Chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from Lappajärvi Crater, Finland

Abstract

Plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks (kärnäite) were sampled from the coherent melt sheet located in the central crater area of the Lappajärvi meteorite crater and analyzed by optical, electron optical, X ray, and microprobe methods. Both types of feldspars display strong chemical, textural, and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt. Increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt. In a lower degree of shock both primary plagioclase and microcline transform to ‘checkerboard’‐textured grains that consist of 5–10–μm‐sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz. The subgrains are strongly zoned, disordered labradorite to andesine and rimmed by sanidine. In a high degree of shock the subgrains are larger and randomly oriented. The subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place. On the basis of model calculations it is concluded that the temperature of the kärnäite melt at the time of the incorporation of the clastic debris was in the range of 1800° to 2100°C. The equilibration temperature of the clast‐melt mixture is estimated accordingly to be at least in the range of 1100° to 1140°C or higher, depending on the magnitude of the clast's postshock temperature. This temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars. The clast‐laden melt was at rest when the equilibrium temperature was reached, suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours.