Quaternary Sediment Provenance
- Provenance means source/origin
- Provenance indicators, any proxy that allows provenance to be identified or reconstructed
- The methods we are going to use are;
- Assemblages and morphologies of certain 'heavy' minerals that form in association with common minerals (such as quartz and feldspars) in source rocks
- U-Pb dating of zircon minerals, which will tell us when the zircon grew
- Fission track thermochronology of zircon minerals, this will tell us the last time the zircons were at 200 degress C
- U-Pb dating of rutile minerals which will give the age of when the rutile was last at 500 degrees C
- Ar-Ar dating of hornblende minerals, this will give the date that the hornblendes were last at 500 degrees C
- Geochemistry of garnet minerals, this will tell us what environment the garnets grew in
- Isotopic ratios of Nd, Hf, and Sr in bulk sediments. Different source areas have different isotopic characteristics so the loess data can be compared with the sources to establish which source is most likely.
- Provenance generally means source area, but can also mean mode of transport
Provenance can tell us about past flow indicators, for example for this project it will give information about past wind patterns. It can also be used to provide more information on river histories in terms of catchment area, climate and tectonic influence. Provenance can also tell us about what is being eroded, so what is uplifting and/or how is climate/ processes (i.glaciation) affecting the landscapes.
Zircon U-Pb = One grain, one source
Single grain analysis gives the geochemical property of one particular mineral. This is often diagnostic of source rocks that this mineral came from. Given this, looking at the properties of single mineral grains allows us to trace their origins, and there, dust sources. Data comes from measuring multiple individual grains from the same sample to build up a statistically robust data set as a fingerprint of the source regions for that sample. This data set can then be matched to potential source regions to establish provenance. One example of this is using individual zircon U-Pb ages. The U-Pb age of a zircon usually dates the time at which that zircon grain crystallised and so can tell us the ages of likely source rocks. If we can match these ages to zircons from source rocks or sediment then this can allow us to trace the provenance of dust.
The disadvantages of using zircon U-Pb are that you are only looking at one aspect of the sediment. Zircons normally grow in granitic melts and rarely grow in other types of rocks so by only studying them you excluding input to your sediment from other possible source rocks. The other disadvantage is that zircons are heavy minerals and are very robust, so you are not looking at the light minerals and you may only be looking at a very early input to the sediment, for example material that has undergone many phases of transport, deposition and erosion. Despite this zircon U-Pb is an extremely useful tool and example of how it can be used is shown below.
Figure 1 shows zircon U-Pb age spectra from samples from the Mu Us desert north of the Chinese Loess Plateau. The samples on the western side of the desert show two age peaks under 500 Ma, while the eastern samples only show one. Figure 2 shows zircon U-Pb data from the Qaidam basin, which also shows two peaks under 500 Ma. This suggests that the sediments in the west of the Mu Us are at partly sources from the Qaidam basin area.
Fig 1. Figure edited from Stevens et al. 2012 showing the zircon U-Pb spectra for most of the samples from the Mu Us desert. The red line splits the samples which have two peaks less than 500 Ma from those that only have one peak.
Fig. 2. Showing potential source area for samples from the western side of the Mu Us desert.
Stevens, T., Carter, A., Watson, T.P., Vermeesch, P., Andò, S., Bird, A.F., Lu, H., Garzanti, E., Cottam, M.A., Sevastjanova, I., in press. Genetic linkage between the Yellow River, the Mu Us desert and the Chinese Loess Plateau. Quaternary Science Reviews.
Enkelmann, E., Weislogel, A., Ratschbacher, L., Eide, E., Renno, A., & Wooden, J. 2007. How was the Triassic Songpan-Ganzi basin filled? A provenance study. Tectonics, 26.
Lease, R.O., Burbank, D.W., Gehrels, G.E., Wang, Z., & Yuan, D. 2007. Signatures of mountain building: Detrital zircon U/Pb ages from northeastern Tibet. Geology, 35, 239-242.
Weltje, G.J. & von Eynatten, H. 2004. Quantitative provenance analysis of sediments: review and outlook. Sedimentary Geology, 171, 1-11