Effects of shock‐induced incongruent melting within Earth’s crust: the case of biotite melting

Abstract

Terra Nova, 23, 225–231, 2011 Abstract Biotite is an abundant hydrous silicate in the Earth’s crust and plays a critical role in the formation of felsic magmas via incongruent melting. Although biotite incongruent melting reactions have been described for various contact and regional metamorphic environments, the effects of impact‐induced melting of biotite remain poorly understood. Our observations from the 40‐km‐wide Araguainha impact structure indicate that biotite can undergo incongruent melting during impact processes, producing aluminous melts that can accumulate as pockets or fracture networks in crystalline target rocks. We observed that as much as 9% of the crystalline target at Araguainha is a crystallization product after biotite breakdown. Given the relatively low shock pressures recorded in the Araguainha central uplift (20–25 GPa), our results imply that impacts in the size of Araguainha or larger are capable of generating substantial volumes of incongruent aluminous melts in crystalline crust underlying large (>40 km diameter) impact structures.

Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater, Brazil

“…Analyses carried out by electron microscopy (Silva et al. ) showed that these dark areas are regions of resorption at the grain boundaries and are characterized by the replacement and intergrowths of alkali‐feldspar with Fe‐Ti oxides and quartz (Schneider ).…”

“…According to Silva et al. () and Machado et al. () feldspar and biotite go through a selective melting process during the impact.…”

“…In other samples of the granite, biotite is marked by dark domains along the edges of the grains and cleavage planes. Analyses carried out by electron microscopy (Silva et al 2011) showed that these dark areas are regions of resorption at the grain boundaries and are characterized by the replacement and intergrowths of alkali-feldspar with Fe-Ti oxides and quartz (Schneider 1974). Accessory minerals such as zircon, monazite, and oxides of Fe and Ti were also affected by the shock wave.…”

“…For example, melting along pseudotachylitic breccias veins is rather selective, affecting predominantly minerals with low nominal melting points (e.g., Spray 1998;Machado et al 2009). According to Silva et al (2011) and Machado et al (2009) feldspar and biotite go through a selective melting process during the impact. For having lower melting points than quartz, the selective melting of these minerals leads to an increased mobility of major elements such as K, Na, and Ti and Fe relative to Si, thus generating differentiated products compared to the preserved granite.…”

“…; Silva et al. ) show that the uplifted granitic basement is well exposed and accessible at the center of the Aragauinha impact structure, and it may provide significant information about the processes of crustal melting during impact events. This granitic basement, known as the “Araguainha Granite,” records pre‐impact original textures, as well as features associated with shock metamorphism and impact melting.…”