rare gas systematics on mid atlantic ridge (37–40°n)

Rare gas systematics on Mid Atlantic Ridge (37^40‡N)

Manuel Moreira �, Claude-Jean Alle'greLaboratoire de Ge¤ochimie et Cosmochimie, Institut de Physique du Globe et Universite¤ Paris 7-Denis Diderot, CNRS,

4 place Jussieu, 75252 Paris Cedex 05, France

Received 4 September 2001; received in revised form 7 January 2002; accepted 5 February 2002

Abstract

Rare gas concentrations and isotopic compositions in 19 glasses from the North Atlantic ridge have been analyzed.Sixteen of them are located between 36.8 and 39.9‡N on the Azores plateau, three others are located between 21.25and 30.2‡N and may represent normal North Atlantic Mid Ocean Ridge basalts. Helium concentrations decrease bythree orders of magnitude between 37‡ and 38.5‡N. These low He concentrations (1038 cm3 STP/g) are attributed todegassing before eruption. The 4He/3He ratios decrease from 90 000 at 37‡N to 75 000 at 38.5‡N and then increase to100 000 at 40.5‡N. The low 4He/3He ratio measured on the ridge at 38.5‡N indicates a plume^ridge interaction withthe Terceira^Pico^Sa‹o Jorge plume, which has a 4He/3He lower than 50 000 (R/Ras 15). More radiogenic 4He/3Heratios (up to 110 000) can also be found in this segment of the ridge and may indicate either in situ production of 4Heby radioactive decay or atmospheric contamination of very low helium content samples. The 20Ne/22Ne ratiosdecrease from a mantle-like value (s 12.0) between 20 and 37‡N to the atmospheric ratio (9.8) at 38.5‡N. The 40Ar/36Ar ratios show the same tendency as the neon isotopic ratios (atmospheric-like at 38.5‡N). This may indicatecontamination in shallow degassed magma chambers. However, because the maximum measured 20Ne/22Ne ratios arestrongly correlated to the 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd ratios, an alternative explanation for the atmospheric20Ne/22Ne is the presence of an atmospheric component in the source. However, due to the large He loss (andtherefore Ne and Ar), atmospheric contamination is the more probable explanation. ß 2002 Elsevier Science B.V.All rights reserved.

Keywords: noble gases; Azores; degassing; helium; isotopes

1. Introduction

Rare gas studies have shown that the uppermantle, which is the source of Mid Ocean Ridgebasalts (MORB), is degassed at more than 99%[1,2]. The mean MORB 4He/3He ratio is around

90 000 (R/Ra = 8, where R is the 3He/4He ratioand Ra is the atmospheric ratio of 1.384U1036)[3]. Most of the hotspots show 4He/3He ratioslower than the MORB ratios. Ratios as low as19 000 (R/RaW37) were measured on basaltsfrom Iceland [4,5] and re£ect the existence of aless degassed reservoir (‘primordial’), locateddeep in the Earth’s mantle. This di¡erence in the4He/3He ratio between the MORB reservoir(V90 000) and the reservoir source of the OceanIsland basalts (OIB) (V20 000) can be explained

0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 1 9 - 8

* Corresponding author. Tel. : +33-1-4427-4915;Fax: +33-1-4427-3752.

E-mail address: [email protected] (M. Moreira).

EPSL 6155 26-4-02

Earth and Planetary Science Letters 198 (2002) 401^416

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easily by a two-layer mantle model, where thelower mantle is the source of plumes and theupper mantle is the source of MORB. The heliumsystematics are con¢rmed by the study of neonisotopes [6^14]. Neon has three stable isotopes,20Ne, 21Ne and 22Ne. The 20Ne and 22Ne isotopesare virtually ‘primordial’ whereas 21Ne has ameasurable nucleogenic component (e.g.,18O(K,n)21Ne, 24Mg(n,K)21Ne) due to its lowabundance compared to the two other isotopes[15]. Sarda and collaborators [6] have shownthat MORB fall on a line in a three-neon-isotopediagram, interpreted as a mixing line between ahigh 20Ne/22Ne (V13) and 21Ne/22Ne (V0.07)end-member and an AIR-like end-member (9.8and 0.029). Honda et al. [7], followed by di¡erentgroups [8^14], have shown that the source of ‘high3He’ hotspots also has a solar-like 20Ne/22Ne ratio(s 12.5) but a less nucleogenic 21Ne/22Ne (0.04^0.05) than the MORB source reservoir (0.07).This is due to the less degassed character of theplume source mantle.

Kurz et al. [16] have proposed the existence of agroup of hotspots called ‘low 3He’ because themeasured 4He/3He ratios are higher than themean MORB ratio. Tristan, Gough, St. Helena,Canaries and the Tubuai Islands belong to thisgroup (4He/3Hes 120 000) [16^20]. They pro-posed that this radiogenic component may resultfrom 4He production in a high (U+Th)/3He ma-terial stored during a long time in a thermalboundary layer. This component could be sedi-ments and/or degassed oceanic crust injectedinto the mantle through subduction zones. Analternative view for these radiogenic isotopic ra-tios was proposed by Hilton et al. [21], followingthe initial proposition of Condomines et al. [22]and Zindler and Hart [23], suggesting that theradiogenic helium component re£ects a shallowdepth contamination or a radioactive productionof 4He in degassed magma chambers.

Based on radiogenic 4He/3He (s 110 000; R/Ra6 6) measured on MORB from the Mid At-lantic Ridge between 35 and 50‡N, and supposinga plume^ridge interaction, Kurz et al. [24] pro-posed that the Azores hotspot is a ‘low 3He’ hot-spot. Moreira et al. [25] have analyzed heliumisotopes in basalts from the Azores archipelago

islands (Fig. 1) and have observed three di¡erenthelium isotopic signatures: MORB-like, primitive(‘high 3He’) and radiogenic. They observed theradiogenic component in central and east Sa‹o Mi-guel island. All the 4He/3He ratios are higher than120 000 (R/Ra6 5.9) with a maximum value of482 000 (R/Ra = 1.9). These radiogenic helium iso-topic ratios are associated with radiogenic 87Sr/86Sr (0.705), 206Pb/204Pb (20) and 207Pb/204Pb(15.8) isotopic ratios [26^28], which may re£ectan old and radiogenic component located in themantle. The primitive component observed inTerceira and Pico islands has 4He/3He ratiosdown to 65 000 (R/Ra = 11.5) and re£ects a lowermantle component. Recent measurements on Sa‹oJorge island have given ratios as low as 45 000 (R/Ra = 15.9), con¢rming the previous suggestionthat the Azores are a ‘high 3He’ hotspot (Chadu-teau and Moreira, unpublished data). Therefore,the Azores hotspot is the second hotspot showingsuch duality in the helium signature (with Heardisland in the Indian Ocean [21]).

In order to characterize the origin of the di¡er-ent components observed in the Azores archipel-ago, we have performed a rare gas study onMORB dredged from the Azores plateau between37 and 40‡N.

2. Geologic settings, sample locations andanalytical procedure

The Azores archipelago is composed of nineislands and is the surface expression of a largesubmarine plateau (Fig. 1). Three islands (Ter-ceira, Graciosa and Sa‹o Miguel) are aligned ona spreading center called the ‘Terceira Rift’. Otherislands are on the south of this rift, except Corvoand Flores, which are on the American Plate. TheMid Atlantic Ridge crosses the Azores archipela-go (Fig. 1) which o¡ers us the opportunity tohave fresh and zero age samples close to a hot-spot. The MORB coming from 36 to 41‡N showhigh 87Sr/86Sr, 206Pb/204Pb and La/Sm ratios, re-£ecting an interaction between the ‘Azores hot-spot’ and the MOR [29^32]. However, the notionof the ‘Azores hotspot’ is ambiguous because ofthe existence of at least three di¡erent isotopic

EPSL 6155 26-4-02

M. Moreira, C.-J. Alle'gre / Earth and Planetary Science Letters 198 (2002) 401^416402

signatures in the Azores archipelago (MORB-like,Terceira^Pico^Sa‹o Jorge and Sa‹o Miguel compo-nents) [25]. The observed isotopic ratios of Sr andPb on the Mid Atlantic Ridge close to 39‡N aresimilar to the Terceira^Pico^Sa‹o Jorge compo-nent [32] whereas the Sa‹o Miguel component islocalized in the east of Sa‹o Miguel island [27,28].

Two series of samples from the North AtlanticRidge were studied. The ¢rst were dredged by theFrench oceanographic missions CH30, CH77,CH97, CH98 and ARP74. These samples were

given to us by the IFREMER. The second setof samples (‘A127’ cruise) was dredged by theRV Atlantis II (FAZAR expedition) and were giv-en by C. Langmuir (LDEO). Major and traceelements of these samples can be found in [33].Isotopic compositions of Sr, Nd and Pb can befound in [32]. Geophysical data and tectonic in-terpretations on the structure of the Mid AtlanticRidge close to 39‡N are published in [34].

The samples are located between 21.25‡N and39.91‡N on the Mid Atlantic Ridge (Table 1). We

Fig. 1. The Azores archipelago is composed of nine islands, the surface expression of a vast submarine plateau. The islands, ex-cept Corvo (C) and Flores (Fl), are located between the so-called Terceira rift, joining Sa‹o Miguel island (SMi) and Graciosa(G), and the East Azores Fracture Zone. The actual triple junction seems to be on the Mid Atlantic Ridge at 38.5‡N, based ongeophysical evidence [54]. T is for Terceira, SJ for Sa‹o Jorge, P for Pico, F for Faial, SMa for Santa Maria.

EPSL 6155 26-4-02

M. Moreira, C.-J. Alle'gre / Earth and Planetary Science Letters 198 (2002) 401^416 403

analyzed samples far from the Azores plateau tohave a ‘baseline’ of isotopic ratios and concentra-tions of MORB from the North Atlantic. Depthsvary between 4416 m and 340 m. Sample loca-tions close to 39‡N are given in Fig. 2.

All the samples were fresh glasses and we usedpieces of 0.5^1 cm in size for experiments. Sam-ples A127 17-3, 18-1 and 21-3 were small pieces ofglasses (1 to 3^4 mm). Samples were ultrasonicallycleaned using distilled water, ethanol and acetone.

We used step heating and crushing to extractthe gas. For the step heating, we used two steps(600 and 1500‡C) except for sample A127 D15-1,for which we used three steps (550, 650 and1500‡C). During analyses, the blanks at low tem-perature were 2.0 þ 0.2U1038 ccSTP 4He,1.00 þ 0.07U10311 ccSTP 20Ne, 2.9 þ 1.2U10312

ccSTP 36Ar, 1.2 þ 0.3U10313 ccSTP 84Kr and1.10 þ 0.08U10314 ccSTP 132Xe. The high temper-ature blanks were 1.9 þ 0.2U1038 ccSTP 4He,1.5 þ 0.6U10311 ccSTP 20Ne, 3.0 þ 1.3U10312

ccSTP 36Ar, 1.0 þ 0.3U10313 ccSTP 84Kr and1.1 þ 0.2U10314 ccSTP 132Xe (errors are standard

deviation for all the blanks). Blank isotopic ratiosare atmospheric within uncertainties. Crusherblanks were half lower than the low temperatureblank.

In some cases, we also used the method formeasuring the helium of [25] making it possibleto have a 4He blank of 5U10310 ccSTP (for sam-ples A127 D17-3, D18-1, D21-3, D22-5 andCH30). Sample A127 D21-3 has also been ana-lyzed at Woods Hole Oceanographic Institution(WHOI) with a 4He blank of 3U10311 ccSTP.

3. Results

The He, Ne, Ar, Kr and Xe concentrations andisotopic compositions are given in Table 2

3.1. Helium

The 4He concentrations show a wide range ofvariations between 1.2U1038 (A127 D21-3) and7.1U1035 ccSTP/g (C23). Concentrations of

Table 1Weight, location and depth of the samples analyzed in this study

Sample Weight Latitude Longitude Depth(g) (‡N) (‡W) (m)

A127 D36-24 1.02 37.26 32.28 2274A127 D15-1 1.92 37.30 32.28 1706A127 D17-3 0.40 37.84 31.52 1009A127 D18-1 0.49 37.98 31.47 1725A127 D21-3 0.76 38.49 30.27 2000A127 D21-3 duplicate 1.00 38.49 30.27 2000A127 D21-3* 0.11 38.49 30.27 2000A127 D22 SG 0.91 39.04 30.03 1380A127 D22-5 1.47 39.04 30.03 1380A127 D22-5 duplicate 1.32 39.04 30.03 1380A127 D29-SG 1.33 39.44 29.85 1850A127 D27-3 1.66 39.51 29.74 2200A127 D27-5 1.86 39.51 29.74 2000A127 D26-5 0.56 39.91 29.68 2100C23 0.25 21.25 45.77 ?CH77 DR5-105 2.07 23.42 44.98 4416CH98DR12 2.62 30.17 41.92 4200ARP74 10-14 2.10 36.83 33.27 2523ARP74 12-19 2.30 36.86 33.26 2400CH30 DR24-07 1.22 38.96 29.87 340CH30 DR24-07 duplicate 1.35 38.96 29.87 340CH97 DR2-101 2.02 39.78 29.64 ?CH97 DR2-103 0.99 39.78 29.64 1939

EPSL 6155 26-4-02


V1038 or 1037 ccSTP/g are low relative toMORB concentrations which show a content of1035^1034 ccSTP/g [35,36]. The variation of the4He concentration with latitude is given in Fig. 3.This ¢gure shows that the He-poorest samples arelocated between 37 and 40‡N. This was alreadynoted by Kurz et al. [24] (white dots in Fig. 3).Sample CH30 gives similar concentrations by thetwo methods of extraction (2.5U1038 ccSTP/g bymelting and 3.8U1038 by crushing) indicatingthat the helium is essentially located in thevesicles. Sample A127 D22-5 also shows thesame results by the two extraction procedures(2.4U1037 and 1.6U1037 ccSTP/g). SampleA127 D22-sg, coming from the same dredge as

A127 D22-5, yields a similar concentration(1.5U1037) which may indicate homogeneity ina single dredge for helium content. The three sam-ples poorest in helium from [24] (TR154-10D-3,TR154-21D-4 and TR154-8D-3) have concentra-tions of 7.0U1038, 3.0U1038 and 4.4U1037

ccSTP/g, of the same magnitude as our He-poorsamples.

We will use the following total 4He/3He ratiosto avoid a bias due to mass fractionation duringstep heating. 3He can di¡use faster than 4He,leading to di¡erences in the isotopic ratio betweenthe ¢rst and second steps. This con¢rms the ex-perimental approach of Trull and Kurz [37] thatshows such a mass fractionation for helium.

Fig. 2. Location of the MORB samples analyzed in this study (dots). Squares represent three samples analyzed by Kurz et al.[24].

EPSL 6155 26-4-02


Tab

le2

Rar

ega

sco

ncen

trat

ions

(in

ccST

P/g

)an

dis

otop

icco

mpo

siti

ons

ofth

eM

idA

tlan

tic

MO

RB

Sam

ple

Step

4H

e20

Ne

36A

r84

Kr

130X

e4H

e/3H

ec

20N

e/22

Ne

c21

Ne/

22N

ec

38A

r/36

Ar

c40

Ar/

36A

rc

A12

7D

36-2

460

0‡C

2.1U

103

74.

1U10

311

2.0U

103

115.

3U10

313

7.3U

103

1577

325

112

09.

930.

070.

0271

0.00

090.

1880

0.00

2229

13

1500

‡C5.

2U10

36

9.3U

103

113.

2U10

311

6.9U

103

137.

8U10

315

8465

091

010

.75

0.06

0.03

650.

0005

0.18

740.

0013

241

020

Tot

al5.

4U10

36

1.3U

103

105.

2U10

311

1.2U

103

121.

5U10

314

8434

087

010

.48

0.05

0.03

350.

0005

0.18

760.

0012

159

512

A12

7D

15-1

550‡

C2.

1U10

37

1.2U

103

112.

6U10

311

4.0U

103

133.

9U10

315

8178

01

870

9.64

0.11

0.02

900.

0017

0.18

650.

0022

282

365

0‡C

9.1U

103

61.

2U10

310

1.4U

103

102.

3U10

312

1.9U

103

1489

640

125

011

.05

0.03

0.03

910.

0004

0.18

740.

0010

893

515

00‡C

2.6U

103

63.

5U10

311

4.3U

103

117.

6U10

313

1.0U

103

1484

460

870

11.0

10.

080.

0390

0.00

090.

1871

0.00

1196

25

Tot

al1.

2U10

35

1.7U

103

102.

1U10

310

3.4U

103

123.

4U10

314

8545

569

510

.94

0.03

0.03

830.

0004

0.18

720.

0008

831

4A

127

D17

-3cr

ush

1.4U

103

7na

nana

na79

700

278

0A

127

D18

-1cr

ush

1.3U

103

7na

nana

na76

030

171

0A

127

D21

-3cr

ush

1.2U

103

81.

8U10

310

3.8U

103

10na

na74

915

801

59.

750.

050.

0292

0.00

030.

1871

0.00

2028

94

A12

7D

21-3

crus

h2.

3U10

38

nana

nana

8120

07

300

A12

7D

21-3

*18

00‡C

5.7U

103

879

420

610

A12

7D

22-s

g60

0‡C

4.6U

103

81.

8U10

310

1.4U

103

103.

5U10

312

3.9U

103

1485

928

578

09.

920.

040.

0295

0.00

030.

1865

0.00

1129

02

1500

‡C1.

0U10

37

4.2U

103

109.

5U10

310

1.6U

103

111.

3U10

313

8457

02

780

9.84

0.03

0.02

870.

0003

0.18

720.

0004

290

2T

otal

1.5U

103

75.

9U10

310

1.1U

103

92.

0U10

311

1.7U

103

1385

000

262

09.

860.

020.

0289

0.00

020.

1871

0.00

0329

02

A12

7D

22-5

600‡

C1.

9U10

37

6.3U

103

106.

2U10

310

1.5U

103

111.

7U10

313

8205

01

310

9.91

0.03

0.02

980.

0003

0.18

760.

0006

296

215

00‡C

4.1U

103

85.

2U10

311

4.9U

103

111.

5U10

312

3.5U

103

1477

195

545

09.

920.

070.

0291

0.00

070.

1886

0.00

1833

63

Tot

al2.

4U10

37

6.9U

103

106.

7U10

310

1.7U

103

112.

0U10

313

8116

01

490

9.91

0.02

0.02

980.

0003

0.18

770.

0006

299

2A

127

D22

-5cr

ush

1.6U

103

7na

nana

na83

850

111

0A

127

D29

-sg

600‡

C4.

6U10

37

9.6U

103

122.

1U10

312

1.4U

103

13na

8656

01

665

9.93

0.23

0.03

430.

0023

0.18

880.

0168

346

2315

00‡C

3.1U

103

78.

4U10

312

1.0U

103

115.

3U10

313

na92

740

178

010

.62

0.40

0.03

150.

0042

0.19

500.

0475

790

170

Tot

al7.

7U10

37

1.8U

103

111.

3U10

311

6.7U

103

1388

930

124

010

.24

0.22

0.03

300.

0025

0.19

400.

0398

713

141

A12

7D

27-5

600‡

C1.

5U10

35

8.9U

103

111.

1U10

311

2.1U

103

134.

8U10

315

8352

01

170

9.98

0.04

0.03

000.

0004

0.18

960.

0028

200

028

1500

‡C2.

0U10

36

3.5U

103

118.

0U10

312

1.9U

103

135.

8U10

315

8739

01

050

10.4

60.

060.

0332

0.00

090.

1904

0.00

223

890

40T

otal

1.7U

103

51.

2U10

310

1.9U

103

114.

1U10

313

1.1U

103

1485

785

780

10.1

20.

030.

0308

0.00

040.

1899

0.00

182

796

23A

127

D27

-360

0‡C

9.8U

103

71.

7U10

311

4.6U

103

123.

8U10

313

2.0U

103

1479

570

108

510

.28

0.22

0.03

310.

0032

0.18

830.

0060

580

1515

00‡C

2.0U

103

62.

3U10

311

7.8U

103

123.

5U10

313

5.6U

103

1585

390

102

011

.22

0.15

0.04

180.

0025

0.18

800.

0023

201

030

Tot

al3.

0U10

36

4.0U

103

111.

2U10

311

7.3U

103

132.

6U10

314

8339

076

010

.80

0.13

0.03

790.

0021

0.18

810.

0026

148

020

A12

7D

26-5

600‡

C1.

7U10

36

8.3U

103

111.

4U10

310

1.7U

103

129.

9U10

315

8617

01

270

10.0

10.

140.

0316

0.00

110.

1873

0.00

1470

55

1500

‡C1.

5U10

35

2.1U

103

102.

6U10

310

4.9U

103

125.

0U10

314

8528

087

010

.96

0.09

0.03

970.

0010

0.18

850.

0009

335

020

Tot

al1.

6U10

35

2.9U

103

104.

0U10

310

6.6U

103

126.

0U10

314

8537

079

510

.67

0.08

0.03

720.

0009

0.18

810.

0008

242

413

C23

crus

h7.

1U10

35

3.7U

103

102.

1U10

310

na9.

7U10

314

7963

589

011

.98

0.13

0.05

560.

0020

0.18

540.

0034

1911

028

0C

H77

DR

5-10

560

0‡C

3.7U

103

61.

1U10

311

4.3U

103

123.

1U10

313

3.8U

103

1578

356

790

11.1

00.

300.

0424

0.00

570.

1892

0.00

7914

150

430

1500

‡C2.

1U10

35

1.1U

103

105.

7U10

311

3.1U

103

124.

4U10

314

8550

085

512

.15

0.15

0.05

960.

0019

0.18

620.

0018

1760

014

0T

otal

2.5U

103

51.

2U10

310

6.1U

103

113.

4U10

312

4.8U

103

1484

365

845

12.0

60.

140.

0581

0.00

180.

1864

0.00

1817

358

134

CH

98D

R12

600‡

C7.

7U10

37

9.3U

103

121.

0U10

311

na2.

7U10

315

7996

01

275

10.0

20.

330.

0314

0.00

610.

1889

0.00

7128

411

1500

‡C1.

7U10

35

1.3U

103

108.

1U10

311

5.7U

103

136.

8U10

314

8220

01

010

12.1

30.

080.

0516

0.00

100.

1853

0.00

2314

726

162

Tot

al1.

8U10

35

1.4U

103

109.

1U10

311

5.7U

103

137.

1U10

314

8210

01

025

11.9

70.

070.

0501

0.00

100.

1857

0.00

2213

139

144

AR

P74

10-1

460

0‡C

6.2U

103

71.

3U10

310

3.2U

103

106.

5U10

312

4.6U

103

1484

145

214

09.

690.

050.

0285

0.00

060.

1879

0.00

0429

31

1500

‡C8.

3U10

36

1.5U

103

101.

7U10

310

6.7U

103

125.

0U10

314

9062

01

865

11.1

80.

060.

0407

0.00

090.

1876

0.00

071

722

7T

otal

8.9U

103

62.

8U10

310

4.9U

103

101.

3U10

311

9.5U

103

1490

135

188

510

.44

0.04

0.03

470.

0005

0.18

780.

0004

789

3A

RP

7412

-19

600‡

C2.

5U10

36

5.0U

103

112.

0U10

311

3.7U

103

131.

8U10

314

8391

51

410

10.6

90.

090.

0358

0.00

120.

1865

0.02

597

852

769

1500

‡C6.

1U10

37

1.1U

103

114.

1U10

312

1.7U

103

131.

4U10

314

9228

52

140

16.1

41.

930.

0632

0.01

230.

1874

0.10

7224

730

994

0T

otal

3.1U

103

66.

1U10

311

2.5U

103

115.

4U10

313

3.2U

103

1485

415

127

011

.32

0.24

0.03

890.

0018

0.18

670.

0282

1072

31

807

EPSL 6155 26-4-02


The total 4He/3He isotopic ratios are between74 915 for A127 D21-3 (R/Ra = 9.7) and 110 695for CH30 (R/Ra = 6.5). The ¢rst value is lowerthan the mean MORB value of 90 000 [3] whereasthe second value is higher.

Sample A127 D21-3 has 4He/3He ratios of81 200 þ 7300 (R/Ra = 8.9 þ 0.8) and 74 915 þ 8015(R/Ra = 9.6 þ 1.0). It was analyzed a third time atthe WHOI and gave a 4He/3He ratio of79 420 þ 610 (R/Ra = 9.10 þ 0.07), similar to ourmeasurements. In the following, we will take theratio measured at WHOI for this sample becauseof a better accuracy.

Sample CH30 has a helium isotopic ratio(110 695) similar to the three samples from Kurzet al. [24] located near the CH30 dredge (blacksquares in Fig. 2). These samples have 4He/3Heratios of 99 010, 111 111 and 96 154. Fig. 4 showsthe 4He/3He variation with latitude. If the fourradiogenic samples are not considered (CH30,

Fig. 3. Variation of depth and 4He with latitude. Note thelog scales. Minima of concentration are observed atV38.5‡N.

Tab

le2

(Con

tinu

ed)

Sam

ple

Step

4H

e20

Ne

36A

r84

Kr

130X

e4H

e/3H

ec

20N

e/22

Ne

c21

Ne/

22N

ec

38A

r/36

Ar

c40

Ar/

36A

rc

CH

30D

R24

-07

600‡

C1.

4U10

38

2.4U

103

101.

0U10

39

3.6U

103

113.

6U10

313

dl9.

860.

040.

0304

0.00

050.

1875

0.00

0628

71

1500

‡C1.

1U10

38

1.8U

103

93.

5U10

39

7.4U

103

113.

3U10

313

dl9.

780.

030.

0289

0.00

030.

1880

0.00

0429

11

Tot

al2.

5U10

38

2.0U

103

94.

5U10

39

1.1U

103

106.

9U10

313

dl9.

790.

020.

0290

0.00

030.

1878

0.00

0329

01

CH

30D

R24

-07

crus

h3.

8U10

38

nana

nana

1106

954

695

CH

97D

R2-

101

600‡

C3.

2U10

37

1.1U

103

107.

3U10

311

1.5U

103

129.

8U10

315

8712

02

020

9.85

0.07

0.02

950.

0007

0.18

730.

0019

289

315

00‡C

1.3U

103

66.

9U10

311

4.3U

103

112.

8U10

312

4.1U

103

1493

375

110

510

.29

0.07

0.03

400.

0012

0.18

580.

0034

609

10T

otal

1.7U

103

61.

8U10

310

1.2U

103

104.

3U10

312

5.1U

103

1492

095

133

010

.02

0.05

0.03

120.

0006

0.18

680.

0017

408

4C

H97

DR

2-10

360

0‡C

1.0U

103

61.

3U10

310

nana

na90

560

242

09.

880.

060.

0314

0.00

13na

nana

na15

00‡C

1.2U

103

62.

1U10

311

nana

na93

170

193

011

.44

0.38

0.04

490.

0057

nana

nana

Tot

al2.

2U10

36

1.5U

103

10na

nana

9191

52

180

10.0

70.

070.

0331

0.00

13na

nana

na

Unc

erta

inti

eson

conc

entr

atio

nsar

e5%

.A

127

D21

-3*

was

anal

yzed

inth

eW

HO

I.na

mea

nsno

tan

alyz

edan

ddl

mea

nsde

tect

ion

limit

.

EPSL 6155 26-4-02


and the three samples from [24]), the 4He/3Heratio is MORB-like (V90 000) between 35 and37‡N, decreases to 75 000 (R/Ra = 9.7) close to38.5‡N, and increases to V95 000 after 40‡N(Fig. 4).

3.2. Neon

Total 20Ne concentrations are between1.8U10311 and 2.0U1039 cm3 STP/g, which aretypical glassy margin MORB concentrations [6].There is no systematic variation of 20Ne concen-trations with latitude as we have seen before forhelium.

Total isotopic ratios 20Ne/22Ne (sum of the twoor three steps) are between the atmospheric ratio(9.8) and 12.06. Total 21Ne/22Ne ratios vary be-tween the air value (0.029) and 0.0581. The ratiosof the ¢rst steps are between the air value and11.10 for the 20Ne/22Ne and the air ratio and0.0424 for the 21Ne/22Ne. The maximum ratiosare associated to the high temperature steps(9.8^12.15 and 0.029^0.0596). Sample A127D15-1, analyzed in three steps, shows the same20Ne/22Ne for the two last steps (11.05 and11.01), suggesting that this value may be the max-imum for this sample.

Fig. 5 shows the maximum 20Ne/22Ne versusthe maximum 21Ne/22Ne for the MORB analyzedin this study. The dots representing our samples

are consistent with the MORB line de¢ned bySarda et al. [6], in accordance with the close-to-MORB 4He/3He ratios even if it appears thatsamples fall above the MORB line. The threesamples with the highest 20Ne/22Ne (white dotsin Fig. 5) are samples far away from the AzoresPlateau (6 31‡N) and may represent a baseline ofthe North Atlantic (20Ne/22NeW12).

Fig. 6 shows the variation of the maximum

Fig. 5. Three-neon-isotope diagram showing our MORB re-sults (maximum). The MORB line is from Sarda et al. [6].The Hawaii^Iceland ¢eld is from [7,11^14]. Solar value isfrom [55].

Fig. 4. Variations of the total 4He/3He ratios with latitude showing a minimum close to 38.5‡N. This might result from plume^ridge interaction with the Terceira^Pico^Sa‹o Jorge hotspot, which also shows primitive helium [25].

EPSL 6155 26-4-02


measured 20Ne/22Ne ratio with latitude. The 20Ne/22Ne ratios have a minimum close to 38.5‡N (sim-ilar to the atmospheric value) and the ratios arebetween 10.5 and 11.2 south and north of 38.5‡N.

3.3. Argon

Total 36Ar concentrations are between1.2U10311 and 4.5U1039 cm3 STP/g. Maximummeasured concentrations were obtained for sam-ple CH30 DR24-07. The 36Ar concentrations arestrongly correlated with 20Ne contents.

Total 40Ar/36Ar ratios vary between the air ra-tio (295.5) and 19 100 þ 280. Maximum 40Ar/36Arare between air and 24 730 þ 9940 (19 100 is thesecond highest ratio). This last value is close tothe highest ratio measured on MORB from theNorth Atlantic (V28 000) [38] but still lowerthan the measurement on one single vesicle [39]or the predicted value of V44 000 [40]. The 38Ar/36Ar ratios are all atmospheric within the errorbars (air = 0.1880). Fig. 6 shows the maximum40Ar/36Ar as a function of latitude. One can ob-serve that the variation with latitude is similar to

the neon isotopic variations, where the ratios be-come atmospheric at 38.5‡N.

The 4He/40Ar* ratios measured on the MORBsamples from this study vary between 11.9 and358 where 40Ar* is the argon corrected for aircontamination. This range is observed on mostof the MORB [35,36]. Fig. 7 shows the 4He/40Ar* ratio variation with latitude. One can ob-serve a ‘baseline’ of V15^20 before the AzoresPlateau and a peak at 38^39‡N with values about10 times higher than the ‘baseline’.

3.4. Krypton and xenon

Total concentrations of 84Kr are between4.1U10313 and 1.1U10310 cm3 STP/g and thoseof 130Xe are between 1.1U10314 and 6.9U10313

cm3 STP/g. These concentrations are correlatedwith neon concentrations.

4. Discussion

We will focus on the plume^ridge interactionand the consequences for the helium isotopic ra-tios, on the very low concentration of helium insamples from latitudes close to 38.5‡N and ¢nallyon the existence of atmospheric neon and argonat 38.5‡N, associated with relatively primitive he-lium.

Fig. 6. Variations with latitude of the 20Ne/22Ne and 40Ar/36Ar ratios (maximum measured ratios). We do not representthe sample with 40Ar/36Ar = 24 727 þ 9939 because of its largeerror bar. The minimum is observed at 38.5‡N where theminimum of the 4He/3He ratio is observed.

Fig. 7. Variations with latitude of the 4He/40Ar* ratio. Apeak at V39‡N can be observed. Such variations re£ect de-gassing processes that can fractionate rare gases (see text).

EPSL 6155 26-4-02


4.1. Plume^ridge interaction at 38.5‡N

The minimum of the 4He/3He ratios is observedat 38.5‡N. Moreira et al. [25] have shown thatTerceira and Pico islands have the most primitive4He/3He of the Azores archipelago (V70 000; R/Ra = 10.5). Recent measurements on Sa‹o Jorgehave con¢rmed the primitive nature of the centralgroup of islands. 4He/3He ratios as low as 45 000(R/Ra = 15.9) have been measured on Sa‹o Jorge(Chaduteau, Moreira and Kurz, unpublisheddata). It is therefore correct to think there is aplume^ridge interaction between the Terceira^Pico^Sa‹o Jorge plume and the Mid AtlanticRidge, and that the primitive ratios observed onthe ridge basalts result from this interaction. Thisinterpretation is con¢rmed by the Sr^Nd^Pb iso-tope systematics [32]. In isotopic diagrams, the35^40‡N MORB trend can be attributed to a mix-ing between the Terceira^Pico^Sa‹o Jorge compo-nent and a normal MORB [32].

Moreover, it appears that the samples fallabove the MORB line in the neon diagram ofFig. 5 also suggesting a plume^ridge interaction[10].

4.2. Loss of He, Ne and Ar

To explain the very low concentration of heli-um (1038 ccSTP/g against 1035) in the basaltglasses from 39‡N, Kurz et al. [24] proposedthat helium was lost by degassing during the erup-tion by vesicles bursting because of the low erup-tion depth. They also observed that the samplesfrom the Azores Plateau are very vesicular (up to25^30% in volume), and that high vesicularity isassociated with low helium content, which maycorroborate their hypothesis.

We will propose a di¡erent explanation usingthe He^Ar systematics. The 4He/40Ar* is close toV200 at V39‡N but is close to 15^20 to thesouth (Fig. 7). The upper mantle production ratio4He/40Ar* can be estimated between 2 and 4 [38].Because the ‘popping rock’ sample 22D43 hassuch a ratio [35,38^41], we will also use the mea-sured concentration in this sample (V1034

ccSTP/g 4He [40]) for the upper mantle heliumcontent. A source origin for the high 4He/40Ar*

ratio at 39‡N can be ruled out by a simple calcu-lation on the production rate. If a relativelyyoung component is injected in the source of theAzores the present 4He/40Ar* ratio should beequal to the instantaneous production ratio.With a ‘mantle’ K/U ratio of 12 700 [42], the in-stantaneous production ratio is V4 in the mantle,far from the ratio of 200 measured at 39‡N. Toobtain a ratio of 200, a low K/U material shouldexist beneath 39‡N which is not the case [33]. Forthis reason, we prefer a process of fractionationby degassing to explain the high 4He/40Ar* ratio.

During vesiculation, Ar is incorporated in thegas phase more easily than He because Ar is lesssoluble in magma than helium (by a factor ofV10) [43]. This physical property leads to largeelemental fractionation during vesiculation pro-cesses. Two simple models are possible for suchvesiculation. One consists of a vesiculation in anopen system (Rayleigh distillation) whereas thesecond one is a vesiculation in a closed system.

The simplest approach would be the Rayleighdistillation (continuous nucleation and loss ofvesicles) for which the equation is :

4He40Ar

� �¼

4He40Ar

� �0Wf K31

where f = 40Ar/40Ar0 and K is the ratio of the sol-ubilities (K= 0.105) [43]. Such a distillation pro-cess has been proposed for MORB degassing byBurnard [44]. However, distillation cannot explainthe 3He/22Ne corrected for atmospheric contami-nation in MORB (i.e., the ratios are too high)because distillation does not fractionate He andNe enough [45].

Using (4He/40Ar)0 = 2, and taking 4He/40ArV358, we can estimate the remaining fractionof Ar of fW3U1033. It corresponds to a remain-ing fraction of He of V0.5. The model of Ray-leigh distillation is not consistent with the heliumdata (samples have lost at least 99.9% of the pri-mordial helium (Fig. 3). In other words, distilla-tion is not a good process for helium loss becauseof the high helium solubility in magma comparedto argon.

A single-stage vesiculation in a closed systemwill not signi¢cantly fractionate the 4He/40Ar* ra-

EPSL 6155 26-4-02


tio from the upper mantle value of 2 (i.e., all thegases are in vesicles) [43]. 4He/40Ar* ratios higherthan 200 are measured. Vesiculation can produceelemental fractionation in large proportion in theresidual phase (magma). The 4He/40Ar ratio in theglass after vesiculation (vesicularity s 5%) is :

4He40Ar

� �glass ¼

4He40Ar

� �0WSHe

SArW10W

4He40Ar

� �0

while the ratio in vesicles is very close to the ini-tial ratio (SHe and SAr are the solubilities of heli-um and argon). If most of the vesicles formedduring this ¢rst stage of vesiculation are lost, asecond stage of vesiculation will produce the fol-lowing ratios in the magma:

4He40Ar

� �glass

W100W4He40Ar

� �0W200

This is what we observe for most of the samples(Fig. 7), except one sample with a 4He/40Ar* ratioof 358. This sample may have su¡ered distillationduring eruption that has increased the 4He/40Ar*ratio compared to other samples. The 4He/3Hemeasured at 38.5‡N is not so di¡erent from theMORB ratio. Therefore, for undegassed MORBparameters we will take the measured He and Arcontents on the popping rock 22D43 obtained by[40], which are 4He0W1034 ccSTP/g and40Ar*0W5U1035 ccSTP/g. The maximum He/He0 ratio is V1034 (Fig. 3). With such heliumloss, we can estimate an Ar/Ar0 ratio of 1036.The most degassed samples for helium have atmo-spheric argon isotopic ratios. This huge argon lossmay explain the measured close-to-atmosphericratios.

So, using a three-stage vesiculation model (withtwo vesicle losses), we can deduce that the sam-ples of the summit have been subjected to onemore stage of vesiculation than the normalMORB, which have su¡ered only two stages ofvesiculation (and one vesicle loss), except for thepopping rock, which has not su¡ered loss. The¢rst two vesiculations could have been producedin magma chambers and the third one duringtransfer to the surface. Normally, after a step ofvesiculation, most of the CO2 is in the gas phase,

thus after degassing, a second vesiculation cannotoccur if the pressure is the same. But a secondvesiculation in the same magma chamber can beproduced after crystallization that enriches themagma in CO2. CO2 is an incompatible element(e.g., [46]). Another possibility for the second ve-siculation is the existence of a second magmachamber at lower depth, permitting a new vesicu-lation. Kingsley and Schilling [47] have proposedthat samples from 39‡N are more degassed thanN-MORB because, paradoxically, the magmasource is richer in CO2, which permits a higherdegassing rate. Our conclusions are in agreementwith Kingsley’s model.

4.3. The radiogenic samples at 39‡N and atlatitude s 40‡N

We have excluded in the discussion four sam-ples from 39‡N with radiogenic 4He/3He (CH30and three samples from [24]) (Fig. 4). Three pos-sible processes for the origin of these radiogenicratios are proposed.

4.3.1. Atmospheric contamination during eruptionor in magma chamber

We will suppose that sample CH30 (and thethree others from [24]) has been subjected to at-mospheric contamination during the eruption,which modi¢ed the 4He/3He from V85 000 to111 000. The mixing equation between the MORBand atmosphere components is :

4He3He

� �measured

¼4He3He

� �MORBþ

K

4He3He

� �SW

34He3He

� �MORB

� �

where K is the proportion of 3He coming fromseawater (SW). Taking 4He/3HeMORB as 85 000(MORB) and 111 000 for the measured ratio, weestimate the 3He coming from seawater as 4%(1.4U10314 ccSTP in the sample). This corre-sponds to a 20Ne concentration of V3.5U1038

ccSTP in the sample (we measure 2U1039

ccSTP/g) and a 36Ar concentration of V2.8U1037

EPSL 6155 26-4-02


ccSTP (we measure 4.5U1039). Unless to supposea preferential contamination of the helium com-pared to the other noble gases (by di¡usion forexample), the expected neon and argon concentra-tions in the samples are much higher than themeasured concentrations. This contaminationcan also have occurred in the magma chamber.

4.3.2. In situ radioactive productionWe will suppose here that the radiogenic 4He/

3He of the CH30 sample (and others from [24]) isdue to 4He production by U and Th decay aftereruption, with an equilibrium between glass andvesicles (because the isotopic ratios are measuredby crushing and the (U+Th,K) reactions occur inthe glass). The equation of 4He production in aclosed system is (for a young age):

4He3He

� �t¼

4He3He

� �0þ 2:8U1038 4:35þ Th

U

� �½U�½3He�Wt

where [U] is in ppm, 3He is in ccSTP/g and t is thetime in Ma.

Sample A127 D22-5, located at the same lati-tude as sample CH30, has a U content of 0.63ppm and a Th/UW3.5 [33]. We will supposethat these two values are the same for sampleCH30. We can calculate t = 64 000 years. This val-ue is reasonable, but equilibrium between glassand vesicles is required because the U and Thradioactive decay produces 4He in the glass. It isnot clear if this process is possible (by di¡usion orby K recoil?). A second alternative can be pro-posed: the radioactive decay occurred in a de-gassed magma chamber. This magma chamberhas to be completely degassed and this degassingis clearly to associate to the phenomena discussedin Section 4.2. Therefore, a magma residence timeof 64 000 years can be proposed.

4.3.3. Source e¡ectwe can also admit that the 37‡^40‡ area is het-

erogeneous and that this radiogenic helium iso-topic ratio re£ects another component, distinctfrom those of Terceira. This could be the Sa‹oMiguel component that has very radiogenic iso-topic ratios (4He/3Hes 200 000) [25]. This is not

the most probable interpretation for the 37‡^40‡area, considering the low He contents. However,in Fig. 4, one can observe that after 40‡N, sam-ples show 4He/3HeW100 000 and normal concen-tration (1036^1035 ccSTP/g) [24]. Therefore, theseradiogenic ratios, compared to normal MORB(V90 000), can certainly not be attributed to ra-diogenic production after degassing or atmospher-ic contamination. Yu et al. [31] have discussed thepossibility of the existence of a hotspot at 44‡N(Altair and Antialtair seamounts) which could beresponsible for the enrichment of trace elementsand isotopes at the 43‡N and 46‡N anomalies.This hotspot could also be responsible for theradiogenic helium isotopic ratios at latitudes high-er than 40‡N if it belongs to the ‘low 3He’ hotspotfamily, by opposition to the Terceira^Pico^Sa‹oJorge plume.

4.4. Neon: contradiction with helium?

We have seen that the 20Ne/22Ne ratios are at-mospherically close to 38.5‡N (Fig. 6), but thatthe 4He/3He ratios are more primitive than nor-mal MORB. Di¡erent noble gas studies of ‘high3He’ hotspots located on MORs [9,48,49] haveshown that the 20Ne/22Ne is close to 12.5 whenthe 4He/3He is primitive. There is an importantdi¡erence between these results and those ob-tained in these studies. To solve this contradic-tion, we propose two models. (1) The Terceira^Pico^Sa‹o Jorge plume has a 4He/3He ratio of65 000 but an atmospheric 20Ne/22Ne ratio be-cause a neon-rich component with atmosphericrare gas isotopic compositions is located in thesource or on the way of the plume and ‘masks’the primordial neon. (2) The Terceira^Pico^Sa‹oJorge source has a 4He/3He ratio of 65 000 anda mantle 20Ne/22Ne ratio but the magmas eruptedclose to 38.5‡N are contaminated, either duringthe eruption, or in a degassed magma chamberby a component with atmospheric rare gas com-positions.

There are three constraints to these two hy-potheses. (1) The samples are more degassed atthe top of the anomaly (Fig. 3). (2) The maximum20Ne/22Ne ratio is well correlated to the 87Sr/86Sr,143Nd/144Nd and 206Pb/204Pb ratios (e.g., the Ne^

EPSL 6155 26-4-02


Pb isotopic diagram in Fig. 8). (3) For the Shonaridge and Discovery anomalies, in the South At-lantic, the 20Ne/22Ne ratio is not in£uenced bydepth [9,48] (Fig. 9).

Model 1: The Terceira^Pico^Sa‹o Jorge sourcehas an atmospheric 20Ne/22Ne ratio. In this case,the correlation between 20Ne/22Ne ratios and 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb ratios (Fig. 8)re£ects mixing between the Terceira^Pico^Sa‹oJorge end-member (206Pb/204PbW20 and 20Ne/22Ne = 9.8) and the normal MORB (206Pb/204PbW18 and 20Ne/22Ne = 12.5^13.8).

We now have to explain how a ‘high 3He’ hot-spot (V65 000) has an atmospheric, and not man-tle-like, 20Ne/22Ne ratio whereas the other knownhotspots have a mantle-like 20Ne/22Ne ratio. Toexplain this atmospheric signature, we can suggesta mixing in the Terceira^Pico^Sa‹o Jorge sourcebetween lower mantle material (4He/3HeW20 000and 20Ne/22Ne = 13.8) and sediments or alteredoceanic crust (4He/3Hes 108 and atmospheric20Ne/22Ne). If an atmospheric gas-rich componentis recycled in the mantle, the proposed mixing willgive large mixing hyperbolic curvature in a He^Ne isotopic diagram because, due to its low abun-dance in the atmosphere, helium is not subductedin the mantle. This mixing would mask the prim-

itive neon but not the primitive helium. It shouldbe mentioned that volatiles are abundant in thelocal North Atlantic upper mantle [47] and thatthe atmospheric component could be related tothat. Old and altered oceanic crust, which maycontain atmospheric noble gases [50], probablyexists in the source of the Terceira^Pico^Sa‹oJorge hotspot as observed with the lead system-atics, which re£ect the existence of a ‘high W’ ma-terial (206Pb/204Pbs 20) [51].

The atmospheric component could also be lo-cated at relatively shallow depth and could beassociated with the Sa‹o Miguel component. Inthe Azores archipelago, it has been shown that athird component (together with MORB and Ter-ceira^Pico^Sa‹o Jorge) exists and it has particularfeatures. It has, for example, high 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios (respectively20, 15.8 and 39.4) and also high 87Sr/86Sr(s 0.705). This component is associated withSa‹o Miguel Island and its origin is associatedwith delamination of the continental mantle,probably stored at relatively low depth in themantle or at the 670 km discontinuity [25]. Thiscomponent has a high 4He/3He ratio (s 140 000)[25] and may also have atmospheric Ne and Arisotopic ratios. If the ‘high 3He’ and mantle-like

Fig. 8. 20Ne/22Ne (maximum data) against 206Pb/204Pb. Leaddata are from Dosso et al. [32]. The correlation re£ects eithera mixing or an induced correlation (see text).

Fig. 9. 20Ne/22Ne (maximum data) against the depth of sam-pling for both samples of the Azores Plateau and South At-lantic hotspots (Shona and Discovery) [9,48].

EPSL 6155 26-4-02


neon Terceira^Pico^Sa‹o Jorge plume has incorpo-rated some volatiles during its way to the surfacefrom this volatile-rich component, it could havegiven the observed atmospheric-like neon isotopiccomposition.

Model 2: Shallow contamination. The atmo-spheric 20Ne/22Ne isotopic ratios measured inthe samples can be due to atmospheric contami-nation either during eruption or in a shallowdepth magma chamber.

Degassing in magma chambers has decreasedthe primitive rare gas content in the magma bythree orders of magnitude. Atmospheric contam-ination in this degassed magma chamber couldhave decreased the mantle-like 20Ne/22Ne to airratio (9.8) (and the same for Ar) without a¡ectingthe helium isotopic ratio because of the low con-tent of helium in the atmosphere, except perhapsfor some special cases like sample CH30. Thevariation of 20Ne/22Ne with latitude in Fig. 6can be explained by mixing between the de-gassed^contaminated ‘3He-rich’ magma and thelocal upper mantle (less degassed and less conta-minated).

Atmospheric contamination during eruptioncan also explain the relationship between 20Ne/22Ne and 206Pb/204Pb (Fig. 8). However, a prefer-ential contamination of the most primitive sam-ples in helium (the most radiogenic in lead) isnecessary. This could have occurred if the con-tamination in seawater is easier in the more de-gassed samples, which have the lowest eruptiondepths and highest lead ratios. Therefore, the cor-relation between 20Ne/22Ne and 206Pb/204Pb (Fig.8) would be an induced correlation:

ð206Pb=204Pb3depthÞþ

ðdepth3contamination of neonÞD

ð20Ne=22Ne3206Pb=204PbÞ

One argument against this proposition could bethe correlation between the 20Ne/22Ne ratio andthe depth in the South Atlantic (Shona Ridge andDiscovery anomalies [9,48]) (Fig. 9), where the20Ne/22Ne does not show correlation with depth.However, the degassing conditions are not the

same in the two areas. In the South Atlantic,the loss of helium is only a factor of 10, while itis a factor of V1000 in the Azores (for similardepths). Such a strong helium degassing is in fa-vor of this contamination model (i.e., neon isprobably also strongly degassed). Such an in-duced correlation has already been proposed byBurnard [52] for the 40Ar/36Ar^206Pb/204Pb corre-lation found in Atlantic MORB by Sarda et al.[53] who rather proposed a recycled oceanic crustcomponent in the source of these MORB.

5. Conclusions

Mid Atlantic Ridge samples from 37 to 40‡Nshow for helium the in£uence of the Terceira^Pico^Sa‹o Jorge plume, con¢rming the plume^ridge interaction observed for other isotopic sys-tems. These samples have lost at least 99.9% oftheir helium, probably in shallow depth magmachambers. The 20Ne/22Ne ratio shows the moreatmospheric signature (9.8) where the 4He/3He isminimum (38.5‡N). The 20Ne/22Ne is also verywell correlated with the 206Pb/204Pb, 87Sr/86Srand 143Nd/144Nd. These correlations can be inter-preted as mixing between a MORB-like compo-nent (with high 20Ne/22Ne and low 206Pb/204Pband 87Sr/86Sr ratios) and a Terceira^Pico^Sa‹oJorge component (with high 206Pb/204Pb and87Sr/86Sr, and atmospheric 20Ne/22Ne). This atmo-spheric component is quite surprising because theTerceira^Pico^Sa‹o Jorge plume has a low 4He/3He signature and should thus have a mantle-like 20Ne/22Ne ratio. We propose that some atmo-spheric Ne, Ar and Xe could have been incorpo-rated (via subduction or locally?) into the sourceof the Terceira^Pico^Sa‹o Jorge hotspot and‘masked’ the primordial neon. Alternatively, con-tamination in highly degassed magma chamberscould also have produced the observed system-atics. Due to the large He loss (therefore Neand Ar), this last suggestion is the more probable.

Acknowledgements

We thank C. Langmuir for providing ‘A127’

EPSL 6155 26-4-02


samples and the IFREMER for other samples.We also thank J. Kunz, P. Sarda, R. Doucelanceand B. Bourdon for discussion. M.D. Kurz isthanked for discussions and access to unpublisheddata. S. Niederman, P. Burnard and an anony-mous reviewer are thanked for their constructiveand helpful review. This is IPGP contribution No.1801.[AH]

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