1.6a Questions

The following questions are designed to help you think about the information that can be deduced from volcanic rocks collected from the sea floor on the Juan de Fuca (JdF) ridge. Images of petrographic thin sections made from several JdF samples can be accessed from this list. Photomicrographs of the samples in plane polarized light (ppl) and crossed polarized light (xpl) are available, both large area images and selected closeup images. You can pan and zoom the images with your mouse and mousewheel. And in many cases, higher resolution images are provided by clicking "medRes" or "hiRes" buttons. Note that higher resolution images may take some time to load, so be patient if you want to see all the details.

1.01 ALV-2268-1
  1. What are the two major microphenocryst phases? Detail the observations you made to draw this conclusion.
  2. Several phenocrysts show a texture of cotectic (simultaneous) crystallization. Find an example of this and draw it.
  3. Examine the overall population of phenocrysts. Which of these phases do you think may have started crystallizing first? Explain the observation(s) that led you to this conclusion. (Tip: Consider differences in crystal morphology the associations in which crystals occur relative to one another when trying to determine the sequence of crystallization.)
  4. The groundmass texture is difficult to see because of the dark glass color. Look at the closeup view in ALV-2268_009. For this image, the "xpl" button shows the image with the light intensity increased significantly. Describe (draw) and identify the texture you observe.
1.02 DR12A
  1. Examine the groundmass mineral phases and matrix. Use the more brightly illuminated closeup view (DR12A_002) to see the goundmass. Describe and draw any textural features that are indicative of rapid quenching of this lava.
  2. This sample only has almost only one phenocryst phase. What is it?
  3. Olivine occurs as a fine-grained groundmass phase. Is the sequence of crystallization in this sample consistent with the sequence you determined for sample ALV-2268-1? Why or why not?
1.03 T179-G10 This MORB is composed primarily of three silicate mineral phases, opaque minerals and glass (both of the latter as part of the groundmass).
  1. Identify the three silicate phases that occur as phenocrysts or microphenocrysts. You may need to work a bit to distinguish the two mafic phases from one another. Be sure to note the observations you made to correctly identify the phases; an interference figure may be diagnostic in distinguishing the two mafic phases. (Tip: The best interference figures are seen in grains that don’t change their appearance and color greatly with stage rotation in cross-polars. Look for a grain that is big enough and has low-order interference colors. Your high power objective should be fairly well centered to get a good interference figure on a small grain.)
  2. Describe the textural relationship(s) that exist between the phenocryst phases? What might you infer about the relative timing of crystallization of these phases?
  3. Describe and draw any textural features that are indicative of rapid quenching of this lava.
  4. How does the crystallinity and relative abundance of phenocryts in this sample compare to that of DR12A? What might you infer about these two rocks from your observations? (Hint: think pre-eruption cooling history, not so much post eruption cooling history. You might also consider how the chemical compositions of these two samples differ, and a possible cause of that difference.)