Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components1,2 hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa3. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume4,5,6,7,8,9,10. Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high µ = 238U/204Pb) and DM (depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low-3He/4He components in the plume mixing with a common high-3He/4He component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high-3He/4He plume matrix that hosts, and mixes with, several distinct low-3He/4He components.
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We thank J. Natland, P. Hall and M. Regelous for discussions, and R. Carlson for access to analytical facilities. Comments from B. Hanan and K. Harpp improved the manuscript. M.G.J. acknowledges grants from the NSF that funded this research: OCE-1061134, OCE-1153894, EAR-1348082 and EAR-1145202.
The authors declare no competing financial interests.
Extended data figures and tables
The range of ages for each location (subaerial or submarine dredge) is provided in a box at the periphery of the map, and a yellow line connects each location with the respective age data; not all samples with geochemical data have age data (indeed, most Samoan samples with geochemical data, submarine and subaerial, do not have age constraints). Dredge locations are labelled with a red line: dredges from the 1999 AVON2/3 cruise aboard the RV Melville17 have dredge numbers less than 100, and dredges from the 2005 ALIA cruise aboard the RV Kilo Moana18,19,39 have dredge numbers greater than 100. Samples collected on land were taken from the five Samoan islands (and are labelled with yellow stars: Savai’i subaerial, Upolu subaerial, Tutuila subaerial, Ta’u subaerial and Ofu subaerial). Malumalu and Vailulu’u seamount ages are based on uranium-series disequilibrium, and therefore maximum ages are provided65,66. Upolu subaerial lavas include both rejuvenated series (which bracket the younger limit of ages) and the shield series (which bracket the older limit of ages); poor outcrop exposure on the highly vegetated Samoan islands can make designation of the volcanic stages difficult (particularly if geochemical data are not available for the hand sample), and an average age for the rejuvenated or shield stages on Upolu is therefore not provided. Rejuvenated lavas are present on Tutuila, but ages are not available in the literature. All reported subaerial lavas from Savai’i are rejuvenated, indicating that the island has been covered with a veneer of rejuvenated volcanism21,28. Rejuvenated volcanism has been observed during historical times on Savai’i, which was last active from 1905–1911 (ref. 67); error bars are not provided for the oldest Savai’i subaerial sample in ref. 17. Submarine samples dredged off the coast of Savai’i (D114, D115 and D128) and from Tisa seamount were dredged distal to the Upo lineament and may not belong to this lineament. All available ages for Samoan islands and seamounts are provided in refs 17, 18, 39, 40, 41, 65, 66, 68 and 69. (Ma, million years.)
Extended Data Figure 2 A three-dimensional presentation of the Pb-isotopic groups shows that they converge on the high 3He/4He common component region.
99% confidence intervals (appearing as ‘tubes’) around the best-fit lines through each of the four data groups—Malu lineament (pink tube), Vai lineament (dark blue), subaerial Upo lineament (yellow) and rejuvenated lavas (light blue)—are shown in three-dimensional Pb-isotopic space. The composition of the common component region is modelled as an ellipsoid (grey) that is defined by the 2σ variance around the average in the Pb-isotopic compositions for samples with 3He/4He >20 Ra. In three-dimensional Pb-isotopic space, the 99% confidence intervals around each of the best-fit trend lines overlap with the ellipsoid that encompasses the common component region. Each tube represents an estimate of the error around the best-fit trend to the data defining each geographic lineament. The tube therefore encloses the set of all possible mixing arrays associated with a given geographic lineament. Since all the tubes intersect the ellipsoid of the common component region, statistically a range of mixing arrays exists for each geographic lineament that passes through the common component region. This result is consistent with the compositional data of the four lineaments mixing with the high-3He/4He common component.
Extended Data Figure 3 The isotopic composition of the four Samoan data groups are shown in Nd and Pb isotopic spaces.
In both panels, the high-3He/4He common component region is modelled by a grey ellipse that defines the 2σ variance around the average of the heavy radiogenic isotopic compositions of Samoan lavas with 3He/4He > 20 Ra. The left panel shows the four data groups identified in Pb-isotopic space (Fig. 2) in a plot of 143Nd/144Nd versus 206Pb/204Pb. Samples for which Pb-isotopic ratios were measured by high-precision techniques (Pb-spiked samples run by TIMS and samples run using Tl-addition by MC-ICP-MS) are shown as large symbols (where estimated 2σ external uncertainties are smaller than the symbols17,19,20,21,29,38), and unspiked Pb-isotopic TIMS data are shown as small symbols (where estimated 2σ external uncertainties are equal to or better than ±0.076 for the 208Pb/204Pb ratio, as shown14,17,48,51). The right panel shows the four data groups identified in Fig. 2 in a plot of 206Pb/204Pb versus 207Pb/204Pb. Samples for which Pb-isotopic ratios were measured by high-precision techniques (Pb-spiked samples run by TIMS and samples run using Tl-addition by MC-ICP-MS) are shown as large symbols (where estimated 2σ external uncertainties are smaller than the symbols, except for samples run on the P54 at Carnegie, where estimated 2σ external precision error bars are shown on the individual data points, as reported in the Methods); unspiked Pb-isotopic TIMS data are shown as small symbols (where estimated 2σ external uncertainties are equal to or better than ±0.019 and ±0.023 for 206Pb/204Pb and 207Pb/204Pb, respectively, as shown). 99% confidence intervals around the best-fit lines through each data group overlap with the high-3He/4He common component region. Symbols are the same as in Fig. 2 of the main text. The MORB average composition is from ref. 55. The HSDP-2 drill core data are from refs 24 and 30. See Supplementary Table 4 for a compilation of the Samoan data shown; Alexa data are from ref. 29.
Extended Data Figure 4 Various geochemical signatures show clear trends with increasing distance from the common component region in Pb-isotopic space.
The top panel shows a plot of versus Ti/Ti*. Samoan lavas with the highest 3He/4He have the highest Ti/Ti* and the lowest values; this follows from an earlier observation that Ti/Ti* correlates with 3He/4He in Samoan lavas, and the high-3He/4He mantle reservoir has elevated Ti/Ti* (ref. 60). Ti/Ti* is defined in ref. 19. Only lavas with MgO > 7 wt% are shown, to avoid the effects of fractional crystallization of trace phases that might fractionate the trace element ratios. A sample with high MnO from Soso (ALIA110-39) is excluded owing to a high degree of alteration. ALIA-115-07, which is highly altered, is also excluded, as are all samples from ALIA Dredge 118. Samples with He concentrations <10−9 cm3 of 4He at STP per gram of sample (olivine) are excluded. Additionally, only shield-stage lavas are plotted. The middle panel shows versus 143Nd/144Nd. 143Nd/144Nd shows systematic behaviour in each data group moving away from the common component region (that is, with increasing ) in Pb-isotope space. Data from subaerial Upo-lineament lavas (yellow) exhibit increasing 143Nd/144Nd with increasing distance (higher ) from the common component region, and this supports the hypothesis that the subaerial portion of the Upo lineament samples a depleted mantle (DM) component similar to that found in the Alexa seamount and Hawaii. The other data groups (from the rejuvenated lavas and the Vai and Malu volcanic lineaments) all exhibit lower (more enriched) 143Nd/144Nd with increasing distance from the common component region. Finally, the middle panel shows that 143Nd/144Nd exhibits the least amount of variability in the common component region—where is zero—as the four isotopic groups converge on a common component with relatively homogeneous isotopic characteristics. The bottom panel shows versus 3He/4He (also shown in Fig. 2 of the main text) for comparison with the other panels. Symbols are the same as in Fig. 2. The MORB average composition is from ref. 55. The HSDP-2 drill core data are from refs 24 and 30. When calculating Ti/Ti*, only data obtained by ICP-MS (except Ti, which is measured by X-ray fluorescence) are used. See Supplementary Table 4 for sources of the Samoan data; Alexa data are from ref. 29.
Vai-lineament lavas exhibit the highest (U+Th)/Pb values in Samoa, consistent with a HIMU signature. Such high (U+Th)/Pb values are consistent with the radiogenic Pb-isotopic compositions in Vai-lineament lavas and similar to the high (U+Th)/Pb values observed in HIMU lavas. Samoan rejuvenated lavas, which have an EM1 signature, have high Ba/Th (and Ba/Sm and Ba/Nb); this Ba-enrichment matches the positive Ba-anomalies observed in EM1 endmember lavas from Pitcairn29. Highly altered samples and samples with low MgO are excluded (as described in Extended Data Fig. 4). Ref. 17 identified Upolu sample U10 as an outlier in many isotope and trace element spaces. Symbols are the same as in Fig. 2 of the main text. Only data obtained by ICP-MS are shown. See Supplementary Table 4 for a compilation of the Samoan data shown.
Extended Data Figure 6 The 3He/4He ratios of Samoan lavas are shown in colour (warmer colours represent higher 3He/4He) to show the distribution of 3He/4He ratios in 208Pb/204Pb versus 206Pb/204Pb isotopic space.
Lavas with the highest 3He/4He tend to cluster near the region in Pb-isotopic space where the four Pb-isotopic data groups converge, and lavas with lower 3He/4He tend to plot farthest from the common component region. Samples with <10−9 cm3 of 4He at STP per gram of sample (olivine) are excluded. See Supplementary Table 4 for a compilation of the Samoan data shown.
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Jackson, M., Hart, S., Konter, J. et al. Helium and lead isotopes reveal the geochemical geometry of the Samoan plume. Nature 514, 355–358 (2014). https://doi.org/10.1038/nature13794
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