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Sr and Nd isotopic compositions of the single Meiji sample analysed by Keller et al.

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Age-corrected Pb isotope compositions lie at the unradiogenic end of the array defined by young lavas from the Hawaiian Islands in Fig. The Meiji sample analysed by Keller et al. The age correction for the Meiji lavas is relatively large 85 Ma , and some uncertainty in the age correction for these highly altered and strongly leached samples may account for the apparent scatter in Fig. Initial Pb isotope compositions of leached Emperor Seamount lava samples. Stars represent data for unleached rock powders of samples , and , and connecting lines indicate data for leached powders of the same samples.

Also plotted are conventional Pb isotope data from Keller et al. Small triangles and small circles are data from Keller et al. Mahoney et al. New high-precision triple-spike isotope measurements Abouchami et al. Galer et al. Our Sr isotope data for these seamounts are similar to those measured by Lanphere et al. This is probably due to the fact that our measurements were carried out on leached sample powders, and therefore more closely reflect the original magmatic values.

The single Suiko sample analysed by Keller et al. Alkalic lavas generally have less radiogenic Pb isotope compositions than tholeiitic lavas from the same seamount. An alkalic basalt from Suiko Seamount, as well as tholeiitic and alkalic lavas from seamounts younger than 60 Ma Ojin, Koko, Yuryaku and Daikakuji lie within the field for Hawaiian lavas. Tholeiites from Meiji Seamount 85 Ma have low concentrations of incompatible elements, and depleted trace element and Sr isotope ratios, compared with young Hawaiian Islands tholeiites.

At 81 Ma, when Detroit Seamount was built, Hawaiian magmatism was even more depleted. Incompatible trace element compositions and Sr and Nd isotope compositions of Detroit tholeiites are unlike those of all other Hawaiian lavas, and are similar to those of Pacific N-MORB. Tholeiitic and alkalic lavas from Emperor Seamounts younger than 65 Ma Suiko, Nintoku, Ojin, Koko, Kimmei, Yuryaku and Daikakuji have trace element compositions that lie within the range of young lavas from the Hawaiian Islands. Between 42 Ma and the present, there has not been any systematic temporal variation in trace element or isotopic composition of Hawaiian magmatism.

The isotope variation within lavas from the Hawaiian Islands has often been explained in terms of mixing of least three end-member components Staudigel et al. However, recent high-precision Pb isotope data indicate that, in detail, many more than three end-members would be required Abouchami et al. The same sources may therefore have contributed to Hawaiian magmatism during this period. However, Abouchami et al. Our Pb isotope data suggest further that the Kea component is unlikely to be aged Pacific lithosphere, as suggested by Tatsumoto , because Suiko Seamount was built on oceanic lithosphere that was only 40 Myr old compared with 90— Myr old beneath Mauna Kea.

If the depleted compositions of the oldest Emperor lavas are due to mixing of Hawaiian plume material with the upper-mantle source of MORB, either by entrainment of Pacific upper mantle into the Cretaceous Hawaiian plume, or by plume—spreading ridge interaction see below , then depleted Pacific-type upper mantle will be one component in the source of these lavas.

However, the isotopic compositions of the Meiji and Detroit tholeiites are not entirely consistent with such mixing. In summary, the differences in isotopic composition between the depleted Detroit tholeiites and lavas from both active and extinct Pacific spreading centres are not readily explained by mixing between Hawaiian plume mantle and depleted Pacific upper mantle. We suggest that the depleted mantle component that contributes to Detroit and Meiji lavas may be intrinsic to the Hawaiian plume. A depleted plume component has also been identified in intra-plate lavas from the Galapagos Hoernle et al.

A possible explanation for the unusual compositions of the oldest Emperor Seamount lavas is that the composition of the mantle ascending in the Hawaiian plume has changed over time. On the other hand, tholeiitic lavas as depleted as those from Detroit Seamount are rare on other intra-plate seamount chains, and have not been found on the Hawaiian Chain. Nevertheless, it is difficult to rule out a change in source composition as an explanation for the temporal variations in Hawaiian magmatism. Class et al.


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The Hawaiian Islands and the Hawaiian Seamounts are situated on oceanic lithosphere that was 80— Myr old at the time of intra-plate magmatism Caplan-Auerbach et al. The age of the ocean floor beneath the northernmost Emperor Seamounts is not well constrained, because it was formed during the Cretaceous Quiet Period and therefore lacks magnetic lineations. Data from Caplan-Auerbach et al. Plumes are known to influence the composition of lavas erupted at nearby spreading ridges Schilling et al.

However, the physical process of plume—ridge interaction is not well understood. The ages of individual seamounts of the Emperor Chain increase progressively from south to north, which indicates that the oldest seamounts were not formed in a plume channel. Bi-directional flow and mixing of material between the plume and the ridge axis is an unlikely explanation for the compositions of the Meiji and Detroit tholeiites, because the highly depleted trace element and isotopic compositions of the latter Fig.

Alternatively, increased entrainment of depleted upper-mantle material into a plume may occur when the plume is close to a spreading centre Keller et al.

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However, neither of these mechanisms can explain satisfactorily the isotopic compositions of the oldest Emperor lavas. As discussed previously, assuming that the North Pacific mantle at 80 Ma was similar in composition to that sampled along the length of the EPR and taking into account the effects of radioactive decay , the combined Sr—Nd and Nd—Hf isotopic compositions of Detroit and Meiji lavas appear to be inconsistent with mixing between Pacific depleted upper mantle and Hawaiian plume mantle.

This age difference would correspond to a plume—ridge distance of over km at the time the southern Emperor Seamounts were constructed. Intra-plate lavas erupted onto younger, thinner lithosphere are produced by larger mean degrees of melting, at shallower average depth, than melts produced beneath thicker lithosphere.

Both the degree and the depth of melting influence the chemistry of intra-plate magmas. Variations in the depth of melting may also influence the trace element chemistry of intra-plate lavas, according to how much of the melting occurs within the stability field of garnet Ellam, ; Haase, The trace element compositions of the Detroit tholeiites indicate that they are the product of relatively high degrees of mantle melting, and that much of the melt was generated at low pressure, within the stability field of spinel Fig. Larger mean degrees of melting beneath thin oceanic lithosphere may to some extent explain the low incompatible element concentrations of the Detroit and Meiji lavas.

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On the other hand, there is no indication from the volumes of Hawaiian—Emperor volcanoes that the average degree of mantle melting was greater before 60 Ma Fig. Clearly, variable melting of a homogeneous source cannot explain the variations in highly incompatible trace element and isotope ratios in Emperor Seamount lavas. We suggest instead that the temporal variations in Emperor lava composition may be the result of variable degrees of disequilibium melting of heterogeneous plume mantle as a result of variations in the thickness of the overlying lithosphere Fig.

Phipps Morgan has argued that disequilibrium melting of a heterogeneous mantle may account for some of the isotopic heterogeneity observed in the lavas from individual oceanic islands, and the fact that these often define tube-like arrays in three-dimensional isotope space.

This hypothesis predicts variations in basalt chemistry with lithospheric thickness that are qualitatively similar to those observed along the ESC. In detail, the melt extraction trajectories created during this process are sensitive to the compositions and ease of melting of the various source components, although it is interesting that the Detroit lavas plot close to the end of the melting trajectory estimated for Hawaiian plume mantle by Phipps Morgan We suggest that the depleted compositions of the Detroit lavas are the result of melting a relatively refractory component contained within the ascending plume mantle Fig.

This depleted component does not contribute to younger Hawaiian lavas, which were formed by lesser degrees of melting beneath thicker lithosphere. Interestingly, a refractory plume component, which is chemically and isotopically depleted yet distinct from the upper-mantle source of MORB, has been identified in lavas from the Galapagos Hoernle et al. Schematic diagrams illustrating how the thickness of the lithosphere may have influenced the incompatible trace element and isotope compositions of Hawaiian—Emperor lavas. A greater proportion of the refractory, incompatible element depleted material within the ascending plume mantle is melted, when the plume is situated beneath younger, thinner lithosphere.

If lithospheric thickness is an important control on the trace element and isotope composition of intra-plate lavas, then geochemical variations should also occur along other seamount chains that were built upon lithosphere of variable age. Younger lavas from the Kerguelen Archipelago, which were erupted onto older, thicker lithosphere, are generally more alkalic and have more enriched trace element and isotope compositions Gautier et al. The age of the underlying crust is not well known, but increases from 1—4 Ma at Easter Island to 8—10 Ma at Salas y Gomes.

Sr, Nd and Pb isotope compositions of the lavas from seamounts at the eastern end of the chain, which were formed on older, thicker lithosphere, tend to be more enriched Cheng et al. The age of the underlying lithosphere, and even the exact location of the plume, is not well known. Since 33 Ma, the Louisville plume has been situated on oceanic crust that was 45—52 Myr old at the time of magmatism, whereas estimates of the age of the crust underlying the older 33—66 Ma seamounts at their time of formation range from 50 to 80 Ma Lonsdale, b ; Watts et al.

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The age-corrected isotopic compositions of lavas from the Louisville Seamounts show little variation Cheng et al. The isotope compositions of lavas erupted along the Pukapuka Ridge in the South Pacific vary with the age of the underlying sea floor, which was between 0 and 25 Myr old, at the time of intra-plate volcanism Janney et al. Lavas erupted onto younger sea floor were formed by larger degrees of melting, at lower pressures, and have more depleted compositions than lavas erupted onto older lithosphere. The Pukapuka Ridge was not formed by hotspot activity, but by lithospheric extension Sandwell et al.

Instead, these might be the result of variations in distance from the enriched South Pacific Superswell Janney et al. Alternatively, they could be the result of differences in the degree of melting of heterogeneous mantle beneath lithosphere of variable thickness. Intra-plate lavas associated with the Canary hotspot in the NE Atlantic were erupted upon sea floor that was between 90 and Myr old, and show little variation in isotopic composition Geldmacher et al. In summary, the geochemical variations along other seamount chains are consistent with lithospheric thickness being an important control on the extent of melting of plume mantle, and hence the compositions of the lavas produced.

However, detailed geochemical and geochronological studies of other seamount chains, together with more precise constraints on the age of the underlying sea floor, are required, before the influence of lithosphere thickness on the compositions of intra-plate lavas can be fully quantified.

Our hypothesis predicts that intra-plate lava chemistry will vary with the thickness of the lithosphere rather than with plume—ridge distance, as required by plume—ridge interaction. This implies that systematic geochemical and isotopic variations may be found along seamount chains that were constructed several thousand kilometres from the nearest spreading axis. Lavas from the oldest seamounts sampled Meiji and Detroit have depleted incompatible trace element and Sr—Nd isotopic compositions, compared with those of young lavas from the Hawaiian Islands.

Age-corrected Pb isotope compositions of most Emperor lavas lie within the field of young lavas from the Hawaiian Islands. The trace element and isotope compositions of these lavas vary with the age of the underlying oceanic Pacific lithosphere at the time of seamount magmatism.


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The oldest Emperor Seamount lavas, which were erupted onto relatively young lithosphere close to a former spreading centre, have relatively depleted incompatible trace element and isotope compositions. In contrast, younger Hawaiian—Emperor lavas were erupted onto older lithosphere, and have more enriched compositions.

Major and trace element compositions of Meiji and Detroit Seamount tholeiites indicate that they were formed by relatively large degrees of mantle melting, at lower pressures, compared with younger ESC tholeiitic lavas. We suggest that variable degrees of melting of a heterogeneous mantle may explain the temporal compositional changes in Hawaiian magmatism. When the Hawaiian plume was situated beneath young, thin lithosphere, melting was more extensive and extended to shallower depths. The melts produced had relatively depleted trace element and isotope compositions, because incompatible element depleted, more refractory source materials contributed more to the melting.

In contrast, lavas from the younger seamounts, which were built on older, thicker crust, are more enriched because they were produced by smaller degrees of melting, and so the compositions of the melts were dominated by the contribution from incompatible-element-rich, easily melted mantle materials.

Description

E-mail: m. We thank F. Frey, M. Garcia, an anonymous reviewer, and the editor M. Thirlwall for constructive comments, which improved the manuscript.


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Niu, J. Lassiter and I. Greig for the trace element analyses, and S. Bederke-Raczek and H. Feldmann for technical assistance. Oxford University Press is a department of the University of Oxford.

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