Residual Strains in Magnetite and Zircon Record Volcanic Processes

Abstract:

Quantitative constraints on the processes and physical conditions in magma reservoirs are important for understanding their formation, evolution, and eruption. Crystals stressed by forces from magmatic processes develop lattice strains that are locked into place upon cooling. We use X-ray Laue microdiffraction (µXRD) to measure residual strains preserved in zircon and magnetite from explosive and effusive eruptions from Yellowstone and Long Valley magmatic systems. Strain was mapped across mineral grains and used to construct stress maps by calculating the stress necessary to produce the measured strain with Hooke’s Law.

Our measurements on effusive samples reveal mean residual stresses of 183-260 MPa for zircon and 121-222 MPa for magnetite from Yellowstone. Mean stresses are 231-312 MPa for zircon and 294-458 MPa for magnetite from Yellowstone tuff deposits and 237-344 MPa for zircon from Bishop Tuff. Strain microstructures suggest two main processes contribute to the preserved strain in effusive eruptions: 1) temperature and pressure of mineral formation and 2) heterogeneous distribution of lithostatic pressure across a crystal mush. Differences between magnetite and zircon for effusive deposits likely reflect different formation depths. High strain values correlated with microstructures indicative of high strain rates suggest that rapid decompression is a third process contributing to residual strains in explosive eruptions. Different stresses for zircon and magnetite in tuff deposits likely reflect the different plastic deformation dynamics of the two minerals. Our results show that residual stresses may yield quantitative insights into pressures, temperatures, and strain rates in magma systems.