Geochemistry of continental subduction-zone fluids

Abstract

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

Multiple episodes of anatexis in a collisional orogen: Zircon evidence from migmatite in the Dabie orogen

“…On the other hand, the anatexis of metamorphic rocks generates anatectic melts that have not separated from parental rocks and thus did not achieve the saturation of incompatible trace elements by fractional crystallization (Zheng and Hermann, 2014). With transport and accumulation of the hydrous melts, the anatectic melts would evolve to the magmatic melts to achieve the saturation of water and incompatible trace elements.…”

Two types of the crust-mantle interaction in continental subduction zones

“…This means that subducting crustal rocks may not experience obvious dehydration at shallow depths (<80 km) but become significantly dehydrated and even partially melted at deep depths (80-130 km) in cold continental subduction zones. Thus, it failed to trigger syn-subduction arc volcanism because the overlying subcontinental lithospheric mantle (SCLM) is colder than the mantle wedge overlying oceanic slabs (Zheng and Hermann, 2014). Nevertheless, the slab-derived fluid/melt not only results in amphibolite-facies retrogression and veins with various compositions inside the subducted continental crust, but also metasomatizes the overlying mantle wedge peridotite of overlying continental slab.…”

“…However, this overlooked the fact that the continental crust is generally enriched in SiO 2 and K 2 O in lithochemistry (Rudnick and Gao, 2003). Because Si and K are the most active components of crustal rocks (Zheng and Hermann, 2014), the resulted products of crustal contamination should exhibit positive correlations between ( 87 Sr/ 86 Sr) i and SiO 2 or K 2 O, and negative correlations between ε Nd (t) values and SiO 2 or K 2 O. (3) Mafic igneous rocks with flat REE distribution patterns were interpreted to be MORB, overlooking the fact that back-arc basin basalts are generally composed of two-stage volcanics, in which early stage rocks exhibit geochemical features like island arc basalts whereas late stage rocks have geochemical characteristics similar to MORB (e.g., Lawton and McMillan, 1999;Taylor and Martinez, 2003;Pearce and Stern, 2006).…”

“…During slab subduction/exhumation, materials in subduction channel undergo deformation, metamorphism and even partial melting. Aqueous fluids, hydrous melts and even supercritical fluids can be liberated from subducting/exhuming crustal rocks, which then alter or metasomatize the overlying mantle wedge peridotite to form fertile and enriched metasomatites (Zheng and Hermann, 2014). Partial melting of the mantle metasomatites gives rise to various mafic igneous rocks above subduction zones.…”

“…Thorough and refined studies of the relevant tectonic processes have now been developed toward understanding of the crust-mantle interaction in subduction zones (Zheng and Hermann, 2014). Mafic igneous rocks in fossil subduction zones contain large amounts of information on the mantle, and they are the important carriers for studying the mantle geochemistry.…”