Geochronological evolution of HP metamorphic rocks of the Adula nappe, Central Alps, in pre-Alpine and Alpine subduction cycles

<p>SHRIMP dating of zircon from eclogites, their country rock gneisses and quartz veins from the northern and middle Adula nappe, Central Alps, accompanied by cathodoluminescence imaging and REE geochemistry of these dated zircon crystals revealed that the Adula nappe has been affected by two pre-Alpine high-pressure (HP) metamorphic events (northern: <em>c</em>. 330–340 Ma; middle: <em>c</em>. 370 Ma). In the northern Adula nappe, loss of radiogenic Pb of the <em>c</em>. 330–340 Ma zircon domains in combination with previous geochronological data argue for an overprinting Alpine HP event. In the middle Adula nappe, metamorphic zircon domains record in addition a 33–32 Ma event, interpreted to reflect HT metamorphism, already identified in the southern part of the nappe, where such SHRIMP ages have been related to HT (Lepontine) metamorphism. The Alpine HP event indicated by previous SHRIMP data in the southern Adula nappe at <em>c</em>. 35 Ma is not recorded in the zircons of the rocks dated herein. The absence in this dataset is probably because either no HP (<em>c</em>. 35 Ma) zircon domains were formed or such domains were completely reset during the 33–32 Ma HT event. Protolith ages of orthogneisses and eclogites are 470–457 Ma and 595–587 Ma, respectively, and maximum sedimentation ages of paragneisses 500–460 Ma. The lack of Hercynian protolith zircon ages implies pre-Hercynian formation for the precursors of the dated HP rocks. Our data suggest that the Adula nappe contains various Hercynian basement slivers that underwent subduction to HP conditions at <em>c</em>. 330–340 Ma and <em>c</em>. 370 Ma prior to the Alpine orogeny, similar to many other parts of the Hercynian basement in Central and Southern Europe. </p

Electron Microscopic Study of Soot Particulate Matter Emissions from Aircraft Turbine Engines

The microscopic characteristics of soot particulate matter (PM) in gas turbine exhaust are critical for an accurate assessment of the potential impacts of the aviation industry on the environment and human health. The morphology and internal structure of soot particles emitted from a CFM 56-7B26/3 turbofan engine were analyzed in an electron microscopic study, down to the nanoscale, for ∼100%, ∼65%, and ∼7% static engine thrust as a proxy for takeoff, cruising, and taxiing, respectively. Sampling was performed directly on transmission electron microscopy (TEM) grids with a state-of-the-art sampling system designed for nonvolatile particulate matter. The electron microscopy results reveal that ∼100% thrust produces the highest amount of soot, the highest soot particle volume, and the largest and most crystalline primary soot particles with the lowest oxidative reactivity. The opposite is the case for soot produced during taxiing, where primary soot particles are smallest and most reactive and the soot amount and volume are lowest. The microscopic characteristics of cruising condition soot resemble the ones of the ∼100% thrust conditions, but they are more moderate. Real time online measurements of number and mass concentration show also a clear correlation with engine thrust level, comparable with the TEM study. The results of the present work, in particular the small size of primary soot particles present in the exhaust (modes of 24, 20, and 13 nm in diameter for ∼100%, ∼65% and ∼7% engine thrust, respectively) could be a concern for human health and the environment and merit further study. This work further emphasizes the significance of the detailed morphological characteristics of soot for assessing environmental impacts