Geochemical features of carbonatites – derivatives of mantle magmas with different Ca-K-Na ratio
Institute of geology of ore deposits, petrography, mineralogy & geochemistry, Russian Academy of Sciences, Moscow, Russia
Trace-element fractionation during the differentiation of mantle-derived high-Mg parental magmas, with different Ca contents, resulted from different conditions at the site of melting. In turn, the specific trace-element signatures of carbonatites could be due to their derivation from each of the possible parental magmas.
It is known that the mantle is heterogeneous both laterally (Kogarko, 1984) and vertically: deeper mantle melts have pervasively higher K/Na ratios (Perchuk, 1973; Ryabchikov, Boettcher, 1980), and the Ca contents of the melts increase with pressure (Ryabchikov, 1984; Kravchenko et al. 1992).
In particular, the lateral compositional variations in the mantle result in that alternatively Na- or K-richer alkaline ultrabasic magmas are produced, a process reflecting various styles of mantle alterations.
The vertical heterogeneity of the mantle results in carbonatites produced by deeper seated and more calcic, or more potassic, magmas being enriched in refractory components, particularly Zr. The stronger Zr enrichment of higher temperature magmas is also confirmed by experimental studies (Kogarko et al., 1988) Our studies of the alkaline-ultramafic association with K/Na < 1 (in alkaline rocks) indicate that it comprises two differentiation series whose parental magmas, possessing different Ca contents, originate from different depths (Kravchenko, Rass, 1985; Kravchenko et al. 1992) A separate primitive, high-Ca parental magma for rocks of the melilite-bearing series does exist. Theoretical and experimental investigations of phase equilibria in SiO2-undersaturated magmas have shown that such melts can exist at mantle temperatures and pressures (Kogarko, Green, 1998).This separate primitive magma, essentially richer in Ca and poorer in Si, that is parental for melilite-bearing series, was derived at ≤40 kbar and was originally enriched in CO2, Sr, REE, and Nb.
This magma fractionated, during crystallization of melilite-bearing differentiates, at shallower depths, 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 primitive melilitite melts are compared with the compositions of IB kimberlites as possible parental magmas for melilite-bearing rocks in alkaline ultramafic – carbonatite complexes.
The differences in compositions of the initial magmas, generated in 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 titanite-nepheline-diopside (Veksler, Teptelev, 1990), crystallization should proceed in a different sequence, e.g., earliar crystallization of perovskite or magnetite, perovskite or pyroxene, and melilite or pyroxene.
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)].
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 parental alkaline-ultramafic magmas, which fractionate differently in the Earth’s crust.
High contents of simultaneously Nb and REE in carbonatites of the Tomtor complex (Polar Siberia) may be easily explained with a supposition that a separate K-Ca primitive magma was possible. The occurrence of lamproites (Kravchenko, 2003; 2006/2007) and of melilite dikes (Porshnev, Stepanov, 1980) makes this assumption quite realistic. The geochemistry of carbonatites in the Fort Portal complex, Uganda, (Eby et al., 2009) looks like that in the Turii Mys, Russia (Rass, 1998). Then, a comparison of compositions of melt inclusions in olivines from rocks of the Mushugai-Khuduk carbonatite complex, Mongolia, without any melilite-bearing rocks (Andreeva, Kovalenko, 2003) and from the Vulture alkaline igneous complex, Italy (Solovova et al., 2005) also with K>Na in silicate rocks, but containing rocks affiliated to potassic-melilitite (kamafugite) series, seems to show that the parental magma of the latter was close or identical to the composition of the established one for high-Ca series in the alkaline-ultramafic association with K/Na < 1.
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