TY - JOUR
T1 - Diffusive loss of argon in response to melt vein formation in polygenetic impact melt breccias
AU - Mercer, Cameron M.
AU - Hodges, Kip
N1 - Funding Information:
This work was funded by the NASA grants NNX11AB31G and NNX14AQ04G to K.V.H. and the NASA Earth and Space Science Fellowship to C.M.M. (NNX12AH82H). This work is theoretical, and there are no associated data available to download. The algorithms and model parameters that are needed to duplicate our work are described in detail in this manuscript and that by Watson and Cherniak []. The code used here is available by request from C.M.M.
Publisher Copyright:
©2017. American Geophysical Union. All Rights Reserved.
PY - 2017/8
Y1 - 2017/8
N2 - 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.
AB - 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.
KW - Ar/Ar geochronology
KW - argon
KW - diffusion
KW - impact cratering
KW - polygenetic impact melt breccia
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U2 - 10.1002/2017JE005312
DO - 10.1002/2017JE005312
M3 - Article
AN - SCOPUS:85026532555
SN - 2169-9097
VL - 122
SP - 1650
EP - 1671
JO - Journal of Geophysical Research: Planets
JF - Journal of Geophysical Research: Planets
IS - 8
ER -