Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120–180 km depth

Abstract

Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120-180 km and temperatures of 700-1,200 degrees C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa.

Sources of post-orogenic calcalkaline magmas: The Arrochar and Garabal Hill–Glen Fyne complexes, Scotland

“…Sr is not carried into the mantle by low-density fluids at T < 900 °C, but is efficiently transported in the higher-T, low-density fluids, melts and high-density fluids, with a partition coefficient of about 20. Schmidt et al (2004) demonstrated that subducted MORB basalts, pelites and greywackes will all develop similar eclogitic mineral assemblages and that the coexisting fluids will be rich in Si and K. Schmidt et al (2004) and Ishikawa et al (2005) both determined that supercritical, solute-rich fluids/melts evolved from the slab would be aqueous and strikingly enriched in B, Ba, Rb and K. It therefore seems likely that the Kessel et al (2005) partitioning relationships will be valid for a wide range of subducted rocks.…”

“…At these great depths the fugitive phase, whether low-density fluid, melt or high-density fluid, would always be highly enriched in Rb and Ba. Fluid/melt-mineral partition coefficients (Kessel et al, 2005) are of the order of 100 and 40 respectively, implying efficient scavenging of these elements from the slab into the mantle. Sr is not carried into the mantle by low-density fluids at T < 900 °C, but is efficiently transported in the higher-T, low-density fluids, melts and high-density fluids, with a partition coefficient of about 20.…”

“…Portnyagin et al (2007) The experiments of Rapp et al (1999) and the geochemical and theoretical studies of Xiong et al (2006) suggest that fertile mantle, metasomatised by high-T melts, would consist of sodic amphibole pyroxenite, with phlogopite pyroxenite formed by interaction with lower-T melts. Kessel et al (2005) measured the compositions of fluids and melts that had been experimentally equilibrated with a basaltic eclogite at P-T conditions relevant to subduction zones. At depths up to 120 km they showed that either dehydration or partial melting could occur, depending on T, with melts and fluids showing contrasting behaviour of U, Th, Sr, Ba, Be and LREE.…”

Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc

“…Although it has been proposed that the Nd and Hf isotopic compositions of northern Tonga-southern Kermadec lavas represent pre-existing mantle wedge heterogeneities (for example, refs 12,40,41), the corresponding Sr and Pb isotopic compositions are interpreted as reflecting slab-derived, fluidtransported Sr and Pb from altered oceanic crust and sediment cover into the overlying mantle wedge (for example, refs [42][43][44]. This dichotomy allows us to track slab-derived inputs through the 'subduction factory'.…”

“…Incorporation of volcaniclastic material into the mantle source beneath the central TongaKermadec arc should result in elevated Sr isotope values, which is only observed to a minor extent in lavas from 'U' and 'V'. Pb and Sr each have different mobility in aqueous fluids (for example, refs 43,44) and may, therefore, be decoupled. However, as we observe only a minor increase in Sr, we favour a predominant influence through dehydration of the altered Louisville crust.…”