Hermann reported in 2002 that 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 jC doped with trace elements demonstrate that accessory allanite forms at the expense of the major mineral zoisite at temperatures above 700 jC and 2.0 GPa.
Allanite is stabilized by the incorporation of light rare earth elements and was found up to temperatures of 1050 jC 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 jC, 2.0 GPa yield DLREE all/melt of about 200, DTh all/melt of 60 and DU all/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 jC for metabasalts and metasediments, respectively, at V10% 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.
Hermann concluded that:
1) Accessory minerals play an important role in the trace element budget of subducted crust. In two eclogites from the Dora-Maira massif, which experienced metamorphic conditions off 4.0 GPa, 730 jC, more than 95% of Ti, Nb and Ta is incorporated in rutile, more than 95% of Zr and Hf is incorporated in zircon and more than 90% of LREE and Th is incorporated in allanite. Allanite and zircon host nearly all U and P is mainly stored in apatite. Among the major minerals, phengite is the most interesting one because it incorporates more than 95% of the bulk rock Rb, Ba and Cs.
2) Piston cylinder experiments demonstrate that allanite is expected as a residual phase in subducted crust in the region of liquid extraction. Therefore, allanite will control the LREE and the Th content of subduction zone liquids.
3) On the basis of experiments, LREE saturation levels of subduction zone liquids coexisting with allanite are estimated to be 1/4 the primitive mantle value at 600 jC, 12 times at 800 jC and 180 times at1000 jC. This demonstrates that significant LREE(and Th) mantle metasomatism is only possible if the subduction zone liquid is extracted at high temperature.
4) Residual allanite and phengite in the subducted rocks control the composition of the subduction zone liquid. This liquid is probably a supercritical fluid at low temperature, characterized by low LREE contents and a high LILE/LREE ratio and a hydrous granitic melt at high temperature, characterized by higher LREE contents and a lower LILE/LREE ratio.
5) Experimentally determined partitioning reveal that DTh all/melt>DU all/melt. Therefore, residual allanite could participate in the production of subduction zone liquids with low Th/U ratios.
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