Journal of Petrology Advance Access originally published online on September 9, 2004
Journal of Petrology 2004 45(12):2573-2612; doi:10.1093/petrology/egh063
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Journal of Petrology 45(12) © Oxford University Press 2004; all rights reserved
The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths
1 PHYSICS OF GEOLOGICAL PROCESSES, UNIVERSITY OF OSLO, PO BOX 104, BLINDERN, NO-0316 OSLO, NORWAY
2 GEMOC ARC NATIONAL KEY CENTER, DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MACQUARIE UNIVERSITY, SYDNEY, N.S.W. 2109, AUSTRALIA
3 CSIRO EXPLORATION AND MINING, NORTH RYDE, N.S.W. 2113, AUSTRALIA
RECEIVED SEPTEMBER 1, 2003; ACCEPTED JUNE 24, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Laser ablation microprobe data are presented for olivine, orthopyroxene and clinopyroxene in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro, and Lanzarote, and new whole-rock trace-element data for xenoliths from Hierro and Lanzarote. The xenoliths show evidence of strong major, trace element and Sr isotope depletion (87Sr/86Sr
0·7027 in clinopyroxene in the most refractory harzburgites) overprinted by metasomatism. The low Sr isotope ratios are not compatible with the former suggestion of a mantle plume in the area during opening of the Atlantic Ocean. Estimates suggest that the composition of the original oceanic lithospheric mantle beneath the Canary Islands corresponds to the residues after 25–30% fractional melting of primordial mantle material; it is thus significantly more refractory than ‘normal’ mid-ocean ridge basalt (MORB) mantle. The trace element compositions and Sr isotopic ratios of the minerals least affected by metasomatization indicate that the upper mantle beneath the Canary Islands originally formed as highly refractory oceanic lithosphere during the opening of the Atlantic Ocean in the area. During the Canarian intraplate event the upper mantle was metasomatized; the metasomatic processes include cryptic metasomatism, resetting of the Sr–Nd isotopic ratios to values within the range of Canary Islands basalts, formation of minor amounts of phlogopite, and melt–wall-rock reactions. The upper mantle beneath Tenerife and La Palma is strongly metasomatized by carbonatitic or carbonaceous melts highly enriched in light rare earth elements (REE) relative to heavy REE, and depleted in Zr–Hf and Ti relative to REE. In the lithospheric mantle beneath Hierro and Lanzarote, metasomatism has been relatively weak, and appears to be caused by high-Si melts producing concave-upwards trace element patterns in clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-rich harzburgites/lherzolites, dunites, wehrlites and clinopyroxenites formed from mildly alkaline basaltic melts (similar to those that dominate the exposed parts of the islands), and appear to be mainly restricted to magma conduits; the alkali basalt melts have caused only local metasomatism in the mantle wall-rocks of such conduits. The various metasomatic fluids formed as the results of immiscible separations, melt–wall-rock reactions and chromatographic fractionation either from a CO2-rich basaltic primary melt, or, alternatively, from a basaltic and a siliceous carbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism
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INTRODUCTION |
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The Canary Islands form a roughly east–west-trending ocean island chain close to the margin of western Africa (Fig. 1). The lithosphere beneath the Canary Islands formed during the opening of the Central Atlantic Ocean about 180–150 Myr ago (Verhoef et al., 1991
; Roest et al., 1992; Hoernle, 1998). The intraplate magmatic event represented by the Canary Islands started >24 Myr ago (Abdel-Monem et al., 1971, 1972; Schmincke, 1982; Balogh et al., 1999). Because the trend of the Canary Islands is normal to the passive margin of the African continent, east–west variations in the chemistry and structure of different parts of the lithosphere are of great interest as they may throw light on the effects of the ocean–continent transition on intra-plate processes. It has, furthermore, been proposed that a mantle plume was located below western Africa during the opening of the Central Atlantic Ocean 200 Myr ago (Ernst & Buchan, 1997; Wilson & Guiraud, 1998), and may have contributed to the formation of the continental-margin lithosphere.
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Mantle and crustal xenoliths have been found entrained in primitive magmas in all the large islands, providing important information about the upper mantle beneath individual islands. However, no attempt has, thus far, been made to put these data together and relate them to distance from the continent–ocean transition in the area. The available data include petrographic descriptions, whole-rock compositions, and major element mineral compositions for xenoliths from La Palma, Hierro, Gomera, Tenerife, Gran Canaria and Lanzarote (e.g. Sagredo Ruiz, 1969
; Muñoz, 1973; Muñoz & Sagredo, 1974; Amundsen, 1987; Johnsen, 1990; Neumann, 1991; Siena et al., 1991; Rolfsen, 1994; Neumann et al., 1995, 2002; Wulff-Pedersen et al., 1996). These papers conclude that the upper mantle beneath the Canary Islands consists of oceanic lithospheric mantle later metasomatized during the Canary Islands intraplate event. Trace element data on minerals have been presented only for veined xenoliths from La Palma (Vannucci et al., 1998; Wulff-Pedersen et al., 1999) and for xenoliths from Tenerife (Neumann et al., 2002). It is the aim of this paper to expand the database on the Canary Islands with laser ablation trace element data for minerals from mantle xenoliths from La Palma, Hierro and Lanzarote, as well as new whole-rock trace element data for Hierro and Lanzarote. We also present laser ablation Sr isotope data on clinopyroxenes from several samples. Tables comprising all whole-rock and mineral data on mantle xenoliths from the Canary Islands published by E.-R. Neumann and coworkers, together with some unpublished data, are available as Electronic Appendices, which may be downloaded from the Journal of Petrology web site at http://www.petrology.oupjournals.org/. The expanded dataset is used to (1) establish processes in the lithospheric mantle that are caused by the Canarian intraplate event, (2) establish variations in the intensity of different mantle processes along the island chain, (3) discuss the possible causal mechanisms of these variations, and (4) test the hypothesis of a mantle plume in the area at the time of opening. |
GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The Canary Islands are situated close to the continental margin of NW Africa (Fig. 1). Magnetic anomalies M22–M25 (145–148 Ma) have been traced towards the westernmost islands (La Palma, Hierro), which thus clearly rest on oceanic crust (e.g. Verhoef et al., 1991
; Roest et al., 1992, and references therein). The eastern islands, Lanzarote and Fuerteventura, are located on thicker lithosphere in the Jurassic magnetic quiet zone (e.g. Dash & Bosshard, 1968; Hayes & Rabinowitz, 1975; Banda et al., 1981; Weigel et al., 1982; Verhoef et al., 1991; Araña et al., 1993). It has been debated whether this part of the lithosphere represents thickened oceanic crust, or a Palaeozoic–Precambrian continental basement (e.g. Rothe & Schmincke, 1968; Dietz & Sproll, 1970; Goldflam et al., 1980; Robertson & Bernoulli, 1982; Roeser, 1982; Araña & Ortiz, 1991; Verhoef et al., 1991). However, the presence of magnetic anomaly S1, located between the easternmost Canary Islands (Lanzarote and Fuerteventura) and the coast of Africa (Roeser, 1982; Verhoef et al., 1991; Roest et al., 1992) (Fig. 1), and the oceanic nature of the gabbroic and ultramafic xenoliths exhumed by the Lanzarote basalts (Siena et al., 1991; Neumann et al., 1995, 2000; Schmincke et al., 1998) imply a (relatively) sharp ocean–continent transition east of the Canary Islands.
Magmatism in the Canary Islands is generally divided into two main stages, an older shield-building stage, which, after a period of quiescence and erosion, was followed by a younger period of activity leading to voluminous volcanic sequences in some of the islands (e.g. Teide in Tenerife), as well as numerous cinder cones (e.g. Schmincke, 1982). The magmatic activity is dominated by basaltic lavas, but includes felsic (trachytic to phonolitic) magmas. The mafic magmatism ranges from hypersthene-normative tholeiitic basalts to strongly silica-undersaturated nephelinites, but is dominated by TiO2-rich alkali basalts (e.g. Schmincke, 1982). In Fuerteventura the oldest volcanic complex is intruded by a series of rock types that include carbonatites and ijolites (e.g. Le Bas et al., 1986; Balogh et al., 1999). There is a westwards decrease in age of the oldest exposed lavas of the shield-building stage from >20 Ma in the easternmost Canary Islands (Lanzarote and Fuerteventura) to 1·1 Ma in the western islands (La Palma and Hierro) (Abdel-Monem et al., 1971, 1972; Schmincke, 1982; Balogh et al., 1999; Fig. 1). Eruptions have taken place in historical time in La Palma, Tenerife and Lanzarote (e.g. Carracedo & Day, 2002). Mantle xenoliths are mainly found in alkali basaltic lavas and dykes belonging to the younger period of magmatic activity.
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ANALYTICAL TECHNIQUES |
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Modal compositions were determined by point counting; between 2000 and 4500 points were counted in each thin section. For some of the most coarse-grained samples point counting was performed in two thin sections.
Minerals were analysed for major elements using an automatic wavelength-dispersive CAMECA Sx100 electron microprobe fitted with a LINK energy dispersive system at the Mineralogical–Geological Museum, University of Oslo. An acceleration voltage of 15 keV, sample currents of 20 nA for Na-poor (ol, px, sp) and 10 nA for Na-rich phases (plag), and counting times of 100 s were used. Oxides and natural and synthetic minerals were used as standards. Matrix corrections were performed by the PAP-procedure in the CAMECA software. Analytical precision (2
error) evaluated by repeated analyses of individual grains is better than ±1% for elements in concentrations of >20 wt % oxide, better than ±2% for elements in the range 10–20 wt % oxide, better than 5% for elements in the range 2–10 wt % oxide, and better than 10% for elements in the range 0·5–2 wt % oxide.
Trace element data on minerals were obtained with the laser ablation microprobe (LAM) housed in the Geochemical Analysis Unit, GEMOC Key Centre, Macquarie University. The LAM used in this study is a custom-built UV (266 nm) laser microprobe coupled to an Agilent 7500 s inductively coupled plasma-mass spectrometry (ICPMS) system. A detailed description of the laser system has been given by Norman et al. (1996). The laser was operated at a repetition rate of 10 Hz and typical energy of 0·5–1 mJ per pulse, allowing data collection from individual grains in polished thick sections (100 µm) for at least 100 s. All analyses were carried out using Ar as the carrier gas with a flow rate of 
1·5 l/min. The Agilent 7500 s was operated without the shield torch option and a forward power of 1350 W, and tuned to give oxide production <0·5% (measured as Th:ThO). Data collection was monitored in time-resolved format and the data were processed on-line using GLITTER, a data reduction software package developed at GEMOC (www.es.mq.edu.au/GEMOC). The time-resolved signals were selectively integrated to ensure processing of the most representative portion of the ablation signal. This procedure is important as it allows anomalies in the signal to be assessed and interpreted using analytical and mineralogical criteria. Calibration was based on the NIST 610 trace element glass standard with reference values from Norman et al. (1996). Calcium was used as the internal standard for quantification of clinopyroxene analyses, magnesium for olivine and orthopyroxene. The calibration protocol involves standardization at the beginning, middle and end of each analytical run to correct for instrumental drift during the run. During each run BCR2G was analysed as an unknown. The accuracy and reproducibility of the analyses were given by Neumann et al. (2002). The new trace element data on minerals are presented in Tables 1–3. The figures and discussion include previously published trace element data obtained by ion probe (Vannucci et al., 1998; Wulff-Pedersen et al., 1999) and LAM (Neumann et al., 2002).
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87Sr/86Sr ratios were measured on a Nu Plasma (UK) laser ablation ICPMS microprobe (LAM-ICPMS) instrument in the GEMOC Geochemical Analysis Unit, Macquarie University. Masses 83, 84, 85, 86, 87, 88 are measured simultaneously in Faraday collectors and all measurements are made in static mode. Corrections for the mass fractionation of Sr and Rb isotope ratios are made using an exponential law, with a normalizing value for 86Sr/88Sr = 0·1194. Any interference of 87Rb on 87Sr is corrected by measuring the intensity of the interference-free isotope 85Rb and using a 85Rb/87Rb value of 0·38632. This value was obtained by doping the QCD Analysts Sr standard with Rb (Plasmachem Lot No. S4JS3700) and making repeated measurements to refine the value of 85Rb/87Rb necessary to give the true 87Sr/86Sr. The maximum 87Rb/86Sr ratio of the spiked solutions used in the determination of the 85Rb/87Rb ratio was 0·3977. Although 83Kr was monitored, the need to correct for 86Kr interference on 86Sr was eliminated by measuring the background on peak and thus removing the gas blank from the signal. Repeated solution analysis of the NBS987 standard using these techniques gave a value for 87Sr/86Sr of 0·710263 ± 0·000038 (2 SD; n = 71). Laser ablation was performed using a Merchantek/New Wave LUV213 nm microprobe, based on a Lambda Fysik laser. Ablations were carried out at 4 Hz, using typical laser power of 1–2 mJ/pulse. These conditions typically yielded total Sr signals of (2–4) x 10–11 A. All ablations were carried out in He carrier gas, which is mixed with Ar before introduction to the ICP torch. The typical spot size was c. 80–100 µm; the large size was required by the low Sr contents of the pyroxenes in the peridotites. All analyses were carried out using the Nu Plasma's time-resolved analysis mode, in which the signal for each mass is monitored as a function of time. This allows the immediate identification of areas of anomalous elemental composition (i.e. high Rb) or anomalous isotopic composition. After analysis the software allows selection of the portions of the signal to avoid such anomalies; the integrated time interval is divided automatically into 40 replicates for the calculation of standard errors. To monitor the accuracy and precision of the laser microprobe analysis, we analysed, under similar analytical conditions, a series of natural minerals with Sr contents ranging from 1700 to 7800 ppm, and a synthetic fluorite with c. 190 ppm Sr, all of which had been analysed by standard thermal ionization mass spectrometry (TIMS) procedures (Table 4). The Batbjerg clinopyroxene standard was run seven times with the samples, and Sr contents were estimated by comparison of signal sizes with this standard.
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For whole-rock analyses pieces of the central parts of xenoliths were cut out and crushed by hand in steel mortars. Major elements were analysed on fused Li-tetraborate pellets, minor elements (Ti, K, P) on pressed powder pellets. The analyses were performed on a Philips PW 1400 X-ray fluorescence spectrometer at the Department of Biology and Geology, University of Tromsø, and the Institute of Geology, University of Oslo. Whole-rock trace element concentrations (Table 5) were obtained by ICPMS at ACTLABS, Ancaster, Ontario, Canada (La Palma) and at the GEMOC Key Centre, Macquarie University, Sydney, Australia (Hierro and Lanzarote). In addition, a number of samples from La Palma were analysed by epithermal instrumental neutron activation analysis (INAA) at the Mineralogical–Geological Museum, University of Oslo, using the method described by Brunfelt & Steinnes (1969)
. The international rock standards BCR-1, BHVO-1 and G-2 were used for calibration [using standard values recommended by Govindaraju (1989)], and included as unknowns in each run. The data are presented in Table 5.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The xenolith collections from each of the Canary Islands show clear similarities. With the exception of La Gomera, all the islands are dominated by Cr–Mg series spinel harzburgites (Fo89–93; Fig. 2). Spinel lherzolites are rare, but more common among xenoliths retrieved from Tenerife than from the other islands. Cr–Mg series spinel dunite is the second most common rock type, whereas Cr–Mg series wehrlite is relatively rare. The xenolith collection includes rare Ti–Al series harzburgites and lherzolites with Fe-rich olivine (Fo83–85), and relatively Ti–Al-rich clinopyroxene and spinel. These xenoliths, which are relatively small, exhibit a mixture between porphyroclastic and magmatic textures and have probably reacted with the host magma during transport to the surface. Ti–Al series dunites, wehrlites and clinopyroxenites (Fo
86) are common in Hierro and Gomera, but rare in the other islands (Figs 2 and 3). Wehrlites and clinopyroxenites sometimes occur as veins and veinlets cutting harzburgites and xenoliths. A summary of petrographic descriptions (based on data by Hansteen et al., 1991; Neumann, 1991; Frezzotti et al., 1994, 2002a, 2002b; Andersen et al., 1995; Neumann et al., 1995, 2000, 2002; Wulff-Pedersen et al., 1996, 1999; E.-R. Neumann, unpublished data, 2002), is given below.
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Spinel harzburgites and lherzolites
With few exceptions the olivine–orthopyroxene–clinopyroxene relationships of the Cr–Mg series xenoliths from the Canary Islands are similar to those found in North Atlantic spinel peridotites (based on data from Dick et al., 1984
; Michael & Bonatti, 1985; Juteau et al., 1990; Komor et al., 1990). Most harzburgites and lherzolites have porphyroclastic to protogranular textures, and exhibit two generations of mineral growth. There are, however, textural differences, which have caused us to divide the harzburgites and lherzolites into three main groups.
The majority of the harzburgites (and a few lherzolites) belong to a group referred to as HEXO (harzburgites with exsolved orthopyroxene). This group contains deformed porphyroclasts of olivine (Fo89·7–92·5), and orthopyroxene with exsolution lamellae of spinel or clinopyroxene. Occasionally the porphyroclast assemblage includes large, rounded grains of spinel, commonly with spongy rims with inclusions dominated by silicate glass. Strong deformation is reflected in undulatory extinction in olivine and orthopyroxene, and bent and broken exsolution lamellae in orthopyroxene. A second generation of grains is represented by mildly deformed to undeformed neoblasts of olivine, orthopyroxene, Cr-diopside and chromite, which partly occur in polygonal clusters, partly as irregular, interstitial grains. Cr-diopside most commonly occurs along the boundaries of, and as irregular inclusions in, orthopyroxene porphyroclasts, but occasionally it forms interstitial grains enclosing vermicular chromite. Chromite commonly forms vermicular inclusions in, or intergrowths with, Cr-diopside. A high proportion of the samples collected in La Palma and Tenerife contain phlogopite, generally in trace amounts as parts of polyphase inclusions in olivine and orthopyroxene porphyroclasts, but interstitial phlogopite neoblasts are occasionally seen. In xenoliths from Hierro and Lanzarote phlogopite is very rare, but Sagredo Ruiz (1969) has reported spinel harzburgite with up to 9 vol. % phlogopite from Lanzarote. Rare phlogopite-bearing harzburgites are also found in Gran Canaria (Amundsen, 1987; Fig. 2). To differentiate between the islands where phlogopite is common and those in which it is rare, we will refer below to the two groups as hydrous and ‘dry’, respectively. The HEXO group was interpreted by Neumann et al. (2002) as the least metasomatized type of mantle rocks in the Canary Islands.
Another xenolith group, referred to as HLCO (harzburgites and lherzolites containing only ‘clear’ orthopyroxene; that is, without visible exsolution lamellae) consists of spinel lherzolites and harzburgites from Tenerife with poikilitic textures (Neumann et al., 2002). These xenoliths are characterized by large, poikilitic, ‘clear’ orthopyroxene grains (6 mm in diameter), enclosing numerous rounded to irregular grains of olivine and Cr-diopside (<0·5 mm in diameter), clusters of irregular to vermicular chromite, and single, rounded to equant chromite grains. The ‘clear’ orthopyroxene shows minor or no indications of strain, whereas coexisting olivine porphyroclasts (Fo89·9–90·3) are strongly strained, as in the HEXO group. Also Cr-diopside commonly forms poikilitic grains (2 mm in diameter) that enclose rounded neoblasts and blebs of olivine, blebs or irregular grains of orthopyroxene, and irregular to vermicular chromite. Cr-diopside is also present in clusters of neoblasts (cpx ± ol) enclosed by ‘clear’ orthopyroxene. Olivine neoblasts, particularly olivine blebs enclosed by poikilitic orthopyroxene, may contain linear rows of minute spinel inclusions. In Tenerife, all lherzolites belong to the HLCO group. Small, poikilitic clinopyroxenes that resemble those in HLCO xenoliths from Tenerife are occasionally seen in xenoliths from the other islands. The HLCO group was interpreted by Neumann et al. (2002) as highly metasomatized peridotites.
Some harzburgites contain both exsolved orthopyroxene porphyroclasts and poikilitic orthopyroxene. These are termed HTR (transitional harzburgite). In these samples the exsolution-free domains in some exsolved orthopyroxene porphyroclasts appear to have expanded into large, clear domains of orthopyroxene enclosing rounded inclusions of olivine + Cr-diopside. The HTR group is moderately metasomatized (Neumann et al., 2002).
The rare Ti–Al series spinel harzburgites and lherzolites show ‘mixed’ textures, which include both porphyroclastic/protogranular and magmatic elements; sometimes the magmatic elements are concentrated along narrow zones that may represent veinlets.
Dunites
Cr–Mg-series dunite xenoliths (Fo87–92; Figs 2 and 3) have been sampled in all the islands. They exhibit porphyroclastic to granoblastic textures, and consist of moderately to highly strained olivine together with interstitial Cr-diopside and chromite. Minor amounts of orthopyroxene are present in some samples. Plagioclase is occasionally observed together with spinel in samples from Lanzarote. Phlogopite is a common accessory mineral (generally <<1 vol. %) in dunites from La Palma and Tenerife (Table 1).
Ti–Al-series dunites (Fo76–86) have equigranular textures, but domains exhibiting magmatic textures such as poikilitic clinopyroxene and spinel are common. The olivine is mildly strained to unstrained, and is accompanied by augitic clinopyroxene and Ti–Fe3+-rich spinel; some rocks contain titanomagnetite and/or magnesian ilmenite.
Wehrlites
The Cr–Mg series spinel wehrlite xenoliths from the Canary Islands have similar textures to the Cr–Mg series dunites, but differ from those by somewhat higher modal proportions of Cr-diopside, the presence of rare phlogopite in wehrlites from Tenerife, and the common presence of kaersutite in wehrlites from La Palma. The mineral chemistry is similar to that in the Cr–Mg-series dunites. Ti–Al series wehrlites have textural characteristics and mineral chemistry similar to Ti–Al dunites. Ti–Al series wehrlites from Hierro contain no hydrous minerals, but kaersutite is common in samples from La Palma.
Other xenolith types
All clinopyroxenite xenoliths (Fo70–80) collected by us belong to the Ti–Al series. In Gomera Ti–Al series wehrlite and clinopyroxenite commonly occur as veinlets crosscutting Ti–Al series dunite xenoliths (Rolfsen, 1994). Rare orthopyroxenite xenoliths have been reported from Gran Canaria (Amundsen, 1987), whereas rare olivine websterite xenoliths (Fo76–79; Ti–Al series) have been recovered in Hierro (Neumann, 1991).
Fluid inclusions
Three main types of fluid inclusions were identified in mineral phases in the Cr–Mg series spinel harzburgite and lherzolite xenoliths from Tenerife: (1) pure (or nearly pure) CO2; (2) carbonate-rich CO2–SO2 mixtures; (3) polyphase inclusions dominated by silicate glass ± spinel ± clinopyroxene ± phlogopite ± sulphide ± carbonates (magnesite and dolomite) ± CO2 (Frezzotti et al., 2002a; Neumann et al., 2002). CO2 and CO2-bearing polyphase inclusions rich in silicate glass are also present in spinel harzburgites and lherzolites from the other Canary Islands (Hansteen et al., 1991; Neumann et al., 1995; Wulff-Pedersen et al., 1996, 1999; Neumann & Wulff-Pedersen, 1997). Inclusion types (1) and (3) are common in all the islands. Silicate glass in inclusions shows a wide range in compositions, with 45–71 wt % SiO2 in spinel harzburgites and lherzolites, and 46–65 wt % SiO2 in dunites and wehrlites (Neumann & Wulff-Pedersen, 1997). In xenoliths from Tenerife CO2 inclusions commonly exhibit a ‘coating’ a few millimetres thick on the inclusion wall, consisting of an aggregate of a platy, hydrous Si–Mg–Fe phase, probably talc, together with very small amounts of halite, dolomite and other phases. Larger crystals [e.g. (Na,K)Cl, dolomite, spinel, sulphide and phlogopite] may be found between the ‘coating’ and the inclusion wall, or towards the inclusion centre. Fluid inclusions are particularly common as secondary trails in olivine porphyroclasts and in exsolved orthopyroxene porphyroclasts (HEXO xenoliths). Exsolved orthopyroxene porphyroclasts (HEXO xenoliths) commonly have a mottled appearance as a result of the presence of abundant, randomly distributed, irregular shaped inclusions consisting of silicate glass ± olivine ± clinopyroxene ± spinel ± vapor. The exsolved orthopyroxene porphyroclasts locally exhibit domains without visible exsolution lamellae associated with fluid inclusion trails. These domains may contain rounded blebs or neoblasts of olivine and clinopyroxene. Fluid or glass inclusions are very rare in poikilitic orthopyroxene and clinopyroxene in HLCO xenoliths. Olivine neoblasts commonly contain scattered fluid (and solid) inclusions, or concentric sets of inclusions, that are interpreted as primary inclusions trapped during crystal growth. The various fluids present in fluid inclusions in Tenerife are interpreted as the result of immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle (Frezzotti et al., 2002a; Neumann et al., 2002). In addition to the inclusion types described above, harzburgite and lherzolite xenoliths from Hierro contain inclusions of devitrified ultramafic glass and polyphase inclusions consisting of basaltic glass + spinel + clinopyroxene ± sulphide ± CO2, interpreted as trapped ultramafic and basaltic melts (Hansteen et al., 1991).
The Cr–Mg series dunites and wehrlites contain the same types of fluid inclusions as the harzburgites and lherzolites from the same islands, but the inclusions are generally smaller and less common than in harzburgites and lherzolites. In addition, spinel dunites from Lanzarote contain mixed N2–CO2 inclusions, ranging from pure N2 to pure CO2 (Andersen et al., 1995).
Ti–Al series dunites, wehrlites and clinopyroxenites from Gomera contain two types of inclusions (Frezzotti et al., 1994, 2002b). Primary silicate glass + CO2 inclusions containing Cr-spinel and clinopyroxene daughter minerals compositionally similar to those in the dunite are interpreted as remnants of the magma from which the dunites formed. Secondary silicate glass inclusions, mixed silicate glass + carbonate (Mg-rich calcite or dolomite) inclusions and CO2 inclusions occur together along a network of late veinlets. These inclusions are believed to represent trapping of a homogeneous, volatile-rich, CO2-saturated melt that was present in the upper mantle during the Canary Islands volcanism (Frezzotti et al., 1994, 2002b). Silicate glass + CO2 inclusions are also present in Ti–Al series xenoliths from Hierro (Hansteen et al., 1991). In both Cr–Mg series and Ti–Al series xenoliths from Hierro the densest CO2 inclusions have molar volumes of c. 39 cm3/mol, which corresponds to a pressure of 1·2 GPa at c. 1000°C (Hansteen et al., 1991).
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MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Spinel harzburgites and lherzolites
Olivine
Systematic differences in mineral chemistry are observed between the Canary Islands. Olivine porphyroclasts in Cr–Mg series spinel harzburgite and lherzolite xenoliths from Hierro and Lanzarote show somewhat higher Fo contents (Hierro: Fo90·9–92·5; Lanzarote: Fo89·7–92·1) than North Atlantic peridotites, whereas those from La Palma and Tenerife are similar to slightly lower (La Palma: Fo89·8–91·2; Tenerife: Fo89·0–91·2; Fig. 3; data from Neumann, 1991
; Neumann et al., 1995, 2002; Wulff-Pedersen et al., 1999). Olivine in Ti–Al series lherzolites and harzburgites (Hierro and Tenerife) falls in the range Fo83–85.
In Cr–Mg series harzburgites and lherzolites olivines with different Fo contents are characterized by different trace element compositions. The highly magnesian olivine porphyroclasts in harzburgite xenoliths from Lanzarote tend to be enriched in Sc and Co, and depleted in Cr, compared with xenoliths from the other islands (Table 1). The olivine is characterized by concave-upwards PM-normalized trace element patterns [where PM is primordial mantle, as defined by McDonough & Sun (1995); Fig. 4] with similar or higher enrichment factors (concentration/PM) for large-ion and high-valency elements than for heavy rare earth elements (HREE). Olivines from La Palma, Hierro and Lanzarote are depleted in REE, V and Cr relative to those from Tenerife; the former commonly have REE concentrations below the detection limit. Among the xenoliths from Tenerife olivine shows no overall differences in trace element concentrations, except for Ti and Al, which are significantly lower in the HEXO than in the HLCO and HTR samples (Table 1, Fig. 4). Viti & Frezzotti (2000) showed that olivines in harzburgite xenoliths from Tenerife and Hierro are rich in submicroscopic fluid ± glass inclusions. It is, therefore, likely that, although we did our best to avoid fluid inclusions while analysing olivines, the high, and highly variable, concentrations in Rb–Pr relative to middle REE (MREE) in olivine (Fig. 4) are due to the presence of glass inclusions.
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Orthopyroxene
Orthopyroxene in Cr–Mg series harzburgites and lherzolites has mg-number [cation ratio Mg x 100/(Mg + Fetotal)] in the range 89–92. Most samples have <0·10 wt % TiO2, but a few samples from Hierro and Tenerife have higher concentrations (
0·32 wt %). The xenoliths are also generally depleted in Al2O3 (3·5 wt %; Fig. 5), but there are significant differences associated with textural type. Orthopyroxene porphyroclasts with exsolution lamellae tend to have higher Al contents than grains and domains without visible exsolution lamellae (Fig. 5). Very low Al2O3 contents (<1 wt %) are common in xenoliths from La Palma and Tenerife (HLCO) but rare in xenoliths from Hierro and Lanzarote (Fig. 5). Orthopyroxenes in Ti–Al series harzburgites and lherzolites have lower mg-number (84–87; Fig. 5; Table 2), and tend towards higher TiO2 than those in Cr–Mg series rocks.
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Also, the trace element compositions of orthopyroxenes in Cr–Mg series xenoliths fall in two distinct groups (Table 2). In all the islands the exsolved orthopyroxenes have very low enrichment factors for the MREE and HREE (Sm 0·01–0·05; Lu 0·2–0·4), and positive Ti-anomalies (Fig. 6). Like the olivines (Fig. 4), the orthopyroxenes show a wide scatter in enrichment factors, and a tendency for higher enrichment factors for the most strongly incompatible elements than for MREE (Fig. 6a–c). In general, poikilitic orthopyroxene in HLCO and HTR xenoliths from Tenerife are less depleted in MREE and HREE than exsolved orthopyroxene porphyroclasts (Sm 0·2–0·5; Lu 0·4–0·9; Fig. 6), they are depleted in Sr and Zr–Hf relative to MREE, and have similar or lower enrichment factors for Sc and Cr to the HREE. However, highly depleted domains have also been analysed in poikilitic orthopyroxene in TF14-36 (HLCO). It should be noted that unlike the exsolved orthopyroxenes, the ‘clear’ ones in the HLCO xenoliths show a general trend of decreasing enrichment factors from Lu to Rb. Orthopyroxenes in xenoliths from La Palma have intermediate trace element patterns. The less depleted trace element patterns (La Palma and Tenerife) are thus associated with the lowest Al2O3 contents. Neumann et al. (2002)
observed that exsolved orthopyroxenes in HEXO xenoliths from Tenerife are so rich in densely spaced silicate glass inclusions that it is virtually impossible to obtain analyses of pure orthopyroxene. This is also true for exsolved orthopyroxene in xenoliths from the other islands. The observed trace element patterns for exsolved orthopyroxene therefore probably reflect a combination of depleted orthopyroxene and enriched silicate glass inclusions.
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Clinopyroxene
Clinopyroxenes in Cr–Mg series harzburgites and lherzolites are Cr-diopsides, most of which are depleted in Al2O3, and enriched in Cr2O3 and Na2O (Canary Islands: 0·1–4·5 wt % Al2O3, 0·2–3·9 wt % Cr2O3, 0·2–2·5 wt % Na2O; data from Neumann, 1991
; Neumann et al., 1995, 2002; Wulff-Pedersen et al., 1996; Fig. 7) relative to clinopyroxenes in depleted MORB-source mantle (DMM: 2·6–8·1 wt % Al2O3; 0·7–1·9 wt % Cr2O3; <0·1–1·5 wt % Na2O; data from Johnson et al., 1990; Bonatti et al., 1992; Johnson & Dick, 1992). The highest concentrations in Cr2O3 and Na2O are found in HLCO xenoliths from Tenerife; in general, high Cr2O3 and Na2O contents are coupled with low Al2O3 and TiO2 contents. The Cr–Mg series harzburgites and lherzolites in each island show clear trends of increasing Al2O3, Cr2O3 and Na2O with decreasing mg-number. The Ti–Al series harzburgites and lherzolites plot between the field of Cr–Mg series harzburgites and that of Tenerife basalts (Fig. 7; Table 3).
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The various textural groups also show contrasting trace element characteristics (Table 3; Fig. 8). Because of the small size of clinopyroxenes in the xenoliths from Hierro and Lanzarote few trace element analyses were obtained; however, the grains analysed are the most depleted ones that we found. These clinopyroxenes exhibit concave-upwards trace element patterns depleted in REE relative to PM (e.g. LaN 0·2–6; SmN 0·1–0·6), in MREE relative to Rb, Ba, Nb, Ta, light REE (LREE) and HREE ([Sm/Yb]N = 0·2–0·5), and have negative Zr- and Ti-anomalies. Clinopyroxenes in Tenerife xenoliths, in contrast, are highly enriched in REE (e.g. LaN [Lacpx/LaPM] = 6–275; SmN 1–74), enriched in LREE and MREE relative to HREE (e.g. [Sm/Yb]N = [Smcpx/SmPM]/[Ybcpx/YbPM] = 1·7–6·1), and strongly depleted in Sr, Zr–Hf and Ti relative to REE. The highest degree of enrichment is found in the HLCO xenoliths. Clinopyroxenes in xenoliths from La Palma show a range in trace element compositions from those similar to the highly enriched clinopyroxenes in xenoliths from Tenerife, to those with REE concentrations similar to PM (e.g. LaN 1–20; [Sm/Yb]N = 1–3·2). Also clinopyroxenes in xenoliths from La Palma are depleted in Zr and Ti relative to REE; the degree of depletion appears to increase with increasing REE content.
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It is important to note that whereas olivine appears to be homogeneous on the scale of a thin-section and is strongly depleted in LREE relative to HREE (Table 1; Fig. 4), the same is not true for the pyroxenes. In many samples orthopyroxene and clinopyroxene show a wide range in trace element compositions, including LREE/HREE ratios (different compositions are denoted I, II and III in Tables 2 and 3, and Figs 6 and 8), indicating disequilibria at the scale of a thin-section. Such variations are particularly common in xenoliths from La Palma and Tenerife.
Spinel
Spinel in Cr–Mg series harzburgites and lherzolites is Cr–Al-rich and Ti–Fe3+-poor, plotting slightly below the Al2–Cr2 line in Fig. 9; cr-number [= cation proportion Cr x 100/(Cr + Al)] is 39–93; Fig. 9; Table 4). The highest cr-numbers are found in pitted rims with numerous glass inclusions on large spinel grains, and, in some cases, in vermicular spinel. The pitted rims are believed to represent a late stage of heating, possibly during ascent. The TiO2 contents fall in the range <0·1–4·2 wt %, but are generally <1 wt %. The ratio Fe3+ x 100/(Al + Cr + Fe3+) is generally in the range 5–15. In general the lowest proportions of Ti and Fe3+ are found in harzburgites from Lanzarote, whereas the highest proportions of Ti, Cr and Fe3+ are found in HLCO xenoliths from Tenerife and in harzburgites/lherzolites from La Palma. Spinel in Ti–Al series harzburgites and lherzolites is significantly richer in Ti and Fe3+ than that in the Cr–Mg series rocks (Fig. 9).
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Other xenolith types
Olivine
Olivine in Cr–Mg dunites and wehrlites falls in the ranges Fo87·2–91·5 and Fo89·4–90·3, respectively, whereas olivines in the Ti–Al series dunites are Fo76–86, and in the wehrlites Fo76·5–83·4 (Fig. 3). Olivine has only been analysed for trace elements in one dunite and one wehrlite, both Cr–Mg series, from Tenerife (Table 1; Fig. 4). These have U-shaped PM-normalized trace element patterns, which essentially fall within the range of spinel harzburgites and lherzolites from the same island.
Pyroxenes
With respect to Ti and Al, orthopyroxene in Cr–Mg series dunites and wehrlites fall essentially within the range of harzburgites and lherzolites from the same island (Fig. 5). Orthopyroxenes in Ti–Al series rocks (harzburgites/lherzolites, dunites, wehrlites and clinopyroxenites) have lower mg-number, and, in Hierro xenoliths, define a trend of increasing TiO2 and Al2O3 with decreasing mg-number parallel to that defined by Cr–Mg series xenoliths. Orthopyroxene in Ti–Al series rocks has not been analysed for trace elements.
Also, clinopyroxenes in Cr–Mg series dunites and wehrlites fall essentially within the range of those in harzburgites and lherzolites from the same islands, but tend towards lower mg-number, Cr2O3 and Na2O, and higher TiO2 and Al2O3 (Fig. 7). Clinopyroxenes in Cr–Mg series dunites and wehrlites have only been analysed in xenoliths from Tenerife. These have similar trace element patterns that are parallel to, and fall within or close to, the range of patterns defined by clinopyroxenes in Tenerife harzburgites and lherzolites (Fig. 10).
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Clinopyroxenes in Ti–Al series dunites, wehrlites and clinopyroxenites, in contrast, have lower mg-number than those in the Cr–Mg series xenoliths and define trends of increasing TiO2 and Al2O3, and decreasing Cr2O3 and Na2O, with decreasing mg-number (Fig. 7). These trends are markedly different from the trends defined by Cr–Mg series harzburgites and lherzolites, but partly overlap with those of clinopyroxenes in basaltic rocks from Tenerife. Ti–Al series harzburgites/lherzolites and many Cr–Mg series dunites and wehrlites fall between the two trends. The trace element compositions of clinopyroxenes in the Ti–Al series dunites and wehrlites are closely similar to clinopyroxene phenocrysts in basaltic lavas from Tenerife, although the latter are slightly more enriched in MREE relative to HREE (Fig. 10). These clinopyroxenes show clear affinity to those in the basaltic lavas.
Spinel
Like spinel in Cr–Mg series harzburgites and lherzolites, those in Cr–Mg series dunites and wehrlites fall close to the Al2–Cr2 line in Fig. 9. However, they differ from those by tending towards lower cr-number ratios (32–92), and higher Ti and Fe3+ proportions (Fig. 9).
Spinel in Ti–Al series dunites, wehrlites and clinopyroxenites is enriched in Ti and Fe3+ relative to Cr and Al <1–20 wt % TiO2, 9–36 wt % Fe2O3 estimated on the basis of stoichiometry (Fig. 9).
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Sr ISOTOPES |
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LAM-ICPMS analyses of clinopyroxene in mantle rocks from the Canary Islands give a range in 87Sr/86Sr ratios (Table 4; Fig. 11), which is clearly related to petrographic type and degree of metasomatism. The lowest ratios were obtained for clinopyroxene in the HEXO group spinel harzburgites H1-12 and TF14-52, 0·70266 ± 0·00029 and 0·70276 ± 0·00017, respectively. The HLCO xenoliths TF14-38 and TF14-36 and the HTR sample TF14-58 gave somewhat higher ratios, 0·70288 ± 0·00039, 0·70310 ± 0·00017 and 0·70314 ± 0·00011, respectively, whereas the highest ratio, 0·703286 ± 0·000035, was obtained for the strongly trace element enriched amphibole wehrlite PAT2-36. The 87Sr/86Sr ratios obtained by point analyses of clinopyroxenes in HEXO rocks fall within the depleted part of the range covered by Mid-Atlantic Ridge basalts (MAR). The rest of the samples fall within the range of Canary Islands basalts (Fig. 11).
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WHOLE-ROCK CHEMISTRY |
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Spinel harzburgites and lherzolites
Most of the Cr–Mg series spinel harzburgite and lherzolite xenoliths from the Canary Islands fall within a restricted ranges of concentrations with respect to MgO (42–47 wt %), TiO2 (
0·2 wt %), Al2O3 (0·4–1·1 wt %), and CaO (0·4–2·4 wt %), have somewhat wider ranges in SiO2 (40–45 wt %) and FeOtotal (7·3–10·1 wt %), and a wide range in Na2O (<0·1–0·6 wt %; Table 5; Fig. 12). There are rough trends of increasing TiO2 and CaO with decreasing MgO, but no correlation between MgO and other major elements (Fig. 12). The major element compositions mostly reflect differences in modal compositions. The highest concentrations in MgO (44·7–47·0 wt %) and SiO2 (42·9–45·4 wt %), and lowest in TiO2 (0·03 wt %) and CaO (0·4–0·7 wt %) are shown by xenoliths from Lanzarote, which have very low modal contents of clinopyroxene (0·04 vol. %) and relatively high contents of orthopyroxene (12–35 vol. %). HLCO xenoliths from Tenerife with 1·5–9·4 vol. % clinopyroxene and 7–17 vol. % orthopyroxene have lower MgO (41·9–45·4 wt %) and SiO2 contents (40·9–42·8 wt %), and higher concentrations of TiO2 (0·05–0·2 wt %) and CaO (1·0–2·3 wt %). However, the significant differences in Na2O contents observed among the Cr–Mg series xenoliths appear to be independent of both MgO and mineral assemblage.
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The bulk-rock xenoliths include wide range in incompatible trace element concentrations. Xenoliths from La Palma, Hierro and Lanzarote have S-shaped PM-normalized trace element patterns with the highest enrichment factors for Th, U and Nb (ThN up to 8; Fig. 13), and the lowest for HREE (e.g. YbN 0·01–0·5). The strongest depletion in incompatible trace elements is found in clinopyroxene-poor xenoliths from Lanzarote (LA1-7, LA1-9, LA2-7, LA6-35, LA6-38, LA8-5: 0·0–0·4 vol. % clinopyroxene) which show enrichment factors <1·0 for all the incompatible trace elements (e.g. LaN 0·03–0·3; DyN 0·01–0·05; Fig. 13), weakly negative Zr–Hf-anomalies, no Ti-anomalies, and a tendency for positive Sr-anomalies. Strong depletion in MREE and HREE (DyN 0·02–0·06) is also exhibited by the dry, clinopyroxene-poor samples from La Palma (PAT2-29, 2-31, 2-68: 1·2–2·2 vol. % clinopyroxene; Fig. 13), but also these samples are enriched in LREE relative to HREE. The samples most strongly enriched in incompatible trace elements are the HLCO xenoliths from Tenerife (1·7–9·4 vol. % clinopyroxene; LaN 2–20; DyN 0·7–2·0). REE-enriched xenoliths show negative Zr–Hf- and Ti-anomalies, and the HLCO xenoliths also have negative Sr-anomalies. The xenolith suites from La Palma and Tenerife are enriched in Rb relative to Ba, whereas those from Hierro and Lanzarote are relatively depleted in Rb relative to Ba (Fig. 13).
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Dunites and wehrlites
The Cr–Mg series dunites and wehrlites show similar compositional characteristics to the harzburgites and lherzolites, e.g. strong depletion in TiO2 (<0·2 wt %) and Al2O3 (0·2–2·5 wt %) and a relatively wide range in Na2O (<0·1–0·4 wt %), and high concentrations in MgO (42–50 wt %; Fig. 12).
With respect to incompatible trace element compositions, the Cr–Mg series dunites and wehrlites show clear similarities to the harzburgites and lherzolites (Fig. 14). The most refractory compositions are exhibited by dunites from Lanzarote, whereas those from Tenerife are the most highly enriched. Like the coexisting harzburgites, the dunites and wehrlites show S-shaped trace element patterns. Many of the most depleted samples have positive Ti-anomalies, whereas the most enriched samples exhibit weak negative anomalies for Sr and Ti, and a tendency for depletion in Zr and Hf relative to LREE.
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The Ti–Al series rocks cover a wide range in MgO contents (10–43 wt %; only samples with MgO >15 wt % are shown in Fig. 12), and define trends of increasing TiO2, Al2O3, CaO and Na2O, and decreasing FeO with decreasing MgO. The Na2O–MgO diagram shows clearly that the Cr–Mg and the Ti–Al series rocks represent two separate groups. The Cr–Mg series rocks show strong variations in Na2O within a restricted range in MgO, whereas the Ti–Al series rocks show a trend of increasing Na2O from the Na-depleted part of the Cr–Mg series domain towards the domain of mafic aphyric lavas (e.g. basalts from Tenerife with >7 wt % MgO contain 2–3 wt % Na2O; Neumann et al., 1999
). The Ti–Al series rocks are enriched in LREE relative to HREE (Fig. 14). Both the concentrations in incompatible elements and the enrichment in LREE relative to HREE are considerably stronger in the wehrlites than the dunites, clearly because of their higher modal proportions of clinopyroxene. Both dunites and wehrlites show a tendency for negative Nb–Ta anomalies; the dunites also have positive Ti-anomalies.
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DISCUSSION |
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Partial melting and depletion processes
The observations and data presented above imply that the lithospheric mantle beneath the Canary Islands was strongly depleted before becoming metasomatized. Depletion is discussed here, metasomatism is discussed below.
Evidence of strong depletion includes low Al2O3 and high Cr2O3 in pyroxenes and spinel in Cr–Mg series xenoliths from the Canary Islands (Figs 5, 7 and 9). Partial melting experiments have shown that the concentrations of Al2O3 in residual spinel, orthopyroxene and clinopyroxene decrease, and that of Cr2O3 increases, with progressive partial melting of peridotite at upper-mantle pressures (e.g. Mysen & Kushiro, 1977; Jaques & Green, 1980). In spite of enrichment in LREE relative to MREE in harzburgite xenoliths from the Canary Islands, many of the xenoliths have concentrations in MREE and HREE that are 1–2 orders of magnitude below those found in average depleted MORB mantle (DMM; Fig. 13). Furthermore, in all the islands we have found highly refractory orthopyroxene porphyroclasts, which show decreasing enrichment factors from Lu towards the LREE (Fig. 6). We proposed above that the high, and highly variable concentrations in the most strongly incompatible elements (Rb–Pr) in olivine and exsolved orthopyroxene (Figs 4 and 6) are due to the common presence of small fluid inclusions dominated by silicate glass, implying that the true concentrations of the most strongly incompatible elements in the olivine and orthopyroxene lattices are significantly lower than the measured values. In xenoliths from Hierro and Lanzarote we have also found clinopyroxenes with refractory REE patterns (Fig. 8). We believe the most refractory rocks and minerals to represent the best preserved remnants of a strongly depleted oceanic mantle that existed in the area before metasomatism took place, and that their HREE and MREE characteristics are close to the original composition.
The low 87Sr/86Sr ratios exhibited by the HEXO samples and HLCO sample TF14-38 (0·7027–0·7029; Table 4, Fig. 11) are lower than any ratio obtained previously for mantle xenoliths from the Canary Islands (>0·7030; Vance et al., 1989; Rolfsen, 1994; Ovchinnikova et al., 1995; Whitehouse & Neumann, 1995; Neumann et al., 2002). All earlier analyses (performed on whole rocks and clinopyroxene separates), including HEXO samples, give 87Sr/86Sr and 143Nd/144Nd ratios that lie essentially within the range of Canary Islands basalts. A few samples with 87Sr/86Sr >0·704 are clearly contaminated. A problem with analyses of whole rocks and clinopyroxene separates is that all the mantle rocks have suffered some degree of metasomatism. This is demonstrated by their bulk-rock trace element patterns, which show enrichment in LREE (and other strongly incompatible elements) relative to HREE (Figs 13 and 14). Like the trace element data, the isotope data may easily be strongly influenced by metasomatic fluids with enriched Sr and Nd isotope compositions, trapped as fluid inclusions, many of which are too small to be removed by acid washing. An additional problem with mineral separates is that it is possible to obtain clinopyroxene separates only for those rocks that are richest in clinopyroxenes; these rocks generally also have clinopyroxenes that have formed as the results of interaction between metasomatic fluids and mantle wall-rock minerals (see discussion below). The isotope ratios obtained for clinopyroxene separates are therefore likely to reflect the metasomatic fluids rather than the pre-metasomatic isotope chemistry. The possibility of obtaining 87Sr/86Sr ratios by laser technique has allowed us to analyse clinopyroxene in spinel harzburgites that have been only mildly metasomatized and have such low modal proportions of clinopyroxene that it is impossible to obtain clinopyroxene separates. We have thus been able to ‘look behind’ the metasomatic processes towards the initial isotope composition of the oceanic lithospheric mantle. We interpret the obtained data to indicate that the upper mantle in the area formed as DMM-type oceanic mantle lithosphere with 87Sr/86Sr 0·7027.
To obtain additional information on the original composition of the lithospheric mantle beneath the Canary Islands, it is necessary to try to ‘strip away’ the effects of the later metasomatism. We assume that the oceanic lithospheric mantle represents parts of the convecting mantle that has ‘frozen’ to the base of the lithosphere, and that its composition is basically the result of repeated stages of partial melting at mid-ocean ridges. The original composition of the lithospheric mantle in the area of the Canary Islands, just after its formation 150–180 Myr ago, may thus best be assessed by comparing those xenolith data that appear to be least affected by the metasomatic processes with partial melting trends. We have chosen the Primordial Mantle as starting material because that is assumed to represent the original mantle composition. We have used two methods to estimate the degree of depletion in the upper mantle beneath the Canary Islands. The first method (Niu & Batiza, 1991; Niu, 1997) is based on whole-rock major element relationships, allowing continuous adjustment of the bulk distribution coefficients as the modal composition of the mantle residue changes in response to progressive partial melting. As starting material we used a primitive mantle composition given by Niu (1997), and bulk rock/melt distribution coefficients given by Niu & Batiza (1991) and Niu (1997). In Fig. 12 the estimated major element compositions of mantle residues formed by up to 25% batch and fractional melting at 1 GPa are compared with the whole-rock compositions of the xenoliths from the Canary Islands. With the exception of Al2O3 and CaO, the Canary Islands xenoliths show wide ranges in major elements, which are independent of variations in MgO (Fig. 9). In general, xenoliths from La Palma and Tenerife that are enriched in highly incompatible elements plot away from the partial melting trends, whereas the highly refractory xenoliths from Hierro and Lanzarote plot close to these trends. The compositions of the latter correspond to primordial mantle that has undergone c. 18–25% partial melting (Fig. 9). We interpret the high contents of TiO2, FeO and Na2O, and low contents of SiO2 relative to the partial melting trends, exhibited by many xenoliths, to reflect the metasomatic processes (addition of Ti, Fe and Na, and depletion in Si).
We have also compared the whole-rock trace element compositions of Canarian mantle xenoliths with estimated partial melting trends based on whole-rock trace element relationships (Fig. 15). The primordial mantle (PM) of McDonough & Sun (1995) was chosen as starting material, together with mineral/melt partition coefficient recommended by Nielsen et al. (1992), Beattie (1994), Salters & Longhi (1999) and Green et al. (2000). We have used the method of Niu (1997) and Niu & Batiza (1991) to change the bulk distribution coefficient in harmony with changes in modal composition in mantle undergoing progressive partial melting. The results for batch melting and fractional melting are indistinguishable. With the exception of HLCO xenoliths from Tenerife, the Yb–Y relationships of the analysed xenoliths fall close to the estimated partial melting trends. The most highly refractory xenoliths from La Palma and Lanzarote fall close to the area of 30–32% partial melting. The HLCO xenoliths from Tenerife fall to the enriched side of PM in all the diagrams, clearly reflecting the metasomatic processes that have affected these xenoliths. As far as we can tell on the basis of our data, after initial formation at the opening of the central Atlantic Ocean, and before the onset of the Canary Islands magmatism, the degree of depletion in the lithospheric mantle was relatively uniform from west to east, with strongly REE-depleted orthopyroxenes found in xenoliths from both Lanzarote and Hierro (Fig. 6).
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We conclude that the original composition of the lithospheric mantle beneath the Canary Islands corresponds to Primordial Mantle that has been subjected to (at least) 25–30% depletion. Experimental partial melting of primordial mantle indicates that, at upper-mantle pressures, >25% depletion leaves a residue consisting of olivine + orthopyroxene + spinel (e.g. Jaques & Green, 1980
; Johnson et al., 1990; Kostopoulos, 1991; Elthon, 1993; Niu & Hékinian, 1997). Spinel harzburgite and lherzolite xenoliths from La Palma, Hierro and Tenerife contain a few volume percent clinopyroxene, but show no correlation between the modal proportions of clinopyroxene and olivine, as is expected by partial melting trends (Fig. 16). In xenoliths from Lanzarote, in contrast, most xenoliths with >70 vol. % olivine contain <1% clinopyroxene (mainly occurring along the boundaries of orthopyroxene porphyroclasts), whereas the least olivine-rich samples have higher clinopyroxene contents. Neumann et al. (1995) interpreted the small amounts of clinopyroxene in xenoliths with >70% olivine in Lanzarote xenoliths as the result of exsolution from orthopyroxene and subsequent recrystallization. We support this view, and regard the higher proportions of clinopyroxene in xenoliths from the other islands (Fig. 16) as the result of formation of clinopyroxene through metasomatic reactions. In general, the spinel harzburgites from Lanzarote appear to be least affected by metasomatic processes. We therefore believe that, with the exception of Na2O, the average major element composition of spinel harzburgites from Lanzarote is close to the original composition of the mantle lithosphere beneath all the Canary Islands.
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Evidence of metasomatism
As indicated above, spinel harzburgite and lherzolite xenoliths from the Canary Islands exhibit considerable evidence of metasomatic processes. The Cr–Mg series rocks are characterized by enrichment in strongly incompatible trace elements (including LREE and MREE) relative to HREE (Fig. 13), and high contents of TiO2, FeOtotal and Na2O relative to the partial melting trends (Fig. 12). Most clinopyroxenes in Cr–Mg series xenoliths from La Palma and Tenerife are highly enriched in REE, and in LREE relative to HREE (Fig. 8), and have 87Sr/86Sr ratios that are significantly higher than those obtained for clinopyroxenes in the least metasomatized samples (Fig. 11). Even clinopyroxene in xenoliths from Hierro and Lanzarote that are generally depleted in REE are enriched in Th, U, Nb, Ta and LREE relative to MREE (e.g. Fig. 8). Metasomatism is also reflected in the presence of phlogopite in many samples, particularly among xenoliths from La Palma and Tenerife. Finally, xenoliths from Tenerife and La Palma show a wide range in CaO/Al2O3 ratios (1·1–3·2; Neumann et al., 2002
); the lowest ratios are found among the HEXO xenoliths and the highest ratios among the HLCO xenoliths. This strongly suggests that the metasomatic processes involve addition of CaO, which is in agreement with the interpretation of Boyd (1996) that CaO/Al2O3 >> 1·0 is the result of ‘Ca-metasomatism’. Reactions between the metasomatic agent(s) and wall-rock minerals appear to have led to the formation of olivine and clinopyroxene at the expense of orthopyroxene, formation of large poikilitic clinopyroxene grains, and recrystallization of orthopyroxene porphyroclasts to large poikilitic orthopyroxene grains. The range in trace element compositions for single phases found within many of the samples (Figs 4 and 6) implies that equilibrium with respect to trace elements has not been reached. These features show that the upper mantle beneath all the Canary Islands has been subjected to metasomatic processes, but to different extents and of somewhat different style in each island. The strongest degree of metasomatism among the Cr–Mg series rocks is seen in xenoliths from Tenerife that show a combination of cryptic, modal and Ca-metasomatism, and slightly lower mg-number than those from Hierro and Lanzarote. The xenoliths from La Palma show the same types of metasomatism as xenoliths from Tenerife, but to a lesser extent. The most obvious evidence of metasomatism in xenoliths from Hierro and Lanzarote is cryptic metasomatism, and rare occurrences of phlogopite indicate that also limited modal metasomatism has taken place. Xenoliths from Hierro show a complex picture compared with those from the other islands, being relatively Mg-rich (Figs 3 and 12), and having pyroxenes strongly depleted in MREE and HREE (Figs 6 and 8) combined with significantly more enriched whole-rock trace element patterns than many xenoliths from La Palma (Fig. 13). This suggests that a large proportion of the incompatible trace elements in the Hierro xenoliths are located in glass inclusions and in interstitial glass. One possible explanation is that there has not been enough time for the minerals to equilibrate with these melts. Xenoliths from Lanzarote consistently show the weakest degree of metasomatism of the islands involved in this study.
Metasomatic agents
To obtain further insight into the metasomatic processes we will first try to establish the types of fluids that have been involved; afterwards we will discuss the various processes that have been in operation, including the genetic relationship between the fluids.
To throw light on the types of melts that have caused metasomatism in the upper mantle beneath the Canary Islands we have estimated the trace element compositions of melts in equilibrium with clinopyroxenes of different trace element compositions, using partitioning coefficients for clinopyroxene/carbonatitic melt [Adam & Green (2001); and Klemme et al. (1995) for V and Ni], clinopyroxene/basaltic melt (Hart & Dunn, 1993; Foley et al., 1996), and clinopyroxene/Si-rich melts (SiO2 >60 wt %; Ionov et al., 1994; Chazot et al., 1996). Those results that most closely resemble known melt compositions (carbonatitic melts, basaltic rocks from the Canary Islands, silicic melt inclusions in Canarian mantle xenoliths) are presented in Fig. 17.
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Available data imply that the upper mantle beneath Tenerife was metasomatized by fluids or melts strongly enriched in highly incompatible elements relative to PM, but depleted in Sr, Zr, Hf and Ti relative to other incompatible trace elements (Figs 8 and 12); the fluids or melts also carried K and H2O (forming phlogopite). The combination of low Ti–Al, and high Na–Cr contents in clinopyroxene cores (Fig. 7), strongly suggests formation from Na-rich, Ti–Al-poor fluids. The common presence of CO2 inclusions, and association of CO2, carbonates, silicate glass and silicate minerals, observed both in polyphase inclusions and in many CO2-dominated inclusions, imply that the metasomatizing fluid(s) also carried Si and C (Frezzotti et al., 2002a
; Neumann et al., 2002). Marked depletion in Zr–Hf and Ti relative to REE (Fig. 12), and enrichment in LREE relative to HREE, and enrichment in Na and Ca relative to Al is generally regarded as evidence of carbonatite metasomatism (e.g. Rudnick et al., 1994; Klemme et al., 1995; Coltorti et al., 1999, 2000). This is strongly supported by estimated trace element compositions of the melts with which the clinopyroxenes last equilibrated. Melt compositions estimated for harzburgites and lherzolites from Tenerife (Fig. 17a and b; based on PCcpx/carbonatite melt) closely resemble the trace element patterns of carbonatites in the basal complex of Fuerteventura (Hoernle et al., 2002) and average carbonatites (Fig. 17e; Woolley & Kempe, 1989). Patterns based on PCcpx/basaltic melt or PCcpx/Si-richmelt, in contrast, show no similarity to relevant silicate melt compositions, including nephelinites in Gran Canaria [data from Hoernle & Schmincke (1993)]. This strongly suggests that the clinopyroxenes in the xenoliths from Tenerife formed in the presence of, or equilibrated with, carbonatitic melt(s). The trace element compositions of clinopyroxenes in Cr–Mg series dunites and wehrlites from Tenerife fall essentially within the range of clinopyroxenes in the HLCO group (Fig. 8), indicating that they have been metasomatized by the same types of fluids that have affected the HLCO harzburgites and lherzolites.
For Hierro and Lanzarote, the best fit between estimated and known melt compositions was obtained using PCcpx/silicicmelt (Fig. 17d). The estimated concave-upwards trace element patterns closely resemble the patterns obtained from silicic glass inclusions (c. 57 wt % SiO2) in olivine porphyroclasts in unveined harzburgite xenoliths from La Palma (Wulff-Pedersen et al., 1999; Fig. 17c). Concave-upwards trace element patterns were also found among high-Si glasses (>60 wt % SiO2) formed through extensive reactions between basaltic melts and spinel harzburgite and dunite in veined xenoliths from La Palma (Wulff-Pedersen et al., 1996, 1999). It thus seems most likely that the upper mantle beneath Hierro and Lanzarote was metasomatized by high-Si melts. Trace element patterns estimated from clinopyroxenes in mantle xenoliths from La Palma show a range intermediate between the patterns obtained from clinopyroxenes in xenoliths from Tenerife and Hierro–Lanzarote.
We showed above that the Ti–Al series rocks (dunites, wehrlites, clinopyroxenites, and rare harzburgites and lherzolites) and their clinopyroxenes show strong affinity to basaltic lavas and their phenocrysts with respect to major and trace element compositions (Figs 7–14). Melt compositions estimated on the basis of Ti–Al series clinopyroxenes, using PCcpx/basaltic melt, are shown in Fig. 17f. These estimated trace element patterns are closely similar to those of mildly alkaline basaltic lavas from the Canary Islands (e.g. Neumann et al., 1999). The main difference is the significantly higher Th and U and lower Nb contents in the estimated melts than in the lavas. It should be noted, however, that the concentrations of these elements in Ti–Al series clinopyroxenes are similar to the concentrations in the clinopyroxene phenocrysts in the Tenerife basalts (Fig. 7). The deviation between the estimated and observed basalt trace element patterns is, therefore, probably caused by analytical error or inaccurate partition coefficients.
These results support the evidence from fluid inclusion studies that carbonatitic, basaltic, as well as Si-rich silicate melts have been present in the upper mantle beneath the Canary Islands. However, the fluid inclusion studies show extensive evidence that at least some of these fluids are genetically related (Hansteen et al., 1991; Frezzotti et al., 1994, 2002a, 2002b). Frezzotti et al. (2002a) proposed the following scenario for the formation of the various types of fluid inclusions observed in the xenoliths from Tenerife. An initial volatile-rich, siliceous alkaline carbonatite melt undergoing immiscible separations and wall-rock reactions gave rise to a mixed CO2–H2O–NaCl fluid and a silicate or a silicocarbonatite melt. The latter reacted with wall-rock minerals, primarily orthopyroxene, and eventually unmixed into a carbonaceous silicate melt, and a CO2-rich fluid. The carbonaceous silicate melt continued to react with the wall-rock minerals, giving rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. Wulff-Pedersen et al. (1996, 1999) observed, in veined xenoliths from La Palma, a gradual transition from basaltic melts with semi-linear patterns typical of Canarian alkali basalts (Fig. 17f), to high-Si melts with concave-upwards patterns similar to those exhibited by high-Si glasses (Fig. 17c). This transition was interpreted as the result of melt–wall-rock reactions. The decrease in MREE from the basaltic to the most silicic melts in the vein system is more than an order of magnitude, whereas the changes in the most strongly incompatible elements and in HREE are minor.
Genetic relationships between different types of fluids or melts are supported by a recent study by Bodinier et al. (2004), who presented evidence that coexisting silicate, hydrous and carbonate melts may form through interaction between mantle wall-rock and a hornblendite melt. Close to hornblendite veins Bodinier et al. (2004) found amphibole harzburgite containing clinopyroxene with mildly upwards convex REE patterns mildly enriched in LREE relative to HREE, and similar enrichment factors for Zr as for neighbouring REE. At greater distances from the veins they found anhydrous harzburgite with clinopyroxene strongly enriched in LREE relative to HREE and depleted in Sr, Zr and Ti relative to REE. These trace element characteristics resemble those of clinopyroxenes in basalts and HCLO xenoliths, respectively (Figs 8 and 10). On the basis of mathematical modelling, Bodinier et al. (2004) showed that the trace element heterogeneity observed in the Lherz harzburgites may be explained by a single stage of reactive porous flow involving emplacement of a silicate melt (in the vein), which invaded the adjacent peridotite wall-rock where chromatographic fractionation and reactions led to the formation of a residual carbonate melt that migrated into the more distant wall-rocks.
We are left with two possible scenarios for the relationships between different types of melts in the upper mantle beneath the Canary Islands. In Scenario I, metasomatism was caused by two types of primary melt: one was a siliceous carbonatite or carbonaceous silicate melt, the other a basaltic melt. Both gave rise to a variety of melts through liquid immiscibility as indicated by the fluid inclusion data. In Scenario II, metasomatism was caused by the derivatives of a single type of primary magma, a CO2-rich basaltic melt that gave rise to all other types of melts or fluids. The overlap in Sr–Nd isotopic ratios between metasomatized mantle rocks and basaltic lavas (Fig. 11) implies that in both scenarios the primary melt(s) originated in the Canarian mantle plume. At our present level of knowledge there is no direct evidence that favours one scenario over the other. The gradual transitions between different trace element patterns, observed for example among clinopyroxenes in Cr–Mg series xenoliths from La Palma and HLCO xenoliths from Tenerife (Fig. 8), are interpreted as the results of progressive changes in the compositions of the metasomatizing fluids through immiscible separations, melt–wall-rock reactions, chromatographic fractionation and mixing. The Ti–Al series harzburgites and lherzolites are interpreted as Cr–Mg series harzburgites extensively infiltrated by basaltic magmas.
The origin of dunites and wehrlites
As shown above, Cr–Mg series dunites and wehrlites show compositional affinities to the Cr–Mg series harzburgites, although the dunites and wehrlites tend towards higher TiO2, Al2O3 and FeOtotal, and lower Cr2O3 and Na2O in minerals and rocks (Figs 5, 7 and 12). The trace element compositions of clinopyroxenes in Cr–Mg series dunites and wehrlites from Tenerife fall essentially within the range of clinopyroxenes in the HLCO group (Fig. 8). The whole-rock trace element compositions closely resemble those of the spinel harzburgites and lherzolites from the same islands (Fig. 13). It thus seems likely that the Cr–Mg series dunite and wehrlite xenoliths in each of the islands have been infiltrated by the same types of fluids that have affected the harzburgites and lherzolites, causing metasomatism and reactions leading to the formation of olivine and clinopyroxene at the expense of orthopyroxene. Dungan & Avé Lallemant (1977) and Kelemen (1990) have proposed that dunites may form from harzburgites as the result of reactions between mantle wall-rock and basaltic melts. We propose that the Cr–Mg series dunites and wehrlites may also form in response to reactions between peridotite and carbonatitic melts.
The Ti–Al series rocks (dunites, wehrlites, clinopyroxenites, and a few harzburgites and lherzolites) and their clinopyroxenes show strong affinity to basaltic lavas and their phenocrysts with respect to major and trace elements as well as Sr–Nd isotope ratios (Figs 7–13). The estimated melt patterns (Fig. 17f) mainly differ from those of mildly alkaline basaltic lavas from the Canary Islands by significantly higher Th and U and lower Nb contents. It should be noted, however, that the Ti–Al series clinopyroxenes resemble the clinopyroxene phenocrysts in the Tenerife basalts with respect to these elements (Fig. 10). The deviation between the estimated and observed basalt trace element patterns is, therefore, probably caused by analytical error or inaccurate partition coefficients. In spite of the differences in Th, U and Nb, we conclude that the Ti–Al series xenoliths have formed from mildly alkaline basaltic melts similar to those that form the main lava series in many of these islands. The Ti–Al series wehrlites are mainly restricted to veins cutting harzburgite and lherzolite, implying that they represent cumulates formed in conduits cutting through the lithosphere. Basaltic melts passing through the mantle are believed to have given rise to some of, or all, the various melts or fluids that have invaded the upper mantle beneath the Canary Islands. The primary silicate glass + spinel + clinopyroxene + CO2 in Ti–Al series rocks in Gomera (Frezzotti et al., 1994, 2002b) may thus represent trapped droplets of the primary basaltic melt indicated in Scenarios I and II above.
Timing of events
It has been proposed that a mantle plume was located below western Africa about 200 Myr ago (Ernst & Buchan, 1997; Wilson & Guiraud, 1998). However, the 87Sr/86Sr 0·7027 obtained for clinopyroxenes in the least metasomatized spinel harzburgites (Fig. 11), as well as the depleted REE compositions of olivine and pyroxenes represent robust evidence that the upper mantle in the area formed as N-MORB type oceanic lithospheric mantle. The lower crust has N-MORB characteristics (Neumann et al., 2000; E.-R. Neumann & Abu El-Rus, unpublished data, 2004). We have found no evidence that supports the presence of a mantle plume. As far as we can tell on the basis of our data, after initial formation at the opening of the central Atlantic Ocean and before the onset of the Canary Islands magmatism, the degree of depletion in the lithospheric mantle was relatively uniform from west to east.
The metasomatism appears to be a relatively recent event. Significant ranges in trace concentrations among mineral grains of the same phase imply that at the time of transport of the xenoliths to the surface the upper mantle beneath the Canary Islands had not had time to reach trace element equilibrium, even on the scale of a few millimetres. It is also significant that the large, poikilitic orthopyroxene and clinopyroxene grains that formed as the result of the metasomatism by carbonaceous melts (HLCO samples) are undeformed or very mildly deformed, in contrast to the exsolved orthopyroxene and olivine in all samples, which both belong to an older generation of grains. This is also strong evidence that the metasomatic processes are recent. This implies that the metasomatism is significantly younger than the formation of the oceanic lithospheric plate in the area. Finally, as indicated above, the Sr–Nd isotopic compositions of the various rocks link all the metasomatizing melts and fluids to the Canary Islands event (Fig. 11). The conclusion that the metasomatism is part of the Canary Islands intraplate event thus seems robust.
Carbonatitic melts and ocean islands
Our data imply the presence of carbonatitic and silicic carbonatite/carbonaceous silicate melt in the upper mantle beneath the Canary Islands. In global context, carbonatites represent a relatively rare type of magmatic activity, mainly found in the continents (e.g. Woolley & Kempe, 1989). Only two oceanic localities are known, in the Cape Verdes (e.g. de Assuncao et al., 1966; Silva et al., 1981; Jørgensen & Holm, 2002) and the Canary Islands (Fig. 1; e.g. Fùster et al., 1968; Ahijado & Hernández-Pacheco, 1990; Cantagrel et al., 1993; Balogh et al., 1999; Hoernle et al., 2002). Studies of mantle xenoliths suggest, however, that carbonatitic melts are more common in oceanic intraplate magmatism than indicated by the rare occurrences of carbonatitic rocks at the surface. Metasomatism by carbonatitic or carbonatite–silicate melts has been identified in, for example, Savai'i (Samoa; Hauri et al., 1993), Tubuai (Austral Islands; Hauri et al., 1993), Kerguelen (Schiano et al., 1994), and Grand Comore (Coltorti et al., 1999), in addition to La Palma and Tenerife in the Canary Islands (Neumann et al., 2002; this study), implying the presence of carbonate-rich melts in the upper mantle during the formation of all these islands. The rare presence of carbonatitic rocks at the surface of an ocean island, but extensive evidence of carbonatite metasomatism in the underlying upper mantle, may be the logical consequence of formation of carbonate-rich melts from basaltic primary melts through reactions between melts and mantle wall-rocks, and immiscible separation(s). The consequence of such processes would be concentration of the basaltic melts in larger conduits with easy passage to the surface whereas the carbonate-rich derivatives would be restricted to the mantle wall-rocks. Infiltration experiments have demonstrated that carbonatitic melts can percolate along grain boundaries and fractures in polycrystalline olivine at rates of several millimetres per hour (e.g. Hammouda & Laporte, 2000). This is several orders of magnitude higher than percolation rates found for basalt infiltration in mantle lithologies. Hammouda & Laporte (2000) proposed that in a system undergoing a combination of infiltration and compaction, carbonatite melts can travel upwards in the mantle over hundreds to thousands of metres on time scales of 0·1–1 Myr. In the lithospheric mantle, carbonatite melts may thus infiltrate large volumes of peridotite by a combination of lateral and vertical infiltration, and be very efficient metasomatic agents (e.g. Yaxley et al., 1998). Chemical changes caused by basaltic melts, in contrast, appear to be significant only in the close vicinity to melt conduits (e.g. Wilshire & Shervais, 1975; McPherson et al., 1996; Wulff-Pedersen et al., 1996).
East–west-related chemical variations
As shown above, the most REE-depleted orthopyroxene porphyroclasts in xenoliths from the islands of La Palma, Tenerife and Lanzarote have similar MREE and HREE (Fig. 6), and cores of olivine porphyroclasts in xenoliths from Hierro and Lanzarote (Fig. 4) have similarly low MREE and HREE contents. Although orthopyroxenes in xenoliths from Hierro are somewhat more depleted in HREE and MREE than xenoliths from the other islands, these similarities strongly suggest that before metasomatism the lithospheric mantle was essentially uniformly depleted from east to west beneath the Canary Islands chain.
The average degree of metasomatism in the upper mantle clearly differs from island to island. The lowest degrees are seen in xenoliths from Hierro, furthest west, and Lanzarote, furthest east; the highest degree is found in xenoliths from Tenerife in the middle of the Canary Islands chain. This implies differences in the intensity of the metasomatic processes on a large scale (tens of kilometres). The presence within the same island of xenoliths showing different degrees of metasomatism (e.g. Tenerife) implies that metasomatism is unevenly distributed also on a relatively small scale (possibly on a metre scale). The small-scale variations probably reflect decreasing intensity of the metasomatism with increasing distance from the fluid conduits. The causes of the large-scale variations are less obvious. It is possible that xenoliths from different islands are collected from different depths and that the observed differences in the degree of metasomatism reflect a depth layering. An alternative possibility is that the lateral variations in degree of metasomatism are caused by different intensities in fluid transport through the lithosphere along the island chain, and/or variations in the availability of fluid conduits. Tenerife, the island beneath which we have found the most extensive degree of metasomatism, is the largest and highest island, reflecting the extensive magmatism in this area. The restricted degree of metasomatism seen in the upper mantle and lower crust beneath Lanzarote may be due to a concentration of fractures along the ocean–continent transition, allowing easier passage of fluids through the lithosphere here than further west. A high degree of fracturing close to the ocean–continent transition, giving easy passage for magmas to the surface, may also explain the fact that the exposed lavas in Lanzarote are more primitive than in the other islands. It is, however, not clear why Ti–Al series xenoliths are common in Hierro and La Gomera and not in the other islands.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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We have arrived at the following conclusions concerning the nature and evolution of the lithospheric mantle beneath the Canary Islands.
(1) Cr–Mg series spinel harzburgite and lherzolite xenoliths from La Palma, Hierro, Tenerife and Lanzarote originally formed as highly refractory oceanic lithospheric mantle during the opening of the central Atlantic Ocean. The original composition corresponds to that expected in a residue formed after about 25–30% partial melting of primordial mantle. The lithospheric mantle beneath the Canary Islands is thus more depleted than ‘normal’ MORB source mantle. There is no evidence of east–west-related variations in the degree of depletion.
(2) The 87Sr/86Sr ratios of 0·7027 obtained by point measurements in clinopyroxene in the most refractory xenoliths are strong evidence against the proposed presence of a mantle plume in the area at the time of opening of the Atlantic Ocean.
(3) The oceanic lithosphere beneath the Canary Islands was metasomatized during the Canary Islands intraplate event. The metasomatic agents include siliceous carbonatite or carbonaceous silicate melts, carbonatites, and high-Si melts. Many or all of the melts or fluids that have been present in the upper mantle formed through immiscible separations, melt–wall-rock reactions and chromatographic fractionation, either from a single type of CO2-rich basaltic primary melt or possibly from two primary melt types, one basaltic, the other silicic carbonatite.
(4) The most extensive metasomatism was caused by carbonatite or silicic carbonatite melts in the lithospheric mantle beneath La Palma and Tenerife. The upper mantle beneath Hierro and Lanzarote was subjected to mild metasomatism, probably caused by high-Si melts. Basaltic melts appear mainly to have given rise to Ti–Al series mantle rocks in magma conduits and melt pockets, and to have caused very little metasomatism.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESPETROGRAPHYMINERAL CHEMISTRYSr ISOTOPESWHOLE-ROCK CHEMISTRYDISCUSSIONCONCLUSIONSSUPPLEMENTARY 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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This project was made possible through grants from the Norwegian Research Council for Science and Humanities (NAVF). We are grateful to Dr Joan MartÕ for help to obtain permits for sampling and exporting rocks in Tenerife, and to the Timanfaya National Park in Lanzarote, and to Ayuntamiento de Fuencaliente de La Palma for permissions to collect xenoliths for scientific studies. Ashwini Sharma is gratefully acknowledged for help with the analytical work. The analytical work was performed as part of the international exchange activities at the GEMOC Key Centre, Macquarie University, Sydney, Australia. Funding sources included a Macquarie University Visiting Scholar Grant and a Large ARC Grant to S.Y.O'R. and W.L.G. for the work at GEMOC. This is publication number 360 from the GEMOC ARC National Key Centre (www.es.mq.edu.au/GEMOC/). This paper has benefited from constructive reviews by J.-L. Bodinier, M. Grégoire and G. Yaxley, and editorial comments by M. Wilson.
* Corresponding author. E-mail: e.r.neumann{at}geologi.uio.no
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REFERENCES |
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