Origin and implications of two Verwey transitions in the basement rocks of the Vredefort meteorite crater, South Africa

Earth and Planetary Science Letters 251 (2006) 305 – 317 www.elsevier.com/locate/epsl

Origin and implications of two Verwey transitions in the basement rocks of the Vredefort meteorite crater, South Africa Laurent Carporzen a , Stuart A. Gilder a,⁎, Rodger J. Hart b a

Institut de Physique du Globe de Paris, Equipe de Paléomagnétisme, 75252 Paris, Cedex 05, France b iThemba Labs, P. Bag 11 Wits 2050, Johannesburg, South Africa Received 3 May 2006; received in revised form 26 July 2006; accepted 5 September 2006 Available online 12 October 2006 Editor: C.P. Jaupart

Abstract Two populations of magnetite exist in the shocked basement rocks of the Vredefort meteorite impact crater: one associated with original crustal genesis and metamorphism around 3.0 Ga, and the other related to the impact itself at 2.02 Ga. Pre-impact magnetite is mostly micron to millimeter in size, lying within the multidomain to pseudo-single domain range. The second population of magnetite is less than 10 μm in size and formed within the interstices of planar deformation features or within the reaction rims of biotite, both of which were created during impact. Our study shows that each of these populations possesses specific Verwey transition temperatures: one around 124 K associated with pre-impact magnetite and the other around 102 K associated with impact-related magnetite. The high temperature Verwey transition is attributed to stoichiometric magnetite while the low temperature Verwey transition to non-stoichiometric magnetite. Pre-impact rocks containing both Verwey transitions are ubiquitous throughout the crater. Pseudotachylites formed during impact have a single Verwey transition spanning temperatures from 94 to 111 K. Heating the basement rocks above ∼ 550–600 °C for 3 min or above ∼ 500 °C for 1 h irreversibly modifies the 124 K Verwey transition by shifting it to lower temperatures. Based on these findings, it is possible that no wholesale heating of the crater occurred above 550–600 °C for 3 min or above 500 °C for 1 h during or since the time of impact, although some places of more localized heating are identified. An unresolved problem remains to reconcile these data with temperatures thought to exist in the crust during and after impact. © 2006 Elsevier B.V. All rights reserved. Keywords: Verwey transition; impact crater; rock magnetism; susceptibility

1. Introduction Magnetite (Fe 2+ Fe23+ O4) has an inverse spinel structure with Fe3+ cations located in tetrahedral sites and both Fe3+ and Fe2+ cations located in octahedral sites. Below the Curie temperature of 580 °C, indirect ⁎ Corresponding author. E-mail addresses: [email protected] (L. Carporzen), [email protected] (S.A. Gilder), [email protected] (R.J. Hart). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.09.013

electron exchange occurs between the Fe cations via oxygen atoms in ordered magnetic sublattices, giving rise to a spontaneous magnetization. The organization of the magnetic sublattices yields a ferrimagnetic character in magnetite, where the magnetic moments generated in two octahedral magnetic sublattices are oriented opposite that of the magnetic moment in a single tetrahedral sublattice [e.g., 1]. The electric and magnetic properties of magnetite change radically around 120 K, due to a slight distortion

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of the crystallographic lattice [e.g., 2–7]. This is called the Verwey transition (Tv), based on a study by Verwey [8] who found that electrical conductivity rises by two orders of magnitude when cooling through 120 K. Similarly, when heating through the Tv, magnetic remanence decreases by 20 to 80% and magnetic susceptibility increases by 20 to 80% [e.g., 9–15], likely due to a modification of the magnetic anisotropy energy [16]. Tv temperatures for pure magnetite typically lie between 119 and 124 K [e.g., 4,11,12,17–21]. The temperature of the Tv as well as the sharpness and degree of the transition depends on many factors. In nonstoichiometric magnetite (Fe 3(1− δ) O 4 ), increasing amounts of cation vacancies (δ) decrease the Tv temperature linearly from 120 K to ∼ 110 K for lattice vacancies up to δ ≈ 0.004, then with a slightly lower slope from ∼110 K to ∼ 80 K for lattice vacancies from δ ≈ 0.004 to δ ≈ 0.012; Tv completely disappears above δ ≈ 0.012 [22–25]. The transition becomes more diffuse with increasing non-stoichiometry. Superficial oxidation of magnetite, called maghemitization, has a similar effect on Tv [26]. Cation substitution in octahedral sites (Fe3−x XxO4 where X = Ti, Al, Zn, etc.) also decreases the Tv temperature, by about 1° per tenth of a percent metal cation until X attains about 4%, above which the transition disappears [9,27–29]. Although debated, pressure appears to decrease the Tv temperature by about −3 K/ GPa [30–32]; however, recent experiments have noted that a coordination crossover occurs under pressure, resulting in an increase of magnetite's isotropic point by more than 10 K/GPa [16,33]. Sharpness and degree of the Tv also depend on domain state and whether magnetite is cooled through the Tv in the presence or absence of an applied magnetic field [14,18,34,35]. For example, when cooled in a null field (called zero field cooling or ZFC), given a saturation isothermal remanent magnetization (SIRM), then heated through the Tv, the amount of remanence lost at Tv increases with increasing grain size [1,34]. When a moment is applied during cooling (called field cooling or FC) from room temperature to below Tv, magnetizations below the Tv are higher than those of ZFC magnetizations for single domain grains, with the opposite being true for multidomain grains (ZFC moments are greater than FC moments) [36]. In this paper, we report the existence of two distinct Verwey transitions in the basement rocks of the Vredefort (South Africa) meteorite impact crater, which is among the largest (original diameter ∼300 km) impact structures known on Earth. Although two Verwey transitions have been found in Chinese loess [37], to our knowledge this is the first study documenting two distinct Verwey transi-

Fig. 1. Photomicrograph of shocked Vredefort granite showing a large Archean multidomain magnetite grain (opaque mineral) surrounded by quartz (transparent). The quartz contains planar fractures and planar deformation features (PDFs) filled with micron-sized magnetite that formed during the impact event [38].

tions in the same crystalline rock. We relate the two transitions to two generations of magnetite known to coexist in the Vredefort rocks (Fig. 1) [38]: (1) relatively large (several microns to millimeter) magnetite that originally formed within the basement rocks (granite, gneiss, granulite, etc.) at ca. 3.0 Ga [39] and (2) relatively small (less than 10 μm) magnetite that crystallized in the alteration halos of biotite or within planar deformation features (PDFs), both during the impact event at 2.02 Ga [40]. The origin and implications of the two Verwey transitions are discussed. 2. Frequency dependence and the Verwey transition of bulk rock material Our original intention was to better understand the origin of the extremely high magnetic remanences and Koenigsberger (Q) ratios of the Vredefort basement rocks [41,42]. The presence of newly formed magnetite within thin PDF planes led us to question whether superparamagnetic grains were also present in the rocks, which would mean that the extremely high Q ratios would actually be underestimated. We also wanted to know if magnetic interactions occur between magnetite in the PDFs, where single domain grains are crystallographically aligned with respect to each other and have a uniform direction of magnetization [38]. Thermally activated

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magnetization kinetics, common to superparamagnetic grains or in cases of magnetic interaction, decelerate in proportion to alternating current (AC) field frequency [43– 45]. Thus, our first step was to measure the frequency dependence of the magnetization at low temperatures. AC magnetic susceptibility measurements have an in-phase, mathematically real, component k′=Xcosφ and an out-ofphase (or quadrature), mathematically imaginary, component k″=Xsinφ, calculated from the magnitude of the bulk susceptibility (X) and the phase shift, φ. For the AC frequency experiments, 24 samples were crushed to a coarse powder, about 300 mg of which was placed in a Quantum Design, Magnetic Property Measurement System (MPMS) cryogenic susceptometer or in a LakeShore Cryotronics alternating current susceptometer. AC susceptibilities (peak AC fields are 279 μT for the Quantum Design and 200 μT for the Lakeshore) were measured at five different frequencies (1, 6, 32, 178 and 997 Hz on the Quantum Design and 40, 140, 400, 1000 and 4000 Hz on the Lakeshore) every 10° between 20 and 300 K. We selected samples from the Vredefort crystalline basement rocks (granite, gneiss and granulite) that previously underwent alternating field demagnetization and had also been analyzed for a number of magnetic properties [42]. Samples were selected based on their position in the crater (Fig. 2), those that exhibit end-member values of bulk

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susceptibility (137 ×10− 6 to 45× 10− 3 SI), natural remanent magnetization (0.01 to 100 A/m) and Q ratios (0.1 to 595.7), as well as their hysteresis parameters and their magnetic anisotropy characteristics. Only one of the 24 samples exhibited a frequency dependence of AC susceptibility and, with one exception, none had perceptible out-of-phase (k″) components, suggesting that the Vredefort rocks are largely void of superparamagnetic grains and that magnetic interactions between magnetite grains are negligible [44]. Surprisingly though, 15 of the 24 samples possessed two distinct, frequency independent, inflections in k′ at about 100 K and 120 K (Fig. 3a and Supplementary Table 1). The rest had a single k′ inflection, again frequency independent, either at around 100 K or around 120 K (Fig. 3b). To better understand the nature of the k′ data, we performed low temperature, direct current (DC) magnetic measurements on the Quantum Design MPMS. Sample measurements were made in a null field once every 5° without pause during heating or cooling between 20 and 300 K in the sweeping mode. Warming rates were 5° per minute. Temperatures vary ≤0.6 K during an individual measurement and the temperature gradient within the sample region is ±0.1 K, with absolute temperatures known to a precision of ≤ 1 K.

Fig. 2. Simplified geologic map of the Vredefort crater. Only bulk rock samples whose Verwey transitions were measured via zero field cooling (ZFC) are shown. 2 Tv = two Verwey transitions identified in the same sample; LT–Tv = sample possesses only a low temperature (b110 K) Verwey transition; HT–Tv = sample possesses only a high temperature (N110 K) Verwey transition; no Tv = no detectable Verwey transition. Note that rocks possessing a single Verwey transition lie along the limit between the intermediate and lower crust (granite–gneiss to granulite transition zone) where aeromagnetic anomalies are the greatest (− 2000 nT at 150 m altitude).

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Fig. 3. (a) AC in-phase susceptibility (k′) showing two distinct Verwey transitions Tv. Out-of-phase susceptibility (k″ or quadrature) is insignificant for 23/24 samples. (b) AC in-phase susceptibility for samples showing a single distinct Verwey transition at low (100 K) and high (120 K) temperatures.

The Quantum Design MPMS has a sensitivity of 10− 8 emu. We carried out ZFC measurements using a 2.5 T field to impart an SIRM at 20 K on 45 bulk rock powders (∼300 mg/sample) from granite, gneiss, granulite or pseudotachylite lithologies (Supplementary Table 1). After warming to 300 K, magnetizations were between 1.1 × 10− 2 and 2.5 × 10− 2 emu/g, several orders of magnitude higher than the sensitivity of the MPMS. The Tv temperature was defined as the peak in the derivative of the change in moment (M) with respect to temperature (T) where dM / dT(T1) = (MT1 − MT0) / (T1 − T0). In some cases, the ZFC moments decrease over a broad temperature range and the derivatives are equally diffuse, yet contain maxima that were interpreted as the Tv for the sample. These cases are called “not sharp” in the comment column in Supplementary Table 1. Two distinct Tv temperatures were characterized in 31 of the 45 samples (Fig. 4), and both low and high temperature Verwey transitions defined two significantly different populations at 102.4 ± 3.1 K and 124.3 ± 2.6 K (Fig. 5; statistics exclude two samples whose Tv temperatures were not well defined—see Supplementary Table 1). All 31 samples with two peaks come from shocked basement rocks located throughout the crater (Fig. 2). Six granite–gneiss samples carried a single peak at 123.2 ± 2.5 K. All but one of these samples come from the transition zone between the granite–gneisses and granulites that marks the boundary between the intermediate and lower continental crust (Fig. 2). Six

granulite samples and a pseudotachylite sample possessed a single peak at 100.9 ± 4.1 K. The granulite samples with the low temperature peak also come from the granite–gneiss to granulite contact zone, but shifted slightly closer to the center of the crater than the population with the single high temperature Tv (Fig. 2). Only a single sample (V260A) possesses a quasi-linear SIRM demagnetization curve between 20 and 300 K, with no apparent Verwey transition. This sample is the only one possessing a frequency dependent AC susceptibility. It too is located at the granite–gneiss to granulite transition zone (Fig. 2).

Fig. 4. Comparison between field cooling (FC) and zero field cooling (ZFC) measurements for the same sample.

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Fig. 5. Histogram of Verwey transition temperatures of samples with two distinct transitions identified via zero field cooling (ZFC) measurements (data from Supplementary Table 1 excluding two samples with ‘not sharp’ notations in the comment column). Mean values of low and high temperature populations are 102.4 ± 3.1 K and 124.3 ± 2.6 K (N = 58).

For each sample, we calculated the delta (δ) parameter, where δ = (M80 −M150) /M80, with M being the magnetic moment at 80 or 150 K [34] (Supplementary Table 1). For samples with two Tv, we calculated the relative contribution of each Tv by defining the low temperature Tv (LT–Tv) at temperatures below 110 K and the high temperature Verwey transition (HT–Tv) above 110 K, leading to δLT–Tv = (M80 − M110) /M80 and δHT–Tv = (M110 −M150) /M80. The relative percentages of each are then %δLT–Tv = (M80 −M110) / (M80 −M150) and %δHT– Tv = (M110 −M150) / (M80 −M150) (Supplementary Table 1). The percentage contribution of both Tv to the total signal ranged from nearly 50–50% to as low as 14–86%. It is possible that even more unequal percentages exist but do not express themselves readily in the derivate curves. Potential cases are signaled in the comment column in Supplementary Table 1. We did not find any correlation between the various Verwey parameters (δLT–Tv, etc.) with the rock magnetic parameters (Mrs, Q etc.) nor the sample's locality within the crater. 3. Grain size dependence on the Verwey transition temperature: part I The high temperature (ca. 124 K) transition (HT–Tv) is characteristic of impurity-free, stoichiometric magnetite, whereas the low temperature (ca. 102 K) transition

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(LT–Tv) could be attributed to impurities, non-stoichiometry, or high pressure (strain) effects as discussed above. Knowing that the Vredefort rocks possess two generations of magnetite [38], we naturally suspected that each generation gave rise to a specific Verwey transition. The question then became, which generation of magnetite (pre-impact or syn-impact) contributed to which Tv? Magnetite's response when cooled in an applied field (called field cooling or FC) or in the absence of an applied field (called zero field cooling or ZFC) varies as a function of domain state. For example, Moskowitz et al. [34] found that δFC/δZFC ∼ 1 for multidomain magnetite and δFC/δZFC N 1 for single domain magnetite. We thus performed FC experiments on six samples exhibiting both Verwey transitions to compare against the ZFC data. For the six Vredefort bulk rock samples, three had equivalent FC and ZFC moments, δFC/δZFC ∼ 1, while the other three had ZFC moments much higher than FC moments below Tv, suggesting that all six samples contain multidomain magnetite [36]. Unfortunately, the FC/ZFC experiments were inconclusive regarding which generation contributed to which Tv. In order to better understand the origin of the Verwey transitions, we separated the magnetite from the Vredefort rocks into two size fractions by crushing the bulk rock, soaking the powder in acetone, then extracting the magnetic particles with a hand magnet. After drying, the magnetic fraction was sieved in nylon meshes of 100 μm, 15 μm and 10 μm. The ≥100 μm fraction underwent a final selection by hand picking under a binocular microscope; hand picking was not employed for the ≤10 μm size fraction. We could not obtain enough of the ≤10 μm fraction for a few samples, so a size fraction of ≤15 μm was accepted for them (Supplementary Table 1). Thus, for each sample, two magnetic fractions of ∼1 mg were obtained: one ≥100 μm and one ≤10 μm (except for those few with ≤15 μm). We determined the domain state of the size fractions by measuring their hysteresis parameters with a Princeton Applied Research vibrating sample magnetometer (Fig. 6; Supplementary Table 1). Hysteresis parameters for 21 ≥ 100 μm samples are 2.5 ≤ Hcr/ Hc ≤ 25.0 and 0.01 ≤ Mrs/Ms ≤ 0.20, consistent with multidomain to pseudo-single domain grain sizes. Hysteresis parameters for 20 ≤ 10 μm and ≤ 15 μm samples are 1.9 ≤ Hcr/Hc ≤ 2.9 and 0.14 ≤ Mrs/ Ms ≤ 0.26, corresponding to pseudo-single domain grain sizes [46]. Thus, as above, the ≥ 100 μm fraction should be representative of multidomain magnetite formed in basement rocks before impact while the ≤ 10 μm fraction should be more indicative of single

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domain magnetite formed during impact (Fig. 1). Although the ≤ 10 μm fraction could be contaminated by some pulverized multidomain magnetite, it is unlikely the ≥ 100 μm fraction would be contaminated by single domain magnetite. However, the hysteresis experiments raise the possibility that some fine grained magnetite crystallized within deformation fractures in the large magnetite grains (M. Cloete, personal communication, 2004) or that the shock broke the large magnetite into clusters of smaller domains. We measured the ZFC magnetizations of 22 ≥ 100 μm samples and 20 ≤ 10 μm samples. The majority (16/22) of the ≥100 μm samples have a dominant Tv at 121.6 ± 4.2 K within uncertainty limits of the HT–Tv measured in their bulk rocks (Fig. 7, Supplementary Table 1). Five ≥100 μm samples possessed a single LT–Tv. They were extracted from bulk rocks either possessing a single LT–Tv, or had potentially two Tv but the HT–Tv was small compared to the LT–Tv. One ≥100 μm sample possessed the same two Tv characterized in its bulk rock. Two Verwey transitions at 101.5 ± 3.2 K and 122.4 ± 3.3 K were found in 18 of 20 ≤ 10 μm samples, similar, to that found in the ZFC curves of their corresponding bulk rocks (Fig. 7, Supplementary Table 1). Two ≤10 μm samples displayed broad derivate curves making it difficult to compare against their bulk rocks (Supplementary Table 1). FC measurements were performed on seven samples of both size fractions to compare their ZFC and FC characteristics (Fig. 8). At temperatures below Tv, the ≥ 100 μm samples were more magnetized after a ZFC

Fig. 7. Normalized ZFC curves (at 20 K) measured on a bulk rock as well as on ≤ 15 μm and ≥100 μm magnetic fractions separated from the bulk rock. Inset shows normalized (to maximum value) derivative curves. The two Verwey transitions found in the ≤15 μm samples are similar to those of the corresponding bulk rock sample, while the ≥100 μm sample has a dominant Tv corresponding to the HT–Tv seen in the bulk rock.

than after a FC (Fig. 8a), whereas the ≤ 10 μm samples had ZFC moments slightly lower or nearly equivalent to FC moments (Fig. 8b). This again indicates that ≥ 100 μm samples are characterized mostly by multidomain magnetite grains while ≤10 μm samples are skewed closer to single domain grain sizes [34–36]. In conclusion, for the majority of the samples, large grains (≥ 100 μm) are associated with the high temperature Verwey transition (HT–Tv), whereas small (≤ 10 μm) grains have the same two Tv as the bulk rock samples. We interpret this as a contamination of multidomain grains in the ≤10 μm fraction that gives rise to the HT– Tv while the single domain grains carry the LT–Tv. 4. Grain size dependence on the Verwey transition temperature: part II

Fig. 6. Day plot [53] of the magnetite fractions separated from the Vredefort bulk rocks. Limits between single domain (SD), pseudosingle domain (PSD) and multidomain (MD) follow Dunlop [46]. Symbols indicate the magnetic size fraction and gray-shades designate whether the fraction had two Verwey transitions (2 Tv), a single low temperature (LT–Tv) or a single high temperature (HT–Tv) Verwey transition determined via zero field cooling (ZFC).

To glean further information on the origin and nature of the two Verwey transitions in the Vredefort rocks, we turned to the AGICO CS-L KappaBridge susceptibility meter. This instrument measures the bulk susceptibility (χ) of rock powders over temperatures spanning ca. 80 K to almost 1000 K. Magnetic extracts were dispersed in a silica matrix to avoid contact with air. Absolute temperatures, including temperature variations occurring during a measurement, are accurate to ± 2°. Temperature gradients in the sample region are likely b0.5°. The measurement interval is higher for the CS-L than the MPMS, being about one measurement per 1° to 3° for the former as opposed to one measurement per 5°

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Fig. 8. Comparison of ZFC and FC experiments (a) for a ≥100 μm magnetite fraction from sample V021A and (b) for a ≤ 10 μm magnetite fraction from sample V317A. These results are consistent with greater amounts of larger magnetite grains in (a) than in (b) [35,36].

for the latter, or one per 10° for the AC susceptibility experiments. To measure χ at low temperatures (b273 K), the cryostat sleeve surrounding the vial holding the sample is filled with liquid nitrogen until the sample reaches ∼ 80 K. The liquid nitrogen is then drained off with argon gas before the measurement series begins. Because the χ of liquid nitrogen is high (tens of 10− 6 SI) in comparison with the χ of the samples in our study, and because some liquid nitrogen remains to about

90 K before completely boiling away, it is difficult to obtain reliable measurements below ∼90 K. Another caveat is that a gap in measurements exists around 273 K when the cryostat sleeve must be exchanged for a furnace between low (b 273 K) and high (N273 K) temperature measurements. Fig. 9 shows a comparison between the MPMS and the CS-L for a bulk rock sample with two clearly visible Verwey transitions. The derivative curves are shifted to

Fig. 9. Comparison of the data acquired from the MPMS with that from the CS-L for the same specimen (V344A): (a) measured values and (b) normalized (to maximum values) derivative curves (absolute values) of the data from (a). The maxima of the derivative curves are about 5° hotter for the MPMS than the CS-L for this sample and 2° on average.

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higher temperatures by about 5° for the MPMS data than for the CS-L data. As discussed below, this is mostly due to a coarser sampling interval for the MPMS. We measured χ for four pseudotachylites, four ≥ 100 μm samples and five ≤ 10 μm samples by (step 1) cooling to ∼ 90 K then heating to ∼ 200 K, (step 2) heating from room temperature to above 853 K (the Curie temperature of Fe-pure, stoichiometric magnetite) then cooling back down to room temperature, and finally (step 3) cooling to ∼ 90 K then heating to ∼ 200 K a second time. Derivatives of the thermomagnetic curves for the five ≤10 μm samples measured before heating (step 1) define a dominant peak at 101.4 ± 1.1 K and a second, much smaller peak at 120.2 ± 3.7 K (Fig. 10). Upon heating through the Curie temperature (step 2), one observes a χ maximum at about 524 K with an inflection at 694 K and that the thermomagnetic cycle is irreversible with about half the χ lost upon cooling (Fig. 10). This behavior is usually attributed to the presence of non-stoichiometric magnetite, such as maghemite, which is converted to hematite plus magnetite or hematite plus oxygen at high temperature, depending on the degree of oxidation [47,48]. Both maghemite and hematite were observed in Vredefort rocks via X-ray diffraction and scanning electron microscopy, respectively [38]. After the second cooling (step 3), the LT–Tv disappears and is replaced by a HT–Tv at 120.8 ± 4.9 K (Fig. 10). Derivatives of the thermomagnetic curves for the four ≥ 100 μm samples measured before heating (step 1) define a single HT–Tv at 117.3 ± 5.7 K with no indication of a LT–Tv (Fig. 11). In contrast to the ≤ 10 μm samples,

Fig. 10. CS-L thermomagnetic measurements for a ≤10 μm magnetite fraction from sample V271A with normalized (to maximum value) derivative curves corresponding to the Tv (bottom left), to the Curie point of magnetite (bottom right) and to the heating part of step 2 (middle top). Note the poor reversibility during step 2.

Fig. 11. Thermomagnetic measurements from the CS-L for a ≥ 100 μm magnetite fraction for sample V006A with normalized (to maximum values) derivative curves corresponding to the Tv (left) and to the Curie point of magnetite (right). Note the good reversibility during step 2.

heating through the Curie temperature then cooling back to room temperature (step 2) of the ≥ 100 μm size fraction shows a high degree of reversibility (Fig. 11). However, upon cooling a second time (step 3), the Tv occurs at 112.5 ± 5.2 K (Fig. 11), some 5° lower than initial conditions. Two pseudotachylite bulk rocks had very diffuse transitions while two others had a single Tv at 107 and 111 K, intermediate between the LT–Tv and the HT–Tv of the granites and granulites. Recalling that the ZFC and AC results defined a single Tv temperature at 94 and 100 K suggests the Tv of the pseudotachylites are skewed toward the LT–Tv. Step 2 measurements of the pseudotachylites defined Curie temperatures around 853 K, characteristic of Fe-pure magnetite. Like the b10 μm fractions, step 2 heating cycles display evidence for non-stoichiometric magnetite that disappears on the cooling cycle of step 2. Also like the b 10 μm fractions, Tv temperatures from the step 3 measurements on the pseudotachylites were higher than those determined during step 1, tending toward stoichiometric values of 120 K (Supplementary Table 1). Thus the magnetite in the pseudotachylites resembles the second generation of magnetite in the basement rocks that formed during impact. For completeness, we note that pseudotachylite sample V5013 displays a potential HT–Tv (Supplementary Table 1). It may not be surprising to find evidence for a HT–Tv in some pseudotachylites because they often contain clasts of basement material that potentially carry the HT–Tv. The sum of the experimental results suggests that the small size fraction (≤ 10 μm) is composed of non-

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stoichiometric magnetite associated with the LT–Tv while the HT–Tv originates from stoichiometric magnetite. The LT–Tv is dominated by single to pseudo-single domain grains that crystallized during impact and the HT–Tv is dominated by pseudo-single to multidomain grains that formed in the pre-shocked basement. That the pseudotachylite dykes, which are clearly associated with the meteorite impact, have low to intermediate Verwey transition temperatures supports this conclusion. This could imply that the oxygen activity of the fluid from which the new magnetite crystallized was relatively high. This fluid must have been broadly dispersed yet inhomogeneous in space based on the fact that some areas in the crater are void of non-stoichiometric magnetite while others have only non-stoichiometric magnetite. Moreover, the fact that heating the ≥ 100 μm size fraction above 1000 K diminishes its Tv lends support that the crater as a whole was never heated above 1000 K. This final point is important because the magnetic remanence directions of some meteorite impact craters are homogeneous, suggestive of a complete resetting of the remanence directions during impact [49]. At Vredefort, Carporzen et al. [42] found that impactrelated dykes and pseudotachylites, which cooled through the Curie temperature after meteorite impact, had homogeneous directions. However, shocked basement rocks were found to possess randomly oriented and very strongly magnetized remanence directions that cannot be explained by a thermal remanent magnetization. Our new finding that the pre-impact, ≥ 100 μm size fraction was never heated above 1000 K is compatible with this interpretation. And to further explore this hypothesis we conducted a series of experiments as described below. 5. Temperature of the Vredefort crater as seen by the Verwey transition We verified the CS-L results on the MPMS by heating two bulk rock samples, both having previously defined two Tv in the ZFC experiments, for 1 h at 700 °C in an open-air atmosphere, then repeating the ZFC measurements a second time (Fig. 12). After heating, only a single Tv existed, 5° to 6° lower than the HT–Tv of the non-heated bulk rock (Supplementary Table 1). The disappearance of the LT–Tv, together with the presence of a sharply defined Tv (Fig. 12) argues against oxidation of the magnetite during heating as maghemitization tends to smear out the transition [26,36]. We are not sure what contributes to the lowering of the HT–Tv. Because oxidization lowers the Tv temperature and decreases the sharpness of the transition, partial oxidization during

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Fig. 12. Comparison of ZFC curves before and after heating to 700 °C for 1 h in air.

heating is not apparent (Fig. 11). As above for the LT–Tv, conversion of maghemite to hematite plus oxygen could account for the loss in remanence [48], recalling that the saturation magnetization per unit cell of maghemite (380 kA/m) is less than magnetite (480 kA/m) but much greater than that of hematite (2 kA/m) [1]. However, we did not identify the presence of a Morin transition or a Néel point to confirm the presence of hematite after converting the maghemite, possibly because the signal would be much weaker than that of the magnetite and thus difficult to resolve. On the other hand, the LT–Tv could arise from cation vacancies and not maghemitization, but we are unaware of published work describing how the magnetic properties of such material behave at high temperature. To estimate the maximum temperature experienced in the crater, we took a ≥ 100 μm size fraction from sample V021A, which is known to possess only a HT–Tv, measured its Tv, then heated it to 399 K for 3 min, cooled back to room temperature, then measured its Tv again. We repeated this exercise at successively higher temperatures (511, 595, 693, 789 and 895 K) each time remaining at the maximum temperature per given run for about 3 min. A second sample went through the same procedure at 817 and 888 K again for about 3 min. We then took a new ≥ 100 μm size fraction and repeated the same experimental procedure at maximum temperature per given run for 1 h, this time to temperatures of 674 ± 2, 752 ± 3, 768 ± 8 and 827 ± 3 K. In each case, we calculated the difference in the Tv temperature at a given temperature with respect to the initial Tv temperature (HT–Tv(T = n) − HT–Tv(T = initial)) (Fig. 13). For the threeminute runs, Tv decreases by 1° between 790 and 817 K and an additional 3° between 817 and 888 K. For the onehour runs, the lowering of Tv commences between 752

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Fig. 13. Difference in the Verwey temperature before heating (HT–Tv(T = initial)) and after heating (HT–Tv(T = n)) for 3 min (black curve) and for 1 h (gray curve) to a temperature (n) indicated by the data point.

and 768 K. Assuming the redox conditions in the experiments were the same as those at Vredefort during impact, these new findings imply that the crater was not globally heated above ∼ 550–600 °C for more than a few minutes or ∼ 500 °C for over an hour following impact. Of course, local patches attained those temperatures, which might be signaled in places with only a single Tv. 6. Discussion and conclusions We set off to determine whether the shocked basement rocks of the Vredefort meteorite crater contained superparamagnetic magnetite or highly interacting magnetite grains. Except for one sample, AC susceptibility measurements found no evidence for either, yet did reveal the presence of two Verwey transitions in 15 of 24 samples. We then verified the existence of two Verwey transitions through zero field cooling (ZFC) experiments on 45 samples, for which 31 had two distinct transitions at 102.4 ± 3.1 K and 124.3 ± 2.6 K. Since earlier work identified two different generations of magnetite in the Vredefort basement rocks [38], we focused our attention on understanding which generation was responsible for which Verwey transition, knowing that the pre-impact magnetite was larger (close to multidomain in size on average) and that the impact-related magnetite was smaller (closer to single domain in size on average). We then separated the magnetite from the rock and sieved it into ≥ 100 μm and ≤ 10 μm size fractions. Performing ZFC and FC experiments on the magnetite extracts and comparing with the bulk rock data

established that the large grains were associated with the high temperature (124 K) Verwey transition while small size fraction had both, but that the low temperature (102 K) transition was more pronounced than in the whole rock. Further experimentation with the CS-L KappaBridge determined that the small size fraction was dominated by non-stoichiometric magnetite. Because non-stoichiometry in magnetite leads to lower Verwey transition temperatures, we concluded that the low temperature (102 K) Verwey transition came from the impact-related magnetite, which is much smaller on average than the pre-impact magnetite, which in turn is associated with the high temperature (124 K) Verwey transition. The CS-L experiments led us to discover that, after heating above 700 °C, the Verwey transition temperature diminishes in large grains and rises or completely disappears in small grains. Experimentation with the MPMS confirmed this finding. We then reasoned that temperatures in the crater never exceeded ∼ 700 °C (globally) because the pre-impact magnetite had Verwey transition temperatures typical of stoichiometric, relatively Fe-pure magnetite [e.g., 4,11,12,19–21]. If true, we then sought out to determine the maximum possible temperature required to lower the Verwey transition temperature of the pre-impact magnetite, taking thermal kinetics into account. These new experiments suggested that the Vredefort crater was never globally heated above ∼ 550–600 °C for 3 min or ∼ 500 °C for 1 h during or since the time of impact. If the conditions of our experiments were the same as those acting during impact, these data have important consequences for the numerical modeling of meteorite impacts and their thermal consequences. Our study and interpretations also raise some problems or require further explanation. For example, one should ask whether the LT–Tv could be due to recent alteration and surface weathering, rather than being acquired during impact. However, if the LT–Tv is due to a recent maghemitization, then why would it systematically affect the small grains and not the large ones? Indeed, the impact-related magnetite should even be better protected from chemical alteration by the host quartz [50]. Moreover, the core samples were drilled 3 to 8 cm into the rock. Except in some cases where the upper few millimeters have a weathering rind, visual inspection shows no discoloration in the lower parts of the core where the samples were derived. The rocks were so hard in fact, that drilling them with a non-sharpened drill bit could take up to 5 min per centimeter. Samples from quarries provided the same results as samples drilled on the surface. If recent alteration gave rise to the LT–Tv, we

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would expect oxidation levels to vary and thus display a wide range of Tv temperatures, rather than a well-defined peak as observed in Fig. 5. Finally, remanence acquired during recent maghemitization should be oriented parallel to the present Earth magnetic field. This is not observed [42]. Other problems surround the HT–Tv. For one, we know of no published work reporting Verwey temperatures higher than 124 K, yet 23 samples in our study exceed 124 K, with one attaining 127 K. However, a more careful analysis of the data (spline fitting, etc.) shows that these high values are mostly due to the relatively crude, 5° temperature steps employed by the MPMS and the way that the maximum of the inflection of the derivative is based on the second (hotter) of the two temperature steps used to calculate the derivative. That the CS-L data, which are based on a finer sampling interval, yield slightly lower (2° on average) Verwey temperatures compared against the MPMS supports this interpretation. Interestingly, Mössbauer and other experiments on magnetite under pressure (to our knowledge, no Mössbauer study exists upon pressure release) find that the Tv temperature decreases with pressure (stress). Because the stresses experienced by the basement rocks during impact were huge (N30 GPa [51,52]), we expected to see only low Verwey transition temperatures throughout the crater, consistent with the Mössbauer data. The presence of Verwey transition temperatures N 120 K implies that the pressure memory of the Verwey transition is not retained—a point worthy of further exploration. Resolving the Verwey data with the geology raises important aspects needing reflection. The fact that the atypical samples (e.g., those with one or no Tv) come mostly from the granite–gneiss to granulite transition zone, which represents the transition from intermediate to lower continental crust, can be explained by focusing or defocusing of shock waves at a rheologic interface. It is likely no coincidence that the magnetic anomalies within the crater are the greatest in that area. Moreover, it seems logical that rocks possessing only the LT–Tv lie closer to the crater's center where temperatures should have been the hottest. Following this logic, the two samples (V227A and V225A) from the innermost parts of the crater with two Verwey transitions are difficult to explain (Fig. 2, Supplementary Table 1). Given a geothermal gradient of 25°/km, and estimating the preimpact depth of the crater's center at 30 km, imposes temperatures of 750 °C at the center at the time of impact. If heating does indeed lower the HT–Tv and remove the LT–Tv, then the center should be void of rocks with two Tv and instead should have a single Tv around 115–

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118 K. In addition, the proportion of HT–Tv to LT–Tv is fairly equal throughout the crater, e.g., we observe no systematic difference in %δHT–Tv/%δLT–Tv as a function of distance from the center. However, no systematic variation in the recorded shock pressure, calculated on the basis of PDF orientations in quartz, has been observed as a function of position in the crater [52]. Further investigation well within the granulite facies rocks should be undertaken to resolve this discrepancy with the heating experiments. Acknowledgements Measurements on MPMS, Lakeshore and Princeton VSM were carried out during two visits to the Institute of Rock Magnetism, University of Minnesota. LC thanks Brian Carter-Stiglitz, France Lagroix, Pete Sølheid and Jim Marvin for help with running the instruments and Mike Jackson for arranging his visits to the IRM. Measurements on the AGICO CS-L were carried out at the IPGP paleomagnetic laboratory at Saint Maur-les-Fossés. Hélène Bouquerel and Yves Gamblin helped design the sieves. We also thank Marthinus Cloete, Vincent Courtillot and two anonymous reviewers for helpful comments. This work was supported by grants to SG and RH from the French Ministries of Foreign Affairs and Education and Research, CNRS-INSU and the South African National Research Foundation. IPGP publication #2153. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. epsl.2006.09.013. References [1] D.J. Dunlop, Ö. Özdemir, Rock Magnetism, Cambridge Univ. Press, Cambridge, 1997. [2] E.J. Verwey, P.W. Haayman, F.C. Romeijn, Physical properties and cation arrangement of oxides with spinel structures — II. Electronic conductivity, J. Chem. Phys. 15 (1947) 181–187. [3] T. Toyoda, S. Sasaki, M. Tanaka, Evidence of charge ordering of Fe2+ and Fe3+ in magnetite observed by synchrotron X-ray anomalous scattering, Am. Mineral. 84 (1999) 294–298. [4] P. Novák, H. Stepánková, J. Englich, J. Kohout, V.A.M. Brabers, NMR in magnetite below and around the Verwey transition, Phys. Rev., B 61 (2000) 1256–1260. [5] J. García, G. Subías, M.G. Proietti, J. Blasco, H. Renevier, J.L. Hodeau, Y. Joly, Absence of charge ordering below the Verwey transition temperature in magnetite, Phys. Rev., B 63 (2001) 054110. [6] J.P. Wright, J.P. Attfield, P.G. Radealli, Charge ordered structure of magnetite Fe3O4 below the Verwey transition, Phys. Rev., B 66 (2002) 214422.

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