1. I once attributed "angular with irregular shape" of zircon to "relatively short-distance transportation and weak rounding". However, now I think the crushing procedure is very likely to produce irregular zircons by breaking them into pieces. So how can I eliminate one of the two possibilities or indentify which possibility playes the main role?
2. What does Index of Compositional Variability (ICV; Cox et al. 1995) really reflect? I know that high ICV values possibly imply a chemically immature source in active tectonic settings while low ICVs are opposite. However, does ICV have a relationship with the distance of transportation of the sedimentary materials. I mean, can high ICV values reflect short distance of transportation? Does ICV only reflect the information of the source area not that of the transportation process ? If not, can you suggest one or two index to measure the distance of transportation of the sedimentary materials, or give me some advice on how to prove that the sedimantary materials of one area have experienced a short not long distance of transportation.Thank you!
Dear Mrs. Shan,
you have to dig a bit deeper and try and decipher the original morphology of zircon, otherwise you will get erroneous results upon erosion and transport. Further literature as to the provenance, transport and attrition you will find in the publications below.
DILL, H.G. (1998) A review of heavy minerals in clastic sediments with case studies from the alluvial fan through the near-shore marine environments.- Earth Science Review, 45: 103-
132.
DILL, H.G. (2007) Grain morphology of heavy minerals from marine and continental placer deposits, with special reference to Fe -Ti oxides.- Sedimentary Geology, 198: 1-27.
DILL, H.G. and KLOSA, D. (2011) Heavy-mineral-based provenance analysis of Mesozoic continental-marine sediments at the western edge of the Bohemian Massif, SE Germany: With special reference to Fe-Ti minerals and the crystal morphology of heavy minerals. - International Journal of Earth Sciences, 100: 1497-1513.
DILL, H.G., WEBER, B. and KLOSA, D. (2012) Crystal morphology and mineral chemistry of monazite–zircon mineral assemblages in continental placer deposits (SE Germany):Ore guide and provenance marker.- Journal of Geochemical Exploration, 112: 322-346.
You can down load a copy of each paper from RG
Best regards
H.G.Dill
Not at all! An angular and irregular shaped zircon may result from unsteady growth. They not necessary indicate sedimentary origin, but unstable conditions of zircon growth, for example, from a basic magmas. It also not necessary resulted from crushing procedure. It may be a real form of zircons, just try to see the shape of grains under electron microscope.
Dear colleagues,
it is correct that angular grains of zircon which resulted from breaking apart of zircon XX during whatever reason can neither be used for an assessment of the transport distance nor for provenance studies. It is, however, not correct to deny the value of morphological studies of zircon in sedimentary deposits. The background studies of zircon in metamorphic and magmatic rocks were performed in an excellent way so that detrital grains can be used for sedimentary studies in the scope of provenance analyses. With this in mind these grains become also a tool for the assessment of the distance of transport. In the papers mentioned above there are several images which underscore this. You need to address this issue taking a crystal-morphological approach labeling the faces as precise as possible. A classification like rounded, subrounded, subangular... is not precise enough to tackle this problems. The radiation damage and resultant metamictization of zircon and its decay on transport will be exaggerated. This is also true for xenotime and monazite, the Th content of which is much higher than the U content of zircon.
Best regards
Harald G. Dill
Dear Harald,
the problem is that angular zircon in metamorphosed sediments need not be a sign of breaking of lager zircon grains during trasportation. See Vavra et al, 1999 for example. It may suggest that detritial zircon has been strongly dissolved during melting or high-grade metamorphic overprint.
Best regards
Nik
Dear Nik,
unfortunately based on my experience (see publications above) I cannot go along with this idea. Zircon has such a good preservation potential that you may even use some of its X habits as an ore guide, e.g., for pegmatites.
In the present situation I guess we are dealing with a sedimentary environment and a provenance of metamorphic, magmatic or reworked/sedimentary origin. Here the morphological approach should work well.
Best regards
Harald
Dear Harald,
I just try to show a problem, nothing else. That is why I said "need not". If you want I cfny show you alot of zircon grains having an angular metamorphic overgrowth.
best regards
Nik
Dear colleagues,
take a look at a beach or coastal placer of zircon across the globe and compare it with a zircon grain in the clastic halo of a rock or deposit, pertaining to an alluvial-fluvial apron and you will see the difference by your own eyes, how shape and grain size vary. I can send you more than one picture.
As to the metamictization, it makes a difference whether you create or provoke artificially radiation damage by irradiation- see e.g. "quality enhancement" in gemology, which may be reversible in some cases. If the cause or source for radiation damage is inherent to the host mineral and the radioactive element accommodated in the lattice of monazite a life-long the situation is conspicuously different. Pleochroic haloes persist in this case, even around rock-forming minerals of moderate contents of Th and U.
Best regards
H.G.Dill
Dear Professors,
I'm sure that these zircons come from sedimentary rocks according to other evidence. My question in fact lies in that I don't know whether the angular shape is caused by the short-distance transportation or mechanical crushing during the zircon separation.I am sorry for the ambiguity of the question .
This is a fascinating discussion, especially as there have been 2.02 Ga impact-shocked detrital zircons that have survived transportation up to 2000 km from the source (Vredefort Dome). Coming from a different angle, would it be possible to liberate zircons from the sedimentary rock using non-rock-crushing techniques (e.g., selFRAG/ high voltage pulsed treatment). This has had excellent results yielding idiomorphic zircons for use in U-Pb dating) and may answer your question if the zircons can be liberated intact using this technique.
Conference Paper Detrital Shocked Zircons Detected 2,000 km Downriver from th...
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