Geology Reference
In-Depth Information
7.2. BACKGROUND AND GEOLOGIC CONTEXT
EETA79001, 2
Mars has been the focus of NASA's planetary explora-
tion program for several decades, and staggering amounts
of information have been collected by orbiting spacecraft,
landers, and rovers (a comprehensive topic edited by
Bell
[2008] provides thorough reviews). The planet's surface
can be divided into ancient, heavily cratered highlands in
the southern hemisphere and younger northern lowlands
composed of layered sediments and volcanics. Four mag-
matic centers host enormous volcanoes with eruptions
apparently extending to recent times. Mars lacks any con-
clusive evidence of plate tectonics; rather the volcanoes
are products of plume activity. Martian stratigraphy is
defined within three time periods. The oldest (Noachian) is
characterized by possibly episodic warmer, wetter environ-
ments that produced sediments, as well as pervasive
impacts. This period was followed by the drier Hesperian
and subsequently by the youngest units (the Amazonian);
by Amazonian time, Mars had become an arctic desert.
All of the known martian meteorites except NWA 7034
are igneous rocks, and the consensus among the majority
of scientists is that, with two exceptions, they are relatively
young, with crystallization ages within the last third of
martian history. Radiometrically determined crystalliza-
tion ages range from 165 to 575 million years for the sher-
gottites, 1260 to 1380 million years for the nakhlites and
chassignites (ages and references are summarized by
McSween
[2008]), and 4.09 billion years for ALH 84001
[
Lapen et al.
, 2010]. The newly discovered NWA 7034 brec-
cia has an age of 4.4 billion years [
Agee et al.
, 2013], which
the authors interpret as a crystallization age. This bias
toward young and relatively strong lithologies is thought to
be a selection effect favoring those rocks that can remain
intact during ejection by impact and disallowing typical
sedimentary rocks or weak impact breccias [
Head et al.
,
2002]. ALH 84001 has a Noachian age, but it is an excep-
tionally strong breccia with clasts cemented by later min-
erals, and thus may be an exception to the trend.
By its nature, the impact mechanism that delivers
martian rocks to Earth removes their geographic context;
the locations on Mars from which the meteorites were
derived are therefore unknown. The most promising
launch sites are probably recent rayed craters identified in
thermal infrared images [
Tornabene et al.
, 2006]. These
impacts occurred on young volcanic surfaces and offer
direct evidence of large-scale transport of ejecta, but with
few constraints on lithology, individual craters cannot yet
be correlated with specific meteorites. The launch ages for
martian meteorites can be determined from measurement
of cosmogenic nuclides, produced through bombardment
by cosmic rays when the meteorites were unshielded in
space. These ages (ages and references are summarized
by
McSween
, [2008]) occur in clusters (Figure 7.2), with
1.1 cm
Figure 7.1a.
Dark gray pockets of impact melted glass in the
EETA 79001 shergottite.
18
CO
2
Trapped atmospheric
gases in EET A79001
16
40
Ar
N
2
14
20
Ne
84
Kr
12
36
Ar
10
Log particles/cm
3
129
Xe
132
Xe
10
12
14
16
18
EETA79001 shergottite glass
Figure 7.1b.
Trapped gas composition in impact glass, compared
to martian atmosphere analyzed by the
Viking
lander. This
correspondence provides the best evidence for martian origin
of SNC meteorites.
In this chapter, we will focus on a subset of martian
meteorites in the U.S. Antarctic collection chosen because
they have significantly influenced our understanding of
the formation, differentiation, and magmatic evolution
of Mars. The meteorites will be discussed in chronolog-
ical order of their recovery. Photographs of these meteor-
ites are presented in the topic plates, and additional
technical information such as subsampling strategies is
given in chapter 3.
