Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada

Abstract

Abstract-Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm À3 . The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.

Hyperspectral imaging of drill core from the Steen River impact structure, Canada: Implications for hydrothermal activity and formation of suevite‐like breccias

“…These assemblages constrain the shock pressures experienced by these rocks (~14–20 GPa); additional work on the response of biotite to shock supports these constraints (Walton et al. ). Walton and colleagues (Walton and Long 2017; Walton et al 2017) studied the lower half of core ST003 (>242 m depth).…”

“…Previous studies on the SRIS crater‐fill impact breccia focused on ST003, characterizing the lower portion (242–381 m; Walton et al. , , , ; MacLagan et al. ).…”

Investigation of impact melt in allochthonous crater‐fill deposits of the Steen River impact structure, Alberta, Canada

“…Shock veins in this granite are associated with the solid‐state transformation of amphibole and biotite to a high‐pressure majoritic garnet, and crystallization of majoritic garnet from shock melt (Walton et al. , ). Planar deformation features in quartz and feldspar, as described by Walton et al.…”

Colour Transformation and Textural Change in Biotite: Some Remarks for the Interpretation of Firing Technology in Greyware Pottery Thin-Sections

“…However, interlayer K depletion is a very common phenomenon among the transformations of biotite in weathering [27] (p. 217) or increasing-temperature contexts as well, together with hydrogen loss and concomitant oxidation of Fe 2+ to Fe 3+ [39,59]. These initial reactions begin at temperatures ≥400 • C [38] (p. 89), although the resulting transformations might not be visible yet. As shown in the results section above, interlayer K depletion in the origin of biotite laths separating along cleavages is observable in samples fired at c. 900 • C or above, and it is self-evident in most biotite inclusions in the samples above 950 • C. At a range of 800-850 • C, some samples might show this 'open-cleavage' effect depending on their size.…”

“…Despite this, the mechanisms of biotite breakdown under pressure and heat conditions-which are closer to those in pottery fabrication than mere weathering reactions-have been widely explored within the domain of metamorphic petrology [34][35][36][37]. Shock-induced pressure-dependent transformations concomitant to impact metamorphism can lead to the formation of new minerals such as garnet or vesiculated glass [38], but the most common affectations of this mineral are those related to thermal [39][40][41][42] or hydrothermal alteration [43,44].…”