An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary M.J. Genge1,2, J. Larsen3, M. Van Ginneken4, and M.D. Suttle1,2 1Department of Earth Sciences and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK 2Department of Earth Science, Natural History Museum, Cromwell Road, London SW7 2BT, UK 3Project Stardust, Oslo, Norway 4Département des Géosciences, Université Libre de Bruxelles, Avenue FD. Roosevelt, 2 B-1050 Bruxelles, Belgium ABSTRACT SAMPLES AND METHODS We report the discovery of significant numbers (500) of large micrometeorites (>100m m) Particles were collected from accumu- from rooftops in urban areas. The identification of particles as micrometeorites is achieved on lated sediments in the gutters of roofs mostly the basis of their compositions, mineralogies, and textures. All particles are silicate-dominated in Oslo, Norway, although one is from Paris, (S type) cosmic spherules with subspherical shapes that form by melting during atmospheric France. The total mass of gutter sediment col- entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite, and lected was 300 kg from a total roof catchment iron-bearing olivine within glass. Four particles also contain Ni-rich metal-sulfide beads. area of ~30,000 m2; the sediment samples were Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile, processed by magnetic separation, washing with and siderophile elements, as observed in micrometeorites from other sources. The reported water, and size fraction separation. Particles were particles are likely to have fallen on Earth in the past 6 yr and thus represent the youngest selected under a binocular microscope on the large micrometeorites collected to date. The relative abundance ratio of barred olivine to basis of several criteria: (1) spherical or sub- cryptocrystalline spherule types in the urban particles of 1.45 is shown to be higher than a spherical shape; (2) color and luster (black vitre- Quaternary average of ~0.9, suggesting variations in the extraterrestrial dust flux over the ous, black to gray metallic, and translucent vitre- past 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational ous particles were selected); and (3) the presence perturbation during transport to Earth are suggested to be responsible. Variations in cosmic of surface dendrites or metallic surface protru- spherule abundance within the geologic column are thus unavoidable and can be a conse- sions. Among 500 particles, 48 were embedded quence of dust transport as well as major dust production events. in resin and polished for further mineralogical and compositional characterization by scanning INTRODUCTION their parent bodies. The preservation of such electron microscopy (Zeiss EVO 15LS) and elec- Micrometeorites (MMs) are extraterrestrial particles in sediments also provides a record of tron microprobe analysis (Cameca SX100) at dust particles that survive atmospheric entry events occurring beyond our planet over geo- the Imaging and Analysis Centre of the Natural and reach Earth’s surface (Genge et al., 2008). logical time (Dredge et al., 2010; Tomkins et History Museum (London, UK); standard matrix A popular belief among amateur astronomers al., 2016). corrections were applied. Bulk compositions is that modern-day extraterrestrial dust can be Micrometeorites are thought to include mate- were determined by averaging 5–20 wide beam collected on roofs in urban environments. Stud- rials derived from both asteroids and comets (10 µm) electron microprobe analyses. ies by Nininger (1941) reported large numbers (Genge et al., 1997; Genge, 2008; Noguchi et of magnetic spherules collected in urban areas; al., 2015). Although a proportion of smaller RESULTS however, later studies showed that the abun- dust particles survive atmospheric entry with- The studied particles are all spherules rang- dance of magnetic particles decreases away out significant heating, the majority of particles ing from 300 to 400 mm in diameter and are from urban areas, and that urban spherules undergo melting during their passage of the identified as MMs on the basis of mineralogy, are largely artificial in origin (Buddhue, 1950; atmosphere (Love and Brownlee, 1991). Most texture, and bulk composition. The identity and Handy and Zimmerman, 1953). Despite these abundant, particularly at large sizes, are cos- abundance of spherules are shown in Table 1. studies, amateur collection projects in built-up mic spherules, i.e., completely melted droplets areas have been common, even though most dominated by quench textures. These spherical Mineralogy and Texture of Silicate- researchers in MMs consider this occurrence particles provide a useful proxy for the total Dominated Particles an urban myth. Collection of MMs for research flux of dust because they are relatively easy to Three textural and mineralogical groups has focused on environments where terrestrial identify (Maurette et al., 1991). of silicate-dominated spherule are recognized: sedimentation rates and input of artificial par- We report the results of a study of 500 MMs (1) porphyritic olivine spherules dominated by ticles is minimal, including deep-sea sediments collected among particles recovered by Project phenocrysts of olivine within glassy mesosta- and Antarctic ice and snow (Brownlee and Bates, Stardust (Oslo, Norway; http://project-stardust. sis (Figs. 1A–1C), (2) barred olivine spherules 1983; Maurette et al., 1991; Taylor et al., 2000; com) in urban areas. A subset of 48 of these dominated by parallel growth dendrites of oliv- Duprat et al., 2007; Rochette et al., 2008). These particles are shown here to be cosmic spherules ine with interstitial glass (Figs. 1D–1F), and (3) samples of the extraterrestrial dust flux allow on the basis of their compositions, mineralogies, cryptocrystalline spherules dominated by radiat- study of the dust population within the early and textures, and represent the youngest large ing clusters of fine olivine dendrites within glass solar system and the nature and evolution of MMs yet recovered. (Figs. 1G and 1H). GEOLOGY, February 2016; v. 44; no. 2; p. 1–4 | doi:10.1130/G38352.1 | Published online XX Month 2016 GEOLOGY© 2016 The Authors.| Volume Gold 44 |Open Number Access: 2 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 1 TABLE 1. RELATIVE ABUNDANCE OF PARTICLE TYPES IN PERCENT FOR SEVERAL DIFFERENT COLLECTIONS Subtype Urban Cap South Larkman Prudhomme* Pole Nunatak§ water well† BO 48 23 54 27 C33271631 PO 19 50 30 42 BO/C 1.45 0.9 3.4 0.9 relicts 10 ––– metal 10 ––– vesicles15––– Note: Cap Prudhomme, South Pole Water Well, and Larkman Nunatak are all Antarctic collections. The abundance of particles with relicts, metal, and vesicles is also shown for the urban spherules. BO— barred olivine; C—cryptocrystalline; PO—porphyritic olivine. Dashes represent no data. *Genge et al. (1997). Figure 1. Backscattered electron images of urban cosmic spherules. REL—relict grain, SUL— †Taylor et al. (2000). §Suttle et al. (2015). sulfide, MET—metal, MGR—magnetite rim, OLR—olivine rim. A–C: Porphyritic spherules. D–F: Barred olivine spherules. G, H: Cryptocrystalline spherules. Nine spherules have porphyritic textures In ovoid spherules the width of olivine dendrites an approximate trend line between a forsterite and are dominated by phenocrysts of olivine is often smallest at the high curvature end of the composition and pure Fe. within an Na-poor aluminosilicate glassy meso- particle, with the largest either at the median of stasis that also often contains magnetite (Figs. the particle or at the low-curvature face (Fig. DISCUSSION 1A–1C). The abundance of olivine within these 1D). The widest olivine bars, in P502, give com- particles varies from ~45 to 70 vol%. Olivine positions of Fa22. Identification of Micrometeorites crystals vary from equant (Fig. 1A) to skeletal We identified 15 cryptocrystalline spherules; All 48 spherules have mineralogies, textures, (Fig. 1B), including those arranged in domains they are dominated by radiating clusters of oliv- and compositions consistent with those of S-type of parallel crystals. Olivine phenocrysts exhibit ine dendrites with interstitial glass (Figs. 1G and cosmic spherules recovered from Antarctica and normal zoning with magnesian-rich cores (fay- 1H). Most particles also have interstitial submi- deep-sea sediments. All spherules are dominated alite, Fa; >Fa16) and iron-rich rims (Fa47). Phe- cron equant magnetite crystals, although these by olivine phenocrysts or dendrites within a nocrysts can also contain relict magnesium-rich are absent in three particles. Olivine dendrites glassy mesostasis, usually with accompanying inclusions (Fa1.5–8) with discontinuous zoning mostly have widths <1 mm; however, several magnetite. Previous studies of cosmic spher- with the enveloping crystals (Figs. 1A and 1B). particles exhibit coarser crystals, often increas- ules noted the dominance of olivine as a quench Relict cores range in size from 24.0 to 1.5 mm ing to 5 µm across a particle and forming areas phase due to crystallization during rapid cooling and are present in apparent abundances of <4 with textures similar to barred olivine spherules. after entry heating in the atmosphere. Olivine vol%. Rounded iron-bearing olivines (Fa25) are Three cryptocrystalline spherules have numerous preferentially forms because pyroxene