Description: The mixing of magmas of distinct temperature, bulk composition, mineralogy, and physical properties plays a central role in explaining the diversity of magma types on Earth and in explaining the growth of continental and oceanic crust. Magma mixing is also of practical importance. For example, the mixing of distinct magmas has been cited as an important process in creation of economically important horizons in layered intrusions as well as a triggering mechanism for initiation of volcanic eruptions. The motivation for better quantifying the dynamics and thermodynamics of magma mixing and its attendant plutonic and volcanic products is clear. The degree of magma mixing, which spans a continuum from mingling to complete hybridization, depends upon initial and boundary conditions, magma properties, driving forces, and time available for mixing. Magma mingling produces a heterogeneous mixture of discrete clumps of the end-member magmas, whereas complete hybridization involves the thermodynamic equilibration of two distinct magmas to form a third. Qualitatively, mixing occurs via reduction in the size of compositional heterogeneities (i.e., clumps) through stretching and folding by viscous flow, followed by homogenization, once shear has reduced the size of compositional anomalies to diffusive length scales. Quantification of this process relies on two statistical measures: the linear scale of segregation () defined as the spatial integral of the compositional correlation function related to the size-distribution of the segregated clumps within the mixture, and the intensity of segregation ( I ) a measure that quantifies how much the composition at each location differs from the average. The mixing dynamics of a layered system are analyzed in terms of the parameters governing mixing (Rayleigh, Lewis, and buoyancy numbers and viscosity ratio) to estimate how the timescale for magma hybridization, H , compares to solidification, recharge, diffusive, and assimilation timescales. This analysis illustrates that hybridization times can be shorter than or comparable to thermal, solidification, and replenishment timescales; thus, formation of hybridized or nearly hybridized magmas is one anticipated outcome of mixing. The machinery of thermodynamics can be used to compute the hybrid magma state. An exploratory model for the thermochemistry of hybridization is developed based on binary eutectic phase relations and thermodynamics. Eight thermodynamic parameters define the phase diagram and associated energetics, and six parameters (initial temperatures, compositions, mass ratio of mixing magmas, and an enthalpy parameter) are necessary and sufficient to determine the state of hybrid magma uniquely. While relevant combinations of 14 thermodynamic and mixing parameters might suggest that the number of mixing outcomes (i.e., products) is too high to systematize, Monte Carlo simulations using the exploratory model document how millions of arbitrary initial states evolve into five possible final (mixed) states. Such an analysis implies that a magma mixing taxonomy that defines possible mixed product states can be developed and tied to petrologic indicators of mixing. Additional insights gained from this exploratory model that are supported by independent results from a multicomponent, multiphase thermodynamic model of magma mixing (Magma Chamber Simulator) include: (1) the proclivity of invariant point hybrid states, which may explain some instances of compositionally monotonous melts associated with mixed magma eruptions; (2) a surprising thermal effect such that the temperature of hybridized magma can be significantly less than the initial temperature of either of the mixing magmas. This type of magma mixing may result in crystal resorption, thus invalidating an assumption that resorption textures in crystals are typically the result of a magma heating event; (3) illustration of the differing effects of stoped block temperature and composition on hybrid magma temperature and phase state; and (4) illustration of a cessation of crystallization effect that may pertain to the MORB pyroxene "paradox." Differences between adiabatic or R-hybridization and diabatic or RFC-hybridization are also explored. The model can be used to elucidate the thermodynamic principles underlying magma mixing in the hybridization limit. These principles are of general applicability and carry over to more compositionally complicated systems.

Description: Understanding the thermodynamics of liquid silicates at high pressure and temperature is essential for many petrologic problems, and sodium aluminosilicates are an important component of most magmatic systems. We provide a high-pressure equation of state (EOS) for liquid NaAlSi 3 O 8 based upon molecular dynamics (MD) simulations. The resulting thermodynamic properties have changes in pressure and temperature correlative to trends in diffusion and atomic structure, giving insight to the connections between macroscopic and microscopic properties. Internal pressure shows a maximum in attractive interatomic forces at low pressure, giving way to the dominance of repulsive forces at higher pressure. Self-diffusion coefficients ( D ) typically order D Na 〉 D Al 〉 D O 〉 D Si . At the lowest temperature, self-diffusivity (anomalously) increases as pressure increases up to ~5–6 GPa for Al, Si, and O. Diffusion data outside this "anomalous" region are fit by a modified Arrhenius expression, from which activation energies are calculated: 85 kJ/mol (Na) to 140 kJ/mol (Si). The amount of AlO 4 and SiO 4 polyhedra (tetrahedra) decreases upon compression and is approximately inversely correlated to the abundance of five- and sixfold structures. Average coordination numbers for Al-O, O-O, and Na-O polyhedra increase sharply at low pressure but start to stabilize at higher pressure, corresponding to changes in interatomic repulsion forces as measured by the internal pressure. High-pressure repulsion also correlates with a close-packed O-O structure where ~12 O atoms surround a central O. Self-diffusivity stabilizes at higher pressures as well. Relationships between the internal pressure, self-diffusion, and structural properties illustrate the link between thermodynamic, transport, and structural properties of liquid NaAlSi 3 O 8 at high pressure and temperature, shedding light on how microscopic structural changes influence macroscopic properties in molten aluminosilicates.

Description: 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 x ) 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.

Description: When analyzed using secondary ion mass spectrometry, dust-sized (〈63 µm) zircon in distal ash deposits of the Tierra Blanca Joven (TBJ) eruption of Ilopango Volcano (El Salvador) yielded results consistent with ages obtained from those in proximal deposits. This finding indicates insignificant age sorting of zircon crystals during their dispersal in the TBJ ash plume. As a result, analysis of zircons may permit reliable source identification of distal tephra marker beds commonly found in terrestrial and marine environments. This technique was applied to test whether an enigmatic volcanic ash used to manufacture Late Classic Maya pottery from El Pilar is from distal TBJ ash deposits, a hypothesis supported by the location, extent, and timing of the TBJ eruption, and the matching high silica content and trace element ratios between TBJ glass and glass in the archaeological samples. The exclusively older than 1 Ma ages of the archaeological zircons compared with the dominantly ca. 0–30 ka ages of the TBJ zircons, however, rule out the TBJ eruption as the source of the pottery ash. The three analyzed archaeological pottery samples define two distinct zircon age distributions, indicating that the ash in the Maya pottery must be from multiple sources, which currently remain unidentified.

Description: Non-equilibrium molecular dynamics (NEMD) simulations are used to compute the phonon thermal conductivity ( k ) for liquids and glasses of composition Mg 2 SiO 4 , CaMgSi 2 O 6 , and NaAlSi 3 O 8 at 2000–4500 K and 0–30 GPa based on classical potentials. These compositions span the range of melt polymerization states in natural systems at ambient pressure. The NEMD results compare well with available laboratory measurements on molten NaAlSi 3 O 8 and CaMgSi 2 O 6 at 1 bar. Thermal conductivities decrease with increasing temperature ( T ), increase with increasing pressure ( P ), and at low pressure increase slightly as the mean coordination number of Si and Al around oxygen increases, in the sequence Mg 2 SiO 4 , CaMgSi 2 O 6 , and NaAlSi 3 O 8 . At 3500 K, the thermal conductivity of CaMgSi 2 O 6 at 0, 10, 20, and 30 GPa is 1.1, 2.1, 2.5, and 3 W/mK, respectively. At ambient pressure (0.2 ± 0.15 GPa), k = 1.2 and 0.5 W/mK at 2500 and 4500 K, respectively, for CaMgSi 2 O 6 . For NaAlSi 3 O 8 composition, k varies from 1.7 to 2.7 W/mK at 3050 K for pressures of 6 and 30 GPa, respectively. Mg 2 SiO 4 liquid at ambient pressure (0.07 ± 0.16 GPa) is found to have thermal conductivities of 1.36 and 0.7 W/mK at 2500 and 4500 K, respectively. Tables giving computed k values for all compositions are included for state points studied. The trade-off between T and P implies that the phonon thermal conductivity of silicate liquids at mantle depths increases substantially (factor of 2–3) along isentropes.