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.
We report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the Western Gneiss Region (WGR) garnet peridotite, Otrøy, Norway. For both clinopyroxene (Cpx) and orthopyroxene (Opx), the same basic crystallographic relationship is found with the host garnet: (100) py ⁄ ⁄ {112} grt , (010) px ⁄ ⁄ {110} grt and (001) px ⁄ ⁄ {111} grt for the majority (>90%) of its intracrystalline pyroxene rods. In addition, this pattern is also exhibited by some interstitial Opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern. The results provide quantitative microstructural evidence demonstrating an exsolution (precipitation) origin of both the intracrystalline Cpx and Opx and the small interstitial Opx crystals. The reconstructed precursor majoritic garnet, taking into account both the intracrystalline pyroxenes and interstitial Opx, was characterized by Si = $3.07 cation per formula unit that corresponds to a minimum pressure of 7.7 GPa ($250 km depth). We also deduce from the observation of Opx being the majority of intracrystalline precipitates and 100% of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than $10 GPa ($300 km depth). A multistage decomposition hypothesis is proposed for this WGR majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes.
Nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav massif, Kazakhstan, have been examined utilizing electron microscopy and focused ion beam techniques. Host garnet and zircon contain numerous pockets of multiple inclusions, which consist of 1-3 diamond crystals intergrown with quartz, phengite, phlogopite, albite, K-feldspar, rutile, apatite, titanite, biotite, chlorite and graphite in various combinations. Recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt% of SiO 2 , suggesting a relatively low-temperature fluid rather than a melt. Transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides, rare carbonates and silicates. Within the 15 diamond crystals studied, abundant inclusions were found of SiO 2 , TiO 2 , Fe x O y , Cr 2 O 3 , ZrSiO 4 , and single grains of Th x O y , BaSO 4 , MgCO 3 , FeCr 2 O 4 and a stoichiometric Fe-rich pyroxene. The diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the Kokchetav diamond crystallized from a fluid containing variable amounts of Si, Fe, Ti, Cr, Zr, Ba, Mg and Th and other minor components such as K, Na, P, S, Pb, Zn, Nb, Al, Ca, Cl. Most of the components in crystals included in diamond appear to have their origin in the subducted metasediments, but some of them probably originate from the mantle. It is concluded that Kokchetav diamond most likely crystallized from a COH-rich multicomponent supercritical fluid at a relatively low temperature (hence the apparently low content of rock-forming elements), and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components.
Metasedimentary rocks, a major component of the continental crust, are abundant within ultra-high pressure (UHP) metamorphic terranes related to continental collisions. The presence of diamond, coesite, and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150-250 km. Reconnaissance experiments at 9-12 GPa and 1000-1300°C on compositions corresponding to felsic rocks from diamond-bearing UHP terranes of Germany and Kazakhstan show that at higher pressures they consist of majoritic garnet, Al-Na-rich clinopyroxene, stishovite, solid solution of KAlSi 3 O 8 -NaAlSi 3 O 8 hollandite, topaz-OH, and TiO 2 with a-PbO 2 structure. Comparison of our data with experiments conducted by others at similar P-T conditions shows differences, which are due to variations in bulk chemistry and the type of starting material (gel, oxides, minerals). These differences may affect correct establishment of the Ôpoint of no returnÕ of subducted continental lithologies. This paper discusses the implication of the experimental data with regard to naturally existing UHP metamorphic rocks and their significance for our understanding of the deep subduction of continental material.
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