13.4 Subduction zone processes

Subduction related magmatism occurs in one of the most complex environments of magma genesis, since there are numerous potential sources of magmas and fluids in the subducting slab, the mantle wedge, and the overlying arc crust (section 6.6). In this situation, evidence from U-series nuclides can provide additional constraints, especially on the timing of fluid metasomatism and magma genesis.

13.4.1 U–Th evidence

The first comprehensive study of Th isotope systematics in subduction-related magmas was undertaken by Hemond (1986). This work revealed a distribution clustering near the equiline on the U)Th isochron diagram, but with strong departures from the Th)Sr isotope mantle array.

The incoherent behaviour of Th)Sr isotope systematics suggested more complex processes at subduction zones, compared with ridges and plumes. With additional work (Gill and Williams, 1990; McDermott and Hawkesworth, 1991) systematic variations began to emerge on the U)Th isochron diagram (Fig. 13.36), where continental arcs tend to lie on the U-depleted side of the equiline, whereas oceanic arcs lie to the U-enriched side. High elemental U/Th ratios (low Th/U) have long been known in the island arc tholeiite series (Jakes and Gill, 1970), so the distribution of these data to the right of the equiline is not surprising. This effect is seen clearly in oceanic arcs such as Tonga and the Marianas, but is small or absent in associated back-arc volcanics. For these reasons, the effect is best explained by U metasomatism from the subducted slab into the melting zone in the overlying mantle wedge.

Fig. 13.36. U)Th isochron diagram showing representative data for subduction-related magmas. Star = average crust. After Hawkesworth et al. (1991).

The slab-derived uranium flux can be seen most clearly when the overlying wedge is LIL-depleted (e.g. Tonga, Marianas), but in arcs with a more enriched wedge, the uranium flux may be swamped by U and Th derived from normal partial melting processes. Hence, continental arcs, underlain by enriched mantle lithosphere, tend to exhibit U/Th behaviour similar to within-plate basalts. This effect can be seen by plotting the ²³⁸U/²³⁰Th activity ratio (1/r in section 13.3.1) against the total Th content of the rock (Fig. 13.37), thus showing that arcs with strong U/Th enrichment are characterised by low total Th contents (McDermott and Hawkesworth, 1991).

Fig. 13.37. Plot of ²³⁸U/²³⁰Th activity (= U/Th fractionation from the equiline) against total Th content of arc volcanics. Open symbols are from the Tonga and Mariana arcs. After McDermott and Hawkesworth (1991).

Magmas to the right of the equiline in Fig. 13.36 must reflect U/Th fractionation shortly before eruption, in order to preserve isotopic disequilibrium. Independent evidence for the role of slab-derived fluids in this process comes from correlated ¹⁰Be/⁹Be and ²³⁸U/²³⁰Th ratios in the Southern Volcanic Zone of the Andes (Sigmarsson et al., 1990). Because ¹⁰Be is a cosmogenic isotope, it can only be introduced into the melting zone of arc magmas by subduction of ocean floor crust and sediment (section 14.3.6). Therefore, a positive correlation between ¹⁰Be/⁹Be and ²³⁸U/²³⁰Th ratios suggests a similar location for uranium enrichment of arc magmas.

Several of the Southern Volcanic Zone samples have ²³⁸U/²³⁰Th activity ratios close to the equiline (in common with other continental arcs), but these all have elevated levels of cosmogenic beryllium (Fig. 13.38). Therefore, if we project the array in Fig. 13.38 back to zero cosmogenic beryllium, we can estimate the ²³⁸U/²³⁰Th activity of the wedge-derived component in subduction-related magmas. This has a value of 0.8 ) 0.9 which is typical of asthenosphere-derived magmas from ocean ridges. This implies that the type of decompression melting seen at ridges (due to mantle diapirism) also occurs to some extent under arcs. This model is supported by ²³¹Pa data, which exhibit Pa/U enrichments resembling MORB in most arcs (Pickett and Murrell, 1997). These ²³¹Pa/²³⁵U enrichments can be explained by mild mantle upwelling (ca. 1 cm/yr), and may reflect gravitational instability in the source caused by fluid metasomatism (Turner, 2002).

Fig. 13.38. Plot of ¹⁰Be/⁹Be ratio against ²³⁸U/²³⁰Th activity ratio in the Southern Volcanic Zone of the Andes, showing correlated enrichments. The open circle indicates a sample that has been perturbed by contamination in the continental crust. Modified after Sigmarsson et al. (1990).

A second distinct feature of subduction-related magmas on the U)Th isochron diagram is the extension of some arc suites to ²³²Th-enriched compositions towards the origin (e.g. Philippines, Indonesia, Fig. 13.36). Because these suites lie close to the equiline, it appears that U/Th fractionation was relatively ancient. This is confirmed on a plot of 6_Pb against 6_Th in Fig. 13.39. Correlated increases in these values must reflect ancient U)Th fractionation events, since 6_Pb is controlled by long-lived U and Th isotopes. The best explanation of these data is contamination of arc magmas by partial melts of subducted sediments, which have appropriate compositions on both the U)Th isochron diagram and the 6)6 diagram (McDermott and Hawkesworth, 1991). Some sediment inevitably escapes the melting process, and is then recycled into the deep mantle.

Fig. 13.39. Plot of 6_Th against 6_Pb to show correlated high values in some arcs and in ocean floor sediments ( ! ). Fields for OIB, MORB, altered MORB, and marine sediments are shown for reference. After McDermott and Hawkesworth (1991).

Processes of fluid metasomatism and sediment contamination in arcs can be summarised on a Th activity versus Sr isotope plot (Fig. 13.40). On this diagram, the Aeolian arc defines a horizontal array which is consistent with two-component mixing between normal depleted mantle and subducted sediment. In contrast, Tongan data trend upwards towards altered MORB and marine carbonate, from where the U-enriched fluid flux is probably derived. A final note must be made regarding the Nicaraguan data. These have very high ²³⁰Th contents, reminiscent of a metasomatised source, but actually fall on the U-depleted side of the equiline (Fig. 13.36). This unusual signature is probably best attributed to a source which suffered U metasomatism some time prior to magma generation.

Fig. 13.40. Th)Sr isotope diagram showing possible mixing models to explain the departure of arc magmas from the mantle array of MORB and OIB compositions (shaded). After McDermott and Hawkesworth (1991).

Recent studies on arc lavas have attempted to put tighter limits on the timing of fluid influx and magma genesis in the mantle wedge. Evidence relevant to the timing of fluid influx came from new studies by Turner et al. (1997) and Elliott et al. (1997) on the Tonga – Kermadec and Marianas arcs. These new data sets revealed low positive slopes on the U–Th isochron diagram, especially when the lower bound to each data set was considered (Fig. 13.41). This lower bound is believed to be most indicative of the typical time from fluid metasomatism of the mantle source to the time of eruption. In contrast, individual data points lying above the line are attributed to magma batches that had longer residence times in crustal magma chambers. The lower bounds to each data set gave slope ages of 30, 50 and 30 kyr respectively for the Tonga, Kermadec and Marianas arcs, suggesting quite rapid processes of magma genesis and ascent in oceanic arcs. However, because of its half life of only 1600 yr, ²²⁶Ra analysis offers the opportunity of seeing even more short-term processes in arc magmas.

Fig. 13.41. Data arrays from the Tongan arc ( solid ) and Kermadec arc ( o ) on a U – Th isochron diagram, showing arrays with possible age significance. After Turner et al. (1997).

13.4.2 Ra–Th evidence

Early work on the Ra–Th systematics of arc lavas (Gill and Williams, 1990) revealed large ²²⁶Ra excesses in many lavas, with a weak positive correlation between ²²⁶Ra and ²³⁰Th enrichment. This led Gill and Williams to speculate that ²²⁶Ra enrichment might be linked to subduction-related fluids. However, subsequent studies (e.g. Chabaux and Allegre, 1994) were equivocal. Furthermore, the discovery of tight correlation lines on the U–Th isochron diagram, implying that fluid enrichment around 30 kyr ago, suggested that any ²²⁶Ra introduced by subduction-related fluids would have decayed away before eruption. Therefore, Turner et al. (1997) suggested that the ²³⁸U and ²²⁶Ra excesses in arc magmas were due to separate processes: the former due to enrichment of the mantle wedge by subduction-related fluids, and the latter from the subsequent melting of this metasomatised mantle.

A study on several different volcanic centres from the Lesser Antilles arc (Chabaux et al., 1999) reversed this interpretation by showing that variations between ²²⁶Ra/²³⁰Th and ²³⁸U/²³⁰Th activity ratios were well correlated, suggesting that both ²³⁸U and ²²⁶Ra enrichment were caused by the subduction-related fluids themselves. This model was confirmed in a study of the Tonga – Kermadec arc (Turner et al., 2000), which showed that the largest ²²⁶Ra excesses were found in the most depleted basalts with the highest Ba/Th ratios (Fig. 13.42). Turner et al. also found that as the silica contents of the magmas increased, ²²⁶Ra excesses declined, reflecting increasing residence times in differentiating magma chambers.

Fig. 13.42. Plot of Ba/Th weight ratio against ²²⁶Ra/²³⁰Th activity ratio showing a positive correlation between ²²⁶Ra enrichment the trace element indicator of enrichment by subduction-related fluids. ( solid ) = basalts; ( o ) = evolved lavas?? After Turner et al. (2000).

Evidence that ²²⁶Ra enrichment is caused by fluid metasomatism is problematical because the earlier U–Th evidence from the Tongan arc suggested that fluid enrichment occurred ca. 30 kyr ago. To overcome this problem, Turner et al. suggested that multiple fluid injections could have invaded the mantle wedge (Fig. 13.43). According to this model, an earlier event would have introduced both ²³⁸U and ²²⁶Ra into the wedge, but the latter nuclide soon decayed away. Meanwhile, as ²²⁶Ra was decaying in the wedge, it was being replenished in the Th-rich subducting slab. Therefore, when a second episode of fluid metasomatism occurred, the replenished inventory of ²²⁶Ra was again released into the mantle wedge. On this second occasion the metasomatic event was quickly followed by the extraction of a basaltic melt, which therefore picked up ²³⁸U from the first event and ²²⁶Ra from the second event.

Fig. 13.43. Plot of excess activities of ²²⁶Ra and ²³⁸U against time to show two-stage metasomatism proposed to explain the enrichment of Tongan lavas in U-series nuclides from subduction-related fluids. After Turner et al. (2000).

Further confirmation of the role of fluid metasomatism in causing ²²⁶Ra enrichments was provided by the observation of a positive correlation with beryllium isotope ratios in Andean lavas (Fig. 13.44). Since cosmogenic ¹⁰Be can only originate from subducted pelagic sediment (section 14.3.6), the correlation with ²²⁶Ra provides strong evidence for the origin of these enrichments in subduction-related fluids (Sigmarsson et al., 2002). However, Sigmarsson suggested that the data arrays with shallow slopes on the U–Th isochron diagram might be mixing lines produced by a multi-stage extraction of fluids from the slab, rather than isochrons dating the first stage of a two-phase process of fluid metasomatism.

Fig. 13.44. Plot of atomic beryllium isotope ratio against ²²⁶Ra/²³⁰Th activity ratio for lavas from the Southern Volcanic Zone of the Andes. Open symbols are lavas believed to have lost their excess ¹⁰Be and ²²⁶Ra during a complex magmatic history in the continental crust. After Sigmarsson et al. (2002).

As well as throwing more light on the timing of fluid metasomatism, ²²⁶Ra evidence from arc basalts also provides new constraints on the rates of ascent of basic magmas. This is because the process of fluid metasomatism must occur at nearly 100 km depth, since the fluids are released by pressure-induced breakdown of amphibole in the subducting slab (section 6.6). In contrast, it is widely argued that ²²⁶Ra enrichments under ocean ridges occur at quite shallow depths, due to ingrowth in the upwelling mantle source as a result of equilibrium porous flow (section 13.3.5).

To investigate these constraints in more detail, Turner et al. (2001) measured Ra–Th activities in seven additional arc systems, for comparison with the Tongan data. It was found that these other arc systems display behaviour similar to that of the Tongan arc, but less extreme in magnitude. Thus the other arcs also displayed positive correlations of initial ²²⁶Ra/²³⁰Th activity ratios with ²³⁸U/²³⁰Th activity and with Ba/Th weight ratio, and inverse hyperbolic relationships with silica content. These findings confirm that ²²⁶Ra enrichments in arcs are a general product of metasomatism by slab-derived fluids, but they also place tight constraints on rates of magma ascent, suggesting that primary subduction-related magmas probably rise at rates approaching 1km/year from their mantle source at ca. 100 km depth. This implies that these magmas rise through open channel-ways rather than by melt percolation. However, it remains unclear as to whether this behaviour is peculiar to fluid-rich subduction-related magmas, or whether MORB melts might also ascend more rapidly than previously thought.

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