Core Analysis

To give an example what can be learned from core analysis, let's look at a simple to imagine scenario: the Dakota Hogback. During the Larmide Orogeny ~50 million years ago, the Rockies were uplifted, exposing many different geologic formations. Here, we see the Mesozoic (Jurassic and Cretaceous periods, 150 - 80 million years ago) line up. Each color band seen in the rock formation records a different environment. The age goes from right to left - the layers to the right of the photograph are much older (150 million years) than the layers to the left (80 million years). The environment to the right in the Jurassic (soft grey rocks) represents an environment not too dissimilar to today's Mississippi river. The formations to the left in the Cretaceous

What can we learn about them using XRF?
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Over time, potassium (K) gradually left the system, as you can see from the highK% from right to left. However, less K gets deposited in large bodies of water - could we be looking at the shift from land to sea. If we switch and look at niobium (Nb) to rubidium (Rb), we can see if we can test this idea.
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It certainly looks like there is a relationship! As potassium (K) falls, the ratio fo niobium (Nb) to rubidium (Rb) increases. Nb is thought to reflect the circulation of water, and this would confirm that idea. But, these are just wavy lines - can we see if there is a meaningful relationship?
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If we plot Nb/Rb vs K, we can see that yes, the difference is meaningful. The Jurassic land deposits show lots of potassium, while the Nb/Rb is much higher in the interior seaway that predominated much of North America. This suggests we can see differences in these two elements to track what kind of environment was present in Golden, Co. XRF can also identify other events.

If you look back to the photo of the formations, you'll see a dark grey layer that doesn't seem to look like sea or land. What element is present there? One useful element to identify these kinds of changes is sulfur (S), which can indicate a very different environment.
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Here, we see that the dark grey formation has lots and lots of sulfur in it - as much as 15% in some places! This indicates that there was very low oxygen in these conditions - sulfur tends to precipitate when there is less water circulation. You can see that where the sulfur increase is, the Nb/Rb ratio drops.

If we take a step back, we can see the evolution of Colorado from a complex river system to a sea, the growth of severe anoxic conditions, and then a recover. All just on the side of the highway.


Economic Geology

Understanding the Earth's past requires detailed geochemical analysis This means understanding not only the compositional materials like SiO2 and CaCO3, but also trace elements like Mo and U which can indicate the reduction sequence in marine environments.







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At PRI, we use an in-line XRF system to do continues step analysis of cores, enabling us to process data rapidly and accurate. We follow the methods and employ the standards used in Rowe et al. 2012. With a full range of elements, complex paleoclimatic interpretation is possible, which relevance to reconstructing geochemical formation processes.

In this example, a calcium carbonate (Ca~30%) predominates, formed largely by the precipitation of dead plankton on the sea floor. In an ocean with plenty of oxygen, the carbon is recycled to the biosphere via the activity of bacteria; with only remnants of being the shells of the microscopic organisms. However, to the right of the graph, the calcium concentration drops while a trace metal, molybedenum (Mo) increases. Key to understanding this change is knowing that Mo oxide is soluble in water while Mo sulfide is not. In anoxic conditions, the Mo will precipitate out. These same anoxic conditions also prevent the growth of bacteria, thus in turn keeping the carbon from being recycled to the biosphere. These are ideal conditions for the preservation of organic carbon, and can be used to identify fossil fuel deposits.

By pairing XRF data with targeted XRD and FTIR application, we can provide complete analysis to contextualize findings and help guide decision making.


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