Subcratonic and tectonic evolution of pyroxenite and eclogite with lamellar inclusions in garnet, Western Gneiss Region, Norway

Oriented lamellar inclusions of pyroxene and rutile in mantle garnet often serve as evidence for majoritic and titaniferous precursor garnets, respectively. We investigated ten new such microstructure-bearing samples from six orogenic peridotite bodies in SW Norway, which originated in the E Greenland mantle lithosphere, petrologically and thermobarometrically. All pyroxenite (nine) and eclogite (one) samples have large (mainly porphyroclastic) garnet containing silicate and oxide inclusions with shape-preferred orientation relationship. These inclusions vary – dependent on their size – systematically in shape (acicular to subprismatic), width (∼50 μm to submicron size), spacing (several 100 to ∼10 μm) and phase (pyroxene to Ti-oxide ± pyroxene). Smaller inclusions can fill the space between larger inclusions, which support the idea of consecutive generations. The larger, early formed lamellae occur least frequent and are most poorly preserved. A younger generation of other inclusions decorates healed cracks cutting across cores but not rims of garnet. These inclusions comprise oxides, silicates, carbonates (aragonite, calcite, magnesite) and fluid components (N2, CO2, H2O). The older, homogeneously distributed inclusions comply texturally and stoichiometrically with an origin by exsolution from excess Si- and Ti-bearing garnet. Their microstructural systematic variation demonstrates a similar early evolution of pyroxenite and eclogite. The younger inclusions in planar structures are ascribed to a metasomatic environment that affected the subcratonic lithosphere. The microstructure-bearing garnets equilibrated at ∼3.7 GPa (840 °C) and ∼3.0 GPa (710 °C), at a cratonic geotherm related to 37–38 mW m−2 surface heat flow. Some associated porphyroclastic grains of Mg-rich pyroxene have exsolution lamellae of Ca-rich pyroxene and vice versa that indicate a preceding cooling event. Projected isobaric cooling paths intersect isopleths for excess Si in garnet at ∼1550 °C, if an internally consistent thermodynamic data set in the system Na2O–CaO–MgO–Al2O3–SiO2 (NCMAS) is applied (or ∼1600 °C if using CMAS). This temperature may confine the crystallisation of the unexsolved garnets at 100–120 km depths of the E Greenland subcratonic lithosphere. Tectonism is indicated in coastal and hinterland samples by porphyroclastic orthopyroxene with Al2O3 concentrations showing W-shaped profiles. Cores of associated large (>200 μm) recrystallised grains have low Al2O3 contents (0.18–0.23 wt.%). Both characteristics typify relatively short intracrystalline Al diffusion lengths and a prograde metamorphism into the diamond stability field. We assign this event to subduction during the Scandian orogeny. Porphyroclastic orthopyroxene in other samples shows U-shaped Al2O3 concentration profiles paired with long Al diffusion lengths (several 100 μm) that exceed the radius of recrystallised grains. Their cores contain high Al2O3 contents (0.65–1.16 wt.%), consistent with a diffusional overprint that obliterated prograde and peak metamorphic records. Unlike Al2O3, the CaO content in porphyroclastic orthopyroxene cores is uniform suggesting that early exhumation was subparallel to Ca isopleths in pressure–temperature space. The depth of sample origin implies that rock bodies of Scandian ultra-high pressure metamorphism occur in nearly the entire area between Nordfjord and Storfjord and from the coast towards ∼100 km in the hinterland, i.e. in a region much larger than anticipated from crustal eclogite.

Petrology of sub-cratonic pyroxenite and eclogite containing lamellae-bearing garnet, Western Gneiss Region, Norway

<p>Garnet from the lithospheric mantle underneath cratons can contain oriented lamellar inclusions of pyroxene and oxides like rutile as a result of exsolution of majoritic and titaniferous components due to cooling and/or decreasing pressure. We investigated ten new such microstructure-bearing samples of pyroxenite and eclogite from six peridotite bodies in SW Norway, which were once located in the E Greenland mantle lithosphere. The lamellar inclusions occur in porphyroclastic garnet and vary – dependent on their size – systematically in shape, (acicular to short-prismatic), width (~50 μm to sub-micron size), spacing (several 100 to ~10 μm), and phase (pyroxene to pyroxene + Ti-oxides to Ti-oxides). Smaller lamellae can fill the space between larger lamellae, which support consecutive generations. The larger (early formed) lamellae are more poorly preserved and more difficult to locate in the suite of samples than the smaller (lately formed) exsolutes. A younger generation of lamellar and other inclusions occur lined-up along healed cracks cutting across cores but not rims of garnet. These inclusions comprise oxides, silicates, carbonates (aragonite, calcite, magnesite) and fluid inclusions (N<sub>2</sub>, CO<sub>2</sub>, H<sub>2</sub>O). Their origin either relates to the Precambrian rock history and/or to a hydrous environment as typical for mantle wedge metasomatism prior to Scandian recrystallisation. Mineral chemistry suggests that the lamellae-bearing garnet grains equilibrated at two discrete depth levels, corresponding to ~3.7 GPa (850 °C) and ~3.0 GPa (710 °C), at a cratonic geotherm corresponding to 38 mW/m<sup>2</sup> surface heat flow. Five samples contain porphyroclastic orthopyroxenes with Al<sub>2</sub>O<sub>3</sub> concentration showing W-shaped profiles and/or very low Al<sub>2</sub>O<sub>3</sub> content (0.18–0.23 wt%) in cores of large (>200 µm) recrystallised grains. Both characteristics typify short intracrystalline diffusion lengths and are consistent with an early prograde metamorphic evolution into the diamond stability field. This evolution is related to subduction during the Scandian orogeny. Porphyroclastic orthopyroxenes in other samples show U-shaped Al<sub>2</sub>O<sub>3</sub> concentration profiles and long diffusion lengths of several 100 μm, i.e. longer than the grain radius of the recrystallised grains. Their cores contain high Al<sub>2</sub>O<sub>3</sub> contents (0.65–1.16 wt%) consistent with a diffusional overprint that followed partial rock recrystallisation and obliterated pro- and peak metamorphic records. The presence of systematic exsolution microstructures in all samples demonstrates a similar early evolution of pyroxenite and eclogite in all six peridotite bodies. The wide distribution of our samples across the Western Gneiss Region indicates that (1) majoritic and titaniferous garnet occurred widespread in the E Greenland lithospheric mantle and (2) rock bodies of Scandian ultra-high pressure metamorphism can be found in nearly the entire area between Nordfjord and Storfjord and from the coast towards ~100 km in the hinterland, i.e. in a region much larger than previously anticipated.</p>

Lamellar Inclusions within Hyperplastic Endoplasmic Reticulum in Benign Mesothelial Cells

<b><i>Introduction:</i></b> In effusion cytology, mesothelial cells can occasionally present with striking intracytoplasmic accumulation of rod- and crystal-like cytoplasmic lamellar inclusions (LIs). Their nature and function are poorly understood, and their diagnostic relevance is unknown. <b><i>Objective:</i></b> The aim of this study was to explore the nature of LIs in mesothelial cells and determine their prevalence and diagnostic utility in routine practice. <b><i>Material and Method:</i></b> We reviewed a consecutive series of cytological specimens of reactive (<i>n</i> = 102) and malignant effusions (<i>n</i> = 90), respectively. Malignant effusions included malignant mesotheliomas (<i>n</i> = 63) and carcinomas (<i>n</i> = 27). LIs of one effusion were analyzed by electron microscopy (EM). <b><i>Results:</i></b> LIs were found exclusively in benign mesothelial cells in 14% (14/102) of reactive and in 4% (1/27) of malignant effusions with carcinomatosis. They were absent in effusions of malignant mesothelioma. EM revealed mainly straight lamellar, less tubular, structures in cisternae of the hyperplasic rough endoplasmic reticulum (rER). <b><i>Conclusion:</i></b> Cytoplasmic LIs located within hyperplastic rER can be found in up to 14% of effusions restricted to benign mesothelial cells. They can be used as an indirect morphological clue favoring the diagnosis of benign effusion and helping the cytologist to differentiate between reactive and malignant mesothelial cells in daily practice.