05 Classification of.. - Department of Earth and Planetary Sciences

110
Classification of Meteorites
Figure 21 Photomicrographs in transmitted light with crossed polars (a, b, d) and reflected light (c) of ureilites
Kenna (a), LEW88774 (b), ALH83014 (c), and Hughes 009 (d). (a) Kenna contains coarse-grained olivine and
pigeonite in equilibrated (note abundant triple junctions), foliated texture. Abundant black material along olivine
grain boundaries and in crosscutting veins consists of fine-grained graphite and reduction rims/veins characteristic
of ureilites.(b) LEW88774 has large oikocryst of low-Ca pyroxene (now orthopyroxene with augite exsolution
lamellae, oriented diagonally upper right to lower left) enclosing irregularly shaped, embayed and corroded
grains of augite (blue) which are in optical continuity with one another (as can be seen in the similar orientation
of shock-produced twins) and appear to be remnants of once single crystals of primary augite which reacted
with the liquid from which the oikocryst crystallized. (c) In ALH83014, ureilite with very low shock level,
graphite grains occur along grain boundaries and within olivine grains as large, euhedral crystals rather than
the fine-grained masses seen in most ureilites. (d) The olivine–augite–orthopyroxene ureilite Hughes 009
shows highly equilibrated texture typical of olivine–pigeonite ureilites. Olivine contains magmatic inclusions
(center of crystal, bottom).
(1979, 1997) in his layered crust model of the
HED parent body.
Howardites. These are lithified regolith from
the surface of the HED parent body, as indicated
by high contents of solar-wind-implanted noble
gases in their fine-grained, clastic, impactproduced
matrices (e.g., Suess et al., 1964)
(Figure 22(a)). They are polymict breccias
and consist mostly of eucritic and diogenitic
material with .10 vol.% of orthopyroxene as well
as some olivine. In addition, they contain
abundant impact-melt clasts and breccia
fragments, and occasionally carbonaceous
chondrite xenoliths (e.g., Metzler et al., 1995;
Pun et al., 1998).
Eucrites. These are pyroxene–plagioclase
basalts and are subdivided into three major
subclasses: the noncumulate eucrites (basaltic
eucrites), the cumulate eucrites, and the polymict
eucrites.
Polymict eucrites (Figure 22(b)) are polymict
breccias consisting mostly of eucritic material, but
they also contain ,10 vol.% of a diogenitic
component in the form of orthopyroxene.
Noncumulate eucrites originally formed
as quickly cooled surface lava flows (unequilibrated
noncumulate eucrites), but most were
subsequently metamorphosed (metamorphosed
noncumulate eucrites). Unequilibrated noncumulate
eucrites, also referred to as the unmetamorphosed
or least-metamorphosed noncumulate
eucrites or Pasamonte-type eucrites, are surface
lava flows (Figure 22(c)) that cooled quickly
(Walker et al., 1978). As a result of their fast
cooling, their pyroxenes (pigeonite of Mg#
,70–20) are zoned, and exsolution lamellae are
only visible by TEM. These rocks have experienced
only minor metamorphism (e.g., Takeda
and Graham, 1991).
Metamorphosed (equilibrated) noncumulate
eucrites are also collectively referred to as the
ordinary eucrites. They include the Juvinas type
(main group) and the Stannern and Nuevo Laredo
types. They are unbrecciated or monomictbrecciated,
metamorphosed basalts (Figure 22(d))
and contain homogeneous low-calcium pigeonite
(Mg# ,42–30) with fine exsolution lamellae
of high-calcium pyroxene. The high abundance