Alexei A. Ariskin

COMAGMAT: a Fortran program to model MAGMA differentiation processes

Abstract The purpose of this paper is to present a general model (COMAGMAT) for the calculation of equilibrium temperatures and phase relations at a given extent of crystallization or melting in natural magmatic systems. This model is based on a set of empirical expressions that describe mineral-melt equilibria for major and trace elements in terms of pressure (up to 12 kbar), temperature, and liquid compositions for systems ranging from primitive basalts to dacites. These expressions are in the form of empirically calibrated mineral-melt geothermometers for Olivine, Augite, Pigeonite (Opx), Plagioclase, Ilmenite, and Magnetite used to develop the algorithm simulating multiply saturated magmatic melts. The results of the program are in the form of calculated liquid lines of descent, plus the equilibrium mineral proportions and compositions. The phase equilibria calculations form the core of a model that allows the user to simulate processes ranging from simple isobaric crystallization to in situ differentiation processes resulted from crystal settling, and polybaric fractionation.

An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 2. Fe-Ti oxides

Abstract In order to develop models simulating the crystallization of Fe-Ti oxides in natural lavas, we have processed published experimental data on magnetite-melt and ilmenite-melt equilibria. These data include 62 Mt-melt and 75 Ilm-melt pairs at temperatures 1040–1150 °C, oxygen fugacities from IW to NNO+2, and bulk compositions ranging from ferrobasalts to andesites and dacites. Five major cations (Fe3+, Fe2+, Ti4+, Mg2+ and Al3+) were considered for the purpose of describing Fe-Ti oxide saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure. The empirically calibrated mineral-melt expression based on multiple linear regressions is: ln Di = a/T + blog fO2 + c + d1XNa + d2XK + d3XP, where Di represents molar distribution coefficients of the given cations between Mt/Ilm and melt; XNa, XK, and XP are the molar fractions of Na, K, and P in the melt. The empirically calibrated Mt-melt and Ilm-melt equilibria equations allowed us to develop two models for calculating crystallization temperatures of the Fe-Ti oxides in the melts with an accuracy of 10–15 °C, and compositions with an accuracy of 0.5–2 mol%. These models have been integrated into the COMAGMAT-3.5 program, improving our ability to study numerically the effects of temperature and oxygen fugacity on the stability and phase equilibria of Fe-Ti oxides. Application of this approach to the tholeiitic series of Chazhma Sill from Eastern Kamchatka (Russia) indicates oxygen fugacity conditions near NNO + 0.5. Numerical simulation of fractional crystallization of an iron-enriched basaltic andesite parent at these oxidizing conditions accurately reproduces the FeO-SiO2 relations observed in the Chazhma suite.

Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition

In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were ∼1,150°C, 2 kbar, and fO2 slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)→olivine+plagioclase+magnetite (1,146°C, WM+0.5)→olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo66.7±1.1 and An64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo69.1±2.8) and the average composition of plagioclase core (An66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987).

INFOREX-3.0: a database on experimental studies of phase equilibria in igneous rocks and synthetic systems II: data description and petrological applications

The purpose of the paper is to provide details of the application of the INFOREX-3.0 database, a package designed to store, retrieve, and process phase equilibria information. This most recent release of the system accesses data of 162 experimental studies, conducted from 1962 to 1994, including a total of 6174 experiments with 5188 addressed to natural igneous rocks and 986 runs carried out in synthetic systems, mostly CMAS. The total database was divided into 3893 “dry” conditions experiments, and 2281 runs performed in the presence of H2O and/or CO2: 1618 of the “wet” runs represented are water saturated. The number of 1 atm experiments (3750) is greater than the number of high-pressure runs (2474). The INFOREX database contains 8311 coexisting phase compositions: 3197 for glasses, 1247—olivine, 1429—pyroxenes, 501—spinels, 842—plagioclase. One block of the INFOREX information includes 298 liquid compositions where the Fe3+Fe2+ ratio was determined. Data for sulfur and water solubility experiments also have been systematized. The INFOREX data management system allows users to find and print out data on a specific set of mineral-melt or two mineral equilibrium experiments requested for a given range of temperatures, pressures, oxygen fugacities, and compositions in a matter of a few seconds. In addition, one can use subsets of the data to develop mineral-melt geothermometers for equilibria including olivine, plagioclase, pyroxenes, and spinels for any specific system type. Two examples illustrate the use of INFOREX for testing empirical equations proposed for the calculation of water solubility and Fe3+Fe2+ ratio in basic to acid melts.

INFOREX-3.0: a database on experimental studies of phase equilibria in igneous rocks and synthetic systems I: datafile and management system structure

Abstract INFOREX-3.0 is a sophisticated database manager for igneous petrologists and geochemists. It is designed as an aid in searching the published experimental database, and extracting specific subsets of that data. In addition, the program allows the user to derive numerical mineral-melt expressions based on any dataset. Its major function is as a computerized melting-experiment reference manual that presents data in a fixed format, with key word search functions and flags as an integral part of the program. At present, the system accesses information on 162 experimental studies carried out from 1962 to 1994. The database includes 6174 individual runs with more than 8300 coexisting phase compositions for 30 minerals plus melt. Most of the data are “dry”, but about 2200 include volatile components, such as H 2 O and CO 2 . INFOREX is hot-key driven, allowing users to sort easily and quickly through the data using any set of experimental conditions or rock type parameters. The main options include: (1) configuration of INFOREX; (2) extraction of general information on the current state of the system; (3) updating and editing the database files; (4) selection of run conditions of interest (setting rock types plus a range of pressures, temperatures, oxygen fugacities, run duration as well as types of containers and resultant phase assemblages); (5) setting phase assemblages (e.g., olivine-melt or garnet-spinel) for the selected set of experiments; (6) export/import operations; and (7) calculation of mineralmelt geothermometers. The INFOREX database is a powerful tool for the use in the development of petrogenetic models by providing the ability to manipulate easily accessed mineral-melt equilibria data.

Magma dynamics at the base of an evolving mafic magma chamber: Incompatible element evidence from the Partridge River intrusion, Duluth Complex, Minnesota, USA

Abstract A characteristic feature of the Partridge River intrusion of the Keweenawan Duluth Complex is the approximately fivefold to ninefold increase in the concentrations of incompatible elements in the lower zone compared with cumulates stratigraphically higher. The concentrations of incompatible elements decrease from the lower zone upward to steady state values, which is ascribed to variations in the proportions of trapped liquid rather than variable degrees of fractional crystallization of a single parental magma. The calculated average composition of trapped liquid using our algorithm is similar to typical Keweenawan low-alumina, high Ti P basalts associated with the Duluth Complex but is different from the leading edge ferrodioritic liquid quenched in the chilled margin of the intrusion. This difference suggests that the chilled margin does not represent the original (parental) magma composition from which the whole intrusion solidified, and that the enrichment of incompatible elements may be related to the local flotation of magmatic suspensions. To test the latter hypothesis numerically, we have used heat-mass transfer models, assuming a sheet-like magma chamber, to calculate the parameters of the model that best reproduce the observed distribution of incompatible elements in a mush zone at the base of the Partridge River intrusion. The results indicate that a mush zone enriched in the incompatible elements is produced if the velocity of movement of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush. The dynamic parameters that best reproduce the observed distribution of incompatible elements include a magma emplacement pressure of 2 kbar, critical crystallinities of 50–68% in the mush zone from which the liquid is being expelled, and an emplacement temperature of ∼ 1160°C for the initial magma.

Application of Computer Simulation of Magmatic Processes to the Teaching of Petrology

A series of computer programs called COMAGMAT-3.0 has been developed to simulate basaltic magma differentiation processes. The programs are designed for use by igneous penologists to show the chemical consequences of magma crystallization. This package is a powerful tool that can be used to help students better understand how pressure, oxygen fugacity, and initial melt composition effect the resultant liquid lines of descent for 10 major and 20 trace elements. The model calculates the mineral proportions and compositions of olivine, plagioclase, pyroxenes and iron-ore phases crystallizing at different pressure and redox conditions. The effect of each parameter (including mineral-melt distribution coefficients) can be investigated numerically and output to the screen or a printer. Moreover, new methods of igneous rock interpretation can be applied to magmatic systems based on the package. The COMAGMAT programs operate in a user-friendly environment with the built-in help, and graphics support makes it usef...