6.2 The Nd)Sr isotope diagram

In the mid 1970s, studies of the origins of mantle heterogeneity were revolutionised by the application of Nd isotope analysis to young volcanic rocks (DePaolo and Wasserburg, 1976; Richard et al., 1976). DePaolo and Wasserburg plotted ¹⁴³Nd/¹⁴⁴Nd isotope ratios, in the form of , Nd (section 4.2), against ⁸⁷Sr/⁸⁶Sr, and found a negative correlation between them in oceanic and some continental igneous rocks (Fig. 6.11). Based on this evidence, they suggested that the formation of magma sources in the mantle involved the coupled fractionation of Sm)Nd and Rb)Sr, whereas some continental samples (which lay to the right of the main correlation line) could have been contaminated by radiogenic Sr in the crust.

Fig. 6.11. Plot of , Nd against Sr isotope ratio for ocean floor, ocean island, and continental basalts analysed before 1976. Arrow shows estimated Bulk Earth strontium. After DePaolo and Wasserburg (1976).

On the basis that the ‘Bulk Earth’ has a chondritic Sm/Nd ratio (section 4.2), DePaolo and Wasserburg used the intersection of the chondritic (zero) , Nd line with the mantle Nd)Sr correlation line to calculate an unfractionated mantle (i.e. Bulk Earth) ⁸⁷Sr/⁸⁶Sr ratio of 0.7045 (Fig. 6.11). By using the initial ⁸⁷Sr/⁸⁶Sr ratio of the solar nebula (0.699), calculated from the ‘Basaltic Achondrite Best Initial’ (BABI, section 3.2.4), and the present-day value from Fig. 6.11, they deduced an Rb/Sr ratio for the unfractionated mantle (now referred to as the Bulk Silicate Earth) of 0.029.

O’Nions et al. (1977) extended the ¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr correlation line in oceanic volcanics by analysing a larger suite of ocean island basalts. This included two samples from Tristan da Cunha with ¹⁴³Nd/¹⁴⁴Nd ratios lower than the Bulk Earth, indicative of a mantle source which is slightly enriched in light rare earths (LREE) relative to Bulk Earth. O’Nions et al. argued that enrichment of some mantle sources in Nd/Sm and Rb/Sr (and depletion of others such as MORB) could be explained by trace element partition between the melt phase and residual mantle during partial melting. In view of the long half-lives of Rb and Sm, they concluded that these heterogeneities had existed for long periods of time.

6.2.1 Box models for MORB sources

The observed depleted nature of the MORB source relative to Bulk Earth has very important implications for the evolution of the mantle, and is attributed to extraction of the continental crust from the mantle. This was first modelled by Jacobsen and Wasserburg (1979) and O’Nions et al. (1979), using calculations commonly termed ‘box models’.

In a box model, the Earth is divided into chemical reservoirs which may exchange matter, grow, shrink, etc., and whose evolution is modelled over the Earth’s 4.5 Byr history. Typical reservoirs or ‘boxes’ are the crust, mantle and core, although these may be subdivided, e.g. into upper and lower mantle. The Earth’s evolution is portrayed in some alternative box models in Fig. 6.12, which will be briefly discussed.

Fig. 6.12. Box models for the geochemical evolution of the mantle. Models 1a and 1b correspond to whole- or half-mantle depletion due to the extraction of continental crust. Models 2a and 2b show alternative types of mantle evolution: progressive growth of a constantly depleted mantle or progressive depletion of a constant-volume mantle.

O’Nions et al. (1979) examined two models of mantle differentiation and crustal growth (1a and 1b in Fig. 6.12). These were based on the numerical solution of upward and downward transport coefficients for several elements in 90 steps, each corresponding to 50 Myr of Earth history. The model was constrained by boundary conditions in the form of the composition of the primitive chondritic mantle 4.55 Byr ago and the estimated composition of the outermost 50 km of the Earth (including the continental and oceanic crust) at the present day. In Fig. 6.13 the results for ⁸⁷Sr/⁸⁶Sr evolution are shown for cases where (a) the whole mantle is depleted by the extraction of the upper 50 km layer; and (b) only the upper half of the mantle is depleted. (These scenarios correspond to models 1a and 1b in Fig. 6.12). Model (b) is found to yield a much better approximation to the present ⁸⁷Sr/⁸⁶Sr ratio of the depleted (MORB) source.

Fig. 6.13. Plots of Sr isotope evolution against time to compare the effects of a) whole-mantle or b) half-mantle convection on the degree of depletion predicted for the residual (MORB) reservoir. Hatched area is the present-day composition of MORB. After O’Nions et al. (1979).

Jacobsen and Wasserburg (1979) used box models to examine another aspect of global differentiation (2a and 2b in Fig. 6.12). They simplified their treatment by considering only unidirectional transport of species from the mantle to generate the crust continuously over geological time, and solved the transport equations algebraically. In model 2a (Fig. 6.12) melts are extracted from the primitive mantle and generate the continental crust and a depleted mantle, both of whose volumes grow over geological time. However, the elemental composition of the depleted mantle remains constant through time. Mass balance calculations based on Sm)Nd data led Jacobsen and Wasserburg to calculate that only 33% of the mantle need be depleted to generate the continental crust, corresponding to the formation of a depleted MORB reservoir occupying approximately the upper 650 km of the mantle.

In Jacobsen and Wasserburg’s second model (2b in Fig. 6.12), the crust is extracted from a fixed volume of mantle which therefore becomes more and more depleted through geological time. The mass of this depleted mantle needed to generate the crust was calculated as only 25% of the total. In this model the isotopic composition of new continental crust will reflect a derivation from depleted mantle, whereas in model 2a new continental crust will have a chondritic (primitive mantle) isotopic signature. On the basis of Nd isotope data available to them at the time (section 4.2.1), Jacobsen and Wasserburg preferred model 2a. However, more recent Nd isotope evidence (section 4.2.2) strongly favours model 2b. The different estimates of O’Nions et al. (1979) and Jacobsen and Wasserburg (1979) for the volume of the depleted mantle reflect the uncertainties involved in estimating the trace element and isotopic composition of the crust.

DePaolo (1980) studied a model similar to 2b, but with the possibility of crustal recycling into the mantle, and again concluded that only 25)50% of the mantle need be depleted to generate the continental crust. It appears that a modest amount of crustal recycling has relatively little effect on mantle Nd)Sr isotope systematics, but a large effect on Pb (section 5.4.3).

The box model approach has been used in numerous more recent papers, e.g. Allegre et al. (1983). These authors used the so-called ‘total inversion method’ to attempt to choose between different models, but the uncertainties in the data did not allow significant extra information to be gained. In fact, more recent work (see below, and also section 4.4.3) has suggested that the subduction of oceanic crust and sea floor sediments may have created an enriched reservoir in the lower mantle as important as continental crust. This suggests that the above modelling has probably underestimated the size of the depleted reservoir at the present day, which may actually comprise more than 50% of the volume of the whole mantle.

6.2.2 The mantle array and OIB sources

The Nd)Sr isotope correlation in oceanic rocks was first referred to as the ‘mantle array’ by DePaolo and Wasserburg (1979). They attributed the OIB which form most of this array to a chondritic lower mantle source contaminated by mixing with melts from the depleted MORB source during ascent. Indeed, much of the early discussion about the Nd isotope systematics of ocean island basalts attempted to explain their composition in terms of the two major reservoirs discussed above, namely Bulk Earth and depleted mantle. Little attention was given to the problem of generating enriched oceanic mantle, since Tristan da Cunha was regarded as more-or-less representing a primitive mantle composition similar to the Bulk Earth (Allegre et al., 1979; O’Nions et al., 1980).

Kamber and Collerson (1999) recently ‘rediscovered’ and attempted to revive this model for OIB sources involving simple mixing between a primordial Bulk Earth reservoir and the MORB source mantle. This proposal was based largely on recent Pb isotope evolution models discussed in section 5.4.3. However, Kamber and Collerson also claimed that there was “growing geophysical and geochemical evidence in support of this original ‘standard’ model”. Nevertheless, the present author is in complete disagreement with this claim, believing that a simplistic model of this type is not consistent with the geophysical and geochemical evidence reviewed below.

The first persuasive evidence against simple mixing between Bulk Earth and MORB source mantle was provided by the extension of the mantle array into the ‘enriched’ lower right quadrant of the Nd)Sr isotope diagram. This was convincingly demonstrated in a study of the Kerguelen Islands (Dosso and Murthy, 1980), shown in Fig. 6.14. More recent work has shown that part of the Kerguelen Plateau is underlain by a fragment of continental lithosphere, which has imparted enriched isotopic signatures to the magmas erupted through it (section 6.3.2). However, the lavas of the Kerguelen Islands themselves are not affected by this phenomenon, and are still attributed by most workers to an enriched source in the deep mantle (e.g. Neal et al., 2002; Mattielli et al., 2002).

Fig. 6.14. Plot of Nd versus Sr isotope compositions for oceanic volcanics showing extension of the ‘mantle array’ into the ‘enriched’ quadrant relative to Bulk Earth, based on Kerguelen data ( > ), and the extension of Sao Miguel data into the upper right quadrant. Modified from Dosso and Murthey (1980).

Another enriched mantle source was revealed by alkali basalts from Sao Miguel in the Azores (Hawkesworth et al., 1979a). However, in this case the data trended towards enriched ⁸⁷Sr/⁸⁶Sr compositions lying to the right of the mantle array. The Sao Miguel trend was subsequently extended by data from Samoa and the Society Islands (White and Hofmann, 1982), breaking the simple Nd)Sr isotope correlation in OIB into a ‘mantle disarray’ (White, 1981). Further disarray is caused by the existence of other ocean islands with compositions to the left of the mantle array, such as St Helena (White and Hofmann, 1982).

Hawkesworth et al. (1979a) discovered an additional problem when they compared isotopic and trace element data in OIB samples. Iceland, Hawaii and Sao Miguel basalts plot in the upper left quadrant of the Nd)Sr isotope diagram, together with MORB (Fig. 6.14), indicating derivation from a source with a time-integrated depletion in light REE relative to Bulk Earth for all of these samples, and depletion in Rb (relative to Sr) for most. However, when trace element abundance ratios in these samples are examined (Fig. 6.15), only MORB samples plot completely in the depleted quadrant relative to Bulk Earth. Iceland reaches into the LREE enriched (i.e. low Sm/Nd) quadrant, while some Hawaiian samples also reach into the Rb/Sr enriched quadrant, and all Sao Miguel samples plot in the LREE- and Rb/Sr-enriched quadrant.

Fig. 6.15. Plot of Sm/Nd versus Rb/Sr elemental ratios for basic volcanic suites relative to the calculated ratios for the Bulk Earth. After Norry and Fitton (1983).

Hawkesworth et al. examined several possible ways of generating the isotopic and trace element features of the Azores data. They ruled out interaction with seawater because this would displace points horizontally to the right on the Nd)Sr isotope diagram (section 6.6). They recognised that ocean floor sediments have suitable Nd and Sr isotope compositions to generate the Sao Miguel array by contamination, but they ruled out this model for two reasons. Firstly, a 1-km deep hole drilled into the pillow lavas which build the island did not encounter any sediments. Secondly, the primitive chemistry of the basalts shows no sign of crustal contamination. Hence, by a process of elimination, Hawkesworth et al. attributed the isotopic signatures to the mantle sources of the basalts.

One model that can explain both the isotopic and trace element characteristics of the Azores data is mantle metasomatism. Hawkesworth et al. suggested that this caused LIL-element enrichment of the source a few tens of millions of years before generation of the Azores magmas. This can explain how a mantle source with a long-term depletion in light REE relative to Bulk Earth (as indicated by Nd isotope compositions, Fig. 6.14) can nevertheless be enriched in LIL trace elements (Fig. 6.15). If the source was only recently enriched in LREE, there would be insufficient time to affect its Nd isotope signature. On the other hand, if the metasomatising fluids came from a region with long-term Rb/Sr enrichment relative to Bulk Earth, they would also carry a radiogenic Sr isotope signature which they could impart to the melting region of the Sao Miguel basalts.

In subsequent work, Hawkesworth et al. (1984) argued that mantle metasomatism could have widely affected the sources of both continental and ocean island basalts. This was a controversial proposal, and caused much debate at the time. Recent work (Widom et al., 1997) has supported the metasomatism model for the mantle source of Sao Miguel, but has also indicated that this represents a delaminated fragment of African continental lithosphere, and is therefore not representative of OIB sources in general. Meanwhile, the paradox of deriving other LIL-enriched magmas from a source with long-term LIL depletion has been solved by new views of the nature of mantle melting. It is now proposed that very small degree (LIL-enriched) melts can be extracted from the mantle under conditions of low-degree partial melting (section 13.3.1). This avoids the need to invoke widespread metasomatic enrichment of mantle sources prior to magmatism.

As an alternative to the metasomatism model, Hofmann and White (1980, 1982) proposed that recycling of ancient oceanic crust into the OIB source could explain relatively enriched trace-element and isotopic compositions within the mantle array. Similarly, the deviation of Azores, Samoa and Society Islands basalts to the right of the mantle array could be explained by the addition of subducted sediment to the recycling of oceanic crust (Fig. 6.16).

Fig. 6.16. Plot of Nd versus Sr isotope composition for oceanic volcanics showing two arrays. The main mantle array was attributed to recycling of magmatically fractionated material such as oceanic crust. The shallow mixing line was attributed to sediment re-cycling. After Hofmann and White (1982).

In this model, subducted ocean crust was believed to descend to a density compensation level where it was stored and reheated for 1)2 Byr before returning to the surface in a plume. Hofmann and White suggested that this was the core)mantle boundary, but Ringwood (1982) advocated the 670 km phase transition as the compensation depth where oceanic crust resides. Seismic evidence for depression of the 670 km mantle phase boundary under subduction zones supports this model by suggesting that the descending slab may be deflected horizontally at this level (Shearer and Masters, 1992). However, other evidence suggests that the density contrast is too small to impede convective transport across this boundary and prevent slab penetration into the lower mantle (Morgan and Shearer, 1993). Furthermore, there is an increasing amount of tomographic evidence showing that some slabs sink to the bottom of the mantle (van der Hilst et al., 1997) and that some plumes rise from the same place (e.g. Bijwaard and Spakman, 1999).

6.2.3 Mantle convection models

Over the past couple of decades, the convective structure of the mantle has been much debated, with geophysicists generally advocating a single layer of convection cells in the mantle, while geochemists have generally advocated two-layer convection. Some geochemists (e.g. Allegre, 1997) have continued to support a fairly rigid two-layer model, but there are increasing suggestions that a compromise position should be taken. This is based on new evidence that the viscosity of the mantle increases by nearly two orders of magnitude from top to bottom (e.g. Bunge et al., 1996). This variable viscosity model seems to imply a convective regime somewhere between the two extremes of single and double layer convection. In principle the lower mantle is part of the main convective system of the mantle, but in practice its high viscosity may isolate large bodies of it from the more rapidly convecting upper mantle. This style of mantle convection is illustrated by a conceptual diagram in Fig. 6.17.

Fig. 6.17. Cartoon to show the predicted effect of a variable viscosity mantle, resulting in a homogeneous upper mantle and a more heterogeneous lower mantle where large ‘blobs’ of material can resist entrainment into the convective system. After Becker et al. (1999).

In an early evaluation of variable viscosity mantle circulation models, Manga (1996) suggested that lower mantle blobs of high viscosity may actually aggregate together over time, unlike the convective regime in the upper mantle, which is able to streak out and homogenise heterogeneities over time. The ‘blob’ model of the lower mantle was also supported by Becker et al. (1999), who suggested that blobs of primitive mantle with a viscosity 100 times larger than the surrounding depleted mantle could be the cause of the variations of mantle viscosity with depth. However, they could not account for the origin of these blobs. The opposite view was taken by van Keken and Zhong (1999), who argued that mantle convection would efficiently mix and eradicate mantle ‘blobs’ so that it would not be possible for large volumes of the lower mantle to remain isolated over the lifetime of the Earth. These conflicting interpretations illustrate the limitations of theoretical modelling of mantle structure, since the models are not sufficiently well constrained to yield definitive results.

A model involving a heterogeneous lower mantle was supported by new tomographic evidence for the gross structure of the mantle (van der Hilst and Karason, 1999). Variations in seismic properties were represented by two measures of the radial (vertical) structure of the mantle (Fig. 6.18). The first (radial correlation) is a measure of the vertical continuity of structure, while the second (radial variation) is a measure of the variability of seismic velocities. These measures show that the mid mantle is relatively homogeneous, whereas the upper and lower mantle are more heterogeneous. In the upper mantle this is due to rapid phase changes with depth, and to the large temperature variations between plumes, slabs and asthenospheric mantle. In the lower mantle it is presumed to correspond to chemical heterogeneity between different domains of primordial, recycled and mixed material.

Fig. 6.18. Cross section of the whole mantle showing the variation of two measures of heterogeneity as a function of depth. Modified after van der Hilst and Karason (1999).

The model of van der Hilst and Karason (1999) was developed by Kellogg et al. (1999) to reinstate a sharper division between upper- and lower-mantle domains. The division between these domains was placed at 1600 km depth, at the top of the heterogeneous lower mantle domain identified by van der Hilst and Karason. The main feature of the new model was a hypothetical reservoir of ‘intrinsically dense’ material at the bottom of the mantle, perhaps reflecting Fe enrichment. It was suggested that this material originated early in Earth history, perhaps from a magma ocean or by recycling of particularly mafic oceanic crust in the Archean. Subducted slabs could penetrate this material, but would normally collect on its upper surface, as illustrated in Fig. 6.19.

The greater density of the hypothetical lower layer obviously stabilises it against entrainment into the convective system of the upper mantle. Because this layer is also enriched in incompatible elements relative to the upper mantle, it can supply large amounts of heat. It can also satisfy Nd isotope box models, because the large volume of enriched material at the base of the mantle compensates for the large volume of the depleted upper mantle. However, the main problem with this model is the vagueness of the nature and origin of the hypothetical ‘intrinsically dense’ reservoir. Until more concrete geochemical evidence is provided, this reservoir is only a convenient theoretical construct.

Fig. 6.19. Cartoon to show a hypothetical mantle with ‘intrinsically dense’ material at its base, upon which recycled slab material would collect. After Kellogg et al. (1999).

Forte and Mitrovica (2001) made a more quantitative evaluation of mantle structure by using global geophysical data to constrain convective flow models. The data included global-scale free air gravity anomalies, observed plate motions, and dynamic topography of the core–mantle boundary and the Earth’s surface (in response to convective flow). Forte and Mitrovica were able to use these data to constrain convective flow in the mantle, and thereby predict the depth dependence of mantle viscosity. The result was a surprising stratification of mantle viscosity (Fig. 6.20), with two zones of very high viscosity at depth of ca. 1000 km and 2000 km. These results appear to support a ‘megablob’ model of the lower mantle by suggesting that some very viscous domains are present in the lower half of the mantle, some of which may represent primordial mantle. In addition, it is possible that the viscosity peak at 1000 km depth represents subducted ‘megaliths’ as suggested by Ringwood (1982). No doubt these new geophysical constraints will be further refined. However, they appear at present to leave plenty of room for the long-term preservation of primordial and recycled mantle domains, as implied by geochemical evidence.

Fig. 6.20. Predicted variation of mantle viscosity with depth, based on geophysical constraints on two different mantle convective flow models. Dashed line = average upper and lower mantle viscosity. After Forte and Mitrovica (2001).

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