrest or modifying ongoing activity. However, despite many observations, it is uncertain whether these phenomena can actually interfere with magma chamber dynamics since it is not clear why some volcanoes are more subjected to these interactions than others. In order to determine whether external stresses interfere with volcanic activity, a viscoelastic 3D Finite Element Mogi-based model of Kīlauea volcano's magma chamber was implemented. First, the model was used to replicate an inflation cycle without external stresses. Its results were then compared with the ones obtained if the same model was subjected to tidal stress modulation and a strong (Mw=7.7) tectonic earthquake. The model showed that tidally-induced pressurization is not sufficiently large to modify the pressure in a 5km deep volcanic magma chamber, but it suggested how the magma chamber pressure build-up rate can be influenced by tidal pressurization and thus why some volcanoes seem to experience tidal interferences more than others. Furthermore, the model's results suggested why magma chambers are about the same size as calderas both on the Earth and on other Solar System silicate planets. System. Finally, it was used to propose an explanation of why a short-lived eruption at Kīlauea volcano, Hawai’i, began 30min after the 1975 magnitude 7.7 (Mw) Kalapana earthquake.

MODELING VOLCANIC PROCESSES: THE PHYSICS AND MATHEMATICS OF VOLCANISM

Edited by Sarah A. Fagents, Tracy K.P. Gregg, and Rosaly M.C. Lopes. (2013) Cambridge University Press, U.K., 431 p. $80.00 (hardback). ISBN 978-0-521-89543-9. $62.00 Adobe eBook also available. Modeling Volcanic Processes is a collection of 17 chapters, each written by different expert(s), which together cover a broad range of physical processes from magma chamber dynamics to tephra sedimentation to volcano acoustics. The perspective is physical with a focus on modeling. Each chapter not only provides a clear and accessible introduction to the topic before moving on to more quantitative aspects, but also brings the reader up to date and considers future directions in the field. All the material is well referenced, which allows the reader to follow-up on particular facts and topics; there are also exercises to try, with solutions provided along with other supporting material on a website supported by Cambridge University Press. When I first flipped through the book and spotted the exercises at the end of each chapter, I thought that it was probably aimed at students …

Lithospheric flexure and volcano basal boundary conditions: keys to the structural evolution of large volcanic edifices on the terrestrial planets

Abstract Large volcanic edifices constitute enormous loads at the surfaces of planets. The lithosphere, the mechanically strong outer layer of a planet, responds to growing edifice loads by flexing. The shape of this lithospheric flexure and the resulting stress state exert critical influences on the structure of the evolving edifices, which in turn feed back into the flexural response. Flexural subsidence of the lithosphere forms topographical moats surrounding volcanoes that are partially to completely filled by landslide debris, volcaniclastic materials and sediments, or by relatively flat aprons of volcanic flows. Flexure creates a characteristic ‘dipole’ state of stress that influences subsequent magma ascent paths and chamber dynamics in the lithosphere. Compression in the upper lithosphere can inhibit magma ascent and favour the development of oblate magma chambers or sill complexes. This compression can be transferred into the edifice unless a décollement allows the volcano base to slip over the underlying lithosphere; generally, basal décollements are found to operate via high pore-fluid pressure in a clay sediment-based layer. Volcanoes lacking such a layer, regardless of the thickness of the basal sediments, lack basal décollements and, thus, tend to be limited in size by compressive stresses adverse to magma ascent.

Magma chamber dynamics recorded by oscillatory zoning in pyroxene and olivine phenocrysts in basaltic lunar meteorite Northwest Africa 032

Oscillatory zoning in silicate minerals, especially plagioclase, is a common feature found in volcanic rocks from various terrestrial tectonic settings, but is nearly absent in the lunar environment. Here we report backscattered electron images, quantitative wavelength-dispersive spectrometry (WDS) analyses, and qualitative WDS elemental X-ray maps that reveal oscillatory zoning of Mg, Ca, Fe, Ti, Al, Cr, and Mn in euhedral pyroxene phenocrysts, and faint oscillatory zoning of P in olivine phenocrysts in basaltic lunar meteorite Northwest Africa (NWA) 032. This is only the third known occurrence of oscillatory zoning in lunar silicate minerals. Zoning bands in pyroxene range from ~3–5 μm up to ~60 μm in width, but are typically ~10–20 μm in width. Oscillatory bands are variable in width over short distances, often within a single grain. Most oscillatory bands preserve a euhedral form and have sharp edges; however some bands have jagged or uneven edges indicative of resorption surfaces. The short-scale oscillatory nature of the zoning in pyroxene is overprinted on longer-scale core to rim normal magmatic zoning from pigeonite to augite compositions. Oscillatory zoning of P in olivine is faint and only resolvable with high beam current (400 nA) mapping. Bands of higher P are typically only a few micrometers in width, and although they preserve a euhedral form, they are not traceable around the full circumference of a grain and have variable spacing. Resorption surfaces, longer-scale normal magmatic zoning, and relatively thick oscillatory bands are indicative of the formation of these chemical oscillations as a result of variable magma composition. Pyroxenes likely experienced variable liquid compositions as a result of convection in a differentially cooling, chemically stratified magma chamber. Periodic replenishments of progressively decreasing volumes of primitive parental magma are also permissible and may have enabled convection. In a convection model, Mg-rich bands reflect growth in the lower, warmer, more crystal-poor regions of the chamber, whereas Ca-Al-Ti-Cr-rich bands reflect growth in the upper, cooler, more crystal-rich regions of the chamber. The limited duration of crystallization in the magma chamber and the slow diffusion rates of multiple elements among multiple crystallographic sites in clinopyroxene, combined with fast cooling upon eruption, act to preserve the oscillatory zoning. Oscillatory zoning of P in olivine is a product of solute trapping resulting from the slow diffusion of P in silicate melts and minerals, and relatively fast magma cooling rates that may be related to magma chamber convection. Differential cooling of the chamber and the fast cooling rates within the chamber are likely a product of the thermal state of the lunar crust at 2.93 Ga when NWA 032, which is currently the youngest dated lunar igneous rock, erupted onto the surface of the Moon.

Bowen, Dufek, and Shelly Receive 2012 James B. Macelwane Medals: Citation

Joe Dufek is an extraordinary young scientist whose research is characterized by deep intellectual content and rigor. He has contributed significantly to our understanding of the dynamics and petrology of magmatic systems, and to physical volcanology, tectonophysics, and planetary volcanism. Even at such an early stage in his career, Joe's contributions have transformed our understanding of magma chamber dynamics and the physics of volcanic eruption.