Role of rutile in subduction zones

Rutile may play a key role in subduction zone processes in retaining high field strength elements (HFSE) during dehydration reactions, thereby explaining the omnipresent negative Nb-anomalies in island arc volcanoes (Nicholls

& Ringwood, 1973; Saunders et al., 1980). For this reason intensive studies have focussed on the partitioning behaviour of a number of trace elements (Nb, Ta, Zr, Hf, U, Th) between rutile and fluid (Brenan et al., 1994;

Stalder et al., 1998) as well as between rutile and melt (Jenner et al., 1993) These experimental results are essentially consistent with earlier models showing that Nb and Ta behave as highly compatible elements in the presence of rutile.

Zr and Hf are not considered in the following discussion since they are expected to be controlled by zircon and not by rutile (Fig. 14). However, we point out that subduction zone dehydration models for Zr and Hf based on rutile/fluid partition coefficients must, therefore, yield erroneous results. The incorporation of zircon in dehydration models might lead to an explanation of why island arc magmas almost always show negative Nb-Ta anomalies, but sometimes lack negative Zr-Hf anomalies (Briqueu et al., 1984).

Nb/Ta ratios have recently received considerable attention following the finding that these elements maybe significantly fractionated from each other by Ti-rich phases (Green & Pearson, 1987; Brenan et al., 1994).

Subduction-related rocks have Nb/Ta ratios spanning a range from subchondritic to superchondritic (8-33; Stolz et al., 1996; Münker, 1998) and seem to follow at least two different trends (Münker, 1998). The interpretation, however, is controversial. Green (1995) and Münker (1998) referred to the difference of DNb/DTa partition

coefficients between rutile/fluid and rutile/melt, where the presence of rutile produced melts with elevated Nb/Ta and fluids with lowered Nb/Ta ratios. Stolz et al. (1996) also favoured the argument that elevated Nb/Ta ratios in high-K lavas were produced by a melt component from the subducted slab, and in addition they questioned the capacity of fluids to change significantly the Nb/Ta ratio in the source area of more common low-K and calc-alkaline lavas. Low Nb/Ta ratios can be explained by re-melting of already depleted mantle (Eggins et al., 1997), where a difference in DNb/DTa between clinopyroxene and melt (Jenner et al., 1993) causes a lowering of the Nb/Ta ratio in the residuum. The key difference between the two models is that fluid loss from subducting oceanic crust will either produce a superchondritic Nb/Ta ratio in the eclogitic residuum in the former case, but will leave no impact in the latter case. Our data seem to support the latter model. Starting from a low-temperature hydrothermally altered basalt (Chapter 2), we infer that eclogites from Trescolmen have lost between 2-4% H2O by dehydration (Schmidt & Poli, 1998), similar to subducted oceanic crust underneath volcanic arcs. Rutiles from this locality have largely chondritic or slightly subchondritic Nb/Ta ratios, with an average of 17.4 (Table 4). One sample (77-5) has a significantly higher Nb/Ta ratio of 31.7 and excluding this sample we obtain an average value of only 15.9, with a range of 9 eclogitic rutiles from 13.6 to 17.4. This value is identical to Nb/Ta ratios from rutiles in similar continental settings (16.0; Rudnick, 1998). However, superchondritic rutiles (with a median Nb/Ta ratio of 33) supporting the first model have been found in eclogitic xenoliths (Rudnick, 1998) so the debate remains open.

As pointed out by Fitton (1995) and McDonough (1999), Nb and Ta systematics tell only part of the rutile story since rutile can also be important for other HFSE. We have shown that besides Nb and Ta, rutile is an important phase for Sb, W, Mo and Sn in metabasitic and -pelitic systems at conditions relevant for subducting slabs underneath volcanic arcs (Schmidt & Poli 1998). From the elastic strain model of Blundy & Wood (1994) and Beattie (1994), it should be expected that in terms of rutile-fluid partitioning, Nb, Ta, Sb (and W?) behave as one group and Mo and Sn as another group in subduction zone environments, taking into consideration ionic radius and charge (Fig. 9). This would be consistent with the trace element systematics of sample 50-13, as described earlier (Fig. 14). Nevertheless, this applies only as long as the crystal structure dominates the trace element partitioning e.g.

in rock samples with several solid phases.

We can now investigate the behaviour of the extended HFSE group in subduction zones with respect to the role of rutile. Fitton (1995) showed that subduction-related lavas from the western US are strongly depleted in Nb and Mo compared to similarly incompatible elements (La, Nd), as well as being highly correlated with each other, and concluded that rutile retains both Nb and Mo in the downgoing slab during dehydration. No clear trend can be found for Sn and W in subduction-related lavas, showing neither pronounced enrichment nor depletion in terms of Sn/Sm and W/Th ratios (Jochum et al., 1993; Noll et al., 1996). Instead Sb is one of the most enriched elements in volcanic arc rocks in terms of its ratio with Pr and follows similar enrichment trends to B, As, Tl and Pb (Ryan et al., 1995;

Noll et al., 1996; Jochum & Hofmann, 1997).

Trace elements enriched in volcanic arc rocks are inferred to be carried by a fluid from the dehydrating slab.

Assuming that light REE (LREE, La-Sm) as well as Th are moderately mobilized in these fluids (Brenan et al., 1995, Stalder et al., 1998), it can be said that Nb, Ta and Mo are less enriched in the fluid, Sn and W similarly, and Sb highly enriched compared to corresponding LREE (and Th). Sb is highly enriched in pelagic sediment and altered oceanic crust (Jochum & Verma, 1996), both likely source rocks for fluids in dehydrating slabs. Nevertheless high Sb concentration in the fluid caused only by an anomalously enriched source can be ruled out for the following

reason. If Sb were as mobile or even less mobile than LREE in dehydrating fluids, Sb/LREE ratios would be expected to be higher than in MORB, but should stay constant (or slightly increase) in across-arc traverses, as normally observed for Nb/La ratios. However, the contrary is observed in across-arc studies, where Ryan et al.

(1995) and Noll et al. (1996) showed for the Japan and Kurile arc transects that Sb/Ce ratios continuously decrease in volcanic arc rocks with increasing depths to the subducting slab.

Combining the observations of subduction zone studies with our own results, we can conclude that neither an enriched source area nor the mineral assemblage of the downgoing slab (dominated by rutile) can be solely responsible for the extreme enrichments of Sb in subduction-related magmas. An alternative might be found in the behaviour of the aqueous fluid itself, with the ability to fractionate Sb from Nb, which otherwise show similar partitioning behaviour in eclogite assemblages and are not fractionated from each other in magmatic systems.

Experimental studies on rutile/fluid partitioning of the extended HFSE group are required to test such a statement and should give some highly desired insight on fluid behaviour under upper mantle conditions. Rutile/fluid experiments would have the advantage of forming a relatively simple system stable under a wide range of PT-conditions. One of the most challenging questions concerns whether complexing systematics in low pressure environments can be applied to upper mantle conditions. Noll et al. (1996) discussed the complexing properties of Mo, Sn, W and Sb, and concluded that subduction zone systematics could be explained by preferentially transporting Sb in a H2S-rich and Cl-poor, reducing and acidic hydrothermal fluid. This is problematic with respect to the observation of Cl-enrichment in island arc volcanics in relation to MORB (Ito et al., 1983) and calculations of Manning (1998) that showed that high pressure fluids in equilibrium with blueschists/eclogites have rather basic properties. Nevertheless, this discussion shows the prospect of characterizing fluid properties coming from dehydrating oceanic crust and better evaluating the role of rutile in fractionating element ratios on a global scale.

Chapter 4. Evaluating hydrous eclogites from Trescolmen