During the Concordia expedition to Antarctica in the year 2000, French scientist Dr. Michel Maurette and his two then students, now colleagues Drs. Cecile Engrand and Jean Duprat discovered a new type of micrometeorite. In their publication they describe a variety of low heated porphyritic olivine (PO-type) micrometeorites that are characterized by a low degree of elemental differentiation. It is without aerodynamic properties, but surface tension has pulled the semi-liquid rock into a sub-spherical shape due to the flash heating that occurred during atmospheric entry. On the surface are chunks of nickel-iron and a surplus of iron sulfide. Holes from degassing surrounded that are also surrounded by iron sulfide are present, too. In Photo 1 is a black and white scanning electron microscope (SEM) image of this type of micrometeorite NMM 2015. The matrix is olivine (forsterite) crystals, which often are white or colorless, in contrast to the black color of ordinary PO-type micrometeorites. Whether or not this is a clue to their precursor in Space is an open question.
PHOTO 1, NMM 2015
The presence of iron sulfide is interesting because sulphur is typically an element that rapidly escapes with other volatile elements. The fact that it has not escaped indicates a specific peak temperature during formation. Furthermore, sulfide is rapidly weathered down on Earth, so its presence on a retrieved cosmic spherule indicates low terrestrial age. It is for this reason that PO-type micrometeorites of this variety were not been discovered earlier. The micrometeorites retrieved on Earth until then, twenty-two years ago, had been old; thousands to millions of years old, and all sulfide weathered away. Today, on the other hand, we find fresh micrometeorites of this type frequently in urban dust. In the Atlas of Micrometeorites you will find more beautiful hi-res color photos of these fascinating treasures from space by Jan Braly Kihle and me.
Last month, I found a new micrometeorite of this type, which is presented in Photo 2. Note the huge “sulfide crater” at the top. I assume that this is from degassing but am not sure. Note also how the highly viscous iron sulfide has floated over the brim.
Photo 2, NMM 3790
A couple of weeks ago, again I found an interesting variety of this type. A PO-type micrometeorite with medium sized golden brown olivine crystals almost entirely covered with a thin layer of iron sulfide (see Photo 3 and the detailed image in Photo 4). Even though “crater”-like formations are typical features of these micrometeorites, I do not yet understand the dynamics of how they are formed. This will, therefore, be a topic that will be investigated further. Stay tuned!
The featured image at the top of this blog post is a new collage with nine similar cases from the Project Stardust collection, for comparison. They do have a bizarre kind of beauty.
Photo 3, NMM 4035
Photo 4, NMM 4035 Detailed View
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