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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