by Russell Kempton, New England
Meteoritical Services
Meteorite classification systems exist for one purpose -
to help establish a baseline of order from a collective of unknowns. Research
enables us to group different rocks and chunks of iron into neat little piles
of similar nature.
Deciphering the record preserved in each of these
groupings helps us to understand their origins, and perhaps, ours. But
sometimes they are not so neat. Sometimes we simply cannot understand the
conditions in which they formed. Thus we enter the weird realm of a rare group
of meteorites -- the Enstatite chondrites.
Alberta, Canada, 1952
On the evening of June 9th at 11:05 PM MST, people living
in Alberta, Canada witnessed a fireball moving swiftly across the sky from the
Northwest. Detonations and concussions were reported as the fireball proceeded
through the atmosphere to extinction. Moments later, the largest known
enstatite chondrite arrived on Earth.
Five days later in the town of Abee, 90 km north of
Edmonton, Harry Buryn, while tending his new wheat field, found a hole 2ft. to
3ft. in diameter, six feet deep, inclined at an angle of 25.5 degrees to the
vertical. At the bottom, sat a single 107 kg blackened, fusion-crusted stony
mass.
Eugene Poitevin, Chief of the Mineral Division and
Collection Curator of the Geological Survey of Canada (GSC), determined it to
be an enstatite chondrite -- a rare type of stone meteorite. Poitevin offered
$1,062 (Canadian) or $10 per kg to Buryn and Abee, the third largest meteorite
to be found in Canada, was acquired by the GSC on December 9, 1952.
EH and EL Chondrites
Based upon bulk compositional differences and what appear
to be separate metamorphic differences, enstatite chondrites are separated into
two classes: high-iron (EH) and low-iron (EL) (Keil, 1968) similar to the H and
L designations of ordinary chondrites.
But they are much different from H and L chondrites and
all other meteorites -- they lack an oxygen abundance. In fact, so little
oxygen is present that virtually all of the iron occurs as metal or as sulfide.
These highly reduced rocks, consisting of a mineral assemblage dominated by the
magnesium silicate mineral enstatite (MgSiO3), represent less than
2% of all known meteorites.
The lack of oxygen, during their formation has made them
the host for some weird, rare, oxygen depleted minerals, among them - sinoite
(Si2N2O), perryite (Ni,Fe)5(Si,P)2,
and djerfisherite (K3CuFel2SI4). When they
formed 4.49 billion years ago, it was within an unknown oxygen-poor area of the
solar nebula.
The Abee Enstatite Chondrite
Abee is an impact-melt breccia, EH chondrite. Its internal
structure is a myriad of granulated metal-rimmed, varying-sized clasts embedded
in a dark gray, fine-grained groundmass. "How?" and "what?" are the first two
words that come to mind when one sees this meteorite.
Since its initial description (Dawson, 1960) Abee has been
the subject of more than 31 studies focusing on brecciation, diamonds
reportedly of solar nebula origin (Russell et al., 1992), the oxygen depleted
environment where it formed (Clayton et al., 1984) and many parent body
studies. The latest (Rubin and Scott, 1996) is an exhaustive treatment of
Abee's mineralogy and its implications. Rubin and Scott argue effectively for
two distinct impact events with an intervening period of brecciation through
meteoroid bombardment to explain the diamonds and partially melted clasts
embedded within the matrix of fine-grained but similar material.
To date, all studies have been conducted on small pieces
originally sectioned from one end of the meteorite around 1960. Recently,
however, another section -- the oriented end piece -- has been made available
for study. The surface dimensions in both axes are 38 cm x 34 cm offering
approximately 1,020 cm2 for a rare "macro" view of the interior that
differs in some ways from other reported sections.
Textural Observations
The most obvious structure in this end piece is a huge
clast 22 cm x 14 cm covering approximately one-third of the surface. This clast
and three others in this specimen continues through the meteorite to the
exterior surface. The fusion crust is eroded around these clasts with the
result being a clastic outline on the crusted exterior. The breccia displays a
distribution of irregular shaped clasts in a 50/50% volume representation of l
mm to 4 cm clasts and eight larger 6 cm to 22 cm clasts. This is different from
the irregularly shaped 1 mm to 6 cm size clasts reported by Rubin and Scott
(1996) and Wacker (1982). These two clastic groups, found in this end piece 21
cm from sections previously studied, indicate that Abee may be a transitional
rock displaying the beginning of a bimodal distribution of fragments.
Chondrules are minimal within the clasts supporting the
observations of Rubin and Keil (1983). Two clasts, 5.5 cm x 4 cm and 5 cm x 3.5
cm, differ from previous reports. Both have a higher abundance of chondrules in
comparison to the other clasts and may be similar to those reported in Adhi-Kot
(Rubin, 1983). All clasts are outlined with coarse-grained, rind-like metal
rims of the nature reported by Dawson (1960).
Everything about this meteorite is interesting. As an
enstatite chondrite, it represents one of the rarest known groups of
meteorites. Abee's brecciated structure is a vivid representation of a violent
and complex sequence of impacts -- large angular clasts of partly melted
material with igneous oldhamite-rich dark inclusions, all embedded within a
previously melted, but similar, groundmass.
The mineralogy of enstatite meteorites is lacking the
oxygen abundance found in all other meteorites; how and where did they form? It
must have been in an oxygen-poor area of the nebula. Can a core-to-surface
model of the E parent body be constructed? What if it was ... Mercury?
An Outrageous Hypotheses
As the solar nebula cooled, specific minerals would have
formed relative to the pressure and temperature of the nebula. The cooling gas
would condense nickel and iron before any silicates. In a discoid-shaped nebula
the gas pressure would be highest at the center decreasing outward to the edge.
High pressure and temperature would have prevailed from Mercury's orbit inward
and metal may have condensed long before the silicates. Metal at 1000 C is
"sticky". The core of Mercury may have accreted from these metal grains well
before any condensation of stony silicates and perhaps formed core-first
through gravitational influence. As the nebula cooled, the core could have been
covered in layers with stony, chondritic material.
Currently, Mercury represents a special case in our solar
system -- its high density iron to silicate ratio (core/mantle) is about twice
that of the other terrestrial planets. A current hypotheses to explain this is
collisional removal of the mantle from impact during the closing stages of
planetary accretion.
In the early solar system the orbit of Mercury inward
would have been a high energy environment. Comets or other objects in eccentric
orbits around the Sun accelerate as they approach. Impacts on the mercurian
surface, then and now, occur with great energy. A cometary object impacting
Mercury, prior to or during its eventual complete differentiation from heat
might have blasted pieces of both the outer chondritic layer and core into
eventual parking or holding orbits within the asteroid belt that later crossed
Earth's path.
Improbable? Perhaps. Curiously, the study of light
reflected from Mercury's surface indicates that it is iron-rich and oxygen-poor
-- characteristics shared with E chondrites. But, if Abee represents a piece of
early Mercury where has it been for over four billion years? Computer models of
Earth-crossing asteroids predict an impact with a planet or another asteroid
within 100 My and even less for objects with Mercury-crossing orbits (Gladman
et al., 1996).
Breccias are from the surface of things. Is Abee a piece
of the primordial exterior surface of Mercury prior to differentiation? Or, is
its home an undifferentiated asteroid-sized body in Mercury's vicinity? Love
and Keil (1995) studied the impact launch and orbital dynamics of possible
mercurian meteorites. Interestingly, they indicated a 10% chance that a
mercurian rock could exist in current meteorite collections.
Current studies seem to present difficulties for other
than an interesting connection between enstatites, the inner solar system, and
Mercury. Is the connection provable or simply an anomalous curiosity? I don't
know. But absence of proof is not proof of absence. We learned that from the
Albion iron.
Acknowledgments
I am indebted to Carl Francis, Richard Herd, Timothy Grove
and Richard Binzel for their valuable discussions; to Alan Rubin for direction
and helpful criticism; and to Joel Schiff and Stan Love for their thoughtful
review.
The Author recognizes the controversial nature of
possible Mercurian meteorites and recognizes the arguments for and against a
connection between E chondrites, the inner solar system., and Mercury.
References
Clayton, R.N., Mayeda, T.K., and Rubin, A.E., Oxygen
isotopic compositions of enstatite chondrites and aubrites, Proc. Lunar
Planet. Sci. Conf. 15th, C245-C249. 1984.
Dawson, K.R., Maxwell, J.A., Parsons, D.E., A
description of the meteorite which fell near Abee, Alberta, Canada,
Geochim. Cosmochim. Acta 21., 127-144, 1960.
Gladman, B.J., Burns, J.A., Duncan, M., Lee, P., Levison,
H.F., The exchange of impact ejecta between terrestrial planets,
SCIENCE 271, 1387-1392, 1996.
Keil, K., A Mineralogical and chemical relationships
among enstatite chondrites, J. Geophys. Res. 73, 6945-6976, 1968.
Love, S. G., Keil, K., Recognizing Mercurcian
Meteorites, Meteoritics 30, 269-278, 1995.
Rubin, A.E., Keil, K., Mineralogy and Petrology of the
Abee enstatite chondrite breccia and its dark inclusions, Earth Planet.
Sci. Lett. 62, 118-131, 1983.
Rubin, A.E., Scott, E.R.D., Abee and related EH
chondrite impact-melt breccias, Geochim. Cosmochim. Acta., Preprint, to be
published
Russell, S.S., Pillinger, C.T., Arden, J.W., Lee, M.R.,
and Ott, U., A new type of meteoritic diamond in the enstatite chondrite
Abee, Science 256, 206-209, 1992.
Wacker, J.F., Composition of noble gases in the Abee
meteorite, and the origin of the enstatite chondrites. Ph.D. thesis, Univ.
Arizona. Tucson, Arizona 1982
Wasson, J.T., The building stones of the planets. In
Mercury, Vilas, Chapman and Matthews Eds., University of Arizona
Press, 1988.
Russell Kempton is the Director of New England
Meteoritical Services based in Mendon, Massachusetts, USA
Back to the Abee Photo
Page
Back to NEMS
Publications
New England Meteoritical
Services
