Quartz exsolution in clinopyroxene of high-pressure granulite from the Münchberg Massif

Quartz exsolution topotaxy in clinopyroxene from the UHP eclogite of Weihai, China

“…Evidence against an UHP origin includes oriented quartz needles/rods within clinopyroxene from rocks which have experienced only HP but no UHP conditions (Gayk et al, 1995;Konzett et al, 2008b;Miller et al, 2005;Page et al, 2005;Proyer et al, 2009). By an experimental study to 15 GPa, Konzett et al (2008a) reported that the content of CaEs component in eclogitic clinopyroxene is a function of temperature rather than pressure, and they argued that oriented silica precipitations in clinopyroxene is a cooling rather than a decompression microstructure.…”

“…The host rocks can be classified into four types: UHP metamorphic eclogite (Dobrzhinetskaya et al, 2002;Katayama et al, 2000;Liu et al, 2011;Smith, 1984;Tsai and Liou, 2000;Zhang et al, 2002), HP metamorphic eclogite (Miller et al, 2005;Page et al, 2005), eclogite xenolith in kimberlite pipe (Smyth, 1980;Sobolev et al, 1968) and HP granulite (Gayk et al, 1995). The oriented silica phases in clinopyroxene are generally α-quartz (Dobrzhinetskaya et al, 2002;Katayama et al, 2000;Proyer et al, 2009;Schmädicke and Müller, 2000;Smith, 1984;Smith and Cheeney, 1980;Z.M.…”

“…The most influential explanation for oriented quartz needles/rods in jadeitic clinopyroxene is that they are derived from precursor nonstoichiometric supersilicic clinopyroxene (e.g., Bakun-Czubarow, 1992;Gayk et al, 1995;Liati et al, 2002;Liou et al, 1998;Smith, 1984Smith, , 1988Tsai and Liou, 2000;Zhang et al, 2002). This argument is based on the assumption that oriented silica precipitates are exsolution products from former supersilicic clinopyroxene by the breakdown of a non-stoichiometric Ca-Eskola's component (Ca 0.5 □ 0.5 AlSi 2 O 6 , CaEs) to form a Ca-Tschermak's component (CaAl 2 SiO 6 , CaTs) while releasing SiO 2 , as the following retrograde reaction: 2Ca 0:5 □ 0:5 AlSi 2 O 6 ¼ CaAl 2 SiO 6 þ 3SiO 2 where □ represents a cation vacancy in a clinopyroxene structure (Smyth, 1980).…”

The Alpe Arami story: Triumph of data over prejudice

“…Despite extensive previous study of alpine eclogites, evidence for Si-rich clinopyroxene had not been described previously from AA nor, to our knowledge, from elsewhere in the Alps. However, omphacite with SiO 2 precipitates was already well-known from xenoliths in kimberlite pipes (e.g., Smyth, 1980) and from many eclogites of other UHPM belts, e.g., the Western Gneiss Region, Norway (Smith, 1988;Smith and Cheeney, 1980), Bohemian massif, (e.g., the Sudetes, Poland; BakunChubarow, 1992) and Münchberg massif, Germany (Gayk et al, 1995), the Sulu region of China (Zhang et al, 1995), and the Kokchetav massif, Kazakhstan (Katayama et al, 2000). Tsai and Liou (2000) also reported abundant SiO 2 rods in sodic augite/ Na-bearing diopside (Fe : Mg : Ca=14.2 : 39.3 : 46.5; Na 2 O=1.55 wt.%) from eclogite of the Dabie Mountains, China.…”

Ultrahigh-pressure metamorphism and multistage exhumation of eclogite of the Luotian dome, North Dabie Complex Zone (central China): Evidence from mineral inclusions and decompression textures

“…These are typically interpreted as the result of cooling and decompression of the host minerals (e.g., Smith, 1984;Gayk et al, 1995;Zhang et al, 1995Zhang et al, , 2002bZhang et al, , 2005Katayama et al, 2000a;Tsai and Liou, 2000;Ye et al, 2000a;Dobrzhinetskaya et al, 2002;Song et al, 2003Song et al, , 2005Malaspina et al, 2006;Nakano et al, 2007 and references therein). This peculiar microtexture is generally used to infer the primary conditions under which eclogites and related metamorphic rocks were formed.…”