Magma Chamber Evolution Research Articles

Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei, Italy)

The Campi Flegrei (Campanian Region, Italy) experienced two cataclysmic caldera-forming eruptions which produced the Campanian Ignimbrite (39 ka, CI) and the Neapolitan Yellow Tuff (15 ka, NYT). We studied the minor eruptions before both these large events to understand magma chamber evolution leading towards such catastrophic eruptions. Major, trace element, and Sr and Nd isotope compositions of pre-Campanian Ignimbrite and pre-Neapolitan Yellow Tuff products define distinct geochemical groups, which are here interpreted as distinct magma batches. These batches do not show any transitional trend towards the CI and NYT eruptions. The CI and NYT systems are decoupled geochemically and isotopically. At least one of the pre-CI and one of the pre-NYT erupted magma batches qualifies as mixing endmembers for the large CI and NYT eruptions, and thus, must have been stored in reservoirs for some time to remain available for the CI and NYT eruptions. The least evolved, isotopically distinct magma compositions that are typical of the last phases of the NYT and CI eruptions did not occur before caldera-forming events. Based on the new data, we propose the following scenario: Multiple magma chambers with distinct compositions existed below the Campi Flegrei before the CI and NYT eruptions and remained generally separated for some time unless new magma was recharged. In each case, one of the residing magma reservoirs was recharged by a new large-volume magma input of intermediate composition from a deeper differentiating magma reservoir. This may have triggered the coalescence of the previously separated reservoirs into one large chamber which fed the cataclysmic caldera-forming eruption. Large magma chambers in the Campi Flegrei may therefore be ephemeral features, interrupted by periods of evolution in individual, separated magma reservoirs.

Bulk-rock geochemistry and plagioclase zoning in lavas exposed along the northern flank of the Western Blanco Depression (Northeast Pacific): Insight into open-system magma chamber processes

The present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the Blanco Transform Fault, Northeast Pacific. The studied area consists of a small basin, the Western Blanco Depression (WBD), extending from the southern end of the Juan de Fuca ridge to a pseudofault trace 60 km eastward. The Northern Scarp of the WBD comprises a volcanic unit overlying a sheeted-dike complex. Major and trace element data, coupled with Sr–Nd isotope ratios, reveal a two-component mantle source, composed by an isotopically depleted matrix variably veined by more enriched material. One chemical group (NS2), indistinguishable from the other Northern Scarp samples on the basis of trace element data, has an unusually depleted isotopic composition typical of a nearly pure mantle end-member. Some cogenetic samples of the Northern Scarp have been used to constrain the differentiation modalities. Anorthite and MgO content profiles in plagioclase xenocrysts and phenocrysts reveal (i) the existence of H 2O-bearing evolved melts in the mushy zones and (ii) the occurrence of mixing process between these melts and anhydrous mafic liquids. The hydration is supported by other petrographic features such as high magmatic fO 2 values, calculated from Fe–Ti oxide pairs, and the presence of pyroxene inclusions in plagioclase phenocrysts. Mixing, consistent with the existence of Ni-rich ferrobasalts, is interpreted to be the consequence of the reservoir refilling by mafic liquids (Mg# = 70). These petrological and geochemical evidences are combined with the evolution of Mg# with depth to suggest a periodic open-system magma chamber evolution beneath the southern end of the Juan de Fuca ridge.

Short-term (1998–2006) fluctuations of Pb isotopes at Piton de la Fournaise volcano (Reunion Island): Origins and constraints on the size and shape of the magma reservoir

Major-trace element concentrations and Pb isotopic compositions of 64 lava samples erupted between November 2002 and July 2006 at Piton de la Fournaise volcano are used to better document temporal–compositional trends since the start of the eruptive cycle (March 1998). After 6 years of continuous drift towards less radiogenic values, the temporal trend in Pb isotopes suddenly reversed in 2005 and rapidly reached the signature of the first lavas erupted in 1998. In agreement with previous inferences, the 1998–2004 variations may reflect increasing contamination of the lavas, probably through deepening of magma storage from within the edifice down to the oceanic basement. The 2005 isotopic shift is interpreted as refilling the plumbing system with less-contaminated, presumably deeper magmas. The volume of lava erupted between 1998 and 2004 (0.16 km 3) would thus provide a lower limit for magma reservoir size. Although periods of quietness dominate between 1998 and 2006 (80% of time), Pb isotopic compositions plotted versus cumulated eruption duration reveal remarkably smooth variations, which are not compatible with assimilation in an evolving magma chamber. Instead, the temporal trends could result from progressive emptying of a vertically zoned magma conduit, the compositional variations with depth reflecting the heterogeneity of the bed-rocks with which magmas interacted. The 1998–2004 compositional evolution of Piton de la Fournaise lavas likely results from injection of a homogenous batch of plume melts within a dyke system followed by depth-dependent crustal contamination, the late-erupting lavas stored at the level of the oceanic crust being the most contaminated. However, preserving the imprint of depth-dependent contamination in temporal–compositional trends is possible only if the time of storage is long compared to the time it takes to magmas to migrate from their storage location to the surface. Extended magma storage may have occurred during the nearly 6 years long period of quietness that preceded the 1998 eruptive cycle.

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from Rishiri Volcano, northern Japan, by dating of these lavas using U^Th radioactive disequilibrium and C dating methods, in combination with theoretical analyses. We obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by C dating of charcoals. The eruption age of the andesitic lavas was estimated to be 20 2 3 1ka, utilizing a whole-rock isochron formed by U^Th fractionation as a result of degassing after lava emplacement. Because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber, the difference of the ages (i.e. 9 kyr) is a timescale of magmatic evolution.The thermal and chemical evolution of the Rishiri magma chamber was modeled using mass and energy balance constraints, as well as quantitative information obtained from petrological and geochemical observations on the lavas. Using the timescale of 9 kyr, the thickness of the magma chamber is estimated to have been about 1 7 km.The model calculations show that, in the early stage of the evolution, the magma cooled at a relatively high rate (40 18C/year), and the cooling rate decreased with time. Convective heat flux from the main magma body exceeded 2W/m when the magma was basaltic, and the intensity diminished exponentially with magmatic evolution. Volume flux of crustal materials to the magma chamber and rate of convective melt exchange (compositional convection) between the main magma and mush melt also decreased with time, from 0 1m/year to 10 3 m/year, and from 1m/year to 10 2 m/year, respectively, as the magmas evolved from basaltic to andesitic compositions. Although the mechanism of the cooling (i.e. thermal convection and/or compositional convection) of the main magma could not be constrained uniquely by the model, it is suggested that compositional convection was not effective in cooling the main magma, and the magma chamber is considered to have been cooled by thermal convection, in addition to heat conduction.

Uranium-series dating applications in natural environmental science

Uranium-series (U-series) analyses are an essential component of many research projects in Earth and environmental science, oceanography, hydrology and science-based archaeology. Topics range from magma chamber evolution and volcanic hazard prediction, global climatic change through dating of authigenic carbonate deposits, human evolution through dating of bone, to the study of groundwater evolution. The U-series decay chains contain many elements that can be fractionated in environmental and geological processes. Half-lives of radioactive isotopes of such elements range from seconds to many millennia and application depends on the natural timeframe of the process or the elapsed time. This review will be limited to some aspects of the 238U– 234U– 230Th– 226Ra system with half-lives of 245 kyr, 76 kyr and 1.6 kyr, respectively. In environmental systems, fractionation of uranium and thorium is a very efficient process because thorium is extremely insoluble while hexavalent uranium in oxidising conditions is relatively soluble. Many authigenic precipitates of calcite contain virtually no thorium and inorganic precipitates have U/Ca ratios very similar to the ratio of dissolved uranium and calcium. Almost no radiogenic 230Th in the precipitate means that the radiogenic clock starts effectively at zero. However, pure authigenic precipitates are rare and many contain some allogenic material, mostly silicate with 238U in secular equilibrium with significant 230Th. Some of the characteristics of different types of samples and various methods to accommodate or correct for ‘inherited’ 230Th will be discussed. Authigenic material should remain a ‘closed system’ with respect to the relevant isotopes but that requirement is sometimes difficult to maintain because radioactive decay results in damage to the crystal lattice of the host mineral. Consequences of this ‘recoil effect’ and correction schemes will be discussed. Dating archaeological bone based on the systematics of diffusion and adsorption to effectively model ‘open system’ behaviour is also included.

Midcrustal emplacement of the Sausfjellet pluton, central Norway: Ductile flow, stoping, and in situ assimilation

Midcrustal (25–30 km) emplacement of the dioritic Sausfjellet pluton into rocks of the Helgeland Nappe Complex, central Norway, occurred in two stages. Stage 1 consists of two-pyroxene hornblende gabbro and diorite. Stage 2 is asymmetrically zoned, with a modally layered central zone of diorite and anorthosite and a western/annular zone of quartz-bearing monzodioritic rocks. Igneous layering was locally attenuated, folded, and boudinaged in the hyper-solidus state. The magmatic foliation trajectory pattern in the pluton defi nes a shallowly southwest-plunging synform that crosscuts compositional zones. Mineral lineations plunge shallowly to moderately to the southwest. The pluton intruded a major lithologic boundary within the nappe; migmatitic pelitic gneisses are the main host rocks to the western part of the pluton, whereas the eastern and central parts are hosted predominantly by metacarbonate rocks. Calc-silicate, marble, quartzo-feldspathic, and dioritic xenoliths (up to 200 m in length) are present throughout the pluton; they are most common in Stage 1 and the central zone of Stage 2. Metapelitic xenoliths are conspicuously absent. Ductile fl ow during emplacement produced an ~1-kmwide structural aureole in which host-rock structures were defl ected into subparallelism with the steeply inward-dipping margin of the pluton. Tight antiforms developed along the northeastern, southeastern, and southwestern margins. Amphibolite-grade shear zones in the host rocks preserve plutonside-up kinematic indicators. In addition to the abundance of dioritic, calc-silicate, and quartzo-feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western/annular zone, regional discordance of the pluton–host-rock contacts indicates that stoping was also an important emplacement process at midcrustal depths. Following magma emplacement, foundering of the central portion of the chamber combined with possible ca. 445 Ma regional contraction produced the map-scale synform defi ned by magmatic foliations and igneous layers. This study demonstrates that stoping and assimilation may occur simultaneously with host-rock ductile fl ow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent.

Petrologic evolution of Pre-Unzen and Unzen magma chambers beneath the Shimabara Peninsula, Kyushu, Japan: Evidence from petrography and bulk rock chemistry

The volcanic history of the Shimabara Peninsula, Kyushu, Japan, is divided into two stages; Pre-Unzen volcano stage (4 Ma-500 ka) and Unzen volcano stage (500 ka-present). Pre-Unzen volcanic rocks comprise olivine basalt and two-pyroxene andesite lava flows and pyroclastics. The similarity of trace elements chemistry indicates that Pre-Unzen basalts evolved from different primary magmas originated in the same mantle source. They were differentiated by olivine-dominant fractional crystallization and crustal assimilation. This process produced parental magmas for Pre-Unzen andesite. The evolution of Pre-Unzen andesites can be explained by the combination of plagioclase + pyroxenes + magnetite fractional crystallization and crustal assimilation. Unzen volcanic rocks are composed of hornblende andesite to dacite lava domes, lava flows and pyroclastics. They have petrographical and geochemical features indicating that they were produced by magma mixing. Mafic inclusions are commonly included in Unzen volcanic rocks and show evidence of hybridization between aphyric basalt and phenocryst-rich dacite magma in various ratios. The existence of mafic inclusions with positive Nb anomalies in spidergram indicates an injection of ocean-island type basaltic magmas. The estimated mafic endmember for the dacite magma is an evolved basalt with MgO = 5.5 wt.%. This corresponds to the most evolved composition of the Pre-Unzen basalt and suggests continuous basaltic magma plumbing system throughout the eruptive history of the Shimabara peninsula.

Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA FC)

Significant petrogenetic processes governing the geochemical evolution of magma bodies include magma Recharge (including formation of ‘quenched inclusions’ or enclaves), heating and concomitant partial melting of country rock with possible ‘contamination’ of the evolving magma body (Assimilation), and formation and separation of cumulates by F ractional Crystallization (RAFC). Although the importance of modeling such open-system magma chambers subject to energy conservation has been demonstrated, the effects of concurrent removal of magma by eruption and/or variable assimilation (involving imperfect extraction of anatectic melt from wall rock) have not been considered. In this study, we extend the EC-RAFC model to include the effects of Eruption and variable amounts of assimilation, Ax. This model, called EC-E 0RAcFC, tracks the compositions (trace elements and isotopes), temperatures, and masses of magma body liquid (melt), eruptive magma, cumulates and enclaves within a composite magmatic system undergoing simultaneous eruption, recharge, assimilation and fractional crystallization. The model is formulated as a set of 4 þ t þ i þ s coupled nonlinear differential equations, where the number of trace elements, radiogenic and stable isotope ratios modeled are t, i and s, respectively. Solution of the ECE 0RAcFC equations provides values for the average temperature of wall rock (Ta), mass of melt within the magma body (Mm), masses of cumulates (Mct), enclaves (Men) and wall rock (M o a) and the masses of anatectic melt generated (M a ) and assimilated (cM a ). In addition, t trace element concentrations and i þ s isotopic ratios in melt and eruptive magma (Cm, em, dm), cumulates (Cct, em, dm), enclaves (Cen, er , d o r ) and anatectic melt (Ca, ea, d o a) as a function of magma temperature (Tm) are also computed. Input parameters include the (user-defined) equilibration temperature (Teq), a factor describing the efficiency of addition of anatectic melt (c) from country rock to host magma, the initial temperature and composition of pristine host melt (Tom, C o m, em, d o m), recharge melt (T o r , C o r , er , dor ) and wall rock (T o a, C o a, ea, d o a), distribution coefficients (Dm, Dr, Da) and their temperature dependences (DHm, DHr, DHa), latent heats of transition (melting or crystallization) for wall rock (Dha), pristine magma (Dhm) and recharge magma (Dhr) as well as the isobaric specific heat capacity of assimilant (Cp,a), pristine (Cp,m) and recharge (Cp,r) melts. The magma recharge mass and eruptive magma mass functions,Mr(Tm) andMe(Tm), respectively, are specified a priori. Mr(Tm) and Me(Tm) are modeled as either continuous or episodic (step-like) processes. Melt productivity functions, which prescribe the relationship between melt mass fraction and temperature, are defined for end-member bulk compositions characterizing the local geologic site. EC-E 0RAcFC has potential for addressing fundamental questions in igneous petrology such as: What are intrusive to extrusive ratios (I/E) for particular magmatic systems, and how does this factor relate to rates of crustal growth? How does I/E vary temporally at single, long-lived magmatic centers? What system characteristics are most profoundly influenced by eruption? What is the quantitative relationship between recharge and assimilation? In cases where the extraction efficiency can be shown to be less than unity, what geologic criteria are important and

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex II: evidence for magma mixing and magma chamber evolution

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350 MPa, temperatures of around 1,000 °C and water contents in excess on 6 wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250 MPa, 950 °C and water contents of 5.5 wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150 MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3 m.y.). The mixing process probably triggered eruptions as a result of volatile exsolution.

Magmatic differentiation examined with a numerical model considering multicomponent thermodynamics and momentum, energy and species transport

Magmatic differentiation processes in a cooling magma body were examined using a numerical model considering multicomponent thermodynamics and momentum, energy and species transport. The model accounts for melt transport induced by its density variation, resulting primarily from solid–liquid phase change. The equilibrium mineral assemblages, their mass fractions and their chemical compositions are determined by multicomponent thermodynamic models, and these parameters are linked with the calculations of velocity, pressure, temperature and species fields at each iteration and time step. For simplicity, solid phases are assumed to be stationary, and only olivine and plagioclase that are the earliest crystallization phases in common basaltic magmas are considered as fractionating phases. Application of the model to natural magmatic system shows that crystallization occurs selectively along the chilled boundaries soon after the cooling, and the magma body is separated into high-crystallinity mush zones and mostly crystal-free main magma. Because of the melt exchange between the mush zones and the main magma by compositional convection, the main magma evolves progressively in composition and the spatial chemical heterogeneity is grown with time. The melt segregated from the sidewall mush zone is accumulated below the roof mush zone, forming compositional stratification in the upper part of the main magma. On the other hand, the fractionated melt from the floor mush zone principally mixes with the overlying ambient magma in the middle and lower parts of the main magma body. This study shows that the direct treatment of nonlinear coupling among momentum, energy and species transport provides useful information of the thermal and chemical evolution of magma chambers as a function of time and space.

The Neogene to Recent Rallier-du-Baty nested ring complex, Kerguelen Archipelago (TAAF, Indian Ocean): stratigraphy revisited, implications for cauldron subsidence mechanisms

Abstract The Kerguelen Archipelago is made up of a stack of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres at various times since 30 Ma ago. In the SW, the Rallier-du-Baty Peninsula is mostly occupied by two silicic ring complexes, each with an average diameter of 15 km, comprising dissected calderas cross-cut by subvolcanic cupolas. Previous radiometric determinations yield ages ranging from 15.4 to 7.4 Ma in the southern centre, and 6.2 to 4.9 Ma in the northern one. The felsic ring dykes were injected by coeval mafic magmas, forming, successively, swarms of early mafic enclaves, disrupted synplutonic cone sheets, and late cone-sheets. After the emplacement and subsequent unroofing of the plutonic ring complexes, abundant and thick trachytic pyroclastic flows and falls were emitted from the younger caldera volcanoes, while hawaiite and mugearite lava flows were erupted from marginal maars and cones. Huge trachyte ignimbritic flows filled the glacial valleys in the central Peninsula, and capped lacustrine deposits and older lava flows, while related pumice falls are widespread throughout the archipelago. This powerful plinian eruption took place after the network of glacial valleys was established, but before the Little Ice Age that occurred during the last centuries. In the south of the peninsula, even younger trachytic formations are exposed, and fumarolic vents are still active. The growth mechanisms of a caldera-related ring complex can be explained as a repetitive sequence of two eruptive episodes. The first episode of hydrofracturing, induced by volatile exsolution within the evolving magma chamber, creates a vertical circular fracture zone, along which highly vesiculated magmas are emitted during explosive eruptive events occurring at the surface in a caldera volcano. It is followed by a second episode of cauldron-subsidence of a crustal block down to the degassed magma chamber, induced by pressure release. Downward movement of the crustal block favours the emplacement at shallow depths within the older caldera-filling formations, of discrete magmatic sheets characterized by a 16-km mean diameter and a 1-km mean thickness, corresponding to an average unit volume of 200 km 3 . Actually, the estimated volumes of the different igneous episodes within the Rallier-du-Baty nested ring complex vary from 60 to 900 km 3 , and correspond to the production during 15 Ma of about 2800 ± 850 km 3 of new materials and a net crustal growth of about 100 ± 30 × 10 3 m 3 per year.

The Origin of Marginal Compositional Reversals in Basic-Ultrabasic Sills and Layered Intrusions by Soret Fractionation

Marginal reversals—a common feature of many basic differentiated igneous bodies regardless of their size and bulk composition—are remarkable in being a mirror of the Layered Series. These are distinguished by: (1) an apparent lack of mass balance between the lower part of the marginal reversals, including chilled margins, and the bulk composition of the intrusions; (2) mineral crystallization sequences and (3) mineral compositional trends, which are both essentially the opposite of those in the Layered Series; (4) the cotectic composition of rocks composing the marginal reversals; (5) the capacity to form from both phenocryst-rich and phenocryst-free parental magmas; (6) the capability to develop along the floor, subvertical walls and even the roof of magma chambers. None of the current models of magma chamber evolution can provide an adequate explanation for the characteristic features of the marginal reversals. The problem can be resolved in the context of a model combining Soret diffusion in thin liquid boundary layers at the magma chamber margins and vigorous convection in the main magma body. The key proposal is that the formation of marginal reversals takes place through the non-equilibrium evolution of liquid boundary layers as a result of a temperature gradient imposed by the cold country rock. The fundamental explanation for the mirror image of a marginal reversal is that the non-equilibrium Soret fractionation works in a manner opposite to that of the equilibrium crystal–liquid fractionation that produces the Layered Series.

On magma chamber evolution during slow effusive eruptions

We explore some of the controls on the evolution of a slow effusive eruption imposed by processes in a shallow magma reservoir. We envisage that the emplacement of basaltic magma into a shallow, evolved crustal reservoir generates sufficient overpressure to trigger the slow effusive eruption of the overlying layer of evolved magma. The subsequent evolution of the system then depends on a number of factors including (1) the volatile contents of the two magmas; (2) the rate of any continuing influx of basalt; (3) the cooling/heating rates of the two layers of magma; and (4) the eruption rate of the silicic magma, where we assume that the crystals and bubbles remain in the layer in which they formed. Any exsolved volatiles in the magma increase the compressibility of the magma. In a waning system, this tends to increase the volume of material, which needs to be erupted to relieve a given overpressure by an amount of order 10–100, and hence increases the duration of the eruption. Typical calculations suggest that eruptions from magma chambers of size 1–10 km3 may persist for times of order 1–10 years if the main body of silicic magma is saturated in volatiles. In addition, cooling of the basalt leads to formation of dense crystals, which lowers the pressure. However, if the magma becomes saturated, exsolution of volatile gases occurs, tending to increase the pressure. Again, in a waning system, this tends to extend the eruption duration and increase the mass erupted from the chamber. For chamber volumes of 1–10 km3, cooling rates of 10−6 K/s can increase the eruption volume by a factor of order 10. Rapid cooling may also lead to an initial increase in the eruption rate, while the chamber pressure is controlled by the rate of production of new bubbles. If there is an influx of basalt to the chamber which is maintained during the eruption, then the evolution of the eruption is primarily dependent on this input rate, although the cooling of the basalt may increase the eruption rate to values in excess of this input rate, especially at an early stage in the eruption. The complex and nonlinear evolution of the eruption rate predicted by our simple model illustrates the sensitivity of the eruption rate to the magma volatile content and magma cooling rates. Such understanding is key in developing quantitative models for hazard assessments of slow effusive eruptions.

Forecasting volcanic eruptions

Forecasting is a central goal of volcanology. Intensive monitoring of recent eruptions has generated integrated time-series of data, which have resulted in several successful examples of warnings being issued on impending eruptions. Ability to forecast is being advanced by new technology, such as broad-band seismology, satellite observations of ground deformation and improved field spectrometers for volcanic gas studies, and spectacular advances in computer power and speed, leading to improvements in data transmission, data analysis and modelling techniques. Analytical studies of volcanic samples, experimental investigations and theoretical modelling are providing insights into the dynamics of magmatic systems, giving a physical framework with which to interpret volcanic phenomena. Magmas undergo profound changes in physical properties as pressure and temperature vary during magma chamber evolution, magma ascent and eruption. Degassing and cooling during magma ascent induce crystallisation and increases of viscosity, strength and compressibility, commonly by several orders of magnitude. Active magmatic systems also interact strongly with their surroundings, causing ground deformation, material failure and other effects such as disturbed groundwater systems and degassing. These processes and interactions lead to geophysical and phenomenological effects, which precede and accompany eruptions. Forecasting of hazardous volcanic phenomena is becoming more quantitative and based on understanding of the physics of the causative processes. Forecasting is evolving from empirical pattern recognition to forecasting based on models of the underlying dynamics. The coupling of highly non-linear and complex kinetic and dynamic processes leads to a rich range of behaviours. Due to intrinsic uncertainties and the complexity of non-linear systems, precise prediction is usually not achievable. Forecasts of eruptions and hazards need to be expressed in probabilistic terms that take account of uncertainties.

Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift: Insights into the four‐dimensional character of crustal construction at fast spreading mid‐ocean ridges

An investigation of ∼1‐m.y.‐old dikes and lavas from the north wall of the Hess Deep Rift (2°15′N, 101°30′W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east‐west flow line, representing ∼370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (∼25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift‐dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed two‐dimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt

The fractionation of B and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 °C and 100–500 MPa. Fluid-melt partition coefficients are 1 for rhyolite melt. This shows that B is not always strongly extracted from melts into hydrous fluids. Boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems (especially at high T ) and is most simply explained by differences in coordination (trigonal vs. tetrahedral) among coexisting phases. Combined with earlier measurements on illite-water (300–350 °C), B isotopic fractionation defines a temperature-dependent trend from 300 to 1100 °C. Because of the large magnitude and apparent low sensitivity to bulk composition, B isotopic fractionation can be readily applied to studies of diagenesis, hydrothermal alteration of planetary bodies, subduction-zone processing and arc magma generation, and magma chamber evolution.

Late Silurian bimodal volcanism of southwestern New Brunswick, Canada: Products of continental extension

Silurian–Devonian bimodal volcanism—a common feature of the northern Appalachians of the northeastern United States and eastern Canada—forms major belts of igneous rocks including the Coastal volcanic belt of New England and New Brunswick. The Coastal volcanic belt is one of the largest bimodal volcanic provinces in the world. Controversy remains regarding the tectonic setting of this volcanism; suggestions range from volcanic arc, to extensional continental rift, transpressional basin, backarc, or plume-modified subduction zone. The mapped Passamaquoddy Bay volcanic sequence of the Coastal volcanic belt of southwestern New Brunswick has a minimum thickness of 4 km and comprises 63 lithologic units that are divided into four distinct cycles of mafic-felsic volcanism. A preliminary U-Pb zircon date indicates an age of 423 Ma, similar to the age of the Cranberry Island bimodal volcanic series of the Coastal volcanic belt in Maine. Our analysis of 115 volcanic rocks for major and trace element concentrations indicates that the sequence is subalkalic and bimodal (basalt-rhyolite) with within-plate tectonic affinities. Most mafic rocks range in composition from basalt to basaltic andesite. The Mg# [defined as 100 × MgO/(MgO + FeO), where FeO = FeO T × 0.9] of the basalts ranges from 30 to 70, and there is a trend toward more primitive compositions upward in the section. The light rare earth element abundances of the basalts are enriched to levels of 30–130 times chondritic values. The flows are enriched in incompatible elements with respect to primitive mantle and show distinct Nb, Hf, and Sr anomalies. We interpret the basalts to be mantle melts modified by crustal contamination and mantle metasomatism from a previous subduction event. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers. The rhyolitic rocks are crustal melts, modified by crystal fractionation. Within each of the rhyolitic cycles there is an upward trend toward more evolved compositions as a result of an evolving magma chamber and magma-chamber zonation. We suggest that the rocks were erupted in an extensional environment and that there was a significant zone of continental extension during the Late Silurian in the northern Appalachians. However, controversy remains regarding the precise nature of the tectonic setting and the duration and extent of this volcanism.

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, South Greenland, is composed of various types of alkali granite and silica-undersaturated alkaline to agpaitic nepheline syenites related to three subsequently intruded magma batches. Mineral chemistry indicates continuous fractionation trends within each rock type, but with distinct differences among them. The last, peralkaline magma batch is the most fractionated in terms of XFemafic mineral, feldspar composition and mineral assemblage. This indicates that an evolving magma chamber at depth discontinuously released more highly fractionated alkaline melts. Fluid inclusions in some sodalites record a pressure drop from 3·5 to 1 kbar indicating that crystallization started during magma ascent and continued in the high-level magma chamber. On the basis of phase equilibria and preliminary fluid inclusion data, crystallization temperature drops from >1000°C (augite syenite liquidus) to <500°C (lujavrite solidus) and silica activity decreases from ∼0·8 to <0·3. An almost pure methane fluid phase at high temperatures and an almost pure aqueous fluid phase in the last crystallization stages of the agpaitic rocks indicate a strong increase in water activity. NaCl activity drops from 0·4 during magmatic sodalite crystallization to <0·01 (3 wt % NaCl equiv) in the late magmatic aqueous fluids. Relative oxygen fugacity [ΔFMQ, where FMQ is fayalite–magnetite–quartz)] depends on silica and water activity via two solid–solid buffer reactions. It decreases during fractionation in the augite syenite from about FMQ – 1 to below FMQ – 4, but increases in the peralkaline stage. The extreme peralkaline fractionation trend appears to be governed by low water activity and low SiO2 activity in the parental melt. Only then is methane a stable fluid phase during most of the crystallization history, which prevents early unmixing of an aqueous NaCl-bearing fluid phase.

B29 箱根新期外輪山・中央火口丘溶岩斑晶化学組成から見たマグマ溜りの時間変化

B29 箱根新期外輪山・中央火口丘溶岩斑晶化学組成から見たマグマ溜りの時間変化

Time Scales of Crystal Fractionation in Magma Chambers—Integrating Physical, Isotopic and Geochemical Perspectives

A simple heat balance model for an evolving magma chamber is used to make predictions of the time scales for magma differentiation, which are compared with geological and isotopic constraints on the rates of crystallization and differentiation. In a 10 km3 magma chamber releasing thermal energy at a rate of 100 MW, basalt and rhyolite magmas should reach 50% crystallization after 2500 and <1500 years, respectively. The tendency for phenocrysts to remain suspended in a cooling magma increases with melt viscosity and hence the degree of magma differentiation. The time scales of crystallization and differentiation may be estimated on the basis of the U-series isotope compositions of separated crystals and bulk rocks, crystal size distributions, and trace element and Sr isotope profiles in phenocryst phases. The last of these indicate crystal residence ages in the melt of tens of years up to 100 years. Short residence and even differentiation times are also obtained from Ra–Th isotope studies of alkali feldspars, and rocks that experienced alkali feldspar fractionation. However, much older ages of 103–105 years for separated phases have been reported from recent volcanic centres in St Vincent in the Lesser Antilles, Vesuvius in Italy, the Kenya Rift Valley and Long Valley, California. These old crystal ages are all from relatively evolved igneous rocks, as predicted from their higher melt viscosities and simple models of cooling and crystal settling. However, the old ages are also typically obtained for complex minerals that are not in bulk equilibrium with their host rocks, and so, apart from offering a minimum estimate of the age of the particular magmatic system, their significance for models of differentiation of the host magmas is not clear. An alternative approach is therefore to determine the variations in U–Th–Ra isotope compositions of bulk rocks reflecting different degrees of magma differentiation, and such data indicate that differentiation in more mafic magmas takes much longer than in more evolved magmas. For example, 50% fractional crystallization of basanite to produce phonolite on Tenerife took 105 years, whereas a further 50% fractional crystallization to generate the more evolved phonolites occurred within a few hundred years of eruption. On Tenerife the more mafic magmas fractionated at greater depths, and the rates of fractional crystallization were higher in the more evolved magmas studied. This is readily explained by a cooling model in which a large volume of primitive magma deep in the crust has a longer cooling time than a smaller body of differentiated magma at shallower depths in the crust.