I have a batch of elemental data of the marine sediments and need to constrain the provenance. Could any rock and/or sediment geochemists give me some suggestions and simple yet good geochemical indices to better understand the original rock types, basic immediate or acid. It seems a 'simple and stupid' question for the rocks, but for the sediments I am not sure any special things?
Dear Mr. Li,
I repeat it time and again, it is the wrong approach taken to immediately shift your thinking to the chemical composition of sediments during provenance analysis.
Better use the items arranged in the order of decreasing particle size and significance
1. Lithoclasts (thin section, EMPA, SEM-EDX, Micro-Raman)
2. Heavy minerals (+ light minerals) (thin section, EMPA, SEM-EDX, Micro-Raman)
3. Clay minerals (XRD + Rietveld)
The chemical composition is taken as some kind of philosopher´s stone but nothing else as poking around in the fog.
Try and learn how to recognized minerals, patterns, textures, structures etc so as to get more experienced in mineralogy and petrology, then you are on the save side. Chemistry be it major-, trace elements or isotopes are only a supportive tool and useful for age dating.
If you only want to feed some triplots for a paper you are on the right track.
You can use Ca, Mg, Cr, V, Ni…. for the basic end of the Bowens reaction rule and K, Na, F, for the opposite end of it.
But you can also examine a thin section and try and find relic granite or basalt lithoclasts or olivine (still stable) and tourmaline varieties ….
The chemical composition of the sediments only whispers.
The mineralogical/petrological composition tells you a story
Since 44 years I have been using the tripartite sequence: Geology-Mineralogy-Geochemistry.
Good luck ! Harald G. Dill
I would like to emphasise Prof. Dills answer. To interpret provenance you have to use a multitude of methods where petrography and mineralogy should be the starting point. How can you be sure that the relevant source is an igneous rock at all?
Typically, a sediment have mixed sources with exeptions for a few tectonic settings.
Cheers
Trond Harstad
Dear Mr. Li
Geochemical studies of lithogenic fraction (free of carbonates, sea-salt, Fe-Mn oxides etc.) in combination with mineralogy will help assessing the provenance of marine sediments. Unless, the sediments have unusually high sand content (for example, paleo-shorelines), the clay-sized fraction (
THANK YOU ALL for your constructive comments and suggestions! Harald G. Dill Trond Svånå Harstad Nagender Nath Bejugam I spent much time to explain my "odd" geochemical data (clay mineral assemblages, trace elements, Sr-Nd-Li isotopes, etc.) from a drilling core with various sedimentary facies of fluvial, lacustrine, floodplain and shallow marine. The quite negative Nd isotopes looks like indicate a quite old craton endmember while the smectite% is quite high (60% to nearly 100%) during the same period, so I want to know what rocks or sediments could provide the high smectite. Could the major and trace elements tell me some clues on the sources? On the other hand, the smectite could also be authigenic origin, which makes the question more complex...anyway, I will follow your ideas and look more at the mineralogy things. Thanks again!
Dear Mr. Li,
I have tried to arrange the various mineralogical tools in order of decreasing significance for provenance analysis from 1. to 3.
If you take steps 1 and 2 and use the pertinent methods then I am sure that at the very end you will not need Sr-Nd-Li isotopes to substantiate your results remaining in this case of provenance analysis nothing more than a garnish.
Sorry for my blunt way of expression but I am a trained economic geologist:
Do not waste time and money because there are always new and nice projects looming !
I wish you much success
H.G.D.
Dear Dr. Li
Smectite is usually a product of basic rocks. But as you mentioned, smectite can form authigenically. But, there are means to distinguish them. And your Li isotope data can be indicative of authigenic formation. How negative are your Nd isotopic values? Products of felsic rocks can have more negative. What are your SiO2, Al2O3 and TiO2 concentrations? They may, in combination with La-Th-Sc and A-CN-K plots can give you an idea to distinguish felsic from mafic sources. Hope, you have considered the effect of size-segregation and carbonate dilution for deciding the methodology while performing the geochemical and isotopic analyses.
Sincerely yours
Nagender
Nagender Nath Bejugam Thank you for your suggestions. My epsilon Nd value is about -15. That's too negative for the mafic rocks? The old craton could have similar value and for my study I guess maybe from North China Craton. The SiO2 is 60% in average, Al2O3 16% and TiO2 0.9%. My Li isotopes have quite scattered data, two have +5-8 per mil values and others have 0-+3 per mil values (bulk samples). I do the A-CN-K plot and it suggests moderately-to-strong weathering conditions so I am not sure this plot could give some information on bedrock?
My first interpretation would be tilting towards a provenance of intermediate or mixed composition. Have you plotted the shale-normalized REE patterns? How do they look like. Look at the ratios of La/Sc, Th/Sc, La/Th diagnostic of sources (see McLennan's papers and ours too). But the smectite mineralogy has to be explained.
- Badges
- Science topic
- Similar topics
- Computer Communications (Networks)
- Simulators