Tell me who you are, I will tell you where do you come from

There is an important question to answer for the sedimentologists: where do the sediments come from?

One way to answer to this question is to take a look at the sands in the cores. It is one of the jobs of Kitty Milliken on the JR.

But, if you want the sands to talk to you, you have to prepare a smear slide first.

Recipe for a smear slide:

1. Look at the core and find an intersting place to analyze,

2. With a toothpick, remove a little bit of sediment,

3. Drop some distilled water on the sample (not too much, two drops is usually enough!)

4. Mix the sediment with the water and spread it out very cautiously. Be careful: if you want make good observations, you can’t have little blobs of mud!

5. Next, put your glass slide on a hot plate and let it dry.

6. Add a couple of drops of a special adhesive: it is a glue which gets hard when exposet to a UV lamp.

7. To finish, put the cover slip on and press the bubbles out.

Here you have a nice smear slide that you will be able to keep for many years!

You can now study it with a polarizing microscope! This microscope is specific for observing rocks. It has 2 polarizers which can be crossed. Each mineral behaves in a specific way under polarized light. You have to observe it in two differents ways: you begin with only on polarizer, the with both of them. If you are good enough in that and if you have a lot of practice (unlike me!), you can easily identify each type of mineral.

Take a look at the sand of our core (the U1480):

smear slide

As you see in the picture, you can find minerals that compose igneous rocks (like granite for example). But, we can’ say if the quartz and the felspar are coming form an igneous rock instead of a sedimentary one.

Most of the times, grains are single minerals but sometimes, pieces of sand are actually pieces of rocks! They are called lithic fragments.

This is an example of a lithic fragment:

lithic-fragment

When you find a lithic fragment, you feell very happy because you get more information about the original rock. In that example, this lithic fragment contains quart and muscovite. It comes probably from a muscovite schist. So, you have to look for the source of the sand in an area with metamorphic rocks!

After this cruise, in their labs, the scientists will perform more accurate analyses on many samples to constrain their chemistry and the ration of the different minerals (quartz and felspars) for example. They also will try to find precisely THE original rocks that have been eroded and gave that sand.

How? By studying the sands on land, in the rivers and in the deltas and then, by comparing their composition with their data. If you have the same mineralogy, the same proportion of each mineral, gotcha (Brian taught me that!), you have your rock!

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