CRITICAL COMPARISON OF TEXTURES OBSERVED IN IMPACT MELT AND VOLCANIC ROCKS

Impact glasses, melt rocks, and hot breccias are a special class of igneous rocks that are formed (and emplaced) as the result of the vaporization, fusion and ejection of crustal material in response to shock metamorphism by asteroid and comet collisions. They contain a unique range of macroscale and microscale textures indicative of the broad range of thermodynamic and rheological parameters available during their petrogenesis. Taken without context (e.g. an unambiguous impact crater or meteorites), however, individual textures are not unique to impact rocks; some are especially recognizable to igneous petrologists familiar with disequilibrium systems. Yet a number of these textures (e.g. schlieren, ballen quartz, and checkerboard feldspar) often are cited by impact petrologists as diagnostic of an origin by impact processes. Moreover, terrestrial petrologists might see the same textures and compare the latter two to fish-scaled cristobalite and sieve-textured feldspar observed in normal volcanic rocks.

Although there may be substantive differences among similar textures, they often are subtle, not easily quantifiable, and even unconvincing in the absence of context. Impact petrologists undoubtedly over interpret the importance of some textures in their petrogenetic models, whereas our endogenic colleagues equally overlook impact materials in volcanic terrains. There is considerable need for more communication between the two communities in order to better understand these rocks. This is especially as we look forward to sample-return missions from worlds on which impacts are common, often in volcanic crusts. Toward that end we present a compendium of similar textures that occur in endogenous and exogenous igneous rocks and discuss their interpretation.