Achondrites and Irons: Products of Magmatism on Strongly Heated Asteroids - 35 Seasons of U.S. Antarctic Meteorites - page 202

reflecting mode of formation from orthopyroxene + chro-
mite ± olivine cumulates, through pigeonite + plagioclase
cumulates to ferroan quenched melts. In the same
sequence, Na, Al, Ca, and Ti generally increase
(Figure 5.8), although some cumulate eucrites have higher
Al than basaltic eucrites, a consequence of plagioclase
accumulation. Incompatible lithophile trace elements
follow the trend of Ti. The polymict breccias form linear
trends on element-element diagrams of nonvolatile litho-
phile elements between diogenite and basaltic eucrite
end-members (Figure 5.8). Moderately volatile elements,
typified by the alkali elements, and volatile elements, such
as Bi, Te, Se, Cd, In, Tl, and halogen elements, are at low
abundances in HEDs [ Laul et al. , 1972; Paul and Lipschutz ,
1990; Wolf et al. , 1983]. Highly siderophile elements are at
extremely low abundances in HEDs. In basaltic eucrites
the Ir contents are at 10 −5 to 10 −6 × CI abundances [ Warren
et al. , 1996]. Diogenites and cumulate eucrites have Ir
contents between 10 −4 to 10 −6 × CI. The contents of Re and
Os are similarly in the range of 10 −4 to 10 −6 × CI in igneous
lithologies [ Warren et al. , 2009]. Cobalt, a moderately
siderophile element, is at higher abundances of ~10 −1 to
10 −2 × CI for diogenites and 10 −2 × CI for basaltic and
cumulate eucrites. Howardites contain generally higher
siderophile element contents because of the admixture of
chondritic debris.
The HED meteorites have been intensively studied since
the 1960s and models for HED meteorite genesis were
already fairly mature by the time systematic meteorite
searches in Antarctica began. There are two competing
models for eucrite genesis: (a) they represent primary
partial melts of chondritic source regions [ Stolper , 1977],
and (b) they represent residual melts left after
crystallization of the global magma ocean [ Ikeda and
Takeda , 1985; Righter and Drake , 1997; Ruzicka et al. ,
1997]. Currently, the consensus favors that latter scenario.
In this model, the HED parent asteroid would have a
metallic core, a dunitic and harzburgitic mantle which
transitions into a harzburgitic/orthopyroxenitic lower
crust, topped by a cumulate and basaltic eucrite upper
crust. The impact-produced debris layer is represented by
howardites, polymict eucrites, and polymict diogenites.
Antarctic HEDs (Plates 55 to 62) have influenced our
understanding of HEDs largely through the wider range
in lithologic types first recognized among the Antarctic
suite, which then influenced interpretations of magmatic
and impact processes on their parent asteroid. One
example is the identification of some eucrites and diogen-
ites as being polymict breccias. There had been a few
HEDs that kept bouncing back and forth between being
classified as howardites or eucrites. With the renewed
interest in HED petrology prompted by the large number
of new Antarctic HEDs, it was recognized that the line bet-
ween eucrite and howardite (and diogenite and howardite)

was indistinct; polymict breccias form a continuum from
eucrites to diogenites [ Delaney et al. , 1983; Delaney et al. ,
1984; Miyamoto et al. , 1978]. Prior to 1976, howardites
were considered a distinct rock type; compositional gaps
separated them from eucrites and diogenites. They were
thought to be regolith breccias by analogy with lunar
regolith breccias. With the recognition of the polymict
breccia continuum, thinking on the nature of the debris
layer on the HED parent asteroid has evolved. It became
clear that some polymict breccias are simply the products
of individual large impacts mixing different lithologies
excavated from the crust, and that regolith gardening was
not required [e.g., Nyquist et al. , 1986]. Currently, how-
ardites are recognized as of two fundamentally different
types; fragmental howardites formed from debris from a
limited number of impacts, and regolithic howardites
formed from well-mixed regolith [ Warren et al. , 2009].
Recent work on a suite of howardites and polymict dio-
genites from the Pecora Escarpment icefield has shown
that these breccias came from a single, petrologically het-
erogeneous meteoroid roughly a meter in size [ Beck et al. ,
2012], giving us a clearer glimpse of the decimeter scale
variation of HED polymict breccias. The new view of
howardites, in large part due to studies of ANSMET
HEDs, is changing our understanding of impact
gardening and regolith formation on asteroids, which is
different from that on the Moon.
In the mid-1970s, basaltic eucrites were recognized as
forming two distinct compositional trends, the Stannern
trend with varying incompatible lithophile element con-
tents with nearly uniform mg#, and the Nuevo laredo
trend with concomitantly decreasing mg# and increasing
incompatible lithophile element contents [ Stolper , 1977].
These trends were identified as arising from partial
melting to form a suite of primitive melts and fractional
crystallization to form a suite of residual melts, respec-
tively. The Stannern trend was sparsely populated;
Stannern, Ibitira, Cachari, Haraiya, Juvinas, and Sioux
County were its members. The latter four also occupied
the primitive end of the Nuevo laredo trend. With the
advent of abundant Antarctic basaltic eucrites, and redef-
inition of Ibitira as an ungrouped basaltic achondrite not
part of the HED suite [ Mittlefehldt , 2005], the Stannern
trend now appears to be subparallel to the Nuevo
laredo-trend eucrites [ McSween et al., 2011]. Thus, the
partial melting trend as originally defined was a result of
too few meteorites and misclassification of one mete-
orite. The new Stannern trend may be a fractional
crystallization trend of parent magmas distinct in com-
position from those of the Nuevo laredo trend [ Hewins
and Newsom , 1988; McSween et al. , 2011], but that
remains to be investigated. An intriguing model is that
Stannern-trend eucrites represent basalts that interacted
with the earliest basaltic crust to form hybrid magmas