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131.
Tine?Derez Tom?Van Der?Donck Oliver?Plümper Philippe?Muchez Gill?Pennock Martyn?R.?Drury Manuel?SintubinEmail author 《Contributions to Mineralogy and Petrology》2017,172(7):56
Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz. 相似文献
132.
Sarah?K.?FortnerEmail author Martyn?Tranter Andrew?Fountain W.?Berry?Lyons Kathleen?A.?Welch 《Aquatic Geochemistry》2005,11(4):391-412
We have investigated the geochemistry of supraglacial streams on the Canada Glacier, Taylor Valley, Antarctica during the
2001–2002 austral summer. Canada Glacier supraglacial streams represent the link between primary precipitation (i.e. glacier
snow) and proglacial Lake Hoare. Canada Glacier supraglacial stream geochemistry is intermediate between glacier snow and
proglacial stream geochemistry with average concentrations of 49.1 μeq L−1 Ca2+, 19.9 μeq L−1 SO42−, and 34.3 μeq L−1 HCO3−. Predominant west to east winds lead to a redistribution of readily soluble salts onto the glacier surface, which is reflected
in the geochemistry of the supraglacial streams. Western Canada Glacier supraglacial streams have average SO42−:HCO3− equivalent ratios of 1.0, while eastern supraglacial streams average 0.5, suggesting more sulfate salts reach and dissolve
in the western supraglacial streams. A graph of HCO3− versus Ca2+ for western and eastern supraglacial streams had slopes of 0.87 and 0.72, respectively with R2 values of 0.84 and 0.83. Low concentrations of reactive silicate (> 10 μmol L−1) in the supraglacial streams suggested that little to no silicate weathering occurred on the glacier surface with the exception
of cryoconite holes (1000 μmol L−1). Therefore, the major geochemical weathering process occurring in the supraglacial streams is believed to be calcite dissolution.
Proglacial stream, Anderson Creek, contains higher concentrations of major ions than supraglacial streams containing 5 times
the Ca2+ and 10 times the SO42−. Canada Glacier proglacial streams also contain higher concentrations (16.6–30.6 μeq L−1) of reactive silicate than supraglacial streams. This suggests that the controls on glacier meltwater geochemistry switch
from calcite and gypsum dissolution to both salt dissolution and silicate mineral weathering as the glacier meltwater evolves.
Our chemical mass balance calculations indicate that of the total discharge into Lake Hoare, the final recipient of Canada
Glacier meltwater, 81.9% is from direct glacier runoff and 19.1% is from proglacial Andersen Creek. Although during a typical,
low melt ablation season Andersen Creek contributes over 40% of the water added to Lake Hoare, its overall chemical importance
is diluted by the direct inputs from Canada Glacier during high flow years. Decadal warming events, such as the 2001–2002
austral summer produce supraglacial streams that are a major source of water to Lake Hoare. 相似文献