HP and UHP garnet peridotites and pyroxenites

HP and UHP garnet peridotites and pyroxenites

Mantle wedge The least known piece of the subduction factory Mantle-wedge peridotites emplace within subducting continental crust (Brueckner, 998; van Roermund et al., ) Metasomatism affects the mantle wedge peridotites and ultramafites s.l. tectonically engaged in the subucting crust These rocks are clues to an understanding of metasomatism of mantle domains percolated by fluids and melts arising from subducting slabs Brueckner, 998

The Ulten study case In the Ulten Zone peridotites are tectonically coupled with high-pressure gneiss and kyanitemigmatites of Variscan age. The ultramafic rocks range from spinel to garnet + amphibole peridotites. They represent previous mantle wedge which underwent cooling, increase in pressure and hydration due to corner flow motion and tectonic uptake inside subducted continental crust. Details and reference are in Nimis and Morten (2000, J. Geodynamics). The garnet and amphibole peridotites are metasomatized by the influx of aqueous fluids rich in incompatible elements (Rampone and Morten, 200). Enrichment is coheval with partial melting of the associated migmatized gneiss at eclogite-facies (Tumiati et al., 2003)

Spinel peridotite with coronitic garnet Mylonitic garnet + amphibole peridotite Opx Amphibole peridotite with mega opx High pressure kyanite migmatite

Sample / PM 0, 0, 373 ULT6 DMM Bulk rocks Spinel lherzolite 0,0 La Ce Pr Nd Sm Eu Gd TbDy Er Yb Lu 0,0 Li Cs Ba Th Nb La Pr Sr Zr Sm Ti Tb Y Yb Sc Be Rb Pb U P Ce Nd Hf Eu Gd Dy Er Lu Sample /PM MK5C ULT2 DMM Garnet + amph peridotite 0, 0, 0,0 La Ce Pr Nd Sm Eu Gd TbDy Er Yb Lu 0,0 Li Cs Ba Th Nb La Pr Sr Zr Sm Ti Tb Y Yb Sc Be Rb Pb U P Ce Nd Hf Eu Gd Dy Er Lu Sample /PM 300B MO DMM Amph peridotite 0, 0, 0,0 La Ce Pr Nd Sm Eu Gd TbDy Er Yb Lu 0,0 Li Cs Ba Be Rb Pb Th Nb La Pr Sr Zr Sm Ti Tb Y Yb Sc U P Ce Nd Hf Eu Gd Dy Er Lu

Opx Opx Minerals 0, 0,0 0, Spinellherzolites 0,00 La Ce Pr NdSm Eu Gd Tb Dy Er Yb Lu 0,0 Li Cs Ba Be Rb K Pb U Th Ta Nb La Pr P Ce Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc Cpx Cpx Sample / PM 0, 0, 0, 0,0 0,00 La Ce Pr Nd Sm Eu Gd Tb Dy Er Yb Lu Gnt La Ce Pr N Sm Eu Gd Tb Dy Er Yb Lu 0,0 0,00 0, 0,0 0,00 0,000 Li Cs Ba Be Rb K Li Cs Ba Be Rb K Pb U Th Pb U Th Ta Nb Ta Nb La Pr P Ce La Pr P Ce Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc Gnt Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc Garnet: in disequilibrium with the spinel cpx and opx. The trace element erichment of pyroxenes pre-dates garnet

CPX CPX 0, Minerals 0, 0,0 S a m ple/ PM 0, La Ce Pr Nd Sm Eu Gd Tb Dy Er Yb Lu Gnt 0,0 Li 0 0, 0,0 0,00 Cs Ba Pb U Ta La Pr Be Rb K Th Nb P Ce Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc Gnt Gnt+amph peridotites 0,000 0,0 La Ce Pr Nd Sm Eu Gd Tb Dy Er Yb Lu 0,0000 Li Cs Ba Pb U Ta La Pr Be Rb K Th Nb P Ce Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc Amph 0 Amph S a m p le / P M 0, La Ce Pr Nd Sm Eu Gd Tb Dy Er Yb Lu 0, Li Cs Ba Pb U Ta La Pr Be Rb K Th Nb P Ce Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc

Minerals Amphibole peridotites Opx Opx 0, 0,0 S a m p le/ P M 0, 0,0 0,00 La Ce Pr Nd Sm Eu Gd Tb Dy ErYb Lu 0,00 0 Li Cs Ba Be Rb Pb U K Th Ta Nb La P Ce Pr Nd Hf Eu Gd Dy Er Lu Sr Zr Sm Ti Tb Y Yb Sc S a m p l e / P M Amph S a m p le /P M Amph 0, La Ce Pr Nd Sm Eu Gd Tb Dy ErYb Lu 0, Li Cs Ba Be Rb K Pb U Th Ta Nb P La Pr Nd Hf Eu Gd Dy Er Lu Ce Sr Zr S Ti Tb Y Yb c

Main result: spinel- and amphibole + garnet peridotites have comparable metasomatic imprints, enriched in LREE and LILE. Amphibole + garnet peridotites: influx of aqueous fluids. Absence of amphibole in spinel lherzolites: no aqueous fluid. Fluid influx at the spinelfacies stage (T = 200 C; P =.5 Gpa): peridotite partial melting and incompatible element loss into the melt. Criptic metasomatism of the spinel peridotites: driven by a melt agent. Pb, U, Cs, Li enrichment of Ulten spinel lherzolites: interaction with a subduction-related metasomatic melt

Li systematic suggests that a mafic melt was the agent responsible for the early spinel-facies metasomatic imprint. Average Li ol (ppm) 6 5 4 3 2 0 Carbonatite Mafic melt 0 2 4 6 8 2 4 6 Average Li cpx (ppm) Sp-lherzolite Gnt-peridotite S&W, peridotite S&W, metas. peridotite Compared with the study of Seitz and Woodland () the Ulten spinel lherzolites (brown symbols) display the same array of peridotites metasomatized by mafic melts. The garnet + amphibole peridotites (red symbols) have a different Li distribution between olivine and cpx. Spinel lherzolites cpx/opx DLi = 3 7 cpx/ol DLi = 2.7-8. Garnet-amphibole peridotites cpx/amph DLi = 0.7-0.8 cpx/ol DLi = 0.7-0.8 Mantle olivine equilibrated with fluid: important Li repository

80 70 60 Olivine P (ppm) 50 40 30 20 0 373-3 ol ULT6 Sp lherz ol MK5D ol MK5C Gnt+amph ol ULT2 peridol 300B Amph MO peridol 2 3 4 5 6 7 Li (ppm) Olivine Monitor of changes in the agent. Li increases from spinel, to garnet + amphibole, to amphibole peridotites. Olivine equilibrated with a fluid phase: good Li repository

Transformation of spinel-peridotites into amphibole + garnet peridotites : Corner flow movement in the mantle wedge (Nimis and Morten, 2000) Transition from spinel to amphibole + garnet peridotites: change from melt to fluid as metasomatic agent Spinel lherzolite rich in Pb, U, Cs, Li: subduction-related melt. They formed a mantle wedge layer percolated by subduction magmas sourced at depth. Garnet + amphibole peridotites: infiltration of aqueous fluid at eclogitefacies. The signature remains unchanged during retrogression and further hydration. During the entire history the Ulten peridotites were fluxed by agents sourced by the same subducting crustal reservoir At eclogite-facies the migmatized gneiss is melting. How to get out a fluid?

opx Depth Km 0 50 250 C 750 C 0 C 200 C 400 C Mantle wedge Melt Fluid Sp-facies 2 750 C Grt + amph 3 Amph peridotite Opx 50 200 Subducting continental crust after Scambelluri et al., 2005, submitted 50 0 C 200 C fluids Partial melting melts

Proposal A possible explanation of the fluid-related metasomatic imprint of the garnet + amphibole peridotites may require formation of an orthopyroxe (+ garnet) layer. This would accomplish precipitation of the Si and Al load of hydrous melts uprising from the slab, production of an aqueous trace element-enriched fluid which enhances hydrous metasomatism in overlaying levels of the mantle wedge.