Comparison of calcite compositions from extrusive carbonatites at Kaiserstuhl, Germany and Calatrava, Spain: implications for mantle carbonate
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Carbonatite
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Abstract Calcite-rich carbonatites are commonly attributed to calcitization of alkalic carbonatite of Oldoinyo Lengai type. The interpretation arises from the presumption that magmatic crystallization of calcite at atmospheric pressure is not possible. We show that only a small percentage of fluorine, a common element in carbonatite magmas, permits such crystallization, and we argue that most of the calcite in extrusive carbonatites is magmatic. The presence of any more than minor apatite precludes an alkalic carbonatite parentage. While not denying that calcification of alkalic carbonatite can occur, we suggest that it is not generally responsible for the formation of extrusive calcific carbonatites.
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Extrusive carbonatites are rare igneous rocks with just 37 known localities. The
majority are calciocarbonatites, the principal exception being those of the active
volcano, Oldoinyo Lengai, which are strongly alkaline. Unresolved questions
concerning extrusive carbonatites include:
1. Why are extrusive carbonatites at Lengai chemically different from all others?
2. Could the extrusive calciocarbonatites originally have had alkaline compositions?
In order to address these questions extrusive carbonatites from both Lengai and the
adjacent volcano, Kerimasi, were collected and compared. A compilation of all the
available data on known extrusive carbonatites is also presented.
The major results documented in this thesis are:
1. Alkali carbonatites from Oldoinyo Lengai erupted in 1993 contain petrographic
evidence for an origin by liquid immiscibility from a highly fractionated peralkaline
silicate melt (wollastonite nephelinite).
2. The suite of silicate rocks at Kerimasi are derived from a primary olivine nephelinite
by fractional crystallisation and cumulus processes.
3. Extrusive carbonatites at Kerimasi are not genetically related to the silicate suite. By
contrast intrusive sovites also present, originated by liquid immiscibility from a
primitive silicate magma, equivalent to a melilite, nephelinite, at low pressure.
4. Extrusive calciocarbonatites from Kerimasi were erupted directly from the mantle.
They contain phenocrysts (previously interpreted as pseudomorphs after alkali
carbonate) which are now thought to have been dolomite containing calcite exsolution
lamellae.
5. Of the 35 other extrusive carbonatite occurrences, none show any petrographic or
geochemical evidence of having originally being alkaline. Therefore extrusive
carbonatites from Oldoinyo Lengai are thought to be unique.
6. Of the 37 extrusive carbonatites, 50% are associated with melilitites or melilitebearing
rocks, 27% are associated with nephelinites and the remaining 23% were
erupted with no associated silicate magmas.
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Abstract The island of São Vicente has the most abundant carbonatite outcrops in the Cape Verde Islands. A field survey of the main outcrops has shown that they consist of extrusive carbonatites, carbonatite dykes and small apophyses of intrusive carbonatite. These outcrops are spatially related to nephelinites. The compositions of the extrusive carbonatites and dykes plot close to, and within, the magnesiocarbonatite field. In contrast, the intrusive carbonatites are calciocarbonatites, with similar average strontium contents to those of extrusive carbonatites and dykes (around 4000 ppm), but remarkably low barium, niobium and total rare earth element concentrations. Whole-rock geochemistry indicates a strong affinity between the nephelinites and intrusive carbonatites, such that the latter could represent fractionation products of the same parental magma. This is in agreement with radiogenic isotope geochemistry, which shows a very restricted range of compositions in the Sr, Nd and Pb systems. Fractionation from a common parental magma occurred in two main steps: high-temperature nephelinite crystallization and high-temperature carbonatite immiscibility. The carbonatitic melts crystallized in two different environments, as follows: (1) a shallow intrusive environment, giving rise to the early calciocarbonatite cumulates; and (2) a vapour-dominated, extrusive environment, producing the later magnesiocarbonatites.
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