Comment on "Bedout: a possible end-Permian impact crater offshore of Northwestern Australia".

Nearly a decade ago, Gorter (1) suggested that the Bedout basement high, offshore Western Australia, might represent an impact structure, in view of the site_s well-defined circular Bouguer anomaly and seismic reflection data indicating a possible ring syncline. Recently, Becker et al. (2), as part of the search for the cause or causes of the mass extinction that marks the Permian-Triassic (P-T) boundary (3), examined the P-T boundary breccia cored at a depth of 3044 m in the Bedout-1 well and with an age (using plagioclase Ar/Ar dating) of 250.7 T 4.3 million years (My)—within experimental error from the age of the P-T extinction event (251.4 T 0.4 My) and Siberian Norlisk volcanism E251.7 T 0.4 to 251.1 T 0.3 Ma (4)^. Becker et al. (2) suggested the presence in the Bedout breccia of shocked mineral grains, diaplectic plagioclase glass (maskelynite), and impact melt glass. Drawing an analogy between the Bedout breccia and the suevite melt breccia at Chicxulub and Sudbury, they attributed the origin of the breccia to the melting of Mg-rich sedimentary materials, although they noted the presence of some basalt in the target material (2). The diagnostic hallmarks of extraterrestrial impacts (5, 6) include (i) shocked minerals displaying planar deformation features (PDFs)—for example, in quartz, feldspar, and zircon; (ii) the presence of high-pressure mineral polymorphs such as coesite, stishovite, and diamond; (iii) megascopic shock structures (for example, shatter cones and melt breccia); and (iv) chondritic chemical signatures—in particular, platinum group elements and other siderophile elements (Ni, Co) (7). A study by the author of this comment (8) suggested that the volcanic breccia samples in the interval from 3035.8 to 3044.95 m in the Bedout-1 core meet none of these criteria. The rocks consist of metaglass-bearing hydromagmatic mafic volcanic breccia dominated by fragments of basalt and dolerite set in mafic pyroclastic matrix (Fig. 1), including vesicular volcanic lapilli, closely akin to hydromagmatic spilites described in detail by Amstutz (9). The figures presented by Becker et al. (2) provided no suggestion of intracrystalline PDF elements, nor are measurements of crystal orientations reported. Instead, the principal argument presented by Becker et al. for impact effects hinges on the suggested existence of diaplectic feldspar glass (maskelynite) around and within plagioclase Efigures 6 and 8 in (2)^. No criteria allowing discrimination between maskelynite and volcanic glasses are indicated. Because the plagioclaseto-maskelynite transformation occurs at a pressure of 35 to 45 GPa (5, 6)—after the development of PDFs, which occurs at 10 to 35 GPa—identification of maskelynite is closely related to the presence of PDFs, which Becker et al. have not documented. Nor do these authors offer any criteria for discrimination between volcanic metaglass and maskelynite. Any presence of pristine unaltered glass, a highly metastable component under hydrous conditions, requires tests by transmission electron microscopy and infrared spectroscopy. The intercrystaline near-isotropic regions shown by Becker et al. Efigure 6 in (2)^ compare well with the cryptocrystalline chlorite/albite–dominated alteration zones (7) common in hydromagmatic altered spilites (9). The intracrystalline nearisotropic regions within calcic plagioclase Efigure 8 in (2)^ correspond to recrystallization and alteration of internal euhedral crystal sectors of oscillatory reverse-zoned magmatic plagioclase, under high water pressures associated with hydromagmatic processes (9). By contrast, in impact-related rocks, maskelynite irregularly overprints PDF-bearing crystalline relics Esee, for example, figure 4.31 in (5)^. The excellent preservation in the Bedout-1 rocks of igneous ophitic and microlitic textures in little-deformed dolerite and basalt fragments, and of calcic plagioclase that retains primary prismatic (euhedral) crystals and albite twinning (Fig. 1), is hardly consistent with the combination of deformation and the hydrothermal effects that commonly accompany impact (5). The presence of euhedral chlorite pseudomorphs after ophitically enclosed mafic phases and euheTECHNICAL COMMENT