1.6b Questions (continued)

1.04 DR13-3 and T178-G4
  1. Complete full thin-section descriptions for DR13-3 and T178-G4. Be sure to note the following in your descriptions: phenocryst mineralogy, your best guess of the groundmass mineralogy, the approximate modal % of each phenocryst phase, and any textures indicating disequilibrium, condition/timing of crystallization or post eruptive cooling. It is generally a good idea to start your examination and description using the lowest magnification view and then proceed to view the higher magnification images. Be sure to use both plain- and crossed-polars.
1.05 T735-G12 This rock is also from the Juan de Fuca Ridge, but it is quite rare and unusual. Examine the major element chemistry in Table 1.
  1. Classify the rock type for this lava using the TAS Classification diagram and the whole rock chemistry for T735-G12 given in Table 1.01.
  2. Would you expect to find olivine in this rock? Explain?
  3. Describe the mineralogy of this lava. Using optical techniques, try to determine the composition of plagioclase present in this sample. Compare this plagioclase composition to the composition observed (or presumed) in one of the previously examined basalt samples. (Tip: The Michel-Levy Technique can be used to determine the composition of plagioclase feldspar. A description of this technique can be found in most Optical Mineralogy Textbooks.)
  4. Examine the vesicles. How do they compare in size and overall modal percent to the more primitive MORB samples? Are your observations consistent with the chemistry of this rock? Why or why not?
  5. Examine the groundmass. Note that overall it appears to be glassier compared to the more variolitic and fascicular textures seen in the basalts. Hypothesize a possible explanation for this observation?
  6. This lava has a number of xenoliths and xenocrysts included in it. Find one (or more, preferably) and examine it. What is the mineralogy of the xenoliths? What do you think these xenoliths represent and where did they originate? Justify your answer.
  7. Compare the textures observed in the xenoliths compared to the host rock. Describe each. Also closely examine the texture along the contacts between xenolith and host rock. Note any observations you make.
  8. Hypothesize a possible origin for the xenoliths.


>