The ability to apply automated quantitative mineralogy (AQM) on metamorphic rocks was investigated on samples from the Fiskenæsset complex, Greenland. AQM provides the possibility to visualize and quantify microstructures, minerals, as well as the morphology and chemistry of the investigated samples. Here, we applied the ZEISS Mineralogic software platform as an AQM tool, which has integrated matrix corrections and full quantification of energy dispersive spectrometry data, and therefore is able to give detailed chemical information on each pixel in the AQM mineral maps. This has been applied to create mineral maps, element concentration maps, element ratio maps, mineral association maps, as well as to morphochemically classify individual minerals for their grain shape, size, and orientation. The visualization of metamorphic textures, while at the same time quantifying their textures, is the great strength of AQM and is an ideal tool to lift microscopy from the qualitative to the quantitative level.
The significance of mineralogical maturity as a provenance indicator has long been debated and we use this study to demonstrate that it can indeed be a powerful tool to track the distribution of sandstone reservoirs. We investigate the cause of the pronounced geographic and stratigraphic differences in mineralogical composition that are found in the Upper Triassic–Lower Jurassic Gassum Formation across the Norwegian–Danish Basin and surrounding areas. Zircon U-Pb dating of 46 sandstone samples including analysis of 4816 detrital grains are combined with quantifications of the detrital mineralogical composition and placed in a sequence stratigraphic framework. The results show that the Gassum Formation can be divided into a southeastern region with high mineralogical maturity and a less mature region to the northwest with more feldspars, rock fragments, micas, and heavy minerals. Both the mineralogical assemblage and the provenance signature have been thoroughly homogenized in the SE region where sediment supplies from the Fennoscandian Shield and the Variscan Orogen are evident. In the NW region, sediment was initially supplied from Fennoscandia only, but the provenance abruptly changed from the Telemarkia Terrane to comprising also the more distant Caledonian Orogen resulting in a different mineralogical assemblage. The change occurred during a basinwide regression and may be caused by tectonic movements in the hinterland that permanently changed the composition of the sediment supplied to the basin.
Meta-sedimentary rocks recovered beneath Palaeogene basalts near the base of Ocean Drilling Program (ODP) Leg 152-917A offshore South-East Greenland were thought to be of Late Cretaceous age. This interpretation, however, has several inconsistencies as it requires a tectono-metamorphic event during the Cretaceous not recognized in the North Atlantic region, and the presence of a wide Mesozoic sedimentary basin that extended from SE-Greenland to the Rockall Plateau, for which there is currently no evidence. Here, we report a Neoproterozoic U/Pb apatite age of 905 ± 21 Ma and a younger 40Ar/39Ar isochron whole-rock age of 820 ± 40 Ma for an altered tuff layer that occurs in the upper part of the meta-volcaniclastic sequence recovered from hole 917A. The 40Ar/39Ar step-heating ages on biotite and whole-rock mini-cores from deeper in hole 917A yielded Palaeoproterozic dates that cluster around 1950 to 1850 Ma, pointing toward a Palaeoproterozoic source. The U/Pb apatite date is interpreted as the eruption age of the tuff layer, whereas the younger whole-rock 40Ar/39Ar age is consistent with low-temperature greenschist alteration of volcanic glass and secondary mineral growth during sedimentary burial in an extensional regime. The c. 905 Ma age for the tuff provides the first evidence for Neoproterozoic rocks offshore South-East Greenland and suggests a correlation between this sequence and the Torridon Group in the Hebridean Foreland of the Scottish Caledonides. The calc-alkaline nature of the volcaniclastic rocks and the age of the tuff layer point toward a source area with arc-magmatism related to the Renlandian event of the Valhalla Orogeny.
The geochemical composition of glauconitic clasts is a provenance marker to distinguish the origin of gravity flows in sedimentary basins. Their geochemical variation is visible in major and trace elements composition, where the former can be visualized by element mapping of glauconitic clasts. By applying automated quantitative mineralogy on a Scanning Electron Microscope equipped with the ZEISS Mineralogic™ software platform, we developed a new way to visualize the element distribution in selected minerals, while masking out the other minerals simultaneously. This software applies energy dispersive spectroscopy spectrum deconvolution for each analysis point, therefore quantitative concentrations (wt%) of each element are determined for each pixel and visualized in the false-coloured element map with reproducible results for individual grains and zonations in these grains. The investigated glauconitic clasts were collected from eleven drill cores, covering a 1400 m-depth interval, and crosscutting four different gravity flow members. The clasts show three different trends: first, cores of glauconitic clasts show a variation of compositions that are mainly dependent on the conditions during their formation. This is most visible for Si and Al. Secondly, the composition of the glauconitic clasts changes with depth, their cores become first more, than less Fe-rich and more K-rich. This is probably an alteration effect, depending on temperature, and therefore mineral stability. And, thirdly, most glauconitic clasts are zoned, where rims are richer in Al and poorer in Fe and K. The visualization of the chemical variation in the glauconitic clasts proofs to be a useful tool to separate these conflicting element exchange processes.
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