6 Isotope geochemistry of oceanic volcanics

Some of the most important questions in geology concern the processes which operate in the Earth’s mantle. Mantle convection is clearly the driving force behind plate tectonics (e.g. Turcotte and Oxburgh, 1967), but the details of its operation are still unclear. The depth of mantle convection cells, the fate of subducted lithosphere, and the source of upwelling mantle plumes are all questions that remain poorly understood. Isotope geochemistry may help to answer these questions by revealing the progress of mantle differentiation into different reservoirs and the extent to which these reservoirs are re-mixed by convective stirring.

The inaccessibility of the mantle presents a severe problem for geochemical sampling. However, mantle-derived basic magmas provide a prime source of evidence about the chemical structure of the mantle. Isotopic tracers represent a particularly powerful tool for such studies, because unlike elemental concentrations, isotope ratios are not affected by crystal fractionation. However, isotope ratios are susceptible to contamination in the continental lithosphere. Therefore the simplest approach to studying mantle chemistry through basic magmas is to analyse oceanic volcanics, which are expected to have suffered minimal contamination in the thin oceanic lithosphere.

Isotope analysis of ocean island basalts (OIB) was first used to demonstrate the existence of mantle heterogeneity (Faure and Hurley, 1963; Gast et al., 1964). Subsequently, variations were found between the isotopic compositions of mid ocean ridge basalts (MORB) and OIB (Tatsumoto, 1966). Isotopic analysis of oceanic basalts can be used both to probe the structure of the mantle and to model its evolution over time. The approach taken here will be to examine the constraints on mantle structure from single isotopic systems (mainly Sr and Pb), then to examine the constraints on mantle evolution from multiple isotopic systems (Sr)Nd), (U)Th)Pb) and (Sr)Nd)Pb). Evidence from other systems will be examined in later chapters.

6.1 Isotopic tracing of mantle structure

6.1.1 Contamination and alteration

Before oceanic volcanics can be used to deduce mantle compositions, we must examine and quantify the amounts of alteration and contamination which could occur during magma transport and eruption on ocean islands or the ocean floor.

Sub-solidus alteration of analysed samples could result from hydrothermal interaction with seawater, in the case of submarine basalts, or sub-aerial weathering, in the case of ocean island basalts. For example, Dasch et al. (1973) found a positive correlation between ⁸⁷Sr/⁸⁶Sr and water content in dredged oceanic basalts of various ages (Fig. 6.1). Samples with more than 1% H₂O had almost invariably suffered contamination with Sr from seawater, but those with less than 1% alteration appeared to be uncontaminated.

Fig. 6.1. Plot of strontium isotope ratio against water content in ocean floor basalts. Vertical arrows show the effect of leaching before analysis. Dashed arrow shows the effect of smectite removal from an altered sample. After Dasch et al. (1973).

Sub-solidus alteration in submarine samples can reliably be avoided by analysing 100% fresh MORB glasses (Cohen et al., 1980). Where crystalline rock must be analysed (e.g. White et al., 1976), alteration can be avoided by analysing fresh material dredged from the median valley of the ocean ridges, where very young, unmetamorphosed basalts outcrop. Alternatively, leaching of crystalline samples before analysis may remove contaminated alteration minerals, also yielding results which are consistent with glasses (Dupre and Allegre, 1980). Unaltered ocean island basalts are easily obtained by sampling only fresh lavas.

Once sub-solidus alteration of samples has been excluded as a cause for isotopic variations, the next possibility that must be considered is contamination in the oceanic lithosphere. Although this is normally much thinner than the continental lithosphere, some ocean islands could be located on micro-continents or some other kind of abnormal lithosphere.

In their early work on Ascension and Gough islands, Gast et al. (1964) considered the possibility of contamination of the analysed lavas by a crustal micro-plate. They tested this possibility by analysing a range of lavas at variable degrees of magmatic differentiation (Fig. 6.2). The lack of any correlation in all but the most evolved rocks was argued to rule out crustal contamination. High Sr isotope ratios in the highly evolved rocks were attributed to radioactive growth after eruption, since these rocks have very high Rb/Sr ratios. No age corrections could be applied to these lavas since their ages were unknown. Similar problems have been encountered in more recent studies of Ascension lavas (Harris et al., 1983). However, most oceanic basalts require no age correction since they have very low Rb/Sr ratios.

Fig. 6.2. Sr isotope ratios in lavas from Gough and Ascension islands plotted against an index of magmatic differentiation. Radiogenic Sr in highly evolved lavas (open symbols) is attributed to radioactive growth since eruption. Arrows show estimated age corrections. After Gast et al. (1964).

Some workers, most notably O’Hara, suggested that isotopic variations in MORB and OIB could be explained by fractionation or contamination processes affecting magmas during their ascent through oceanic crust. In his early papers on the subject, O’Hara (1973, 1975) suggested that variations in ⁸⁷Sr/⁸⁶Sr ratio could be generated by physical fractionation of the isotopes during magmatic differentiation. This is a misconception, since ⁸⁷Sr/⁸⁶Sr ratios are always fractionation-corrected to the standard ⁸⁸Sr/⁸⁶Sr ratio of 8.37521 (section 2.2.3) to eliminate both natural and analytical mass-dependent fractionation. Subsequently, O’Hara and Mathews (1981) argued that large ion lithophile (LIL) elements (including strontium) could be perturbed by contamination with altered oceanic crust in a periodically tapped, periodically re-filled, long-lived magma chamber’. This model is now ruled out by the evidence from U-series isotopes, which severely limits the time between generation and eruption of ocean floor basalt, and hence the ability of an open-system magma chamber to overprint the source isotopic signatures in the erupted products (section 13.3).

More recently, renewed attention has been paid to the possibility that some OIB may have been contaminated by mantle lithosphere. In the case of Hawaii it was proposed some time ago (Chen and Frey, 1983) that late magmatism on Oahu involved partial melting of the LIL-depleted oceanic lithosphere, and that these melts were variably mixed with melts from a plume source. However, it is only recently that strong evidence has been amassed for the contamination of some plume sources by old continental lithosphere. Within the ocean basins, other islands now thought to be underlain by fragments of old lithosphere include Grande Comore (Class et al., 1998), Sao Miguel in the Azores (Widom et al., 1997) and the Canaries (Widom et al., 1999), all of which surround the African continent. Therefore, it has been suggested that fragments of African sub-continental lithosphere were delaminated during continental rifting and left as scattered remnants under new oceanic crust (Widom et al., 1997; Moreira et al., 1999). The case of Sao Miguel will be discussed in section 6.2.2.

6.1.2 Disequilibrium melting

Following the discovery of ‘mantle heterogeneity’ under the oceans, various workers (e.g. Harris et al., 1972; O’Nions and Pankhurst, 1973; Flower et al., 1975) suggested that mantle temperatures might not be high enough to ensure diffusional homogenisation of Sr isotope ratios between different mantle minerals. In that case, grains with higher Rb/Sr ratios (such as the magnesian mica, phlogopite) could develop more radiogenic ⁸⁷Sr/⁸⁶Sr compositions over geological time. ‘Disequilibrium’ melting of such phases could then bias the isotopic composition of a melt towards higher ⁸⁷Sr/⁸⁶Sr compositions. Small degree partial melts would tend to be enriched in Rb/Sr and ⁸⁷Sr/⁸⁶Sr relative to large degree partial melts, due to the tendency of high Rb/Sr phases such as phlogopite to enter the melt first.

Harris et al. (1972) argued in favour of disequilibrium melting during basalt genesis, based on evidence of isotopic disequilibrium in mantle xenoliths carried to the surface in basic magmas. Isotopic disequilibrium in ultramafic xenoliths is very widespread (section 7.1), but such cases represent samples of the solid lithosphere. It is questionable whether these observations can be extrapolated to the higher temperature environment of basaltic magma genesis in the convecting asthenosphere.

Hofmann and Hart (1978) examined data for the diffusion of Sr in mantle silicates in order to determine the rates at which isotopic disequilibrium could be eradicated at various temperatures. In Fig. 6.3, values of diffusivity (D) are used to calculate times for effective equilibration of a species between a sphere of 1 cm diameter and an infinite reservoir such as a slowly moving melt. These times are roughly those taken for diffusion over a ‘characteristic transport distance’ of 0.25 cm, using the equation X = (Dt)^½.

Fig. 6.3. Plot of diffusivity against 1/temperature, showing experimental results for the diffusion of Sr, Ar, Ni, Ca and oxygen in different types of material. Times for effective equilibration are based on 1 cm grain size. Modified after Hofmann and Hart (1978).

Using the lower of the measured diffusivities, it would take millions of years to eradicate Sr isotope heterogeneity between large grains of phlogopite and clinopyroxene (cpx) in solid lithospheric mantle at, say, 600 ^oC. Even in a solid mantle at 1000 ^oC, equilibration could take millions of years if the phlogopite and clinopyroxene grains were separated by intervening olivine or orthopyroxene, which effectively contain no Sr but lengthen the diffusion pathways between phlogopite and cpx. However, as soon as a melt is present, the surface of each crystal is in diffusional contact with nearby (ca. 2 cm distant) grains over a period of a few years. Therefore isotopic disequilibrium between phlogopite and cpx could be eradicated in a few thousand years at temperatures above the basalt solidus (ca. 1000)1200 ^oC). Nevertheless, diffusion over long distances, even in a partially molten mantle, is still slow.

Hofmann and Hart (1978) concluded that the evidence favoured ‘local equilibrium in a partially molten mantle, local disequilibrium in a completely crystalline mantle, and regional disequilibrium in any mantle that convects only slowly in large convection cells’. This suggests that disequilibrium melting does not preferentially sample isotopic mantle heterogeneity at the mineralogical scale. However, it might well sample heterogeneity between different petrological source types, even if these are streaked out by convection into thin bands (see below).

6.1.3 Mantle plumes

Following the acceptance of the plate tectonic model, it was realised that the tectonic setting of basic volcanism was a crucial factor in determining the nature of the mantle source being tapped, and consequent magma chemistry. Morgan (1971) proposed that the different chemistry of MORB and OIB could be explained if the former were derived directly from the asthenospheric upper mantle, whereas the latter were generated by upwelling plumes from the lower mantle. Evidence in support of this model was provided by elemental analysis of Iceland basalts (Schilling, 1973). These data suggested a region of mixing between plume (OIB source) and depleted upper mantle (MORB source) on the Reykjanes Ridge south of Iceland. Sr isotope data for the Reykjanes Ridge (Hart et al., 1973) were slightly more equivocal, since they showed a step like feature in the data (Fig. 6.4). Hart et al. interpreted the data as a mixing phenomenon, but some workers (e.g. Flower et al., 1975) interpreted this step as resulting from disequilibrium melting of a mantle with variable phlogopite contents.

Fig. 6.4. Plot of Sr isotope ratio against latitude for basalts from the Mid Atlantic Ridge. ( + ) = Iceland ) Reykjanes ridge. Age correction of Sr isotope data is unnecessary, due to the low Rb/Sr ratios and young ages of analysed material. After White et al. (1976).

White et al. (1976, 1979) extended the Sr isotope data set by analysing dredged samples from the axial valley of the Mid Atlantic Ridge (MAR) between 29 and 63 ^oN, and by sampling across the Azores platform. Isotopic data are plotted against latitude down the MAR in Fig. 6.4, and against longitude across the Azores Plateau in Fig. 6.5. There are large variations in the strontium isotope ratio of MORB samples along the MAR, but where MORB and OIB are erupted alongside each other (the Azores Plateau), they have very similar isotope ratios (with the exception of Sao Miguel). Because tholeiitic (MORB) and alkaline (OIB) magmas are attributed to different degrees of mantle melting, the overlap of their compositions across the Azores Plateau is evidence against sampling of isotopic heterogeneities on a mineralogical scale.

Fig. 6.5. Plot of strontium isotope ratio against longitude for basalt samples from the Azores Plateau. ( H ) = dredged basalts. Other symbols represent individual islands. After White et al. (1979).

The plume)asthenosphere mixing model for the Reykjanes Ridge was strongly confirmed by Pb isotope analysis (Sun et al., 1975), which revealed a smooth compositional variation down the ridge (Fig. 6.6a). In contrast, Pb isotope analysis of basalts from the Kolbeinsey Ridge, north of Iceland, did not reveal any contamination of this ridge segment with plume material (Mertz et al., 1991). These differences in mixing style north and south of Iceland can be attributed to asymmetrical distortion of the plume by a regional southerly flow of asthenospheric mantle (Fig. 6.6b). Non-uniform Pb isotope contamination of ridge segments has also been observed in the South Atlantic, caused by the off-axis St Helena plume (Hanan et al., 1986).

Fig. 6.6. Interpretation of isotopic data for the Iceland plume: a) compilation of Pb data from Iceland ( " ), Reykjanes ridge ( <> ) and Kolbeinsey ridge ( € ); b) model cross-section of the upper mantle. After Mertz et al. (1991).

6.1.4 Plum pudding mantle

Many workers have questioned whether there might be an intermediate scale of mantle heterogeneity between rare large plumes and mineralogical disequilibrium. Even in their early elemental studies of the Faeroes ‘plume’, Schilling and Noe-Nygaard (1974) recognised that this structure need not be a continuous column, but could have the form of a train of ‘blobs’. Later workers (e.g. Allegre et al., 1980) developed the idea that trains of blobs need not simply pass in streams from a (hypothetical) lower mantle reservoir through the asthenosphere, but could be part of the convecting asthenosphere itself. Allegre identified three alternative models for ‘blob heterogeneity’ of the asthenosphere (Fig. 6.7).

Fig. 6.7. Hypothetical scales of mantle heterogeneity. a) small scale; b) large scale; c) large and small scale. After Allegre et al. (1980).

In an analysis of basaltic glasses from the ocean basins, Cohen and O’Nions (1982) showed that the (comparatively) very large ranges of Pb isotope variation seen in Atlantic MORB were not equalled on the East Pacific Rise. Rather than attributing these differences to a smaller degree of mantle heterogeneity beneath the Pacific, Cohen and O’Nions argued that approximately equal degrees of heterogeneity in the Atlantic and Pacific upper mantle were homogenised in the large magma chamber associated with its fast-spreading ridge. Support for this model came from the observation by Zindler et al. (1984) that seamounts near the East Pacific Rise exhibited much more variation than the adjacent ridge.

Batiza (1984) confirmed the inverse effect of ridge spreading rate on isotopic heterogeneity by plotting total ranges of ⁸⁷Sr/⁸⁶Sr ratio ( ) ) for various mid ocean ridges against their spreading rate (Fig. 6.8). He attributed the small range of compositions on the fast-spreading ridges to homogenisation, during the melting process, of a mantle that was ‘ubiquitously heterogeneous on a small scale’. Low isotopic variation on some slow-spreading ridges (e.g. Juan de Fuca) was attributed to either their short length or limited sampling. Batiza adopted the more gastronomically elegant term of ‘plum pudding’ mantle to describe this blob-bearing asthenosphere. Allegre et al. (1984) also found an inverse correlation between ridge spreading rate and isotopic variation, but argued that homogenisation must be primarily by (solid state) mantle convection rather than magma mixing.

Fig. 6.8. Total ranges of Sr isotope ratio ( ) ) for MORB glasses or leached whole-rocks from a given ridge, plotted against spreading rate on that ridge (JDF = Juan de Fuca; EPR = East Pacific Rise). Figures in brackets indicate number of analyses. After Batiza (1984).

In order to express the idea that plume and plum-pudding models should not be thought of as mutually exclusive, but rather as a continuum of phenomena, Sun (1985) coined the term ‘plume pudding’ mantle (sic). Plums and plumes might originate from a variety of phenomena. However, this question cannot effectively be answered by the application of single isotopic systems, and will be discussed below on the basis of co-variations in multiple isotopic systems.

6.1.5 Marble cake mantle

Fluid dynamic modelling of the convecting asthenosphere (e.g. Richter and Ribe, 1979; McKenzie, 1979) has suggested that discrete structures in the mantle (e.g. blobs, plums, etc.) cannot remain undeformed for long periods in the convecting asthenosphere. They will tend to be elongated and sheared until they are eventually physically homogenised with the depleted reservoir. Polve and Allegre (1980) argued that evidence of this process was provided in orogenic lherzolites (Fig. 6.9), which contain alternating bands of (depleted) lherzolite and (enriched) pyroxenite. They suggested that this banding might have been generated by convective ‘stirring’ and stretching of a two-part sandwich of oceanic crust and underlying residual lherzolite, which is recycled back into the mantle by subduction. Allegre and Turcotte (1986) coined the term ‘marble cake’ mantle to describe this concept, and argued that it is representative of the structure of much of the upper mantle.

Fig. 6.9. Schematic illustration of ‘marble cake’ mantle consisting of pyroxenite (shaded) and lherzolite layers in the Beni Bousera peridotite of Morocco. After Allegre and Turcotte (1986).

Prinzhofer et al. (1989) argued that random mixing between partial melts of pyroxenite and peridotite in a marble cake mantle could generate the large ranges of incompatible element concentrations and the moderate range of radiogenic isotope ratios seen in lavas from a small (40 H 10 km) area of the East Pacific Rise. However, mixing in the magma chamber is not capable of explaining the length dependence of large-scale isotopic anomalies on ridges (Kenyon, 1990). For example, the isotopic ‘texture’ of the South Atlantic Ridge requires convective homogenisation over distances up to 1000 km (Fig. 6.10). This is too large for a magma chamber, since it is more than the length of ridge segments between transform faults. Hence it follows that homogenisation must occur at a deeper level, either by solid-state convection of the marble cake mantle, or during magma ascent from the partial melting zone under the ridge (e.g. section 13.3).

Fig. 6.10. Curve-fit for mixing of isotopic heterogeneity, compared with empirical data for amplitude versus wavelength of Sr isotope variation on the South Atlantic Ridge. After Kenyon (1990).

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