Pinpointing the thermal history of lunar basaltic meteorites in a nutshell

Domain: earth_science
Published: 2025-05-01
Editorial outcome: minor_revision

Abstract
Native metals of igneous origin are recognized as one of the distinctive features of basalts from the Moon. Our study demonstrates that the tiny Fe-Ni metal blebs nucleated from lunar magma exhibit unanticipated, nutshell-like, crystallographic microstructures, that can be used in conjunction with local variations in chemical compositions to trace the complete thermal history of the host rock. This encompasses rapid cooling on the lunar surface, shock reheating upon impact, and post-shock cooling in the outer space. The Fe-Ni metal blebs indicate that the lava cooled in two stages, first down to 90–160 °C during lunar daytime, and then, to −160 °C during nighttime. Upon shock, the (near-)equilibrium peak temperature of 660–690 °C was reached shortly after release to zero pressure, mainly by thermal conduction from hot vesicular plagioclase. Here, we show that intense shock heating is compatible with the small impact scenario put forward for lunar meteorites and that metals are pioneering full-range geothermometers capable of tracing the complete thermal history of achondritic planetary materials.

Introduction
Fe-Ni alloys of extraterrestrial origin have been the subject of extensive scrutiny over the last decades, to gain insight into the evolution of the early solar system1–6. The majority of these studies have focused on iron and iron-rich meteorites cooled at extremely low rates and under near-equilibrium conditions. The goal was to better constrain the phase boundaries and the stability fields of the Fe-Ni phase diagram. At the opposite end of the spectrum, the thermal history of stony meteorites, such as the planetary and asteroidal achondrites, has been deciphered from very different geothermometers, based either on the partitioning of chemical elements or isotopes between co-existing silicate minerals7,8, or on the site occupancy of specific atoms within the crystal lattice9. While these geothermometers have proven effective in determining the closure temperatures in near-equilibrium systems with well-known diffusion rates and partition coefficients, they have shown significant limitations when applied to shocked rocks, i.e., extremely chaotic systems in which nearly instantaneous and highly heterogeneous heating upon impact is the norm. Moreover, since standard geothermometers are dependent on the diffusion of chemical elements, which limits their use below closure temperatures, they are unable to record the cold stages of rock history.

To address the potential of Fe-Ni metal blebs as geothermometers for achondritic rocks, we investigated a subsample of meteorite NWA 479, an exceptionally fresh and unbrecciated mare basalt10 paired with meteorite NWA 03211,12. Meteorite NWA 479 exhibits large phenocrysts of chromite, olivine, and pyroxene, within a fine-grained groundmass of intergrown pyroxene and plagioclase10 (see Supplementary Section 1). The dating indicates a crystallization age of 2.8 Ga13–15 and an ejection age of ~50 ka16,17 (Moon-Earth transfer plus terrestrial residence time).

Here, we show that the complete thermal history of lunar meteorites can be elucidated in detail, encompassing processes such as igneous cooling and shock reheating upon impact, as well as post-shock cooling in the outer space. The analysis of sub-microstructural features and chemical compositions retrieved from the Fe-Ni metal blebs, in conjunction with a refined version of the T-X Fe,Ni phase diagram, indicates that high equilibrium peak-shock temperatures are compatible with a small impact scenario involving a shallow ejection depth from a small crater excavated by a moderately energetic impactor.

Results
Nutshell-like microstructure of Fe-Ni metal blebs
The Fe-Ni metals in meteorite NWA 479 form elliptical to sub-rounded blebs, with a diameter ranging between 5 and 30 μm. Most of them are located within the igneous groundmass, in direct contact with large euhedral chromite, or, in a few instances, as inclusions within olivine phenocrysts (Fig. S2). These features indicate that nucleation and growth occurred at depth, at an early stage in the crystallization sequence of the magma (see Supplementary Section 1). The Fe-Ni metal blebs studied here should not be confused with those of meteoric origin that have been frequently reported in the lunar regolith18,19.

The apparent homogeneity of the Fe-Ni metal blebs in standard field emission-scanning electron microscope (FE-SEM) backscattered electron (BSE) images is challenged by electron backscatter diffraction (EBSD) crystallographic orientation maps, which reveal a previously unknown, core-rim, microstructure, surprisingly reminiscent of a nut nestled within its shell (Fig. 1a). The cores of the Fe-Ni metal blebs are polycrystalline and primarily composed of sub-μm to μm-sized, twinned, and rather lath-shaped body-centered cubic (hereafter bcc) grains, along with tiny interstitial face-centered cubic (hereafter fcc) grains. The cores are typically encased in a single-crystal fcc rim that exhibits smooth and gradual misorientations of up to ~50°, which suggests that the material was deformed under a plastic regime. The sporadic occurrence of sharper misorientations across (sometimes concentric) fractures indicates that the rim was also subject to moderate semi-brittle deformation. The recrystallization of the rim into twinned and deformation-free fcc grains is uncommon and occurs only on a local scale. The Fe-Ni metal blebs with a thin fcc rim (less than one-third of the bleb radius) show a structurally chaotic intermediate band, up to 0.3 μm in thickness, between the core and the rim (Fig. 1a, and blebs 2 and 6 in Figs. S3 and S4, respectively). The intermediate band is made of tiny bcc and fcc grains, measuring between 0.1 and 0.3 μm. These are approximately one order of magnitude smaller than the bcc grains observed in the core, and no clear crystallographic preferred orientations could be identified. No intermediate band was observed in blebs with a thick fcc rim (bleb 3 in Fig. S3, and blebs 4 and 5 in Fig. S4). The different mechanical behavior of the fcc rims and bcc cores, with the prevalence of pervasive plastic deformation in the former and the near absence thereof in the latter, can be attributed to the lower critical resolved shear stress (CRSS) of the fcc lattice than the bcc one20–22, and the massive decrease in ductility with grain size23.

Parent grain identification using the ARPGE software24 indicates that the bcc grains in the core have originated from a single fcc crystal having the orientation of the present rim (Fig. 1b and Figs. S3–S5). The preferred distribution of poles for bcc grains, with clusters forming threefold stars and bowtie patterns, typically reflects the Kurdjumov-Sachs25 and Nishiyama-Wassermann26,27 inter-grain orientation relationships widely reported for fcc-to-bcc transformations in steels. Ultra-high resolution crystallographic orientation mapping (ACOM) in the field emission-transmission electron microscope (FE-TEM) indicates that the fcc rim exhibits similar crystallographic orientations to the majority of the interstitial fcc grains present in the core and the intermediate band (Fig. 1c). This suggests that these grains have been retained after incomplete transformation of the parent fcc single crystal.

Distinct dislocation substructures are present from the core to the rim (Fig. 1d). In the core center, regular planar arrays of dislocations form subgrain boundary walls meeting at triple junctions, while in the core outer border, most grains show straight and parallel dislocations crossing at right angles. The rim is characterized by a poorly defined, cell-like, structure delimited by a network of dislocations exhibiting varying degrees of density and entanglement. In the intermediate band, the dislocation distribution is chaotic, though obtaining a definitive picture of it is hindered by the tiny size of the grains (i.e., significantly smaller than the thickness of the TEM foil).

Cavities of up to several micrometers in diameter and well-documented in the metallurgical literature as Kirkendall voids28 have been observed in the core of some Fe-Ni metal blebs. In many cases, these coalesce to form a single, larger cavity, resulting in hollow blebs that are suggestive of nut-free shells. Hollow Fe-Ni metal blebs typically exhibit the smallest bcc grains, with a size of <1 μm. No systematic correlation has been identified between the nature (hollow or solid) of the blebs, and the occurrence of an intermediate band between the core and the rim. In contrast to the solid blebs, however, the hollow ones are commonly filled or lined with Fe-Ni-rich (>50 vol%) glass, which occasionally spreads outward to form a distinct layer between the bleb and the surrounding phases (Fig. 2a). The interface between the Fe-Ni metal and the Fe-Ni-rich glass is uneven and often propagates within the bleb rim to form strings of globular melting features or more sizable clusters (Fig. 2b). The Fe-Ni-rich glass itself is devoid of vesicles, though it is sometimes fractured. Despite the wealth of information they provide, the hollow blebs have been deemed ineligible as thermal indicators, particularly during the shock pulse (see Supplementary Section 2).

Additionally, porosity occurs in the form of contraction gaps, measuring between 0.05 and 0.5 μm in thickness, between the Fe-Ni metal blebs and the surrounding basalt (Fig. 1a and Figs. S3–S5). The formation of contraction gaps is attributed to the pronounced shrinkage of Fe-Ni metals on cooling, which is in line with their high thermal expansion coefficients (except below 100 °C for alloys with an Invar composition) compared to those of the surrounding minerals29–31.

Other noteworthy characteristics not previously documented in earlier petrographic descriptions of meteorites NWA 479 and NWA 032 include the widespread vesiculation of plagioclase glass, overpressure cracks linking pairs of vesicles nucleated on either side of pyroxene grains, and Si-rich liquids forming mushroom-shaped or rounded blobs that are immiscible with plagioclase glass. Local melting of refractory phases occurs, including ilmenite (which should be distinguished from the ilmenite dendrites grown on igneous cooling) and pyroxene in contact with the plagioclase glass (Figs. 3 and S6). All these features are observed throughout the rock, at distances of up to hundreds of micrometers away from shock melt veins or pockets, which precludes the possibility of formation through friction heating.

An abrupt step disrupting the smooth core-to-rim chemical zoning
Energy-dispersive X-ray spectroscopy (TEM-EDS) maps and profiles performed in bleb 1 (Fig. 4a) indicate chemical zoning, with a gradual decrease in Fe and a corresponding increase in Ni (and Co) from the core to the rim. The strength of this gradient is not constant along the bleb radius, exhibiting a steeper slope in the inner portion of the rim than in the core center and the external portion of the rim. In addition to this long-range (μm-scale) core-to-rim zoning, the bcc grains of the core display a short-range (nm-scale) depletion of Ni towards grain boundaries, while interstitial fcc grains are noticeably enriched in Ni (Fig. 4b). Importantly, an unusually abrupt change in chemical composition occurs across the interface between the core and the intermediate band, i.e., with values for the bcc grains jumping from 77.6 wt% Fe, 19.3 wt% Ni, 3.2 wt% Co to 74.0 wt% Fe, 22.7 wt% Ni, 3.4 wt% Co, respectively (see Table S1). No such prominent compositional step has been observed across the intermediate band and the rim, with a composition on either side of the interface of ~67.8 wt% Fe, ~28.6 wt% Ni, and ~3.6 wt% Co.

The chemical compositions obtained from in-situ analyses of seven Fe-Ni metal blebs (Table 1) using wavelength-dispersive X-ray spectroscopy (WDS) are in good agreement with the TEM-EDS data of bleb 1, indicating that core-to-rim zoning is a common trait across all blebs of lunar meteorite NWA 479. A correlation is observed between the thickness of the rim and the occurrence of an intermediate band on the one hand, and the composition of the outer border of the rim on the other hand. The Fe-Ni metal blebs with a thin fcc rim and an intermediate band consistently show (Fe/Ni) ratios greater than 1.2, whereas those with a thick fcc rim and no discernible intermediate band are below this value. It is hypothesized that the flatter core-to-rim zoning profiles observed in the former allow for sufficient resolution of the intermediate band by EBSD. However, this assertion cannot be fully verified due to the variable accessibility of the core centers for WDS chemical measurements (peripheral sectioning, absence of a core center in hollow blebs), or the fortuitous incorporation of shallow interstitial Ni-rich fcc grains within the X-ray emission volume. Furthermore, WDS analyses indicate that the total concentration of the trace and minor elements P, Cr, S, F, Cl, Ti, Si, and Ca is consistently below 1.5 wt%. The most abundant of these elements is Cr, with concentrations of 0.50–0.99 wt% in the cores, and 0.25–0.64 wt% in the outermost portions of the rims (with the exception of bleb 7, which grew at a distance from chromite; see Table 1 and Fig. S5). As is the case with any lunar basalts containing native metals, the carbon content is expected to be extremely low. This is in accordance with the strong reduction of the magma at depth, which involved the scavenging of oxygen to form carbon monoxide and dioxide at an early stage of the igneous history32,33.

Towards a new geothermometer for lunar basalts
The complete thermal history of meteorite NWA 479, illustrated in the T-X Fe,Ni phase diagrams of Fig. 5, was retrieved from the coupling of the sub-microstructures and the chemical compositions of the Fe-Ni metal blebs with the temperatures of the fcc-to-bcc (martensitic) and bcc-to-fcc (austenitic) finish transformation curves (labeled MF and AF). The methodology used to refine the position and shape of MF and AF (originally established at ambient pressure and for binary, coarse-grained, Fe-Ni alloys34), with the microstructural and chemical attributes of the Fe-Ni metals blebs studied here is detailed in Supplementary Section 4. In brief, we show that the modifications and shifts that must be applied to the key points in the MF and AF curves are primarily influenced by the presence of alloying elements, with a minor contribution from the grain size in the inner portion of the intermediate band. Conversely, the initial particle size and the compression regime induced by (largely relaxed) differential volumetric changes have a negligible effect on MF and AF.

The question of whether the transformations should be better designated as martensitic or bainitic is not crucial in this context, first because martensite and bainite start (and finish) temperature curves are equivalent for Fe-Ni alloys containing more than 5 wt% Ni35, and then because some ambiguity still persists between the dominant schools of thought36 regarding the nomenclature to be used in the phase transformations of carbon-free or -poor Fe-Ni metals. For the sake of simplicity, the transformations were named simply as fcc-to-bcc (or bcc-to-fcc in the case of back transformations, also referred to as reverse or austenitic). A general explanation of martensitic transformations can be found in Supplementary Section 4.

Igneous origin of core-to-rim Fe-Ni zoning
During the initial, high-temperature (~1500 °C), stage of cooling, the chemical composition of the blebs was homogeneous, with constant Fe/Ni (and Fe/Co) ratios from the core to the rim. As the temperature decreased, the growth of chromite phenocrysts resulted in a notable depletion of Fe in the melt. A chemical gradient developed between the Fe-rich metal blebs and the comparatively Fe-poor melt, resulting in the diffusion of Fe from the blebs towards the melt and the formation of the long-range core-to-rim zoning still visible today. Alternatively, the loss of Fe from the rim of the blebs may also have been facilitated by a shift of the melt towards more oxidizing conditions during fractional crystallization, as reported elsewhere33. The observation that the distinctive bleb that grew away from chromite (bleb 7; Fig. S5) is also the least depleted in Fe (Table 1) and the largest in size (i.e., the one having the smallest surface-to-volume ratio, and therefore, the lowest predisposition for massive Fe loss) lend support to this hypothesis. The diffusion of Fe occurred over a period of several days during the slow and gradual cooling of the melt37 at depth, at a rate of less than 2 °C/h10. In contrast to Fe, Ni did not diffuse out of the blebs due to its strong siderophile character and its comparatively low chemical affinity among the crystallizing phases. Given that Fe diffusion in metals is predominantly a vacancy mechanism and that Fe loss was not compensated by a corresponding intake of Ni, vacancies progressively accumulated in the core of some of the blebs, leading to the formation of one or more large cavities (Figs. 2 and S4).

The eruption of basaltic lava onto the lunar surface was accompanied by a more rapid cooling rate of 20–60 °C/h10. Initially consisting of large fcc single crystals, the Fe-Ni metal blebs started to transform into small bcc grains when crossing the fcc-to-bcc transformation curve (Fig. 5). The transformation started in the Ni-poor core centers at a temperature of ~550 °C and proceeded radially outwards with further cooling. The rejection of Ni during the fcc-to-bcc transformation is evidenced by its depletion in the crystallizing bcc grains, and its subsequent enrichment in the retained interstitial fcc ones, as well as beyond the core-rim (today’s core-intermediate band) interface (Fig. 4). Despite the relatively low temperatures, short-range, nm-scale, Ni diffusion was enabled by the high density of subgrain boundaries and defects in the bcc grains. These features served as fast diffusion pathways, consistent with the observation that Ni diffusion coefficients are several orders of magnitude higher38–41 in defect-rich Fe-Ni alloys than in the bulk42. In other words, Ni atoms did not accumulate at the bcc grain boundaries, as they were continuously removed and stored in the interstitial fcc grains of the core and beyond the core-rim interface. This process may have been facilitated by the volumetric changes associated with the fcc-to-bcc transformation, which, at least temporarily, exerted pressure on the bcc grain boundaries.

The Ni-rich outermost portions of the blebs did not undergo any fcc-to-bcc transformation, as they never crossed MF throughout the entirety of the cooling process (Fig. 5). Instead, they were stabilized as single-crystal fcc rims, thereby preserving the crystallographic orientation of the parent blebs. One exception is bleb 7, which underwent complete transformation into fcc upon igneous cooling due to its anomalous Ni-poor periphery (see Table 1 and Fig. S5).

Two-stage day- and nighttime surface cooling of lava
The timing of surface igneous cooling could be refined in a serendipitous way based on the microstructures and the chemical compositions observed in the Fe-Ni metal blebs. For the normalized Ni concentration of 19.9 wt% and 23.5 wt% measured on each side of (what was still at that time) the core-rim interface (Table S1), MF yields a cooling temperature ranging between 90 °C and 160 °C, which is in excellent agreement with the present day-time equatorial to near-equatorial surface temperature of 120 °C reported from direct orbital measurements and computer modeling43. This range also aligns with the conditions that prevailed 2.8 Ga ago, when the amount of thermal energy emitted by the Sun was 20% lower but equally counterbalanced by a higher dose of infrared irradiation generated by the hotter Moon’s interior. With the sudden temperature drop at lunar dusk, the fcc-to-bcc transformation resumed, pushing the reaction front further outwards. The normalized Ni concentration of 29.7 wt% (Table S1) measured at the interface between the (today’s) intermediate band and the rim indicates a temperature of −160 °C at MF. This value is ~20 °C higher than the present equatorial to near-equatorial surface temperature reported by the same authors43, but it is very well in line with the idea of enhanced irradiation from the Moon’s interior during the early Eratosthenian period. Hence, our data indicate that the core-rim interface, which shifted from its inner to its outer position at dusk, corresponds to the locations of the daytime and nighttime fcc-to-bcc reaction fronts, respectively. Based on the post-eruption cooling rates of 20–60 °C/h reported in the literature10, it can be deduced that basalt NWA 479 required 16 to 48 h to reach the daytime surface temperature of 90–160 °C. Furthermore, given the length of lunar days (~350 h), it is probable that this initial surface cooling stage was separated from the second by a rest period during which no fcc-to-bcc transformation occurred.

The two-stage surface cooling process had significant implications for the history of basalt NWA 479. First, it induced the decoupling of the fcc-to-bcc reaction front from the abrupt compositional step (Fig. 4), which was formed by Ni rejection during daytime cooling. The occurrence of such a marked change in composition is important, as it will constitute a major hindrance for the reverse bcc-to-fcc transformation of the major part of the core during shock reheating (see below). Then, the two-stage cooling process indicates that the post-eruption burial depth for basalt NWA 479 was very shallow since lunar day- and nighttime temperatures are homogenized to ~−30 °C at a depth of only 0.4 to 0.8 m43,44. Had the rock been located deeper into the lava flow, no shift of the reaction front would have occurred. This view fits well with the high post-eruption cooling rate of 20–60 °C/h reported for this rock10, and with the very shallow shielding depth of 42 g/cm2 (equivalent to ~0.1 m considering a density of 3.3 g/cm3 for mare basalts) deduced from isotopic analyses of stable cosmogenic noble gases16. Furthermore, this interpretation is not in conflict with the complex multi-stage cosmic-ray exposure history retrieved from cosmogenic radionuclide concentrations14,15,17,45. It is noteworthy, however, that the remarkable freshness of basalt NWA 479 suggests that the rock was likely shielded shortly after crystallization, within the first 10 Ma or so. This could have been achieved by the emplacement of very thin younger lava flows, or overturned regolith layers resulting from the reworking of the uppermost stratigraphic units of the lunar surface46.

During igneous cooling, the Fe-Ni metal blebs were only mildly affected by deformation. Weak plastic, then semi-brittle, deformation occurred in the rim, first as a consequence of differential contraction47 of its medial portion (which has an Invar composition) with respect to its inner and outer portions (with ~29.7 wt% and ~44.8 wt% Ni, respectively; see Table S1), and then because of the ~4% volumetric increase accompanying the formation of the bcc core. It is still unclear whether the formation of the intermediate band was influenced by the presence of a compression zone between the growing bcc core and the fcc rim. Nevertheless, we argue that the effect of compression sensu stricto was, at most, marginal. This interpretation is first supported by the presence of a contraction gap between the blebs and the surrounding basalt, which provides some flexibility to accommodate volumetric changes and release pressure. More importantly, intermediate bands have also been observed in hollow blebs (see bleb 4 in Fig. S4), in which the large internal cavity can readily accommodate the volumetric change resulting from the fcc-to-bcc transformation of comparatively small core fragments. This observation is incompatible with the hypothesis that the intermediate bands are formed by compression.

Therefore, at the end of igneous cooling, the Fe-Ni metal blebs consisted of a core of small bcc (and retained fcc) grains, a prominent compositional step disrupting the smooth and gradual core-to-rim Fe-Ni zoning, and further outwards, a mildly deformed single-crystal fcc rim. In this regard, the broad outlines of the nutshell-like microstructure were already established at the end of the igneous cooling phase.

Long incubation time inhibiting full bcc-to-fcc reversal upon shock
The tremendous amount of energy released by the impactor during the shock pulse had a very significant influence on the microstructures of the Fe-Ni metal blebs. The features attributed to shock metamorphism include the closure of the contraction gaps, the strong plastic deformation of the fcc rims, the thermal annealing of bcc grains in the core centers, and the sporadic recrystallization of the fcc rims into deformation-free twin lamellae. The initial response to shock, however, differed significantly between the hollow and the solid Fe-Ni metal blebs (see Supplementary Section 2). In the former, the large internal cavities collapsed entirely upon shock, inducing local melting and quenching into Fe-Ni-rich glass, and the complete reversal of the bcc cores into fcc. In contrast, the cores of the solid blebs did not transform back into fcc (except in their outermost portions, where they formed the intermediate bands; see below). The complete reset of the microstructures in the hollow blebs made it clear that they were no longer suitable for use as thermal indicators. We therefore focused our attention exclusively on the solid ones.

At the onset of shock, the closure of the contraction gaps surrounding the solid Fe-Ni metal blebs locally absorbed part of the shock energy, thereby compensating for the transient high-pressure spikes occurring in the first tens of nanoseconds after impact. However, this process was much too ephemeral to withstand the high-pressure conditions that persisted throughout the shock pulse. It is well established that athermal transformations (both forward and reverse) occur almost instantaneously once initiated, at a velocity ranging between one-fourth and the speed of sound in matter. Given the velocity at which this process occurs in Fe-Ni alloys (i.e., between ~0.25 and ~3.0 × 103 m/s)48–51, it was to be expected that the cores would undergo a reverse transformation into hexagonal close packing (hereafter hcp) during the time interval spent at high pressure. However, this view is not supported by TEM analyses. In particular, the cores of the solid blebs do not display the high-pressure indicators typically observed in similar structures (Fig. 1). The absence of features such as remnants of mechanical twins of specific variants at the intersection between habit planes52, a dense network of straight dislocations in glide configuration coupled with the presence of stacking faults53, or aligned bcc grains transformed back from hcp laths54, and a reduced grain size compatible with the hypothesis of back-and-forth transformation-induced grain size reduction55–57, indicates that a reversal into hcp did not occur. The lack of evidence for a reverse transformation is puzzling, given that the pressure (and/or temperature) conditions experienced by the Fe-Ni metal blebs undoubtedly resulted in a brief excursion outside the bcc stability field during the shock pulse.

This conundrum may be explained by considering the amount of time, referred to as the incubation period, required for the reverse transformation to be initiated, vs the time spent by the rock under pressure. In contrast to pure Fe and most lean-alloyed steels58, forward and reverse transformations in Fe-Ni alloys may occur isothermally59,60. This suggests that, under specific conditions, transformation only starts after the incubation period has elapsed beyond the transition curve. Additionally, isothermal transformation is characterized by a certain degree of sluggishness at the outset of crystal growth, with velocities as low as 10−5 m/s61–63. This process has been demonstrated convincingly for decades by the shift and flattening of the transformation curves in diagrams plotting time vs transformed fractions, primarily in Mn-bearing Fe-Ni alloys64–66, but also in Mn-free Fe-Ni metals67. Isothermal transformations in Fe-Ni alloys are reinforced by the presence of very low carbon concentrations61,62, small particle or grain sizes (with incubation periods up to ~104 s for diameters smaller than 5 μm)68, and hydrostatic pressure (with incubation periods up to ~103 s for samples pressurized below 1 GPa)69,70. It should be noted that this latter argument is controversial, as hydrostatic pressure and shock loading (which is of particular importance in the context of meteorite impact) generally show the exact opposite effect on incubation times. In essence, while hydrostatic pressure increases the incubation time, shock loading tends to reduce it71–76. The sole shock-loading experiment conducted to quantify the incubation period in the bcc-to-fcc reversal of Fe-Ni alloys corroborates this statement77, with nucleation starting after only ~10−7 s. However, these results were obtained from comparatively coarse-grained Fe-Ni alloys. In light of the major influence of grain size on the duration of incubation periods in Fe-Ni alloys68, and, to a lesser extent, the low carbon content derived from the strongly reduced parent magma, we favor the hypothesis of delayed and sluggish nucleation and reverse transformation hindered by the occurrence of a protracted incubation period.

In order to substantiate this hypothesis, it is necessary to weigh the duration of the incubation period against that of the shock, which is defined here as the time spent by the rock at high pressure. The fundamental premise is that, if the incubation period exceeded the duration of the shock pulse, no reverse transformation of the cores could have occurred. Recent numerical simulations have shown that, for a given impact, the shock duration is primarily influenced by the burial depth (especially in shallow rocks78), and, to a lesser extent, the shock pressure and the impactor size. The role of the impactor velocity is increasingly regarded as inconsequential or secondary79. The available data indicate that the burial depth of basalt NWA 479 never exceeded a few meters16,17,45, which is consistent with the long-standing observation that all the lunar meteorites studied to date were excavated from surprisingly shallow grounds80. It is, however, indisputable that the rock experienced a complex, and as yet unclear, multi-stage cosmic-ray exposure history. The residual concentrations of cosmogenic radionuclides17,45 indicate a burial depth of ~1100 g/cm2, which has been interpreted as the ejection depth by the same authors. When converted into metric units, this corresponds to ~6.1 m (assuming that the overlying material is solely made of compacted regolith; ρ=~1.8 g/cm3), ~4.3 m (with equal proportions of regolith and basaltic lava; ρ=~2.6 g/cm3), or ~3.3 m (with basalt only; ρ=~3.3 g/cm3). In contrast to the first value, the second is more closely aligned with the commonly reported maximum ejection depth of ~4.0 m for lunar meteorites81. The third value is contingent upon the improbable scenario in which the regolith would be entirely absent. An additional possibility is that the meteorite was ejected from the uppermost layer of uncompacted regolith at a depth of ~42 g/cm2, i.e., ~0.3 m (with ρ=~1.5 g/cm3)82, as suggested from noble gas stable isotopic analyses16. Therefore, despite the remaining uncertainty regarding the ejection depth, which obscures the ejection process (i.e., jetting83,84, spallation85,86, vs standard excavation), the evidence does not invalidate the hypothesis of a shallow origin for basalt NWA 479.

The correlation between ejection depth and shock duration has been recently investigated using hydrocode simulations79. Caution is required in their use here, however, as some of the initial assumptions may not be directly applicable to lunar impacts. In essence, while the hydrocode simulations currently available have focused on vertical impacts on Mars-sized planetary bodies having the mechanical strength of basalt79, a more robust determination of the shock duration in our case would require consideration of several additional factors. These include the stronger ejection efficiency81 and higher probability87 of oblique impacts, the poorer mechanical strength of loose regolith compared to competent basalt82, and the lower lunar escape velocity (2.38 km/s vs 5.03 km/s). The incorporation of these inputs into hydrocode simulations is currently underway, but the results are not yet available. It is currently believed that obliquity moderately increases the shock duration, potentially by a factor of two or three79, since oblique impacts significantly increase the maximum ejection depth81. Loose or weakly cemented materials reduce the maximum ejection depth88, but the change in shock duration is negligible when shallow grounds are considered (B.C. Johnson, personal communication). Finally, a reduction in the escape velocity exerts a significant influence on shock duration, enabling ejection from depths that are comparatively greater. In consideration of the available evidence79, it can be posited that the lower lunar escape velocity would result in a slight increase in the shock duration, potentially by a factor of two or less. In conclusion, it can be proposed that the combined effects of obliquity and a lower escape velocity have increased the shock duration, although to a lesser degree than that observed in available hydrocode simulations79. Based on realistic impactor radii (5–15 m depending on the conditions80,81) compatible with the small size89 of young lunar craters (ejection of basalts NWA 479 and NWA 032 dates back to ~50 ka16,17), and taking into account the specificities of oblique lunar impacts, it can be estimated that the shock duration in basalt NWA 479 lasted between 10−4 and 10−2 s. This time interval is in good agreement with the “small impact” scenario80 and the early numerical simulations of impacts and meteorite trajectories. These studies have demonstrated that all the lunar meteorites studied to date were initially slow-moving (2.38–3.00 km/s) ejecta90 excavated from craters as small as 450 m in diameter91. In addition, it should be noted that the aforementioned interval encompasses the pressure decline, which means that the duration at maximum pressure is even shorter, probably in the range of 10−4 to 10−3 s. Therefore, despite the remaining uncertainties, there is little doubt that basalt NWA 479 was ejected from shallow grounds during a very short shock pulse. The brevity of the shock in comparison to the considerably longer duration of the incubation period (see above) suggests that no reverse transformation, either into hcp, or into fcc, occurred in the core of the solid Fe-Ni metal blebs at this early stage of the shock history.

Deferred peak-shock heating from vesicular plagioclase melt
With a brief shock pulse of only 10−4 to 10−3 s, the bulk of the solid Fe-Ni metal blebs only began to heat significantly at the end of the pressure decline and continued to do so after release to the near-perfect vacuum of the outer space. As anticipated based on their inherently high shock impedance, the blebs remained at a low temperature for the first tens of nanoseconds after impact. The nearly instantaneous thermal anomaly was extremely local and brief, making it unable to significantly heat the bulk of the blebs (see Supplementary Section 2). Consequently, the transfer of heat towards the blebs only became substantial after a brief delay, initiated by thermal conduction from the hot, vesicular, plagioclase melt. Heat transfer from the other phases of the rock also occurred, albeit at a much lower rate and down a smoother gradient. This is consistent with their weak to moderate shock impedance and inability to act as efficient heat sources.

The exact duration of heat transfer through conduction is uncertain, but it likely persisted at least until the restoration of the contraction gaps. A lower bound can be inferred from the vesiculation of plagioclase, which unambiguously indicates that it was still molten, and, therefore, in close contact with the blebs, after decompression to zero pressure, i.e., ~10−3 s after shock at least (see above). This minimum value is supported by heat transfer simulations using the Energy2D software92, which show that plagioclase reached the melt-glass transition temperature, ~1450 °C at 0 GPa, in <~10−2 s. As the temperature decreased further, the mode of heat transfer shifted from conduction to radiation. This transition was initiated by the reopening and widening of the contraction gaps between the blebs and the surrounding phases (except in the rare locations where the blebs were still in contact with the nearby phases). The change in the mode of heat transfer and the drastic reduction in contact areas resulted in a less effective and heterogeneous heat flux. It can thus be concluded that most of the heat was transferred to the blebs by conduction from the hot plagioclase melt within the first ~10−2 s after shock.

In this interpretation, the intermediate band is the result of a reverse transformation of the core outer border from fcc to bcc. The interface between the core and the intermediate band, initially delineated during daytime igneous cooling, represents the maximum inwards extension of the bcc-to-fcc reaction front. The occurrence of daytime igneous cooling and shock reheating reaction fronts at the very same location is not fortuitous, as this corresponds to the abrupt compositional step that formed early in the magmatic history of the blebs. During shock reheating, this step acted as a barrier to the further inward bcc-to-fcc reverse transformation of the cores. By integrating the effects of alloying elements93 and grain size94 in the T-X Fe,Ni binary phase diagram, temperatures of 660 °C at 23.0 wt% Ni and 690 °C at 17.2 wt% Ni (Fig. 5) were obtained (see also Supplementary Section 4.4). This interval, corresponding to the location of the bcc-to-fcc reaction front at the core-intermediate band interface, is interpreted as the equilibrium peak-shock temperature reached shortly after shock by thermal diffusion.

Shock reheating followed by rapid post-shock cooling
The time required to achieve full peak-shock thermal equilibrium throughout the rock (including the thick shock melt veins, the plagioclase glass, the rock-forming silicates, and the Fe-Ni metal blebs) was substantial, as it followed a logarithmic equilibration path. The calculations based on the Stefan problem95 indicate that the duration was at least 1 s (see Supplementary Section 5). It is also noteworthy that the heating curve of the blebs is likely to have varied slightly from one location to another. These discrepancies can be attributed to several factors, including local differences in modal composition, the heterogeneous distribution and size of additional heat sources (shock melt veinlets, veins or pockets), the occurrence of thermal conduction barriers (cracks, interphase boundaries, planar defects), the processes consuming or releasing heat (plastic deformation, phase transitions, etc.), and the insulation effect induced by the large phenocrysts having moderate shock impedances.

In addition, it must be remembered that meteorite NWA 049 was ejected from the lunar surface as a very small rock12, ~2.9 to ~7.0 × 10−2 m in radius depending on the type of pairing with meteorite NWA 032, and the percentage of mass lost through ablation upon atmospheric entry81. Using numerical simulations96 and assuming the smallest meteorite size (i.e., the most dynamic scenario), it can be calculated that meteorite NWA 049 may have cooled down from its equilibrium peak-shock temperature (660–690 °C) to its post-shock residence temperature in space in a few seconds only (see below and Supplementary Section 5). This time-lapse was certainly shorter near the meteorite surface, where the cooling rate was higher. In light of the extended period of shock reheating and the sudden post-shock cooling, it cannot be definitively asserted that the blebs located at the periphery of the meteorite or trapped within large insulating phenocrysts have even reached complete peak-shock thermal equilibrium.

Unequivocal evidence for which the shock reheating curve would have been superseded by post-shock cooling has not been found. There are two main reasons for this. Firstly, the former meteorite outer envelope was ablated on atmospheric entry, thus escaping analysis forever. Secondly, no Fe-Ni metal bleb trapped in the center of a large phenocryst has been observed in the subsample studied here. Nevertheless, the features observed in blebs 3 (Fig. S3) and 7 (Fig. S5) lend some credibility to this hypothesis. Bleb 3 is the only bleb fully encapsulated within a phenocryst, thereby isolated from the groundmass. Given the unusually high Ni concentration measured in its center (Table 1), it should display a fully back-transformed, sub-microgranular, intermediate band-like core. This is, however, not the case here. One possible explanation is that the equilibrium peak-shock temperature was not reached in this bleb due to the insulating effect provided by the phenocryst. An alternative hypothesis is that the feature is an artifact resulting from peripheral sectioning of the bleb, with the X-ray emission volume of WDS analyses sampling both the core and the underlying rim (plus shallow retained fcc grains). This geometry would result in an erroneous increase in the Ni content, making it incoherent with the observed microstructures. In view of the unusual thickness of the fcc rim and the peripheral entrapment of this bleb within the olivine phenocryst (thereby minimizing any potential insulating effect), we conclude that the observed feature is an artifact, and that there is no compelling evidence for unachieved thermal equilibrium. Nevertheless, the residual uncertainty that full equilibrium may not have been reached everywhere in the rock indicates that the widely accepted term “equilibrium peak-shock temperature” used throughout the paper may actually reflect a “near-equilibrium” situation. In light of the thermal equilibration time estimated by others97 and considering the logarithmic behavior of thermal diffusion processes, however, it can be stated with confidence that meteorite NWA 049 was, in the worst case, very close to thermal equilibrium when post-shock cooling started.

Bleb 7 is intriguing in many ways (Fig. S5). In contrast to the other blebs, it is not rimmed by an fcc single crystal, but rather by a composite fcc-bcc rim that is structurally similar to the intermediate bands described earlier. The smooth Fe-Ni zoning and the anomalously high peripheral Fe content (Table 1) convincingly account for this peculiarity. In addition, the composite fcc-bcc rim is the thickest when in contact with plagioclase glass, and the thinnest when separated from the moderate shock-impedance minerals (pyroxene phenocrysts) by a contraction gap. The uneven advance of the bcc-to-fcc transformation front upon shock provides evidence that thermal diffusion occurred preferentially from the hot plagioclase melt and markedly slowed down with the restoration of the contraction gap. This may also indicate that the final heating of the bleb was abruptly interrupted by post-shock cooling of the meteorite in space. In this interpretation, bleb 7 can be regarded as an illustrative example of thermal “near-equilibrium”, indicating that shock reheating did not fully reach completion. However, the distance between the location of bleb 7 and the primitive (pre-ablation) meteorite surface is unknown, thus rendering any conclusive statement purely conjectural. Besides this, bleb 7 displays additional characteristics that suggest a complex shock history. The small size of the bcc grains in the core and the spread of Kurdjumov-Sachs25 and Nishiyama-Wassermann26,27 inter-grain orientation relationships are usually observed in hollow Fe-Ni metal blebs that have undergone a complete back transformation during shock. The presence of these characteristics in bleb 7 suggest that it also underwent back-and-forth recrystallization, however not because of pore collapse, but possibly because of its unusual location within the rock. Bleb 7 is caught between two pyroxene phenocrysts and along the trajectory of thin shock melt veins (Fig. S2). In comparison to the other solid blebs, it was subject to a significantly greater degree of shear, with limited space to accommodate it. We therefore propose that bleb 7 underwent a complete back-and-forth transformation at the onset of shock as a consequence of lattice strain induced by intense shearing, rather than as a result of pore collapse. This assumption is supported by the ARPGE reconstructions, which indicate the presence of deformation twinning (see the [111] fcc pole figures and the parent orientation map in Fig. S5). Added to this, the parent grain distribution suggest that the bleb has an upper and lower portion, with a boundary roughly parallel to the borders of the adjacent phenocrysts and on the trajectory of the shock melt veins. At the end of the shock stage, the small, freshly reset, bcc grains that constituted the entirety of bleb 7 were subjected to the same heat flux from the neighboring vesiculated plagioclase glass, which resulted in the formation of the composite fcc-bcc rim still visible today.

To sum up, while the observed features in blebs 3 and 7 have not the value of evidence for incomplete thermal equilibrium, they clearly suggest that the Fe-Ni metals trapped in large phenocrysts or located at the periphery of the meteorite may not be suitable for geothermometry. The post-shock residence temperature reached after ejection into space can also be inferred from Fig. 5, albeit with less accuracy than the values obtained for the other thermal stages of the rock history. This is because the fcc-to-bcc transformation front corresponding to the post-shock thermal equilibrium could not be identified with certainty within the intermediate band. With grains often smaller than the thickness of the TEM foil itself, the method clearly reached its limits here. In the absence of a clear reaction front corresponding to this stage, the best possible temperature range that can be deduced from Fig. 5 corresponds to the intersection of MF with the inner and outer interfaces of the intermediate band, namely at −120 °C and 90 °C, respectively. This interval is robust despite its wide span, as it encompasses the much narrower range (0–50 °C) obtained from the well-established calculation method based on solar absorptance and hemispherical emittance98 (see Supplementary Section 5).

High peak-shock temperature and the small impact scenario
To recapitulate, the combination of sub-microstructural features with geochemical data derived from the Fe-Ni metal blebs, coupled with the use of refined T-X Fe,Ni phase diagrams, allowed us to explain the thermal history of lunar basalt/meteorite NWA 049 in great detail (Fig. 5). Our findings indicate that the cooling of the lava occurred in two stages: first down to 90–160 °C during lunar daytime, and then down to −160 °C during lunar nighttime. Following the collision with a moderate-velocity impactor, the equilibrium peak-shock temperature of 660–690 °C was reached after pressure release via conduction from hot vesicular plagioclase. Post-shock cooling in outer space did not last more than seconds or minutes, depending on the ejection size of the meteorite. The residence temperature in space was between −120 °C and 90 °C (refined to 0 °C and 50 °C from calculations by other means).

The peak-shock temperature of 660–690 °C reported here is well above the value of 200 °C reported in a recent review of the paired meteorite NWA 03299. One explanation could be that meteorites NWA 479 and NWA 032 are launch pairs, ejected from the lunar surface upon the same impact but as separate rocks, rather than fall pairs, broken apart on atmospheric entry. However, the occurrence of vesicles was not identified in the aforementioned study99, whereas they are pervasively present in meteorite NWA 049 and in at least one subsample of meteorite NWA 032 (see Fig. S6c). Given the pivotal role of vesicular plagioclase in estimating peak-shock conditions, we hypothesize that the misidentification of the nature of plagioclase glass may have prompted the authors to underestimate the equilibrium peak-shock temperature99.

In any case, the results presented here are in excellent agreement with the microstructures observed in our subsample of meteorite NWA 479, both inside and outside the Fe-Ni metal blebs. The presence of plastically deformed fcc rims, occasional recrystallization of the rim into deformation-free fcc twin lamellae, and annealing of bcc grains within the cores provide evidence that the equilibrium peak-shock temperature was several hundreds of degrees Celsius. In addition, the pervasive occurrence of vesicles and Si-rich blobs in plagioclase glass, which constitutes 20–30 vol% of the groundmass10, indicates that a significant portion of the rock was still above ~1450 °C immediately after pressure release. Moreover, the abundance of shock melt veins, with the thickest measuring over 1 mm (Fig. S1), is indicative of sustained heating, albeit on a more localized scale. In conjunction with the presence of strong mosaicism and planar deformation features in olivine, the arguments above collectively point to intense shock heating.

In conclusion, our results suggest that high peak-shock temperatures can be reached in meteorites excavated during brief impacts (~10−3 s), from very shallow grounds (a few meters at most) and small craters (up to a few hundred meters in diameter). This finding challenges the long-held assumption that equilibrium peak-shock temperatures are necessarily associated with strong events, high impactor velocities, and shock energies. According to the most up-to-date shock classification scales78,100, the microstructural features observed in meteorite NWA 479 point to a “strong” shock. This diagnostic, unquestionable from a microstructural point of view, modifies our perspective on the “small impact” scenario put forward over the years80 for recent lunar impacts, including the one that excavated meteorite NWA 479. It is beyond the scope of this study to determine whether this apparent contradiction will be resolved by ongoing and future hydrocode simulations. There is no doubt, however, that incorporating variables such as impact obliquity onto grounds similar to the lunar surface will enhance our comprehension of shock-induced thermal processes, including those observed for meteorites excavated by jetting or spallation. Thus, our results stress that extreme caution is required when correlating impactor size and velocity, burial depth, crater size, and peak-shock pressures and temperatures, with the observed microstructures. This is particularly relevant for meteorites containing vesicular plagioclase glass, for which, by the own admission of one of its strongest advocates101, the occurrence of high-pressure mineral assemblages is unreliable.

The data presented here indicate that iron-rich metals offer valuable advantages over standard geothermometers for tracking the full thermal history of extraterrestrial rocks. The distinction between diurnal and nocturnal cooling on the lunar surface, the disentangling of the very short-lived processes occurring during shock reheating, and, to a lesser extent, post-shock cooling have demonstrated that Fe-Ni alloys are highly effective geothermometers applicable to a very wide range of temperatures and timescales. It is, therefore, to be expected that the microstructures of native metals occurring in terrestrial rocks, such as in (rare) basalts, peridotites, kimberlites, or metallogenic hydrothermal deposits, will gain in popularity in the next few years. Finally, our study emphasizes the importance of standardizing the use of sub-microstructural techniques often overlooked in planetary sciences, such as EBSD or ACOM, as they offer a promising avenue for resolving contentious issues.

Methods
SEM imaging
A subsample of lunar meteorite NWA 479 was hot pressed in conductive resin using a Struers CitoPress-1 apparatus. The sample surface was stepwise diamond-polished down to a grade of 0.5 μm and coated with a 15-nm carbon layer to prevent charging under the electron beam.
Backscatter electron (BSE) images were obtained at the Institute of Earth Sciences at the University of Lausanne (UNIL) using a Tescan Mira II LMU FE-SEM operated at an acceleration voltage of 20 kV and a probe current of ~0.4 nA. The nutshell-like microstructures of the Fe-Ni metal blebs were imaged in high-contrast BSE mode using a probe current value of ~15 nA and a very long dwell time to maximize the signal-to-noise ratio.

EBSD crystallographic orientation mapping
As EBSD requires a surface of analysis that is free of mechanical damage, the SEM mount was polished in an alkaline (pH=9.8) aqueous suspension of colloidal silica. No carbon coating was applied in order to ensure optimal pattern quality. EBSD analyses were carried out in the Tescan Mira II LMU FE-SEM operated at an acceleration voltage of 20 kV, a probe current of ~1.1 nA, and a dwell time of 0.15 s. The sample surface was tilted at 70° with respect to the horizontal and placed at a working distance of 23 mm. The EBSD patterns were acquired using a Nordlys S camera and the AZtec software package from Oxford Instruments. The Fe-Ni metals were indexed using a refined version of the structure files of fcc- (a=3.58 Å) and bcc- (a=2.86 Å) iron, which were generated with the Twist software included in the Channel 5 software package. The raw data consisted of orientation matrices, 300×400 pixels in size, collected at step sizes ranging between 25 and 50 nm.
Post-processing started with the removal of the poorly indexed data points, i.e., those exhibiting a mean angular deviation >1.3° or a band contrast <20). The matrices were noise-reduced in accordance with the established wildspike correction methodology and a six-neighbor zero solution extrapolation. The processed data were represented as pole figures of Euler angle triplets (φ1, Φ, φ2), and the maps were color-coded according to the phase, inverse pole figure, or cumulative misorientation schemes. The black lines on the maps delimit misorientations exceeding 1° between neighboring pixels, thereby emphasizing the presence of subgrain and grain boundaries. For further details regarding the determination of parent grain orientations using the ARPGE software, please refer to the relevant literature24.

EPMA-WDS chemical analyses
The chemical compositions of the trace, minor, and major elements in the Fe-Ni metal blebs were determined at UNIL through wavelength dispersive spectrometry (WDS). The WDS system was attached to a JEOL JXA-8530F HyperProbe electron probe microanalyzer (EPMA), operated at an acceleration voltage of 15 kV and a probe current of 15 nA. X-ray intensities and peak-to-background ratios were measured from Kα and Lα lines using a combination of standard and large LIF, PET, TAP, and LDE crystals to enhance sensitivity and counting statistics.

FIB preparation of the TEM foil
The preparation of a site-specific, electron-transparent, thin foil was a prerequisite for the planned analyses in the transmission electron microscopes (TEM). The selection of Fe-Ni metal bleb 1 (Figs. 1, 4, and S3) was based on two criteria. Firstly, the absence of a central cavity made it suitable for use as a geothermometer (for further details, please refer to the main text and Supplementary Sections 2.2 and 2.3). Secondly, the nearby phase association was deemed to be representative of the majority of the Fe-Ni metal blebs in meteorite NWA 479, particularly in terms of proximity to chromite.
The clearance of the foil from the bulk sample and its thinning to electron transparency was conducted in a Zeiss nVision 40 dual beam focused ion beam (FIB) FE-SEM at the Interdisciplinary Center for Electron Microscopy (CIME) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) using the sputtering action of heavy gallium ions accelerated at a voltage of 30 kV. Prior to the milling process, the area of interest was coated in situ with a protective, 3 μm-thick, carbon layer obtained from the dissociation of organic molecules injecting as gaseous form into the FIB-FE-SEM chamber. The abrasion rate of the sample was monitored by adjusting the ion probe current intensity, first stepwise down from 45 nA to 1.5 nA to facilitate the clearance of the foil, and then down to 80 pA during the thinning step to limit amorphization of the foil surface. During the fine milling procedure, the Gaussian profile of the ion beam was compensated by tilting the specimen at an angle of 1.5° to 2.0° with respect to the incident ion beam. Because of the markedly disparate abrasion resistance of the carbon protective layer, the Fe-Ni metal bleb, and the surrounding silicate minerals, the specimen tilt was meticulously and consistently adjusted throughout the process. The final step in the preparation of the foil entails cleaning under smooth ion beam conditions (2 kV and 25 pA) at a tilt angle of 3°. The TEM-ready foil, approximately 15×10×0.1 μm in size, was transferred onto a FIB lift-out grid using Kleindiek micromanipulators. For a comprehensive account of the FIB technique, please refer to other sources102–106.

TEM imaging and EDS chemical analyses
Bright-field (BF), dark-field (DF), and high-angle annular dark-field (HAADF) images were obtained at EPFL from the FIB thin foil using an FEI Tecnai Osiris FE-TEM equipped with an ultra-high brightness source. The instrument was operated at an acceleration voltage of 200 kV. The TEM diffraction patterns were collected on a Gatan Orius CCD digital camera and indexed offline using the JEMS electron microscopy simulation software107.
The concentrations of Fe, Ni, and Co were determined using the same instrument in scanning TEM (STEM) mode at a spatial resolution of 1–2 nm. A twin-pole piece with a Super-X EDS module containing four silicon drift detectors was used to ensure high-speed and high-sensitivity measurements. The raw EDS data were then post-processed offline using the Bruker Esprit 1.9 software package in order to obtain quantified chemical abundances.

ACOM crystallographic orientation analyses
Ultra-high spatial resolution ACOM was performed at EPFL using a JEOL JEM 2200FS FE-TEM equipped with the NanoMegas ASTAR system108. The instrument was operated in nano-beam diffraction (NBD) mode, with the objective of reducing the size of the electron probe to a minimum. The acquisition parameters included an acceleration voltage of 200 kV, a C2 condenser lens aperture of 10 μm, an excitation of the condenser minilens corresponding to an alpha value of 3, and a camera length of ~180 mm. These settings yielded a highly focused electron probe with a diameter of 2 nm and a convergence semi-angle of 0.8 mrad. Orientation maps comprising 800×800 pixels were collected at a step size of 2 nm and a dwell time of ~0.01 s, resulting in acquisition times of <2 h per map. For each point of the scan, a diffraction pattern was displayed on a fluorescent screen, captured by a digital CCD camera, and stored for offline indexing. In contrast to the indexing routine of EBSD patterns, which require the prior identification of Kikuchi band triplets, the procedure used in ACOM is based on template matching, that is the assessment of the goodness-of-fit between the measured and the simulated diffraction patterns. The ASTAR software package was used to generate a template library comprising simulated diffraction patterns for all possible crystallographic orientations. The input data for fcc and bcc iron were the existing CIF files with unit cell dimensions of 3.58 Å and 2.86 Å, respectively.
The raw orientation data were imported into MTEX, an open-source MATLAB toolbox designed for data treatment and texture analysis109. The procedure followed was very similar to that described in full detail elsewhere110,111. In brief, post-processing consisted of a rotation of the dataset to align the ASTAR and MTEX orientation frames. A filtering routine was executed to reduce noise, and another one to correct for the typical 180° ambiguity frequently encountered in ASTAR. Finally, the data were exported as ANG files and imported into the Channel 5 software suite, in which a noise-reduction routine similar to that applied to EBSD data was implemented.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled \"Pinpointing the Thermal History of Lunar Basaltic Meteorites in a Nutshell\" by Pierre Vonlanthen and\nco-authors presents a comprehensive investigation into the thermal history of lunar basaltic meteorite NWA 479 using Fe-Ni\nmetal blebs. The authors used the microstructural and chemical characteristics of these metal blebs to trace the complete\nthermal evolution of the meteorite, including magmatic crystallization, shock reheating, and post-impact cooling. Their main\nfindings reveal that the Fe-Ni metal blebs can offer new insights into both igneous and impact-induced thermal events in\nplanetary bodies, even distinctions between lunar day- and nighttime cooling. Additionally, the thermal history inferred from\nthe blebs' microstructures and chemical compositions supports a small impact scenario for the meteorite's ejection from the\nlunar surface.\nThis study provides novel insights into the thermal history of lunar basaltic meteorites, particularly through the innovative use\nof Fe-Ni metal blebs as geothermometers. While Fe-Ni alloys have been previously studied in meteoritic research, this\npaper advances the field by combining microstructural and chemical analyses to reconstruct a meteorite's thermal history\nwith remarkable detail. The supplementary materials also provide extensive additional data. However, several major issues\nneed to be addressed, as outlined below.\nMajor comments:\n1. Estimation of Shock Conditions\n  The study uses plagioclase melting as key evidence to estimate peak shock conditions, which are linked to the formation of\ncertain microstructures in Fe-Ni metal blebs. While plagioclase melting is evident from some figures in the supplementary\ninformation, the petrographic evidence for plagioclase melting in Figure 3a may be attributed to silicate liquid immiscibility\ncommonly observed in the mesostasis of lunar basalts. In this case, the dark vesicles are likely Si-rich liquids, while the\nbrighter domains at the vesicle peripheries are Fe-rich conjugate liquids. Element mapping of this area could help confirm\nthis interpretation, and it is unlikely that the observed phenomena are due to impact-induced plagioclase melting.\n  The authors estimate a peak-shock temperature of 660-690 °C and suggest that this estimate is consistent with the\nobserved microstructures, such as “occasional recrystallization of the rim into deformation-free fcc twin lamellae, annealing\nof bcc grains within the cores” (Line 568-570). However, the presence of high-pressure minerals like ringwoodite and\nwadsleyite in the same meteorite (Barrat et al., 2005) complicates this estimation. While using these high-pressure minerals\nto estimate peak shock conditions may be questionable, they can set an upper limit on the post-shock temperature, as they\nwould retrograde to low-pressure polymorphs if temperatures were sufficiently high after pressure release (Hu and Sharp,\n2022). It would be beneficial to assess whether the estimated peak-shock temperature aligns with the preservation of high-\npressure minerals in the meteorite.\n  The authors refer to “thermal equilibrium” or “equilibrium peak-shock temperature” to describe the thermal state of the\nmeteorite. It is important to note that temperature equilibration occurs on a timescale much longer than pressure\nequilibration, with millimeter-scale temperature heterogeneities equilibrating on a timescale of seconds (Sharp and DeCarli,\n2006). Although the authors acknowledge the substantial time required to reach peak-shock thermal equilibrium, their model\noversimplifies the situation by estimating an unrealistic equilibration time of “~1 s or more”. Given the proposed scenario of\n“shock reheating followed by rapid post-shock cooling,” it is questionable whether true thermal equilibrium was ever\nachieved throughout the rock. Therefore, the term “equilibrium” should be used with caution in this context.\n2. I strongly recommend adding explanations for key terminology (e.g., bcc, fcc, hcp) and phase transformations (e.g., bcc to\nfcc, bcc to hcp, and martensitic transformation) in the supplementary information. Many planetary scientists may not be\nfamiliar with these terms. For instance, martensitic transformation is uncommon in silicate minerals, making it challenging for\nreaders to understand how this transformation could kinetically occur under extremely low temperatures (i.e., < -100 °C, Line\n279-285, Figure 5a). Providing these explanations will enhance the accessibility of the manuscript.\n3. The authors derive several critical temperatures based on the phase diagram, such as 90-160 °C during rapid daytime\ncooling and -160 °C during rapid nighttime cooling. However, uncertainties in these temperature estimates could arise from\nmodifications and shifts in the MF and AF curves, conversions from ternary to binary concentrations, and the use of\naveraging rather than interface compositions. While it is challenging to determine the exact uncertainties, an approximate\nestimation of these uncertainties would be valuable.\n4. The manuscript describes various stages of deformation of the Fe-Ni blebs. However, it is not clear how the authors\ndifferentiate between deformation features related to igneous cooling (Line 305-321) and those resulting from subsequent\nshock imprints. Providing an explanation of how these features are distinguished would be beneficial for understanding the\ndeformation processes.\n5. The manuscript includes a depiction of the fcc-to-bcc transformation of Fe-Ni blebs upon cooling in space (Figure 5). This\ntransformation is relevant to the section on “Shock Reheating Followed by Rapid Post-Shock Cooling.” However, the text\ndoes not address this transformation process. Including a discussion on this transformation in the relevant section would\nprovide a more comprehensive understanding of the observed phenomena.\nMinor comments:\nLine 23: A space is needed before “°C” here and also at other places.\nLine 26-27: “peak temperature was reached…mainly by thermal conduction from hot vesicular plagioclase”. See the major\ncomment 1\nLine 28-29: The phrase \"We propose… for lunar meteorites\" could be rephrased to clarify the connection between the\nfindings and previous models, as it feels slightly abrupt.\nLine 196 thick enough?\nLine 205-207: references are needed here. The graphite oxidation process is also suggested to be responsible for the\nformation of Fe-Ni metals in lunar picritic magmas (e.g. Rutherford and Papale, 2009).\nLine 240-242 The early crystallized olivine and pyroxene would preferentially incorporate Mg over Fe. This selective\nincorporation of Mg actually leads to a relative enrichment of Fe in the remaining liquid melt.\nLine 246-249 As early crystallization phases, the current relative positions of Fe-Ni blebs and chromite can not provide\nevidence of their locations during the growth process.\nLine 264-267 it would be better to provide an explanation here for why the Ni-rich part did not experience back\ntransformation.\nLine 283-285 This sentence is a little difficult to read, I would suggest changing it to “Hence, our data indicate that the core-\nrim interface, which shifts from its inner to its outer position at dusk, corresponds to the locations of the daytime and nighttime\nfcc-to-bcc reaction fronts, respectively.”\nLine 584-586 Consider splitting the sentence or using a conjunction.\nReference\nBarrat, J.A., Chaussidon, M., Bohn, M., Gillet, P., Göpel, C., Lesourd, M., 2005. Lithium behavior during cooling of a dry\nbasalt: An ion-microprobe study of the lunar meteorite Northwest Africa 479 (NWA 479). Geochim. Cosmochim. Acta 69(23),\n5597-5609. doi.org/https://doi.org/10.1016/j.gca.2005.06.032.\nHu, J., Sharp, T.G., 2022. Formation, preservation and extinction of high-pressure minerals in meteorites: temperature effects\nin shock metamorphism and shock classification. Progress in Earth and Planetary Science 9(1), 6. doi.org/10.1186/s40645-\n021-00463-2.\nRutherford, M.J., Papale, P., 2009. Origin of basalt fire-fountain eruptions on Earth versus the Moon. Geology 37(3), 219-222.\ndoi.org/10.1130/g25402a.1.\nSharp, T.G., DeCarli, P.S., 2006. Shock effects in meteorites, in: Lauretta, D.S., McSween, H.Y., Binzel, R.P. (Eds.),\nMeteorites and the Early Solar System II. University of Arizona Press, Tucson, pp. 653–677.", "reviewer_2": "The paper Pinpointing the thermal history of lunar basaltic meteorites in a nutshell by Vonlanthen et al. presents microscopic\nmetallurgical characterization of the iron-nickel alloy blebs in lunar mare basalt NWA 479. They observed nanoscale fcc-to-\nbcc transition in the alloy and proposed a martensitic transformation model to constrain the thermal history of lava on lunar\nsurface. The authors further described the bcc-to-fcc inversion (as austenitic) and used that to constrain the potential shock\nheating of the sample. While the data are detailed and figures / cartoons are very well made, there are a few uncertainties\nthat could jeopardize the development of Fe-Ni phases into a robust geothermometer. The findings, although being quite\ninteresting and worth publication, has yet to demonstrate universal usefulness for geological samples as high-impact results.\nMy questions about the results and interpretations are listed below:\n1) One key conclusion of the work is the confirmation of a small-impact launch of the sample, suggested by the shallow\ndepth of its pre-impact setting, which in turn is interpreted from the assumption that the sample experienced lunar day-dusk\ntemperature difference that results in the fcc-to-bcc transition at blow 0 °C on 29.7 wt% Ni. If the model is correct, thus\nobservation should be consistent through the whole sample and backed by more than one singular FIB section of alloy.\n(It looks like bleb 3 was FIB cut, but no profile data.) And I am not sure about the meaningfulness of averaging 25 data points in\na profile of continuously changing compositions…\nAlso, if a few wt% of Fe/Ni would be important for the conclusion, the 4 wt% cobalt need to be treated more carefully, instead\nof projecting the Fe/Ni ratio directly on the binary phase diagram.\nIn Fig. 5, the reference phase boundaries are clearly shown in at% space while the axes and chemical data are reported in\nwt%. The martensitic phase boundary, e.g. in Kaufman and Cohen 1956, was also reported in at% space originally. Please\ndouble check the translation to wt% is correct.\n2) In my understanding, EBSD orientation processing requires the user to manually input whether a high misorientation\nangle indicates local deformation of one crystal grain or separate grains. It looks like e.g. bleb 5 rim could represent a bunch\nof non-twinned grains. And I cannot quite understand the map of bleb 4, in which there are areas of no-solution in the alloy\nand with bcc identification in the silicate glass.\nIt would also be clearer to include more experimental details such as mapping step, noise reduction in processing, whether\nmodeled contour is used in the final map and showing a few representative diffraction patterns.\n3) The abrupt step in composition at intermediate band-rim is interpreted as an igneous feature that stops the shock-induced\ninversion. Does that mean the Fe-Ni segregation/diffusion is also expected spontaneously with martensitic transition?\nDoes the transition create an interface-controlled diffusion that leads to the abrupt step? Again, more profile data would be helpful.\n4) The post-shock heating of above 600 °C locally in one bleb/feldspar does not conflict with a lower overall post-shock\ntemperature estimated from the bulk Hugoniot. Unless it was meant to say the vesicular feldspars are pervasive and the bulk\npost-shock temperature is that high of >600 C, which would make a significant difference for the metamorphic petrography of\nthe sample…"}, "author_response": "Response to Reviewer #1\nThe manuscript titled \"Pinpointing the Thermal History of Lunar Basaltic Meteorites in a Nutshell\" by\nPierre Vonlanthen and co-authors presents a comprehensive investigation into the thermal history of\nlunar basaltic meteorite NWA 479 using Fe-Ni metal blebs. The authors used the microstructural and\nchemical characteristics of these metal blebs to trace the complete thermal evolution of the meteor-\nite, including magmatic crystallization, shock reheating, and post-impact cooling. Their main findings\nreveal that the Fe-Ni metal blebs can offer new insights into both igneous and impact-induced thermal\nevents in planetary bodies, even distinctions between lunar day- and nighttime cooling. Additionally,\nthe thermal history inferred from the blebs' microstructures and chemical compositions supports a\nsmall impact scenario for the meteorite's ejection from the lunar surface. This study provides novel\ninsights into the thermal history of lunar basaltic meteorites, particularly through the innovative use\nof Fe-Ni metal blebs as geothermometers. While Fe-Ni alloys have been previously studied in mete-\noritic research, this paper advances the field by combining microstructural and chemical analyses to\nreconstruct a meteorite's thermal history with remarkable detail. The supplementary materials also\nprovide extensive additional data. However, several major issues need to be addressed, as outlined\nbelow.\nComment (1a). The study uses plagioclase melting as key evidence to estimate peak shock condi-\ntions, which are linked to the formation of certain microstructures in Fe-Ni metal blebs. While plagi-\noclase melting is evident from some figures in the supplementary information, the petrographic evi-\ndence for plagioclase melting in Figure 3a may be attributed to silicate liquid immiscibility commonly\nobserved in the mesostasis of lunar basalts. In this case, the dark vesicles are likely Si-rich liquids,\nwhile the brighter domains at the vesicle peripheries are Fe-rich conjugate liquids. Element mapping\nof this area could help confirm this interpretation, and it is unlikely that the observed phenomena\nare due to impact-induced plagioclase melting.\nWe agree and thank Reviewer #1 for this comment. Figs 3 and S6 (and the corresponding cap-\ntions/text) have been updated accordingly (lines 145-161, S318-S331). The plagioclase glass in me-\nteorite NWA 479 contains both vesicles and immiscible Si-rich liquids, which are pervasive throughout\nthe rock. We were misled by the observation that these liquids also form embayment fillings or\nelongated strips within plagioclase, rather than irregular to mushroom or rounded shapes. Vesicles\ncan be distinguished from Si-rich liquid blobs based on a number of characteristics, including their\nmuch darker tone in high-contrast BSE images, the occurrence of overpressure cracks in nearby\npyroxene formed during pressure release, and the absence of brighter Fe-rich haloes.\nWe can confirm that the occurrence of late-stage mesostasis pockets in many lunar basalts has a\npurely igneous origin. The whole-rock composition of basalt NWA 479, with Mg# = 39.6 (Fagan et\nal., 2002), is also consistent with the “immature” residual pockets described by Pernet-Fisher et al.\n(2014). However, the features shown in Figs 3b and S6d stand out for several reasons (see also the\nadditional images below): (1) the occurrence of Si-rich liquid blobs within (or right next to) molten\nvesicular plagioclase, (2) the very irregular shapes and extremely uneven distribution of Si-rich liquid\nblobs, (3) the associated melting of Fe-rich pyroxene and ilmenite grains, and (4) the absence of\nresidual mineral growth, e.g., fayalite with symplectitic “sieve” texture. We therefore argue that the\nobserved features reflect shock-induced melting rather than liquid immiscibility during igneous cool-\ning. For instance, the Si-rich liquids haloed by more Fe-rich areas are generated at the interface\nbetween plagioclase glass and Fe-rich pyroxene/ilmenite. This observation is more consistent with a\nmelting process involving the excess heat generated by large differences in shock impedance be-\ntween pyroxene/ilmenite and plagioclase, than with a “dissociation” of pyroxene or ilmenite into\ndifferentiated liquids on late-stage igneous cooling. Then, the Si-rich liquids assume the form of\nmushroom-shaped blobs reminiscent of “diapiric” intrusions into plagioclase melt. The irregular\nshapes and uneven distribution indicate extreme textural immaturity, in good agreement with very\nrapid quenching immediately after shock. In the event of comparatively slow igneous cooling, we\nwould have anticipated more “stable” blob shapes and textures. In addition, the conjugate melting\nof Fe-rich phases (pyroxene rims, ilmenite) and plagioclase results in the formation of immiscible\nliquids, as experimentally shown by Rutherford et al. (1974) for other purposes. Finally, the associ-\nation of Si-rich blobs with plagioclase glass observed here is not comparable to the “sieve” texture\nresulting from the entrapment of Si-rich melt droplets during late-stage crystal (e.g., fayalite) growth\nin mesostasis pockets (Pernet-Fisher et al., 2014).\nAdditional BSE images from our subsample of meteorite NWA 479. a Field of view containing vesicles (associated with overpres-\nsure cracks, as in Figs 3a and S6a,b) and Si-rich liquid blobs (in medium gray within light-gray plagioclase glass, as in Figs 3b\nand S6d). b Partial melting of (idiomorphic) pyroxene grains in contact with plagioclase glass. The Si-rich liquid blobs are sur-\nrounded by slightly brighter Fe-rich haloes. c Close-up view of b displaying the extremely irregular shapes and uneven distribution\nof the Si-rich liquid blobs generated at the interface with the Fe-rich pyroxene rims. d Replica of Figs 3b and S6d summarizing\nmost of the features shown from a to c. Note that the black spot labeled “vesicle” crosscuts the Si-rich liquid blob to the right, in\ngood agreement with the idea of shock-induced melting of plagioclase (and nearby pyroxene) associated with vesiculation upon\npressure release. Unfortunately, it is no longer possible to obtain a close-up image of this specific location due to the additional\npolishing step (involving the removal of a layer of material) required for EBSD analyses (see lines S738-S739). Accordingly, the\n“vesicle” was not labeled in Figs 3b and S6d.\nIn conclusion, we do not contest the characteristics of the late-stage mesostasis pockets as re-\nported in the literature, but we believe that the features shown in Figs 3b, S6d (and above) are of a\ndifferent nature. Our analysis indicates that these features reflect shock-induced melting of Fe-rich\nminerals (pyroxene, ilmenite) in contact with hot vesiculated plagioclase glass in a process facilitated\nby large differences in shock impedance. In this respect, the presence of the Si-rich liquid blobs\nserves to reinforce our interpretation and the scenario of a high (near-)equilibrium peak-shock tem-\nperature.\nComment (1b). The authors estimate a peak-shock temperature of 660-690 °C and suggest that this\nestimate is consistent with the observed microstructures, such as “occasional recrystallization of the\nrim into deformation-free fcc twin lamellae, annealing of bcc grains within the cores” (Line 568-570).\nHowever, the presence of high-pressure minerals like ringwoodite and wadsleyite in the same mete-\norite (Barrat et al., 2005) complicates this estimation. While using these high-pressure minerals to\nestimate peak shock conditions may be questionable, they can set an upper limit on the post-shock\ntemperature, as they would retrograde to low-pressure polymorphs if temperatures were sufficiently\nhigh after pressure release (Hu and Sharp, 2022). It would be beneficial to assess whether the\nestimated peak-shock temperature aligns with the preservation of high-pressure minerals in the me-\nteorite.\nThis is a highly pertinent and timely issue. Two additional paragraphs have been included in the\nsupplementary materials (lines S345-S362).\nIt is true that Barrat et al. (2005, p. 5600) identified ringwoodite and wadsleyite in some olivine rock\nfragments entrained in the shock melt veins of meteorite NWA 479. However, “despite [the] acquisi-\ntion of >40 [Raman] spectra within [the] shock melted regions”, Mijajlovic et al. (2020, p. 2278) did\nnot document any such high-pressure minerals in paired meteorite NWA 032, while Miyahara et al.\n(2019, p. 6014) reported the occurrence of ringwoodite in close proximity to shock-melt veins in the\nvery same rock. While the results differ, we believe that none of these authors are incorrect or have\noverlooked the petrological aspects of their work. In our opinion, this is an excellent illustration of\nthe importance of the P-T-t path is, and how it can vary on a very local scale within the same (or\npaired) sample(s). This is particularly relevant for high-pressure minerals, as their formation and\npreservation are closely tied to shock melt veins and pockets, in which the P-T-t path can vary dras-\ntically and over very short distances. Thus, we fully agree with Hu and Sharp (2022) and with the\ngrowing consensus that the local P-T-t path is a crucial consideration when studying the shock history\nof meteorites. Actually, the potential shortcomings of high-pressure minerals and the necessity for\nmore reliable indicators were key factors that prompted the present study.\nIn meteorite NWA 479, the time of “~1 s or more” (now “at least 1 s”, lines 497 and S678) to\nreach the equilibrium (or near-equilibrium, see below) peak-shock temperature of 660-690°C\nthroughout the rock (including the thickest shock melt veins, the plagioclase glass, the rock-forming\nsilicates, and the Fe-Ni metal blebs), was calculated by solving the Stefan problem (lines S658-\nS679). Given that the entire shock pulse only lasted between 10-4 to 10-2 s (lines 408-439), the\nequilibrium peak-shock temperature (660-690°C) was reached after pressure release (lines 458-\n468). Our calculations also indicate that post-shock cooling from the equilibrium peak-shock temper-\nature (660-690°C) to the residence temperature in space (0-50°C) lasted between seconds and\nminutes depending on the initial ejection size of the meteorite (lines 504-514, S711-S722). Further\nrefinement of this range to tens of seconds is possible when considering the most probable scenario,\nin which the percentage of mass loss on atmospheric entry was closer to the average value (70%)\nthan the end-members (50% and 90%). As stated by Hu and Sharp (2017, p. 277), “experimental\nstudies of transformation kinetics show that wadsleyite transforms to olivine in seconds at tempera-\ntures above 1200 K (~927°C) and in hours at temperatures between 900 K (~627°C) and 1200 K\n(~927°C)” at ambient pressure. Similar results for ringwoodite are presented in Table 3 (p. 287). We\ntherefore argue that our scenario is very well in line with the data presented by Hu and Sharp (2017,\n2022) and the preservation of ringwoodite and wadsleyite reported by Barrat et al. (2005). Given\nthat the equilibrium peak-shock temperature (660-690°C) was reached after “at least 1 s”, and that\nthe post-shock cooling residence temperature in space (0-50°C) was reached after tens of seconds\n(following a logarithmic path), it can be concluded that the critical temperature of ~627°C (900 K)\nwas crossed very early, after a few seconds at most. This time lapse is two to three orders of mag-\nnitude shorter than the necessary duration reported by Hu and Sharp (2017) to retrograde ring-\nwoodite and wadsleyite into olivine at ambient pressure. Therefore, the local preservation of ring-\nwoodite and wadsleyite is consistent with our interpretation and scenario.\nConversely, providing a definitive explanation for the crystallization of high-pressure minerals\nduring such a brief shock pulse may be more challenging. Our interpretation is that the exceptionally\nrapid crystallization of these high-pressure polymorphs required very strong kinetics. The tempera-\nture of the shock melt veins was likely very high, providing sufficient thermal energy to initiate such\nrapid phase transformations. Unfortunately, there is no indication regarding the size or preservation\nstate of ringwoodite and wadsleyite in Barrat et al. (2005). This is because the high-pressure mineral\nassemblage was apparently identified through Raman spectra only. This significantly restricts the\nability to ascertain the local prograde and retrograde P-T-t paths from these data. Nevertheless, we\nsuggest that the high energy required to form high-pressure minerals in such a short time frame\nprovides further evidence that a globally high-temperature scenario occurred during shock.\nComment (1c). The authors refer to “thermal equilibrium” or “equilibrium peak-shock temperature”\nto describe the thermal state of the meteorite. It is important to note that temperature equilibration\noccurs on a timescale much longer than pressure equilibration, with millimeter-scale temperature\nheterogeneities equilibrating on a timescale of seconds (Sharp and DeCarli, 2006). Although the\nauthors acknowledge the substantial time required to reach peak-shock thermal equilibrium, their\nmodel oversimplifies the situation by estimating an unrealistic equilibration time of “~1 s or more”.\nGiven the proposed scenario of “shock reheating followed by rapid post-shock cooling,” it is ques-\ntionable whether true thermal equilibrium was ever achieved throughout the rock. Therefore, the\nterm “equilibrium” should be used with caution in this context.\nWe agree. The issue of “thermal equilibrium” or “equilibrium peak-shock temperature” was addressed\nin four paragraphs (lines 515-579) in the original manuscript. We stated that even if “unequivocal\nevidence for which the shock reheating curve would have been superseded by post-shock cooling\nhas not been found” (lines 515-516), “it cannot be definitively asserted that the blebs located at the\nperiphery of the meteorite or trapped within large insulating phenocrysts have even reached com-\nplete peak-shock thermal equilibrium” (lines 512-514). Accordingly, we acknowledge that the term\n\"equilibrium\" may be interpreted as \"near-equilibrium\". This has now been clarified in a dedicated\nsection of the text (lines 531-536).\nThat said, the equilibration time of “at least 1 s” (following a logarithmic cooling path) calculated\nby solving the Stefan problem (lines S658-S679) is in line with the estimate of Sharp and deCarli\n(2006, p. 653) that “temperature heterogeneities equilibrate on a timescale of seconds”. Then, from\na terminological standpoint, the term “equilibrium peak-shock temperature” is commonly (and per-\nhaps abusively) used in the planetary science literature to describe the temperature reached after\nthermal equilibration of the rock. This is a generic term that is widely recognized and used by most\nplanetary scientists, regardless of whether full thermal equilibrium has been reached or verified. In\nparticular, it allows for the differentiation between the various thermal states (or stages) of the shock\nhistory. These include the pre-shock residence temperature on the planetary body, the instantaneous\nand transient temperature anomalies on impact (resulting from pore collapse, friction heating, dif-\nference in shock impedance, planar defects, etc.), and the post-shock thermal equilibrium reached\nafter cooling in the outer space. Incidentally, it should be noted that thermal equilibrium is reached\nasymptotically and theoretically after an infinite period of time, which makes the term somewhat\nmisleading anyway. In the updated version of the manuscript, we propose to retain the well-known\nand widely used expression “equilibrium peak-shock temperature” (together with the explanation in\nlines 531-536), even if we acknowledge that full thermal equilibrium might not have been reached.\nBy doing so, we avoid the risk of confusing the readers even more with a new term of our own, or\nwith a shortened wording (such as “peak-shock temperature”) that might be misinterpreted with\ntransient thermal peaks.\nComment (2). I strongly recommend adding explanations for key terminology (e.g., bcc, fcc, hcp)\nand phase transformations (e.g., bcc to fcc, bcc to hcp, and martensitic transformation) in the sup-\nplementary information. Many planetary scientists may not be familiar with these terms. For instance,\nmartensitic transformation is uncommon in silicate minerals, making it challenging for readers to\nunderstand how this transformation could kinetically occur under extremely low temperatures (i.e.,\n< -100 °C, Line 279-285, Figure 5a). Providing these explanations will enhance the accessibility of\nthe manuscript.\nWe agree. The acronyms bcc, fcc, and hcp have been spelled out in full for the first time they are\nmentioned in the manuscript (lines 66-67, 352-353). A widely accessible piece of text on martensitic\ntransformations in the broad sense has been added to the supplementary materials (lines S408-\nS418).\nComment (3). The authors derive several critical temperatures based on the phase diagram, such as\n90-160 °C during rapid daytime cooling and -160 °C during rapid nighttime cooling. However, uncer-\ntainties in these temperature estimates could arise from modifications and shifts in the MF and AF\ncurves, conversions from ternary to binary concentrations, and the use of averaging rather than\ninterface compositions. While it is challenging to determine the exact uncertainties, an approximate\nestimation of these uncertainties would be valuable.\nThe extent of the shift applied to MS, MF, AS, and AF was addressed and calculated with regards to\nalloying elements (lines S461-S495), particle size (lines S497-S526), grain size (lines S528-S562),\nand compression (lines S564-S589). A new paragraph was added to address the influence of Co,\nwhich has the most significant impact on MF and AF. During shock heating for example, a 1.0 wt%\nchange in Co results in a shift of ~30°C of the transformation curves (lines S479-S488).\nPlease note that the Fe and Ni concentrations at the interfaces (Fig. 5) are not average values.\nInstead, they have been retrieved from the TEM-EDS profiles shown in Fig. 4 (lines 164-165, 446-\n447). Only trace elements were averaged (from WDS analyses), first because the TEM-EDS detection\nlimit in profile mode insufficient for quantifying the trace elements with enough counting statistics,\nand then because long analyses in spot mode (or on tiny areas) were found to damage the TEM foil.\nFor further information on statistical significance and representativeness, please refer to the new\nsection in the supplementary materials (lines S591-S653).\nIn our opinion, the primary (but moderate) source of uncertainty is the limited data available for\nMF and AF in Fe-Ni alloys, while MS and AS are much better documented. This issue is thoroughly\ndiscussed from a qualitative standpoint (lines S439-S457), but there is not much more we can do to\nquantify uncertainties resulting from a lack of data. We believe that the exceptional fit between the\ntemperatures obtained for igneous cooling and the day- and nighttime temperatures expected for\nthe early Eratosthenian period (lines 287-294) and that the validation of the high equilibrium peak-\nshock temperature scenario by other means (lines S622-S645) are good indicators of reliability.\nHowever, we agree that the precision of our model could be enhanced through further research, such\nas the analysis of Fe-Ni metal blebs trapped in returned lunar samples. Given the well-established\nlocation and depth of these samples, they would be ideal for fine-tuning the igneous cooling temper-\natures. Nevertheless, in light of the aforementioned discussion and the new sections added to the\nsupplementary materials, we think that the issue of uncertainties has been addressed as thoroughly\nas possible.\nComment (4). The manuscript describes various stages of deformation of the Fe-Ni blebs. However,\nit is not clear how the authors differentiate between deformation features related to igneous cooling\n(Line 305-321) and those resulting from subsequent shock imprints. Providing an explanation of how\nthese features are distinguished would be beneficial for understanding the deformation processes.\nThe differentiation between igneous and shock deformation features was primarily based on the\nmagnitude and nature (e.g., concentric fracturing) of deformation (see also the sketches and related\ntexts in Fig. 5). We acknowledge that unequivocal distinction at the local scale is difficult because\nthe slip systems that were activated during igneous cooling were probably reactivated during shock.\nNevertheless, even if some overlap probably exists, the notable differential volumetric changes of\nthe Fe-Ni metal blebs on igneous cooling cannot be inconsequential on the crystal lattice (see below\nand lines 313-330). However, they were much too weak to explain cumulative misorientations of up\nto 50° degrees observed in the rims (Figs S3 to S5).\nIt is important to note that the crystal lattices of silicates and metals do not respond to cooling in\nthe same way, in accordance with the amount of internal strain underwent by either type of materials.\nIn the former, igneous processes rarely result in significant lattice deformation because the strain\ninduced by magmatic cooling is low. As an example, basalt exhibits a volumetric change between\n1000°C and 0°C of ~1-2% (Hartlieb et al., 2016), which means that the minimal differential stress\nbetween minerals can be accommodated by other means, such as grain boundary sliding and/or\nfracturing. With metals, however, the situation is different. First, the volumetric change within the\nsame temperature range of 1000°C to 0°C is much larger, ~10% in pure iron for example (Basinski\net al., 1955). Then, the expansion coefficients at a given temperature can vary significantly for the\nsame type of alloys. For instance, Fe-Ni metals with an Invar composition (~64 wt% Fe, ~36 wt%\nNi) demonstrate minimal volumetric change when exposed to temperature variations below 100°C.\nIn contrast, Fe-Ni alloys with compositions other than Invar demonstrate significant expansion and\ncontraction. Finally, the martensitic transformations commonly observed in metals are also accom-\npanied by sudden volumetric changes, in the order of ~4 % in Fe-Ni alloys. Therefore, metals are\nsubject to stronger internal strain on cooling than silicates, resulting in deformation of a few degrees\nof the crystal lattice. Deformation can be accommodated plastically at high temperatures and in a\nmore and more fragile way upon cooling. In the present case, the contraction gaps provided some\nadditional flexibility to accommodate volumetric changes, but not enough to fully compensate for the\ninternal deformation of the Fe-Ni metal blebs.\nIn contrast, cumulative misorientations up to several tens of degrees, such as those observed in\nthe Fe-Ni metal rims, require strong perturbations of the stress field. In the absence of regional\ntectonics and metamorphism, these conditions can only result from the shock waves generated on\nimpact. The resulting features are large cumulative misorientations within grains, which are still\nvisible today in the rims (Figs S3 to S5).\nComment (5). The manuscript includes a depiction of the fcc-to-bcc transformation of Fe-Ni blebs\nupon cooling in space (Figure 5). This transformation is relevant to the section on “Shock Reheating\nFollowed by Rapid Post-Shock Cooling.” However, the text does not address this transformation pro-\ncess. Including a discussion on this transformation in the relevant section would provide a more\ncomprehensive understanding of the observed phenomena.\nWe agree (lines 570-579).\nLine 23: A space is needed before “°C” here and also at other places.\nAccording to the “Chicago Manual of Style – The Essential Guide for Writers, Editors and Publishers”,\ncomprehensive 1200-page reference on English writing: “a space is normally used between the nu-\nmeral and the unit of measure, except in a few cases, for example degree, percent and prime sym-\nbols: 50 km, 21 ha, 4.5 l, but 10.5°C”. I am afraid that there is no need for a space here, even\nthough the SI system allows for one. Of course, a space is needed when written in full: “10.5 degrees\nCelsius”.\nLine 26-27: “peak temperature was reached…mainly by thermal conduction from hot vesicular pla-\ngioclase”. See the major comment 1\nSee reply to Comment 1\nLine 28-29: The phrase \"We propose… for lunar meteorites\" could be rephrased to clarify the con-\nnection between the findings and previous models, as it feels slightly abrupt.\nWe agree. The abstract has been fully rephrased to comply with the 150-word policy (lines 13-23).\nLine 196 thick enough?\nWe agree. The sentence has been rephrased (line 193-194).\nLine 205-207: references are needed here. The graphite oxidation process is also suggested to be\nresponsible for the formation of Fe-Ni metals in lunar picritic magmas (e.g. Rutherford and Papale,\n2009).\nWe agree. (line 204).\nLine 240-242 The early crystallized olivine and pyroxene would preferentially incorporate Mg over\nFe. This selective incorporation of Mg actually leads to a relative enrichment of Fe in the remaining\nliquid melt.\nWe agree. Fe-Ni metal is the first phase to crystallize, together with chromite (another Fe-bearing\nmineral). The growth of olivine and pyroxene phenocrysts only starts later. The term “but also olivine\nand pyroxene” has been removed from the updated version (line 239).\nLine 246-249 As early crystallization phases, the current relative positions of Fe-Ni blebs and chro-\nmite cannot provide evidence of their locations during the growth process.\nI am afraid that we have to disagree on this point. As reported in an number of studies (Reid et al.,\n1970; Schreiber & Haskin, 1976; Roeder & Reynolds, 1991; Fogel & Rutherford, 1995; Finnigan et\nal., 2008) and in the supplementary file (lines S75-S84), ” Fe-Ni metal blebs are often associated\nwith chromite phenocrysts (Fig. S2), in good agreement with previous observations4-7. During\ncrystallization from the melt, chromite preferentially incorporates the oxidized cations Fe3+ and Cr3+\nat the expense of their divalent counterparts. This selective uptake occurs at the crystal-melt\ninterface, forming a boundary layer with conditions that are even more reduced than in the bulk.\nConsequently, the local redox conditions permit the reduction of Fe2+ into Fe0, thereby facilitating the\nnucleation and growth of native Fe-Ni metals in proximity to chromite phenocrysts8”. The proximity\nof Fe-Ni metals to chromite phenocrysts is therefore not a mere coincidence.\nLine 264-267 it would be better to provide an explanation here for why the Ni-rich part did not\nexperience back transformation.\nWe agree (lines 269-273).\nLine 283-285 This sentence is a little difficult to read, I would suggest changing it to “Hence, our\ndata indicate that the core-rim interface, which shifts from its inner to its outer position at dusk,\ncorresponds to the locations of the daytime and nighttime fcc-to-bcc reaction fronts, respectively.”\nWe agree (lines 289-291).\nLine 584-586 Consider splitting the sentence or using a conjunction.\nWe agree (lines 619-623).\nReferences cited in the responses to Reviewer #1\n- Barrat, J.A., Chaussidon, M., Bohn, M., Gillet, P., Göpel, C. & Lesourd, M. Lithium behavior during cooling of a dry basalt: An\nion-microprobe study of the lunar meteorite Northwest Africa 479 (NWA 479). Geochim. Cosmochim. Acta 69, 5597-5609.\n(2005).\n- Basinski, Z.S., Hume-Rothery, W. & Sutton, A.L. The lattice expansion of iron, Proc. Roy. Soc. London 229, 459-467 (1955).\n- Butler, C.P. Temperature of meteoroids in space. Meteoritics 3, 59-70 (1966).\n- Finnigan, C.S., Brenan, J.M., Mungall, J.E. & McDonough, W.F. Experiments and models bearing on the role of chromite as a\ncollector of platinum group minerals by local reduction. J. Petrol. 49, 1647-1665 (2008).\n- Fogel, R.A. & Rutherford, M.J. Magmatic volatiles in primitive lunar glasses: I. FTIR and EPMA analyses of Apollo 15 green and\nyellow glasses and revision of the volatile-assisted fire-fountain theory. Geochim. Cosmochim. Acta 59, 201-215 (1995).\n- Hartlieb, P., Toifl, M., Kuchar, F., Meisels, R. & Antretter, T. Thermo-physical properties of selected hard rocks and their relation\nto microwave-assisted comminution, Mineral. Engin. 91, 34-41 (2016).\n- Hu, J. & Sharp, T.G. Back-transformation of high-pressure minerals in shocked chondrites: Low-pressure mineral evidence for\nstrong shock. Geochim. Cosmochim. Acta 215, 277-294 (2017).\n- Hu, J. & Sharp, T.G. Formation, preservation and extinction of high-pressure minerals in mete-orites: temperature effects in\nshock metamorphism and shock classification. Prog. Earth Planet. Sci. 9, 6 (2022).\n- Mijajlovic, T., Xue, X. & Walton, E. A revised shock history for the youngest unbrecciated lunar basalt - Northwest Africa 032\nand paired meteorites. Met. Planet. Sci. 55, 2267-2286 (2020).\n- Miyahara, M. et al. Shock-induced melting and high-pressure polymorphs in lunar basaltic meteorites. Proc. Ann. Meet. Met.\nSoc. 82, 6014 (2019).\n- Pernet-Fisher, J.F., Howarth, G.H., Liu, Y., Chen, Y., Taylor, L.A. Estimating the lunar mantle water budget from phosphates:\nComplications associated with silicate-liquid-immiscibility, Geochim. Cosmochim. Acta 144, 326-341 (2014).\n- Reid, A.M., Meyer, C., Harmon, R.S. & Brett, R. Metal grains in Apollo 12 igneous rocks. Earth Planet. Sci. Lett. 9, 1-5 (1970).\n- Roeder, P.L. & Reynolds, I. Crystallization of chromite and chromium solubility in basaltic melts. J. Petrol. 32, 909-934 (1991).\n- Rutherford, M.J., Hess, P.C. & Daniel, G.H. Experimental liquid line of descent and liquid immiscibility for basalt 70017. Proc.\nLunar Planet. Sci. Conf. 5, 569–583 (1974).\n- Schreiber, H.D. & Haskin, L.A. Chromium in basalts: Experimental determination of redox states and partitioning among synthetic\nsilicates phases. Proc. Lunar Sci. Conf. 7, 1221-1259 (1976).\n- Sharp, T.G. & De Carli, P.S. Shock effects in meteorites. In: Meteorites and the Early Solar System II, 653-677 (University of\nArizona Press, Tucson, 2006).\n- University of Chicago: Chicago Manual of Style - The Essential Guide for Writers, Editors and Publishers (Seventh Edition), The\nUniversity of Chicago Press, 1141 pp (2017)\nResponse to Reviewer #2\nThe paper Pinpointing the thermal history of lunar basaltic meteorites in a nutshell by Vonlanthen et\nal. presents microscopic metallurgical characterization of the iron-nickel alloy blebs in lunar mare\nbasalt NWA 479. They observed nanoscale fcc-to-bcc transition in the alloy and proposed a marten-\nsitic transformation model to constrain the thermal history of lava on lunar surface. The authors\nfurther described the bcc-to-fcc inversion (as austenitic) and used that to constrain the potential\nshock heating of the sample. While the data are detailed and figures / cartoons are very well made,\nthere are a few uncertainties that could jeopardize the development of Fe-Ni phases into a robust\ngeothermometer. The findings, although quite interesting and worth publication, has yet to demon-\nstrate universal usefulness for geological samples as high-impact results. My questions about the\nresults and interpretations are listed below:\nComment (1) One key conclusion of the work is the confirmation of a small-impact launch of the\nsample, suggested by the shallow depth of its pre-impact setting, which in turn is interpreted from\nthe assumption that the sample experienced lunar day-dusk temperature difference that results in\nthe fcc-to-bcc transition at blow 0 °C on 29.7 wt% Ni. If the model is correct, thus observation should\nbe consistent through the whole sample and backed by more than one singular FIB section of alloy.\n(It looks like bleb 3 was FIB cut, but no profile data.)\nTo address this important issue, a new section has been added to the supplementary materials (lines\nS591-S653). While we agree in principle with the above comment that the more analyses done, the\nmore robust the interpretation, we demonstrate that additional high-end analyses (FIB thinning, TEM\nBF/DF/HAADF imaging, quantitative TEM-EDS, and ultra-high resolution ACOM/ ASTAR mapping) are\nnot scientifically justified. Our results and interpretation can be successfully validated (and even\nreinforced) using the data (EBSD mapping, WDS analyses, etc.) collected in other eligible blebs, in\naddition to being supported by typical microstructural features of the rock. We also show that attain-\ning statistical significance using high-end analytical tools is technically unrealistic (statistics and com-\nplexity are inversely correlated), but that this inherent limitation is largely offset by the unprece-\ndented results they provide (see also the comments of Reviewer #1). This is particularly the case\nwhen the interpretation and scenario can be validated by other means. The key points are as follows:\na) Our subsample of meteorite NWA 479 contains a total of only seven Fe-Ni metal blebs (Figs S1 to\nS5). Four of them were deemed ineligible for use as geothermometers (and therefore excluded\nfrom the high-end analyses), either because they were hollow (i.e., subjected to disproportionate,\nshock impedance-related, thermal reaction upon impact; blebs 4, 5, and 6; lines 119-130, 337-\n344, S226-S300, Fig. 2), or completely non-representative because of an exceptional size, unu-\nsual chemical composition and texture (bleb 7; lines 537-566). Of the three remaining blebs, two\nwere fibbed, with a successful outcome for bleb 1 (Figs 1, 4, and S3) and a fatal ending for bleb\n3 (see point c below and footnote in Table 1). Therefore, of the seven initial blebs, only one, bleb\n2, remains as a potential geothermometer.\nb) Table 1 shows that bleb 2 is chemically zoned (as are all the Fe-Ni metal blebs in meteorite NWA\n049), with a core center and a rim containing 12.30 wt% and 33.18 wt% Ni, respectively (see\nFig. S3 for the locations of WDS analyses). Upon conversion to binary compositions, the Ni con-\ncentration of the rim is equivalent to ~34 wt%. Then, Fig. S3 illustrates that bleb 2 has a compa-\nrable nutshell-like microstructure to that observed in other blebs, comprising a core surrounded\nby an intermediate band and a rim. Based on these observations, we can state with a high degree\nof confidence that the Ni content at the intermediate band-rim interface is notably smaller than\n~34 wt%, and even more so at the core-intermediate band interface. It is therefore very reason-\nable (and probably conservative) to assume that the Ni content at the core-intermediate band\ninterface is ~30 wt% (or less). When plotted in Fig. 5a (the 6th T-XFe-Ni diagram), and after fine-\ntuning AF following the method described in Supplementary Section 4, the composition of 30 wt%\nNi indicates an equilibrium peak-shock temperature of ~600°C. Being a bit less conservative with\nregard to the Ni content, the value moves towards (or falls within) the range of 660-690°C derived\nfrom the high-resolution TEM-EDS profiles of bleb 1. Even in the unlikely (actually very improba-\nble) event that the Ni content at the core-intermediate band interface would be identical to that\nmeasured in the middle of the rim (~34 wt%), the temperature would still be ~540°C. In conclu-\nsion, the temperature estimates retrieved from the data already available from bleb 2 are in very\ngood agreement with the results obtained from the high-end analyses of bleb 1. Both support the\nhigh equilibrium peak-shock temperature and the scenario proposed in our study. Furthermore,\nthis convergence of results demonstrates that our interpretation is independent of the location of\nthe blebs within the meteorite. This provides further evidence that validates the use of Fe-Ni metal\nblebs as whole rock geothermometers, and to the interval of 660-690°C as equilibrium (or near-\nequilibrium; lines 531-536) peak-shock temperature for meteorite NWA 479.\nc) It is also worth noting that the high-temperature scenario upon shock is also supported by other\nfactors, namely, the pervasive distribution of vesicular plagioclase glass (lines 145-157, lines\nS306-S317, Figs 3a and S6a,b,c), the occurrence of irregular, mushroom-shaped, Si-rich liquid\nblobs (lines 147-148, S318-S323, Figs 3b and S6d), the local but frequent melting of ilmenite and\npyroxene (Fig. S6e,f), well-developed planar deformation and mosaicism in olivine (lines S379-\nS386), unequivocal evidence for massive crystal-plastic deformation in all the types of pheno-\ncrysts (see below), and large shock melt veins.\nd) Please also consider the non-standard character of the sophisticated analytical techniques used\nhere. We invested a (really) huge amount of time and resources to ensure the successful collection\nof the results presented in the manuscript. As an example, the successful clearance of a homo-\ngeneously thin TEM-transparent FIB foil from a small Fe-Ni metal bleb embedded in silicates with\na contraction gap in between is a tremendously difficult task (note that the foil trace shown in Fig.\n1a represents the portion of the foil shown in Fig. 1c, and not the much larger entire foil, as\nindicated in lines S786-S787 and specified in the new version of the figure). Silicates and metals\nexhibit markedly different abrasion resistance to ions (while the contraction gap has none), which\nmeans that the settings appropriate for one material almost invariably lead to the amorphization\nof the other. This is what happened to bleb 3, that was also fibbed and observed by TEM… just to\nrealize that it had been completely amorphized by the thinning process (footnote in Table 1). It is\nonly after many trial-and-error sessions on blank samples that we finally succeeded in preparing\none foil ready for TEM imaging/-EDS and ACOM/ASTAR mapping (bleb 1). Then, the lift-out of the\nTEM foil from a bleb, which was essentially “suspended in a vacuum” (the contraction gap between\nmetal and silicates was only disrupted by rare contact points), presented a challenge too. In\nsummary, the foil of bleb 1 was on a knife edge throughout the entire FIB session. The complexity\nof the sample preparation process is likely a contributing factor to the fact that this approach has\nnot been attempted previously. A similar observation can be made with regard to ACOM/ASTAR.\nWhile this technique is extremely powerful and crucial for obtaining nm-scale crystallographic\norientation mapping, the access to it is limited by its complexity. As a reminder, the objective of\nthis paper is to pave the way for new solutions to long-standing and contentious issues. We rec-\nognize that these high-end analyses cannot be performed on a routine basis.\ne) The use of sophisticated analytical techniques is inherent to lower statistical significance, given\nthe inverse correlation between the two. Nevertheless, we believe that this drawback is largely\noffset by the unparalleled results they provide. This is particularly the case when the interpretation\ncan be validated by other means, showing that low- and high-end techniques are perfectly com-\nplementary. We also We also emphasize that to achieve true statistical significance, the entire set\nof analyses conducted in bleb 1 would need to be repeated in tens of Fe-Ni metal blebs eligible as\nfull geothermometers, collected from as many different subsamples as possible. Such an approach\nis clearly unrealistic in terms of the requisite machine time, laboratory access, costs, and so forth.\nf) Finally, it should be kept in mind that FIB is destructive: once it is done, the sample has gone\nforever. With only one eligible bleb left (bleb 2), and given the high risk of amorphization, we\nstrongly recommend that it be archived and preserved.\nComment (2) And I am not sure about the meaningfulness of averaging 25 data points in a profile\nof continuously changing compositions…\nThere seems to be a misunderstanding here. The values in Table S1 were averaged over 25 points\nseparately for each of the six selected portions of the profiles, i.e., the internal part of the core (Ibcc\ncore), the very external part of the core (Ebcc core), the internal part of the intermediate band (Ibcc\nIB), the external part of the intermediate band (Ebcc IB), the very internal part of the rim (Ifcc rim),\nand the external part of the rim (Efcc rim) (see the locations of * and † signs in Table S1). Therefore,\nthe compositions are not “continuously changing”, but constant over the selected portions. As an\nexample, the composition of the 25 points averaged from the internal part of the core (Ibcc core) is\nfound to be within the range of 85.0 and 86.5 wt%, with the exception of two “outliers” at ~84 and\n~87 wt% (average: 85.8 wt%).\nComment (3) Also, if a few wt% of Fe/Ni would be important for the conclusion, the 4 wt% cobalt\nneed to be treated more carefully, instead of projecting the Fe/Ni ratio directly on the binary phase\ndiagram.\nThat is precisely what we have done! The effect of Co and the other alloying elements Cr, Ca, and Ti\non MS MF, AS, and AF was detailed in the supplementary materials (lines S461-S488, Table S2). Of\ncourse, the results were used to refine the positioning of the transformation curves in Fig. 5. In\nregard to the potential errors associated with the Co content, we have included additional information\nin the supplementary file (lines S479-S488). Finally, minor (and even moderate) changes in Fe and\nNi concentrations does not at all invalidate the scenario of a high equilibrium peak-shock temperature\n(as outlined in point b of the response to comment 1 and lines S622-S645).\nComment (4) In Fig. 5, the reference phase boundaries are clearly shown in at% space while the\naxes and chemical data are reported in wt%. The martensitic phase boundary, e.g. in Kaufman and\nCohen 1956, was also reported in at% space originally. Please double check the translation to wt%\nis correct.\nThe caption of Fig. 5 has been updated to clarify (lines 447-448) that all the data presented in tables\nor figures (including MS, MF, AS, and AF) were systematically converted into wt% in the original man-\nuscript. This also applies to the background boundary lines shown as references in light gray in Fig.\n5, which were sourced from Swartzendruber et al. (1991) and Yang et al. (1996) precisely because\nthey were displayed in wt% (see the figure below). Both papers were referenced in the caption of\nFig. 5. We agree that the field boundary lines displayed in Fig. 5 bear a striking resemblance to those\nreported in at% in the literature (including in the paper by Kaufman and Cohen, 1956). However,\nthis is simply a consequence of the very close atomic weight of Fe and Ni. As an example, a compo-\nsition of 60.0/40.0 wt% Fe/Ni is equivalent to 61.1/38.9 at% Fe/Ni.\nFe-Ni phase diagram showing the boundary lines refer-\nenced in light gray in Fig. 5 (from Yang et al., 1996). Note\nthat the compositions are in wt% (as everywhere in our\npaper).\nComment (5) In my understanding, EBSD orientation processing requires the user to manually input\nwhether a high misorientation angle indicates local deformation of one crystal grain or separate\ngrains. It looks like e.g. bleb 5 rim could represent a bunch of non-twinned grains. And I cannot quite\nunderstand the map of bleb 4, in which there are areas of no-solution in the alloy and with bcc\nidentification in the silicate glass.\nAn explanation has been added to the supplementary materials (lines S753-S754) to clarify the\nmisorientation angle defining boundary lines in EBSD maps.\nThe issue regarding bleb 5 is explained in the introduction (lines 67-70): “The cores are typically\nencased in a single-crystal fcc rim that exhibits smooth and gradual misorientations of up to ~50°,\nwhich suggests deformation under a plastic regime. The sporadic occurrence of sharper misorienta-\ntions across (sometimes concentric) fractures indicates that the rim was also subject to moderate\nsemi-brittle deformation”. The “bunch of non-twinned grains” reported in the above comment actu-\nally represents the subgrains created by semi-brittle deformation. Upon examination of the EBSD\nmisorientation map - fcc (Fig. S4), it is evident that the deformation of the rim of bleb 5 is of a dual\nnature. The smooth change in crystallographic orientations on the left hand side of the bleb is indic-\native of a plastic regime. In contrast, the more acute changes between subgrains on the top and\nright sides indicate a more brittle regime (even though the subgrains themselves are plastically de-\nformed, hence the use of the term “semi-brittle”). In addition, the overwhelming majority of these\nsubgrains display respective misorientations smaller than 15° (as indicated by colors ranging be-\ntween greenish and yellowish). This indicates that they were generated from the single-crystal rim\n(see also the ARPGE results) when the conditions for purely plastic deformation were no longer met.\nSimilarly, blebs 4 and 6 exhibit comparable characteristics (Fig. S4). The more chaotic microstructure\nof the hollow blebs was presented as one of the arguments explaining their collapse upon shock and\ntheir ineligibility as full geothermometers (lines 123-130, S226-S300).\nThe misunderstanding regarding bleb 4 can be easily resolved by referring to the caption of Fig.\nS3 (lines S152-S156), which states, “The minor inconsistencies in bleb morphology observed in the\nBSE images and the EBSD maps are due to the additional polishing step required for EBSD analyses”.\nThis was repeated in the captions of Figs S4 and S5 (lines S204, S220) as follows: “Layout as in Fig.\nS3”. The same statement is applicable to all the Fe-Ni metal blebs. As detailed in the supplementary\nmaterials (lines S738-S729), electron backscatter diffraction (EBSD) requires a surface of analysis\nthat is free of mechanical damage. This involves an additional chemomechanical polishing using al-\nkaline (pH=9.8) aqueous suspension of colloidal silica. It is inevitable that the removal of material\n(likely in the range of 100s of nanometers in this case) will become visible when comparing the BSE\nimages and the EBSD maps acquired before and after the additional polishing step, respectively. It\nis important to note that the distinction is readily apparent, given the small scale of the features in\nquestion. If the blebs had been larger (in the range of 100s of micrometers) and imaged at a lower\nmagnification, the material removed by EBSD polishing would have likely gone unnoticed.\nComment (6) It would also be clearer to include more experimental details such as mapping step,\nnoise reduction in processing, whether modeled contour is used in the final map and showing a few\nrepresentative diffraction patterns.\nStep sizes for EBSD (25-50 nm), TEM (1-2 nm), and ACOM/ASTAR (2 nm) maps were already spec-\nified in the supplementary materials (line S747, S796, S808). The noise reduction methods used for\nEBSD (lines S748-S752) have been standard since the 1990s. Those used for ACOM/ASTAR are out-\nlined in the text (lines S818-S824) and described in greater detail in two dedicated papers (supple-\nmentary references 81 and 82).\nRegarding the addition of diffraction patterns, please consider the following. Since the early de-\nvelopments of EBSD in the 1970s and 1980s, the technique has primarily been applied to (bcc and\nfcc) metals. The main reason is that metals have historically been and continue to be crucial industrial\nmaterials. Other reasons include the fact that metals are conductive (no charging issue, except when\ntrapped in non-conductive materials such as silicates) and have a high backscatter coefficient (an\nasset when the detectors, or films, had very low sensitivity). During the 1980s and 1990s, tons of\nEBSD patterns of fcc and bcc metals were published. We therefore believe that doing so in a Nature\nCommunications paper to be published in the years 2024-2025 would be inappropriate, outdated,\nand unnecessary for the comprehension of the paper.\nComment (7) The abrupt step in composition at intermediate band-rim is interpreted as an igneous\nfeature that stops the shock-induced inversion. Does that mean the Fe-Ni segregation/diffusion is\nalso expected spontaneously with martensitic transition? Does the transition create an interface-\ncontrolled diffusion that leads to the abrupt step? Again, more profile data would be helpful.\nSorry, but there is no abrupt step in composition at the intermediate band-rim interface (Fig. 4, Table\nS2). We guess that this comment is related to the compositional step at the core-intermediate band\ninterface. That said, it is right that the martensitic transformations in Fe-Ni metal are accompanied\nby a process of “rejection” (or “discharge”) followed by diffusion of Ni towards interstitial fcc grains\nof the core and beyond the core outer border. Further clarification has been provided in the revised\nmanuscript (lines 264-268). This process is also well documented in the steels, where carbon is\nexpelled during martensitic transformations regardless of the direction of the compositional gradient.\nIt is also interesting to note that Ni did not effectively “segregate” at grain boundaries (at least\nnot for an extended period) and was rapidly removed from there (profile BB’ in Fig. 4b). Alternatively,\nwe could say that “segregation” was transitional and short-lived because Ni diffused rapidly along\ngrain boundaries (fast diffusion pathways) and accumulated in the interstitial fcc grains of the core\n(profile AA’ in Fig. 4a) and beyond the core outer border (profile CC’ in Fig. 4b).\nComment (8) The post-shock heating of above 600 °C locally in one bleb/feldspar does not conflict\nwith a lower overall post-shock temperature estimated from the bulk Hugoniot. Unless it was meant\nto say the vesicular feldspars are pervasive and the bulk post-shock temperature is that high of >600\nC, which would make a significant difference for the metamorphic petrography of the sample…\nThe high-temperature scenario is not only supported “locally in one bleb”, but also by additional\nfactors. These include the widespread (i.e., “pervasive” but not “ubiquitous”) distribution of vesicular\nplagioclase glass (lines 145-147, lines S306-S317, Figs 3a and S6a,b) in meteorite NWA 479 but also\nNWA 032 (Fig. S6c), the occurrence of Si-rich liquid blobs (lines 147-148, S318-S322, Figs 3b and\nS6d), and the local but frequent melting of ilmenite and pyroxene (Fig. S6e,f). This indicates that\nplagioclase (which constitutes 20-30 vol% of the rock) was still molten and hot (>1450°C) immedi-\nately after pressure release (lines 600-610). Then, an additional bleb has confirmed an equilibrium\npeak-shock temperature >600°C (lines S622-S645 and point b of the response to comment 1).\nFinally, the usual indicators of strong shock are numerous, including well-developed planar defor-\nmation and mosaicism in olivine (lines S384-S385), unequivocal evidence for massive crystal-plastic\ndeformation in all the types of phenocrysts (see below and lines S385-S386), and large shock melt\nveins. In conclusion, the observations indicate that the equilibrium peak-shock temperature in me-\nteorite NWA 479 was high. Even the local preservation of high-pressure minerals does not invalidate\nthis statement (lines S333-S362 and the response to Reviewer #1).\nRegarding the crystal-plastic deformation of silicates and oxides, we would like to bring to your\nattention the fact that we have carried out EBSD maps on 11 olivine, 8 pyroxene, 7 ilmenite, and 6\nchromite phenocrysts from our subsample of meteorite NWA 479 (not shown as dedicated to a spe-\ncific paper). The majority of the phenocrysts show smooth, gradual, crystal-plastic deformation with\ninternal cumulative misorientations of up to several tens of degrees. These results provide further\ncompelling evidence that the rock experienced high peak-shock temperatures. Based on a compre-\nhensive review of the available literature and my own experience, it is very unlikely that these min-\nerals can undergo plastic deformation at low temperatures, particularly if the time is short. A specific\npaper on the deformation of those phenocrysts will be submitted in the near future to confirm the\nhigh peak-shock temperature proposed here. The decision to focus on the Fe-Ni metal blebs in the\npresent paper is a deliberate one, justified by the fact that the information is already very dense,\nwith a substantial amount of new data supported by a detailed supplementary file. Including EBSD\nmaps of silicates and oxides would densify the information even more, blur the message, and cer-\ntainly not help to keep the reader on edge.\nFinally, it is important to note that the reliability of whole-rock temperature estimates obtained\nfrom the bulk Hugoniot is increasingly being questioned (Asimow et al., 2018; Hu and Sharp, 2022,\nand references therein), especially for complex systems, in good agreement with the general com-\nments of Reviewer #1.\nReferences cited in the responses to Reviewer #2\n- Asimow, P.D. Melts under extreme conditions from shock experiments. In: Kono, Y. & Sanloup, C. (eds) Magmas Press. Elsevier,\nAmsterdam, pp 387–418 (2018).\n- Hu, J. & Sharp, T.G. Formation, preservation and extinction of high-pressure minerals in meteorites: temperature effects in\nshock metamorphism and shock classification. Prog. Earth Planet. Sci. 9, 6 (2022).\n- Kaufman, L. & Cohen, M. The martensitic transformation in the iron-nickel system. J. Met. 206, 1393-1401 (1956).\n- Swartzendruber, L.J., Itkin, V.P. & Alcock, C.B. The Fe-Ni (iron-nickel) system. J. Phase Equilib. 12, 288-312 (1991).\n- Yang, C.W., Williams, D.B. & Goldstein, J.I. A revision of the Fe-Ni phase diagram at low temperatures (≤400°C). J. Phase\nEquilib. 17, 522-531 (1996)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided detailed and thoughtful responses, along with revisions that significantly improve the manuscript.\nAll my comments and concerns have been fully addressed. I have no further comments and recommend accepting the\nmanuscript for publication.", "reviewer_3": "The answers of the authors to the comments of the reviewers are exceptionally extensive, very detailed, well\nunderstandable. I will mainly comment the replies of the authors to the two reviewers.\nReply to reviewer 1\nComment 1a: Mesostasis vs impact melting: The textural arguments are convincing, specially because the authors confirm\nthe presence of both vesicles and immiscible Si-rich liquids. They give also characteristics how to distinguish between both\nfeatures on BSE images.\nComment 1b: Presence of high pressure phases and the indicated peak pressure. The inhomogenous presence of the latter\nfrom previous studies underlines Al Goresy’s et al. (2013) red flag to use them for peak pressure and the presence of very\ndifferent P-T-t paths on a local scale.\nComment 1c: Thermal equilibrium: It seems and the authors convingly show the “sloppy” usage uf the therm “equilibrium” in\nthe planetary science community. added text is sufficient to explain the situation.\nComment 2: In my opinion, bcc, fcc. Martensitic transformation etc. should be terms which should be also common terms in\nthe space science community and a reference to one of the many material science / metallography textbook should have\nbeen sufficient. The explanation in the supplementary material is ok, except that I would add rhat martensitic transformation\nhave a low activation energy and need a small driving force.\nA critical own comment: The authors use the term “lattice”, but meaning “structure”. This error is wide spread and usually\naccepted. I would change nothing.\nComment 3: Influence of allloying elements, use of average composition, statistical significance:\nThe treatment of alloying elements, grain size and compression are well done and understandable. The authors have\nclarified that they have used average compositions not over the whole profile but for plateaus along rhe profile.\nStastical significance: It is clear may one of three blebs the stastical significance is somewhat reduced, but to prepare more\nFIB samples is not realistic. What may be critizided that no error is given for the chemical analyses nor for the contstants in\nthe equation 1-3 in the supplimentary material, therefore no error propagation calculation possible.\nComment 4: Deformation process due to magmatic cooling vs. impact: The difference in the material response of stress\nbetween metals vs. minerals and the actual observations in the sample are convincing evidence that the authors are able to\ndistinguish the results of magmatic cooling vs. impact.\nComment 5: ok\nLine 196: Rephrasing is ok.\nLine 205-207: References have been added.\nLine 246-249: “but also olivine and pyroxene” has been removed.\nLine 246-249: The autors show convincingly that the reviewer is wrong!\nLine 264-267: The authors give a good explanation why the back transformation fcc to bcc is hindered in the Nj-rich fcc\nparts.\nLine 283-285: The sentence havs been revised.\nLine 584-586: The sentence has been split.\nReviewer 2:\nReply to reviewer 2\nComment 1: The comprehensible reduction to one measurable metal bleb out of 7blebs is explanation enough for the limited\nstatistical significance for the results from the sophisticated analytical methods. The validation of the latter results by more\nconventional WDS and EBDS methods reinforces the statistical significance.\nI agree with the authors that to use FIB on many samples is unrealistic- For the nature of the sample, metal embedded in\nceramic, the fibbing is a lottery (I speak from experience).\nComment 2: see reply on comment 3 of reviewer 1\nComment 3: see reply on comment 3 of reviewer 1\nComment 4: at% vs. wt%: authors have adjusted all results and diagrams to wt%.\nReply to this comment by the authors: “……showing the boundary lines referenced in light gray in Fig. 5 (from Yang et al., 1996).” I cannot see the “boundary lines referenced in light gray”!\nComment 5: EBSD and misorientation: Automatic misorientation over boundary may only possible for small misorientation\nangles, but “manual” determination is possible based on crystallographic orientation of the individual grainsfor any\nmisorientation angles!\nComment 6: More experimental details, diffraction patterns: experimental details see supplementary materials. The not\nincluding of diffraction patterns is well argumented and ok.\nComment 7: Abrupt compositional step: Error of reviewer2!\nComment 8: high-temperature scenario: Yes there is anew high-temperature scenario already shown with the data\npresented in this paper and which will be reinforced by a following paper on the deformation features found in this meteorite.\nI have no new own comments."}, "author_response": "Response to Reviewer #3\nThe answers of the authors to the comments of the reviewers are exceptionally extensive, very\ndetailed, and well understandable. I will mainly comment on the replies of the authors to the two\nreviewers.\nReviewer #1:\nComment 1a: Mesostasis vs impact melting: The textural arguments are convincing, especially be-\ncause the authors confirm the presence of both vesicles and immiscible Si-rich liquids. They give also\ncharacteristics of how to distinguish between both features on BSE images.\nComment 1b: Presence of high-pressure phases and the indicated peak pressure. The inhomogene-\nous presence of the latter from previous studies underlines El Goresy et al. (2013) red flag to use\nthem for peak pressure and the presence of very different P-T-t paths on a local scale.\nComment 1c: Thermal equilibrium: It seems, and the authors convincingly show the “sloppy” usage\nof the term “equilibrium” in the planetary science community. Added text is sufficient to explain the\nsituation.\nComment 2: In my opinion, bcc, fcc. Martensitic transformation etc., should be also common terms\nin the space science community and a reference to one of the many material science / metallography\ntextbook should have been sufficient. The explanation in the supplementary material is ok, except\nthat I would add that martensitic transformation have a low activation energy and need a small\ndriving force.\nSpelling out the acronyms bcc, fcc, and hcp was a special request of Reviewer #1. We suggest\nkeeping the latest version unchanged (i.e., with the acronyms explained and the dedicated paragraph\nin the supplementary materials), so that the unfamiliar readers have the information on hand. Then,\nit is true that martensitic transformations generally have low activation energies and usually need a\nsmall driving force to be initiated. However, as explained in lines 307-328, Mn-bearing but also Mn-\nfree Fe-Ni alloys are exceptions on that score, especially when fine-grained, C-poor and compressed\nlike here. In short, martensitic transformations in Fe-Ni materials need longer incubation times, and\nhigher activation energies and driving forces than other materials. Put differently: on an absolute\nscale, the activation energy (in Joule) of martensitic transformations is low when compared to other\nphysicochemical reactions, but non-negligible when Fe-Ni alloys are considered.\nA critical own comment: The authors use the term “lattice” but meaning “structure”. This error is\nwidespread and usually accepted. I would change nothing.\nReviewer #3 is right with this comment, but the use of “lattice” instead of “structure” is deliberate\nhere. The reason is that, in the EBSD community, the term “structure”, or sometimes “texture”, has\nthe meaning of “crystallographic preferred orientation (CPO)”, also referred to as… “lattice preferred\norientation (LPO)” in many texts. For example, saying that a given mineral phase has “a strong\ntexture/structure/CPO/LPO” means that the pole figures for that mineral show “strong clusters of\naxes/poles”. From this perspective, there is no wording making consensus in all fields of Earth and\nmaterials sciences. We also suggest not changing anything, as recommended by Reviewer #3.\nComment 3: Influence of alloying elements, use of average composition, statistical significance. The\ntreatment of alloying elements, grain size and compression are well done and understandable. The\nauthors have clarified that they have used average compositions not over the whole profile but for\nplateaus along the profile.\nStatistical significance: The statistical significance is somewhat reduced, but preparing more FIB\nsamples is not realistic. What may be criticized is that no error is given for the chemical analyses nor\nfor the constants in the equation 1-3 in the supplementary material, therefore no error propagation\ncalculation possible.\nTrue, but uncertainties are discussed in lines S698-S707 and S710-S714. For example, we consider\nranges of values for solar absorptance, hemispherical emittance, orbital distance from the Sun, and\nmass loss on atmospheric entry. In addition, we address the hypotheses of launch vs fall pairing to\ncalculate the meteorite mass. We also want to recall that equations 1-3 have been developed by\nothers with no clue on how to deal with error calculations in complex cases like here. In short, we\ntackled the possible errors and uncertainties from a qualitative point of view, because doing it quan-\ntitatively would require a dedicated study.\nComment 4: Deformation process due to magmatic cooling vs. impact: The difference in the material\nresponse of stress between metals vs. minerals and the actual observations in the sample are con-\nvincing evidence that the authors are able to distinguish the results of magmatic cooling vs. impact.\nComment 5: ok\nLine 196: Rephrasing is ok.\nLine 205-207: References have been added.\nLine 246-249: “but also olivine and pyroxene” has been removed.\nLine 246-249: The authors show convincingly that the reviewer is wrong!\nLine 264-267: The authors give a good explanation why the back transformation fcc to bcc is hindered\nin the Ni-rich fcc parts.\nLine 283-285: The sentence has been revised.\nLine 584-586: The sentence has been split.\nReviewer #2:\nComment 1: The comprehensible reduction to one measurable metal bleb out of 7 blebs is explana-\ntion enough for the limited statistical significance for the results from the sophisticated analytical\nmethods. The validation of the latter results by more conventional WDS and EBDS methods reinforces\nthe statistical significance.\nI agree with the authors that using FIB on many samples is unrealistic. For the nature of the sample,\nmetal embedded in ceramic, the fibbing is a lottery (I speak from experience).\nComment 2: see reply on comment 3 of reviewer #1\nComment 3: see reply on comment 3 of reviewer #1\nComment 4: at% vs. wt%: authors have adjusted all results and diagrams to wt%. Reply to this\ncomment by the authors: “……showing the boundary lines referenced in light gray in Fig. 5 (from\nYang et al., 1996).” I cannot see the “boundary lines referenced in light gray”!\nThe boundary lines shown in light gray in Fig. 5a are present indeed. We have checked the original\nAdobe Illustrator and EPS files and there is neither transparency nor any other insidious option that\ncould make them disappear when converted into PDF. We assume that the misunderstanding is\ncaused by the absence of gray lines in the 4th and 5th T-XFe-Ni diagram from the left. Both illustrate\ntransient disequilibrium stages for which referring to “equilibrium boundary lines” would be a non-\nsense. In contrast, gray lines should come out in the 1st, 2nd, 3rd, 6th and 7th T-XFe-Ni diagrams. We\nwill double-check that specific point in the proof and make you aware in case a misdisplay issue is\nstill there.\nComment 5: EBSD and misorientation: Automatic misorientation over boundary may only be possible\nfor small misorientation angles, but “manual” determination is possible based on crystallographic\norientation of the individual grains for any misorientation angles!\nComment 6: More experimental details, diffraction patterns: experimental details see supplementary\nmaterials. The absence of diffraction patterns is well explained and ok.\nComment 7: Abrupt compositional step: Error of Reviewer #2!\nComment 8: high-temperature scenario: Yes, there is a new high-temperature scenario already\nshown with the data presented in this paper and which will be reinforced by a following paper on the\ndeformation features found in this meteorite."}]

Decision letter