Helium, argon and lead isotopic composition of volcanics from Santo Antão and Fogo, Cape Verde Islands

Chemical Geology 178 Ž2001. 127–142 www.elsevier.comrlocaterchemgeo

Helium, argon and lead isotopic composition of volcanics from Santo Antao ˜ and Fogo, Cape Verde Islands Birgitte Printz Christensen a,) , Paul Martin Holm a,1, Albert Jambon b,2 , J. Richard Wilson c,3 a

Geological Institute, UniÕersity of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Laboratorie Magie, UniÕersite` Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France c ˚ Department of Earth Sciences, UniÕersity of Aarhus, DK-8000 Arhus C, Denmark

b

Received 12 May 2000; accepted 18 December 2000

Abstract Helium, argon and lead isotopic ratios have been determined for volcanics from two of the youngest Cape Verde Islands, Santo Antao ˜ and Fogo. Helium isotopic ratios range from radiogenic 4 Her3 He values of 224,000 Ž3.2 RrRa. to more primitive values of 52,000 Ž13.8 RrRa., which suggest a contribution from different reservoirs to the magmatism at the Cape Verde Islands. 40Arr36Ar isotopic ratios range from slightly higher values than atmospheric up to around 1250. Pb isotopic ratios are relatively radiogenic: 206 Pbr204 Pb s 18.90–19.63, 207 Pbr204 Pb s 15.526–15.621 and 208 Pbr204 Pb s 38.694–39.272, with volcanics from Santo Antao ˜ being the most radiogenic. Several factors like high 4 Her3 He ratios, relatively radiogenic Pb isotopes, high CerPb, ZrrHf, low BarLa, LILErNb and low 40Arr36Ar indicate a significant contribution from a HIMU-type source. A lower mantle contribution is indicated from the low 4 Her3 He ratios in some samples. Moreover, relatively low 40Arr36Ar ratios, low 4 Her3 He ratios, negative D7r4 Pb values and HIMU-characteristic trace element ratios preclude the DMM as a significant contributor to the Cape Verde magmatism. The HIMU component is concluded to represent subducted oceanic crust recycled to a boundary layer separating the lower mantle from the upper degassed mantle, from where new plumes originate. The observed isotopic variation can be explained by a lower mantle contribution to the recycled component either through rising of small plumes, entrainment of lower mantle material due to instabilities in the boundary layer or He migration from the lower mantle. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Cape Verde Islands; Helium; Argon; Lead; HIMU; Recycling

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Corresponding author. Fax: q45-33148433. E-mail addresses: [email protected] ŽB.P. Christensen., [email protected] ŽP.M. Holm., [email protected] ŽA. Jambon., [email protected] ŽJ.R. Wilson.. 1 Tel.: q45-35322426; fax: q45-35322440. 2 Tel.: q33-144275135; fax: q33-144273911. 3 Tel.: q45-89422526; fax: q45-89422525.

1. Introduction The origin of noble gas signatures in Ocean Island Basalts ŽOIB. and Mid-Ocean Ridge Basalts ŽMORB. have been increasingly debated in the last two decades and controversy still exists. MORB basalts represent a relatively homogenous source region with

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 1 . 0 0 2 6 1 - 3

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a mean 3 Her4 He value of 8 " 1 RrRa, where Ra is the atmospheric 3 Her4 He ratio of 1.39)10y6 . OIB basalts by contrast show a very large variation in He isotopic ratios and have been grouped into high and low 3 Her4 He hotspots ŽKurz et al., 1982.. High 3 Her4 He hotspots, such as Hawaii, Iceland, Reunion, Samoa, Galapagos and the Ethiopian Rift have 3 Her4 He ratios ranging from MORB-like to ) 30 RrRa as observed at Loihi Seamount and Iceland Že.g. Kurz et al., 1983; Hilton et al., 1999.. Low 3 Her4 He hotspots comprise EM- and HIMUOIBs such as Tristan da Cunha, Gough Island, Saint Helena, Tubuaii and Mangaia with 3 Her4 He ratios as low as 4.3 RrRa ŽGraham et al., 1992.. Recently, 3 Her4 He isotopic ratios both lower, similar and higher than MORB values have been reported from Heard Island ŽHilton et al., 1995. and the Azores Archipelago ŽMoreira et al., 1999.. The uniform He isotopic ratios of MORB contra the diverse OIB values was explained by Allegre` et al. Ž1983, 1986. by a two layer mantle model, where MORBs are generated from an upper degassed and well mixed layer. The high 3 Her4 He hotspots were thought to have their origin from a less degassed reservoir, rich in primordial volatiles deeper in the mantle, presumable in the lower mantle. The radiogenic He signal of many oceanic islands has been explained to be a product of recycling of oceanic crust or sediments into the mantle Že.g. Kurz et al., 1982; Vance et al., 1989; Graham et al., 1992; Hanyu and Kaneoka, 1997.. Due to outgassing of helium during magmatism and subduction, altered subducted oceanic crust will have high ŽU q Th.r3 He, which with time will result in high 4 Her3 He ratios through a-decay of U and Th. Other explanations for radiogenic He have included metasomatic processes in the subcontinental lithosphere or intra-mantle metasomatism Že.g. Hanyu and Kaneoka, 1997.. Yet another explanation for the radiogenic He signatures is that of Condomines et al. Ž1983., who suggested that the radiogenic He isotopic ratios of Icelandic volcanics is due to shallow crustal contamination processes. This view is shared by Zindler and Hart Ž1986a,b. and most recently debated by Hilton et al. Ž1995. based on results from Heard Island. Hilton et al. Ž1995. report RrRa from 0.4 to 18.3 and argue that all hotspots in fact have 3 Her4 He ratios close to or higher than MORB and claim that

He isotopes cannot be used to infer the provenance of Aenriched materialsB in mantle plumes. Moreira et al. Ž1999. have observed three distinct He-isotopic signatures from the Azores: primitive, MORB and radiogenic values. They consider that the radiogenic signature cannot represent post-eruptive decay of U and Th, because melting and crushing experiments on olivines yielded the same helium isotopic results. Moreover, Moreira et al. Ž1999. preclude shallow contamination processes for the radiogenic He isotopic signature observed on Sao ˜ Miguel and propose instead a model involving delamination and recycling of enriched subcontinental lithosphere. There is agreement that the 40Arr36Ar in the MORB source is as high as about 28,000 Že.g. Staudacher et al., 1989.. By contrast, the debate about the 40Arr36Ar ratio in the undepleted mantle is controversial. Several authors Že.g. Allegre et al., ` 1983; Staudacher et al., 1986; Sarda et al., 1988. have, on the basis of high 3 Her4 He ratios and atmospheric-like heavy noble gas data ŽAr, Ne, Xe and Kr. from Loihi, suggested the existence of an undegassed deep mantle reservoir. In the case of an atmospheric-like 40Arr36Ar ratio in the lower mantle, high 40Arr36Ar ratios in basalts must result from contamination by DMM during magma ascent ŽKaneoka and Takaoka, 1985; Staudacher et al., 1991.. Another view is that the 40Arr36Ar in the undepleted mantle is much higher than atmospheric. For instance, Poreda and Farley Ž1992. reported 40 Arr36Ar ratios up to 12,000 in Samoan xenoliths and suggest a lower mantle 40Arr36Ar ratio in excess of 5000. Most recently Valbracht et al. Ž1997. suggested a lower mantle 40Arr36Ar ratio between 2500 and 6000 on the basis of measurements on both glasses and olivines from Loihi Seamount. Fisher Ž1985., Jambon et al. Ž1985., Patterson et al. Ž1990, 1991. and Farley and Craig Ž1994. argue that low 40Arr36Ar ratios in basalts are the result of contamination with air-derived noble gases through interaction with seawater. The reason why helium is unaffected by atmospheric contamination is that it continuously escapes into space, resulting in very low abundances of helium in seawater. In this study we report the first He and Ar isotope analyses from volcanic rocks from the Cape Verde Islands of Santo Antao ˜ and Fogo, together with new Pb isotopic and trace element data.

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A previous model for the Cape Verde Islands based on trace elements and Sr, Nd, Pb isotopes is that of Gerlach et al. Ž1988., who proposed a model for the source of the northern islands Žincluding Santo Antao ˜ . to be a mixture of a HIMU component and the DMM, whereas the southern islands Žincluding Fogo. also contain an enriched component ŽEM.. Kokfelt et al. Žsubmitted. propose a mixing model for Fogo involving a Ayoung HIMUB component developed through carbonatitic metasomatism and an EM1 component. They argue that negative D7r4 values Žvertical deviation from the Northern Hemisphere Reference Line ŽHart, 1984a.. indicate the presence of a young HIMU component, and find concordant ages of ; 150 Ma from arrays in 206 Pbr204 Pb vs. m Ž238 Ur204 Pb. and 208 Pbr204 Pb vs. v Ž232 Thr204 Pb. diagrams. We present evidence rejecting the HIMU–DMM mixing model for the northern Cape Verde Islands

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and argue instead for a model concerning entrainment of lower mantle material into subducted and recycled oceanic crust, which started to form new rising plumes from the base of the upper mantle.

2. Geological setting The Cape Verde Islands are situated in the Atlantic Ocean about 500 km west of Senegal on approximately 125 Ma old ocean floor according to magnetic anomalies ŽHayes and Rabinowitz, 1975.. The archipelago consists of 10 major islands aligned along three ridge segments arranged in a WNW–ESE trend ŽSanto Antao–Sao ˜ ˜ Nicolau., an almost N–S trend ŽSal–Maio. and a WSW–ENE trend ŽBrava– Santiago., ŽFig. 1.. Volcanism has been active for at least 20 Ma and possibly for 40–50 Ma ŽCourtney

Fig. 1. Map showing the position of the Cape Verde islands in the Atlantic Ocean. Ages of magnetic anomalies are M2 s 127 Ma, M11 s 136 Ma, M20 s 147 Ma and M25 s 155 Ma ŽHayes and Rabinowitz, 1975.. Insert map shows the arrangement of the Cape Verde Islands.

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and White, 1986.. With time magmatism has migrated westwards, so that the youngest island are located in the west. Historic eruptions have only been reported from Fogo ŽHolm et al., 1997.. During the time of volcanism, the Cape Verde Islands have been located close to the pole of rotation of the African plate, resulting in almost stationary magmatism ŽDuncan, 1981; Pollitz, 1991.. Sleep Ž1990. describes the Cape Verde hotspot as a weak mantle plume, with a buoyancy flux of approximately 1r5 of that of Hawaii. Rock samples from two of the youngest islands Santo Antao ˜ and Fogo have been used in this study. At these two islands, a high degree of exposure is maintained as a result of a dry climate, keeping the lavas unaltered and free of growth, which in combination with a high topographic relief provides an excellent opportunity to study large lava sequences.

3. Sample details Exposed lavas on Fogo and Santo Antao ˜ are all quite young with maximum ages of 350 ka ŽKokfelt et al., submitted. and 7.5 Ma ŽPlesner et al., in preparation., respectively. The age of lavas used in this study range from less than 870 ka to historic. Lavas are olivine andror clinopyroxene porphyritic, with accessory amounts of Fe–Ti–Cr oxides, hauyne, ¨ amphibole, nepheline, plagioclase and apatite. The lavas range from aphyric to ankaramitic rock types, where many lavas display a seriate or porphyritic texture of the most abundant minerals. Two samples from Fogo were analysed for He and Ar, both olivine and clinopyroxene separates were used. Olivine separates were analysed for He and Ar in six samples from Santo Antao. ˜ The following is a short description of samples used in this study. SA-110062: Cryptocrystalline groundmass with 8 vol.% up to 3 mm sub-euhedral or skeletal olivine phenocrysts. Rare clinopyroxene. SA-111822: Up to 5 mm sub-euhedral olivine Ž7 vol.%. and clinopyroxene Ž8 vol.%. phenocrysts in a microcrystalline groundmass composed of olivine, clinopyroxene, Fe–Ti oxides and plagioclase. Many clinopyroxenes are zoned and some form glomerocrysts.

SA-111827: Two olivine generations comprising 10 vol.% surrounded by a crypto- to microcrystalline groundmass of olivine, clinopyroxene and Fe–Ti oxides. Large olivines are an- to subhedral, 2–3 mm and slightly resorbed. Small olivines are - 1 mm and sub- to euhedral. - 1 vol.% clinopyroxene. SA-111832: 20 vol.% an- to euhedral olivine phenocrysts up to 3 mm with chromite inclusions. Minor Fe–Ti–Cr oxide microphenocrysts. Crypto- to microcrystalline groundmass composed of olivine, clinopyroxene and Fe–Ti–Cr oxides. SA-111837: Phenocrysts of olivine and clinopyroxene in a microcrystalline groundmass of olivine, clinopyroxene and Fe–Ti oxides. Olivine Ž10 vol.%. up to 3 mm with an-, sub- and euhedral shapes. Clinopyroxenes Ž3 vol.%. are sub-euhedral, up to 2 mm and zoned. Some have green cores rich in Na 2 O and FeO. Minor Fe–Ti oxide microphenocrysts. SA-111940: About 20 vol.% olivine in a microcrystalline groundmass of clinopyroxene, Fe–Tioxides, olivine and feldspatrfeldspatoid. Resorbed mantle xenoliths of olivine and clinopyroxene occur. Olivine phenocrysts are an-subhedral, up to 2 mm, and a part shows embayed edges, indicating disequilibrium and probably an origin by disaggregation of xenoliths. Rare clinopyroxene phenocrysts and minor Fe–Ti-oxide microphenocrysts. F-106403: Phenocrysts of clinopyroxene Ž20 vol.%. and olivine Ž4 vol.%. in a crypto- to microcrystalline groundmass of Fe–Ti-oxides, clinopyroxene and interstitial glass. Olivine is sub-euhedral and up to 2 mm. Many grains have iddingsitized rims. Clinopyroxene is fresh, sub-euhedral, zoned and up to 3 mm. Some form glomerocrysts. 2 vol.% Fe–Tioxide microphenocrysts. F-106444: Phenocrysts of clinopyroxene Ž15 vol.%. and olivine Ž5 vol.%. in a microcrystalline groundmass of clinopyroxene, Fe–Ti-oxides and plagioclase. Olivine and clinopyroxene are sub-euhedral and up to 3 and 5 mm, respectively. Aggregates of olivine and clinopyroxene occur. 2 vol.% Fe–Tioxide microphenocrysts.

4. Analytical procedures Noble gas analyses was performed mainly on separated olivine phenocrysts, but also on two

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clinopyroxene separates. Crushing, sieving and mineral separation were carried out at the Geological Institute, University of Copenhagen. The grain sizes of the phenocrysts range from 0.5 to 2 mm, mostly 0.5 to 1 mm. Sample selection was based on geochemistry, rock freshness and the size, shape and amount of relevant phenocrysts. Primitive samples ŽMgO G 9 wt.%. were preferred as we expect them to have spent the shortest storage time in magma chambers. The hand specimens were jaw crushed and sieved, then ultrasonically cleaned several times in deionized water. Phenocrysts were separated in three steps using a CW Cook vertical magnetic separator, a Frantz-Isodynamic separator and finally hand-picked under a binocular microscope to avoid iddingsitized grains or groundmass adhered on grain surfaces. The final separate was cleaned in acetone. Rare gas measurements were performed on a Fisons VG5400 Instrument sector-type mass spectrometer at Universite´ Pierre et Marie Curie in Paris. Gases were extracted by vacuum crushing. In this way, magmatic gases trapped in melt inclusions are released Že.g. Graham et al., 1992; Hanyu et al., 1999; Kurz, 1986a,b.. About 0.2 g of sample was placed in each of three stainless steel crushers with the possibility of crushing 0.6 g of material of the same sample. Before analysis, the crushers and the whole gas extraction line was baked overnight at ; 1008C. Blanks were measured before each sample analysis and were on average in the order of 3.0)10y1 0 cm3 STP 4 He and 4.5)10y9 cm3 STP 40 Ar. An atmospheric composition was assumed for the helium blank, as it was not possible to detect 3 He. The released gas was expanded in a stainless steel vacuum line and cleaned by two hot Ti-getters. The residual gas was then adsorbed on a cold finger held at 19 K. At this temperature Ar is adsorbed, whereas He is expanded into the mass spectrometer. After He analysis, Ar was desorbed and analysed at 858K. 4 He and 40Ar were analysed using a Faraday detector and 3 He, 36Ar, 38Ar by a Daly detector. Two analytical procedures have been used in order to optimise the analytical conditions for the different isotopes. 4 He and Ar isotopes were thus analysed first with a low trap current of 400 mA. Because of the very small amounts of gas released, samples with high amounts of 4 He were selected for He isotopic analysis and measured with a higher trap current of

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800 mA to increase the sensitivity and under cooling of the photomultiplier, to increase the signal significantly and permit 3 He analysis. Further analytical procedures are identical to those described by Marty et al. Ž1993.. Lead isotope analyses were carried out at the Geological Institute, University of Copenhagen. Whole rock powders were dissolved using HBr followed by HF and HNO 3 . Pb was extracted using the HBr standard technique.

5. Geochemical characteristics Volcanic rocks from Fogo and Santo Antao ˜ are all silica undersaturated with 37 to 49 wt.% SiO 2 classifying mainly as basanitesrtephrites or foidites in a Total Alkali–Silica ŽTAS. diagram ŽKokfelt et al., submitted; Christensen, unpubl... Fogo and Santo Antao ˜ show a similar overall variation in major element composition, whereas a difference between the southern and northern Cape Verde Islands exist in Sr, Nd and Pb isotopic composition and incompatible trace element ratios as noted by Gerlach et al. Ž1988. and Davies et al. Ž1989.. The Cape Verde Islands display, most pronounced in the northern islands, several geochemical features similar to extreme HIMU islands like Saint Helena, Mangaia and Tubuaii. Fig. 2 shows a primitive mantle normalised multi-element plot for selected rocks from Santo Antao ˜ and Fogo together with an extreme HIMU sample from Saint Helena ŽChaffey et al., 1989; Thirlwall, 1997. and a typical EM1-type basalt from Gough ŽWeaver et al., 1987.. The overall pattern is the same as the HIMU sample and the characteristic enrichment in Nb and Ta together with depletion in K, Pb and HREEs is obvious. The relative enrichment of HFSE compared to LREE and LILE give rise to low LarNb and LILErNb ŽWeaver, 1991.. Santo Antao ˜ shows mean ratios of LarNb Ž0.76., KrNb Ž120. and BarNb Ž6.9. ŽChristensen, unpubl.., which is within the range of extreme HIMU given by Weaver Ž1991.. The difference between the southern and northern Cape Verde Islands is distinct in incompatible trace element ratios concerning Ba. For instance, BarNbf 15 for the southern islands, which suggests contribution from an enriched component ŽGerlach et al., 1988;

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Fig. 2. A primitive mantle normalised multi-element plot comparing selected rocks from Santo Antao ˜ and Fogo with a HIMU basalt from St. Helena and a typical EM1-type basalt from Gough. Data sources: Santo Antao: ˜ Christensen Žunpublished.; Fogo: Kokfelt et al. Žsubmitted.; Typical HIMU ŽSH-8.: Chaffey et al. Ž1989. and Thirlwall Ž1997.; Typical EM1 ŽG93.: Weaver et al. Ž1987., except estimated Pb and Sm values. Trace elements from Santo Antao ˜ are measured by XRF and INA analyses. Primitive mantle normalising values are from Sun and McDonough Ž1989..

Davies et al., 1989.. Other HIMU features ŽChauvel et al., 1992; Thirlwall, 1997; Kokfelt et al., submitted. observed on the Cape Verde Islands include high CerPb, ZrrHf and low ZrrNb, BarLa. Kokfelt et al. Žsubmitted. pointed out several similarities in many incompatible trace element ratios between carbonatites and HIMU rocks from Fogo, consistent with the presence of carbonatite-metasomatised mantle xenoliths on Santiago ŽDavies and Mendes, 1991. and carbonatitic rocks on several of the Cape Verde Islands. The involvement of a HIMU source in the Cape Verde magmatism is consistent with the Sr, Nd and Pb isotopic data of Gerlach et al. Ž1988. and Kokfelt et al. Žsubmitted..

6. Results 6.1. Noble gases Helium and argon concentrations and isotopic ratios measured on olivine and clinopyroxene phenocrysts are listed in Table 1. Helium and argon concentrations obtained by crushing range from

0.67)10y8 cm3 STPrg 4 He and 1.9)10y8 cm3 STPrg 40Ar for SA-111822 to 6.14)10y8 cm3 STPrg 4 He and 84.7)10y8 cm3 STPrg 40Ar for SA-111827a. He isotopic ratios vary from primitiverunradiogenic values of 4 Her3 He ; 52,000 ŽRrRas 13.8. for SA-111837 to more radiogenic compositions with 4 Her3 He ; 224,000 ŽRrRa s 3.2. for SA-111822. The He results from Fogo are within the range of Santo Antao. ˜ The 40Arr36Ar isotopic ratios range from slightly higher values than atmospheric Ž295.5. to 1250. Sample F-106403 displays the lowest 40Arr36Ar ratio Ž356.. The highest argon isotopic ratios of around 1250 are measured in samples SA-111822, SA111832 and SA-111940. When duplicate analyses are considered, concentrations of 4 He appear differently. This feature is expected, as noble gases in phenocrysts are known to reside in meltrfluid inclusions, which are broken on crushing. Their abundance may differ in samples of a few hundred milligrams. In addition, crushing yield may vary from one sample to another and the amounts of gas recovered are therefore expected to

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Table 1 Helium and argon concentrations Žin 10y8 cm3 STPrg. and isotopic ratios Sample

Mineral

Weight Žmg.

4 He Ž10y8 .

4

Santo Antao ˜ SA-110062 SA-111822 SA-111827a SA-111827b SA-111832a SA-111832b SA-111837 SA-111940

ol ol ol ol ol ol ol ol

612 611 591 283 591 675 621 626

3.65 0.67 6.14 5.18 0.95 0.78 2.20 1.41

Fogo F-106403 F-106403 F-106444 F-106444

ol cpx ol cpx

322 600 – 600

1.60 3.16 – 1.81

Her 3 He

Arr36Ar

RrRa

Weight Žmg.

40 Ar Ž10y8 .

40

61,718 224,311 118,345 91,134 74,351 82,568 52,088 78,912

11.7 " 0.5 3.2 " 1.7 6.1 " 0.3 7.9 " 0.6 9.7 " 1.7 8.7 " 1.8 13.8 " 0.7 9.1 " 1.0

242 205 202 – 203 – 209 204

7.23 1.90 84.7 – 16.8 – 38.9 11.4

608.0 " 10.1 1246 " 50 837.2 " 7.3 – 1234 " 10 – 448.0 " 2.9 1256 " 27

104,002 94,499 – 59,423

6.9 " 2.2 7.6 " 0.5 – 12.1 " 0.9

208 – 201 –

25.8 – 4.36 –

356.4 " 3.0 – 639.2 " 18.2 –

All analyses have been performed on olivine and clinopyroxene phenocrysts by the crushing method at Pierre et Marie Curie Universite´ de Paris. RrRa is the 3 Her4 He ratio normalized to the air value of 1.39)10y6 . The uncertainties are expressed as 1 s and are calculated by error propagation. The uncertainty includes the within run precision of the mass spectrometer analysis and error on the blank Žexcept 3 He., gain calibration and weighing. Blank levels are described in the text. A –B not analysed.

vary even in duplicate analyses. He isotopic ratios for SA-111827 and SA-111832 vary only slightly between duplicate analyses and the He isotopic ratios measured for olivine and clinopyroxene in F-106403 are similar within uncertainties. 6.2. Lead Whole rock lead isotopes have been measured on the same samples as the rare gases ŽTable 2.. The

new lead isotope analyses from Santo Antao ˜ are relatively radiogenic and show 206 Pbr204 Pb in the range 19.17–19.63, 207 Pbr204 Pb s 15.55–15.62 and 208 Pbr204 Pb s 38.82–39.27. The volcanics from Fogo have less radiogenic lead with 206 Pbr204 Pb s 18.8 – 19.4, 207 Pbr 204 Pb s 15.52 – 15.59 and 208 Pbr204 Pb s 38.7–39.1 ŽKokfelt et al., submitted.. Pb isotopic ratios from Santo Antao ˜ coincide with previous measurements ŽGerlach et al., 1988; Davies et al., 1989.. As mentioned by both Gerlach et al.

Table 2 Lead isotopic ratios determined for the Cape Verde samples Sample

206

Pbr 204 Pb

207

Pbr 204 Pb

208

Pbr 204 Pb

D7r4

D8r4

Santo Antao ˜ SA-110062 SA-111822 SA-111827 SA-111832 SA-111837 SA-111940

19.59 19.57 19.45 19.17 19.60 19.63

15.61 15.60 15.62 15.55 15.61 15.62

39.13 39.27 39.03 39.16 38.82 39.22

y0.5 y1.2 2.2 y0.9 y2.1 y0.3

y17.9 y1.0 y11.3 y15.9 1.3 y14.5

Fogo F-106403a F-106444a

19.40 18.90

15.57 15.53

39.03 38.69

y2.2 y1.4

y4.9 21.2

Isotope analyses were performed in Copenhagen on a VG Sector 54-30 multicollector instrument. Pb isotope analyses were corrected for mass fractionation using measured values of NBS981 ŽTodt et al., 1993. and amounted to 0.12 " 0.02 Ž1 s .%rAMU. a Previously published by Kokfelt et al. Žsubmitted..

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Ž1988. and Davies et al. Ž1989., isotopic differences exist between the northern and southern Cape Verde Islands, as the northern islands exhibit more radiogenic Pb and Nd isotope- and less radiogenic Sr isotope signatures.

7. Discussion Before discussing geodynamic or petrological aspects of the data, we examine possible late processes that may have affected the initial isotopic signatures. He produced by post eruptiÕe decay of U and Th: Sample SA-111822 exhibits the lowest measured wHex and is therefore most susceptible to contamination processes. However, the radiogenic He-signature of SA-111822 is very unlikely to be explained by post eruptive decay of Th and U after eruption, because of the young age of this lava Ž0.4–0.9 Ma, Plesner et al., in preparation.. The He-contents in the other analysed samples are higher, and younger ages, combined with a relatively low diffusion rate of He in olivine at low temperatures, do not permit significant ingrowth of radiogenic He in the inclusions originating from decay of U in or outside the inclusions. Moreover, as inherited magmatic He is thought to be preferentially released by the crushing method Že.g. Graham et al., 1992; Hanyu et al., 1999., radiogenic 4 He produced in the rock matrix after eruption is unlikely to account for the radiogenic He-signature. Contribution from crustal components: Another possible source of radiogenic He is from crustal fluids in shallow magma chambers Že.g. Condomines et al., 1983; Hilton et al., 1995.. Both Santo Antao ˜ and Fogo are young islands, where magmatism has persisted for probably - 10 Ma. The islands are situated on ca. 120 Ma old incompatible element depleted MORB crust. The lavas chosen are all very primitive in composition, indicating short crustal residence times, if any, for both phenocrysts and melts. Xenocrystal olivine would have a longer crustal residence time. If we assume residence of the magma in the crust before eruption, a rough estimate of the temperature in the wall rocks around a body of magma is half the initial magma temperature ŽBest, 1982.. If we suppose a maximum temperature of 7008C at the boundary between the magma and the

surrounding crustal rocks, contamination with He from the crust into the crystals at the interface seems unlikely considering the low diffusion coefficient for He in olivine Ž D s 2.47)10y9 cm2rs at 7008C, calculated from data in Hart, 1984b.. Surface conditions also prohibit significant He-diffusion into olivine. Cosmogenic He: Subaerial lavas exposed to cosmic rays for sufficient time, especially at high altitude, may exhibit excess 3 He resulting in high 3 Her 4 He isotopic ratios. Several experiments performed on olivine and clinopyroxene phenocrysts exposed to cosmic rays over long periods have shown that, by crushing in vacuum, only magmatic helium contained in vesicles is released, in contrast to step heating experiments where both cosmogenic and radiogenic helium is released ŽCraig and Poreda, 1986; Kurz, 1986a,b.. On this basis, we believe that the measured 3 Her4 He ratios are representative of the inherited magmatic helium. Atmospheric contamination has been suggested to explain low 40Arr36Ar ratios Že.g. Fisher, 1985; Jambon et al., 1985; Patterson et al., 1990, 1991; Farley and Craig, 1994.. In the present case, significant amounts of atmospheric argon adsorbed on grain surfaces is precluded as a result of baking the samples and gas extraction by crushing. In previous studies on OIB lavas, Ar isotopic analyses have mainly been performed on basalt glasses, where contamination with atmospheric Ar during eruption in seawater seems likely. The high vesicularity and outgassed state of the lavas makes them more susceptible to contamination, than in the subaerially erupted lavas used in this study ŽFarley and Craig, 1994.. Moreover, the present analyses are made on olivine and clinopyroxene phenocrysts, phases that presumably are least affected by atmospheric contamination ŽFarley and Craig, 1994.. In fact Staudacher et al. Ž1990. exclude atmospheric Ar contamination in Reunion olivine samples due to low diffusion rates. Farley and Craig Ž1994., however, conclude that the large Ar isotopic variation Ž400– 7700. in olivine phenocrysts from a single lava flow from Juan Fernandez hotspot is due to addition of air-derived noble gases. We consider this unlikely because of the slow diffusion rate for Ar Ž- 0,15 mmra at 12008C. and a higher closure temperature than for He ŽStaudacher et al., 1990.. We believe

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that the Ar isotopic ratios measured in this study most likely resemble the Ar signature of the parental mantle-derived magmas, as the phenocrysts did not reside in shallow magma chambers for long due to the primitive character of all analysed minerals ŽMga ) 80. and lavas ŽMgO ) 9 wt.%..

the low 4 Her3 He in Cape Verde samples as evidence for a lower mantle involvement. As mentioned previously, the radiogenic component has various explanations to which we will return later on.

7.1. Helium

The available argon data from hotspot volcanics is limited to Hawaii, Reunion, Iceland, Samoa, Afar, Juan Fernandez, Cook Austral and Society Islands Že.g. Staudacher et al., 1986, 1990; Harrison et al., 1999; Burnard et al., 1994; Poreda and Farley, 1992; Marty et al., 1993; Farley and Craig, 1994; Hanyu et al., 1999.. These data display a large range of 40 Arr36Ar ratios: from near atmospheric in Loihi basalt glasses Že.g. Staudacher et al., 1986. up to 12,000 in Samoan xenoliths ŽPoreda and Farley, 1992.. All of the reported argon data, except from Cook Austral and Society Islands, correspond to low 4 Her3 He data ŽFig. 3., which make the argon signature of the lower mantle a subject of great debate. In Fig. 3, 4 Her3 He vs. 40Arr36Ar data from Fogo and Santo Antao ˜ are compared with MORBs and other OIBs. Santo Antao ˜ and Fogo show an Ar isotopic variation very similar to Reunion Island, where two possibilities have been discussed to explain the observed variation ŽKaneoka et al., 1986; Staudacher et al., 1990.. Either the 40Arr36Ar ratio of the source is higher than the highest measured ratio and the low ratios are due to atmospheric Ar contamination, or the source 40Arr36Ar ratio is low and the highest measured ratios are due to entrainment of MORBderived Ar. Of these two possibilities, at least the first is unlikely due to the proposed lack of atmospheric Ar contamination at both the analytical and magmatic stage. A contribution from the lower mantle, according to 4 Her3 He ratios lower than MORB values, could suggest a low 40Arr36Ar signature in the lower mantle. The limited and low variation in Ar isotopic ratios on Fogo and Santo Antao ˜ Žfrom 350 to 1300., however, seems to eliminate the depleted MORB mantle as an important contributor to the Cape Verde volcanism. A third explanation for low 40Arr36Ar could perhaps be the role of subducted oceanic crust contaminated by atmospheric Ar ŽSarda et al., 1999., although large amounts of atmospheric Ar have not been recycled to the mantle as mantle-derived mate-

A notable feature of the He isotopic ratios measured at the Cape Verde Islands is the occurrence of ratios both higher and lower than the MORB range. This was also observed at Heard island and the Azores archipelago ŽHilton et al., 1995; Moreira et al., 1999., whereas most previous studies on oceanic islands yielded ratios either higher or lower than MORB. Reunion island, for instance, shows almost constant helium isotopic ratios between 12 and 14 RrRa over a long time scale Že.g. Staudacher et al., 1990; Graham et al., 1990., indicating a uniform mantle source region with respect to helium. The variation at some Hawaiian volcanoes ŽHaleakala, Mauna Loa, Mauna Kea. is thought to be age-correlated due to a decreasing contribution from the plume with time and therefore a decreasing 3 He concentration ŽKurz et al., 1987, 1996.. The variation at Santo Antao ˜ and Fogo does not show any consistent variation with age, but indicates instead both a contribution from sources like those feeding Hawaii and Reunion giving low 4 Her3 He, and a component with high 4 Her3 He as observed at, for example, St. Helena or Gough island. In a two-layered mantle model Že.g. O’Nions and Oxburgh, 1983; Allegre ` et al., 1983, 1986; Kellogg and Wasserburg, 1990; O’Nions and Tolstikhin, 1994; Porcelli and Wasserburg, 1995., the presence of several samples from the Cape Verde Islands with 4 Her3 He ratios lower than MORB would indicate an involvement of the lower mantle. A 3 He contribution from the lower mantle either suggests that the Cape Verde plume rises directly from this reservoir ŽFarley et al., 1992. or that the plume rises from the base of the upper mantle, in which case helium has to migrate into the plume from the lower mantle, or alternatively, that lower mantle material becomes entrained due to instabilities in the mesosphere boundary layer ŽAllegre ` and Turcotte, 1985; Allegre, ` 1987; O’Nions and Tolstikhin, 1994.. We interpreted

7.2. Argon

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Žstepheating on olivine.. Analyses Fig. 3. 4 Her3 He vs. 40Arr36Ar. All data are analysis of gas released by crushing except from Reunion ´ from Iceland and Hawaii were performed on glasses and from Samoa on xenoliths. Polynesia, Afar and a few analyses from Iceland are like Cape Verde data from olivine and clinopyroxene separates. Note that He-isotopic ratios performed on clinopyroxene are plotted vs. Ar-isotopic ratios from olivine for samples F-106444 and F-106403. The MORB field between the stipled lines represent 8 " 1 RrRa. Pol. stands for Polynesian. Data sources: Polynesia: Hanyu et al. Ž1999.; Samoa: Poreda and Farley Ž1992.; Reunion: Staudacher et al. Ž1990.; ´ Afar: Marty et al. Ž1993.; Iceland: Burnard et al. Ž1994. and Harrison et al. Ž1999.; Loihi and Hualalai: Staudacher et al. Ž1986..

rials may have high 40Arr36Ar ratios ŽStaudacher and Allegre, 1988.. Sarda et al. Ž1999. have recently ` shown a broad positive correlation between 40Arr36Ar and 206 Pbr204 Pb in mid-Atlantic ridge basalts with 4 Her3 He ) 75,000 f 9.5 RrRa. Samples with 4 Her3 He - 75,000 were not used in the study to avoid any influence from primitive plumes. They explain the correlation as mixing between the DMM and a mantle component with radiogenic lead, which is thought to be recycled in origin. This recycled component is characterised by 206 Pbr204 Pb ratios between 19 and 21 and low 40Arr36Ar ratios of 300 to 1000. The extreme HIMU islands Mangaia, Tubuaii and Rurutu have recently provided 40Arr36Ar ratios in the range of atmospheric to 1400 coupled with radiogenic He isotopes ŽHanyu et al., 1999.. Moreover, Mangaia basalts are characterised as having the most radiogenic lead isotopes observed in OIB magmatism, and the origin of the corresponding HIMU source has been related to recycling of oceanic

crust ŽChauvel et al., 1992; Woodhead, 1996.. These new observations could suggest a recycled origin for the low Ar isotopic ratios from Santo Antao ˜ and Fogo. The samples showing higher 40Arr36Ar ratios can be explained by contamination of phenocryst olivine separates with olivines from disaggregated xenoliths, as suggested by Staudacher et al. Ž1990. for Reunion samples. For sample SA-111940, this ´ reasoning is in agreement with petrography as the lava contains mantle xenoliths. Sample SA-111822 and SA-111832 are very phenocryst-rich, but show no signs of mantle olivines, although it cannot be ruled out that some olivines may represent disaggregated xenoliths. 7.3. Lead Lead isotopes from Fogo and Santo Antao ˜ show a good correlation of both 207 Pbr204 Pb and 208 Pbr 204 Pb with 206 Pbr204 Pb ŽFig. 4., which could be

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Fig. 4. Ža. 206 Pbr204 Pb vs. 207 Pbr204 Pb, Žb. 206 Pbr204 Pb vs. 207 Pbr204 Pb. Fields for selected OIB and MORB, the Northern Hemisphere Reference Line ŽNHRL. as defined by Hart Ž1984a. and the fields representing the theoretic components C ŽHanan and Graham, 1996., FOZO ŽHauri et al., 1994. and PHEM ŽFarley et al., 1992. are also shown. The Azores field excludes Sao ˜ Miguel and MORB 10–178N excludes the 148N anomaly. Small triangles represent data from the northern Cape Verde Islands: Santo Antao ˜ and Sao ˜ Vicente; small plus signs represent southern Cape Verde Islands: Fogo, Santiago and Maio. Note the location of the majority of the Santo Antao ˜ and northern Cape Verde Islands data points inside the limits of the C component in both diagrams. Data sources: Mangaia: Woodhead Ž1996.; St. Helena: Graham et al. Ž1992.; Azores: Moreira et al. Ž1999.; MORB: Dosso et al. Ž1991.; Loihi: Eiler et al. Ž1998.; Cape Verde Islands: Gerlach et al. Ž1988. and Davies et al. Ž1989..

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explained simply by binary mixing, like the model of Gerlach et al. Ž1988. for the origin of the northern Cape Verde Islands, as mixtures between HIMU and DMM. Thirlwall Ž1997. precludes this HIMU–DMM mixing model for several OIBs due the existence of negative D7r4 Pb values, which cannot be generated by mixing of extreme HIMU Ž206 Pbr204 Pb ) 20.5. and DMM. Although some extreme HIMU samples from Polynesia have negative D7r4 Pb values, mixing with MORB would yield maximum negative D7r4 Pb values of y1 for 206 Pbr204 Pb - 20 ŽThirlwall, 1997.. In addition, extreme HIMU– MORB mixtures cannot produce higher 87 Srr86 Sr ratios, similar or higher CerPb and similar or lower KrNb ratios than those of extreme HIMU, ruling out the development of negative D7r4 Pb OIBs as extreme HIMU–MORB mixtures ŽThirlwall, 1997..

Vidal Ž1992., Chauvel et al. Ž1992. and Thirlwall Ž1995. suggest the existence of young high-m mantle to explain the low 207 Pbr204 Pb to a given 206 Pbr 204 Pb or 208 Pbr204 Pb. On the basis of several incompatible trace element ratios characteristic of HIMU lavas, relatively radiogenic Pb isotopes and slightly negative D7r4 Pb values Žmax. y2. at Cape Verde Islands, we suggest a significant contribution from a HIMU mantle source, probably young in age. Lead isotopic ratios in samples from Santo Antao ˜ and Fogo display a correlation with 4 Her3 He that is almost within the variation observed at the Azores, though with less radiogenic Pb ŽFig. 5.. Mixing of a extreme HIMU Ž206 Pbr204 Pb ) 20.5. source with the DMM cannot explain the primitive He isotopic ratios observed on both Fogo and Santo Antao. ˜ Instead, mixing of a young HIMU source having

Fig. 5. 4 Her3 He vs. 206 Pbr204 Pb for Santo Antao ˜ and Fogo samples. Fields for selected OIB and MORB are shown. Azores field excludes Sao ˜ Miguel and MORB 10–178N excludes the 148N anomaly. Note that mixing of DMM and a HIMU component Žrepresented by the St. Helena field or situated even further to the right. cannot give rise to the low 4 Her3 He ratios common among the Cape Verde samples. Data sources: Loihi: Eiler et al. Ž1998.; Azores: Moreira et al. Ž1999., a part of the data are read from Fig. 5 in Moreira et al. Ž1999.; St. Helena: Graham et al. Ž1992.; MORB: Dosso et al. Ž1991. and Staudacher et al. Ž1989..

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radiogenic He and radiogenic but not extreme Pb isotopic ratios and a source with less radiogenic lead and a primitive He isotopic signature are possible explanations. Sample SA-111822 is below the Heisotope MORB interval Ž8 " 1 RrRa. even with its large analytical uncertainty, and indicate a contribution from a radiogenic component. 7.4. Composition and origin of the Cape Verde source A lower mantle component contributing to the Cape Verde magmatism is evident from the observed lower 4 Her3 He ratios than MORB. Several workers, recently Hilton et al. Ž1999., have argued for the existence of a fifth mantle end member common to many OIBs and named it FOZO ŽHart et al., 1992; Hauri et al., 1994., PHEM ŽFarley et al., 1992. and C ŽHanan and Graham, 1996., respectively. FOZO, PHEM and C are defined by roughly similar intermediate Sr, Nd and Pb isotopes. Also common to FOZO and PHEM are primitive high 3 Her4 He values, whereas C can have both higher and lower 3 Her4 He values than MORB. The northern Cape Verde Islands fall almost within the field of C with regard to Sr, Nd and Pb isotopes ŽFig. 4.. The high 4 Her3 He ratios also observed on the Cape Verde Islands indicates a component like those in EM1- or HIMU-OIBs. Moreira et al. Ž1999. argue that the very radiogenic Pb and He isotopes from eastern Sao ˜ Miguel is due to incorporation of Jurassic delaminated subcontinental lithosphere. Kokfelt et al. Žsubmitted. use delaminated Jurassic subcontinental lithosphere instead as an EM1-end-member for the genesis of the southern Cape Verde Islands. An EM1 component is not obvious in the northern Cape Verde Islands, due to less radiogenic Sr and more radiogenic Pb and Nd isotopic ratios and certain trace element ratios ŽGerlach et al., 1988; Davies et al., 1989.. The EM1 component is therefore rejected as being responsible for the radiogenic He isotopes from Santo Antao, ˜ but cannot be excluded for Fogo. Several factors, in addition to high 4 Her3 He ratios, such as relatively radiogenic Pb isotopes, some incompatible trace element ratios and low 40Arr36Ar ratios, indicate a significant contribution from a HIMU component at Cape Verde. The HIMU com-

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ponent is probably young in age due to negative D7r4 Pb values coupled with relatively high, but not extreme, 206 Pbr204 Pb, and trace element ratios typical of HIMU OIB. Thirlwall Ž1997. notes the Sr, Nd and Pb isotopic similarities between young HIMU islands and the C component of Hanan and Graham Ž1996., and concludes that all negative D7r4 Pb OIBs can be generated from young HIMU mantle with only minor admixtures. In fact, Hanan and Graham Ž1996. propose a recycled origin for the C component and suggest a location in a transition zone below which the high 3 Her4 He reservoir is situated. We find the young high-m mantle model of Thirlwall Ž1997. appropriate to explain the observed trace elements and Sr, Nd and Pb isotopic variation at Santo Antao. ˜ However, we believe that even young high-m mantle sources must be expected to have lower 3 Her4 He than MORB, as the 3 Her4 He ratio of fresh MORB will decrease from 8 to 5 Ra in only 100 Ma ŽGraham et al., 1992.. A He contribution from the lower mantle is therefore needed to explain the primitive He signatures in several Cape Verde samples, which may result from either entrainment of lower mantle material or He migration. We propose a model involving primordial lower mantle He and a young high-m source to account for the observed combination of high and low 4 Her3 He ratios, relatively radiogenic Pb isotopes, negative D7r4 Pb values and HIMU-like trace element ratios. A similar explanation could be proposed for Fogo according to He and Ar isotopes. However, the EM1-type enrichment in the source, which is indicated by Sr, Nd and Pb isotopes and incompatible trace elements, suggest a more complex process with the involvement of an additional component, which we refrain from elaborating on due to the small number of He and Ar analyses presently available from Fogo. On the basis of this study, it is impossible to estimate the 40Arr36Ar signature of the lower mantle. Involvement of low 4 Her3 He coupled with low 40 Arr36Ar could suggest a low Ar signature of the lower mantle; another possibility is helium migration from the lower mantle without Ar ŽAllegre, 1987.. ` We believe that the observed Ar-variation can be explained from either the HIMU source alone or as a mixture of HIMU and lower mantle Ar, perhaps with minor modification from the DMM. The HIMU–

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DMM mixing model for the northern Cape Verde Islands of Gerlach et al. Ž1988. is ruled out on the basis of the samples with low 4 Her3 He ratios, low 40 Arr36Ar ratios, negative D7r4 Pb values and HIMU-like trace elements, which cannot be produced by mixing of such end-members.

8. Conclusions We have reported helium isotopic signatures from the Cape Verde Islands both lower, within and higher than the MORB range, together with a limited 40 Arr36Ar isotopic variation from atmospheric to 1250. Lead is relatively radiogenic and Pb isotopic ratios show a weak positive correlation with 4 Her 3 He. A HIMU component in the volcanics of the Cape Verde Islands is evident from high 4 Her3 He, relatively radiogenic Pb isotopes, low 40Arr36Ar and HIMU trace element characteristics such as: HFSE enrichment, low LILErNb, BarLa, ZrrNb and high CerPb, ZrrHf. The HIMU component, moreover, is perhaps young in age due to slightly negative yD7r4 Pb values. A contribution from a relatively undegassed reservoir, probably the lower mantle, is indicated by the common low 4 Her3 He ratios. Contribution to the samples of low 4 Her3 He coupled with low 40Arr36Ar suggests that the lower mantle has relatively unradiogenic Ar, but the exact 40Arr36Ar signature is impossible to clarify. Another possibility is that only He migrates from the lower mantle to the boundary layer and that the observed Ar isotopic variation has an origin by recycling. We propose that the He and Ar isotopic variations can be explained by mixing of HIMU with or without lower mantle Ar and only a minor, if any, contribution from the DMM. A DMM–HIMU mixing model is, in the light of low 40 Arr36Ar ratios, low 4 Her3 He ratios, negative D7r4 Pb values and HIMU-like trace elements, rejected for the northern Cape Verde Islands. We suggest a model for Santo Antao ˜ related to recycling of oceanic crust to a thermal boundary layer separating the lower undegassed and upper degassed mantle. Low 4 Her3 He "40Arr36Ar from the lower mantle contributes to this layer either

through rising of small plumes or migration of helium, before new plumes consisting mainly of recycled crust ascend from the boundary layer.

Acknowledgements We gratefully thank Nicole Vassard for her technical support during performing and improving the noble gas analyses at Universite` Pierre et Marie Curie. We thank Malene Hein for her assistance during sampling at Santo Antao, ˜ and Thomas F. Kokfelt and Rikke Pedersen for donating samples from Fogo and Santo Antao. ˜ John Bailey and Raymond Gwozdz are thanked for the XRF and INA analyses on trace elements. We thank Ph. Sarda and an anonymous reviewer for improving the manuscript through their comments. The Natural Sciences Research Council supported the Cape Verde project, Grant no. 9401647 to PMH, and Hotelejer Mansson ˚ og hustrus Legat, William og Anna Evers Legat and the Faculty of Natural Sciences travel grant supported BPC.

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