Investigation of a large number of samples during this project led to development of the following processing routine.
Curation of samples was performed at the University of California, Los Angeles. As appropriate, subsamples were subsequently distributed to PPRG members for analyses to be carried out at their home institutions. A flow chart outlining the various procedures involved is shown in Figure 15.2 and is summarized below.
The initial curation for every incoming rock sample consisted of assigning a PPRG Sample Number (e.g., “1001”). Pertinent geological information was compiled and entered into the databases “Inventory,” “Site,” and “Strat” (see Chapter 21).
For paleontological and mineralogical studies, petrographic thin sections were prepared of each sample: for microfossil studies, a 150µi-thick “paleo”-section (“1001-1-A”); and for petrographic studies, either a standard 30 µm-thick section for non-carbonates (“1001-1-B”) or a 5 to 15 µm-thick section for carbonates (“1001-1-C”). In addition, large-area thin sections (150 µm-thick, “1001-1-STROM”) were prepared of selected stromatolitic samples.
Sample processing for geochemical and/or palynological studies was initiated by discarding any weathered surface or secondarily emplaced vein material and generating a mass of clean interior rock chips ≤ 1 cm in diameter (“1001-1-RC”). Chipping of small samples was performed using a geologic hammer; larger samples were chipped with a jawbone (i.e., “chipmunk”) crusher. In order to remove any organic contaminants, the chips were etched in a 20% HF-10% HC1 solution, then rinsed with large volumes of distilled water and dried in a drying oven at 75° C.