Partial melting and reaction along deformation features in plagioclase

Abstract

Geological processes involving deformation and/or reactions are highly influenced by the rock grain size, especially if diffusion-controlled processes take place such as metamorphic reactions and diffusion creep. Although many processes, inducing grain-size reduction, are documented and understood at relatively high stresses and low temperatures (e.g., cataclasis) as well as at lower stress and higher temperature conditions (e.g., bulging and subgrain rotation), deformation twinning, a plastic deformation mechanism active in various minerals at lower temperatures, has been neglected as nucleation site for melting and reaction and thus as a cause for grain-size reduction so far. We conducted experiments on natural plagioclase-bearing aggregates at 2.5 to 3 GPa confining pressure and temperatures of 700 C to 950 C using two different deformation apparatus, a deformation multianvil apparatus (DDIA) and a Griggs press, as well as a piston-cylinder apparatus. Regardless of the apparatus type, we observe the breakdown of plagioclase into an eclogite-facies paragenesis, which is associated with partial melting in the high temperature domain of the eclogite facies. Partial melting mostly takes place along the grain and interphase boundaries. However, several melt patches or plagioclase decomposition products coincide with the occurrence of deformation twins and grain-scale microcracking in plagioclase indicating intracrystalline melting and reaction in addition to melting and reaction along grain and interphase boundaries. In the present study, we demonstrate how the interplay between brittle microcracking and plastic deformation twinning can cause intracrystalline melting and/or reaction, which has the potential to lower the effective grain size of plagioclase-rich rocks and thus impacts their reactivity and deformation behaviour.

How Strong/Weak Is Epidote Relative to Plagioclase?

“…We therefore expect that the high normal stress acting on these structures is likely to influence the amount of fracturing as it has been previously shown by for example, Mair and Abe (2011). In addition, the presence of two perfect cleavage planes in plagioclase—or feldspar in general—leads to easy fracturing along these crystallographic planes (Incel et al., 2020; Incel, Baïsset, et al., 2023; Marshall & McLaren, 1977; J. Tullis & Yund, 1992), which could further enhance grain comminution. In fact, amorphization in feldspar due to mechanical grinding is quite common and pointed out in several studies in the past (e.g., Pec & Al Nasser, 2021; Marti et al, 2020; Yund et al., 1990).…”