Chemical-microstructural changes in eclogite-facies shear zones (Monviso, Western Alps, north Italy) as indicators of strain history and the mechanism and scale of mass transfer

International audienceVariations of eclogite-facies mineral chemistry are related to the textural characteristics of shear zones in FeTi metagabbros from the Monviso meta-ophiolitic complex in the Italian Western Alps. Three different kinds of microstructural domains are described: (1) small lenses of low-strain rocks with preserved igneous textures; (2) larger high-strain domains where deformation caused pervasive mylonitic foliation (S1) and a stretching mineral lineation (L1); and (3) two types of eclogite-facies veins. The latter can be subdivided into veins composed of only Na-pyroxenes filling small tension gashes and dilatation fractures and veins composed of a Na-pyroxene-garnet-rutile-apatite assemblage occupying large fractures that randomly cross-cut the mylonites.In low-strain domains and mylonites the Na-pyroxenes are omphacites and the garnets are almandine rich. In some mylonite layers containing phengite and blue amphibole crosscutting S1, chloromelanite (iron-rich omphacites) exsolved from omphacite. In type-1 veins the pyroxene is either omphacite or concentric and sector-zoned omphacite-chloromelanite. In type-2 veins pyroxenes are omphacite and/or concentric and sector-zoned omphacite-chloromelanite and/or zoned kosmochlor (chromium-rich omphacite)-omphacites. Associated garnets are richer in pyrope component than those from low-and high-strain domains.Possible origins of both sector and concentric zoning and exsolution are discussed. It is emphasized that a miscibility gap between the omphacite and acmite end-members partly controlled the omphacite-chloromelanite distribution. We define a chemical vector coat constrains the strain history in eclogite-facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing.P-T conditions are calculated for each microstructural domain. The garnet-pyroxene thermometer yields scattered temperature values. These values relate to the dependence of temperature estimates on the amount of Fe3+ in the pyroxene and possibly on non-ideal substitution in omphacite-acmite solid solution

Improving dynamic process stability in finishing of thin-walled workpieces by optimal selection of stock shape

In 5-axis milling of thin-wall parts, flexibility of the in-process-workpiece (IPW) governs static and dynamic deflections, especially at the semifinishing and finishing stages. Thus, the near net shape preform, i.e. the stock shape left for semi/finishing is crucial for chatter stability. In this study, a methodology is presented to design the stock shape for improved stability. Coupled process simulation, which includes material removal, finite element analysis (FEA) and process stability simulations, is used to show the effect of stock shape on chatter stability. Spindle speed is optimized along the toolpath based on the varying process dynamic behavior for selected cutter locations (CL). The proposed method is experimentally demonstrated on case studies