Calcite Carbonatite Research Articles

Pyrochlores from the Lueshe carbonatite complex (Democratic Republic of Congo): a geochemical record of different alteration stages

Magmatic pyrochlores from the Lueshe syenite–carbonatite complex from the northeastern part of Democratic Republic of Congo (ex-Zaı̈re) are characterized by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonatite (sövite), silicate xenoliths (nodules) to syenite. Substitutions involving Nb, Ta, Ti and REE have been precisely described. Hydrothermal alteration of Lueshe pyrochlore involves the substitution of Na ++F −=VA+VY and Ca+O=VA+VY (VA=A-site vacancy and VY=Y-site vacancy). In calcite carbonatite, hydrothermal alteration of pyrochlore took place during and after the precipitation of ancylite-(Ce), strontianite, celestite, baryte and fayalite according to a fluid composition of relatively low pH, a Na + , a Ca 2+ and a HF, and high a Sr 2+ and a LREE 3+ . The supergene alteration is characterized by complete leaching of Na, Ca and F and partial incorporation of K, Ba, Sr and Ce resulting in the formation of kali-, bario-, strontio- and ceriopyrochlore respectively. The Na-poor pyrochlore may be an intermediate variety corresponding to an alteration stage between the hydrothermal and weathered pyrochlores. The IR spectroscopic study has indicated that the weathered pyrochlore is a hydrated variety containing two bands of OH vibration modes at 3413 and 1630 cm −1. During hydrothermal and supergene alterations, the cations at B-site remain relatively constant. The variable chemical compositions of the pyrochlores from the Lueshe complex represent geochemical memories of the different alteration conditions including the variation in the oxidation–reduction environment.

Rayleigh fractionation of stable isotopes from a multicomponent source

A formulation of the Rayleigh equation for the stable isotopic evolution of a multicomponent source reservoir is presented. Its applicability to the carbon and oxygen isotopic evolution of a fluid-rich carbonate magma and the crystallizing calcite carbonatite is demonstrated using data from the Amba Dongar carbonatite complex, Deccan province, India. The initial δ 13C of the parent magma for this complex has been estimated to be −5.3 ± 0.2‰ relative to V-PDB.

Evolution of carbonatite complexes of the Deccan flood basalt province: Stable carbon and oxygen isotopic constraints

The stable carbon and oxygen isotopic composition of carbonatites from three carbonatite‐alkaline complexes (Amba Dongar, Mundwara and Sarnu‐Dandali) of Deccan province, India, can be placed into two groups, primary and secondary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely composed of a mantle having δ18O similar to that of mean upper mantle (5 to 8‰) but higher δ13C (>−5.5‰). The higher δ13C, particularly that of a batch of parent magma for Amba Dongar (−3.4‰), suggests the incorporation of recycled inorganic crustal carbon in these carbonatites. As in the plume model for generation of continental flood basalts and associated carbonatite‐alkaline complexes, the fluid‐rich rim of the plume head acts as the source for the carbonatites, so the above incorporation of crustal carbon could have been facilitated through the migration of CO2‐rich, 13C enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correlated variations of δ13C and δ18O in unaltered calcite carbonatites in all three complexes are consistent with the formation of these rocks by fractional crystallization from CO2‐rich carbonate magmas, derived from parent carbonated silicate magmas through liquid immiscibility. The extreme enrichment of δ13C and δ18O in some carbonatites and metasomatic rocks is a result of postcrystallization alteration process caused by CO2‐bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a magmatic‐hydrothermal fluid could have caused the alteration, whereas rocks of Mundwara and Sarnu‐Dandali appear to have been altered by meteoric‐hydrothermal fluids.

The Petrogenetic Association of Carbonatite and Alkaline Magmatism: Constraints from the Spitskop Complex, South Africa

The 1341 Ma old Spitskop Complex in South Africa is one of a series of intrusions of alkaline affinity, which were emplaced into the central Kaapvaal Craton over the time period 1.4–1.2 Ga. Spitskop contains calcite and dolomite carbonatite closely associated with pyroxenite, ijolite and nepheline syenite, and provides an ideal opportunity to study the petrogenetic relationships between alkaline silicate and carbonatite magmatism. The pyroxenites are not alkalic and are preserved as xenoliths within a plug-like intrusion of ijolite. Nepheline syenites are highly peralkaline, though not agpaiitic, and intrude the ijolites as a series of sheets. These units are cut by a plug of carbonatite composed of an incomplete marginal zone of calcite and dolomite–calcite carbonatite, and a larger central zone of ferroan dolomite carbonatite. Clinopyroxene compositions change systematically from diopside-rich compositions in the pyroxenites to aegirine-augite–hedenbergite in the ijolites to acmite-dominated compositions in the nepheline syenites. Whole-rock chemical data indicate, however, that the nepheline syenites and ijolites are unlikely to be related through fractional crystallization of any reasonable combination of their component minerals (clinopyroxene, nepheline, perthite) from a common parental magma. Low total rare earth element (REE) concentrations and flat to convex-up normalized patterns in the syenites contrast strongly with the steep, light REE (LREE)-enriched patterns in the ijolites. The silicate and carbonatite components differ markedly in their εSr–εNd compositions, the carbonatites having more depleted values (εSr –10 to +10; εNd –1 to –8) than the silicates (εSr 0 to +33; εNd –8 to –13). In addition, the calcite-rich carbonatites have more negative εNd (–6 to –8) than the dolomite carbonatites (–1 to –4). Contrasting isotopic compositions along with the geochemical variations within and between the silicates and carbonatites argue against them being derived from conjugate immiscible liquids. Instead, it is proposed that the carbonatites evolved from primitive carbonate liquids produced directly by low-degree melting of carbonated mantle peridotite. A preliminary model is presented to explain how mantle carbonatite melts can ascend through the mantle and into the crust. It is proposed that the silicate magmatic rocks associated with the carbonatite are produced by melting of enriched mantle lithosphere induced by the influx of deeper-sourced carbonatite melts.

Mineralogy of Crystallized Melt Inclusions from Gardiner and Kovdor Ultramafic Alkaline Complexes: Implications for Carbonatite Genesis

Cumulus olivine, clinopyroxene, melilite and perovskite from silicate rocks and carbonatites of the Gardiner (East Greenland) and Kovdor (Kola Peninsula) ultramafic alkaline complexes contain primary melt inclusions crystallized into aggregates of daughter minerals. The petrography and homogenization temperatures of the inclusions constrain the fractionation paths and the formation of carbonatites in the complexes. Carbonated melanephelinite was parental to both complexes and early cumulates (dunite, olivinite, peridotite and melteigite) are comparable. The common occurrence of phlogopite and amphibole in the inclusions and in the host rocks indicates that these were important liquidus phases. In both complexes the fractionation of phlogopite and amphibole drove the melts towards Ca-rich (larnite-normative) compositions. At the ijolite stage the evolutionary trends are believed to separate and the evolved larnite-normative melt produced calcite-bearing ijolite in Kovdor and melilitolite in Gardiner. The two assemblages are related by decarbonation reactions. A fractional crystallization origin is suggested for the Kovdor carbonatite, whereas the Gardiner carbonatite formed by liquid immiscibility in the course of melilite fractionation. Na–K–Ca and Na–Mg carbonates are common daughter phases and especially abundant in late-stage inclusions. Thus, all carbonatite melts in the two complexes are alkaline. Calcite carbonatites appear to be cumulates.

Pb/Pb age determinations on the Newania and Sevattur carbonatites of India: evidence for multi-stage histories

The ages of Indian carbonatites are still controversial. Most of the earlier datings were done by K/Ar methods. We therefore analysed Pb/Pb ratios in carbonatites from carbonatite-alkalic complexes of Newania (NW India, Rajasthan State) and Sevattur (SW India, Tamil Nadu State) to constrain the age and geological history of these rocks. Newania carbonatites are intrusive into Precambrian Untala granite-gneiss and mainly dolomitic in composition (rauhaugite) followed by a later phase of ankerite carbonatite, while thin calcite carbonatite (sövite) dykelets are the youngest in the sequence. The analysed whole-rock samples are characterised by 206Pb/ 204Pb ratios between 60 and 176 and 207Pb/ 204Pb ratios between 22 and 40, which are extremely high in comparison to common igneous rocks and even for carbonatite compositions. One sample, New 37, shows the extreme ratios of 206Pb/ 204Pb = 574 and 207Pb/ 204Pb = 73. This requires a μ-value of about 2000 for the last 1550 Ma. If the samples are classified according to their petrographic/geochemical characteristics this results in an isochron age of 1551 ± 46 Ma for the ankerite carbonatites (six samples). The dolomites (6 samples) yield an isochron age of 2.27 Ga. Although these results fit quite well into the geological evolution scheme of the area, the extreme long age hiatus between dolomite carbonatite and ferrocarbonatite formation events raises severe problems for their petrologic interpretation. The Proterozoic Sevattur carbonatite complex (SCC, Tamil Nadu) was emplaced contemporaneously with a large number of carbonatite complexes within the Precambrian gneissic terrane of the Eastern Ghats Mobile Belt. The main mass is composed of dolomite carbonatite (rauhaugite) with a few dikes of calcite carbonatite (sövite) and ankerite carbonatite within it. All eight samples together yield an isochron of 805 ± 10 Ma. This isochron is mainly determined on ankerite carbonatites with μ-values up to 1900 for the last 800 Ma. Taking only ankerite carbonatites into account, the resulting age is 801 ± 11 Ma. The 206Pb/ 204Pb and 207Pb/ 204Pb ratios of these samples are similar to the main group of Newania and far beyond the isotopic composition of common igneous rocks. Our investigations show that in carbonatitic rock systems extremely high lead isotopic ratios can be established due to the crystallization of uranium-rich mineral phases. In both cases the observed high to extremely high initial Pb isotope ratios require the residence of the lead in intermediate high-μ reservoirs either within the upper mantle or the crust prior to the carbonatite formation. A high-temperature event, which completely reset the Rb/Sr and K/Ar isotopic systems of Nevania carbonatites, seems to have no influence on the lead isotopic systematics.

The origin of dolomitic carbonatites: field and experimental constraints

Carbonatites are most commonly either calcitic or dolomitic/ankeritic with very few types in between - what is generally referred to as the bimodal distribution. There is a widely held view that dolomitic carbonatites are less abundant than calcitic carbonatites and that these arise as sub-solidus alteration products of primary calcitic carbonatites. It is demonstrated here that dolomitic types are far more common than is sometimes appreciated and that they are particularly abundant in old Precambrian cratonic regions such as the Zimbabwean and Kaapvaal Cratons and the Archaean parts of the Canadian Shield. The field, petrographic and chemical features favour these dolomitic carbonatites being magmatic rather than arising from sub-solidus replacement of calcitic carbonatites. Experimental studies show that partial melting of carbonated mantle peridotite produces a carbonate liquid with high Mg# and MgO content and an alkali content of up to 6%. On ascent through the mantle from its original generation site this primitive carbonatite will be destroyed by reaction with lherzolite and harzburgite if it remains in equilibrium with the surrounding mantle. If, however, the melt is shielded from the surrounding mantle by a lining of metasomatic wehrlite on the conduits or if it rises too rapidly for equilibrium to be maintained, it is able to escape the mantle and rise into the crust. Reaction between primitive magnesian carbonate melt and wall-rock wehrlite shifts the composition of the melt to more Ca-rich (“calcitic”) compositions. It is argued that such liquids are capable of generating the complete range of carbonatite compositions recognised at the surface. Dolomite melts incongruently at low pressures and so will only crystallise from a magnesian carbonate magma at temperatures below the dolomite dissociation reaction. These conditions are dictated by the P-T trajectory of the ascending melt as well as the nature and concentration of minor “fluxing” constituents in the melt such as fluorine and alkalis. As a result calcite is the liquidus phase over a wide range of P-T-X conditions. Several authors have suggested that many calcite-rich carbonatites formed as cumulate-enriched crystal mushes. Such calcite mushes could be readily generated from magnesian, essentially “dolomitic”, parental magmas. It is argued that no reasonable petrogenetic mechanism exists whereby magnesian carbonatite magmas could be generated from calcitic parental melts: it is argued that the reverse is true.

Compositional variation in pyrochlore from the Bingo carbonatite, Zaïre

The composition of pyrochlore from the calcite carbonatite and its associated laterite at Bingo, Zaïre was determined and compared with pyrochlore from the nearby and geologically-similar carbonatite at Lueshe. Large compositional variations exist in the Bingo pyrochlore which relate both to primary magmatic zonation and, more commonly, a secondary alteration process. Altered pyrochlore is rich in Ba and Si. At Lueshe an identical primary magmatic trend is observed, but secondary alteration is subordinate. Pyrochlore from the Bingo laterite is bariopyrochlore, whereas at Lueshe the predominant variety is kalipyrochlore.

Carbon and oxygen isotope variations in southern African carbonatites

The C and O isotope composition of 54 carbonate samples from 18 carbonatite localities throughout southern Africa was determined. Sample frequency distributions indicate a dependence of the δ 13C and δ 18O values of southern African carbonatites on their carbonate mineralogy. The calcite carbonatite samples fall into the range −8% to −4% for δ 13C and 6% to + 10% for δ 18O, which is defined as the primary igneous carbonatite field (Taylor et al., 1967). Those samples with higher δ 13C and δ 18O values are either dolomite/ankerite carbonatites or contain dolomite/ankerite, possibly indicating the involvement of different magmatic or hydrothermal phases in the carbonatite formation. Emplacement level, type of country rock intruded and secondary alteration by e.g. hydrothermal fluids, seem to be the most important parameters which affected the C and O isotope patterns found in southern African carbonatite complexes. Magmatic processes like fractional crystallisation and liquid immiscibility are of minor importance in controlling C and O isotope ratios. An alteration model of at least two stages is proposed for the evolution of the C/O isotope patterns observed in southern African carbonatites. The first stage involved magmatic-metasomatic processes during or shortly after the formation of carbonatites. These processes affected both the C and O isotope composition of carbonatites by exchange with a H 2O/CO 2-rich fluid and led to higher δ 13C and δ 18O values when compared to primary igneous values. The elevated 18O content of carbonatite carbonates was introduced by a second stage (or more stages) of post-magmatic, hydrothermal alteration and/or interaction with ground water at low (<200–250°C) temperatures. These alterations seem to be accompanied by a growing mineralogical diversity, which reflects, to some extent, the degree of alteration affecting the carbonatites. Information on stable isotope heterogeneities of the igneous sources of southern African carbonatites is obscured. Small scale C isotope heterogeneities within the igneous sources of carbonatites may be found, provided that the effects of a first stage, magmatic-metasomatic alteration can be eliminated. Changes in δ 18O values of carbonatites by sub-solidus second stage alteration processes within the crust further complicate an estimation of stable isotope variations of igneous carbonatite sources.

Dawson's Oldoinyo Lengai calciocarbonatite: a magmatic sövite or an extremely altered natrocarbonatite?

AbstractIn 1962 Dawson described a calcite carbonatite (specimen BD83) from the Tanzanian volcano Oldoinyo Lengai as a sövite, thus implying that at an earlier stage in its evolution this volcano had crystallized magmatic calciocarbonatites as well as the highly alkalic natrocarbonatite lava that has been erupted in more recent times. This proposition is difficult to reconcile with the currently fashionable hypothesis whereby the natrocarbonatite lava separated immiscibly from a type of nephelinite magma, most recently thought to be a wollastonite nephelinite. In 1993 Dawson sought to discredit the magmatic origin of this sövite specimen by arguing that it was derived from natrocarbonatite lava through extreme alteration (calcitization) in which process the original nyerereite was replaced by calcite in near-perfect pseudomorphs. We suggest that the arguments advanced in support of this concept are unconvincing and that the specimen is exactly what it was originally described as, namely a magmatic sövite in which the calcite crystallized from a magma rather than having replaced nyerereite. We do not seek to discredit the liquid immiscibility hypothesis but do believe that whatever process is responsible for the Oldoinyo Lengai natrocarbonatites and silicate rocks must also allow for the crystallization of calciocarbonatite.

Parental melts of melilitolite and origin of alkaline carbonatite: evidence from crystallised melt inclusions, Gardiner complex

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na2O + K2O/Al2O3 > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai.

Genesis of the carbonatite-hosted fluorite deposit at Amba Dongar, India; evidence from fluid inclusions, stability isotopes, and whole rock-mineral geochemistry

The Amba Dongar carbonatite complex is located approximately 400 km northeast of Bombay, India, and consists of a carbonatite ring dike and a number of syenitic intrusions which were emplaced into Late Cretaceous Bagh sandstones and Early to Late Eocene Deccan volcanic rocks. Hydrothermal activity associated with intrusion of the carbonatite was responsible for fenitization of the surrounding sandstones, and deposition of economic quantities of fluorite (11.6 Mt of 30% CaF 2 ). Fluorite mineralization occurs as veins and vug fillings, localized along fractures within the calcite carbonatite, near its contact with the sandstone.Fluid inclusions in fluorite indicate a low temperature-low salinity ( -42 and <3.5, respectively. The presence of Al and S in the fluid, the molar equivalence of Na and Cl, and the positive deviation of 18 O and D from the meteoric water line point to a small contribution from orthomagmatic fluids.A model is proposed in which the intrusion of a carbonatite magma at high crustal levels caused faulting and fracturing of the surrounding country rocks, and was accompanied by the release of orthomagmatic fluids, expressed as extensive K and Na metasomatism of the surrounding sandstones. With the decline of orthomagmatic activity, a meteoric water-dominated hydrothermal system was initiated by the heat of the intrusion. The interaction of Ca-bearing meteoric fluids with the last vestiges of F-bearing orthomagmatic NaCl brines caused deposition of large quantities of fluorite at the site of mixing.

The Bingo carbonatite-ijolite-nepheline syenite complex, Zaire: geology, petrography, mineralogy and petrochemistry

The Bingo complex, on which little information has been published heretofore, is located in northeastern Zaire, 30 km west of the western branch of the East African rift. Although outcrop is poor, fresh float has enabled construction of a geological map showing that the intrusion consists of ijolites cut and net-veined by nepheline and sodalite syenites. The ijolites include wollastonite-bearing varieties while some feldspathic; urtite also occurs. Calcite carbonatites underline some 8 km 2 and fenites are present. The rocks are described petrographically and microprobe analyses of most mineral phases, including the rare mineral götzenite, are given. The pyroxenes define a complete series in terms of diopside-hedenbergite-aegirine and range from sodic diopside/hedenbergite to aegirine-augite in ijolite pyroxene cores and rims with aegirine-augite and aegirine in the nepheline syenites. The presence of götzenite and eudialyte attests to the high peralkalinity of the nepheline syenites. The analyses, including many trace elements, of igneous silicate rocks and three carbonatites are given. As is typical for intrusions of this type, there is considerable incoherance in the chemical data of the igneous silicate rocks, but they do give some evidence of a continuous series from the ijolites to the nepheline syenites, which, together with the pyroxene data, are taken to indicate a fractionation series. There is no direct evidence of a genetic relationship between the carbonatite and the igneous silicate rocks.

The chemistry of hydrothermal fluids in carbonatites: Evidence from leachate and SEM-decrepitate analysis of fluid inclusions from Oka, Quebec, Canada

Apatite and calcite from calcite carbonatites at Oka, Quebec, Canada, contain primary, liquid-vapour and liquid-vapour-halite fluid inclusions. Leachate analysis of these inclusions indicate that they contain significant concentrations of Na, K, and Cl and lower concentrations of Fe, Mg, and probably Ca. Energy-dispersive X-ray (SEM) analysis of decrepitates from these inclusions confirm that Na, K, and Cl are abundant. In addition, significant S was detected in the decrepitates. A correction was applied to the spectra from decrepitates on apatite in order to subtract the contribution from the substrate (apatite). The reliability of a decrepitate analysis depends on the concentration of an element in the decrepitate relative to that in the substrate and to the relative contributions to the spectrum of decrepitate and substrate. Each elemental analysis was screened for its reliability based on the concentration of the element in the mixed analysis and its concentration in the substrate. The only reliable results were for Na, K, Cl, and S. Both the leachate and decrepitate analyses indicate that Na > K. K Na + K ratios fall within a narrow range, from 0.01-0.11. Potassium was a more important constituent in the LV than in LVS inclusions. Coupled with high salinities indicated from microthermometic data, these data confirm that aqueous fluids with high concentrations of alkalis may have evolved from low-alkali carbonatitic magmas. In contrast to the K Na + K ratios, Cl Cl + S ratios cover a wide range, from 0.17 to 0.93. The presence of sulphate minerals indicates that the sulphur existed as SO;- (and associated species) in the fluids. Chlorinities based on microthermometric data range from 2.5–11.4 m, which indicates SO;- molalities of 1–22.7. Charge imbalances indicate that other anions, probably HCO 3 − and CO;-, are important constituents of the fluids. Fenites around the Oka complex are characterized by the replacement of quartz-alkali feldspar gneisses by aegirine, nepheline, and K-feldspar. The dominance of Na and lack of Si in the fluid inclusions is consistent with the precipitation of predominantly nepheline and aegirine in the fenites. Phase equilibria in the K 2ONa 2OAl 2O 3SiO 2 system indicate that the development of K-feldspar in the fenites requires lower NaCl KCl ratios and higher silica activities in the fluids that can only be explained by fluid-rock interaction at low fluid/rock ratios. The nature of the fluid-rock reactions that result in fenitization may play a more important role than has previously been recognized in controlling the development of potassic versus sodic fenites and on the spatial and temporal evolution of fenites.

Hydrothermal rare earth mineralisation in carbonatites of the Tundulu complex, Malawi: Processes at the fluid/rock interface

The Tundulu carbonatite complex in southeastern Malawi was intruded during the late Jurassic to early Cretaceous over three episodes. During the first and second episodes, the major rock types were calcite carbonatites, ankerite carbonatites, and apatite carbonatites. These rocks experienced hydrothermal alteration at the close of the second episode, during which quartz-baryte veins containing a significant level of rare earth fluorocarbonates were deposited. Veins in calcite carbonatites contain abundant synchysite [(La, Ce, Nd)Ca(CO 3) 2)F] with subordinate amounts of parisite [(La, Ce, Nd) 2Ca(CO 3) 3F 2] and bastnäesite [(La, Ce, Nd)CO 3F], crystallising in the order synchysite parisite → bastnäesite. Some of the parisite is retrograde in origin, having formed as an alteration product from synchysite. Only synchysite has been identified in veins hosted by apatite carbonatites. However, bastnäesite predominates the hydrothermal assemblage in ankerite carbonatites. Parisite and synchysite have been found only as a fibrous fringe between wallrock ankerite and bastnäesite. The crystallisation sequence seen in calcite carbonatites represents a progressive depletion of both Ca 2+ and CO 2− 3 in the fluids from which this mineral suite precipitated, with Ca 2+- and CO 2− 3-poor phases precipitating last. Clearly, the predominance of bastnäesite in veins hosted by ankerite carbonatites suggests insufficient Ca 2+ and CO 2− 3 in the fluids for the ubiquitous precipitation of synchysite and parisite. These observations are consistent with a model in which hydrothermal fluids reacted with the various wallrocks, which then released different amounts of Ca 2+ and CO 2− 3, that subsequently reacted with REEs in the fluid to form the various fluorocarbonates. As such, the Tundulu carbonatites provide a natural laboratory in which compositional phase relationships of rare earth fluorocarbonates can be related to variations in the activities of Ca 2+ and CO 2− 3 within the Ln(CO 3)F-CaCO 3-F 2(CO 3) −1 system.

Dolomite-calcite textures in early carbonatites of the Kovdor ore deposit, Kola peninsula, Russia: their genesis and application for calcite-dolomite geothermometry

In early calcite carbonatites of the Kovdor ore deposit four morphological types of dolomite are represented. In the first type, dolomite microcrystals occur as lamellae enclosed by optically continuous calcite. In the second, dolomite microcrystals occur as segmented rods, plates and xenomorphic grains, enclosed by optically discontinuous calcite, and in the third, dolomite is represented by grains of various morphologies, situated along calcite grain boundaries. The fourth type of dolomite occurs as a fine-grained aggregate, which develops along grain boundaries and cleavage cracks of calcite. From microscopic, scanning electron microscope and microprobe studies of these different types of dolomite microcrystals, as well as the calcite associated with them, it can be concluded that the first type of dolomite was exsolved from magnesian calcite during cooling. The second, and the third types of dolomite microcrystals were formed by recrystallization. The fourth type of dolomite was formed by metasomatic dolomitization. As the result of these two processes-recrystallization and metasomatic dolomitization-early dolomite microcrystals seldom occur. The composition of the early-formed primary magnesian calcite yielded temperatures of exsolution of dolomite from magnesian calcite between 665 and 700°C.

PHOSCORITES OF THE MAYMECHA-KOTUY IJOLITE-CARBONATITE ASSOCIATION

A classification and nomenclature for phoscoritic rocks is proposed, based on the relative contents of the essential minerals olivine, magnetite, and apatite. There is much variation in the mode of occurrence and time of emplacement of these rocks within the complexes. Most phoscorites were emplaced early, after alkalic syenites and before calcite carbonatite, but a smaller fraction, mostly olivine-poor varieties, was emplaced at later stages. Phoscorites, although commonly affected by postmagmatic processes, are clearly magmatic. The genesis of the melts, however, in particular their relation to carbonatite melts, remains unclear.

Zr-rich garnet and Zr- and Th-rich perovskite from the Polino carbonatite, Italy

AbstractTh- and Zr-bearing perovskite and Zr-rich Ti garnet are described from the Polino calcite carbonatite, Italy. The garnets contain 6.05-7.50 wt.% ZrO2 in cores and up to 15.80 wt.% in rims. Silicon is low in these garnets and substantial Al and Fe 3+ has been assigned to the tetrahedral sites. A strong correlation of Si with Zr from published data suggests the possibility that some Zr may also be tetrahedrally coordinated. However, the chemistry alone does not constrain the site distribution of Fe, Ti, Zr and Mg unambiguously. The perovskites are very unusual in containing 2.81-3.27 wt.% ZrO2 and 1.48-1.71 wt.% ThO2. For comparison, new analyses are presented of the zirconium garnet kimzeyite and coexisting perovskite from the type locality at Magnet Cove.

Carbonatite dykes at bayan Obo, inner Mongolia, China

Calcite-rich dykes occur in the thrust fold belt near the Bayan Obo rare earth element (REE) deposit. They cut a thrust inlier of granitic migmatite within folded Bayan Obo Group sediments of Proterozoic age. Cathodoluminescence, X-ray fluorescence and microprobe studies show that the rock is a calcite carbonatite with Sr-Mn-bearing calcite, magnesio-riebeckite, apatite, pyrochlore, K-feldspar and biotite. One dyke was chosen for detailed analysis. Its margin is strongly REE-mineralized with much monazite developed adjacent to zoned apatite. Secondary alteration is marked by the introduction of Fe and Mn. The adjacent migmatite is fenitized to a magnesio-riebeckite-albite rock. The sedimentary dolomite of the Bayan Obo Group is composed mainly of Mn-Sr-RE-hearing ferroan dolomite and contains bands of opaque grains, apatite, monazite, fluorite and taeniolite. Many trace element and isotope similarities between the carbonatite dyke and the sedimentary dolomite are revealed, and the evidence supports the possibility that the dolomite is a dolomitized carbonatite tuff. The Bayan Obo REE mineralization also shows geochemical similarities with the mineralization seen in the carbonatites, and a possible genetic connection is presented.

Nephelinite-carbonatite liquid immiscibility at Shombole volcano, East Africa: Petrographic and experimental evidence

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 sovitic (calcite carbonatite) in composition.