[meteorite-list] Bedout Structure and Maskelynite

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Sat Apr 23 15:32:13 2005
Message-ID: <DIIE.0000003300003751_at_paulinet.de>

R.L. Fleischer (2004) Does maskelynite imply anything
about impact craters? (MAPS 39-12, 2004, p. 2055):

Becker et al. (2004a) interpreted the Bedout structure offshore near northwest
Australia to be an impact crater, partly on the basis that maskelynite (a glass
with the composition of the crystalline mineral plagioclase) was present. The
paper received multiple comments and questions in the October 22 issue of Science
(Wignal et al. 2004; Renne et al. 2004; Glickson 2004). The fact that, in defending
their hypothesis, Becker et al. (2004b) placed strong emphasis on the observed
presence of maskelynite implied that impact is the sole way of producing this
glass.

I take no position on whether Bedout is an impact crater, but I do have 40-year-old evi-
dence that shock is not required for production of maskelynite. It caught our attention
because it is notably present in the Shergotty meteorite. To investigate this glass, P.B.
Price, R.M. Walker, and I had a melt prepared of a glass of Shergotty composition for
studies of nuclear track registration at the General Electric Research Laboratory some
years ago.

The sample has a specific gravity of 2.74, a plagioclase composition, a mass of 950
grams, and is glassy throughout. Major cooling of so large a piece of glass would
require 15 to 60 minutes. The existence of this glass is thus clear evidence that
flash melting and quenching is not needed to produce maskelynite. So one piece of
Becker et al.'s supporting evidence is not decisive. Therefore, the answer to the
title question is "no."

As clarification, it may be that the presence of maskelynite with a particular combination
of other phases, with deformation indicators, in a particular morphology may result from
impact events; but the mere presence of maskelynite is not decisive.

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Best wishes,

Bernd
Received on Sat 23 Apr 2005 03:32:11 PM PDT