Carbonatite Rocks Research Articles

The Characteristics of the Hydrothermal Exhalative Dolostone of Lucaogou Formation in Santanghu basin and its Geology Setting Indication

The NW‐SE trending Santanghu basin is located in Xinjiang, NW China, between Tianshan Mountains and Altai Mountains. The tectonic evolution history of north Xinjiang has long been debated, especially the question when did that area began its intraplate rift evolution stage. This abstract discusses the features and possible origin of the hydrothermal exhalative dolostone of Lucaogou Formation in Santanghu basin, aiming to provide relevant information about this question.The target dolostone are characterized by extreme thin lamina of 0.3cm – 1 cm thick. Dolomite, ankerite, potassium feldspar and analcite are dominant composition. Most dolomite is poor crystallized and is in anhedral or subhedral shapes. Ankerite is generally porphyritic, few is in irregular shapes. It has to be noted that the porphyritic ankerite commonly develop zonal textures characterized by rings with various FeO content. Potassium feldspar and analcite (<50%) are commonly observed in dolostones. Thin sections show some analcite is tetragonal trisoctahedron with harbour – like corrosion rims. Besides, few fragments of carbonatite rocks are observed in Lucaogou Formation and they perform intermittent or scattered distribution in rock layers.Previous work done by our study team shows δ 13C PDB values are 4.8 ± ∼11.4±, 6.93± on average. The δ18 O PDB values are between −5± ∼ −21.1±, −10.94 ± on average. Paleosalinity calculated by δ 13C and δ 18 O is between 128.35 and 136.81, 134.42 on average. Sr content of dolostones in study area reaches to 164 × 10−6∼537 × 10−6 (379.1 × 10−6 on average), most Sr/Ba > 1 (4.05 on average). 87Sr/86Sr of iron–rich dolostone in target area are between 0.70457∼0.706194, average at 0.705360.Considering either accompanying residual tuff or residual leucite is found via electron probe, this indicates the analcite may derive form analcime phonolite. Furthermore, fragments of carbonatite rocks proves mantle‐derived magmatism exist. The rapid variation of δ18 O indicates multiple interactions of hydrothermal fluids with lake water during Lucaogou sedimentary period. 87Sr/86Sr values are much lower than that of crustal silica rocks while are more close to mantle derived rocks, which indicates diagenetic hydrothermal fluid is mantle‐origin.It has been reported that dolostone does not intergrow with iron‐rich dolostone in study area. According to the latest X diffraction results of cores, iron‐rich dolostone distributes rather locally. Given that ankerite commonly develop zonal textures characterized by various FeO content, we confirm it is due to multiple hydrothermal fluids activities in different hydrothermal vents.There should be abundant hydrothermal vents during Lucaogou period. Deep faults and its relevant fractures built plenty paths for mantle‐origin hydrothermal fluids to run up. Different vents owned various fluid properties, so did relevant sedimentation products.Hence we inclined to believe Santanghu basin was at intraplate rift evolution stage, under regional extension condition in late Carboniferous. During early‐mid Permian the basin was a starved, deep lacustrine intracontinental rift basin. Accordingly, in northern Xinjiang, ocean basin may close in late Carboniferous and started continental sedimentary development in early Permian.

The use of carbonatite rock powder as a liming agent

AbstractCarbonatite rock powder, originating from the Lillebukt Alkaline Complex at Stjernøy in northern Norway, can potentially be used as a slow‐releasing lime and potassium (K) and magnesium (Mg) fertilizer due to a high concentration of the easily weathered minerals calcite (42%) and biotite (30%). However, the enrichment of barium (Ba) and strontium (Sr) may cause an undesired uptake to plants when carbonatite is applied to agroecosystems. A pot experiment was designed to investigate the liming and fertilization effects of carbonatite and the potential mobilization of Ba and Sr compared to a dolomite lime commonly used in Norwegian agriculture. These liming agents were mixed with a sandy soil applied to different amounts of peat, and the uptake of Ba, Sr, calcium (Ca), Mg, and K by Festuca arundinácea Schreb. Kora (tall fescue) and Trifolium repens L. Milkanova (white clover) was evaluated. The liming agents were generally incapable of buffering the acidifying effect from increased applications of peat, while the plant dry mass was unaffected. Compared to pots given dolomite and soluble K, the availability of K from carbonatite to plants was equally high or higher, and no difference in the K:(Ca + Mg) ratio in plants was observed. Carbonatite was a significant source to plant Ba and Sr, and the uptake seemed to follow the Ca uptake. Addition of peat amplified the uptake of Ba, Sr, Ca, Mg, and K to plants, probably an effect of organic acid‐induced weathering of carbonatite. White clover took up Ba, Sr, and Ca more effectively than tall fescue, but the Ba and Sr concentrations in plants were relatively moderate compared to concentrations reported from field investigations.

Ultrasound-assisted extraction of rare-earth elements from carbonatite rocks

In view of the increasing demand for rare-earth elements (REE) in many areas of high technology, alternative methods for the extraction of these elements have been developed. In this work, a process based on the use of ultrasound for the extraction of REE from carbonatite (an igneous rock) is proposed to avoid the use of concentrated reagents, high temperature and excessive extraction time. In this pioneer work for REE extraction from carbonatite rocks in a preliminary investigation, ultrasonic baths, cup horn systems or ultrasound probes operating at different frequencies and power were evaluated. In addition, the power released to the extraction medium and the ultrasound amplitude were also investigated and the temperature and carbonatite mass/volume of extraction solution ratio were optimized to 70°C and 20mg/mL, respectively. Better extraction efficiencies (82%) were obtained employing an ultrasound probe operating at 20kHz for 15min, ultrasound amplitude of 40% (692Wdm−3) and using a diluted extraction solution (3% v/v HNO3+2% v/v HCl). It is important to mention that high extraction efficiency was obtained even using a diluted acid mixture and relatively low temperature in comparison to conventional extraction methods for REE. A comparison of results with those obtained by mechanical stirring (500rpm) using the same conditions (time, temperature and extraction solution) was carried out, showing that the use of ultrasound increased the extraction efficiency up to 35%. Therefore, the proposed ultrasound-assisted procedure can be considered as a suitable alternative for high efficiency extraction of REE from carbonatite rocks.

Experimental constraints on the stability of baddeleyite and zircon in carbonate- and silicate-carbonate melts

Carbonatites are rare igneous carbonate-rich rocks. Most carbonatites contain a large number of accessory oxide, sulfide, and silicate minerals. Baddeleyite (ZrO 2 ) and zircon (ZrSiO 4 ) are common accessory minerals in carbonatites and because these minerals host high concentrations of U and Th, they are often used to determine the ages of formation of the carbonatite. In an experimental study, we constrain the stability fields of baddeleyite and zircon in Ca-rich carbonate melts with different silica concentrations. Our results show that SiO 2 -free and low silica carbonate melts crystallize baddeleyite, whereas zircon only crystallizes in melts with higher concentration of SiO 2 . We also find that the zirconsilicate baghdadite (Ca 3 ZrSi 2 O 9 ) crystallizes in intermediate compositions. Our experiments indicate that zircon may not be a primary mineral in a low-silica carbonatite melt and care must be taken when interpreting zircon ages from low-silica carbonatite rocks.

Light rare earth element systematics as a tool for investigating the petrogenesis of phoscorite-carbonatite associations, as exemplified by the Phalaborwa Complex, South Africa

In-situ trace element analyses of fluorapatite, calcite, dolomite, olivine, and phlogopite have been undertaken on representative phoscorite and carbonatite rocks of the Palaeoproterozoic Phalaborwa Complex. Textural and compositional characterization reveals uniformity of fluorapatite and calcite among most of the intrusions, and seems to favor a common genetic origin for the phoscorite-carbonatite association. Representing major repositories for rare earth elements (REE), fluorapatite and calcite exhibit tightly correlated light REE (LREE) abundances, suggesting that partitioning of LREE into these rock forming minerals was principally controlled by simple igneous differentiation. However, light rare earth element distribution in apatite and calcite cannot be adequately explained by equilibrium and fractional crystallization and instead favors a complex crystallization history involving mixing of compositionally distinct magma batches, in agreement with previously reported mineral isotope variability that requires open-system behaviour.

Apatite in carbonatitic rocks: Compositional variation, zoning, element partitioning and petrogenetic significance

Apatite-group phosphates are nearly ubiquitous in carbonatites, but our understanding of these minerals is inadequate, particularly in the areas of element partitioning and petrogenetic interpretation of their compositional variation among spatially associated rocks and within individual crystals. In the present work, the mode of occurrence, and major- and trace-element chemistry of apatite (sensu lato) from calcite and dolomite carbonatites, their associated cumulate rocks (including phoscorites) and hydrothermal parageneses were studied using a set of 80 samples from 50 localities worldwide. The majority of this set represents material for which no analytical data are available in the literature. Electron-microprobe and laser-ablation mass-spectrometry data (~600 and 400 analyses, respectively), accompanied by back-scattered-electron and cathodoluminescence images and Raman spectra, were used to identify the key compositional characteristics and zoning patterns of carbonatitic apatite. These data are placed in the context of phosphorus geochemistry in carbonatitic systems and carbonatite evolution, and compared to the models proposed by previous workers. The documented variations in apatite morphology and zoning represent a detailed record of a wide range of evolutionary processes, both magmatic and fluid-driven. The majority of igneous apatite from the examined rocks is Cl-poor fluorapatite or F-rich hydroxylapatite (≥0.3apfu F) with 0.2–2.7wt.% SrO, 0–4.5wt.% LREE2O3, 0–0.8wt.% Na2O, and low levels of other cations accommodated in the Ca site (up to 1000ppm Mn, 2300ppm Fe, 200ppm Ba, 150ppm Pb, 700ppm Th and 150ppm U), none of which show meaningful correlation with the host-rock type. Silicate, (SO4)2− and (VO4)3− anions, substituting for (PO4)3−, tend to occur in greater abundance in crystals from calcite carbonatites (up to 4.2wt.% SiO2, 1.5wt.% SO3 and 660ppm V). Although (CO3)2− groups are very likely present in some samples, Raman micro-spectroscopy proved inconclusive for apatites with small P-site deficiencies and other substituent elements in this site. Indicator REE ratios sensitive to redox conditions (δCe, δEu) and hydrothermal overprint (δY) form a fairly tight cluster of values (0.8–1.3, 0.8–1.1 and 0.6–0.9, respectively) and may be used in combination with trace-element abundances for the development of geochemical exploration tools. Hydrothermal apatite forms in carbonatites as the product of replacement of primary apatite, or is deposited in fractures and interstices as euhedral crystals and aggregates associated with typical late-stage minerals (e.g., quartz and chlorite). Hydrothermal apatite is typically depleted in Sr, REE, Mn and Th, but enriched in F (up to 4.8wt.%) relative to its igneous precursor, and also differs from the latter in at least some of key REE ratios [e.g., shows (La/Yb)cn≤25, or a negative Ce anomaly]. The only significant exception is Sr(± REE,Na)-rich replacement zones and overgrowths on igneous apatite from some dolomite(-bearing) carbonatites. Their crystallization conditions and source fluid appear to be very different from the more common Sr-REE-depleted variety. Based on the new evidence presented in this work, trace-element partitioning between apatite and carbonatitic magmas, phosphate solubility in these magmas, and compositional variation of apatite-group minerals from spatially associated carbonatitic rocks are critically re-evaluated.

Magnetotelluric Imaging of a Carbonatite Terrane in the Southeast Mojave Desert, California and Nevada

The southeast Mojave Desert hosts one of the world’s largest rare earth element (REE) deposits at Mountain Pass, California. Although surface geology has been studied, a full understanding of the carbonatite and associated intrusive suite complex requires subsurface geophysical characterization. In this study, a combination of geophysical methods, including magnetotelluric (MT), magnetics, and gravity are used to create a two-dimensional (2D) geophysical model to a depth of about 10 km. An electrically conductive body is found 2–3 km below and west of the deposit that is associated with a magnetic high that could be connected to a deeper (10 km) conductive body related to possible intrusions or hydrothermal systems. The carbonatite body coincides with a steep magnetic gradient and a bench or terrace in the gravity data that may reflect relative lower-density intrusive rocks. Although carbonatite rocks are typically magnetic, the carbonatite rocks, associated intrusive suite, and host rocks in this area are essentially non-magnetic. Combined geophysical data indicate that the enriched REE deposit may be related to a regional extensive hydrothermal alteration event.

Vermiculite expansion through non-artificial processes in pyroclastic carbonatites from Catanda (Angola)

AbstractThe pyroclastic carbonatitic rocks from the Catanda volcanic area (Angola) contain annite and phlogopite grains which have been affected to a significant extent by vermiculitization. A mineralogical study of these micas has revealed that the annite was altered to K-vermiculite while vermiculitization of phlogopite generated Ca-vermiculite. Intermediate alteration products such as intergrowths of phlogopite/Ca-vermiculite were also found, but interstratified crystals of phlogopite/ vermiculite, a common intermediate product during vermiculitization processes, are not reported in the Catanda rocks. The two types of vermiculitized micas show ‘accordion’ textures, related to the expansion of vermiculite. To the authors’ knowledge, the present study is the first report of expanded vermiculites described in natural rocks and not generated as a consequence of industrial processing.

Geochemistry of barium in soils supplied with carbonatite rock powder and barium uptake to plants

Apatite-biotite-carbonatite (short form: carbonatite) rock powder originating from Lillebukt alkaline complex, N-Norway is a potential liming and multi-nutrient fertilizer. However, the elevated contents of potentially bio-toxic barium (Ba) (up to 5.5 g kg−1) is a matter of concern. A pot experiment was conducted in order to investigate the geochemistry of Ba in a sandy peat-containing growth media treated with carbonatite, with special focus on Ba-speciation in the pore water. These results were correlated with Ba uptake in Trifolium repens L. (white clover) and Fesctua arundinácea L. (tall fescue). The Ba-speciation and bioavailability as affected by treatment and plant growth was conducted using WHAM/Model VI version 7.0.4 and the diffusive gradients in thin films (DGT) technique, respectively. The increasing cation exchange capacity (CEC) in the growth media, and colloidal fulvic acids (FA) decreased the Ba2+ concentration in pore water. Both the Ba detected in DGT (Badgt), and exchangeable Ba (Baexc) were higher in pots containing rock powder. A strong relationship between Baexc and diffusively bound Ba to FA (Badfa) with plant uptake was evident. The latter was likely initiated by organic material mobilization of Ba from carbonatite. The mobilized Ba bound to FA (Bafa) in pore water was mainly bound diffusively, and the Badfa fraction was thus made available to plants. Finally, the Badgt was increased in pots grown with white clover compared to tall fescue. Further investigation is required to determine if this is related to increased rhizosphere activity by plants or to the nitrogen fixating plant-microbe relationship.

Formation of gold mineralization in ultramafic alkalic magmatic complexes

Study of mineral inclusions within alluvial gold particles of the Guli Complex (East Siberia) and findings of lode gold in rocks of the same intrusion have demonstrated that gold mineralization occurs in interstitions of both early high–magnesium rocks (dunite) and later alkalic and carbonatite rocks. In dunite the native gold occurs in association with Fe–Ni sulfides (monosulfide solid solution, pentlandite, and heazlewoodite). Formation of the gold-bearing alloys took place under a low oxygen potential over a broad range of temperatures: from those close to 600°C down to below 400°C.

Zirconium and hafnium fractionation in differentiation of alkali carbonatite magmatic systems

Zirconium and hafnium are valuable strategic metals which are in high demand in industry. The Zr and Hf contents are elevated in the final products of magmatic differentiation of alkali carbonatite rocks in the Polar Siberia region (Guli Complex) and Ukraine (Chernigov Massif). Early pyroxene fractionation led to an increase in the Zr/Hf ratio in the evolution of the ultramafic–alkali magmatic system due to a higher distribution coefficient of Hf in pyroxene with respect to Zr. The Rayleigh equation was used to calculate a quantitative model of variation in the Zr/Hf ratio in the development of the Guli magmatic system. Alkali carbonatite rocks originated from rare element-rich mantle reservoirs, in particular, the metasomatized mantle. Carbonated mantle xenoliths are characterized by a high Zr/Hf ratio due to clinopyroxene development during metasomatic replacement of orthopyroxene by carbonate fluid melt.

Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada

AbstractAlkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U–Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U–Pb age of ~1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ~0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ~+3.5 and ~+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.

Geochemical and mineralogical characteristics of weathered ore in the Dalucao REE deposit, Mianning–Dechang REE Belt, western Sichuan Province, southwestern China

The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite–carbonatite rocks that intrude a Proterozoic quartz–diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200–400m long, 180–200m wide, and extending to >450m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55–175m and 14–58m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite+barite+celestite+bastnäsite (i.e., Type I), whereas the No. 3 orebody is characterized by an assemblage comprising fluorite+celestite+pyrite+muscovite+bastnäsite+strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60wt.% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning–Dechang REE Belt.Faulting and cryptoexplosive breccia events, possibly linked to movement on the Panxi Fault, were more common in the No. 1 orebody than in the No. 3 orebody. This facilitated the introduction of ore-forming hydrothermal fluids and provided space for the precipitation of REE minerals. Based on the present results, we infer that the Dalucao deposit was the product of multiple stages of ore formation. REE minerals formed in envelopes around, or fractures within, quartz, fluorite, calcite, barite, and celestite in the brecciated ores. The main REE minerals were deposited from hydrothermal fluids within cryptoexplosive breccia, followed by weathering that increased the ore grade. Petrographic studies and X-ray powder diffraction (XRD) analyses indicate that the weathered ore contains 5–60% REE minerals (including bastnäsite, parisite, and monazite), together with gangue (quartz, barite, celestite, and fluorite), large amount of clay minerals (smectite, illite, kaolinite, and sepiolite), and relict igneous minerals (quartz, albite, and K-feldspar). The weathered samples are strongly enriched in La (up to 92,390ppm), Ce (up to 103,500ppm), Pr (up to 8006ppm), and Nd (up to 16,690ppm) compared with the unweathered brecciated ores. Conversely, Sr concentrations are significantly more enriched in the brecciated ores (up to 256,500ppm) than in the weathered ores (generally less than 2671ppm with one exception of 37,850ppm) due to less celestite. Calcite is largely absent from the weathered ores (except one sample with up to 30% mode), which contrasts with the brecciated ores that contain up to 75% calcite. The effects of weathering, oxidation, loss of ions, and hydration on the brecciated ores led to the refertilization of the REEs and an increase in the grade of the ore deposit.

Petrology of potassic alkaline ultramafic and carbonatitic rocks from Planalto da Serra (Mato Grosso State), Brazil

Abstract The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90–80 Ma we question the subsistence of a stationary mantle plume for so long time.

Extreme chemical conditions of crystallisation of Umbrian Melilitolites and wealth of rare, late stage/hydrothermal minerals

Abstract Melilitolites of the Umbria Latium Ultra-alkaline District display a complete crystallisation sequence of peculiar, late-stage mineral phases and hydrothermal/cement minerals, analogous to fractionated mineral associations from the Kola Peninsula. This paper summarises 20 years of research which has resulted in the identification of a large number of mineral species, some very rare or completely new and some not yet classified. The progressive increasing alkalinity of the residual liquid allowed the formation of Zr-Ti phases and further delhayelitemacdonaldite mineral crystallisation in the groundmass. The presence of leucite and kalsilite in the igneous assemblage is unusual and gives a kamafugitic nature to the rocks. Passage to non-igneous temperatures (T<600 °C) is marked by the metastable reaction and formation of a rare and complex zeolite association (T<300 °C). Circulation of low-temperature (T<100 °C) K-Ca-Ba-CO2-SO2-fluids led to the precipitation of sulphates and hydrated and/or hydroxylated silicate-sulphate-carbonates. As a whole, this mineral assemblage can be considered typical of ultra-alkaline carbonatitic rocks.

Lamprophyres of Italy: early Cretaceous alkaline lamprophyres of Southern Tuscany, Italy

Alkaline lamprophyres from southern Tuscany are early Cretaceous, ultrabasic, primary mantle melts that have not undergone significant magmatic differentiation. New mineralogical, geochemical and Sr, Pb and Nd isotopic evidence shows contrasting geochemistries with “crustal-like” and mantle geochemical features. High Mg#'s, however, coupled with high compatible elements, rule out any notable mantle melt dilution by bulk crustal contamination. Variations of Zr/Hf and Ta/Nb indicate a source containing residual titanates while their REE geochemistry suggests low degrees of partial melting and possible metasomatism of their source by carbonatitic melts. Arrays in isotope ratio diagrams are consistent with mixing between two distinct mantle end members. One is FOZO-like in character, and supports the involvement of asthenosphere, while the second has 87Sr/86Sr initial values that are much higher than FOZO (>0.70641). High 87Sr/86Sr signatures are present in the other Italian alkaline lamprophyres as well as other potassic–ultrapotassic and carbonatitic rocks of Italy. Our preferred model involves melting of a two component, metasomatised mantle, which can be tied into plume-related magmatism.

Investigaton of Kpong carbonatite as a potential source for rare earth elements (REEs) using instrumental neutron activation analysis (INAA)

Abstract Instrumental neutron activation analysis (INAA) was used to investigate REEs in carbonatite from Kpong southeastern, Ghana. Total rare earth element (TREEs) obtain were in the range of 540 mg/kg to 705 mg/kg. The total number of rare earth elements (REEs) determined by INAA in the carbonatite rocks from Kpong were 11, namely; La, Ce, Nd, Sm, Eu, Tb, Dy, Ho, Tm, Yb, Lu. The INAA results from the carbonatite show a high enrichment of light rare earth elements (LREEs) deposits, marking the Kpong carbonatite as a potential REE source.

Petrological and geochemical characteristics of Paleoproterozoic ultramafic lamprophyres and carbonatites from the Chitrangi region, Mahakoshal supracrustal belt, central India

A number of ENE–WSW trending Paleoproterozoic dykes and plugs of mafic, ultramafic, alkaline and carbonatite rocks intrude Mahakoshal supracrustal belt (MSB), which is a part of the Central Indian Tectonic Zone (CITZ). Best exposures of these intrusions are found in the eastern parts of the MSB, particularly in and around Chitrangi area. Many of these intrusions have greenschist facies mineral composition and show sharp contact with supracrustal rocks. However, igneous textures, such as porphyritic/glomeroporphyritic, are still preserved in the form of partly pseudomorphed olivines, phlogopites and pyroxenes. Striking feature observed in some ultramafic samples is the presence of melanite garnet and rounded or elliptical carbonate ocelli. The petrographic characteristics suggest occurrence of carbonate-rich ultramafic lamprophyres; close to aillikite composition. Coarse-grained carbonatites show hypidiomorphic texture and mostly composed of calcite with appreciable amount of silicate minerals like clinopyroxene, phlogopite and olivine (often pseudomorphed by calcite, amphibole and chlorite). It is difficult to establish any direct genetic relationship between carbonatite and ultramafic lamprophyre samples on the basis of their chemistry; they were likely derived from distinct parental melts. High Mg# (up to ~78), and high Ni and Cr contents (up to ~1700 and ~1100, respectively) and low HREE concentration in few ultramafic lamprophyre samples apparently suggest their derivation from a near-primary mantle-derived melts originated at great depths. Geochemistry and presence of carbonate ocellae in ultramafic lamprophyre samples suggest genesis of these silicate rocks and associated carbonatites through liquid immiscibility, however possibility of their derivation through vein-plus-wall-rock melting model cannot be ignored. A multi-stage veined mantle melting model is suitable in the latter case. It is suggested that early stages of rifting in the Mahakoshal region due to lithospheric thinning caused by possible plume activity provided suitable conditions for the genesis of ultramafic lamprophyre (possibly aillikitic) and carbonatitic melts which ultimately crystallized as dykes and plugs.

Different zircon recrystallization types in carbonatites caused by magma mixing: Evidence from U–Pb dating, trace element and isotope composition (Hf and O) of zircons from two Precambrian carbonatites from Fennoscandia

Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of the U–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0Ga old Tiksheozero complex underwent a coupled dissolution–reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state as well as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.

Geochemical temporal evolution of Brava Island magmatism: Constraints on the variability of Cape Verde mantle sources and on carbonatite–silicate magma link

Elemental and isotopic (Sr, Nd and Pb) analyses of silicate and carbonatite rocks from Brava, the southernmost island of the Cape Verde archipelago, are reported here to assess the nature, origin and location of the main contributors to magma composition and to evaluate the chemical evolution of magma through time. In contrast to other islands of the archipelago where a continuous range is usually observed, Brava is clearly characterized by two groups of distinct isotopic compositions. Rocks from the Upper Unit (<0.5Ma) are less Nd- and Pb-radiogenic and more Sr-radiogenic than rocks belonging to the Basal Complex (~3 to 1.4Ma), which are also characterized by more unradiogenic He signatures. Here, the chemistry of the Basal Complex is mainly explained by the mixing of a high-μ (HIMU)-type local end-member, which represents an ancient (~1.3Ga) recycled oceanic crust, with the lower mantle. For the Upper Unit, elemental and isotopic signatures suggest the involvement of an additional local end-member akin to the EM1-type. Such differences in the contributing end-members to the Upper Unit and Basal Complex sources mirror those usually assigned to the southern and northern Cape Verde islands, respectively. This temporal evolution is discussed in light of plume interaction with a shallow EM1-type domain, which is considered to represent the subcontinental lithosphere floating in the asthenosphere. Brava carbonatites define two distinct groups with similar isotopic ranges as those observed for the coeval alkaline silicate rocks. This observation suggests that, in each of the volcano-stratigraphic units, carbonatite and silicate magmas are ultimately derived from the same sources. We propose that calciocarbonatites from either the Basal Complex or the Upper Unit resulted from nephelinite–carbonatite liquid immiscibility, while the subordinate Basal Complex magnesiocarbonatites represent residual liquids after calcite fractionation from carbonatite magma.