U. Mass Lowell |
Prof. Nelson Eby | Department of Environmental, Earth, & Atmospheric Sciences |
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Carbonatites |
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Carbonatites are primarily composed of calcite (or dolomite) of igneous origin. Silicate minerals (such as phlogopite and sodic pyroxenes) are variably present. Apatite is often an essential mineral and a variety of exotic minerals, such as pyrochlore, perovskite, bastnaesite, etc., frequently occur in carbonatites. They are often, but not always, associated with silica-undersaturated alkaline rocks. Carbonatites are geographically widely distributed (over 500 occurrences have been identified). While of minor areal extent, because of their unusual petrogenesis and exotic mineralogy (in some cases giving rise to rare-earth, niobium, copper, etc. ore deposits), carbonatites have been extensively studied. Investigation of the geochemistry and geochronolgy of carbonatites is an ongoing research topic. Publications resulting from this research are listed below. |
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Sovite (calcite carbonatite) photomicrograph. Crossed polars. Width of field of view, 5 mm. The blue birefringent mineral is niocalite (a Nb-silicate) from the Bond Zone of the Oka carbonatite complex, Quebec. |
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Eby, G. N. (2000) Geochronology, geochemistry and petrogenesis of the Arkansas Alkaline Province. Geological Society of America Abstracts with Programs 32, 3, p. A9.
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Petrogenesis of the Fort Portal, Uganda, Extrusive Carbonatite Eby, G. N., Lloyd, F. E. and Woolley, A. R. The Quaternary Fort Portal volcanic field occurs at the northern end of the Western Rift in Uganda. The eruptive phases consist of (1) early carbonatite tuff cones followed by (2) a blanket carbonatite tuff (the major unit of the field) and finally (3) a small volume of calciocarbonatite lava. Mantle and crustal xenoliths are found in all eruptive phases and silicate (melilitite) lapilli are associated with the blanket tuffs. Major and trace element geochemistry was done on 37 samples from all three eruptive phases plus 10 crustal xenoliths. The silicate lapilli contain carbonate ocelli and abundant lithic fragments. Major and trace element geochemistry was determined for the melilitite lapilli matrix and carbonate ocelli, plus inter-lapilli carbonates, using a variety of microbeam techniques. The chemistry of the tuffs was modeled using a 5 component system – carbonatite lava, melilitite lapilli matrix, low REE and HFSE carbonate associated with the silicate lapilli, and felsic and mafic crustal xenoliths. Mixing calculations for individual tuff samples show that the major components are carbonatite lava and xenolithic material. The melilitite lapilli component varies from 0% (cone tuffs) to 27% in the blanket tuffs. The low REE and HFSE carbonate component is generally of minor importance. The carbonate phases found in the tuffs and lapilli are formed by a
variety of processes. The carbonate ocelli in the silicate lapilli, on the basis
of major element chemistry and texture, could represent an immiscible liquid
separated from melilitite melt. However, the trace element data strongly support
a model involving the separation of a late stage fluid phase from the
crystallizing melilitite. The inter-lapilli carbonate is the result of deuteric
processes. Both major element and trace element data support an origin of the
carbonatite lava as an immiscible liquid separated from a carbonated melilitite
melt at high pressure (>0.6GPa). Melilitite at Fort Portal, Uganda: Another Dimension to the Carbonate Volcanism Bailey, K., Lloyd, F., Kearns, S., Stoppa, F., Eby, N., and Woolley, A. Because the calciocarbonatite lavas at Fort Portal were the first ever described they have received great attention, with the pyroclastic rocks being relatively neglected. Volumetrically the lavas are minute, and the major deposit is a 2 m thick blanket of "flaggy" tuffs, long regarded as carbonatite tuff with crustal debris. Fresh examination shows these tuffs to contain melilitite previously unreported from Fort Portal. The rock is a mix of crust and mantle debris with near-isotropic lapilli, set in a matrix composed predominantly of carbonatite. The low birefringence parts of the lapilli are devitrifed melilitite glass. Compound lapilli are abundant, containing aggregates of globules, together with xenolithic/crystic fragments. In some, there are concentric zones of more carbonate rich material alternating with melilitite; tangential phlogopite flakes mark the outer zones, in marked contrast to their planar distribution through the enclosing rock matrix. Euhedral titano-magnetite (10-15%) is the most obvious cognate mineral. Devitrified melilitite contains abundant small crystals and microlites of melilite, apatite, magnetite, and carbonates, mostly formed during disequilibrium quench crystallisation. Because of this, and widespread fine grained accidental debris, a precise bulk melt composition is hard to obtain, but the average is close to melilitite with high P2O5. Mantle debris is largely disaggregated magnetite-phlogopite clinopyroxenite, which could give a bulk composition close to the melt. Low Mg and high Mg calcite are present in the melilitite lapilli, and in the enclosing carbonate rich matrix. Previously, high Mg calcite was reported only as cement in lapilli tuffs, while the lavas contain only low Mg calcite in the assemblage calcite-periclase (consistent with low pressure carbonate melt crystallisation). Carbonatite-melilitite magma left the mantle carrying restite debris. Melt fragmentation took place in the deep crust, with rapidly quenched droplets enclosing crust debris. Chemical covariations within the flaggy tuffs are uniform and explicable as carbonatite-melilitite plus a thoroughly mixed combination of crust and mantle debris. New links are indicated with the alkaline ultramafic-carbonate volcanism to the south, in Uganda, and parallel with that in Italy. Eby, G. N., Lloyd, F. E., Woolley, A. R., Stoppa, F., and Weaver, S. D. Samples from the various volcanic fields in the Uganda portion of the western branch of the East African rift system were analyzed for major and trace elements. The northernmost Fort Portal field consists of extrusive carbonatite tuffs and lavas. All these samples are mixtures of carbonatite, basement rock fragments and peridotite xenoliths. The central fields, Katwe-Kikorongo and Bunyaraguru, and Kasenyi Crater, are ultrapotassic, but detailed sampling indicates that the degree of K enrichment, with respect to Na, varies geographically with Bunyaraguru and Kasenyi Crater showing the greatest enrichment. The southern field, Bufumbira, while also potassic shows a much lower degree of K enrichment. In terms of a variety of trace elements, and trace element ratios, various mantle domains can be identified that gave rise to the magmas in each of the volcanic areas. The data indicate that the subcontinental mantle under this portion of Uganda has undergone variable degrees of metasomatism. A complete description of the character of the primary magmas requires consideration of both the degree of metasomatism and the degree of melting of the garnet-bearing source. Geochemistry and Mantle Source(s) of Carbonatitic and Potassic Lavas from SW Uganda Eby, G. N., Lloyd, F. E., Woolley, A. R., and Stoppa, F. The western branch, in SW Uganda, of the East African rift system is one of the classic localities for potassic alkaline magmatism. From north to south the province consists of carbonatite lavas at Fort Portal, ultrapotassic mafic rocks in the central Katwe-Kikorongo and Bunyaruguru fields and potassic mafic-felsic flows in the Bufumbira field. In this study we report the results of a major and trace element study of the lavas of these various fields. The Fort Portal extrusive carbonatites consist of both pyroclastic deposits and lava flows. Basement xenoliths are common in all samples. In agreement with previous authors, the chemistry of the various units can be explained by simple mixing between carbonatite and basement xenoliths. Assuming that the original carbonatite melt contained no alumina, extrapolation of these mixing curves yields an original composition for the carbonatite magma that is broadly similar to that of OIB (2x OIB for the HFSE to 10x OIB for the LIL elements), with notable depletions in Rb, K, Hf and Ti and significant enrichment in Cs. In the Katwe-Kikorongo and Bunyaruguru fields the lavas are potassic to ultrapotassic in composition. There is a well-defined regional variation with lavas from the eastern portion of the Katwe-Kikorongo field and the Bunyaruguru field (which lies to the east) showing ultrapotassic characteristics (K2O/Na2O = 3 to 12), which presumably reflects a mantle source significantly enriched in potassium. Pyroxene and olivine are the common phenocryst phases in these lavas. Both of these volcanic fields show OIB-normalized spider diagrams sloping upwards from HFSE to LIL elements. The only significant variation from a relatively smooth trend is a pronounced Cs enrichment. In the Bufumbira field, K2O/Na2O < 2. The common phenocryst phases in these lavas are pyroxene and olivine. On OIB-normalized spider diagrams the enrichment in LIL elements is not as pronounced as for the other volcanic fields, and there is no Cs anomaly. Phase equilibra relationships and trace element data indicate that the chemical evolution of the silicate magmas can largely be explained by fractional crystallization of pyroxene and olivine, with minor contributions from micas and opaque oxides. Plagioclase, and other feldspar and feldspathoid minerals, were not significant phases in the early evolution of the magmas. Regional variations in the composition of the subcontinental mantle are reflected by variations in both major element compositions and trace element ratios. The Bufumbira lavas consistently plot within the OIB fields for basaltic rocks while the ultrapotassic lavas from Katwe-Kikorongo and Bunyaruguru fall outside these fields. Each volcanic field plots in a different area on a Y/Nb versus Nb/Zr diagram which indicates both the effect of different degrees of melting and variations in source composition. Other elemental ratios, such as Nb/Ta and Zr/Hf, are broadly similar for both the ultrapotassic and potassic lavas. We conclude that the eastern Katwe-Kikorongo and Bunyaruguru lavas were derived from a chemically distinct mantle source. The potassic and ultrapotassic magmas can be related through different degrees of melting of a garnet lherzolite mantle (classic crossing REE patterns). Thus, the major regional differences in lava chemistry may be due to variable degrees of potassium metasomatism of a garnet lherzolite subcontinental mantle. Lloyd, F. E., Woolley, A. R., Stoppa, F., and Eby, G. N. Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (>12 wt% Al2O3), typically contain significant levels of Cr (up to 1.1 wt% Cr2O3), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al2O3 and TiO2 compositional field of the xenolith mica, and on the basis of Mg# and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonate tephra. Higher Al2O3 distinguishes Fort Portal carbonatite xencorysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carboantite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Stoppa, F., Woolley, A. R., Lloyd, F. E., and Eby, N. A group of carbonate-rich tuffs are described from the Murumuli crater, Katwe-Kikorongo volcanic field, SW Uganda which contain abundant carbonatite pelletal lapilli, together with melilitite lapilli and a range of xenocrysts and lithic fragments including clinopyroxenites considered to be of mantle origin. The carbonatite lapilli consist essentially of Sr-bearing calcite and Mg-calcite which form quench-textured laths. The lapilli contain microphenocrysts of Ti-magnetite, perovskite, apatite, clinopyroxene, sanidine and altered prisms of melilite. A 7 cm long dolomite carbonatite bomb is described which displays a form typically assumed by lava clots erupted in a molten state. Chemical analyses of a tuff, the bomb and a range of minerals are presented. Carbonatite clearly played an important role in the Katwe-Kikorongo magmatism and it is suggested that carbonatite magma evolved from carbonate-bearing melilitite. |
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Geochronology, Geochemistry and Petrogenesis of the Arkansas Alkaline Province Eby, G. N. The intrusives of the Arkansas Alkaline Province (AAP) lie in a northeast trending belt along the northern edge of the Mississippi Embayment. There is a spatially-related variation from lamproites at the southwestern end through several carbonatite complexes, a lamprophyre dike swarm, and then at the northeastern end several large syenite bodies. Fission-track titanite and apatite ages suggest two discrete periods of magmatic activity: one ca. 100 Ma which includes most of the intrusions and the second at ca. 88 Ma which is the time of emplacement of the syenite bodies. In general the fission-track ages are in agreement with radiometric ages obtained by other methods. Of note is that for all the intrusions, with the exception of Magnet Cove, titanite and apatite ages are indistinguishable indicating rapid cooling for these intrusions. At Magnet Cove an age differential of ca. 6 million years (titanite = 102 Ma and apatite = 96 Ma) suggests slower cooling and a greater depth of emplacement. At present there is no convincing evidence for a regular time progression that would indicate a hot-spot trace. Existing Sr, Pb and Nd isotopic data indicate that the source of the various magmas was a LIL-depleted mantle, as seen for other alkaline provinces. There is a good correlation between Zr and Hf for all the intrusives and Zr/Hf = 44, typical of a LIL-depleted mantle source, for the province. Nb/Ta and Th/U ratios are highly variable indicating late- and post-magmatic redistribution. Modelling based on phase equilibria constraints and trace element data suggest that the lamproites and mafic dikes of the various intrusions can be related through variable degrees of partial melting of a garnet lherzolite mantle. The syenites are seen to have a separate source and differentiation most likely occurred at depth with emplacement of the felsic residua at high levels. Within the syenite bodies differentiation was largely controlled by feldspar fractionation and accumulation. The existing data support the idea that the emplacement of the various intrusions of the AAP should be viewed as two separate events separated by approximately 12 million years. Eby, G. N. and Mariano, A. N. Apatite and titanite fission-track ages have been determined for carbonatite complexes and associated alkaline rocks of the Paranaíba province, Brazil, and the Amambay and Concepción provinces, Paraguay. Close agreement between previously determined K-Ar ages and the apatite fission-track ages indicates that the complexes were emplaced at shallow levels and quickly cooled to the low temperatures (<70oC) required for the retention of fission tracks in apatite. The complexes in Paranaíba province, with the exception of the somewhat older Cataläo I (114 Ma), were emplaced between 79 Ma and 88 Ma, in two apparently separate pulses of activity ca. 80 Ma and ca. 88 Ma. The alkaline complexes of the Amambay and Concepción provinces exhibit a greater range of emplacement ages, from 86 Ma to 147 Ma, with most of the complexes emplaced between 111 Ma and 126 Ma. At both Chiriguelo and Cerro Sarambi, much younger ages obtained for transgressive carbonatitic units indicate a renewal of the igneous activity that led to mineralization. Eby, G. N. Rare-earth (REE) and yttrium abundances were determined, by an ion-exchange - X-ray fluorescence procedure, for whole-rock (14) and mineral (87) samples from the Oka carbonatite complex. Whole-rock and mineral data indicate a trend of total REE + Y enrichment, and relative enrichment in light REE, in the order: ultrafenites < ijolites < okaites. The sövites may show wide variations in total REE + Y concentrations, but relative REE abundance patterns will be similar. The greatest REE and Y concentrations occur in apatite, niocalite, perovskite and pyrochlore. Many of the minerals show europium anomalies (both positive and negative), and these are believed to be the result of closed system competition between the various minerals for divalent Eu. The partition coefficients for mineral pairs are quite variable, indicating that the Oka rocks were emplaced through a wide-range of physicochemical and/or nonequilibrium conditions. A reasonable model for the origin of the complex involves a limited partial melting of mantle material, emplacement of the melt in a magma chamber, crystallization of mafic minerals resulting in a residual liquid which produced ijolite and subsequently okaite, and crystallization of the carbonatites from a volatile-rich, possible immiscible, phase. Scandium Geochemistry of the Oka Carbonatite Complex, Oka, Quebec Eby, G. N. Scandium abundances were determined by X-ray fluorescence for rocks (14 samples) and minerals (84 samples) from the Oka carbonatite complex, Oka, Quebec. Whole-rock data indicate a general trend of scandium enrichment in the order: urtites < ijolites < okaites < sövites < alnöites. The ore minerals (niocalite, perovskite, and pyrochlore) and apatite are the principal carriers of scandium; ferromagnesian minerals contain less scandium and show the order of scandium uptake: biotite < pyroxene < garnet. The scandium content of the calcites is quite variable. Monticellite, melilite, nepheline, albite, the zeolites, and the feldspathoids contain negligible amounts of scandium. Scandium and REE concentrations vary sympathetically for the ore minerals, calcites, and apatites but not for any of the other minerals. Partitioning coefficients for various coexisting phases indicate that, in general, equilibrium was not attained by the Oka rocks. The silicate rocks of the complex crystallized from a differentiating alkali peridotite magma, and the sövites formed from a volatile-rich phase. The selective partitioning of scandium, possibly in the form of complex ions, into the volatile-rich phase may account for the concentration of scandium in the sövites. |
