The Origin of “toasted” Quartz in Impactites Revisited

Introduction: Quartz is one of the most abundant minerals in terrestrial crustal rocks. Viewed in planepolarized light with a petrographic microscope, quartz is clear (i.e., transparent). In cross-polarized light, quartz typically displays gray to white interference colors. Quartz (and other minerals) is deformed and transformed when subjected to strong shock waves, as a consequence of impact events. Shock-induced deformations and transformations in quartz include changes in refractive index, and formation of planar fractures (PFs), planar deformation features (PDFs), high-pressure polymorphs, diaplectic glass, and lechatelierite [e.g., 1]. Another feature, so-called “toasted” quartz [2] (quartz grains of orange-brown to grayish-reddish brown appearance, Fig. 1), which has been so far only reported in rocks affected by shock metamorphism [3, 4], is possibly related to post-shock temperatures, but the exact formation mechanism for “toasted” quartz is still unresolved. To assess this enigmatic phenomenon, hundreds of thin sections of samples from several impact structures were studied by optical microscopy. In addition, investigations of thin sections of Bosumtwi and Chesapeake Bay impactites by electron microscopy and electron microprobe (EMP) analysis were carried out. Previous work: Toasted quartz has been considered a post-shock feature, probably the result of “hydrothermal or other post-shock modification” [3], or resulting from “the exsolution of water from glass, primarily along PDFs, during heat-driven recrystallization” [4]. It was also noted by [4] that “no compositional origin for the browning is evident”, and they concluded that the brownish appearance of quartz was caused by a high proportion of very small fluid inclusions mainly located along decorated PDFs. Results and Discussion: In addition to the 15 impact structures from which toasted quartz was previously described by [3, 4], we can add 11 further impact structures (Aorounga, Boltysh, Bosumtwi, Chesapeake Bay, Ilynets, Paasselkä, Puchezk-Katunki, Santa Fe, Shoemaker, Ternovka, and Upheaval Dome) to this list. Toasted quartz grains were observed in various lithologies, mainly in clasts of impact melt rocks and suevite. At Bosumtwi, toasting occurs in up to 41 rel% of the quartz grains in impact breccia samples from the deep crater moat [5], and ~20 rel%, on average, of the shocked quartz grains from the basement metagraywacke in drill core LB-08A are toasted [6]. Quartz grains are partially (i.e., only locally) or totally toasted. Even though commonly, as also noted by [5], toasted appearance does indeed occur in close association with PDFs, frequently toasted quartz grains are barren of PDFs.