# The Oxygen Isotope Compositions of Large Numbers of Small Cosmic Spherules: Implications for Their Sources and the Isotopic Composition of the Upper Atmosphere

N. G Rudraswami, Matthew J Genge, Yves Marrocchi, Johan Villeneuve, S.

Taylor

► To cite this version:

N. G Rudraswami, Matthew J Genge, Yves Marrocchi, Johan Villeneuve, S. Taylor. The Oxygen Isotope Compositions of Large Numbers of Small Cosmic Spherules: Implications for Their Sources and the Isotopic Composition of the Upper Atmosphere. Journal of Geophysical Research. Planets, 2020, 125 (10), 10.1029/2020JE006414. [https://doi.org/10.1029/2020JE006414](https://doi.org/10.1029/2020JE006414)

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Title:

The oxygen isotope compositions of large numbers of small cosmic spherules:

Implications for their sources and the isotopic composition of the upper atmosphere

N. G. Rudraswami¹*, Matthew J. Genge², Yves Marrocchi³, Johan Villeneuve³, S. Taylor⁴

¹National Institute of Oceanography (Council of Scientific and Industrial Research),
Dona Paula, Goa 403004, India

²Department of Earth Science and Engineering, Imperial College London, London, UK

³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, F-54501, France

⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New
Hampshire 03755-1290, USA

Corresponding Author: rudra@nio.org; rudraswami@gmail.com

Date: May 2020
30 **Key Points:**

31 • The ranges of different types of cosmic spherules suggest increasing mass
fractionation consistent with estimates of peak temperature from textures.

33 • Porphyritic and barred spherules have derivation of a significant proportion
34 mainly from ordinary and carbonaceous chondrites, respectively.

35 • G-types have experienced little mass fractionation with oxygen isotope composition
36 similar to carbonaceous chondrite precursors.

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41 Abstract:

42 Cosmic spherules are micrometeorites that melt at high altitude as they enter Earth's
43 atmosphere and their oxygen isotope compositions are partially or completely inherited from
44 the upper atmosphere, depending on the heating experienced and the nature of their precursor
45 materials. In this study, the **three oxygen isotope** compositions of 137 **cosmic spherules are**
46 determined using 277 in-situ analyses by ion probe. Particles of each different type of cosmic
47 spherule (scoriaceous, porphyritic, cryptocrystalline, barred, glass, calcium aluminium and
48 titanium (CAT), G-type and I-type) in the diameter range ~52–480 µm were analysed. The
49 results confirm that the **three** oxygen isotope compositions of melted **micrometeorites**
50 reflect a combination of their precursor composition, exchange with the atmosphere and mass
51 fractionation owing to evaporation during entry heating. The data **appear to** reveal an
52 increase in average δ¹⁸O values of silicate dominated (S-type) spherules in the series
53 scoriaceous<porphyritic<barred<glass<CAT spherules (~20, 22, 25, 26 and 50‰) that is
54 consistent with the evolution of oxygen isotopes by mass fractionation owing to increased
55 average entry heating. The trend of δ¹⁷,¹⁸O is broadly parallel to the terrestrial fractionation
56 line and thus suggests mass fractionation dominates changes in isotopic composition, with
57 atmospheric exchange a less significant effect. The Δ¹⁷O values of spherules, therefore, are
58 mostly preserved and suggest that ~80% of particles are related to the carbonaceous
59 chondrites (CC) and are probably samples of C-type asteroids. The genetic relationships
60 between different S-types can also be determined with scoriaceous, barred and
61 cryptocrystalline spherules mostly having low Δ¹⁷O values (≤0‰) suggesting they are mainly
62 derived from CC-like sources, whilst porphyritic mostly have positive Δ¹⁷O (>0‰)
63 suggesting they are largely from ordinary chondrite (OC)-like sources related to S(IV)-type
64 asteroids. Glassy and CAT-spherules have Δ¹⁷O values suggesting they formed by intense
entry heating of both CC and OC-like materials. I-type cosmic spherules have a narrow range
of δ¹⁷O (~20–25‰) and δ¹⁸O (~38–48‰) values, with Δ¹⁷O (~0‰) indicating their oxygen is
obtained entirely from the Earth’s atmosphere, albeit with significant mass fractionation
owing to evaporation during entry heating. The observed range of δ¹⁸O with size is suggested
here to reflect entry angle with high values representing enhanced heating at high angle.
Finally, G-type **cosmic spherules** have unexpected isotopic compositions suggesting little
mass-fractionation from a CC-like source and are suggested to have sulphide-silicate
precursors with relatively low melting temperatures. The results of this study provide **vital**
**assessment** of the **wider** population of extraterrestrial dust arriving **to the** Earth.

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75 **Plain Language Summary**

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**Extraterrestrial dust particles are often intensely heated during atmospheric entry,**
**inducing significant changes of their mineralogy and chemical and isotopic**
**compositions. Such heating processes make difficult to identify the nature of their**
**precursors, and thus the sources of extraterrestrial dust falling on Earth. In this study,**
**we report the oxygen isotope compositions of a large collection of cosmic spherules to**
**assess their sources in the solar system. Previous studies have shown that oxygen**
**isotopes are altered by heating and mixing with the atmosphere, but have yet analysed**
**relatively small numbers of particles. Our results suggest that 80% of particles are**
**related to carbonaceous chondrites, and are thus samples of C-type asteroids, with most**
**of the remainder being related to ordinary chondrites that sample S-type asteroids. We**
**also show, for the first time, that G-type cosmic spherules have a C-type asteroid source.**
**We conclude that oxygen isotopes provide a valuable probe of the sources of**
**extraterrestrial dust despite the changes that occur during atmospheric heating.**

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1. INTRODUCTION

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Micrometeorites are the most abundant extraterrestrial materials reaching the Earth's surface on an annual basis (Love and Brownlee, 1993). Cosmic spherules are **extraterrestrial** particles that undergo significant melting during atmospheric entry and represent **micrometeorites** that have experienced high temperature during **passage through the atmosphere**. Although a large fraction of this material never makes it to Earth's surface and is evaporates (e.g. Love and Brownlee, 1991), **surviving particles still** represent the most significant contributors of **extraterrestrial** material to the Earth's surface (Taylor et al., 1998; Yada et al., 2004; Plane, 2012; Prasad et al., 2013). Micrometeorites provide information on the nature of their **Extraterrestrial** parent bodies, the processes operating during atmospheric entry and the composition of the Earth's atmosphere. Extraterrestrial material that enters the upper atmosphere at >100 km altitude experiences gas drag heating thereby changing their original chemical and isotopic properties (Kurat et al. 1994; Beckerling and Bischoff 1995; Brownlee et al. 1997; Greshake et al. 1998; Engrand et al., 2005; Genge et al., 2008; Cordier et al., 2011; Rudraswami et al., 2012, 2015, 2016). The oxygen isotope compositions of particles that survive atmospheric entry are thus related to those of both their precursors and the upper atmosphere. **Currently, the oxygen isotope composition of the upper atmosphere (i.e. > 60 km) has not been determined yet and any isotopic information above this altitude need to be deciphered from micrometeorites as most of these particles have undergone heating leading to isotopic exchange between ~80 and 120 km** (Thiemens et al., 1995; Pack et al., 2017).

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The present study undertakes oxygen isotope analyses of different types of **micrometeorites** in large numbers from partially heated scoriaceous **micrometeorites** to the
most heated **cosmic spherules** is an attempt to constrain the contribution of heating,
exchange with atmosphere, and precursor to their isotopic composition. The ideal **cosmic
spherules** for deciphering the composition of the upper atmosphere are I (iron) type
spherules since all of their oxygen is acquired from the atmosphere (Pack et al., 2017). The
analyses of Pack et al., (2017) suggest that it was possible to identify the $\delta^{17,18}\text{O}$ in the upper
atmosphere and concluded it is similar to the troposphere. The current study employs a larger
set of I-type **cosmic spherules** to better evaluate their oxygen isotope composition and
alteration during atmospheric entry.

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Unmelted micrometeorites and interplanetary dust particles have experienced
minimal chemical and isotopic alteration during atmospheric entry, due to high zenith
angle and low entry velocity. Hence, they are invaluable witnesses for estimating the
pre-atmospheric oxygen isotopic composition of the altered S-type cosmic spherules and
scoriaceous micrometeorites. The mineralogy and bulk chemical compositions of cosmic
spherules are changed significantly, but previous studies suggest that the $\Delta^{17}\text{O}$ abundance is
largely preserved in some significantly altered cosmic spherules (Engrand et al., 2005; Yada
et al., 2005; Suavet et al., 2010; Cordier and Folco, 2014; Rudraswami et al., 2015, 2016;
Goderis et al., 2019). Here we report the in-situ analyses of 277 spots for oxygen isotopes
from selected 137 cosmic spherules of different types (S, I and G-type) using an ion
microprobe to outline the variation during atmospheric entry.

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## 2. SAMPLE COLLECTION

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The **cosmic spherules** are collected from the central Indian Ocean from the depth of
~5200 m using a grab sampler having size 50×50 cm (length × breadth). This grab can
penetrate to a depth of ~15 cm and pick up ~ 45 kg of wet sediments. The details of the
sampling methodology have been reported in previous research publications, namely,
Rudraswami et al., 2011, 2012; Prasad et al., 2013. Antarctica samples were collected from
South Pole Water Well (SPWW), Amundsen-Scott South Pole Station, below ~100 m from
the snow surface. The well dimension are ~24 m in diameter with water depth of ~15 m with
a capacity of ~ 5000 m³ (Taylor et al., 1998, 2000). In addition, further Antarctica samples
were collected near Indian research station Maitri by melting blue ice and sieving using ~50
µm mesh. Nearly ~50 tons of blue ice was melted to extract the trapped cosmic dust particles
that are expected to exhibit minimal terrestrial alteration (Rudraswami et al. 2018). The
selection of 137 particles were made after investigating ~3000, ~1500, ~2000 cosmic
spherules from deep-sea, SPWW, Maitri collection, respectively. The collected samples
were mounted on epoxy resin for polishing to be able to use for further electron microscopy
and isotopic studies using ion microprobe. Silicate (S) type particles analysed are as follows:
scoriaceous (13), porphyritic (11), barred (18), cryptocrystalline (22), glass (37). A further I-
type (15), G-type (19) and CAT (Calcium-Aluminium-Titanium, 3) spherules were also
measured (Table 1 and Appendix A). Twenty-three spherules with metal bead are reported in
Table 2 and Appendix A.

### 3. INSTRUMENTAL TECHNIQUES

Prior to oxygen isotope analyses by ion microprobe, documentation of the texture,
chemical composition and type of spherules was performed on polished sections. The
scanning electron microscope (SEM, JEOL JSM-IT300LV at National Institute of
Oceanography, Goa) was used to recognise the texture and type of spherules, while an
electron probe micro analyzer (EPMA, Cameca SX5 at National Institute of Oceanography,
Goa) was used to acquire the major and minor elemental composition. Most cosmic spherules are in the size range of less than a few hundred µm thus the instrument can perform a detailed study of specific areas in particles. Chemical analyses were performed using electron microprobe on selected cosmic spherules phases with accelerating voltage ~15 kV, ~ beam current ~12 nA, ~1–2 µm beam diameter for spot analyses, and ~5 µm beam diameter for bulk analyses (Rudraswami et al., 2019). Multiple measurements obtained using different standards are < 1% error for major elements and for minor elements are within a few percent.

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A Cameca 1270 E7 ion microprobe at CRPG-CNRS (Nancy, France) was used for oxygen isotope analyses on selected phases or areas. The selected phases were analyzed by $Cs^+$ beam having primary current of ~1 nA and beam spot of ~5 µm. Faraday cups were used to measure secondary ions $^{16}O^-$ and $^{18}O^-$ with mass resolving power of ~2500, while $^{17}O^-$ was measured using mono-electron multiplier with mass resolving power ~7000 to resolve the $^{16}OH^-$ contribution. The isotopic errors (2σ), considering the measurement errors as well as standard external reproducibility, are estimated to be ~0.7, 0.6 and 0.8‰ for $\delta^{18}O$, $\delta^{17}O$, and $\Delta^{17}O$ (= $\delta^{17}O$ – 0.52 × $\delta^{18}O$), respectively. All the analyzed phases were examined under SEM to discard those data that falls on visible cracks, pits or regions that were not part of this study. The instrumental mass fractionation in olivine grains was corrected using San Carlos olivine, and for I-type and some G-type spherules that were dominated by magnetite we have used terrestrial magnetite standard. Further, detailed technical description of the SEM, electron microprobe and ion microprobe is given elsewhere (Rudraswami et al., 2019).

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## 4. RESULTS
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We have classified 137 cosmic spherules and partially melted scoriaceous micrometeorites based on classification scheme by Genge et al. (2008) in order to understand the nature of oxygen isotope composition. The oxygen isotope study of cosmic spherules includes: scoriaceous: 13 particles, porphyritic: 11, barred: 18, cryptocrystalline: 22, glass: 37 and CAT: 3, along with I-type: 15 and G-type: 18 (Table 1 and 2). The back-scattered electron images of nine S-type spherules are shown in Fig. 1, while the bulk chemical compositions of all the spherules are summarized in Appendix A. Some of the spherules analysed have FeNi metal bead – these include one porphyritic, three barred, two cryptocrystalline, fourteen glass and one G-type (Fig. 2). Four spherules that have metal beads separated from the main silicate body have a narrow range of oxygen isotope compositions, except MS-I3-P55 (Table 2). The chemical compositions of FeNi beads present in various types of spherules (Fig. 2) are given in Appendix A. The bulk chemical compositions, however, does not show any correlation with oxygen isotope data. Finally, CAT spherules are formed by extreme heating and evaporation during atmospheric entry (Taylor et al., 2000, 2005) and thus provide an endmember of atmospheric entry behaviour.

The oxygen isotope compositions of the analysed cosmic spherules are shown by spherule type in Figure 3 and 4. The over-all compositional range is δ¹⁷O –6 to +36‰ and δ¹⁸O –3 to +72‰ and forms a field scattered mostly around the terrestrial fractionation line (TFL) with some adjacent to carbonaceous chondrite anhydrous mineral (CCAM) line. The S-type spherules show average δ¹⁸O values that increase in the series porphyritic < barred < cryptocrystalline < glass with values of ~20, 22, 25 and 26‰, respectively. The δ¹⁸O range is similar for all groups; however, porphyritic spherules exhibit the smallest range and lowest
values at ~0.7‰. The highest δ¹⁸O values are exhibited by the CAT spherules with an average of ~50‰, maximum of ~72‰ and a minimum of ~30‰. Scoriaceous **micrometeorites** have the lowest average, maximum and minimum values at ~11, 20 and –3‰, respectively.

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The Δ¹⁷O compositions of S-type particles show significant variations (Figure 5). Porphyritic spherules have a higher average and smaller range at –3 and 2‰ compared with barred (–6 and 2‰) and cryptocrystalline spherules (–5 and 2‰), the majority of barred and cryptocrystalline spherules plot below the TFL in the three isotope plot, whilst the porphyritic spherules scattered closely around this line, with 7 out of 11 plotting above. For barred and cryptocrystalline spherules the highest Δ¹⁷O values are those particles with the highest δ¹⁸O and plot above the TFL. Glass spherules have a broadly similar average to barred, porphyritic and cryptocrystalline combined, and a slightly smaller range at –4 and 3‰. Scoriaceous **micrometeorites** have an average Δ¹⁷O most similar to barred and cryptocrystalline spherules, but a smaller range at –5 to 0.2‰. The smallest range of Δ¹⁷O amongst silicate dominated particles is exhibited by the CAT spherules and these all plot close to the TFL.

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I-type **cosmic spherules** have oxygen isotope compositions distinct from the S-types and scoriaceous **micrometeorites** having a small range of high δ¹⁸O values of ~39 to 48‰. The Δ¹⁷O ratios of I-types are very low and they all fall within ~1‰ of the TFL. None of the I-type spheres measured for oxygen isotopic composition contained metal beads that could be seen in the polished section. However, spherule AAS-62-61-P87 has a void space indicating that bead has been plucked off from the spherules probably during polishing leaving behind void space (Fig. 2). This particle has a thinner magnetite rim (~3µm) than others. The trend of the magnetite thickness **versus δ¹⁸O from the present study, along with that observed by**

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Engrand et al. (2005), suggests an increase in δ¹⁸O with thickness (width) of the rim;
however, there is no such strong correlation observed with diameter of the I-type spherules
(Fig. 6).

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In contrast to I-type spherules, G-types have lower δ¹⁸O values with a range from ~7
to 35‰, with maximum values smaller than any spherule of I-type. The average Δ¹⁷O of G-
types ranges between -6 and 1‰ and thus plots mainly below the TFL in a three isotope plot
(Fig. 5). The Δ¹⁷O values of G-types increase towards the TFL with increasing δ¹⁸O. There is
only one G-type spherule SP005-P207 that has FeNi bead in it (Fig. 2).

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The porphyritic spherule SP005-P1078 has two beads with a significant abundance of
sulfur (~17 wt%). Sulfur at ~5 wt% was also seen in cryptocrystalline spherules SP005-
P1176. Few glass spherules have metal beads with sulfur (Appendix A): SP005-P157 (~33
wt%), SP005-P1071 (~15 wt%), SP005-P34 (~6 w%), SP005-P294 (4 wt%) and SP005-P296
(~1wt%). We have also compiled the data of all metal bead spherules (one porphyritic, three
barred, two cryptocrystalline, fourteen glass and one G-type) **to explore** the trend of Ni
(wt%) with δ¹⁸O values with different texture (Fig. 7). **Two data sets were examined from
different cosmic spherules**, one from cryptocrystalline SP005-P1176 and one from glass
MS-I35-P39, most of them have nickel composition <30 wt% and there is no visible trend of
Ni vs. δ¹⁸O. Similarly, **we investigated whether δ¹⁸O varies with size of cosmic spherules
(Fig. 8) with the expectation that heating, and thus mass fractionation, may increase
with particle size. No correlation, however, was observed with the size of particles.**
**Finally the δ¹⁸O and Δ¹⁷O data was compared with those of meteorites as shown in Figure 9
since previous studies have suggested that the oxygen isotope compositions of spherules
can be related to those of their precursors by consideration of the nature of exchange**

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processes occurring during entry heating Suavet et al. (2010).

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## 5. DISCUSSION

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### 5.1. Oxygen isotope of I-type cosmic spherules

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I-type **cosmic spherules** are thought to form by the atmospheric entry of FeNi metal interplanetary dust particles rather than by separation from silicate particles. **Their origin as metal grains is supported by their sizes, which are larger than immiscible metal beads present in S-type cosmic spherules. Furthermore, the flux of micrometeorites is significantly larger than that of meteorites, implying that I-types are not separated from these larger meteoroids** (e.g. Genge et al., 2017). The presence of the spallogenic isotope $^{10}$Be within I-type spherules confirms they were exposed in space as metal grains, since this highly lithophile element would have been removed if they separated from silicates (Yiou et al., 1985). The textures and mineralogies of I-type **cosmic spherules** are also consistent with their formation by progressive oxidation with the formation of an oxide mantle at the expense of the FeNi metal bead (Brownlee et al., 1984; Engrand et al., 2005; Genge et al., 2017b). **I-type cosmic spherules, therefore, could be used as a proxy for the isotopic composition of the thermosphere/mesosphere and stratosphere. Since this requires an independent assessment of the degree of evaporation, and thus analyses of $\delta^{56}$Fe** (Engrand et al. 2005; Pack et al., 2017) **it was not possible in the current study.**

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Although most are formed by melting of metal grains, there are probably some I-type particles formed by metal separation from S-types, as suggested by Brownlee et al. (1984), since metal-beads are observed to migrate to the margins of these spherules during
deceleration (Genge and Grady, 1998). These are likely to be in the minority, and likely to be
of smaller size (<50 µm in diameter). In such cases, there is bound to be some iron isotope
fractionation during immiscibility **that could potentially influence the use of δ<sup>56</sup>Fe as a
proxy for evaporation. Fractionation is, however, likely minor during immiscibility** in
comparison to evaporation simply because the mass difference is small and diffusion rates in
silicate melts are more controlled by electronic structure and speciation rather than atomic
mass. In the present study, we consider all I-type particles **to have had FeNi metal
precursors**.

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The oxygen isotope of 15 I-types has δ<sup>18</sup>O ranging between 38.5–48.4‰, while for
δ<sup>17</sup>O of 20.0–25.2‰ with Δ<sup>17</sup>O value within analytical uncertainty of the terrestrial
fractionation value. The spread in δ<sup>18</sup>O is ~10‰, while that in δ<sup>17</sup>O are ~5‰. These isotopic
values cover a larger range and are heavier than those used by Pack et al., (2017) as a proxy
for mesospheric composition (δ<sup>17</sup>O= 19.3–21.6‰, δ<sup>18</sup>O= 37.8–42.5‰); albeit on only four
**cosmic spherules** between ~400 and 550 µm in size. In contrast Engrand et al (2005)
obtained a wider range of values of (δ<sup>17</sup>O= 19.1 to 29.4‰, δ<sup>18</sup>O= 38.4 to 57.4‰) for a set of
twelve I-type spherules ranging in size from ~408 to 575 µm. They showed an increase in
δ<sup>18</sup>O with the diameter of the spherule which is also true for the thickness of the magnetite
rim (Fig. 6). The thickness of rim correlation seen in many I-type spherules also holds true
for the current study where rim thickness is much smaller (<11 µm). **As the oxidised rim
thickness increases so does the δ<sup>18</sup>O value, however, this correlation also holds over the
entire diameter range of the spherules.**

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Engrand et al. (2005) indicated that δ<sup>18</sup>O correlates with size for particles larger than
>400 µm. I-type particles from the present study are less than 300 µm and have a smaller
range and average of δ¹⁸O than the larger spherules studied by Engrand et al., (2005), but
have a similar minimum δ¹⁸O. **An increasing trend of δ¹⁸O with size was observed except**
**for** few outlier datasets such as AAS-38-167-1-P115 and AAS-38-167-1-P116 (circled in Fig.
6). The oxygen isotope data values did not correlate with the total abundance of magnetite in
I-type spherules, **which is likely to be a proxy for total oxidation**, nevertheless, δ¹⁸O
strongly correlated with the thickness of the magnetite rim (Fig. 6 and Table 2). The
thickness of the rim is large in spherules dominated by magnetite unlike those that are
wustite-dominated. This undoubtedly relates to increased mass fractionation as a result of
larger degrees of evaporation from larger particles – largely a consequence of their higher
peak temperatures (Genge, 2016). The increase in the range and maximum value of δ¹⁸O with
particle size is consistent with the variation of their peak temperatures with entry parameters
as modelled numerically by Genge (2016). At near vertical entry angle peak temperature
increases **rapidly** with particle size, resulting in significant increases in mass fractionation,
whilst at low entry angles peak temperature increases less with size, causing smaller changes
in mass fractionation with size. **The range of the data thus agree with theoretical
constraints on the entry heating of these particles.**

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I-types with metal beads and smaller magnetite rims have been shown to have lower
δ¹⁸O due to lower degrees of heating than those with no bead and thick magnetite rim
(Engrand et al., 2005); however, we cannot verify this as no metal bead-bearing I-types were
analysed in the current study. Furthermore, without δ⁵⁶Fe measurements we cannot
independently determine the degree of evaporation, and thus the magnitude of mass
fractionation in oxygen cannot be determined, except by Rayleigh fractionation with an
assumed atmospheric composition. The range of mass fractionation in I-type spherules is
nevertheless consistent with previous studies. These spherules have experienced a similar

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range of evaporation, albeit greater than the spherules observed by Pack et al. (2017) and less
than some of those analysed by Engrand et al. (2005).

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At altitudes >100 km in the thermosphere, increases in δ¹⁷,¹⁸O of spherules could be
caused by dissociation and recombination rates of oxygen molecule resulting in enrichment
of heavier isotopes by **mass independent fractionation** (Colegrove et al. 1965; Thiemens et
al., 1995). The I-type spherules in this study **have oxygen isotope compositions** within
uncertainty of the TFL; hence the average atmospheric oxygen have acquired Δ¹⁷O = 0.3 ‰
within the uncertainty of ~0.6‰ (2 standard deviation). We can, therefore, safely say that if
the thermosphere has a Δ¹⁷O value greater than 1‰ of the troposphere, then only a small
proportion of this oxygen is incorporated into I-type spherules and **may be** further diluted by
oxygen acquired at peak deceleration at lower altitudes in the mesosphere. Higher precision
oxygen isotope compositions such as reported by Pack et al. (2017) might be capable of
resolving Δ¹⁷O differences arising from capture of thermospheric gas with oxygen isotope
compositions that diverge from the TFL. However, these high precision data reported no
thermospheric oxygen with higher Δ¹⁷O involved in the oxidation of the I-type cosmic
spherules. We would predict that those spherules with the lowest entry angles are most likely
to exhibit such divergences since at any particular size these undergo more of their
deceleration at higher altitude. These spherules are **likely** to exhibit the lowest δ¹⁸O values for
their size since low angle particles have lower peak temperatures and thus **experience** less
evaporation and mass fractionation.

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## 5.2. Oxygen isotope of S-type cosmic spherules

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The oxygen isotope compositions of S-type **cosmic spherules** and scoriaceous
**micrometeorites** differ from those of I-type spherules since oxygen is present as a significant

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component of their pre-atmospheric precursors. Silicate **micrometeorites**, therefore, inherit the isotopic composition of their precursor which is then modified during atmospheric entry by exchange with atmospheric oxygen and mass-fractionation owing to partial evaporation on heating. These two processes cause oxygen isotope compositions to evolve in different ways on an oxygen three isotope plot. Mass-fractionation owing to preferential loss of light oxygen during partial evaporation causes changes in composition according to relative mass of the isotope. On a three isotope plot, therefore, oxygen isotope compositions **under a mass-dependent regime** evolve parallel to the TFL along a slope 0.52 line towards $\delta^{18}$O-rich values (Figs. 3 and 4). In contrast exchange with air causes isotope compositions to converge towards the oxygen isotope composition of the atmosphere resulting in $\Delta^{17}$O values **approaching** zero whilst the $\delta^{18}$O value trends towards that of the air. These two processes have been previously discussed by Sauvet et al. (2010), Cordier et al (2011) and Van Ginneken et al. (2017) and are superimposed on the isotopic variability of the original precursors. (Fig. 9).

Two broad genetic groups of S-type cosmic spherule can be recognised by their $\Delta^{17}$O values within the analysed particles. Porphyritic spherules have higher average $\Delta^{17}$O values scattered at small values ($\Delta^{17}$O= ~ $\pm$2‰) above and **similarly for** below the TFL, whilst barred and cryptocrystalline spherules have lower average $\Delta^{17}$O values that fall mainly below the TFL. Porphyritic spherules have $\Delta^{17}$O similar to Ordinary, Enstatite, R and CI chondrites, whilst barred and cryptocrystalline spherules have lower values consistent with CM, CR, CV and CO chondrites (Fig. 9). Although broadly consistent with these meteorite groups, the isotope compositions of porphyritic, barred and cryptocrystalline spherules significantly overlap. The small number of barred and cryptocrystalline spherules (~10%) with the highest
389 $\Delta^{17}$O values, however, are consistent with evolution from CI and ordinary chondrite-like materials, whilst those porphyritic spherules with $\Delta^{17}$O < -0.5‰ is consistent with CM, CR and perhaps CV, CK sources, particularly if their $\Delta^{17}$O has evolved towards zero as a result of exchange with air. Eighteen percent of spherules from porphyritic, barred, cryptocrystalline, glass, scoriaceous micrometeorites have $\Delta^{17}$O > -0.5‰ and $\delta^{18}$O less than CI chondrite and can only be derived from precursors with oxygen isotopes similar to Ordinary or Enstatite chondrites as previously suggested by Cordier et al. (2011). Glassy spherules analysed in this study have a broader range of $\Delta^{17}$O than barred, cryptocrystalline and porphyritic spherules, which nearly matches the range of all three groups suggesting these particles can be derived from any of the chondrite groups.

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400 Previous studies on coarse-grained porphyritic spherules have indicated that they are derived from fragments of chondrules, principally from ordinary chondrites, whilst fine-grained porphyritic spherules can be correlated with fine-grained matrices having affinities to CM and CR chondrites (Van Ginneken et al., 2017). In addition, the presence of relict grains, major and minor elements, and oxygen isotope studies support these conclusions (e.g. Cordier et al., 2011, Rudraswami et al., 2015; Van Ginnken et al., 2017). Our data is consistent with suggestion that barred and porphyritic spherules are mostly genetically different particles, which are predominantly derived from coarse-grained, chondrule-derived materials, whilst barred spherules are principally derived from fine-grained materials such as chondrite matrix.

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411 Van Ginneken et al (2017) found one cryptocrystalline spherule $\Delta^{17}$O < -2‰ (Group 2 of Van Ginneken et al., (2017)), whilst 6 particles had $\Delta^{17}$O > -0.5‰ (Group 3 and 4). The

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current study reveals that the oxygen isotopes of cryptocrystalline spherules cover a wide

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range rather **than previous observed with** the majority (~70%) having $\Delta^{17}O < -0.5‰$. **These**

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**are** thus similar to barred olivine spherules and derived from CR, CM, CO or CV chondrites

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(Fig. 5 and 9). This implies that cryptocrystalline spherules **are formed** by increased heating

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of **precursors similar to those of** barred olivine spherules, a suggestion consistent with the

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higher average $\delta^{18}O$ of cryptocrystalline particles observed in the current study. Likewise, the

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broad range of $\Delta^{17}O$ in glassy spherules and their higher average $\delta^{18}O$ is consistent with their

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formation by increased heating of **precursors similar to both** porphyritic olivine and

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cryptocrystalline spherules.

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The **increase** in $\delta^{18}O$ in S-type cosmic spherules, relative to their precursors,

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resulting from entry heating of S-type cannot be directly related to the degree of heating

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and evaporation **without knowledge of the degree of atmospheric mixing and the**

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composition of the precursor., Alexander et al. (2002), **however, have** shown that **cosmic**

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spherules that suffered the highest peak temperatures also **exhibit** the largest increase in $\delta^{18}O$

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values, **implying that evaporation dominates $\delta^{18}O$ evolution.** Exchange with air makes it

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more challenging to assess $\delta^{18}O$ in terms of heating since it will also cause increases in $\delta^{18}O$

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for particles with values less than that of air ($\delta^{18}O$= 23.5‰; Thiemens et al., 1995). Exchange

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with air means the $\delta^{18}O$ cannot be directly related to heating alone. Whether $\delta^{18}O$ increase is

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largely the result of exchange of atmospheric oxygen in silicate melt or mass fractionation by

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evaporation depends on which of these two processes dominate.

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The average $\delta^{18}O$ observed in the current study increases in the series porphyritic<

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barred< cryptocrystalline< glassy< CAT spherules. The textures of this series of spherules

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has been interpreted as representing increasing peak temperature, **with texture controlled by**
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crystallisation under increasing super-cooling due to progressive destruction of crystallisation
nuclei with increasing temperature (Taylor et al., 1991; Genge et al. 2008). The increase in
average δ¹⁸O through this series is, therefore, evidence that mass fractionation owing to
heating dominates over exchange with the atmosphere. The approximately equal maximum
and minimum values of Δ¹⁷O with δ¹⁸O, shown by the band of data points in a three isotope
plot around TFL (Fig. 5), also testifies to the dominance of mass fractionation. If exchange
with air was dominant, in contrast, Δ¹⁷O would become less with increasing δ¹⁸O values up
until those of air.

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**The** extreme heating of the calcium aluminium titanium-rich (CAT) spherules
enriched in refractory elements such as Ca, Al, and Ti makes **their** oxygen isotope
composition very interesting (Taylor et al., 2000; Genge et al., 2008). The oxygen isotope
composition of δ¹⁷,¹⁸O is much **heavier in these particles** than any other spherules indicating
not only atmospheric exchange but also coupled evaporation during entry. CAT spherules
with the highest average δ¹⁸O values of the analysed particles are consistent with the extreme
partial evaporation implied by their refractory compositions (Taylor et al., 2000).

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Previous bulk oxygen isotope measurements of high Ca and Al spherules made by
Cordier et al. (2011) have shown significantly lower δ¹⁸O values **than observed here**, having
**a** range of ~15 to 19‰, **with only one having a value of ~28‰**. It is possible **the** spherules
**analysed by Cordier et al., (2011) are fragments of** Ca-Al-rich precursors, rather than **the**
**refractory products of** extreme evaporation. **One** CAT spherule (MY240184) analysed by
Yada et al. (2005) **yielded a** high δ¹⁸O value of ~93‰. This value is much higher than our
highest δ¹⁸O value of ~71‰, and our lowest value is also higher than given by Cordier et al.
(2011). The extremely high value of δ¹⁸O indicates excessive heating and evaporation for an

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extended period during atmospheric entry, which results in **evaporative loss of moderately volatile elements**, such as Fe, followed by Si and Mg, **and a corresponding enrichment of** the remaining Ca, Al and Ti (Wang et al., 2001, Yada et al., 2005). That one of the CAT spherules (SP007-P236) has the highest δ¹⁸O value (~71‰) of any of the S-type spherules **in our study** supports the above assertion.

All CAT spherules have small Δ¹⁷O values close to the TFL largely similar to porphyritic spherules, suggesting their precursors might be derived from a similar source (Fig. 5). That all the CAT spherules are derived from the same source is surprising since extreme partial evaporation **might** be expected to largely source independent, if most likely for those sources with higher average entry velocities, and thus eccentricities. **Nevertheless, Cordier and Folco (2014) analyzed several CAT spherules with most of them have oxygen isotopic composition below TFL line, although as suggested above these may have been derived from refractory precursors and thus different to typical CAT spherules.** The lack of isotopic diversity in CAT spherules **observed in this study** could, however, be due to the low number of particles and poor sampling statistics **compared to other types of cosmic spherules (Fig. 9).** At present, it seems that there are few CAT spherules above and some below TFL, indicating two different populations that do not overlap each other. **Based on the above/below TFL dichotomy it is likely that CAT spherules can originate from both CC and OC bodies, and can be formed either by extreme evaporation of chondritic precursors, or by melting of refractory precursors, presumably including fragments of CAIs.**

The present study of scoriaceous, porphyritic, barred, cryptocrystalline, glass, and CAT spherules indicate that S-type spherules in this size range (most have diameters of
60–300 µm) have sources dominated (~80%) by carbonaceous chondrites, with the remainder derived from sources similar to ordinary chondrites. Experimental oxygen isotope analyses of different type of S-type **cosmic spherules** has demonstrated an increase in δ¹⁷,¹⁸O due to alteration during atmospheric entry (Fig. 3). The alteration in barred, cryptocrystalline, glass spherules are similar as they fall on TF line. Some of the porphyritic spherules seem to be **preserved δ¹⁷,¹⁸O and** does seem to alter significantly like glass, barred, cryptocrystalline spherules. This is also observed in previous studies that have shown some data on CCAM line (Rudraswami et al., 2015, 2016). Among the S-type **cosmic spherules**, the scoriaceous spherules **appear to have been the least affected by atmospheric entry since their** oxygen isotope fall **mainly** on CCAM line (Fig. 4). The bulk of the data from scoriaceous and porphyritic have Δ¹⁷O <0‰ similar to those seen in carbonaceous chondrites (Fig. 5), **although with significant numbers of porphyritic spherules having higher values**. Similarly barred, cryptocrystalline, and G-type **mostly have lower Δ¹⁷O than the TFL and are likewise related to carbonaceous chondrite. By comparison glassy spherules have a range of compositions.** The data suggest that Δ¹⁷O does not alter much irrespective of altitude, but variation in δ¹⁷,¹⁸O is visible. Although the Δ¹⁷O tends to increase with increase in δ¹⁷,¹⁸O values (Rudraswami et al., 2016), most of the partially altered scoriaceous particles show Δ¹⁷O <0‰, **suggesting** a carbonaceous chondrite origin as seen in Figs. 4 and 5, **rather than as a consequence of heating.** The origin of these particles from precursors consisting of hydrated fine-grained matrix is supported by their textures and mineralogy (Genge et al., 2008; Taylor et al., 2012; Badyukov et al., 2018; Suttle et al., 2019) and the presence of relict cores of matrix (Genge, 2006) and fragmented olivine within some particles (Genge et al., 2017a).

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Finally, no significant difference was noted between the isotopic compositions of S-
type spherules with and without metal-beads. The lack of discrete compositions for spherules
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with FeNi metal beads suggests that these can be present within spherules derived from any
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chondritic source, in contrast to the suggestion of Cordier et al., (2011b **based on** the Ni-
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contents of olivines that **argued** metal separation is most common for those particles derived
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from carbonaceous chondrite-like sources. The relatively small number of these particles
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analysed, however, may not sufficiently reveal whether metal beads are more common from
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one source or another. The separation of metal owing to deceleration during flight, or the lack
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of exposure of metal beads on the plane of section, are likely to make this comparison less
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than rigorous.
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Some the particles that have metal beads need to be looked for source related
variation, such as, whether they are abundant in CC-derived (below TFL), OC-derived
(above TFL), or both. Also, it will be remarkable to see if the metal beads more or less
abundant in δ¹⁸O-rich particles that have experienced more intense heating and
probably higher decelerations. The presence of metal beads is related to low δ¹⁸O in I-type
spherule compared to those with magnetite rich I-type particles demonstrating the mass-
dependent fractionation by evaporation for particles during atmospheric entry (Engrand et al.,
2005). Metal beads are present in all types of spherules in small number, except scoriaceous
micrometeorites, and rarely in porphyritic spherules (Rudraswami et al., 2014). The oxygen
isotope data on barred spherules with **at least one** metal bead has shown low δ¹⁸O values of
~12–16‰, except MS-I3-P21 that has ~23‰ probably due to larger size resulting in
enrichment of heavier oxygen isotope during entry. This variation in oxygen isotope
composition in barred spherules is similar to the I-type spherule correlation as suggested by
Engrand et al. (2005). The Ni content in MS-I3-P21 is less (~19 wt%) compared to other

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three barred spherules (23–30 wt%) indicating no relation as far as barred spherules are
considered, however, we cannot draw a statistically significant conclusion with few particles
(Fig. 7).

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Glass spherules have undergone more heating than barred spherules and often have
metal beads. The δ¹⁸O values in metal-bearing glass spherules have shown scatter with the
increase in Ni content in metal bead (Fig. 7). The Fe is more **volatile** than Ni and having
large presence of Ni in metal bead, and those spherules showing low δ¹⁸O is surprising and
has to do primarily with precursor properties and entry parameters. However, if the oxygen
isotope composition of precursor were similar, then Fe would have got evaporated
proportionately showing relation to δ¹⁸O. MS-I35-P39 is the only glass spherule where it has
shown high Ni content of ~64 wt%; one of the reasons is a different type of precursors that is
contributed and also the entry parameter as indicated earlier. (Fig. 7). We **have measured a
single** G-type spherule with bead that has very low nickel content (~10 wt%) and δ¹⁸O value
of ~28‰ which fit in range of bead bearing glass spherules. The metal bead in porphyritic
spherule is strange, and SP005-P1078 has two metal bead where its average composition has
shown presence of sulfur (~17 wt%) along with Fe and Ni (Appendix A) indicating that these
metals are not due to entry phenomena but were existing in its precursor, **and most likely
have got altered due to oxidation during entry** (Fig. 2a). The δ¹⁸O composition of ~32‰ is
much larger and comparable to other spherules that have shown some sulfur composition in
glass spherules (SP005-P157, SP005-P1071, SP005-P34, SP005-P294, and SP005-P296).
The lone cryptocrystalline spherule also has low δ¹⁸O values compared to high Ni content of
~64 wt%. Four particles, namely two from glass, one from CAT and one from I-type, has
shown metal bead escaped from the spherules (Fig. 2). The δ¹⁸O of the **spherules that have
escaped segregated** metal bead is ~35–40‰, except MS-I3-P55 with low values. However,

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there are exception and the general trend favours increase in δ¹⁸O values with the high Ni content (Fig. 7). **This is due to heating and melting of the particles that lead to evaporation thereby increasing the δ¹⁸O, and oxidation resulting in increase of Ni in the residual metal.**

No systematic variation of δ¹⁸O of scoriaceous **micrometeorites** with size is observed, but for porphyritic spherules there is slight increase in the maximum and minimum δ¹⁸O with increasing size of particles, albeit with significant scatter (Fig. 8). There is no clear pattern when it comes to G-type particles. Nevertheless, the particle size has a greater role in elemental loss, **owing to increased heating**, and also changes the isotopic pattern as it enters the Earth's atmosphere (Rudraswami et al., 2015, 2016). It is likely that the lack of well-defined correlations between size and oxygen isotope composition for S-type particles, in contrast to I-types, relates to isotopic variability in the precursors that will partly obscure any systematic variations with entry parameters. **Additionally, carbonaceous chondrite precursors are more abundant for smaller size cosmic spherules, with ordinary chondrites contributing more at larger sizes (e.g. Cordier and Folco, 2014). Changes in the sources of cosmic spherules with size complicate evaluating changes in isotopic composition owing to heating since both contribute to final distributions of isotopic composition.**

## 5.3. Oxygen isotope of G-type Cosmic Spherules

G-type particles have unique textures largely dominated by magnetite dendrites within a silicate glass mesostasis that are intermediate between S- and I-type **cosmic spherules** (Taylor et al., 1991; Genge et al., 2008). The oxygen isotope composition of G-types are
similar to barred, cryptocrystalline and glass **cosmic spherules**, and very different from I-
types having low δ¹⁸O values and a wider range of Δ¹⁷O. The intermediate chemical nature of
G-types would imply that their precursors have abundant FeNi metal with sub-ordinate
silicate. The low δ¹⁸O values of most G-types, however, it is not consistent with the
significant accretion of atmospheric oxygen and large mass-fractionation as observed for I-
types. Furthermore, the δ¹⁸O range of G-types is smaller than porphyritic, barred, and
cryptocrystalline spherules suggesting less significant mass-fractionation.

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The isotopic compositions of G-types are not what would be expected for dense,
metal-dominated particles. Some of the observed G-types have compositions close to the
CCAM line and thus their precursors are likely to be similar to that observed for porphyritic
spherules. Compositions close to the likely precursors suggests minimal mass fractionation
and thus partial evaporation of silicates. One possible explanation for the formation of these
particles might be if they were mixtures of iron-sulphide and silicates, rather than metal. The
low melting temperatures of sulphides may allow spherule formation at a temperature low
enough to prevent significant partial evaporation of silicates, precluding significant mass
fractionation. Sulphides would react with atmospheric oxygen removing sulphur as SO₂ and
leaving iron-rich silicate. A similar explanation was proposed earlier for the lack of iron-
sulphides in I-type **cosmic spherules**, with vesicles present in metal beads, and extrusion of
metal into the surrounding oxide mantle as evidence for oxidative loss of sulphides (Genge et
al., 2017). The lack of abundant vesicles in G-types, however, isn't entirely compatible with
this interpretation and thus their origins remain enigmatic.

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5.4. Implications

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This study has shown that the analysis of the oxygen isotope compositions of large numbers of spherules is required to enable the interpretation of their sources and the processes affecting them during atmospheric entry. In particular comparisons of the isotope systematics correlated with textural type allows the genetic relationships between different types of particles to be determined. High precision studies of smaller numbers of large particles such as Cordier et al. (2011) and Van Ginneken et al (2017) successfully showed that both mass-fractionation by partial evaporation and exchange with air significantly modify the precursor compositions of **cosmic spherules** and introduce an inherent uncertainty in the parent body affinity. Cordier et al. (2011) established that those spherules with $\Delta^{17}\text{O}$ values >-0.5 ‰ and $\delta^{18}\text{O}$ values less than CI chondrites are related to the Ordinary, Enstatite or R chondrites. Van Ginneken et al. (2017) showed that barred and porphyritic spherules have sufficiently different $\Delta^{17}\text{O}$ values to necessitate various broad sources. Whilst high precision oxygen isotope studies, provide insights into the relationships between small numbers of large particles, ion microprobe analyses allow comparison of larger populations of particles, including smaller particles down to diameters of ~60 µm providing complementary information. In this study, the large number of particles analysed enables the majority of cryptocrystalline particles to be related to the barred olivine spherules through increased heating, whilst glass spherules can be shown to be derived by extensive superheating and nuclei destruction from similar precursors to porphyritic, barred and cryptocrystalline spherules.

The complexity of oxygen isotope evolution during entry heating is a consequence of the simultaneous operation of both mass fractionation and exchange with air. These two processes, however, are likely to be related by the entry parameters of **micrometeorites** (i.e. entry angle and entry velocity) as suggested by Sauvet et al (2010; 2011). Mass fractionation
by evaporation is mostly dependent on peak temperature and the duration of the heating pulse. Peak temperature increases with entry velocity or with entry angle with high velocity, high entry angle particles experiencing larger peak temperatures and evaporation. The duration of the heating pulse, conversely increases with decreasing entry angle. In two particles experiencing the same peak temperature, for example, the particle with the lower entry angle (which would necessarily have a higher entry velocity to attain the same peak temperature) would experience a larger degree of partial evaporation owing to the longer time spent at high temperature. The time spent above the solidus has been shown to increase from 1-2 s at steep entry angles to >12 s at shallow entry angles (e.g. Genge, 2016, 2017). Other properties of particles might, however, influence mass fractionation of oxygen. Volatile- or moderately volatile-rich precursors may undergo more significant partial evaporation than those particles with refractory starting compositions, increasing the mass fractionation of oxygen. The degree of mass-fractionation may, therefore, in part be source dependent, albeit with significant variation related to the different components present, such as fine-grained, volatile-rich matrix, chondrules and calcium-aluminium-inclusions.

The exchange of air with **cosmic spherules** during atmospheric entry may also depend on entry parameters to some extent. At high entry velocity, direct implantation of oxygen into the surface layers of particles is possible since incident energies exceed those of covalent bonding for silicates. At lower velocities the exchange with air is likely to be dominated by chemical reactions, with iron-dominated metal rapidly oxidising by reaction with atmospheric oxygen increasing the degree of exchange. The original oxidation state of the precursor may also be a factor with oxidised materials such as oxide-bearing type II chondrule fragments reacting less with atmospheric oxygen than reduced type I chondrules. Furthermore, reactions that incorporate some of the incident atmospheric oxygen into a gas
phase may reduce exchange with air by loss of the gas. The reaction of sulphides to produce SO₂, for example, is likely to minimize the atmospheric oxygen retained by **cosmic spherules** and might in part explain the low δ¹⁸O observed within G-type spherules in this study.

The oxygen isotope compositions of I-type cosmic spherules have been determined by several studies (e.g. Engrand et al., 2005; Pack et al., 2017) and have been suggested to be a proxy for mesospheric atmospheric compositions. In these studies, however, the Δ¹⁷O of particles is usually sufficiently close to TFL to be consistent with an atmospheric composition lacking significant non-mass fractionation effects. An independent means of identifying the degree of evaporation, such as δ⁵⁶Fe, is required to use the oxygen isotope compositions as a proxy for those of the atmosphere. Differences in the degree of evaporation suggested by δ⁵⁶Fe and δ⁶⁰⁻⁶⁴Ni, however, question the validity of this method (e.g. Engrand et al., 2005).

Given the expectation that non-mass fractionation effects should become important in the thermosphere, it is expected that spherules that undergo deceleration at the highest altitudes are most likely to show anomalous oxygen that does not fall on the TFL. Particles with the lowest entry angles are most likely to exhibit this behaviour and maybe only a very small proportion of I-types – emphasising the importance of studies of large numbers of particles. Since deceleration altitude is related to particle size, as well as entry velocity, small I-types are most likely to sample thermospheric atmosphere. These particles, which will prove invaluable in probing isotopic composition of the current and past atmosphere of our planet, are currently challenging to analyse at sufficient precision to provide rigorous results.
687 **6. CONCLUSIONS**

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The oxygen isotope compositions of cosmic spherules are a complex signatures of parent body along with atmospheric entry processes arising due to both mass fractionation and isotopic exchange with terrestrial oxygen. We performed 277 in situ oxygen isotope analyses using ion microprobe in 137 micrometeorites collected from deep-sea and Antarctica along with many spherules that have metal bead to understand the isotopic pattern. There is an increase in average δ¹⁸O from scoriaceous< porphyritic< barred< cryptocrystalline< glass< CAT which is clearly related to mass-fractionation and evaporation, and thus peak temperature the particle has acquired during atmospheric entry. It also suggests that mass-fractionation due to heating dominates over exchange with air for S-types. Porphyritic spherules have small Δ¹⁷O scattered around the TFL, barred and cryptocrystalline spherules fall mainly below the TFL confirming that porphyritic are primarily derived from chondrule fragments, many from OCs, whilst barred and cryptocrystalline are from carbonaceous chondrite matrix. Cryptocrystalline spherules have mostly negative Δ¹⁷O similar to barred olivine spherules suggesting they form by further melting at higher peak temperature of these particles. It suggests these too are mainly derived from fine-grained carbonaceous chondrite matrix. Glass spherules have Δ¹⁷O with a range similar (but not quite as large) as porphyritic, barred and cryptocrystalline spherules together. This suggests these form by extensive melting of these types. CAT spherules have small Δ¹⁷O scattered around TFL and making close associates of porphyritic spherules. G-types **that are** expected to have significant exchange of atmospheric oxygen and mass-fractionation at high temperatures; however, their small range of δ¹⁸O suggests the opposite. The G-type with unique dendrites of magnetite and silicate appears to be dominated by carbonaceous chondritic precursors. These show the genetic relationships with precursor materials very well and allow some
712 quantitative estimates of precursor affinities. The progressive heating of different S-type

713 spherules with bead and non-bead do have some correlation with oxygen isotope enrichment

714 of $\delta^{18}$O.

715
716 ACKNOWLEDGMENT

This research work is supported by GEOSINKS, MOES-PMN, and the PLANEX
717 project. The sample collection from the South Pole water well was funded by National
718 Science Foundation, while from Maitri station and deep-sea sediments collection is funded by
719 MOES-NCAOR and MOES-PMN, respectively. NGR acknowledges the support of CSIR-
720 Raman research fellowship to CRPG-CNRS, Vandoeuvre-les-Nancy, France. We are
721 **thankful for the constructive reviews by M. D. Suttle and one anonymous reviewer.** This
722 is NIO's contribution No. xxxx.
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724

725 APPENDIX A. SUPPLEMENTARY DATA

726 Supplementary data related to this article is provided in the electronic annex.

727
728 REFERENCES

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870

871
872 **Figure Captions**

873

874 **Figure 1.**

875 The representative back-scattered electron image of different type of **cosmic spherules**
876 selected for the oxygen isotope studies: a) scoriaceous, b) porphyritic, c) barred, d)
877 cryptocrystalline, e) glass, f and g) CAT, along with h) I-type and i) G-type. **The back-**
878 **scattered images scale bars indicates 50 µm.** The detail bulk chemical compositions of the
879 particles are given in Appendix A.

880

881 **Figure 2.**

882 The backscattered electron image of polished sections of the **cosmic spherules** with metal
883 bead where oxygen isotope analyses were performed on silicate. (a) SP005-P1078 is one
884 porphyritic spherules that have two metal **beads**. (b) MS-I3-P59, (c) MS-I3-P39, and (d)
885 SP005-P10 are glass spherules with the metal bead. (e) SP007-P4, (f) SP005-P1176 and (i)
886 SP005-P207 is barred, cryptocrystalline and G-type with metal bead. (g) SP007-P255 and (h)
887 AAS-62-61-P87 are CAT and I-type spherules have metal bead **void formed either by the**
888 **dissolution of the metal bead whilst on the seafloor or by plucking during the polishing**
889 **process.** The back-scattered images scale bars indicates 50 µm.

890

891 **Figure 3.**

892 The oxygen isotope compositions of **cosmic spherules**: a) porphyritic, b) barred, c)
893 cryptocrystalline, d) glass, e) I-type and f) G-type obtained using ion microprobe. The oxygen
894 isotope data are given in Appendix A. The solid lines represents the terrestrial fractionation
895 (TF) and the carbonaceous chondrite anhydrous mineral (CCAM) line with slope~0.52
896

and~0.94, respectively (Clayton et al., 1977; Clayton and Mayeda, 1999). All error bars

897

shown are smaller than the size of symbol.

898

899

**Figure 4.**

900

The oxygen isotope compositions of least altered and most altered **cosmic spherules**,

901

scoriaceous and CAT spherules analyzed using ion microprobe. The oxygen isotope data are

902

given in Appendix A. The solid lines represent TF and CCAM line with slope~0.52

903

and~0.94, respectively (Clayton et al., 1977; Clayton and Mayeda, 1999). All error bars are

904

small than the size of the symbol.

905

906

**Figure 5.**

907

Plot of Δ¹⁷O for different types of spherules analysed in this study. Most of the data are below

908

the TF line suggesting **cosmic spherules** are contributed by carbonaceous chondrites.

909

Conversely, I-type spherules are near the TF line. **The number at the corner of each plot is**

910

**the average value of the Δ¹⁷O along with error.** All error bars shown are 2σ.

911

912

**Figure 6.**

913

(a) The plot of δ¹⁸O vs. magnetite rim thickness of I-type spherules appears to increase with

914

δ¹⁸O values. (b) The plot of δ¹⁸O versus the diameter of I-type spherules with sizes less than

915

300 µm from this study. Results of I-type spherules data for both magnetite thickness and

916

diameter from Engrand et al. (2005) have been included in the plot for comparison. The plots

917

indicate that with increase in δ¹⁸O values the magnetite rim and diameter of spherule **range**

918

also increases. The circle marked in b is the data seems **anomalous and is** slightly far from

919

the trend.
920

921

**Figure 7.**

922

923

924

925

926

927

928

The variation of Ni (wt%) in metal bead and δ¹⁸O from different types (porphyritic, barred, cryptocrystalline, glass, G-type) of spherules. Glass spherule has good statistics where we can suggest increase trend in Ni content with respect to δ¹⁸O. Other spherules do not have good statistical representation. Few particles excluded from this plot are MS-I3-P55 (glass), MS-I35-P22 (glass), SP007-P255 (CAT) and AAS-62-61-P87 (I-type). They have shown void space due to removal of metal bead **either by the dissolution of the metal bead during its residence time on seafloor, during sample polishing, or** during atmospheric entry.

929

930

**Figure 8.**

931

932

933

934

The behaviour of δ¹⁸O from different types (porphyritic, barred, cryptocrystalline, glass, G-type) of spherules with respect to the diameter of the spherules. There seems to overall increase in δ¹⁸O values with the increase in diameter but not uniform probably due to complexity involved in entry parameter and precursor original oxygen isotope composition.

935

936

**Figure 9.**

937

938

939

940

941

942

943

Compiled oxygen isotopic compositions Δ¹⁷O vs. δ¹⁸O values of different types of **cosmic spherules** analyzed in this study using ion microprobe. The range of oxygen isotope composition of various chondritic parent bodies (such as CI, CM, CV, CO, CK, CR, H, L, LL, EC, and R) are illustrated (Clayton et al., 1991; Clayton and Mayeda, 1999). The shaded region is the value of the isotopic composition of the micrometeorite from the various precursor. The different group 1, 2, 3, 4 with a diverse isotopic range of Δ¹⁷O and δ¹⁸O is earlier defined by Suavet et al. (2010).

944