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Mass-balance melting model for the Lanzo peridotites (Bodinier,1988)

Chemical and mineralogical variations in mantle rocks: traditionally ascribed to partial melting , but. ol±sp. dunites. harzburgites. ol+opx+sp. lherzolites. ol+opx+cpx+s p. Melting experiments: Tinaquillo lherzolite (Jaques & Green, 1980). pristine mantle source at ~ 4% Al2O3

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Mass-balance melting model for the Lanzo peridotites (Bodinier,1988)

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  1. Chemical and mineralogical variations in mantle rocks: traditionallyascribed to partial melting, but... ol±sp dunites harzburgites ol+opx+sp lherzolites ol+opx+cpx+sp Melting experiments: Tinaquillo lherzolite (Jaques & Green, 1980) pristine mantle source at ~ 4% Al2O3 (= Primitive Mantle) Mass-balance melting model for the Lanzo peridotites (Bodinier,1988)

  2. Someinconsistencies in the melting model ‘melt impregnation’ textures in plagioclase peridotites harzburgites-lherzolites layering Othris peridotite (Greece) (Dijkstra et al., 2001) Lanzo massif (Boudier & Nicolas, 1972) Horoman massif (Japan) (Takazawa et al., 1996) Oman ophiolite (Boudier & Nicolas, 1995) Melt redistribution due to compaction: layering = frozen compaction waves lherzolites = melt-rich zones (Obata & Nagahara, 1987) Plg-rich peridotites = refertilized by crystallization of trapped melts or reaction with percolating melts (e.g., Nicolas & Dupuy, 1984; Rampone et al., 1994) • chemical variations also result from reactive melt transport

  3. Melt-rock interactions: magmatic rejuvenation or "asthenospherization" of the lithospheric mantle the Lherz massif, France (lherzolite type-locality) • thermo-chemical "erosion" of an old • subcontinental mantle lithosphere • interactions between melt transport & deformation Le Roux et al. (2007) The Lherz spinel lherzolite: refertilized rather than pristine mantle. EPSL, 259: 599-612.

  4. Refertilization in Lherz: structural evidence harzburgites' foliation & lineation: constant orientation over the entire massif & crosscut, at the contacts by the websteritic banding & lherzolites' foliation, which are steeply deeping and mould the contacts

  5. Refertilization in Lherz: geochemicalevidence • Constant mineral compositions in lherzolites • Positive correlation between Cr and Al • REE enrichment at harzburgite-lherzolite contacts • partial melting • refertilization reactions melt1 + ol = melt2 + cpx + sp ± opx Harzburgite Websterite components Lherzolite Le Roux et al. (2007) EPSL

  6. Refertilization in Lherz: structural evidence contacts = changes in composition (+px,+sp,-ol) grain growth, annealing dispersion and reorientation of olivine CPO lherzolites =new fertile "asthenosphere" formed at the expense of old refractory subcontinental mantle lithosphere= harzburgites by reactive percolation of basaltic magmas + deformation?

  7. Lherz: feedback between melt percolation and deformation websteritic (cpx+opx+sp rich) lenses flattened in the harzburgites' foliation plane in the harzburgites: • deformed olivine (inherited) • deformed opx with en echellon fractures filled by undeformed cpx • undeformed sp • percolation of small melt fractions controlled by the harzburgite foliation: anisotropic percolation under static (?) conditions Le Roux et al. EPSL, 2008

  8. Lherz: feedback between melt percolation and deformation • at the contacts: • websteritic layers parallel to the contact • melt accumulation levels: compaction waves? hz incipient foliation in lherzolites websterites sp alignment + weak elongation ol coarse grains, variable deformation: def. & undef. px; undeformed sp Le Roux et al. EPSL 2008

  9. Lherz: feedback between melt percolation and deformation changes in olivine crystal preferred orientations at the contacts 1. dispersion 2. delayed reorientation (≥1m from the contact only!) incipient deformation of lherzolites Le Roux et al. EPSL 2008

  10. Lherz: feedback between melt percolation and deformation in the lherzolites strain  with  distance from harzburgites contact (percolation front) proximal lherzolite distal lherzolite higheststrains = most fertile lherzolites • melt-induced strain localization? Kohlstedt & Mackwell 2008 Le Roux et al. EPSL, 2008

  11. Lherz: feedback between melt percolation and deformation strain in the lherzolites  with  distance from harzburgites contact (percolation front), BUT olivine CPO in lherzolites and websterites becomes progressively weaker • strong contribution of diffusional processes to deformation

  12. Lherz: feedbacks between melt percolation and deformation layered lherzolites = melt segregation favored by shearing? Holtzman & Kohlstedt 2007 JPet Le Roux et al. EPSL, 2008

  13. What have we seen in the Lherz massif? field evidence: ≠ melt migration mechanisms? by Ben Holtzman low shear stress + permeability anisotropy? strain gradient in lherzolites : melt-controlled strain localization? layering in lherzolites : melt segregation favored by shearing ?

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