# Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa

Jörg Fritz, Roald Tagle, Luisa Ashworth, Ralf Thomas Schmitt, Axel Hofmann, Béatrice Luais, Phillip Harris, Desirée Hoehnel, Seda Özdemir, Tanja Mohr-Westheide, et al.

► To cite this version:

Jörg Fritz, Roald Tagle, Luisa Ashworth, Ralf Thomas Schmitt, Axel Hofmann, et al. Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa. Meteoritics and Planetary Science, 2016, 51 (12), pp.2441-2458. 10.1111/maps.12736. hal-01772415

HAL Id: hal-01772415

https://hal.univ-lorraine.fr/hal-01772415v1

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Distributed under a Creative Commons Attribution 4.0 International License
Nondestructive spectroscopic and petrochemical investigations of Paleoarchean
spherule layers from the ICDP drill core BARB5, Barberton Mountain Land,
South Africa

Jörg FRITZ<fnref n="1" />,<fnref n="2" />,*, Roald TAGLE<fnref n="3" />, Luisa ASHWORTH<fnref n="4" />, Ralf Thomas SCHMITT<fnref n="2" />, Axel HOFMANN<fnref n="5" />,
Béatrice LUAIS<fnref n="6" />, Phillip D. HARRIS<fnref n="4" />, Desirée HOEHNEL<fnref n="2" />,<fnref n="7" />, Seda ÖZDEMIR<fnref n="8" />,
Tanja MOHR-WESTHEIDE<fnref n="2" />,<fnref n="9" />, and Christian KOEBERL<fnref n="8" />,<fnref n="10" />

*(Received 29 May 2015; revision accepted 12 August 2016)*

**Abstract**—A Paleoarchean impact spherule-bearing interval of the 763 m long International Continental Scientific Drilling Program (ICDP) drill core BARB5 from the lower Mapepe Formation of the Fig Tree Group, Barberton Mountain Land (South Africa) was investigated using nondestructive analytical techniques. The results of visual observation, infrared (IR) spectroscopic imaging, and micro-X-ray fluorescence (μXRF) of drill cores are presented. Petrographic and sedimentary features, as well as major and trace element compositions of lithologies from the micrometer to kilometer-scale, assisted in the localization and characterization of eight spherule-bearing intervals between 512.6 and 510.5 m depth. The spherule layers occur in a strongly deformed section between 517 and 503 m, and the rocks in the core above and below are clearly less disturbed. The μXRF element maps show that spherule layers have similar petrographic and geochemical characteristics but differences in (1) sorting of two types of spherules and (2) occurrence of primary minerals (Ni-Cr spinel and zircon). We favor a single impact scenario followed by postimpact reworking, and subsequent alteration. The spherule layers are Al₂O₃-rich and can be distinguished from the Al₂O₃-poor marine sediments by distinct Al-OH absorption features in the short wave infrared (SWIR) region of the electromagnetic spectrum. Infrared images can cover tens to hundreds of square meters of lithologies and, thus, may be used to search for Al-OH-rich spherule layers in Al₂O₃-poor sediments, such as Eoarchean metasediments, where the textural characteristics of the spherule layers are obscured by metamorphism.

INTRODUCTION

The Earth-Moon system experienced episodes of
heavy bombardment by fragments of planetary bodies

including asteroids and comets during its first ~1.5 Ga
(Koeberl 2006; Fernandes et al. 2013; Fritz et al. 2014).
To date, the most precise data pertaining to the
intensity of the impact flux on the Earth-Moon system

<footnote n="1">¹Saalbau Weltraum Projekt, Wilhelmstrasse 38, 64646 Heppenheim, Germany</footnote>

<footnote n="2">²Museum für Naturkunde—Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany</footnote>

<footnote n="3">³Bruker-Nano GmbH, Am Studio 2D, 12489 Berlin, Germany</footnote>

<footnote n="4">⁴GeoSpectral Imaging, Office E6 Block E, Somerset Office Estate, 604 Kudu Street, Allens Nek, 1737 Johannesburg, South Africa</footnote>

<footnote n="5">⁵Department of Geology, University of Johannesburg, PO Box 524, Auckland Park, 2006 Johannesburg, South Africa</footnote>

<footnote n="6">⁶Centre de Recherches Pétrographiques et Géochimiques, CRPG UMR 7358 CNRS-UL, 15 rue Notre Dame des Pauvres, 54500 Vandœuvre les Nancy, France</footnote>

<footnote n="7">⁷Institut für Erd und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany</footnote>

<footnote n="8">⁸Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria</footnote>

<footnote n="9">⁹Institut für Geologische Wissenschaften, Freie Universität Berlin (FU Berlin), Malteserstrasse 74-100, D-12249 Berlin, Germany</footnote>

<footnote n="10">¹⁰Natural History Museum, Burgring 7, 1010 Vienna, Austria</footnote>

<footnote>*Corresponding author. E-mail: joerg.fritz@kino-heppenheim.de</footnote>
(bombardment timeline) has been determined for the lunar Imbrian System, particularly for the Lower Imbrian epoch (~3.7–3.2 Ga): various volcanic surfaces of well-defined radiometric ages and crater densities provide detailed information (Wilhelms 1987; Neukum et al. 2001). Following the Orientale impact, the Lower Imbrian crater record on the Moon includes three impact craters larger than 150 km diameter, the Tsiolkovskiy (180 km Ø), Humboldt (207 km Ø), and Iridum (260 km Ø) craters. The global lunar impact record, together with a Moon/Earth impact rate of ~1/20 (Öpik 1960) implies that an intense bombardment of Earth must have occurred during the Eo- and Paleozoarchean (the latter ranging from 3.6 to 3.2 Ga; Gradstein et al. 2004).

On Earth traces of large impact events as old as 3.47 Ga have been preserved in impact spherule-bearing layers within Paleozoarchean sedimentary rocks of the Kaapvaal (South Africa) and the Pilbara (Western Australia) cratons (e.g., Lowe and Byerly 1986; Kyte et al. 1992; Koeberl and Reimold 1995; Reimold et al. 2000; Shukolyukov et al. 2000; Simonson and Harnik 2000; Simonson and Glass 2004; Hofmann et al. 2006; Glass and Simonson 2013; Lowe et al. 2014). Up to eight different Paleozoarchean spherule layers have been reported from stratigraphically different positions in the Barberton Greenstone Belt of the Kaapvaal craton (Lowe et al. 2003, 2014). Spherule layers from Paleozoarchean sedimentary successions on Earth allow the study of impacts on the early Earth, and the nature of some of the asteroids or comets that were on Earth-crossing orbits during that distant time.

Fresh samples of Paleozoarchean spherule layers in the Barberton Mountain Land were obtained through the International Continental Scientific Drilling Program (ICDP); five drill cores denoted BARB1 to BARB5 were retrieved from the central part of the Barberton Mountain Land during the “Barberton Drilling Project: Peering into the Cradle of Life” campaign (Arndt et al. 2012, 2013). The BARB5 core includes several intersections of Paleozoarchean spherule layers interbedded with Fig Tree Group metasediments that were deposited in a marine setting well below the storm-weather wave base (Lowe and Nocita 1999). The present contribution investigates the spherule layers by means of noninvasive methods including optical inspection, infrared imaging, and spatially resolved micro-X-ray fluorescence (μXRF) spectrometry.

## Noninvasive Analytics

Infrared imaging of drill cores is a nondestructive technique whereby core trays and/or samples are imaged by cameras measuring across the infrared range

of the electromagnetic spectrum. This is done for mineral identification, mapping, and the monitoring of mineral composition (e.g., Thompson et al. 1999; Harraden et al. 2013; Tappert et al. 2013). It is based on the principle that certain wavelengths of light are absorbed by minerals because of electronic and vibrational processes occurring within them at the molecular level (Clark 1999; Doublier et al. 2012). Absorption features associated with overtones and combinations are measureable in the shortwave infrared (SWIR) range between 1000 nm and 2500 nm. These effects are primarily attributable to vibrational processes occurring in the hydroxyl and CO₃²⁻ ions, and metal-OH (e.g., Al-OH, Fe-OH, Mg-OH) cations (Clark 1999). For this reason, minerals containing these components, such as phyllosilicates, carbonates, sulfates, and amphiboles, can be identified.

Spatially resolved μXRF is a nondestructive technique that is here used to scan cut surfaces of quarter core samples. This technique combines the sensitivity of conventional XRF spectrometry with the spatial resolution provided by an X-ray beam strongly focused with polycapillary collimating optics (Yan and Gibson 2002). Compared to an electron beam used in scanning electron microscopy or electron microprobe analysis, the X-ray beam offers a lower detection limit for a range of heavy elements; does not require the sample to be coated with a conductive layer such as carbon; and can be operated at atmospheric pressures, although measuring elements lighter than potassium requires the analysis to be conducted in a vacuum environment.

Together with optical inspection and routine geochemical analysis these noninvasive methods are applied here to characterize the ICDP BARB5 drill core section hosting several Archean spherule layers.

## GEOLOGY OF THE BARBERTON GREENSTONE BELT

The Barberton Greenstone Belt (BGB; Fig. 1) comprises the Barberton Supergroup, a well-preserved volcano-sedimentary succession of mostly low metamorphic grade that is subdivided into the Onverwacht, Fig Tree, and Moodies Groups (e.g., Brandl et al. 2006; Lowe and Byerly 2007). Strata dip steeply due to tight to isoclinal folding. The greenstone belt is intruded along its margin by different generations of granitoids. Up to eight different spherule layers, named S1 to S8, have been proposed in the 3.55–3.23 Ga old volcano-sedimentary rocks of the Onverwacht and Fig Tree groups (Lowe et al. 2014).

BARB5 was drilled into the lower Mapepe Formation of the Fig Tree Group, on the eastern limb
<figure><img src="image_2.png" /><figcaption>Fig. 1. Geological map of the southwestern part of the Barberton Greenstone Belt (modified from Hofmann and Harris 2008). TTG = tonalite–trondhjemite–granodiorite.</figcaption></figure>

of the southern Barite Valley Syncline (Fig. 2) (Hofmann et al. 2006; Hofmann and Bolhar 2007; Mason et al. 2013). The stratigraphy of this region has been described in detail by Lowe and Nocita (1999). According to these authors, the Fig Tree Group strata consist of a coarsening-upward sedimentary sequence of ~230 m thickness of shale, with some jaspilite beds in the lower part and a parallel-laminated tuffaceous siltstone to fine-grained sandstone facies in the middle part (Fig. 2). Near the base of the sandstone facies, a bed of massive, densely packed impact spherules occurs, which is ~20 cm thick, and has been correlated with the S3 layer by Lowe et al. (2003). A poorly exposed spherule bed was also described at the base of the Fig Tree Group in this area, and has tentatively been correlated with S2 (Lowe et al. 2003). The upper part of the sedimentary sequence consists of interstratified sandstone, locally cross bedded chert-pebble conglomerate, banded black and gray chert, and jaspilite, and is capped by barite. This is followed by a fining-upward sequence of chert-pebble conglomerate and sandstone, grading into parallel-laminated tuffaceous sandstone and siltstone, which have been dated at 3227 ± 4 Ma (Kröner et al. 1991). Lowe and Nocita (1999) interpreted the Fig Tree Group strata in the Barite Valley Syncline to represent fan-delta and

coastal deposits in an area of reduced subsidence or along the flanks of structural highs.

In the eastern limb of the Barite valley, the Archean spherule layer S2 is observed at the base of the Mapepe Formation, and spherule layer S3 occurs 10–30 m above it (Lowe et al. 2003; Hofmann et al. 2006) (Fig. 2). The BARB5 spherule-layer bearing section might be related to either of these two stratigraphic horizons.

## ANALYTICAL METHODS

The obtained spherule layer bearing section occurs from 510 to 512 m in the 763 m long BARB5 core. This section is archived in core box 55, currently hosted at the Department of Geology, University of Johannesburg, South Africa. Infrared spectroscopic (IR) imaging of the entire drill core was conducted by GeoSpectral Imaging (Johannesburg, South Africa). Spatially resolved µXRF spectrometry scans of the obtained quarter core samples were conducted at Bruker Nano Analytics in Berlin-Adlershof, Germany. In addition to these noninvasive approaches, routine destructive XRF measurements were made on powdered samples for analysis of major and some minor elements at the Museum für Naturkunde Berlin (Table 1). Details on reference samples, the limits of detection,
<figure><img src="image_3.png" /><figcaption>Fig. 2. Geological map of the southern part of the Barite Valley Syncline (for location see black rectangle in Fig. 1; Hofmann et al. 2006), and stratigraphic log for the eastern limb of the Barite Valley Syncline (modified after Lowe and Nocita 1999). BARB5 was drilled in the east limb, but did not reach the base of the Fig Tree Group. The occurrence of the spherule layer investigated in drill core is shown schematically in the log.</figcaption></figure>

and accuracy for routine XRF analysis in this laboratory are given by Raschke et al. (2013). Additional major element analyses were performed at the Service d'Analyses des Roches et des Minéraux (SARM-CNRS) at the CRPG (Nancy) using an emission spectrometer ICP-OES Thermo Fisher ICap 6500 and ICP-MS (X series), respectively, with analytical precisions better than 2%, except for TiO₂ and P₂O₅ (<10%) and Na₂O (<15%). These XRF and ICP-OES and ICP-MS results are given in Table 1.

## Infrared Scanning and Processing

The five drill cores from the ICDP Barberton Drilling were imaged by GeoSpectral Imaging in February 2013, using a Specim SWIR camera, and a TVI RGB camera. The wavelength calibration of the spectrometer was monitored using a fluorescent lamp and a sample of Styrofoam, which exhibits strong, characteristic spectral features in the measured

wavelength range. Samples in core trays were illuminated using halogen lamps, and a Spectralon® panel was used as a white reference. In addition a dark reference, that is, measurement of spectrometer-related background noise, was captured for each image. Spectral data were captured across 256 bands at a spectral resolution of 10 nm at 6.3 nm band spacing, and a spatial resolution of 1.6 mm.

The data captured for each drill core were normalized to reflectance, eliminating instrument and illumination effects, using the white and dark references collected simultaneously with each core tray during data acquisition. In addition to this, the data were filtered and smoothed. Further processing was done to extract absorption feature depth and wavelength information in order to map the presence of minerals responsive in the SWIR, and to constrain their spectral characteristics.

All IR images for the BARB1 to BARB5 drill cores are available online at www.geospectralimaging.com/intellicoresvr (e-mail info@geospectral.co.za to acquire login access).

## Micro XRF Analysis and Processing

Flat unpolished surfaces of the cut quarter cores were analyzed using a spatially resolved µXRF Bruker M6-Jetstream and a Bruker M4-Tornado spectrometer. The M6-Jetstream is an open-beam (atmospheric pressure) system that allows measuring samples of various sizes up to 800 × 600 mm in a single run. Samples of up to 20 cm in size were measured with the M4-Tornado in a vacuum chamber at 20 mbar to improve the detection of elements lighter than potassium. Measurements were carried out using an X-ray source with a Rh-anode (max. power 30 W) operated at 50 kV and 600 µA. The M6-Jetstream data file from a 30 cm long and 2 cm wide quarter core surface contains 0.55 million pixels, each collected with a dwell time of 2 ms/pixel, and a pixel size of 100 µm. A M4-Tornado data file of a spherule bed sample is composed of 1.4–2.2 million pixels, collected with a dwell time of 3 ms/pixel and a pixel size of 25 µm. The data files were processed with the M4-Tornado software.

Elemental distribution mapping of the samples follows the principle of position-tagged spectroscopy, whereby for each pixel (X-Y-location) a complete fluorescence spectrum (z-location) is stored in a data cube (XYZ). The postprocessing of the acquired information (data mining) was conducted to visualize the stored information as line scans, multicolor elemental distribution maps, phase identification maps, relative element ratio plots, and sum spectra of pixels from selected areas.
<table><caption>Table 1. Chemical composition determined by powder XRF§ and ICP-OES* (major elements) and ICP-MS* (trace elements) of marine sediments in the logged interval of the BARB5 drill core. Sample name is BARB5 followed by core depth in meter and centimeter. The spherule layers are SL-A (512-30) and SL-D (511-35) and sample 511-33 is the sediment sample between spherule layer D and E.</caption><thead><tr><th>BARB5</th><th>505_14§</th><th>507_63§</th><th>510_03§</th><th>511_33*</th><th>511_35*</th><th>512_30§</th><th>513_62§</th><th>515_33§</th><th>518_31§</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>42.1</td><td>56.9</td><td>37.3</td><td>34.8</td><td>52.3</td><td>54.2</td><td>61.4</td><td>56.5</td><td>51.2</td></tr><tr><td>TiO<sub>2</sub></td><td>0.36</td><td>0.58</td><td>0.29</td><td>0.24</td><td>1.18</td><td>1.53</td><td>0.34</td><td>0.6</td><td>0.42</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>6.1</td><td>10.8</td><td>5.2</td><td>5.6</td><td>17.7</td><td>22.1</td><td>6.1</td><td>10.7</td><td>8.8</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>25.9</td><td>9.31</td><td>29.4</td><td>30.6</td><td>9.4</td><td>3.0</td><td>11.0</td><td>9.7</td><td>19.6</td></tr><tr><td>MnO</td><td>0.59</td><td>0.29</td><td>0.45</td><td>0.61</td><td>0.05</td><td>0.01</td><td>0.33</td><td>0.25</td><td>0.38</td></tr><tr><td>MgO</td><td>3.84</td><td>3.91</td><td>3.82</td><td>5.0</td><td>1.47</td><td>1.3</td><td>3.81</td><td>3.93</td><td>4.74</td></tr><tr><td>CaO</td><td>3.59</td><td>4.17</td><td>2.49</td><td>0.54</td><td>0.48</td><td>0.12</td><td>4.46</td><td>4.19</td><td>3.1</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.06</td><td>0.08</td><td>0.11</td><td>0.02</td><td>0.05</td><td>0.09</td><td>0.05</td><td>0.06</td><td>0.08</td></tr><tr><td>K<sub>2</sub>O</td><td>1.87</td><td>3.88</td><td>2.02</td><td>1.96</td><td>9.2</td><td>11.9</td><td>1.96</td><td>4.14</td><td>1.35</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.10</td><td>0.08</td><td>0.08</td><td>0.08</td><td>0.20</td><td>&lt;0.01</td><td>0.05</td><td>0.07</td><td>0.08</td></tr><tr><td>LOI</td><td>15.5</td><td>9.6</td><td>18.2</td><td>n.d.</td><td>n.d.</td><td>3.3</td><td>9.9</td><td>9.0</td><td>9.6</td></tr><tr><td>Sum</td><td>100.0</td><td>99.6</td><td>99.4</td><td>79.5</td><td>92.0</td><td>97.6</td><td>99.4</td><td>99.1</td><td>99.4</td></tr><tr><td>V</td><td></td><td>124</td><td></td><td>151</td><td>426</td><td></td><td>77</td><td></td><td>122</td></tr><tr><td>Cr</td><td></td><td>911</td><td></td><td>370</td><td>2745</td><td></td><td>557</td><td></td><td>762</td></tr><tr><td>Co</td><td></td><td>23</td><td></td><td>43</td><td>371</td><td></td><td>15</td><td></td><td>28</td></tr><tr><td>Ni</td><td></td><td>263</td><td></td><td>582</td><td>2992</td><td></td><td>186</td><td></td><td>357</td></tr><tr><td>Cu</td><td></td><td>58</td><td></td><td>111</td><td>258</td><td></td><td>49</td><td></td><td>28</td></tr><tr><td>Rb</td><td></td><td>127</td><td></td><td>73</td><td>270</td><td></td><td>55</td><td></td><td>30</td></tr><tr><td>Sr</td><td></td><td>92</td><td></td><td>14</td><td>28</td><td></td><td>119</td><td></td><td>84</td></tr><tr><td>Y</td><td></td><td>17</td><td></td><td>8</td><td>23</td><td></td><td>10</td><td></td><td>14</td></tr><tr><td>Zr</td><td></td><td>91</td><td></td><td>30</td><td>91</td><td></td><td>53</td><td></td><td>70</td></tr><tr><td>Ba</td><td></td><td>3038</td><td></td><td>1933</td><td>8912</td><td></td><td>1952</td><td></td><td>1452</td></tr></tbody></table>

Total Fe given as Fe<sub>2</sub>O<sub>3</sub>; LOI = loss on ignition. Oxide concentrations are given in wt% and trace element data in ppm. n.d. = not determined.

The element maps of those elements with a low peak to background ratio (here the minor elements Zr, Cr, Ni, S, and As) were obtained by subtracting the background from each spectrum. The background signal in the spectrum was defined by selecting an energy range in the spectrum close to the peak of interest. The intensity at the selected range was then subtracted from the intensity of the peak for each pixel. This procedure is ideal to visualize minor element distributions, as the background intensity of the spectra can vary with density of the analyzed spot, and the count number acquired for each pixel is relatively low because of the fast data acquisition (3 ms per pixel).

## RESULTS

### Optical Inspection of the Core Section

Fourteen meters from the BARB5 core (517–503 m) were logged in detail (Fig. 3) as they include the investigated spherule layers. Planar to lenticular laminated, medium-gray shale (Fig. S1a in supporting information) forms the lower part of the succession, and extends farther downhole for several tens of meters. Fine-grained sandstone forms lamina typically 1 mm thick and

lenses (<3 mm thick) that rarely show cross lamination. A greenish-gray shale, showing a more pronounced lamination, is interbedded with this facies in the central part (514–506 m) of the logged interval. It frequently contains lamina to thin beds (<2 cm thick) of fine- to medium-grained sandstone that are massive or normally graded, and some of them show erosive scours (Fig. S1b in supporting information). Besides siliciclastic material, this facies contains some carbonate concentrated in more light-colored lamina. A few centimeter thick beds of laminated white chert are associated with this facies. These beds show folding/boudinage or form fragments floating in a ductile deformed shale matrix suggesting different rheological behavior as compared to shale during compaction. The upper part of the logged interval is characterized by Fe-rich shale and banded iron formation consisting of 1 mm microbands of red jasper and light brown carbonate (Fig. S1c in supporting information). This 5 m zone at 506–501 m is overlain by ca. 6 m of predominantly greenish-gray shale and followed by gray shale, both of which were not investigated further.

The logged section is characterized by folding and local shearing. Much of the deformation probably developed during regional deformation of the succession, although an earlier event of soft-sediment
<figure><img src="image_4.png" /><figcaption>Fig. 3. Stratigraphic column of the BARB5 drill core from 517 to 503 m through the spherule-bearing core interval. Numbers refer to depth in meters in the core. Note that undisturbed strata are shown with horizontal bedding, although bedding is actually dipping relative to the core margins.</figcaption></figure>

slumping may also have affected the rocks. Deposition of the spherule layer may have been associated with strong disturbance of the seafloor sediments during the passage of tsunami waves and submarine slumping triggered by seismic activity (Hassler and Simonson

2001). Evidence of syn-depositional deformation has been observed at the top of the spherule-bearing interval in the form of a small-scale intraformational unconformity (Fig. S1d in supporting information). While the logged section is strongly deformed, the rocks in the core above and below are clearly less disturbed.

Several spherule layers and occurrences were found in the logged interval (denoted A-H, Fig. 3, and displayed in Figs. S1e-g in supporting information). Layer A occurs in a fold hinge and is only partially preserved. It resembles the lower zone of layer B due to spherule type, clast support, and lack of sedimentary structures.

In layer B, five zones characterized by compositional layering of different clast-supported particle types can be discriminated optically (Fig. S1e in supporting information). The largest spherules are circular in cross section on each side of the quarter core, white colored, and form a densely packed zone at the bottom. Above is a zone of grayish deformed spherule layer material followed by a thin zone of smaller white and circular spherules, and a green zone of deformed spherule layer material. The deformed spherules in the grayish and greenish zone are mostly flattened in the plane of bedding. This sequence is capped by a zone with dispersed circular spherules set in a dark matrix.

Layer C consists of two separate layers, a lower clast-supported layer and an upper layer containing matrix-supported spherules. Layers D and E are massive and consist of clast-supported spherules. Layers B to E are separated by laminated shale and fine-grained sandstone, which show current-ripple lamination between layers B and C. Occurrences F and G (Fig. 3; Figs. S1e-g) are lenticular thin beds (<2 cm) to lamina of massive fine-grained sandstone with <15 vol% disseminated spherules. Layer H marks the occurrence of isolated spherules.

## Spherule Layer Localization by IR Spectroscopy

Lithologies in the BARB5 core comprise mainly sandstones, conglomerates, and carbonaceous shales. These lithologies are composed predominantly of minerals that are unresponsive in the short wave infrared (SWIR). Thus, the spectral data for BARB5 overall have a low spectral contrast, and do not show distinct absorption features. However, the volcaniclastic unit occurring at ~260-107 m shows greater spectral contrast, including hydroxyl and water absorptions at 1406 nm and 1907 nm, respectively, as well as Al-OH absorptions at 2200 nm, and carbonate/Mg-OH absorptions at 2340 nm.
<figure><img src="image_5.png" /><figcaption>Fig. 4. Results from the infrared imaging of the cores in box 55 are displayed. Downhole plot of depth and wavelength positions of the 2200 nm absorption feature, illustrating how this feature in the spherule layers contrasts sharply with that of the surrounding carbonaceous shales, and showing that the wavelength position of this feature is also longer than that of the shales. In the D2200 inset, red delineates the area where this absorption feature is prominent, and in the W2200 nm inset, red illustrates that the absorption feature occurs at a higher wavelength position relative to the same feature in adjacent lithologies. W is the wavelength position of the absorption feature and D is depth of the absorption feature in a spectrum after the Hull Quotient has been applied and the spectrum has been background subtracted to zero; the unit for depth of an absorption feature is arbitrary.</figcaption></figure>

The spherule layers occur within spectrally unresponsive carbonaceous shale (Fig. 4; Fig. S2 in supporting information). They are characterized by the distinctive features of OH absorptions at 1412 nm, prominent Al-OH absorptions at 2220 nm, and a weak water absorption at 1907 nm (Figs. 4–5; Fig. S2). The intense Al-OH infrared signal from these sections corresponds to the high Al₂O₃ content of ~22 wt%, as determined by powder XRF analyses of spherule layer A, or of ~18 wt%, as determined by bulk ICP-MS analyses of spherule layer D (Table 1; Figs. S1e–g). These values are much higher than the 6–8 wt% Al₂O₃ obtained from sections between the spherule layers B to E by µXRF and ICP-OES, and 6–11 wt% Al₂O₃ as determined by powder XRF in the marine sediments between 518 and 503 m core depth (Table 1).

The spectra obtained from the spherule layers in BARB5 are characteristic of sericite, although the wavelength position of the Al-OH absorption feature is at longer wavelengths (2220 nm) than that of typical

potassic micas, whose Al-OH feature normally occurs ~2200 nm. Variation in the wavelength position of the Al-OH absorption is caused by compositional variation in the mica (Beran 2002; Van Ruitenbeek et al. 2005). The wavelength position of the Al-OH absorption feature (>2220 nm) in the BARB5 spherule layers suggests that the sericite mapped is likely to be a phengitic (Fe-rich) variety (Duke 1994; Pontual et al. 1997; Clark 1999; Duke and Lewis 2010); however, this determination would need to be corroborated using mineral chemical analysis.

In addition to the distinct Al-OH feature, a shoulder feature at 2252 nm, associated with the Fe-OH absorption (Clark 1999), suggests that minor chlorite may also be present in the spherule layers.

## Micro-XRF Line Scans

In the following the results of data mining of the data cubes (X-Y area directions and energy spectrum in Z direction) obtained by µXRF measurements across the
<figure><img src="image_6.png" /><figcaption>Fig. 5. Representative infrared spectrum from spherule layer E, showing a sharp OH absorption feature at 1412 nm, weak water absorption at 1907 nm, and distinct Al-OH absorption feature at 2220 nm.</figcaption></figure>

quarter core hosting the four closely spaced spherule layers B to E (511.2–511.5 m) are presented (Figs. 6–7; Figs. S3–S7 in supporting information). The elemental distribution line scans along the quarter core section, for one surface only, are depicted in Fig. 6 and illustrate the main chemical trends identified. The results are presented for various elements as counts normalized to 100%, that is, the respective strongest recorded signal.

The Ca distribution varies along the core section without showing a clear trend with respect to spherule and shale layers, although Ca content is highest near the base of layer B. In contrast, the elements Al, Si, Ti, K, Rb, and Zr are enriched in all spherule layers compared to the surrounding shale (Fig. 6; Table 1). In layer B, comparatively higher Rb abundances were observed in the regions composed of deformed spherule layer material. Rubidium enrichment appears unrelated to K abundance, but rather with Ti-rich material. In contrast, the spherule layers are all depleted in Fe and Mn that are strongly enriched in shales.

The distribution of Ni, Cr, S, As, and Cu appears to be more diverse. Two types of Ni enrichment can be discriminated, as Ni correlates either with Cr, or with elements such as S, As, and/or Cu. Nickel and Cr show a positive correlation and elevated abundances in layers C to E with a progressive decrease in the Ni and Cr concentration from layers C to E. The correlation between Ni and Cr are investigated in more detail in a later section of this contribution. In general, Ni and Cr are low in abundance in the shales and the main parts of layer B. The highest Ni abundances occur in distinct bands, in association with high S, As, and/or Cu

abundances. At the top of layer C, an increase in Ni, As, and Cu, and in part Ca, but not in S, can be observed. Directly above layer E, the relative abundance of S, but not of As, Ni, or Cu increases. Layers C to E (between 8 cm and 27 cm; Fig. 6) show more variation and higher S abundances compared to layer B.

The section at the top of layer B (corresponding to 22–23.5 cm; Fig. 6) is illustrated with complex Ni, Cr, S, and As line-scan data (Fig. S3). At the top of layer B Ni and Cr have elevated abundances and are positively correlated. Below is a thin zone with Ni, S, As, and Cu enrichments, in which substantial pyrite was identified by optical microscopy. Further below, in layer B (22–23.5 cm; Fig. 6; Fig. S3), the green zone that is exclusively composed of deformed spherules is enriched in Cr but not in Ni. That contrasts with the white spherule zone just below, which exhibits much lower content in Cr and is slightly higher in Ni.

## General Petrographic and Geochemical Characteristics of the Sedimentary Units Using Elemental Maps

The general structure of the spherule layers and intercalated sediments can be further visualized with 2-D distribution maps for different elements and element pairs (Fig. 7). All maps displayed in Fig. 7 are based on four (one per spherule layer) µXRF measurements acquired using the M4-Tornado. To improve the count statistics the element maps and the Ni and Cr mineral phase map (Fig. 7) are displayed using the sum of three and five adjacent pixels, respectively. The threshold value for the element concentration in the maps was selected to optimize the presentation of the different components in the sections; for example, the shales contain significant amounts of SiO₂ but generally map as black in the Si element map in contrast to the spherule layers that are comparatively enriched (Table 1). The highest Si concentrations occur in a thin silica vein below layer B. Abundances of major elements are displayed in element maps. Minerals that are rich in both Ni and Cr are shown in the Ni and Cr phase map displaying only those pixels that are high both in Ni and Cr.

As already indicated by the line scan plots (Fig. 6) and the quantified element composition data (Table 1), the spherules and the matrix between spherules, are distinctly different in chemical composition from shale separating these spherule layers. In particular, the concentrations of the major elements Si, Al, K, and Ti, and the minor elements Rb and Zr (Figs. 6–7) are higher in the spherule layers, and the shales have higher Fe and Mn concentrations.

The Si, Ti, Al, and Zr maps (Fig. 7) indicate the distribution of particles apparently grading from the
<figure><img src="image_7.png" /><figcaption>Fig. 6. Visible light image and line scans displaying the relative abundances of Ca, Ti, K, Rb, Zr, Fe, Mn, Ni, Cr, S, As, and Cu. The line scan displays the sum spectra from a rectangle that covers the full width of the quarter core and 15 pixels in length. Data were collected at 1 bar atmospheric pressure with the Bruker M6-Jetstream µXRF and are displayed as distance in cm versus counts normalized to 100% for all elements.</figcaption></figure>

spherule layer into interbedded shale. But no spherules are observed in separating layers between C, D, and E, and only one isolated spherule is observed in shale separating layers B and C. Irregular shaped Si-rich particles outline the cross lamination between layer B and C. The distribution of the irregular shaped Si-rich particles apparently resembles a depositional environment with water current activity. From the spherule layers C to E, these irregular shaped Si-rich particles grade into the shale from the top of layer C, the bottom of layer D, and the top of layer E. A sharp contact to the shale is developed at the top of layer D and bottom of layer E. Iron may be used as an indicator for the occurrence of background sediments within spherule layer deposits, and this sedimentary contribution appears to be higher in layers C to E than in layer B. Layer A has significantly lower Fe abundances than layer D (Table 1). The Fe-rich material may represent some kind of rip-up or intraclasts of local sediments. In contrast to layer C to E the main part of layer B does not contain rip-up clasts, irregular Si-rich particles or Zr-rich particles.

## Differences in Spherule Populations and Geochemical Similarities of Layers B to E

The composition of individual spherules is also elucidated by element maps (use zoom function in the electronic version of the article). In comparison to the more diffuse matrix, the round and well-preserved spherules are characterized by higher K and Si concentrations and lower Ti and Al concentrations (Fig. 7). Titanium is concentrated at the rims of particles that are now either deformed or retain their original spherical shape. Circular spherules, and more diffuse spherule material, are evenly distributed through layers C to E. In layer B, the K-rich round spherules are concentrated at the bottom and in the middle part of the horizon, and the more diffuse appearing Al-rich spherule material is concentrated between and above these zones. These two zones of high-Al-bearing material differ in optical appearance and chemical composition. The upper zone is green and has high Cr abundance. This zone is capped by a thin layer of roundish spherules embedded in shale
<figure><img src="image_8.png" /><figcaption>Fig. 7. Micro-XRF elemental distribution maps for the 22 cm long quarter core section (511.44–511.22 m) containing the four spherule layers B to E. The M4-Tornado µXRF data were collected at low vacuum and 4 images are stitched together to cover the full length of the core section. The color scheme of the displayed elements and element combinations are indicated at the top of the column. The Ni and Cr mineral phase map is constructed by selecting all spectra from the data cube with high Ni and high Cr peaks. Each pixel represents the sum spectra of five adjacent pixels to improve the count statistics. The concentration of a displayed element above a threshold value in each pixel is represented by one color. For each element the threshold value is selected to optimize visualization of the sedimentary structures.</figcaption></figure>

that is K-, Al- and Ti-rich, and displays a slightly higher Fe content compared to the spherule zones below.

Compositional sorting in layer B allows further visualization of the chemical signatures of the two types of material by plotting the element abundances of Rb
versus K, and Rb versus Al from different sections of layer B (Fig. S4). Rubidium was selected because the line scan shown in Fig. 6 indicates an increased Rb abundance in the region of the deformed Al- and Ti-rich spherules. Figure S4 shows that the Al and Rb abundances are positively correlated throughout layer B. The Rb and K abundances display different populations, with low K, high Rb material occurring in the region composed of deformed spherules, and high K, low Rb material occurring at the bottom of layer B that is mainly composed of round, K-rich spherules.

These differences are further enhanced by comparing the sum X-ray spectra of selected areas from the different parts of layer B. Figure S5a shows the sum spectra from the two parts with round and the two parts with deformed spherules. The displayed spectra are arranged with the background at 14.5 kV at the same height. This permits a comparison between the peak intensities of the displayed elements. An increase in the modal abundance of the deformed Al-rich material correlates with an increase in the Rb, Y, and Zr peaks, and a decrease in the Sr peak. Thus, the deformed Al- and Ti-rich spherules are the main host of Rb.

Notably the sum spectra of layers B to E (Fig. S6b) show that the relative proportions of Rb, Sr, Y, and Zr are similar to even identical. Differences between layer B and layers C, D, and E depend on the modal abundance of the Al- and K-rich zones included in the sum spectra of layer B. In summary, spherule layers B to E appear chemically related.

## Differences in the Distribution of Primary Particles in Spherule Layers B to E

Despite the chemical similarities between layers B to E, layer B differs from layers C to E with respect to the occurrence of Zr-rich particles, and Ni- and Cr-rich phases. The Zr-rich phases occurring in layers C to E partly extend into the surrounding shale, and with low abundances extends through the cross bedded shales separating layers C and B, but they are seemingly of lower abundance, or even absent in the other shale intervals (Fig. 7). The line scans (Fig. 6) show that all layers have, within the limits of analytical resolution, similar Zr abundances. Notably, the main part of layer B either does not contain these Zr-rich phases, or they are too small to be resolved on the available sections. The short line scans across Zr-rich particles in layer E are displayed in Figs. S6a and S6b. They show that the Zr-rich spots in the map of layers C to E are indeed Zr-rich particles and not diffuse local enrichments of this element. From the line scans of other elements from these spots (not displayed) it appears that these <100 µm sized particles are Zr-rich and most likely

zircon, which has also been reported from other Archean spherule layers (Byerly et al. 2002). Note that the modal abundance of zircon in these projected 2-D maps is higher than what is expected based on observations using electron microscopy or optical thin section studies. That is because with increasing atomic mass the depth in the sample from which the characteristic X-rays can reach the detector increases from a few micrometers for K to a millimeter for Zr.

The Ni and Cr mineral phase map (Fig. 7) displays pixels representing the sum of five adjacent pixels (to improve the count statistics) characterized by high Ni and Cr values. The sum spectrum (data not shown) of the spectra from all yellow pixels in each layer of the Ni and Cr phase map displays additional enrichments in Zn, Cu, Ni, Co, Cr, and V, and these elements correspond to those elements reported for Ni-Cr spinel of the Barberton spherule layers (Byerly and Lowe 1994; Krull-Davatzes et al. 2010; Mohr-Westheide et al. 2015). Notably, in this section Ni-Cr spinel occurs only in layers C, D, and E, and at the top of layer B—but not within layer B.

A few phases displayed in the Ni and Cr phase map may actually resemble an intergrowth of a Ni- and a Cr-rich phase that both contribute to the five pixel sum spectrum. To test this, examples of the identified Ni and Cr phases from layer E are displayed as short line scans (Figs. S7a–d) to show the spatial correlation of Ni and Cr abundances. Most line scans show a strong spatial correlation between Ni and Cr (Figs. S7a–c), indicating the occurrence of Ni and Cr in one mineral. Nevertheless, Ni-rich and Cr-poor phases (Fig. S7d) are also present in these samples indicating the occurrence of different minerals. The yellow spots in the Ni and Cr phase map are also visible in the Cr element map displayed to the right of the phase map in Fig. 7. Most of the highest Cr concentrations are correlated with the occurrence of Ni-Cr-spinel, with the exception of those in the greenish zone in layer B which is high in Cr, but not in Ni (Figs. 6–7; Fig. S3).

# DISCUSSION

## General Properties and Sedimentary Environment of Spherule Layers in BARB5

The Fig Tree Group strata in the Barite Valley Syncline in the Barberton Mountain Land are considered to represent fan-delta and coastal deposits (Lowe and Nocita 1999). The spherule layer is hosted by marine sediments represented by fine-grained sandstones and shales, chert, jasper, and banded iron formation. In general, the sediments in the BARB5 drill core are less disturbed than the strongly deformed
spherule layer-hosting section between 517–503 m core depth. Much of the deformation probably formed during regional deformation of the succession, accentuated by the presence of BIF in the spherule-bearing domain that may have behaved rheologically different upon deformation. Nevertheless, an earlier event of soft-sediment slumping, possibly shortly after the impact spherule deposition affected the rocks, as indicated by a small-scale intraformational unconformity at the top of the spherule-bearing interval.

The shales and fine-grained sandstones are relatively rich in MnO and Fe₂O₃, and have moderate Al₂O₃ concentrations. In this succession the spherule layers stand out as discrete bands composed of discrete particles that are either zircons or impact derived TiO₂, K₂O, and Al₂O₃ rich spheres, or Ni- and Cr-rich spinels. Ni-rich spinels are of unusual composition and, thus, are interpreted as primary phases that formed during an impact event on Earth (Robin et al. 1992; Byerly and Lowe 1994; Kyte and Bostwick 1995; Gayraud et al. 1996).

The high K concentrations in the spherule layers of up to 11 wt% K₂O (Table 1) have been interpreted as a result of alteration and replacement processes in the Archean marine environment; that is, replacement of particles of originally basaltic composition by authigenic K-feldspar (Simonson et al. 1999). Titanium is concentrated along the rims of flattened and circular spherules. The concentration of this refractory element at the rim was interpreted as due to early formation of crystallites by either devitrification or growth from the melt.

In contrast, the relatively high Al₂O₃ concentrations of the spherule layers seem in part a primary feature of these deposits, because Al behaves as a rather immobile element under many conditions. In the altered metasediments, the high Al₂O₃ values are now hosted by feldspar and phyllosilicates. The well-preserved texture of the spherule layers places constraints on the effect of passive Al₂O₃ enrichment, by removal of mobile elements (Krull-Davatzes et al. 2012; Belza et al. 2015). But spherule layers, as well as some volcanoclastic layers in Archean metasediments, were interpreted as being deposits having basaltic composition, followed by diagenetic Al₂O₃ enrichments due to Al migration along distances on the order of decimeters in the sedimentary successions (Hassler and Simonson 1989; Kohl et al. 2006; Hassler et al. 2011). Chemical modifications of ejecta layers are also described from Cretaceous-Paleogene (K/Pg, formerly K/T) sections of the Chicxulub impact event (Smit and Klaver 1981; Hecht et al. 2004). But high Al₂O₃ contents of ~15–20 wt%, and occasionally up to 32 wt

%, have also been reported for well-preserved Phanerozoic impact spherules, microtektites, and microkrystites (Alvarez et al. 1992; Vonhof and Smit 1999; Glass 2002; Glass et al. 2004; Kelly and Elkins-Tanton 2004; Folco et al. 2009). Notably, such Al₂O₃ concentrations are similar to the 16 wt% Al₂O₃ typical for the modern oceanic and continental crust from which the more recent microtektites and microkrystites are derived. Either way the Al-rich Archean spherule layers are composed of secondary minerals (K-feldspar and phyllosilicates) with a chemical and mineralogical composition distinctly different to the surrounding marine metasediments of the BARB5 core.

## Single or Multiple Impact Origin of the BARB5 Spherule Layers

In the BARB5 drill core, eight spherule-bearing layers (A to H, Fig. 3) are observed within a stratigraphic interval spanning <1.5 m and four of these eight layers occur within a short 23-cm interval. The formation of each layer by a separate impact cannot be excluded. By using the sedimentary and compositional characteristics described above and by applying Occam's razor, that is, preferring the hypothesis based on the fewest assumptions (Hoffmann et al. 1997), we aim to constrain the minimum number of impact events involved.

Repetition of spherule layers due to folding may be the case for layer A due to its occurrence in a fold hinge and its similar composition to layer B (Table 1). Evidence for sedimentary reworking is observed above the deformed top of layer E. There, diluted, sporadic, or lenticular shaped spherule occurrences are observed (layers F to H; Fig. 3), which may suggest redeposition.

Closely spaced and densely packed layers B to E exhibit more complex structures. The matrix of spherule layers B to E displays a different chemical composition compared to the interbedded shales. The matrix is enriched in Al, Ti, Si, Rb, and Zr compared to the Fe- and Mn-rich shales, and, therefore, is interpreted to be composed of altered spherules. Occasionally, Ti-rich rims outline deformed spherules. Only layer B displays complex compositional zoning, and is interpreted as original undisturbed deposit. The grayish and the greenish zone of this layer B contain spherules that are flattened parallel to bedding, which is likely a result of sediment compaction. This compaction did not affect the apparently more resistant K- and Si-rich round spherules. The four zones of spherules in layer B apparently comprise distinct populations of spherules that differ in either chemical composition or degree of crystallization at the time of deposition. The spherules and the Ni-Cr spinel is interpreted as particles that
formed during the impact event, and the zircon is considered as a mineral that formed earlier on Earth and delivered into the spherule layer either by ejection from the crater or, more likely, by redeposition of local crustal material (see also Byerly et al. 2002). That means the spherules, Ni-Cr spinel, and zircon are primary constituents of the spherule layer deposit and did not form later by secondary processes. The main part of the stratified layer B is characterized by a lack of Ni-Cr spinels, and redeposited terrestrial material such as zircon, Si-rich irregular particles, and Fe- and Mn-rich sedimentary material, including rip-up or intraclasts. Thus, layer B shows significant textural differences compared to the apparently redeposited layers C to H. Only the top part of layer B shows some enrichments in Fe- and Mn-bearing sedimentary material, zircons, Si-rich irregular particles, and, in addition, the uppermost 300 µm of layer B are enriched in S and As. These S and As enrichments may be analog to the detritally reworked pyrite described from the Neoarchean Monteville spherule layer (Simonson et al. 1999; Simonson and Harnik 2000). Thus, we argue that the main part of the BARB5 layer B represents an undisturbed fallout layer, with its top part affected by reworking.

In this interpretation, the sorting of larger and round K-rich and the smaller and deformed Al- and Ti-rich, spherules, and the occurrence of Ni-Cr spinel at the top of layer B would be a consequence of the impact emplacement process and the size-dependent differential settling of various particles through the atmosphere and water column (e.g., Manville and Wilson 2004; Artemieva and Morgan 2009; Schulte et al. 2009). Such a depositional sequence would be similar to the situation described from the K/Pg ejecta layer in Haiti and Wyoming (Bohor and Glass 1995), and the western Atlantic (Schulte et al. 2009).

Layer B is separated from layer C by current-ripple lamination. This suggests episodic current or wave activity that apparently formed discrete layers C to E of homogenized spherules, and Ni-Cr spinel and zircon with a minor contamination of sedimentary material (minor Fe and Mn enrichments) and/or irregular Si-rich particles, which are tentatively interpreted as detrital quartz. Notably, spherule layers C to E do not display internal sorting or lamination.

Layer D shows contacts between the spherule layer material and the adjacent shale layers that is gradual at the top and sharp at the bottom (see Si, Al, and Ti map; Fig. 7). Layer E shows similar features but appears upside down. In addition, the close spatial relation between spherule layers B to E (Figs. 3 and 7); the cross-bedding observed between layers B and C (compare the Si map in Fig. 7); and the overall

similarity with respect to Rb, Sr, Y, and Zr element ratios (Fig. S5b) support the interpretation that these spherule layers are part of the same impact event.

We have argued that the stratified layer B represents the undisturbed impact fallout deposit composed of discrete zones of chemically different spherules, with Ni-Cr spinel concentrated toward the end of the depositional sequence. Layer B might be entirely composed of ejecta material with no admixed detrital material like zircon or irregular Si-rich particles. This may be analog to the Paraburdoo spherule layer in Archean-Proterozoic boundary sediments (Hassler et al. 2011). Layers C to E represent mixed layers that are interpreted as locally redeposited and homogenized ejecta including detrital material of likely terrestrial origin such as zircon, irregular Si-rich particles (maybe detrital quartz), and Fe- and Mn-rich sedimentary material. The reworked BARB5 spherule layers C to E show textural similarities to the reworked spherule layers close to Archean-Proterozoic boundary (Simonson et al. 1999; Simonson and Harnik 2000).

## Large Area Noninvasive Analytics on Micrometer to Kilometer Scale

Due to the high Al concentrations, which correspond to the presence of K-feldspar and Al-rich phyllosilicates, the BARB5 spherule layers are clearly recognizable by their distinct Al-OH absorption in the shortwave infrared imagery. This emphasizes the relatively high spatial resolution of the IR spectrometry method, at the subcentimeter scale. Also, such data sets might aid in locating Al-rich spherule layers in those cases where the textural features most easily recognized during routine core logging are obscured by diagenetic, deformational, and metamorphic overprints. Differentiation between volcanic and impact-derived debris would, however, not be possible using IR alone, thus necessitating additional petrological and/or chemical analysis. A further screening for samples of interest could be accomplished by looking for high Ni and Cr values using µXRF or even portable XRF in the field and by searching for Ni-Cr spinel using the µXRF phase identification feature and select pixels with high values for Cr and Ni.

Suitable targets for the search of spherule layers using IR are drill cores or large surface exposures such as open pit mines of sedimentary rocks deposited under low energetic conditions and far from clastic, Al-rich input. These would include banded iron formations, specifically the thick iron formations of the Superior-type (Bekker et al. 2014), banded cherts so widespread in Palaeoarchean greenstone successions (Hofmann et al. 2013), and carbonates, such as the
thick carbonate platform deposits of the Neoarchean (Beukes 1987). Note that some of the Superior-type iron formations are now considered to be granular iron formations deposited in shallower, more energetic paleoenvironments (Simonson 2003). Nevertheless, in high energetic paleoenvironments, for example, represented by sandstones or oolitic carbonates, the spherules are likely to be dispersed and would not retain their identity as discrete layers. Discrete layers can be visualized with large area scans due to the chemical and mineralogical differences between Archean spherule layers and their host rocks. Such an approach could also be used to target the earliest traces of impacts onto Earth in strongly metamorphosed Eoarchean strata such as the Isua and Akilia successions of West Greenland. Locating these early traces of impacts would be of outstanding relevance for study of the type(s) of projectiles that impacted during part of the Heavy Bombardment eon (Fritz et al. 2014).

CONCLUSIONS

The BARB5 drill core hosts eight impact spherule layers and occurrences in the interval between 511–512.6 m core depth. The spherule layers occur in a section from 517 to 503 m where the rocks are significantly more disturbed than the rocks in the core above and below.

The µXRF data revealed geochemical trends at the micrometer to meter scale. Similarities and differences between spherules, the matrix between the spherules, and the sediments separating the spherule layers in a 30 cm long drill core section were characterized. At least two types of spherules were identified: K- and Si-rich spherules that are circular in cross section on each side of the quarter core, and Al-, Ti-, Rb-, and Zr-rich, flattened spherules with some of them being also rich in Cr. These morphological differences could be related to differences in the initial chemical composition, or degree of crystallization (e.g., microkrystites and microtektites) that resulted in different alteration products.

Separating only those spectra with high Ni and high Cr peaks resulted in a phase map of Ni- and Cr-rich particles that additionally are enriched in Zn, Cu, Co, and V. This elemental composition resembles the composition reported for Ni-Cr spinels in Paleoarchean spherule layers. The µXRF data allowed to map the distribution of tens of micrometer-sized primary phases such as Ni-Cr spinel and zircon.

The BARB5 spherule layers share general characteristics such as spherule size and shapes, and certain geochemical characteristics, while they differ in the abundance and distribution of the “primary”

compounds Ni-Cr spinel and zircon, as well as the distribution of Fe- and Mn-rich sedimentary material and irregularly shaped Si-rich particles. Based on the data obtained so far, we cannot exclude a multiple impact origin for some or all of these spherule layers. However, a single impact emplacement followed by reworking of the material on the Archean sea floor could also account for the observed similarities and differences and the sedimentary structures of the layers. In this scenario we assume an initial deposition of at least two chemically different types of spherules followed by Ni-Cr spinels (associated with high Ir concentrations; Mohr-Westheide et al. 2015) toward the top of layer B. Such a sedimentary sequence would be similar to the situation described in proximal and marine ejecta deposits of the Chixculub impact event that terminated the Mesozoic era (e.g., Bohor and Glass 1995; Smit 1999; Schulte et al. 2009; Artemieva and Morgan 2015).

Notably, despite differences in the composition of the Archean and Phanerozoic crust the high bulk Al₂O₃ content of up to 22 wt% in the Archean spherule layers is similar to those values reported for Phanerozoic spherule layers (Alvarez et al. 1992; Vonhof and Smit 1999; Glass 2002; Glass et al. 2004; Kelly and Elkins-Tanton 2004; Folco et al. 2009). Alternatively, the high Al₂O₃ might be related to diagenetic enrichment (Smit and Klaver 1981; Hassler and Simonson 1989; Hecht et al. 2004; Kohl et al. 2006; Hassler et al. 2011). Either way, due to their unusually high Al₂O₃ concentrations, these spherule layers can be located by characteristic absorption features, using infrared imagine methods. Such data sets can cover kilometers of core material, such as those produced for the BARB1 to BARB5 drill cores. Thus, infrared imaging techniques can be used to image strongly metamorphosed Al₂O₃ poor Archean sediments deposited in a low energy paleoenvironment that may contain spherule layers in which the petrographic hallmarks of spherules have been obliterated, or in large outcrops of Archean to Proterozoic Al₂O₃ poor marine sediments.

*Acknowledgments*—Financial support of JF by Saalbau goes Science. Financial support for DH and TMW, sample acquisition and powder XRF analyses by the Deutsche Forschungsgemeinschaft (DFG) through DFG —RE 528/14-1 formulated by JF and allocated to W. U. Reimold, JF, and RTS. We are grateful to the PIs of the ICDP drilling project for providing the sample material and sampling support. Special thanks to K. Krahn and H.-R. Knöfler for the sample preparation, and to L. Tissandier (CRPG-Nancy) for assistance in sample preparation. We thank an anonymous reviewer, Bruce Simonson, and Tim Jull for constructive
comments that helped to improve an earlier version of
the manuscript.

*Editorial Handling*—Dr. A. J. Timothy Jull

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article:

**Fig. S1.** Visible light images displaying typical rock types and spherule layer section encountered in the BARB5 core. a) Planar laminated medium gray shale (517.0 m). b) Greenish gray shale facies with abundant thin sandstone beds (512.8 m). c) Folding in banded iron formation (503.7 m). d) Small-scale intraformational unconformity in gray shale facies indicating soft-sediment deformation prior to deposition of the overlying bed (510.4 m). e–f) Quarter core samples with their three surfaces from the drill core BARB5 at depth of 511.6–511.0 m in the core. The spherule layers and occurrences are labeled with A.

**Fig. S2.** Red-green-blue (RGB) and shortwave infrared (SWIR) image data for BARB5 drill core box 55 showing a) natural color RGB image; b) false color composite image generated from 3 bands of the spectral data; c) map illustrating the depth, after background subtracting the data using the Hull Quotient, of the 2200 nm absorption feature. Note the color scale of the image; cold colors indicate shallow absorption features, while warm colors are indicative of deeper absorption features at the given wavelength position. The four closely spaced spherule layers B-E are indicated by a green rectangle.

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**Fig. S3.** Visible light image and corresponding element distribution of Ni, Cr, S, and As for spherule layer B. Two different Ni-rich zones are identified that either correlate with high Cr contents or high S and As abundances. In addition to the high Ni and Cr zone another Cr-rich zone occurs in the upper deformed spherule layer and this zone displays low values for Ni, S, and As. The S and As rich part is high in Ni but low in Cr.

**Fig. S4.** Element ratios for Rb/Al and Rb/K of texturally different parts for the sorted spherule layer B. The Al/Rb trend through the whole spherule layers shows a positive correlation between Al and Rb. In contrast the zone with K-rich circular spherules contains less Rb than the two zones with deformed spherules.

**Fig. S5.** Micro-XRF energy spectra of spherule layers a) showing four sections of layer B and b) showing bulk data for the layers B to E. The four sections of layer B (a) represent the lowest part with K-rich spherules (27–26 cm), the transition zone with K-rich spherules and deformed spherules (26–25 cm), and the two regions (24.5–24 cm; 23.5–23 cm) where only deformed spherules and no round K-rich spherules are present. The region between 23.5–23 cm is enriched in Cr. All energy spectra are arranged with the Zr peak at the same height.
**Fig. S6.** Line scans across two Zr-rich spots from layer E given in variations of the relative abundances to show that the identified spots are individual small particles highly enriched in Zr. Line scans across individual particles covers the full width of the particle of interest but measures only one pixel in length in order to conserve the spatial resolution of the data set.

**Fig. S7.** Line scan across the Ni- and Cr-rich spots from layer E as identified in the Ni & Cr phase map of Fig. 7. display special correlations between Ni and Cr. Line scan across selected particles cover the full width of the particle of interest but only one pixel in length in order to conserve the spatial resolution of the data set.