Unmelted FeNi Metal Micrometeorites from the Novaya Zemlya Glacier

Introduction: The micrometeorite flux onto the Earth amounts to ~30,000 t/y [1] and overwhelms the meteorite flux that makes up ~53 t/y [2]. Ice of Antarctic and Arctic glaciers is a well-known collector of dust [3], part of which are micrometeorites (MMs). During the last 35 years, a huge number of MMs were collected from glaciers of Greenland and Antarctica [3]. The main fraction of MMs appears to be related to carbonaceous chondrites and ordinary chondrites seem to be rare [4,5] – as are particles of achondritic origin [6,7]. Here we report on the texture and mineralogy of two FeNi metal and one metal-chromite particles, which possibly are the first unmelted metal micrometeorites found so far. Results: Particles NZ6-4-4,59, NZ6-4-4,10, and NZ6-2-4,20 were extracted from a cryoconite sample collected at the northern passive margin of the Novaya Zemlya glacier [8] during the 2006 field season. The sample preparation included wet sieving and a separation of >50 μm particles under an optical microscope. The operations were performed in a room where no meteorites have been handled before. After extraction, particles were mounted in epoxy resin, sectioned, and studied using routine optical, ASEM, and electron microprobe techniques. Particle NZ6-4-4,59 consists of Ni,Co-bearing Fehydroxides with elongated and worm-like inclusions of kamacite and taenite (Fig.1). Taenite is more abundant than kamacite and has a low Co content compared to kamacite (Table 1). There are at least three types of iron hydroxides with low analyses totals, two of which contain NiO and CoO (Table 2). Ni-free Fe-hydroxide forms a crust at one side of the particle (Fig. 1). According to the analyses totals and the BSE image contrasts the Ni-containing oxides (1 and 2 in Table 2) have different water contents. The metal particle NZ6-2-4,20 (Fig.2) has a scalelike shape with a notched margin. Metal is chemically inhomogeneous with Ni contents varying from 13 to 17 wt% (Table 1). The surface of the particle is enriched in Ni and the core seems to be composed of a mixture of very fine-grained Ni-rich and Ni-poor phases (kamacite and taenite?). Particle NZ6-4-4,10 (Fig. 3) consists of chromite and Fe-Ni-Cr metal. Tiny subhedral chromite grains form a non-compact, porous aggregate. Chromite has the composition: 56.5 61.2 wt% Cr2O3, 23.9 28.0 FeO, 5.8 7.7 TiO2, 5.6 6.9 MnO, 1.1 1.7 Al2O3, and < 0.1 wt% MgO. Metal (Table 3, 1) is homogeneous and contains small inclusions (Table 3, 2; bright in Fig.3) that are rich in Cu. Systematically rather low analyses totals (~ 97 %) might be due to the presence of a light element such as C, although we cannot exclude other reasons – e.g., analytical error.