EXPLOADING ZIRCONS: ORIGIN AND SIGNIFICANCE

Some zircons in pyroclastic rocks have distinctive bowl-shaped fractures with smaller empty cavities at the bottom. Essentially identical features were described earlier for pyroclastic quartz (Triplehorn, 2008), where they are easier to recognize because the quartz grains are much larger. These features were discovered while observing loose grains: they probably cannot be seen in thin section. The empty cavities represent gas or liquid-filled inclusions and such “exploding” quartz and zircon are apparently produced by rapid decompression during an eruption, resulting in sufficient pressure increase in an inclusion to blast out a lens-shaped fragment. Evidence for the internal origin of the force consists of the shape of the crater, the occurrence and position of the cavity, as well as concentric and radiating fractures around the cavity. Such features have been observed in a number of pyroclastic rocks, and their somewhat modified recycled equivalents appear in sandstones and shales. Photographs and SEM images are shown of exploding zircons from the Late Cretaceous Black Peaks Formation, Big Bend National Park, Texas (Lehman & Busben, 2007). Recognition of such features may permit identification of pyroclastic zircons as opposed to those of other origins in sedimentary rocks. Age-dating of such exploding zircons may permit selection of those produced by volcanic eruptions with the same age as the depositional age of the enclosing sediment. This would be most useful for carbonates and fine-grained shales unlikely to contain significant detrital zircons; i.e., of greater age.