A carbonate source for carbonatites: Whether mantle is progressively enriched in δ13C over geological time scale?

Carbonatites are of economic importance as they contain high concentrations of Rare Earth Elements (REE), niobium, tantalum, zirconium and platinum group elements (PGE). Moreover, carbonatites contain >50% carbonates and its exposure to the Earth's surface through volcanic activity and further weathering has the potential to impact climate change. Hence, understanding their origin is essential.

Carmody (2012) has found that the isotopic analysis of fumarole gases at the world's only active carbonatite volcano, Oldoinyo Lengai, Tanzania, yielded δ13 C values of -7.37 to -4.46‰ and concluded that the carbonatites are of mantle origin. Shavers et al. (2016) also expressed the view that the carbon isotopic composition of carbonatites indicates their mantle origin. However, Hulett et al. (2016) found that the temporal evolution of boron isotopic composition of carbonatites, found worldwide with an emplacement time between ~40 and ~2,600 Ma, indicated evidence for ingestion of subducted crustal components by carbonatite melts. Thus, the sedimentary carbonates may become a source of the carbon (or, carbonate) for the carbonatites - though exact mechanisms are not discernible.

The primary barrier in invoking sedimentary carbonates as a source of carbon for carbonatites through a subduction process arises from the low δ13C values of primary mantle-derived carbonatites (between -4‰ and -8‰; Xu et al., 2018) compared to crustal sedimentary carbonates (~0 ± 1.5‰; Veizer et al., 1992). For a discussion on δ13C values of various carbon reservoirs, please refer, Cartigny et al., 1998). A possible mechanism should be proposed to explain the observed discrepancy in the δ13C signatures of carbonatites and the sedimentary carbonates. Such a proposition mandates that 1) the subduction results in the dissolution of sedimentary carbonates; 2) the dissolved carbonates become the source of carbon (or, carbonate) for the carbonatites by specific reactions, 3) during such a process, carbon isotopic fractionation occurs that explain the observed low δ13C of the carbonatites compared to sedimentary carbonates.

Now consider a subduction zone, defined as the convergent boundary of tectonic plates where one plate moves under another so that the sinking plate finally reaches the mantle. Usually, the denser oceanic plate moves under, the lighter continental plate. During such subduction, the sediments deposited on the oceanic plate, especially authigenic and biogenic carbonates, are transferred to the mantle. The subducted carbonates, along with the sediments, are either decarbonated or dissolved. The decarbonation is a process by which the carbonates react with the silicates to release CO2 to the atmosphere through volcanic eruptions. Instead, if carbonates in the subduction slab dissolve, they are transferred to the mantle. The evolution of CO2, after carbonate dissolution and transfer to the mantle, can only occur if the subduction fluid is subjected to appropriate P-T conditions at a later stage.

In scientific literature two schools of thoughts prevail - one favouring decarbonation of the subducted carbonates and the other dissolution. The dissolution of Mg and Ca carbonates in the subduction zone has been widely studied using carbonatite and related samples from the field. Hoernle et al. (2001) found that calcio-carbonatites had mantle-like stable isotopic compositions. Moreover, they have suggested that these calcio-carbonatites result from the melting of recycled carbonated oceanic crust with a recycling age of 1.6 Ga. Such a long recycling age can be inferred from the subduction-related long-term evolution of boron isotopic composition of the carbonatites over Earth's history (Hulett et al., 2016). 

There is substantial evidence suggesting that the subducted carbonates are getting dissolved at shallower depths, and that serve as a source of carbon to the mantle (Frezzotti et al., 2011). Such a dissolution of carbonates under the conditions prevailing in the upper mantle is also inferred for MgCO3 from ab initio molecular dynamics by Pan et al. (2013). The dissolved carbonates in the subduction zone either return to the atmosphere as volcanic emanations of CO2 or, accumulated and transferred to the deep mantle. A recent study proposed that almost all such subducted carbon return to the Earth's crust and atmosphere (Kelemen and Manning, 2015). 

Apart from carbonate dissolution, evidence for subducting carbonates as a source of volcanic gaseous CO2 has emerged. From the study of Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece, Ague and Nicolescu (2016) argued that fluid-mediated carbonate mineral dissolution results in silicate precipitation and the release of CO2. If carbonate dissolution occurs along the subducting slab, then metasomatic reactions are expected to occur along the subducting slab resulting in elemental mobilisation. The evidence was found by Li et al. (2018) by an investigation of the mélange rocks of the ultramafic blocks in the Franciscan Complex of California for Mg-isotopic signature that recorded fluid metasomatism along the slab-mantle interface in the subduction zone. They have concluded that multi-stage fluid-rock interaction is driving Mg-isotopic variation during fluid metasomatism, and the Mg in the subducted carbonate becomes mobile within the subduction zone. The overall picture seems to emerge from the work of Boudoire et al. (2018) using CO2-He-Ar systematics that revealed extensive degassing in the upper mantle with multiple steps of magmatic differentiation beneath the Piton de la Fournaise oceanic basaltic volcano of the La Réunion Island. During such a magmatic differentiation, there is an isotopic fractionation that results in lower (<-6 per mil) δ13C values of volcanic CO2 compared to primary phase. Whether such δ13C depleted CO2 are the emanations from crystallising carbonatite magma or, derived by the decarbonation reactions is an unresolved question. Hopefully, high temperatures and oxidising conditions favour decarbonation. The dissolution of carbonates may proceed under oxygen-deficient conditions.

If sedimentary carbonates are the source of the carbon for the carbonatites, then the next fundamental question is when it all started? The study of the eclogite xenolith in Paleoproterozoic carbonatite in North China (Xu et al., 2018) argues for cold subduction as early as 1.8 Ga. The Paleoproterozoic carbonatite of North China has a carbonate carbon signature and Sr-Nd composition indicative of the ocean crustal source. From this study, we understand that the deep carbon cycle has been in operation over an extended geological period influencing the mantle oxidation state and its compositional heterogeneity. Combining the results of Xu et al. (2018) with the evolution of boron isotopic composition (Hulett et al., 2016), we can conclude that the subduction process and its influence on the composition of the mantle can be as old as 2.6 Ga.

From the preceding discussion, it is clear that the sedimentary carbon may constitute a part of carbonatite carbon. The following processes may be thought to prevail:

1) Subduction of the slab and dissolution of sedimentary carbonates possibly aided by lowering of the dielectric constant of the water (Cline et al., 2018).

2) Transfer of the dissolved fluid along with the subducting slab deep into the mantle (Frezzotti et al., 2011).

3) The reaction of carbonate fluid with iron and silicate that results in the replacement of oxygen by carbon in the silicate structure (Sen et al., 2013) and the formation of Fe-Si-carbides (Horitaa and Veniamin, 2015).

4) Upwelling of these materials towards the upper mantle by convection, the oxidation of carbon-rich phases to carbonatite (Horitaa and Veniamin, 2015) due to the high oxygen fugacity of the upper mantle (Cline et al., 2018) that generates CO2 degassing through the volcanoes (Boudoire et al., 2018) and results in 13C isotopic fractionation (Horitaa and Veniamin, 2015).

5) The carbon isotopic fraction during the formation of carbonatite melt results in the low δ13C values typical of the mantle within the carbonatite melt. As the carbonatite magma ascents, it loses fluids and volatiles to crystallise carbonatites with low δ13C values.

If these assumptions are correct, then the δ13C of the mantle should move towards more positive values (i.e., 13C/12C of the mantle should increase) over geological periods. Because the mantle is magmatically differentiated to form the carbonatite melt enriched in 12C due to isotopic fractionation, the mantle becomes comparatively rich in 13C. The Paleoproterozoic carbonatites in North China Craton (Xu et al., 2018) have higher positive δ13C values (-5.7‰ to -1.6‰) compared to the typical mantle-derived primary carbonatites (-8‰ to -4‰). If the Paleoproterozoic mantle was rich in 12C than it is today, then the carbonatites of that time may have higher δ13C values. In that case, assimilation of subducted sediments may have contributed little to the observed deviation in the carbon isotopic composition of these carbonatites. After emplacement, the carbonatite weathering and its return to the mantle as sedimentary carbonates (i.e., carbonatite recycling) shall further enhance the isotopic fractionation over geological periods. There might also be a Ca isotope fractionation, and the samples collected by Hulett et al. (2016) can provide essential clues for testing the δ13C and Ca isotopes evolution of carbonatites over geological periods.

From the temporal distribution of carbonatites, there is an exponential increase in the frequency distribution of carbonatites emplacement at the least since 1.8 Ga, which was alternatively interpreted as due to the probability of carbonatite preservation against weathering (Veizer et al., 1992). If the frequency of carbonatite emplacement is on the rise, then we shall observe a systematic fall in the δ13C of carbonatites with geological time. Thus, age-old carbonatites are expected to be comparatively rich in δ13C than the recent carbonatites. Such assumptions are applicable only if the 13C/12C variations are affected by the carbonatite formation. However, in nature, other biogeochemical processes may have an impact.