Shocked Biotite in Natural and Experimental Systems

English Abstract:

Shocked anhydrous minerals (e.g., plagioclase, Kspar, quartz) has been well documented using both natural and experimental samples. The goal has been to determine the initial shock and/or shock recovery pressure under which such minerals break down as a means to estimating the shock pressure conditions incurred by impacted materials. However, the behaviour of hydrous minerals, such as biotite, under shock conditions has not been well characterized. Hydrous minerals are common constituents in terrestrial rocks, and their presence may influence the shock recovery process within the bulk rock, as both a source of shock impedance contrast and fluids in the system. Research on the high-grade metapelites of the Vredefort Dome has suggested that the breakdown of hydrous minerals such as biotite within the host rock may have been a significant source of fluids in the shocked target rock.

Shock recovery experiments, using samples of migmatitic metapelites from the Etive aureole in Scotland as an analogue for the Vredefort Dome metapelites, were carried out at the Ernst-Mach-Institut, Germany. SEM-BSE data indicate that the breakdown of biotite to release fluids facilitated shock-related deformation by explosively injecting contiguous minerals (garnet, plagioclase, kspar, quartz) with fluids, locally fracturing some and facilitating the fusion of others. The biotite itself formed shock fractures perpendicular to (001), as well as having undergone intragranular vesiculation, sintering, shredding and extreme strain localization at increasing degrees at each shock pressure tested (from 12.5, 18, 25 34, 40 to 56 GPa). Shredding of the biotite structure appears to be due to a combination of shearing perpendicular to (001) and internal shock reflections that generated complex interaction between melt and kink bands. In comparison with natural samples from the Vredefort Dome, which are thermally overprinted, the microtextures of biotite shocked from 34 GPa upwards are comparable. Whether biotite occurs as an accessory mineral or as a principal mineral in the target rock assemblage, its break down under shock is likely to be a significant source of fluids in the system and a facilitator of damage to proximal phases under high secondary shock pressures.