IMCA Insights – September 2011
Brian Mason Award 2011,
by Florian J. Zurfluh
The author in the field
(Photo courtesy of the author)
As recipient of the
Brian Mason travel award for the Annual Meeting of the Meteoritical
Society 2011, it is a great pleasure to write some lines about myself.
Currently I'm a PhD student at the University of Bern, Switzerland, and
involved in the Omani-Swiss meteorite search and research project. This
includes field work in the hot desert of the Sultanate of Oman followed
by investigation of the returned samples in the laboratory. The main
focus of my studies is the weathering and contamination of ordinary
chondrites.
I grew up in the heart of Switzerland surrounded by high mountains. With
my family, I often went hiking in the mountains and occasionally looked
for crystals. My favorite mineral was the black variety of
quartz-morion, or smoky quartz. After high school I decided to study
geology - I was interested to learn more about the rocks we live on and
was fascinated by the unique blend of classes, fieldtrips, and labwork
this study offers. As a consequence of my interest in dark minerals and
rocks, I studied volcanic rocks for my bachelor's thesis. This work was
done with a colleague and we performed fieldwork for five days in a pit
in the Eifel, Germany.
Until this stage of my studies, meteorites were only treated
peripherally in some geochemistry or mineralogy classes. For my master's
studies, I took a chance and chose a topic where I could study these
fascinating extraterrestrial rocks in more depth. In 2007 I could joined
the field team of the Omani-Swiss meteorite search project for the first
time. I was still looking for dark minerals and rocks, but now in a
completely flat environment: the hot desert of Oman. After the
completion of my master's thesis, I got the chance to do a PhD in the
same project. In this context I was back in Oman for several weeks
during the winters 2009 and 2010.
As mentioned above, the topic of our work is mainly the weathering and
contamination of ordinary chondrites. We are interested in the
interaction of the environment with the meteorite. The influences of the
local soil composition, climate, and biology on the weathering are
studied.
Careful planning of the fieldwork is fundamental. Suitable surfaces for
meteorite recovery are selected using satellite images available from
Google Earth. Based on our experience from former campaigns (the first
expedition was performed in 2001), we are able to interpret the
satellite images accurately and to plan our routes along ideal surfaces.
During the campaigns in which I was involved, we mainly followed routes
from the coast towards the interior of the country. The idea behind this
was to find meteorites at various distances from the sea in a transect
to study the influence of the distance to the sea on the weathering. The
search for meteorites is performed visually by car or on foot. To get an
idea of the find
density, we systematically searched several quarters of a square
kilometer on foot.
When a meteorite is found, we record its coordinates by GPS, take a
photograph with label, estimate the degree of burial, collect all
fragments, and weigh the five largest fragments and note the total mass.
Samples are wrapped without touching in polypropylene bags to avoid
contamination. The recording of the coordinates is essential for further
studies to find density and to answer the question, which stones are
paired, i.e., belonging to the same fall event. In hot deserts,
meteorites usually are found in the place where they have fallen. This
allows us to reconstruct meteorite strewn fields.
When the meteorites reach the Natural History Museum of Bern, we unpack
them, again without touching, clean them with pressurized air, count the
number of fragments, and once more take the weights. I have to note here
that we have been loaned the samples just for study: they remain
property of the Sultanate of Oman. After a macroscopic description of
weathering features such as wind ablation, the meteorites are cut and
thin sections for classification are produced. The degree of shock and
weathering and the petrologic type of the chondrites are determined
individually through the use of optical microscopy in reflected and
transmitted light by at least two persons for verification. Afterwards I
measure the composition of the minerals with the electron microprobe to
assign the group. Most of the meteorites are ordinary chondrites of the
groups H or L. After classification, the pairing of meteorites is
determined by comparing meteorites with similar classifications and
close geographical provenance with each other. The whole procedure of
classification and checking for pairing is very time consuming and not a
trivial issue, but it is necessary. We have now collected and analyzed a
large number of meteorite samples from a significant area, which allow
us to do statistics on a unique and very significant meteorite
population.
The classification by microscopy is one of the most intensive steps, but
one is rewarded by the beautiful colors of olivine and pyroxene under
crossed nicols in transmitted light. But most of the meteorites from
Oman are moderately to heavily weathered, resulting in brownish staining
and onset of networks of iron hydroxide veins. One of our goals is to
detect which features overprint the pristine signatures of the
meteorites. We do mineral characterization in reflected light by using a
scanning electron microscope with an energy dispersive spectrometer and
X-ray diffraction. We determine the chemical composition with a
hand-held X-ray fluorescence device (HHXRF). You can see me using this
instrument in the picture above the article. It allows us to measure the
composition of rocks non-destructively, even in the field, which
supports a fast classification. While in the field, we were able to
identify the rock I analyzed in the picture as a diogenite, an
orthopyroxene-rich rock probably derived from asteroid 4 Vesta.
In the study I received the award for, we focused on the strontium
contamination of ordinary chondrites found in Oman. We measured a large
amount of our collected meteorites for their chemical composition and
observed strontium accumulations up to 200 times the initial value. The
Sr content of unaltered ordinary chondrite is between 9 and 11 ppm. We
measured up to 2200 ppm! Inside the meteorites we measured
concentrations up to 888 ppm. It is certain that the Sr is derived from
outside. But from where? To solve this question we performed 87Sr/86Sr
ratio analyses of three meteorites with various distances from the sea.
In addition, corresponding soil samples were analyzed. The results
showed the local soil to be the source of the strontium, since the
87Sr/86Sr ratio of the three soil samples is different for the three
geographical localities and is similar to the corresponding meteorite.
With this method we can exclude sea spray as an important source of Sr
for the contamination of meteorites in Oman.
Cut surface of an
ordinary chondrite from Oman
with efflorescence of hygroscopic salts (Photo by the author)
The
contamination of Sr is a continuous process and it shows a positive
correlation to the terrestrial residence time. My goal is to be able to
estimate the terrestrial age of a meteorite from Oman based on visual
weathering effects, the amount of accumulated Sr, and the degree of
weathering. For this purpose we have slightly refined the usually
applied weathering scale.
Our work shows that it is worthwhile and important to classify and study
every ordinary chondrite and that recording of the find location is
indispensable.
On weekends I'm still often in the mountains hiking and climbing with
eyes open for black rocks. So far I have found several pieces which had
impacted on Earth - but unfortunately they all also started here: they
were all of military origin. But nevertheless, I still walk with open
eyes on the glaciers and scree slopes and I hope to one day run across a
“Swiss” meteorite. When I have “failed” on the weekends, I can go during
work to the Natural History Museum where, beside some nice quartz
crystals from my place of origin, meteorites from all over the world
(including Oman and Switzerland) are on display.
Finally, I would like to thank some people who helped me during the
project: First of all my supervisors Beda Hofmann, Edwin Gnos, and Urs
Eggenberger. Then I acknowledge the effort of Igor Villa, Dea Vögelin,
and Nicolas Greber who made the strontium isotope analyses possible and
Tim Jull who determined the terrestrial ages of the meteorites. Roland
Bächli and Marc Dupayrat helped us with the handling of the Niton HHXRF.
I would also like to thank Ali Al-Rajhi from the Ministry of Commerce
and Industry, Sultanate of Oman, who enabled us to work in Oman and
loaned the samples for study. My studies are financed by the Swiss
National Science Foundation (SNF), grant 200020-119937. And ultimately,
I appreciate the IMCA for providing students the Brian Mason travel
award.
Florian J. Zurfluh
This
article has been edited by Anne Black and Norbert
Classen
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