143 research outputs found
Life-cycle analysis of coesite-bearing garnet
Detrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultrahigh-pressure conditions, exhumation to Earth’s surface, erosion, and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in 14 medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very fine-sand fraction. To understand this imbalance, we analyze the role of source rock lithology, inclusion size, inclusion frequency, and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that small coesite inclusions <9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz, and garnet fracturing, fluid infiltration, and the coesite-to-quartz transformation is a late process during exhumation taking place at ~330°C. A model for the disintegration of coesite-bearing garnet enables explaining the heterogeneous grain27 size distribution by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration
Garnet major‑element composition as an indicator of host‑rock type: a machine learning approach using the random forest classifier
The major-element chemical composition of garnet provides valuable petrogenetic information, particularly in metamorphic rocks. When facing detrital garnet, information about the bulk-rock composition and mineral paragenesis of the initial garnet-bearing host-rock is absent. This prevents the application of chemical thermo-barometric techniques and calls for quantitative empirical approaches. Here we present a garnet host-rock discrimination scheme that is based on a random forest machine-learning algorithm trained on a large dataset of 13,615 chemical analyses of garnet that covers a wide variety of garnet-bearing lithologies. Considering the out-of-bag error, the scheme correctly predicts the original garnet host-rock in (i) > 95% concerning the setting, that is either mantle, metamorphic, igneous, or metasomatic; (ii) > 84% concerning the metamorphic facies, that is either blueschist/greenschist, amphibolite, granulite, or eclogite/ultrahigh-pressure; and (iii) > 93% concerning the host-rock bulk composition, that is either intermediate–felsic/metasedimentary, mafic, ultramafic, alkaline, or calc–silicate. The wide coverage of potential host rocks, the detailed prediction classes, the high discrimination rates, and the successfully tested real-case applications demonstrate that the introduced scheme overcomes many issues related to previous schemes. This highlights the potential of transferring the applied discrimination strategy to the broad range of detrital minerals beyond garnet. For easy and quick usage, a freely accessible web app is provided that guides the user in five steps from garnet composition to prediction results including data visualization
Detrital garnet petrology challenges Paleoproterozoic ultrahigh-pressure metamorphism in western Greenland
Modern-style plate tectonics is characterised by the
global operation of cold and deep subduction involving blueschist facies and
ultrahigh-pressure metamorphism. This has been a common process since the
Neoproterozoic, but a couple of studies indicate similar processes were
active in the Paleoproterozoic, at least on the local scale. Particularly
conspicuous are extreme ultrahigh-pressure conditions of ∼ 7 GPa at thermal gradients < 150 ∘C GPa−1 proposed for
metamorphic rocks of the Nordre Strømfjord shear zone in the western part
of the Paleoproterozoic Nagssugtoqidian Orogen of Greenland. By acquiring a
large dataset of heavy minerals (n = 52 130) and garnet major-element
composition integrated with mineral inclusion analysis (n=2669) from
modern sands representing fresh and naturally mixed erosional material from
the metamorphic rocks, we here intensely screened the area for potential
occurrences of ultrahigh-pressure rocks and put constraints on the
metamorphic evolution. Apart from the absence of any indications pointing to
ultrahigh-pressure and low-temperature–high-pressure metamorphism, the
results are well in accordance with a common Paleoproterozoic
subduction–collision metamorphic evolution along a Barrovian-type
intermediate temperature and pressure gradient with a pressure peak at the
amphibolite–granulite–eclogite-facies transition and a temperature peak
at medium- to high-pressure granulite-facies conditions. In addition, we
discuss that all “evidence” for ultrahigh-pressure metamorphism proposed
in the literature for rocks of this area is equivocal. Accordingly, the
Nordre Strømfjord shear zone is not an example of modern-style plate
tectonics in the Paleoproterozoic or of very low thermal gradients and
extreme pressure conditions in general.</p
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