Properties of Carbonatite Magma and Processes in Carbonatite Magma Chambers

Treiman, Allan H.; Schedl, Andrew

doi:10.1086/628789

Cited by 93 publications

(26 citation statements)

References 32 publications

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“…Such needle-like, extremely elongate crystals arranged in a fabric resembling the spinifex texture of komatiites (micro-spinifex) are strongly indicative of rapid quenching and rapid crystallization of a primary carbonate melt. In volcanic carbonatites, growth of dendritic crystals and quenching may be important (Treiman, 1989), leading to the formation of carbonatites similar in texture to spinifex-textured komatiites (Treiman and Schedl, 1983;Treiman, 1989;Barker, 1989). (Fig.…”

Section: Textural Evidence For Carbonate-silicate Liquid Immiscibilitymentioning

confidence: 99%

“…In contrast to polymerized silicate melts, carbonate melts are essentially unpolymerized ionic liquids or fused salts with individual anions and cations bound by ionic forces (Treiman and Schedl, 1983;Treiman, 1989). The carbonate anions form metal-carbonate complexes (Treiman and Schedl, 1983; Genge et af., 1995) that do not associate with the silicate anions in the melt.…”

Section: Physical Properties and Conditions Of Carbonate Melts: Resulmentioning

confidence: 99%

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Carbonate‐silicate liquid immiscibility upon impact melting: Ries Crater, Germany

Graup¹

1999

Meteorit & Planetary Scien

163

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Abstract-The 24 km diameter Ries impact crater in southern Germany is one of the most studied impact structures on Earth. The Ries impactor struck a Triassic to Upper Jurassic sedimentary sequence overlying Hercynian crystalline basement. At the time of impact (14.87 ? 0.36 Ma; Storzer el ul., 1995), the 350 m thick Malm limestone was present only to the south and east of the impact site. To the north and west, the Malm had been eroded away, exposing the underlying Dogger and Lias. The largest proportion of shocked target material is in the impact-melt-bearing breccia suevite. The suevite had been believed to be derived entirely from the crystalline basement. Calcite in the suevite has been interpreted as a postimpact hydrothermal deposit.From optical inspection of 540 thin sections of suevite from 32 sites, I find that calcite in the suevite shows textural evidence of liquid immiscibility with the silicate impact melt. Textural evidence of liquid immiscibility between silicate and carbonate melt in the Ries suevite includes carbonate globules within silicate glass, silicate globules embedded in carbonate, deformable and coalescing carbonate spheres within silicate glass, sharp menisci or cusps and budding between silicate and carbonate melt, fluidal textures and gas vesicles in carbonate schlieren, a quench crystallization sequence of the carbonate, spinifex textured quenched carbonate, separate carbonate spherules in the suevite mineral-fragment matrix, and inclusions of mineral fragments suspended in carbonate blebs. Given this evidence of liquid immiscibility, the carbonate in the suevite therefore has-like the silicate melt-a primary origin by impact-shock melting.Evidence of carbonate-silicate liquid immiscibility is abundant in the suevites from the southwest to east of the Ries crater. The rarer suevites to the west to northeast of the crater are nearly devoid of carbonate melts. This correspondence between the occurrence of outcropping limestones at the target surface and the formation of carbonate melt indicates that the Malm limestones are the source rocks of the carbonate impact melt. This correspondence shows that the suevites preserve a compositional memory of their source rocks. From the regional distribution of suevites with or without immiscible carbonate melts, it is inferred that the Ries impactor hit the steep Albtrauf escarpment at its toe, in an oblique impact from the north.

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Section: Textural Evidence For Carbonate-silicate Liquid Immiscibilitymentioning

confidence: 99%

Section: Physical Properties and Conditions Of Carbonate Melts: Resulmentioning

confidence: 99%

Carbonate‐silicate liquid immiscibility upon impact melting: Ries Crater, Germany

Graup¹

1999

Meteorit & Planetary Scien

163

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“…The homogenization of these domains would replenish the host fluid with Nb and U, and repetition of this process during emplacement could produce the concentric and rhythmic zonation observed. However, carbonatitic magma has low viscosity and would flow turbulently during emplacement, especially in relatively thin dykes (Treiman, 1989;Hodgson, 1985). Under such turbulent flow the re-equilibration of such domains would be rapid (Treiman margin of crystal A is best explained by minor late-stage or secondary alteration, which belongs to a separate process from the fractionation being discussed here.…”

Section: Discussion Of Cryptic Geochemical Zonationmentioning

confidence: 99%

“…Primordial mantle-normalized element distribution ratios for carbonatites 82LV43, 51 and 55 from San Vicente, Cape Verde Islands (XRF analyses). and Schedl, 1983). Therefore crystal growth would be unlikely to deplete the magma at a rate that exceeded magma homogenization, lnstead, it is envisaged that the crystals grew during gravitational settling or during circulation within a chemically stratified magma chamber.…”

Section: Discussion Of Cryptic Geochemical Zonationmentioning

confidence: 99%

The geochemistry and cryptic zonation of pyrochlore from San Vicente, Cape Verde Islands

Hodgson

Bas

1992

Mineral. mag.

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Zr-rich pyrochlore crystals in carbonatite from San Vicente (Cape Verdes Islands) show cryptic, concentric and rhythmic chemical zonation with Ca increasing and Ti, U and Zr decreasing towards the rims. In one carbonatite, taken from the Camile dyke, previously undocumented cryptic sector zonation of Ti, U and Zr is also observed in these crystals. The chemical variation is investigated by wavelength-dispersive electron microprobe, with both single spot and crystal map analyses.The concentric and rhythmic zonation, marked by element substitution, was generated by magma heterogeneity and/or element diffusion kinetics, but it is suggested that the sector zonation, marked by differential site substitution, was governed by protosite variation between octahedral and cubic faces.

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“…Compared to silicate melts, the less viscous carbonate melts can remain physically unchanged even under the upper crustal conditions and easily ascend into the crust (Harmer and Gittins 1997). The emplacement of the carbonatite magma with its initial crystallized products may have resulted in rapid cooling and end of convection in the core of the magma chamber (Treiman and Schedl 1983;Andersson et al 2013). The type-1 calcite and dolomite may have crystallized at similar conditions within the crust.…”

Section: Primary Crystallization and Subsolidus History Of Carbonatitesmentioning

confidence: 99%

Primary and secondary textures of dolomite in Eppawala carbonatites, Sri Lanka: implications for their petrogenetic history

Madugalla¹,

Pitawala²,

Manthilake³

2017

J. Geosci.

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Textural studies of carbonate minerals over the past three decades revealed that their textures are useful tool for understanding of petrogenesis of carbonatites. Petrographic, cathodoluminescence (CL) and electron-microprobe studies on textures of calcite and dolomite were performed for interpretation of evolution of Eppawala carbonatites in Sri Lanka. The studied carbonatites are dominated by calcite with subordinate dolomite. Calcites occur in two different morphological forms, reflecting two generations: as grains with dolomite inclusions (type-1) and dolomite-free (type-2) ones. Dolomites were subdivided into five distinct morphological types: randomly distributed, coarse-grained dolomite (type-1), rod--shaped or vermicular dolomite microcrysts within the type-1 calcite (type-2), inclusions of dolomite within the type 1 calcite forming plug-or wedge-shaped arrangements (type-3), dolomite microcrysts along the grain boundaries of the type 1 calcite (type-4) and clusters of dolomite crosscutting the type 1 calcite (type-5).The geochemical results indicate that these five morphological types accounts for three different generations of dolomites. Type-1 dolomite and type-1 calcite are interpreted as primary magmatic. Type-2 and type-3 represent exsolved dolomite formed by exsolution from type-1 calcite. Type-4 and type-5 dolomites are recrystallized and reorganized dolomites of exsolved type-2 and type-3 dolomites. Type-2 calcite reflects later recrystallization event. The composition of type-1 calcite indicates minimum temperatures of exsolution of c. 650 °C. The exsolution and recrystallization kinetics reflected the equilibration of carbonatite magma at two crustal depths during the petrogenesis of Eppawala carbonatite. The re-localization may have been related to the deformations experienced by the country rocks. : carbonatites, calcite, dolomite, exsolution, recrystallization Received: 15 March 2016; accepted: 24 August 2017; handling editor: V. Rapprich (Cooper and Reid 1991) have been well studied. Such studies revealed that textures of carbonate minerals are important tools to interpret the specific conditions of super cooling, cooling rates and super-saturation prevailing in the corresponding magmatic body. Additionally, the intergrowth textures of calcite and dolomite, where dolomite forms vermicules, plates, rods, segmented rods, blebs and xenomorphic grains have been reported from certain carbonatites (Van der Veen 1965;Zaitsev and Polezhaeva 1994). These textures yielded the composition of the primary carbonate phases, information on crystal-melt segregation and crystallization temperature as well as depth and timing of sub-solidus processes that have modified them (Van der Veen 1965;Cooper and Reid 1991;Wall et al. 1993;Zaitsev and Polezhaeva 1994). In the past several decades, intergrowth formation mechanisms have been subject to discussion. Exsolution (Goldsmith 1983;Puhan 1984) or metasomatism with recrystallization (Zaitsev and Polezhaeva 1994) could lead to the formation of such textures....

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Properties of Carbonatite Magma and Processes in Carbonatite Magma Chambers

Cited by 93 publications

References 32 publications

Carbonate‐silicate liquid immiscibility upon impact melting: Ries Crater, Germany

Carbonate‐silicate liquid immiscibility upon impact melting: Ries Crater, Germany

The geochemistry and cryptic zonation of pyrochlore from San Vicente, Cape Verde Islands

Primary and secondary textures of dolomite in Eppawala carbonatites, Sri Lanka: implications for their petrogenetic history

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