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Our research includes the in-situ study of trace element
and isotopic compositions of meteorites and lunar samples to
better understand their formation and to gain insights into
the chemical nature of their parent materials. Trace
elements, and particularly the rare-earth elements (REE),
are sensitive indicators of igneous differentiation
processes, as they typically partition strongly into either
the liquid or crystal phase of a magma system. We have used
this approach to investigate a wide variety of
extraterrestrial samples from large planetary bodies (lunar
samples, SNC meteorites from Mars) as well as smaller
asteroidal bodies (eucrites, ureilites, aubrites and
angrites). Current projects are described in more detail
under the links listed below.
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Primitive Achondrites
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Primitive achondrites are an important source of
information about early differentiation processes in
asteroidal bodies. These meteorites typically exhibit
non-chondritic textures, but have retained at least some
primitive compositional characteristics.
The acapulcoites and lodranites come from a common parent
body that experienced variable degrees of heating, resulting
in complex partial melting and melt migration processes.
These include simple melt formation and removal (or
crystallization in situ), interaction of melts with
residual minerals, formation of overgrowth rims, and
intrusion of partial melts into distant regions of the
parent body. In order to better understand the partial
melting processes that took place on the
acapulcoite-lodranite parent body, we are measuring the
trace element distributions of individual minerals in both
acapulcoites and lodranites. Our results show that there are
systematic trace element differences between the
acapulcoites and lodranites, which are consistent with the
removal of silicate partial melts in the latter. These
results are complicated by the fact that some lodranites and
acapulcoites have highly elevated trace element abundances
in their silicate phases, which may be the result of complex
interactions of incipient silicate partial melts with
residual minerals in the rocks. In addition, some samples
exhibit characteristics intermediate between acapulcoites
and lodranites and at least some meteorites currently
classified as lodranites do not appear to belong to this
group at all. Our current work focuses on identifying the
most primitive members of this group in order to constrain
the precursor material from which these meteorites
originated. These meteorites are also good candidates to
search for 26Mg excesses resulting from the decay
of 26Al.
The winonaites are another group of primitive
achondrites, which may be related to the silicate inclusions
in IAB iron meteorites. Most winonaites are fine- to
medium-grained with equigranular textures, but several
contain mm-sized areas that differ substantially in grain
size and/or mineralogy from the surrounding matrix. These
include fine-grained plagioclase and pyroxene rich areas in
Pontlyfni that may represent partial melts, coarse-grained
olivine clumps in Winona and Mt. Morris interpreted as
partial melt residues, and large poikilitic calcic pyroxenes
enclosing smaller olivine, orthopyroxene and plagioclase
grains in Tierra Blanca. Some winonaites, most notably NWA
1463, contain relict chondrules, attesting to the chondritic
origin of this meteorite group. Unique among the winonaites
is HaH 193. It is texturally unusual, with large poikiliitc
orthopyroxenes and, moreover, contains large grains of the
amphibole fluoro-edenite (see Figure) that appear to be of
pre-terrestrial origin. Despite the mineralogical and
textural indications, our initial trace element data are
more consistent with metamorphic equilibration of initially
heterogeneous minerals in compositionally distinct regions
of the winonaite parent body than with partial melting
processes. One possibility is that metamorphism following
impact mixing may be obscuring or modifying trends due to
partial melting.
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Basaltic Achondrites
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Melting of asteroids was a common process in the early
solar system, and has resulted in differentiated planetary
bodies containing a core, mantle and silicate crust. There
are a large number of magmatic iron meteorite groups, which
are thought to represent the cores of differentiated
asteroids. However, there are relatively few groups of
meteorites that correspond to the silicate differentiates
that made up the mantles and crusts of these asteroids. The
largest group is the HED meteorites or the howardites,
eucrites and diogenites. One thing that makes this group of
meteorites interesting is that they are the only group of
meteorites that have been fairly convincingly associated
with a particular asteroid, 4 Vesta. The eucrites are
plagioclase-pyroxene rocks that probably formed through the
eruption of basaltic lavas on the surface of their parent
body.
NWA 011 was originally thought to be a eucrite, but it
was soon recognized that this meteorite represents a unique
parent body and, therefore, was of particular importance as
an example of a new type of basaltic achondrite. We carried
out a detailed petrographic, mineralogical and trace element
study of NWA 011. Our work showed that this meteorite bears
many similarities to the eucrites that it was initially
identified with. Like many eucrites, NWA 011 crystallized
from a source with chondritic proportions of the REE,
although a slightly LREE-enriched bulk composition with a
small positive Eu anomaly, as well as highly fractionated
Fe/Mg ratios suggest that the source region from which NWA
011 originated experienced some pyroxene and/or olivine
fractionation. However, oxygen isotopes rule out a genetic
relationship with the eucrites (see Figure) and suggest
instead, that the material from which NWA 011 originated may
have been like some members of the CR chondrite clan, which
have similar oxygen isotopic compositions. The NWA 011
parent body is probably of asteroidal origin, based on the
age of this meteorite and its cosmic ray exposure age.
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Angrites
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The angrites are a small group of achondrites that are
characterized by unusual enrichments of refractory elements
such as Ca, Al and Ti. Chronology studies show that the
angrites are very old, indicating that asteroidal
differentiation began very early in solar system history.
The original angrite, Angra dos Reis, which fell in Brazil
in 1869, consists of more than 90 % fassaitic pyroxene and
has been interpreted as a pyroxene cumulate, although other
interpretations have also been suggested. The later
discovery of three angrites from Antarctica, however, has
shown that Angra dos Reis is in many ways an atypical
angrite. The Antarctic angrites (LEW861010, LEW87051 and
Asuka 881371) consist of variable amounts of anorthite,
fassaite and olivine (including the Ca-rich endmember,
kirschsteinite), and represent crystallized melts with only
limited amounts of crystal accumulation.
Two new angrites have recently been discoved, Sahara
99555 (see Figure) and D'Orbigny, bringing the total number
of meteorites in this rare group up to six. Preliminary
examination of their petrology and mineralogy suggest
similarities to the Antarctic angrites. We investigated the
trace element systematics of Sahara 99555 and D'Orbigny in
order to clarify their relationship to the other known
angrites. Our results show that both of these angrites are
very closely related to two of the Antarctic angrites,
LEW87051 and Asuka 881371. All four angrites crystallized
rapidly under near closed system conditions and appear to be
comagmatic. The trace element systematics of LEW 86010 are
different and, although this angrite also crystallized from
a melt, the source magma must be different from that of the
other angrites. Angra dos Reis remains anomalous and it has
been suggested that it may have formed on a separate parent
body.
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