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He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components
Earth and Planetary Science Letters, 113 (1992) 487-493
487
Elsevier Science Publishers B.V., Amsterdam [MK]
He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components R.J. Poreda and H. Craig Isotope Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA Received September 14, 1992; revision accepted September 21, 1992
ABSTRACT Helium isotope ratios in Lau Basin back-arc basalts range from 7 to 22 times the atmospheric value (RA), i.e. from ratios typical of MORB (Depleted Mantle) helium ( R / R A = 8 + 1) to ratios similar to 'high-3He ' hotspots as observed in the Hawaiian, Icelandic, and nearby Samoan plume ( R / R A = 24). Along the Central Lau Basin spreading axis and its northward extension in the region around Niuafo'ou Volcano, 3He/aHe ratios have typical MORB values (range = 7.5-8.6), but on Rochambeau Bank, the southern flank of a large seamount, ratios up to 22 R A occur. These high 3He/aHe ratios are extrema of linear arrays (11-22 R A) of He vs. Sr, Nd and Pb isotope ratios, between a Depleted Mantle (MORB) end-member and a Primitive Helium Mantle component (PHEM). PHEM is the Enriched Mantle end-member for the 'depleted' array formed with the DM component, and at the same time the Depleted end-member for the 'enriched' array formed with 'EM' the EM2-type end-member for Masefau Bay, Samoan basalts, as these two binary arrays intersect at its composition. Sr and Nd isotopic arrays vs. each other and vs. 3He are consistent with these binary 'mirror arrays' for Lau and Masefau basalts. The 3He data show unequivocally that deep-mantle plume material is present at Rochambeau Bank, and to some extent in the leaky transform/spreading axis along Peggy Ridge. We suppose that the Samoan plume component regards itself as an 'off-ridge' hotspot relative to the nearby Lau spreading axis, and that some of its material is channeled toward Peggy Ridge in a manner similar to the channeling we observe at the Galapagos and Pascua (Easter Island) hotspots.
I. Introduction One of the major problems of mantle geochemistry is the question of the minimum number of sources required to generate the isotopic arrays found in Ocean Island Basalts (OIB) erupting from the suboceanic mantle. At least four 'exterior' components, defined by the extrema bounding the Sr-Nd arrays, are required as mixing end-members [1,2], but the existence of an 'interior' array component, the 'primitive mantle' (PM) source, cannot be established from these radiogenic isotope arrays because it plots inside the arrays. For the definition of such 'interior' array components it is to the 3 H e / 4 H e ratios that
we must look. Ratios much greater than the widespread Mid-Ocean Ridge Basalt (MORB) value of 8 R A (the atmospheric ratio) are found in 'hotspot' helium [3], but correlations of 3 H e / 4He with 'enriched' 87Sr/86Sr ratios in OIB lavas have so far been reported only in the shield lavas of the Samoan hotspot [4,5]. In this paper we describe a similar correlation in 'depleted' lavas of the back-arc Lau Basin, close to the Samoan plume but on the opposite side of the Tonga Trench, that appear to be a mixing array between a depleted source and a primitive mantle component very similar to the 'high-3He ' source defined by the enriched Samoan OIB array.
2. Regional geology
Correspondence to: R.J. Poreda, Department of Geological Sciences, University of Rochester, Rochester, N.Y. 14627, USA.
The Lau Basin (Fig. 1) is one of a number of small back-arc basins along the suture between the Pacific and Indo-Australian plates that are generating new basaltic crust [6,7]. The active
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
488
R.J. POREDA AND H. CRAIG
spreading axis lies in the central part of the basin (18-20°S) and is propagating southward [7], while the broad northern section contains numerous volcanic loci [8,9] that are heterogeneous in Sr and Nd isotope ratios, with affinities to Samoan lavas [10,11]. The first studies of 3 H e / 4 H e in back-arc basins [12,13] showed that the dominant helium source resembles M O R B He ( R / R A = 8), and that in the northern Lau Basin near R o c h a m b e a u Bank a high-3He plume component is present, surprisingly like the high-3He Samoan plume now found on the opposite side of the Tonga Trench. Niuafo'ou is an active volcano (that last erupted in 1946) on the northern edge of the Lau Basin: we collected gases in the crater lake Vai Kona from a submerged bubbling gas vent (27.5°C) close to the caldera wall on Leg V of the PAPATUA Expedition [14]. The gases were collected in evacuated 1720 glass-flasks with sampling lines flushed by water displacement, and were returned to the laboratory for analysis.
in vacuum to ~ 10 /~m to release He trapped in vesicles using standard procedures [13-15]. The T W D basalt data are from Poreda [13]: H e in these samples was released by fusion at 1600°C. Niuafo'ou gas samples were processed in a highvacuum system [16]. 3 H e / 4 H e ratios were measured on a 25-cm split-tube mass spectrometer ( G A D ) using Yellowstone Park H e ( ' M M ' = 16.45 R A) as a secondary standard [15]: the ratios are reported relative to air H e (R A) and are accurate to 2% or better. Neon was removed from He on activated charcoal at 37 K for all samples and standards. He and Ne concentrations were determined by peak-height comparison to air standards, and are accurate to 3 and 10% respectively. All data are corrected for the mean system-blank: 1.0 × 10 -9 cm 3 of 4He, with 3 H e / 4 H e = 2 R A, and 2 . 0 × 1 0 -9 cm 3 of Ne. Neon values shown with a ' < ' sign have blank corrections of > 75% of the total Ne signal. In most cases the blank correction for helium was < 1%.
3. Analytical methods
4. Results
Fresh basalt glass (2 m m chips) from the margins of pillow fragments was cleaned ultrasonically in water and dried at 100°C. Glass with surface alteration was rejected under a binocular microscope. Glass chips (about 1 g) were crushed
Figure 3He/4He T h e r e is tern: the Northern
'
16os
7:~
'
"-~0.9 ~o~.2 :s. 9.5
::
~
~o.,
72
)
1800
'
178 °w
'
1 and Table 1 show the accumulated data from [13] and the present work. a rather clear three-fold regional patCentral Basin Spreading Center and Basin with R / R A very similar to M O R B '
....
~:~ ': .-:
8.4 .-+
176 °w
174 °w
172°W
Fig. 1. Location map of the Lau Basin with dredge locations and helium R / R A values. Note the highest 3He/4He ratios at Rochambeau Bank (R = 22 R A) in the Lau Basin, and on Tutui|a in the Samoan hotspot (R = 24 R A) [5] on the opposite side of the Tonga Trench, and the M O R B ratios (8 R A) along the Lau spreading centers in the Central and Northern Basins. Niuafo'ou sample = volcanic gases from crater lake.
DEPLETED AND PRIMITIVE MANTLE COMPONENTS IN THE LAU BASIN
values (7.5-8.6 RA), the Rochambeau Bank high3He section [12], now more clearly defined with R up to 22 RA, and a transitional region along Peggy Ridge (7.2-10.9 RA), an elongate topographic high interpreted as a leaky transform fault probably transitional to a spreading center. The 3He data are MORB-Iike on this ridge near the Central Basin axis, with higher than meanM O R B values on the northwest s e c t i o n - - u p to 10.9 R A at the closest approach to Rochambeau Bank. Helium in Niuafo'ou volcanic gases (7.8
489
R A) is is,topically similar to the M O R B - H e in the Central and Northern Basins, and to volcanic gases in most arc and back-arc terrains (mean R / R A = 7.5) [13,14,16,17]. Rochambeau Bank, the flank of a large seamount, has plume-like 3 H e / n i l e ratios very similar to He at the Hawaiian [15,19,20], Icelandic [21,22], and Samoan [4,5] hotspots. The 3 H e / a H e ratio here is close to the maximum of 24 R A observed on the Samoan hotspot ~ 600 km east-northeast across the northern part of the
TABLE 1 Helium and strontium isotope ratios in Lau Basin basalts Sample
Lat (*S)
Long (*W)
3He/4He (R/R A)
He (/~cc/g)
Ne (ncc/g)
87Sr/86Sr*
ROCHAMBEAU BANK TWD-106-1 PPT-4-23-2 PPT-4-23-3 PPT-4-24-1 PPT-4-24-3
15"10' 15"12' 15" 12' 15"26' 15"25'
176"38' 176"38' 176"38' 176016 ' 176016 '
11.0 14.1 12.3 22.1 21.9
1.13 0.44 1.00 0.18 0.18
2.1 0.9 1.2 2.6 3.0
0.70329 0.70334
16"10' 16"18' 16"20' 16"23' 16"55' 17"02' 17"21'
177"22' 177053 ' 177"30' 177"28' 176"50' 176"31' 176"26'
10.9 9.5 10.4 9.5 7.2 9.0 8.4
3.89 0.60 2.03 0.075 0.042 1.66 0.88
1.4 2.1 2.2 2.2
15"20' 15"29' 15035 '
175"20' 175"34' 174"42'
7.5 8.1 7.7
0.79 0.37 0.21
< 1.1 1.7 8.1
0.70388 0.70428
17"58' 18"40' 18"52' 18"52' 19"10'
175"53' 176"26' 176"36' 176"35' 176"33'
7.5 8.4 8.6 8.0 8.6
1.51 3.95 4.04 0.24 0.15
3.1 <3.2 1.8 3.0 2.3
0.70360 0.70319 0.70318 0.70304 0.70344
0.70400 0.70401
PEGGY RIDGE PPT-4-19-1 PPT-4-21-1 PPT-4-20-5 TWD-95-P TWD-86-P PPT-4-17-1 TWD-79-1
1.2 4.5
0.70342 0.70348 0.70380 0.70393 0.70340 0.70294
NORTHERN BASIN PPT-4-6-1 PPT-4-11-1 PPT-4-5-2 C E N T R A L BASIN PPT-4-28-2 PPT-4-31-1 PPT-4-32-1 PPT-4-33-1 PPT-4-36-1
N I U A F O ' O U VOLCANO Volcanic gas, Lake Vai Kona
7.8
[He] = 4 8 0 p p m
PPT-4 basalts were dredged on s~o PAPATUA Expedition Leg 4, on R.V. Washington. T h e second n u m b e r in the sample listing is dredge number; the final n u m b e r s define individual basalt pieces from a given dredge haul. T W D basalts were dredged on the 7-TOW Expedition [9]. T h e Niuafo'ou volcanic gases were collected by us from the main crater lake of Vai Kona, on Leg 5 of PAPATUA Expedition [14]. * 87Sr/86Sr ratios are from [9] ( T W D basalts) and [10] (PPT basalts). For plotting purposes the ratio PPT-4-23-3 was assumed to be the same as in the companion piece 23-2.
the slo the for
490
R.J.
Tonga Trench [5], which strongly suggests a physical relationship with the Samoan plume high-3He component.
AND
H.
CRAIG
~Nd -0.4
+3.5
+ 7.4 i
+ 11.3 i
i
40 ~
5. Helium-strontium isotopic relationships Figure 2 shows the 3 H e / 4 H e vs. Sr isotope ratios [10] together with data for Samoan shield lavas [5] and other hotspots. The most striking feature of the Lau Basin data is the positive correlation of He and Sr ratios at Rochambeau Bank, which appear to lie on a mixing trajectory between a depleted M O R B (DM) or 'back-arc' source (8 R A, 87Sr/86Sr = 0.7025-0.7030) and a 'primitive' component ( > 4 0 R A, 87Sr/86Sr= 0.7045-0.7050). This binary mixing trajectory is complementary to the across-trench binary mixture of primitive mantle He and enriched mantle
P O R E D A
~
" < 30
Loihi
=_.
(D Samo~ ~u
Ix: v
"I,~
20
"1"
10
DM
ATristan
EM ....
i ....
0.5126
f ....
I ....
0.5128
I ....
I ....
0.5130
I ....
I ....
0.5132
143Nd/144Nd Fig. 3 . 3 H e / 4 H e ratios vs. Nd isotope ratios for the Lau Basin and Masefau Bay [5] basalts. Symbols and references are the same as in Fig. 2. The He-Nd arrays are "mirror arrays" of DM-PHEM and EM-PHEM "branches", like the He-Sr arrays in Fig. 2.
40
Loihi ~ n" n-
T
7 / /
3O
Kauai
20
I
\i
/
~idge~,~/ Bank Reykj Iceland anes
- - -~~
Rochambeau
10 ~ O u
0
. . . .
0.702
I
. . . .
0.703
•
I
,
0.704
87Sr /
i
0.705
0.706
0.707
86Sr
Fig. 2. 3 H e / 4 H e ratios ( R / R A values) vs. S7Sr/S6Sr ratios in the Lau Basin, basalts. • = Rochambeau Bank; 0 = Central Basin; • = Northern Basin; • = Peggy Ridge. Other data from our literature [e.g., 2,5]. The Samoan field encompasses the Tutuila Masefau Bay data [5]. In this figure one can see for the first time He-Sr isotopic trajectories for a Primitive Helium Mantle (PHEM) component mixing with an Enriched Mantle source (EM) on one side of a trench, and with a depleted (MORB-like) mantle source (DM) on the other side, ~ 600 km apart. These "mirror arrays" for He-Sr (and He-Nd and He-Pb, see text) attest to the actual existence of a PM component, the Holy Grail of mantle geochemists [e.g., 2], a mantle end member that cannot be identified in Sr-Nd-Pb isotopic arrays because the PM source always plots within the array boundaries as a virtual (i.e., being in force or effect although not directly observed) component.
EM components that make up the Masefau Bay sequence of Samoan shield lavas [5], and these two mixing arrays intersect at, and reasonably well define, the PHEM, or Primitive Helium Mantle component, in 3He-87Sr space. P H E M thus defines an 'interior' component in Sr-Nd and other binary radiogenic isotope arrays, observable only when the 3 H e / a H e data are used to provide a third dimension to these OIB arrays. Figures 3 and 4 show the 3 H e / a H e ratio plots vs. the Nd isotope ratios [10] and the Pb isotope data [D. Macdougall, pers. commun.], and the same arrays for the Samoan Masefau Bay sequence [5]. One sees that these isotopic arrays show precisely 'the same picture as the He-Sr array in Fig. 2: a 'depleted branch' for the Rochambeau lavas, mappable as a binary mixture of P H E M with a Depleted Mantle source, intersecting at the P H E M composition an 'enriched branch' mappable as a binary mixture of P H E M with an 'enriched' source like EM2. (The ' E M I l ' component of Zindler and Hart [2] was defined with far too low a Nd ratio of 0.5120: as can be seen in their Fig. 5.20 this ratio for EM2 must be about 0.5126, and this matches the Masefau Bay
DEPLETED
AND
PRIMITIVE
MANTLE
COMPONENTS
IN
THE
LAU
40
~
L°ihi
3O
he"
~
ID
-i,~
Rochambeau
20
Samoa
~
Bank
"I10 v
LiE
DM 0
....
HIMU
EM2 I ....
J ....
i ....
i ....
I ....
I , i i i
17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 206Pb/204pb Fig. 4. 3 H e / 4 H e ratios vs. Pb isotope ratios [D. Macdougall, unpublished Pb isotope data]. Symbols as in previous figures. The He-Pb "mirror arrays" are similar to the He-Sr and He-Nd branched arrays. T h e three figures together show that despite the fact that there are only two R o c h a m b e a u rocks from a single dredge haul, all isotopic data are in agreement as to the singularity represented by this lava. It is clear that further dredging studies of the area around R o c h a m b e a u are of the utmost interest for the study of these closely associated, yet so different, mantle arrays.
data and the Samoa data in general.) These He vs. Sr, Nd and Pb isotopic arrays all resemble simple 'mirror binaries' with an D M - P H E M mixture on a depleted branch, and a PHEM-EM(2) mixture on the opposite enriched branch, strong evidence for simple paired mixing diagrams. A second linear array can be seen in Fig. 2 in the Central and Northern Basin basalt data which, remarkably, display a range of 878r/86Sr ratios (0.7030-0.7043) at a constant 3 H e / 4 H e ratio ( R / R A = 8.0 _+ 0.5, n = 8) precisely equal to the mean ratio in MORB. The highest 87Sr/86Sr ratios are in the vicinity of Niuafo'ou in the Northern Basin, and in the Central Basin at opposite ends of the sample array along the spreading axis [10]. The Sr isotope data in these samples form a linear array with the Nd isotope ratios, with the depleted end-member being enriched in 87Sr/86Sr relative to Pacific M O R B of the same Nd ratio. It has been proposed that this 87Sr enrichment is due to ancient recycling of altered ocean crust
BASIN
491
deep within the mantle [9,10]. However, it is difficult to account for the highly uniform MORB-like 3 H e / 4 H e ratios with no evidence of radiogenic He (R ~ 0). This problem requires further study with a complete array of He and other isotopes. The Peggy Ridge basaits have a relatively large range in both S7Sr/S6Sr (0.7029-0.7038) and 3 H e / 4 H e (7.2-10.9 R A) and show a trend in Fig. 2 intermediate between the Rochambeau and the Central Basin arrays. As noted above there is a maximum 3 H e / 4 H e ratio at the closest approach to Rochambeau Bank: this intermediate array perhaps represents a ternary mixture of DM, PHEM, and the EM2 source or material in the enriched Masefau branch. As noted earlier the Sr-Nd isotopic arrays for the Rochambeau and Masefau Bay lavas are, like the 3He-radiogenic isotope arrays, consistent with simple depleted and enriched binary arrays with P H E M (PM?) as a common end member. Sr-Pb and Nd-Pb arrays for the Rochambeau lavas also form linear arrays between the DM depleted end member and PHEM, with the 22 R A Rochambeau sample as the extremum at the P H E M side. Similarly, for the Masefau Bay lavas the 87Sr2(~Pb array [5] is a pure binary with the enriched end member close to the EM2 source [1,2] and a trajectory through the field of Samoan shield lavas directly to the P H E M (or in this case PM) component. Thus the Rochambeau and Masefau Bay arrays are 'branched' and intersect at PHEM, as in the case of the 3He-Sr arrays. However, the Nd-Z°6pb array for Masefau Bay is not so simple: one has first to correct the H a r t / Z i n d l e r Nd isotope ratio used for their EM2 source (see above) to a ratio of ~ 0.5126 which, in fact, lies close to the enriched end of the Masefau Bay array trending toward P H E M (PM). The H a r t / Z i n d l e r Nd ratio defined for EM2 is not a possible end member for Samoan shield lavas. The Pb-Pb arrays are similar: both the 2°8pb2°6pb and the 2°7pb-2°6pb arrays for the Rochambeau lavas are linear and relatively enriched in 208 and 207 relative to the Masefau Bay samples [5]. Thus, it is in the Nd-Pb and Pb-Pb arrays that a detailed study will shed more light on the exact nature of these components, and more Rochambeau Bank samples are badly needed for isotopic and trace element studies.
492
7. Discussion When the He-Sr isotopic trends in Rochambeau Bank and Samoan lavas are compared, they point to a common 'Primitive Helium' component with 3 H e / 4 H e of ~ 35-50 R A and 87Sr/86Sr of ~ 0.7045-0.7050. Similar plots are observed in He-Nd and He-Pb space: in the Nd array the 'enriched' and 'depleted' trends intersect at a P H E M component with ~ 35 R A and 0.51278, assuming roughly equal H e / N d ratios in the two end-member components. William of Occam's philosophy ("Give unto them the Razor: Sell unto them the Blades") makes a logical argument for associating P H E M with the long-sought Primitive Mantle (PM) component, and very likely these are indeed one and the same. But the Pb paradox (as it has been called) confuses this simple equality and requires special processing of one kind or another [5]. Time will no doubt sort out these processes. This P H E M / P M mantle source mixes with a highly radiogenic, enriched mantle component (EM) beneath Samoa, and with a Depleted Mantle source (DM) at Rochambeau Bank. The physical process by which material related to the Samoan plume can mix with Depleted Mantle beneath the northern end of the Lau Basin is speculative, like most postulates concerned with the mantle. In contrast to the vigorous high-3He plumes in Hawaii, Samoa and Iceland that have existed for more than 50 m.y., Rochambeau Bank is a relatively minor feature in an undernourished back-arc basin. Vertical trajectories for rising plumes are presumably blocked by the descending Pacific Plate to depths of at least 700 km. Thus the plume material rising beneath Samoa must be channeled to the southwest toward Rochambeau Bank by the effect of crustal extension on the, spreading axis in the central Lau Basin. Channeling mechanisms from off-ridge hotspots toward spreading axes are described by Morgan [23] and Schilling et al. [24] for the Galapagos Hotspot and Spreading Center, and, specifically for 3He-Sr isotopic effects on the Easter Microplate, by Poreda et al. [25]. Rochambeau Bank lavas are unique in being the only plume material so far known with R / R A > 15 for Sr ratios between Loihi Seamount values and the P H E M / P M component. It is very
R.J. P O R E D A A N D H. CRA I G
important, therefore, that this region be subjected to intensive tectonic and isotopic studies in future work. If primitive-mantle domains such as Rochambeau Bank are consistently found in back-arc basins adjacent to hotspots, the associated physical mechanisms dominating the subcrustal flow of material will surely become much better defined.
Acknowledgements We thank J. Hawkins for providing the Lau Basin samples and information thereon, K. Farley for a critical and helpful review, D. Macdougall for an advance look at his Pb data and a careful review, E. Hernandez for maintaining the Isotope Laboratory, and R. Comer for assistance on and about Niuafo'ou. This research was supported by NSF Marine Geology and Geophysics Program grants to the Isotope Laboratory, SIO.
References 1 W.M. White, Sources of oceanic basalts: radiogenic isotope evidence, Geology13, 115-118, 1985. 2 A. Zindler and S. Hart, Chemical Geodynamics, Annu. Rev. Earth Planet. Sci. 14, 493-571, 1986. 3 H. Craig and J.E. Lupton, Helium-3 and mantle volatiles in the ocean and the oceanic crust, in: The Sea, Vol. 7, The Oceanic Lithosphere, C. Emiliani, ed., pp. 391-428, Wiley, 1981. 4 W. Rison and H. Craig, Helium 3: Coming of age in Samoa, EOS 63, 1144, 1982. 5 K.A. Farley, J. Natland and H. Craig, Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas, Earth Planet. Sci. Lett. 111, 183-199, 1992. 6 L.A. Lawyer, J.W. Hawkins and J.G. Sclater, Magnetic anomalies and crustal dilation in the Lau Basin, Earth Planet. Sci. Lett. 33, 27-33, 1976. 7 L.M. Parson, J.A. Pearce, B.J. Murton, R.A. Hodkinson and R.R.S. Charles Darwin Scientific Party, Role of ridge jumps and ridge' propogation in the tectonic evolution of the Lau back-arc basin, southwest Pacific, Geology 18, 470-473, 1990. 8 J.W. Hawkins, Petrologyand geochemistryof basaltic rocks of the Lau Basin, Earth Planet. Sci. Lett. 28, 283-297, 1976. 9 J.W. Hawkins and J.T. Melchior, Petrology of Mariana Trough and Lau Basin basalts, J. Geophys. Res. 90, 11,431-11,468, 1985. 10 A.M. Volpe, J.D. Macdougall and J.W. Hawkins, Lau Basin basalts: trace element and Sr-Nd isotopic evidence for heterogeneity in backarc basin mantle, Earth Planet. Sci. Lett. 90, 174-186, 1988.
DEPLETED AND PRIMITIVE MANTLECOMPONENTS IN THE LAU BASIN 11 E. Wright and W.M. White, The origin of Samoa: new evidence from Sr, Nd and Pb isotopes, Earth Planet. Sci. Lett. 81, 151- 162, 1986. 12 J.E. Lupton and H. Craig, Excess 3-He in oceanic basalts: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 26, 133-139, 1975. 13 R. Poreda, Helium-3 and deuterium in back-arc basalts: Lau Basin and the Mariana Trough, Earth Planet. Sci. Lett. 73, 244-254, 1985. 14 H. Craig and R. Poreda, PAPATUA Expedition Legs V and VI: Studies of methane and helium in hydrothermal vent plumes, spreading-axis basalts, and volcanic island lavas and gases in Southwest Pacific marginal basins, Scripps Inst. Oceanogr. Ref. 87-14, 77 pp., 1987. 15 W. Rison and H. Craig, Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths, Earth Planet. Sci. Lett. 66, 407-426, 1983. 16 R. Poreda and H. Craig, Helium isotope ratios in CircumPacific volcanic arcs, Nature 338, 473-478, 1989. 17 D. Hilton and H. Craig, A helium isotope transect along the Indonesian Archipelago, Nature 342, 906-908, 1989. 18 H. Craig, Helium isotope distribution in mantle hot spots, EOS 71, 1669, 1990. 19 H. Craig and J.E. Lupton, Primordial neon, helium, and hydrogen in oceanic basalts, Earth. Planet. Sci. Lett. 31, 369-385, 1976.
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20 M.D. Kurz, W.J. Jenkins, S.R. Hart and D. Clague, Helium isotope variations in volcanic rocks from Loihi Seamount and the island of Hawaii, Earth Planet. Sci. Lett. 66, 388-406, 1983. 21 M. Condomines, K. Gronvold, P.J. Hooker, K. Muehlenbachs, R.K. O'Nions, N. Oskarsson and E.R. Oxburgh, Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics, Earth Planet. Sci. Lett. 66, 125-136, 1983. 22 R. Poreda, H. Craig, S. Arnorsson and' J.A. Welhan, Helium isotopes in Icelandic geothermal systems I: 3He, gas chemistry and 13C relations, Geochim. Cosmochim. Acta., in press, 1992. 23 W.J. Morgan, Rodriguez, Darwin, Amsterdam...: a second type of hotspot island, J. Geophys. Res. 83, 5355-5360, 1978. 24 J.-G. Schilling, G. Thompson, R. Kingsley and S. Humphris, Hotspot-migrating ridge interaction in the South Atlantic: geochemical evidence, Nature 313, 187191, 1985. 25 R.J. Poreda, J.-G. Schilling and H. Craig, Helium isotope ratios in Easter Microplate Basalts, Earth. Planet. Sci. Lett., in press.
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Elsevier Science Publishers B.V., Amsterdam [MK]
He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components R.J. Poreda and H. Craig Isotope Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA Received September 14, 1992; revision accepted September 21, 1992
ABSTRACT Helium isotope ratios in Lau Basin back-arc basalts range from 7 to 22 times the atmospheric value (RA), i.e. from ratios typical of MORB (Depleted Mantle) helium ( R / R A = 8 + 1) to ratios similar to 'high-3He ' hotspots as observed in the Hawaiian, Icelandic, and nearby Samoan plume ( R / R A = 24). Along the Central Lau Basin spreading axis and its northward extension in the region around Niuafo'ou Volcano, 3He/aHe ratios have typical MORB values (range = 7.5-8.6), but on Rochambeau Bank, the southern flank of a large seamount, ratios up to 22 R A occur. These high 3He/aHe ratios are extrema of linear arrays (11-22 R A) of He vs. Sr, Nd and Pb isotope ratios, between a Depleted Mantle (MORB) end-member and a Primitive Helium Mantle component (PHEM). PHEM is the Enriched Mantle end-member for the 'depleted' array formed with the DM component, and at the same time the Depleted end-member for the 'enriched' array formed with 'EM' the EM2-type end-member for Masefau Bay, Samoan basalts, as these two binary arrays intersect at its composition. Sr and Nd isotopic arrays vs. each other and vs. 3He are consistent with these binary 'mirror arrays' for Lau and Masefau basalts. The 3He data show unequivocally that deep-mantle plume material is present at Rochambeau Bank, and to some extent in the leaky transform/spreading axis along Peggy Ridge. We suppose that the Samoan plume component regards itself as an 'off-ridge' hotspot relative to the nearby Lau spreading axis, and that some of its material is channeled toward Peggy Ridge in a manner similar to the channeling we observe at the Galapagos and Pascua (Easter Island) hotspots.
I. Introduction One of the major problems of mantle geochemistry is the question of the minimum number of sources required to generate the isotopic arrays found in Ocean Island Basalts (OIB) erupting from the suboceanic mantle. At least four 'exterior' components, defined by the extrema bounding the Sr-Nd arrays, are required as mixing end-members [1,2], but the existence of an 'interior' array component, the 'primitive mantle' (PM) source, cannot be established from these radiogenic isotope arrays because it plots inside the arrays. For the definition of such 'interior' array components it is to the 3 H e / 4 H e ratios that
we must look. Ratios much greater than the widespread Mid-Ocean Ridge Basalt (MORB) value of 8 R A (the atmospheric ratio) are found in 'hotspot' helium [3], but correlations of 3 H e / 4He with 'enriched' 87Sr/86Sr ratios in OIB lavas have so far been reported only in the shield lavas of the Samoan hotspot [4,5]. In this paper we describe a similar correlation in 'depleted' lavas of the back-arc Lau Basin, close to the Samoan plume but on the opposite side of the Tonga Trench, that appear to be a mixing array between a depleted source and a primitive mantle component very similar to the 'high-3He ' source defined by the enriched Samoan OIB array.
2. Regional geology
Correspondence to: R.J. Poreda, Department of Geological Sciences, University of Rochester, Rochester, N.Y. 14627, USA.
The Lau Basin (Fig. 1) is one of a number of small back-arc basins along the suture between the Pacific and Indo-Australian plates that are generating new basaltic crust [6,7]. The active
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
488
R.J. POREDA AND H. CRAIG
spreading axis lies in the central part of the basin (18-20°S) and is propagating southward [7], while the broad northern section contains numerous volcanic loci [8,9] that are heterogeneous in Sr and Nd isotope ratios, with affinities to Samoan lavas [10,11]. The first studies of 3 H e / 4 H e in back-arc basins [12,13] showed that the dominant helium source resembles M O R B He ( R / R A = 8), and that in the northern Lau Basin near R o c h a m b e a u Bank a high-3He plume component is present, surprisingly like the high-3He Samoan plume now found on the opposite side of the Tonga Trench. Niuafo'ou is an active volcano (that last erupted in 1946) on the northern edge of the Lau Basin: we collected gases in the crater lake Vai Kona from a submerged bubbling gas vent (27.5°C) close to the caldera wall on Leg V of the PAPATUA Expedition [14]. The gases were collected in evacuated 1720 glass-flasks with sampling lines flushed by water displacement, and were returned to the laboratory for analysis.
in vacuum to ~ 10 /~m to release He trapped in vesicles using standard procedures [13-15]. The T W D basalt data are from Poreda [13]: H e in these samples was released by fusion at 1600°C. Niuafo'ou gas samples were processed in a highvacuum system [16]. 3 H e / 4 H e ratios were measured on a 25-cm split-tube mass spectrometer ( G A D ) using Yellowstone Park H e ( ' M M ' = 16.45 R A) as a secondary standard [15]: the ratios are reported relative to air H e (R A) and are accurate to 2% or better. Neon was removed from He on activated charcoal at 37 K for all samples and standards. He and Ne concentrations were determined by peak-height comparison to air standards, and are accurate to 3 and 10% respectively. All data are corrected for the mean system-blank: 1.0 × 10 -9 cm 3 of 4He, with 3 H e / 4 H e = 2 R A, and 2 . 0 × 1 0 -9 cm 3 of Ne. Neon values shown with a ' < ' sign have blank corrections of > 75% of the total Ne signal. In most cases the blank correction for helium was < 1%.
3. Analytical methods
4. Results
Fresh basalt glass (2 m m chips) from the margins of pillow fragments was cleaned ultrasonically in water and dried at 100°C. Glass with surface alteration was rejected under a binocular microscope. Glass chips (about 1 g) were crushed
Figure 3He/4He T h e r e is tern: the Northern
'
16os
7:~
'
"-~0.9 ~o~.2 :s. 9.5
::
~
~o.,
72
)
1800
'
178 °w
'
1 and Table 1 show the accumulated data from [13] and the present work. a rather clear three-fold regional patCentral Basin Spreading Center and Basin with R / R A very similar to M O R B '
....
~:~ ': .-:
8.4 .-+
176 °w
174 °w
172°W
Fig. 1. Location map of the Lau Basin with dredge locations and helium R / R A values. Note the highest 3He/4He ratios at Rochambeau Bank (R = 22 R A) in the Lau Basin, and on Tutui|a in the Samoan hotspot (R = 24 R A) [5] on the opposite side of the Tonga Trench, and the M O R B ratios (8 R A) along the Lau spreading centers in the Central and Northern Basins. Niuafo'ou sample = volcanic gases from crater lake.
DEPLETED AND PRIMITIVE MANTLE COMPONENTS IN THE LAU BASIN
values (7.5-8.6 RA), the Rochambeau Bank high3He section [12], now more clearly defined with R up to 22 RA, and a transitional region along Peggy Ridge (7.2-10.9 RA), an elongate topographic high interpreted as a leaky transform fault probably transitional to a spreading center. The 3He data are MORB-Iike on this ridge near the Central Basin axis, with higher than meanM O R B values on the northwest s e c t i o n - - u p to 10.9 R A at the closest approach to Rochambeau Bank. Helium in Niuafo'ou volcanic gases (7.8
489
R A) is is,topically similar to the M O R B - H e in the Central and Northern Basins, and to volcanic gases in most arc and back-arc terrains (mean R / R A = 7.5) [13,14,16,17]. Rochambeau Bank, the flank of a large seamount, has plume-like 3 H e / n i l e ratios very similar to He at the Hawaiian [15,19,20], Icelandic [21,22], and Samoan [4,5] hotspots. The 3 H e / a H e ratio here is close to the maximum of 24 R A observed on the Samoan hotspot ~ 600 km east-northeast across the northern part of the
TABLE 1 Helium and strontium isotope ratios in Lau Basin basalts Sample
Lat (*S)
Long (*W)
3He/4He (R/R A)
He (/~cc/g)
Ne (ncc/g)
87Sr/86Sr*
ROCHAMBEAU BANK TWD-106-1 PPT-4-23-2 PPT-4-23-3 PPT-4-24-1 PPT-4-24-3
15"10' 15"12' 15" 12' 15"26' 15"25'
176"38' 176"38' 176"38' 176016 ' 176016 '
11.0 14.1 12.3 22.1 21.9
1.13 0.44 1.00 0.18 0.18
2.1 0.9 1.2 2.6 3.0
0.70329 0.70334
16"10' 16"18' 16"20' 16"23' 16"55' 17"02' 17"21'
177"22' 177053 ' 177"30' 177"28' 176"50' 176"31' 176"26'
10.9 9.5 10.4 9.5 7.2 9.0 8.4
3.89 0.60 2.03 0.075 0.042 1.66 0.88
1.4 2.1 2.2 2.2
15"20' 15"29' 15035 '
175"20' 175"34' 174"42'
7.5 8.1 7.7
0.79 0.37 0.21
< 1.1 1.7 8.1
0.70388 0.70428
17"58' 18"40' 18"52' 18"52' 19"10'
175"53' 176"26' 176"36' 176"35' 176"33'
7.5 8.4 8.6 8.0 8.6
1.51 3.95 4.04 0.24 0.15
3.1 <3.2 1.8 3.0 2.3
0.70360 0.70319 0.70318 0.70304 0.70344
0.70400 0.70401
PEGGY RIDGE PPT-4-19-1 PPT-4-21-1 PPT-4-20-5 TWD-95-P TWD-86-P PPT-4-17-1 TWD-79-1
1.2 4.5
0.70342 0.70348 0.70380 0.70393 0.70340 0.70294
NORTHERN BASIN PPT-4-6-1 PPT-4-11-1 PPT-4-5-2 C E N T R A L BASIN PPT-4-28-2 PPT-4-31-1 PPT-4-32-1 PPT-4-33-1 PPT-4-36-1
N I U A F O ' O U VOLCANO Volcanic gas, Lake Vai Kona
7.8
[He] = 4 8 0 p p m
PPT-4 basalts were dredged on s~o PAPATUA Expedition Leg 4, on R.V. Washington. T h e second n u m b e r in the sample listing is dredge number; the final n u m b e r s define individual basalt pieces from a given dredge haul. T W D basalts were dredged on the 7-TOW Expedition [9]. T h e Niuafo'ou volcanic gases were collected by us from the main crater lake of Vai Kona, on Leg 5 of PAPATUA Expedition [14]. * 87Sr/86Sr ratios are from [9] ( T W D basalts) and [10] (PPT basalts). For plotting purposes the ratio PPT-4-23-3 was assumed to be the same as in the companion piece 23-2.
the slo the for
490
R.J.
Tonga Trench [5], which strongly suggests a physical relationship with the Samoan plume high-3He component.
AND
H.
CRAIG
~Nd -0.4
+3.5
+ 7.4 i
+ 11.3 i
i
40 ~
5. Helium-strontium isotopic relationships Figure 2 shows the 3 H e / 4 H e vs. Sr isotope ratios [10] together with data for Samoan shield lavas [5] and other hotspots. The most striking feature of the Lau Basin data is the positive correlation of He and Sr ratios at Rochambeau Bank, which appear to lie on a mixing trajectory between a depleted M O R B (DM) or 'back-arc' source (8 R A, 87Sr/86Sr = 0.7025-0.7030) and a 'primitive' component ( > 4 0 R A, 87Sr/86Sr= 0.7045-0.7050). This binary mixing trajectory is complementary to the across-trench binary mixture of primitive mantle He and enriched mantle
P O R E D A
~
" < 30
Loihi
=_.
(D Samo~ ~u
Ix: v
"I,~
20
"1"
10
DM
ATristan
EM ....
i ....
0.5126
f ....
I ....
0.5128
I ....
I ....
0.5130
I ....
I ....
0.5132
143Nd/144Nd Fig. 3 . 3 H e / 4 H e ratios vs. Nd isotope ratios for the Lau Basin and Masefau Bay [5] basalts. Symbols and references are the same as in Fig. 2. The He-Nd arrays are "mirror arrays" of DM-PHEM and EM-PHEM "branches", like the He-Sr arrays in Fig. 2.
40
Loihi ~ n" n-
T
7 / /
3O
Kauai
20
I
\i
/
~idge~,~/ Bank Reykj Iceland anes
- - -~~
Rochambeau
10 ~ O u
0
. . . .
0.702
I
. . . .
0.703
•
I
,
0.704
87Sr /
i
0.705
0.706
0.707
86Sr
Fig. 2. 3 H e / 4 H e ratios ( R / R A values) vs. S7Sr/S6Sr ratios in the Lau Basin, basalts. • = Rochambeau Bank; 0 = Central Basin; • = Northern Basin; • = Peggy Ridge. Other data from our literature [e.g., 2,5]. The Samoan field encompasses the Tutuila Masefau Bay data [5]. In this figure one can see for the first time He-Sr isotopic trajectories for a Primitive Helium Mantle (PHEM) component mixing with an Enriched Mantle source (EM) on one side of a trench, and with a depleted (MORB-like) mantle source (DM) on the other side, ~ 600 km apart. These "mirror arrays" for He-Sr (and He-Nd and He-Pb, see text) attest to the actual existence of a PM component, the Holy Grail of mantle geochemists [e.g., 2], a mantle end member that cannot be identified in Sr-Nd-Pb isotopic arrays because the PM source always plots within the array boundaries as a virtual (i.e., being in force or effect although not directly observed) component.
EM components that make up the Masefau Bay sequence of Samoan shield lavas [5], and these two mixing arrays intersect at, and reasonably well define, the PHEM, or Primitive Helium Mantle component, in 3He-87Sr space. P H E M thus defines an 'interior' component in Sr-Nd and other binary radiogenic isotope arrays, observable only when the 3 H e / a H e data are used to provide a third dimension to these OIB arrays. Figures 3 and 4 show the 3 H e / a H e ratio plots vs. the Nd isotope ratios [10] and the Pb isotope data [D. Macdougall, pers. commun.], and the same arrays for the Samoan Masefau Bay sequence [5]. One sees that these isotopic arrays show precisely 'the same picture as the He-Sr array in Fig. 2: a 'depleted branch' for the Rochambeau lavas, mappable as a binary mixture of P H E M with a Depleted Mantle source, intersecting at the P H E M composition an 'enriched branch' mappable as a binary mixture of P H E M with an 'enriched' source like EM2. (The ' E M I l ' component of Zindler and Hart [2] was defined with far too low a Nd ratio of 0.5120: as can be seen in their Fig. 5.20 this ratio for EM2 must be about 0.5126, and this matches the Masefau Bay
DEPLETED
AND
PRIMITIVE
MANTLE
COMPONENTS
IN
THE
LAU
40
~
L°ihi
3O
he"
~
ID
-i,~
Rochambeau
20
Samoa
~
Bank
"I10 v
LiE
DM 0
....
HIMU
EM2 I ....
J ....
i ....
i ....
I ....
I , i i i
17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 206Pb/204pb Fig. 4. 3 H e / 4 H e ratios vs. Pb isotope ratios [D. Macdougall, unpublished Pb isotope data]. Symbols as in previous figures. The He-Pb "mirror arrays" are similar to the He-Sr and He-Nd branched arrays. T h e three figures together show that despite the fact that there are only two R o c h a m b e a u rocks from a single dredge haul, all isotopic data are in agreement as to the singularity represented by this lava. It is clear that further dredging studies of the area around R o c h a m b e a u are of the utmost interest for the study of these closely associated, yet so different, mantle arrays.
data and the Samoa data in general.) These He vs. Sr, Nd and Pb isotopic arrays all resemble simple 'mirror binaries' with an D M - P H E M mixture on a depleted branch, and a PHEM-EM(2) mixture on the opposite enriched branch, strong evidence for simple paired mixing diagrams. A second linear array can be seen in Fig. 2 in the Central and Northern Basin basalt data which, remarkably, display a range of 878r/86Sr ratios (0.7030-0.7043) at a constant 3 H e / 4 H e ratio ( R / R A = 8.0 _+ 0.5, n = 8) precisely equal to the mean ratio in MORB. The highest 87Sr/86Sr ratios are in the vicinity of Niuafo'ou in the Northern Basin, and in the Central Basin at opposite ends of the sample array along the spreading axis [10]. The Sr isotope data in these samples form a linear array with the Nd isotope ratios, with the depleted end-member being enriched in 87Sr/86Sr relative to Pacific M O R B of the same Nd ratio. It has been proposed that this 87Sr enrichment is due to ancient recycling of altered ocean crust
BASIN
491
deep within the mantle [9,10]. However, it is difficult to account for the highly uniform MORB-like 3 H e / 4 H e ratios with no evidence of radiogenic He (R ~ 0). This problem requires further study with a complete array of He and other isotopes. The Peggy Ridge basaits have a relatively large range in both S7Sr/S6Sr (0.7029-0.7038) and 3 H e / 4 H e (7.2-10.9 R A) and show a trend in Fig. 2 intermediate between the Rochambeau and the Central Basin arrays. As noted above there is a maximum 3 H e / 4 H e ratio at the closest approach to Rochambeau Bank: this intermediate array perhaps represents a ternary mixture of DM, PHEM, and the EM2 source or material in the enriched Masefau branch. As noted earlier the Sr-Nd isotopic arrays for the Rochambeau and Masefau Bay lavas are, like the 3He-radiogenic isotope arrays, consistent with simple depleted and enriched binary arrays with P H E M (PM?) as a common end member. Sr-Pb and Nd-Pb arrays for the Rochambeau lavas also form linear arrays between the DM depleted end member and PHEM, with the 22 R A Rochambeau sample as the extremum at the P H E M side. Similarly, for the Masefau Bay lavas the 87Sr2(~Pb array [5] is a pure binary with the enriched end member close to the EM2 source [1,2] and a trajectory through the field of Samoan shield lavas directly to the P H E M (or in this case PM) component. Thus the Rochambeau and Masefau Bay arrays are 'branched' and intersect at PHEM, as in the case of the 3He-Sr arrays. However, the Nd-Z°6pb array for Masefau Bay is not so simple: one has first to correct the H a r t / Z i n d l e r Nd isotope ratio used for their EM2 source (see above) to a ratio of ~ 0.5126 which, in fact, lies close to the enriched end of the Masefau Bay array trending toward P H E M (PM). The H a r t / Z i n d l e r Nd ratio defined for EM2 is not a possible end member for Samoan shield lavas. The Pb-Pb arrays are similar: both the 2°8pb2°6pb and the 2°7pb-2°6pb arrays for the Rochambeau lavas are linear and relatively enriched in 208 and 207 relative to the Masefau Bay samples [5]. Thus, it is in the Nd-Pb and Pb-Pb arrays that a detailed study will shed more light on the exact nature of these components, and more Rochambeau Bank samples are badly needed for isotopic and trace element studies.
492
7. Discussion When the He-Sr isotopic trends in Rochambeau Bank and Samoan lavas are compared, they point to a common 'Primitive Helium' component with 3 H e / 4 H e of ~ 35-50 R A and 87Sr/86Sr of ~ 0.7045-0.7050. Similar plots are observed in He-Nd and He-Pb space: in the Nd array the 'enriched' and 'depleted' trends intersect at a P H E M component with ~ 35 R A and 0.51278, assuming roughly equal H e / N d ratios in the two end-member components. William of Occam's philosophy ("Give unto them the Razor: Sell unto them the Blades") makes a logical argument for associating P H E M with the long-sought Primitive Mantle (PM) component, and very likely these are indeed one and the same. But the Pb paradox (as it has been called) confuses this simple equality and requires special processing of one kind or another [5]. Time will no doubt sort out these processes. This P H E M / P M mantle source mixes with a highly radiogenic, enriched mantle component (EM) beneath Samoa, and with a Depleted Mantle source (DM) at Rochambeau Bank. The physical process by which material related to the Samoan plume can mix with Depleted Mantle beneath the northern end of the Lau Basin is speculative, like most postulates concerned with the mantle. In contrast to the vigorous high-3He plumes in Hawaii, Samoa and Iceland that have existed for more than 50 m.y., Rochambeau Bank is a relatively minor feature in an undernourished back-arc basin. Vertical trajectories for rising plumes are presumably blocked by the descending Pacific Plate to depths of at least 700 km. Thus the plume material rising beneath Samoa must be channeled to the southwest toward Rochambeau Bank by the effect of crustal extension on the, spreading axis in the central Lau Basin. Channeling mechanisms from off-ridge hotspots toward spreading axes are described by Morgan [23] and Schilling et al. [24] for the Galapagos Hotspot and Spreading Center, and, specifically for 3He-Sr isotopic effects on the Easter Microplate, by Poreda et al. [25]. Rochambeau Bank lavas are unique in being the only plume material so far known with R / R A > 15 for Sr ratios between Loihi Seamount values and the P H E M / P M component. It is very
R.J. P O R E D A A N D H. CRA I G
important, therefore, that this region be subjected to intensive tectonic and isotopic studies in future work. If primitive-mantle domains such as Rochambeau Bank are consistently found in back-arc basins adjacent to hotspots, the associated physical mechanisms dominating the subcrustal flow of material will surely become much better defined.
Acknowledgements We thank J. Hawkins for providing the Lau Basin samples and information thereon, K. Farley for a critical and helpful review, D. Macdougall for an advance look at his Pb data and a careful review, E. Hernandez for maintaining the Isotope Laboratory, and R. Comer for assistance on and about Niuafo'ou. This research was supported by NSF Marine Geology and Geophysics Program grants to the Isotope Laboratory, SIO.
References 1 W.M. White, Sources of oceanic basalts: radiogenic isotope evidence, Geology13, 115-118, 1985. 2 A. Zindler and S. Hart, Chemical Geodynamics, Annu. Rev. Earth Planet. Sci. 14, 493-571, 1986. 3 H. Craig and J.E. Lupton, Helium-3 and mantle volatiles in the ocean and the oceanic crust, in: The Sea, Vol. 7, The Oceanic Lithosphere, C. Emiliani, ed., pp. 391-428, Wiley, 1981. 4 W. Rison and H. Craig, Helium 3: Coming of age in Samoa, EOS 63, 1144, 1982. 5 K.A. Farley, J. Natland and H. Craig, Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas, Earth Planet. Sci. Lett. 111, 183-199, 1992. 6 L.A. Lawyer, J.W. Hawkins and J.G. Sclater, Magnetic anomalies and crustal dilation in the Lau Basin, Earth Planet. Sci. Lett. 33, 27-33, 1976. 7 L.M. Parson, J.A. Pearce, B.J. Murton, R.A. Hodkinson and R.R.S. Charles Darwin Scientific Party, Role of ridge jumps and ridge' propogation in the tectonic evolution of the Lau back-arc basin, southwest Pacific, Geology 18, 470-473, 1990. 8 J.W. Hawkins, Petrologyand geochemistryof basaltic rocks of the Lau Basin, Earth Planet. Sci. Lett. 28, 283-297, 1976. 9 J.W. Hawkins and J.T. Melchior, Petrology of Mariana Trough and Lau Basin basalts, J. Geophys. Res. 90, 11,431-11,468, 1985. 10 A.M. Volpe, J.D. Macdougall and J.W. Hawkins, Lau Basin basalts: trace element and Sr-Nd isotopic evidence for heterogeneity in backarc basin mantle, Earth Planet. Sci. Lett. 90, 174-186, 1988.
DEPLETED AND PRIMITIVE MANTLECOMPONENTS IN THE LAU BASIN 11 E. Wright and W.M. White, The origin of Samoa: new evidence from Sr, Nd and Pb isotopes, Earth Planet. Sci. Lett. 81, 151- 162, 1986. 12 J.E. Lupton and H. Craig, Excess 3-He in oceanic basalts: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 26, 133-139, 1975. 13 R. Poreda, Helium-3 and deuterium in back-arc basalts: Lau Basin and the Mariana Trough, Earth Planet. Sci. Lett. 73, 244-254, 1985. 14 H. Craig and R. Poreda, PAPATUA Expedition Legs V and VI: Studies of methane and helium in hydrothermal vent plumes, spreading-axis basalts, and volcanic island lavas and gases in Southwest Pacific marginal basins, Scripps Inst. Oceanogr. Ref. 87-14, 77 pp., 1987. 15 W. Rison and H. Craig, Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths, Earth Planet. Sci. Lett. 66, 407-426, 1983. 16 R. Poreda and H. Craig, Helium isotope ratios in CircumPacific volcanic arcs, Nature 338, 473-478, 1989. 17 D. Hilton and H. Craig, A helium isotope transect along the Indonesian Archipelago, Nature 342, 906-908, 1989. 18 H. Craig, Helium isotope distribution in mantle hot spots, EOS 71, 1669, 1990. 19 H. Craig and J.E. Lupton, Primordial neon, helium, and hydrogen in oceanic basalts, Earth. Planet. Sci. Lett. 31, 369-385, 1976.
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20 M.D. Kurz, W.J. Jenkins, S.R. Hart and D. Clague, Helium isotope variations in volcanic rocks from Loihi Seamount and the island of Hawaii, Earth Planet. Sci. Lett. 66, 388-406, 1983. 21 M. Condomines, K. Gronvold, P.J. Hooker, K. Muehlenbachs, R.K. O'Nions, N. Oskarsson and E.R. Oxburgh, Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics, Earth Planet. Sci. Lett. 66, 125-136, 1983. 22 R. Poreda, H. Craig, S. Arnorsson and' J.A. Welhan, Helium isotopes in Icelandic geothermal systems I: 3He, gas chemistry and 13C relations, Geochim. Cosmochim. Acta., in press, 1992. 23 W.J. Morgan, Rodriguez, Darwin, Amsterdam...: a second type of hotspot island, J. Geophys. Res. 83, 5355-5360, 1978. 24 J.-G. Schilling, G. Thompson, R. Kingsley and S. Humphris, Hotspot-migrating ridge interaction in the South Atlantic: geochemical evidence, Nature 313, 187191, 1985. 25 R.J. Poreda, J.-G. Schilling and H. Craig, Helium isotope ratios in Easter Microplate Basalts, Earth. Planet. Sci. Lett., in press.