Summary
We summarize the evidence for silicate-carbonate liquid immiscibility in two nephelinite lavas from Shombole volcano, East Africa, and discuss its significance for carbonatite petrogenesis. The nephelinite lavas contain spherical to irregular globules ≤ 0.5 cm containing low-Sr calcite, Sr-Ca and K-Ba zeolites, fluorite, aegirine, strontianite, and fluorapatite. The globules are interpreted to be magmatic in origin, and represent quenched immiscible carbonate liquid. Most phases in the globules form an interlocking mosaic of euhedral crystals, however, rare blebby intergrowths of calcite and strontianite indicate eutectic crystallization from a melt. The phase assemblages and respective compositions of minerals in the globules and silicate groundmass are nearly identical, indicating that the samples were quenched when two liquids were in near-equilibrium. Experiments with the samples at 200–500 MPa and 975–925 °C have reproduced the natural assemblages (phenocrysts + 2 liquids) exactly and the compositions of experimentally generated solid phases closely match the original phenocrysts. The natural and experimentally produced carbonatites are both sövitic (calcite carbonatite) in composition.
The two-liquid experimental data from Shombole are compared with the 300 MPa experimental data ofFreestone andHamilton (1980) andHamilton et al. (1989), who utilized strongly peralkaline bulk compositions typical of the lavas erupted at Oldoinyo Lengai. Both data sets are nearly coplanar in the tetrahedron Si-(Ca + Mg + Fe2+)-(Al + Fe3+)-(Na + K) (SCAN), but the tielines have different orientations and the Oldoinyo Lengai bulk compositions generate alkali-rich carbonatitic liquids, rather than sövitic liquids. At both volcanic centers, only one type of extrusive carbonatite is known, and crystal fractionation schemes to generate one carbonatite from another are not supported by the data. Experiments illustrate that the full range of Ca-Mg-(Na + K) carbonatites can be generated by liquid immiscibility from nephelinitic magmas of different compositions.
Zusammenfassung
Wir fassen die Hinweise auf fehlende Mischbarkeit von Silikat-Karbonatschmelzen in zwei Nephelinitlaven des Shombole-Vulkans, Ostafrika, zusammen und diskutieren die Bedeutung der Ergebnisse für die Genese der Karbonatite. Die Nephelinitlaven enthalten rundliche bis unregelmäßig geformte Einschlüsse von bis zu 0,5 cm Durchmesser, die Sr-armen Kalzit, Sr-Ca und K-Ba Zeolite, Fluorit, Aegirin, Strontianit und Fluorapatit enthalten. Diese Einschlüsse (“Globules”) sind magmatischen Ursprungs und stellen rasch abgekühlte unmischbare Karbonat-Schmelze dar. Die meisten Phasen in den Einschlüssen bilden ein vernetztes Mosaik idiomorpher Kristalle. Selten kommen auch tröpfchenförmige Verwachsungen von Kalzit und Strontianit vor, die auf eutektische Kristallisation aus einer Schmelze hinweisen. Die Assoziationen der Phasen, und die Zuammensetzungen der Minerale in den Einschlüssen und in der silikatischen Grundmasse sind fast identisch, und weisen darauf hin, daß die Proben rasch abgekühlt wurden als beide Schmelzen beinahe im Gleichgewicht waren. Experimente mit den Proben bei 200–500 MPa und 975–925°C haben die natürlichen Assoziationen (idiomorphe Kristalle und zwei Schmelzen) genau wiedergegeben und Zusammensetzungen der experimentell hergestellten festen Phasen stimmen sehr gut mit denen der ursprünglichen idiomorphen Kristalle überein. Die natürlichen und die experimentell hergestellten Karbonatite sind sövitischer Zuammensetzung (Kalzit-Karbonatit).
Die experimentellen Daten vom Shombole werden mit den bei 300 MPa durchgeführten experimentellen Daten vonFreestone undHamilton (1980) undHamilton et al. (1989) verglichen; letztere benützten stark peralkalische Gesamtzusammensetzungen die typisch für die Laven des Oldoinyo Lengai-Vulkans sind. Beide Datengruppen sind beinahe koplanar in den Tetraedern Si-(Ca + Mg + Fe+2)-(Al + Fe +3)-(Na + K) (SCAN), aber die Konoden haben verschiedene Orientierungen und die Oldoinyo Lengai-Zusammensetzungen erzeugen alkalireiche karbonatitische Schmelzen und nicht sövitische. In beiden vulkanischen Zentren ist nur ein Typ von Karbonatiten bekannt und Fraktionierungs-Mechanismen, die einen Karbonatit aus dem anderen ableiten könnten, werden von den erarbeiteten Daten nicht gestützt. Experimente zeigen, daß das volle Spektrum möglicher Ca-Mg(Na + K) Karbonatite durch Unmischbarkeit (immiscibility) aus nephelinitischen Magmen verschiedener Zusammensetzung abgeleitet werden kann.
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Kjarsgaard, B., Peterson, T. Nephelinite-carbonatite liquid immiscibility at Shombole volcano, East Africa: Petrographic and experimental evidence. Mineralogy and Petrology 43, 293–314 (1991). https://doi.org/10.1007/BF01164532
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DOI: https://doi.org/10.1007/BF01164532