He and Pb Isotopes in Azores

7/30/2019 He and Pb Isotopes in Azores

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ELSEVIER Earth and Planetary Science Letters 169 (1999) 189205

Helium and lead isotope geochemistry of the Azores Archipelago

Manuel Moreira a,b,, Regis Doucelance a, Mark D. Kurz b, Bernard Dupre a,c,Claude Jean Allegre a

aLaboratoire de Geochimie et Cosmochimie, URA CNRS 1758, Institut de Physique du Globe de Paris,

Universite Denis Diderot Paris 7, 4 Place Jussieu, 75252, Paris Cedex 05, Franceb Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, MS25,

Woods Hole, MA 02543, USA

cLaboratoire de Geochimie, OMP, UMR-CNRS 5563, 38 rue des 36 ponts,31400, Toulouse, France

Received 21 April 1998; revised version received 10 March 1999; accepted 11 March 1999

Abstract

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores

have 4He=3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are

observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which

are identified by data from Sao Miguel and Terceira. The eastern part of Sao Miguel island displays radiogenic helium

(4He=3He > 140,000, R=Ra < 5:1) and lead (20.00, 15.75 and 40.33 for206Pb=204Pb, 207Pb=204Pb and 208Pb=204Pb). The

207Pb=204Pb and 208Pb=204Pb ratios for Sao Miguel are unusually radiogenic for oceanic basalts. Terceira basalts contain

relatively unradiogenic=primitive 4He=3He ratios, with a minimum value of 64,000 (R=Ra D 11:3), and relatively high lead

isotopic ratios (206Pb=204Pb D 20.02, 207Pb=204Pb D 15.64 and 208Pb=204Pb D 39.35). We propose that the Terceira source

has a composition produced by a mixing between recycled oceanic crust (high 206Pb=204Pb) and entrained lower mantle

(high 3He) material. The Sao Miguel island isotopic signature may be due to sampling of local (km-size) heterogeneity

located at relatively shallow depth. The preferred origin of this heterogeneity is the Jurassic delamination of subcontinental

lithosphere, which occurred during rifting and opening of the North Atlantic. The primitive helium ratios were also

observed on the Mid Atlantic ridge at 38.5N, reflecting plumeridge interaction, whereas radiogenic ratios (>100,000)

were observed at latitude higher than 40N and may reflect the influence of the Sao Miguel component at the ridge.

1999 Elsevier Science B.V. All rights reserved.

Keywords: helium; lead; isotope ratios; Azores; magmas; mixing

1. Introduction

Helium isotopic ratios measured on Oceanic Is-land Basalts (OIB) display very large variations

Corresponding author. Fax: C1 508 457 2193; E-mail:

[email protected]

compared to Mid Oceanic Ridge Basalts (MORB).The relatively homogeneous MORB source reser-voir has a 4He=3He mean value of 88,000 5000(R=Ra D 8) [1] whereas the

4He=3He isotopic ratios

observed in OIB range from 25,000 (R=Ra D 30)for Loihi seamount samples [2] to values higher than150,000 (R=Ra D 4:8) for the GoughTristan da

0012-821X/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 7 1 - 0


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190 M. Moreira et al. / Earth and Planetary Science Letters 169 (1999) 189205

CunhaSt Helena island group [35]. A two-layeredmantle best explains these results, where the uppermantle represents the MORB source, which is highlydegassed and well mixed by convection [1], whereas

the primordial signal observed in Loihi seamountbasalts is attributed to the presence of a less de-gassed reservoir, which is probably located in thelower mantle [2,6].

The significance of the radiogenic helium signa-tures observed in the GoughTristan da CunhaStHelena island group remains controversial. Thesesignatures have been attributed to the injection,through subduction, of oceanic crust and sedimentsinto the upper mantle, which will present high (U CTh)=3He ratios and be stored for some time at the670 km boundary layer, prior to eruption [3,5,7]. Analternate view attributing the high 4He=3He ratiosto shallow, magma chamber contamination processes(unrelated to the mantle), originally discussed byCondomines et al. [8] and Zindler and Hart [9,10],was more recently defended by Hilton et al. [11].

Based on results from Heard island (Indian Ocean),where both primitive and radiogenic helium isotopicratios have been measured, Hilton and co-work-ers propose that most the hotspots are in fact low4He=3He hotspots, and the observed radiogenic he-lium signature may reflect shallow depth contamina-

tion, either by interaction of the plume with oceaniccrust or radiogenic 4He production in a degassed

magma chamber.Lead isotopes also display different histograms

for MORB and OIB. Ocean island samples usuallypresent more radiogenic signatures and, again, ex-hibit more variability, than lavas from spreading cen-ters [10,12,13]. Differences in lead isotopic composi-tions between oceanic islands are still debated. Theycould reflect plume source heterogeneities [14,15],

material entrainment during plume ascent [16,17] or

interaction between plumes and lithospheric mantle[18]. Due to the different decay constants, lead iso-topes are less sensitive to in situ production, andcannot be related to magma chamber outgassing.

This study focusses on the Azores archipelago,which is considered to belong to the high 4He=3Hehotspot category [1921], and presents new he-

lium and lead isotopic results for six islands fromthe Azores archipelago: Faial, Graciosa, Pico, SaoMiguel, Santa Maria and Terceira. Good covaria-

tions between helium and lead isotopes are observedat the scale of the archipelago, and suggest thecoexistence of two components in the Azores; thetwo end members are best identified by data from

Sao Miguel and Terceira islands. The Eastern partof Sao Miguel island displays radiogenic helium(4He=3He > 140,000) and lead (20.00, 15.75 and40.33 for 206Pb=204Pb, 207Pb=204Pb and 208Pb=204Pb)signatures, whereas Terceira basalts present primi-tive 4He=3He ratios, with a minimum value around64,000 (R=Ra D 11:3), similar

206Pb=204Pb ratio(20.02), but significantly lower 207Pb=204Pb (15.64)and 208Pb=204Pb (39.35). All other island results are

interpreted in terms of binary mixing between thelocal MORB mantle source and these two plumecomponents.

2. Geographical situation and sample locations

The Azores archipelago is composed of nine is-lands (Corvo, Faial, Flores, Graciosa, Pico, SantaMaria, Sao Jorge, Sao Miguel and Terceira), whichrepresent the emerged part of a large oceanic plat-form (Fig. 1). Located at the triple junction be-tween the Eurasian, African and American plates,the plateau is crossed over by the Mid-Atlantic

ridge; seven islands (Faial, Graciosa, Pico, SantaMaria, Sao Jorge, Sao Miguel and Terceira) standat the East of the ridge, two (Corvo and Flores) arelocated to the west, on the American plate.

The oldest rocks are observed on Santa Maria(8 Myr) and Sao Miguel (4 Myr for the Easternpart) islands [2225]. Other volcanics, including thecentral part of Sao Miguel, were erupted duringthe Pleistocene and Holocene. Historic lavas havebeen found in most areas of the archipelago (in-cluding Sao Miguel island), and the most recent

volcanism occurred on Faial in 1957=58 (Capelinhossite). The lava compositions range from basalts totrachy-andesites and some peridotitic nodules havebeen observed on Pico, Faial and Terceira islands[26,27].

Samples from the islands of Santa Maria, SaoMiguel, Terceira, Pico, Graciosa and Faial, wereanalyzed. Sample locations are given on Fig. 2 andin Appendix A.


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M. Moreira et al. / Earth and Planetary Science Letters 169 (1999) 189205 191

F

Fl

C

P

G

T

SMi

SMa

Triplejunction

Terceirarift

M.A.

R

east Azores fracture zone

2000m

42 N

40 N

38 N

AZORES

32 W 28 W

?

o

o

o

o

o

Fig. 1. Location map of the Azores archipelago (modified after Turner et al. [34]). Triple junction has been proposed to be located at

38.5N [41,57], rather than in the straight continuity of the Terceira rift. Abbreviations for the island names: Santa Maria ( SMa), Sao

Miguel (SMi), Terceira (T), Graciosa (G), Pico (P), Faial (F), Flores (Fl) and Corvo (C).

3. Analytical procedure

Helium analyses were performed on millimeter

sized olivine and pyroxene phenocrysts separatedfrom fresh lavas. Samples from the Eastern sideof Sao Miguel island were ankaramites with largeolivine and pyroxene crystals, up to 2 cm in size;most of the other lavas were alkali-basalts. Measure-ments were preferentially done by crushing, in orderto avoid possible in-situ radioactive decay or produc-tion of 3He cosmogenic and to obtain the inherited

helium trapped in inclusions [3,28]. The powder

of the crushed olivines and pyroxenes from sampleACO95-68 (ankaramite from Sao Miguel) were alsomelted in vacuo to release the gases located in thematrix, which allows an evaluation of the contribu-tion from radiogenic and cosmogenic helium.

Most of the helium measurements were per-formed in Paris using a glass mass spectrometer

and extraction line. Due to diffusion, prior mea-surements with two purifications using hot titaniumgetters and trapping on a cold head in our KO-

VAR glass ARESIBO II mass spectrometer yielded

significant 4He blanks for very low helium con-centration samples such as phenocrysts (olivines,

pyroxenes) from oceanic basalts [29,30]. Thus, wedeveloped a shorter crushing procedure which nowyields a 4He blank of 5 1010 0.5 cm3 STP,a significant improvement to previous blank valuesof 1:5 0:5 108 cm3 STP. A charcoal trap wasinstalled close to the crusher which retains all ac-tive gases, except He and Ne, during 30 minutes ofcryo-pumping at liquid nitrogen temperature. 3He is

analysed on an electron multiplier and 4He, depend-

ing of the absolute amount of gas, on a Faraday cup,or on the electron multiplier. Using only 30 minutesof purification in a very small volume, the helium in-troduced into the extraction line by diffusion is verysmall, leading to low blanks. This procedure makespossible the measurement on this mass spectrometerof very low level samples using 1 or 2 g of olivine or

pyroxene.This new procedure was tested by repeated analy-

sis of a oceanite sample from Reunion island (Indian


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T2

T7

T9

Terceira Island

T28

Pico

ACO95-33

S14

S9

S6

ACO95-30

ACO95-20

Pico Island

ACO95-14

ACO95-16

ACO95-9

ACO95-10

ACO95-11

ACO95-12

Faial Island

So Miguel Island

Santa Maria Island

ACO95-3ACO95-56

ACO95-66ACO95-62

ACO95-47ACO95-49

ACO95-53

ACO95-s

ACO95-52

FA7

FA6

FA12

FA3

FA2

FA1

ACO95-55

ACO95-68

SM7 SM319

SM139

FA10

SM169

SM150

FA44

ACO95-26

-40

1718-1720

1563

1761

nordeste

Capelinhos

5km

5km

5km

5km

5km

Fig. 2. Faial, Pico, Santa Maria, Sao Miguel and Terceira island

sampling maps, modified after Forjaz et al. [67].

ocean), whose olivines are known to have a veryhomogeneous 4He=3He isotopic ratio of 56,000 1000 (R=Ra D 12:9 0:2) [31,32]. Using differentamounts of olivines (0.5, 0.8 and 1.3 g, respectively),

we obtained reproducible 4He=3He results of 55,296 3254, 55,918 4152 and 55,591 2740.

Some helium analyses were also performed inthe Woods Hole Oceanographic Institution where theblank is typically 3 to 5 1011 cm3 STP [33];these data are italicized in Table 1. There is a goodagreement between the data obtained in the two lab-oratories. Sample ACO95-3, which was collected atthe 1563 lava flow from Queimado peak eruption,have given 4He=3He ratios of 149,600 and 133,830similar to the ratio of 133,800 of one sample fromthe same lava flow analyzed by M.D. Kurz (sam-ple #SM88-45, unpublished results) indicating goodinter-laboratory agreement.

Lead chemical separation was performed on 0.5g of powdered samples using the procedure of Man-hes et al. [34], in a clean room under controlled at-mosphere. This permits one to maintain the total Pbblank below 0.35 ng. Lead isotopic ratios were mea-sured on a Thomson THN 206 mass spectrometerand calibrated against SRM981 NBS [35] standardmeasurements, with a statistical mass discriminationfactor of 1 0:3 per amu. The total precision,

tested by duplicate analysis, is better than 0.05%(2) per mass unit difference in the isotopic ratio.

4. Results

Results are given in Tables 1 and 2. Heliumconcentrations obtained by crushing range from

Footnote to Table 1: R=Ra

is the 3He=4He ratio normalized to

the AIR value of 1:384 106. is the uncertainty. Uncertainty

in the helium concentrations is approximately 5%. The last col-

umn gives the percentage of 4He blank contribution (which was4He 51010 cm3 STP in Paris and 3 to 51011 at WHOI).

Data in italics were analyzed in the Woods Hole Oceanographic

Institution isotope facilities. Measurements were conducted on

olivine (Ol.) and clinopyroxene (CPX). Re-crush means that

samples were crushed a second time, sequentially, in an attempt

to insure complete extraction, and also to resolve any contri-

butions from radiogenic helium. The experiments conducted by

melting of powder remaining after crushing are abbreviated mp.


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Table 1

Helium concentration (in 108 cm3 STP=g) and isotopic ratios (corrected for blank) for Azores samples

Sample Mineral Weight 4He R=Ra4He=3He Blank

(g) (108) (%)

Faial

ACO95-sand Ol. 1.04 4.9 7.5 0.5 95960 6630 1

Ol. 1.33 6.8 8.0 0.2 90890 1930 14

ACO95-47 Ol. 1.79 0.53 8.3 1.2 87160 12090 4

ACO95-49 Ol. 1.58 43 8.7 0.1 86320 1210 3

ACO95-52 Ol. 1.08 4.5 7.2 0.4 99940 5940 1

ACO95-53 Ol. 0.38 1.2 8.4 0.1 85970 700 1

ACO95-55 Ol. 1.01 0.54 5.6 1.7 129720 40520 11

FA10 Ol. 1.13 2.0 7.8 0.5 92280 5780 3

FA12 Ol. 0.19 1.2 8.0 0.1 89601 2378 2

Pico

ACO95-20 Ol. 0.35 1.7 10.0 0.1 72545 415 2

ACO95-20 CPX 1.84 3.6 10.0 0.2 72110 1730 1

ACO95-26 Ol. 1.32 1.4 10.0 0.3 72550 2480 2ACO95-30 Ol. 0.96 36 8.8 0.1 81820 870 0.2

ACO95-30 b Ol. 1.01 13 10.3 0.1 70150 890 1

ACO95-30 b CPX 1.08 7.6 9.7 .0.1 74320 1070 1

ACO95-33 Ol. 1.17 3.2 8.9 0.5 80910 4620 1

ACO95-40 Ol. 0.97 1.0 7.5 0.9 95960 10830 3

Santa Maria

S6 Ol. 0.17 0.19 7.7 0.3 94376 3242 17

Sao Miguel

ACO95-3 Ol. 1.19 1.4 4.8 0.6 149600 17650 2

Ol. 1.46 1.8 5.4 0.5 133830 13490 3

ACO95-56 Ol. 1.02 0.50 4.5 1.2 162070 43520 29

ACO95-62 Ol. 1.97 0.23 6.1 1.9 118450 36900 12

ACO95-62 CPX 1.07 0.60 3.0 1.1 240850 88310 9ACO95-66 Ol. 1.42 0.33 5.9 2.0 121640 41160 16

ACO95-68 Ol. 0.33 0.20 4.1 0.1 174317 5047 7

Re-crush 0.06 3.8 0.4 189644 20408 25

mp 0.31 0.37 4.4 0.1 163180 4310 4

ACO95-68 CPX 1.61 3.3 2.6 0.1 276840 11670 1

CPX 2.82 3.4 4.0 0.3 180550 11910 1

ACO95-68 (melt) CPX 3.02 3.3 4.4 0.7 163470 24600

ACO95-68 CPX 0.32 2.4 4.06 0.03 178200 1360 1

Re-crush 0.02 4.3 0.8 170000 30520 82

mp 0.18 0.48 1.6 0.1 461980 18610 6

Terceira

T2 Ol. 1.08 0.41 10.4 1.6 69530 10570 11

Ol. 1.88 0.59 11.3 0.8 63780 4620 8

T7 Ol. 1.02 1.5 11.0 0.4 65510 2140 3

T9 Ol. 0.34 1.4 9.3 0.1 77550 430 1

T28 Ol. 1.74 0.23 7.0 1.8 103070 26910 9

T28 Ol. 0.30 0.13 8.5 0.2 85165 2350 13

ACO95-9 Ol. 0.10 1.6 9.5 0.1 75858 709 3

ACO95-10 Ol. 0.20 0.93 9.4 0.1 76965 680 3

ACO95-11 Ol. 1.60 2.2 9.4 0.5 77200 3710 2

ACO95-12 Ol. 0.27 0.14 9.7 0.3 74604 2326 13

ACO95-14 Ol. 0.99 1.2 8.0 0.8 90420 8520 5

ACO95-16 Ol. 0.14 0.50 10.3 0.2 70437 1415 7

Graciosa

G1 Ol. 0.19 0.26 8.1 0.2 89600 2380 10


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Table 2

Lead isotopic compositions determined for some Azores samples

Sample 206Pb=204Pb 207Pb=204Pb 208Pb=204Pb

Faial

FA1 19.332 15.555 38.796

19.318 15.541 38.746

FA2 19.607 15.574 38.943

FA3 19.137 15.601 38.814

19.130 15.605 38.816

FA6 19.471 15.619 39.123

FA7 19.607 15.642 39.314

FA10 19.707 15.635 39.296

FA12 a 19.20 15.60 38.89

FA44 a 19.60 15.59 39.10

Graciosa

G1 19.80 15.60 39.18

PicoP1 a 19.96 15.61 39.37

Santa Maria

S6 a 19.14 15.56 39.07

S9 a 19.01 15.56 39.01

S14 a 19.28 15.58 39.30

Sao Miguel

West

SM7 a 19.47 15.59 39.32

SM319 a 19.48 15.62 39.37

Nordeste

SM139 a 20.00 15.78 40.33

SM150 a 19.96 15.75 40.19

SM169 a 19.96 15.74 40.31

a Previously published by Dupre (PhD thesis).

1:7 109 cm3 STP=g for olivines from sampleS6 (Santa Maria) to 4:3 107 cm3 STP=g forolivines analysed in a xenolith from Faial island(ACO95-49). Helium isotopic ratio vary between63,780 (R=Ra D 11:3) and 276,840 (R=Ra D 2:6) inT2 (Terceira) and ACO95-68 (Sao Miguel) samples

respectively.

Helium concentrations obtained for melting aftercrushing of sample ACO95-68 were 3:7 109 cm3

STP=g for the olivines, with a 4He=3He ratio of163,180 4310 (R=Ra D 4:40:1) which is similarto the crushed ratio (4:1 0:1), and 4:8 109

cm3 STP=g for the pyroxenes with a helium ratioof 461,980 18,610 (R=Ra D 1:6 0:1). This

ratio is much more radiogenic than the crushedratio (4:060:03). The agreement between 4He=3Heratios obtained by crushing in this sample, despite

the presence of radiogenic 4He in the clinopyroxenestrongly suggest, that the crushing data reflects themagmatic helium. This point is illustrated in Fig. 3c,where good agreement in 4He=3He ratios is obtained

for several different extractions and over a widevariation in 4He concentrations.

Three distinctive island groups have been dis-tinguished on the basis of helium isotopic results(Fig. 3):

(1) Sao Miguel island, for which 4 samples havebeen analysed, has the most radiogenic 4He=3He ra-tios. One sample (ACO95-3) was from a historicallava flow (Queimado peak eruption, 1563) whereasthe 3 others are Pliocene ankaramites [23]. Theyall display very radiogenic 4He=3He ratios, from121,640 41,160 to 276,840 11,670, with aweighted mean value of 140,660 for the histori-cal sample ACO95-3 (crushed samples). These dataagree with previous measurements from Sao Miguelby Kurz (fig. 8.10 of [36]).

(2) Faial, Graciosa and Santa Maria islands all ex-

hibit a MORB-like helium signature, with 4He=3Heratios ranging from 85,970 1210 to 94,380 3240 (Faial ACO95-55 sample displays an higherbut uncertain value of 129,720 40,520).

(3) Pico and Terceira islands have a more dis-persed helium signature and range from typical

MORB ratios to relatively primitive values, suchas 63,780 4620 for Terceira T2 basalt and 70,150

890 for Pico ACO95-30 sample.Lead isotopic results (Table 2) for the Azores

islands are all relatively radiogenic in compositioncompared to local MORB. However, the respectivepositions of the five islands in PbPb space suggestsa similar grouping of Azores islands (Fig. 4), withgood helium and lead covariations at the scale of thearchipelago:

(1) Eastern Sao Miguel samples have relatively

high206

Pb=204

Pb ratios, around 20, and very ra-diogenic 207Pb=204Pb and 208Pb=204Pb values, upto 15.78 and 40.33 respectively, which are clearlydifferent from those measured on other islands.

(2) Faial and Santa Maria basalts represent theleast radiogenic compositions of the archipelago,with a mean value of 19.50 for 206Pb=204Pb, 15.60

for 207Pb=204Pb and 39.00 for 208Pb=204Pb.(3) Pico and Terceira samples exhibit lead isotope

ratios ranging from FaialSanta Maria values (19.50,


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10-9 10-8 10-7

10-9 10-8 10-7

4He (ccSTP/g)

Terceira

Sao Miguel

Pico

Kurz (unpublish)

rc

rc

rc

mp

c

c

c

mp

mp

100000

150000

100000

150000

200000

300000

400000

4H

e/3He

4He/3H

e

4He/3He

ACO95-68CPX

Olivine

MORB

MORB

radioge

nic

crush

re-crush

melt powder

inherited helium

Graciosa

Santa Maria

Faial

R/Ra

R/Ra

R

/Ra

7.2

4.8

7.2

4.8

14.5

14.5

3.6

2.4

1.8

Fig. 3. 4He=3He ratios vs. 4He concentrations (in cm3 STP=g) for Azores samples. Grey lines correspond to the MORB 4He=3He

mean value of 90,000 [1,68]. Top figure represents the Pico, Terceira and Sao Miguel islands. Some Sao Miguel data from Kurz

(unpublished) were added. Middle figure shows Faial, Santa Maria and Graciosa basalts. One can see the isotopic ratios are exactly

the same as the mean MORB ratio. Bottom figure represents two experiments on the ankaramite ACO95-68 from the Nordeste area on

Sao Miguel. Step crushing and powder melting extractions were performed on olivines and clinopyroxene to constrain the post eruption

radioactive production of 4He. The results show that the 4He=3He ratios are always the same during crushing (olivine and CPX), but

different for CPX melting due to the higher U content of CPX. This implies the 4He=3He ratios obtained by crushing are representative

of the magmatic values, and have not been altered by post-eruptive radiogenic helium (see text).

15.60 and 39.00) to radiogenic compositions simi-

lar to Sao Miguel basalts for 206Pb=204Pb (20.02),lower for 207Pb=204Pb (15.64) and 208Pb=204Pb(39.35) [37,38]. Moreover, similarly to helium,

PicoTerceira and FaialSanta Maria associated

fields are not totally distinctive.These three groups were also proposed by Turner

et al. [38] based on trace element-isotope systematics


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TT

SM

T

SM

SMSM

15.5

15.6

15.7

15.8

17.5 18.5 19.5 20.5 21.5

17.5 18.5 19.5 20.5 21.5

207Pb/204Pb

208Pb/204Pb

206

Pb/204

Pb

38.0

38.5

39.5

40.5

37.5

39.0

40.0

M

M

Tu

Tu

BE

MORB

36-40N

MORB

36-40 N

MORB36-40N

MORB36-40 N

FaialPicoSanta MariaSao MiguelPrevious studies

Terceira

Graciosa

FaialPicoSanta MariaSao MiguelPrevious studies

Terceira

M

M

Tu

Tu

T

BE

NHRL

NHRL

NHRL

15.5

15.6

15.7

15.8

17.5 18.5 19.5 20.5 21.5

17.5 18.5 19.5 20.5 21.5

206

Pb/204

Pb

38.0

38.5

39.5

40.5

37.5

39.0

40.0

BE

ENA

ENA

BE

a

b d

c

o

o

Fig. 4. Lead isotopic diagrams, showing end-member positions of Sao Miguel and Terceira [37] islands. (a) 206Pb=204Pb vs. 207Pb=204Pb.

(b) 206Pb=204Pb vs. 208Pb=204Pb. Also reported are previous Sao Miguel [38,48] results, the Northern Hemisphere Reference Line

(NHRL, as defined in [69]), Bulk Earth average value (BE) [58], and representative fields for local MORB [7073], Tubuaii and Mangaia

islands [7476]. SM and T points are postulated to be representative of the Azores plume compositions. (c) 206Pb=204Pb vs. 207Pb=204Pb.

(d) 206Pb=204Pb vs. 208Pb=204Pb. ENA fields (East North America tholeiitic samples [59]) can be interpreted as reflecting binary mixingbetween a end-member with low lead isotopic ratios and Sao Miguel end-member SM.

where for example, the Th=Nb is higher for SaoMiguel island [38].

Fig. 5 shows the 4He=3He ratio against the 206Pb=204Pb ratios for the different Azores archipelagoislands. The three groups can be easily distinguishedon this figure.

5. Discussion

5.1. The Eastern Sao Miguel radiogenic helium

signature

Since Sao Miguel island is one of the oldest is-lands in the archipelago (up to 4 Myr [23,25]), high


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18

19

20

21

206P

b/204Pb

50,000 100,000 150,000 200,000

14.5 7.2 4.8 3.6

4He/3He

R/Ra

Gough

Loihi

Heard 1

Heard 2

St Helena

MORB 35-40NReunion

Recycled oceaniccrust

Faial - St Maria-Graciosa

Terceira

Faial

Santa Maria

Terceira

Graciosa

Sao Miguel (Kurz)

Sao Miguel

n-MORBs

Fig. 5. 4He=3He ratios versus 206Pb=204Pb ratios for Azores Archipelago basalts and some other islands. Some Sao Miguel data from

Kurz (unpublished) were added. Other data are from the literature.

4He=3He ratios measured in some samples could

be explained by post-eruptive decay of Th and U,

particularly for samples with low helium and highU and Th contents. Sample crushing procedures areperformed to liberate the inherited gases contained influid and melt inclusions, but 4He production couldoccur in the matrix with subsequent diffusion into theinclusions, or partial release by crushing. However,both the low concentration Pliocene ankaramites (2.3

to 6:0 109 cm3 STP=g for ACO95-62, 66 and 68)and historical lava ACO95-3 (434 yr), whose olivinescontain around 1:6 108 cm3 STP=g 4He, display

radiogenic helium isotope ratios, which strongly sug-

gests that the unique radiogenic signature of Sao

Miguel cannot totally result from such a decayprocess. Only old samples could have been mod-ified, starting from a minimum ACO95-3 olivineslike radiogenic value (140,000, we will consider,in the following, as the Nordeste Sao Miguel he-lium signature) to reach ratios as high as 276,840(ACO95-68 CPX) as illustrated by the melting ex-

periment (Fig. 3).The melt of the ACO95-68-CPX powder has

given a 4He=3He ratio of 461,980 18,610, much


10/17


more radiogenic than the crushed value of 178,200.This can easily be explained by post-eruptive 4Heproduction since this sample has an age between 2and 4 Ma [23]. Using a U concentration of 0.012

ppm and a Th=U ratio of 2.9 for CPX [39] we canobtain a U=He age of 1.2 Ma for sample ACO95-68.This age is in good agreement with the K=Ar age[23] considering the uncertainty on the U contentand the Th=U ratio. However, for olivines, crushingand melting have given the same helium isotopicratios (Fig. 3), indicating that 4He production from Uand Th decay is negligible for olivines. This showsthat the radiogenic helium isotopic ratios observed inthe samples from the Nordeste area do not representpost-eruptive decay of Th and U.

Zindler and Hart [9,10] and Hilton et al. [11] haveproposed that high 4He=3He ratios measured in someOIB could reflect shallow depth contamination athigh levels in the crust. The presence of ankaramitesamples with low pressure pyroxenes (high total Aland low AlVI) on Sao Miguel island is consistent

with the existence of shallow magma chambers be-neath the Azores archipelago [23]. Plume sourcecontamination is modeled by a two-component mix-ing and the expression of the mass ratio betweenplume (Mi) and contaminant (oceanic crust) material(Mc) is then given by:

Mc

Mi

8>>>>>:

4He3He

SM

4He3He

i

4He3He

c

4He3He

SM

9>>>=>>>;

3He

i

3He

c

where .4He=3He/SM, .4He=3He/i and .

4He=3He/crefers to the Sao Miguel island, Sao Miguel plumeand contaminant representative helium values, [3He]iand [3He]c to the plume and contaminant concentra-

tions (cm3 STP=g).

We used results obtained by Staudacher and Al-legre [40] on 22 My old oceanic crust (which corre-sponds to the approximate age of the nearby Azorescrust [41]) as a contaminant material ([3He]c D51014 cm3 STP=g and .4He=3He/c D 210

7), thevalue of 140,000 for Sao Miguel island composition,and Terceira-like 4He=3He ratio (60,000) for Sao

Miguel plume. The 3He concentration in the plumematerial is thought to be intermediate between lower(2:5109 cm3 STP=g) [42] and upper mantle (1:1

1010 cm3 STP=g) [43,44] values because the heliumratio we took is very different from the lower mantleratio of 20,000. Let us take 5 1010 cm3 STP=g forthe 3He content for the Terceira plume. The Mc=Mi

result (4000%) suggests that important magmachamber outgassing, at least 99.9%, is required tobe consistent with a plume contamination scenario.

In the previous calculations, we do not considerthe possibility of radiogenic decay in a degassedmagma chamber. Using 3He concentration estimatedabove (5 1010 cm3 STP=g) in the following 4Heradioactive production equation (t in Ma, U in ppmand He in cm3 STP=g):

4He3He

t

D

4He3He

0

C

2:8 108

4:35CTh

U

[U]

[3He] t

we can determine the time interval t necessary toincrease the initial helium ratio (4He=3He)0 from

60,000 (Terceira less primitive value) to the present-day value .4He=3He/t of 140,000. Assuming a Th=Uratio of 2.75 [38] and a U concentration of 0.040ppm (we postulate Terceira plume uranium to beintermediate between lower mantle material [6] andrecycled altered oceanic crust [15] based on lead iso-

tope systematics, cf. Section 5.2), we need to invokevery large outgassing (at least 99.999%) to obtain

magma chamber interval time consistent with classi-cal estimates of magma residence time (between 10and 10,000 yr [45,46]).

Similar calculations can be done with lead iso-topes. However, using a Terceira-like isotopic com-position (207Pb=204Pb D 15.64) for the Sao Miguelplume and the most radiogenic value (207Pb=204PbD 15.64) measured in old oceanic crust (1437 My)

near the Azores archipelago [47] as a contaminant

does not allow to obtain207

Pb=204

Pb ratios that areas high as those determined on Sao Miguel sam-ples (207Pb=204Pb D 15.78). Contamination by recentAtlantic sediments is also unlikely, given the high208Pb=204Pb ratios [48].

There is no evidence for such an extensive out-gassing in the Sao Miguel olivine samples, which

show similar He content to olivines from other is-lands, in particular from Terceira. Kurz et al. [20]have observed a variation of the 4He=3He ratio with


11/17


the longitude on Sao Miguel island (with a decreasewestward), similar to the 87Sr=86Sr and 206Pb=204Pbvariations [20,48,49]. Because these variations andcorrelations are difficult to explain by shallow depth

contamination, we propose a deeper origin for thehigh 4He=3He ratios measured in Sao Miguel basalts.A source effect also appears to be consistent with theheliumlead covariations observed at the scale of thearchipelago. Kurz et al. [3], Vance et al. [4], Grahamet al. [5], Hanan and Graham [16] and Hanyu andKaneoka [7] have postulated that the radiogenichelium measured in Gough, Tristan, St Helena andsome Pacific islands can be explained by recyclinginto the mantle of oceanic crust or sediments, andreinjection of this material in plume sources. Sub-ducted plates should have high (U C Th)=3He ratiosbecause helium is outgassed to the atmosphere dur-ing ridge magmatism and during subduction. Afterstorage for some time in the mantle, this will givehigh 4He=3He ratios by U and Th -decay. Lead andstrontium isotope signatures are consistent with such

a scenario [38,49], especially the high 207Pb=204Pb ra-tios which imply old U=Pb fractionation. Recyclingof subducted altered oceanic crust and sedimentsallows to increase the U=Pb ratio [15,50], and thengive very radiogenic 207Pb=204Pb ratios with time.

5.2. The Terceira helium signature

Kurz et al. [19] first proposed the presenceof a high 4He=3He plume beneath the Azoresarchipelago, based on helium isotopes measurementsin MORB glasses near the Azores platform. How-ever, it turns out that the Azores hotspot is alsocharacterized by low 4He=3He, since primitive val-ues (4He=3He 65,000) have been measured in thisstudy in some Terceira and Pico samples. Produc-

tion of cosmogenic helium [28] in Terceira or Pico

samples remains very unlikely, as all the analysedbasalts are relatively young (some are historical,see Table 1 and Appendix A) and were sampledat low altitude, generally in freshly exposed sur-faces. Moreover, we have analyzed these samplesby crushing which primarily releases the inheritedhelium trapped in inclusions; only a small fraction

of the cosmogenic helium is released by crushingand should be negligible for these samples. There-fore, the primitive helium has to originate within

the source of the plume. It may come from thelower mantle, source of primitive helium for most ofoceanic islands (Hawaii, Iceland, Galapagos or Reu-nion islands). The [3He]LM=[

3He]UM concentration

ratio between the lower and upper mantle shouldbe roughly 10 [6,51] so even a small amount ofentrained lower mantle could transfer a primitivehelium isotopic signal. A helium signature similarto Terceira was found on nearby Atlantic MORBglasses (Fig. 6, [52]) thereby confirming the plumeridge interaction already observed for REE, Sr, Ndand Th=U systematics [5356]. The somewhat dif-ferent helium observations by Kurz et al. [19], couldsimply be due to the different sampling scale ofthis study. For example, most of the MORB samplesfrom 39N are very degassed, and may have beendredged off the ridge axis since the ridge is not welldefined in this area. In fact, the helium signaturerecorded by MORB glasses displays very localizedprimitive values (around 38.5N; Moreira and Alle-gre [52]) which seems to correspond to the locationof the triple junction [41,57] (Fig. 6).

Most primitive helium ratios measured on Ter-ceira samples could be explained by mixing be-tween primitive mantle having high 3He (4He=3He

D 20,000) and two other discrete mantle sources:MORB mantle (4He=3He D 90,000) and a low 3He

plume best represented by data from Sao Miguel(4He=3He D 140,000). Helium alone cannot resolve

between the two hypotheses. However, lead isotopicratios (Fig. 4a,b) show that the Terceira signaturecannot result from binary mixing between the SaoMiguel plume and lower mantle material. The pre-sent-day composition of the lower mantle certainlycorresponds to a binary mixing between Bulk Earthmaterial and about 17% of depleted mantle [58],and then is few different from Bulk Earth composi-

tion. The Terceira signature probably reflects mixing

of high material, lower mantle material andentrained depleted asthenospheric material, whereasthe observed trend for Terceira island may representshallow interaction between this specific materialand the local MORB mantle source.

5.3. TerceiraSao Miguel: same hotspot?

Since we excluded a shallow contamination originfor explaining the He data in Sao Miguel island, the


12/17


60,000

80,000

100 000

120,000

36 38 40 42 44 46

LATITUDE ( N)

4He/3He

N MORB

o

Fig. 6. 4He=3He ratios measured in MORB as a function of the latitude along mid Atlantic ridge between 35 and 40N, modified after

Kurz et al. [19] (crosses) and Moreira and Allegre [52] (diamonds). Note that the minimum of the 4He=3He ratio is close to 38.5N,

which is the supposed triple junction between African, American and European plates [41,57].

Azores archipelago helium signature, with coexis-

tence of both radiogenic and primitive ratios, seemsto be paradoxical. Identification of recycled terrige-nous sediments in the source of Sao Miguel basaltsdoes not give information about the depth of thisspecific material and there are two models for thegeneration of the Azores islands.

The first one proposes the Azores plume deepsource may be isotopically heterogeneous, contain-

ing a mixture of recycled subducted terrigenoussediments, altered oceanic crust and lower mantlematerial. A low degree of melting has permittedto exhibit the sedimentary signature (radiogenic) ofthe source (the fertile part of the mixture, i.e. sed-iments melt preferentially) in Sao Miguel basalts(Nordeste), whereas other islands result from higherextent of melting and reflect mixing of recycledoceanic crust, lower mantle and upper mantle mate-

rials. The origin of this heterogeneous source mayresult of the entrainment by a plume coming fromthe lower mantle of material stored at the 670 kmdiscontinuity or higher or reflects the heterogeneityof the source itself.

The second model proposes the Azores plumedeep source is isotopically homogeneous, with a Ter-ceira like signature (mixing of lower mantle mate-rial and himu material). Sao Miguel island basaltssample a very localized upper mantle heterogene-

ity, melted by the rising of the Terceira plume orblob. This heterogeneity could correspond to sub-ducted oceanic crust and sediments stored underthe north america continent during the pre-Grenvillesubduction [59], recycled into the upper mantle bydelamination occurring during the opening of theNorth Atlantic ocean (Fig. 4c,d and Fig. 7). Again,

intermediate helium and lead compositions reflect in-teraction with the local MORB mantle source (Faial,Graciosa, Pico : : : ). Such a model can be validatedby the fact that East North American basalts (ENA),that correspond to the nearest subcontinental mate-rial from the Azores, show linear trend in PbPbdiagrams (Fig. 4b) which could reflect mixing be-tween a source with low 206Pb=204Pb 207Pb=204Pband 208Pb=204Pb ratios, which could be subconti-nental mantle [60,61], and a component with Sao

Miguel-like signature which could be a mixing of

sediments and recycled oceanic crust. We proposethis component was stored below the North Amer-ica before injection in the North Atlantic mantleby delamination during the opening of the NorthAtlantic. This model is similar to the proposal ofWidom et al. [48], involving shallow interaction ofthe plume source with delaminated subcontinental

lithosphere enriched by metasomatism. Radiogenichelium ratios measured in some continental xeno-liths or continental basalts that sample the subcon-


13/17


ENA High Atlas

MAR

MARAmerica

America

Africa

Africa

SMiT

>1Ga

1Ga

200 Ma

200 -10 Ma

10 Ma

Pre-Grenville subduction

Grenville continent - continent collision

ENA / High Atlas

delamination

Rifting

Sea floor spreading

a

b

c

d

e

"Terceira"

plume or blob

Fig. 7. Schematic evolution of the North Atlantic to explain the Azores isotopic data (model 2). (a) Pre-Grenville subduction [59] brings

some oceanic crust and sediments into the subcontinental lithospheric mantle. (b) End of the subduction with the Grenville collision. (c)

and (d) Opening of the Atlantic ocean is accompanied by some delamination, which provides regional enrichment of the Azores mantle.

(e) Sao Miguel volcanism samples some localized km-size heterogeneity corresponding to old recycled material.


14/17


tinental lithosphere can account for such a model[62,63].

In both model 1 and model 2, we postulate a re-gional enrichment of the asthenosphere beneath the

Azores. The observed radiogenic helium signature isnot restricted to Sao Miguel island. Kurz et al. [19]have measured 4He=3He ratios higher than 100,000in North Atlantic MORB glasses (for latitudes com-prised between 40N and 50N; Fig. 6), which areconsistent with contamination of the local MORBmantle source by delaminated subcontinental litho-sphere (occurring during the opening of the Atlanticocean). Such a hypothesis has been already proposedto explain specific Pb and Sr radiogenic signaturesof the Oceanographer transform zone basalts [64] at35N, and could also be proposed to explain simi-lar isotopic anomalies identified on the Mid AtlanticRidge at 14N [43,65], 43N and 46N [56]. Recentdiscovery of very old zircons in the Mid AtlanticRidge are also consistent with such a model [66].

6. Conclusions

The Azores archipelago corresponds to the sec-ond hotspot where three distinctive helium isotopicsignatures have been observed: MORB values found

at Santa Maria, Graciosa and Faial islands, primitivevalues for some Terceira and Pico basalts, and radio-genic values in Eastern Sao Miguel. The only otheroceanic island where 4He=3He ratios both higher andlower than MORB are found is Heard Island [11].The helium data are correlated with lead isotopiccompositions observed on the same islands and areinterpreted in terms of source variations. The exis-tence of such a multiple isotopic signature can beexplained by two scenarios. The first model pro-

poses that the heterogeneous Azores plume source

has evolved from Sao Miguel to Terceira composi-tion, by various degrees of melting of heterogeneousmantle. The second hypothesis is that the Azoresplume is homogeneous with a Terceira like com-position. The origin of the Sao Miguel signaturewould then correspond to sampling of very localkm-size heterogeneities, resulting from the delami-

nation of enriched subcontinental lithosphere, whichoccurred during the Jurassic opening of the NorthAtlantic.

Acknowledgements

M.M. would like to thank Pascale Louvat verymuch for his help during the mission in Azores and

patience during rock sampling, and the team of theEarth Sciences department from Universidade dosAcores, in particular Pr. Victor Forjaz and J.C. Nunesfor discussions and help during the Azores trip. B.Bourdon, J. Kunz and Th. Staudacher are thankedfor discussion and improvement of the manuscript.D. Graham, I. Kaneoka and R. Poreda helped to im-prove the quality of the manuscript by their reviewcomments. Josh Curtis is thanked for his help duringanalyses in the Woods Hole Oceanographic Insti-tution. This is IPGP contribution 1574 and WHOIcontribution 9893. [CL]

Appendix A. Sample locations

Sample locations are also given on Fig. 2. Some location arein [37].

Faial

ACO95-sand Olivine rich sand from the Capelinho place. Re-

sults from projection of submarine eruption during

the 1957 eruption.

ACO95-47 Collected on the chimney in the rim that separates

the historical eruption of the Capelinho (1957=58)

from the Complexe do Capelo (


15/17


Nordeste

ACO95-62 Ankaramite collected near the Guihermo ou dos

Moinhos river. South of Lomba da Fazenda.

ACO95-66 Ankaramite collected near the Mulher river. South

of Santana.

ACO95-68 Ankaramite collected near Faial da Terra.

Terceira

ACO95-9 1761 lava flow at the bottom of the Pico do Fogo.

ACO95-10 Collected near the road between Biscoitos and

Angra Do Heroismo (near the Pico do Fogo).

ACO95-11 Collected in the Guiherme Moniz Caldeira (north).

ACO95-12 Collected near Faieis in a river (Rib. das Pedras).

ACO95-14 Collected near the road between Sao Sebastiao and

Feteira (near Boavista).

ACO95-16 Collected near the road near Rossio de Sant Ana.

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He and Pb Isotopes in Azores

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