Crystal Histories and Crustal Magmas: Insights into Magma Storage from U-Series Crystal Ages

Abstract:

The dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas, and crystals in volcanic rocks can provide a temporally-constrained archive of these changing environments. A new compilation of ²³⁸U-²³⁰Th ages of accessory phases and ²³⁸U-²³⁰Th-²²⁶Ra ages of bulk mineral separates of major phases documents that crystals in individual samples often have ages spanning most of the history of a volcanic center. Somewhat surprisingly, this observation holds for surface analyses as well as interior analyses, indicating that the latest stages of growth took place at different times for different grains. Nevertheless, average ages of surfaces are younger than interiors (as expected), and the dominant surface age population is often within error of eruption age. In contrast to accessory phase ages, less than half of the bulk separate ²³⁸U-²³⁰Th-²²⁶Ra ages for major phases are more than 10 kyr older than eruption. This suggests that major phases may in general reflect a later stage of development of an eruptible magma body than do accessory phases, or that the extent of discordance between ages of major and accessory phases reflects the extent to which a crystal mush was remobilized during processes leading to eruption. Crystal ages are most useful for illuminating magmatic processes when combined with crystal-scale trace-element or isotopic data, and I will present several case studies where such combined data sets exist. For example, at Yellowstone and at Okataina Caldera Complex, New Zealand, the combination zircon surface and interior analyses (of age, Hf isotopic, and trace-element data) with bulk dating and in-situ trace-element and isotopic compositions of feldspar allows a comparison of the early history of storage in a crystal mush with the later history of melt extraction and further crystallization prior to eruption, thus tracking development of erupted magma bodies from storage through eruption.