PSJC #44 Sep 11 2009
Enstatite Chondrite Physical Properties
Enstatite chondrites (ECs) are highly reduced
meteorites marked by an abundance of enstatite and a depletion
or absence of olivine. Of the many attempts to sort these meteorites
on the basis of petrography or trace elements, the most
widely used system divides the ECs into two groups, EH and EL,
based on iron content, noting differences between the groups
in siderophile trace elements and silicon in the metal.
But do the two groups really differ in iron content? In many
cases, the bulk compositions were determined from samples significantly
smaller than 10g, while ECs are rich in large clasts and
can be heterogeneous at this scale.
Grain density and magnetic susceptibility measurements of
larger samples can resolve this issue, as they provide a quick and
non-destructive measure of average whole-rock iron content.
Using techniques reported previously we have
measured density, porosity and magnetic susceptibility of 26
stones from 16 different ECs: 7 EH (4 falls, 3 finds) and 9 EL (5
falls, 4 finds). Grain density grouped between 3.45 and 3.75
g/cm3, with a few outliers at slightly lower density and one
(Khairpur [EL]) with a density of 4.17 g/cm3. Average grain density
for EH is 3.61±0.14 g/cm3, and for EL is actually slightly
higher, at 3.65±0.24 g/cm3; statistically the
two groups are indistinguishable.
Considering only falls from the main group, both
EH and EL have the same average grain density of 3.64 g/cm3.
Porosities from all but one sample were between 0 and 6.4%.
Most samples had a magnetic susceptibility between a log χ
of 5.35 and 5.64, with three having much lower susceptibilities.
Average log χ for EH is 5.21±0.46 and for EL is 5.38±0.30.
Eliminating outliers, average log χ's for EH and EL agree at
The EH and EL chondrites are indistinguishable
in all of the physical properties tested: density, porosity, and
magnetic susceptibility. We conclude that there is no systematic
difference in iron content between EH and EL chondrites.
Still, it is apparent that individual ECs show clear mineralogical
differences that cannot be explained by metamorphism;
indeed there may be multiple EC parent bodies. A reexamination
of these important meteorites with modern analytical
tools, cognizant of their large heterogeneities, should allow a
better understanding of trace element trends and their origins.
[This is a recap of a presentation made at
Meteoritical Society Meeting.]