The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths

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1 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 PAGES doi: /petrology/egh063 The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths ELSE-RAGNHILD NEUMANN 1 *, WILLIAM LINDSEY GRIFFIN 2,3, NORMAN J. PEARSON 2 AND SUZANNE YVONNE O REILLY 2 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 ADVANCE ACCESS PUBLICATION SEPTEMBER 9, 2004 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 ( 87 Sr/ 86 Sr 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 midocean 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 CO 2 -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 INTRODUCTION 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 Myr ago (Verhoef et al., 1991; Roest et al., 1992; Hoernle, 1998). The intraplate *Corresponding author. e.r.neumann@geologi.uio.no Journal of Petrology vol. 45 issue 12 # Oxford University Press 2004; all rights reserved

2 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Fig. 1. Map of the Canary Islands showing bathymetry (with 1000 m contours) and magnetic anomalies [simplified after Verhoef et al. (1991) and Roest et al. (1992)]. 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. 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; Mu~noz, 1973; Mu~noz & 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 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 The Canary Islands are situated close to the continental margin of NW Africa (Fig. 1). Magnetic anomalies M22 M25 ( 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; Ara~na 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; Ara~na & 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 2574

3 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS hypersthene-normative tholeiitic basalts to strongly silicaundersaturated nephelinites, but is dominated by TiO 2 - 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 11 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. ANALYTICAL TECHNIQUES 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 (2s 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 wt % oxide, better than 5% for elements in the range 2 10 wt % oxide, and better than 10% for elements in the range 05 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 05 1 mj per pulse, allowing data collection from individual grains in polished thick sections (100 mm) for at least 100 s. All analyses were carried out using Ar as the carrier gas with a flow rate of 15 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 <05% (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 ( 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). 87 Sr/ 86 Sr 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 86 Sr/ 88 Sr ¼ Any interference of 87 Rb on 87 Sr is corrected by measuring the intensity of the interferencefree isotope 85 Rb and using a 85 Rb/ 87 Rb value of 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 85 Rb/ 87 Rb necessary to give the true 87 Sr/ 86 Sr. The maximum 87 Rb/ 86 Sr ratio of the spiked solutions used in the determination of the 85 Rb/ 87 Rb ratio was Although 83 Kr was monitored, the need to correct for 86 Kr interference on 86 Sr 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 87 Sr/ 86 Sr of (2 SD; n ¼ 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) A. All ablations were carried out in He carrier gas, which is mixed with Ar before introduction to the 2575

4 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 1: Trace element compositions of olivine porphyroclasts in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro and Lanzarote Island: La Palma Hierro Rock type: Sp harzburgite Sp harzburgite Sample: PAT2-75 PAT2-83 PAT2-86 H1-4 H1-12 H1-39 H1-43 n: SiO FeO total MnO MgO NiO CaO Sum Fo Li B Al Ca Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb < < Sr Y <0.004 <0.004 <0.002 Zr < < <0.02 Nb <0.003 <0.004 <0.004 Cs 0.01 <0.02 < <0.02 <0.03 <0.019 Ba 0.03 <0.01 <0.01 <0.01 <0.02 <0.02 <0.01 La <0.002 <0.002 <0.002 Ce <0.002 <0.002 Pr <0.001 < < <0.001 Nd <0.008 < <0.009 <0.01 <0.010 Sm <0.003 <0.006 <0.006 <0.007 <0.009 <0.01 <0.007 Eu <0.002 <0.003 <0.002 <0.02 <0.003 <0.004 <0.003 Gd <0.003 <0.005 < <0.007 <0.006 <0.008 Tb <0.002 <0.003 <0.004 <0.003 <0.004 <0.004 <0.004 Dy <0.008 <0.01 <0.02 <0.01 <0.002 <0.02 <0.01 Ho <0.001 < <0.002 <0.002 <0.002 Er < <0.008 <0.008 <0.01 Tm <0.001 < <0.002 <

5 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Island: La Palma Hierro Rock type: Sp harzburgite Sp harzburgite Sample: PAT2-75 PAT2-83 PAT2-86 H1-4 H1-12 H1-39 H1-43 n: Yb < <0.01 <0.008 <0.012 Lu < <0.002 <0.003 Hf <0.004 <0.005 <0.005 <0.01 <0.008 <0.009 Ta <0.002 <0.002 <0.001 <0.003 <0.001 <0.003 Th <0.002 < <0.003 <0.003 <0.005 Island: Rock type: Lanzarote Sp harzburgite Sample: LA1-7 LA1-13 LA6-35 LA6-38 n: SiO FeO total MnO MgO NiO CaO Sum Fo Li B Al Ca Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb 0.29 <0.03 <0.03 <0.001 Sr < Y < < Zr <0.007 < Nb <0.003 <0.005 <0.004 <0.002 Cs <0.02 < <0.008 Ba 0.42 <0.07 <0.01 <0.01 La <0.002 <0.001 <0.003 <0.002 Ce < Pr <0.01 <0.002 <0.002 <

6 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 1: continued Island: Rock type: Lanzarote Sp harzburgite Sample: LA1-7 LA1-13 LA6-35 LA6-38 n: Nd <0.008 <0.009 <0.01 <0.009 Sm <0.006 <0.01 <0.007 <0.008 Eu <0.002 <0.003 <0.004 <0.003 Gd <0.007 <0.008 <0.01 <0.006 Tb <0.003 <0.003 <0.004 <0.005 Dy <0.01 <0.01 <0.02 <0.02 Ho <0.001 <0.002 <0.003 <0.001 Er <0.007 <0.01 <0.008 <0.007 Tm <0.002 <0.003 <0.002 <0.002 Yb <0.007 <0.008 <0.02 <0.007 Lu <0.002 <0.003 <0.003 <0.002 Hf <0.005 <0.009 <0.008 <0.007 Ta <0.002 <0.003 <0.003 <0.002 Th <0.003 <0.005 <0.004 <0.002 n, number of trace element analyses; Fo, forsterite content. The high, and highly variable concentrations in the most strongly incompatible elements (e.g. Rb, Nb, Cs, Ba, La Pr, Ta, Th) compared with HREE are interpreted as the result of the common presence of sub-microscopic fluid inclusions dominated by enriched silicate glass. Data on these elements are shown in italics. ICP torch. The typical spot size was c mm; 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. 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 Litetraborate 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. PETROGRAPHY 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 (Fo ; Fig. 2). Spinel lherzolites are rare, 2578

7 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Table 2: Trace element analyses of orthopyroxene porphyroclasts in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro and Lanzarote Island: La Palma Hierro Rock type: Sp harz Sp harz Sample: PAT2-75 PAT2-83 PAT2-86 H1-4 H1-12 H1-39 H1-43 n: SiO TiO Al 2 O Cr 2 O FeO total MnO MgO NiO CaO Na 2 O Sum mg-no Li B Al Ca Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb 0.23 < <0.04 < Sr Y Zr Nb Cs <0.02 <0.08 <0.01 <0.2 < Ba La Ce Pr <0.002 <0.001 <0.001 Nd <0.01 <0.006 <0.007 Sm < <0.008 <0.005 <0.006 Eu <0.003 <0.002 <0.003 Gd <0.007 <0.007 <0.007 Tb <0.003 <0.003 <0.003 Dy <0.02 <0.008 <

8 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 2: continued Island: La Palma Hierro Rock type: Sp harz Sp harz Sample: PAT2-75 PAT2-83 PAT2-86 H1-4 H1-12 H1-39 H1-43 n: Ho Er Tm Yb Lu Hf < <0.01 <0.007 <0.005 <0.006 Ta < <0.003 <0.001 <0.003 Th <0.003 Island: Rock type: Lanzarote Sp harz Sample: LA1-7 LA1-13 LA6-35 LA6-38 n: SiO TiO Al 2 O Cr 2 O FeO total MnO MgO NiO CaO Na 2 O Sum mg-no Li B Al Ca Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb 0.06 < <0.011 Sr

9 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Island: Rock type: Lanzarote Sp harz Sample: LA1-7 LA1-13 LA6-35 LA6-38 n: Y Zr Nb Cs <0.02 <0.02 <0.02 <0.007 Ba <0.02 <0.02 <0.02 <0.01 La < Ce Pr Nd Sm < Eu Gd Tb < <0.004 Dy Ho Er Tm Yb Lu Hf <0.008 <0.009 <0.01 <0.006 Ta <0.003 <0.003 <0.04 <0.002 Th <0.005 <0.004 <0.06 <0.002 Sp harz, spinel harzburgite; n, number of trace element analyses. The high, and highly variable concentrations in the most strongly incompatible elements (e.g. Rb, Nb, Cs, Ba, La Pr, Ta, Th) compared with HREE in the most refractory orthopyroxenes are interpreted as the result of the common presence of sub-microscopic fluid inclusions dominated by enriched silicate glass. Data on these elements are shown in italics. 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 (Fo ), 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. 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 2581

10 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 3: Trace element analyses of clinopyroxene porphyroclasts in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro and Lanzarote Island: La Palma Sample: PAT2-56 PAT2-68 PAT2-75 PAT2-86 Rock type: sp harz sp harz sp harz sp harz Population: I* II* I II I II* III* n: mg-no Li Be B Al P Sc Ti V Co Ni Cr Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho n.d Er Tm Yb Lu Hf Ta Pb Th U

11 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Island: La Palma Hierro Lanzarote Sample: PAT2-36 PAT2-52 H1-12 H1-43 LA1-13 Rock type: sp wehrlite sp dunite sp harz sp harz Sp harz Population: I* II* III* * I II n: mg-no Li Be < B <1 0.9 Al P Sc Ti V Co Ni Cr Ga Rb 0.22 <0.07 <0.08 <0.07 Sr Y Zr Nb Cs <0.02 <0.03 <0.04 <0.03 Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf <0.01 <0.01 <0.02 <0.01 Ta < Pb Th <0.005 <0.02 <0.005 U One analysis made by ion probe (PAT2-68, in italics) from Wulff-Pedersen et al. (1999) is repeated here for completeness. I, II and III are populations with different compositions within the same sample; sp harz, spinel harzburgite; lherz, spinel lherzolite; n.d., not detected; n, number of analyses. Open space means not analysed. Analyses of clinopyroxene in xenoliths from Tenerife have been given by Neumann et al. (2002). *Analyses made on an older LAM instrument that gives fewer elements and somewhat lower precision than the one used for the other analyses. 2583

12 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 4a: Rb-Sr isotope ratios measured by LAM-ICPMS analyses on clinopyroxene (cpx) and phlogopite (phlog) in mantle xenoliths from the Canary Islands Sample Rock type Phase n 87 Rb/ 86 Sr 95% conf. 87 Sr/ 86 Sr 95% conf. Sr (ppm) La Palma PAT2-36 amph wehr cpx PAT2-36 amph wehr phlog PAT2-75 sp harz (HTR) phlog PAT2-86 sp harz (HTR) phlog Hierro HI-12 sp harz (HEXO) cpx Tenerife TF14-36 sp lherz (HLCO) cpx TF14-38 sp harz (HLCO) cpx TF14-52 sp harz (HEXO) phlog TF14-52 sp harz (HEXO) cpx TF14-58 sp harz (HTR) cpx Sr contents were estimated from signal, compared with the Batbjerg clinopyroxene standard. The 95% conf. values are given as error-weighted means with 95% confidence limits where number of points is >1, otherwise as 1 standard error. The terms HEXO, HTR and HLCO are explained in the legend of Table 1. Table 4b: Analyses of standards by LAM-ICPMS at Macquarie University, compared with data obtained by thermal ionization mass spectrometry analyses (TIMS) at other universities Sample, mineral Rb (ppm) Sr (ppm) n 87 Sr/ 86 Sr 95% conf. 87 Sr/ 86 Sr 2s TIMS data source ID ID LAM LAM LAM TIMS TIMS MF1 (10317/1), anorthoclase Univ. of Oslo MF2, anorthoclase Univ. of Oslo 45066/1, anorthoclase Univ. of Oslo Batbjerg-1, Cr-diopside < Danish Lithosphere Centre LS245235, apatite Monash University CaF 2, synthetic fluorite Monash University ID, isotope dilution. The LAM data are given as error-weighted means with 95% confidence limits. with exsolved orthopyroxene). This group contains deformed porphyroclasts of olivine (Fo ), 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) 2584

13 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Table 5: New trace element data (ppm) for Cr Mg series spinel harzburgite and lherzolite xenoliths from La Palma, Hierro and Lanzarote Island: La Palma Sample: PAT2-29 PAT2-31 PAT2-37 PAT2-41 PAT2-68 PAT2-70 PAT2-75 PAT2-85 PAT2-90 Rock type: sp hz sp hz sp ph hz sp ph hz sp hz sp ph hz sp ph hz sp ph hz sp ph hz SiO TiO Al 2 O FeO tot MnO MgO CaO Na 2 O K 2 O P 2 O LOI Sum Sc Ti V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu Hf n.d n.d. Ta n.d. n.d. n.d. n.d. n.d n.d Th U n.d. n.d. n.d. n.d. n.d

14 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 5: continued Island: Hierro Lanzarote Sample: H1-4 H1-7 H1-11 H1-12 H1-19 LA1-7 LA1-9 LA2-4 Rock type: harz lherz harz harz harz harz harz harz SiO TiO Al 2 O FeO tot MnO MgO CaO Na 2 O K 2 O n.d n.d. P 2 O n.d n.d. n.d. Sum Li Be n.d Sc Ti V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Mo Cd Sn Sb Cs n.d Ba La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er

15 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Island: Hierro Lanzarote Sample: H1-4 H1-7 H1-11 H1-12 H1-19 LA1-7 LA1-9 LA2-4 Rock type: harz lherz harz harz harz harz harz harz Yb Lu Hf Ta Pb Th U Island: Lanzarote Sample: LA2-7 LA6-35 LA6-38 LA8-4 LA8-5 LA8-6 LA11-1 Rock type: harz harz harz harz harz lher harz SiO TiO n.d Al 2 O FeO tot MnO MgO CaO Na 2 O K 2 O n.d P 2 O Sum Li Be n.d Sc Ti V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Mo Cd Sn Sb Cs

16 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Table 5: continued Island: Lanzarote Sample: LA2-7 LA6-35 LA6-38 LA8-4 LA8-5 LA8-6 LA11-1 Rock type: harz harz harz harz harz lher harz Ba La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Yb Lu Hf Ta Pb Th U Major element data (wt %) from Neumann (1991), Neumann et al. (1995) and Wulff-Pedersen et al. (1999) are added for completeness. Harz, spinel harzburgite; lherz, spinel lherzolite; (ph), traces of phlogopite; LOI, loss on ignition. 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 (<05 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 (Fo ) 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 þ 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 2588

17 NEUMANN et al. UPPER MANTLE BENEATH CANARY ISLANDS Fig. 2. Modal olivine orthopyroxene clinopyroxene relationships in mantle xenoliths from the Canary Islands compared with those in peridotite xenoliths collected along the North Mid-Atlantic Ridge (grey field). Data are from the following sources: Hierro: Neumann (1991); La Palma: Wulff-Pedersen et al. (1996); Tenerife: Neumann et al. (2002); Gran Canaria: Amundsen (1987); Lanzarote: Sagredo Ruiz (1969) and Neumann et al. (1995); the North Mid-Atlantic Ridge: Dick et al. (1984), Michael & Bonatti (1985), Juteau et al. (1990) and Komor et al. (1990). The figure includes unpublished data on Canary Islands xenoliths. porphyroclastic/protogranular and magmatic elements; sometimes the magmatic elements are concentrated along narrow zones that may represent veinlets. Dunites Cr Mg-series dunite xenoliths (Fo ; 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 (Fo ) 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 Fe 3þ -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 (Fo ) 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 (Fo ; 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) CO 2 ; (2) carbonate-rich CO 2 SO 2 mixtures; (3) polyphase inclusions dominated by silicate glass spinel clinopyroxene phlogopite sulphide carbonates (magnesite and dolomite) CO 2 (Frezzotti et al., 2002a; 2589

18 JOURNAL OF PETROLOGY VOLUME 45 NUMBER 12 DECEMBER 2004 Fig. 3. Forsterite contents in olivine in various types of mantle xenoliths from the Canary Islands compared with peridotite xenoliths collected along the North Mid-Atlantic Ridge. Data are from the following sources: Hierro: Neumann (1991); La Palma: Wulff-Pedersen et al. (1996); Tenerife: Neumann et al. (2002); Gran Canaria: Amundsen (1987); Lanzarote: Siena et al. (1991) and Neumann et al. (1995); the North Mid- Atlantic Ridge: Dick et al. (1984), Michael & Bonatti (1985), Juteau et al. (1990) and Komor et al. (1990). The figure includes unpublished data on Canary Islands xenoliths. The arrows show the average for olivine in North Mid-Atlantic Ridge peridotites. Neumann et al., 2002). CO 2 and CO 2 -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 wt % SiO 2 in spinel harzburgites and lherzolites, and wt % SiO 2 in dunites and wehrlites (Neumann & Wulff-Pedersen, 1997). In xenoliths from Tenerife CO 2 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 2590