Paul D. Asimow

Algorithmic modifications extending MELTS to calculate subsolidus phase relations

Abstract Algorithmic modifications to the MELTS software package are presented in order that calculations of heterogeneous phase equilibria can be performed in the subsolidus. Methods are presented for: (1) selecting an ‘‘initial guess assemblage’’ that satisfies the bulk composition constraints; (2) detecting saturation of new phases (including liquid) in an assemblage; (3) adding and removing phases from the assemblage without adjusting the system bulk composition; and (4) constraining the assemblage to a fixed fO₂ . These methods have been added to MELTS, allowing it to calculate heterogeneous phase equilibria with or without liquid, closed or open to O, and with fixed intensive variables (P,T), (P,S), (P,H), or (V,T). Applications include fractional melting calculations, metamorphic phase equilibria, and geophysical models of subsolidus regions of the Earth.

The importance of water to oceanic mantle melting regimes

The formation of basaltic crust at mid-ocean ridges and ocean islands provides a window into the compositional and thermal state of the Earths upper mantle. But the interpretation of geochemical and crustal-thickness data in terms of magma source parameters depends on our understanding of the melting, melt-extraction and differentiation processes that intervene between the magma source and the crust. Much of the quantitative theory developed to model these processes has neglected the role of water in the mantle and in magma, despite the observed presence of water in ocean-floor basalts. Here we extend two quantitative models of ridge melting, mixing and fractionation to show that the addition of water can cause an increase in total melt production and crustal thickness while causing a decrease in mean extent of melting. This may help to resolve several enigmatic observations in the major- and trace-element chemistry of both normal and hotspot-affected ridge basalts.

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile-enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ~1450–1500°C in the Atlantic and 1500–1600°C in the Pacific, substantially greater than ~1350°C for ambient mantle. Lavas from the Galapagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

Adiabat_1ph: A new public front-end to the MELTS, pMELTS, and pHMELTS models

The program adiabat_1ph is a simple text-menu driver for subroutine versions of the algorithms MELTS, pMELTS, and pHMELTS [Asimow et al., 2004; Ghiorso et al., 2002; Ghiorso and Sack, 1995]. It may be used to calculate equilibrium assemblages along a thermodynamic path set by the user and can simultaneously calculate trace element distributions. The MELTS family of algorithms is suitable for multicomponent systems, which may be anhydrous, water-undersaturated, or water-saturated, with the options of buffering oxygen fugacity and/or water activity. A wide variety of calculations can be performed either subsolidus or with liquid(s) present; melting and crystallization may be batch, fractional, or continuous. The software is suitable for Linux, MacOS X, and Windows, and many aspects of program execution are controlled by environment variables. Perl scripts are also provided that may be used to invoke adiabat_1ph with some command line options and to produce output that may be easily imported into spreadsheet programs, such as Microsoft Excel. Benefits include a batch mode, which allows almost complete automation of the calculation process when suitable input files are written. This technical brief describes version 1.04, which is provided as ancillary material. Binaries, scripts, documentation, and example files for this and future releases may be downloaded at http://www.gps.caltech.edu/~asimow/adiabat. On a networked computer, adiabat_1ph automatically checks whether a newer version is available.

A hydrous melting and fractionation model for mid‐ocean ridge basalts: Application to the Mid‐Atlantic Ridge near the Azores

The major element, trace element, and isotopic composition of mid-ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H2O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge-hotspot interaction therefore requires a comprehensive model that accounts for the effect of H2O in the source on melting behavior and for the effect of H2O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ~700 ppm H2O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H2O-rich melting regimes. An on-axis thermal anomaly of ~35°C in potential temperature explains the full crustal thickness increase of ~4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO2, trace element and isotopic composition of the H2O-rich component in a way that self-consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.

Hydrogen incorporation in olivine from 2–12 GPa

Abstract We performed new experiments on incorporation of hydrogen in olivine at high pressures (2-12 GPa) and temperatures (1000-1300 °C). OH concentrations were calculated using the Bell et al. (2003) calibration applied to principal-axis infrared absorption spectra synthesized from polarized measurements on randomly oriented grains. Starting materials for the experiments included both fine-grained powders and larger single crystals. Hydrogen was incorporated during grain growth in the former case and by volume diffusion in the latter. The spectra of Fe-bearing olivines exhibit similar structure regardless of the starting material, and are dominated by bands in the wavenumber range from about 3500 to 3650 cm-1. We do not observe bands at 3525 and 3573 cm-1, which are predominant in many natural olivines as well as olivines annealed in experiments at lower pressures, and are attributed to humite-related defects. Furthermore, bands between 3300 and 3400 cm-1, attributed to high silica activity or high oxygen fugacity, are weak or non-existent. Our measurements indicate that OH solubility in Fe-bearing olivine is 2.5-4 times higher than that measured by Kohlstedt et al. (1996). Although this is largely due to the use of a new calibration in our study, correction of previous values is not straightforward. In the pure Mg-system, in contrast to Fe-bearing olivine, order-of-magnitude apparent differences in OH solubility can be obtained using different experimental procedures. This raises questions about attainment of equilibrium in experimental studies of hydrogen incorporation in nominally anhydrous minerals, particularly when crystals are grown from a hydrous melt.

An analysis of variations in isentropic melt productivity

The amount of melt generated per unit pressure drop during adiabatic upwelling, the isentropic melt productivity, cannot be determined directly from experiments and is commonly assumed to be constant or to decrease as melting progresses. From analysis of one– and two–component systems and from calculations based on a thermodynamic model of peridotite partial melting, we show that productivity for reversible adiabatic (i.e. isentropic) depressurization melting is never constant; rather, productivity tends to increase as melting proceeds. Even in a one–component system with a univariant solid–liquid boundary, the 1/T dependence of (∂S/∂T)P and the downward curvature of the solidus (due to greater compressibility of liquids relative to minerals) lead to increased productivity with increasing melt fraction during batch fusion (and even for fractional fusion in some cases). Similarly, for multicomponent systems, downward curvature of contours of equal melt fraction between the solidus and the liquidus contributes to an increase in productivity as melting proceeds. In multicomponent systems, there is also a lever–rule relationship between productivity and the compositions of coexisting liquid and residue such that productivity is inversely related to the compositional distance between coexisting bulk solid and liquid. For most geologically relevant cases, this quantity decreases during progressive melting, again contributing to an increase in productivity with increasing melting. These results all suggest that the increases in productivity with increasing melt fraction (punctuated by drops in productivity upon exhaustion of each phase from the residue) predicted by thermodynamic modelling of melting of typical mantle peridotites using MELTS are neither artifacts nor unique properties of the model, but rather general consequences of adiabatic melting of upwelling mantle.

THE EFFECT OF PRESSURE-INDUCED SOLID-SOLID PHASE TRANSITIONS ON DECOMPRESSION MELTING OF THE MANTLE

Pressure-release melting of the earths mantle is thought to be an isentropic process. The intersection of an isentropic melting path with a solid-state phase transition affecting the residual minerals must result in some change in melting rate unless the entropy of reaction of the phase transition is exactly zero. Furthermore, both phase transitions of primary interest for peridotite melting in the upper mantle (garnet-spinel peridotite and spinel-plagioclase peridotite) have positive Clapeyron slopes, and hence the lower-pressure assemblage has a higher molar entropy. Thus, these phase transitions must retard isentropic, decompression melting, or even lead to freezing. There cannot be enhanced melting accompanying such phase transitions, even if there is a cusp in the solidus. Model calculations in simple one-component and two-component systems demonstrate the effect of solid-solid phase transformations on isentropic decompression melting. Conversion of low entropy solids to high entropy solids in the presence of liquid results in crystallization; the amount of crystallization depends on the relative molar entropies of the solid and liquid phases, on the modal abundance of the reacting solid phases, and on the proportion of liquid present when the reaction is initiated. The effect of solid-solid phase transitions on freezing is more pronounced for fractional fusion than for batch fusion in that melting ceases for a finite pressure interval at pressures below the invariant point where melt and the solids involved in the phase transition coexist. Isentropic upwelling calculations for a model nine-component peridotite using a modified version of the MELTS potential minimization algorithm (Ghiorso et al., 1994; Ghiorso and Sack, 1995) verify that the simple-system behavior can be extended to multicomponent, mantle-like systems: melt production is suppressed during the transformation from garnet to spinel peridotite and for batch melting there is freezing during the transformation from spinel to plagioclase peridotite; these effects are exaggerated during fractional fusion and barren zones are produced as in the simple systems. These results imply that melt production during upwelling may be highly nonuniform. Very slow melt production in the spinel-garnet transition region may enhance development of U-Th disequilibria. If significant contributions of melt from garnet peridotite are needed to account for the Lu-Hf systematics and REE patterns in MORB, melting must begin deep within the garnet stability zone. If plagioclase ever appears in the melting residue, this event is likely to end decompression melting despite further upwelling. Regions such as the garnet-spinel and spinel-plagioclase peridotite transitions may serve as nucleation sites for solitary waves in porous flow or regions of fracture formation and enhanced melt segregation.

Shock‐induced melting of MgSiO3 perovskite and implications for melts in Earth's lowermost mantle

[1] New shock wave equation of state (EOS) data for enstatite and MgSiO3 glass constrain the density change upon melting of Mg-silicate perovskite up to 200 GPa. The melt becomes denser than perovskite near the base of Earth’s lower mantle. This inference is confirmed by shock temperature data suggesting a negative pressure-temperature slope along the melting curve at high pressure. Although melting of Earth’s mantle involves multiple phases and chemical components, this implies that the partial melts invoked to explain anomalous seismic velocities in the lowermost mantle may be dynamically stable. INDEX TERMS: 3919 Mineral Physics: Equations of state; 3944 Mineral Physics: Shock wave experiments; 8124 Tectonophysics: Earth’s interior—composition and state (1212). Citation: Akins, J. A., S.-N. Luo, P. D. Asimow, and T. J. Ahrens (2004), Shock-induced melting of MgSiO3 perovskite and implications for melts in Earth’s lowermost mantle, Geophys. Res. Lett., 31, L14612, doi:10.1029/2004GL020237.

PRIMELT3 MEGA.XLSM software for primary magma calculation: Peridotite primary magma MgO contents from the liquidus to the solidus

An upgrade of the PRIMELT algorithm for calculating primary magma composition is given together with its implementation in PRIMELT3 MEGA.xlsm software. It supersedes PRIMELT2.xls in correcting minor mistakes in melt fraction and computed Ni content of olivine, it identifies residuum mineralogy, and it provides a thorough analysis of uncertainties in mantle potential temperature and olivine liquidus temperature. The uncertainty analysis was made tractable by the computation of olivine liquidus temperatures as functions of pressure and partial melt MgO content between the liquidus and solidus. We present a computed anhydrous peridotite solidus in T-P space using relations amongst MgO, T and P along the solidus; it compares well with experiments on the solidus. Results of the application of PRIMELT3 to a wide range of basalts shows that the mantle sources of ocean islands and large igneous provinces were hotter than oceanic spreading centers, consistent with earlier studies and expectations of the mantle plume model.