Journal of Petrology Advance Access originally published online on July 18, 2005
Journal of Petrology 2005 46(12):2465-2493; doi:10.1093/petrology/egi061
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Kimberlite-like Metasomatism and ‘Garnet Signature’ in Spinel-peridotite Xenoliths from Sal, Cape Verde Archipelago: Relics of a Subcontinental Mantle Domain within the Atlantic Oceanic Lithosphere?
EARTH SCIENCE DEPARTMENT, UNIVERSITY OF FERRARA, VIA SARAGAT, 1 44100 FERRARA, ITALY
RECEIVED NOVEMBER 12, 2004; ACCEPTED JUNE 16, 2005
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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A mantle xenolith suite from two Late Tertiary necks on Sal Island (Cape Verde Archipelago) consists of nearly equivalent amounts of anhydrous spinel-bearing lherzolites and harzburgites, in which secondary metasomatic textural domains are superimposed on the original protogranular textures. Detailed petrographic studies, coupled with in situ major and trace element analyses of the constituent minerals and interstitial glasses, reveal the complex evolutionary history of the Cape Verde lithospheric mantle, from depletion in the garnet facies to re-equilibration and re-enrichment in the spinel stability field. Low CaO (16·4–18·0 wt %) and heavy rare earth element (HREE; Ybn = 2·4–4·8), and high Cr2O3 (1·06–1·84 wt %) contents in the clinopyroxenes of the lherzolites can be quantitatively accounted for by (1) low-degree (
4%) partial melting of a Primitive Mantle-like garnet lherzolite followed by (2) partial re-equilibration of the melting residuum from the garnet to the spinel stability field. This model is further supported by thermobarometric estimates (T = 975–1210°C; P = 1·3–2·1 GPa), which cluster around the spinel–garnet boundary in the peridotite system. Secondary parageneses, regardless of the primary lithologies, are characterized by (1) two clinopyroxenes, cpx2-O and cpx2-C, respectively related to orthopyroxene and clinopyroxene destabilization after reaction with metasomatic fluids, and (2) glasses with anomalously high, even for continental settings, K2O contents (up to 8·78 wt %), together with K-feldspar. Major and trace element mass balance calculations between the primary and secondary parageneses suggest infiltration of a kimberlite-like metasomatizing agent (on volatile-free basis, MgO 17–27 wt %; K2O/Na2O 1·6–3·2 molar; (K2O + Na2O)/Al2O3 1·1–3·0 molar; Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-like metasomatism in the Cape Verde lithospheric mantle, together with the presence of lherzolitic domains, partially re-equilibrated from the garnet to the spinel stability field, may suggest the presence of subcontinental mantle lithosphere relicts left behind by drifting of the African Plate during the opening of the Central Atlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Mantle xenoliths from two Late Tertiary volcanic necks on Sal Island were investigated to define the petrological characteristics and the thermobarometric and compositional evolution of the Atlantic lithospheric mantle beneath the Cape Verde Archipelago. Although lavas from Cape Verde are distinguished by the presence of highly alkaline products, including carbonatites, which at present are known within the ocean basins only from the Cape Verde and the Canary Archipelagos (Hoernle et al., 2002
; Jørgensen & Holm, 2002), petrological studies on both lavas and their associated mantle xenoliths are rather scarce (De Paepe & Klerkx, 1971; Klerkx & De Paepe, 1976; Gerlach et al., 1988; Ryabchikov et al., 1995; Mendes & Silva, 2001).
The Cape Verde Archipelago, situated 500 km west of the Senegal coast (15–17°N, 23–26°W; Fig. 1), consists of 10 major islands divided into a northern group (Santo Antão, São Vicente, Santa Luzia, São Nicolau), an eastern group (Sal, Boa Vista) and a southern group (Brava, Fogo, Santiago, Maio) of islands. They occupy an area of 4033 km2 and are aligned along three segments of uplifted oceanic crust (known as the Cape Verde Rise) defining WNW–ESE (Santo Antão–São Vincente–São Nicolau), almost north–south (Sal–Boa Vista) and WSW–ENE (Brava–Fogo–Santiago–Maio) trends. Relics of this oceanic crust, made up of Jurassic pillow lavas with mid-ocean ridge basalt (MORB) affinity, occur on Maio and Santiago (Stillman et al., 1982). Volcanism has been active for the last 20 Myr and possibly since 40–50 Ma (Courtney & White, 1986); the most recent eruptions have been reported from Fogo in 1995 (Holm et al., 1997). The easternmost islands of the archipelago (Sal and Boa Vista) are deeply eroded, as are the islands of São Nicolau and São Vincente in the northern part of the archipelago, compared with Santo Antão and Fogo. These morphological features agree with recent Ar–Ar geochronological studies on Maio, Fogo and Santo Antão, which suggest progressive rejuvenation of volcanic activity westward (Plesner et al., 2002).
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Although each island is distinctive for its volcanic stratigraphy, relatively primitive SiO2-undersaturated volcanic products occur in most islands; these are predominantly basanites, nephelinites, alkali basalts, melilitites and tephrites, with minor differentiated trachytes and phonolites. Associated plutonic rocks are mainly syenites and essexites. Carbonatites are found on Fogo, Santiago, Maio, São Vicente and Santo Antão (Gerlach et al., 1988
; Christensen et al., 2001; Jørgensen & Holm, 2002). The overall geochemical characteristics of the Cape Verde lavas indicate that partial melting commonly occurred within the garnet stability field at depths >60–70 km for the most alkaline basic magmas (Gerlach et al., 1988). Compared with most other oceanic islands, the Cape Verde Islands exhibit significant radiogenic isotope variations, which have been explained by variable mixing of HIMU (‘high µ’), Enriched Mantle (EM)I and Depleted Mantle (DM) components. In the northern islands only the HIMU and DM components are present, whereas in the southern islands EMI products are also found (Gerlach et al., 1988; Jørgensen & Holm, 2002; Doucelance et al., 2003). This has been explained by variable degrees of partial melting of a heterogeneous mantle plume (HIMU + DM) in the northern islands, which appears to interact with lithospheric (EMI) melts in the southern islands (Gerlach et al., 1988).
Based on He–Ar–Pb isotopic compositions, Christensen et al. (2001) concluded that the HIMU component represents subducted oceanic crust recycled in the boundary layer between the lower and upper mantle at 1·6 Ga, whereas the He–Ar isotopic signature could be inherited from the lower mantle, thus precluding the depleted mantle component from playing a significant role. More recently, Escrig et al. (2002), on the basis of the Os, Sr, Nd and Pb isotopic compositions of basalts and carbonatites from Fogo, recognized a less extreme HIMU mantle end-member together with an enriched component identified as subcontinental lithospheric mantle (SCLM), which shares some trace element and isotopic similarities with kimberlites and lamproites. This SCLM component has been interpreted as a geochemical remnant of the African subcontinental mantle, which was entrained in the Cape Verde plume during the opening of the Atlantic Ocean (Doucelance et al., 2003). Similar features were pointed out by Coltorti et al. (2000a) and Bonadiman et al. (2002), who invoked a reaction between the Cape Verde lithospheric mantle and a K-lamprophyre metasomatic melt to account for the anomalously K2O-rich glasses found in some mantle xenoliths from Sal.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Mantle xenoliths of Sal occur in two adjacent necks (Morrinho do Açurar and Morriho do Filho) in the northern part of the island. Sample locations are shown in Fig. 1. The volcanic rocks hosting the xenoliths consist of nephelinite to basanite lavas. One hundred very fresh xenoliths were collected, and 60 of them were analysed for whole-rock and mineral major element compositions (the dataset is available as an electronic appendix at http://www.petrology.oupjournals.org). They include almost exclusively peridotite varieties, apart from rare wehrlites and clinopyroxene megacrysts, which are not included in the dataset. The few samples showing textural evidence of host basalt infiltrations were disregarded.
The studied mantle xenoliths are anhydrous spinel-bearing peridotites, with modal compositions ranging from lherzolite (7–13 vol. % of cpx) to harzburgite (0·6–3 vol. % of cpx) (Table 1). Xenolith sizes range from 5 to 20 cm and generally show sharp-edged contacts with the host basalts. Most of them are characterized by primary coarse-grained protogranular textures with olivine (ol1), orthopyroxene (opx) and clinopyroxene (cpx1) crystals up to 1–2 mm across; rare spinel (sp1) (<1 vol. %) typically occurs as interstitial, vermicular-shaped, 0·04–0·1 mm crystals, sometimes also included in orthopyroxene (Fig. 2c). Primary clinopyroxenes (cpx1) (Fig. 2a and b) vary in grain size from 0·50 to 1 mm. They often have distinctive secondary rims, which partially, or sometimes completely, replace the primary crystals (Fig. 2b and c). The replacement of cpx1 usually results in the formation of finely disseminated secondary clinopyroxene (cpx2-C) and olivine (ol2), locally associated with glassy patches [Type B and C textures of Coltorti et al. (1999) and Bonadiman et al. (2001)]. These textures are very common in the lherzolites.
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Porphyroclastic textures, with large tabular orthopyroxene (1–2 mm in size), partly or totally replaced by fine intergrowths of recrystallized ol (ol2), sp (sp2), cpx (cpx2-O) and rare glass, are commonly found in harzburgites but rarely in lherzolites [Type A texture of Coltorti et al. (1999)
and Bonadiman et al. (2001); Fig. 2d and e]. Both lherzolites and harzburgites contain glassy patches (<1 vol. %) surrounding orthopyroxene relicts and containing secondary clinopyroxene, olivine, tiny subidiomorphic spinel and rare K-feldspar [Type B texture of Coltorti et al. (1999) and Bonadiman et al. (2001); Fig. 2e and f]. |
ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Whole-rock X-ray fluorescence (XRF) analyses for major, minor and some trace elements (Ba, Co, Cr, Nb, Ni, Rb, Sr, V, Y and Zr) were performed on pressed powder pellets at the Department of Earth Sciences, University of Ferrara, on a Philips PW1400 spectrometer using standard procedures (Franzini et al., 1975
; Leoni & Saitta, 1976). Typical uncertainties are <3% for Si, Ti, Fe, Ca and K, and 7% for Mg, Al, Mn, Na and P; uncertainties for trace elements (above 10 ppm) are <7% for Rb, Sr and V and 15% for Ba, Ni, Co and Cr. Rare earth elements (REE) and Y were determined at the Centre de Recherches Pétrographiques et Géochimiques of Nancy (France) by inductively coupled plasma atomic emission spectrometry (ICP-AES) with an accuracy of 15% for Yb and Lu and better than 8% for the other REE (Roelandts & Michel, 1986). Electron microprobe analyses were carried out with a Cameca CAMEBAX in the Istituto di Geoscienze e Georisorse, CNR, Padua (Italy), using both energy- and wavelength-dispersive spectrometry (EDS and WDS). Routine WDS analyses were obtained on carbon-coated polished thin sections using an accelerating voltage of 20 kV, 15 nA beam current and a counting time of 20 s for each element. Both natural and synthetic standards were used for calibration. Major element contents in glasses were determined using a defocused beam (5–10 µm in diameter) at 15 kV and 10 nA.
The concentrations of trace elements in minerals and glasses and Ca in olivines were obtained by secondary ionization mass spectrometry (SIMS) using a Cameca IMS 4f ion microprobe located at CNR-IGG (Pavia). The primary beam consisted of mass filtered 16O– and was focused on a spot of 10–20 µm diameter. Analytical conditions were 10 nA beam current and 15 keV total impact energy. The ions sputtered from polished gold coated thin sections were transferred to the mass spectrometer by the 25 µm optics and energy filtered of –100 V offset voltage with an energy band width of +25 eV. Quantification was carried out by means of Si-normalized working curves obtained from natural mineral and glass standards. The details of the analytical procedures have been reported by Ottolini et al. (1993) and Bottazzi et al. (1994). The analytical accuracy and precision have been assessed by analysing natural and synthetic standards. The reliability of Ca quantification has been tested on San Carlos olivine [reference values by Köhler & Brey (1990)]. The estimated accuracy is 10% relative for all trace elements with absolute concentrations at the ppm level.
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WHOLE-ROCK COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element analyses of representative lherzolites and harzburgites from Sal island are reported in Table 1; all data are plotted in the major element diagrams of Fig. 3 and available from http://www.petrology.oupjournals.org. On these diagrams curves for isobaric batch melting at pressure of P0 = 2 GPa and polybaric near fractional melting (1% melt porosity) at P0 = 1·5–2·5 GPa, calculated after the Niu (1997)
model, are also plotted. Two fertile mantle compositions were chosen as starting points: the preferred source (PS) proposed by Niu (1997) and the fertile lherzolite SG3 (modal cpx 21%) from the Veneto Volcanic Province, Italy (Beccaluva et al., 2001a; Bonadiman et al., 2001). The lherzolites (8–13% modal cpx) and harzburgites (<4% modal cpx) are clearly separated in the major element variation diagrams. As usually observed in progressive mantle depletion by extraction of basaltic components, modal cpx, SiO2, CaO and Al2O3 decrease as MgO increases, overlapping the theoretical partial melting curves. TiO2 also decreases from lherzolites to harzburgites and parallels the same curves but at higher values, suggesting a different starting composition. Several harzburgites have MgO contents higher than 45%, suggesting they are a residuum after partial melting degrees higher than 25% (limit of the depletion curves starting from the PS and SG3 modelled sources). However, the trace element distribution, particularly heavy REE (HREE), suggests that the compositional variation from lherzolites to harzburgites is difficult to explain with a common progressive melt depletion process. In fact the Yb contents of the lherzolites (cpx 8–13%; Ybn, 0·6–1 times C1 chondrite) is anomalously low for fertile sp-lherzolites and is similar to that of harzburgites from Cape Verde and other oceanic mantle xenolith occurrences (modal cpx <4%; Ybn = 0·1–1 times C1 chondrite) (e.g. Canary Islands, Siena et al., 1991; Neumann et al., 2005; Kerguelen Archipelago, Delpech et al., 2004). This reflects the peculiarly low HREE contents of clinopyroxenes in Cape Verde lherzolites (see the mineral chemistry section). On the other hand, Lan of the lherzolites only varies from 2·5 to 10·4 times C1 chondrite, whereas the harzurgites display a larger range of variation with Lan = 0·7–21·4 times C1 chondrite, indicating a greater metasomatic effect in the latter, as usually recorded in more restitic mantle lithologies (e.g. Bedini et al., 1997) (Table 1).
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MINERAL AND GLASS COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major elements
Olivines in the Cape Verde peridotites range from Fo 86·7 to Fo 91·5, with the highest values occurring in the harzburgites (Table 2). Fo contents in lherzolites, with similar olivine modal contents, are comparable with those of olivines from anhydrous spinel- and garnet-bearing off-cratonic lherzolites (Boyd, 1989
; Glaser et al., 1999; Ionov, 2004). No significant differences were noted between primary and secondary olivines.
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Orthopyroxene occurs only as a primary phase. Its Al2O3 content ranges from 3·65 to 6·17 wt % with mg-number [Mg/(Mg + Fe) x 100] varying between 87·6–91·3 in lherzolites and 91·3–93·0 in harzburgites (Table 2). In a CaO vs Cr2O3 plot (Fig. 4) opx compositions from the Cape Verde lherzolites are compared with those from other unmetasomatized anhydrous spinel and garnet lherzolites, the latter subdivided as cratonic (Archaean in age) and off-cratonic. The distinctive, high, CaO and Cr2O3 contents of the primary unreacted opx in the Cape Verde lherzolites are clearly evident (Fig. 4). The majority have CaO contents comparable with those of the most fertile sp-lherzolites, with Cr2O3 contents very similar to those of off-cratonic gt-lherzolites. They show some similarities to enstatite grains found in C1 achondrites and chondrites (Mittlefehldt et al., 1996
; McCoy et al., 1997).
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Large protogranular primary cpx (Fig. 2a) and unreacted cores of spongy cpx (cpx1) (Fig. 2b) are all Cr-rich augites (Morimoto et al., 1988
; endiopsides following Deer et al., 1983) and rather uniform in composition (Table 2). A slight decrease of Al2O3 and TiO2 contents with increasing mg-number (88–92·1) can be observed (Table 2). As for opx, the Cape Verde cpx1 have a distinctive composition with anomalously low CaO (16·4–18·01 wt %) and high Cr2O3 (1·06–1·84 wt %) contents when compared with worldwide spinel-bearing lherzolites (Table 2 and Fig. 5). In Fig. 5, despite the spinel-bearing (garnet-absent) modal assemblage, the clinopyroxene compositions of the Cape Verde lherzolites fall within the field of four-phase, fertile, gt-bearing (cratonic and off-cratonic) lherzolites (cpx >8%; mg-number 88–92). Thus (1) the discrepancy between cpx (and opx) compositions, matching those of pyroxenes in equilibrium with garnet, and (2) the absence of garnet are striking features.
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Secondary clinopyroxenes (cpx2-C and cpx2-O) have higher mg-number with respect to cpx1 (Table 2) and plot within distinct fields in the Na2O vs SiO2/Al2O3 diagram (Fig. 6). Cpx2-C and cpx2-O have lower Na2O contents than cpx1, with variable SiO2/Al2O3 ratios. Cpx2-O, associated with opx, have the highest SiO2/Al2O3 ratios (Fig. 6).
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Primary spinel (sp1) (Table 2) compositions are uniform within each sample, with cr-number [cr-number = Cr/(Cr + Al) x 100] increasing from lherzolites (cr-number 15–34) to harzburgites (cr-number 46–79). There are no significant compositional differences between the larger vermicular crystals and the small grains included in pyroxenes. Rare secondary spinels (sp2), mainly associated with the fine intergrowth surrounding primary pyroxenes, have higher cr-number and lower mg-number with respect to the primary crystals (Table 2).
Feldspars occur only within metasomatic patches in the Cape Verde mantle xenoliths and are always associated with glass and secondary minerals (Fig. 2e and f, and Table 2). Their composition falls within the range An0·38–8·8, Ab6·6–24·0, Or72·0–89·1 and shows an unusual enrichment in the orthoclase component (Fig. 7). In Fig. 7 the compositions of feldspars from both continental (Veneto Volcanic Province, Italy, Beccaluva et al., 2001a; Bonadiman et al., 2001; Hamar-Dablam, Mongolia, Ionov et al., 1995; Yitong, China, Xu et al., 1996) and oceanic mantle xenoliths (Kerguelen Archipelago; Grégoire et al., 2000; Delpech et al., 2004) are also illustrated. Feldspars from the Kerguelen oceanic mantle, which occur intimately associated with Ti-rich oxide minerals, have potassium-rich compositions, although they do not reach the extreme enrichment of the Cape Verde feldspars.
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Silica-rich glasses occur only in a few xenoliths (Table 2). Regardless of the textural setting, the glasses are rather homogeneous in composition, and are characterized by relatively high SiO2 (55·73–67·13 wt %), Al2O3 (14·33–21·4 wt %) and alkali contents (Na2O 2·49–7·14 wt %; K2O 5·50–8·78 wt %). Their compositions are similar to those of most mantle xenolith glasses worldwide, apart from the exceptionally high K2O contents, which have never been found in oceanic settings before and have rarely been matched even in continental xenoliths (Coltorti et al., 2000b
, and references therein).
Trace elements
Trace element compositions of Cape Verde mantle xenolith minerals and glasses are reported in Table 3 and Figs 8 and 9. In a chondrite-normalized trace element diagram the REE (plus Sr, Zr, Ti and Y) distribution in the opx shows fractionated patterns, systematically depleted in light REE (LREE), and characterized by positive Zr and Ti anomalies, with the exception of CV43, which lacks a titanium spike (Fig. 8a). This may indicate that CV43 suffered distinct metasomatic processes and/or inherited a different ‘original’ source characteristic resulting in Ti and Zr decoupling (see below). The HREE contents of the Cape Verde opx plot at around 0·7 times C1 chondrites, intermediate between the fields of worldwide unmetasomatized garnet and spinel lherzolites (at comparable cpx modal content) (Fig. 8a). Because the distribution of HREE in orthopyroxene is clearly controlled by the presence of garnet or spinel (Eggins et al., 1998; Glaser et al., 1999; Green et al., 2000), the observed patterns further support the hypothesis that a ‘garnet-facies signature’ is still preserved in the Cape Verde orthopyroxenes (Fig. 8a).
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As already shown for major elements, the primary and secondary clinopyroxenes display distinct trace element compositions (Figs 8b and 9a). Primary Cpx1 in lherzolites show extremely homogeneous convex upward REE patterns (Lan = 3·6–5·3; Smn = 8·3–12·8; Ybn = 2·4–4·8), coupled with a negative Zr anomaly (Fig. 8b). Such profiles are unusual for clinopyroxenes in spinel-bearing lherzolites at comparable cpx modal percentages; these are usually characterized by strong LREE depletion and higher, almost flat, HREE distributions, associated with distinct Zr and Ti negative anomalies (i.e. Johnson & Dick, 1992
; Norman, 1998; Xu et al., 2000; Beccaluva et al., 2001a, 2004). On the other hand, patterns similar to those of the Cape Verde cpx1 are typical of clinopyroxenes from anhydrous garnet peridotites, residual after low degrees of partial melting (Walter, 1998; Kempton et al., 1999a; Xu et al., 2000; Norman, 2001) (Fig. 8b). Secondary clinopyroxenes (cpx2-C and cpx2-O) have trace element concentrations that are significantly higher with very different trace element profiles from cpx1, with enrichment of light and middle REE (MREE; Lan = 7·9–64·1; Smn = 36·6–95·4) and positively fractionated HREE patterns (Ybn = 4·9–13·1). Most of the analysed cpx2 have negative Ti and Zr anomalies, which tend to be more pronounced in cpx2-O (Fig. 9a).
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The analyses of Cape Verde K-feldspars coexisting with glasses are characterized by an extreme enrichment in Rb and K with slightly LREE depleted patterns (Fig. 9b) at about 2–3 times C1 chondrite. Compared with feldspars from other alkali–silicate metasomatized mantle xenoliths, such as those of the Veneto Volcanic Province, Italy (Beccaluva et al., 2001a
), these display higher Rb and K, lower LREE, and comparable Ba and Sr contents. However, the Cape Verde feldspars do not show a Ba, K and Sr antithetic distribution with the coexisting glasses, testifying to attainment of equilibrium, as observed in the Veneto Volcanic Province (Bonadiman et al., 2001). Thus the Cape Verde feldspars seem to represent a disequilibrium crystallite.
Silicate glasses in Cape Verde lherzolites have positive fractionated HREE patterns similar to those of the coexisting secondary clinopyroxenes, but display distinct LREE enrichment. Glasses in harzburgite CV15 show parallel patterns, but with higher trace element concentrations, particularly for the most incompatible elements (Rb, Ba, Nb and K) (Fig. 9b). Unlike the secondary clinopyroxenes, the glasses do not show any significant Ti and Zr negative anomalies, as are usually observed in mantle glasses related to alkali–silicate metasomatism worldwide (Coltorti et al., 2000b).
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P–T ESTIMATES |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Both cpx1 and opx within individual samples have homogeneous major element compositions, particularly regarding mg-number, and, together with ol1, they show mutual equilibrium relationships, as indicated by the Fe/Mg exchange equilibria of Brey & Köhler (1990)
(Fe/Mgopx/cpx = 1·08, Fe/Mgol/cpx = 1·22, Fe/Mgol/opx = 1·09). On this basis, estimates of the P–T conditions of equilibration of the protogranular assemblage can be obtained. To test the validity of such estimates, temperatures were calculated using both the two-pyroxene geothermometer of Brey & Köhler (1990) (TBK), and the opx–cpx Cr-exchange of Seitz et al. (1999) (TS). Only cpx1 core and opx core pairs were considered in the calculations. Pressures were estimated based on the Ca distribution in olivine (Köhler & Brey, 1990), with accurate Ca determinations obtained by SIMS. Considering the high diffusion rate of Ca in olivine compared with that in pyroxenes (Brady & McCallister, 1983), the reliability of this barometer is strongly dependent on the state of equilibrium for each ol–cpx pair (Witt-Eickschen & Kramm, 1997). For this reason pressure estimates were calculated for the ol1 cores, combined with the opx–cpx1 TBK thermometer, to yield realistic pressures for ‘equilibrated’ parageneses. Nominal equilibration temperatures (for a pressure arbitrarily assumed to be 1·5 GPa), for the lherzolites, range from 975 to 1210°C and from 995 to 1155°C for the TBK and the TS geothermometers, respectively (Table 4). The good agreement (<50°C) between TBK and TS should be stressed, especially taking into account that the determinations were based on relatively independent approaches (Fe–Mg, vs Cr distribution in opx–cpx pairs). Lherzolite CV43 shows comparatively lower temperature ranges (TBK 930–1150°C; TS 805–1025°C), which could be interpreted as related either to diffusion effects owing to incipient metasomatism, or to derivation from an original, shallower mantle domain (Table 4).
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Pressure estimates are constrained between 1·3 and 2·1 GPa, overlapping the transitional spinel–garnet peridotite boundary in the CMAS system (Klemme & O'Neill, 2000
) (Table 4, Fig. 10). Unrealistically low pressures were obtained for lherzolite CV43 (<0·2 GPa, not reported in Table 4) and clearly reflect the anomalous low temperature values used for the determination. This pressure range is in agreement with the distinctive opx and cpx1 compositions (Figs 5 and 6), which suggest a ‘fossil’ equilibrium in the garnet lherzolite field. The P–T trend is sub-parallel, at higher pressures, to the geotherm calculated from surface heat flux measurements over a distance of 450 km across the Cape Verde hotspot swell (68 mW/m2) (Courtney & White, 1986), which are the highest heat flow values in the North–Central Atlantic (Turcotte & Schubert, 1982) (Fig. 10). The P–T estimates seem to indicate that at least part of Cape Verde mantle lithosphere approaches, but does not completely attain, the thermobarometric conditions of the present-day hotspot swell geotherm. However, many more data are required to unravel and define the thermobarometric history of the mantle section underlying the Cape Verde Archipelago.
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MODELLING DEPLETION AND ENRICHMENT PROCESSES |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Depletion of garnet-bearing lherzolites and re-equilibration in the spinel stability field
Unlike other oceanic mantle xenolith suites in which harzburgites represent the most common lithotype (i.e. Canary Islands, Siena et al., 1991
; Neumann et al., 2005; Kerguelen, Delpech et al., 2004), the Cape Verde spinel-bearing xenolith population is characterized by the presence of nearly 40% lherzolites, with distinctive pyroxene compositions. These pyroxenes provide evidence of: (1) depletion related to melt extraction within the garnet stability field conditions; (2) partial re-equilibration in the spinel stability field.
To model the above processes we focus on the chemical characteristics of cpx1, as the most sensitive phase that controls the incompatible trace element whole-rock budget in unmetasomatized spinel peridotites. As shown above, cpx1 are characterized by distinctive HREE-fractionated trace element patterns coupled with very high Cr2O3 contents, both features being difficult to reconcile with typical sp-bearing lherzolite parageneses. Several models were used to reconstruct the geochemical characteristics of the primary Cape Verde cpx1 (Johnson et al., 1990; Norman, 1998; Xu et al., 2000; Zou & Reid, 2001), but none was able to reproduce the observed trace element patterns. Therefore, we tested the hypothesis that the Cr-rich HREE-fractionated cpx1 compositions could be consistent with a prior partial melting event in the garnet stability field. An inferred primitive clinopyroxene composition coexisting with garnet (CpxPM-Gt) was calculated based on (1) the major element and REE (plus Zr and Ti) Primitive Mantle composition of McDonough & Sun (1995); (2) the mineral major element compositions of the most primitive (volatile-rich) C1 chondrites (Brearley & Jones, 1998); (3) the ol/cpx, opx/cpx and gt/cpx distribution coefficients of Zack et al. (1997), Takazawa et al. (2000) and Green et al. (2000). The bulk-rock mineral mode of 53% ol, 21% opx, 15% cpx, and 11% gt was obtained through mass balance calculations and the resulting CpxPM-Gt trace element pattern is reported in Fig. 11.
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The calculated Primitive Mantle mode compares favourably with the primitive mantle modal composition of Ionov (2004)
[source (3): 57% ol; 21% opx; 13% cpx; 9% gt] and Johnson et al. (1990) (55% ol; 20% opx; 15% cpx; 10% gt). However, it differs from the estimate obtained by Francis (2003), who, following a similar approach, calculated an ‘original’ garnet content of 17·2% for the Bulk Silicate Earth composition of O'Neill & Palme (1998). The use of this higher garnet modal proportion would, however, lead to a partial melting degree (c. 10%) higher than those calculated by Xu et al. (2000) (3–5%) for the most fertile off-cratonic xenoliths from Nushan and Mingxi (China) (Fig. 12a). For the few off-cratonic garnet lherzolites with modal gt percentages >15% (i.e. Pali Aike, Kempton et al., 1999a; Nushan, Mingxi, Xu et al., 2000; Vitim, Glaser et al., 1999; Litasov et al., 2000; Ionov, 2004; Patagonia, T. Ntaflos, personal communication, 2004) it is reasonable to consider these high garnet domains as the result of small-scale heterogeneity, probably owing to local new impregnation (Vitim, Ionov, 2004) or multiple metasomatic events caused by melts generated from fertile garnet peridotites (Pali Aike, Kempton et al., 1999b).
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The calculated CpxPM-Gt has a REE profile (Fig. 11) with distinctly higher LREE (Lan = 18·2) and comparable positively fractionated HREE contents (Ybn = 0·93), with respect to clinopyroxenes from worldwide garnet lherzolites. This is consistent with experimental cpx/garnet partition coefficient data (Sweeney et al., 1995
; O'Reilly & Griffin, 1996; Takazawa et al., 1996, 2000; Green et al., 2000; Van Westrenen et al., 2000; GERM dataset: http://earthref.org/GERM/main.htm), which favours LREE and HREE partitioning in clinopyroxene and garnet respectively.
Both fractional and batch partial melting models of clinopyroxene based on the equation of Johnson et al. (1990), starting from CpxPM-Gt, were used to calculate the composition of clinopyroxene after partial melting. Batch melting models led to better results than those obtained by fractional melting, although no great differences can be noted for melting degrees around or lower than 4% (Figs 11 and 12a,b). No trapped melt (residual porosity) in equilibrium with clinopyroxene was assumed in the whole-rock, because it has been recently demonstrated that it significantly affects MREE and HREE residual clinopyroxene contents only at partial melting degree >15% (Takazawa et al., 2000; Zou & Reid, 2001; Hellebrand et al., 2002).
The REE (plus Ti and Zr) pattern of CpxPM-Gt after 4% of batch melting matches fairly well with the upward-convex patterns of the Cape Verde primary clinopyroxenes, except for the HREE (Fig. 11). This discrepancy may be explained by taking into account the re-equilibration path of the Cape Verde lherzolites from the garnet to the spinel stability field modelled using the Hauri & Hart (1994) and Takazawa et al. (1996) subsolidus equations. In this way the REE hosted in the residual garnet were redistributed between spinel and pyroxenes, enhancing the HREE content in cpx1 (Fig. 12b), thus reproducing the humped Cape Verde cpx1 REE profile (Fig. 11). In this framework, this geochemical model may be supported by the recent finding on the Island of Santiago (Evans et al., 2004) of clusters of spinels, either skeletal or strongly embayed with small clinopyroxenes, interpreted as garnet relics.
Ti and Zr partitioning in Primitive Mantle clinopyroxenes
Analogous to the Cape Verde primary cpx1 (Fig. 8), a Zr negative anomaly, not accompanied by a comparable Ti anomaly, is observed both in the model CpxPM-Gt pattern {Zrn/[(Ndn + Smn)/2] = 0·54; Tin/[(Eun + Gdn)/2] = 0·95} and in clinopyroxene from the most fertile (modal cpx >10%) unmetasomatized garnet-bearing anhydrous lherzolites (Fig. 11) and diamond-bearing eclogites from Newlands kimberlites (Menzies et al., 2003; not shown for sake of clarity). The decoupling between Zr and Ti negative anomalies may be due to the different partitioning behaviour of Zr and Ti with respect to the neighbouring elements in clinopyroxene coexisting with garnet or spinel. Ti and Zr retained in orthopyroxene will in fact contribute to the whole-rock Zr and Ti budget by no more than 10% and 30% respectively (Rampone et al., 1991; Eggins et al., 1998), in both garnet and spinel facies. It follows that the presence of the two negative anomalies in clinopyroxene strictly depends on the nature of the aluminous phases (garnet or spinel). Zr is preferentially partitioned into garnet with respect to the coexisting cpx (
), whereas Sm and Nd favour cpx over garnet (
; 
). On the other hand, the partitioning between Ti (
) and the adjacent REE (
and 
) is comparable (Sweeney et al., 1995; O'Reilly & Griffin, 1996; Takazawa et al., 1996, 2000; Green et al., 2000; Hill et al., 2000; Van Westrenen et al., 2000; GERM dataset: http://earthref.org/GERM/main.htm). This subsolidus elemental distribution leads to a profile with a distinctive negative Zr anomaly, but a weak or absent negative Ti anomaly. However, the Ti distribution in clinopyroxene changes rapidly when melting begins, as Ti is more incompatible in silicate melts than the adjacent REE [
; 
; 
(Sweeney et al., 1995; Hill et al., 2000)], as evidenced by the calculated clinopyroxene composition after 4% partial melting of the CpxPM-Gt (Fig. 11). Thus the almost ubiquitous Zr but variable Ti negative anomalies in clinopyroxene from dry garnet peridotites can be easily accounted for (Figs 8 and 11).
Unlike the CpxPM-Gt, the inferred Primordial Mantle clinopyroxene composition in the spinel facies (CpxPM-Sp) (Fig. 11) shows well-defined, comparable Zr and Ti negative anomalies {Zrn/[(Ndn + Smn)/2] = 0·76; Tin/[(Eun + Gdn)/2] = 0·60; Johnson et al., 1990; Beccaluva et al., 2001a; Norman, 2001}, consistent with an analogous fractionation of Zr and Ti with respect to their neighbouring elements (
; 
; 
; 
; 
; 
) and accompanied by similar clinopyroxene Zr, Ti (and their neighbouring elements) partitioning behaviour during melting (O'Reilly & Griffin, 1996; Takazawa et al., 1996; Adam & Green, 1994; GERM dataset: http://earthref.org/GERM/main.htm).
Nature of Cape Verde metasomatic melts
The occurrence of secondary minerals (olivine, clinopyroxene, spinel, K-feldspar) and glass, texturally and chemically distinct from the protogranular peridotite paragenesis (Fig. 2c–f), suggests incomplete reactions between primary phases and metasomatic melt(s). The coexistence of primary and secondary phases permits the quantitative modelling of the metasomatizing melt(s).
The hypothetical composition of this melt(s) has been obtained assuming an instantaneous equilibrium between the melt and peridotite micro-textural domains, using mass balance calculations based on the following equation:
 |
The unknown factor in this general equation is the composition of the metasomatizing fluid (MELT in the equation). The resulting melts obtained for all samples where glassy patches occur, together with the compositions of the phases acting as reactants and products, are reported in Table 5. Mass balance coefficients are summarized in the following equations: CV15 harzburgite
 |
 |
 |
 |
 |
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The mass balance coefficients represent the relative percentages of reacting and recrystallized phases, which are always qualitatively in agreement with the modal amounts of metasomatic assemblages. These coefficients are in turn applied to the trace element contents of both reactants (orthopyroxene and clinopyroxene) and products (clinopyroxene, K-feldspar and glass) to reconstruct the trace element compositions of the metasomatizing agent (Table 6, Fig. 13). Olivine and spinel incompatible trace element contents are assumed negligible.
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Melt compositions vary from sample to sample, although they share some distinctive geochemical characteristics (Table 5, Fig. 13). On a volatile-free basis, they have high MgO contents (17–27 wt %), FeO <10 wt %, CaO <9 wt %, highly variable TiO2 (1·5–4·8 wt %) and K2O/Na2O >1 (1·6–3·2 molar %), (K2O + Na2O)/Al2O3 >1 (1·1–3·0 molar %) (Table 5). They are also characterized by high and rather variable low field strength and high field strength element abundances: Rb 91–165 ppm; Ba 318–1333 ppm; Sr 313–1612 ppm; Zr 194–238 ppm; Nb 38–260 ppm. On the whole, these compositions correspond to ultrabasic, peralkaline and potassic–ultrapotassic melts and according to Mitchell (1995)
, following the IUGC Subcommission on the Systematics of Igneous Rocks (Woolley et al., 1996), they may be defined as kimberlite-like or lamproite melts. On a chondrite-normalized multi-element diagram (Fig. 13) they plot between the trace element patterns of Group I and Group II primary kimberlites (Fraser & Hawkesworth, 1992; Le Roex et al., 2003; Harris et al., 2004), deduced from aphanitic, uncontaminated samples from various South Africa pipes (Group I: Bultfontein, Dutoitspan, Big Hole, Wesselton, De Beers and Uintjiesberg Pipes, Le Roex et al., 2003, Harris et al., 2004; Group II: Finsch Mine, Fraser & Hawkesworth, 1992). The pattern of the average of the leucite–olivine-bearing lamproites from Gaussberg (Antarctica) has been also reported for comparison (Murphy et al., 2002).
We conclude that the Cape Verde metasomatizing melts display intermediate geochemical features between Group I and Group II kimberlites. Group II kimberlites have geochemical characteristics comparable with those of lamproites (e.g. Gaussberg, Fig. 13). They have high K2O (and SiO2) contents, with K/Ti ratios approaching those of Group II kimberlites, but high HREE abundances and Nb/Zr and Ba/Nb ratios, more similar to Group I kimberlites (Table 6) (Taylor et al., 1994; Agashev et al., 2001; Le Roex et al., 2003).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Mantle xenoliths from Sal consist of sp-bearing lherzolites and harzburgites, mainly with protogranular textures, and exhibit clear evidence of metasomatic enrichment. Whole-rock and pyroxene trace element characteristics, particularly HREE abundances, suggest that the compositional variation from lherzolite to harzburgite cannot be explained by a simple partial melting process in the spinel stability field.
The high Cr2O3 contents and distinctive REE patterns of both orthopyroxenes and clinopyroxenes in the spinel-bearing lherzolites compare favourably with those of worldwide, four-phase, garnet lherzolites, and can be quantitatively modelled by: (1) low-degree (4%) partial melting of a Primitive Mantle-like garnet lherzolite protolith, with a clinopyroxene composition (CpxPM-Gt) calculated from the Bulk Silicate Earth of McDonough & Sun (1995), followed by (2) subsequent partial re-equilibration of the melting residuum from the garnet to the spinel stability field. Cape Verde lherzolites record relatively high equilibration temperatures (975–1210°C), and pressures in the range 1·3–2·1 GPa, overlapping the spinel–garnet boundary in the CMAS system (Klemme & O'Neill, 2000). This suggests that a significant portion of the Cape Verde lithospheric mantle, specifically the lherzolites, was not formed as a residuum of mid-ocean ridge partial melting processes, as could be the case of the harzburgites, which are known to represent the most abundant lithology of the uppermost oceanic mantle, as testified by mantle peridotites recovered from oceanic ridge–fracture zones and mantle xenoliths from oceanic settings (i.e. Canary Islands, Siena et al., 1991; Neumann et al., 2005). Whereas the harzburgite portion of the Cape Verde lithosphere could be satisfactorily interpreted as a residuum after MORB extraction, as also suggested by the presence of Late Jurassic MORB at Maio and Santiago, the same origin cannot be attributed to the lherzolites.
The distinctive metasomatic parageneses characterized by the interstitial high-potassium glass (and K-feldspar) suggest that the metasomatic melt(s), affecting both lherzolites and harzburgites of the Cape Verde mantle lithosphere, are kimberlitic in nature. Kimberlite(lamproite)-like melts, commonly observed in subcratonic mantle, are unknown in oceanic settings, with the exception of the highly speculative suggestion based on the nature of spinels in a composite clinopyroxenite xenolith from Hawaii (Keshav & Sen, 2003). Therefore the results of this study may suggest a subcontinental lithospheric mantle origin for the ‘Cape Verde kimberlite/lamproite’, which was also considered to be the source of carbonatites and alkali basalts on Fogo (Jørgensen & Holm, 2002; Doucelance et al., 2003). This may further support a genetic link between ultrapotassic and carbonatitic magmas as also suggested by experimental work (Dalton & Presnall, 1998). Moreover, recent 3D tomographic mapping of the African and adjacent Atlantic lithosphere indicates the presence of a high-velocity body beneath the Cape Verde Archipelago, reaching at least 170 km in depth, indicating a mantle dynamic setting very different from that observed in the Central Atlantic and Canary Archipelago (Griffin et al., 2004). All these facts strongly suggest that subcontinental mantle domains are still preserved within the Cape Verde oceanic lithosphere. They may represent relicts left behind by the drifting process of the African Plate during the opening of the Central Atlantic Ocean. The coexistence of ‘enriched continental’ and ‘depleted oceanic’ mantle domains may also explain the main geochemical components so far recognized in the Cape Verde magmatism (Gerlach et al., l988; Doucelance et al., 2003).
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONPETROGRAPHYANALYTICAL TECHNIQUESWHOLE-ROCK COMPOSITIONSMINERAL AND GLASS COMPOSITIONSP-T ESTIMATESMODELLING DEPLETION AND...CONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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The authors thank Alberto Zanetti and Luisa Ottolini for their invaluable assistance during SIMS analyses. Further thanks go to Raul Carampin for his assistance during electron microprobe analyses. The authors are deeply grateful to Michel Grégoire, Else-Ragnhild Neumann and Marjorie Wilson for their very constructive comments and for their support and encouragement of our work. Marjorie Wilson is further acknowledged for the meticulous and appropriate additional editorial work.
* Corresponding author. Telephone: +39 0532293753. Fax: +39 0532210161. E-mail: bdc{at}unife.it
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