The isotopic composition of carbonatites – what they tell us about mantle evolution
Dept. of Earth Sciences, Carleton University, Ottawa, Ontario K1S 5B6, Canada
The widespread distribution of carbonatites on all continents, coupled with their high abundances of Sr and the REEs, and their range in age from the Archean to the present, make carbonatites ideal for monitoring the chemical evolution of the sub-continental mantle. Especially significant among the results that have emerged from the numerous Sr, Nd, Pb and Hf isotopic studies of carbonatites is the similarity in isotopic compositions of carbonatites to some of the mantle components that characterize oceanic island basalts (OIBs). The isotopic signatures of FOZO, HIMU and EMI have been recognized in most young (<200 Ma) carbonatites (e.g. Bell and Tilton, 2004), leading to the conclusions that: i. parts of the sub-oceanic and sub-continental mantles have similar isotopic compositions, and ii. by implication, the sources for the carbonated parental melts are sub-lithospheric. Thus, isotopic similarities between OIBs and carbonatites suggest an involvement of mutual mantle sources on a global scale. With the possibility of having molten carbonatitic melts present within the mantle (e.g. Gaillard et al., 2008), such isotopic signatures introduce some interesting implications for mantle metasomatism and partial melting.
Another important finding is that some carbonatites (e.g. those from East Africa and the Kola region, Russia) require the involvement of more than one distinct mantle component during their formation. In the case of the former, only two components are needed, and in the latter at least three (Bell and Rukhlov, 2004). Mantle end-members for East Africa are represented by HIMU and EMI that form a linear array in Nd-Sr space (the East African Carbonatite Line; EACL - Bell and Blenkinsop, 1987); data from Oldoinyo Lengai, Earth’s only active carbonatite volcano, cluster midway along this line. Still unresolved is the question of whether these isotopic variations reflect: i. reaction between asthenosphere and continental lithosphere, ii. melting of layer-cake mantle at sub-lithospheric depths or iii. an isotopically inhomogeneous, plume head.
Because carbonatites range in age from 3 Ga old (Tupertalik, Greenland; Bizzarro et al., 2002) to the present (Oldoinyo Lengai, Tanzania), it becomes possible to take ‘snapshots’ of the isotopic composition of the sub-continental mantle over this considerable interval of time. This feature is helped by the fact that carbonatites contain high abundances of Sr and REEs that help buffer their inherited mantle isotopic compositions against possible contamination during melt migration and emplacement. The Sr isotope values for carbonatites from the eastern part of the Canadian Shield, which range in age between ~2.7 Ga and ~0.1 Ga, have convincingly established the presence of a depleted (and old) mantle, i.e. one that evolved with a time-integrated Rb/Sr ratio less than that of bulk Earth. However, additional Sr isotope data for carbonatites, including some from the Baltic Shield, indicate the presence of two mantle sources - one depleted and one enriched (Bell and Rukhlov, 2004). Both of these sources were generated about 3.0 Ga, and have been intermittently sampled since that time. The same, however, is not true for the Lu-Hf and Sm -Nd systems. Both the Nd and Hf isotope values for carbonatites from Canada and South Africa appear to be coupled, and both reflect a depleted mantle that was established well before 3 Ga (Rukhlov and Bell, 2003). Further evidence for mantle depletion is shown by Hf isotopes from Khibina, Kola, (Kogarko et al., 2010), and carbonatites from the Labrador coast (Tappe et al., 2007). In spite of mantle depletion early in Earth history as proposed by some workers, Hf isotope values obtained for minerals such as baddeleyite and zircon in carbonatites from southwestern Greenland (Bizzarro et al., 2002) and South Africa (Phalaborwa; Scherer et al., 2001) also indicate an enriched reservoir preserved in the deep mantle for at least 3 Ga old (Bizzarro et al., 2002). This ancient reservoir is considered to be the result of subduction of enriched lithosphere into the deeper parts of the mantle.
Assessment of recycling of upper continental crust based on Li isotopes from carbonatites did not detect any significant isotopic variations in mantle Li since 2.8 Ga (Halama et al., 2008). Compared to radiogenic isotopes, the limited δ7Li variations were attributed to the more effective homogenization of Li than for its radiogenic equivalents. The small variations noted were attributed to isotopic fractionation associated with shallow-level processes such as assimilation and/or isotopic fractionation.
Significant findings of mantle evolution brought out by the isotopic data from carbonatites include: i. a marked depletion event > 3.0 Ga indicated by the Sm-Nd and Lu-Hf systems, ii. an ancient enriched reservoir, perhaps the result of subduction of fertile lithospheric mantle at some time prior to 3.0 Ga, iii. mantle sources that have remained relatively well-behaved relative to Rb/Sr, marking a differentiation event at ca. 3.0 Ga (assuming that the bulk Earth is meaningful), and iv. a similarity in isotopic compositions of young carbonatites to OIBs. These conclusions are consistent with strong element fractionation during the Earth’s early history, a less severe event affecting only the Rb-Sr system, and the retention of distinct and different isotopic reservoirs in spite of mantle convection. Sulphur isotopic compositions from Proterozoic carbonatites from the Superior Province, Canada, are consistent with the preservation of small mantle domains of only several km scale (Farrell et al., 2010).
Using carbonatites it may be possible to separate out first- and second-order chemical effects in mantle processes, i.e. global vs local, and to map out the size of isotopic domains using different isotope systems. The limitations imposed by using basalts (relatively low Sr and REE values, susceptibility to weathering and alteration) in monitoring the secular evolution of the mantle are more than overcome by utilizing carbonatites. The insights provided by carbonatites, in spite of their rarity and their small volumes, make these rocks ideal for monitoring isotopic changes of the mantle over a considerable part of Earth history.
Bell K, Blenkinsop J (1987) Nd and Sr isotope systematics of East African carbonatites: implications for mantle heterogeneity. Geology 15: 99-102
Bell K, Tilton GR (2002) Probing the mantle: the story from carbonatites. EOS, American Geophys Union 83:273, 276-277.
Bell K, Rukhlov AS (2004) Carbonatites from the Kola Alkaline Province: Origin, evolution and source characteristics. In: Zaitsev A, Wall F, (eds) Phoscorites and Carbonatites from Mantle to Mine: the key example of the Kola Alkaline Province, Miner Soc Series 10, London, pp 421-455.
Bizzarro M, Simonetti A, Stevenson RK, David J (2002) Hf isotope evidence for a hidden mantle reservoir. Geology 30: 771-774.
Farrell S, Bell K, Clark I (2010) Sulphur isotopes in carbonatites and associated silicate rocks from the Superior Province, Canada. Miner Petrol 98: 209-226.
Gaillard F, Malki M, Iacono-Marziano G, Pichavant M, Scaillet B (2008) Carbonatite melts and electrical conductivity in the asthenosphere. Science 322:1363-1365.
Halama R, McDonough WF, Rudnick RL, Bell K (2008) Tracking the lithium isotopic evolution of the mantle using carbonatites. Earth Planet Sci Lett: 265:726-742.
KogarkoLN, Lahaye Y, Brey GP (2010) Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics. Miner Petrol 98: 197-208.
Rukhlov A and Bell K (2003) Depleted mantle: the story from Hf isotopes in zircon and beddelyetyite from carbonatites. Geophys Res Abstracts 5, 13944-13945.
Scherer E, Münker C, Mezger K (2001) Calibration of the lutetium-hafnium clock. Science 293: 683-687
Tappe S, Foley SF, Stracke A, Romer, RL, Kjarsgaard, BA, Heaman LM, Joyce N (2007) Craton reactivation on the Labrador Sea margins; 40Ar/ 39Ar age and Sr-Nd-Hf-Pb isotope constraints from alkaline and carbonatite intrusives. Earth Planet Sci Lett 256, 433-454.