Tracing.

What did you do today? Yesterday? The past three years?

Tracing crystals. It’s one of the boulders I’ve been pushing uphill since the beginning of my graduate research. Why? What is it all for? Three little words: crystal size distributions. CSDs are a method to quantify rock textures from thin sections. Let’s look at one of my samples, Apollo 17 basalt 75015,52:

You can readily describe the overall texture of a thin section, and estimate phase proportions and morphology, etc. The coarse-grained nature and large crystals are obviously the result of slow cooling. Ilmenite (FeTiO3) makes up the majority of the opaque phase and varies in morphology from generally euhedral, lath-like crystals to skeletal and almost amoeboidal in some areas. CSDs are a way to (attempt to) address questions such as:
– Whether these phases all one population of crystals (i.e. a single crystallization sequence with larger crystals being the earliest formed) or a mixed population (perhaps large skeletal ilmenite crystals formed prior to eruption or elsewhere during flow and were later mixed with smaller euhedral ilmenite crystals) or were they affected by some other process;
– How fast the sample cooled, and whether it was constant or variable
– Whether this basalt is related to other basalts from the same area.

Crystal size distributions are an objective method of assessing crystal size, shape, abundance, and distribution. It’s basically a three step process. Step 1 is to make a mosaic of the thin section (as above). Step 2 involves tracing individual crystals (we use a tablet), which takes anywhere from a couple of days to a couple of weeks. I trace different phases on separate layers in Photoshop, with the final result looking like this:

Gray is the sample area, blue is pyroxene, yellow is plagioclase, and black is ilmenite.  For this sample I traced ~1500 individual crystals.

The rest is easy. We have a slew of programs to measure length, width, area, and position of the crystals (NIH ImageJ), estimate crystal habit short/intermediate/long axis dimensions, and population density (CSDCorrections).

See the website of M.D. Higgins for more information on quantitative analysis by CSDs.

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