Traceelement chemistry of primitive arc volcanics

Trace elements in arc volcanics: comparison with oceanic basalts

The trace-element abundance patterns of primitive arc volcanics highlight first the overall difference between arc rocks and MORBs and second the similarity of different arc rocks to each other in respect of some particular trace-element features (Fig. 25.1). Arc rocks display spectacular enrichments in the large-ion lithophile elements (LILEs) Rb, Ba, K and also Pb and Sr. A common property of these elements is that they are highly soluble in aqueous fluids, and therefore this general feature of the arc trace-element patterns is in full accord with the hypothesis that fluid advection plays an important role in arc magmatism. On the one hand, Th and U, which are abundant in sediments, are also enriched compared with MORBs (Table 24.1); these enrichments, however, are not enough to supply the continental crust with these elements. On the other hand, all arc magmatic rock types are depleted in high-field-strength elements (HFSEs) such as Ta, Nb and Ti.

The rare earth elements also show a very specific abundance pattern. The LREEs are slightly more concentrated than in N-MORBs, whereas the rare Earth elements heavier than Sm are all relatively depleted in both basalts and andesites (Table 24.1) and show a concave-up distribution. Such a pattern is typical for melts that have equilibrated with amphibole, in which the HREEs are all only slightly incompatible; generally the D(mineral/melt)HREE values are within 0.3 to 1 (Dalpe and Baker, 2000). In the hydrated mantle wedge this hydrous silicate has a very large field of stability at pressures between ~ 0.2 and 3 GPa (i.e. at almost 100 km depth, Niida and Green, 1999).

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Rb Th K La Pb Sr Zr Sm Tb Dy Yb Y Ba U Nb Ce Pr Nd Hf Eu Ti Er Lu Ni

Fig. 25.1 Trace-element abundances in primitive arc volcanics and continental rocks. Crustal composition from Rudnick and Fountain (1995). Be, B, Nb and Ta abundances from McLennan (2001), N-MORB composition used for normalization from Sun and McDonough (1989). After Kelemen et al. (2003), © Elsevier Science 2003, reproduced by permission.

Rb Th K La Pb Sr Zr Sm Tb Dy Yb Y Ba U Nb Ce Pr Nd Hf Eu Ti Er Lu Ni

Fig. 25.1 Trace-element abundances in primitive arc volcanics and continental rocks. Crustal composition from Rudnick and Fountain (1995). Be, B, Nb and Ta abundances from McLennan (2001), N-MORB composition used for normalization from Sun and McDonough (1989). After Kelemen et al. (2003), © Elsevier Science 2003, reproduced by permission.

The depletion of Nb relative to La and MORBs (Fig. 25.1), also a fingerprint common to all arc magmatic rocks, is a further pointer to subduction-zone processes. The high La/Nb ratios of arc rocks exceed the ratios in primitive mantle, MORB-source mantle and all known mantle derivatives, which reflect the similar incompatibilities of La and Nb in common mantle minerals (Fig. 26.6). As the concentration of La fits with that of the other LREEs, it must be Nb that is behaving in an anomalous manner. Further, the negative Nb anomaly cannot be attributed to the fluid insolubility of Nb, as the LREEs are also not transported by fluids. A possible solution is that arc magmas are generated in equilibrium with a mineral or assemblage in which Nb is less incompatible than the LREEs, or even compatible. As Ti is also depleted in arc magmas (Table 24.1) and Nb is compatible in rutile (TiO2), this is the phase that is widely thought to cause the Nb anomaly (Kelemen et al., 2003). However, Foley et al. (2002) carried out experimental studies on the rutile-melt and amphibole-melt partitioning of Nb, La and Ta and found that amphibole also lowers the Nb/La ratio in the melt: the partition coefficient ratio DNb/DLa is ~ 2. Equilibration with amphibole would produce a low Nb/Ta ratio in the melt, as is indeed observed in many arc magmas and in the continental crust. This indicates that in the formation of arc basalts (and continental crust), the equilibration of melt with amphibole-bearing assemblages was probably an important factor, whether this took the form of the partial melting of amphibolite or the fractional crystallization of an amphibole-rich cumulate. A further examination of the abundance patterns reveals differences between basalts and andesites: the latter show the greatest enrichments in the LILEs as well as in U and Th. This is also true for basalts and andesites from the Aleutian arc, where no continental crust is present. These trace-element characteristics shed light on the complicated combination of fluid-releasing and partial-melting processes in subduction zones. It is particularly useful to compare the behaviour of fluid-insoluble elements with that of soluble elements.

Thorium is a suitable element for such a comparison (Straub et al., 2004). Experimental studies of eclogite-aqueous-fluid and peridotite-aqueous-fluid partitioning show that Th is poorly soluble. However, Th is highly incompatible during partial melting. Another highly incompatible element, Ba, is much more readily partitioned into a fluid phase than Th, and therefore the Ba/Th ratio of volcanics is predicted to be enhanced and variable if a fluid phase is involved in magma genesis. The Ba/Th ratios in arc volcanics are indeed variable by a factor of up to 100, reaching values as high as ~ 1000 (Fig. 25.2). For comparison, the average Ba/Th value inferred for MORBs is - 70 (Hofmann, 1988), for OIBs it is - 80 (see Sun and McDonough, 1989) and for the global average of subducted sediments (GLOSS) it is — 100 (Plank and Langmuir, 1998), and these values are much less variable.

The highest Ba/Th ratios are found in arc rocks with relatively low [Th] values and 143Nd/144Nd ratios similar to those in MORBs (Fig. 27.2(a)). Most arc volcanics show enhanced Th concentrations, resulting in lower Ba/Th ratios (but still higher than those of MORBs, OIBs and GLOSS). With increasing Th content their 143Nd/144Nd ratios approach those of GLOSS (Fig. 25.2), which indicates a large contribution from subducted sediments. As Nd is also insoluble in fluids, this combined trend indicates that the sediment contribution to these Th-rich arc volcanics resulted from partial melting of the sediments rather than their dehydration.

Uranium-thorium fractionation also occurs during slab dehydration: arc volcanics generally have higher U/Th ratios than both MORBs and sediments. This feature has also generally been considered as the result of preferential U transfer by fluids from the subducting slab into the mantle wedge; fluid-insensitive Th remained in the slab rocks, at least as long as these did not melt. According to the available fluid-mineral partition coefficients, U/Th ratios as high as — 40 are expected in slab-derived fluids, much exceeding those observed in any mantle and crustal rocks (Table 24.1, Turner et al., 2003). Addition of such a fluid should indeed increase the U/Th ratio in any reservoir.

The observed abundances of fluid-soluble trace elements in arc volcanics, along with estimates of the rates of volcanism in given arc segments, allow the fluxes of

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