Geology Reference
In-Depth Information
5
Achondrites and Irons: Products of Magmatism
on Strongly Heated Asteroids
David W. Mittlefehldt
1
and Timothy J. McCoy
2
5.1. INTRODUCTION
suites of samples representing different portions of the
parent asteroid must be studied. Thus, one key to making
progress in understanding magmatic processing on aster-
oids is having a range of samples available that can be
used to make robust models for styles of melting, magma
migration, collection and eruption, and crystal fraction-
ation in metallic cores and silicate magma chambers.
Delivery of meteorites to Earth is a stochastic process,
and for this reason, the likelihood of having especially
diagnostic samples for study is directly related to the
number of meteorites available.
Antarctica provides an ideal location for collecting large
numbers of meteorites. The combination of ice flow, flow
barriers, and an ablation mechanism provide locations
where meteorites that have fallen over wide areas get
concentrated and stranded. The low temperatures and
entombment in ice retard terrestrial alteration mechanisms
and provide for a long meteorite accumulation period.
Together, these environmental factors allow for regions
where vast numbers of meteorites can be harvested
within relatively short time spans. A typical six-week U.S.
Antarctic Search for Meteorites (ANSMET) field season
can yield more than 1,000 meteorites from particularly
fruitful locations.
The two most well-sampled strongly heated asteroids
are the parents of the howardite-eucrite-diogenite (HED)
clan (1,135 named meteorites) and the ureilite group (315
named meteorites; census as of November 2012). Prior to
the start of systematic meteorite searches in Antarctica
(the 15th JARE in 1973-1975 and the first U.S. Antarctic
Search for Meteorites in 1976-1977), these two asteroids
were represented by 62 and 9 meteorites, respectively.
Over the following ~25 years, the numbers of meteorites
recovered from all locations in Antarctica from each
asteroid reached ~75%-80% of the totals, the propor-
tions only decreasing with the advent of “commercial
The systematic search and recovery of meteorites from
Antarctica, precipitated by the serendipitous discovery of
nine meteorites on glacial ice near the Yamato Mountains
by the 10th Japanese Antarctic Research Expedition
(JARE), has been a great boon for understanding the
origin of the solar system. This has been especially true for
an understanding of strongly heated asteroids. Meteorites
from such asteroids have experienced heating to the point
where melting has taken place. For some, only incipient
melting has occurred, while for others, total melting may
have occurred.
In the former case, nascent melts were able to collect
and migrate even in the low-gravity environments of
asteroids. The results of this process are primitive achon-
drites that show varying degrees of grain coarsening and
mineralogical and elemental fractionation from that of
their chondritic precursors. In the latter case, gross-scale
chemical segregation occurred due to density-driven sep-
aration of immiscible metallic and silicate melts. Some
of the melts cooled slowly enough that gravity-induced
crystal settling took place, resulting in additional miner-
alogical and chemical fractionation, in some cases on
much smaller scales. Impact-induced fragmentation and
debris gardening engendered a heterogeneous debris layer
on asteroidal surfaces composed of the disparate prod-
ucts of asteroidal magmatism.
Because of these processes, strongly heated asteroids
show extreme heterogeneities on scales of hundreds or
tens of kilometers down to meters or decimeters. In order
to understand the differentiation and regolith processes,
1
Astromaterials Research Office, NASA Johnson Space Center
2
Department of Mineral Sciences, National Museum of
Natural History, Smithsonian Institution
,
