Gladstone Geochronology – Update 6

I wrote yesterday about the insights gained from looking at thin sections to better understand the bulk composition of rock packages. Today, I want to write about one final key piece of the summer research puzzle: geochronology. Geochronology is a field of geology concerned with dating rocks, fossils, geologic events, sedimentation rates, and other geologic or geomorphological processes. As you can imagine, geochronological methods take many forms. For my project, I am primarily concerned with something known as Argon-Argon dating. Ar-Ar is a form of radiometric dating that examines the ratio of 40Ar, an unstable isotope with a half-life of 1.25 billion years, and 39Ar, Argon’s stable isotope.
There are other forms of geochronological radiometric dating, like the decay of Uranium to Lead, but that method is not helpful to me in Gladstone. Why? Well, as often is the case in geology: minerals. Different minerals form at different temperatures and have wide ranges in elemental composition. Zircons, for example, which contain uranium isotopes useful to U-Pb dating, only crystallize at about 1600-830 degrees Celsius. While these temperatures are certainly reached from a source melt (the magma that hardens to create an igneous rock), they were not reached during the deformation and metamorphism of Gladstone rocks. How do I know this? Simple: the metamorphic rocks found in Gladstone do not contain the mineral assemblages that indicate temperatures of that magnitude. I would explain in more detail how geologists know which minerals form at what temperatures, but this would then be less of a blog post and more of a metamorphic petrology lesson. For now, just remember: minerals!
Returning to Ar-Ar dating, there is one particular mineral found in abundance in Gladstone rocks that could provide us with the necessary radiometric samples: muscovite. Muscovite, or white mica, can be formed in both metamorphic and igneous environments. It has a low closure (when the mineral stops growing and traps the potassium (which is radiated to produce argon) into place) temperature, making it a good tool to uncover the thermal and geologic history of an area. I will be using Ar-Ar dating to uncover the geochronological history of different rock packages in Gladstone. These histories can be compared with the literature and will contextualize Gladstone in the over geological evolution of Virginia.