INTRODUCTION

Studies of cosmic dust falling onto the Earth provides valuable information on the composition of various objects in the Solar System and outer space. Much cosmic material reaching the Earth’s surface consists of microscopic-sized objects (0.01–2 mm), which are referred to as micrometeorites (Rubin and Grossman, 2010). Particles larger than 50 μm melt when entering the Earth’s atmosphere and are then quickly quenched after deceleration. The melting parameters depend on the size, composition, and physical properties of the original particle and also on its entry angle and velocity (Brownlee, 1985). The heating and cooling of a micrometeorite usually takes as long as approximately 10–15 s (Love and Brownlee, 1991). When a micrometeorite is decelerated to a velocity of <3 km/s and its temperature decreases to approximately 900°C, it is already a solid object (Rudraswami et al., 2012). Less than 1% particles larger than 300 μm can survive (Lowe and Brownlee, 1991). Complete melting and subsequent quenching result in cosmic spherules, which are beads with quench textures that are classified according to their inner structure. The I-type spherules consist of Fe oxides, sometimes possess a Fe–Ni core, or are metallic Fe–Ni beads devoid of an outer oxide coating. The G-type spherules consist of a silicate glass matrix with crystals and dendrites of Fe oxides, and the S-type ones are made up of silicate glass and olivine, sometimes with magnetite (Genge et al., 2008).

Representative collections of cosmic spherules were gathered by various researchers in Antarctica (Goderis et al., 2020; Maurette et al., 1991; Rochette et al., 2008; Taylor et al., 2000), Greenland (Maurette et al., 1987), and the Novaya Zemlya (Badjukov and Raitala, 2003; Khisina et al., 2016) or were separated from deep sea sediments (Brownlee et al., 1984; Millard and Finkelman, 1970; Rudraswami et al., 2011, 2014, 2014b) and Fe–Mn nodules and crusts found on the seafloor (Finkelman, 1970; Halbach et al., 1989). Although deep sea sediments contain higher contents of the spherules than in terrestrial rocks, hundredweights and even tons of these sediments shall be processed to obtain a representative collection of the cosmic material (Brownlee, 1985). The highest concentrations of the spherules were found in oceanic Fe–Mn crusts, which is explained by their very low growth rates (<10 mm/Ma). Such crusts coat surfaces devoid of sediments on seamounts and oceanic ridges at depths of 400 to 7000 m (Hein et al., 2013). A reasonably good collection of the spherules can sometimes obtained from less than 1 kg of the material of a crust (Halbach et al., 1989).

It is thought that most of the spherules originated from chondrite bodies that have lost some elements because of their evaporation after atmospheric entry (Rudraswami et al., 2012 and references therein). An important feature of cosmic spherules is that they contain refractory metals, including the platinum-group elements (PGE) (Brownlee et al., 1984; Rudraswami et al., 2011, 2012; 2014a, 2014b). The elevated PGE concentrations were explained by the differentiation of metals at their oxidation when the spherule melted at atmospheric entry (Brownlee et al., 1984).

Collections gathered in polar areas contain modern micrometeorites. Cosmic spherules from oceanic sediments, nodules, and crusts have ages up to dozens of million years (Brownlee, 1985; Vonderhaar and McMurtry, 1990). Cosmic spherules were also found in sediment sequences dated at tens to hundreds of million years (Sungatullin et al., 2017; Suttle and Genge, 2017; Taylor and Brownlee, 1991). Magnetic spherules separated from old rocks provide information on the composition of cosmogenic materials coming to the Earth when the spherules were formed, and in prospect, shall enable studying changes in a complex meteoroids during the geological history (Taylor and Brownlee, 1991).

This paper presents data obtained by studying 2720 cosmic spherules separated from two samples of Fe–Mn crusts at two guyots of the Magellan Seamounts in the Northwest Pacific. We have focused mainly on the Fe–Mn cores and PGE inclusions in the spherules, because their silicate constituents have long been affected by seawater, and the composition of the spherules has therewith been significantly modified. The composition of the numerous relict metallic cores of the cosmic spherules allowed us to identify trends and relations that cannot be found by studying the silicate components of the spherules.

STARTING MATERIALS AND METHODS

Material for this study was obtained from two samples of Fe–Mn crusts dredged from guyots in the Magellan Seamounts. The samples from the Fedorov Guyot (35D159) was made available for us by courtesy of T.E. Puzankova, and the sample from Alba Guyot was from V.A. Rashidov’s collection.

The sample from Fedorov Guyot is a crust consisting of three layers. Approximately 900 magnetic spherules were separated from 400 g of the material of the upper two layers (massive and porous) of the crust, 5.5 cm in total thickness. The spherules range from 25 to 250 μm across and are mostly 50–150 μm. The extraction techniques are described in (Savelyev et al., 2020). These techniques are based on the fact that cosmic spherules contain magnetic minerals and are different in this from the nonmagnetic authigenic matrix of the Fe–Mn crusts. The spherules are hosted mostly in the porous layer of the crust, which corresponds to layer II of Fe–Mn crusts at the Magellan Seamounts (Mel’nikov, 2005; Mel’nikov and Pletnev, 2013) and which is dated in this sample (35D159) at the Middle Miocene (Mel’nikov, 2005).

The sample from Alba Guyot is also a crust consisting of three layers, whose total thickness is 10–12 cm. The lower layer (4–6 cm in thickness) is a brecciated rock, with phosphatized brownish yellow and brown clay fragments up to 5 cm across, cemented with brown Fe–Mn material. The structure of the middle layer is massive, patchy, and locally columnar. The sample is dominated by black and brownish black ore material. The nonore phosphate–clay brown material is relatively scarce (5–10%), is usually porous, and occurs as elongate segregations between Mn and Fe oxide columns. This layer hosts single unrounded brown yellow fragments 1–2 mm across, which are analogous to those in the lower layer. The thickness of the middle layer is 4 cm, and it can be paralleled with layer II of Fe–Mn crusts at the Magellan Seamounts (Mel’nikov, 2005; Mel’nikov and Pletnev, 2013). The upper layer is black, slightly brownish, its material is massive, and its thickness is 1.5–2 cm. The surface of this layer is bumpy, botryoidal (with small bumps), and this layer corresponds to layer III of Fe–Mn crusts at the Magellan Seamounts. A sample of this crust (270 g) was used to separate 1800 magnetic spherules. The latter are most abundant in the middle layer (brownish black patchy and columnar material). This layer contained more than 10 magnetic spherules per 1 g of the dry material.

Some of the spherules separated from the Fe–Mn crusts (270 spherules) were placed onto conductive scotch tape to study under an electron microscope. The rest of the spherules (2450) were arranged in rows on 1-in carbon-fiber collars, poured over with epoxy resin, and polished upon its solidification using diamond sandpapers and pastes. The spherules were studied under a VEGA3 electron microscope equipped with an X-MAX80 analytical system at the Institute of Volcanology and Seismology, Far East Branch, Russian Academy of Sciences, in Petropavlovsk-Kamchatsky. The calibration standards were synthetic Ni and FeCoNi. The analysis was carried out at an accelerating voltage of 20 kV and beam current of 1 nA on pure Ni. The composition of the metallic phases was determined as an average of three to five individual analyses over an area of 3 × 3 μm.

RESULTS

More than half of the 2450 spherules studied in polished samples belongs to I-type, approximately 30% is of G-type, and less than 20% is of S-type (they are made up of silicate glass). These proportions of spherules of different type differ from those in collections of modern cosmic spherules from Antarctica and the Novaya Zemlya: these collections are dominated by S-type spherules (Taylor et al., 2000; Badjukov and Raitala, 2003). The fact that our collection is dominated by I-type spherules is explained by the conditions under which the material was buried, with the silicate constituents of the spherules more rapidly decomposed in seawater (Rudraswami et al., 2012), in contrast to what is typical of the Antarctic glacial moraine, in which the metallic type is more resistant to oxidation (Van Ginneken et al., 2016). The abundance of I-type spherules in our collection makes it similar to collections from deep-sea sediments (Rudraswami et al., 2014b) and offers a unique chance to study many of the metallic cores and PGE nuggets, which occur mostly in the spherules of this type.

More than 400 of our spherules contained a metallic Fe–Ni core. The ratios of the linear sizes of the metallic core and outer shell seen in thin sections vary from 1 : 1.2 to 1 : 10 (Fig. 1), and some of the spherules are metallic cores without oxide shells. It is not always clear whether these shells were destroyed when the samples were prepared or the cores were buried in this form in the Fe–Mn crusts. The outer shells around the cores usually consist of magnetite or are mixture of magnetite and wüstite, although some Fe–Ni cores were hosted in G-type spherules, in shells consisting of magnetite in a Fe-oxide matrix with silicate admixtures (Fig. 1e). Some of the metallic cores were partly replaced by secondary minerals in the course of younger processes on the seafloor (Fig. 1f). The metallic cores of some of the spherules are completely replaced by mixtures of clay minerals and oxides. Rare metallic cores in oxide shells host small irregularly shaped metallic inclusions near the core (Fig. 1b).

Fig. 1.
figure 1

Fe–Ni cores in the cosmic spherules: (a–d, f) in I-type spherules, (e) G-type spherule. Phases: Wu—wüstite, Mt—magnetite, Fe + Ni—Fe–Ni alloy, and Sil—silicate phase.

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We managed to qualitatively analyze [analytical totals Fe + Ni + Co (sometimes + PGE) = 100 ± 1.5%] 406 metallic cores or their fragments. We have not detected any zoning of these cores (with regard to the analytical uncertainties). The composition of the cores is illustrated in Figs. 2 and 3. One of the important results of our study was that we have found a few spherules whose metallic cores were of unusual composition: they were richer in Co (6–15 wt %) (Table 1, Fig. 3). Alloys of such composition have never before been documented in cosmic spherules.

Fig. 2.
figure 2

Ni–Co × 10–Fe and Ni–Co–Fe (wt %) diagrams for the composition of the metallic cores of the cosmic spherules in two Fe–Mn crusts from the Magellan Seamounts.

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Fig. 3.
figure 3

Fe–Co diagram for the composition of the metallic cores of the cosmic spherules from two Fe–Mn crusts from the Magellan Seamounts. Triangles show the composition of kamacite and taenite as averages for 40 meteorites according to (Goldstein et al., 2014). The gray line with an arrowhead is the compositional evolution of the Fe–Mn cores in the course of Fe oxidation at passage through the atmosphere.

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Table 1.   Composition (wt %) of the Co-rich metallic cores and the types of the host spherules
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Two spherules of I-type were found out to possess Fe–Ni cores rich in PGE (the analytical totals of PGE were as large as 2.5 wt %). These spherules were noted for that their cores are small compared to their oxide shells (Fig. 4a).

Fig. 4.
figure 4

PGE in the spherules: (a) spherule with a Fe–Ni core enriched in PGE, (b and c) micrometer-sized PGE nuggets (see analyses 3 and 4, respectively, in Table 2 for the composition of the nuggets), (d) nanometer-sized nuggets of Rh-bearing Pt. Phases: Fe + Ni—Fe–Ni alloy, Wu—wüstite, Mt—magnetite, Pt—nuggets of Rh-bearing Pt, Sil—silicate phase, PGE—PGE nuggets.

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Some of the studied spherules contained PGE nuggets: we have identified such nuggets in 23 spherules. The 1- to 3-μm nuggets were found in the G-type spherules (Figs. 4b, 4c), and contained all six PGE. A 3-μm nugget contained Os, Ir, Ru, Rh, and Pt in roughly chondritic proportions but was strongly depleted in Pd (Table 2, Fig. 5). A nugget 1 μm across and consisting of Ir, Os, and Ru was identified on the surface of a G-type spherule. In addition to these, 20 I-type spherules hosted nanometer-sized nuggets (<0.5 μm) of rhodium platinum. These nuggets were found exclusively in the oxide matrix of the spherules that consisted of wüstite and magnetite, in spherules devoid of a metallic core. The Pt inclusions are hosted mostly in magnetite, although wüstite also sometimes contains them. A single section of a spherule usually shows more than one such nuggets (from 1 to 20), which are unevenly distributed over the surface of the section (Fig. 4d). The small size of these nuggets did not allow us to analyze them by microprobe because of the excitation of the host oxides. The Pt/Rh ratios of the inclusions were usually greater than the chondritic ones: Pt/Rh = 9–20 for analyze with Pt > 10 wt %.

Table 2.   Composition (wt %) of Ni-rich metallic cores rich in PGE and nuggets (1 and 3 μm)
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Fig. 5.
figure 5

CI chondrite-normalized (Anders and Grevesse, 1989) PGE patterns of a metallic core and PGE nuggets in cosmic spherules: (1) in the Ni-rich core, (2) in a 3-μm nugget, (3) in nugget AAS-26-D1#1-P8 from (Rudraswami et al., 2011).

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DISCUSSION

The most interesting result of our study is the identification of spherules with Co-rich Fe–Ni cores (Table 1, Fig. 3). We failed to find available published data on spherules with cores of such composition. In our opinion, the two reasons for these are either that the number of so far analyzed modern spherules is still inadequately small or that the composition of cosmic dust coming to the Earth’s surface has been modified. The highest Co concentrations of the metallic cores are, according to literature data, 4.4 wt % (Rudraswami et al., 2014b), with a similar value of 4.05 wt % reported in (Dekov et al., 2007). Both selections pertain to spherules from the seafloor, with the richest of the collections on which data are published (Rudraswami et al., 2014b) comprising analytical data on 55 metallic cores. We have analyzed 406 metallic cores (including spherules without oxide shells), and six of them contained >5 wt % Co (Figs. 2, 3). The Ni, Fe, and Co proportions of the cores of the cosmic spherules depend on the original composition of the micrometeorite and the degree of oxidation of the spherule (the volume proportion of the oxide shell and residual metallic core). In the course of their oxidation when passing through the atmosphere, the spherules started to oxidize from their surface, and thereby siderophile elements more “noble” than Fe were concentrated in the core (Brownlee et al., 1984). As a result, Ni and Co concentrations in the metallic cores increased and the Fe concentration simultaneously decreased. As follows from the composition of the Ni-rich cores (Ni > 90%, Fe < 10%), the latest oxidation was associated with enrichment of the cores only in Ni and PGE, whereas Co was oxidized simultaneously with Fe (Fig. 3, Table 2). The evolutionary trends of the metallic cores are shown in the Fe–Co/Ni diagram, which was presented in (Bi et al., 1993) and is shown in Fig. 6.

Fig. 6.
figure 6

Fe–Ni/Co diagram for the composition of the cosmic spherules and metal phases of meteorites: (1) metallic cores of the spherules from Fe–Mn crusts from the Magellan Seamounts (this publication), (2, 3) composition of kamacite and taenite, respectively (averages for 40 meteorites from Goldstein et al., 2014), (4) spherical particles from the Tunguska area (Badjukov et al., 2011), (5) metallic spherules from Pleistocene sediments in Alberta, Canada (Bi et al., 1993), (6) cores of spherules from deep-sea sediments (Bi et al., 1993), (7) metallic cores of spherules form Fe–Mn crusts and nodules (Finkelman, 1970; Kosakevitch and Disnar, 1997), (8) cores of cosmic spherules from metal-bearing sediments (Dekov et al., 2007), (9) cores of cosmic spherules from deep-sea sediments form the Indian Ocean (Rudraswami et al., 2014b), (10) metallic phase of chondrites (Krot et al., 2000). Arrows show the hypothetical compositional evolution of the metallic cores at the oxidation of the spherules in the atmosphere.

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As seen in this diagram, the starting compositions of cosmic particles defining distinct trends was different, with the Co/Ni ratios of the original particles being several times different. The composition of the Fe–Ni cores at low degrees of oxidation (Fe > 90%) is close to kamacite (Figs. 3, 6). This composition was the starting one for most of the metallic cores of cosmic spherules (Fig. 6). Apparently, metallic cores of this composition were formed in most cosmic spherules at the initial stage of differentiation.

The Co and Ni ratios of most of our metallic cores do not any significantly differ from the chondritic ones (0.46 according to (Anders and Grevesse, 1989)), which is consistent with data in (Rudraswami et al., 2014b). In addition to the Ni-rich cores, which have low Co concentrations (Ni > 90%, Co < 1%), a group of the cores is characterized by low Co concentrations and Co/Ni ratios lower than the chondritic ones (Figs. 3, 6). This may be explained either by that Co concentrations in the original micrometeorites were relatively low or by that the metallic spherules were formed not from chondritic dust but immediately from iron meteorites or the iron–nickel phase of chondrites, as was suggested for metallic spherules found in sedimentary sequences in Alberta, Canada (Bi et al., 1993). The Fe–Ni phase of unoxidized meteorites of chondrite composition contains less Co (close to 0.5% Co at 81–92% Fe) (Krot et al., 2000).

The composition of metallic cores containing more than 5% Co cannot be explained by the evolution of the spherule that originated from a micrometeorite of chondrite composition. At the oxidation of a metallic bead in the atmosphere, Co enriches the core less intensely than Ni does (Kosakevitch and Disnar, 1997), and hence, the source of the spherules with Co-rich cores was significantly enriched in Co. Such a source for meteorites studied so far might have been Co-kamacite [which is listed in the catalogue of meteorites (Ivanov et al., 2019)]. This phase is very rare and was described in LL chondrites (Affittalab and Wasson, 1980; Rubin, 1990) and in a single diogenite (Ramdohr and El Goresy, 1969). Our finds (six Co-rich cores of the 406 studied ones) led us to hypothesize that meteorites with the Co-kamacite phase were more abundant when some layers of the Fe–Mn crusts in the Magellan Seamounts were formed than now.

A number of groups can be distinguished according to Co concentrations in the Fe–Mn crusts (Fig. 3). In addition to the cores whose composition points plot within the main swarm of the data points (originally chondrite composition) and the aforementioned Co-rich (6–15% Co) and Co-poor (<1% Co at Fe < 70%) cores, some cores contain only slightly elevated Co concentrations (3–4.5%). Cores of this composition have also been described by other authors (Bi et al., 1993; Dekov et al., 2007; Rudraswami et al., 2014b). They may correspond to the composition of the original bodies with higher Co/Ni ratios than the chondritic one. Although Co concentration in the metallic core changes in the course of oxidation of a cosmic spherule, we propose to use this parameter to identify the sources of cosmic dust other than chondritic. For example, this method makes it possible to identify cosmic objects that were rich in Co in the past.

Another important find was metallic cores with high PGE concentrations. We have identified two such cores, whose ratios of their core diameter to the diameter of their oxide shell are 1 : 5 and 1 : 10 (Fig. 4a). The core richest in PGE (the total PGE concentration is 2.5 wt %) contained all of the PGE (identified under an electron microscope; Table 2), although Os and Rh concentrations in some of the analyses were lower than the detection limits. The average (of six analyses) PGE composition of this core shows a chondrite-normalized pattern close to the chondritic one (×10 000), except Os (which is likely explained by the insufficient analytical sensitivity to Os). No Fe–Ni cores with such high PGE concentrations have been described in the literature before. The PGE concentrations and composition in the cores confirm the model (Brownlee et al., 1984), according to which PGE nuggets were produced in cosmic spherules by the gradual oxidation of the Fe–Ni cores and PGE concentrated in the remaining metallic phase. The cores described herein reflect an intermediate stage of the process: the enrichment of the core in PGE before the complete oxidation of Fe and Ni. Both cores in which PGE were found contain >90% Ni (Table 2). These results are consistent with data in (Rudraswami et al., 2014b) on a positive correlation between Ni and PGE in the metallic cores. The PGE patterns in these cores are almost parallel to those of chondrites, which confirms the conclusion (Rudraswami et al., 2014b) that the source of the spherules was chondritic and indicate that PGE were accumulated in the metallic cores at oxidation without losses of more volatile PGE.

The composition of PGE nuggets found in the spherules corresponds to those in (Brownlee et al., 1984; Rudraswami et al., 2011; Rudraswami et al., 2014a). Similar to what has been done in earlier studies, we found nuggets of two types: larger (1–3 μm) nuggets that contain five or six PGE (all PGE or all but Pt) and nanometer-sized nuggets with rhodium platinum. Finds of rhodium platinum (which was found only in I-type spherules) are fully consistent with observations in (Rudraswami et al., 2011; Rudraswami et al., 2014a). Compared to what has been done by other researchers, we identified relatively few spherules with nanometer-sized nuggets (only 20 of the 2450 spherules studied in polished sections). This is explained by that we did not try to find all PGE nuggets and studied each of the spherules in a single section, whereas spherules are usually successively studied in seven to ten cross-sections spaced 5–10 μm apart to find as many as possible nuggets (Rudraswami et al., 2014a).

The PGE concentrations of a 3-μm nugget that we found almost exactly coincide with the concentrations in nugget AAS-26-D1#1-P8 in (Rudraswami et al., 2011) (Fig. 5). The PGE proportions of this nugget are close to chondritic, with a slight depletion in Ru, Rh, and Pt and significant Pd deficit. Such PGE proportions (Pd deficit and near-chondrite proportions of other PGE) are in good agreement with the model (Brownlee et al., 1984), according to which Pd is the first PGE to start evaporating after Fe and Ni have been lost.

Our study confirms that cosmogenic material played an important role in PGE accumulation in Fe–Mn crusts and nodules. Some researchers believe that cosmogenic Pt may make up as much as 25% of the total Pt concentrations in the Fe–Mn crusts (Halbach et al., 1989).

CONCLUSIONS

(1) 2720 cosmic spherules were extracted from Fe‒Mn crusts dredged at Fedorov and Alba guyots and were studied in polished sections under an electron microscope equipped with an EDS analyzer. Compared to collections of modern cosmic spherules, our collection is significantly richer in spherules of type I and is, as of now, the richest collection of the spherules of this type among all such collections described in the literature.

(2) We have analyzed compositions of the 406 metallic cores of the cosmic spherules. Six of the cores were found out to be significantly enriched in Co (>5 wt %). No spherules of such composition have been documented previously. These high Co concentrations in the cores cannot be explained by the evolution of the spherule originating from a micrometeorite of chondrite composition.

(3) Our collection comprises spherules with elevated and relatively low Co concentrations compared to those on the evolutionary trend of spherules produced during passage through the atmosphere. They correspond to the different composition of the sources of cosmic dust. These observations make it possible to use the composition of the cores of cosmic spherules to understand how compositions of cosmic dust coming to the Earth evolved with time.

(4) We have found a few spherules that possessed relatively small cores as compared to their oxide shells (less than 1/10 of the diameter). These cores are noted for containing much PGE (the maximum total PGE concentration is 2.4 wt %). The averaged composition of the core richest in PGE corresponds to a chondrite-normalized pattern close to the chondrite one.

(5) Our collection contains 23 spherules that host PGE nuggets. Among others, three spherules of type G hosted micrometer-sized PGE nuggets (one Ir + Os + Ru nugget and two nuggets with all PGE). The chondrite-normalized PGE pattern of the largest (3 μm) nugget is nearly parallel to the chondrite pattern, except only strong depletion in Pd.

(6) The cosmic spherules had a source of mostly chondrite composition, but some spherules in our collection originated from a source different from chondrite one.