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“I’ll take a fraction of those zircons, make thin sections of them—slices of mineral thirty micrometers thick, roughly as thick as a hair, that are mounted on glass—and get an idea of what they look like in terms of zoning pattern, whether they underwent multiple episodes of growth, how simple or complex they are,” says Henry.He passes this information along to Mueller, along with the sample’s geological context.“I also look at a thin section of the rock to learn something about the framework in which the zircon occurs. Or is it in a metamorphic rock that has had a more complex history? By knowing its history, we can interpret the age of the rock much better.”“To understand the relative geologic history of a rock, Darrell uses thin sections because he’s interested in the relations among all the minerals, which make up the rock,” explains Mueller.“However, for geochronology, we’re interested in the minerals that make up one tenth of one percent or less.” He looks at the zircon using various techniques—“light reflected off the grains, light transmitted through them, cathodoluminescent light resulting from hitting the zircon with an electron beam”—to establish the scale at which the zircon grains should be analyzed.Carbon-14 dating can go no further back than about 70,000 years, because the half-life of carbon-14 is only 5,730 years.
From the types of minerals and their distributions in the rocks he reconstructs a relative sequence of events that reflects the change over time of parameters like pressure, temperature, and deformation. Henry, “I can use the types of minerals and their chemistry to determine the conditions that the rock had experienced at some point in its history.“For example, there may be a granite which contains pieces of other types of rocks enclosed in the granite.Because of their position, we know that the rocks enclosed in the granite have to be older.” Geologists map an area to identity these relative age relationships.They may survive many geologic events, which can be recorded in rings of additional zircon that grow around the original crystal like tree rings.Like a tiny time capsule, the zircon records these events, each one of which may last hundreds of millions of years.