Spinel group minerals in dunites of Guli massif.  II Numerical modeling of maimechite, pictite and khatangite crystallization.

Zaitsev V.A. Ermolaeva V.N.

Vernadsky Institute of Geochemistry and Analytical Chemistry

The spinel-group minerals are believed to be the most informative phase of ultramafic rocks. Their composition is a complex function depending of the magma composition, volatile elements concentration, oxygen fugacity, P-T parameters of crystallization, cooling rate and history of postmagmatic reequilibration (Kamenetsky et al., 1991).

In this study we try to answer the question about the possible primary melt for Guli massif, comparing the composition of spinel-group minerals from the dunites of Guli massif with model compositions of spinel-group minerals, which are in equilibrium in system, chemically corresponding with primary melt. We consider three candidates to be a primary melt for Guli massif: meimechite, picrite and khatangite.

Using the MELTS Web-applet (Ghiorso, Sack, 1995, Asimow, Ghiorso 1989) we carried out the numerical experiment of equilibration of systems with meimechite, picrite and khatangite bulk composition in wide range of conditions: T=1200-1400єC, P=1-10 kbar, Fe²⁺/Fe^Total = 0.99-0.50 (it is correspond to log(fO2) = –15.50 +4.48 relative QFM buffer) and H₂O content 1% (for meimechite composition we also used  also 0.2, 2 and 5%). The average compositions of meimechite, picrite and hatangite were estimated from published analyses (Kogarko, Ryabchikov, 1999).

The results of modeling allow to estimate the influence of the main parameters of magma crystallisation to the composition of spinel-minerals. In all cases the temperature decreasing results in the decrease of chromium concentration in the spinel minerals, this trend is subparallel to the Fe-Cr side. System oxidation leads to the Fe³⁺ enrichment of chromite at a constancy or slightly reduction of the Al/Cr ratio. Pressure increasing results in increasing of Fe³⁺ content in chrome-spinels together with slight increasing of Al-content. Increasing of water content results in decreasing of Al-content in spinel minerals with slight increasing of Fe³⁺ content. The influence of this factor is strongly limited by the H₂O solubility in the melt.

Comparing of composition fields of model chrome-spinels and chrome-spinel from the Guli massif shows, that model compositions are much richer in Mg: all of they are Mg-dominant, unlike the natural compositions which are Fe-dominant (except the central part of  chromite crystals in maimechite and the shlire chromite in one dunite). We can explain this fact by the post-crystallization re-equilibration, because the diffusion coefficient of Mg and Fe in chromite are quite high, to provide the noticeable changing of Fe/ Mg ratio in conditions of subaeral lava cooling (Kamenetsky et al., 1991). The diffusion coefficients for Cr and Al are much lower, and Cr/Al ratio is preserving practically constant in postcrystallization history even for intrusive bodies (Roeder, Cambell, 1985). It is reason, why Al-Fe-Cr diagram is very convenient for chromite.

Fig.1 The model compositions of spinel-group minerals in the systems with meimechite (A), picrite (B) and khatangite (C) bulk composition on the Al-Fe-Cr diagram

The composition field of model composition of spinel-minerals (see fig.1) in system with picritic bulk composition corresponds to the most aluminum-rich part of natural chrome-spinels from the Guli massif (fig. 1 from part I). The composition fields of model composition of spinels in systems with maimechite and khatahgite bulk composition are much closer to the Al-angle of diagram.

At the same time, Al content in the real chromite from meimechite decrease from core to rim. This difference in composition can be result from joint changing of pressure of crystallization and water content in the melt. For example, it can be explained by crystallization of central parts of chromite grains in the high depth with further moving to upper horizon and covering here by the chromite with lower Al-content. The composition of such chromite is similar to the composition of chromites from r. Vostochny, Ingarynga, Khaluda regions of Guli massif.

Thus both two-stage meimechite crystallization with melt separating from earlier chromite crystals (formed in high-pressure and low water-content conditions), and crystallization of pictrite may result in formation of chrome-spinel composition, which is observed in Guli massif.

References

Egorov L.S. Ijolite-carbonatite plutonism (on the sample of Maymecha-Kotuy complex, Polar Siberia) // Leningrad: Nedra, 1991. 260 p.

Roeder P.L. Cambell I.H. The effect of pustcumulus reaction of chrome-spinels from the Jimberlana intrusion // J. of Pertrology. 1985. Vol. 26. Part 3. Pp.763-786.

Kogarko L.N., Ryabchikov I.D. Geochemical evidence for meimechite magma generation in the subcontinental lithosphere of Polar Siberia // Journal of Asian Earth Sciences. 1999. P. 1-9.

Kamenetsky V.S. Craford A.J., Meffre S. Factors controlling chemistry of magmatic spinel: an empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks.// J. Of petrology, 2001, vol.42 No 4 pp. 655-671.

Ghiorso, M. S., Sack R. O. Chemical Mass Transfer in Magmatic Processes. IV. A Revised and Internally Consistent Thermodynamic Model for the Interpolation and Extrapolation of Liquid-Solid Equilibria in Magmatic Systems at Elevated Temperatures and Pressures. Contributions to Mineralogy and Petrology, 1995 Vol. 119, pp. 197-212.

Asimow P.D, Ghiorso M.S. Algorithmic Modifications Extending MELTS to Calculate Subsolidus Phase Relations. American Mineralogist, 1998, Vol. 83, pp. 1127-1131.