P-wave velocities across the α → β quartz transition at lower
continental crust pressure and temperature conditions
Abstract
The quartz α → β transition is a displacive phase transition associated
with a significant change in elastic properties. However, the elastic
properties of quartz at high-pressure and temperature remain poorly
constrained experimentally, particularly within the field of β-quartz.
Here, we conducted an experimental study on the quartz α → β transition
during which P-wave velocities were measured in-situ at pressure (from
0.5 to 1.25 GPa) and temperature (200 to 900 °C) conditions of the
continental lower crust. Experiments were carried out on samples of
microcrystalline material (grain size of 3-6 μm) and single-crystals. In
all these, the transition was observed as a minimum in P-wave
velocities, preceded by an important softening, while P-wave velocities
measured in the β-quartz field were systematically lower than that
predicted by thermodynamic databases. Additional experiments during
which acoustic emission (AE) were monitored showed no significant peak
of AEs near or at the transition temperature. Microstructural analysis
nevertheless revealed the importance of microcracking while Electron
Back-Scatter Diffraction (EBSD) imaging on polycrystalline samples
revealed a prevalence of Dauphiné twinning in samples that underwent
through the transition. Our results suggest that the velocity change due
to the transition known at low pressure might be less important at
higher pressure, implying a change in the relative compressibilities of
α and β quartz. If true, the velocity changes related to the α → β
quartz transition at lower crustal conditions might be lower than that
expected in thickened continental crust.