Geology Reference
In-Depth Information
the compositional differences between lEW 86010 and
lEW 87051. Note that as originally promulgated, the
olivine-control model considers large magnesian olivine
grains in A-881371 and lEW 87051 to be xenocrystic
[
Mikouchi et al.
, 2004]. Thus, a mixing process is implied.
Angra dos Reis remains an anomaly. Petrologic consid-
erations show that it was derived from a distinct source
[
Longhi
, 1999], and compositionally it is distinct from all
other angrites [
Mittlefehldt et al.
, 2002;
Riches et al.
,
2012]. Tungsten isotopic data show that it, along with
NWA 2999 and paired NWA 4931, were derived from a
source with a much higher Hf/W ratio than were other
angrites [
Kleine et al.
, 2012]. The W isotopic data suggest
that core formation was not a single, asteroid-wide event
but rather was localized. Thus, there is still much that is
unknown regarding the differentiation of the angrite
parent asteroid.
Perhaps the most significant contribution of the study of
angrites to our understanding of the solar system lies in the
ability to determine very precise absolute ages for their
formation and tie these to several short-lived chronometers.
This allows much greater confidence in placing asteroidal
magmatism into the context of early solar system processes,
first made possible by the recovery of lEW 86010.
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5.7. SUMMARY
The collection of meteorites from Antarctica has
greatly expanded the numbers and diversities of achon-
drite and iron meteorites available for study. This has
allowed for the development of more robust models for
magmatic processes on strongly heated asteroids. In
particular, the acapulcoite-lodranite clan could only be
defined once numerous examples were available, and
their study led directly to development of models for
silicate and metallic melt migration on asteroids that
were only partially melted. Recovery of angrites has
permitted the integration of the very early events in the
history of the solar system as defined by nebular mate-
rials with the differentiation processes that occurred
on some asteroids. The growth of the HED clan and
ureilite group through Antarctic meteorite recoveries
has helped us refine our knowledge of magmatic processes
on asteroids through providing previously unknown or
underrepresented lithologies. All of these are ongoing
endeavors that will continue to yield new insights into
solar system history as new meteorites are returned
from Antarctica.
Acknowledgements.
The senior author was supported by
funding from the NASA Cosmochemistry Program. We
thank J.-A. Barrat, whose review was particularly helpful
to us while formulating our revisions. An anonymous
reviewer also provided a few useful suggestions.
