Allanite: thorium and light rare earth element carrier in subducted crust

Abstract

The investigation of deeply subducted eclogites from the Dora-Maira massif, Western Alps reveals that accessory minerals are important hosts for trace elements. Rutile contains most of the bulk rock Ti, Nb and Ta while zircon hosts nearly all Zr and Hf. More than 90% of the bulk rock light rare earth elements (LREE) and Th and about 75% of U are incorporated in accessory allanite. Phengite is the most interesting major mineral because it hosts more than 95% of Rb, Ba and Cs. Synthesis piston cylinder experiments in a model crustal composition in the range 2.0-4.5 GPa, 680-1150 °C doped with trace elements demonstrate that accessory allanite forms at the expense of the major mineral zoisite at temperatures above 700 °C and 2.0 GPa. Allanite is stabilised by the incorporation of light rare earth elements and was found up to temperatures of 1050 °C and to pressures of at least 4.5 GPa. Disappearance of allanite is caused by dissolution in a coexisting hydrous granitic melt and is not related to the breakdown of any major phase. Experimentally determined element partitioning between allanite and a hydrous granitic melt at 900 °C, 2.0 GPa yield DLREEall/melt of about 200, DThall/melt of 60 and DUall/melt of 20. These results, combined with literature data, are used to estimate light rare earth element saturation levels of subduction zone liquids as a function of temperature. Allanite is a residual phase up to temperatures of at least 900 and 1000 °C for metabasalts and metasediments, respectively, at ≤10% partial melting. Allanite is therefore capable of controlling LREE and to a lesser extent Th contents for the critical temperature range of subduction zone liquid extraction. Low temperature fluids coexisting with allanite are not capable of transporting significant amounts of light rare earth elements and therefore arc lavas showing enrichment of these elements probably originate from a mantle source that experienced metasomatism by a hydrous granitic melt.