Clinoenstatite in Alpe Arami Peridotite: Additional Evidence of Very High Pressure

Abstract

Observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the Alpe Arami garnet lherzolite of the Swiss Alps. The simplest interpretation of the orientation, crystallography, and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high-pressure C-centered form of clinoenstatite. These results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision.

Microstructural Indications for Protoenstatite Precursor of Cometary MgSiO 3 Pyroxene: A Further High-Temperature Component of Comet Wild 2

“…The structural transformation from enstatite belonging to the C2/c space group (HT-CLEN and HP-CLEN) to the P2 1 /c modification (LTLP-CLEN) will lead to the formation of antiphase domains (APDs) due to the loss of a translational symmetry component (e.g., Bozhilov et al 1999). …”

Very high-pressure orogenic garnet peridotites

“…Despite considerable controversy, subsequent experiments under these conditions have confirmed that Ͼ1 vol % of TiO 2 can be accommodated in olivine (46,47). The additional observation of exsolved Ca-poor pyroxene displaying antiphase domains in diopside (8,48,49) also supports the idea of an extremely deep origin (Ͼ300 km) for the Alpe Arami garnet peridotite.…”

“…Intergrowths of ortho-and clinoenstatite lamellae are common within Chinese orogenic garnet peridotites. Clinoenstatite lamellae in orthoenstatite may have formed either by inversion from orthoenstatite or by a displacive transformation from VHP clinoenstatite during decompression (8,12). Experiments indicate that orthoenstatite transforms to VHP clinoenstatite at P Ͼ 8 GPa, 900°C, corresponding to a mantle depth of Ϸ300 km (e.g., refs.…”

“…[2][3][4][5][6]. Some peridotites preserve a record of ultradeep origin revealed by mineral exsolution and the persistence of VHP polymorphs (6)(7)(8)(9)(10)(11)(12)(13)(14), and several peridotites contain dense hydrous magnesian silicates (DHMS) that are stable only at mantle depths (15,16). It was also found that some garnet peridotites, and their host continental crust, underwent coeval subduction-zone VHP metamorphism under pressure-temperature (P-T) conditions characterized by low thermal gradients (Յ5°C/km), based on sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon separates from both rock types (e.g., refs.…”

Rapid Eocene erosion, sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance

“…Many studies have demonstrated that continental crust can be subducted to depths in excess of at least 80 km, possibly to depths of 200 to 350 km (Xu et al, 1992;Yang et al, 1993;Dobrzhinetskaya et al, 1996;Bozhilov et al, 1999;Ye et al, 2000;Zheng, 2003;Liu et al, 2007aLiu et al, , 2008a. During the deep subduction, continental crust underwent high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism and resulted in the formation of various HP and UHP metamorphic rocks in the orogen.…”