High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project

Chemical Geology 169 Ž2000. 187–209 www.elsevier.comrlocaterchemgeo

High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project W. Abouchami ) , S.J.G. Galer, A.W. Hofmann Max-Planck-Institut fur ¨ Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Received 10 August 1999; accepted 15 May 2000

Abstract We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project ŽHSDP-1. core at Hilo. These data were obtained with an external precision of ; 100 ppm Ž2 sext. . on the ratios 206 Pbr204 Pb, 207 Pbr204 Pb and 208 Pbr204 Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section Ž1200 to 280 m. of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207 Pbr204 Pb– 206 Pbr204 Pb and 208 Pbr204 Pb– 206 Pbr204 Pb isotope spaces. There is a suggestion that Mauna Loa Ž0 to 280 m depth. also displays such linear arrayŽs.. However, analysis of the Mauna Loa samples is complicated by residual contamination andror sample heterogeneity. While these latter data exhibit a satisfactory array in 208 Pbr204 Pb vs. 206 Pbr204 Pb, there still remains scatter in 207 Pbr204 Pb– 206 Pbr204 Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207 Pbr204 Pb– 206 Pbr204 Pb systematics correspond to an age of ; 1.9 Ga. Comparison of measured ThrU ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the ‘‘Kea’’, ‘‘Loihi’’, and ‘‘Koolau’’ components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space Ž2D and 3D.. Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise ŽEPR. MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of ´ Nd and ´ Hf values as a function of age. This may in part be explained by

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Corresponding author. Tel.: q49-6131-305260; fax: q49-6131-371051. E-mail address: [email protected] ŽW. Abouchami..

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 3 2 8 - 4

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differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead Žas well as Rb andror Sr. during the ancient differentiation process that created the differences in parent–daughter ratios. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Lava; Lead isotope systematics; Hawaiian Scientific Drilling Project; Hawaii; Mauna Loa; Mauna Kea

1. Introduction The Hawaiian plume, whose center is presently located between the volcanoes Mauna Loa and Kilauea and Loihi seamount ŽFig. 1., produces lavas with a wide range of isotopic compositions. These isotopic variations, in principle, hold the key to understanding the origin of components present in the Hawaiian plume source, how these components came to be incorporated there, and when. There are several features of the published radiogenic isotope data that now seem to be well established. These concern the temporal evolution at a single location, the spatial variation, and the number of distinct source components involved in the Hawaiian plume. First, it is clear that late-stage, post-shield alkali basalts and nephelinites consistently have lower 87 Srr86 Sr and higher 143 Ndr144 Nd ratios than tholeiitic lavas from the shield-building stage of each volcano. This observation has been ascribed to increasing contributions of melts from depleted upper

Fig. 1. Map of the Hawaiian chain showing the location of the Hilo drill hole.

mantle sources Ži.e. non-plume., as each volcano moves away from the plume axis with time ŽChen and Frey, 1983.. Second, two isotopically distinct trends have been identified — the ‘‘Loa’’ and ‘‘Kea’’ trends — which have been interpreted as sampling the central ŽLoa. and more peripheral ŽKea. parts of the plume, and have led to the idea that the Hawaiian plume is concentrically zoned in chemical and isotopic composition ŽHauri et al., 1996; Kurz et al., 1996; Lassiter et al., 1996.. Third, West et al. Ž1987. have argued that three isotopically distinct components must be present in the Hawaiian plume to account for the variation observed. This suggestion has recently found support in a principal component analysis of major element and published isotope data on Hawaiian shield lavas ŽEiler et al., 1996; Hauri, 1996; Eiler et al., 1998.. This analysis indicates that three components — called Loihi, Koolau and Kea — are required to account for data from the shield lavas alone and, further, that the Kea source component has low d18 O and thus may represent a gabbroic source located within the underlying lower oceanic crust ŽEiler et al., 1998.. However, on the basis of trace element characteristics of Hawaiian tholeiites, Hofmann and Jochum Ž1996. have argued that a recycled gabbroic source component is present in all Hawaiian volcanoes. In this study, we present high precision Pb isotope data on Mauna Loa and Mauna Kea lavas from the Hilo drill hole, drilled by the preliminary phase of the Hawaiian Scientific Drilling Project ŽHSDP-1.. The Pb isotope data were obtained using a Pb triple spike ŽTS. to correct for instrumental mass bias ŽGaler, 1999.. The TS technique allows Pb isotopic compositions to be determined with external precisions of F 100 ppm Ž2 sext. . ŽGaler and Abouchami, 1998; Abouchami et al., 1999a,b. — an order of magnitude better than conventional analyses. Lead isotopic compositions of Hawaiian lavas have the potential of yielding the maximum amount

W. Abouchami et al.r Chemical Geology 169 (2000) 187–209

of information on the age and origin of source components present in the Hawaiian plume. Although other radiogenic isotope ratios, such as 143 Ndr144 Nd and 87 Srr86 Sr, are indicative of those in the sources, they cannot yield age constraints on the source. By contrast, age information can in principle be derived from 207 Pbr204 Pb– 206 Pbr204 Pb systematics. A further useful property that lead isotope ratios possess is that binary mixtures form straight linear arrays in 207 Pbr204 Pb– 206 Pbr204 Pb and 208 Pbr204 Pb– 206 Pbr204 Pb space, and deviations from such a linear array automatically imply mixtures involving more than two components. These inherent advantages of Pb isotopes notwithstanding, it has proven difficult to exploit Pb isotopes fully because of the relatively low precision of measurement. In most cases, the analytical error is limited to ; 0.05% per AMU because of mass-dependent, instrumental isotope fractionation. In the case of 207 Pbr204 Pb ratios, for example, the analytical error is frequently of the same order of magnitude as, or larger than, the natural variations in a given sample suite. The Pb isotope data presented here are the first obtained on ocean island basalt ŽOIB. lavas using the triple spike technique, and therefore offer a first glimpse into the structure of Pb isotopic variations present in OIB at a 100 ppm level of precision. Mauna Kea lavas define two distinct linear Pb isotopic arrays, which most likely reflect binary mixing. Each Kea array points to two isotopically distinct components present within the plume source, indicating that the isotope systematics of a single volcano are more complex in detail than can be accomodated by current two or three-component models proposed for the Hawaiian islands as a whole.

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H 2 O to free the residue of any remaining leachate. This procedure resulted in a loss of about 50% of the sample Pb. Following leaching, the samples were dissolved in a mixture of HF and HNO 3 , and the Pb separated by anion exchange using HBr–HNO 3 mixtures as eluent, as has been described previously ŽAbouchami et al., 1999a.. The eluent from the Pb chemistry was retained for Sr and REE chemical separations. Lead blanks during the course of this study were between 15 and 50 pg and are negligible. Following elution of Pb, a small aliquot of the sample Pb Ž; 5%. was taken. A triple spike amount, estimated to be optimal, was then added to this aliquot and the resultant mixture homogenized by drying down. The spiked and unspiked aliquots were loaded separately onto Re filaments along with silica

2. Analytical methods All samples were subjected to the following cleaning and leaching procedure prior to dissolution. Chips were used whenever possible for reasons that will be discussed below. The samples were washed and ultrasonicated in ultrapure water, then ultrasonicated in 6 N HCl Ž15 min., and finally leached in hot 6 N HCl for 1 h. The 6 N HCl leachate was discarded and the residue washed repeatedly with

Fig. 2. External reproducibility on Pb isotope ratios using the Pb triple spike technique illustrated by the deviation from the mean 206 Pbr204 Pb and 207 Pbr204 Pb ratios Žin ppm. of replicate measurements on the same dissolution and from separate dissolutions. In the latter case, the deviations for some Mauna Loa samples are greater than the external reproducibility for replicate measurements from the same dissolution. This may be caused by either incomplete removal of Pb contamination or intrinsic sample heterogeneity. For example, replicate dissolutions of sample R-117 Žout of scale. differ by 3.5‰ in 206 Pbr204 Pb ŽTable 1..

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Table 1 Pb, Sr and Nd isotope data for lavas HSDP-1 Depth Žm. Mauna Loa R68-res. R68-6 N HCl leach. R75 R83 R83a R91 R91a R103-a1 R103-a2 R103-mean R110 R117 R117a R121-res.-a1 R121-res.-a2 R121-res. mean R121-6 N HCl leach. R128 R129 R129a R142 R150 R150 R153 Mauna Kea R158-a1 R158-a2 R158-mean R159 R160-a1 R160-a2 R160-mean R164 R166 R167 R168 R171 R177 R180 R185 R189 R193 R197 R202 R208-a1 R208-a2 R208-mean R212$ R229-a1 R229-a2 R229-mean

87

Srr86 Sr

143

Ndr144 Nd

´ Nd

206

Pbr204 Pb

207

Pbr204 Pb

208

Pbr204 Pb

15.4460 " 41 15.5307 " 28 15.4722 " 25 15.4712 " 18 15.4604 " 11 15.4648 " 19 15.4607 " 12 15.4604 " 13 15.4606 " 11 15.4605 15.4604 " 12 15.4263 " 26 15.4571 " 9 15.4502 " 11 15.4508 " 18 15.4505 15.4615 " 27 15.4504 " 27 15.4320 " 38 15.4386 " 10 15.4536 " 14 15.4558 " 15

37.8041 " 102 38.2149 " 74 37.8909 " 73 37.8813 " 51 37.8524 " 33 37.9704 " 50 37.9580 " 33 37.9563 " 38 37.9569 " 31 37.9566 37.9084 " 37 37.7440 " 46 37.9074 " 28 37.9346 " 32 37.9349 " 46 37.9347 37.9651 " 71 37.8442 " 80 37.7924 " 115 37.8089 " 31 37.9156 " 42 37.9529 " 38

94.1

0.703792

0.512927

5.63

104.8 117.1

0.703796 0.703776

0.512918 0.512897

5.46 5.05

129.2

0.703668

0.512957

6.22

147.7

0.703649

0.512966

6.40

164.0 184.4

0.703697 0.703710 0.703621 0.703689

0.512937 0.512919

5.83 5.48

0.703761 0.703809 0.703763 0.703691 0.703625 0.703618 0.703618

0.512970 0.512950 0.512960

6.48 6.09 6.28

18.0990 " 47 18.4360 " 30 18.1087 " 24 18.1107 " 19 18.1071 " 10 18.2593 " 20 18.2539 " 11 18.2507 " 12 18.2508 " 11 18.2507 18.1665 " 13 18.0403 " 13 18.1670 " 8 18.2225 " 11 18.2228 " 19 18.2226 18.2289 " 30 18.0579 " 26 18.0424 " 36 18.0472 " 10 18.1779 " 13 18.2458 " 16

0.512955

6.18

18.2480 " 11

15.4580 " 12

37.9558 " 35

18.4287 " 28 18.4281 " 22 18.4284 18.4234 " 7 18.4219 " 19 18.4218 " 18 18.4218 18.3849 " 7 18.4095 " 32 18.4080 " 8 18.4048 " 35 18.4107 " 18 18.4367 " 11 18.4358 " 13 18.4344 " 10 18.5003 " 13 18.4066 " 35 18.4656 " 13 18.4658 " 18 18.5446 " 12 18.5423 " 13 18.5435 18.4289 " 47 18.4855 " 29 18.4876 " 23 18.4866

15.4822 " 35 15.4817 " 27 15.4819 15.4826 " 8 15.4765 " 19 15.4764 " 18 15.4764 15.4770 " 7 15.4753 " 38 15.4763 " 8 15.4732 " 33 15.4603 " 18 15.4824 " 11 15.4852 " 13 15.4851 " 11 15.4912 " 13 15.4756 " 32 15.4831 " 12 15.4801 " 17 15.4831 " 12 15.4822 " 12 15.4826 15.4815 " 58 15.4870 " 25 15.4885 " 20 15.4878

38.0174 " 113 38.0164 " 85 38.0169 38.0120 " 27 37.9957 " 55 37.9955 " 56 37.9956 37.9722 " 22 37.9809 " 122 37.9855 " 25 37.9725 " 89 37.9365 " 52 38.0243 " 31 38.0129 " 36 38.0052 " 31 38.0788 " 36 37.9606 " 87 38.0317 " 34 38.0274 " 47 38.1141 " 34 38.1096 " 34 38.1118 38.0207 " 188 38.0603 " 63 38.0623 " 51 38.0613

191.8

205.7 210.8 241.1 260.0 268.2

281.3

285.5 289.0

299.0 306.0 307.7 312.8 320.5 331.7 342.0 354.5 367.6 378.3 389.4 400.3 415.7

424.1 472.6

0.703500 a 0.703538

7.47 a

0.703505a 0.703522 0.703491 0.703515 0.703468 0.703500 0.703560 a 0.703502 0.703484 0.703516 a 0.703554 0.703509 a 0.703558

7.50 a

0.703538 0.703563 a

0.703537 a

7.20 a

0.513015

7.35 7.35a 7.50 a

0.512991

6.89 7.45a

7.47 a

W. Abouchami et al.r Chemical Geology 169 (2000) 187–209

191

Table 1 Ž continued . Depth Žm. R229 b R243 R286 R303 R365$ R395 R395$ ) R466-a1 R466-a2 R466-mean R466$-a1 R466$-a2 R466$-mean R466$ ) R466$q HSDP-2 bulk mud HSDP-2 mud-res. HSDP-2 mud-6 N HCl leach.

513.3 590.1 623.4 781.2 855.9 855.9 1052.4

87

Srr86 Sr

143

Ndr144 Nd

´ Nd

0.703589 a

7.21a

0.703561a 0.703522 a 0.703527 0.703530

7.29 a 7.19 a 6.71

0.512982

0.703596 a

0.708505

6.66 a

0.512207

y8.41)

206

Pbr204 Pb

18.4902 " 16 b 18.5529 " 14 18.5239 " 11 18.4994 " 14 18.6066 " 36 18.5360 " 43 18.5361 " 15 18.5622 " 15 18.5580 " 25 18.5601 18.5608 " 12 18.5585 " 12 18.5596 18.5657 " 14 18.5797 " 18 19.6614 " 10 19.7017 " 7 19.6561 " 17

207

Pbr204 Pb

15.4860 " 14 b 15.4953 " 13 15.4892 " 11 15.4723 " 16 15.5003 " 41 15.4929 " 47 15.4927 " 16 15.4803 " 16 15.4770 " 26 15.4787 15.4779 " 14 15.4751 " 13 15.4765 15.4895 " 16 15.5032 " 17 15.7217 " 11 15.7237 " 7 15.7235 " 20

208

Pbr204 Pb

38.060 " 4 b 38.1364 " 36 38.1054 " 33 38.0437 " 50 38.1957 " 130 38.1611 " 144 38.1513 " 49 38.1520 " 49 38.1435 " 77 38.1477 38.1405 " 44 38.1313 " 42 38.1359 38.1785 " 50 38.1956 " 50 39.5445 " 34 39.5550 " 23 39.5538 " 67

Pb isotopic compositions are corrected for instumental mass fractionation using a Pb triple spike ŽGaler, 1997, 1999; Galer and Abouchami, 1998.. Sr and Nd isotopic compositions were measured using static multicollection on a Finnigan MAT 261. 87 Srr86 Sr are normalized to 86 Srr88 Sr of 0.1194 and the NBS-987 standard yielded a value of 0.710232 " 32 Ž2 sext., N s 9.. Nd isotopic compositions are normalized to 146 Ndr144 Nd of 0.7219 and La Jolla standard yielded 143 Ndr144 Nd s 0.511848 " 17 Ž2 sext., N s 7.. All samples are chips and were leached in hot 6 N HCl unless designated otherwise. a: Duplicate dissolution; a1 and a2 refers to two analyses on the same dissolution; $: powder, $ ) : powder leached with 2 N HCl; $ q : unleached powder. Samples R-171, R-208, R-303 and R-466 belong to the ‘‘anomalous’’ Kea array. a Data from Lassiter et al. Ž1996. and Lassiter and Hauri Ž1998.. b Data from Thirlwall Ž2000. determined using a 207 Pb– 204 Pb double spike.

gel–H 3 PO4 activator and run on a Finnigan MAT 261 mass spectrometer in static multicollection mode. Procedures for combining the results from the two runs to yield the bias-corrected Pb isotopic composition have been described elsewhere ŽGaler, 1997, 1999; Galer and Abouchami, 1998.. Based upon duplicate analyses of eight samples Žsame dissolution., the external reproducibility on 206 Pbr204 Pb, 207 Pbr204 Pb and 208 Pbr204 Pb is ; 100 ppm ŽFig. 2, Table 1.. All analyses are referenced to values of 206 Pbr204 Pb, 207 Pbr204 Pb and 208 Pbr204 Pb of 16.9405 " 15, 15.4963 " 16, 36.7219 " 44 Ž2 sext. ., respectively, for the NBS-981 Pb standard ŽGaler and Abouchami, 1998..

top part of the drill core Ž0 to 280 m. samples lava series from Mauna Loa, whereas Mauna Kea lavas occur from 280 m down to the maximum depth of penetration of 1200 m ŽStolper et al., 1996.. Two aspects of the Pb isotope data are discussed below. First, leaching experiments were performed to evaluate the extent to which the samples faithfully record the Pb isotopic compositions as erupted, or whether the magmatic ‘‘signal’’ was corrupted by minor alteration. Second, the Pb isotopic variation and covariation between the three Pb isotope ratios that can be seen at the high precision afforded by the Pb triple spike is discussed and compared with published Pb isotope data obtained by conventional methods.

3. Results

3.1. Leaching experiments

Lead isotope data on lavas from the HSDP drill hole are presented in Table 1, together with new Sr and Nd isotopic compositions on some samples. The

Although the Pb triple spike allows an accurate isotopic assay of the Pb extracted from a sample, this may not necessarily reflect the true Pb isotopic com-

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position of the lavas as erupted. It is well known that crystalline basaltic samples may sometimes have been severely contaminated by ‘‘exotic’’ Pb even when they appear fresh ŽMcDonough and Chauvel, 1991.. In many cases it remains unclear whether samples were contaminated during sample handling Že.g. contamination, crushing or storage., by surficial weathering, or by hydrothermal alteration in the geological environment; nevertheless, the end result is that a fraction of the Pb within a sample is not magmatic in origin. In general, this problem is tackled, first by preselecting fresh samples andror chips for analysis and, second, by leaching the samples prior to dissolution. The rationale of leaching is that any contaminating components present will be ‘‘labile’’ to acid attack, leaving essentially the unaltered, resistant mineral components. Because different leaching procedures have been used in previous HSDP studies ŽHauri et al., 1996; Lassiter et al., 1996; Lassiter and Hauri, 1998., we performed a series of leaching experiments in order to investigate whether leaching can lead to reliable recovery of the original magmatic signature. The effects of leaching were found to be variable and contrasting between different samples. The first leaching experiment consisted of increasing the strength of the HCl used. HSDP sample R-466 ŽMauna Kea. was analysed unleached Žpowder., leached in 2 N HCl Žpowder., and leached in hot 6 N HCl Žpowder and chips. ŽTable 1.. Pb isotopes were measured on the residues after leaching and the results are shown in Fig. 3. There is a systematic decrease in 206 Pbr204 Pb, 207 Pbr204 Pb and 208 Pbr204 Pb isotope ratios passing from the unleached to the hot 6 N HCl leached split, with intermediate ratios observed in the milder 2 N HCl leached split. However, the Pb isotope ratios of the 6 N HCl leached chips and leached powder agree within analytical uncertainties. The second sample, R-395 ŽMauna Kea., exhibits a quite different behaviour upon leaching ŽTable 1.. In particular, Pb isotope ratios after leaching in 2 N HCl and 6 N HCl are similar and do not show the pronounced change seen for R-466 ŽFig. 3.. Thus, the level of contamination is variable between HSDP samples. Replicate dissolutions after leaching in hot 6 N HCl in some cases yielded inconsistent results: for

Fig. 3. Leaching experiments on HSDP Mauna Kea sample R-466 plotted as 207 Pbr204 Pb Ža. and 208 Pbr204 Pb Žb. vs. 206 Pbr204 Pb ratios. Pb isotope ratios of the residues decrease with increasing strength of HCl used for leaching. Note that the Pb isotope ratios on the powder leached in 6 N HCl are in good agreement with those obtained on chips using the same leaching procedure, whereas the unleached and 2 N HCl leached powder have systematically higher Pb isotope ratios than the 6 N HCl leached powder and chips.

example, both the Pb and Sr isotopic compositions of Mauna Loa sample R-117 do not agree within analytical uncertainty ŽTable 1.. These differences could reflect either sample heterogeneity or incomplete removal of extraneous Pb despite the strong leaching procedure used. Lead isotope ratios of the 6 N HCl leachates can provide some indication of those of the contaminant, as well as the direction of the isotopic shift involved by the latter. The results for the leachate and residue of two Mauna Loa samples, R-121 and R-68, are quite different: 206 Pbr204 Pb ratios in the residue and leachate of sample R-121 agree within analytical uncertainty, while 207 Pbr204 Pb and 208 Pbr204 Pb ratios are slightly higher in the leachate than in the residue. In contrast, Pb isotope ratios of sample R-68 are systematically more radiogenic in the leachate than in the residue ŽTable 1.. In both cases, the

W. Abouchami et al.r Chemical Geology 169 (2000) 187–209

contaminant appears to have a radiogenic Pb isotopic composition, causing a shift toward higher Pb isotope ratios, most noticeable on the 207 Pbr204 Pb ratio ŽFig. 4.. Highly radiogenic Sr has been reported in the leachates of HSDP samples and attributed to interac-

193

tion with sea water, on the basis that addition of drilling mud would have altered the Nd isotopic composition ŽHauri et al., 1996; Lassiter et al., 1996.. Sr and Pb isotopes were analysed on a mud sample from the second phase of HSDP drilling ŽHSDP-2. ŽTable 1.. There is evidence, based on Sr isotopes at

Fig. 4. Ža. 207 Pbr204 Pb vs. 206 Pbr204 Pb plot showing analyses of the residue ŽR. and leachate ŽL. of two Mauna Loa samples ŽR-68 and R-121., and results for Mauna Kea sample R-466 unleached, leached in 2 N and 6 N HCl. Potential sources of Pb contamination are also plotted. These include the HSDP-2 drilling mud from the on-going phase of drilling Žthis study., bulk and carbonate fraction of eolian dust ŽJones et al., 2000. and the field of Pacific Fe–Mn nodules, representative of Pacific seawater Pb ŽAbouchami and Galer, 1998.. In Žb., the whole HSDP Pb isotope dataset is plotted together with potential contaminants. This plot shows that HSDP-2 drilling mud cannot be responsible for the Pb contamination present in HSDP-1 lavas and, further, does not lie on the extension of the Mauna Kea and Mauna Loa arrays.

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least, that the mud used during the two phases of drilling is different. The 87 Srr86 Sr in HSDP-1 mud unleached is 0.704673 ŽJ. Lassiter, 1999, pers. com.. while HDSP-2 mud has a higher value of 0.708505 ŽSr s 207 ppm., which is similar to the present-day value of seawater. Unfortunately, no Pb isotope measurements are available on HSDP-1 mud. The Pb isotope ratios in HSDP-2 mud are highly radiogenic with 206 Pbr204 Pb s 19.6613 " 10; 207 Pbr204 Pb s 15.7217 " 11 and 208 Pbr204 Pb s 39.5445 " 34 ŽTable 1.. We applied the hot 6 N HCl treatment to a drilling mud sample in order to test the effectiveness of leaching in removing any mud present in the lavas. During this treatment, a substantial fraction of the mud went into solution. The Pb isotopic compositions of the residue and leachate of the mud are similar, with a slightly higher 206 Pbr204 Pb ratio in the residue than in the leachate ŽTable 1.. Fig. 4 shows the results of the leaching experiments together with potential sources of contamination, which include: Ž1. the drilling mud from the second drill hole, Ž2. Pacific pelagic sediments ŽBen Othman et al., 1989. and manganese nodules ŽAbouchami and Galer, 1998., indicative of seawater Pb, and Ž3. eolian dusts blown into the Pacific from western Asia ŽJones et al., 2000.. Eolian detritus blown onto flow tops during periods between eruptions might be an important contaminant, given that more than half of the Sr present in Hawaiian soils and weathering horizons is derived from atmospheric sources — in other words has a predominantly continental origin ŽKennedy et al., 1998.. Contamination could also have taken place below sea level. Although seawater itself does not contain significant Pb Ž; 1 pgrg., deposition of marine sediment onto a flow top accompanied by minor alteration, for example, might result in significant Pb addition, and particularly so if associated with palagonitic alteration or precipitated as Fe–Mn oxides. The average isotopic composition of Pacific Fe–Mn deposits is 206 Pbr204 Pb s 18.65, 207 Pbr204 Pb s 15.63 and 208 Pbr204 Pb s 38.70 ŽAbouchami and Galer, 1998.. Addition of such material would shift the 207 Pbr204 Pb in the lavas towards higher values without affecting much the 206 Pbr204 Pb ratio. Lastly, contamination could conceivably have taken place during sample handling, crushing or

grinding. In the case of the Hilo drill core samples ŽHSDP-1., incorporation of traces of drilling mud must be considered and cannot be ruled out until Pb isotope data on HSDP-1 mud become available. Fig. 4 shows that the HSDP-2 drilling mud does not have the required Pb isotopic composition to lie on the trend defined by the progressive leaching of HSDP sample R-466. However, the carbonate fraction of eolian dust lies on the radiogenic extension of this trend, and thus may represent the Pb contaminant present in these lavas. Such contamination would not be unreasonable given that the carbonate fraction is probably the most labile component of eolian dust. On the other hand, the 6 N HCl leachate of sample R-68 points toward the field of Pacific Fe–Mn nodules, in other words seawater Pb ŽAbouchami and Galer, 1998.. Taken together, these results demonstrate that the level of extraneous, non-magmatic Pb contamination present in the HSDP samples is highly variable, but appears to be common. It is by no means certain that the hot 6 N HCl leaching procedure can fully rid some samples of their non-magmatic Pb components. In such cases, incomplete removal would result in significant isotopic scatter outside the error of measurement achievable using the triple spike technique. In all cases, the lavas appear to have been contaminated by Pb that is more radiogenic than the magmatic lead from the respective lavas. The fact that the contaminant has an elevated 207 Pbr204 Pb suggests that this Pb is ultimately derived from continental sources. In summary, the leaching results show that both instrumental Žmass bias fractionation. and chemical Žsample alteration andror contamination. effects are responsible for some of the spread in 207 Pbr204 Pb and 208 Pbr204 Pb ratios observed in literature Pb data on oceanic basalts. 3.2. Pb isotope data On the basis of the leaching experiments, the leaching procedure was standardized for all HSDP samples: all measurements were performed on samples that were leached in hot 6 N HCl prior to dissolution. Twenty four samples from Mauna Kea and eleven samples from Mauna Loa were analysed

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for Pb isotopes, and Sr and Nd isotopes on samples for which data were previously unavailable ŽTable 1.. Our results are compared with literature data on Mauna Kea and Mauna Loa in Fig. 5. The Pb TS data for Mauna Kea form narrow and tightly constrained linear arrays in Pb isotope space; by comparison, the previous conventionally analysed data form a ‘‘cloud’’ with considerable scatter. We infer that this is the result of the increased precision afforded by using the triple spike and the strength of leaching used in this study. In support of this, recent double spike measurements on sample HSDP R-229 by Thirlwall Ž2000., using a similar leaching procedure, are in agreement within analytical uncertainty with our TS measurements Žsee Table 1 and Fig. 5c.. For Mauna Loa, the picture is less clear since there is significant scatter in the data in 207 Pbr 204 Pb– 206 Pbr204 Pb space. Nevertheless, in 208 Pbr 204 Pb– 206 Pbr204 Pb space, the data form a reasonably well defined array which suggest that the 207 Pbr 204 Pb– 206 Pbr204 Pb scatter is related to sample contamination or heterogeneity. In this connection, we note that some Mauna Loa samples failed to duplicate satisfactorily on separate dissolutions ŽTable 1. but are nonetheless plotted in Fig. 5. The comparison on a sample-by-sample basis, shown in Fig. 5c for Mauna Kea, illustrates the significant improvement in precision of the Pb TS method, compared to that of conventional Pb isotope measurements. The Mauna Kea Pb TS data define two distinct linear arrays in both 207 Pbr204 Pb and 208 Pbr204 Pb vs. 206 Pbr204 Pb plots ŽFig. 6.. Even though the second array is defined by only four samples ŽR-171, R-208, R-303 and R-466. — referred to hereafter as ‘‘anomalous’’ Mauna Kea — the two Kea arrays are clearly resolved within our analytical uncertainty. In addition, the agreement between Pb isotopic measurements on replicate dissolutions of sample R-466 ŽTable 1., which belongs to the ‘‘anomalous’’ Mauna Kea array, confirms that the second Kea array is not an analytical artefact. Further support is provided by HSDP sample R-333, analysed by Thirlwall Ž2000. using a double spike method, which plots directly on the anomalous Kea array ŽFig. 5c.. It is peculiar that there is no systematic correlation between the two Kea trends and the stratigraphic position of the samples.

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Fig. 5. Ža. and Žb. Comparison of the Pb TS data with literature Pb isotopic data obtained by conventional methods. The Mauna Kea Pb TS data exhibit two well-constrained linear arrays Žsee also Fig. 6. which contrast with the ‘‘cloud’’ defined by conventional Pb isotopic measurements ŽLassiter et al., 1996; Lassiter and Hauri, 1998.. Žc. A comparison on a sample-by-sample basis for Mauna Kea lavas is shown in 207 Pbr204 Pb vs. 206 Pbr204 Pb space. Also shown are analyses performed by Thirlwall Ž2000. using a 207 Pb– 204 Pb double spike on two HSDP samples ŽR-229 and R-333.. Note the good agreement between the TS and double spike analyses of R-229.

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Fig. 6. Pb isotopic covariations in Mauna Kea. Two distinct and well-constrained linear Pb isotopic arrays are resolved outside analytical uncertainties. Duplicate dissolutions of sample R-466, which belongs to the ‘‘anomalous’’ Kea array, agree within analytical uncertainty, confirming the significance of this array. The position of a given sample on either array does not show any obvious relationship to its stratigraphic position. Samples belonging to the ‘‘anomalous’’ Kea array are: R-171, R-208, R-303, and R-466.

In general, the TS 206 Pbr204 Pb ratios agree well with those previously reported for HSDP Mauna Loa and Mauna Kea samples. In contrast, the previously reported range in 207 Pbr204 Pb ratios of 15.43 to 15.52 for Mauna Kea lavas is reduced to only 15.46 to 15.50 ŽFig. 5a,c.. The deviation in 207 Pbr204 Pb can be as high as ; 3‰ from the originally reported values. Such deviations are not uncommon ŽFig. 5c. and are clearly outside the usual quoted external reproducibility of ; 0.1% for conventional analyses Žsee Thirlwall Ž2000. for discussion.. In the case of Mauna Loa sample R-129, two separate dissolutions yielded reproducible and lower Pb isotope ratios than those reported by Hauri et al. Ž1996.. This difference could be due to either sample heterogeneity or interlaboratory bias. The fact that sample R-128, from a flow unit adjacent to that of R-129, also displays low Pb isotope ratios compared with the rest of the samples analysed, and that melt inclusions with anomalous Sr excesses and distinct Pb isotope ratios

Fig. 7. Correlations between 206 Pbr204 Pb and 87 Srr86 Sr, ´ Nd , ´ Hf and 187Osr188 Os for Mauna Kea and Mauna Loa. Data sources: Pb isotope ratios Žthis study.; 87 Srr86 Sr ratios Žthis study; Lassiter et al., 1996; Lassiter and Hauri, 1998.; ´ Nd Žthis study; Lassiter et al., 1996; Lassiter and Hauri, 1998.; ´ Hf ŽBlichert-Toft et al., 1999. and 187Osr188 Os ŽHauri et al., 1996; Lassiter et al., 1996; Lassiter and Hauri, 1998..

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occur in sample R-129 ŽSobolev et al., 1999. suggests that such relatively low Pb isotope ratios may be a significant feature of Mauna Loa. 3.3. Isotopic correlations Correlations between Pb isotopes and other radiogenic isotope systems ŽSr, Nd, Hf and Os. are shown in Fig. 7. The 87 Srr86 Sr ratios are generally higher in Mauna Loa than in Mauna Kea, in agreement with previously reported data for HSDP samples ŽHauri et al., 1996. ŽTable 1.. There is a negative correlation between 206 Pbr204 Pb and 87 Srr86 Sr ratios in Mauna Loa ŽFig. 7a., as has previously been recognized ŽKurz et al., 1995; Rhodes and Hart, 1995; Hauri et al., 1996.. In contrast, the Mauna Kea section shows little or no correlation between 206 Pbr204 Pb and 87 Srr86 Sr — the alkali basalts, which form the upper part of the section, have lower 206 Pbr204 Pb than in the lower tholeiitic section for a given 87 Srr86 Sr ratio ŽFig. 7a.. On Mauna Loa, Nd and Hf isotopes display a positive correlation which reverses itself on Mauna Kea, with the alkali basalt having higher ´ Nd and ´ Hf values than the tholeiites ŽFig. 7b,c.. In contrast, Os and Pb isotopes are negatively correlated in both Mauna Loa and Mauna Kea, Mauna Loa lavas having higher 187Osr188 Os and lower Pb isotope ratios than those from Mauna Kea ŽFig. 7d., in agreement with previous observations ŽHauri et al., 1996; Lassiter and Hauri, 1998.. This negative correlation is also observed in 207 Pbr204 Pb– and 208 Pbr204 Pb– 187Osr188 Os space Žnot shown.. There are insufficient He and O isotope data on the samples analysed in this study to draw any significant conclusions.

4. Linear arrays: mantle isochrons or mixing lines? The isotopic variations seen in Hawaiian lavas have been interpreted in terms of mixing between two ŽBennett et al., 1996; Lassiter and Hauri, 1998; Blichert-Toft et al., 1999. or three mantle components ŽChen and Frey, 1983; West et al., 1987; Hauri, 1996; Eiler et al., 1996; Eiler et al., 1998.. The new Pb TS isotopic results demonstrate that on the scale of an individual volcano — Mauna Kea —

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Pb isotopes can display tightly constrained linear arrays ŽFig. 6.. It is unclear, at present, whether the Pb isotopic compositions from Mauna Loa also fall on linear arrays, given the problems encountered with replicate dissolution of some samples. Nevertheless, the linear arrays of Mauna Kea are parallel in 207 Pbr204 Pb– 206 Pbr204 Pb space and could reflect true isochrons or, alternatively, binary mixing lines.

4.1. Mantle isochrons The significance of arrays in 207 Pbr204 Pb– Pbr204 Pb space in oceanic island basalts, whether from individual islands or island groups, has already been discussed ŽTatsumoto, 1978; Chase, 1981.. Taken as a whole, ocean island basalts define a broad linear array whose slope implies an age of ; 1.8 Ga ŽTatsumoto, 1978.. Results from linear regression of the HSDP Pb isotope data are presented in Table 2. In 207 Pbr204 Pb– 206 Pbr204 Pb space, the ‘‘normal’’ Mauna Kea array has a slope of 0.1085 " 0.0089 which is within error of that of the ‘‘anomalous’’ Kea array Ž0.126 " 0.013.. These slopes, if interpreted as isochrons, correspond to ages of 1.78 " 0.14 and 2.05 " 0.18 Ga, respectively ŽTable 2.. In the case of Mauna Loa, the data display considerable scatter ŽFig. 5a,b. and do not provide age constraints, while for Mauna Kea the existence of two clearly distinct arrays ŽFig. 6. renders regression of the dataset as a whole questionable. Both Kea arrays are thus consistent with a common ‘‘age’’ of ; 1.9 Ga which might reflect the differentiation age of the Kea sourceŽs.. In principle, it is possible to test whether a linear array in 207 Pbr204 Pb– 206 Pbr204 Pb space represents an isochron using the additional information available from 208 Pbr204 Pb– 206 Pbr204 Pb systematics. If the 207 Pbr204 Pb– 206 Pbr204 Pb array represents a true isochron, the position on the array is dictated by the m value Ž'238 Ur204 Pb atomic. of the source. Further, if the 208 Pbr204 Pb– 206 Pbr204 Pb array is also linear, this means that either k Ž'232 Thr238 U atomic. is constant for all samples on the array, or that k varies linearly as a function of m ŽWhite et al., 1987.. 206

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Table 2 Derived Pb isotope regression parameters

Mauna Loa Mauna Kea Anomalous Mauna Kea Mauna Kea Žall.

n

207

Pbr206 Pb slope

xr2

t ŽGa.

208

Pbr206 Pb slope

xr2

k

k)

17 21 6 27

0.0811 " 0.0057 0.1089 " 0.0085 0.126 " 0.013 0.0527 " 0.0062

5.65 1.43 1.86 3.75

1.22 " 0.14 1.78 " 0.14 2.05 " 0.18 0.31 " 0.27

0.799 " 0.016 1.004 " 0.025 1.311 " 0.041 1.006 " 0.018

4.00 1.84 3.10 2.90

2.678 " 0.055 3.467 " 0.087 4.60 " 0.14 3.210 " 0.057

3.43 " 0.57 a – – 3.27 " 0.50 a,b

Regressions performed using the method of Williamson Ž1968.. Mauna Kea data include two double spiked analyses by Thirlwall Ž2000.. n: number of samples; xr2 : reduced chi-squared ŽMSWD.; k ) : atomic 232 Thr238 U ratio derived from measured ThrU Žwt... Average k ) in Hawaiian lavas is 3.18 " 0.40 ŽHemond et al., 1994; Cohen and O’Nions, 1994; Jochum and Hofmann, 1995; Sims et al., ´ 1995.. a Data source: Hofmann and Jochum Ž1996.. b Data source: Albarede ` Ž1996..

In principle, these possibilities can be evaluated by comparing k values inferred from Pb isotope systematics with 232 Thr238 U ratios derived from the measured Th and U concentrations in the lavas, designated here as k ) ŽTable 2, Fig. 8.. The ratio-

nale is that if k equals the source 232 Thr238 U ratio, such a source must be capable of generating lavas with the observed k ) upon melting. Trace element patterns of Hawaiian basalts seem to require that partial melting occurs in the presence of garnet Že.g.

Fig. 8. Evolution of k Žs232 Thr238 U. as a function of age, calculated from Pb isotope systematics, in Mauna Kea and Mauna Loa ŽTable 2.. The solid lines correspond to k values inferred from the 208 Pbr204 Pb– 206 Pbr204 Pb slopes of the two Mauna Kea arrays and Mauna Loa. The 207 Pbr204 Pb– 206 Pbr204 Pb age range for the two Kea arrays is highlighted by a vertical stippled band. Also shown Žopen circles. are 232 Thr238 U ratios, designated as k ) , calculated from measured concentrations of Th and U in HSDP lavas ŽML: Mauna Loa; MK: Mauna Kea. ŽHofmann and Jochum, 1996; Albarede, ` 1996.. The horizontal stippled band represents the range of k ) values found in Hawaiian lavas ŽHemond et al., 1994; Cohen and O’Nions, 1994; Sims et al., 1995; Jochum and Hofmann, 1995.. ´

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Hofmann, 1984; Feigenson et al., 1996.. Experimental garnetrmelt partition coefficients for Th and U Že.g. Beattie, 1993. predict that the melt ThrU ratio should be greater or equal to that in the source — in other words, k ) G k — and this sense of ThrU fractionation in Hawaiian lavas has been shown from U-series disequilibrium studies ŽCohen and O’Nions, 1994; Hemond et al., 1994; Sims et al., 1995.. ´ Fig. 8 shows that for a given 208 Pbr204 Pb– 206 Pbr204 Pb slope, k is actually not very sensitive to the age chosen. Using the 207 Pbr204 Pb– 206 Pbr204 Pb ages, we obtain for Mauna Loa a k value of 2.678 " 0.055, lower than the k ) value of 3.43 " 0.57 measured in the HSDP lavas ŽTable 2. ŽHofmann and Jochum, 1996.. In the case of Mauna Kea, the k values are 3.467 " 0.087 and 4.60 " 0.14 for ‘‘normal’’ and ‘‘anomalous’’ Mauna Kea, respectively ŽTable 2.. While the k value for ‘‘normal’’ Mauna Kea is consistent with the k ) value of 3.27 " 0.50 ŽAlbarede, ` 1996; Hofmann and Jochum, 1996., k of ‘‘anomalous’’ Mauna Kea is considerably higher. Taken together, the Pb isotope systematics of the two Kea arrays are puzzling. On the one hand, the 207 Pbr204 Pb– 206 Pbr204 Pb slopes of both arrays are within error, indicating a common ‘‘age’’ of ; 1.9 Ga. On the other hand, the inferred source k value of the ‘‘anomalous’’ array Ž; 4.6. is larger than k ) Ž; 3.5. measured in the lavas — that is, inconsistent with the expected sense of ThrU fractionation upon melting which requires that k ) G k . Consequently, the ‘‘anomalous’’ Kea array is unlikely to reflect a true isochron, throwing into question the notion of a common ‘‘age’’ of 1.9 Ga. A particularly important additional constraint comes from the average k ) of non-HSDP Hawaiian lavas which is 3.16 " 0.40 Ž2 s.d., n s 72., though this value is admittedly largely constrained by data from Kilauea and Mauna Loa ŽCohen and O’Nions, 1994; Hemond et al., 1994; Jochum and Hofmann, ´ 1995; Sims et al., 1995.. For any reasonable degree of partial melting, the uniformity of k ) suggests that source 232 Thr238 U ratios are relatively constant as well, lying close to ; 3.2. Thus, it appears that k for the three arrays plotted in Fig. 8 exhibit too much scatter overall to be consistent with a relatively uniform source 232 Thr238 U ratio. Therefore, the Pb isotope arrays are more reasonably interpreted in

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terms of binary mixing between components in a heterogeneous source rather than true isochrons. 4.2. Mixing lines The existence of two distinct linear arrays in Pb isotope space appears to indicate the presence of four endmember components in the source of Mauna Kea alone ŽFig. 6.. Such an inference is, however, in contradiction with current two component ŽBennett et al., 1996; Lassiter and Hauri, 1998; Blichert-Toft et al., 1999. or three component models for the Hawaiian plume ŽChen and Frey, 1983; West et al., 1987; Eiler et al., 1996; Hauri, 1996; Eiler et al., 1998.. Below, we consider a three-component model for Mauna Kea by, first, examining the relationships between Pb isotope ratios in 3D and, second, performing a principal component analysis of this dataset. Three different perspective views in 3D space are shown in Fig. 9. These views were chosen to illustrate the form of the Pb isotopic variation in Mauna Kea, and the relationships between the two Kea arrays defined in 2D plots ŽFig. 6.. In the view of Fig. 9a, the array appears to lie along a single ‘‘line’’, but in Fig. 9b, this ‘‘line’’ is resolved into two roughly parallel lines, which are seen in an end-on view in Fig. 9c. If both lines are considered together, the Mauna Kea data form a plane ŽFig. 9a.. This demonstrates, in the simplest interpretation, that at least three components are necessary to account for the Pb isotopic variation of Mauna Kea. In order to move and statistically quantify the Pb isotopic variation, we performed a principal component analysis ŽPCA. of the Mauna Kea dataset following the methods outlined by Albarede ` Ž1995.. Principal component analysis allows examination of the statistical variance and ‘‘clustering’’ of data in multi-dimensions, and therefore cannot be used directly to define the endmember components of a mixing model. For three independent Pb isotope ratios, PCA yields three orthogonal eigenvectors that describe the principal directions of variation in the dataset. Each of these eigenvectors passes through the point corresponding to the arithmetic mean of the dataset. In addition, the proportion of the total variance associated with each eigenvector — v1 , v2 or

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Fig. 9. Evaluation of a three component model from 3D plots of the Mauna Kea Pb TS data. Three perspectives are shown: Ža. the two Kea arrays form a plane seen side-on; Žb. the two Kea arrays are parallel; and Žc. the two Kea arrays seen edge-on form two distinct blobs.

v3 — is simply related to its corresponding eigenvalue ŽAlbarede, ` 1995.. If a single eigenvector explains a large fraction of the dataset variance, it would indicate that the data form a ‘‘line’’ in 3D, and can be modelled well as mixtures of two endmember components. This is actually what is found when the main Mauna Kea array and the anomalous Mauna Kea array are treated separately. For the main array, 96.8% of the dataset variance can be explained by the first vector v1 , with only 2.5% and 0.7% associated with v2 and v3 . For the anomalous Mauna Kea array, the corresponding values are similar: 97.5%, 2.5% and 0.1% of the variance. The situation is different if both arrays are taken together. The variances associated with v1 , v2 and v3 are: 87.0%, 12.4% and 0.7%, respectively. Thus, more variance is associated with v2 and slightly less with v1 , while v3 makes a negligible contribution and can reasonably be considered as ‘‘noise’’. Fig.

10 shows the data transformed to lie perpendicular to each one of the eigenvectors. The large circles represent one standard deviation of the dataset, and the projected locations of the three original Pb isotope axes are also plotted Žsee Albarede ` Ž1995... It can be seen that the principal direction of variation v1 is common to both Mauna Kea arrays, while the ‘‘offset’’ between the two arrays is mostly in the v2 direction. The ‘‘offset’’ occurs mainly in the 207 Pbr204 Pb ratio, as can be appreciated from the location of the projected 207 Pbr204 Pb axis in Fig. 10c or, more readily, from Fig. 6. On the basis of these considerations, a three component mixing model Žtwo significant eigenvectors. can adequately account for the observed Pb isotope variation in Mauna Kea ŽFigs. 9 and 10.. Note that PCA provides no information on the location or relative geometry of the putative three components involved but only stipulates that they lie on a plane containing the eigenvectors v1 and v2 .

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Fig. 10. Principal component analysis ŽPCA. of the Mauna Kea Pb isotope dataset. In Ža., the data are plotted projected into the plane containing the two principle eigenvectors v1 and v2 , and perpendicular to the third v3 . In Žb. and Žc., views are shown containing eigenvectors v1 and v3 , and v2 and v3 , respectively. The mean of the dataset projects to the origin, while circles of unit radius represent one standard deviation. Also plotted are the directions corresponding to the original axes in Pb isotope space Žsee: Albarede ` Ž1995. for details..

Because the two arrays form nearly parallel lines in Pb isotope space ŽFigs. 6, 9c and 10c., a three

component mixing model would only be tenable under rather special conditions, such as have been

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recently discussed by Douglass and Schilling Ž2000.. For example, pre-mixing of the three components in constant proportions could create four intermediate endmembers, which then mix in pairs, in a second stage, to produce the two observed Kea arrays. Since the two arrays are also parallel, the pre-mixed endmembers of the two arrays must differ only in their proportion of one endmember, lying in the v2 direction ŽFig. 10a,c.. Given the condition of pre-mixing required by such a three-component model, the question of whether three or four components exist would be largely semantic. Alternatively, the linear arrays could be created without an initial stage of pre-mixing between the three components. If the proportions of the mixing endmembers changed in a highly systematic and progressive fashion, this could lead to linear subarrays within the ternary mixing plane which do not necessarily point towards any of the endmember compositions Žsee Douglass and Schilling, 2000.. However, such mixing could only occur under exceptional circumstances, and any suitable underlying physical mechanism would be highly contrived. A three-component mixing model therefore requires special, ad hoc circumstances to work, though it is certainly correct from a mathematical stand point. To avoid special pleading, we argue that the two Mauna Kea arrays are far more easily and naturally explained as due to mixing, in pairs, between four source endmembers. The fact that ; 97% of the variance of each Kea array can be ascribed to a single eigenvector strongly suggests that each array is a mixture of only two discrete components. The implication is that either the melting of the source or the volcanic plumbing system beneath Mauna Kea taps only two source components at a given time. Furthermore, this pair of source components must be able to switch in order to generate both Mauna Kea Pb isotope arrays.

5. Nature and origin of the CKea componentD We have shown that the Pb isotopic variations in Mauna Kea alone can be accounted for by mixing between three components provided that mixing occurs under special circumstances. The questions that

can be raised now are Ž1. whether the ‘‘Kea component’’, as defined by Eiler et al. Ž1996, 1998., could explain the isotopic variability observed in Mauna Kea, and Ž2. whether this component can be identified with the present-day Pacific oceanic crust or lithosphere. Fig. 11 shows the Pb isotopic compositions of the endmember components defined by Eiler et al. Ž1998, 1996., together with the Pb TS data on HSDP lavas, and those obtained on East Pacific Rise ŽEPR. MORB glasses ŽGaler et al., 1999.. Three observations can be made: Ž1. Most of the Mauna Kea lavas have higher 207 Pbr204 Pb and 208 Pbr204 Pb than those of the ‘‘Kea component’’, even taking into account the analytical uncertainty of conventional Pb isotopic measurements Ž; 1‰ relative. on which the characteristics of the latter are based; Ž2. A significant portion of the TS Pb data lies outside the triangular space defined by the three components ‘‘Kea’’, ‘‘Loihi’’ and ‘‘Koolau’’; Ž3. For a given 206 Pbr 204 Pb, EPR MORB have higher 207 Pbr204 Pb and lower 208 Pbr204 Pb than any of the Mauna Kea lavas analysed. Fig. 11 shows that, in order to preserve a specific ‘‘Kea component’’ for Hawaiian volcanism in general, this component would have to be moved to a substantially higher 206 Pbr204 Pb value, along with some corresponding changes in 207 Pbr204 Pb and 208 Pbr204 Pb. We intend to address this issue more specifically elsewhere, where we will present high precision Pb isotope data for several other Hawaiian volcanoes Žcf. Abouchami et al., 1999a,b.. Here we will address the question whether the actual Pb isotopic compositions found on Mauna Kea could be derived from the present-day Pacific lithosphere. It is clear from Fig. 11 that there exists not only a significant mismatch between the new, high precision EPR data and the Mauna Kea data, but also there is virtually no overlap. Thus, unless the Pacific upper mantle beneath Hawaii is isotopically quite different from EPR samples, it cannot be the source of Mauna Kea lavas. Whether a lower oceanic crust, evolved from such an EPR region over ; 100 Ma, could be similar to Mauna Kea compositions, as proposed by Eiler et al. Ž1996., is slightly more difficult to evaluate. Such a crust would have to have a systematically higher UrPb but a lower ThrPb ratio than the underlying mantle. This is by no means

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Fig. 11. Comparison of the three component model for the Hawaiian plume with the new high precision data from Mauna Kea and Mauna Loa and East Pacific Rise MORB glasses ŽGaler et al., 1999.. Ža. 207 Pbr204 Pb vs. 206 Pbr204 Pb, Žb. 208 Pbr204 Pb vs. 206 Pbr204 Pb. The three endmember components — Loihi ŽL., Koolau ŽKo. and Kea ŽK. — derived from principal component analysis of published literature data on shield tholeiites are from Eiler et al. Ž1996, 1998.. A typical uncertainty of ; 0.05% per AMU is shown for the Kea component for comparison with the Pb TS data. A significant portion of the data lies outside the isotope space defined by the principal component analysis of previously published data ŽEiler et al., 1996, 1998.. Moreover, the endmember components defined by this principal component analysis bear no obvious relationship to the endmember compositions of the binary mixtures defined by the new high precision Pb data. Furthermore, these data provide no support for involvement of EPR-type oceanic lithosphere in the source of the Mauna Kea lavas.

impossible, but it would again require special pleading to preserve the high degree of linearity observed for Mauna Kea in Pb isotope space ŽTable 2., with slopes that are similar Žthough not identical. to those

of the EPR data. We therefore consider this possibility a remote one. On the basis of a multi-isotope ŽOs, Sr, Nd, Pb and O. dataset on gabbroic xenoliths from Hualalai

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Volcano, representative of the Pacific middle to lower oceanic crust, Lassiter and Hauri Ž1998. suggested that the underlying oceanic crust does not in fact provide an adequate endmember for Mauna Kea lavas. Furthermore, the low ´ Hf of the Kea component has been used to argue against the involvement of upper mantle material in the Hawaiian plume ŽBlichert-Toft et al., 1999.. In summary, the Pb isotopic data presented here show that the ‘‘Kea component’’, as defined by Eiler et al. Ž1996, 1998., cannot account for the Pb isotope characteristics of Mauna Kea lavas; in turn, Kea lavas are unlikely to represent melts derived from the present-day Pacific oceanic crust or lithosphere. Rather, binary mixing between four isotopically distinct Pb components seems at present to provide the best explanation for Mauna Kea, implying substantial heterogeneity within the Hawaiian plume and systematic tapping of these heterogeneities on the scale of a single volcano. Melting taps this heterogeneous source on a sufficiently small scale so that most of the time only two specific sources contribute to a given melt. The much longer stratigraphic record expected from HSDP-2 should reveal more clearly the approximate length Žor width. scale of these heterogeneities within the plume.

6. Isotope stratigraphy The pilot hole ŽHSDP-1. has offered the possibility of evaluating temporal geochemical variations, particularly in the Mauna Kea section which provides a ; 200 ka record of chemical and isotopic fluctuations in the sourceŽs. of the erupted lavas. Previously published isotopic data have shown that there are limited isotopic variations with depth in the Mauna Kea section of the core ŽLassiter et al., 1996; Lassiter and Hauri, 1998; Blichert-Toft and Albarede, ` 1999., compared to the large isotopic variability in the shorter ; 30 ka record of Mauna Loa ŽKurz et al., 1996.. Our Pb and Sr isotopic data complete the previous record of the HSDP core, providing a time series of isotopic variations to 1200 m shown in Fig. 12, together with the Nd and Hf isotopic stratigraphy for Mauna Kea ŽLassiter et al., 1996; Lassiter and Hauri, 1998; Blichert-Toft and Albarede, ` 1999..

Fig. 12. Depth variations of 206 Pbr204 Pb Ža., 87 Srr86 Sr Žb. and ´ Nd and ´ Hf Žc. values in the Mauna Kea section of HSDP-1 core. The error bars shown correspond to "0.3 ´ Nd units, "0.5 ´ Hf units and "4=10y5 for 87 Srr86 Sr; for 206 Pbr204 Pb, the error bar is smaller than the symbol size. 206 Pbr204 Pb ratios exhibit a general increase with increasing depth. Superimposed on this trend are fluctuations in 206 Pbr204 Pb ratios which contrast with the monotonic increase of ´ Nd and ´ Hf values from bottom to top of the core. Despite the 206 Pbr204 Pb variations as a function of depth, all Mauna Kea samples still fall on one of the two arrays shown in Fig. 5. Data sources: 206 Pbr204 Pb, 87 Srr86 Sr and ´ Nd : this study, Lassiter et al. Ž1996. and Lassiter and Hauri Ž1998.; ´ Hf : Blichert-Toft and Albarede ` Ž1999..

In the Mauna Kea section, fluctuations in Pbr204 Pb ratio, which display a total range from 18.35 to 18.60, are superimposed on a general trend of increasing ratios from top to bottom of the core ŽFig. 12.. The alkali basalts, which form the upper 50 m of the section, have generally lower 206 Pbr 206

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Pb ratios than the tholeiites, but still lie on the same Pb isotopic array ŽFig. 5.. Qualitatively similar fluctuations are seen in 87 Srr86 Sr. In contrast, ´ Nd Žq6.7 to 7.7. and ´ Hf Žq11.4 to q13.1. values appear to decrease monotonically from the top to the bottom of the section without Žanalytically significant. fluctuations ŽLassiter et al., 1996; Lassiter and Hauri, 1998; Blichert-Toft and Albarede, ` 1999.. The total range of ´ Nd and ´ Hf values for Mauna Kea is only about five times the analytical error. In contrast, this total range is about 25 times the analytical error for 207 Pbr204 Pb and about 140 times the analytical error for 206 Pbr204 Pb. Therefore, the apparently monotonic changes in Hf and Nd isotopes may in part reflect the limited analytical resolution of the latter systems. The stratigraphic distribution of 206 Pbr204 Pb ratios for Mauna Loa is shown in Fig. 13 for comparison. The amplitude of 206 Pbr204 Pb variation in Mauna Loa Žfrom 18.00 to 18.30. is similar to that seen in Mauna Kea lavas Ž18.35 to 18.60. and this variation seems to occur on even shorter length-scales than in Mauna Kea, in agreement with the large observed 3 Her4 He variability ŽKurz et al., 1996.. However, this may be an artefact resulting from the lower sampling resolution of the Mauna Loa section. If the sources of these volcanoes are produced by mantle recycling of ancient crustal material ŽHofmann and White, 1982., the apparently contrasting behaviour of fluctuating Pb and perhaps Sr isotopes

Fig. 13. 206 Pbr204 Pb variations as a function of depth for Mauna Loa section. Sample R-117, for which replicate dissolutions Žreferred as D1 and D2. yielded inconsistent results, is also plotted.

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on the one hand, and the monotonically changing Nd and Hf isotopes on the other, may be caused by the relative mobility of Pb, Sr andror Rb in the former crust during ridge-crest hydrothermal circulation ŽMichard et al., 1983; Michard and Albarede, ´ 1985; Peucker-Ehrenbrink et al., 1994; Chauvel et al., 1995; Valsami-Jones and Ragnarsdottir, 1997.. The linear correlation of 208 Pbr204 Pb vs.206 Pbr204 Pb ŽFig. 6b. indicates that the variability in 206 Pbr204 Pb and 208 Pbr204 Pb ratios is mostly the result of ancient Pb mobility, while Th and U remained relatively immobile during hydrothermal processes. Lutetium, Hf, Sm, and Nd are also relatively immobile, and the variations in LurHf and SmrNd ratios were probably caused by the relatively more subtle magmatic fractionation only.

6.1. Time series Significant discontinuities in the incompatible element ratio time series from the Mauna Kea section have been found to occur with a recurrence period of ; 30 ka ŽAlbarede, ` 1996., similar to those inferred from Pb and He isotopic variations ŽDePaolo, 1996.. Using the higher density sampling now available from the Kea section, we performed a spectral analysis of the radiogenic isotope data. The technique used was a Lomb periodogram ŽPress et al., 1992., which takes into account the uneven sample spacing, and also allows an estimate to be made of the significance levels of any spectral peaks found. Unfortunately, the limited age controls ŽSharp et al., 1996. permit analysis of only the lower, tholeiitic section of the HSDP core. Fig. 14 shows the significance levels obtained for the 206 Pbr204 Pb data for the lower part Ž380 to 1050 m. of the Mauna Kea section ŽFig. 13., using the age model outlined by DePaolo Ž1996.. Two peaks with significance levels greater than 0.7 are observed corresponding to periods of ; 200 and ; 50 ka. By comparison with Fig. 12a, the longer period peak reflects the general trend of increasing 206 Pbr204 Pb ratio from top to bottom of the Kea section, whereas the ; 50 ka peak accounts for most of the fluctuations superimposed on this trend. The ; 50 ka peak probably corresponds to the ; 30 ka period peak

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Fig. 14. Significance levels of spectral peaks plotted against frequency Žkay1 . for the 206 Pbr204 Pb variation in the Mauna Kea section Ž380–1050 m. shown in Fig. 12. Depths were converted into ages using the age model of DePaolo Ž1996., with the dataset comprising 29 points spanning an age of ; 128 ka. The power spectrum was calculated using a Lomb periodogram, since this approach allows handling of unevenly spaced data Žsee Press et al. Ž1992. for details.. The significance levels represent the probability that the spectral power at a given frequency is not ‘‘noise’’. The Nyquist frequency f c of the dataset is ; 0.113 kay1 but some information may be present above f c due to the random data spacing in time ŽFig. 13.. Two significant peaks seem to be present with periods of ; 200 and ; 50 ka.

identified by DePaolo Ž1996. using Fourier analysis on a more limited dataset interpolated onto a coarser, evenly spaced grid. The only significant spectral peak for Nd and Hf isotopes Žgiven the error of measurement. has a long period Ž) 150 ka. reflecting the increase in ´ Nd and ´ Hf towards the top of the core ŽFig. 12c.. The spectrum of 87 Srr86 Sr shows a strong peak at ; 5.0 ka Žnot shown., which is predominantly caused by the fluctuations in the bottom of the section ŽFig. 12b.. None of the spectra are continuous or are at all ‘‘fractal’’ in character. A more detailed analysis must await data from the HSDP-2 core, which will provide a longer stratigraphic record.

7. Conclusions This study reports high precision Ž2 sext.f 100 ppm. Pb isotopic data on Mauna Kea and Mauna

Loa samples from the Hawaiian Scientific Drilling Project. The data for the Mauna Kea section exhibit two distinct linear arrays in Pb isotope space, with ; 20% of samples falling on the second array. In the case of Mauna Loa, it remains unclear whether the Pb isotopic scatter observed reflects intrinsic sample heterogeneity or incomplete removal of Pb contamination. The linear Pb isotope arrays exhibited by the lavas from Mauna Kea are at present best accounted for by binary mixing, despite the apparent age of ; 1.9 Ga inferred from 207 Pbr204 Pb– 206 Pbr204 Pb relationships. The Pb isotopic structure exhibited by Mauna Kea lavas — namely two parallel arrays in both 2D and 3D space — combined with a principal component analysis, further demonstrate that a minimum of three source components are required to explain the Mauna Kea Pb isotope dataset. However, any three-component model would require very special mixing conditions. Therefore, a physically more

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plausible mixing model employs four components, two of which contribute to each of the two linear Pb isotopic arrays. The distinctive Pb isotopic characteristics of Mauna Kea lavas and EPR MORB glasses rule out any contribution from EPR-type oceanic mantle lithosphere to Mauna Kea. A contribution from aged, lower Pacific crust cannot be completely ruled out on the basis of Pb isotope data alone, but it would require special pleading with regard to the UrPb and ThrU ratios of this crust. Finally, the ‘‘Kea component’’ defined in the literature does not match the isotopic characteristics of the lavas studied here, nor does it correspond to the present-day Pacific lithosphere. These Pb isotopic measurements draw into question the validity of two and three component models for the Hawaiian plume, which have been proposed for explaining the isotopic variability of Hawaiian lavas. The three component endmembers described in the literature provide an inadequate description of the Pb isotope systematics of Mauna Loa and Mauna Kea. The stratigraphic variation of isotopic composition shows periodic fluctuations for Pb Žwith a significant spectral peak of period ; 50 ka. and, to a lesser extent, for Sr. These fluctuations contrast with the more monotonic stratigraphic changes of Nd and Hf isotopes. This difference may be related, in part, to the greater analytical resolution of the TS Pb isotope data, but it may also reflect the greater mobility of Pb Žand Sr or Rb. during processes that generated the ancient fractionation of parent– daughter ratios. The on-going phase of drilling ŽHSDP-2. is expected to provide a longer stratigraphic record of Mauna Kea, which should reveal the longer term history and evolution of the sources of this volcano.

Acknowledgements We dedicate this paper to Jerry Wasserburg whose constant quest for ever higher quality geochemical data has permanently changed the way geochemists value the quality of isotope analyses. In the spirit of this, we wish to contribute a small piece of Bernachon chocolate in return for a very large Hershey

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bar. We would like to thank D. DePaolo and E. Stolper for making available the HSDP samples, M. Sharma for extra sampling of the HSDP core, C. Jones for a preprint, and F. Albarede ` and J. Eiler for their thoughtful and critical reviews. S. BederkeRackzek and H. Feldmann are acknowledged for their technical assistance during this project. During this study, W. Abouchami was supported by DFG grant Ho1026-10.

References Abouchami, W., Galer, S.J.G., 1998. The provinciality of Pb isotopes in Pacific Fe–Mn deposits. Mineral. Mag. 62A, 1–2. Abouchami, W., Galer, S.J.G., Koschinsky, A., 1999a. Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: proxies for trace metal paleosources and paleocean circulation. Geochim. Cosmochim. Acta 63, 1489–1505. Abouchami, W., Hofmann, A.W., Galer, S.J.G., 1999b. Lead isotope anatomy of the Hawaiian plume. EoS, 1999 Fall Meeting 80, f1183. Albarede, ` F., 1995. Introduction to Geochemical Modeling. Cambridge Univ. Press, 543 pp. Albarede, ` F., 1996. High resolution geochemical stratigraphy of Mauna Kea flows from the Hawaiian Scientific Drilling Project. J. Geophys. Res. 101, 11841–11853. Beattie, P., 1993. Uranium–thorium disequilibria and partitioning on melting of garnet peridotite. Nature 363, 63–65. Ben Othman, D., White, W.M., Patchett, J., 1989. The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling. Earth Planet. Sci. Lett. 94, 1–21. Bennett, V., Esat, T.M., Norman, M.D., 1996. Two mantle–plume components in Hawaiian picrites inferred from correlated Os– Pb isotopes. Nature 381, 221–224. Blichert-Toft, J., Albarede, ` F., 1999. Hf isotopic compositions of the Hawaii Scientific Drilling Project core and the source mineralogy of Hawaiian basalts. Geophys. Res. Lett. 26, 935– 938. Blichert-Toft, J., Albarede, ` F., Frey, F., 1999. Hf isotopic evidence for pelagic sediments in the source of Hawaiian basalts. Science 285, 879–882. Chase, C.G., 1981. Oceanic island basalt Pb: two-stage histories and mantle evolution. Earth Planet. Sci. Lett. 52, 277–284. Chauvel, C., Goldstein, S.L., Hofmann, A.W., 1995. Hydration and dehydration of oceanic crust controls Pb evolution in the mantle. Chem. Geol. 126, 65–75. Chen, C.Y., Frey, F.A., 1983. Origin of Hawaiian tholeiite and alkalic basalt. Nature 302, 785–789. Cohen, A., O’Nions, R.K., 1994. Melting rates benath Hawaii: evidence from uranium series isotopes in recent lavas. Earth Planet. Sci. Lett. 120, 169–175. DePaolo, D.J., 1996. High-frequency isotopic variations in the

208

W. Abouchami et al.r Chemical Geology 169 (2000) 187–209

Mauna Kea tholeiitic basalt sequence: melt zone dispersivity and chromatography. J. Geophys. Res. 101, 11855–11864. Douglass, J., Schilling, J.-G., 2000. Systematics of three-component, pseudo-binary mixing lines in 2D isotope ratio space representations and implications for mantle plume–ridge interaction. Chem. Geol. 163, 1–23. Eiler, J.M., Farley, K.A., Stolper, E.M., 1998. Correlated helium and lead isotope variations in Hawaiian lavas. Geochim. Cosmochim. Acta 62, 1977–1984. Eiler, J.M., Farley, K.A., Valley, J.W., Hofmann, A.W., Stolper, E.M., 1996. Oxygen isotope constraints on the sources of Hawaiian volcanism. Earth Planet. Sci. Lett. 144, 453–468. Feigenson, M.D., Patino, L.C., Carr, M.J., 1996. Constraints on partial melting imposed by rare earth element variations in Mauna Kea basalts. J. Geophys. Res. 101, 11815–11829. Galer, S.J.G., 1997. Optimal triple spiking for high precision lead isotope ratio determination. Terra Nova 9, 441. Galer, S.J.G., 1999. Optimal double and triple spiking for high precision lead isotopic measurement. Chem. Geol. 157, 255– 274. Galer, S.J.G., Abouchami, W., 1998. Practical application of lead triple spiking for correction of instrumental mass discrimination. Mineral. Mag. 62A, 491–492. Galer, S.J.G., Abouchami, W., Macdougall, J.D., 1999. East Pacific Rise MORB through the Pb isotope looking-glass. EoS 80, F1086. Hauri, E., Lassiter, J.C., De Paolo, D.J., 1996. Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii. J. Geophys. Res. 101, 11793–11806. Hauri, E.H., 1996. Major-element variability in the Hawaiian mantle plume. Nature 382, 415–419. Hofmann, A.W., Feigenson, M.D., Raczek, I., 1984. Case studies on the origin of basalt: III. Petrogenesis of the Mauna Ulu eruption, Kilauea, 1969–1971. Contrib. Mineral. Petrol. 88, 24–35. Hemond, C., Hofmann, A.W., Heusser, G., Condomines, M., ´ Raczek, I., Rhodes, J.M., 1994. U–Th–Ra systematics in Kilauea and Mauna Loa basalts, Hawaii. Chem. Geol. 116, 163–180. Hofmann, A.W., Jochum, K.P., 1996. Source characteristics derived from very incompatible trace elements in Mauna Loa and Mauna Kea basalt, Hawaiian Scientific Drilling Project. J. Geophys. Res. 101, 11831–11839. Jochum, K.P., Hofmann, A.W., 1995. Contrasting ThrU in historical Mauna Loa and Kilauea lavas. Mauna Loa Revealed: Structure, Composition, History and Hazards. In: Rhodes, J.M., Lockwood, J.P. ŽEds.., Geophys. Monogr. Ser. vol. 92 Amer. Geophys. Union, Washington DC, pp. 307–314. Jones, C.E., Halliday, A.N., Rea, D.K., Owen, R.M., 2000. Eolian inputs of lead to the North Pacific. Geochim. Cosmochim. Acta 64, 1405–1416. Kennedy, M.J., Chadwick, O.A., Vitousek, P.M., Derry, L.A., Hendricks, D.M., 1998. Changing source of base cations during ecosystem development, Hawaiian islands. Geology, 1015–1018. Kurz, M.D., Kenna, T.C., Kammer, D.P., Rhodes, J.M., Garcia, M.O., 1995. Isotopic evolution of Mauna Loa volcano: a view

from the submarine Southwest Rift zone. Mauna Loa Revealed: Structure, Composition, History and Hazards. In: Rhodes, J.M., Lockwoods, J.P. ŽEds.., Geophys. Monogr. Ser. 92 Amer. Geophys. Union, Washington DC, pp. 289–306. Kurz, M.D., Kenna, T.C., Lassiter, J.C., DePaolo, D.J., 1996. Helium isotopic evolution of Mauna Kea Volcano: first results from the 1-km drill core. J. Geophys. Res. 101, 11781–11791. Lassiter, J.C., DePaolo, D.J., Tatsumoto, M., 1996. Isotopic evolution of Mauna Kea volcano: Results from the initial phase of the Hawaiian Scientific Drilling Project. J. Geophys. Res. 101, 11769–11780. Lassiter, J.C., Hauri, E.H., 1998. Osmium–isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume. Earth Planet. Sci. Lett. 164, 483–496. McDonough, W.F., Chauvel, C., 1991. Sample contamination explains the Pb isotopic composition of some Rurutu island and Sasha seamount basalt. Earth Planet. Sci. Lett. 105, 397–404. Michard, A., Albarede, ` F., 1985. Hydrothermal U uptake at ridge crests. Nature 317, 244–246. Michard, A., Albarede, ` F., Michard, A., Minster, J.-F., Charlou, J.L., 1983. Rare-earth elements and uranium in high temperature solutions from East Pacific Rise hydrothermal vent field Ž138N.. Nature 303, 795–797. Press, W.H., Teukolsky, S.A., Vetterling, W.T., Flannery, B.P., 1992. Numerical recipes in C. 2nd edn. Cambridge Univ. Press. Peucker-Ehrenbrink, B., Hofmann, A.W., Hart, S.R., 1994. Hydrothermal lead transfer from mantle to continental crust: the role of metalliferous sediments. Earth Planet. Sci. Lett. 125, 129–142. Rhodes, J.M., Hart, S.R., 1995. Episodic trace element and isotopic variations in historical Mauna Loa lavas: implications for magma and plume dynamics. Mauna Loa Revealed: Structure, Composition, History and Hazards. In: Rhodes, J.M., Lockwood, J.P. ŽEds.., Geophys. Monogr. Ser. vol. 92 Amer. Geophys. Union, Washington DC, pp. 263–288. Sims, K.W.W., De Paolo, D.J., Murrell, M.T., Baldridge, W.S., Goldstein, S.J., Clague, D.A., 1995. Mechanisms of magma generation beneath Hawaii and Mid-Ocean ridges: uraniumrthorium and samariumrneodymium isotopic evidence. Science 267, 508–512. Sobolev, A.V., Hofmann, A.W., Nikogosian, I.K., Shimizu, N., Chaussidon, M., 1999. New evidence for oceanic crust recycling in the Hawaiian plume: trace elements and isotopes of melt inclusions in olivines from Mauna Loa. J. Conf. Abs. 4, 344. Stolper, E.M., DePaolo, D.J., Thomas, D.M., 1996. Introduction to special session: Hawaiian Scientific Drilling Project. J. Geophys. Res. 101, 11593–11598. Tatsumoto, M., 1978. Isotopic compositions of lead in oceanic basalt and its implication to mantle evolution. Earth Planet. Sci. Lett. 38, 63–87. Thirlwall, M.F., 2000. Inter-laboratory and other errors in Pb isotope analyses investigated using a 207 Pb– 204 Pb double spike. Chem. Geol. 163, 299–322. Valsami-Jones, E., Ragnarsdottir, K.V., 1997. Controls on

W. Abouchami et al.r Chemical Geology 169 (2000) 187–209 uranium and thorium behaviour in ocean floor hydrothermal systems: examples from the Pindos ophiolite, Greece. Chem. Geol. 135, 263–274. West, H.B., Gerlach, D.C., Leeman, W.P., Garcia, M.O., 1987. Isotopic constraints on the origin of Hawaiian basalts from the Maui volcanic complex. Nature 330, 216–220.

209

White, W.M., Hofmann, A.W., Puchelt, H., 1987. Isotope geochemistry of Pacific mid-ocean ridge basalt. J. Geophys. Res. 92 ŽB6., 4881–4893. Williamson, J.H., 1968. Least-squares fitting of a straight line. Can. J. Phys. 46, 1845–1847.