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doi:10.2204/iodp.proc.345.102.2014

Collection of core saw cuttings

Following standard IODP procedures, all cores collected during Expedition 345 were cut in half lengthwise to produce an archive half and a working half. The sawing process leaves a ~1.5 mm gap of material ablated by the saw. These saw cuttings should constitute a representative sample of the entire coherent core recovered because the saw does not discriminate between different lithologies. Arguably, this sample may have some advantages for determining bulk core properties over point sampling techniques. This is because the point samples are few in number, preferentially taken on fine-grained, homogeneous and plentiful lithologies in the core, generally avoiding altered, delicate, and “interesting” intervals whose minerals, textures, and structures are of interest for microscopic study, whereas the saw takes everything. Rough calculations show that ~300 g of rock per meter of cut core is produced during the sawing process. The major caveat is that the sawing process itself may introduce some contamination, an effect that should be investigated quantitatively postcruise. The fragments produced by this process are typically <200 µm in diameter, with some fragments reduced to clay size (<4 µm or +8Φ [phi] units). Saw cuttings from each core section cut generated a poorly sorted bulk or “mud” sample (more than ~4 µm), a less than ~4 µm filtrate sample, and a ~30–10 µm filtered sample (Fig. F25).

After the core is removed from its plastic casing, the core pieces are washed and rinsed of all drilling mud and air-dried. The core is then cut using a hand-driven shuttle, forcing the individual pieces through a diamond saw blade lubricated with distilled water. The entire saw and shuttle assembly are enclosed on five sides by a plastic housing. Together with the spray guard at the front of the saw, this ensures that the water and cuttings sprayed from the sample and the drippings from the sample are contained within the housing.

After all pieces in a section (typically 0.5–1.5 m long) are cut, the cuttings are collected. Some of the cuttings lie on the floor of the plastic housing, and as much of this material as possible as is collected using a spatula. The remaining cuttings from the shuttle, mounts, and farther reaches of the housing are rinsed using a strong spray of deionized water and flushed to the drain of the enclosure. This rinse water flows into a sink in which a temporary sampling drain replaces the original drain so that rinse water and cuttings can be diverted into 10 L plastic containers. A final rinse ensures maximum capture of the total cuttings of all rocks cut in any given section of core and also leaves the saw and housing clean for cutting the next section of core.

Next, the slurry of cuttings and rinse water is decanted from the plastic containers into five 1 US gallon plastic buckets and allowed to settle for at least 12 h to allow most particles to fall to the bottom. In this time, all silt-sized particles (those >4 µm) should settle to the bottom of the 37 cm tall bucket (e.g., Lewis, 1984). The supernatant suspension is then decanted from the bucket, leaving a thick slurry of particles from ~200 to <10 µm in size (Fig. F25A). This slurry, our mud sample, is then air-dried and archived.

The supernatant is free of most silt-sized particles and is passed through a 10 µm Dutron 10 sieve to capture any remaining coarser particles (Fig. F25B). Where a >10 µm fraction is captured, the material is sampled as “>10 µm.” In practice, no material was captured in samples where a settling time was 12 h or more, and in no sample did the >10 µm fraction exceed 0.1 g.

A 2 L aliquot of the filtered supernatant with its clay-sized suspended material was centrifuged in 50 mL tubes at 2000 rpm for 10 min, causing the clay-sized particles to settle out. This clay-sized material was then washed in alcohol, dried in air, and weighed. The weight was recorded as grams of suspended material per 2 L of supernatant. The volume of the remaining supernatant was also measured in a burette to ±5 mL. The total amount of clay-sized particles was then calculated as follows:

This clay-sized sample (Fig. F25B) typically constitutes ~25% of the total weight of the sample.

To test whether any fractionation occurred between the coarser mud sample and the clay-sized sample, both aliquots were analyzed on board by XRD using a Brucker D-4 Endeavor diffractometer with a Vantec-1 detector and nickel-filtered CuKα radiation. Samples were mounted as smear slides or pressed into depressions in sample holders if the amount of sample was sufficient. Mineral identification was achieved with the interactive Diffrac.Suite Eva, version 1.4, software package (2010 release) using the powder diffraction file database associated with the program. Identifications were based on multiple peak matches. Instrument conditions were the same as those listed in “X-ray diffraction.”

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