Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth’s mantle

Up to 10 per cent of the ocean floor consists of plateaux--regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.

Up to 10 per cent of the ocean ¯oor consists of plateaux 1 Ðregions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes.Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent.Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle.The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments.The basalts' low CaO/Al 2 O 3 and Ce/Pb ratios, their unradiogenic 206 Pb and 208 Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material.Also, the high Th/U ratios of the mantle source resemble those of plume magmas.Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments 2,3 .We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux.The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.
Variations of d 18 O in Hawaiian basalts indicate that their mantle source contains material that was once exposed to alteration at low to medium temperatures.The positive correlation between d 18 O and the 187 Os/ 188 Os ratio for the different Hawaiian volcanoes strongly suggests that the component altered at the lowest temperature (Koolau) has the largest time-integrated Re/Os ratio and therefore represents the upper layers of an ancient oceanic crust 4 .The other end-member of this correlation (Mauna Kea) was identi®ed with the lower gabbroic oceanic crust, a conjecture strongly supported by the discovery of Sr anomalies in melt inclusions from Mauna Loa lavas 5 .In addition, the rather radiogenic Hf in the Koolau component and the curvature of the Hf±Pb isotope correlation for Hawaiian volcanoes require the presence of ancient pelagic sediments in the source of Hawaiian basalts 6 .
More components have been identi®ed in the source of other hotspot basalts.The large U/Pb ratio (`HIMU'; that is, `high m', where m = 238 U/ 204 Pb) characteristic of St Helena and some Polynesian basalts has been ascribed to the presence of ancient sea-¯oor basalts depleted in Pb and enriched in seawater U by hydrothermal activity 3 .Yet other hotspots, such as Pitcairn, contain a more controversial component (EM-1) that has some trace-element and isotope characteristics reminiscent of pelagic and/or metalliferous sediments 2,3 .Alternatively, EM-1 has been compared with melts from the subcontinental lithospheric mantle (see discussion in ref. 7).Because of its nearly ubiquitous character in ocean island NATURE | VOL 408 | 7 DECEMBER 2000 | www.nature.combasalts, EM-1 is central to the understanding of mantle component topology 8,9 .Here we report an extensive set of geochemical data on Pleistocene basalts from Logudoro that represent an extreme case of EM-1 basalts.
The Logudoro volcanic ®elds are located in northwestern Sardinia, where the local thickness of the continental crust is 20±30 km (ref .10).In order to assess whether the underlying continental crust affected the geochemistry of these volcanic rocks, a contemporaneous sample from the Tyrrhenian Sea, very similar to the Logudoro samples and collected during Leg 107 of the Ocean Drilling Project at Site 654 11 , was also analysed.The geochemical characteristics of this sample do not differ from those of the Sardinian samples (see Supplementary Information).The isotopic compositions of Nd and Hf of the Logudoro volcanics and, to a lesser extent, their Sr isotope compositions, plot on the enriched extension of the normal mantle array (Fig. 1).Hf is not particularly radiogenic with respect to Nd and, unlike the situation for Hawaiian basalts 6 , does not plot towards the ®eld of pelagic sediments above the Hf±Nd array.The Nb/U and Ce/Pb ratios have been widely used to discriminate between oceanic basalts and those from the continental crust 12,13 .As such, they have become indicators of continental-crust in¯uence, either in the form of a source component or acquired by shallowlevel contamination.The average Nb/U ratio of Logudoro volcanics (48) is typical of uncontaminated oceanic basalts (47 6 10), whereas the average Ce/Pb ratio is lower (15 with respect to 25 6 5).The Ce/Pb ratios seem to be weakly correlated with 206 Pb/ 204 Pb (correlation coef®cient of +0.5), whereas a negative correlation would be expected from contamination by typical continental material (Ce/Pb , 5, 206 Pb/ 204 Pb .18).Lower continental crust with unradiogenic Pb and low Ce/Pb would be a potentially acceptable contaminant, but the similarity between the Pb isotope compositions of the ODP Leg 107 sample and the samples erupted in Sardinia does not support extensive incorporation of crustal Pb.
A petrogenetic model that does not alter the source geochemical characteristics of the Logudoro basalts through contamination by crust, including the presence of a terrigenous component in the mantle source, must therefore be found.The Logudoro lavas are essentially aphyric alkali basalts and trachybasalts with olivine and plagioclase microphenocrysts.Major-element features include Mg# between 46 and 64 and high Al 2 O 3 contents (15±17%; see Table 1 in Supplementary Information).The remarkably low CaO/Al 2 O 3 ratios (0.4) of these rocks are independent of the MgO contents.Such a relationship excludes substantial pyroxene fractionation during the petrogenesis of the lavas, and signals the abundance of a plagioclase-rich component in the source.We contend that this component consists of metagabbros, that is, ancient plagioclase-rich cumulates entrained to depth by ancient subduction and processed in the deep mantle.The unusual presence of an abundant plagioclase component in the source is also indicated by the excesses of four elements, Ba, Sr, Eu and Pb, that preferentially partition into this mineral during magmatic processes.Ba enrichment is particularly visible in a plot of Th/La versus Th/Ba 14 (Fig. 2), where the Logudoro volcanics fall on the extension of the Bushveld anorthosites 15 .The rather large correlation coef®cient (+0.63) between Sr/Eu* and Eu/Eu* (Fig. 3)Ðwhere Eu* is the Eu concentration estimated by interpolating between the neighbouring trivalent elements Sm and GdÐindicates that the Sr excess correlates with the positive Eu anomaly and therefore suggests control of the source composition by plagioclase fractionation.The 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios plot to the left of the geochron (Fig. 4b).The low time-integrated U/Pb and Th/Pb ratios inferred from the unradiogenic Pb isotope compositions concur with the low Ce/Pb ratios of the rocks, and con®rm that the mantle source of the Logudoro basalts was dominated by a plagioclase-bearing component for a long period of time.
The Pb isotope compositions are among the least radiogenic of all modern terrestrial basalts (Fig. 4).The combination of unradiogenic 206 Pb and normal 208 Pb further requires that the source had a high time-integrated Th/U ratio (k = 4.1), a feature found only in EM-1 basalts such as those from Pitcairn and a few Polynesian islands 3,16 .Pitcairn and Logudoro basalts otherwise have rather similar Sr, Nd and Hf isotope compositions (Fig. 1) and Ce/Pb and Nb/U ratios.Pitcairn has more radiogenic 208 Pb (Fig. 4a) and less radiogenic 207 Pb (Fig. 4b).The relative distributions of the most incompatible elements in the two series are similar.All the incompatible elements, except Ba and Sr, are depleted in Pitcairn relative to Logudoro basalts.We suggest that the source of these two series initially contained various gabbros and dolerites that were related to each other by low-pressure plagioclase fractionation.From the relative abundances of Sr, Ba and Pb in the two series, the protolith appears to be more cumulative and less differentiated for Logudoro than for Pitcairn basalts.The basaltic samples from the Walvis ridge and, to a lesser extent, Tristan da Cunha, resemble those from Pitcairn.
Although the EM-1 basalts from Logudoro and Pitcairn share evidence with some Hawaiian lavas for plagioclase control of their protolith 5 , differences in their isotopic compositions signal that the EM-1 sources formed in another manner.The presence of abundant plagioclase on the liquidus of the igneous rocks that eventually contributed to the source of EM-1 basalts is a strong argument against recycled subcontinental lithospheric mantle.As plagioclase does not reach saturation in basaltic liquids at pressures in excess of 1 GPa (see, for example, ref. 17), it cannot appear at mantle depths below the continental Moho (35±45 km).In contrast with Hawaiian volcanics, which show a relatively strong time-integrated depletion in highly incompatible elements (radiogenic Nd and Hf, unradiogenic Sr), the Logudoro samples, just as those from Pitcairn, show a slight time-integrated enrichment (Fig. 1).The high Th/U ratio of  It is clear that normal oceanic crust (N-MORB), either ancient or recent, is an unsuitable mantle source for the Logudoro and Pitcairn basalts.Because the above discussion has concluded that ancient gabbros and dolerites are components of the source, basaltic series with only a mild enrichment of the most incompatible elements should be sought.Plateau basalts, and, in particular, oceanic plateauxÐobviously abundant and formed continuously throughout the Earth's history, such as those from Ontong-Java or the Caribbean 18 Ðform a possible mantle source component that can account almost ideally for the characteristics of EM-1 basalts.Rareearth-element patterns that range from nearly ¯at to slightly enriched, the lack of a Nb anomaly, and low-pressure conditions of differentiation giving rise to shallow cumulates rich in plagioclase, are all conditions that should bring both the trace-element distributions and long-term isotopic patterns to resemble EM-1 basalt mantle sources.
Another potential mantle source of EM-1 basalts could be anomalous, geochemically enriched, oceanic crust.This material, referred to as E-MORB, does not represent a well de®ned component but is rather the enriched `tail' of the normal population of ridge basalts: ultra-depleted MORB represents the other `tail' of the distribution.E-MORB tends to form shallow ridges 19 and grades into plume material.The most compelling argument in favour of a plume source, and against any type of MORB source, for the EM-1 basalts is their time-integrated Th/U (k) value derived from their Pb isotope compositions; these k values vary between 4.0 and 4.5.In comparison, all fresh MORB glasses form a unimodal population with an average Th/U value of 2.5, in agreement with previous estimates 20 and with 230 Th isotope geochemistry 21 , whereas ocean island basalts have substantially higher values, such as observed for Re Âunion (4.0), Galapagos (3.7), Hawaii (3.0) and Iceland (3.3).Any material that is part of the EM-1 source falls outside the MORB range, whereas normal plume material displays much more suitable Th/U ratios.
In contrast to oceanic crust, large buoyant oceanic aseismic ridges and plateaux are not usually subducted 22 .Such ridges and plateaux are now considered as dominant sources of juvenile material accreting to the continents 1,23 .But small or thin plateaux, associated with minor plume heads, as well as small fragments of larger plateaux attached to large heavy sinking plates, have less buoyancy and may be entrained by subduction into the mantle where they subsequently evolve into a potential mantle component for basaltic melts in much the same way as does normal oceanic crust.As the different altered basaltic sections of the oceanic crust, including its overlying pelagic sediments, have been identi®ed as normal components of the source of hotspot basalts, it is not surprising that plateaux, which are just another common element of oceanic lithospheric plates, are also now found to be a component of oceanic basalt.The `graveyard' of the plates must accommodate all those plume heads formed during the entire history of the Earth that escaped incorporation into the continental crust.We propose here that Logoduro and Pitcairn basalts are melts from precisely the mantle component that represents recycled oceanic plateaux.
The geochemical characteristics of the EM-1 reservoir sampled by Logudoro and Pitcairn basalts can also mitigate the apparent imbalance between terrestrial heat ¯ow and heat production.This imbalance necessitates the presence of U, Th and K in inaccessible deep mantle layers in concentrations higher than those of the depleted upper mantle 24 Ðand even higher than those of the subducted oceanic plates imaged by high-resolution mantle tomography 25 .As discussed above, the EM-1 reservoir is unlike oceanic crust, but is consistent with mildly enriched material formed from plume head magmas similar to those of oceanic plateaux.The different shape and chemistry of plume heads compensate for their apparently small proportion with respect to that of oceanic crust.The crust beneath the Ontong-Java plateau is 4±6 times as thick as normal oceanic crust, and U and Th are 3±5 times more concentrated in its basalts than in normal MORB 18 .The preferential storage of plume material in the deep mantle may be one of the factors that help sustain the chemical and mineralogical zoning that has been inferred from seismic tomography and heat-¯ow measurements.

Figure 1 Figure 2
Figure 1 Plot of 143 Nd/ 144 Nd versus 87 Sr/ 86 Sr for basalts from Logudoro (Sardinia), Site 654 (sample 654-9-CC-00-04) and Pitcairn.Inset, e Hf versus e Nd for basalts from Lugodoro and Site 654 (sample 654-9-CC-00-04).The mantle array is contoured from a large amount of published and unpublished data, too numerous to reference here, but available from J.B.-T.Fields for Mn nodules and pelagic sediments are from refs 26 and 27, respectively.CHUR, chondritic uniform reservoir.

Figure 3
Figure 3 Plot of Sr/Eu* versus Eu/Eu* in the basalts from Logudoro.Eu* is the Eu concentration derived by interpolation between Sm and Gd.The high value of the correlation coef®cient (0.63) indicates that Eu and Sr excesses have a common cause, namely plagioclase accumulation in the gabbro that was the precursor of the mantle source of the Logudoro volcanics.

Figure 4
Figure 4 Lead isotope compositions of basalts from Logudoro (Sardinia), Site 654 (sample 654-9-CC-00-04), Pitcairn, Walvis ridge and Tristan da Cunha.a, 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb. b, 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb.The mantle array is contoured from hundreds of data points from the PETDB database for MORBs (http://petdb.ldeo.columbia.edu/petdb)and the GEOROC database for ocean island basalts.Data for Walvis ridge and Tristan da Cunha are from refs 29 and 30, respectively.The age adopted for the geochron is 4.55 Gyr.The unradiogenic character of 206 Pb and 208 Pb in the Logudoro basalts shows that the source had low U/Pb and Th/Pb ratios, a further indication that it formed by plagioclase accumulation.