ALKALI-FELDSPAR + CLINOPYROXENE CORONAE AROUND QUARTZ CLASTS IN THE MANSON, IA IMPACT STRUCTURE

Unit 1b of the M1 core in the Manson Impact Structure of north-central Iowa contains shocked quartz clasts with clinopyroxene + alkali-feldspar reaction coronae between the quartz clasts and the matrix surrounding them. The clinopyroxene typically rims the quartz clast and appears to replace quartz as small (~20 micron) euhedral crystals. The clinopyroxene rim is usually separated from the plagioclase-rich groundmass by a layer of potassic alkali-feldspar. Some alkali-feldspar crystals in this layer have plagioclase cores suggesting that the layer grew into a plagioclase bearing matrix. Several variations of the qz-cpx-af-plg sequence have have been observed. For example in some cases the cpx layer is missing in the corona while in others in others, the cpx is within the quartz grain. The matrix around the corona is quite variable with patches of Na-rich, Ca-rich, or K-rich fine-grained material dominating in different areas. Thermodynamic calculations indicate that the matrix is quartz undersaturated, a conclusion also reached by Reagan et al. (1996) based on the normative composition of the matrix. Two mechanisms have been previously proposed for the Manson coronas: Koeberl et al. (1996) suggested the cold quartz clasts interacted with hot melt, providing thermal gradients that drove diffusion; alternatively, Reagan et al. (1996) suggested a mechanism where quartz reacted with a quartz-undersaturated matrix via to material transport driven by compositional gradients between the quartz undersaturated matrix and the quartz-saturated region adjacent to the quartz clast. The aim of this study is to test these mechanisms using petrologic data collected through microprobe analysis, x-ray computed tomography, and thermodynamic modeling.