6.6 Island arcs and mantle evolution

Island arcs are central to the understanding of mantle evolution because they represent the site where lithospheric material of various types is returned to the deep mantle. Island-arc magmatism may allow us to sample this material which is in the process of being recycled. Dewey (1980) showed that the volcanic front is always located about 100 km above the descending slab, whatever the angle of subduction. This shows that dehydration of the slab, triggered by pressure, is central to the operation of island-arc magmatism. However, the petrology of island-arc basalts (IAB) precludes their genesis by fusion of subducted oceanic crust (since this would require nearly 100% melting). Therefore, they must be dominantly produced by melting of the ‘mantle wedge’ overlying the subduction zone (e.g. Wyllie, 1984). Hence, the central problem in interpreting island-arc basalts is to identify which signatures are derived from the slab (and subducted sediment) and which are derived from the overlying wedge. We will therefore examine this problem in terms of two-component mixing between the slab and wedge.

6.6.1 Two component mixing models

Island-arc basalts have enhanced levels of ⁸⁷Sr/⁸⁶Sr relative to MORB. However, the origin of these differences is only discernible in the context of other isotope evidence. The first study using combined Sr and Nd isotope data was made by Hawkesworth et al. (1977) on island-arc and back-arc tholeiites from the Scotia Sea (South Sandwich Islands). Analysis of back-arc material provides a control condition because it samples a mantle segment which should be similar to the wedge, but without any slab component.

Hawkesworth et al. found that both island-arc and back-arc samples from the Scotia Sea had identical ¹⁴³Nd/¹⁴⁴Nd ratios, overlapping with those of MORB. However, the island-arc samples had significantly higher ⁸⁷Sr/⁸⁶Sr ratios (Fig. 6.42), which could not be explained by sub-aerial weathering. Therefore, Hawkesworth et al. suggested that the enhanced Sr isotope ratios of the IAB were a product of subducted ⁸⁷Sr from seawater or (alternatively) oceanic sediments. Possible mechanismas proposed were the direct partial melting of altered and subducted oceanic crust, or alternatively, metasomatic contamination of the mantle wedge with elements derived from the ocean crust. It is now generally accepted that the latter model is correct for the Scotia arc (e.g. Pearce, 1983).

Fig. 6.42. Histograms of Sr isotope ratio for basalts from the South Sandwich arc and the (back-arc) Scotia Sea Rise. After Hawkesworth et al. (1977).

The Scotia arc provides an example of the role of slab-derived fluids in an arc with depleted chemistry. However, in arcs with less-depleted chemistry, material contributions from the slab and wedge are more difficult to resolve. The Lesser Antilles (Caribbean) arc provides a good test case for the behaviour of arcs with more enriched signatures, since the chemistry of the arc changes along its length. This may help in resolving the origin of enriched components.

Grenada volcanics display variations in both ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, defining a range similar to those observed by Hawkesworth et al. (1979a) from Sao Miguel in the Azores, but further to the right of the main mantle correlation line (Fig. 6.43). In view of these parallels, Hawkesworth et al. (1979b) attributed the Grenada data to Sr contamination (from the slab) of a heterogeneous mantle wedge with a range of Nd and Sr isotope compositions along the mantle array. However it is not clear why this particular piece of sub-oceanic lithosphere would display such large heterogeneities, since mantle plumes cannot penetrate above a subduction zone. It would be necessary to propose that the mantle wedge in this location happened to contain an enriched ‘plum’, as suggested for the Aleutian arc by Morris and Hart (1983).

Fig. 6.43. Comparison of Sr)Nd isotope systematics in a) the Azores, and b) Grenada, showing possible derivation from enriched mantle sources. After Hawkesworth et al. (1979b).

Isotopic investigation of other islands in the Lesser Antilles (Davidson, 1983) revealed that St Kitts, situated at the northern end of the arc, has a very small range of isotopic composition close to MORB, whereas Martinique, in the centre of the arc, has an extremely large range of isotope composition (Fig. 6.44). If such variations were inherited from the mantle wedge, then ‘gross heterogeneity on a scale of kilometres is implied’. Davidson initially ascribed the variations to contamination of the mantle source with subducted sediment. However, more detailed geochemical studies (Davidson, 1987) revealed positive correlations between Sr isotope ratio, oxygen isotope ratio and silica content in Martinique lavas. These are indicative of crustal contamination of ascending magma in the arc crust, which is thickest in the central region of the arc near Martinique. Such processes will not be detailed here, since they will be covered in the next chapter. However, they serve to exclude Martinique data from considerations of magma petrogenesis in the mantle.

Fig. 6.44. Sr)Nd isotope diagram showing extreme isotopic variation in Martinique lavas ( " ) compared with Grenada and St Kitts (SK). Mixing lines model the effects of contamination by sediments or seawater. After Davidson (1983).

White and Dupre (1986) presented Pb isotope data for representative samples from the whole length of the Lesser Antilles arc, showing that they were generally intermediate between MORB and sediment compositions. There is no evidence that these signatures are derived from magma contamination in the arc crust. For example, sedimentary xenoliths in Grenada lavas actually have unradiogenic Pb, inherited from an earlier location of the arc to the west, above the subducting Farallon plate. In contrast, Atlantic ocean floor sediments in front of the present-day arc have radiogenic Pb signatures.

White and Dupre found a general increase in the Pb isotope ratio of Atlantic floor sediment when going southwards in front of the Lesser Antilles subduction zone, probably reflecting sediment carried onto the sea floor at the south end of the arc by the Orinoco river. This trend was matched by the composition of Lesser Antilles volcanics, suggesting the presence of a subducted sediment component in the arc magmas. This model is supported by the covariation of Pb and Nd isotope data in the volcanics (Fig. 6.45). Two-component mixing between a MORB source and average Atlantic sediment can therefore explain the observed Pb)Nd isotope systematics of Lesser Antilles magmas, avoiding the need to invoke an enriched mantle wedge (Ellam and Hawkesworth, 1988).

Fig. 6.45. Assessment of a sediment)asthenosphere mixing model for Lesser Antilles volcanics, in terms of Pb and Nd isotope systematics. ) 7/4 indicates the ²⁰⁷Pb/²⁰⁴Pb deviation above the ‘Northern Hemisphere Reference Line’ of Hart (1984). After Ellam and Hawkesworth (1988).

Rare earth concentration data may present a problem for this model, since light REE enrichment in some arc volcanics may be too great to be explained by simple mixing between a MORB source and subducted sediment (Hawkesworth et al. 1991). This problem is illustrated in Fig. 6.46 on a plot of Ce/Yb ratio (i.e. REE profile slope) against Sr isotope ratio. LREE enriched basalts and andesites from Grenada, the Sunda arc, and the Aeolian arc of southern Italy fall off the mixing line between depleted arcs and a typical sediment represented by ‘post Archean average shale’ (PAAS). However, White and Dupre (1986) argued that the Pb isotope evidence for sediment involvement in arc magma genesis was so conclusive that it over-rides these trace element problems. Given this constraint, the very steep REE profiles must be due to some feature of the melting process. For example, partial melting of sediment in the presence of residual garnet could elevate light REE abundances in the melt while depressing the abundances of heavy REE.

Fig. 6.46. Plot of Ce/Yb ratio against Sr isotope ratio for island-arc basalts and andesites. ( ! ) = normal arc volcanics; ( Î ) = LREE enriched; ( Q ) = Martinique lavas, contaminated during magma ascent. PAAS (post-Archean average shale) is a typical sediment composition. After Hawkesworth et al. (1991).

Most workers now accept the supremacy of Pb isotope evidence for sediment involvement in IAB genesis. For example, Ben Othman et al. (1989) observed perfect matching of Pb isotope systematics between the West Sunda arc and ocean-floor sediment in front of the arc (Fig. 6.47). Since the Pb contents of arc volcanics are nearly an order of magnitude lower than those of typical sediments, it is unlikely that the sediment signature is itself controlled by erosion of arc volcanics. Therefore, it is most likely that the reverse relationship applies: arc volcanic Pb is controlled by subducted sediment. Further evidence was provided by McDermott et al. (1993), who observed Pb isotopic variations along the North Luzon (Philippine) arc which were correlated with the composition of sediment cores from the South China Sea, in front of the trench.

Fig. 6.47. Pb)Pb isotope plot showing colinearity of the West Sunda arc ( > ) with ocean-floor sediment in front of the trench (solid line). Indian MORB is shown by open symbols. After Ben Othman et al. (1989).

6.6.2 Three component mixing models

In the above examples, two-component mixing between slab and wedge was examined for cases where the slab-derived component (SDC) was dominantly either a fluid or sediment. However, it is clear that in some cases both of these components must be present. Therefore, White and Dupre (1986) and Ellam and Hawkesworth (1988) expanded the two-component models described above into a three component mixing model. This involves contamination of the depleted-mantle source of IAB with partial melts of subducted sediment and large-ion lithophile (LIL) element-enriched slab-derived fluids. Evidence for such a process is seen when abundances of low-field-strength LIL elements such as Sr are ratioed against high-field-strength elements (HFSE) such as REE. Subsequently, it has been shown that the Ba/Th ratio is the perfect monitor of the slab-derived fluid component (e.g. Turner et al., 1996). For example, arc basalts from the Lesser Antilles have elevated ⁸⁷Sr/⁸⁶Sr ratios relative to MORB, implying sediment contamination, but they also have variable Ba/Th ratios that are higher than either MORB or average sediment compositions, implying the addition of a Sr-rich fluid (Fig. 6.48).

Fig. 6.48. Plot of Ba/Th ratio against Sr isotope ratio to show the necessity of three component mixing to explain the geochemistry of Lesser Antilles lavas. After Turner et al. (1996).

The slab derived fluid has a relatively minor effect on radiogenic isotope signatures, apart from causing a modest increase in the Sr isotope composition of the source (Fig. 6.42). However, this component is very strongly resolved by boron isotope evidence (as well as cosmogenic ¹⁰Be, section 14.3.6), which can then be plotted against radiogenic isotope tracers. The use of this technique was demonstrated by Smith et al. (1997) in a study of island arc lavas from Martinique (Fig. 6.49). Since the mantle contains very little boron, mixing lines on this plot extend between the composition of the altered oceanic crust and the subducted sediment component. Smith et al. modelled these mixing lines assuming that both signatures were carried by fluids. However, the availability of both subducted marine and continental sediment means that a fan of mixing lines is produced between three fluid components. In addition, further complexity is caused by subsequent contamination of the arc magmas during their ascent through the arc crust. Detailed analysis of various magma suites using several different tracers can be used to unravel these very complex petrogenetic processes involving multiple components (e.g. Thirlwall et al., 1996). However, it will be more profitable here to examine crustal contamination processes involving fewer end-members. This will be done in the next chapter during a discussion of continental magmatism.

Fig. 6.49. Plot of boron isotope ratio against Nd isotope ratio to show how mixing models involving multiple fluid components can explain lava compositions from Martinique. Numbered ticks indicate percentages of sedimentary component. After Smith et al. (1997).

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