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
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 titanite – nepheline
– diopside (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
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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