[meteorite-list] Shock metamorphism of LA Mars meteorite

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Mon Aug 16 22:18:06 2004
Message-ID: <20040817021805.7278.qmail_at_web51705.mail.yahoo.com>

The latest issue of Geology (Geological Society of
America) has a paper about the age and temperature of
shock metamorphism of the Los Angeles Mars meteorite.
The authors suggest that impact-induced shock
metamorphism was coeval with ejection of the LA
precursor from Mars.

The article is:

Age and temperature of shock metamorphism of Martian
meteorite Los Angeles from (U-Th)/He thermochronometry

Kyoungwon Min, Peter W. Reiners, Stefan Nicolescu and
James P. Greenwood, 2004, Geology. vol. 32, no. 8,
pages 677–680.

http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=32&issue=8

http://www.gsajournals.org/pdfserv/10.1130%2FG20510.1

In part, the abstract stated:

ABSTRACT
Mineralogic features attributed to impact-induced
shock metamorphism are commonly observed in meteorites
and terrestrial impact craters. Partly because the
duration of shock metamorphism is very short,
constraining the timing and temperature of shock
events has been problematic. We measured (U-Th)/He
ages of single grains of merrillite and chlorapatite
from the Martian meteorite Los Angeles (LA).
Merrillite and chlorapatite ages cluster at 3.28 6
0.15 Ma (2s) and 2.18 6 0.19 (2s) Ma, respectively.
The mean age of the merrillites, which are larger than
chlorapatites, is indistinguishable from cosmic-ray
exposure ages (3.1 6 0.2 Ma), suggesting that
impact-induced shock metamorphism was
coeval with ejection of the LA precursor from Mars. To
constrain the initial temperature of shock
metamorphism in the LA precursor body, we modeled
diffusive loss of He from merrillite as a function of
diffusion domain size, LA precursor body size, and
ablation depth. From these calculations, we suggest
that the metamorphic temperature of the shock event
was higher than 450 8C. These results support the idea
that shock pressures of the Martian meteorite
Shergotty were higher than 45 GPa, as inferred from
the presence of post-stishovite SiO2 polymorphs.
Single-grain (U-Th)/He dating of phosphates may
provide unique constraints on the timing and
pressure-temperature dynamics of shock metamorphism in
a wide variety of extraterrestrial materials.

Bob
 V
Received on Mon 16 Aug 2004 10:18:05 PM PDT