First results from EMP

Today I finally got my hands on EMP (electron microprobe), a hi-tech toy for geochemists which allows to determine the chemical compositions for the tiniest amounts of matter (down to the size of 1 micron).  It works by bombarding the rock sample with an electron beam. The beam causes each element to emit X-rays of a characteristic wave length. Then EMP detects the emitted waves and determines the specific composition of each hit point.

My supervisor had a question of the Earth’s origin on his to-do list today. Fortunately my task of the day was only to characterize the mineral composition of a simple granulite.

EMP image where spinel is partially replaced by corundum. The fractures are filled with reaction products. Then the structure is surrounded by a corona of kyanite and amphibole intergrowth. The kyanite needles extend even farer out in the plagioclase. We saw that kyanite and amphibole intergrowth often makes “bridges” connecting more of these larger corona structures. The transport of the elements mostly occurs along fractures and grain boundaries.

(EMP does not use the light so the image represents the average atomic numbers of the elements in the sample. Thus the heavy minerals appear bright (because they have a high atomic number) and light minerals appear dark (because of their low atomic number)).

Optical microscope image of partially replaced spinel inclusions in granulite. EMP section is framed in red. Spinel here is green, corundum is white and plagioclase is pinkish-gray. The orange stuff is amphibole (light) and biotite (dark).

Update: A treat for Ole (from the comment section):

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7 Responses to First results from EMP

  1. Andrew says:

    But what is the material within the fractures in the spinel (pale grey)? Have you tried doing a mass-balance for a reaction between spinel and plagioclase – i.e. do you have to introduce any elements (apart from water for the amphibole)?

    • Liene says:

      We are not sure if the stuff in the fractures is a real mineral or some amorphous substance. We need to do Raman to be sure what it is. Here is the data from EPM (the stuff in fractures):
      SiO2 = 39,1
      Al2O3 = 32,0
      Mg = 7,5
      FeO = 6,7
      MnO = 0,3
      Cr2O3 = -0,02
      Na2O = 0,1
      CaO = 0,9
      K2O = 2,8
      TiO2 = 0,01
      Total = 89,5
      It has a low total so should be some hydrous phase. Maybe chlorite but there is too much alumina for chlorite.
      I am very interested if you have any good ideas?
      Regarding mass balance I will do it later but for now it seems potassium should be introduced from somewhere because neither spinel nor corundum nor plagioclase have can provide it.

      • Andrew says:

        I agree that Raman would be the next step to try to identify that phase.
        Is there also K in the amphibole? Is there any K in the plagioclase?
        It is also interesting that the kyanite extends into the plagioclase … is the plag about An50?
        If the Plag is An50 then the Al:Si ratio is about 3:5 while the kyanite has 2:1, so if the kyanite is replacing plag then Na,Ca and Si have to be removed.
        This would mean quite a local volume reduction …. which presumably does not seem to happen…. interesting thing to think about on a dark and wet Sunday afternoon….

        • Liene says:

          Plagioclase has only 0,2 wt% K2O but amphibole has 2,4-2,8 wt%. So K could come from amphibole but then it should be introduced to make amphibole.
          You are right about plagioclase composition. It is An54. I suppose we blame fluids for this mass transfer :)
          Interesting things to think about, indeed…

          • Ole says:

            You have a biotite in this sample too, that’s even more K2O. It looks to me as if the hornblendic amphibole (brown) has been partially replaced by biotite?

            The kyanite/amphibole fibrous mass most likely has a completely different amphibole composition, I would guess actinolite…

            • Liene says:

              Yes, biotite replaces amphibole and kyanite needles form in amphibole in contact with biotite. Check the picture below the post! Potassium can come from both of them as well as from plagioclase. Kyanite probably balances reaction in Al and Si so the system can be relatively closed.
              Regarding the amphibole, the yellow one is indeed a hornblende but I have no data on the composition of the other one. As it was not the main interest at the time we did just a quick spectroscopy check with EDS. However other studies report pargasite and also omphacite in these kind of reaction rims from nearby localities. It makes sense because we know that these granulites have experienced eclogite facies metamorphism but reactions were very limited and mostly occured along the fluid pathways such as fractures and shear zones. These tiny reaction rims around spinel can also be from eclogite conditions.

  2. Pingback: A day in a lab « Mente et malleo: by thought and hammer

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