Geology Reference
In-Depth Information
There has been considerable controversy about whether
shergottite magmas were dry or hydrous [e.g.,
McSween
et al.
, 2001, and references therein;
Usui et al.
, 2012].
Analyses of apatites in QUE 94201 clearly demonstrate
that the magma that formed this particular meteorite was
hydrous, containing 730 to 2130 ppm H
2
O [
McCubbin
et al.
, 2012].
The crystallization age of QUE 94201 is 327 Ma
[
Nyquist et al.
, 2001], older than most other basaltic sher-
gottites. However, its Mars ejection age groups it with
other basaltic shergottites (Figure 7.2), suggesting deriva-
tion from the same location.
increased equilibration [
Harvey and McSween
, 1992;
Mikouchi et al.
, 2003].
Minor secondary minerals, formed by hydrothermal
alteration, occur within MIL 03346 [
Hallis and Taylor
,
2011]. Like the other nakhlites, most of the alteration
assemblage consists of “iddingsite”: a mixture of Fe-oxides,
sulfate, carbonates, and clays found primarily as alteration
veins in olivine phenocrysts. In MIL 03346 the absence of
siderite and halite suggests a weakly acidic alteration brine
relatively less concentrated and depleted in HCO
3
-
as com-
pared to those projected for some other nakhlites [
Hallis
and Taylor
, 2011]. MIL 03346 and other nakhlites serve as
an intermediate data point within the range of martian
water-rock interactions recorded in martian meteorites,
between the ancient, relatively wet conditions recorded by
ALH 84001 and the young and nearly secondary-mineral-
free basaltic shergottite QUE 94201.
Because it is a cumulate rock, the bulk chemical analysis
of MIL 03346 [
Day et al.
, 2006] does not represent a liquid
composition. The meteorite, like other nakhlites, is highly
enriched in incompatible trace elements. The history of
this meteorite is summarized as follows [
Day et al.
, 2006]:
• Partial melting and ascent of magma
• Slow crystallization of augite and olivine in a shallow
magma chamber
• Eruption or emplacement in a sill of the crystal-laden
magma
• Crystal settling, rapid cooling, and formation of
overgrowths on pyroxenes and skeletal crystals in inter-
cumulus melt
• Secondary alteration by hydrothermal fluid.
7.7. NAKHLITE: MIL 03346
Like QUE 94201, the importance of the Miller Range
03346 nakhlite lies in its place within the range of known
lithologies. The nakhlites preserve a record of martian
magmatic processes that are akin to terrestrial komatiites:
among the earliest lavas formed when the Earth's interior
was hotter than it is today. Their best terrestrial analogs are
voluminous ultramafic flows, deep enough and fluid
enough to allow significant mineral accumulation. variation
among the nakhlites thus mirrors that seen within a vertical
sequence through such a flow, providing a look at the frac-
tionation, crystallization sequence, and equilibration that
would have occurred [
Lentz et al.
, 1999].
MIL 03346 is the largest sample among a set of four
nakhlites all found in close proximity (Plate 74). MIL
03346 was described by
Day et al.
[2006], and
Udry et al.
[2012] showed that MIL 090030, 090032, and 090136 are
petrologically identical to MIL 03346. These paired mete-
orites are the only nakhlites in the U.S. Antarctic collec-
tion. Within error, MIL 03346 shares the same ~1.3 Ga
crystallization and 11 Ma ejection age as the other
nakhlites and the chassignites (Figure 7.2), suggesting all
originate from the same differentiated flow on Mars
[
Murty et al.
, 2005;
Shih et al.
, 2006].
Like other nakhlites, MIL 03346 is composed mostly
of cumulus augite, with minor olivine and about 20% vit-
rophyric intercumulus material, or mesostasis. This meso-
stasis is both more abundant and less crystalline than in
other nakhlites. The pyroxenes have homogeneous cores
but strongly zoned rims. The vitrophyre is composed of
glass with fayalite laths and skeletal grains of iron-tita-
nium oxide and sulfide, and lacks the feathery feldspars
found in other nakhlites. This meteorite is among the
most highly oxidized nakhlites [
Dyar et al.
, 2005;
Righter
et al.
, 2008]. MIL 03346 is less equilibrated than most
other nakhlites, presumably because of faster cooling.
This, along with the glassy mesostasis, suggests fast
cooling near the top of the thick parent lava flow or near-
surface sill, with other nakhlites originating further down
in the sequence and experiencing slower cooling and
7.8. SUMMARY
The U.S. Antarctic meteorite collection has been
invaluable in providing new martian meteorites that have
had a profound effect on our understanding of the Red
Planet. Of the six types of martian meteorites that have
been recognized, three were first recovered by ANSMET.
We have described five of these meteorites, including
• Allan Hills A77005, the first known lherzolitic shergot-
tite, which expanded the varieties of martian cumulate rocks
• Elephant Moraine A79001, the first recognized olivine-
phyric shergottite, which provided the critical evidence that
these meteorites were from Mars
• Allan Hills 84001, the only known orthopyroxenite,
which generated a Mars exploration program focused on
the search for life
• Queen Alexandra Range 94201, a basaltic shergottite
that defines the range of magmatic fractionation in these
lavas
• Miller Range 03346, the only nakhlite (with four
paired meteorites) in the collection, which samples the
top of the nakhlite cumulus pile.
