These investigations into Abu Hureyra meltglass (AH glass) included analyses of sedimentary abundance peaks, morphologies, geochemical composition, crystallinity, melting/boiling temperatures, carbon reflectance, remanent magnetism, and water content. These were accomplished using reflected-light microscopy, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), electron microprobe, reflectance, and transmission Fourier transform infrared spectroscopy (FTIR). Numbers, depths, ages of stratigraphic levels, and impact proxies investigated are listed in Appendix, Table S2.

Abu Hureyra meltglass

Morphology and composition

Peak concentrations of AH meltglass8, magnetic spherules5, and nanodiamonds12 co-occur in and adjacent to the E301 sample at a depth below the surface of ~405 cm (Appendix, Fig. S1). Representative SEM-EDS analyses for AH meltglass are listed in Appendix, Table S3, keyed to figures in this paper; representative microprobe analyses for AH glass and other types of meltglass are listed in Appendix, Table S4.

Meltglass fragments comprise 1.6 wt.% of bulk sediment and range in diameter from <200 μm to ~1.4 cm, with most ranging from 1–2 mm in diameter. Some AH glass fragments are roughly spherical, others are rounded (Fig. 2a–b), and some have complex subrounded shapes (Fig. 2c–f); most typically are found broken and rarely whole. Glass surfaces are generally rough-textured but commonly have small areas with smooth, reflective surfaces. Nearly all fragments larger than several millimeters are highly vesicular (Fig. 3), so that most float on water, which is how they were extracted from AH sediment31. Nearly all vesicles observed in broken AH glass display interior surfaces with one or more of the following features: (i) flow patterns suggesting the material was in motion when it cooled; (ii) small inclusions of melted-to-unmelted minerals, including quartz, magnetite, titanomagnetite, chrome-magnetite, chromite, zircon, and monazite (Figs. 2, 3); and/or (iii) intricate, well-organized, crystalline patterns (Fig. 3). Optical microscopy typically shows that the mineral linings have different colors than the outside of AH glass fragments. Often, these linings are red-colored, indicative of high-Fe content.

Figure 2 Typical examples of Abu Hureyra (AH) meltglass. (a) Photomicrograph of rough 2.5-mm-wide AH glass spheroid from level 457, sample E313, 413 cm depth. The term “level 457” refers to a single stratum as numbered by the original site excavators31. “Sample E313” was extracted from level 457 at 413 cm below a surveyed datum point. Numbers, depths, and ages of stratigraphic levels investigated are listed in Appendix, Tables S1 and S2. Sediment samples discussed in this article are from Trench E unless otherwise noted. (b) SEM image of the same glass fragment as in (a). (c) Photomicrograph of a 2.7-mm-wide fragment from level 449, sample E305, 412 cm depth. Yellow arrows point to inclusions of chromite and monazite. (d) SEM image of the same AH glass fragment as in (c). (e) Photomicrograph of 7.8-mm glass fragment from level 457, sample E313 at a depth of 413 cm depth. (f) SEM image of a different view of the same glass fragment as in (e). Full size image

Figure 3 Examples of inclusions inside vesicles and on outer surfaces of AH glass. (a) Photomicrograph of 3.9-mm fragment of broken meltglass dominated by a large vesicle. The red-brown color (arrow) indicates the lining of vesicle contains high Fe content. (b–f) SEM images of the inner walls of typical glass vesicles. (g) SEM image of flow textures (e.g., at arrow) inside a large vesicle. (h) Common crystalline textures inside glass vesicles formed as the molten glass cooled. (i) Central, cross-shaped, ~5.5-μm-wide embedded crystal composed of Ca, Mg, and Si. Based on SEM-EDS, it likely is a quenched, skeletal diopside crystal (CaMgSi 2 O 6 ) within a glassy Ca-Al-Si matrix. (a,f,g) from level 457, sample E313, 413 cm depth. (b–e,h–i) from level 445, sample E301, 405 cm depth. Full size image

Bunch et al.8 found that most AH glass is a calcium-rich aluminosilicate mix, with an average composition of 59.5 wt.% SiO 2 , 15.2 wt.% CaO, 6.3 wt.% FeO, and 6.0 wt.% Al 2 O 3 , with the remaining 13.0 wt.% as various minor oxides. We report similar compositions of AH glass averaging 50.9 wt.% SiO 2 , 16.6 wt.% CaO, 9.8 wt.% FeO, 9.0 wt.% Al 2 O 3 and the remaining 13.7 wt.% as various minor oxides, excluding carbon. The meltglass is geochemically similar to the local sediment, which is mostly composed of plagioclase, carbonates, gypsum, chert, and quartz with other minor rock components derived from local and upstream bedrock. The HCl treatment of several grams of AH sediment left less than 50% of the original volume due to carbonate dissolution, as indicated by vigorous outgassing. When carbon was measured by SEM-EDS along with oxides, we found typical C concentrations to fall between 25 and 50 wt.%, consistent with high carbonate content.

Crossed polarizers are useful in determining whether a mineral is anisotropic (e.g., quartz, calcite, and tourmaline) or isotropic, a characteristic that occurs in only a few materials (e.g, sodium chloride, anthropogenic glass, tektites, and some impact glass32). Six pieces of AH glass were crushed, and dozens of the thinnest fragments were viewed through crossed polarizers, which revealed that all were isotropic, consistent with being amorphous glass. Fragments of YDB glass samples of an Australasian tektite (Muong Nong) were also isotropic, as were samples from two other YDB sites at Melrose, Pennsylvania, and Blackville, South Carolina.

Furnace and torch heating experiments

To test the hypothesis that local melting produced AH glass, heating experiments were conducted on AH sediment and pieces of excavated AH glass. Laboratory furnaces were used at temperatures varying from 1100° to 1850 °C at 50–150° intervals (Table S5). All experiments used either sediment from Abu Hureyra level 435, sample ES15 at 395 cm depth or YDB AH glass displaying plant imprints (e.g., impressions of leaves and stems) from level 457, sample E313 at 413 cm depth. The highest temperature of 1850 °C achieved in the heating experiments is known to occur in impact events and atomic detonations but does not occur under any other natural terrestrial conditions except lightning strikes.

Melting plant-imprinted AH glass

Baseline temperatures were established by heating fragments of 12,800-year-old, plant-imprinted AH glass until they fully melted. At ~1100 °C, the glass softened but did not completely melt, retaining most of its original morphology. Plant imprints remained visible but became less pronounced. Thirteen original fragments melted within a range of 1200°–1300 °C. It is important to note that this is the minimum temperature to which they may have been exposed, not the maximum.

Melting AH bulk sediment

At 1100 °C, the bulk sediment showed no obvious melting. From 1200° to 1300 °C, the melted portion of AH sediment encapsulated abundant unmelted or partially melted mineral grains (Appendix, Fig. S3). Smaller quartz grains (approximately <50 µm) began partial melting at ~1300 °C, most likely because limestone (CaCO 3 ), soda ash (Na 2 CO 3 ), and other minerals acted as fluxing agents to lower the melting point of quartz. At 1700 °C, clusters of white minerals (identified by SEM-EDS as mostly quartz) remained visible on the solid AH glass surface, while nearly all of the remaining sediment and grains were transformed to transparent-to-translucent amber-brown-colored glass.

In summary, when bulk AH sediment was heated to ~1700 °C, the resulting meltglass had similar morphology and characteristics as AH glass. When cooled from ~1700 °C, this glass remained molten until it solidified at 1200° to 1300 °C. The same was true of actual pieces of AH glass. These results show this as the solidus temperature for both AH glass and laboratory meltglass produced from AH sediment, closely matching that reported by Thy et al.30. However, this is not the maximum temperature range possible, as claimed by Thy et al.30, but rather, the minimum temperature at which molten AH sediment solidifies. These laboratory experiments conclusively show that it is possible for AH glass to reach temperatures to ~1700 °C, much higher than its melting point of ~1200° to 1300 °C.

AH meltglass with carbon-rich siliceous plant imprints

Approximately 40% of AH meltglass pieces display one or more carbon-infused impressions, typically appearing as fine, parallel ridges and grooves (Fig. 4a–k) that closely correspond to the ribbed patterns on plant stems and leaves. This plant-imprinted AH glass typically contains inclusions of charcoal and burned carbon, all indicative of biomass burning closely associated with the formation of AH meltglass. Such plant imprints (with C, O, Mg, Al, Si) have been previously reported by Schultz et al.33, who found that temperatures above 1500 °C were necessary to entrap organics and that lower temperatures resulted in carbonization.

Figure 4 Plant imprints on surfaces of AH glass. (a–c) Photomicrographs of AH glass (AG) with silicified plant imprints (P). (d–k) SEM images of carbon-infused, siliceous plant imprints (P) on AH glass (AG). (i–m) Multi-element EDS maps of same objects in images above. Magenta represents Al, Blue represents Ca, and Gold represents Si. Carbon is not shown, but SEM-EDS indicates it is almost exclusively associated with the Si-rich imprints. From level 457, sample E313, 413 cm depth. (l–n) SEM images of polished charred organics embedded within AH glass. The reflectance-derived temperature is ~421 °C, much lower than the laboratory-measured melting point. AH meltglass from level 445, sample E301, 405 cm. Inferred temperature range of ~1200° to 1300 °C for melting AH glass. Full size image

SEM-EDS analyses indicate the imprints have distinct chemistry, averaging ~80 wt.% SiO 2 (range: 65–100 wt.%) with an average of ~20 wt.% carbon (range: 0–35 wt.%). The composition of non-imprinted AH glass is distinctly different, mainly containing a mix of oxides of Si, Ca, Fe, Al, and Mg, with only a few weight percent of carbon. SEM analyses reveal a three-layered morphology: (i) Ca-rich glass as the matrix; (ii) fused with microns-thick silicified plant material, typically only on one surface; and (iii) with a transition layer between the two.

This morphology may best be explained as resulting when airborne molten Ca-rich glass struck and fused to local plant material and then cooled to preserve the plant imprints. This carbon-infused meltglass is associated with melted magnetic spherules5 and nanodiamonds12, along with abundance peaks in nickel, cobalt, iridium, and platinum, as discussed below.

Reflectance as a temperature proxy

We investigated whether carbon infused into plant-imprinted glass might reveal clues to formation temperatures. To do so, we measured reflectance (%R 0 ), defined as the percentage of vertically incident monochromatic light reflected from a highly polished surface of a sample of carbon calibrated against the light reflected from a carbon standard of known reflectance34,35. Typically, carbon exposed to higher temperatures exhibits higher reflectance values. We determined the average reflectance-derived temperatures for two types of AH carbon: for carbon embedded in AH meltglass from level 445, the reflectance-inferred temperature was ~421 °C (Fig. 4l–m); for loose sedimentary charcoal found associated with AH meltglass, the temperature was ~391 °C. For carbon inside the AH glass, the inferred reflectance-derived temperatures are ~779° to 879 °C lower than the measured laboratory melt temperature range of ~1200° to 1300 °C. This important result demonstrates that melted glass can maintain higher temperatures than that of the charcoal it encloses (Appendix, Table S6). All experimental results for reflectance are shown in Appendix, Figs. S4–S6 and described in Appendix, Text S2.

Silicon-rich minerals as temperature indicators

Light microscopy and SEM-EDS analyses show that AH glass commonly contains several varieties of SiO 2 : (i) thin coatings of vitreous or glassy amorphous SiO 2, often called lechatelierite (Fig. 5a); (ii) subrounded melted glass objects that are nearly 100 wt.% SiO 2 (Fig. 5b); and (iii) partially to fully melted quartz grains (Fig. 5c).

Figure 5 Photomicrographs of varieties of SiO 2 on surfaces of AH glass. (a) ~12.5-mm-wide fragment of AH glass with a thin coating of SiO 2 (lechatelierite) that lines bubble cavities. The AH glass matrix is pale green, and the Si-rich coating is white. Under light microscopy, the surface is highly reflective and mostly vesicle-free with few inclusions. (b) Rounded, vesicular, rough-textured, 21-mm-wide AH glass fragment composed of SiO 2 (white areas) with some inclusions. (c) Dashed circles and arrows indicate partially melted quartz grains embedded in AH glass. (Equilibrium melting point of quartz is 1720 °C, with a flux-adjusted melting point of ~1520 °C. (a) from level 457, sample E313, 413 cm depth. (b–c) from level 445, sample E301, 405 cm depth. Full size image

Melted amorphous silica (SiO 2 as lechatelierite)

SEM-EDS analyses indicate that surfaces and vesicles of AH glass are commonly coated with lechatelierite (Figs. 5, 6), with a typical composition of SiO 2 at 88.3–98.7 wt.%. Other oxides showed an average of 2.9 wt.% for MgO; 2.0 wt.% Al 2 O 3 ; 1.3 wt.% K 2 O; 1.2 wt.% CaO; and 3.8 wt.% FeO. Many fragments of AH glass display sinuous flow marks8, indicating that high temperatures lowered viscosity enough so that the molten glass flowed before cooling very rapidly (Fig. 6a,b). In other cases, AH glass surfaces and vesicles display distinctive, highly organized, moiré-like textural patterns that appear to have formed as the molten glass cooled (Fig. 6c–e). In some examples, surface textures are distorted by irregularities in the glass surface (Fig. 6c,d), indicating that the thin lechatelierite surface coating flowed around topographical irregularities. In other cases, the cooling patterns appear to have conformed to cracks in AH glass that existed prior to deposition and cooling (Fig. 6e). These thin lechatelierite coatings most likely resulted from complete, high-temperature vaporization/deposition of quartz grains and/or of silica-rich plant parts. If so, the moiré-like patterns (Fig. 6c–e) likely formed during a rapid process analogous to vapor deposition (Appendix, Text S3).

Figure 6 SEM images of pure SiO 2 (lechatelierite). (a) Lechatelierite coating with swirling flow marks on an outer surface of AH glass. Sharp lechatelierite spikes are shown at the upper yellow arrow. From level 445, sample E301, 405 cm depth. (b) Lechatelierite coating on an outer surface of AH glass with oriented flow marks at arrows. (c) Distinctive patterns produced during the cooling of lechatelierite. The lechatelierite is a transparent outer layer on the wall of an interior vesicle and covers the Ca-rich aluminosilicate glass matrix. (d) Manually constructed EDS-based phase map shows how the surface irregularity (purple) distorted the thin lechatelierite coating (light red) that flowed over and around it, distorting the textural ridges. (e) Distinctive lechatelierite pattern that appears orthogonally oriented relative to crack in vesicle wall of AH glass. The lechatelierite coating is only several microns thick and these distortions indicate that lechatelierite flowed around the pre-existing grain. Glass samples are from level 457, sample E313, 413 cm depth. Full size image

Melted quartz grains (SiO 2 )

To determine maximum potential temperatures, we examined the surfaces of the AH meltglass for melted quartz (SiO 2 ) that typically melts at ~1720 °C. Light microscopy, SEM-EDS, and microprobe were used on both sectioned and non-sectioned materials to identify partially melted and fully melted quartz grains. One 15-μm-size quartz grain (Fig. 7a) was fully melted but generally retains its original outline; SEM-EDS analyses show that the central melted zone is 100 wt.% SiO 2 (Fig. 7b). Immediately adjacent to the outline of the grain, the diffusion zone (Fig. 7b) is composed of 10.3 wt.% typical sedimentary oxides (MgO, Al 2 O 3 , K 2 O, FeO, and CaO with no other oxides detected). Si and O account for the remaining 89.7 wt.%. If stoichiometric, the predicted amount of O is 58.0 wt.%, but instead, there is only 31.7 wt.% of O. This oxygen concentration is insufficient to produce oxides of the total measured concentrations of Mg, Al, K, Ca, and Si. These measured percentages suggest that the mixture possibly is composed of ~59.4 wt.% SiO 2 and ~30.3 wt.% native (elemental) Si. Native Si occasionally is produced under highly reducing conditions in volcanic exhalations, in some mantle-derived rocks, and in meteorites but is extraordinarily rare in other terrestrial settings.

Figure 7 SEM images of melted AH quartz grain. (a) SEM topographical image of 15-μm-wide grain on the inner wall of an AH glass vesicle. The darker gray central portion of the image is a smooth, melted quartz grain with no apparent crystalline structure. From level 445, sample E301, 405 cm depth. (b) Manually constructed EDS-based phase map with light red representing the aluminosilicate glass matrix, blue representing melted quartz grain, and green representing zone of diffusion of SiO 2 into the matrix. The crack was likely caused by thermal contraction during cooling. Percentages of SiO 2 range from 100 to 68.0 wt.%). (c) SEM topographical image of melted zircon of melted 8-μm-wide zircon grain embedded in an outer surface of AH glass. (d) Manually constructed EDS-based phase map of same grain with the zircon weight percentage ranging from 100 → 90 → 70 → 0 wt.%. (e) SEM image showing the location of zircon grain in melted 2.7-mm-wide AH glass fragment. From level 449, sample E305, 412 cm depth. Full size image

One observed melted example is an unusual 125-μm-wide shattered grain, an example of monomict quartz breccia (Appendix, Fig. S7). This is similar to brecciated rocks found at known impact sites, such as Meteor Crater, Arizona, where the surface impact of a NiFe impactor generated an extreme mechanical and thermal shock that fractured rocks and melted/boiled material along fracture zones. SEM-EDS analyses indicate that SiO 2 percentages range from 100 wt.% to 63 wt.% (Appendix, Fig. S7c).

Another piece of high-temperature AH glass exhibits an area ~260 µm wide in which one or more completely melted quartz grains are diffusing into the aluminosilicate matrix (Appendix, Fig. S8). SiO 2 percentages are in a gradient from 100 to 64.7 wt.%. A large highly vesicular, 1040-μm-wide quartz grain shows evidence that it reached the boiling temperature for quartz of ~2230 °C (Appendix, Fig. S9). The central part of the grain is unmelted quartz displaying a highly fractured and vesiculated surface (Appendix, Fig. S9b), surrounded by an area composed of an oxygen-deficient mix of native Si and SiO 2 (Appendix, Fig. S9b). The high percentage of Si from the original grain and the highly vesicular nature strongly suggest that the edges of the grain were boiling at ~2230 °C. The SiO 2 content continued to decline as the melted grain diffused into the AH glass matrix (Appendix, Fig. S9). A ternary diagram supports a theoretical minimum temperature range of ~1250 °C for the AH glass matrix and ~1720 °C for the quartz grain (Appendix, Fig. S10). Furnace experiments suggest that some fully melted quartz grains embedded in AH meltglass were subjected to temperatures of >1700 °C (Appendix, Fig. S11), whereas other grains show significant outgassing suggesting that they may have reached ≥2230 °C. In addition, the diffusion zones around some quartz grains in AH glass appear to contain elemental or native Si, suggesting that melting occurred in an oxygen-deficient atmosphere.

Because the accuracy of these analyses is crucial to identifying native Si, SEM-EDS analyses were performed on three reference quartz grains. In pure quartz, the expected value for Si is 46.7 wt.%; our average measured value for the three quartz grains using SEM-EDS was 46.1 wt.% (range 45.2 to 46.6 wt.%). The expected value for O is 53.3 wt.% and the average measured value was 53.9 wt.% (range 53.4 to 54.8 wt.%). These analyses indicate that the identification of oxygen-depleted native Si is accurate.

Ca silicate (wollastonite)

Crystals of another Si-rich mineral, calcium silicate (CaSiO 3 , wollastonite) were commonly observed on the outer surfaces and vesicle walls of AH glass but were never found inside the glass or on top of plant imprints (Appendix, Fig. S12). Wollastonite follows the triclinic crystal system, often forming distinctive star-like crystals. This mineral typically melts at 1540 °C with an estimated flux-mediated melting point of ~1340 °C.

Zirconium-rich minerals

Melted zircon, AH meltglass

A surface of one piece of AH glass contained a melted 8-μm-wide zircon grain (Fig. 7c–e). SEM-EDS analyses revealed a composition of Zr at 46.1 wt.%; Si at 15.8 wt.%; O at 36.0 wt.%; and Hf at ~2.1 wt.%. Zircons typically melt at ~1775 °C with a flux-mediated melting point of ~1575 °C. Full melting caused the zircon to lose its characteristic euhedral shape and to diffuse into the aluminosilicate AH glass matrix with weight percentages decreasing from 100 → 90 → 70 → 0 wt.% in the matrix.

Chromium-rich minerals

Excavated AH glass was examined for the presence of Cr-rich minerals, and the SEM-EDS analyses identified three such minerals: isovite ((Cr,Fe) 23 C 6 ) with the highest percentage of Cr = 67.4 wt.%; Fe = 26.8 wt.%; C = 5.8 wt.%; chromite (Fe2+Cr 2 O 4 ) with Cr = 46.5 wt.%; Fe = 24.9 wt.%; O = 28.6 wt.%; chrome-magnetite (Fe2+(Fe3+,Cr) 2 O 4 ) with Cr = 11.3 wt.%; Fe = 60.8 wt.%, and O = 27.9 wt.% (Fig. 8); and chromferide (Fe 3 Cr 1-x (x = 0.6)) with the lowest percentage of Cr at 11.0 wt.% and Fe = 89.0 wt.% (Fig. 9). Chromferide, a very rare mineral, has been previously found in impact melt rocks of the El’gygytgyn Crater, Chukotka, Russia36. Some Cr-rich grains observed in AH glass appear to be intergrowths of chromferide and isovite. The surface of one AH glass sample contains an embedded chromium-magnetite grain that displays flow marks consistent with aerodynamic mechanical deformation (elongation and twisting) while molten (Fig. 9). The SEM topographical image shows surface relief.

Figure 8 Melted chromite grains. (a) Two possible chrome-magnetite grains (Fe2+(Fe3+,Cr) 2 O 4 ), one ~10 µm long and the other ~7.5 µm long, that melted and flowed together on the inner wall of AH glass vesicle. (b) Manually constructed EDS-based phase map of SEM image in (a) with chrome-magnetite in blue and AH glass matrix in light red. Field of view also contains melted titano-magnetite grain (green; equilibrium melting point: 1625 °C) and post-depositional, unmelted calcium silicate in purple. Note melted droplet of AH glass fused to the top of melted chrome-magnetite at upper right, indicating the melted chrome-magnetite was deposited while the AH glass was molten. (c) Irregular chrome-magnetite grain inside AH glass vesicle shows evidence that it melted and flowed as the glass cooled. Chromium is mixed with gold. All grains from level 449, sample E305, 412 cm depth. Full size image

Figure 9 Flow marks in chromferride embedded in AH meltglass. Elongated chromferride grain, 18 μm long by 2.5 μm wide, embedded in the surface of AH meltglass. SEM-EDS indicates it is composed of 88.2 wt.% Fe and 11.8% Cr with almost no detectable oxygen. Grain displays flow marks that likely resulted from mechanical deformation while airborne and molten. Note adjacent darker bubbles in the Al-Si glass matrix. (a) SEM backscatter image. (b) SEM topographical image shows raised relief. (c) Single-element (Cr) EDS map with an intensity scale. (d) Single-element (Fe) EDS map with an intensity scale. From level 449, sample E305, 412 cm depth. Full size image

Iron-rich minerals

SEM-EDS analyses of the outer surfaces of AH glass and interior surfaces of vesicles revealed a high abundance of melted Fe-rich globules, mostly FeO, but sometimes containing native Fe and Fe silicides. Iron globules were only observed on outer surfaces of AH glass and inside vesicles but never on broken interior surfaces. Sometimes, the globules appear to have solidified atop the molten glass (Fig. 10); at other times, the globules are organized in rows (Fig. 10b,c), suggesting that they formed inside vesicles by vapor deposition of Fe. The surfaces of Fe-rich AH glass often display distinctive patterns that appear dendritic (branch-like or feather-like) (Appendix, Fig. S13). During the melting of AH sediment, it appears that these dendritic Fe crystals grew mostly by vapor deposition, as the molten glass cooled rapidly. Other forms of Fe also were observed (Fig. 11a–d), including melted magnetite (Fe2+Fe3+ 2 O 4 ) (Fig. 11a) and melted titanium-magnetite (Fe2+(Fe3+,Ti) 2 O 4 ) (Fig. 11f), containing ~10.1 wt.% Ti. We also identified highly reduced iron (FeO), containing 95.1 wt.% Fe and only 4.9 wt.% O (Fig. 11e),

Figure 10 Fe-rich globules on surfaces and in vesicles of AH glass. (a) SEM image of hundreds of Fe-rich globules on the outer surface of AH glass. Some globules are FeO and others are iron silicides. Note that globules occur only on original surfaces and not on broken ones (upper left and upper right). Gold arrow points to closeup of same region. (b) SEM close-up of globules. Some are round, but most are irregularly shaped. All are flattened, convex globules embedded on the surface of AH glass. (c) Multi-element SEM-EDS map of same area of (b) showing the distribution of multiple elements (see the color legend at bottom left). Fe-rich globules (red) on the AH glass matrix containing Ca and Si, with smaller amounts of Mg and/or Ni. All from level 457, sample E313, 413 cm depth. Full size image

Figure 11 Different forms of iron in Abu Hureyra meltglass. (a) SEM image of melted 23-μm-wide magnetite grain embedded in the outer surface of AH glass. Note crust on grain formed from thermal and chemical alteration. The high degree of vesiculation suggests magnetite possibly boiled at ~2623 °C. (b) Single-element (Fe) EDS map with intensity scale showing the diffusion of Fe into the glass matrix. This map also reveals that Fe combined with S to produce iron sulfide, FeS (discussed below). (c) Single-element (Si) EDS map with intensity scale showing Si-rich composition of the AH glass matrix. (d) Single-element (S) EDS map with intensity scale indicating the location of troilite (FeS). (e) Irregularly shaped, 150-μm-wide Fe globule on the outside surface of AH glass, composed of highly reduced iron at 95.1 wt.% Fe and the remainder as oxygen. (f) Melted 27-μm-wide titano-magnetite grain on the outside surface of AH glass. The equilibrium melting point is 1625 °C. High vesiculation suggests titano-magnetite grain reached or exceeded its boiling point. All samples from level 445, sample E301, 405 cm depth. Full size image

Oxygen fugacity (abbreviated ƒO 2 ) relates to the vapor pressure of oxygen. Under low ƒO 2 , little oxygen is available to combine with other elements to produce oxides and so low-oxygen minerals form37 in an unusual occurrence (Appendix, Text S4). Formational environments of native Fe and magnetite differ by two log units of ƒO 2 and by one to two log units between native Fe (Fe°) and Fe silicides. The oxygen fugacity of Fe-rich objects varied from highly reduced (forming native Fe and Fe silicides) to typical oxidizing conditions (forming magnetite), a range of several log units37. Based on the Fe-bearing mineral phases attached to vesicle walls of AH glass, it is inferred that exposure temperatures range from the equilibrium melting point of magnetite at ~1590 °C to its boiling point at ~2623 °C.

Melted nickel-rich iron

Our SEM-EDS investigations of AH glass identified three types of Ni-enriched minerals: (a) flattened, elliptical globules of Ni-magnetite ((Ni,Fe) 2 O 4 ) are common on vesicle walls inside AH glass (Fig. 12); (b) awaruite (Ni 3 Fe) is commonly found on the surfaces of AH glass spherules; and (c) small NiFe spheres and hemispheres (<200 µm in diameter) are found in linear alignments, in random groupings, and as isolated spheres often bordered by Ni-rich glass created by Ni diffusion (Fig. 12). There are also enrichments in concentrations of nickel (300 ppm), cobalt (68 ppm), and chromium (3750 ppm) in the YDB magnetic fraction of sample E301 at a depth of 405 cm, compared to typical concentrations in background samples of ≤20 ppm for Ni, ≤50 ppm for Co, and <800 ppm for Cr (Appendix, Table S7).

Figure 12 SEM images of Fe enriched in Pt, Ir, and Ni on outer surfaces of AH glass. (a) Melted 2.5-μm-wide AH subrounded glass. Box marks area with abundant NiFe globules. (b) Surface of AH glass with hundreds of NiFe globules with Ni content of ~2.0 wt.%. (c) Inside of vesicle in AH glass shown in (a), containing hundreds of NiFe globules. Middle globule at arrow contains ~19.5 wt.% Ni and ~9.7 wt.% Fe. (d) Multi-element EDS map (see the legend at lower left) of the surface of AH glass displaying numerous NiFe globules (reddish-orange), a combination of Fe (red) and Ni (yellow). Note there are no globules on the broken upper edge, indicating they did not form inside the AH glass, only on the surface, suggesting possible formation by vapor deposition. (e) Thin, flat fragment of AH glass with plant imprints and NiFe globules. Note that plant imprints formed on top of the NiFe, indicating that the imprinting occurred after the glass melted. (f) AH glass with long grain with ~20.9 wt.% platinum and ~6.9 wt.% iridium. (a–d) from level 457, sample E313, 413 cm depth. (e) from level 445, sample E301, 405 cm depth. (f) from level 449, sample E305, 412 cm depth. Full size image

The Ni-rich grains are dominated by three NiFe-rich minerals: kamacite (NiFe with Ni concentrations at 2–6.8 wt.%; Fe = 70–73.8 wt.% and the remainder as oxygen); taenite (NiFe with a higher percentage of Ni at 7.3–28.6 wt.%); and awaruite (Ni 2 Fe to Ni 3 Fe). Ni-magnetite is usually limited to hydrothermally serpentinized mafic zones of subducted ophiolites38 but is also found in some meteorites. These Ni-rich materials are most likely terrestrial in origin but with possible mixing with a small amount of meteoritic material, because most impactites contain very small meteoritic contributions (usually well below 1 wt.%)39.

The Ni-bearing spherules formed under highly reducing conditions at low ƒO 2 of approximately −6 to −8 log units, sometimes only a few microns away from oxidized magnetite that formed at a very different fugacity. This high variation over very short distances indicates highly variable and chaotic formation conditions. Kamacite and taenite usually only form in highly reduced environments with ƒO 2 of approximately −10 log units (iron-wüstite or IW buffer). Native Fe and NiFe are common inside several AH glass vesicles, suggesting temperatures of >1500 °C. Thus, some AH meltglass formed. We propose that the NiFe spherules resulted from the melting at >1500 °C under oxygen-deficient conditions (ƒO 2 between log −6 and −8) of AH bulk sediment with ~0.3 wt.% NiO, also containing ~10% Ni-rich magnetite, potentially mixed with Ni from the impactor.

Melted iron with platinum and iridium

SEM-EDS analyses identified a melted PGE-rich grain embedded in a vesicle that contains ~20.9 wt.% Pt; ~6.9 wt.% Ir; and ~56.3 wt.% Fe (Fig. 12f). This nugget was embedded in the wall of the vesicle when the AH glass was molten. The occurrence of Pt and Ir in AH glass is consistent with neutron activation analyses which showed high Pt concentrations in the YDB layer, in which peak concentrations of 6.2 ppb Pt were measured in bulk sediment and of 8.1 ppb Pt in the magnetic fraction from sample E301, the YDB layer (Appendix, Table S7). Layers above and below showed negligible concentrations of Pt.

Melted iron silicide

SEM-EDS analyses identified globules of iron silicides (FeSi, Fe 2 Si, Fe 3 Si) and native Si embedded in the walls of AH glass vesicles, as well as on outer glass surfaces (Fig. 13). In AH glass vesicles, ≥1-μm-wide spherules of Fe 3 Si (suessite) (Fig. 13) have an average composition of Fe = 24.5 atom% and Si = 75.5 atom% (n = 5; Si/(Si + Fe) = 24.5), consistent with Fe 3 Si. Other spherules in the same vesicle (Fig. 13c) have compositions similar to typical Ca-Al-Si AH glass. Inside another vesicle, an array of FeSi spherules was embedded in SiO 2 -rich silica glass (SiO 2 = 93 wt.%) with the same bulk composition as plant-imprinted AH glass, in which Fe = 48.9 atom%, Si = 51.3 atom% (Si/(Si + Fe) = 50.9 ratio), consistent with FeSi. In several AH glass vesicles, spheres of Fe 2 Si (hapkeite) were observed with an estimated elemental composition of Fe = 79 atom% and Si = 21 atom% (not the same as elemental ratios) with the structural formula of Fe 2 Si. Fe 2 Si Formula = (Fe 1.9 + Ni 0.8 + Cr 0.2 )2 (Si 0.9 + P 0.1 ), where Cr and Ni often substitute for Fe, and P sometimes substitute for Si.

Figure 13 SEM images of iron silicides on the inner walls of AH glass vesicles. (a) Small, rounded, bright blebs or globules are iron silicide (Fe 3 Si); the largest is ~22 µm across. AH glass matrix is 65 wt.% SiO 2. Dark, polygonal, recessed areas are 84 wt.% SiO 2 . (b) Close-up of hexagonal ~4-μm-wide globule of Fe 3 Si shown in (a). (c) Small higher-Z spherules of Fe 3 Si; largest is ~3 µm wide. Fragments at right are AH glass; the largest is ~9 µm wide. From level 457, sample E313, 413 cm depth. Full size image

Suessite (Fe 3 Si) is a very rare mineral on Earth, forming at high temperatures and low ƒO 2 , as for example, in fulgurites40. Suessite, in contrast, is common in meteorites, where the mineral was first discovered. It is commonly found in lunar meteorites; micrometeorite impact pits in Apollo samples; and highly reduced achondrites. Fe silicides in AH glass indicate exposure to extreme reducing atmospheres, 2 to 4 log units lower than the iron-wüstite (IW) buffer with temperatures >2000 °C.

It is likely that these highly reduced minerals originally formed in AH glass that contained vesicles filled with trapped gases. If so, abundant carbon, possibly from incinerated vegetation, created a reducing atmosphere that produced the reduced phases of various minerals, including Fe silicides, native Fe, and native Si.

Sulfide minerals

Melted Fe sulfide (troilite)

In some fragments of AH glass, globules of the mineral troilite (iron sulfide or FeS) were observed with a composition of ~64 wt.% Fe and ~36 wt.% S. FeS globules were observed only on surfaces of AH glass fragments and inside of vesicles (Fig. 14) but not within the meltglass, suggesting that they formed inside vesicles by vapor deposition.

Figure 14 SEM images of iron sulfide (FeS) in AH glass and trinitite. All images from the inner walls of melted glass vesicles. (a) Sub-micron FeS spherules in AH glass matrix; the largest is ~0.4 µm wide. (b) Large, flattened, textured ~45-μm-wide globule of FeS on AH glass. (c) Tiny, 0.15-μm-wide FeS spherules in AH glass along the edge of melted quartz grain. (d) Convex disc of ~20-μm-wide FeS globule on the surface of trinitite glass. Small rounded FeS globule lies on the larger disk. AH glass from level 457, sample E313, 413 cm depth. Trinitite sample provided by co-author R.E.H. Full size image

Melted Titanium sulfide

The inspection of surfaces and vesicles of AH glass revealed melted titanium sulfide (TiS) that is commonly associated with melted magnetite and other high-temperature melted minerals (Appendix, Fig. S14). When oxygen is sufficient the titanium in Ti-rich minerals typically combines with oxygen upon melting, but when there is insufficient oxygen, Ti combines with S to form sulfides and P to form phosphides. Thus, the presence of TiP and TiS in AH glass suggests high-temperature melting under highly reducing conditions.

Rare-earth-rich phosphate (monazite)

Inside some AH glass vesicles, SEM-EDS analyses identified melted monazite, a rare-earth phosphate ((Ce,La,Nd,Th)PO 4 ). In one sample, a faint outline of the original grain is still visible, but the grain has no distinct boundary due to melting and diffusion into the AH glass matrix (Fig. 15). The monazite grain surrounds part of a melted quartz grain that is in partial contact with the AH glass. The large gas bubbles visible in the monazite grain suggest outgassing as the monazite broke down and released oxygen at temperatures of ~2230 °C41. This observation is consistent with rapid cooling from a superheated liquid. As the material cooled, it reached a blocking temperature at which the vapor phase could no longer equilibrate with the liquid phase. At that stage, oxygen from the breakdown of oxides remained in the vapor phase, and the liquid phase became oxygen-deficient. This can only result from heating followed by very rapid cooling. The enclosing quartz grain also contains numerous small vesicles, suggesting that it possibly reached its boiling point of ~2230 °C, the same temperature required to boil the monazite grain.

Figure 15 SEM images of melted monazite in AH glass. (a) Bright material at center is a partially melted ~35-wide monazite grain on the outer surface of AH glass. This uncommon rare-earth phosphate grain has been partially absorbed by AH glass with decreasing EDS-measured percentages of monazite (100 → 54 → 46 → 0 wt.%). The equilibrium melting point of monazite is ~2072 °C. In addition, more than a dozen small, partially-melted, chromite grains are scattered throughout the image, mostly at the lower left. (b) Manually constructed EDS-based phase map showing the remnants of vesicular monazite grain (blue, #1) with diffusion of monazite components (green, #2-#3) into AH glass (light red, #4); globules of chromite (dark red, #5). Melted ~14-μm-wide quartz grain (purple, #6) is highly vesicular, suggesting it reached boiling temperatures of ~2230 °C. From level 449, sample E305, 412 cm depth. Full size image

Remanent magnetism as a formation criterion

We considered whether Fe-rich spherules and meltglass from Abu Hureyra may have been produced by lightning strikes. To investigate, we measured remanent magnetism, defined as the magnetization remaining after molten Fe-rich material cools while exposed to Earth’s magnetic field (e.g. geomagnetism) or some other intense magnetic field. Molten magnetic minerals eventually cool to a blocking temperature (Curie point), the point at which magnetic remanence is essentially fixed. All Fe-rich terrestrial magnetic minerals, whether they have been melted or not, possess a natural remanent magnetization (NRM)42. If those minerals were once melted, the resulting TRM (thermoremanent magnetization) provides a record of the strength of Earth’s natural magnetic field at the time the Fe-rich material cooled. This is the most commonly observed natural magnetization. The magnetic arrangement can vary from nearly perfect (saturation state) to nonexistent (demagnetized state), depending on the level of intensity of the ambient field during the cooling of the magnetic mineral. The efficiency of the magnetic intensity level is referred to as REM43 and follows an empirical magnetic acquisition law44. Some previously molten minerals can hold this magnetic arrangement longer than the age of our solar system45.

If any given material has a remanent magnetic value that differs from that of Earth’s natural ambient field, these measurements provide clues to the formation of that material. During an impact event, the magnetic oxides that formed during the quenching process in melted impact materials typically display low values for remanent magnetism, but they can be higher than normal background values, possibly related to ionization and charge separation within the plasma generated during an impact (or perhaps blast)46,47. Alternately, lightning-produced fulgurites typically display very high values for remanent magnetism.

In these experiments, the sample’s remanent magnetization was compared to its maximum value, called its saturation remanent magnetization. Induced magnetization and susceptibility are dependent terms (Mi = XB, where Mi is induced magnetization, X susceptibility, and B external field), indicative of the “induced magnetization” that characterizes the immediate response of the sample to the current external magnetic field.

Remanent magnetism of AH magnetic spherules

Six representative Fe-rich spherules were extracted from AH sediment, and a superconducting magnetometer was used to measure magnetic remanence (Appendix, Tables S8 and S9). Magnetic efficiency values were obtained48,49, along with efficiency demagnetization spectra (Appendix, Fig. S15a) that indicate levels of both paleomagnetic field intensity and magnetic coercivity, the measure of ferromagnetic material’s ability to withstand an external magnetic field without becoming demagnetized50,51. None of the six Abu Hureyra Fe-rich spherules contained remanent efficiency values within the range characteristic of lightning (Appendix, Tablse S8 and S9). During the demagnetization of these spherules, the magnetic vectors preserved their direction until the maximum level of demagnetization was reached (Appendix, Fig. S15b). This result indicates that the magnetic acquisition of the spherules originated from one direction at the moment the sample cooled through its magnetic blocking temperature while exposed to Earth’s geomagnetic field. These results conclusively indicate that the tested AH spherules did not acquire their remanent magnetism from lightning discharges.

Remanent magnetism of YDB meltglass

Remanent magnetism was measured for 3 samples of AH meltglass (Appendix, Tables S8 and S9). The Abu Hureyra meltglass samples contain normal magnetization with one component, meaning that these samples most likely were not produced by lightning43 (Fig. 16a). However, the results on the magnetic signatures are complicated. On the one hand, the inferred oxygen fugacities and mineralogies indicate intense temperatures and a reducing environment consistent with a plasma, which would have partially blocked (rather than enhanced) the terrestrial field. On the other hand, the results here indicate that some materials examined escaped this environment and cooled under conditions beyond the effects of any plasma. In other words, variable remanent magnetism (enhanced or reduced) in melts could indicate either lightning or a fireball, depending on their location during cooling.

Figure 16 Remanent Magnetism and Water Content (a) Magnetization efficiency of Fe-rich materials. Abu Hureyra closely matches lab-produced anthropogenic NiFe spherules, along with terrestrial magnetite, and titanomagnetite. Meteorites are lower in remanent magnetism and lightning-produced materials are much higher. Red vertical dashed lines indicate typical terrestrial values. (b) Graph and table of the water content of various glasses. The uppermost medium green bar represents water content in Abu Hureyra (AH) glass. The light green bar represents laboratory-produced, high-temperature reed glass. Red vertical dashed lines indicate upper and lower limits of water content in AH glass, which overlaps that of tektites (Australasian field), impact glass, trinitite, and fulgurites. The water content of AH glass is lower than measured examples of volcanic glass, biomass glass, and anthropogenic glass, eliminating those as likely formation mechanisms. Full size image

Water content as a temperature indicator

The water content of a melted material can be diagnostic for inferring origin by high or low temperatures, principally because very high temperatures typically cause outgassing of water and other volatiles, leaving behind ppm concentrations of water52. The H 2 O content was measured in AH glass, using established techniques of FTIR53,54. Measured absorbance was calculated by measuring the detected energy from light passing through the sample and comparing that with a reference spectrum using no sample.

The results are shown in Fig. 16b (Appendix, Table S10). The Abu Hureyra YDB glass was extracted from bulk sediment sample ES15, level 435, at a depth of ~392 cm. Water content ranged from 222–460 ppm (n = 19 measurements). This suggests that plant-imprinted AH glass was subjected to similarly high temperatures. The geochemical composition of samples was determined by microprobe (results in Appendix, Table S4).