Technical Comments

Comment on "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia"

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Science  22 Oct 2004:
Vol. 306, Issue 5696, pp. 613
DOI: 10.1126/science.1100404

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 40Ar/39Ar dating) of 250.7 ± 4.3 million years (My)—within experimental error from the age of the P-T extinction event (251.4 ± 0.4 My) and Siberian Norlisk volcanism [251.7 ± 0.4 to 251.1 ± 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).

Fig. 1.

(A) Ophitic-textured dolerite fragment in Bedout-1 breccia, showing lath-like calcic plagioclase and pseudomorphs of chlorite after pyroxene (pyx) protruding into feldspar in an ophitic texture (oph). The fragment is enveloped and injected by microbreccia (mb). Crossed nicols. (B) Microlite-rich basalt fragment (B-mic) and dolerite fragment (dol) separated by microbreccia (mb). Crossed nicols. (C) Plagioclase (An 52) microphenocryst in chlorite-dominated basalt fragment. Crossed nicols. (D) Backscattered scanning electron microscope image of ophitic dolerite fragment with plagioclase enveloping pseudomorphs of chlorite after pyroxene and accessory rutile. White arrows denote relic ophitic crystal structure (oph) of mafic pseudomorph within calcic plagioclase.

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 [figures 6 and 8 in (2)]. No criteria allowing discrimination between maskelynite and volcanic glasses are indicated. Because the plagioclase-to-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. [figure 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 [figure 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 [see, 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 euhedral Ti-magnetite and ilmenite typical of mafic volcanic rocks (10) are all consistent with burial metamorphism of mafic volcanic breccia, but distinct from features diagnostic of suevite shock-melt breccia (5). The Ries suevite includes shocked PDF-bearing granite clasts and carbonate sediment-derived clasts that display extensive heat-induced plastic deformation of fragments intermeshed with flow-banded melt components [figures 5.9 to 5.15 in (5)], clearly distinct from the angular to subrounded breccia at Bedout.

High degrees of impact melting may result in magmatic-type rocks, for example in the Mistasin and Dellen craters [figures 6.12 to 6.21] and in Vredefort granophyre veins (5). However, these processes commonly result in coarse-grained quench crystallization, not seen in the Bedout breccia.

In the absence of the unique criteria of shock metamorphism, Becker et al. (2) invoke novel petrologic and geochemical arguments for an impact connection for the Bedout breccia, including high-silica glass, spherulitic glass, feldspar compositions, presence of Mg-ilmenite, carbonate clasts with fragmented ooids, and coexistence of Ti-rich silica glass with Ti-poor aluminous silica glass. They describe the mineral parageneses of the breccia as “compositions... unknown and unlikely to exist in terrestrial volcanic agglomerates, lava flows, and intrusive pipes” (2). However, the spherulitic-textured particles [figure 5 and figure 7, A and B, in (2)] are identical to vesicular lapilli fragments in spilitic pyroclastics [pp. 24 and 26 in (9)]. High-silica cryptocrystalline veins form under hydromagmatic conditions, hydrothermal conditions, or both. Mg-ilmenite is known in ultrabasic and alkaline xenoliths (11) and is no criterion for impactites. Likewise, liquid immiscibilities, which are observed in heterogeneous volcanic magmas (12), offer no criterion for impact melting.

Becker et al. (2004) refer to the breccia in part as product of Mg-rich sediments (e.g., dolomites). However, apart from the pristine igneous textures of the breccia, the transition element levels (chlorite in dolerite fragment—Ni 97 to 160 ppm, Co 75 to 152 ppm, Cu 69 to 204 ppm; interfragmental mesostasis—Ni 29 to 45 ppm, Co 18 to 52 ppm, Cu 26 to 110 ppm) are consistent with Fe-rich basalts but exceed common abundances in carbonates and marls (13).

Becker et al. (2) thus have identified no impact effects at Bedout. The circular gravity structure and the presence of flanking rim synclines around the Bedout High (1) justify drilling into the basement that underlies the breccia at Bedout-1 and Legrange-1 to test the origin of this important structure.


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