Mineral-melt partition coefficients of trace-elements

in melilite-bearing and melilite-free rocks of carbonatite complexes

Rass I.T.

Institute of geology of ore deposits, petrography, mineralogy and geochemistry, Moscow, Russia 


     The differences in the concentrations and mineral/melt partition-coefficients of trace elements can be controlled by differences in the concentrations of these elements in the parental melt, the conditions under which these melts were derived in the mantle, by the crystallization sequence of minerals in the course of magma differentiation in the Earth’s crust, and by the conditions of its crystallization [P, T, f(O2), P(CO2), a(SiO2)].

     The problem of the composition of primitive mantle-derived magmas and specific features of their differentiation in the Earth’s crust.is of great importance to research in the genetic relationships between carbonatites and associated silicate igneous rocks in alkaline-ultramafic-carbonatite complexes, as these rocks, along with kimberlites and lamproites, are the products of the deepest mantle-derived magmas known at the Earth’s surface. More then 70% of known carbonatite massifs and kimberlite fields on the Siberian Platform are located along ridges in the Moho discontinuity. Such ridges rise up to 15 km above the average depth of the discontinuity, as established by Chernyshov, Bokaja (1984), and the largest massifs, Tomtor, Guli, Odikhincha (northwestern Siberia) occur at ridge intersections (Kravchenko et al. 1997).

     Melilite-bearing rocks are typical components of the alkaline-ultramafic complexes containing carbonatites. They occur in 10 of 19 massifs of the Maimecha-Kotuy area (northwestern Siberian Platform), the largest alkaline province in the world. Two coexisting series, strongly Ca-enriched melilite-bearing rocks and more common melilite-free alkaline-ultramafic rocks compose the alkaline–ultramafic association (Kravchenko, Rass 1985; Peterson, 1989; Nielsen, 1994). Compositions of rock-forming minerals demonstrate the essential distinctions dependent on their affinity to rocks of the two series: olivine from the series with melilite is enriched in Ca, nepheline of the melilite-bearing rocks has higher Ca and lower K contents, and clinopyroxene from melilite-bearing rocks is also enriched in Ca, compared with clinopyroxene from melilite-free rocks, which is richer in Na content. The REE concentrations of the clinopyroxene are higher in melilite-bearing than in the melilite-free rocks of the same massif. Carbonatites associated with melilite-bearing rocks in alkaline–ultramafic complexes show low P coupled with high contents of Zr, Nb and REE, whereas carbonatites in complexes without melilite-bearing rocks are enriched either in Nb (where alkaline–ultramafic rocks have Na > K), or in REE (if K > Na) (Rass 1998). The trace-element composition of carbonatites from complexes with and without melilite-bearing rocks inherits to some extent the contents of trace elements from the parental alkaline–ultramafic magmas, which fractionate differently in the Earth’s crust.

     The observed different major- and trace-element contents and the zoning patterns in coexisting minerals, trace-element fractionation during differentiation of the parental magmas of these series in accordance with the Rayleigh model with different partition-coefficients, as well as recent results on melt inclusions in olivine of melilite-bearing rocks from the Guli and Kaiserstuhl carbonatite complexes (Rass, Plechov 2000; Solovova et al. 2005), provide strong support for the existence of two different mantle-derived magmas parental for the above series (Kravchenko, Rass 1985). The separate primitive magma, essentially richer in Ca and poorer in Si, that is parental for melilite-bearing series, was derived at ≤40 kbar (Kravchenko et al., 1992; Wilson et al.,1995; Gudfinnsson, Presnall, 2005) and was originally enriched in CO2, Sr, REE, and Nb. This magma fractionated, during crystallization of melilite-bearing differentiates, at shallower depths (<15kb), lower CO2 activity and higher oxygen fugacity, as compared with the conditions of differentiation of the Ca-poor magma. In turn, the fractionation of the Ca-poor magma, parental to melilite-free rocks, could begin at great depths (≥20 kbar), during its ascent toward the surface. The differences in compositions of the initial magmas, generated at different depths from metasomatically and heterogeneously altered sources in the mantle, defined different paths of magma evolution, which eventually led to the crystallization of melilite-bearing or melilite-free rocks. According to the diagram for the system CaO–SiO2–MgO and phase equilibria in the pseudoternary system titanitenephelinediopside (Veksler, Teptelev 1990), crystallization should proceed in a different sequence, e.g., earlier crystallization of perovskite or magnetite, perovskite or pyroxene, and melilite or pyroxene.

     The compositions of the accessory minerals in melilite-bearing (i.e., Ca-rich) and melilite-free (i.e., Ca-poor) series from the massifs Guli, Kugda, Odikhincha, Kara–Meni, and Turiy Cape, have been studied by electron-probe micro-analysis (EPMA) techniques. Minerals from rocks in the above series display distinct compositions and zoning in terms of both major and trace elements.

     Magnetite in the melilite-bearing rocks is noticeably enriched in Al and Mg, and depleted in Mn compared to that in melilite-free rocks. The higher Al content of magnetite from melilite-bearing  rocks, and also the core-to-rim increase of the grossular component in crystals of titaniferous andradite seem to result from higher contents of Al in the initial magma. Magnetite from melilite-free rocks is richer in Ti, decreasing from the core of a crystal toward its margin. Perovskite from melilite-bearing rocks has a higher Fe content than in melilite-free rocks. These features in the melilite-free rocks may be caused by earlier crystallization of magnetite with respect to perovskite. Related differences in Fe zoning in perovskite and in Ti distribution in magnetite of the melilite-bearing and  melilite-free rocks are controlled by the earlier crystallization of perovskite in the melilite-bearing rocks (Rass et al. 1996). The observed difference in magnetite composition may be caused by different fugacity of oxygen in the two magma types, as the Ti content of magnetite is very sensitive to the oxidation conditions. The compositional trends exhibited by perovskite in the melilite-bearing and melilite-free rocks depend not only on the order of crystallization of perovskite and magnetite (Chakhmouradian & Mitchell 1997), but also on higher f(O2), because Fe3+ in perovskite can be used as an oxygen barometer (Canil & Bellis 2007). Therefore, the rocks with more Fe-rich perovskite might be more oxidized than the ones with Fe-poor perovskite.

     The concentrations of Sr in melilite-bearing rocks are an order of  magnitude higher than in melilite-free ones. The melilite/melt partition-coefficient for Sr is about 1 (Kuehner et al. 1989, Krigman et al. 1995). That is why the occurrence of melilite as a rock-forming mineral in all differentiates controls the Sr contents and  the pattern of zoning in other minerals. In the melilite-free rocks, Sr is concentrated in apatite (apatite/melt partition coefficient > 1) and, then, in perovskite. Both minerals in these rocks have higher Sr contents compared to those in melilite-bearing rocks. Concentrations of Sr in apatite decrease from core to rim in melilite-free rocks, whereas there is no Sr zoning in melilite-bearing rocks. The distribution of REE in apatite is also different in melilite-bearing and melilite-free rocks: Ce increases from core to rim in melilite-bearing  rocks, but its level of concentration in melilite-free rocks is below the detection limit (or near it) (Rass, Laputina 1996). The SiO2 content of apatite in melilite-bearing rocks reaches ~1 wt.%, caused by lower CO2 activity, and shows a core-to-rim decrease, whereas in melilite-free MF rocks, its concentrations are noticeably lower.



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