238U–230Th–226Ra fractionation in historical lavas from the Azores: long-lived source heterogeneity vs. metasomatism fingerprints

$Page 1: 238U–230Th–226Ra fractionation in historical lavas from the Azores: long-lived source heterogeneity vs. metasomatism fingerprints$
Ž .Chemical Geology 176 2001 295–310www.elsevier.comrlocaterchemgeo

238U–230Th–226Ra fractionation in historical lavas from theAzores: long-lived source heterogeneity vs.

metasomatism fingerprints

Christelle Claude-Ivanaja,b,), Jean-Louis Joronc, Claude J. Allegreaà Laboratoire de Geochimie et Cosmochimie, URA-CNRS 1758, Institut de Physique du Globe, UFR des Sciences de la Terre,´

UniÕersite Paris VII Denis Diderot, 4 Place Jussieu, 75252 Paris cedex 05, France´b Max-Planck-Institut fur Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany

c Laboratoire Pierre Sue, CErSaclay, B.P. 2, 91190 Gif-sur-YÕette, France¨

Received 24 December 1999; accepted 2 October 2000

Abstract

This study reports238U–230Th–226Ra nuclides measured by mass spectrometry, trace elements and87Srr86Sr isotopes onŽ230 232 . Žhistorical alkali basalts from the Azores region. The Thr Th activity in the alkali basalts spans a range from 0.897 in

. Ž .Sao Miguel to 1.382 in Terceira island that is almost as large as the global variation of mid ocean ridge basalts MORBŽ . Ž230 238 . Ž . Ž . 87 86 Žand oceanic island basalts OIB . The Thr U activity 10–40% , ThrU 2.92–3.74 and the Srr Sr 0.70354–

. Ž226 230 .0.70467 also display large variations. The Rar Th activity varies from 0.84 in Sao Miguel to 1.60 in Pico island.Ž230 238 . Ž226 230 .Samples from Pico, Faial and Terceira islands are found to plot on the negative Thr U vs. Rar Th correlation

w Ž . xdefined by the 1730–1736 eruption of Lanzarote, Canary islands Sigmarsson et al., Earth Planet. Sci. Lett. 162 1998 137 ,except for the sample from Sao Miguel island. We propose that the negative correlation reflects a mixing between two

Ž230 238 . Ž226 230 .end-member melts, one displaying a low Thr U and a high Rar Th and the other end-member displaying aŽ230 238 . Ž226 230 . 226 230high Thr U and a low Rar Th . Numerical model suggests that the high Ra, low Th excesses

end-member represented by samples from Pico island can be explained by partial melting of a garnet lherzolite. The otherend-member represented by the Terceira lava is shown to result from the interaction of low-degree melts with themetasomatized mantle containing phlogopite. Phlogopite crystals in contact with a melt enriched in226Ra and Ba have aŽ226 230 .Rar Th 41 and high BarTh because of crystal affinities for Ra and Ba relative to Th and U. After 8000 years,however, the unsupported226Ra excess will have decreased down to 0 as phlogopite displays little amount of U and Th. So,during the process of interaction with phlogopite crystals, low-degree melts formed in the garnet stability field that are

Ž230 238 . Ž226 230 . 226characterized by high Thr U , Rar Th and BarTh will loose the Ra excess, scavenged by phlogopite whileŽ230 238 .the Thr U and BarTh will remain unaffected. One finds that this process may be valid over 250 years maximum so

Ž230 238 . Ž226 230 .that only the first interacting melts may preserve a high Thr U , low Rar Th activity. On Sao Miguel island, itŽ230 238 .is shown that the low Thr U , while the source is clearly enriched in Th, cannot be explained by a lack of garnet in the

) Corresponding author. Max-Planck-Institut fur Chemie, Abteilung Geochemie, Saarstrasse 23, Postfach 3060, D-55020 Mainz,Germany. Fax:q49-6131-37-10-51.

Ž .E-mail address: [email protected] C. Claude-Ivanaj .

0009-2541r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 00 00406-X

( )C. Claude-IÕanaj et al.rChemical Geology 176 2001 295–310296

226 Ž .mantle neither by a higher degree of melting. The Ra deficit 15% and low BarTh and SrrTh are explained by partialmelting of an enriched mantle in the presence of phlogopite.q2001 Elsevier Science B.V. All rights reserved.

Keywords: 238U–230Th–226Ra disequilibria; Azores; Mantle metasomatism

1. Introduction

The discovery of ubiquitous fractionation ofshort-lived nuclides namely238U, 230Th and 226Ra,has led to significant improvements in the under-standing the magma genesis in various volcanic set-

Žtings Overby and Gast, 1968; Allegre and Con-`domines, 1982; McKenzie, 1985; Condomines et al.,

. 2381988 . Among the conditions to fractionate Ufrom 230Th and 230Th from 226Ra leading to theformation of radioactive disequilibria, the presenceof garnet in the source and the degree of melting are

Ž .particularly important Beattie, 1993a . In oceanicŽ .island basalts OIBs , radioactive disequilibria are in

general better explained by these two factors than inŽ .mid ocean ridge basalts MORBs . Indeed, in MORB

settings the apparent conflicting observation of large238U–230Th and230Th–226Ra radioactive disequilib-ria with high degree of melting gave birth to theelaboration of dynamic models, where the durationof the melting process is another important factorŽMcKenzie, 1985; Williams and Gill, 1989; Spiegel-

.man and Elliott, 1993, Cohen and O’Nions, 1993 .The provenance of OIBs from strongly heteroge-

neous reservoirs is a striking feature. Recent studieshave attempted to consider some correlations be-tween 238U–230Th–226Ra radioactive disequilibriaand the chemical diversity of OIBs. Bourdon et al.Ž .1996a have suggested that the fertility of the sourceor the volatile content, affecting theD rD , mightU Th

have an influence on238U–230Th disequilibria. In theComores, on the basis of226Ra–230Th data, it hasbeen proposed that melts generated by partial melt-ing in the presence of amphibole, stable in thelithospheric mantle, have a lithosperic signatureŽ .Claude-Ivanaj et al., 1998 . On Lanzarote island, ithas been proposed that the large variation in238U–230Th–226Ra disequilibria from a single eruptionmight reflect the mixing of Gt-pyroxenites and Lher-zolites-derived melts, andror contamination by flu-

Ž .ids Sigmarsson et al., 1998 .

The volcanism from the Azores is one examplewhere a very large geochemical diversity is observedboth in terms of radiogenic isotopes, trace elements

238 230 Žand U– Th disequilibria Schilling, 1975; Whiteet al., 1976; Hawkesworth et al., 1979; Dupre et al.,´1982; Widom et al., 1997; Turner et al., 1997;

.Moreira et al., 1999 . Here, we present a combinedstudy of 238U–230Th–226Ra disequilibria, major andtrace elements with a Sr isotopic study on a well-dated lava suite from the Azores in order to addressthe possibility of a relationship between the238U–230Th–226Ra element fractionation and the geochem-ical complexity of the Azores. In particular, a specialinterest is devoted to the use of Ra–Th disequilibriaas a tool to provide information on the short time

Ž .scales processes 100–10000 years .

2. Geological setting and geochemistry of theAzores region

The Azores volcanic chain, located at the triplejunction between the African–Eurasian and North-American plates, emerges from the Azores Plateau, atopographic and gravity high near the Mid Atlantic

Ž .ridge MAR . Both geochemical and geophysicalstudies document evidence for ridge-hot spot interac-

Žtions White and Schilling, 1978; Bourdon et al.,.1996a . It is likely that the actual Sr, Nd and Pb

isotopic compositions of the upper mantle underlyingŽ .the Azores Plateau FAZAR MORBs reflects the

Žcontamination by the Azores plume Schilling et al.,.1983; Dosso et al., 1996 . Similarly, a MORB com-

ponent is present in the source of all islands and isakin to the mean composition of the FAZAR MORBsŽ .Bourdon et al., 1996a; Dosso et al., 1996 . ThisMORB component contributes to the heterogeneityof the Azores volcanics, in addition to two othercomponents typically observed in Terceira and SaoMiguel islands. The Azores plume heterogeneity hasbeen sometimes described in terms ofARegional

( )C. Claude-IÕanaj et al.rChemical Geology 176 2001 295–310 297

Ž .plume componentB Terceira island andALocalisedŽ .Sao Miguel componentB Turner et al., 1997 . In the

Pb–Sr isotopic diagram, samples from Terceira is-land plot between the depleted mantle and the HIMUfield suggesting some recycled component in its

mantle source. It has been proposed that pyroxenites,derived from recycled oceanic crust, could have such

Ž .characteristics Hofmann and White, 1982 . Thisinterpretation contrasts with the recent study of

Ž .Turner et al. 1997 who favor some metasomatised

Ž .Fig. 1. Faial, Pico, Sao Miguel and Terceira islands sampling maps, modified after Forjaz et al. 1990 .


Table 1Ž . Ž .Major elements in wt.% , Ni, Co and Sc contents in ppm in the Azores volcanics. Dates are eruption ages

aSample SiO Al O Fe O tot MnO MgO CaO Na O K O TiO P O Sc Ni Co2 2 3 2 3 2 2 2 2 5

Sao MiguelŽ .ACO95-3 1563 47.81 13.68 11.79 0.16 8.69 9.03 3.18 2.48 2.98 0.46 18.5 178 47.2

TerceiraŽ .ACO95-8 1760 49.03 15.41 12.35 0.21 4.51 8.59 4.46 1.37 3.14 1.27 15.6 17.4 22.7

PicoŽ .ACO95-19 1718 51.51 16.07 9.59 0.17 5.59 7.46 5.26 2.14 1.80 0.48 13.1 97.2 25.2Ž .ACO95-24 1562 48.13 14.98 10.63 O.14 9.26 10.40 3.21 0.94 2.37 0.41 20.1 197 42.6Ž .ACO95-28 1718 46.63 14.15 11.01 0.15 9.88 11.24 3.09 1.00 2.68 0.46 22.2 190 43.3Ž .ACO95-31 1718 46.82 15.03 10.93 0.15 8.18 11.14 3.48 1.15 2.75 0.51 21.2 134 40.0

FaialŽ .ACO95-45 1958 47.78 16.26 10.33 0.14 7.79 9.46 3.89 1.54 2.57 0.52 18.3 126 35.7

lithospheric mantle rather than pyroxenites. The par-ticipation of lower mantle in the source of the Azoresplume has been suggested based on He and Os

Ž .isotopes Widom et al., 1997; Moreira et al., 1999 .In the source of Sao Miguel island, the presence ofan old recycled component has been proposed toexplain very radiogenic isotopic and trace elementenriched signatures. This material could either be

delaminated sub-continental lithosphere or sedimentsŽHawkesworth et al., 1979; White et al., 1976;

.Widom et al., 1997; Turner et al., 1997 . Theseinterpretations also agree with the high4Her3He

Žisotopic compositions found in this island Moreira.et al., 1999 . All the islands, with the exception of

Santa Maria island, display historical volcanismŽ .Flower, 1976 .

Table 2Ž . Ž y1.U, Th, Ba, Sr, Nb, Rb, Zr in ppm and measured Ra in fg g contents, ThrYb, ThrU and BarTh in the Azores volcanics

a a a ba Sample U Th Ra Ba Sr Rb Nb Zr Yb ThrYb ThrU BarTh

Sao MiguelŽ .ACO95-3 1563 1.807 6.709 0.581 349.5 550.5 62.1 61.0 363 2.11 3.18 3.713 52.09

Dupl. 0.578 2.23 3.01

TerceiraŽ .ACO95-8 1760 1.450 4.468 0.668 723.0 768.0 32.1 59.2 293 3.53 1.27 3.081 161.82

Dupl. 3.33 1.34

Pico)Ž .ACO95-19 1718 1.764 5.420 0.973 542.0 518.0 51.8 62.7 347 2.80 1.94 3.073 100.00

Ž .ACO95-24 1562 0.770 2.317 0.450 260.0 547.0 22.1 32.1 186 1.90 1.22 3.009 112.55Ž .ACO95-28 1718 0.859 2.706 0.483 276.0 722.0 21.9 35.8 209 1.93 1.40 3.150 102.00Ž .ACO95-31 1718 0.994 2.903 0.505 321.8 729.0 26.5 40.1 220 2.08 1.39 2.921 110.83

Faial)Ž .ACO95-45 1958 1.081 3.546 0.618 453.0 636.0 36.1 42.6 253 2.03 1.75 3.279 127.75

aICP-MS measurements.bINAA measurements.


3. Sampling

Samples collected for this study are historicalŽbasalts from Sao Miguel erupted in 1563, Pico de

. Ž .Queimado , Faial erupted in 1958, Capelinhos , PicoŽerupted in 1562, Misterio da Prainha and in 1718,

.Misterio de Silveira and Sao Jao and TerceiraŽ . Ž .erupted in 1761, Pico de Fogo islands Fig. 1 .Very fresh lavas were analyzed by TIMS for U-series,Sr isotopes, Sr and Ba concentrations. The tech-niques used for the analyses are described in Ap-pendix A.

4. Results

4.1. Major and trace elements systematics

Major and trace elements are presented in Tables1 and 2 and displayed in Figs. 2–4. Our data plot inthe trends defined by the more substantial data set

Ž . Ž .from Turner et al. 1997 and Widom et al. 1997 .In terms of major elements, most of the samples

Ždisplay a rather unfractionated composition hawai-. Ž .ites Le Bas et al., 1986 . The most evolved lava is

sample ACO95-19 from Pico island which plots in

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Fig. 2. Crystal fractionation in the Azores lavas a Ni in ppm , b Sc ppm vs. MgO wt.% , c Sr ppm vs. Th ppm contents. TheŽbold line corresponds to the fractionation model of 5% ol, 75% cpx and 20% plag removal in Pico samplesK s1.9, K s3,MgOrcpx MgOrOl

Ž . Ž .K s0, K s f MgO , K s0.08 according to Hart and Davis 1978 ,K s2, K s0.37, K s1.8, K sMgOrpl NirOl Nirpl Nircpx Scrpl Scrcpx Scrpl. Ž .0.065, K s2.7–5 . Black symbols are from this study; open symbols are literature data from Turner et al. 1997 and Widom et al.Srrpl

Ž .1997 .


Ž . Ž226 Ž230 . Ž .Fig. 3. a Rar Th disequilibria vs. MgO wt.% . The fractionation model derived from the Ni, Sc and Sr variations cannot explainŽ226 230 .the range in Rar Th . The dotted line is the fractionation model that would fit the Pico data. It corresponds to a bulkD s0.4,Ra

Ž . Ž226 230 .calculated with 75% albiteD s0.6 q25% cpx that would be required to match the decrease in Rar Th down to 1.24. This isRaŽ . Ž .highly unlikely for this type of samples.D in plagioclase An30 has been calculated using elastic moduli Blundy and Wood, 1994 ,Ra

Ž . Ž . Ž . Ž . Ž . . Ž226 . Žother partition coefficients are taken from Irving 1978 and Beattie 1993a,b . b Ba ppm c , BarTh RarBa vs. Th contents in. Ž .ppm . The modeled bold line is as in Fig. 2. d Same symbols as in Fig. 2.

the mugearite field in the Na OqK O vs. SiO2 2 2

plot. Relative to the samples from Pico, the Terceirahawaiite has a lower SiO , Na O, K O, Al O but a2 2 2 2 3

higher Fe O , CaO, TiO and P O contents. De-2 3 2 2 5

spite a more radiogenic87Srr86Sr, the hawaiite fromFaial lies in all the trends defined by Pico samplesŽ .Figs. 2–4 . The Sao Miguel sample, which is clearlyenriched in K O with respect to Terceira, Pico and2

Ž .Faial samples, is a potassic trachy-basalt Table 1 .On Sao Miguel, an increasingly potassic composition

towards the east has been shown by Widom et al.Ž .1997 to be independent of MgO content althoughto reflect a variation in the source composition. Thissample is characterized by an unfractionated compo-

Ž .sition with high MgO 8.7% and transition elementsŽ .contents Table 1 .

Across the archipelago, from Terceira to SaoMiguel islands, a gradual enrichment in trace ele-ments has been described as representative of the

Ž .signature of the Azores plume Flower, 1976 . The


Ž . Ž87 86 . Ž . Ž230 232 . Ž . Ž230 238 . Ž . Ž226 230 .Fig. 4. Variations of a Srr Sr , b Thr Th , c Thr U d Rar Th with the ThrYb in the Azores lavas plottedŽ .with other data from the literature Turner et al., 1997; Widom et al., 1997 . Parentheses denote activity ratio; same symbols as in Fig. 2.

samples from Terceira and Sao Miguel islandsdisplay trace element characteristics typical of theARegional plumeB and ALocalized Sao MiguelB

Ž .compositions, respectively Turner et al., 1997 . TheTerceira lava has incompatible trace element ratiosthat are among the most depleted in this data setŽ .ThrYb, ThrU although is has a very high BarTh

Ž .and SrrTh Table 2, Figs. 3 and 4 . High BarThand SrrTh have already been described as a geo-

Žchemical feature of the Azores plume Weaver, 1991;.Turner et al., 1997 . Relative to the other samples

with a similar degree of differentiation, the SaoMiguel lava is markedly enriched in Th, Nb, Rb andHREE but depleted in U, Zr, Ba and Sr. Notably,

this lava is characterized by the lowest BarTh andŽSrrTh but the highest ThrYb and ThrU Table 2,

.Fig. 4 . These are also trace element features of theŽSao Miguel island source Turner et al., 1997; Widom

.et al., 1997 .

4.2. Th isotopes and 238U–230Th–226Ra disequilibria

The U–Th results are reported in Table 3 andrepresented in the classical U–Th isochron diagramŽ .Fig. 5a . Our measurements are in good agreementwith the literature data, although one sample fromPico and the other sample from Terceira display

Ž230 232 . Ž230 238 .higher Thr Th and Thr U activity. The


Table 387 86 Ž226 230 . Ž226 .U-series measurements and Srr Sr isotopic ratios in the Azores volcanics. Rar Th and Ra rBa are the decay corrected0 0

y1 y1 Ž226 . y1ratios to the date of eruption, usingl s4.332=10 year . Parentheses denote activity ratios. RarBa in dpmmg226 0238 232 230 232 230 238 226 230 226 87 86Ž . Ž . Ž . Ž . Ž .a Sample Ur Th Thr Th Thr U Rar Th Ra rBa Srr Sr0 0

"2s "2s "2s "2s "2s "2s

Sao MiguelŽ .ACO95-3 1563 0.8174"25 0.897"16 1.098"21 0.841"10 3538"16 0.704673"22

Dupl. 0.834"10 3511"15

TerceiraŽ .ACO95-8 1760 0.9849"13 1.382"07 1.403"08 0.970"07 2024"09 0.703539"28

PicoŽ .ACO95-19 1718 0.9860"23 1.331"09 1.350"12 1.238"11 4032"18 0.703794"22Ž .ACO95-24 1562 1.0091"15 1.163"11 1.153"12 1.604"13 4060"30 0.703773"14Ž .ACO95-28 1718 0.9633"20 1.100"07 1.142"08 1.516"22 4144"19 0.703714"17Ž .ACO95-31 1718 1.0386"31 1.188"15 1.145"16 1.355"20 3549"17 0.703840"15

FaialŽ .ACO95-45 1958 0.9253"12 1.130"13 1.221"15 1.392"20 3006"14 0.703975"19

Ž230 232 . Žvariability in the Thr Th is very large 0.897–.1.382 and overlaps almost completely the global

Žvariation of MORBs and OIB Gill and Condomines,.1992 . This range is the largest ever measured within

a single archipelago.On the scale of the archipelago, the Terceira

ŽŽ230 232 . 87 86 .Thr Th s1.382, Srr Srs0.70354 andŽŽ230 232 . 87 86Sao Miguel Thr Th s0.897, Srr Srs

.0.70467 islands samples have the most extreme ThŽ .and Sr isotopic compositions Fig. 5b . Interestingly,

not all the data fall on the global Sr–Th correlationŽ .defined by Condomines et al. 1981 . The Terceira

lava, several FAZAR and one sample from PicoŽ .plots above the Th–Sr trend Fig. 5b . This seems to

be a characteristic feature of part of the Azores lavas.

Ž230 238 .All the samples display Thr U higher than1, as expected ifD -D during partial meltingTh UŽ . 230Fig. 5 . More precisely, the Th excess increasesfrom 10% in Sao Miguel to more than 40% in theTerceira sample. In Pico and Faial samples, theU–Th disequilibria are more akin to the MORBsvalues from this area, whereas on Terceira island, thelarge 230Th excesses is in the range of only a few

Žocean islands Claude-Ivanaj et al., 1998; Thomas et.al., 1999 .

Radium contents vary from 0.450 to 0.973 pgrg.On Pico island, the range of Ra concentrations ex-tends the variation of the whole data set, Faial, SaoMiguel and Terceira samples displaying intermediate

Ž . Ž226 230 .Ra contents Table 2 . The Rar Th activity

Ž . Ž230 232 . Ž238 232 . Ž .Fig. 5. a The Thr Th vs. Ur Th plot Th–U isochron diagram in the Azores together with other data from the literatureŽ .Condomines et al., 1981; Bourdon et al., 1996a; Widom et al., 1997; Turner et al., 1997 and data from Lanzarote and Tenerife islandsŽ Ž . .Claude-Ivanaj, 1997 circled crosses , Sigmarsson et al., 1998 . For comparison, additional MORB data from the Atlantic and the Pacific

Ž .ridges are also reported Condomines et al., 1981; Bourdon et al., 1996b; Lundstrom et al., 1998 . MORBs from the Azores PlateauŽ .FAZAR MORBs plot in between Terceira and Sao Miguel end-member compositions. The field delimiting Morbs from the Mid Atlantic

Ž . Ž . Ž230 232 . 87 86oceanic Ridge MAR is shaded. EPR is for East Pacific Ridge. b Thr Th activity vs. Srr Sr isotopic ratios. For comparison isŽ . Ž . Žalso reported the global Th–Sr correlation from Condomines et al. 1981 shaded . Small black dots are MORBs Condomines et al., 1981;.Bourdon et al., 1996a,b; Dosso et al., 1996; Lundstrom et al., 1998 , other symbols are OIB. For the Azores data, same symbols as in Fig. 2:

Ž . Ž .open symbols are from this study, filled symbols are from Turner et al. 1997 and Widom et al. 1997 ; Crosses are the 1730–1736Ž .Lanzarote eruption Sigmarsson et al., 1998 .


ranges from 0.84 to 1.60. Terceira and Sao MiguelŽ226 230 . Ž .lavas have Rar Th -1 Table 3, Fig. 6 . Based

Ž222 230 .on Rnr Th measurements, Widom et al.

Ž . 2261997 report an average of 20% Ra deficit in thetrachytes from Agua de Pao volcano on Sao Miguel

Ž226 230 .island. In Pico samples, Rar Th increases with


Ž230 238 . Ž226 230 .Fig. 6. The covariations of Thr U vs. Rar Th in the Azores. Same symbols as in Fig. 5. Samples from Pico, Faial andTerceira measured in this study plot on the negative correlation defined by the samples from the 1730–1736 eruption in Lanzarote, Canary

Ž . Ž .islands Sigmarsson et al., 1998 . Other two samples from Tenerife and Lanzarote from Claude-Ivanaj 1997 also plot on the correlation.Ž . Ž .The sample from Sao Miguel clearly plots apart. a Curve 1: partial melting of a gt–lherzolite 0.6 ol, 0.1 cpx, 0.2 opx, 0.1 gt . This model

can explain some of the samples of Pico island but not the negative correlation for the Azore and Canary data. Curve 2: the226Ra deficit inthe Sao Miguel lava is explained by partial melting of a gt–lherzolite containing 10y3 phlogopite. In this calculation, phlogopite has been

226 Žprefered over amphibole as the most efficient way to decrease the Ra excess in the meltsK amphrmelts0.017, K phlgrmeltsRa Ra. y32.85 . A similar effect to what is obtained with 10 phlg. indeed requires 5% amphibole, which appears to be a lot. Phlogopite is

progressively added in the modal proportions and the new bulk partition coefficients are re-calculated to keep constant ol:cpx:opx:gtŽ . Žproportions. The partition coefficients are taken from Elliott 1997 andD in phlogopite has been calculated using elastic moduli BlundyRa

. Ž . Ž226 230 .and Wood, 1994; LaTourrette et al., 1995 . b The negative correlation is explained by mixing between a high Rar Th , lowŽ230 238 . Ž226 230 . Ž230 238 .Thr U end-member represented by Pico samples and a low Rar Th , high Thr U end-member, represented by the

Ž .Terceira sample. The former can be generated using curve 1 Fig. 6a while the latter results from the interaction of low-degree melts with aphlogopite-bearing lherzolite.

ŽMgO contents but decreases with ThrYb Figs. 3a. Ž226 .and 4a while the RarBa displays no systematic

Ž .variations with MgO Fig. 3d .The samples from Pico, Faial and Terceira islands

Ž226plot on the negative correlation between Rar230 . Ž230 238Th and Thr U displayed by samples from

ŽLanzarote Sigmarsson et al., 1998; Claude-Ivanaj,. Ž .1997 and Tenerife islands Claude-Ivanaj, 1997

while the sample from Sao Miguel is clearly plottingapart.

4.3. Sr isotopes

The results are reported in Table 3, together with238U–230Th–226Ra data. The 87Srr86Sr isotopes

Ž .cover a large range of variations 0.70354–0.70467Ž .Fig. 5b from Terceira to Sao Miguel. When com-pared with literature data, the Terceira and SaoMiguel samples display rather typical end-memberSr isotopes, already described as representative of

the Azores plume and localized Sao Miguel compo-Žsitions Dupre et al., 1982; Widom et al., 1997;´

.Turner et al., 1997 .

5. Discussion

In the following we assess the different processesŽcrystal fractionation, partial melting, source hetero-

. 238 230geneity and mixing that may explain the U– Thand230Th–226Ra co-variations in the Azores.

5.1. 226Ra–230Th disequilibria and crystal fractiona-tion

Samples from Faial, Terceira and Sao Miguelislands are rather unfractionated lavas. In contrast,

Ž226 230 .on Pico island the decrease in Rar Th isŽcorrelated with the degree of crystallization Fig. 2,


.Table 1 . Hence, one may check whether the crystal-lization of plagioclase has a significant effect on theŽ226 230 .Rar Th . Taking one plagioclase compositionmakes easy to calculate the partition coefficient of

ŽRa Blundy and Wood, 1994, see also details in.caption of Fig. 2 . On Terceira island, modeling the

fractional crystallization leads to a parental magmaŽ226 230 .composition with an initial Rar Th of 1. On

Pico island, the fractionating assemblage of 5% ol,75% cpx, 20% plag, inferred from the modeling of

Ž .Ni, Sc, Sr and MgO contents Fig. 2a and b onlyŽ226 230 .leads to decrease the Rar Th from 1.6 in the

Ž .basalt to 1.585 in the mugearite Fig. 3a . In order tomatch the decrease from 1.6 to 1.24 in the mugearite,the calculated bulkD should have a value of 0.4.Ra

This bulk D can be obtained with 75% albiteRaŽ . Ž .D s0.6 Blundy and Wood, 1994q25% cpx,Ra

which is highly unlikely given the type of rock.Radioactive decay during magma differentiationseems rather improbable as well because in three of

Ž226 . Žfour samples, the RarBa is fairly constant Fig.. Ž226 .3d . Sample 95ACO28 displays a lower RarBa,

but is also characterized by a distinctly more radio-87 86 Ž .genic Srr Sr Table 3 . This would suggest that

small source heterogeneity may be affecting theŽ226 .RarBa. Therefore on Pico, the decrease inŽ226 230 .Rar Th has to be explained by another pro-cess rather than crystal fractionation and radioactivedecay.

The Sao Miguel sample is characterized by adeficit of 226Ra, despite evidence for source enrich-

Ž . 226ment Table 3 . A similar Ra depletion has al-ready been described in trachytes from the Agua dePao volcano on Sao Miguel island and related toK-feldspar removal from a putative alkali basaltŽ .Widom et al., 1992 . The sample is basaltic incomposition and therefore such a process cannotexplain the 226Ra deficit. In addition, this sample

Ž230 232 . Ž230 238 .displays similar Thr Th and Thr U toŽ .the trachytes from Widom et al. 1992 . As a conse-

quence, the assumptions of the model of evolution ofthe trachytes by crystal fractionation of an alkalibasalt and the corresponding model age of 90 ka

Ž .proposed by Widom et al. 1992 cannot be valid.Therefore on Sao Miguel island, in view of thesearguments, the Ra depletion as well as the lowŽ230 232 . Ž230 238 .Thr Th and Thr U activity were inher-ited before magma differentiation.

5.2. U–Th–Ra disequilibria and partial melting

5.2.1. Pico, faial and terceira samplesŽ226 230 .On Pico island, Rar Th decreases with in-

creasing ThrYb, an index of partial melting giventhe consistency of the87Srr86Sr for those samplesŽ . Ž230 238 .Fig. 4d . Conversely, the highest Thr U ac-tivity is found in the sample displaying the highest

Ž . Ž230 238 .ThrYb Fig. 4c . The large range in Thr UŽdisequilibrium for the Pico samples)50% of the

.total variation suggests that partial melting is anŽ .important factor Fig. 6 . On the other hand, the Pico

samples plot on the negative correlation betweenŽ230 238 . Ž226 230 .Thr U and Rar Th disequilibria de-

Ž .fined by the Canary data Sigmarsson et al., 1998Ž .Fig. 6 . Clearly, one cannot explain an inverse

Ž230 238 . Ž226 230 .co-variation of Thr U and Rar Th dis-Žequilibria by simply melting of a gt–lherzolite see

.curve 1 in Fig. 6a . That would implyD -DTh Ra

and D -D , which is not expected in a sourceTh U

composed of ol, cpx, opx and gt only. Therefore,partial melting is not the favored hypothesis to ex-

Ž230 238 . Ž226 230 .plain the Thr U vs. Rar Th trend inthe Azores and the Canaries.

5.2.2. Partial melting in Sao MiguelIf one considers that Terceira and Sao Miguel

islands are taken as part of the same plume, provid-ing that Sao Miguel island is located in the colderperiphery of the Azores plume, one should expectthe Sao Miguel island lavas to be lower degree meltsand therefore, relatively to the Terceira island lavas,

Ž230 238 . Ž226 230 .to display higher Thr U and Rar Th .This is the opposite of what is observed. In addition,

Ž230 238 . Ž .as Thr U and ThrYb also LarYb are nega-tively correlated, it is difficult to unequivocally infera lack of garnet in Sao Miguel island source rela-

Ž .tively to the Terceira island source Fig. 4 . InŽ230 238 . Ž226 230 .addition, in terms of Thr U and Rar Th

disequilibria, the Sao Miguel sample plots is clearlyapart from the other Azores and Canary islands dataŽ .Fig. 6a . Based on long-lived isotopic, trace element

Ž .and other U–Th data, Turner et al. 1997 describedthe Sao Miguel characteristics altogether under thename ofALocalised Sao Miguel componentB.

In the Azores, numerous amphibole-bearing nod-Ž .ules kaersutite gabbros, pyroxene hornblendites

Ž .Mattioli et al., 1997 have been brought to the


surface while on Sao Miguel island a few metasoma-tised mantle phlogopite-bearing xenotiths have beendescribed in the western most and youngest area.Those are evidences for a hydrous mantle. This has

Ž .led Turner et al. 1997 to propose that on SaoŽ230Miguel relative to Terceira island a lower Thr

238 .U could result from higher degree melts, inducedby the presence of fluids in the mantle source. Onthe other hand, one can expect those fluids to beenriched in mobile elements like U, Ba, Sr and Raand to be susceptible to contaminate the magmas.However, this seems to conflict with the low BarTh,RarTh and SrrTh of the lavas. Thus, the addition of

Ž230 238 .U-bearing fluids might lower the Thr U , butanother process is required to explain the226Radeficit and the low BarTh and SrrTh. Here the

Ž226simplest model that can account for low Rar230 .Th , BarTh and SrrTh in the melts is the pres-ence of residual phlogopite or amphibole during

Žpartial melting of the enriched SAM source Fig. 6a,.curve 2, see details in the caption . This model also

explains other low trace element ratios, for instancethe RbrZr of 0.17.

5.3. 238U–230Th–226Ra fractionation and the Azoresplume heterogeneity

The identification of at least three distinct sampletypes can be derived from binary diagrams involving

Ž .both long- and short-lived isotopes Sr and Th , traceŽelement ratios and U–Th disequilibria Figs. 4 and

. Ž .5 Turner et al., 1997; Moreira et al., 1999 . There-Ž .after, we will refer to them as to be the TC Terceira ,

Ž .the SAM Sao Miguel and the MORB components.Ž230 232 .The TC samples have high Thr Th , BarTh

and NbrTh, high 230Th excesses but low ThrU andrelatively low87Srr86Sr. Conversely, the SAM sam-ple is characterized by an enriched signature com-pared with the TC samples, with a high ThrU but

230 Ž230 232 .low Th excesses and Thr Th . Lately, theMORB component is present in all islands and isakin to the composition of the MORBs from the

ŽAzores plateau Dupre et al., 1982; Bourdon et al.,´.1996a . However, there is no simple relationship

between the degree of enrichment, Sr isotopes, andŽ230 238 . Ž226 230 .Thr U and Rar Th co-variations forthe Azores and Canary lavas. As an example, the

sample from Faial island is characterized by interme-diate 238U–230Th and 226Ra–230Th disequilibria buthas the most radiogenic Sr isotopic composition.This becomes obvious when comparing the87Srr86Sr

230 232 Ž230 238 .vs. Thr Th plot with Thr U vs.Ž226 230 .Rar Th or the Th–U isochron diagram: in thefirst diagram, the single eruption from Lanzarote isrepresented by a narrow but isotopically very distinctfield that can hardly be compared with the Azores

Ž .archipelago as a whole Fig. 5b . In contrast in theŽ230 238 . Ž226 230 .second diagram, Thr U and Rar ThŽ .vary consistently Figs. 5a and 6 . Therefore, alto-

gether, these observations suggest that for the Azoresand the Canaries, the long-lived source hetero-geneities do not explain the238U–230Th–226Ra dise-quilibria.

(230 238 ) (226 230 )5.4. The negatiÕe Thr U Õs. Rar Thcorrelation: a mixing process

The idea of melting differently pyroxenitic andlherzolitic derived melts, in response to their respec-

Žtive degree of enrichment Hirschmann and Stolper,.1996 , has been proposed as an appealing scenario to

Ž .explain the Lanzarote data Sigmarsson et al., 1998 .As the Azores data plot on the correlation defined bythe Lanzarote data, one could attempt to interpretthem using the same model. However, compellingevidences are missing. Firstly, the fractionation be-tween Ra and Th in garnet is not expected to bedifferent in a gt–lherzolite and a gt–pyroxenite.Therefore, even though some recycled pyroxeniticmelts are mixed with lherzolitic melts, a positiverather than a negative correlation should be expected.The range in230Th excess does not seem to be a

Ž230 238 .garnet effect as Thr U and ThrYb are nega-Ž .tively correlated Fig. 3 . By comparison with the

Pico lavas, the TC lavas do not display the requiredcharacteristics in major and trace elements of apyroxenitic component. The Terceira island sampledisplays similar silica and Na O but lower FeO2

contents than the most primitive lavas from Pico.ŽLast, apart from the Pb isotopes Moreira et al.,

.1999 and BarTh, the trace element composition ofTerceira island sample is rather depleted. Of particu-lar interest, the ThrU is low and falls in the range of

Ž .the Pico values Table 2 . Hence, the model pro-


Ž .posed by Sigmarsson et al. 1998 does not explainthe Azores data.

We propose to explain the negative correlation inthe Azores by mixing between two types of melt,

Ž230 238 . Ž226 230 .one with a high Thr U , low Rar ThŽ230 238 .and the other with a low Thr U and high

Ž226 230 .Rar Th , corresponding to lower and higherŽ .degree melts, respectively Fig. 6b . We caution,

however, that our data set is very limited and thatthis model still has to be confirmed by a largernumber of samples.

Ž230 238 . Ž226 230 .The low Thr U and high Rar Thend-member is akin to the composition of Pico sam-ples that can be explained by partial melting of a

Ž .garnet lherzolite curve 1, Fig. 6a . Using the ap-Ž .proach of Spiegelman and Elliott 1993 , the Pico

Ž230 238 . Ž226 230 .Thr U and Rar Th of 1.15 and 1.6,respectively, can be reproduced with a melting rateof 10y4 kg my2 yeary1 and a residual porosity of0.5%. This is consistent with the estimates of Bour-

Ž . Ž .don et al. 1996a and Turner et al. 1997 . However,Ž230 238 . Ž226 230 .explaining the Thr U and Rar Th dise-

quilibria on Terceira island by such a process ismore problematic. The coexistence of 40%230Th butno 226Ra excess would indeed require a minimum of10% residual porosity and a very slow melting rateŽ y7 y2 y1.10 kg year , which would lead to the col-lapse of the melting zone. Another mechanism isthen required to explain the composition of the TC

Ž226 230 .melts. It could be argued that the low Rar ThŽ226 .and RarBa in the Terceira lava might result

from radioactive decay in a magma chamber orŽduring magma transport towards the surface Vigier

. Ž226 230 .et al., 1999 . However, the other low Rar Thsamples plotting on this correlation are either very

Žprimitive samples basanites from Tenerife and Lan-.zarote islands or, like for the Pico samples, without

significant variations of the226RarBa with the de-gree of differentiation. This suggests that the low226Ra content of the Terceira island end-member is aprimitive feature.

We propose to explain the238U–230Th–226Ra ra-dioactive disequilibria of the Terceira lava by aprocess of interaction of low-degree melts character-

Ž226 230 . Ž230 238 .ized by high Rar Th and Thr U andBarTh with the mantle enriched in phlogopite. Phlo-gopite, together with amphibole, are the only miner-

Žals that may fractionate Ra from Ba and ThD -Th

. ŽD -D LaTourette et al., 1995; Claude-IvanajRa Ba.et al., 1998 . Crystals of phlogopite are strongly

Ženriched in Ba and Ra but poor in U and ThK sBa

3.65, K s2.84, K s0.013, K s0.013, La-Ra U Th.Tourette et al., 1995; Blundy and Wood, 1994 . This

implies that phlogopites younger than 8000 years arecharacterized by an excess of226Ra. With time, the226Ra excess tends to be 0, as a result of the low Ucontent of the crystals. Therefore, a melt enriched inU, Th, Ba and Ra interacting with the upper mantlemetasomatized with phlogopite on a time scale lowerthan the time scale of decay of226Ra, will loose allof the 226Ra while the Ba, U and Th contents willremain unaffected. As suggested by the high TCŽ230 238 .Thr U , the interaction may preferentially oc-cur between phlogopite and the lowest degree melts.If the phlogopite crystals are small the interactionshould be more efficient. In addition, this processneeds to be relatively fast, with respect to the decayperiod of 230Th and 226Ra, in order to preserve the230Th excess and preventing any226Ra ingrowth inthe melts.

This idea has been tested using a melt percolationŽ .model Bodinier et al., 1990 . To do so, we have

considered an initial percolating melt displaying theŽ .composition of the Terceira island sample ACO95-8

Ž226 230 .and a Rar Th of 1.6. The composition of theinteracting mantle matrix has been taken to contain5% cpx, 74.5% ol, 20% opx and 0.5% phlogopiteand to be U, Th and Ba enriched and Ra depleted, inequilibrium with the sample from Sao MiguelŽ .ACO95-3 . The numerical experiment is carried outover one km long column characterized by 1%porosity. For the case, taking 10 m yeary1 for themelt velocity in the lithosphere implies a duration of100 years for the initial melt to achieve travellingthrough the mantle column. One finds that only 130

Ž230 238 .years are required to re-equilibrate the Thr Uwith the initial melt composition while 350 years are

Ž226 230 .necessary for the Rar Th . It is only between130 and 250 years that a melt composition display-

Ž230 238 . Ž . Ž226 230 .ing high Thr U 1.4 but low Rar ThŽ .-1 can be preserved. Hence, only the very firstmelts, also likely to be the low-degree melts, areaffected by such a process.

It is worth to point out that the samples fromLanzarote island that plot near the lower degree melt

Ž .end-member TC are basanites while those plotting


Ž .near the higher degree melt end-member Pico areŽ .tholeites Sigmarsson et al., 1998 . This is in agree-

ment with the general knowledge that basanites re-sult from lower degree of melting than tholeites. Wesuggest that mixing between TC and mantle meltsmay be applicable to islands in the sector of theAtlantic Ocean that includes Azores and Canaryislands because there the lithosphere is likely to bemetasomatized and to contain phlogopite.

6. Concluding remarks

The extent of238U–230Th–226Ra radioactive dise-quilibria in historical volcanics from the Azores arecharacterized by a large range of variations that canneither be attributed to crystal fractionation and melt-ing nor reflect source heterogeneities. Rather, the238U–230Th–226Ra data provide information on theprocesses and timing of interactions of melts withthe lithospheric mantle on this sector of the AtlanticOcean.

We propose to explain the unexpected negativeŽ230 238 . Ž226 230 .correlation between Thr U and Rar Th

disequilibria for Terceira, Faial Pico and the Canaryislands to reflect mixing between the Pico type and

Ž230 238 . Ž226the Terceira type melts. Thr U and Rar230 .Th radioactive disequilibria in Pico melts can beexplained by partial melting of a garnet lherzolite.On the other hand, the Terceira type melts are thoughtto result from the interaction in the mantle of low-

Ž230 238 .degree melts characterized by high Thr U andBarTh with crystals of phlogopite. This process isshown to occur over a couple of hundred years andtherefore to strongly affect the first, low-degree melts.

Ž226 230 .In Sao Miguel lava, the Rar Th activity of0.84 is shown to be a primary feature of the meltsresulting from partial melting in the presence ofresidual phlogopite.

Acknowledgements

Samples were kindly provided by M. Moreira.CCI wishes to thank B. Bourdon, G. Suhr and A.Mayer for fruitful discussions and encouragement.

We thank G. Manhes and J.L. Birck for their exper-`tise during analytical development of the U–Th–Ratechnique. This paper benefited from thorough re-views by S. Turner and an anonymous reviewer.During the redaction, CCI benefited from a EuropeanTMR grant at Max Planck Institut fur Chemie¨Ž .Germany . This is IPG contributiona1714.

Appendix A. Analytical techniques

Details of the chemical separation and mass spec-trometry for U, Th have been described elsewhereŽ .Claude-Ivanaj et al., 1998 . Thorium isotope ratiosŽ230 232 .Thr Th were measured with a FinniganMAT262 RPQ thermal ionization mass spectrometer.Except for sample ACO95-3, internal reproducibility

Ž .is better than 1.5% 2s . External precision andaccuracy were controlled by replicate analysis of a

Ž .rock standard Claude-Ivanaj et al., 1998 from ourŽ .laboratory 86KA60, Comores and an inter-labora-

tory standard provided by M. Condomines in Cler-mont-Ferrand. The replicate measurement of stan-

ŽŽ230 232 . .dard ThS1 Thr Th s1.0205"20, ns13 .agrees within error thea-counting value of 1.0211

Ž ."40 M. Condomines, personnal communication .U and Th concentrations were measured by isotopedilution, using a230Th tracer solution, on a VG 354.The chemical separation of U and Th is identical tothat used for isotopic measurements. The final error

Ž238 232 .on the Ur Th activity ratio is about 0.5%Ž .2s . Radium concentrations were measured using a228Ra tracer by isotope dilution on a Finnigan MAT262 RPQ. The separation and purification techniquefor Ra is performed on 2 g of rock are described in

Ž .Claude-Ivanaj et al. 1998 . The overall error on theRa measurement is not greater than 0.5%. Ba con-centrations were measured using a135Ba tracer byisotope dilution on a VG 354 mass spectrometer. Bacontents were obtained with less than 0.3% error.Strontium isotopes and concentrations were mea-sured on a VG-354 mass spectrometer following the

Ž . Ž .techniques of Birck, 1986 and Horwitz et al. 1991 .The replicate measurement of standard NBS 987Ž .0.71025"17 agrees within error the certified value

Žof 0.71025. Major and trace elements except for U,.Th, Ra, Ba and Sr were measured by ICP-emission


Ž .and ICP-MS at C.R.P.G., Nancy France and Yb byŽ .INAA at LPS, Saclay France .

References

Allegre, C.J., Condomines, M., 1982. Basalt genesis and mantle`structure studied trough Th-isotopic geochemistry. Nature 299,21–24.

Beattie, P., 1993a. Uranium–thorium disequilibria and partitioningon melting of garnet peridotite. Nature 363, 63–65.

Beattie, P., 1993b. The generation of uranium series disequilibriaby partial melting of spinel peridotite: constraints from parti-tioning studies. Earth Planet. Sci. Lett. 117, 379–391.

Birck, J.-L., 1986. K, Rb, Sr isotope analysis: application toRb–Sr chronology. Chem. Geol. 56, 73–83.

Blundy, J., Wood, B., 1994. Prediction of crystal-melt partitioncoefficients from elastic moduli. Nature 372, 452–454.

Bodinier, J.-L., Vasseur, G., Vernieres, J., Dupuy, C., Fabries, J.,`1990. Mechanisms of mantle metasomatism—geochemical ev-idence from the Lherz orogenic peridotite. J. Petrol. 31, 597–628.

Bourdon, B., Zindler, A., Langmuir, C.H., 1996a. Ridge-Hotspotinteraction along the Mid-Atlantic ridge between 37830 and40830 N: the U–Th disequilibrium evidence. Earth Planet. Sci.Lett. 142, 175–189.

Bourdon, B., Zindler, A., Elliott, T., Langmuir, C.H., 1996b.Constraints on melting at mid-ocean ridges from global238U–230Th disequilibrium data. Nature 384, 231–235.

Claude-Ivanaj, C., 1997. Systematique238U–226Ra–230Th ap-´pliquee au volcanisme actif oceanique. Contraintes sur la´ ´duree et les processus de formation des magmas. Thesis, Paris,´365 pp.

Claude-Ivanaj, C., Bourdon, B., Allegre, C.J., 1998. Ra–Th–Srìsotope systematics in Grande Comore island: a case study ofplume-lithosphere interaction. Earth Planet. Sci. Lett. 164,99–117.

Cohen, A.S., O’Nions, R.K., 1993. Melting rates beneath Hawaii:evidence from uranium series isotopes in recent lavas. EarthPlanet. Sci. Lett. 120, 169–175.

Condomines, M., Morand, P., Allegre, C.J., 1981.230Th–238U`radioactive disequilibria in tholeiites from the FAMOUS zoneŽ .Mid-Atlantic Ridge, 368508N : Th and Sr isotopic geochem-istry. Earth Planet. Sci. Lett. 55, 247–256.

Condomines, M., Hemond, C., Allegre, C.J., 1988. U–Th–Ra´ `radioactive disequilibria and magmatic processes. Earth Planet.Sci. Lett. 90, 243–262.

Dosso, L., Bonnier, O., Bollinger, C., Bougault, J., Langmuir, C.,1996. Geochemical structure of the mid Atlantic ridge between

Ž .108 and 418N. Conf. Abstr. 1 2 , 784.Dupre, B., Lambret, B., Allegre, C.J., 1982. Isotopic variations´ `

within a single oceanic island: the Terceira case. Nature 299,620–622.

Elliott, T., 1997. Fractionation of U and Th during mantle melt-ing: a reprise. Chem. Geol. 139, 165–183.

Flower, M.F., 1976. Rare earth and other trace elements inhistoric Azorean lavas. J. Volcanol. Geotherm. Res. 1, 127–147.

Forjaz, V., Serralheiro, A., Nunes, J.C., 1990. Carta volcanologicados Acores. Grupo Central, Universidade Dos Açores. 1stedn., Ponta Delgada.

Gill, J.B., Condomines, M., 1992. Short lived radioactivity andmagma genesis. Science 257, 1368–1376.

Hart, S.R., Davis, K.E., 1978. Nickel partitioning between olivineand silicate melt. Earth Planet. Sci. 40, 203–219.

Hawkesworth, C.J., Norry, M.J., Roddick, J.C., Baker, P.E., 1979.143Ndr144Nd, 87Srr86Sr and incompatible element variationsin calc-alkaline andesites and plateau lavas from South Amer-ica. Earth Planet. Sci. Lett. 42, 45–57.

Hirschmann, M.M., Stolper, E.M., 1996. A possible role forgarnet pyroxenite in the origin of the garnet signature inMORB. Contrib. Mineral. Petrol. 124, 185–208.

Hofmann, A.W., White, W.M., 1982. Mantle plumes from ancientcrust. Earth Planet. Sci. Lett. 57, 421–436.

Horwitz, E.P., Dietz, M.L., Fisher, D.E., 1991. Separation andpreconcentration of Sr from biological, environmental andnuclear wast samples by extraction chromatography using acrown ether. Anal. Chem. 63, 522–525.

Irving, A.J., 1978. A review of experimental studies ofcrsytalrliquid trace element partitioning. Geochim. Cos-mochim. Acta 42, 743–770.

Le Bas, M., Le Maıtre, R.W., Streckeisen, A., Zanettin, B., 1986.Â chemical classification of volcanic rocks based on the totalalkali-silica diagram. J. Petrol. 27, 745–750.

LaTourrette, T.Z., Hervig, R., Holloway, J., 1995. Trace elementpartitioning between amphibole, phlogopite, and basanite melt.Earth Planet. Sci. Lett. 135, 13–30.

Lundstrom, C.C., Gill, J., William, Q., Hanan, B.B., 1998. Inves-tigating solid upwelling beneath mid-ocean ridges using U-series disequilibria: II. A local study at 338S Mid-Atlanticridge. Earth Planet. Sci. Lett. 157, 167–181.

Mattioli, M., Upton, B.G.J., Renzulli, A., 1997. Sub-volcaniccrystallization at Sete Citades Volcano, Sao Miguel, Azores,inferred from mafic and ultramafic plutonic nodules. Miner.

Ž .Petrol. 60 1–2 , 1–26.McKenzie, D., 1985.230Th-238U disequilibrium and the melting

processes beneath ridge. Earth Planet. Sci. Lett. 72, 149–157.Moreira, M., Doucelance, R., Dupre, B., Allegre, C.J., 1999.´ `

Helium and lead isotope geochemistry in the Azores. EarthŽ .Planet. Sci. Lett. 169 1–2 , 189–205.

Overby, V.M., Gast, P.W., 1968. Lead isotopic composition anduranium decay series disequilibrium in recent volcanic rocks.Earth Planet. Sci. Lett. 5, 199–206.

Schilling, J.-G., 1975. Azores mantle blob: rare earth evidence.Earth Planet. Sci. Lett. 25, 103–115.

Schilling, J.-G., Zajac, M., Evans, R., Johnston, T., White, J.D.,Devine, J.D., Kingsley, R., 1983. Petrologic and geochemicalvariations along the Mid-Atlantic Ridge from 298N to 738N.Am. J. Sci. 283, 510–586.

Sigmarsson, O., Carn, S., Carracedo, J.C., 1998. Systematics ofU-series nuclides in primitive lavas from the 1730–36 erup-tion in lanzarote, canary Islands, and implications for the role


of garnet pyroxenites during oceanic basalt formations. EarthPlanet. Sci. Lett. 162, 137–151.

Spiegelman, M., Elliott, T., 1993. Consequences of melt transportfor uranium series disequilibrium in youmg lavas. Earth Planet.Sci. Lett. 118, 1–20.

Thomas, L.E., Hawkesworth, C.J., Van calsteren, P., Turner, S.P.,Rogers, N.W., 1999. Melt generation beneath ocean islands: aU–Th–Ra isotope study from Lanzarote in the Canary islands.Geochim. Cosmochim. Acta 63, 4081–4099.

Turner, S., Hawkesworth, C., Rogers, N., King, P., 1997. U–Thdisequilibria and ocean island basalt generation in the Azores.Chem. Geol. 139, 145–164.

Vigier, N., Bourdon, B., Joron, J.L., Allegre, C.J., 1999. U-decay`series and Sr isotope systematics in the 1978 eruption ofArdoukoba, Rep. of Djibouti: timescale of magma crystalliza-tion. Earth Planet. Sci. Lett., in press.

Weaver, B.L., 1991. The origin of ocean island basalt end-mem-ber compositions: trace element and isotopic constraints. EarthPlanet. Sci. Lett. 104, 381–397.

White, W.M., Schilling, J.-G., 1978. The nature and origin ofgeochemical variation in Mid-Atlantic ridge basalts from theCentral North Atlantic. Geochim. Cosmochim. Acta 42, 1501–1516.

White, W.M., Schilling, J.-G., Hart, S.R., 1976. Evidence for theAzores mantle plume from strontium isotope geochemistry ofthe Central North Atlantic. Nature 263, 659–663.

Widom, E., Schmincke, H.-U., Gill, J.B., 1992. Processes andtimescales in the evolution of a chemically zoned trachyte:Fogo A, Sao Miguel, Azores. Contrib. Mineral. Petrol. 111,311–328.

Widom, E., Carlson, R.W., Gill, J.B., Schmincke, H.-U., 1997.Th–Sr–Nd–Pb isotope and trace element evidence for theorigin of Sao Miguel, Azores, enriched mantle source. Chem.Geol. 140, 49–68.

Williams, R.W., Gill, J.B., 1989. Effects of partial melting on theuranium decay series. Geochim. Cosmochim. Acta 53, 1607–1619.

Documents

238U–230Th–226Ra fractionation in historical lavas from the Azores: long-lived source heterogeneity vs. metasomatism fingerprints