A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites

Diverse explanations exist for the large-volume catastrophic eruptions that formed the Bishop Tuff of Long Valley in eastern California, the Bandelier Tuff in New Mexico, and the tuffs of Yellowstone, Montana, USA. These eruptions are among the largest on Earth within the last 2 Myr. A common factor in recently proposed petrogenetic scenarios for each system is multistage processing, in which a crystal mush forms by crystal fractionation and is then remobilized to liberate high-silica liquids. Magma evolves in the lower crust in earlier phases. We have tested these scenarios quantitatively by performing phase equilibria calculations (MELTS) and comparing the results with observed liquid (glass) and phenocryst compositions. Although comparison of tuff samples from each ignimbrite reveals distinct phenocryst compositions and proportions, the computed results exhibit a remarkable degree of congruity among the systems, pointing to some underlying uniform behavior relevant to large-volume silicic ignimbrites. Computed liquid compositions derived from more than ∼25% fractional crystallization of the parental melt in the deep crust are marked by SiO2 concentrations several weight per cent too low compared with the observed compositions, suggesting a limit on the extent of magma evolution by crystal fractionation in the deep crust. In all cases, the phase equilibria results and related considerations point to evolution dominated by crystal fractionation of a water-saturated mafic parental melt at shallow depths (∼5 km). Parental melt compositions are consistent with those of observed regional primitive basalts erupted prior to ignimbrite eruption for each system in each region. Fractional crystallization of water-rich mafic melt at shallow levels leads inherently to destabilization near thermodynamic pseudoinvariant points at around 800°C within the melting interval close to, but above, the solidus. For each system, the magmas evolve to states of high exsolved H2O volume fraction even at 5 km depth, eventually exceeding the criterion for magma fragmentation of ∼60 vol. % near the pseudoinvariant point temperature. Copious exsolution and possible expulsion of fluid occurs at this temperature, where the solid fraction in the magma changes almost discontinuously (isothermally) to significantly higher values. This instability mechanism acts as an eruption trigger by generating a gravitationally unstable arrangement of low-density, water-saturated magma beneath a thin (several kilometres) crustal lid. The trigger mechanism is common to fractional crystallization scenarios based on a variety of conditions, when crystallized solids and/or exsolved fluids are fractionated from residual melt isobarically (constant pressure) or isochorically (constant volume). In a single system, differences in liquid compositions resulting from constant volume versus constant pressure crystallization and expulsion versus retention of exsolved H2O are small compared with those arising from variations in initial water concentration, lithostatic pressure, and oxygen fugacity. It is these latter quantities that lie at the crux of the commonality in large-volume ignimbrite-forming eruptions, with a reasonable range of metamodel parameters. Scale analysis provides thermal timescales for fractional crystallization, including age ranges for discrete crystal populations. For the Bishop Tuff, the overall timescale for the Bishop magma body is >1 Myr. For the Yellowstone Tuffs, calculated thermal timescales are consistent with recurrence intervals of ∼600 kyr between successive caldera collapses. Although it is recognized that petrogenetic processes other than perfect fractional crystallization play a role in ignimbrite petrogenesis, by emphasizing common features the uniqueness of each system can be brought into better focus by sound and quantitative analysi

Energy‐constrained open‐system magmatic processes 3. Energy‐Constrained Recharge, Assimilation, and Fractional Crystallization (EC‐RAFC)

Geochemical data for igneous rock suites provide conclusive evidence for the occurrence of open‐system processes within thermally and compositionally evolving magma bodies. The most significant processes include magma Recharge (with possible enclave formation and magma mixing), Assimilation of anatectic melt derived from wallrock partial melting and formation of cumulates by Fractional Crystallization (RAFC). In this study, we extend the Energetically Constrained Assimilation and Fractional Crystallization (EC‐AFC) model [Spera and Bohrson, 2001; Bohrson and Spera, 2001] to include the addition of compositionally and thermally distinct recharge melt during simultaneous assimilation and fractional crystallization. Energy‐Constrained Recharge, Assimilation, and Fractional Crystallization (EC‐RAFC) tracks the trace element and isotopic composition of melt, cumulates and enclaves during simultaneous recharge, assimilation and fractional crystallization. EC‐RAFC is formulated as a set of 3 + t + i + s coupled nonlinear differential equations, where the number of trace elements and radiogenic and stable isotope ratios modeled are t, i, and s, respectively. Solution of the EC‐RAFC equations provides values for the average wallrock temperature (Ta), mass of melt within the magma body (Mm), mass of cumulates (Mct) and enclaves (Men), mass of wallrock involved in the thermal interaction (Mao), mass of anatectic melt assimilated (M*a), concentration of t trace elements and i + s isotopic ratios in melt (Cm), cumulates (Cct), enclaves (Cen), and anatectic melt (Ca) as a function of magma temperature (Tm). Input parameters include the equilibration temperature (Teq), the initial temperature and composition of pristine melt (Tmo, Cmo, εmo), recharge melt (Tro, Cro, εro), and wallrock (Tao, Cao, εao), temperature‐dependent trace element distribution coefficients (Dm, Dr, Da), heats of transition for wallrock (Δha), pristine melt (Δhm), and recharge melt (Δhr), and the isobaric specific heat capacity of assimilant (Cp,a), pristine melt (Cp,m), and recharge melt (Cp,r). The magma recharge mass function, Mr(Tm), is specified a priori and defines how recharge magma is added to standing magma. The present EC‐RAFC simulator incorporates a weak coupling to major element mass balance and phase relations based on laboratory experiments or Gibbs Energy minimization [e.g., Ghiorso, 1997]. EC‐RAFC can be applied to a variety of magmatic systems including volcanic suites that show evidence of magma mixing, layered mafic intrusions, and granitoid plutons. Predictions for masses, as well as compositions of magmatic products, are part of the EC‐RAFC solution. The “systems” approach provides an opportunity to quantitatively assess the roles of assimilation, fractional crystallization, and magma recharge in magma evolution using trace element and isotopic constraints together with energy conservation

Energy‐constrained open‐system magmatic processes IV: Geochemical, thermal and mass consequences of energy‐constrained recharge, assimilation and fractional crystallization (EC‐RAFC)

A wealth of geochemical and petrological data provide evidence that the processes of fractional crystallization, assimilation, and magma recharge (replenishment) dominate the chemical signatures of many terrestrial igneous rocks. Previous work [Spera and Bohrson, 2001; Bohrson and Spera, 2001] has established the importance of integrating energy, species and mass conservation into simulations of complex magma chamber processes. An extended version of the energy‐constrained formulation, Energy‐Constrained Recharge, Assimilation, Fractional Crystallization (EC‐RAFC), tracks mass and compositional variations of melt, cumulates, and enclaves in a magma body undergoing simultaneous recharge, assimilation, and fractional crystallization [Spera and Bohrson, 2002]. Because many EC‐RAFC results are distinct from those predicted by extant RAFC formulations, the primary goal of this paper is to present a range of geochemical and mass relationships for selected cases that highlight issues relevant to modern petrology. Among the plethora of petrologic problems that have important, well‐documented analogues in nature are the geochemical distinctions that arise when a magma body undergoes continuous versus episodic recharge, the connection between erupted magmas and associated cumulate bodies, the behavior of recharge‐fractionation dominated systems (RFC), thermodynamic conditions that promote the formation of enclaves versus cumulates, and the conditions under which magma bodies may be described as chemically homogeneous. Investigation of the effects of continuous versus episodic recharge for mafic magma undergoing RAFC in the lower crust indicates that the resulting geochemical trends for melt and solids are sensitive to the intensity and composition of recharge, suggesting that EC‐RAFC may be used as a tool to distinguish the nature of the recharge events. Compared to the record preserved in melts, the geochemical and mass characteristics of solids associated with particular RAFC events may record a more complete view of the physiochemical history of an open‐system magma body. The capability of EC‐RAFC to track melts and solids creates a genetic link that can be compared to natural analogues such as layered mafic intrusions and flood basalts, or mafic enclaves and their intermediate‐composition volcanic or plutonic hosts. The ability to quantify chemical and volume characteristics of solids and melts also underscores the need for integrated field, petrologic and geochemical studies of igneous systems. While it appears that a number of volcanic events or systems may be characterized by continuous influx or eruption of magma (“steady state systems”), reports describing compositional homogeneity for products that represent eruptions of more than one event are relatively rare. In support of this, EC‐RAFC results indicate that very specific combinations of recharge conditions, bulk distribution coefficients, and element concentrations are required to achieve geochemical homogeneity during cooling of a magma body undergoing RAFC. In summary, critical points are that EC‐RAFC provides a method to quantitatively investigate complex magmatic systems in a thermodynamic context; it predicts complex, nonmonotonic geochemical trends for which there are natural analogues that have been difficult to model; and finally, EC‐RAFC establishes the link between the chemical and physical attributes of a magmatic system. Application of EC‐RAFC promises to improve our understanding of specific tectonomagmatic systems as well as enhance our grasp of the essential physiochemical principles that govern magma body evolution

Energy-Constrained Recharge, Assimilation, and Fractional Crystallization (EC-RAxFC): A Visual Basic computer code for calculating trace element and isotope variations of opensystem magmatic systems

Volcanic and plutonic rocks provide abundant evidence for complex processes that occur in magma storage and transport systems. The fingerprint of these processes, which include fractional crystallization, assimilation, and magma recharge, is captured in petrologic and geochemical characteristics of suites of cogenetic rocks. Quantitatively evaluating the relative contributions of each process requires integration of mass, species, and energy constraints, applied in a self-consistent way. The energy-constrained model Energy-Constrained Recharge, Assimilation, and Fractional Crystallization (EC-RaxFC) tracks the trace element and isotopic evolution of a magmatic system (melt + solids) undergoing simultaneous fractional crystallization, recharge, and assimilation. Mass, thermal, and compositional (trace element and isotope) output is provided for melt in the magma body, cumulates, enclaves, and anatectic (i.e., country rock) melt. Theory of the EC computational method has been presented by Spera and Bohrson (2001, 2002, 2004), and applications to natural systems have been elucidated by Bohrson and Spera (2001, 2003) and Fowler et al. (2004). The purpose of this contribution is to make the final version of the EC-RAxFC computer code available and to provide instructions for code implementation, description of input and output parameters, and estimates of typical values for some input parameters. A brief discussion highlights measures by which the user may evaluate the quality of the output and also provides some guidelines for implementing nonlinear productivity functions. The EC-RAxFC computer code is written in Visual Basic, the programming language of Excel. The code therefore launches in Excel and is compatible with both PC and MAC platforms. The code is available on the authors’ Web sites http://magma.geol.ucsb.edu/and http://www.geology.cwu.edu/ecrafc) as well as in the auxiliary material

Chaotic thermohaline convection in low-porosity hydrothermal systems

Fluids circulate through the Earth's crust perhaps down to depths as great as 5^15 km, based on oxygen isotope systematics of exhumed metamorphic terrains, geothermal fields, mesozonal batholithic rocks and analysis of obducted ophiolites. Hydrothermal flows are driven by both thermal and chemical buoyancy; the former in response to the geothermal gradient and the latter due to differences in salinity that appear to be ubiquitous. Topographically driven flows generally become less important with increasing depth. Unlike heat, solute cannot diffuse through solid matrix. As a result, temperature perturbations advect more slowly than salinity fluctuations by the factor P, but diffuse more rapidly by the factor U/D and are so smoothed out more efficiently. Here, P is porosity, while U and D denote the thermal and chemical molecular diffusivity, respectively. Double-advective instabilities may play a significant role in solute and heat transport in the deep crust where porosities are low. We have studied the stability and dynamics of the flow as a function of P and thermal and chemical buoyancy, for situations where mechanical dispersion of solute dominates over molecular diffusion in the fluid. In the numerical experiments, a porous medium is heated from below while solute provides a stabilizing influence. For typical geological parameters, the thermohaline flow appears intrinsically chaotic. We attribute the chaotic dynamical behavior of the flow to a dominance of advective and dispersive chemical transfer over the more moderate convective heat transfer, the latter actually driving the flow. Fast upward advective transport and lateral mixing of solute leads to formation of horizontal chemical barriers at depth. These gravitationally stable interfaces divide the domain in several layers of distinct composition and lead to significantly reduced heat flow for thousands of years. The unsteady behavior of thermochemical flow in low-porosity regions has implications for heat transport at mid-ocean ridges, for ore genesis, for metasomatism and metamorphic petrology, and the diagenetic history of sediments in subsiding basins. ß 1999 Elsevier Science B.V. All rights reserved

Long-Term Volumetric Eruption Rates and Magma Budgets

A global compilation of 170 time-averaged volumetric volcanic output rates (Qe) is evaluated in terms of composition and petrotectonic setting to advance the understanding of long-term rates of magma generation and eruption on Earth. Repose periods between successive eruptions at a given site and intrusive:extrusive ratios were compiled for selected volcanic centers where long-term (\u3e104 years) data were available. More silicic compositions, rhyolites and andesites, have a more limited range of eruption rates than basalts. Even when high Qe values contributed by flood basalts (9 ± 2 10[1]1 km3/yr) are removed, there is a trend in decreasing average Qe with lava composition from basaltic eruptions (2.6 ± 1.0 10[1]2 km3/yr) to andesites (2.3 ± 0.8 10[1]3 km3/yr) and rhyolites (4.0 ± 1.4 10[1]3 km3/yr). This trend is also seen in the difference between oceanic and continental settings, as eruptions on oceanic crust tend to be predominately basaltic. All of the volcanoes occurring in oceanic settings fail to have statistically different mean Qe and have an overall average of 2.8 ± 0.4 10[1]2 km3/yr, excluding flood basalts. Likewise, all of the volcanoes on continental crust also fail to have statistically different mean Qe and have an overall average of 4.4 ± 0.8 10[1]3 km3/yr. Flood basalts also form a distinctive class with an average Qe nearly two orders of magnitude higher than any other class. However, we have found no systematic evidence linking increased intrusive:extrusive ratios with lower volcanic rates. A simple heat balance analysis suggests that the preponderance of volcanic systems must be open magmatic systems with respect to heat and matter transport in order to maintain eruptible magma at shallow depth throughout the observed lifetime of the volcano. The empirical upper limit of 10[1]2 km3/yr for magma eruption rate in systems with relatively high intrusive:extrusive ratios may be a consequence of the fundamental parameters governing rates of melt generation (e.g., subsolidus isentropic decompression, hydration due to slab dehydration and heat transfer between underplated magma and the overlying crust) in the Earth

Termodynaamiset rajat silikaattisen kuoren assimilaatiolle primitiivisissä magmoissa

Some geochemical models for basaltic and more primitive rocks suggest that their parental magmas have assimilated tens of weight percent of crustal silicate wall rock. But what are the thermodynamic limits for assimilation in primitive magmas? We pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus on modeling assimilation of wall-rock partial melts, which is thermodynamically more efficient compared to bulk assimilation of stoped wall-rock blocks in primitive igneous systems. In the simulations, diverse komatiitic, picritic, and basaltic parental magmas assimilate progressive partial melts of preheated average lower, middle, and upper crust in amounts allowed by thermodynamics. Our results indicate that it is difficult for any subalkaline primitive magma to assimilate more than 20−30 wt% of upper or middle crust before evolving to compositions with higher SiO2 than a basaltic magma (52 wt%). On the other hand, typical komatiitic magmas have thermodynamic potential to assimilate as much as their own mass (59−102 wt%) of lower crust and retain a basaltic composition. The compositions of the parental melt and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition given typical temperatures. These findings have important implications for the role of assimilation in the generation and evolution of, e.g., ultramafic to mafic trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type anorthosites.Peer reviewe

Syvä avoin varasto ja matala suljettu kuljetussysteemi laakiobasalttisekvenssille paljastui Magmakammiosimulaattorin avulla

Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from similar to 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., epsilon(Nd) from -16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (epsilon(Nd) up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (delta O-18 = 4.4-5.2%; Fo = 78-92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher delta O-18 (6.2-7.5%; Fo = 70-88 mol%); and one of the dike samples gives intermediate compositions (5.2-6.1%, Fo = 83-87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, delta O-18 correlates positively with enrichments in large-ion lithophile elements (especially K) and Os-187. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.Peer reviewe

Thermodynamic Model for Energy-Constrained Open-System Evolution of Crustal Magma Bodies Undergoing Simultaneous Recharge, Assimilation and Crystallization: the Magma Chamber Simulator

The Magma Chamber Simulator quantifies the impact of simultaneous recharge, assimilation and crystallization through mass and enthalpy balance in a multicomponent–multiphase (melt + solids ± fluid) composite system. As a rigorous thermodynamic model, the Magma Chamber Simulator computes phase equilibria and geochemical evolution self-consistently in resident magma, recharge magma and wallrock, all of which are connected by specified thermodynamic boundaries, to model an evolving open-system magma body. In a simulation, magma cools from its liquidus temperature, and crystals ± fluid are incrementally fractionated to a separate cumulate reservoir. Enthalpy from cooling, crystallization, and possible magma recharge heats wallrock from its initial subsolidus temperature. Assimilation begins when a critical wallrock melt volume fraction (0·04–0·12) in a range consistent with the rheology of partially molten rock systems is achieved. The mass of melt above this limit is removed from the wallrock and homogenized with the magma body melt. New equilibrium states for magma and wallrock are calculated that reflect conservation of total mass, mass of each element and enthalpy. Magma cooling and crystallization, addition of recharge magma and anatectic melt to the magma body (where appropriate), and heating and partial melting of wallrock continue until magma and wallrock reach thermal equilibrium. For each simulation step, mass and energy balance and thermodynamic assessment of phase relations provide major and trace element concentrations, isotopic characteristics, masses, and thermal constraints for all phases (melt + solids ± fluid) in the composite system. Model input includes initial compositional, thermal and mass information relevant to each subsystem, as well as solid–melt and solid–fluid partition coefficients for all phases. Magma Chamber Simulator results of an assimilation–fractional crystallization (AFC) scenario in which dioritic wallrock at 0·1 GPa contaminates high-alumina basalt are compared with results in which no assimilation occurs [fractional crystallization only (FC-only)]. Key comparisons underscore the need for multicomponent–multiphase energy-constrained thermodynamic modeling of open systems, as follows. (1) Partial melting of dioritic wallrock yields cooler silicic melt that contaminates hotter magma. Magma responds by cooling, but a pulse of crystallization, possibly expected based on thermal arguments, does not occur because assimilation suppresses crystallization by modifying the topology of multicomponent phase saturation surfaces. As a consequence, contaminated magma composition and crystallizing solids are distinct compared with the FC-only case. (2) At similar stages of evolution, contaminated melt is more voluminous (∼3·5×) than melt formed by FC-only. (3) In AFC, some trace element concentrations are lower than their FC-only counterparts at the same stage of evolution. Elements that typically behave incompatibly in mafic and intermediate magmas (e.g. La, Nd, Ba) may not be ‘enriched’ by crustal contamination, and the most ‘crustal’ isotope signatures may not correlate with the highest concentrations of such elements. (4) The proportion of an element contributed by anatectic melt to resident magma is typically different for each element, and thus the extent of mass exchange between crust and magma should be quantified using total mass rather than the mass of a single element. Based on these sometimes unexpected results, it can be argued that progress in quantifying the origin and evolution of open magmatic systems and documenting how mantle-derived magmas and the crust interact rely not only on improvements in instrumentation and generation of larger datasets, but also on continued development of computational tools that couple thermodynamic assessment of phase equilibria in multicomponent systems with energy and mass conservation