6.4 Mantle reservoirs in isotopic multispace

6.4.1 The mantle plane

The unification of radiogenic ²⁰⁸Pb^*/²⁰⁶Pb^* with other isotope systematics (section 6.3.3) breaks down when Pb isotope ratios involving non-radiogenic Pb are plotted against other isotope systems on bivariate diagrams (e.g. Fig. 6.30). This indicates that the isotope systematics of the mantle cannot be explained by a two-component mixing model. An exception to this general observation is provided by Pb)Sr isotope systematics on the North Atlantic, which do define a coherent positive correlation (Dupre and Allegre, 1980). However, this can be attributed to coincidental contamination of the MORB reservoir in this area with a single compositional type of enriched plume material.

Fig. 6.30. Diagrams to show the decoupling of ²⁰⁶Pb/²⁰⁴Pb from other isotopic systems in oceanic volcanics: a) Sr isotope data, after Sun (1980); b) radiogenic ²⁰⁸Pb^*/²⁰⁶Pb^* data, after Allegre et al. (1986). B.E. = Bulk Earth.

To explain the scatter of data on Fig. 6.30a, Zindler et al. (1982) argued that the Pb)Sr)Nd isotope compositions of oceanic volcanics must be caused by (solid state?) mixing of three mantle components. The proposed end-members were a pristine chondritic mantle with a Pb composition on the geochron, a MORB source depleted by continental crustal extraction, and a reservoir containing recycled MORB. This made Kerguelen the best candidate for a primitive mantle source, while St Helena was regarded as having the greatest amount of recycled MORB material in its source. Zindler et al. argued that average isotopic compositions of ocean ridges and ocean islands displayed very little scatter away from a plane containing these three end-member components (Fig. 6.31).

Fig. 6.31. Three-component mantle mixing model for MORB and OIB sources. Solid and open symbols indicate points respectively above and below the mantle plane. Pa, At, In = Pacific, Atlantic and Indian MORB. Hi= Hiva Oa, Ha= Hawaii, Ic= Iceland, Ea= Easter, Ga= Galapagos, As= Ascension, Ca= Canaries, Az= Azores, Bo= Bouvet, Sh= St Helena, Tr= Tristan, Go= Gough, Kg= Kerguelen. After Zindler et al. (1982).

Zindler et al. justified their three-component model by the high correlation coefficient of 0.98 calculated for their data set. However, such a limited scatter was achieved by excluding some ocean islands. For example, Sao Miguel was not included in the Azores average. However, this signature was argued by White (1985) to be part of a much wider compositional field, including data from the Society Islands, Samoa and Marquesas which extend ‘above’ the ‘mantle plane’ of Zindler et al. (1982). Furthermore, the averaging process also obscured data which lay ‘below’ the mantle plane, such as the Walvis Ridge. Therefore, at least one additional component must be invoked to explain the data.

6.4.2 The mantle tetrahedron

Hart et al. (1986) considered that the mantle plane of Zindler et al. (1982) might really be a ‘co-incidence of similar mixing proportions’ of end-members with more extreme compositions, rather than a discrete entity in its own right. This is illustrated in Fig. 6.32, where samples are plotted in terms of parts per 10⁵ deviation in Nd isotope ratio from the mantle plane, against Pb isotope composition.

Fig. 6.32. Plot of ) Nd (part per 10⁵ deviation in ¹⁴³Nd/¹⁴⁴Nd ratio from the mantle plane of Zindler et al., 1982) against Pb isotope ratio. OIB compositions are plotted both as fields and discrete points. JF= Juan Fernando, Re= Reunion, Gu= Guadeloupe. Other abbreviations as in Fig. 6.31. After Hart et al. (1986).

Hart et al. proposed that the lower bound of individual ¹⁴³Nd/¹⁴⁴Nd sample compositions on the Nd)Sr isotope diagram (Fig. 6.33a) might be a more fundamental topological structure, which they termed the ‘LoNd’ array. The samples which define this array on the Nd)Sr isotope diagram also fall in a line on the Sr)Pb isotope plot (Fig. 6.33b), despite the fact that this cuts across the middle of the OIB field in this diagram. ²⁰⁸Pb/²⁰⁴Pb ratios in these samples are also coherent with the three other isotope systems.

Fig. 6.33. Plots of: a) Sr versus Nd isotope ratio, and b) Sr versus Pb isotope ratio, showing the proposed end-members of a four component mixing system: DMM, HIMU, EMI (= EM1) and EMII (= EM2). Dots are compositions argued to lie on an array between the HIMU and EMI end-members, termed the LoNd array. After Hart et al. (1986).

The LoNd array was itself interpreted as a mixing line between ‘HIMU’ (high U/Pb) and ‘EMI’ (enriched mantle I) end-members (Zindler and Hart, 1986). Other important end-members were defined by the most extreme composition of the MORB field (DMM) and the Societies (EMII). In addition, Zindler and Hart (1986) suggested that three other components might be located inside the tetrahedral mixing space in Fig. 6.33. These are a ‘primordial helium isotope reservoir’, exemplified by Loihi seamount (section 11.1.3); a ‘Bulk Earth’ U)Th)Pb isotope reservoir exemplified by Gough)Tristan (section 6.3.3); and a ‘PREvalent MAntle’ or PREMA component, justified on the grounds that the mixing of discrete components may have reached such a stage of completeness that this mixture itself becomes a recognisable entity. These possible components will be discussed further below.

One of the characteristics of the LoNd array (Hart, 1988) is that island groups are not generally elongated along the proposed mixing line, but often trend obliquely off the line. This was used as evidence that mixing within the LoNd array occurred a long time ago, before secondary mixing with other components lying off the array. In addition, Hart et al. (1986) argued that the straightness of the proposed LoNd mixing line places tight constraints on the nature of the two mixing end-members, by requiring them to have similar Nd)Sr)Pb ratios and an intimately related environment of formation. Since they believed that such conditions would not be expected for mixing between recycled crustal and mantle components, Hart et al. argued that the two end-members must have resulted by different metasomatic enrichment processes in the sub-continental lithosphere.

Hart (1988) identified another two-component mixing line within the OIB data set, by using an upper ⁸⁷Sr/⁸⁶Sr cut-off of 0.703 to exclude all samples with an enriched mantle component. On a diagram of ¹⁴³Nd/¹⁴⁴Nd against ²⁰⁶Pb/²⁰⁴Pb (Fig. 6.34), these island groups with low ⁸⁷Sr/⁸⁶Sr ratios define a so-called ‘no EM’ array between the HIMU and DMM end-members. The straightness of this array again suggests that the end-members had similar Nd/Pb ratios, and hence that DMM, HIMU and EMI all have similar Nd/Pb ratios. However, the geochemical relationship between DMM and HIMU cannot easily be attributed to spatial proximity, as was the EMI)HIMU relationship, because the depleted mantle is a distinct reservoir. This therefore weakens Zindler and Hart’s argument for an intimate genetic relationship between the end-members of the LoNd array. Instead, a more general relationship is possible, whereby the three components are generated by similar mantle differentiation processes, but in different locations.

Fig. 6.34. Nd versus Pb isotope diagram showing the linear array of OIB samples with ⁸⁷Sr/⁸⁶Sr below 0.703, attributed to the ‘No-EM’ mixing line. After Hart (1988).

In contrast to the linear mixing lines described above, mixing with the EMII component tends to generate elongated curved arrays within island groups, as shown in Figs. 6.32 and 6.33. This suggests that elemental ratios between EMII and the other mantle domains were far from unity, which is consistent with a model in which DMM, HIMU and EMI are generated by mantle differentiation processes, but EMII represents recycled continental crust with a very different trace element signature. Hart (1988) went further in his distancing of EMII from the other components, suggesting that mixing with this end-member was a late phenomenon which occurred after other mixing processes. However, Staudigel et al. (1991) found strong evidence for mixing between HIMU and EMII in the South Pacific Isotopic and Thermal Anomaly (SOPITA), particularly on the Sr)Pb isotope diagram (not shown here). In view of the intimate geographical association of HIMU and EMII in the SOPITA case, it is likely that this array was formed prior to mixing with MORB, and it may constitute one of a family of curved ‘HiNd’ mixing lines analogous to the LoNd array.

There is considerable danger in looking at isotope variations in a number of two-component systems, since arrays are projected onto these surfaces from a multi-dimensional mixing polygon, and in this process the true trends of the arrays may be misunderstood. In order to analyse the data in a more objective fashion, Allegre et al. (1987) ran a principal component analysis on a large set of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb data for MORB and OIB samples. This was also performed on an updated sample set by Hart et al. (1992).

Principal component analysis resolves the oceanic data set into five eigenvectors, representing directions in multi-component space which show the greatest percentage of variance in the data. The magnitudes of these vectors (in the calculation by Hart et al.) are approximately 56%, 37%, 4%, 2% and 1%. The pre-eminence of the first two vectors demonstrates the largely planar form of the data set, as emphasised by Zindler et al. (1982). However, there is enough residual scatter in the data that a third vector is necessary in order to represent the mixing process properly. The sum of these three vectors is 97.5% in Hart’s analysis, and 99.2% in Allegre’s analysis. Hence Hart et al. argued that a three-dimensional (four component) analysis is appropriate to analyse the data with a fairly high degree of reliability. However, the eigenvectors are so divorced from the familiar isotope ratios that it becomes difficult to understand the data. Therefore, Hart et al. presented the data in the form of a three-dimensional isotope plot (of ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb), but projected in such a way as to approximate the eigenvector directions (Fig. 6.35).

Fig. 6.35. Two views of a three-dimensional mantle tetrahedron representing the mixing relationships of four isotopically proposed mantle components seen in oceanic volcanics, along with a proposed ‘FOcus Zone’, FOZO. Modified after Hart et al. (1992).

The focussing of data points at the lower corners of the mantle tetrahedron (Fig. 6.35) provides evidence that some of the proposed components are real entities, rather than merely theoretical end-members. This is exemplified by the intersection of the LoNd and No-EM arrays, which provide a relatively strong constraint on the composition of HIMU, suggesting that the ‘pure end-member’ has a composition very similar to the most radiogenic Pb already analysed, from the island of Mangaia. This conclusion is supported by the close agreement between the compositions of the widely separated HIMU islands from the South Atlantic and South Pacific. In contrast, the density of samples lying at the EMI and EMII end-member compositions is much lower, and may suggest that these are the most extreme products yet sampled of enrichment processes, rather than significant mantle reservoirs in their own right (Barling and Goldstein, 1990).

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