[meteorite-list] August Geology Highlights

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 29 17:41:17 2004
Message-ID: <200407292141.OAA03640_at_zagami.jpl.nasa.gov>

http://www.eurekalert.org/pub_releases/2004-07/gsoa-ag072904.php

Public release date: 29-Jul-2004

Contact: Ann Cairns
acairns_at_geosociety.org
303-357-1056
Geological Society of America

August GEOLOGY & GSA TODAY media highlights

Boulder, Colo. - The August issue of GEOLOGY covers a wide variety of
potentially newsworthy subjects. Topics include: water chemistry as a
predictor of earthquakes; insights into Martian surface alteration;
whether many small ruptures on the San Andreas fault might reduce
occurrence of large earthquakes; impact of glacial-interglacial rhythms
on rainforests; evidence of the Chesapeake Bay impact event found in
Georgia; and how Richmond's Goochland terrane originated in Long Island,
went out to sea, and ended up in Virginia. GSA TODAY's science article
describes monitoring volcanic activity with satellite radar technology.

Highlights are provided below. Representatives of the media may obtain
complimentary copies of articles by contacting Ann Cairns at
acairns_at_geosociety.org <mailto:acairns@geosociety.org">. Please discuss
articles of interest with the authors before publishing stories on their
work, and please make reference to GEOLOGY in articles published.
Contact Ann Cairns for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and
Service, gsaservice_at_geosociety.org <mailto:gsaservice@geosociety.org>

GEOLOGY

[snip]

Global geologic context for rock types and surface alteration on Mars
Michael B. Wyatt, Arizona State University, Geological Sciences, Mars
Space Flight Facility, Tempe, AZ 85251, USA, et al. Pages 645-648.

Interpretations of rock compositions from thermal emission spectra from
Mars orbiting spacecraft indicate the widespread occurrence of basaltic
and either andesitic or partly altered basalt surface compositions. It
is important to distinguish andesitic and altered basalt compositions on
Mars because each rock type has implications for planetary physical and
chemical conditions and petrogenetic processes. Here we assess these
conflicting interpretations of martian surface lithologies in light of
new geologic context emerging from the Mars Global Surveyor and Mars
Odyssey missions. Based on global geologic context, we propose a model
to describe the distributions of martian surface compositions and argue
that portions of Mars experienced surface alteration over a cold and
episodically wet geologic history. We propose the formation of altered
units through limited chemical weathering from basalt interactions with
icy mantles deposited during periods of high obliquity. Alteration of
sediments in the northern lowlands depocenter may have been enhanced by
temporary standing bodies of water and ice.

[snip]

Age and temperature of shock metamorphism of Martian meteorite Los
Angeles from (U-Th)/He thermochronometry
Kyoungwon Min, Yale University, Geology & Geophysics, New Haven, CT
06511, USA, et al. Pages 677-680.

All of the martian meteorites are believed to have experienced impact
collision on Mars. Partly because the duration of impact-induced shock
metamorphism is very short, constraining the timing and temperature of
shock events has been problematic. We applied (U-Th)/He dating method to
single grains of phosphates from the Los Angeles Martian meteorite. The
most reliable (U-Th)/He age of 3.28 ? 0.15 Ma (2?) is suggested to
represent the timing of shock metamorphism. This age is identical with
the cosmic-ray exposure ages, suggesting that shock metamorphism was
coeval with ejection of the Los Angeles precursor from Mars. The initial
temperature of the shock metamorphism is deduced by modeling He
diffusion in the phosphates. From these calculations, we conclude that
the metamorphic temperature of the shock event was higher than 450 ?C.
These results support the idea that shock pressures of some Martian
meteorites were higher than 45 GPa, as inferred from the presence of a
high-pressure form of SiO2. 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.

[snip]

An upper Eocene impact horizon in east-central Georgia
R. Scott Harris, Brown University, Geological Sciences, Providence,
RI 02912, USA, et al. Pages 717-720.

The authors report the discovery of shocked quartz grains in an upper
Eocene sand layer exposed in the open-pit kaolin mines of east-central
Georgia. Shocked quartz is formed during cataclysmic events when large
asteroids or comets crash into the crust of the Earth. The shocked
quartz grains in this layer probably represent the fallout of debris
ejected from the impact that formed the Chesapeake Bay crater about
36 million years ago. The authors also suggest that the layer is the
most likely source of the tektites that are scattered across central
Georgia.
Received on Thu 29 Jul 2004 05:41:13 PM PDT