Oceanic Crust or Sub-arc Mantle Melting: 238U-230Th Disequilibria in the Austral and Southern Volcanic Zone, Chile

O. Sigmarsson CNRS URA-10, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Fd., France


J. Knowles CNRS URA-10, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Fd., France

H. Martin CNRS URA-10, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Fd., France

Young arc-lavas show a large range of 238U - 230Th radioactive disequilibrium. Many of them have 238U excesses over 230Th which are thought to reflect addition of slab-derived aqueous fluids to the sub-arc mantle inducing partial melting and often rapid magma ascent. Approximately half of analysed arc-lavas display equilibrium between 238U and 230Th which has been suggested to result from mantle metasomatism older than 0.3 Ma. Excesses of 230Th have been found in a few lavas, some of which are associated with subduction of young oceanic crust.

In the South Andes, the Southern Volcanic Zone (SVZ) and the Austral Volcanic Zone (AVZ) are separated by a volcanic gap related to the subduction of the Chile Rise. The age of the young Antarctica plate subducted under the AVZ increases towards the South (12 - 24 Ma, at the Chile trench). North of the Chile triple point, a significantly older Nazca plate is being subducted below the SVZ.

Dacites and andesite (with adakitic characteristics) from five volcanoes extending over the entire AVZ, are all enriched in 230Th compared to 238U (4 - 27%). The (230Th/238) slightly increases with Th abundances and is correlated with high La/Yb ratios. Furthermore, (230Th/232Th) of the dacites (0.665 - 0.97) are negatively correlated with Th abundances (11.0 - 0.910 ppm) southward along the AVZ. These features are compatible with the following model:

The degree of dehydration of a subducting oceanic crust depends on its thermal regime and consequently on its age. Young crust retains a substantial amount of fluids until it partially melts at the wet solidus. The younger (and hotter) a subducted plate is, the greater will be its dehydration before it reaches the amphibolite - eclogite transition: the degree of melting will thus be lower (case 1) than for an older plate which has retained more fluids (case 2). In contrast, a much older oceanic crust will extensively dehydrate, precluding slab melting at depths relevant to front arc volcanism, and only contributes fluids to the mantle source of arc magmas (case 3).

Dehydrated slabs will have lost mobile elements, such as U, in proportions related to the amount of escaped fluids. After this dehydration, the younger crust subducting under the northern part of the AVZ will be more enriched in 230Th and its (230Th/232Th) will decrease with time compared to the older crust below the southern part of the AVZ. Both high (230Th/238U) and La/Yb ratios of the dacites decrease southward along the AVZ, which may reflect increasing degree of melting of an increasingly old slab (evolution from case 1 to case 2). The residue after the slab melting will consist of clinopyroxene + garnet ± amphibole ± rutile, which imposes the element fractionation observed in the magmas. Possible mantle and crustal assimilation will only dilute, but not change, the geochemical characteristics discussed here.

In contrast to the AVZ, lavas from SVZ have normal and nearly constant arc-like La/Yb with 238U highly enriched over 230Th. In that case, fluid induced melting only occured in the mantle wedge (case 3). In the southern Andes, U-Th systematics thus allows a clear distinction between melting of the subducted oceanic crust (AVZ) and melting of the metasomatized sub-arc mantle (SVZ).