Shock metamorphism of potassic feldspars

Abstract

Hypervelocity meteorite impact produces transient pressures as high as several megabars and temperatures in excess of 1500 ºC. Shock metamorphism describes the effects upon the target rocks, effects most distinctive in the range approximately 100-600kb. Shock deformation produced in potassic feldspars at three terrestrial craters and in experimentally shocked K-spar have been examined. Pressures in natural material were estimated from deformation of coexisting quartz and plagioclase, and in experiments pressures were calculated using impedance matching. At Charlevoix crater, Quebec, pressures above 170kb have induced chemical remobilization partially converting film perthite to spindle microperthite in orthoclase and microcline. In ortho-clase, shock produced cleavages form parallel to (210) and (120) at 170kb, and above 300kb parallel to (ll2), (031), (Oil) and (101). Planar deformation features resulting from lattice gliding, form initially at 200kb, and are well developed at 230-270kb. In orthoclase they are generally parallel to (241) and (241), and (131), (110) and (120) in microcline. Above 300kb some sets become glide twins. Structural state of the feldspars is apparently not affected in the pressure range examined (≤ 360kb).In the breccias of Lac Couture crater, Quebec, strong deformation twins and planar features develop in microcline above ISOkb, and some of the twins may obey a 'diagonal' twin law. Single crystal microcline was experimentally shocked at pressures from 37 to 417kb. Three shock regions were recognized: Regime I (<100kb) normal elastic compression, Regime II (100-300kb) mixed phase region. Regime III (>300kb) high pressure phase region. Shock cleavages, (111), (111) and (hkO), developed initially below 60kb, contain glass at highest pressures. Planar features, generally parallel to (111) and (111) also, appear at 154kb becoming more pronounced with increasing pressure, with poorly defined deformation twins occurring above 175kb. X-ray patterns show that long range structural order diminishes above ISOkb accompanied by formation of a low-birefringent, unidentified phase, possibly a high-pressure, disordered modification of K-feldspar. Differences in the pressure required for formation of particular shock effects between craters, and between natural and experimental systems are due to differences in structural state of the feldspars. At Brent crater, Ontario, partially recrystallized alkali feldspars in shocked basement gneisses show compositional variations due to incomplete homogenization. This is not a shock effect but results from thermal metamorphism by the overlying melt layer. A similar thermal effect has produced chemical variations in some highly shocked orthoclases at Charlevoix.