COMAGMAT: DEVELOPMENT OF A MAGMA CRYSTALLIZATION MODEL AND ITS PETROLOGICAL APPLICATIONS

Abstract

This publication summarizes the results of petrologic studies aimed at the development and appli- cation of computer models simulating equilibrium and fractional crystallization in igneous systems of mafic to intermediate composition. The empiric and thermodynamic basis for the mineral-melt equilibria calculations is given in detail, including descriptions of the algorithm used in the COMAGMAT program to simulate both per- fect and partial magma fractionation. New techniques are presented for the genetic interpretation of igneous rocks from large layered intrusions and Siberian sills and volcanic rocks from the Mid-Atlantic Ridge and East- ern Kamchatka. Using the COMAGMAT crystallization model, a set of low-pressure phase equilibria calcula- tions (called Geochemical Thermometry ) was conducted for the most primitive rocks from the Marginal Series and the Lower Zones of differentiated intrusions (Partridge River and Talnakh) and large layered complexes, such as Skaergaard, Kiglapait, and Yoko-Dovyren. This allowed us to define the range of initial temperatures (1165-1230 ° C) and the intercumulus melt compositions intrinsic to the original crystal mush from which the contact rocks crystallized. Simultaneously, we estimated the original mineral-melt proportions for the parental intrusive magmas. These modal proportions indicate that the magmatic suspensions could contain from a few percent (Siberian traps) to 15-25% (Talnakh and Kiglapait) and even 50-60% crystals (Yoko-Dovyren and Par- tridge River intrusions). Moreover, an impressive potential of a special sort of Geochemical Thermometry was demonstrated, which was designed for the interpretation of differentiated intrusion rocks. Based on the results of the modeling of phase equilibria for 65 rocks from the Layered Series of the Skaergaard intrusion, the first evaluations were obtained for the range of the intercumulus liquid temperatures (1145 to 1085 ° C) and oxygen fugacities (from 1-1.5 log units above QFM to slightly below QFM). The calculated average major-element compositions of the residual liquids demonstrate a trend of continual enrichment in FeO (up to ~18 wt %) and TiO 2 (up to ~5.5 wt %) with only minor variations in the SiO 2 contents (48 to 50 wt %) for the rocks from LZa to UZa. Using the ability of COMAGMAT-3.5 to calculate more accurately the crystallization of magnetite and ilmenite, the effect of oxygen fugacity on basalt magma fractionation trends was studied numerically. The application of this program to the tholeiitic series of Chazhma Sill in Eastern Kamchatka, Russia, points to an oxygen fugacity near NNO + 0.5. The numerical simulation of the fractional crystallization of an iron-enriched basaltic andesite parent under these oxidizing conditions accurately reproduced the FeO-SiO 2 relations observed in the Chazhma suite. Another example of the modeling of basalt magma fractionation includes poly- baric calculations simulating the formation of tholeiitic glasses from the Mid-Atlantic Ridge. A decompression version of COMAGMAT was applied to the genetic interpretation of the volcanic suite of Klyuchevskoi vol- cano, Kamchatka. To identify the processes responsible for the origin of the suite, which is composed of rocks ranging from high-magnesia to high-alumina basalts, we used the model to simulate the isobaric crystallization of a parental HMB magma at a variety of pressures and conducted a separate set of simulations assuming frac- tionation during continuous magma ascent from a depth of 60 km. These results provide evidence that the Kly- uchevskoi trend could be caused by ~40% fractionation of the Ol-Aug- Sp ± Opx assemblage during the ascent of the parental HMB magma within a pressure range of 19-7 kbar at 1350-1110 ° C. The decompression rate was estimated at 0.33 kbar/% crystallized, with ~2 wt % H 2 O in the initial melt and ~3 wt % H 2 O in the resultant high-Al basalt.