Carbonate-rich Magma Research Articles

Comparative mineralogy, geochemistry and petrology of the Beloziminsky Massif and its aillikite intrusions

The Beloziminsky Massif (BZM) is an alkaline ultramafic carbonatite complex that includes carbonatites, ijolites, meltegites, and syenites (abbreviated as the CIMS suite) as well as aillikite intrusions that range in age from 645–621 Ma. Aillikite intrusions also occur in the Yuzhnaya Pipe (YuP), located about 16 km eastward of the BZM. Over 5400 analyses in total were conducted to compare mineralogy and geochemistry of different rock types in this study; of these, 24 CIMS samples (>1100 analyses) and about 16 aillikites (>2300 analyses) were collected from within the BZM; the rest are aillikite mineral samples from pipes and dykes outside the massif (>2000 analyses). The results suggest significant differences in sources for rock-forming minerals, less so for the accessories. The pyroxenes in aillikite correspond either to mantle Cr-diopside xenocrysts or megacrystic augites. Low-Na Ti-augites and diopsides as well as aegirines are prevalent in the CIMS intrusive suite. Amphiboles show a considerably long compositional trend, from hornblendes to richterites. Dolomitic carbonatites include admixtures of Na, K, and Ba while calcium carbonatites often contain Sr. The carbonate-rich aillikitics are enriched either in Mg or Ca. The CIMS rocks, particularly the Ca-Mg carbonatites, often include siderites. Thermobarometry for the YuP samples, collected from outside the BZM and containing Cr-diopsides, Cr-phlogopites and Cr-spinels, suggest a formation pressure of 2–4 GPa and a temperature of 800–1250°C; augite xenocrysts with elevated HFSE, U, Th, and Al-augites trace a 90 mW/m2 geotherm.The huge thermal impact of the plume that triggered the break-up of Rodinia also created a series of ultramafic–alkaline–carbonatite massifs. Initially, the aillikites in the mantle were likely produced by the plume-induced melting of carbonated metasomatites containing ilmenite, perovskites, apatites, amphiboles and phlogopites which, in turn, were created by subduction-related melts. Any additional enrichment in the ore components might have occurred subsequentlty in the lower crust, due to liquation. The aillikites inside the BZM contain low-temperature clinopyroxenes tracing a steep advective geotherm (0.4–1.5 GPa); they also contain clots, related to intermediate depth magma chambers, together with CIMS pyroxenes and amphiboles. This suggests that the liquation of aillikites was accompanied by density separation and assimilation and fractional crystallization (AFC) fractionation with the participation of crustal material. Trace elements (especially REEs) in silicate minerals, carbonates, apatites, and accessories (perovskites, pyrochlores, monazites, columbites, zircons, ancylites, etc.) show a general rise in REE levels and La/Ybn ratios from aillikites to ijolites, and later to Fe- carbonatites. The presence of zircons, monazites, columbite-tantalites, and other Zr-Hf and Ta-Nb minerals like perovskites and tantalites in the BZM aillikites occurred due to the mixing of the silicate melts with carbonate-rich magmas at deeper levels in the crust and later, in the massif. In the aillikites, xenocrysts. Further, apatites and perovskites show high REE levels. The carbonate-silicate magmas likely passed through a system of polybaric magmatic chambers and liquated carbonatites. Consequently, the later-formed aillikites captured and mixed all varieties of xenocrysts.

Ringwoodite and zirconia inclusions indicate downward travel of super-deep diamonds

Abstract Natural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as ~800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic.

Carbonatites: Classification, Sources, Evolution, and Emplacement

Carbonatites are igneous rocks formed in the crust by fractional crystallization of carbonate-rich parental melts that are mostly mantle derived. They dominantly consist of carbonate minerals such as calcite, dolomite, and ankerite, as well as minor phosphates, oxides, and silicates. They are emplaced in continental intraplate settings such as cratonic interiors and margins, as well as rift zones, and rarely on oceanic islands. Carbonatites are cumulate rocks, which are formed by physical separation and accumulation of crystals that crystallize from a melt, and their parental melts form by either ( a) direct partial melting of carbonate-bearing, metasomatized, lithospheric mantle producing alkali-bearing calciodolomitic melts or ( b) silicate-carbonate liquid immiscibility following fractional crystallization of carbonate-bearing, silica-undersaturated magmas such as nephelinites, melilitites, or lamprophyres. Their emplacement into the crust is usually accompanied by fenitization, alkali metasomatism of wallrock caused by fluids expelled from the crystallizing carbonatite.Carbonatites are major hosts of deposits of the rare earth elements and niobium, and the vast majority of the global production of these commodities is from carbonatites. ▪ Carbonatites are igneous rocks formed from carbonate-rich magmas, which ultimately formed in Earth's upper mantle. ▪ Carbonatites are associated with economic deposits of metals such as the rare earth elements and niobium, which are essential in high-tech applications. ▪ There are more than 600 carbonatites in the geological record but only one currently active carbonatite volcano, Oldoinyo Lengai in Tanzania.

Light oxygen isotopes in mantle-derived magmas reflect assimilation of sub-continental lithospheric mantle material

Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.

Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements

The rare earth elements (REE) are critical raw materials for much of modern technology, particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition. Many of the world’s largest REE deposits occur in alkaline rocks and carbonatites, which are found in intracontinental, rift-related settings, and also in syn- to post-collisional settings. Post-collisional settings host significant REE deposits, such as those of the Mianning-Dechang belt in China. This paper reviews REE mineralization in syn- to post-collisional alkaline-carbonatite complexes worldwide, in order to demonstrate some of the key physical and chemical features of these deposits. We use three examples, in Scotland, Namibia, and Turkey, to illustrate the structure of these systems. We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes. It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralization in these settings, with REE, Ba and F partitioning into the carbonate-rich phase. The most significant REE mineralization in post-collisional alkaline-carbonatite complexes occurs in shallow-level, carbothermal or carbonatite intrusions, but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.

Displaced cratonic mantle concentrates deep carbon during continental rifting.

Continental rifts are important sources of mantle carbon dioxide (CO2) emission into Earth's atmosphere1-3. Because deep carbon is stored for long periods in the lithospheric mantle4-6, rift CO2 flux depends on lithospheric processes that control melt and volatile transport1,3,7. The influence of compositional and thickness differences between Archaean and Proterozoic lithosphere on deep-carbon fluxes remains untested. Here we propose that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. Sources and fluxes of CO2 and helium are examined over a 350-kilometre-long transect crossing the boundary between orogenic (Natron and Magadi basins) and cratonic (Balangida and Manyara basins) lithosphere from north to south. Areas of diffuse CO2 degassing exhibit increasing mantle CO2 flux and 3He/4He ratios as the rift transitions from Archaean (cratonic) to Proterozoic (orogenic) lithosphere. Active carbonatite magmatism also occurs near the craton edge. These data indicate that advection of the root of thick Archaean lithosphere laterally to the base of the much thinner adjacent Proterozoic lithosphere creates a zone of highly concentrated deep carbon. This mode of deep-carbon extraction may increase CO2 fluxes in some continental rifts, helping to control the production and location of carbonate-rich magmas.

Aillikites and Alkali Ultramafic Lamprophyres of the Beloziminsky Alkaline Ultrabasic-Carbonatite Massif: Possible Origin and Relations with Ore Deposits

The 650–621 Ma plume which impinged beneath the Siberian craton during the breakup of Rodinia caused the formation of several alkaline carbonatite massifs in craton margins of the Angara rift system. The Beloziminsky alkaline ultramafic carbonatite massif (BZM) in the Urik-Iya graben includes alnöites, phlogopite carbonatites and aillikites. The Yuzhnaya pipe (YuP) ~ 645 Ma and the 640–621 Ma aillikites in BZM, dated by 40Ar/39Ar, contain xenoliths of carbonated sulfide-bearing dunites, xenocrysts of olivines, Cr-diopsides, Cr-phlogopites, Cr-spinels (P ~ 4–2 GPa and T ~ 800–1250 °C) and xenocrysts of augites with elevated HFSE, U, Th. Al-augites and kaersutites fractionated from T ~ 1100–700 °C along the 90 mW/m2 geotherm. Higher T trend for Al-Ti augite, pargasites, Ti-biotites series (0.4–1.5 GPa) relate to intermediate magma chambers near the Moho and in the crust. Silicate xenocrysts show Zr-Hf, Ta-Nb peaks and correspond to carbonate-rich magma fractionation that possibly supplied the massif. Aillikites contain olivines, rare Cr-diopsides and oxides. The serpentinites are barren, fragments of ore-bearing Phl carbonatites contain perovskites, Ta-niobates, zircons, thorites, polymetallic sulphides and Ta-Mn-Nb-rich magnetites, ilmenites and Ta-Nb oxides. The aillikites are divided by bulk rock and trace elements into seven groups with varying HFSE and LILE due to different incorporation of carbonatites and related rocks. Apatites and perovskites reveal remarkably high LREE levels. Aillikites were generated by 1%–0.5% melting of the highly metasomatized mantle with ilmenite, perovskite apatite, sulfides and mica, enriched by subduction-related melts and fluids rich in LILE and HFSE. Additional silicate crystal fractionation increased the trace element concentrations. The carbonate-silicate P-bearing magmas may have produced the concentration of the ore components and HFSE in the essentially carbonatitic melts after liquid immiscibility in the final stage. The mechanical enrichment of aillikites in ore and trace element-bearing minerals was due to mixture with captured solid carbonatites after intrusion in the massif.

Composition and emplacement of the Benfontein kimberlite sill complex (Kimberley, South Africa): Textural, petrographic and melt inclusion constraints

The Benfontein kimberlite is a renowned example of a sill complex and provides an excellent opportunity to examine the emplacement and evolution of intrusive kimberlite magmas. We have undertaken a detailed petrographic and melt inclusion study of the Benfontein Upper, Middle and Lower sills. These sills range in thickness from 0.25 to 5 m. New perovskite and baddeleyite U/Pb dating produced ages of 85.7 ± 4.4 Ma and 86.5 ± 2.6 Ma, respectively, which are consistent with previous age determinations and indicate emplacement coeval with other kimberlites of the Kimberley cluster.The Benfontein sills are characterised by large variations in texture (e.g., layering) and mineral modal abundance between different sill levels and within individual samples. The Lower Sill is characterised by carbonate-rich diapirs, which intrude into oxide-rich layers from underlying carbonate-rich levels. The general paucity of xenogenic mantle material in the Benfontein sills is attributed to its separation from the host magma during flow differentiation during lateral spreading. The low viscosity is likely responsible for non-explosive emplacement of the Benfontein sills, while the rhythmic layering is attributed to multiple magma injections.The Benfontein sills are marked by the excellent preservation of olivine and groundmass mineralogy, which is composed of monticellite, spinel, perovskite, baddeleyite, ilmenite, apatite, calcite, dolomite along with secondary serpentine and glagolevite [NaMg6[Si3AlO10](OH,O)8•H2O]. This is the first time glagolevite is reported in kimberlites. Groundmass spinel exhibits atoll-textures and is composed of a magnesian ulvöspinel – magnetite (MUM) or chromite core, surrounded by occasional pleonaste and a rim of Mg-Al-magnetite. We suggest that pleonaste crystallised as a magmatic phase, but was resorbed back into the residual host melt and/or removed by alteration.Analyses of secondary inclusions in olivine and primary inclusions in monticellite, spinel, perovskite, apatite and interstitial calcite are largely composed of Ca-Mg carbonates and, to a lesser extent, alkali-carbonates and other phases. These inclusions probably represent the entrapment of variably differentiated parental kimberlite melts, which became progressively more enriched in carbonate, alkalis, halogens and sulphur during crystal fractionation. Carbonate-rich diapirs from the Lower Sill contain more exotic phase assemblages (e.g., Ba-Fe titanate, barite, ancylite, pyrochlore), which probably result from the extreme differentiation of residual kimberlite melts followed by physical separation and isolation from the parental carbonate-rich magma. It is likely that any alkali or halogen rich minerals crystallising in the groundmass were removed from the groundmass during syn−/post-magmatic alteration, or in the case of Na, remobilised to form secondary glagolevite. The Benfontein sill complex therefore provides a unique example of how the composition of kimberlites may be modified after magma emplacement in the upper crust.

A MAJOR LIGHT RARE-EARTH ELEMENT (LREE) RESOURCE IN THE KHANNESHIN CARBONATITE COMPLEX, SOUTHERN AFGHANISTAN

The rapid rise in world demand for the rare-earth elements (REEs) has expanded the search for new REE resources. We document two types of light rare-earth element (LREE)-enriched rocks in the Khanneshin carbonatite complex of southern Afghanistan: type 1 concordant seams of khanneshite-(Ce), synchysite-(Ce), and parisite-(Ce) within banded barite-strontianite alvikite, and type 2 igneous dikes of coarse-grained carbonatite, enriched in fluorine or phosphorus, containing idiomorphic crystals of khanneshite-(Ce) or carbocernaite. Type 1 mineralized barite-strontianite alvikite averages 22.25 wt % BaO, 4.27 wt % SrO, and 3.25 wt % ∑ LREE 2 O 3 (sum of La, Ce, Pr, and Nd oxides). Type 2 igneous dikes average 14.51 wt % BaO, 5.96 wt % SrO, and 3.77 wt % ∑ LREE 2 O 3 . A magmatic origin is clearly indicated for the type 2 LREE-enriched dikes, and type 1 LREE mineralization probably formed in the presence of LREE-rich hydrothermal fluid. Both types of LREE mineralization may be penecontemporaneous, having formed in a carbonate-rich magma in the marginal zone of the central vent, highly charged with volatile constituents (i.e., CO 2 , F, P 2 O 5 ), and strongly enriched in Ba, Sr, and the LREE. Based on several assumptions, and employing simple geometry for the zone of LREE enrichment, we estimate that at least 1.29 Mt (million metric tonnes) of LREE 2 O 3 is present in this part of the Khanneshin carbonatite complex.

Kimberlites: Magmas or mixtures?

Although the presence of xenocrystic olivine is widely recognized in kimberlite, there is little consensus about its contribution to the existing estimates for the composition of kimberlite magma. Whole rock geochemistry is critical to the debate regarding the composition of kimberlite magma, however, it has received little attention as an indicator of diamond grade due to conventional thought that diamonds are xenocrysts unrelated to their host kimberlite. The Foxtrot kimberlite Field in Northern Québec is comprised of at least three distinct kimberlite intrusions exhibiting variation in both diamond grade and geochemistry making it an ideal suite with which to test a possible correlation between diamond grade and whole rock composition. Olivine is ubiquitous (30 to 70%) in the Foxtrot kimberlites and exhibits a restricted composition that overlaps that of olivine in harzburgite xenoliths suggesting that the majority of olivine is xenocrystic. Carbonate is also abundant (8 to 35%) in the Foxtrot kimberlites and exhibits magmatic textures requiring that carbon be considered in any petrogenetic model for the Foxtrot kimberlites. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in the Foxtrot kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (~ 80/20), similar to that of cratonic mantle xenoliths. The xenocrystic nature of olivine requires that the contribution of mantle harzburgite must be removed to constrain the composition of the magma. The calculated magma composition that results from the mathematical removal of olivine and orthopyroxene (80/20) from the whole rock compositions is significantly poorer in MgO (15 wt.%) and silica (~ 24 wt.%), but CO 2 rich (~ 17 wt.%) compared to previous estimates for kimberlite magma. The Foxtrot kimberlites are best modelled as mixtures of harzburgite mantle and a relatively carbonate-rich magma. According to this model the correlation between whole rock composition and diamond grade reflects the fact that the diamonds are also xenocrystic and sourced in the harzburgite component.

Kimberlites and aillikites as probes of the continental lithospheric mantle

Although the mantle xenoliths carried by kimberlites are the source of much of our information about the composition of the mantle beneath the continents, the compositions of kimberlites themselves have received little attention for the information they carry about the nature of the lithospheric mantle. This neglect in part reflects their common fragmental, contaminated, and hybrid nature, but also the pervasive view that Group-I kimberlites are sourced in the underlying asthenosphere. Insight into the nature of kimberlites and their relationship to the other alkaline ultramafic rocks, such as aillikites, olivine lamproites, and meimechites, can be obtained by comparing their major element compositions in a way that treats their carbonate content as a primary magmatic phase. Group-I kimberlites and aillikites contain significant magmatic carbonate and their compositions fall to the Si-poor side of the composition of olivine. Group-I kimberlite can be distinguished from aillikite on the basis of Fe content, but there appears to be a gradation between these two end-members. In contrast, olivine lamproites and meimechites contain relatively little primary magmatic carbonate and have compositions that are more Si-rich than olivine. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in hypabyssal kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (∼ 70/30) similar to that of cratonic mantle xenoliths. Much of the olivine is present as xenocrysts, but the orthopyroxene is occult and has presumably been assimilated. The fact that individual fields of alkaline ultramafic rocks are characterized by uniform Fe and Ti contents that can be mapped on a regional scale suggests that the major element composition of these unusual rocks, and Group-I kimberlites in particular, is a reflection of the continental lithospheric mantle with which they have interacted. The association of Fe-rich aillikitic magmas with zones of cratonic rifting, and the requirements of Fe–Mg partitioning indicate that they form deeper than Group-I kimberlites, near the intersection of the 1350 °C mantle adiabat with the CO 2 mantle solidus, at pressures of 8 +GPa. According to our working model, Group-I kimberlites are produced by the influx and reaction of carbonate-rich magmas with the highly magnesian harzburgites of the lithospheric mantle beneath continental cratons. In non-cratonic environments, these rising carbonate-rich magmas evolve into aillikites because of the lower Mg # of the asthenospheric mantle. They rarely reach the surface, however, but become the enriched component incorporated into higher-degree basaltic melts. An inverse correlation between diamond grade and kimberlite Fe and Ti contents may simply reflect the fact kimberlites with the lowest Ti contents have interacted with the most depleted and reduced harzburgites of the lithospheric mantle, where diamonds are most likely to be encountered.

Crystallization of Cr-poor and Cr-rich megacryst suites from the host kimberlite magma: implications for mantle structure and the generation of kimberlite magmas

Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.

Melt inclusions from the deep Slave lithosphere: implications for the origin and evolution of mantle-derived carbonatite and kimberlite

Melt inclusions in clinopyroxenes from lherzolitic xenoliths from the deep lithospheric mantle beneath the Slave Craton (Lac de Gras area, Canada) reveal multiple origins for carbonatitic melts. One type of inclusions consists of a series of silicate–carbonate–silicate concentric layers, interpreted to have unmixed under disequilibrium conditions during rapid ascent to the surface. Bulk major- and trace-element compositions are typical of Group 1 kimberlites and quantitative nuclear microprobe imaging of the globules reveals fractionation of related elements (e.g. F–Br, Nb–Ta) between the silicate and carbonate components. The globules probably formed by partial melting of carbonated peridotite, consistent with results of melting experiments and some models for the generation of kimberlite magmas. They provide evidence for a genetic relationship between some carbonate-rich magmas and ultramafic silicate magmas, and for the possibility of unmixing processes of these melts during their evolution.The second inclusion type comprises carbonate-rich globules interpreted as samples of Mg-carbonatite melt that quenched on ascent to the surface. Bulk major- and trace-element compositions indicate that the melts were derived from a carbonate-rich source and oxygen, carbon, and strontium isotope data are consistent with the involvement of recycled crustal material and suggest that some mantle-derived carbonatites are unrelated to kimberlites.

Melts in the mantle modeled in the system CaO-MgO-SiO2-CO2 at 2.7 GPa

The effect of CO_2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO_2-CO_2 at 2.7 GPa. The experiments provide isotherms for the vapor-saturated liquidus surface, bracket piercing points for field boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics and peritectics). CO_2-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 °C above the solidus, with a transition to silicate liquids (>80% silicate) within ∼75 °C across a plateau on the liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate liquidus field boundary, so the carbonate liquidus field is therefore a forbidden volume for liquid magmas. This confirms the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed for the carbonation and melting reactions for mineral assemblages corresponding to lherzolite, harzburgite, websterite and wehrlite, with carbonate, CO_2 vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing compositions of eutectic and peritectic liquids on the liquidus surface. At an invariant point Q (∼2.8 GPa/1230 °C), all peridotite assemblages coexist with a calcite-dolomite solid solution (75 ± 5% CaCO_3) and a dolomitic carbonatite melt [57% CaCO_3 (CC), 33% MgCO_3 (MC), 10% CaMgSi_2O_6 (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic) at pressures below Q (∼70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q will begin to crystallize, releasing CO_2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.

Model System Controls on Conditions for Formation of Magnesiocarbonatite and Calciocarbonatite Magmas from the Mantle

Experimental data indicate that carbonate-rich magmas may be generated at depths greate than ∼70 km by partial melting of carbonated peridotite. The near-solidus magmas lie on the liquidus field boundary between silicates and carbonates. Liquid compositions are dominated by th system CaCO_3–MgCO_3, and precise compositions (e.g. Ca/Mg) are define by the peridotite mineralogy (e.g. harzburgite, lherzolite, wehrlite); alkali contents reflect directly the peridotite composition. These liquids are dolomitic, with Ca/(Ca + Mg) between 0.7 and 0.5 from 2 GPa to at least 7 GPa. At conditions of mantle melting, there is a large separation between the silicate–carbonate liquid immiscibility volume, the silicate–carbonate liquidus field boundary, and probable liquid paths. The formation of carbonate-rich liquids immiscible with silicate magmas in the mantle is therefore unlikely, which denies the generation of immiscible CaCO_3 ocelli and primary natrocarbonatite magmas. Rising carbonate-rich magmas retaining equilibrium with mantle lherzolite will react, crystallize and release CO_2 vapor at depths of ∼70 km, increasing clinopyroxene/orthopyroxene in the rock. Primary magnesiocarbonatite magmas (dolomitic) can be erupted explosively from this depth. Given sufficient magma, lherzolite can be converted to wehrlite by this decarbonation reaction. At shallower depths, wehrlite (but no other peridotite) can coexist with carbonatite magma relatively enriched in Ca/Mg. If metasomatism of lherzolite to wehrlite can occur through depth of tens of kilometers, our new data at 1 GPa confirm an earlier proposal that primary calciocarbonatite magmas can be generated at some depth between 70 km and 40 km, but indicate considerably higher silicate components. The shallowest magmas contain a maximum of 73 wt % CaCO_3 (equivalent to 89% CaCO_3 in the carbonate components of the liquid) with 18% silicate components at 1 GPa. Phase relations in the system CaO–MgO–CO_2–H_2O show that magnesiocarbonatite magmas can precipitate sovites (calciocarbonatite rocks).

Petrogenesis of Carbonatite Magmas from Mantle to Crust, Constrained by the System CaO-(MgO + FeO*)-(Na2O + K2O)-(SiO2 + Al2O3 + TiO2)-CO2

Experimental results from the systems CaO–MgO–SiO2–CO2, Na2O–CaO–Al2O3–SiO2–CO2, and a primitive magnesian nephelinite mixed with carbonates have been combined for construction of phase diagrams for the pseudoquaternary system CaO–(MgO + FeO*)–(Na2O + K2O)–(SiO2 + Al2O3 + TiO2) with CO2 at 1.0 and 2.5 GPa pressure. These diagrams provide a petrogenetic framework for magmatic processes from mantle to deep crust, with particular reference to the melting products of carbonate peridotite and the paths of crystallization of carbonated silicate magmas toward carbonatite magmas, with or without the intervention of silicate–carbonate liquid immiscibility. Three key features control these processes: (1) the liquidus surface bounding the silicate–carbonate liquid miscibility gap, (2) the silicate–carbonate liquidus boundary surface which separates the liquidus volume for primary silicates from that for primary carbonates, and (3) the curve of intersection of these two surfaces (1 and 2) which defines the coprecipitation of silicates and calcite with coexisting immiscible silicate- and carbonate-rich liquids. The geometrical arrangement of the two surfaces varies as a function of both pressure and bulk composition (e.g. with Si/Al, Na/K, Mg/Fe). Surface (2) is the locus of initial liquids from partial melting of carbonate–silicate assemblages, and the limit for residual liquid compositions derived from silicate–CO2 liquids. The carbonate liquidus volume is a forbidden region for carbonate-rich magmas derived from silicate magmas at the pressures investigated. The immiscible liquids dissolve no more than 80 wt % CaCO3, and the miscibility gap (MG) becomes smaller with increasing Mg/Ca. Extrapolation of experimental data indicates that the MG disappears with more than ∼50 wt % (MgO + FeO*) at 1.0 GPa for the compositions investigated. The distance between the miscibility gap, surface (1), and the silicate–carbonate liquidus surface, surface (2), increases significantly with increasing (MgO + FeO*). This observation, coupled with knowledge of the phase boundaries in the system, allows comparisons with projected rock compositions, and this permits the following conclusions. Calciocarbonatites and natrocarbonatites are excluded as candidates for primary magmas from the mantle, which must have compositions dominated by calcic dolomite. The formation of (equilibrium) carbonate-rich liquids immiscible with silicate magmas in the mantle is unlikely, which denies the formation of CaCO3 ocelli in mantle xenoliths as immiscible liquids. Immiscible carbonate-rich magmas separated from many silicate magmas may tend to be concentrated near calciocarbonatite compositions, with maximum CaCO3 75–80 wt %, low (MgO + FeO*), and (Na,K)2CO3 near 15 wt %. Silicate parents with higher Na/Ca and peralkalinity may yield immiscible magmas approaching natrocarbonatite compositions. Exsolution of immiscible carbonate-rich magma occurs without the coprecipitation of calcite except along the limiting field boundary (3). Only after the carbonate-rich magma is physically separated from the parent magma, and cooled with the precipitation of silicates, does it reach the silicate–carbonate field boundary and precipitate cumulate carbonatites, with inevitable enrichment of residual liquids in alkalis. Calciocarbonatite magmas cannot be derived from natrocarbonatite magmas. Dolomitic carbonatite magmas cannot be formed by liquid immiscibility, but only by fractionation of calciocarbonatites (according to CaCO3–MgCO3), or as primary magmas.

Liquid Immiscibility in the Join NaAlSi3O8−CaCO3 to 2.5 GPa and the Origin of Calciocarbonatite Magmas

Field evidence from intrusive and effusive carbonatites supports the existence of calciocarbonatite magmas. Published experimental evidence in the model system Na_2O−CaO−Al_2O_3−SiO_2−CO_2 indicated the formation of nearly pure (99%) CaCO_3 immiscible liquids from a carbonated silicate liquid. This evidence has been used to support interpretations of extremely CaCO_3-rich calciocarbonatite magmas, and immiscible liquids with compositions of almost pure CaCO_3 in metasomatized mantle peridotite and eclogite. Detailed phase relationships are constructed in the model system, based on phase fields intersected by the join NaAlSi_3O_8−CaCO_3 (Ab-CC) at 1.0, 1.5, and 2.5 GPa between 1100 and 1500°C, and analyzed immiscible liquids. The miscibility gap between silicate-rich liquid and carbonate-rich liquid intersected by the join Ab-CC contracts considerably with decreasing pressure: 2.5 GPa, between Ab_(10)CC_(90) (by wt%) and Ab_(65)CC_(35) above 1310°C; 1.5 GPa, between Ab_(23)CC_(77) and Ab_(43)CC_(57) above 1285°C; 1.0 GPa, not intersected. The liquidus piercing point between calcite and silicates becomes enriched in CaCO_3 with decreasing pressure, from Ab_(80)CC_(20) at 2.5 GPa to Ab_(47)CC_(53) at 1.0 GPa. No immiscible liquid contains more than ∼80% dissolved CaCO_3, and all contain at least 5% Na_2CO_3. A round CaCO_3 phase exhibiting morphology similar to that displayed by immiscible liquid globules is determined to be crystalline calcite under experimental conditions. The topology of the phase fields and field boundaries illustrates the kinds of processes and controls existing in magmatic systems. Calciocarbonatite magmas cannot be produced by equilibrium immiscibility process in the mantle. Carbonated silicate magmas in the crust yield residual calciocarbonatite magmas by fractionation along the silicate-calcite field boundary, reached either directly from the silicate liquidus or more commonly via the miscibility gap. Immiscible carbonaterich magmas when freed from the silicate parent cool down a sleep silicate liquidus until they reach a silicate-carbonate field boundary. There is no experimental evidence for immiscible calciocarbonatite magmas with > 80% CaCO_3, and calcite lapilli cannot be formed from 99% CaCO_3 magmas. Sovites are surely cumulates.

Ca-rich carbonate melts; a regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus

Other| February 01, 1995 Ca-rich carbonate melts: A regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus Allan H. Treiman Allan H. Treiman Lunar and Planetary Institute, Houston, TX, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (1-2): 115–130. https://doi.org/10.2138/am-1995-1-212 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Allan H. Treiman; Ca-rich carbonate melts: A regular-solution model, with application to carbonatite magma + vapor equilibria and carbonate lavas on Venus. American Mineralogist 1995;; 80 (1-2): 115–130. doi: https://doi.org/10.2138/am-1995-1-212 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract A thermochemical model of the activities of species in carbonate-rich melts would be useful in quantifying chemical equilibria between carbonatite magmas and vapors and in extrapolating liquidus equilibria to unexplored PTX. A regular-solution model of Ca-rich carbonate melts is developed here, using the fact that they are ionic liquids, and can be treated (to a first approximation) as interpenetrating regular solutions of cations and of anions. Thermochemical data on systems of alkali metal cations with carbonate and other anions are drawn from the literature; data on systems with alkaline earth (and other) cations and carbonate (and other) anions are derived here from liquidus phase equilibria. The model is validated in that all available data (at 1 kbar) are consistent with single values for the melting temperature and heat of fusion for calcite, and all liquidi are consistent with the liquids acting as regular solutions.At 1 kbar, the metastable congruent melting temperature of calcite (CaCO3) is inferred to be 1596 K, with ΔHfus(calcite) = 31.5 ± 1 kj/mol. Regular solution interaction parameters (W) for Ca2+ and alkali metal cations are in the range −3 to −12 kj/mol2; W for Ca2+-Ba2+ is approximately −11 kj/mol2; W for Ca2+-Mg2+ is approximately −40 kj/ mol2, and W for Ca2+-La3+ is approximately +85 kj/mol2. Solutions of carbonate and most anions (including OH−, F−, and SO42−⁠) are nearly ideal, with W between 0 (ideal) and −2.5 kj/mol2. The interaction of carbonate and phosphate ions is strongly nonideal, which is consistent with the suggestion of carbonate-phosphate liquid immiscibility. Interaction of carbonate and sulfide ions is also nonideal and suggestive of carbonate-sulfide liquid immiscibility. Solution of H2O, for all but the most H2O-rich compositions, can be modeled as a disproportionation to hydronium (H3O+) and hydroxyl (OH ) ions with W for Ca2+-H3O+ ≈ 33 kj/mol2.The regular-solution model of carbonate melts can be applied to problems of carbonatite magma + vapor equilibria and of extrapolating liquidus equilibria to unstudied systems. Calculations on one carbonatite (the Husereau dike, Oka complex, Quebec, Canada) show that the anion solution of its magma contained an OH− mole fraction of ~0.07, although the vapor in equilibrium with the magma had P(H2O) = 8.5 × P(CO2). F in carbonatite systems is calculated to be strongly partitioned into the magma (as F−) relative to coexisting vapor. In the Husereau carbonatite magma, the anion solution contained an F− mole fraction of ~6 × 10−5.Calcite and anhydrite may be present on the surface of Venus, but they would not be molten at ambient surface temperature (660–760 K) because the minimum melt temperature (eutectic) for the calcite + anhydrite system is calculated to be 1250 K. The Venus atmosphere contains 5 ppb HF, which implies that the anion solution of a carbonate-rich magma in equilibrium with the atmosphere would contain a F− mole fraction of ~7 × 10−3, or about 0.1 wt%. Although this proportion of F is much enriched compared with the atmosphere, it would have little effect on phase relations of the carbonatite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.