I am considering the suitability of geochonological studies of heavy mineral concentrates from a sedimentary formation that is devoid of fossils and fall within a "magnetic reversal-quiet zone". One would expect the age of the heavies to be those of the parent rocks (and hence give indication for the provenance materials), but authigenic minerals (diagenetic in origin) could give and indication of sedimentary rocks formation age. The question is, how scientifically justified is this and how can authigenic minerals be distinguished from rock forming minerals when carrying out heavy mineral analysis
An interesting but very difficult and extensive topic. Radiometric dating of some heavy minerals (zircon, apatite, or titanite and monazite) by the U-Pb method using LA is of course possible. However, considerable difficulties of interpretation are to be expected. Samples may contain differently recycled heavy minerals from different sources. Some source rocks, such as zircons, may not even be known.
The dating of common autigenic minerals is not methodically managed and is not commonly performed (probably except for calcite). Probably an authigenic "gel-like" zircon, "hydrozircon" is only a very rare component of some sediments and its dating has not been verified. However, it is probably undated due to too high U concentrations.
To sum it up, dating heavy minerals is a lottery bet. It is relatively expensive, one sample is not enough and the result is not guaranteed.
The radiometric dating of heavy minerals (HM) using, e.g., U/Pb, Sm/Nd, Sr/Sr and K/Ar (Ar/Ar) is as delicate as these methods applied to hard rock samples taken in the source rock or provenance area which should of course be known as to their age data when a correlation of, e.g., HM in stream sediments with potential source rocks is performed.
Dating HM (transparent , e.g., zircon, monazite, apatite, orthite, fluorite and opaque, e.g., columbite-tantalite s.s.s., cryptomelane) needs a twofold approach. Firstly, you have to fuel in all your sedimentological, geomorphological and hydrographic-topographic experience and knowledge and secondly, your mineralogical , and crystallographic-morphological ones. It makes a difference where the allogenic grains have been laid down as to the distance from the potential source rock (fluvial, marine, aeolian) and what the physical-chemical regime was like when it comes to the selection of fluorite, baryte, celestite (Sm/Nd, Sr/Sr) which are common authigenic cement minerals.
The final part will be either a meticulous picking of mineral grains different as to color and grain morphology under the stereo-microscope for the classical methods or a SEM/EDX- and/or EMPA- based microchemical analysis for the Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS). Both preparatory measures pave the way to the singling out of HM communities among the zircon, monazite, fluorite….suites.
Dating detrital minerals such as mica (LM+HM) for basin analysis be it allogenic or authigenic material is state-of-the art. In this case the granulometry of the phyllosilicates targeted upon plays a significant role and grain size separation from sieving to the centrifuge plus settling tube is a must.
The detrital HM and LM approach is very effective for basin analysis provided you have a good experience and knowledge in the disciplines mentioned above. The preparatory work is paramount and requires a good geoscientist, the subsequent radiometric dating can be done by a commercial lab.
The results obtained thereby depend first and foremost on the geoscientific preparatory field and lab works.
HGD
P.S. You can browse my list of publication for U/Pb, Sm/Nd, Ar/Ar-K/Ar, Sr/Sr and Ca/Ca dating as well as studies of the various depositional environments fluvial, marine, and aeolian. Glacial and mass wasting do not play any part considering the issues you referred to in your question.
Many thanks Prof Dill, I have sourced some of your contributions and i am currently redesigning my research based on the lessons learnt (obviously, some were over looked) so far. I sincerely appreciate how you brought your wealth of experience to bear in delivering the answer to my concerns/question. Best Regards.
Dalibor Matýsek Thank you for your contribution. It seems the limitation can be overcome by more advance technological improvements and by using methods that may not be reactive to the applied techniques
Dear Mr. Okon,
You are welcome!
If you need assistance or papers you can contact me via RG.
With kind regards
HGD
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