Thin Section Lab

Luan Heywood

Summary

3 thin sections requests on Exp 379T/JR100, small format. 3 carbonate muds.

5 thin sections of various sample rocks created as sample thin-sections for equipment calibration for Imaging Specialist, small format.

12 thin sections created with and for JR Academy students, small format. These were basalts and gabbros.

LP-50

Not used

PM5 Polisher

No problems

PetroThin

Over many years, considerable amounts of debris have accumulated on the walls of the water trap segment of the Logitech VS2, which is used in between the Buehler PetroThin and the Welch B917 vacuum pump. Logitech VS2 was disassembled and cleaned using a combination of table salt/isopropyl alcohol slurry as an abrasive, and water to clean and rinse. The abrasive slurry seemed to work well when agitated, and in retrospect I would have set it to agitate on either the shaker table or the larger ultrasonicator located in the Paleo Lab. The tubing in the unit was replaced with new Tygon tubing (1/4" internal diameter), as the old was discolored and very brittle. In the future, I recommend performing cleaning/maintenance every few years, including new tubing.

Buehler Lap Wheel

No problems

Epo-tek 301 Epoxy

Chemistry Technician Vincent Percuoco requested that Thin Section Technicians use 1 mL disposable transfer pipettors for Epo-tek 301 epoxy in order to avoid epoxy damage sustained on non-disposable pipettors.

All remaining unexpired Epo-tek 301 epoxy (5 kits) will expire on October 30, 2019 in the middle of Exp 385. This was communicated to current ALOs/LOs and the LO of the upcoming expedition (during which epoxy will expire), however ordering was not done in time for Exp 385 ongoing freight. To hopefully forestall this occurring in the future, end-of-expedition cleanup list was edited to add an action item "Perform a physical account of epoxy and update the AMS check-out sheet, taking into account potential upcoming expiration dates for unopened bottles".

This is part of an ongoing problem with epoxy ordering. Epo-tek 301 resin is currently set to an AMS Restock Level of 6, because the vendor has a minimum order of $250 (5 kits). Typically, we only use 1-3 kits per year, but since the Epo-tek 301 resin only has a shelf life of 12 months we typically throw away several unopened bottles per year. Since we cannot lower the Restock Level, perhaps in the future we should explore alternative epoxies.

Zeiss Petrographic Microscope

Thin-section lab microscope was removed and replaced with microscope previously located in the smear slide area. The new microscope is larger, has more features, and is much cleaner than previous thin-section lab microscope.

Samples and Methods

During this expedition, I conducted experiments to try to modify the SOP for epoxy impregnation of lake sediments from LacCore, the National Lacustrine Core Facility (PDF: http://lrc.geo.umn.edu/laccore/assets/pdf/sops/thinsections.pdf). The LacCore facility (at the University of Minnesota) routinely prepares thin-sections of soft sediments without using time-consuming methods of acetone-replacement. One of the main differences between their SOP and what is done at IODP is that they use a slow embedding process using a low-viscosity epoxy with a high pot life (Spurr’s epoxy; 1-4 days) which soaks into dried sediment over a period of hours-days.

An experiment was devised to see how much Epo-tek 301 resin could be diluted with acetone in order to extend the pot-life and still be usable for thin-sections. It was found that for 4 mL typical epoxy recipes (3 mL part A, 1 mL part B), any more than 0.5 mL acetone dilution resulted in unsightly yellowing, deeming it unsuitable for thin-section preparation. At this dilution, the acetone cures in ~12 hours, whereas undiluted epoxy cures in ~4 hours.  

Sample impregnations were performed with 2 cm long cubes of unconsolidated marine mud (2 cm is the size of the small impregnation molds). These were freeze-dried, then placed in a medium sized impregnation mold, which was already filled with 4 mL of epoxy (undiluted and diluted, variously). These were allowed to cure at room temperature, under intermittent vacuum to help suck the epoxy through the pore space of the sample. Additional epoxy (again, variously undiluted and diluted) was added to the side of the mold after the first epoxy was absorbed. Both cubes were examined after an overnight room temperature cure. For the cube impregnated with undiluted epoxy, epoxy penetrated to an internal depth of 5-8 mm before curing. For the cube impregnated with diluted epoxy, epoxy penetrated throughout almost the entirety of the cube and visibly wet the top surface of the cube.

Considering my findings, I recommend the following approach for embedding soft sediment thin-sections. Freeze dry the sample in the Labconco freeze-drier after freezing briefly in a -80 freezer—this prevents the growth of large ice crystals with can disrupt the internal structure. For embedding, use Epo-tek 301 resin diluted with 0.5 mL acetone per 4 mL normal recipe. Limit the size of mud chunks selected for thin sectioning to <2.5 cm as the smallest dimension. For larger (e.g. standard size) thin sections, we should investigate an alternative resin with a longer pot-life, or procedures where impregnation occurs under vacuum, perhaps using the new vacuum impregnator that arrives on Exp 385.

Thin-Section Lab and X-ray prep

During the 385T Transit, the Thin-Section Lab false floor was removed for cleaning. The floor was left disassembled with grates and wood temporarily stored in the reefer, prior to final decision on whether to set it back up or junk it occurring on Exp 385, following the crew change. At this time, the X-Ray Prep area storage was also extensively renovated. Storage areas were cleaned out, and new shelving was prepared with the help of the ETs. One result of this reshuffling was that the shelf that the stereo was on was removed, with the anticipation that on a further tie-up or transit a new shelf could be built above the sink by the Petrothin or the stereo could be mounted underneath the shelving above the counter in the X-Ray Prep Lab.