A Tunguska sized airburst destroyed Tall el-Hammam a Middle Bronze Age city in the Jordan Valley near the Dead Sea

Melted quartz grains

Crystalline quartz melts between 1670 °C (tridymite) and 1713 °C (cristobalite), and because quartz is pervasive and easily identified, melted grains serve as an important temperature indicator. At TeH, we observed that unmelted potsherds displayed no melted quartz grains, indicating exposure to low temperatures. On the other hand, most quartz grains on the surfaces of pottery, mudbricks, and roofing clay exhibited some degree of melting, and unmelted quartz grains were rare. Nearly all quartz grains found on broken, unmelted surfaces of potsherds were also unmelted. On melted pottery and mudbricks, melted quartz has an estimated density of 1 grain per 5 mm².

Melted quartz grains at TeH exhibit a wide range of morphologies. Some show evidence of partial melting that only melted grain edges and not the rest of the grain (Figs. 22, 23). Others displayed nearly complete melting with diffusion into the melted Ca–Al–Si matrix of pottery or mudbrick (Fig. 22). Melted quartz grains commonly exhibit vesiculation caused by outgassing (Figs. 22, 23), suggesting that those grains rose above quartz’s melting point of ~ 1713 °C.

An SEM–EDS elemental map of one melted grain showed that the quartz had begun to dissociate into elemental Si (Fig. 22b). Another grain (Fig. 23c–e) displayed flow marks consistent with exposure to temperatures above 1713 °C where the viscosity of quartz falls low enough for it to flow easily. Another SEM–EDS analysis confirmed that one agglutinated mass of material is 100 wt.% SiO₂ (Fig. 23f, g), suggesting that this polycrystalline quartz grain shattered, melted, and partially fused again.

Discussion of melted quartz

Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4478″>17} reported that during heating experiments, many quartz grains < 50-µm-wide exhibited partial melting at as low as 1300 °C, but larger grains > 50-µm-wide remained visually unaltered up to ~ 1700 °C. By 1850 °C, all quartz grains fully melted. These experiments establish a particle-size dependency and confirm confirmed the melting point for > 50-µm-wide TeH quartz grains between ~ 1700–1850 °C. Melted > 50-µm-wide quartz grains on the surfaces of melted pottery and mudbrick from the TeH destruction layer indicate exposure to these unusually high temperatures > 1700 °C.

Previously, Thy et al.⁷⁰ proposed that glass at Abu Hureyra did not form during a cosmic impact, but rather, formed in biomass slag that resulted from thatched hut fires. However, Thy et al. did not determine whether or not high-temperature grains existed in the biomass slag. To test that claim, Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4489″>17} analyzed biomass slag from Africa and found only low-temperature melted grains with melting points of ~ 1200 °C, consistent with a temperature range for biomass slag of 1155–1290 °C, as reported by Thy et al.⁷¹. Upon testing the purported impact glass from Abu Hureyra, Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4497″>17} discovered high-temperature mineral grains that melt in the range of 1713° to > 2000 °C, as are also found in TeH glass. These test results suggest that the melted glass from Abu Hureyra must have been exposed to higher temperatures than those associated with fires in thatched huts. Because of the presence of high-temperature minerals at TeH, we conclude that, as at Abu Hureyra, the meltglass could not have formed simply by burning thatched huts or wood-roofed, mudbrick buildings.

Melted Fe- and Si-rich spherules

The presence of melted spherulitic objects (“spherules”) has commonly been used to help identify and investigate high-temperature airburst/impact events in the sedimentary record. Although these objects are referred to here as “spherules,” they display a wide range of other impact-related morphologies that include rounded, sub-rounded, ovate, oblate, elongated, teardrop, dumbbell, and/or broken forms ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4510″>17,72,73,74,75,76,77,78,79,80,81,82}. Optical microscopy and SEM–EDS are commonly used to identify and analyze spherules and the processes by which they are formed. Care is needed to conclusively distinguish high-temperature spherules produced by cosmic impacts from other superficially similar forms. Other such objects that frequently occur in sediments include anthropogenic spherules (typically from modern coal-fired power plants), authigenic framboids (Supporting Information, Fig. S7), rounded detrital magnetite, and volcanic spherules.

Spherules in TeH sediment were investigated from stratigraphic sequences that include the MB II destruction layer at four locations: palace, temple, ring road, and wadi (Fig. 24). For the palace (Field UA, Square 7GG), the sequence spanned 28 cm with 5 contiguous samples of sediment ranging from 3-cm thick for the MB II destruction layer to 13-cm thick for some outlying samples. In the palace, 310 spherules/kg (Fig. 24d) were observed in the destruction layer with none found in samples above and below this layer. For the temple (Field LS, Square 42J), 5 continuous samples spanned 43 cm and ranged in thickness from 6 to 16 cm; the MB II layer contained ~ 2345 Fe- and Si-rich spherules/kg with 782/kg in the sample immediately below and none at other levels (Fig. 24c). Six contiguous samples from the ring road (Field LA, Square 28 M) spanned 30 cm with all 5 cm thick; the MB II destruction layer at this location contains 2150 spherules/kg with none detected in younger or older samples (Fig. 24b). Five discontinuous samples from the wadi spanned 170 cm, ranging from 10-cm thick for the destruction layer up to 20-cm thick for other samples; the MB II destruction layer at this location contained 2780 spherules/kg with none in samples from other levels (Fig. 24a, Supporting Information, Table S3). Notably, when melted mudbrick from the ring road was being mounted for SEM analysis, numerous loose spherules were observed within vesicles of the sample, confirming a close association between the spherules and meltglass. At all four locations, the peaks in high-temperature spherule abundances occur in the MB II destruction layer dating to ~ 1650 BCE.

SEM images of spherules are shown in Figs. 25, 26, 27 and 28, and compositions are listed in Supporting Information, Table S4. The average spherule diameter was 40.5 µm with a range of 7 to 72 µm. The dominant minerals were Fe oxides averaging 40.2 wt.%, with a range of up to 84.1 wt.%; elemental Fe with a range of up to 80.3 wt.%; SiO₂ averaging 20.9 wt.%, ranging from 1.0 to 45.2 wt.%; Al₂O₃ averaging 7.8 wt.% with a range of up to 15.6 wt.%; and TiO₂ averaging 7.1 wt.% with a range of up to 53.1 wt.%. Fourteen spherules had compositions > 48 wt.% of oxidized Fe, elemental Fe, and TiO₂; five spherules contained < 48 wt.% Fe and Ti; and two had > 75 wt.% Fe with no Ti. Eight of 23 spherules analyzed contained detectable levels of Ti at up to 53.1 wt.%.

Two unusual spherules from the palace contain anomalously high percentages of rare-earth elements (REEs) at > 37 wt.% of combined lanthanum (La), and cerium (Ce) (Fig. 26), as determined by preliminary measurements using SEM–EDS. Minor oxides account for the rest of the spherules’ bulk composition (Table S1).

One 54-µm-wide sectioned spherule contains titanium sulfide (TiS) with a melting point of ~ 1780° C. TiS, known as wassonite, was first identified in meteorites (Fig. 27) and has been reported in impact-related material ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4761″>17,81,83}. However, TiS sometimes occurs as an exsolution product forming fine networks in magnetite and ilmenite and can be of terrestrial origin.

One unusual piece of 167-µm-wide Ca–Al–Si meltglass contains nearly two dozen iron oxide spherules on its surface (Fig. 28). The meltglass contains a completely melted quartz grain as part of the matrix (Fig. 28b). Most of the spherules appear to have been flattened or crushed by collision with the meltglass while they were still partially molten (Fig. 28c).

Discussion of spherules and meltglass

Melted materials from non-impact-related combustion have been reported in multiple studies. Consequently, we investigated whether Ca-, Fe-, and Si-rich spherules and meltglass (mudbrick, pottery, plaster, and roofing clay) may have formed normally, rather than from a cosmic impact event. For example, (i) glassy spherules and meltglass are known to form when carbon-rich biomass smolders below ground at ~ 1000° to 1300 °C, such as in midden mounds⁷¹. They also form in buried peat deposits⁸⁴, underground coal seams⁸⁵, burned haystacks⁸⁶, and in large bonfires, such as at the Native American site at Cahokia, Illinois, in the USA⁸⁷. (ii) Also, ancient fortifications (hillforts) in Scotland and Sweden, dating from ~ 1000 BCE to 1400 AD, have artificially vitrified walls that melted at temperatures of ~ 850° to 1000 °C⁸⁸. (iii) Partially vitrified pottery and meltglass derived from the melting of wattle and daub (thatch and clay) with estimated temperatures of ~ 1000 °C have been reported in burned houses of the Trypillia culture in Ukraine^89,90. (iv) Vitrified mudbricks and pottery that melted at < 1100 °C have been reported at Sardis, Turkey, dating to the seventh and sixth centuries BCE⁹¹; in the northern Jordan Valley at an Early-Bronze-Age site called Tell Abu al-Kharaz⁹²; and at Early-Bronze-Age Tell Chuera in Syria⁹³. All these sites describe melting temperatures ranging from ~ 850° to 1300 °C.

In another example of meltglass, vitrified bricks at Tell Leilan in Syria, dating to ~ 2850 to 2200 BCE, are estimated by Weiss⁹⁴ to have melted at ~ 1200 °C, and he attributed high-calcium spherules to low-temperature combustion of thatch roofing materials⁹⁵. However, thatch has low calcium content, leading Courty⁹⁶ to propose that this material formed from melted lime-based plaster during an airburst/impact at Tell Leilan ~ 550 years before the destruction of TeH. Courty⁹⁶ reported aluminosilicate spherules with unusual high-temperature elemental nickel (melting point: ~ 1455 °C); complex vesicular glass particles that contain terrestrially rare unoxidized nickel inclusions; and both single and multiple calcite spherules (melting point = 1500 °C, as measured by experiments in this contribution).

For the melted materials, there is a definitive difference: high-temperature minerals are embedded in meltglass at TeH but none are present at these other sites (except for Tell Leilan). To explore this difference, Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4856″>17} investigated biomass glass from midden mounds in Africa and found no high-temperature minerals. For this contribution, we used SEM–EDS to examine aluminosilicate meltglass from an underground peat fire in South Carolina, USA; meltglass in coal-fired fly ash from New Jersey, USA; and mining slag from a copper mine in Arizona, USA. All these meltglass examples display unmelted quartz and contain no other high-temperature melted grains, consistent with low-temperature melting at < 1300 °C.

At the sites with non-impact meltglass, estimated temperatures were consistently less than 1300 °C, too low to melt magnetite into Fe-rich spherules, e.g., with compositions of > 97% wt.% FeO, as are found at TeH. Nor can these low temperatures produce meltglass and spherules embedded with melted zircon (melting point = 1687 °C), chromite (2190 °C), quartz (1713 °C), platinum (1768 °C), and iridium (2466 °C). Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e4863″>17} confirmed that the melting of these high-temperature minerals requires minimum temperatures of ~ 1500° to 2500 °C.

This evidence demonstrates that although the matrix of the spherules and meltglass at TeH likely experienced incipient melting at temperatures lower than ~ 1300 °C, this value represents only the minimum temperature of exposure, because the high-temperature minerals embedded in them do not melt at such low temperatures. Instead, the spherules and meltglass at TeH must have reached temperatures greater than ~ 1300 °C, most likely involving brief exposure to ambient temperatures of ~ 2500 °C, the melting point of iridium. These temperatures far exceed those characteristic of city fires and other types of biomass burning. In summary, all of this evidence is consistent with very high temperatures known during cosmic impacts but inconsistent with other known natural causes.

Calcium carbonate spherules and plaster

In sediments of the destruction layer, we observed amber-to-off-white-colored spherules (Fig. 29) at high concentrations of ~ 240,000/kg in the palace, ~ 420/kg in the temple, ~ 60/kg on the ring road, and ~ 910/kg in the wadi (Supporting information, Table S2). In all four profiles, the spherules peak in the destruction layer with few to none above or below. Peak abundances of calcium carbonate spherules are closely associated with peak abundances of plaster fragments, which are the same color. By far the most spherules (~ 250× more) occurred in the destruction layer of the palace, where excavations showed that nearly every room and ceiling was surfaced with off-white lime-based plaster. Excavators uncovered high-quality lime plaster fragments still adhering to mudbricks inside the MB II palace complex, and in one palace room, we uncovered fragments of melted plaster (Fig. 29e). In contrast, lime plaster was very rarely used in buildings on the lower tall, including those near the temple.

To explore a potential connection between plaster and spherules, we performed SEM–EDS on samples of the palace plaster. Comparison of SEM–EDS analyses shows that the plaster composition has a > 96% similarity to the spherule composition: CaCO₃ = 71.4 wt.% in plaster versus 68.7 wt.% in the spherules; elemental C = 23.6 versus 26.3 wt.%; SiO₂ = 2.4 versus 1.8 wt.%; MgO = 1.7 versus 2.0 wt.%; and SO₃ = 0.94 versus 1.2 wt.%. The high carbon percentage and low sulfur content indicate that the plaster was made from calcium carbonate and not gypsum (CaSO₄·2H₂O). SEM imaging revealed that the plaster contains small plant parts, commonly used in plaster as a binder, and is likely the source of the high abundance of elemental C in the plaster. Inspection showed no evidence of microfossils, such as coccoliths, brachiopods, and foraminifera. The morphology of the spherules indicates that they are not authigenic or biological in origin.

Discussion of carbonate plaster and spherules

One of the earliest known uses of CaCO₃-based plaster was in ~ 6750 BCE at Ayn Ghazal, ~ 35 km from TeH in modern-day Amman, Jordan⁹⁷. At that site, multi-purpose lime plaster was used to make statues and figurines and to coat the interior walls of buildings. Because the production of lime-based plaster occurred at least 3000 years before TeH was destroyed, the inhabitants of TeH undoubtedly were familiar with the process. Typically, lime powder was produced in ancient times by stacking wood/combustibles interspersed with limestone rocks and then setting the stack on fire. Temperatures of ~ 800–1100 °C were required to transform the rocks into crumbly chalk, which was then mixed with water to make hydrated lime and plastered onto mudbrick walls⁹⁷.

At TeH, fragments of CaCO₃-based plaster are intermixed in covarying abundances with CaCO₃-based spherules with both compositions matching to within 96%. This similarity suggests that the carbonate spherules are derived from the plaster. We infer that the high-temperature blast wave from the impact event stripped some plaster from the interior walls of the palace and melted some into spherules. However, it is difficult to directly melt CaCO₃, which gives off CO₂ at high temperatures and decomposes into lime powder. We investigated this cycle in a heating experiment with an oxygen/propylene torch and found that we could decompose the plaster at ~ 1500 °C, the upper limit of the heating test, and begin incipient melting of the plaster. The heated plaster produced emergent droplets at that temperature but did not transform into free spherules (Supporting Information, Text S2).

Similar spherules have been reported from Meteor Crater, where spherules up to ~ 200 μm in diameter are composed entirely of CaCO₃ formed from a cosmic impact into limestone^98,99. One of several possible hypotheses for TeH is that during the impact event, the limestone plaster converted to CaO with an equilibrium melting point of 2572 °C. However, it is highly likely that airborne contaminants, such as sodium and water vapor, reacted with the CaO and significantly lowered the melting point, allowing spherule formation at ≥ 1500 °C.

The proposed chemical sequence of events of plaster formation and the later impact are as follows:

1. 1.

   Limestone was heated to ~ 800–1100 °C, decomposing to quicklime:

1. 2.

   Quicklime was mixed with water to make a wet plaster:

   $${text{CaO }} + {text{ H}}_{{2}} {text{O }} to {text{ Ca}}left( {{text{OH}}} right)_{{2}}$$

1. 3.

   The plaster hardened and slowly absorbed CO₂ to revert to CaCO₃:

   $${text{Ca}}left( {{text{OH}}} right)_{{2}} + {text{ CO}}_{{2}} to {text{ H}}_{{2}} {text{O }} + {text{ CaCO}}_{{3}}$$

1. 4.

   The high-temperature impact event melted some plaster into spherules:

   $${text{CaCO}}_{{3}} to {text{ CaO }}left( {{text{spherules}}} right) , + {text{ CO}}_{{2}} left( { > {15}00^circ {text{C}}} right)$$

1. 5.

   CaO spherules slowly absorbed CO₂ to revert to CaCO₃:

   $${text{Ca }} + {text{ CO}}_{{2}} to {text{ CaCO}}_{{3}} left( {text{as spherules}} right)$$

General discussion of all spherules

According to the previous investigations ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5405″>17,72,81,82}, Fe-rich spherules such as those found at TeH typically melt at > 1538 °C, the melting point of iron (Table 1). Because of the presence of magnetite (Fe₃O₄) in the REE spherule, its melting point is inferred to be > 1590 °C (Table 1). The Si-rich spherules are similar in composition to TeH sediment and mudbrick, and thus, we propose that they were derived from the melting of these materials at > 1250 °C. The carbonate-rich spherules likely formed at > 1500 °C.

Several studies describe a mechanism by which spherules could form during a low-altitude cosmic airburst^100,101. When a bolide enters Earth’s atmosphere, it is subjected to immense aerodynamic drag and ablation, causing most of the object to fragment into a high-temperature fireball, after which its remaining mass is converted into a high-temperature vapor jet that continues at hypervelocity down to the Earth’s surface. Depending on the altitude of the bolide’s disruption, this jet is capable of excavating unconsolidated surficial sediments, melting them, and ejecting the molten material into the air as Si- and Fe-rich spherules and meltglass. This melted material typically contains a very low percentage (< 1 wt.%) of impactor material¹⁰².

Melted zircon grains

To more accurately determine the maximum temperatures of the destruction layer, we used SEM–EDS to comprehensively investigate melted minerals on the outer surfaces of melted pottery and mudbricks. We searched for and analyzed zircon (melting point: ~ 1687 °C), chromite (~ 2190 °C), and quartz (~ 1713 °C) ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5451″>17}.

Melted zircons in pottery and mudbricks were observed (Fig. 30) at an estimated density of 1 grain per 20 mm². On highly melted surfaces, nearly all zircons showed some degree of melting. In contrast, nearly all zircons found on broken interior surfaces were unmelted (Fig. 30d), except those within ~ 1 mm of melted surfaces. This implies that the temperature of the surrounding atmosphere was higher than the internal temperatures of the melting objects. Unmelted potsherds displayed only unmelted minerals.

The melted zircons in TeH materials exhibit a wide range of morphologies. Most showed evidence of sufficient melting to alter or destroy the original distinctive, euhedral shape of the grains. Also, the grains were often decorated with vesicles that were associated with fractures (Fig. 30a, c).

Stoichiometric zircon contains 67.2 wt.% and 32.8 wt.% ZrO₂ and SiO₂ respectively, but in several TeH samples, we observed a reduction in the SiO₂ concentration due to a loss of volatile SiO from the dissociation of SiO₂. This alteration has been found to occur at 1676 °C, slightly below zircon’s melting point of 1687 °C¹⁰³. This zircon dissociation leads to varying ZrO₂:SiO₂ ratios and to the formation of distinctive granular textures of pure ZrO₂, also known as baddeleyite¹⁰⁴ (Figs. 30, 31, 32). With increasing time at temperature, zircon will eventually convert partially or completely to ZrO₂. Nearly all zircons observed on the surfaces of melted materials were either melted or showed some conversion to baddeleyite. We observed one zircon grain (Fig. 32d–e) displaying granular ZrO₂ associated with three phases that span a wide range of SiO₂ concentrations, likely formed at temperatures above 1687 °C. This extreme temperature and competing loss of SiO over an inferred duration of only several seconds led to complex microstructures, where grains melted, outgassed, and diffused into the surrounding matrix.

Discussion of melted zircon

Zircon grains have a theoretical, equilibrium melting point of ~ 1687 °C. Under laboratory heating ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5631″>17}, zircon grains showed no detectable alteration in shape at ~ 1300 °C but displayed incipient melting of grain edges and dissociation to baddeleyite beginning at ~ 1400 °C with increasing dissociation to 1500 °C ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5635″>17}. Most zircon grains < 50 µm completely melted at a temperature of ~ 1500 °C, while at ~ 1700 °C, the shapes of zircon grains > 120 µm were still recognizable but displayed considerable melting ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5639″>17}. These experiments establish a lower melting range for TeH zircon grains of ~ 1400° to 1500 °C.

Patterson¹⁰⁵ showed that zircon dissociation becomes favorable above 1538 °C and particles between 1 and 100 µm in size melted and dissociated when passing through a plasma, forming spherules with various amounts of SiO₂ glass containing ZrO₂ crystallites ranging in size from 5 nm to 1 µm. The majority of zircon crystals were monoclinic, but tetragonal ZrO₂ was observed for the smaller crystallite sizes. Residence times were in the order of 100 ms, and the specific ZrO₂ to SiO₂ ratio within each spherule depended on the particle’s time at temperature¹⁰⁶.

Bohor et al.¹⁰⁴ presented images of impact-shocked zircons from the K-Pg impact event at 66 Ma that are morphologically indistinguishable from those at TeH. Decorated zircon grains are uncommon in nature but commonly associated with cosmic impact events, as evidenced by two partially melted zircons from the known airburst/impact at Dakhleh Oasis, Egypt (Fig. 30e). The presence of bubbles indicates that temperatures reached at least 1676 °C, where the zircon began to dissociate and outgas. Similar dissociated zircon grains also have been found in tektite glass and distal fallback ejecta (deposited from hot vapor clouds). Granular baddeleyite-zircon has been found in the ~ 150-km-wide K-Pg impact crater¹⁰⁷ and the 28-km-wide Mistatin Lake crater in Canada¹⁰⁷. The dissociation of zircon requires high temperatures of ~ 1676 °C¹⁰⁴, implying that TeH was exposed to similar extreme conditions.

Melted chromite grains

Examples of melted chromite, another mineral that melts at high temperatures, were also observed. Thermally-altered chromite grains were observed in melted pottery, melted mudbricks, and melted roofing clay from the palace. Their estimated density was 1 grain per 100 mm², making them rarer than melted zircon grains. The morphologies of chromite grains range from thermally altered (Fig. 33a) to fully melted (Fig. 33b, d). One chromite grain from the palace displays unusual octahedral cleavage or shock-induced planar fractures (Fig. 33b). The typical chemical composition for chromite is 25.0 wt.% Fe, 28.6 wt.% O, and 46.5 wt.% Cr, although the Cr content can vary from low values to ~ 68 wt.%. SEM images reveal that, as chromite grains melted, some Cr-rich molten material migrated into and mixed with the host melt, causing an increase in Cr and Fe, and corresponding depletion of Si. The ratio of Cr to Fe in chromite affects its equilibrium melting point, which varies from ~ 1590 °C for a negligible amount of Cr up to ~ 2265 °C for ~ 46.5 wt.% Cr as in chromite or chromian magnetite ((Fe)Cr₂O₄), placing the melting point of TeH chromite at close to 2265 °C.

Discussion of melted chromite

Chromite grains theoretically melt at ~ 2190 °C. Moore et al. ^{2200 °C. Sci. Rep. 4185 (2020).” href=”https://www.nature.com/articles/s41598-021-97778-3#ref-CR17″ id=”ref-link-section-d16227753e5750″>17} reported the results of heating experiments in which chromite grains in bulk sediment showed almost no thermal alteration up to ~ 1500 °C (Supporting Information, Fig. S8). At temperatures of ~ 1600 °C and ~ 1700 °C, the shapes of chromite grains were intact but exhibited limited melting of grain edges. These results establish a range of ~ 1600° to 1700 °C for melting chromite grains.

Because chromite typically does not exhibit cleavage, the grain exhibiting this feature is highly unusual. Its origin is unclear but there are several possibilities. The cleavage may have resulted from exsolution while cooling in the source magma. Alternately, the lamellae may have resulted from mechanical shock during a cosmic impact, under the same conditions that produced the shocked quartz, as reported by Chen et al.¹⁰⁸ for meteorites shocked at pressures of ~ 12 GPa. Or they may have been formed by thermal shock, i.e., rapid thermal loading followed by rapid quenching. This latter suggestion is supported by the observation that the outside glass coating on the potsherd does not exhibit any quench crystals, implying that the cooling progressed very rapidly from liquid state to solid state (glass). This is rare in terrestrial events except for some varieties of obsidian, but common in melted material produced by atomic detonations (trinitite), lightning strikes (fulgurites), and cosmic airburst/impacts (meltglass)⁸¹. More investigations are needed to determine the origin of the potentially shocked chromite.

Nuggets of Ir, Pt, Ru, Ni, Ag, Au, Cr, and Cu in meltglass

Using SEM–EDS, we investigated abundances and potential origins (terrestrial versus extraterrestrial) of platinum-group elements (PGEs) embedded in TeH meltglass, in addition to Ni, Au, and Ag. Samples studied include melted pottery (n = 3); melted mudbrick (n = 6); melted roofing clay (n = 1), and melted lime-based building plaster (n = 1). On the surfaces of all four types of meltglass, we observed melted metal-rich nuggets and irregularly shaped metallic splatter, some with high concentrations of PGEs (ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt)) and some nuggets enriched in silver (Ag), gold (Au), chromium (Cr), copper (Cu), and nickel (Ni) with no PGEs (Figs. 34, 35). Importantly, these metal-rich nuggets were observed only on the top surfaces of meltglass and not inside vesicles or on broken interior surfaces.

Using SEM–EDS, we identified variable concentrations and assemblages of PGEs. The metallic particles appear to have melted at high temperatures based on the minimum melting points of the elements: iridium at 2466 °C; platinum = 1768 °C; and ruthenium = 2334 °C, indicating a temperature range of between approximately 1768° and 2466 °C. Our investigations also identified two PGE groups, one with nuggets in which Pt dominates Fe and the other with metallic splatter in which Fe dominates Pt.

Pt-dominant nuggets

We conducted 21 measurements on Pt-dominant TeH nuggets on meltglass (Fig. 34a–c). The nuggets average ~ 5 µm in length (range 1–12 µm) with an estimated concentration of 1 nugget per 10 mm². For these nuggets, Fe concentrations average 1.0 wt.%, Ir = 6.0 wt.%, and Pt = 44.9 wt.% (Supporting Information, Tables S6, S7). The presence of PGEs was confirmed by two SEM–EDS instruments that verified the accurate identification of PGEs through analyses of several blanks that showed no PGE content. Some concentrations are low (< 1 wt.%) with high uncertainties (± 100 wt.%), and elemental concentrations greater than 10 wt.% have certainties of approximately ± 10 wt.%, making these results preliminary and indicating the need for further analyses. However, even though uncertainties are high, the relative elemental relationships of Fe > Pt or Pt > Fe were found to be consistent between the two instruments.

To determine the source of TeH nuggets and splatter, we constructed ternary diagrams. Terrestrial PGE nuggets are commonly found in ore bodies that when eroded, can become concentrated in riverine placer deposits, including those of the Jordan River floodplain. To compare Fe–Ir–Pt relationships among the TeH nuggets, we compiled data from nearby placer deposits in Greece¹⁰⁹, Turkey^110,111, and Iraq¹¹², along with distant placers in Russia^113,114,115, Canada¹¹⁶, and Alaska, USA^117,118. The compilation of 109 Pt-dominant placer nuggets indicates that the average Fe concentration is 8.2 wt.%, Ir = 2.9 wt.%, and Pt = 80.3 wt.%. For the Ir-dominant placer nuggets (n = 104), Fe = 0.4 wt.%, Ir = 47.8 wt.%, and Pt = 5.3 wt.% (Supporting Information, Tables S6, S7). The ternary diagrams reveal that the values for Pt-dominant TeH nuggets overlap with Pt-dominant terrestrial placer nuggets but the Fe-dominant splatter is dissimilar (Fig. 36a).

Fe-dominant splatter

We made 8 measurements on TeH Fe-dominant PGE splatter (Fig. 34d–f). The metal-rich areas average ~ 318 µm in length (range 20–825 µm) with an estimated concentration of 1 PGE-rich bleb per mm², 100× more common than the TeH nuggets. Average concentrations are Fe = 17.5 wt.%, Ir = 4.7 wt.%, and Pt = 1.5 wt.%.

We explored a potential extraterrestrial origin by constructing ternary diagrams for comparison of TeH Fe-dominant splatter with known meteorites and comets (Fig. 36b, c). We compiled data for 164 nuggets extracted from carbonaceous chondritic meteorites (e.g., Allende, Murchison, Leoville, and Adelaide)^{119,120,121,122}, seafloor cosmic spherules^123,124, iron meteorites^122,125, Comet Wild 2¹²⁶, and cometary dust particles¹²⁶. For average weight percentages, see Supporting Information, Tables S6, S7. The Fe-dominant TeH splatter (Fig. 36b) closely matches nuggets from carbonaceous chondrites and cosmic spherules but is a weak match for most iron meteorites (Fig. 36c). In addition, the TeH nuggets are similar to four cometary particles, two of which were collected during the Stardust flyby mission of Comet Wild 2 in 2004¹²⁶. For average weight percentages, see Supporting Information, Tables S6, S7.

To further explore an extraterrestrial connection for TeH Fe-dominant splatter, we compiled wt.% data for TeH PGEs (Rh, Ru, Pd, Os, Ir, and Pt) and normalized them to CI chondrites using values from Anders and Grevasse¹²⁷. We compared those values to CI-normalized nuggets in carbonaceous chondrites, including CV-type chondrites (e.g., Allende) and CM types (e.g., Murchison)^{119,120,122,128,129,130,131}, seafloor cosmic spherules¹²⁴, micrometeorites¹²³, and iron meteorites^122,125. These results are shown in Fig. 36d.

The TeH Fe-dominant splatter closely matches all types of extraterrestrial material with a similar pattern among all data sets: Pd has the lowest normalized values and Os and/or Ir have the highest, closely followed by Pt. The TeH splatter was also compared to the CI-normalized wt.% of bulk meteoritic material from CV- and CM-type chondrites (Fig. 36d). The composition of TeH splatter shows poor correlation with bulk chondritic materials, although the splatter is an excellent geochemical match with the PGE nuggets inside them. In summary, the CI normalization of PGEs suggests an extraterrestrial origin for the Fe-dominant TeH splatter, just as the ternary diagrams also suggest an extraterrestrial source. The correspondence of these two independent results suggests that the quantification of PGEs is sufficiently accurate in this study.

Another unusually abundant element, Mo, is also associated with Fe-dominant splatter but not with Pt-dominant nuggets. Mo averages 0.3 wt.% with up to 1.1 wt.% detected in Fe-dominant splatter but with none detected in TeH Pt-dominant nuggets. Mo also is not reported in any terrestrial placer nuggets and occurs in low concentrations (less than ~ 0.02 wt.%) in iron meteorites. In contrast, Mo is reported at high concentrations in PGE nuggets from carbonaceous chondrites (~ 11.5 wt.%), cosmic spherules (0.6 wt.%), and cometary material (5.8 wt.%). Thus, the Mo content of TeH splatter appears dissimilar to terrestrial material but overlaps values of known cosmic material, suggesting an extraterrestrial origin.

Based on the volume and weight of the meltglass, we estimate that the extraterrestrial-like metallic TeH Fe-dominant splatter represents < 1 wt.% of the total mass of meltglass. This low concentration is typical of low impactor content in impact ejecta (< 1 wt.%)¹⁰².

Ag, Au, Cr, Cu, and Ni in TeH nuggets

We also investigated nuggets that lack PGEs. The geochemistry of these nuggets shows two distinct populations, one Ni-dominant and one Fe-dominant (Fig. 34g–i). Twelve measurements of TeH samples show enrichments in Ag averaging 5.7 wt.%, Au = 0.6 wt.%, Cr = 2.2 wt.%, Cu = 2.8 wt.%, and Ni = 3.7 wt.%. All particles appear to have been melted at high temperatures: silver at ~ 961 °C; gold at 1064 °C; chromium at 1907 °C; copper at 1085 °C; and nickel at 1455 °C.

Ternary diagrams for Cr, Fe, and Ni show that some TeH nuggets exhibit chemical similarities to mineral deposits in Greece, Turkey, and Oman, suggesting that some nuggets are of terrestrial origin. However, other TeH nuggets are chemically similar to materials found in iron meteorites, chondrites, achondrites, and comets (Supporting Information, Fig. S9). When compared to meteoritic material, the Ni-dominant group roughly corresponds to measurements from sulfide inclusions in chondrites¹³², and the Fe-dominant group overlaps both chondritic sulfide inclusions and metal-rich grains from Comet Wild 2¹³³. These results suggest that a small fraction (< 1 wt.% of bulk) of the Cr- and Ni-rich dust found embedded in TeH meltglass is of extraterrestrial origin, most likely ejected from an airburst/impact of either a meteorite or a comet.

Discussion of PGE nuggets

Importantly, the PGE-rich nuggets and splatter were observed embedded only on melted surfaces of the TeH meltglass but not inside the vesicles or within the meltglass. This suggests that the nuggets and splatter were not contained in the original sedimentary matrix but were fused onto the TeH glass while still molten. Geochemical analyses suggest a dual origin. The Pt-dominant nuggets do not match known extraterrestrial material and instead appear to be of terrestrial origin, possibly from placer deposits and regional mines. It is unclear exactly how they became embedded onto but not inside TeH meltglass, but one possibility is that they were originally buried as river-laid, PGE-rich placer deposits. If so, we propose that they were ejected during the impact event and distributed across the molten glass by the impact blast wave. Another possibility is that they derive from jewelry and raw precious metals in the palace complex that were pulverized and dispersed during the high-velocity destruction of the palace.

In contrast, the Fe-dominant nuggets fused into the surfaces of TeH meltglass closely match the composition of nuggets from chondritic meteorites, cosmic spherules, and comets, consistent with an extraterrestrial origin. The data suggest that a carbonaceous chondrite or a comet detonated in the air near TeH, pulverized PGE-rich nuggets within the bolide, accreted terrestrial placer nuggets, and dusted both terrestrial and extraterrestrial material across the surfaces of molten mudbricks, pottery, and building plaster at low concentrations of < 1 wt.%.

Platinum, iridium, and palladium in sediment

For TeH bulk sediment, neutron activation analyses show Pt abundance peaks in the destruction layer of all profiles tested (Fig. 37a–d) at ~ 2× to 8× an average crustal abundance of 0.5 ppb. Sedimentary Ir was below detection at < 0.1 ppb for the palace, temple, and ring road, but unusually high in the wadi, where values peaked at 1.0 ppb, ~ 50× larger than a crustal abundance of 0.02 ppb (Supporting Information, Table S3). Also, Pt/Pd ratios in bulk sediment from the destruction layers are anomalously higher than background layers by ~ 4× to 14× (Fig. 37e–h).

Discussion of Pt, Ir, and Pd

Abundances of Pt, Ir, and the Pt/Pd ratios all peak in sediment at or near the top of the destruction layer, suggesting an influx of those elements at 1650 BCE, most likely from both extraterrestrial and terrestrial sources. Sedimentary concentrations of Ir only peak in the wadi samples and were not detectable at the other three sites for unknown reasons.

Meltglass vesicles lined with metal-rich minerals

The interior portions of many melted pieces of mudbrick, roofing clay, and pottery are highly vesicular, and the walls of these vesicles nearly always display an array of metal-rich crystals (Fig. 38). These include elemental iron and iron oxides, labeled as FeO, but is actually oxidized as hematite (Fe₂O₃) with a melting point of ~ 1565 °C; magnetite (Fe₃O₄) that melts at ~ 1590 °C; and/or elemental iron (Fe) melting at ~ 1538 °C (Fig. 38a, b, d, f, g). Also observed on vesicle walls were crystals of Fe phosphide (Fe₂P) that melts at ~ 1100 °C (Fig. 38c, g), manganese oxide (MnO) at ~ 1945 °C (Fig. 38d), calcium phosphate (Ca₃(PO₄)₂) at ~ 1670 °C (Fig. 38e), and calcium silicate (CaSiO₂) at ~ 2130 °C (Fig. 38e).

Discussion of minerals in vesicles

In some cases, these mineral crystals appear to have crystallized from the molten matrix as it solidified. In other cases, it appears that they plated onto the vesicle surface, suggesting that they may have condensed through vapor deposition from the high-temperature, mineral-saturated atmosphere within the vesicle.

Melted iron and titanium

SEM inspection of the palace mudbrick meltglass revealed partially melted grains of magnetite (Fe₃O₄) with a melting point of ~ 1590 °C (Fig. 39a, b) and titanomagnetite (TiFe₂O₄) with a melting point of ~ 1550 °C (Fig. 39c, d). The latter is an oxyspinel that commonly occurs as discrete grains or as an exsolution product within magnetite. The chemical composition of magnetite at equilibrium is 72.36 wt.% Fe and 27.64 wt.% O. Titanomagnetite is 21.42 wt.% Ti, 49.96 wt.% Fe, and 28.63 wt.% O. SEM–EDS analysis confirms similar compositions for the observed TeH grains.

Discussion of melted Fe and Ti

These grains display bubble-rich features that are commonly associated with grain fractures. There are several possibilities. (i) Approximately 20% of the time, magnetite grains are reported to be naturally overprinted by porous magnetite as a precipitation product, creating a bubble-like texture¹³⁴. (ii) Alternatively, these features may be textures caused by differential dissolution^135,136. (iii) The grains may have been exposed to temperatures equal to or greater than their melting point, causing the outgassing of volatiles or the rapid reduction of iron oxides. Because of the morphological dissimilarity of TeH grains to published examples of grains altered by precipitation and dissolution, we infer that these grains were altered by exposure to high temperatures.

Melted iron sulfide and iron phosphide

Sulfide and phosphide grains were found attached to the walls of the vesicles within mudbrick meltglass. SEM–EDS analyses of palace mudbrick meltglass identified melted Fe sulfide (FeS), also known as troilite (Figs. 40, 41), with a composition of 63.53 wt.% Fe and 36.47 wt.% S and a melting point of ~ 1194 °C. Commonly found associated with the Fe sulfide, Fe phosphide (Fe₂P; Fig. 40b, e) is a nickel-poor variety of barringerite, a mineral first identified at Meteor Crater in Arizona. Another variant of Fe phosphide, Fe₃P, was also identified in melted mudbrick from the palace. Both phosphide variants melt at ~ 1100 °C. Fe phosphide is common in meteorites, but although terrestrially rare, Fe₂P is found in pyrometamorphic rocks, such as are found in the Hatrurim Formation in nearby Israel¹³⁷. Britvin et al.¹³⁷ report that the local Fe phosphide displays averages for Fe at 76.4 wt.% and P at 21.4 wt.%, with small amounts of Ni, Co, and Cr at 2.2 wt.%. The composition of Fe phosphide found at TeH is comparable, averaging 78.3 wt.% Fe and 21.7 wt.% P. However, unlike those Fe₂P grains from Israel, the TeH grains lack detectable Ni, Co, and Cr.

Discussion of iron sulfide and phosphide

Visual inspection indicated that the sulfide and phosphide grains were attached to the inner surfaces of vesicles, and therefore, most likely formed by vapor deposition at > 1100 °C, rather than by crystallization from the melted matrix. Troilite (FeS) is very rare terrestrially but common in meteoritic material^81,138. Harris et al.¹³⁸ reported finding inclusions of troilite (FeS) as meteoritic clasts in Chilean meltglass proposed to have derived from a cosmic airburst that left meltglass on the surface along a 19-km-long stretch of the Atacama Desert approximately 12,800 years ago. At that site, troilite is typically found lining the walls of vesicles in the meltglass, as is the case for TeH meltglass.

Melted calcium phosphide

SEM–EDS analyses also show that vesicles in melted mudbrick from the palace contain calcium phosphide (Ca₃P₂) (Fig. 42). This mineral has a stoichiometric composition of ~ 66.0 wt.% Ca and ~ 34.0 wt.% P and melts at ~ 1600 °C (Table 1). Unlike Fe sulfide and Fe phosphide discussed above that appear to have formed by vapor deposition, these examples most likely crystallized from the molten Ca–Al–Si matrix material.

Melted calcium silicate (wollastonite)

SEM–EDS analyses of palace mudbrick meltglass (Fig. 43a, b) and melted pottery (Fig. 43c) reveal the presence of Ca silicate, also known as wollastonite (CaSiO₃), with a composition of 48.3 wt.% CaO and 51.7 wt.% SiO₂ and a melting point ~ 1540 °C (Table 1). These crystals are mostly found on broken surfaces of the matrix but also are sometimes observed inside vesicles. Unlike Fe sulfide and Fe phosphide above, which appear to have formed by vapor deposition, these crystals appear to have condensed from the molten matrix as it cooled.

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