Monday, October 26, 2009

Parke Creek Tachylyte: Washington's Newest Obsidian Source
The newest addition to our list of obsidian sources in Washington is the Parke Creek tachylyte source. Thanks go to Adam and Joel Castanza for their efforts in tracking down source samples from this new outcrop located in the Parke Creek area northeast of Ellensburg and for sending them to the lab for analysis.

But first off, what's a tachylyte? And how is it different than the usual obsidians that are found in the Northwest? Because obsidian is a textural term, the composition of obsidian glasses can vary considerably. Most commonly, the silica content of obsidian falls into the rhyolitic range and the SiO2 content will be approximately 70 percent. Tachylitic obsidian, on the other hand, has a silica content closer to 50% (basaltic) and is commonly found as chilled margins of flows, sill, or dikes. These basaltic glasses are usually dark and very opaque and are easily distinguished from their rhyolitic counterparts by their elevated contents of titanium, manganese, and iron. In addition, the basaltic glasses are usually full of phenocrysts and most often are poor candidates for the manufacture of artifacts.

What makes the Parke Creek tachylyte unique is its relatively good flaking characteristics compared to those of most basaltic glasses. Interestingly, it turns out that there are several other tachylyte sources in Washington that also share this flaking quality. However, the size of available nodules from all of these Washington tachylyte sources tends to be fairly small and we only very occasionally run across them when analyzing archaeological collections.