Growth of early continental crust controlled by melting of amphibolite in subduction zones

Abstract

It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process.

The formation of Qulong adakites and their relationship with porphyry copper deposit: Geochemical constraints

“…Mid-ocean ridge basalts have fairly constant Nb/Ta of~17 (with Nb/Ta value of 17.7; Sun and McDonough, 1989) whereas the continental crust has Nb/Ta of~12-13 in average and~8 in the lower crust (with Nb/Ta value of 8.0; Rudnick and Gao, 2003). Partial melting of MORB eclogite forms melt with Nb/Ta up to~25 (Foley et al, 2002). Therefore, slab melts with variable crustal contamination plausibly explain the Nb/Ta ratios of Qulong adakites.…”

“…Niobium and Ta were taken as geochemical twins that do not fractionate from each other during mantle magmatisms (with Nb/Ta value of 17.7; Sun and McDonough, 1989). Nevertheless, Nb and Ta do fractionate from each other during crustal processes Foley et al, 2002;Liang et al, 2009;Xiao et al, 2006). Mid-ocean ridge basalts have fairly constant Nb/Ta of~17 (with Nb/Ta value of 17.7; Sun and McDonough, 1989) whereas the continental crust has Nb/Ta of~12-13 in average and~8 in the lower crust (with Nb/Ta value of 8.0; Rudnick and Gao, 2003).…”

Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt

“…Both Nb and Ta show similar geochemical behaviors due to their nearly identical ionic radii and charge (Jochum et al, 1986(Jochum et al, , 1989, so Nb/ Ta value is hardly affected by magmatic processes such as fractional crystallization and partial melting unless a significant volume of rutile and/or low-Mgnumber amphibole is involved in the mantle source (e.g., Ionov et al, 1997;Foley et al, 2000Foley et al, , 2002Tiepolo et al, 2001). Partition coefficients for rutile/ melt from both experimental and natural systems suggest that rutile is a potential phase to fractionate Nb from Ta and produce superchondritic Nb/Ta value in the melt (Foley et al, , 2002. Hence, melt coexistent with residual rutile at the time of melting will have superchondritic rather than subchondritic Nb/Ta value.…”

“…Mass consideration indicates that if only 0.1% rutile is added into the mantle, Nb/Ta ratio in the metasomatized source would be increased by about 40-60%. On the other hand, amphibole of low Mg number (e.g., titanian pargasite and kaersutite) is another phase to fractionate Nb from Ta (Tiepolo et al, 2001;Foley et al, 2002;Grégoire et al, 2000a,b). Generally, amphibole in mantle peridotites generally shows high Mg number (>70) and melting of amphibole-bearing peridotites could not successfully produce melts with low Nb/Ta ratios (e.g., Foley et al, 2002).…”

Partitioning of rare earth and high field strength elements between titanite and phonolitic liquid

“…Fractionation of titanite during magmatic crystallization-differentiation therefore leaves a distinctive imprint of depletion in middle REE and increasing Nb/Ta in the residual liquid (Wolff, 1984;Wolff and Storey, 1984;Glazner et al, 2008). Partly because of the strong geochemical similarity of Nb and Ta, and the restricted stability in P-T-X space of Ti-phases such as titanite and rutile that exert strong leverage on these elements, variable Nb/Ta in different terrestrial reservoirs has been of particular interest (Green and Pearson, 1987;Rudnick et al, 1993;Green, 1995;Lundstrom et al, 1998;Barth et al, 2000;Horng and Hess, 2000;Tiepolo et al, 2000;Wade and Wood, 2001;Foley et al, 2002;John et al, 2010). Titanite also incorporates high concentrations of U and Th, and may be employed in geochronology (Frost et al, 2000).…”