Magma Chamber Dynamics Research Articles

Numerical modelling of convection interacting with a melting and solidification front: Application to the thermal evolution of the basal magma ocean

Melting and solidification are fundamental to geodynamical processes like inner core growth, magma chamber dynamics, and ice and lava lake evolution. Very often, the thermal history of these systems is controlled by convective motions in the melt. Computing the evolution of convection with a solid–liquid phase change requires specific numerical methods to track the phase boundary and resolve the heat transfer within and between the two separate phases. Here we present two classes of method to model the phase transition coupled with convection. The first, referred to as the moving boundary method, uses the finite element method and treats the liquid and the solid as two distinct grid domains. In the second approach, based on the enthalpy method, the governing equations are solved on a regular rectangular grid with the finite volume method. In this case, the solid and the liquid are regarded as one domain in which the phase change is incorporated implicitly by imposing the liquid fraction fL as a function of temperature and a viscosity that varies strongly with fL. We subject the two modelling frameworks to thorough evaluation by performing benchmarks, in order to ascertain their range of applicability. With these tools we perform a systematic study to infer heat transfer characteristics of a solidifying convecting layer. Parametrized relations are then used to estimate the super-isentropic temperature difference maintained across a basal magma ocean (BMO) (Labrosse et al., 2007), which happens to be minute (<0.1K), implying that the Earth’s core must cool at the same pace as the BMO.

Mineralogical and strontium isotopic record of hydrothermal processes in the lower ocean crust at and near the East Pacific Rise

Tectonic exposures of upper plutonics (>800 m) that are part of a contiguous section of young East Pacific Rise (EPR) crust at the Hess Deep Rift provide the first regional-scale constraints on hydrothermal processes in the upper plutonic crust formed at a fast-spreading ridge. Submersible-collected samples recovered over a 4-km-wide region show that the sheeted dike complex is largely underlain by a 150- to 200-m-thick gabbro unit, followed by a more primitive gabbronorite unit. Gabbroic samples are variably altered by pervasive fluid flow along fracture networks to amphibole-dominated assemblages. The gabbroic rocks are significantly less altered (average 11% hydrous phases) than the overlying sheeted dike complex (average 24%), and the percentage of hydrous alteration diminishes with depth. Incipient, pervasive fluid flow occurred at amphibolite facies conditions (average 720°C), with slightly higher temperatures in the lower 500 m of the section. The extent of subsequent lower-temperature alteration is generally low and regionally variable. The gabbroic samples are slightly elevated in 87Sr/86Sr relative to fresh rock values (0.7024) and less enriched than the overlying sheeted dike complex. 87Sr/86Sr for the pervasively altered gabbroic samples ranges from 0.70244 to 0.70273 (mean 0.70257), tonalites is 0.7038, and pyroxene hornfels ranges from 0.70259 to 0.70271. 87Sr/86Sr does not vary with depth, and there is a strong positive correlation with the percentage of hydrous phases. Strontium contents of igneous and hydrothermal minerals, combined with bulk rock 87Sr/86Sr, indicate that Sr-isotopic exchange is largely controlled by the uptake of fluid 87Sr/86Sr in hydrous minerals and does not require Sr gain or loss. The minimum, time-integrated fluid–rock ratio for the sheeted dike complex and upper plutonics is 0.55–0.66, and the fluid flux calculated by mass balance is ~2.1 to 2.5 × 106 kg m−2, 30–60% higher than fluid fluxes calculated in the same manner for sheeted dike complexes on their own at Hess and Pito Deeps, and Ocean Drilling Program Hole 504B. Alteration patterns within the upper plutonics evolved in response to axial magma chamber (AMC) dynamics at the EPR, such that magma replenishment led to assimilation and thermal metamorphism of the country rock, and the position of the hydrothermal root-zone tracked the vertical migration of the AMC. The freshness of the lowermost gabbroic rocks suggests that pervasive fluid flow does not lead to significant fluid and heat fluxes at and near fast-spreading ridges.

Magma chamber dynamics in a silicic LIP revealed by quartz: The Mesoproterozoic Gawler Range Volcanics

Silicic-dominated large igneous provinces (SLIP) represent vast amounts of magma (≥ 10 5 km 3) erupted onto the Earth's surface or injected into the crust over short time spans, and are important components of the continental crust. The conditions of formation and evolution of these large magmatic provinces and their magma chambers are still poorly constrained. In this contribution, we examine cathodoluminescence textures and trace element (Al, Ti, Fe) zoning of quartz in a Mesoproterozoic SLIP, the Gawler Range Volcanics (GRV), South Australia. We describe intra-granular textures such as truncation of growth textures and reverse zoning (rimwards increase of Ti content). These characteristics of quartz, together with remelting of already crystallised portions of the magma chamber (felsic enclaves), suggest a complex history of crystallisation and resorption, and fluctuating magma temperature. Titanium-in-quartz geothermometry indicates that adjacent quartz zones record temperature variations (ΔT) up to 70 °C in volcanic units. We also report contrasting (non-correlatable) zoning patterns amongst quartz crystals, each indicating different crystallisation conditions. The juxtaposition of quartz crystals with contrasting zoning patterns is consistent with a dynamic regime (convection, stirring, overturning) of the GRV magma chamber. These results point to pulsating magmatic conditions, compatible with a non-linear evolution of the GRV magma chamber. Heat, necessary to explain both intra-granular and infra-granular textural variations, may have been provided in different pulses by underplating of mafic magma.

Transition to chaos and implications for time-scales of magma hybridization during mixing processes in magma chambers

In this contribution numerical simulations of magma mixing are carried out with the aim of understanding the physico-chemical conditions that trigger the development of chaotic mixing dynamics. In detail, the mixing process is simulated in a magma chamber where a less evolved magma underplates a more evolved magma. The ‘chaoticity’ of this system is quantitatively estimated via computed finite time Lyapunov exponents. It is shown that magma chamber dynamics are strongly controlled by the Rayleigh number (Ra), whereas other parameters, such as buoyancy ratio and viscosity ratio, play a subordinate role. Results indicate that magma chambers with Ra of the order of 106 are dominated by non-chaotic behaviour. Increasing Ra shifts the system towards an increasing chaotic state and, in particular, above Ra=107 chaotic dynamics are fully developed in the magma chamber. Estimates of possible time-scales of hybridization of magmas during chaotic mixing suggest, for example, that a blob of mafic magma with an initial diameter of 100m within a magma chamber with a width of 1000m is stretched to a filament-like structure with a thickness of 15m after 320years and, after 1900years its thickness is reduced to 4.0mm. Using typical chemical diffusion coefficients for magmas (of the order of 10−14m2/s) it can be calculated that a 4.0mm-thick filament would homogenize by diffusion in ~10years. Such homogenization time-scales are orders of magnitude lower that typical life-times estimated for magma chambers, even in the volcanic environment, indicating that fast hybridization of magmas can be rapidly achieved when chaotic dynamics govern the evolution of the magmatic system.

Magma Solidification Processes beneath Kilauea Volcano, Hawaii: a Quantitative Textural and Geochemical Study of the 1969-1974 Mauna Ulu Lavas

Kilauea volcano is a very intensively studied, active basaltic magmatic system and thus represents an ideal location to study magma solidification processes in a natural environment. Understanding solidification is important in refining models of magma chamber dynamics and its detailed study may improve our knowledge of magma system evolution. In this study magma solidification processes are examined and quantified using samples from the 1969–1974 Mauna Ulu (MU) rift eruption. We have collected major and trace element whole-rock data plus in situ olivine compositions, along with crystal size distribution data on 11 lava samples. The observed whole-rock chemical variation was partly produced by olivine addition within the Kilauea edifice. At least two distinct olivine populations are inferred from quantitative textural analysis: (1) a 3–40-year-old population characterized by a low crystal density, greater crystal length and flatter slopes of the crystal size distributions (CSDs); (2) a 1·5–15-year-old population marked by a high density of smaller crystals and steep CSD slopes. The range in olivine composition suggests that all these crystals grew from a range of different magmas, probably closely related by crystal fractionation. The ubiquitous presence of deformed olivine crystals shows that population 1 reflects a component that must have mostly originated by disruption of a deformed cumulate. This antecrystic olivine population represents an earlier-coarsened and aggregated, cumulate-forming magma component. In contrast, the phenocrystic population 2 represents a late magma component formed in the summit magma storage region. Our results are consistent with the hypothesis that the components of the MU magmas followed two different routes. The deformed-olivine-bearing magma moved along the deep basal décollement then rose through vertical pipe-like conduits under the MU rift. The undeformed-olivine-bearing magma rose via the main conduit to the summit reservoir and then moved out along the rift zone, where the magmas mixed in small chambers. The presence of narrow, reversely zoned olivine rims suggests that the mixing occurred just prior to eruption.

Magma chamber stability in arc and continental crust

The growth and thermo-mechanical stability of magma chambers in Earth's crust dictate the dynamics of volcanism at the surface, and the organization of volcanic plumbing at depth. We analyze a model of magma chamber evolution in which volumetric growth is governed by the mechanical focusing of rising dikes by the magma chamber, “magmatic lensing,” as well as melting and assimilation of country rock. This modeling framework emphasizes the two-way coupling between chamber stresses and thermal evolution with specific compositions of intruding magma and country rock. We consider as end member compositional scenarios a “wet” environment magma chamber, in which basalt with 2 wt.% H 2O intrudes an amphibolitic country rock, and a “dry” chamber consisting of anhydrous basalt intruding tonalitic country rocks. Magma chambers that erupt, freeze, or reach dynamic equilibrium in the crust occupy distinct regions of a parameter space that measures the relative importance of depth, chamber pressurization, wall rock viscoelastic rheology, and thermal viability. Lower crustal melt flux is the most important factor controlling chamber stability, but chamber depth and composition also help determine long-term dynamical behavior. In general, interactions between thermal and mechanical processes exert first-order control on chamber stability, defining four distinct regimes of magma chamber dynamics. In addition to thermally and mechanically unstable (freezing and eruptive) chambers, we find steady-state thermally viable chamber volumes are possible as well as a range of parameters for which chamber growth is roughly exponential in time and mechanically stable (no eruption occurs). Long-lived (> 1 Ma) chambers generally result from lower crustal melt flux values that range from ∼ 10 − 4 to ∼ 10 − 1 m 3/m 2/yr for 20 and 40 km deep chambers and both compositional end members used in this study. However chambers become considerably less stable in cool shallow environments, particularly with anhydrous compositions of magma and country rock. Model predictions in this framework suggest that a range of observed intrusive structures in Earth's crust may be the result of magma chambers in different, clearly defined dynamical regimes.

Application of the multi distribution function lattice Boltzmann approach to thermal flows

Numerical methods able to model high Rayleigh (Ra) and high Prandtl (Pr) number thermal convection are important to study large-scale geophysical phenomena occuring in very viscous fluids such as magma chamber dynamics (104 < Pr < 107 and 107 < Ra < 1011). The important variable to quantify the thermal state of a convective fluid is a generalized dimensionless heat transfer coefficient (the Nusselt number) whose measure indicates the relative efficiency of the thermal convection. In this paper we test the ability of Multi-distribution Function approach (MDF) Thermal Lattice Boltzmann method to study the well-established scaling result for the Nusselt number (Nu ∝ Ra 1/3) in Rayleigh Benard convection for 104 ≤ Ra ≤ 109 and 101 ≤ Pr ≤ 104. We explore its main drawbacks in the range of Pr and Ra number under investigation: (1) high computational time N c required for the algorithm to converge and (2) high spatial accuracy needed to resolve the thickness of thermal plumes and both thermal and velocity boundary layer. We try to decrease the computational demands of the method using a multiscale approach based on the implicit dependence of the Pr number on the relaxation time, the spatial and temporal resolution characteristic of the MDF thermal model.

Concurrent Mixing and Cooling of Melts under Iceland

The compositions of 75 melt inclusions, their host olivines and 49 whole-rock samples of their carrier lavas have been determined. These compositions were added to a compilation of the trace element composition of 243 melt inclusions from 10 eruptions in the neovolcanic zones of Iceland and used to investigate melt mixing processes. Whereas the compositional variability of whole-rock samples from single eruptions is limited, there are significant compositional differences between eruptions. The compositions of inclusions are more variable than those of whole-rock samples of their carrier lava. On a flow-by-flow basis, the average composition of trace element ratios such as La/Yb in the inclusions is similar to that of their carrier lava. These observations indicate that, for each lava flow, melts with compositions similar to those of the inclusions crystallized and mixed to produce the magma that transported the host olivines to the surface. Although many of the olivines are not in Mg–Fe equilibrium with their host melt, they are not accidental xenocrysts because they crystallized from melts similar to those that mixed to form the carrier magma. The trace element variability of melt inclusions drops with decreasing forsterite content of the host olivine. This relationship is observed both within single flows and in the compilation of data from 10 flows. Concurrent mixing and crystallization dominate the compositional evolution of basaltic melts in lower crustal magma chambers. This coupled mixing and cooling is likely to result from convective motions in magma chambers. The rate of change of the mixing parameter, M, with temperature of the melt is dM/dT = 0·0094 ± 0·0036 per °C. This relative rate may be used to constrain the fluid dynamics of basaltic magma chambers.

Deciphering Magma Chamber Dynamics from Styles of Compositional Zoning in Large Silicic Ash Flow Sheets

The understanding of the dynamic processes in magmatic systems has grown and changed markedly in the last decade. Old models for magmatic systems as vats of near-liquidus material have been revised by observations from seismology (Sinton and Detrick 1992), crystal chemistry and zoning (Davidson et al. 2007) and the geochronology and geochemistry of both plutonic and volcanic systems (Hildreth 2004; Charlier et al. 2007; Miller et al. 2007; Peressini et al. 2007; Walker et al. 2007). New views emphasise magmatic systems as temporally dominated by crystal mushes (magma bodies with a high fraction of solid particles; see definition in Miller and Wark 2008), that wax and wane in temperature and crystallinity, and are subject to significant open system processes (Charlier et al. 2007; Hildreth and Wilson 2007; Walker et al. 2007; Bachmann and Bergantz 2008). These processes, such as magma reintrusion, mixing, gas sparging, and subsequent thermal rejuvenation, may be significantly more important in producing the characteristics of a magmatic system than the previous closed-system, near-liquidus behaviour would predict. There are a number of recent reviews that summarize these observations (for example see Eichelberger et al. 2006; Bachmann et al. 2007b; Lipman 2007). Our aim here is to illustrate how this new perspective to magma dynamics is motivated by observations of heterogeneities (or lack thereof) in erupted rocks (and to a lesser amount in plutons). Owing to rapid withdrawal and quenching of magma during explosive volcanic eruptions (hours to a few days), large-volume (>1 km3) pyroclastic deposits (also referred to as ignimbrites or ash-flow tuffs ) provide an instant image of the state of the magma chamber evacuated during eruption. A first-order observation that characterizes these pyroclastic deposits of intermediate to silicic composition is that many do not tap into chemically …

Insights into magma chamber processes from the analysis of size distribution of enclaves in lava flows: A case study from Vulcano Island (Southern Italy)

The size distribution of latitic enclaves dispersed in a rhyolitic lava flow is studied. Enclave size distribution is self-similar, a feature that can be explained by a fractal fragmentation process, and estimated fractal dimension of fragmentation is 2.50. Fragment size distribution of enclaves generated by disruption of viscous fingering structures in granitoid rocks is analyzed. It is shown that this distribution is fractal with a value of fractal dimension of fragmentation of 2.55, in close agreement with the value estimated for enclaves in the lava flow. It is suggested that the fragmentation process producing the size distribution of enclaves in the lava flow may have occurred in response to the disruption of viscous fingering morphologies generated by the injection of the more mafic magma into the felsic one. Accordingly, the size distribution of enclaves dispersed in the lava flow is considered a feature inherited from deep magma chamber processes and gives insights into magma chamber dynamics.

Experimental study of radium partitioning between anorthite and melt at 1 atm

We present the first experimental radium mineral/melt partitioning data, specifically between anorthite and a CMAS melt at atmospheric pressure. ^(226)Ra disequilibria are an important chronometer of recent magmatic activity. Ion microprobe measurement of coexisting anorthite and glass phases produces a molar D_Ra = 0.040 ± 0.006 and D_(Ra)/D_(Ba) = 0.24 ± 0.05 at 1400 °C. Our results indicate that lattice strain partitioning models fit the divalent (Ca, Sr, Ba, Ra) partition coefficient data of this study well, supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values.

Magmatic Stratigraphy of the Tilted Tottabetsu Plutonic Complex, Hokkaido, North Japan: Magma Chamber Dynamics and Pluton Construction

The steeply tilted Tottabetsu plutonic complex (TPC) provides an exceptional cross‐sectional view of a frozen felsic magma chamber. The entire pluton is apparently cylindrical in shape, but the active magma chamber at any one time was much thinner, tabular in shape. The pluton is stratified, with a topmost granitic unit (zone III), an underlying dioritic unit (zone II), and a lowermost gabbroic unit (zone I). Field, petrographic, and geochemical data suggest that zones I and II developed at the aggrading floor of a felsic magma chamber. Only part of zone III appears to have crystallized from the roof. Successive stoping events, recorded as included country‐rock blocks in zones I and II, apparently prohibited growth of the roof solidification zone. The complex record of hotter replenishments and fractional crystallization is preserved in the stratigraphy of zones I and II as either alternating comagmatic sheets and cumulate layers or an upward fractionation trend with a major compositional reversal exhibited by the cumulates. The magmas injected during the development of zone I were basalts to basaltic andesites, while those injected during the development of zone II were andesites to dacites. The overall compositional difference between zones I and II is essentially attributed to the compositional difference of the injected magmas. By contrast, zone III comprises monotonous granites whose geochemical variation suggests that they represent mixtures of fractionates complementary to the cumulates formed from both the resident felsic magma and the recharged intermediate magma. Buoyant fractionates released from the crystallizing recharge magma presumably rose as plumes and mixed with the resident magma. Thus, the TPC reveals the protracted history of pluton construction from a long‐lasting, melt‐rich magma chamber.

U-Pb dating of zircon in the Bishop Tuff at the millennial scale

Zircon from the Bishop Tuff (eastern California) was dated by the U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) method to evaluate time scales of magmatic evolution and applicability of the method to rocks younger than 1 Ma. The 206 Pb/ 238 U dates from 17 of 19 grains are equivalent with a weighted mean of 767.1 ± 0.9 ka that overlaps with the published 40 Ar/ 39 Ar sanidine age, requiring that most zircon crystallized immedi- ately before eruption. Dates from two grains that are 6 and 12 k.y. older indicate minimum times that a small portion of the grains resided in the magma. These fi ndings are consistent with chemical zoning patterns in zircon that suggest a relatively simple crystallization history, yet contrast with previous results of ion microprobe dating of zircon that show 50-80 k.y. of magma residence. The weighted mean 207 Pb/ 235 U date is ~52 k.y. older than the 206 Pb/ 238 U date, a difference that is attributed to excess 207 Pb from initial Pa/U disequilibrium. The results dem- onstrate that high-precision ID-TIMS geochronology can resolve magma chamber dynamics of <1 Ma silicic eruptions at the millennial scale.

Plagioclase microtextures and their importance for magma chamber dynamics examples from Lesvos, Hellas and Teide, Canary Islands

Plagioclase microtextures and their importance for magma chamber dynamics examples from Lesvos, Hellas and Teide, Canary Islands

Anorthite-calibrated backscattered electron profiles, trace elements, and growth textures in feldspars from the Teide–Pico Viejo volcanic complex (Tenerife)

Magma chamber dynamics at the phonolitic to basanitic Teide–Pico Viejo volcanic complex on Tenerife, Canary Islands, have been investigated using feldspar zonation and chemistry. Backscattered electron-image grey value profiles, obtained by electron microprobe and calibrated on anorthite content, allow the investigation of much better spatially resolved zonation profiles than quantitative point analysis alone. On the basis of these zonation patterns and trace element analyses by electron microprobe, magma chamber processes such as magma recharge and mingling, as well as whole-chamber overturn events, can be identified and characterized in more detail. Samples from all investigated eruption centres contain feldspars, which have experienced growth in a deep reservoir, followed by ascent, resorption, and growth of mantles in more shallow levels in the crust just prior to eruption. Two Pico Viejo samples exhibit feldspars with a characteristic drop in anorthite content, which is not accompanied by resorption. Trace element behaviour in these samples shows that cores and mantles have grown from the same magma reservoir: The conclusion from these data is that there occurred complete overturn events in layered magma chambers, presumably induced by cold plumes from the cupola layers. A mass balance calculation shows that the proportions of the layers are ∼ 75 wt.% lower part and ∼ 25 wt.% cupola layer. Correlations between zonation patterns of single crystals in different rocks show that the Teide magma chambers are larger and chemically buffered compared with the smaller Pico Viejo system. Feldspars from Pico Viejo exhibit rather complex anorthite-profiles and more frequently display disturbances likely to be caused by arrival of new magma batches.

Time scale of magma differentiation in arcs from protactinium-radium isotopic data

Abstract Absolute chronology of magma differentiation processes has been a long-desired goal, given its importance in understanding magma chamber dynamics and its connection to a fundamental understanding of the style and frequency of volcanic eruptions. Broad estimates of the duration of magma differentiation and overall crustal residence times have been made based on a variety of indirect approaches, such as physical models of magma chamber cooling, rates of crystal growth and settling, and long-lived radiogenic isotopes. In contrast, combined 231Pa-235U data may provide a robust measure of the time scale of magma differentiation. Based on 231Pa-235U, 230Th-238U and 226Ra-230Th data from Taal volcano, Luzon Arc, Philippine Archipelago, we show that 231Pa-235U data may provide a robust direct measure of the time scale of magma differentiation. A closed-system magma fractionation model gives a 231Pa-235U differentiation time scale in the range of 30 k.y., while the 226Ra-230Th time scale is considerably younger. The time scales are reconciled if we consider either fluid-mixing or magma-mixing models. The fluid-mixing model gives a time scale of differentiation similar to the 231Pa-235U closed-system time scale and is supported by the 230Th-238U data. The magma-mixing model gives a considerably longer time, in the range of 55 k.y. The combined observations support the robustness of the 231Pa-235U chronology, indicating a differentiation time scale in the range of 30 k.y., although this time scale for other volcanoes may vary depending on size and thermal state of the magma chamber. The 226Ra-230Th closed-system model ages, which yield much younger estimates for magma differentiation, are not likely to reflect time scales of magma differentiation.

Solubility of H 2O in rhyolitic melts at low pressures and a new empirical model for mixed H 2O–CO 2 solubility in rhyolitic melts

Experiments of H 2O solubility in synthetic haplogranitic and natural rhyolitic melts were conducted at 700–1200 °C and 0.098–25 MPa. Attainment of equilibrium was proven by reversals using starting materials with different initial H 2O contents. Dissolved H 2O contents were determined using Fourier transform infrared spectroscopy. When temperature decreases from 1000 to 700 °C, the solubility of H 2O increases from 0.100 to 0.124 wt.% at 0.098 MPa, from 0.99 to 1.36 wt.% at 11 MPa, and from 1.46 to 2.17 wt.% at 25 MPa. At 6 MPa, the solubility of H 2O increases from 0.63 to 0.77 wt.% from 1200 to 850 °C. Combining our data with those from the literature on pure H 2O and pure CO 2 solubility, mixed H 2O–CO 2 solubility, and H 2O solubility for H 2O–H 2 gas mixture (240 measurements for H 2O and 59 points for CO 2), we develop a single empirical model for solubility of pure H 2O vapor, that of pure CO 2 vapor, as well as that of mixed H 2O–CO 2 vapor in metaluminous high-silica rhyolitic melt. This model applies to the conditions of 700–1200 °C and 0–500 MPa, and consists of two equations: H 2 O t = ( 354.94 P w 0.5 + 9.623 P w − 1.5223 P w 1.5 ) / T + 0.0012439 P w 1.5 + P CO 2 ( − 1.084 × 10 − 4 P w 0.5 − 1.362 × 10 − 5 P w ) CO 2 = P CO 2 ( 5668 − 55.99 P w ) / T + P CO 2 ( 0.4133 P w 0.5 + 2.041 × 10 − 3 P w 1.5 ) where H 2O t is total dissolved H 2O content in wt.%, CO 2 content is in ppm by mass, and T is temperature in Kelvin, P w= X w f P and P CO 2 = X CO 2 f P (in MPa), where X w f and X CO 2 f are the mole fraction of water and CO 2 in the fluid. The 2 σ relative uncertainty is 15% for the H 2O equation, and 20% for the CO 2 equation. The above H 2O equation also applies to H 2O solubility in mixed H 2O–H 2 fluid for X H 2 f<0.54 at 100 MPa, for X H 2 f<0.49 at 200 MPa, and X H 2 f≤0.44 at 300 MPa (that is, the presence of H 2 in the fluid insignificantly affects H 2O solubility). This empirical model is recommended for the modeling of explosive volcanic eruptions and magma chamber dynamics.

The formation of economic porphyry copper (-gold) deposits: constraints from microanalysis of fluid and melt inclusions

Abstract This paper summarizes our current understanding of the formation of porphyrystyle Cu ± Au ± Mo deposits, in the light of data obtained by direct analysis of the ore metals in individual fluid and melt inclusions using laser-ablation ICP mass spectrometry. An integrated study of the evolution of the calcalkaline Farallón Negro Volcanic Complex hosting the Bajo de la Alumbrera porphyry Cu-Au deposit (Argentina), and supplementary fluid-chemical data from Bingham (Utah) and other examples, permit a quantitative re-assessment of the fundamental processes controlling the key economic parameters of porhyry-style ore deposits. Deposit size (total metal content) is optimized by exsolution of a relatively dense (&gt;0.3 g cm −3 ) single-phase fluid or a two-phase brine + vapour mixture from a moderately large hydrous pluton, possibly with an intermediate step involving the scavenging of the ore-forming elements in a magmatic sulphide melt. Emplacement mechanism, magma-chamber dynamics and possibly an additional source of sulphur are probably more decisive for the formation of a large deposit than sheer pluton volume and elevated Cu contents in the melts. Primary bulk ore grade is determined by temperature-controlled precipitation of ore minerals, which is optimized where a large magmatic fluid flux is cooled through 420–320 °C over a restricted vertical flow distance. Bulk metal ratios of the deposits, exemplified by the economically important Au/Cu ratio in the ore, are primarily controlled by the magmatic source defining the composition of the fluids before they reach the deposit site, although selective precipitation may contribute to metal zoning within orebodies.

Time scale of magma differentiation in arcs from protactinium-radium isotopic data

Research Article| August 01, 2005 Time scale of magma differentiation in arcs from protactinium-radium isotopic data Yemane Asmerom; Yemane Asmerom 1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar S. Andrew DuFrane; S. Andrew DuFrane 1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar Samuel B. Mukasa; Samuel B. Mukasa 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Hai Cheng; Hai Cheng 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA Search for other works by this author on: GSW Google Scholar R. Lawrence Edwards R. Lawrence Edwards 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Yemane Asmerom 1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA S. Andrew DuFrane 1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Samuel B. Mukasa 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Hai Cheng 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA R. Lawrence Edwards 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA Publisher: Geological Society of America Received: 18 Feb 2005 Revision Received: 25 Mar 2005 Accepted: 29 Mar 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2005) 33 (8): 633–636. https://doi.org/10.1130/G21638AR.1 Article history Received: 18 Feb 2005 Revision Received: 25 Mar 2005 Accepted: 29 Mar 2005 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yemane Asmerom, S. Andrew DuFrane, Samuel B. Mukasa, Hai Cheng, R. Lawrence Edwards; Time scale of magma differentiation in arcs from protactinium-radium isotopic data. Geology 2005;; 33 (8): 633–636. doi: https://doi.org/10.1130/G21638AR.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Absolute chronology of magma differentiation processes has been a long-desired goal, given its importance in understanding magma chamber dynamics and its connection to a fundamental understanding of the style and frequency of volcanic eruptions. Broad estimates of the duration of magma differentiation and overall crustal residence times have been made based on a variety of indirect approaches, such as physical models of magma chamber cooling, rates of crystal growth and settling, and long-lived radiogenic isotopes. In contrast, combined 231Pa-235U data may provide a robust measure of the time scale of magma differentiation. Based on 231Pa-235U, 230Th-238U and 226Ra-230Th data from Taal volcano, Luzon Arc, Philippine Archipelago, we show that 231Pa-235U data may provide a robust direct measure of the time scale of magma differentiation. A closed-system magma fractionation model gives a 231Pa-235U differentiation time scale in the range of 30 k.y., while the 226Ra-230Th time scale is considerably younger. The time scales are reconciled if we consider either fluid-mixing or magma-mixing models. The fluid-mixing model gives a time scale of differentiation similar to the 231Pa-235U closed-system time scale and is supported by the 230Th-238U data. The magma-mixing model gives a considerably longer time, in the range of 55 k.y. The combined observations support the robustness of the 231Pa-235U chronology, indicating a differentiation time scale in the range of 30 k.y., although this time scale for other volcanoes may vary depending on size and thermal state of the magma chamber. The 226Ra-230Th closed-system model ages, which yield much younger estimates for magma differentiation, are not likely to reflect time scales of magma differentiation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mineral-scale Sr-isotope constraints on magma evolution and chamber dynamics in the Rum layered intrusion, Scotland

Sr isotopic zoning within single plagioclase crystals from rocks from Unit 9 of the Rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination. The 87Sr/86Sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying Unit 10 peridotite is approached. Models of near-solidus interaction of the cumulate with a fluid or melt, or large scale textural re-equilibration, cannot easily account for the systematic differences in 87Sr/86Sr between small crystals and the rims of larger crystals.