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Iron Isotope Compositions of Carbonatites Record Melt Generation, Crystallization, and Late-Stage Volatile-Transport Processes

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Iron Isotope Compositions of Carbonatites Record Melt Generation, Crystallization, and Late-Stage Volatile-Transport Processes Miner Petrol (2010) 98:91–110 DOI 10.1007/s00710-009-0055-4 ORIGINAL PAPER Iron isotope compositions of carbonatites record melt generation, crystallization, and late-stage volatile-transport processes Clark M. Johnson & Keith Bell & Brian L. Beard & Aaron I. Shultis Received: 6 January 2009 /Accepted: 1 May 2009 /Published online: 30 May 2009 # Springer-Verlag 2009 Abstract Carbonatites define the largest range in Fe iso- fluid-rock or fluid-magma interactions comes from the tope compositions yet measured for igneous rocks, record- common occurrence of Fe isotope disequilibrium among ing significant isotopic fractionations between carbonate, carbonate, oxide, silicate, and sulfide minerals in the oxide, and silicate minerals during generation in the mantle majority of the carbonatites studied. The common occurrence and subsequent differentiation. In contrast to the relatively of Fe isotope disequilibrium among minerals in carbonatites restricted range in δ56Fe values for mantle-derived basaltic may also indicate mixing of phenocyrsts from distinct magmas (δ56Fe=0.0±0.1‰), calcite from carbonatites have magmas. Expulsion of Fe3+-rich brines into metasomatic δ56Fe values between −1.0 and +0.8‰, similar to the range aureols that surround carbonatite complexes are expected to defined by whole-rock samples of carbonatites. Based on produce high-δ56Fe fenites, but this has yet to be tested. expected carbonate-silicate fractionation factors at igneous or mantle temperatures, carbonatite magmas that have modestly negative δ56Fe values of ~ −0.3‰ or lower can Introduction be explained by equilibrium with a silicate mantle. More negative δ56Fe values were probably produced by differen- Stable and radiogenic isotope studies of carbonatites have tiation processes, including crystal fractionation and liquid been used to monitor the secular evolution of the sub- immiscibility. Positive δ56Fe values for carbonatites are, continental mantle (e.g. Bell and Rukhlov 2004), the however, unexpected, and such values seem to likely reflect evolution of carbonated melts as they migrate from mantle interaction between low-Fe carbonates and Fe3+-rich fluids to crustal levels (e.g. Harmer 1999), and sub-solidus at igneous or near-igneous temperatures; the expected δ56Fe cooling and fluid/rock interaction (e.g. Deines 1989). values for Fe2+-bearing fluids are too low to produced the Carbonatites range in age from Archean to present, are observed positive δ56Fe values of some carbonatites, found on all continents (Woolley and Kjarsgaard 2008), and indicating that Fe isotopes may be a valuable tracer of redox have distinct chemical compositions relative to silicate conditions in carbonatite complexes. Further evidence for igneous rocks (e.g. Simonetti et al. 1997; Chakmouradian 2006). Although volumetrically small compared to other Editorial handling: A. Simonetti igneous rocks, carbonatites provide unique probes into the C. M. Johnson (*) : B. L. Beard : A. I. Shultis mantle, and, because their ages extend back into the Department of Geology and Geophysics, Archean, they can be used to monitor the chemical and Lewis G. Weeks Hall for Geological Sciences, isotopic evolution of the mantle over much of Earth’s 1215 W. Dayton Street, history. Radiogenic isotope (Sr, Nd, Pb) compositions of Madison, WI 53706-1692, USA e-mail: [email protected] carbonatites have shown, unequivocally, that carbonatite URL: http://www.geology.wisc.edu magmas are of mantle origin, that many have compositions that are similar to those found in OIBs, and that mixing K. Bell between isotopically distinct, carbonatitic melts is common. Isotope Geochemistry and Geochronology Research Centre, 2117 Herzberg Laboratories, Carleton University, Stable and radiogenic isotope disequilibrium among min- Ottawa, ON K1S 5B6, Canada erals, even within the same sample, demonstrates the 92 C.M. Johnson et al. commonly cumulate nature of carbonatites, and is well expulsion of alkali-rich fluids from the carbonate and/or explained by mixing within magma chambers, as well as silicate magmas into the surrounding country rocks. the effects of intrusion cooling and alteration (e.g. Simonetti In this study, we present the first Fe isotope study of and Bell 1994a; Bizzarro et al. 2003; Haynes et al. 2003). carbonatites, including whole-rocks and mineral phases. We Most young carbonatites (<200 Ma) have isotopic compo- show here that the largest range in Fe isotope compositions sitions that are typical of sub-oceanic mantle, pointing to yet measured in igneous rocks is found in carbonatites. The sub-lithospheric sources, similar to the HIMU, EM I, and relatively large Fe isotope fractionations among carbonates, FOZO components defined by oceanic basalts (Bell and silicates, and oxides at igneous temperatures, coupled with the Tilton 2001; Bell and Simonetti 2009). A substantial body large contrasts in Fe contents among these mineral groups, of isotopic data now exists for carbonatites from East makes Fe isotopes a particularly sensitive tracer of processes Africa, which suggests mixing between the HIMU and EM that are commonly invoked in models for carbonatite genesis I mantle components (e.g. Bell and Dawson 1995; Bell and and evolution, including magmatic and fluid evolution, crystal Simonetti 1996). Noble gas compositions indicate mantle fractionation, and liquid immiscibility, (for review, see Lee sources (e.g. Marty et al. 1998; Tolstikhin et al. 2002), and and Wyllie (1994)). The results of this Fe isotope survey of some carbonatites have Li, C, and O isotope compositions carbonatites suggest that Fe isotope fractionations among that are similar to those of oceanic basalts (e.g. Deines silicates, carbonates, oxides, and sulfides rarely record 1989; Keller and Hoefs 1995; Halama et al. 2008). Among isotopic equilibrium in carbonatites. Rather, the Fe isotope the models proposed for the sources of carbonated melts, compositions measured in these minerals record complex isotopic data generally support one involving mantle differentiation pathways, mixing of phenocrysts from distinct upwelling such as plumes/hot spot activity, accompanied in magmas, and late-stage fluid interactions. some cases perhaps by interaction with the continental lithosphere (see discussion in Bell and Simonetti 2009,and references within). Fenitization and the role of fluids in carbonatite Carbonatite complexes commonly contain a wide variety evolution of rock types, and their close spatial association with deep- crustal fracturing and rifting implies an intimate relation The enormous capacity of carbonated mantle-derived between intrusion and tectonism; this is particularly well magma to dissolve CO2 and H2O, along with other volatiles shown by alkalic-carbonatitic complexes in the East such as Cl, F, and S, requires fluid phases to develop and African Rift Valley System (e.g. Bailey 1993) and the evolve during carbonatite magma differentiation. Fluid Trans-Superior Tectonic and Kapuskasing Structural Zones compositions can be estimated from fenites, fluid inclusion of the Superior Province, Canada (e.g. Sage 1991). Most studies of carbonatites themselves, and mineral chemistry, carbonatite complexes take the form of circular- or tear- especially REEs abundances and their distribution patterns. shaped plutons, many associated with silicate rocks of Because CO2 and H2O were the principal volatiles used in miaskitic affinity. Where related alkalic silicate rocks occur, melting experiments, the concept of a “carbothermal fluid” these form large stocks or ring complexes, and the of varied CO2 and H2O ratio was introduced in the literature, carbonatites generally occur as plug-like intrusive bodies but it was suggested that other components, especially the that have diameters <5 km. Carbonatite complexes may halogens, may be just as important as H2O (Gittins 1989). consist solely of carbonatite, usually dolomitic in compo- Fenites, a term coined by Brögger (1921), generally sition, whereas others are associated with silicate rocks, consist of alkali feldspar, sodic pyroxene, and/or alkali normally undersaturated with respect to silica. Silicate amphibole that formed at sub-igneous temperature. Fenites rocks commonly associated with carbonatites include may be zoned relative to a carbonatite intrusion, with an syenite, nepheline syenite, gabbro, melilitolites, and their innermost part composed of amphibole and pyroxene, and volcanic equivalents, as well as pyroxenites (e.g. King and an outer part rich in biotite (e.g. Le Bas 2008). Metasoma- Sutherland 1960; Le Bas 1977). Calciocarbonatites and/or tised rocks can be broadly divided into sodic or potassic magnesiocarbonatite make up the bulk of the carbonatites varieties, although other, more complicated classification within a given complex, but late-stage carbonatites do schemes have been developed (Morogan 1994). Both occur, where these generally comprise only a few percent of sodium- and potassium-rich fenites can occur around a the total carbonatite volume. The most common, late-stage single intrusion, and it has been suggested that these carbonatites are composed of ankerite or form ankeritic distinctions may be related to depth, where potassium dolomite-bearing carbonatites, enriched in REEs, fluorite, fenitization occurs at the upper levels of a carbonatite and incompatible trace elements such as U and Th (Le Bas complex, and sodic fenitization occurs at greater depths (Le 1989). Carbonatite complexes are commonly surrounded by Bas 1989). Fenitization is envisioned
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