The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono‐thermal evolution of continental crust during the ...Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post‐kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse‐grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain‐fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080
± 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al2O3‐ and FeO‐mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine‐grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open‐system behaviour during this event limits the use of whole‐rock‐based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976
± 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet growth during this hydrothermal event lasted for less than 6 Myr.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
When human monocytes or alveolar macrophages are cultured in the presence of interleukin (IL)-4 or IL-13, the expression of the reticulocyte-type 15-lipoxygenase is induced. In mice a 15-lipoxygenase ...is not expressed, but a leukocyte-type 12-lipoxygenase is present in peritoneal macrophages. To investigate whether both lipoxygenase isoforms exhibit a similar regulatory response toward cytokine stimulation, we studied the regulation of the leukocyte-type 12-lipoxygenase of murine peritoneal macrophages by interleukins and found that the activity of this enzyme is upregulated in a dose-dependent manner when the cells were cultured in the presence of the IL-4 or IL-13 but not by IL-10. When peripheral murine monocytes that do not express the lipoxygenase were treated with IL-4 expression of 12/15-lipoxygenase mRNA was induced, suggesting pretranslational control mechanisms. In contrast, no upregulation of the lipoxygenase activity was observed when the macrophages were prepared from homozygous STAT6-deficient mice. Peritoneal macrophages of transgenic mice that systemically overexpress IL-4 exhibited a threefold to fourfold higher 12-lipoxygenase activity than cells prepared from control animals. A similar upregulation of 12-lipoxygenase activity was detected in heart, spleen, and lung of the transgenic animals. Moreover, a strong induction of the enzyme was observed in red cells during experimental anemia in mice. The data presented here indicate that (1) the 12-lipoxygenase activity of murine macrophages is upregulated in vitro and in vivo by IL-4 and/or IL-13, (2) this upregulation requires expression of the transcription factor STAT6, and (3) the constitutive expression of the enzyme appears to be STAT6 independent. The cytokine-dependent upregulation of the murine macrophage 12-lipoxygenase and its induction during experimental anemia suggests its close relatedness with the human reticulocyte-type 15-lipoxygenase despite their differences in the positional specificity of arachidonic acid oxygenation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
When human monocytes or alveolar macrophages are cultured in the presence of interleukin (IL)-4 or IL-13, the expression of the reticulocyte-type 15-lipoxygenase is induced. In mice a 15-lipoxygenase ...is not expressed, but a leukocyte-type 12-lipoxygenase is present in peritoneal macrophages. To investigate whether both lipoxygenase isoforms exhibit a similar regulatory response toward cytokine stimulation, we studied the regulation of the leukocyte-type 12-lipoxygenase of murine peritoneal macrophages by interleukins and found that the activity of this enzyme is upregulated in a dose-dependent manner when the cells were cultured in the presence of the IL-4 or IL-13 but not by IL-10. When peripheral murine monocytes that do not express the lipoxygenase were treated with IL-4 expression of 12/15-lipoxygenase mRNA was induced, suggesting pretranslational control mechanisms. In contrast, no upregulation of the lipoxygenase activity was observed when the macrophages were prepared from homozygous STAT6-deficient mice. Peritoneal macrophages of transgenic mice that systemically overexpress IL-4 exhibited a threefold to fourfold higher 12-lipoxygenase activity than cells prepared from control animals. A similar upregulation of 12-lipoxygenase activity was detected in heart, spleen, and lung of the transgenic animals. Moreover, a strong induction of the enzyme was observed in red cells during experimental anemia in mice. The data presented here indicate that (1) the 12-lipoxygenase activity of murine macrophages is upregulated in vitro and in vivo by IL-4 and/or IL-13, (2) this upregulation requires expression of the transcription factor STAT6, and (3) the constitutive expression of the enzyme appears to be STAT6 independent. The cytokine-dependent upregulation of the murine macrophage 12-lipoxygenase and its induction during experimental anemia suggests its close relatedness with the human reticulocyte-type 15-lipoxygenase despite their differences in the positional specificity of arachidonic acid oxygenation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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