Geochemical Characteristics of Oceanic Carbonatites: Comparison
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The occurrence of carbonatites in oceanic settings is very rare if compared with their continental counterpart, having been reported only in Cape Verde and Canary Islands. This entry provides an overview of the main geochemical characteristics of oceanic carbonatites, around which many debates still exist regarding their petrogenesis.

  • trace elements
  • noble gases isotopes

1. Introduction

Carbonatites are rare carbonate-rich rocks of igneous origin, with more than 50% modal carbonates [1]. Alkaline–carbonatite complexes are considered one of the main sources of critical metals for key economic sectors in heavily industrialized countries, and their occurrence is almost exclusive in continental regions [2], while in oceanic settings, carbonatites (we refer to carbonatite complexes, occurring beside carbonatite micro-enclaves or veins in mantle xenoliths) are very rare and they are limited to only two occurrences on the western side of the Atlantic Ocean just in front of the African coasts: at Cape Verde Archipelago and Fuerteventura Island (Canary Archipelago).

Carbonatite petrogenesis is still a debated topic, and three petrogenetic processes have been variably proposed, all being related to the formation of a primary carbonate melt derived from a carbonated mantle [3]: (1) low-degree partial melting of a carbonated mantle source [4][5][6]; (2) immiscible separation from a CO2-rich alkaline silicate melt [7][8], and (3) the late-stage result of a fractional crystallization of a carbonated alkaline silicate melt [6][9][10]. Another important petrogenetic process is the (4) assimilation of carbonates in relatively shallow magma chambers (e.g., [11]).

The nature of the mantle that sourced the oceanic carbonatites (shallow convective mantle vs. a deep-seated mantle reservoir, plume-type) is still a debated issue. A shallow origin for oceanic carbonatites was suggested for some oceanic carbonatites [12], with the plume contribution limited to a thermal input, a trigger for low-degree partial melting at the base of the oceanic lithosphere. Another trigger possibility is the “metasomatic underplating” hypothesis, where crustal fractures that developed during hot-spot magma ascent allow seawater to infiltrate and to serpentinize the sub-Moho mantle, with Ca-rich hydrothermal fluids [13]. A thickened and metasomatized oceanic lithosphere is needed [14], and a relatively hot environment could favour both the survival of carbonate melt at shallow mantle depths and the emplacement of carbonatites at or near the surface [15], at the same time becoming more calcic with respect to primary dolomitic melts [15][16]. In addition, the role of the carbonated eclogite for deep global cycling of carbon and the potential source for carbonatites is also recognized [17][18][19]. Other models proposed for the petrogenesis of oceanic carbonatites involve (i) the contribution of marine carbonates recycled via subduction, (ii) carbonates assimilated in shallow magma chambers [11][12][14][20], or (iii) the involvement of primordial deep-mantle carbon [21][22][23].

An increasing number of multi-isotopic studies (C-O-Sr-Nd-Pb) tackled the petrogenesis of oceanic carbonatites and the characterization of their mantle source [12][14][20][21][22][24][25], and more recently, additional constraints were placed by noble gas isotopes [23]. Indeed, their properties (such as a large mass range, high volatility, and chemical inertia) make noble gas geochemical tracers of primary importance [26][27][28] to investigate mantle heterogeneity and degassing and mantle-crust interaction processes occurring in the volcanic plumbing systems (e.g., crustal assimilation). Despite their important role and potential significance for deciphering carbonatite genesis, there are no previous studies of noble gas isotopes in Fuerteventura carbonatites. On the contrary, noble gas isotopes in Cape Verde carbonatites are widely studied [23], also because carbonatite outcrops are present in at least six of the ten Cape Verde Islands (S. Vicente, S. Nicolau, Maio, Santiago, Fogo, and Brava), while on Canary Islands, carbonatites occur only on Fuerteventura (the easternmost island of the Canary Archipelago). Mata et al. (2010) [23] recognized the presence of fluids from a deep and low degassed mantle (He isotopic ratios higher than the typical mid-ocean ridge basalt MORB mantle values of 8 ± 1 Ra) in Cape Verde carbonatites, supporting a deep-rooted mantle plume in the region as inferred by geophysical investigations [29][30]. However, this is not necessarily so as the helium mantle signature can easily survive during the contribution of hydrothermal fluids that can introduce surface-related noble gases, nitrogen, and CO2. This evidence supports the presence of non-recycled carbon in the genesis of the Cape Verde carbonatites.

2. Geological and Geochemical Background

2.1. Regional Geology

2.1.1. Fuerteventura (Canary Islands)

The volcanism of the Canary Archipelago is located 100 km off the northwestern African continental margin (Figure 1a). The age of the oceanic crust beneath the Canary Islands is constrained by the 175 Ma S1 magnetic anomaly separating the easternmost Canary Islands and northwestern Africa and the 156 Ma M25 magnetic anomaly separating the westernmost islands of La Palma and Hierro [31][32][33][34]. The Moho depth decreases from west to east (i.e., towards Africa), varying in depth from approximately 12 km beneath El Hierro and La Palma to 20–30 km beneath Lanzarote and Fuerteventura [35]. Therefore, Fuerteventura is located on a transitional oceanic to continental crust.

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Figure 1. (a) Canary Islands and Cape Verde Archipelago in their geographic contest. (b) Simplified geology map of Fuerteventura showing the location of the carbonatite-studied samples (red arrow) (modified from Sagan et al. 2020 [36]) and (c) detail of the Cape Verde Archipelago, where the islands on which carbonatites are present are shown in white (modified from Mata et al. 2010 [23]).

Among all the Canary Islands, Fuerteventura is one of the few islands (together with La Palma) where the various phases of submarine formation, transitional phases, and subaerial growth are better recognized [37][38][39][40][41][42][43][44][45][46]. This volcanic island consists essentially of Mesozoic sediments, Oligocene submarine volcanic complexes, Late Oligocene transitional volcanic complexes, Miocene subaerial basaltic and trachytic lava flows, ultramafic, mafic to felsic intrusive rocks, and carbonatitic dyke swarms [41][46][47]. These intrusive and carbonatitic dyke swarms, with the submarine, transitional, and earliest subaerial complexes and their associated plutonic bodies, form a lithostratigraphic unit known as the Basal Complex.

Carbonatites and closely associated alkaline-rich-silica undersaturated rocks (clinopyroxenites, melteijites–ijolites, nepheline–sienites, and sienites) form a typical alkaline–carbonatitic association, outcropping in the central western part of the island, in an almost continuous NE–SW direction parallel to the coast. This association represents the first Magmatic Episode during upper Oligocene (≈25 Ma) [44], subsequent to the Submarine and early Transitional Volcanic Episodes. The main exposures of the alkaline–carbonatitic association are the northwestern Montaña Blanca–Esquinzo sector and the central-western Ajuy–Solapa sector (Figure 1b). The Early Miocene intrusion of a gabbroid–pyroxenitic series (Pájara Pluton), after the emplacement of the alkaline–carbonatitic complex, produced bands of thermal metamorphism on host rocks with up to 1 km of lateral extension [48]; thus, the original igneous mineral assemblage within the contact zone was variably overprinted and substituted with a metamorphic association composed of wollastonite, monticellite, diopside, vesuvianite, garnet, calcite, perovskite, alabandite, pyrrhotite, and Nb–Zr–Ca silicates (e.g., cuspidine–niocalite–baghdadite series) [49]. Nevertheless, carbonatite outcrops in Punta de La Nao (Ajuy–Solapa sector) escaped this thermal overprint and preserved their original igneous assemblage and textures [50]. Accordingly, in order to focus the study on well-preserved carbonatite rocks, all samples studied here were collected from Punta de La Nao area (Figure 1b).

After the Miocene subaerial volcanic activity, a period of quiescence occurred until the Pliocene, where renewed volcanic activity produced some basaltic volcanoes on the northern part of the island [40]. During the Plio–Quaternary, alluvial and aeolian complexes were also generated [51].

2.1.2. Cape Verde Archipelago

The Cape Verde Archipelago is located 500 km off the Senegalese coast (Figure 1a). The lithospheric crust beneath the islands is constrained by 140 Ma M16 and 120 Ma M0 magnetic anomalies [52]. In the same way as the Canary Archipelago, the crustal thickness beneath the Cape Verde Islands is also anomalously high (up to 22 km), and this is mainly due to magmatic transfer from the upper mantle to shallow (i.e., crustal) accumulation levels [53], although between the islands of the archipelago the crustal thickness decreases considerably to ca. 7 km [54]. Plume activity beneath the Cape Verde Archipelago is supported by tomographic studies, which show the evidence of a P-wave negative anomaly down to about 1000 km, which includes the Azores and Canary plumes apparently reaching the core–mantle boundary [29].

The archipelago is composed of ten major islands, which can be divided into two groups: (i) northern Islands (S. Antão, S. Vicente, S. Luzia, S. Nicolau, Sal and Boavista) and (ii) southern Islands (Brava, Fogo, Santiago, and Maio).

The lack of a simple age progression of volcanism for the Cape Verde Archipelago indicates that the archipelago is located close to the rotation pole of the slowly drifting African Plate [55]. The oldest exposed magmatic rocks related to Cape Verde hot-spot volcanism occur on Sal Island, which preserves a magmatic history ranging from 25.6 Ma to 0.6 Ma [56], although a 40Ar-39Ar study limits the volcanic evolution for Sal Island from around 15 Ma to 1.1 Ma [55]. Indeed, several age determination studies on the Cape Verde Islands suggest that most of the volcanic activity took place from 16 Ma until the present [55][57][58][59][60], such as the volcanic activity on S. Vicente (6.6 Ma to 0.3 Ma), on S. Antão (7.5 Ma to 0.1 Ma), on Maio (12 Ma–7 Ma) or on Fogo (Quaternary to the present). Currently, at least three islands of the Cape Verde Archipelago are considered volcanically active (S. Antão, Brava, and Fogo), with the latest historical eruptions on Fogo Island, i.e., the 1995 and 2014 events [61].

Cape Verde magmatism is strongly alkaline, as testified by the occurrence of nephelinitic and melilititic rocks in several islands, and all volcanic rocks are silica-undersaturated, with basanites, tephrites, and nephelinites representing the most dominant compositions [62][63]. Carbonatites occur on six out of ten islands (S. Vicente, S. Nicolau, Maio, Santiago, Fogo, and Brava) (Figure 1c) and they are grouped into two main types: (i) Ca-carbonatites (the most dominant type) and (ii) Mg-carbonatites. S. Vicente presents the largest exposed carbonatites, which occur either as an intrusive or extrusive type [22], similar to Brava Island [64]. Carbonatites on Fogo Islands occur in the basement and they are all coarse-grained Ca-carbonatites (sövite), while on Santiago and Maio, Mg-carbonatites are also present [20]. Carbonatites on the Secos islets near Brava Island and on S. Nicolau Island have also been described [65].

2.2. Geochemistry

Despite that both Fuerteventura and Cape Verde oceanic carbonatites are characterized by a large variability in minor and trace element compositions, they show broadly similar trace element characteristics [20]. Indeed, it has been recognized in both the carbonatitic complexes a depletion in Rb, K, Hf, Zr, and Ti, coupled to an enrichment in Ba, Th, U, Sr, Y, LREE, and MREE, especially if compared with OIB, although elements such as Th, U, Nb, and Ta normally show differences in their relative abundances, interpreted as due to pyrochlore fractionation/accumulation [20].

Hoernle et al. (2002) [20] proposed that calcio–carbonatites from Fuerteventura, similar to the calcio–carbonatites from Cape Verde, result from melting of secondary Ca–carbonate belonging to recycled 1.6 Ga oceanic crust. Furthermore, coupling Ce and Nd isotopic data, Doucelance et al. (2014) [14] suggested the recycling of marine carbonates in the source region of the Cape Verde oceanic carbonatites. In the same way, a recent study on Ca isotopes of carbonatites (tracers of subducted sedimentary material) confirmed the recycled marine carbonate contribution to Fuerteventura and Cape Verde carbonatite petrogenesis [66]. Although there are several models that underline the importance of recycling of marine carbonates and the oceanic crust in carbonatite origin, other hypotheses involving a primary deep-seated signature [21][23] suggest a role of primordial carbon in the origin of the oceanic carbonatite source.

As regards the Cape Verde carbonatites, there is no unique viewpoint. De Ignacio et al. (2012) [22] suggest that Ca–carbonatites from São Vicente (northern Cape Verde Islands) represent fractionated melts from parental nephelinitic magma (although the authors also recognize the importance of a high-temperature immiscibility process in the petrogenesis of São Vicente Ca–carbonatites). Other models in which the Ca–carbonatites result from liquid immiscibility (and not from extreme fractionation of carbonated silicate parental magma) were also proposed for Brava, one of the southern Cape Verde Islands [25]. Although in some cases (e.g., Fogo and Santiago islands) isotopic compositions (Sr–Nd–Pb) of carbonatites significantly differ from those of their associated alkaline silicate rocks, the close association of carbonatites with evolved alkaline silica-undersaturated magmas suggests a common origin. Indeed, a recent study proposes that both oceanic carbonatites (Cape Verde and Fuerteventura carbonatites) occur where primitive silicate melts have the lowest silica and highest alkali contents, driving the liquid line of descent into the silicate–carbonatite miscibility gap [67].

With respect to the continental carbonatites, Fuerteventura oceanic carbonatites present a much narrower spread in the 143Nd/144Nd vs 87Sr/86Sr diagram [20][21]. Regarding Pb isotopic compositions, carbonatites instead plot in a relatively large range, and initial 207Pb/204Pb ratios plot below the trend of the Northern Hemisphere Reference Line (NHRL) [67], which is normal for oceanic carbonatites (initial 207Pb/204Pb ratios of the Cape Verde carbonatites also plot below the NHRL (Hoernle et al. 2002 [20], their Figure 6)) and OIB [20][68].

Given the close similarity in isotopic compositions between Cape Verde and Fuerteventura oceanic carbonatites with Tamazert continental carbonatites (Morocco, ca. 970 km north-eastwards from Fuerteventura), a common source for the latter was also proposed [69], emphasizing the invoked eastward deflection of the Canary mantle plume head through a sub-continental lithospheric corridor from the eastern Atlantic to the middle Atlas mountain chain and the western Mediterranean [70].

Lastly, Mata et al. (2010) [23] reported the first noble gas results obtained for Cape Verde carbonatites, and some of the analysed calcite and apatite minerals present high 3He/4He ratios (R/Ra up to 15.5 and 9.76, respectively), suggesting a contribution of He from a deep and more primitive mantle (plume-type) for Cape Verde carbonatites.

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