Diffusive loss of argon in response to melt vein formation in polygenetic impact melt breccias

Abstract

Many planetary surfaces in the solar system have experienced prolonged bombardment. With each impact, new rocks can be assembled that incorporate freshly generated impact melts with fragments of older rocks. Some breccias can become polygenetic, containing multiple generations of impact melt products, and can potentially provide important insights into the extensive bombardment history of a region. However, the amount of chronological information that can be extracted from such samples depends on how well the mineral isotopic systems of geochronometers can preserve the ages of individual melt generations without being disturbed by younger events. We model the thermal evolution of impact melt veins and the resulting loss of Ar from K‐bearing phases common in impact melt breccias to assess the potential for preserving the 40Ar/39Ar ages of individual melt generations. Our model results demonstrate that millimeter‐scale, clast‐free melt veins cause significant heating of adjacent host rock minerals and can cause detectable Ar loss in contact zones that are generally thinner than, and at most about the same thickness as, the vein width. The incorporation of cold clasts in melt veins reduces the magnitudes of heating and Ar loss in the host rocks, and Ar loss can be virtually undetectable for sufficiently clast‐rich veins. Quantitative evidence of the timing of impacts, as measured with the 40Ar/39Ar method, can be preserved in polygenetic impact melt breccias, particularly for those containing millimeter‐scale bodies of clast‐bearing melt products.

End of magmatism in the upper crust of asteroid 4 Vesta

“…The system will then take an additional 98 s to reach 0°C. Those models are simplistic of course, because the temperature is distributed across the entire outcrop systems intruded by a network of veins (hotter ambient temperature) but which will cool at a much accelerated rate due to the presence of many clasts (Mercer and Hodges 2017). Despite the simplicity of our model, our results are surprisingly similar to the somewhat more sophisticated thermal models developed by Shaw and Walton (2013) and Mercer and Hodges (2017), so while the real duration of the cooling period is hard to exactly pinpoint, our estimates should be accurate within an order of magnitude.…”