¹M. D. Suttle,²T. Hasse,^2,3L. Hecht
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13712]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD UK
²Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstr. 43, Berlin, 10115 Germany
³Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, Berlin, 12249 Germany
Published by Arrangement with John Wiley & Sons

We report the recovery and characterization of a new urban micrometeorite collection derived from the rooftop of an industrial building in Germany. We identified 315 micrometeorites (diameter: 55–515 µm, size peak: ˜150 µm, size distribution slope exponent: −2.62). They are predominantly S-type cosmic spherules (97.2%) but also two G-type spherules (0.6%), an unmelted coarse-grained single-mineral micrometeorite, and eight scoriaceous particles (2.5%) or particles transitional between scoriaceous micrometeorites and porphyritic spherules. Their analysis details how the magnetite rim on partially melted micrometeorites is progressively diluted as the melt fraction increases during heating. At least 10 micrometeorites contain platinum group nuggets (PGNs). They have chondritic compositions but are depleted in volatile Pd. However, a single nugget preserves chondritic Pd concentrations. We suggest that an Fe-Ni-S bead originally containing the PGN escaped its host cavity and wet the particle exterior, creating an Fe-rich melt that protected the nugget from evaporation. This melt layer oxidized forming magnetite—indicating that wetting events can affect the texture and composition of micrometeorites. Utilizing the well-constrained surface area (8400 m²) and rooftop age (21 yr), we attempted the first global mass flux estimate based on urban micrometeorite data. This produced anomalously low values (13.4 t yr^–1), even when correcting for losses due to sample processing (<89.7 t yr^–1). Our value is approximately two orders of magnitude lower than previous estimates, indicating that >99% of particles are missing, having been lost via drainage and cleaning. Rooftop collection sites have limited potential for mass flux calculations unless problems of loss can be resolved. However, urban micrometeorite collections have other advantages, notably exceptionally well-preserved particles with extremely young terrestrial ages and the ability to extract many micrometeorites from accessible sites. Urban micrometeorites should be considered complementary to Antarctic and deep-sea collections with potential for citizen science and educational exploitation.

^1,2Takafumi Niihara,^3,4Tatsunori Yokoyama,⁵Tomoko Arai,^3,6Keiji Misawa
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13719]
¹Department of Systems Innovation, School of Engineering, University of Tokyo, Hongo 7-3-1, Tokyo, 113-8656 Japan
²University Museum, University of Tokyo, Hongo 7-3-1, Tokyo, 113-0033 Japan
³Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), 10-3 Midoricho, Tachikawa, Tokyo, 190-8518 Japan
⁴Tono Geoscience Center, Japan Atomic Energy Agency, 959-31 Izumicho Jorinji, Toki, Gifu, 509-5102 Japan
⁵Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba, 275-0016 Japan
⁶National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo, 190-8518 Japan
Published by arrangement with John Wiley & Sons

We have conducted petrological and mineralogical studies on an igneous clast in the Northwest Africa (NWA) 1685 (LL4) chondrite. This meteorite was described in the Meteoritical Bulletin as containing clasts similar to alkali-rich clasts in LL chondritic breccias Yamato (Y)-74442 (LL4), Bhola (LL3-6), and Krähenberg (LL5). We carefully compared the textures, as well as mineral and matrix compositions, of the NWA 1685 clast with those of the previously described alkali-rich clasts in the LL chondritic breccias. Olivine grains are embedded in glassy matrix and have no chemical zoning. Shock melt veins and fractures were observed only in olivine grains and did not continue into matrix. Potassium abundance of matrix glasses of the NWA 1685 clast is lower than those of alkali-rich igneous clasts in Y-74442, Bhola, and Krähenberg, indicating that the igneous clasts in NWA 1685 are different from the alkali-rich clasts previously reported in the LL chondritic breccias. The NWA 1685 clast might have formed during an impact melting and quenching event on the LL-chondrite parent body, and then been incorporated into a breccia.