Geology Reference
In-Depth Information
Cosmochemists generally had a different interpretation
of ureilites and the C-rich materials. They considered
ureilites to be restites remaining after partial melting had
generated metallic melts that segregated into a core, and
silicate melts that erupted as basalts [
Boynton et al.
, 1976;
Wasson et al.
, 1976]. In this scenario, the C-rich material
was injected into the silicate host as “veins” by impact
processes. The noble gases were trapped in the diamonds
produced by the shock that mobilized the C-rich material.
In this way, cosmochemists neatly avoided the conun-
drum of having high noble gas contents in magmatic
rocks. Ureilites were found to contain two siderophile
element components. One was thought to be a compo-
nent remaining in the host ureilite after partial melting
allowed separation of a metallic core. The C-rich “veins”
were also rich in siderophile elements, and the second
component was thought to have been injected along with
the C-rich material.
Thus, there were two competing models for ureilite
genesis circa 1980: (a) fractional crystallization in magma
chambers with entrapment of interstitial graphite and
adcumulus grain growth to expel interstitial silicate
liquid, and (b) partial melting leaving a metal-sulfide
and basalt-depleted restite, followed by injection of
C-rich materials, including rare gases and siderophile
elements, by impact on the parent asteroid. Points of
agreement were that the high Ca contents of olivine indi-
cated high equilibration temperatures for the silicates,
and the reduced rims on olivine were engendered by an
impact that substantially decreased the lithostatic
pressure and allowed redox reactions between the C-rich
materials and the hot silicates to reduce FeO out of the
olivine; CO and/or CO
2
formed by this process escaped
the system.
Paired ureilites Allan Hills (AlH) A78019 and
AlH A78262, among the earliest Antarctic ureilite
recoveries (Plate 44), resolved the issue over internal
versus external origin for the C-rich materials
(Figure 5.4). These are very low-shock-stage ureilites
with well-preserved primary textures of the interstitial
graphite [
Berkley and Jones
, 1982]. graphite occurs as
nearly undeformed euhedral grains interstitial to the
silicate phases, but intergrown with Fe-Ni metal and
sulfide. This clearly established graphite as a primary
phase in ureilites, and not material injected by impacts.
Thus, the rare gases and siderophile element compo-
nents associated with the C-rich materials must be an
integral part of ureilites. In diamond-free AlH A78019,
the noble gases are contained in amorphous C, not the
graphite [
Wacker
, 1986].
Perhaps the most astonishing result to come out of
ureilite studies that was greatly facilitated by the
Antarctic meteorite collections was the recognition
that the ureilite group is very heterogeneous in its
oxygen isotopic composition, and that the differences
are dominated by non-mass-dependent variations. The
first three-isotope O isotopic analyses of ureilites
included only three non-Antarctic meteorites and
showed that they lay along the carbonaceous chondrite
anhydrous mineral (CCAM) line [
Clayton et al.
, 1976].
These three meteorites showed variations in Δ
17
O
(a parameter not yet defined) from −0.61‰ to −1.30‰,
but this was not remarked upon. (For comparison,
twelve samples of four HEDs in the same study showed
variations in Δ
17
O only from −0.18‰ to −0.46‰.) A
subsequent comprehensive study of the O isotopic
composition of ureilites demonstrated that the varia-
tion in Δ
17
O for the group is from −0.23‰ to −2.53‰
[
Clayton and Mayeda
, 1988; 1996]. The extremes of the
range are found in Antarctic ureilites (Figure 5.5). The
oxygen isotopic heterogeneity is a characteristic inher-
ited from the proto-ureilite material accreted to form
the parent asteroid, and the differentiation process did
not homogenize oxygen. The large, non-mass-depen-
dent isotopic variation revealed by Antarctic ureilites
effectively precludes cumulate models for the genesis
of ureilites, and a restite origin is now considered to be
the petrogenetic mechanism.
With the improved sampling of the ureilite parent
asteroid represented by the current collection, the varia-
tion in mg# of olivine cores has come into better focus.
Rather than discrete subgroups, there is a continuum of
compositions from mg# 75 to 94, with a peak in the dis-
tribution at ~78-79. A most significant finding is that
this mg# variation is correlated with the Δ
17
O of the
meteorites (Figure 5.5). The range in olivine core mg#
has been explained by two different mechanisms, and
thus the mg#-Δ
17
O correlation has two interpretations.
One camp considers the olivine core compositions to
reflect redox between silicates and graphite during
partial melting [
Goodrich et al.
, 2007;
Singletary and
Grove
, 2003]. In this scenario, the high mg# ureilites
were formed closer to the asteroidal surface such that
the lower lithostatic pressure permitted a greater degree
of reduction of FeO from the silicates. Material deeper
in the proto-ureilite asteroid had the highest Δ
17
O and
that nearer the surface had the lowest Δ
17
O. The mg#-
Δ
17
O correlation then is a juxtaposition of initial hetero-
geneity and pressure-controlled redox. The second
interpretation is that both the mg# and O isotopic het-
erogeneities reflect those of the proto-ureilite material,
with little modification having occurred during magma
genesis [
Warren and Huber
, 2006]. The mg#-Δ
17
O corre-
lation then is a memory of initial asteroid heterogeneity.
The origin of the mg# variations is an unsettled problem
in ureilite genesis.
The siderophile element contents of ureilites are still
not completely understood, but the improved sampling
