EPSL ELS EV IER Earth and Planetary Science Lett ers 120 (1993) 135- 148

Noble gases in submarine pillow basalt glasses from the Lau Basin: Detection of a solar component in backarc basin basalts

Masahiko Honda a, Desmond B. Patterson a,l , Ian McDougall a , Trevor J . Fa lloon b,2

a Research School of Earth Sciences, The Au stralian National University, Canberra, ACT 0200, Australia, b Department of Geology, University of Tasmania, Hobart, Tasmania 7001, A ustralia

(Received May 3, 1993; revision accepted October 25, 1993)

Abstrac t

Noble gas elementa l and isoto pic abunda nces have been ana lysed in eight samples of youthful basalt ic glass dredged from three different locations within the Lau Backarc Basin: (1) the King's Triple Junction , (2) the Central Lau Spreading Cen tre at 18°S and (3) the Eastern Lau Spreading Cent re at 19°5. Samples from the Lau central and eastern spreading centres have MORB-like helium isotopic ratios of approximatel y 1.2 X 10- 5 (8.5 R/R A ) . In contras t, the samples from the King's Triple Junction yield helium isotopic rat ios averag ing 9.4 ( ±0.8) x 10- 6

(6.7 ± 0.6 R/ RA ) , systematically lower than the MO RB-Iike value, which may be re flecting the add ition of radiogenic 4He re lease d from the descending slab. Neon isotopic ra tios are enriched in 20 Ne and 21Ne with respect to atmospheric ratios by as much as 23% and 62% respectively. Th ese obse rvations fur ther confirm that non-atmo­spheric neon is a common characteristic of samples derived from the mantle. The helium and neon isotopic signa tures in the samples can be explained by mixing of a primordial solar component, radiogenic and nucleogenic components produced by radioactive processes inside the Ea rth, and an atmosphe ric compo nent. This reconnais­sance survey of nob le gases in a backa rc basin indicates that curre nt volcanism is domin ated by magmas from the man tle wedge, a source similar to that from which MO RBs are derived. The heavier noble gases (a rgon, krypton and xenon) , however, show more atmosphere-like composi tions, eithe r indicat ing strong interaction of the magmas with the atmosphere or the presence of a recycled compo nent derived from the und erlying subducting slab.

1. Introduction

Studies of the elemen tal and isotopic composi­tions of noble gases in samples derived from the mantle provide important constraints on the evo­

tcu 1 Present address: Depar tment of Earth and Planetary Physics, Universi ty .of Tokyo, Tokyo 113, Japan 2 Present address: Department of Geo logy, Wills Memorial Building, University of Bristol, Bristo l, BS8 1RJ, UK

lution of the solid Ea rth and its atmosphere. Glassy margins of submarine pillow basalts con­tain a trapped component which commonly yields noble gas isotopic signatures related to their mantle sources. Thus, over the last decade exten­sive noble gas studies, especially on helium and argon, have been carr ied out on MORBs (mid-oc­ean ridge basalts) and on OlEs (oceanic island basalts), the latter notably on Hawaiian samples [e.g., 1-7]. These studies have identified clear differences between the helium isotopic ratios of N-type MORBs and OlEs. Helium in N-type

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136 M. Honda et al. / Earth and Planetary Science Letters 120 (I 993) 135-148

MORB s, away from the influence of mantle C He/4He)observed/ C He / 4He)air, and C He/ 4

plumes, is remarkably uniform, with a 3He / 4He Hejair = 1.4 X 10- 6 [e.g., 8,9]), whereas the 3He / ratio of 1.2 X 10- 5 (or 8.6 R/R A , where R / RA = 4He ratios of OIBs are much more variable. In

15°8

17°

19°

21°

23°

25°

Samoa

11 - 24

100 km

179°W 177° 175°

Fig. 1. Geological sett ing of the Lau Basin and locat ion of th e Mir submersible dive sites (0) where sampl es wer e collected for noble gas analysis. CLSC = Ce ntral Lau Spreadin g Centre; ELSC = Easte rn Lau Spre adi ng Centre; KTJ = King's Triple Junction. Light and darker shading is used to rep resen t de pt hs between 500 an d 2000 m and 0 and 500 m respectively (a fter Hergt an d Hawkesworth [27]). Helium isotopic results are shown as R/ RA values labelled in bold italics close to the dive sites. Whe re duplicated measur ements are ava ilable, averaged he lium da ta are shown in the figure. Th e ran ge of R/ R A values observed in the samples fro m the Rocham beau Ban k [14] and from Samoa [28] are also shown in the figure.

137 M. Honda et al. / Earth and Planetary Science Letters 120 (J993) 135-148

particular, OIBs characterised by 3Hej4He ratios that are higher than the MORB value (e.g., Hawaiian basaltic glasses which have 3Hej4He

ratios as high as 4.5 X 10- 5 or 32 R jRA [3]) are generally believed to be derived from a mantle source which is less outgassed, and therefore contains a higher proportion of trapped primor­dial 3He than the extensively outgassed upper mantle source region for MORB [e.g., 4,10].

In contrast to the relatively numerous noble gas studies of glass samples of MORBs and OIBs, studies of similar samples from backarc basins, formed behind volcanic arcs in regions of exte n­sional tectonics associated with subduction pro­cesses, are limited. Noble gas studies of youthful basaltic glasses from backarc basins may there­fore provide insights as to the composition of the mantle wedge above the subduction zone and possible involvement of the downgoing oceanic slab. Previous noble gas studies of samples from backarc basins focused on helium and argon iso­topic measurements. Samp les from the Mariana Trough and Lau Basin generally show MORB-like helium isotopic ratios, together with 40Ar j 36A r ratios exceeding 5000 [9,11-14]. However, to our knowledge, none of the samples from backarc basins have been analysed for the elemental and isotopic abundances of all five noble gases (helium , neon, argon, krypton and xenon).

A key interest in this contribution is to com­pare the noble gas results for these backarc basalts with those from mid-ocean ridge magmas. Of importance is whether or not the noble gas ele­mental and isotopic abundances differ from the ocean ridge environment, and if so whether these differences can be linked to the subduction envi­ronment.

In this paper we report noble gas studies of fresh basaltic glass samples recently dredged from the Lau Backarc Basin, augmenting the previous helium and argon work. In addition, we report in this paper neon isotope ratios significantly differ ­ent from the atmospheric values . We compare these new neon results from the Lau Basin sam­ples with previo usly available neon isotope data obtained on MORBs and OIBs, and we shall further explore the solar hypothesis which postu­lates that the Earth's primordial noble gas com­

position was solar [e.g., 1,15-23]' From the noble gas geochemistry perspective this paper should be regarded as an initial reconnaissance study of backarc basin basalts.

2. Geological setting and sampling

The Lau Basin is a triangular depression that is 450 km wide in the north, narrowing to 190 km in the south, and is - 1300 km long and sepa­rates the Tonga and Lau ridges [24]. The Lau Basin is an active extensional backarc basin with a half-spreading rate of approximately 38 mmja [25]. It lies to the west of the Tonga Ridge and Trench; the latter formed above the subduction zone marking the convergence of the Pacific and Australian plates. A detailed discussion of the geology, petrology and structure of the basin can be found in Parson et al. [26]. In brief, the basin has opened from north to south with the clock­wise rotation of the Tonga Ridge away from the now abandoned Lau Ridge . Formation of the basin was initially confined to extension and thin­ning of the arc / forearc crust of the original Tonga Arc. At approximately 5.5 Ma a southward prop­agati ng spreading ridge migrated into the basin , possibly from the Peggy Ridge, and new seafloor was formed. This continues today at the Central and Eastern Lau Spreading Centres, and new bas in floor would also appear to be forming at the King's Triple Junction, a nascent triple junc­tion in the northeastern part of the basin , east of Niuafo'ou Island (Fig. n

The samples studied for noble gases were col­lected during the 1990 cruise of the Akademik Mstislav Keldysh using a Mir submersible and are described in detail by Falloon et al. [29]. Glass samples for noble gas analysis were dredged from three different locations within the Lau Basin: (1) the King's Tr iple Junction [30] at 15°S (samples from dive station 2218), (2) the Central Lau Spreading Centre [31] at 18°S (dive station 2231), and (3) the northern end of the Eastern Lau Spreading Centre at 19°5 (dive station 2239). Glass samples for noble gas analysis were se­lected on the basis of availabili ty of materials and their freshness, and were from young basalts,

138 M. Honda et al. / Earth and Planetary Science Letters 120 (1993) 135-148

effectively reflecting products of ma ntle melting processes presently occur ring in the region. All the glasses are petrographically fre sh, were recov­ered from water depths greater than 2100 m, and have generally MORB-like major oxide and tr ace element geochemistry [29]. Locations an d recov­ery depth of individu al samples are listed in Table 1. Pe trograp hic descrip tions of samples used for noble gas analysis are re ported by Falloo n et al. [29]. Vesicularities of the samples are less than 1 vol% [29]. The H 20 content dete rmined by Danyushevsky et al. [32] and Bay Nb ratios by Falloon et al. [29] of all the samples in this study are also listed in Ta ble 1.

Table I Noble gas elemenlal abundances in Lau Basin glass samples a

3. Experimental Procedures

Th e samp les for noble gas analysis were pre­pared from the 2- 10 mm th ick quenched glass rims of fresh pillow basalts. The glasses were lightly crushed and sieved to a 1- 5 mm frac tion, washed ultrasonically in distille d wate r for I h, followed by wash ing in acetone for 1 hour, and then dried in an oven at 60°C for I h.

The procedures for noble gas extraction, pu­rification an d mass spectrometr ic an alysis are es­sentially the same as those describe d by Honda et al. [22]. However, for most of the samples in th is study 36Ar and 38Ar were measured using a Daly

Temperature H2O BalNb

(OCJ (%)

KINGs lEWE JUNCDON

M-2218·2 (I .983g, ANU# 90-079, 150 37.59'5 1740 49.0'W , 2249m) 600 1.32(1 3) 17.6(5) 175(1)

tOOO 0.59( 6 ) 5.7(2) 29(1) 1500 0.004( 1) 0.4(2) 79(1) Total 1.9 1(14) 23.8(6) 283(2)

126(23) n.d,

31(2) 157(32)

5.59(2 5) 0.77(5) 0.40(4) 6.76(26)

0.03(19) 0.1 5(19) 1.20( 19) 1.38(33 ) 1.3 2 23 .9

M-2218-4 (1.696g. ANU# 9O-OS0a, 15039.2'5 1740 49'W. 2234m) 600 2. 10(2) 2.1(1 ) 40(1)

tOOO 14.20( 14) 16.7(6) 100(3) 1500 0.006(2) t.7(1 ) 2 10(6 ) Total 16.30(1 4) 20 .5(6) 351(7 )

6(5 ) 67(7 )

177(7) 250(1 2)

0.69( 15) 2.11(1 5) 3.23(15 ) 6.03 (26)

0.050(23) 0.1 95(23) 0. 623(27 ) 0.869( 43) 0 .38 7 .9

M-2218-4 (2.089g, ANU# 90-080b, 150 39.2'5 1740 49'W. 2234m) 600 2.57(3) 29.4(8 ) 49.3(4)

tOOO 18.70( 19) 28.9(9) 120( 1) 1500 n.d. 0,9(2) 386(3) Total 2 1.30( 19) 59.2(1 2) 556(3)

3(16 ) 74(22)

340(18) 417 (3 3)

1.49(8) 4.12( 19)

n.d. 5.61(20)

n.d, n .d, n.d. n.d. 0 ,38 7 .9

M-2218-9 (1.59 1g, ANU# 9O-OS I, 150 36.7'5 1740 49.0'W, 2221m) 600 4.20(4) 9.7(3) 49.0(4 )

tOOO 6.6 2(7) 29 .3(8 ) 103(1 ) 1500 0.0 05(2) 0.5(2) 74.5(6) Total to .82(8) 39.5(9) 226(1 )

n.d. 42(30) 14(27 ) 56( 40)

0.9 9(7) 2.17(1 1) 0.32(5 ) 3.48(14)

ILd. 0. 16(24 ) 0.02(24) 0.18 (33) 0 .33 6.5

M·2218-to (1.750g, ANU# 9O-OS 2a, 150 36.55'5 1740 49.03'W 2217m) 600 2.32(2) 4.8(2) 69(2)

tOOO 9.66(10) 17.2(6 ) 152(5) 1500 0.007(2 ) 0.4( 1) 79(2) Total 11.92( 10) 22.4(6) 300( 5)

37(5) 119(7) 47(5)

203(10)

1.47(14) 3.09(1 5) 0. 64( 14) 5.20(25)

0.092(22 ) 0.246( 23) 0.122(23) 0.46 1( 39) 0. 83 16.8

M-2218-10 (1.935g, ANU# 9O-082b, 150 36.55'5 1740 49.03'W 2217m) 600 2.87(3) 11.7(4 ) 151(1 )

1000 13,60( 14) 23.7(8) 379(3 ) 1500 0.011(2) 1.0( 2) 165(1) Total 16.47(14) 36.4(9) 696(4)

101( 20) 329(20) 115(20) 546(35)

3.93( 18) 3.96(30) 1.06(6) 8.95(3 6)

0.041 (49) 0.390(50) 0.197(49) 0.629(86) 0.8 3 16.8

M. Honda et al. / Earth and Planetary Science Letters 120 (J993) 135-/48 139

T abl e 1 (continued)

Temperature (OC)

4He (10-6)

22Ne (10-12)

36Arm (10-12)

36Arc (10-12)

84Kr

(l a­ 12)

132Xe

(10-12) H2O (%)

BalNb

rnNJRAL BASIN

M-2231-3 (1.416g, ANU# 90-083a, 180 37.35'S 1760 26.13'W, 2361m) 600 2.65(3) 26.8(9) 30.9(9)

1000 5.45(5 ) 10.5(4) 68(2) 1500 n.d. 0.6(1) 70(2) Total 8.11(6) 37.8(10) 168(3)

n.d, 27.4(83) 29.8(72) 57.3(1 1)

0.67(17 ) 1.44(18 ) 0.13 ( 17) 2.24(30)

0.070(28 ) 0. 150(28) 0.087(28 ) 0.306(48 ) 0.22 6.2

M-2231-3 (1.859g, ANU# 9O-083b, 180 37.35'S 1760 26.13'W, 236 1m) 600 2.81(3) 43.1( 16) 53.2(4)

1000 7.48(7 ) 11.6(6) 52(3) 1500 0.004(2 ) 1.1(2) 74(1) Total 10.29(8) 55.8(17) 179(3)

n.d, n.d.

22(21) 22(21)

0.93( 10) 1.22(7) 0.36 (5) 2.51(13)

n.d. n .d, n.d, n.d. 0.22 6.2

M-2231-5 (1.999g ANU# 9O-084a, 180 36.71'S 1760 25.28'W, 241Om) 600 4.54(5) 2.3(2 ) 31.4(6)

1000 8.40(8 ) 10.7(3) 107(2) 1500 0.004(1) n.d, 49.4(5 ) Total 12.94(9) 13.0(4) 188(2)

n.d, 59(22)

1(22) 61(31)

0.82 (5) 2.10(10) 0.26(4) 3.19( 12)

n.d. 0.269(48)

n.d. 0 .269(48) 0.30 5 .3

M-2231-5 (2.083g, ANU# 9O-084b, 180 36.71'S 1760 25.28'W, 241Om) 600 1.83(2 ) 2.2(2) 93.8(8 )

1000 8.26(8 ) 9.4(4) 137(2) 1500 0.00 2(1 ) n.d, 86. 1(7) Total 10.19(8 ) 11.6(4) 317( 2)

47(22) 9 1(20) 40( 19)

178(35)

1.03(6) 1.98(1 0) 0. 18(4) 3.20( 12)

n.d. 0 .333(47)

n.d. 0 .333(47) 0.30 5.3

M-2231-13 (1.994g. ANU# 90-085, 180 36.43'S 1760 26.6'W. 2242m) 600 2.15(2 ) 0.3(1) 69. 0(5)

1000 2.1 2(3 ) 8.0(3) 133(2) 1500 n.d. 0.2(2) 143(1) Total 4.27(4 ) 8.6(4) 346(2)

21(24) 85(21) 96(22)

200(39)

0.49( 5) 1.24(7 ) 0.16(4) 1.89(9)

n.d, 0.253( 48)

n.d, 0.253( 48) 0. 31 5 .0

FASIERN BASIN

M-2239-2 (2.001g, ANU# 90-086, I ~ 30.84'S 1750 57.65'W, 2750m) 600 2.97(3 ) 3.6(2 ) 20.8(2)

1000 0.7 1(1) 5.2(2) 188(2) 1500 0.0 12( 1) n.d 225(2) Total 3.68(3 ) 8.8(3) 434(2)

n.d, 140(17 ) 176(22) 317(28)

0.68(5) 2.35(1 1) 0.79(5) 3.82(13)

n.d, 0 .922(57) 0.208(48) 1.130(74) 0 .78 20 .0

a No ble gas a mounts are in cm' STP/ g. Quoted e rro rs (J standard deviation ), sho wn in parentheses as th e last d igits, incl ude un certaint ies in the se nsitivity dete rmine d by rep eat ed ana lysis of the sta ndard gas es, as well as unce rta int ies in th e b lank cor rectio n for H e, Ne, Kr and Xe. Becau se o f lar ge variat ion of blank amo unts fo r Ar, precise blank co rr ecti o n co uld not be applied to Ar (se e text). T hus, 36Ar amo unts without and with blank corrections are shown as 36Arm and 36Arc respect ively in the table . Combined H 2 0 con tents are from Danyushevsky e t a l. [32]. Ba z'Nb ra tios a re from Falloon et al. [29]. n .d . = gas amo unt fro m sa m ples be low hot b lank.

multiplier collector so as to increa se the pr eci­ are thu s expected to show significant amounts of sion. mantle-derived nobl e gases [e.g., 33], whereas

Samples wer e analysed in three stages of step­ those in the 1500°C fraction were re leased mostly wise heat ing (600, 1000 and 1500°C) for 30 min. from the glass during melting. Typical procedural The 600°C fraction was intended to remov e possi­ hot blanks, irrespective of the extraction temp er­ble atmospheric con tamina tion on the surface of atures, were 8 X 10- 1°, 2 X 10- 11,3 X 10- 13 and the samples. Noble gases in the 1000°C fraction 4 X 10- 14 crrr ' STP for 4He, ZONe, 84Kr and 132Xe, were probably released mostly from vesicles and respectively, and were constant withi n 20% dur­

140 M. Honda et al. / Earth and Planetary Science Letters 120 (J993) 135-148

ing thi s study. Isotopic ratios in blank runs of Ne, Kr and Xe were atmospheric within uncertain­ties. Atmosph eric isotopic ratios were used for Ne, Kr and Xe hot blank subtraction. No signifi­cant 3H e signal could be detected « 10- 14 em" ST P) in hot blanks, and th e helium isotopic ratio of the hot blanks was assumed to be the atmo­spheric value of 1.4 X 10- 6. On the other hand, although the 40Ar blank amount was typically 3 X 10- 10 cm' STP before sample measurement, the 40Ar blank am ount sometimes increased to as high as 5 X 10- 8 crrr' STP afte r sample measure­ment. The reason for thi s increase is not fully understoo d, but it appears to be related to a reaction between melt, crucible and liner. The 40Ar/ 36A r ratios of all argon blanks were atmo­spheric. The average amount of argon based on several hot blank runs before and after the sam­ple measurement was used for the blank correc­tion . However, because of variability of the argon blank, uncorrected and blank corrected (desig­nated as ' m' and 'c', respectively) argon contents and 4oAr/36Ar ratios ar e listed in Tables 1 and 2.

4. Results

Noble gas elemental abundances and helium, neo n and argon isotopic data for eight Lau Basin basaltic glass samples are list ed in Tables 1 and 2. Fo ur samples were measured in duplicate. Kryp­ton and xenon isotopic ratios are indistinguish­able from the atmosphe ric values within uncer­tainti es, and ar e not tabulated. Uncertainties in the krypton and xenon isotopic ratios, after mass discrimination and blank correction, are typically 1 and 2% (I standard deviation), respectively.

4.1. Helium results

T he abunda nces of helium in the samples (Ta­ble 1) are relatively high and range from 1.9 X

10- 6 to 2.1 X 10- 5 cm ' STP/ g of 4He, similar to those observed in MORB glasses [ef., 9]. Total helium isotopic ratios for the Lau Basi n samples are listed in Table 2. There is some variability in the 3He/4H e results from the stepwise hea ting, but no consistent pattern is evident. T hus, it was decided to use the total helium results, enabling more direct comparison with previously published data. The helium results show a twofold variation

6from 8.3 X 10 - (5.9 R/R A ) to 1.6 X 10- 5 (II R /R A ) . 3He/4H e ratios measured in duplicate (samples M-2218-4, 2218-10, 2231-3 and 2231-5) show some variation, perhaps indicating slight heterogeneity in the samples in terms of helium isotopic signatures. However, as these variation s are small, they are not cri tical to interpreting the hel ium isotope results fro m the Lau Backarc Basin.

In detail, samples from the Lau central and eastern spreading centres (stations 2231 and 2239, respectively; see Fig. 1) have MORB-like heli um isotopic ratios of approximately 1.2 X 10- 5 (8.5 R/R A ) , except for sample M-2231-13 which shows a relative ly high heli um isotopic ra tio of 1.6 X

10- 5 (11 R/R A ) . In contrast, the six sam ples from the King 's Triple Junction (station 2218, Fig. 1) yield helium isotopic ratios averaging 9.4 (±0.8) X 10-6 (6.7 ± 0.6 R/R A ) , systema tica lly lower than the MORB-like values observed in the samples fro m the Lau Basin spreading ce ntres.

In Fig. 2 we plot 3H e/ 4H e ratios against he­lium concentrations observed in our Lau Basin samples together with previously ava ilable data fro m the Lau Basin [9,13,14]. Helium co ncentra­tions in the Lau Basin samples measured in the present study lie at the higher end of the dat aset. There is no obvious correlation be tween helium concentrations and the factor of two variation in helium isotopic ratios in our samples. As dis­cussed by Trull et al. [37] and Hilton et al. [9], ingrowth of radiogenic 4He into evolve d (i .e .,

Note to Table 2 a Quoted errors (J standar d deviation), shown in parentheses as the last digits, include uncertainties in the correction factors for mass discriminat ion, as well as uncert aint ies in the blank corr ection and interference corrections in the case of Ne. R /R A is the He isotopic ratio relat ive to the atmospheric value of 1.4 x 10- 6. Ar isotopic rat ios corrected for blank are shown with the label 'c'. 4oAr/36Ar ratios without blank corre ctions are shown with the label ' rn' (see text). * From Ozima and Podosek [341and references therei n; *from Geiss et al. [35}. §from Kyser and Rison [36].

141 M. Honda et al. / Earth and Planetary Science L etters 120 (1993) 135-148

Table 2

Helium, neon, argon isotop ic data for Lau Bas in glass samples a

Temperature (0C)

3Het'He (10. 6)

R1RA 2~eP2Ne 21NeP2Ne (38Ar/36Arlo ("0 Ar/36Ar)c ("0 Ar/36Ar)m

KINGS DUPLE RJNCOQN

M-2218-2 (1.983S. ANUS 90-(79) 600 8.6(1) 9.991(77) 0.02939(30) 0. IS82(22) 308(2) 304 .6(4)

1000 7.3(1) 10. 138(9 7) 0.0314 7(61) n.d. n.d. 469 .7(33 ) 1300 13.I(S) 9.73(33) 0.0311(39) 0.1921(86) 347(41) 313 .7(18) TOlal 8.3(1) 5.9 9.99 7(62) 0.03007(28) 0.1886 (22) 316(9) 324.3(7)

M-2218-4 (1.696S. ANUS 90-080.) 600 11.0(1) 10.13(17) 0.03188(79) 0.2203(479) 430(130) 3165 (17 )

1000 10.1(1) 11.986(79) 0.04199(48) 0 .132(33) 7810(830) 3300(990) 1300 16.1(37) 9 .46(13) 0.02864(69) 0 .1949(32) 471(8) 442 .1(26 ) Tot_I 10.2(1) 7.3 1l.S 94(70) 0.03989(41) 0 .1841(93) 2430(280) 1820(36 )

M-2218 -4 (2.0898. ANUS 9O-080b) 600 9.6( 1) 9 .792(62) 0.02932(25) 0.187(S4) .s40(1780) 311.3(6)

1000 9.2(1) 10. 873(10 7) 0.03683(35) 0 .1859( 44) 8300(2500) 3201(35) 1500 n.d. 10.60(76) 0.0 309(32) 0 .1895(11) 413(8) 399.2(14) Total 9.2(1) 6.6 10.332(63) 0.03302(23) 0. 1892(11) 1810(570) 1430(13)

M-2218-9 (1.59 1S. ANUS 90-(81) 600 9.2(1) 9.943(S4) 0.02937(33) n.d. n.d, 301.3(13)

1000 8.4(1) 11.070(S3) 0.03606(36) 0 .1868(66) 12000(8400) 3143(33 ) 1300 17.2(73) 12.2(13) 0.012(11) 0. 194(22) 2300(4400) 663 .2(23) TOlal 8.7(1) 6.2 10.807(69) 0.03415(32) 0 .1883(82) 9700(7400) 2623( 20 )

M-2218-10 (1.730S. ANUS 90-0820) 600 10.2(1 ) 9.8 83( 82) 0.02988(66) 0.2075(59) 434(27) 368 .5(8 )

1000 9.8(1) 10.917(44) 0.03731(28) 0 .196( 13) 211(110) 172 1(24 ) 1500 34(27) 10.02(43) 0.03 13(24) 0 .1942( 79) 637(31) 496.7 (16) Total 9.8(1) 7.0 10.680(40) 0.03561(27) 0 .19 76(78) 1469(79) 108 7(13)

M-2218-10 (1.933s. ANU# 9O-082b) 600 10.7 (2) 9.850(88) 0.029 76(3 8) 0 .1887(26) 380(21) 352 .6(16)

10 00 10.4(2) 10.836(94) 0.03681(43) 0 .1897(11) 140(64) 1028 .9(28) 1500 9.4( 12) 9.98( 42) 0.0 312( 19) 0.1895(23) 379(18) 333 .9( 13) Total 10.4(2) 7.4 10.50 8(69) 0.03 439(32) 0 .1895(10) 838(47) 721.8(23)

CEND!A1 BASIN

M-2231-3 (1.416S. ANU# 90-08 3_) 600 12.4(2) 9.81 6(31) 0.02909(22) n.d. n.d. 334.9(30)

1000 11.6(2) 1 1.158( 83) 0.03889(41) 0 .201 4(74) 7200(2200) 3110.4(38) 1300 n.d, 9.7 4(3 1) 0.0270(3 1) 0.197 4(38) 1440(340) 783(115) Tot al 11.S(1) 8.4 10. 188(35) 0.0 3178(22) 0 .1993( 47) 4200(1200) 1634 (20)

M-2231-3 (1.859S. ANU# 9O-083b) 600 13.4(2) 9 .994(66) 0.0301 4(40) n.d. n.d . 300(1)

1000 11.9(2) 10.968(97) 0.03 873( 50) n.d, n.d. 3400(160) 15 00 14 .8(61) 9 .46(25) 0.03402( 194) 0 .1907(1 4) 1300(1200) 590 (5) Total 12.3(2) 8.8 10.148(36) 0.03201(34) 0. 1907(14) 1300(1200) 1310(55)

M-2231-5 (1.999S ANUS 90-084.) 600 12.8(2) 10.01(23) 0.02954( 106) nd. n.d. 30S(3)

1000 12.1(3) 11.38(12) 0.04150(80) 0.1894(7) 2849(1042) 1704(33 ) 1500 16.0(59) n.d. n.d. 0 .23( 147) 4000(8000 0) 376 .8(30) Total 12.4(2) 8.9 11.14(11) 0.039 42(70) 0.190(41) 2900(1800) 1124(20)

M-2231-5 (2.083S. ANUS 9O-084b) 600 14.3(3) 9.93(17) 0.0308 7(9 1) 0 .1886(54) 306(3) 301.1(10)

1000 11.6(2) 11.72( 11) 0.04363(62) 0 .1889(29) 1670(330) 1206(13) 1500 22(16) n.d. n.d 0.1 885(62) 370(43) 330 .1(8) Tota l 12. 1(2) 8.6 11.37( 10) 0.0411 7(5 5) 0 .18 87(2 5) 1020(220) 700 .8(76 )

M-2231 -13 (1.9948. ANUS 90-085) 600 15.3(3) 11.8(15) 0.046(1 3) 0.187(1 4) 298(3) 296. 7(6)

10 00 16.2(3) 12.06(13) 0.04 707( 73) 0.1890(31) 2050(500) 1410(19) 1500 n.d. 9.38(83) 0.02699(637) 0. 1898(2 7) 327(8) 316.6(3) Total 15.7(2) 1 1. 2 11.97(13) 0.04647(97) 0. 1891(24) 1050(270) 732 .9(83)

EAS!EB N BASIN

M-2239-2 (2.oo I S. ANUS 90-(86) 600 12.8(3) 9 .99( 17) 0.0306 4(66) n.d, n.d. 296 .0(15)

1000 11.6(2) 10.93(12) 0.03632(58) 0 .1878(20) 518(38) 461.1 (23) 1500 12.1(14 ) n.d. n.d, 0 .1883( 17) 320(3) 313.2(4) Tot al 12.5(2) 8.9 10.35(10) 0.03400( 45) 0. 1881(13) 408(19) 377.6(11)

Atmosphere " 1. 4 1.0 9 . 8 0.029 0. 1880 295.3

Nucleogenicf 2.3 32

Solar:!: 400 2 86 13.6 0.032

142 M. Honda et al.I Earth and Planetary Science Letters 120 (J993) /3 5-148

3e-5 • 25

20

Q)

J:

~ J: M

2e-5

1e-5

f ~O R B

+ .••+

+++ .

• •.•.. .~ • ~+ . ...

A bAt.

15

10

5

:II-:II l>

10 -8 10 -7 10 -6 10 ·5 10 -4

4He (ccSTP/g)

Fig. 2. Plot of 3He/ 4He ratio against 4He concentration (cm' STPI g) for Lau Basin samples. Thi s study (0 = spreading centres; 1:> = King's Triple Junction); Poreda [I3] (.A); Poreda and Craig [14] ( .); Hilton et al. [9] (+). Avera ge 3He/ 4He ratio of N-type MORB ( =1.19 (±O.04) xlO - 5; 8.5 ( ±O.3) R I R A) compiled by Kurz [8] is also shown in the figure.

well-outgassed) magma, and post-eruptive pro­cesses such as contamination with atmospheric helium from seawater and diffusive loss of mag­matic helium, are effective in lowering 3Hej 4 He ratios in samp les that have low helium abun­dances (e.g., lavas from the Valu Fa Ridge in the southern Lau Basin [9]). However, because of the high helium abundances measur ed in the present study (- 10- 5 ern:' STP j g, Fig . 2) and the fres h­ness of these youthful basaltic glasses, these pro­cesses are unlikely to have significantly affected helium isotopic signatures in the samples. We therefore conclude that the helium isotopic ratios of the Lau Basin samples are likely to reflect the helium isotopic signatures of their magma sources.

Earlier workers have observed MORB-like iso­topic compositions in samples from currently ac­tive regions in the Lau Basin, suc h as the spread­ing centres and Peggy Ridge [9,13,14]. The upper mantle, as samp led by N-type MORB volcanism, is believed to have a fairly un iform 3Hej 4 He isotopic ratio of about 1.2 X 10-5 (8.6 RjR A ;

[8,9]). Thus, the observation of predominantly MORB-like helium in basaltic glasses from backarc bas ins implies tha t, in gen eral, the man­tle source for these basa lts has not been signifi­cantly affected by addition of radioge nic he lium in fluids re leased from the sub ducted slab.

However, although the majority of the Lau Basin glass samples have MORB-like helium compositions (Fig. 2), a few basaltic glass samples show more extreme 3He j 4He ratios, ranging from 3.1 X 10- 5 (22 R jRA ) at Rochambeau Bank [13,14] to 1.7 X 10- 6 (1.2 R jRA ) at Valu Fa Ridge [9]. This variation contrasts strongly with the relatively uniform 3He j 4He ratios of N-type MORB glasses. The observed variability may re­flect the complexities of backarc ba sins. Sample M-2231-13 from the Central Basin has a 3Hej 4 He ratio of 1.6 X 10-5 (11 R jRA ) , a substantially higher valu e than MORB. This may reflect a component of helium derived from the Samoan Hotspot, which has 3He j 4 He ratios of up to 3.5 X 10- 5 (25 R jRA ) [14,28,38]. However, it is somewhat difficult to envisage how high 3Hej4 He helium could be transported over 500 km through the mantle from the Samoan plume to the Lau Basin, across a major plate boundary. Alterna­tively, the high helium isotopic ratio may reflect a separate hot spot signature centred near the Lau Basin, although there is no other geochemical evidence for the presence of such a component in the region [29]. The 3He j 4He ratios lower than the MORB value observed in samp les from the King 's Triple Junction will be discussed later in conjunction with the neon results from the sam­ples (see section 5.2), but are consistent with the findings of Falloon et a1. [29] that the major, trace and isotope geochemistry indicates the presence of a slab-derived component.

4.2. Elem ental abundances of neon, argon, krypton and xenon

In general, the elemental abundances of the noble gases in oceanic basaltic glasses vary by between three and four orders of magnitude (e.g., 84 Kr ranges between about 1 X 10- 12 and 1 X

10- 9 cm' STP j g; [22]), and the abundances of the heavy nobl e gases (neon, argon, krypton and xenon) are linearly correlated, with abundance ratios that approximate atmospheric values [e.g., 5,22]. In comparison, the abundances of heavy nob le gases in the Lau Basin samples vary by less than one order of magnitude, an d fall at the lower en d of the range of abundances in oceanic

143 M. Honda et al. / Earth and Planetary Science Letters 120 (1993) 135-148

basalt glasses (e .g., 84Kr in the Lau Basin glasses ranges from 1.9 X 10- 12 to 8.9 X 10- 12 crrr' STP/ g; Table 1).

Some workers have ar gued that the heavy no­ble gases in oce anic basaltic glasses are a mixture of atmospheric and non -atmospheric compo ­nents, and have suggested that the most likely source for the atmosphere-l ike component is in­teraction with sea water which carries disso lved atmospheric volat iles [e.g., 39,40]. This might oc­cur in a nu mber of ways, including interaction be tween ascending magma and downward perco­lating seawater, and addition of seawater-derived volatiles to the mantle source region of the mag­mas from the downgoing subducted slab .

Seawater has heavy nob le gas abundance typi­cally a few orders of magnitude higher than oceanic glasses (e .g., 132 Xe in sea water is 2.3 X

10- 9 cm' STP / g [10], whereas the average amount of 132 Xe in the Lau Basin basaltic glasses is 5.8 X 10- 13 ern? STP /g; Table 1). Thus, even minor interaction with sea water is likely to swamp any original mantle-derived contribution of the nob le gases in the samples, and it is possible that we might observe some correlat ion be twee n the concentrati ons of heavy noble gases and com­bined water in the samples. Fig. 3 is a plot of 84Kr and 132Xe versus combine d H 20 abu n­dances, and illustrates that there indeed ap pears to be a weak positive correlat ion betwee n the abundances of krypton an d xenon with wate r, although there is a much poorer correlation for neon and ar gon . We acknowledge that th is is a limited data set, but the correla tion is certa inly consistent with the suggestion that a significant component of atmosphere- derived heavy nobl e gases in the samples has been der ived through inte raction with seawater. There also appears to be a positive cor re lation between the noble gas abundances and Bay Nb ratios, which are sensi­tive geochemical tr acers of island-a rc magmas [e.g., 41]. T hese correlations favour the idea that atmosp he re-like noble gases in the backar c basin bas altic glasses ultim ately may be derived from the downgoing subducte d slab. Clearly, fur ther studies are necessary to clarify whether a correla­tion between heavy nobl e gas content and com­bined water is a gen eral characteristic of oceanic

1e -11

8e-12 Ol C3 6e -12 ~ ~ 6 .... ~ v

4e -12 rf"

0

ec 2e-12 eo

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

2.0e-12

:g 1.5e-12

~ § !Q) 1.0e-12 X C\l C') ,.... 4

5. 0e-13 4

0.0 0 .2 0.4 0.6 0.8 1.0 1.2 1.4

H20 (wt %) Fig. 3. Plot of H4Kr and 132Xe conce ntrations (cm' ST P/ g) against water content (wt%). Da ta from the sprea ding cent res and the King's Triple Junction are shown as circles (0) and triangles ( tJ.) respectively.

basaltic glasses, or speci fic to those from the backarc environme nt.

4.3. Neon isotopic ratios

In Fig. 4 the neon isotopic result s from the 1000°C fract ions, which show the highes t neon isotope ratios with the sma llest uncertaintie s of the three temper ature fractions, are plotted in 2oNe/ 22Ne _ 21Ne/22Ne space. Neon isotopic ra­tios observed in the Lau Basin samples are en­riched in 2°Ne and 21Ne by as much as 23 and 62%, respectively (sample M-2231-13), with re­spect to atmospheric rat ios. Th ese are the first neon isotopic compositions diffe ring from atmo­spheric values repor ted from backarc basin sam­ples, further confirming that non-atmospheric neon is a common characteristic of samples de­rived from the mantle [e .g., 1,16-23,38]. In a neon three-isotope plot the results lie on a straight line (the best-fit line, calculated by York's method [42] which takes into account corre late d uncer­

144 M. Honda et al. j Earth and Planetary Science Letters 120 (J993) 135-148

tainties, is 2o Ne/22 Ne = 5.64(±0.24) + 143( ± 7) X CZ1Ne/ 22Ne), MSWD = 2.67 and is shown as a heavy line in Fig. 4).

The neon correlation line for the Lau Basin samples differs slightly from the MORB correla­tion line as given by Sarda et al. [17]. The Lau Basin neo n correlatio n line has a significantly less stee p slope than that observed for the Hawaiian Loihi and Kilauea samples (shown as 'L-K' in the figure as given by Honda et al. [19]) and for the solar- atmosphe re neo n mixing line. Corre la­tion lines in neon thr ee isotope plot s have previ­ously been inte rpreted as mixing lines between a neon component derived from the mantle (com­prising a mixtur e of solar and nucleogen ic neon) and atmospheric neon [1,17- 23].

4.4. A rgon isotopic ratios

The 40Ar/ 36Ar ratio s observed in the Lau Basin samples ran ge from nearl y at mospher ic (= 295.5) to as high as 2430 (sample M-2218-4), except for

13

12 Q)

Z N N 11 Q) -Z 0 N 10

AIR 9

0.0 3 0 .05 0.04

21Ne/22Ne Fig. 4. Neon isotope plots for Lau Basin glass samples with I sta ndard deviation uncertainties. Data for the 1000°C frac ­tions are shown in the figure . Data from the sp reading centres and the King's Triple Jun ct ion are shown as circles (0) and triangles ( ", ) respec tively. Th e heavy line passing th rough all the data is the bes t-fit line based on the equat ion: 20Ne j 22 Ne = 5.64(±O.24)+ 143( ± 7) x CZ1Ne j 22Ne), MSWD = 2.67. Atmospheric neon is shown as ' AIR '; the MORB and L-K (Loihi- Kilauca) cor relation lines are as given by Sarda et at. [17] and Ho nda et at. [191, respecti vely. Hypoth et ical atmo­sphere-mantle neon mixing lines are plotted for the cor re­sponding 3Hej4He rat ios (9 X 10- 6, 1.6 x 10- 5) shown adja­cent to each mixing line (see text).

4000

s C')

-.::: g. 2000 -e­

H20(wt%) Fig. 5. Plot of 40Arj 36Ar rati os and combined H 20 conte nts for Lau Basin glass samp les.

some higher values with very large uncertainties owing to large blank corrections (see section 3). The observed 40Ar/ 36Ar ra tios are significantly lower tha n the maximum 4°Ar/ 36Ar rat io so far observed in MORBs ( = 28,000; [43]), contrasting with the helium and neon results which are gen­erally MO RB-like (see sections 4.1 and 4.3).

Although it is premature to discuss the deta ils based on the limited number of data, significantly lower 40Ar/ 36Ar ra tios than the highest MORB value may reflect the add ition of atmosphere-de­rived argon . Interestingly, there is a wea k nega­tive correlation between the 40Ar/ 36Ar rat ios and combined H 20 conten ts of the samples (Fig. 5), consistent with addition of atmosph ere-derived argon through interaction with seawa te r. How­ever, it is unclear whether th is atmosphere-like argon comp onent is de rived from the subducted slab or from atmospheric contamination of the magmas just prior to eruption in the backarc environment (see section 4.2).

5. Discussion

5.1. The solar hypothesis

Observations of 2° Ne/ 22 Ne ra tios significantly higher than atmos pheric ( = 9.8) in MORBs and OIBs [1,17- 20,22,38,44] led some investigators to suggest that the high 2oNe/ 22Ne ratios reflect a

145 M. Honda et al. / Earth and Planetary Science Letters 120 (J993) 135-148

mixture of solar and atmospheric neon compo­nents [1,17-23,38], because solar neon is the only known component in the solar system with a 2°Ne/22Ne ratio (= 13.6, as observed in solar wind [35]) higher than the atmospheric value. Honda et al. [19,22,23] demonstrated that helium and neon isotopic ratios obtained from MORB and Hawaiian OfB glasses are strongly correlated and reinforce the solar hypothesis, under which it is postulated that an important component of the Earth's primordial gases was solar in composi­tion.

If the Earth's primordial noble gas composi­tion, at least for helium and neon, was solar (3He/4He = 4 X 10-4, 2oNe/22Ne = 13.6, 21Ne/ 22Ne = 0.032, 3He/22Ne = 3.3, as observed in so­lar wind [35]), then we expect to see a correlation between helium and neon isotope systematics in gases extracted from mantle-derived samples. This is because variations in the observed 3He/4He

and 21Ne/22Ne ratios in the mantle result from the time-integrated ingrowth of radiogenic 4He (4He*) and nucleogenic 21Ne (ZINe*), having a constant 21 Ne */4 He * production ratio of ap­proximately 1.0 X 10- 7 [e.g., 36,45,46]. This rela­tionship between helium and neon in the mantle can be explicitly stated in the following form:

(4He /3He)mantle

= (4He/3Heh + C1Ne* / 22 Ne s )

/{(I Ne* /4He*)· CHe/22Ne)s} (1)

where the subscript'S' denotes the solar compo­sition [22].

It is possible to quantitatively deconvolve the relative contribution of each of the three neon components (primordial solar, time-integrated nucleogenic and atmospheric; see Table 2), by using simple mixing equations [23], so that the amounts of solar-derived 22Nes and nucleogenic 21 Ne * can be calculated. Further, by entering the calculated 21Ne */22Nes ratio into Eq. (1) we can predict what the 3He/4He ratio of the sample should be. If the solar hypothesis is correct, the predicted and observed 3He/4He ratios should agree. Honda et al. [23] have shown that for a range of mantle-derived samples with neon iso­topic ratios differing by more than two standard

deviations from atmospheric values the ratios of 3He/4He values calculated through Eq. (I) to observed 3He/4He values are indeed close to one, supporting the solar hypothesis. In the fol­lowing section we discuss the helium and neon results from the Lau Basin samples, especially in relation to the solar hypothesis.

5.2. Mantle helium and neon in the Lau Basin samples

First we shall derive a predicted 3He /4 He ratio based on the mantle-atmosphere neon mix­ing line determined from the Lau Basin samples (Fig. 4). By assuming that the 2oNe/22Ne ratio of the mantle source has the solar value of 13.6, we can calculate the 21 Ne */ 22 Ne , ratio (= 0.024 ± 0.003) of the mantle source for the Lau Basin samples from the slope of the observed neon mixing line. The 3He / 4He ratio calculated for the Lau Basin samples by entering this 21Ne */ 22Nes ratio into Eq. (I) is 1.3 (±0.2) X 10- 5,

close to the 3He/4He ratio (- 1.2 X 10- 5) actu­ally observed in most of the Lau Basin samples (Table 2). For comparison, in Fig. 4 we plot hypothetical mantle-atmosphere neon mixing lines corresponding to 3He / 4He ratios of 1.6 X

10- 5 and 9 X 10- 6, which covers the range of 3He/4He ratios observed in the Lau Basin sam­ples. The neon results from the Lau Basin sam­ples lie between the two calculated mantle-atmo­sphere mixing lines, as expected from the solar hypothesis.

It is apparent that the Lau Basin neon correla­tion line has a slightly different slope from that observed in MORB samples by Sarda et al. [17]. This is unexpected because both MORB and Lau Basin samples have similar 3He / 4He ratios (­1.2 X 10-5 ) . The difference may reflect relatively larger uncertainties in the MORB neon isotope data [17], compared with the present neon data set for the Lau Basin samples, or it may reflect slightly different primordial 3He /22 Ne ratios in the mantle sources of Lau Basin basalts com­pared with MORBs.

In order to further examine mantle helium and neon in the samples, the 3He/4He ratios, based on the amounts of nucleogenic 21Ne * and solar­

146 M. Honda et at. / Earth and Planetary Science Letters 120 (I 993) 135-148

1.6e-5 t-Oi I I

" Q)

c: Q) rJ)

1.4e-5

I I I I I I

.c 0

Ql 1.2e-5 ~ I v '<, Q)

I I I 1.......-6;---i . A .

I '2....

1.0e-5 : : I

r-l:r-i .--..r-o

t--------6----­I------6-----t

8.0e-6

0.5 1.0 1.5 2.0 2.5

(3He/4He)calcu lated /(3He/4He)observed

Fig. 6. Plot of ratio of calculated 3He/4 He value (based on the solar hypothesis) to observed 3He/4 He value for the Lau Basin samples. The results are shown with 1 standard devia­tion uncertainties, derived by deconvolution of the three neon end members (solar , nucleogenic and atmospheric, see text).

derived 22Nes in the total neon data from indi­vidual samples, are calculated through Eq. (1) . The ratios of calculated 3He/4He relative to ob­served 3He/4He (hereafter referred to as Reo) are plotted against observed 3He / 4He ratios in Fig. 6. Although values of Reo for the Lau Basin samples are generally close to one, there does appear to be a systematic trend. For the samples from the Lau spreading centres which show MORB-like 3He/4He ratios values of Reo are close to one, whereas samples from King's Triple Junction (hereafter referred to as KTJ), which have observed 3He/4He ratios lower than the MORB value, have values of which areR co greater than one.

It must be emphasised that the correlation between the helium and neon isotopes described in Eq. (1), and expressed above as the R co value, is insensitive to recent elemental fractionation processes [22]. This is because the degree of elemental fractionation of primordial 3He rela­tive to solar 22 Ne will be essentially the same as the elemental fractionation of time-integrated ra­diogenic 4He * relative to nucleogenic 2lNe *. Thus, the product term (ZINe*/ 4He* ). (3He / 22 Ne)s in Eq, (1) is likely to remain largely unaf­fected by recent elemental fractionation.

Hence, if we accept the solar hypothesis, the values of R co greater tha n one observed in the KTJ samples (Fig. 6) appear to require some form of decoupl ing between helium and neon such that the 3He/4He ratios are lowered from the MORB-like mantle value, but mantle-related neon is not affected. As already discussed in section 4.1, ingrowth of radiogenic 4He * or post­eruptive processes are unlike ly to lower the 3He/ 4He ratios because of the high helium abun­dances in the samples. A possible mechanism would be the addition of radiogenic 4He *, re­leased from the descending slab without the asso­ciated nucleogenic neon, to the parent magma of KTJ samples. By this mechanism the initially MORB-like 3He/4He ratios of the parent mag­mas of the KTJ samples may have been lowered to their observed values without affecting the MORB-like neon compositions. On the basis of the major and trace element geochemistry, Fal­loon et al. [29] concluded that there was some involvement of slab-derived fluid in the KTJ mag­mas, consistent with the observations of lower 3He/ 4He ratios compared with MORBs.

6. Summary

We have measured noble gas elemental and isotopic abundances in eight glassy basalts, with four replicates, dredged from three different lo­cations within the Lau Backarc Basin: (1) the King's Triple Junction, (2) the Central Lau Spreading Centre at 18°S, and (3) the Eastern Lau Spreading Centre at 19°5 (Fig. 1). Samples from the Lau central and eastern spreading cen­tres have MORB-like helium isotopic ratios of approximately 1.2 X 10- 5 (8.5 R/R A ) . In con­trast, the samples from the King's Triple Junction yield helium isotopic ratios averaging 9.4 (± 0.8) X 10- 6 (6.7 ± 0.6 R /R A ) , systematically lower than the MORB-like value.

We have found neon isotopic compositions differing from the atmospheric values with 2°Ne/ 22Ne and 21Ne/ 22Ne isotopic ratios as high as 12.0 and 0.0465 respectively, together with pri­mordial 3He. These observations from backarc basin samples further confirm the notion that

147 M. Honda et al. / Earth and Planetary Science Letters 120 (J993) 135-148

non-atmospheric neon is a common characteristic of samples derived from the mantle, and tha t the observed helium and neon isotopic compositions reinforce the view that the mantle contains a significant primo rdial component of solar compo­sition.

We have attempted to correlate the noble gas results with other geochemical tracers represen­tative of island-arc basalts. However, we have only been able to demonstrate some weak corre­lations, for example between the krypton, xenon abundances, 40Ar j 36A r ratios and H 2 0 contents. Further work is necessary to determine whether the study of noble gases can provide additional information concerning backarc spreading pro­cesses, and whether there is significant involve­ment of noble gases derived from the subduction slab.

Acknowledgements

We thank the captain and crew of the Akademik Mstislao Keldysh j Mir and the chief scientist Alexander Lisitzin . We would also like to thank Peter Valbracht and Bill McDonough for their helpful suggestions and advice. Discus­sions with Janet Hergt are gratefully acknowl­edged in particular. Reviews by David Fisher, Ken Farley and an anonymous reviewer substan­tially improved the original manuscript.

References

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[2] M.D. Kurz, WJ . Jenkins and S.R. Hart, Helium isotopic systematics of oceanic islands and mantle heterogeneity, Nature 297, 43-47, 1982.

[3] M.D. Kurz, WJ. Jenkins, S.R . Hart and D.A. Clague, Helium isotopic variations in volcanic rocks from Loihi Seamount and the island of Hawaii, Earth Planet. Sci. Lett. 66, 388-406, 1983.

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[13] RJ. Poreda, Helium-3 and deuterium in back-arc basalts: Lau Basin and the Mariana Trough, Earth Planet. Sci. Lett. 73, 244-254, 1985.

[14] RJ . Pored a and H. Craig, He and Sr isotopes in the Lau basin mantle: depleted and primitive mantle components, Earth Planet. Sci. Lett. 113, 487-494, 1992.

[15] O.K Manuel and D.O. Sabu, The noble gas record of the terrestrial planets, Geochem. J. 15,245-267, 1981.

[16] M. Ozima and S. Zashu, Solat-type Ne in Zaire cubic diamonds, Geochim. Cosmochim. Acta 52,19-25,1988.

[17] P. Sarda, T. Staudacher and CJ. Allegre , Neon isotopes in submarine basalts, Earth Planet. Sci. Lett. 91, 73-88, 1988.

[18] B. Marty, Neon and xenon isotopes in MORB : implica­tions for the earth-atmosphere evolution, Earth Planet. Sci. Lett. 94,45-56, 1989

[19] M. Honda, I. McDougall, D.B. Patterson, A. Doulgeris and D.A. Clague , Possible solar noble-gas component in Hawaiian basalts, Nature 349,149-151,1991.

[20] H. Hiyagon, M. Ozima, B. Marty , S. Zashu and H. Sakai, Noble gases in submarine glasses from mid ocean ridges and Loihi seamount: constraints on early history of the Earth, Geochim. Cosmochim . Acta 56, 1301-1316, 1992.

[21] KA. Farley and R.J . Pereda, Mantle neon and atmo ­spheric contamination, Earth Planet. Sci. Lett. 114, 325­339, 1993.

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