160 Gpa Shock on the Magnetic Properties of Basalt and Diabase

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Geochemistry, Geophysics, Geosystems

RESEARCH ARTICLE The effects of 10 to >160 GPa shock on the magnetic 10.1002/2016GC006583 properties of basalt and diabase

Special Section: N. S. Bezaeva1,2, N. L. Swanson-Hysell3, S. M. Tikoo3,4, D. D. Badyukov5, M. Kars6, R. Egli7, Magnetism From Atomic to D. A. Chareev1,2,8, L. M. Fairchild3, E. Khakhalova9, B. E. Strauss9, and A. K. Lindquist10,11 Planetary Scales: Physical Principles and Interdisciplinary 1Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia, 2Institute of Geology and Petroleum Applications in Geo- and Technologies, Kazan Federal University, Kazan, Russia, 3Department of Earth and Planetary Science, University of Planetary Sciences 4 California, Berkeley, California, USA, Department of Earth and Planetary Sciences, Rutgers University, New Brunswick, New Jersey, USA, 5V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Key Points: Moscow, Russia, 6Center for Advanced Marine Core Research, Kochi University, Nankoku, Japan, 7Central Institute for Spherical shock experiments result in Meteorology and Geodynamics, Vienna, Austria, 8Institute of Experimental Mineralogy, Russian Academy of Sciences, a range of pressures across a single 9 10 sample Moscow, Russia, Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota, USA, Department of 11 We observed and characterized Geosciences, University of Arkansas, Fayetteville, Arkansas, USA, Department of Environmental Studies, St. Olaf College, shock-induced and temperature- Northfield, Minnesota, USA induced magnetic hardening The described effects likely occur in target rocks during large impacts on Abstract Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk terrestrial bodies magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sam- Supporting Information: Supporting Information S1 ple (ranging between 10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by Correspondence to: pseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magnetic N. S. Bezaeva, properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy [email protected] support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal Citation: effects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments on Bezaeva, N. S., et al. (2016), The effects of 10 to >160 GPa shock on the untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First-order magnetic properties of basalt and reversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock and diabase, Geochem. Geophys. Geosyst., heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that 17, doi:10.1002/2016GC006583. does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomag- Received 8 AUG 2016 Accepted 3 NOV 2016 netic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters. Accepted article online 7 NOV 2016

1. Introduction Hypervelocity impacts represent a major mechanism for the evolution of solid matter in our solar system. Impact shock waves can modify bulk magnetic properties and the remanent magnetization of rocks through a variety of mechanisms [Cisowski and Fuller, 1978; Gattacceca et al., 2007]. Shock-induced heating at sufﬁciently high pressures may induce total or partial melting accompanied by crystallization of new ferromagnetic grains, as well as subsolidus thermal remagnetization of existing grains. Shock can also produce phase transformations of ferromagnetic minerals, grain fracturing, and crystallographic defects (such as point defects and dislocations) within magnetic grains [e.g., Reznik et al., 2016]. Consequently, the investiga- tion of magnetic records in solid bodies in the solar system often involves analyzing rocks whose primary magnetization has been partially erased or completely overprinted by impact events. Understanding the properties of shock-induced remagnetization and changes in magnetic properties of rocks is therefore important for correctly interpreting the crustal magnetism of cratered surfaces on Earth, Mars, the Moon, and asteroids, as well as paleomagnetic records of extraterrestrial materials [Weiss et al., 2010] and rocks from terrestrial impact craters [e.g., Halls, 1975; Louzada et al., 2008; Elbra et al., 2009].

VC 2016. American Geophysical Union. One way to experimentally assess the effects of shock on magnetic minerals is to conduct shock experi- All Rights Reserved. ments wherein rocks are brieﬂy taken to high pressures as an analog for the effect of the passage of shock

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 1 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

waves during a hypervelocity impact. In spherical shock experiments, explosively generated shock waves travel through a prepared spherical rock sample [Bezaeva et al., 2010]. The spherical shock approach has several advantages relative to other types of shock experiments. One advantage is that target rocks are not contaminated by impactor materials. Another advantage is that a single shock experiment produces a wide range of shock pressures and temperatures within the same sample (with shock pulse duration 8 ls), including ultra-high pressure (from 10 to >160 GPa) and temperature (from 200 to >15008C) regimes. Due to the maximum shock wave amplitude in the center of the sphere, shock pressures and temperatures decrease from the center of the sample toward the periphery such that the different regimes can be sub- sampled along the radius of the sphere. In this study, we conducted spherical shock experiments on natural basalt and diabase samples, which can be considered to be analogs of basaltic crusts on differentiated planetary bodies. Our goal was to characterize the mechanisms and extent of shock-induced changes in remanent magnetization and bulk rock magnetic parameters by comparing these properties to those of unshocked sister samples prepared from the same parent rocks.

2. Materials and Methods 2.1. Samples Unoriented block samples were collected from a basalt ﬂow, og-1, (previously studied by Swanson-Hysell et al. [2014] as site SI1-11.8 to 26.4) and a diabase dike, og-2, both from the Osler Volcanic Group, Ontario, Canada (Figure 1). Both the lava ﬂow and the dike were emplaced during magmatic activity associated with the development of the late Mesoproterozoic North American Midcontinent Rift [Cannon, 1992]. These targets were chosen because of the largely single-component nature of their natural remanent magnetization (NRM) [Swanson-Hysell et al., 2014] and because they are unshocked analogs of lithologies that experienced shock within the nearby Slate Islands impact crater (Figure 1). In a study of the remanence and rock magnetic properties of lithologies within the Slate Island central uplift, Halls [1979] hypothesized a rela- tionship between shock pressure and coercivity of remanence, but stated: ‘‘to test whether [remanent coercivity] gives a measure of shock pressure, further experiments, including laboratory shock wave studies, on Slate Islands rocks are obviously required.’’ Low-temperature magnetic properties of unshocked og-1 and

Figure 1. Simplified geological map of the Lake Superior Archipelago showing the locations of sampling sites for og-1 and og-2 (modified from Carter et al. [1973], Sage [1990], and the Ontario Geological Survey [2011]). The location of the geological map is shown in the inset map of North America. Sites og-1 and og-2 belong to the Osler Group, which consists of lava flows and intrusions related to the ca. 1110 to 1085 Ma North American Midcontinent Rift. The outline of the Slate Islands impact structure as inferred by bathymetry is shown by the dashed circle. The Slate Islands archipelago is the remnant of the crater’s central uplift [Halls and Grieve, 1976] and dominantly comprises of Superior Province metamorphic rocks with some exposure of Osler Volcanic Group intrusions and flows [Sage, 1990].

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og-2 samples, along with thermomagnetic experiments, reveal ferromagnetic mineral assemblages dominated by near-stoichiometric magnetite, in the case of og-1, and low-Ti titanomagnetite, in the case of og-2 (estimated as TM08, see section S2 in Supporting Information). Minor hematite contributions in og-1 can be deduced from thermal demagnetization of NRM and are independently conﬁrmed by petrographic investigations (see Supporting Information).

2.2. Spherical Shock Experiments Cubic 1000 cm3 samples from the og-1 basalt and og-2 diabase were cut from the original blocks and shaped into spheres of 49 mm (og-1) and 50 mm (og-2) diameter for the spherical shock experiments. Each sample was encased in a stainless steel spherical container that was vacuum-degassed. The initial (preshock) bulk densities of the spherical samples were 2.80 g/cm3 for og-1 and 2.84 g/cm3 for og-2 [Nikolaev et al., 2015]. Spherically convergent shock waves were generated through detonation of a layer of explosive materials surrounding the stainless steel casing. During and after shock loading, the container remained hermetically sealed and intact, such that the investigated samples were not contaminated by products of the explosion (Figure 2).

Figure 2. Microtomographic images of the spherically shocked samples of basaltic lava ﬂow og-1 (a) and diabase dike og-2 (c), and photo- graphs of equatorial slices of the same samples og-1 (b) and og-2 (d) with labeled specimens. The shock zones discussed in the text are labeled as I, II, III, and IV. Note that the spherical steel container was thinned down prior to cutting the equatorial slice imaged in Figure 2b, so that its initial thickness in Figure 2a does not correspond to its altered thickness in Figure 2b. The total volume of the inner cavity in og-1 is 330 mm3 and the total volume of the inner voids in zone I of og-2 is 196 mm3.

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220 1600 200 2400 200 a 1400 180 b 2200 180 1st peak pressure 160 1st peak pressure 2000 maximum pressure 1200 maximum pressure Temperature (°C) 160 Temperature (°C) post-shock 140 post-shock 1800 Zone I temperature temperature 140 1000 Zone II 120 1600 120 Zone III Zone IV 800 100 1400 100 Zone IV 80 1200 80 600 Zone III Pressure (GPa) Pressure Pressure (GPa) Pressure 60 1000 60 400 40 40 800 200 20 20 Zone II 600 Void Zone I 0 0 0 400 1 3 5 7 9 11 13 15 17 19 21 23 25 24681012 14 16 18 20 22 24 Position from sphere center (mm) Position from sphere center (mm)

Figure 3. Pressure (solid lines) and temperature (dashed lines) proﬁles along radii of shocked (a) og-1 and (b) og-2 samples along with position of the zonal divisions used in the text [Kozlov et al., 2015; E. A. Kozlov, personal communication, 2015]. Diamonds and circles correspond to calculations for radially positioned Lagrangian particles. Solid gray lines correspond to the estimated peak pressure values of the incoming spherically convergent shock wave, and solid black lines correspond to the estimated overall maximum peak pressure. Postshock temperatures are indicated by dashed lines.

2.3. Peak Shock Pressure Estimations The spherical shock experiments explored two different pressure regimes that were achieved by varying the thickness of the stainless steel container, the thickness of the explosive layer, and the chemical composition of the explosive agent. The ‘‘high-intensity’’ regime, used for sample og-2, is similar to that implemented by Bezaeva et al. [2010] and Kozlov and Sazonova [2012]. The ‘‘low-intensity’’ regime, used for sample og-1, was previously implemented by Dobromyslov et al. [2013]. Numerical simulations using the ‘‘VOLNA’’ software package [Kuropatenko et al., 1988a, 1988b] and experimental data on the shock com- pressibility of og-1 and og-2 [Nikolaev et al., 2015] were collectively used to estimate the peak shock pressures and temperatures experienced by different regions within the og-1 and og-2 samples (Figure 3) [Kozlov et al., 2015; E. A. Kozlov, personal communication, 2015]. Calculations were based on the Mie- Gruneisen€ equation of state with ultimate compression values according to Zababakhin [1997]. The parameters for the Mie-Gruneisen€ equation of state were selected from the following relation between mass velocity u and shock wave propagation velocity D: D 5 3.01 1 1.482u for 2 km/s < u < 3.5 km/s [Nikolaev et al., 2015], and initial bulk densities as reported in section 2.2. During spherical shock experiments, the shock wave converges toward the sample center and progressively compresses the samples along the radius as it focuses into the center of the sphere. After the initial passage of the wave, a second wave propagates from the center outward and increases the compression state of the already shock-loaded matter, thereby producing higher peak pressures than the initial incoming wave (Figure 3). Given that regions with higher postshock temperatures have much smaller volumes than those with lower temperatures, the contribution of conductive heating to postshock temperatures is relatively minor. The errors for the reported pressure and temperature estimates are likely 65% and 610%, respectively.

3. Results 3.1. Petrographic Effects of Shock The shocked sample spheres, still encased in their steel jackets, were imaged using X-ray microtomography with a spatial resolution of 39 lm(og-1; Figure 2a) and 33 lm(og-2; Figure 2c), using a XT H 450 microfocus X-ray and CT tomography system by Nikon Metrology in Leuven (Belgium) with assistance from Neva Tech- nology (Moscow, Russia). Three-dimensional reconstructions based on 2400 angle scans per sample were developed with the X-Tek Reconstruction Software. Equatorial planes extracted from 3-D reconstructions of og-1 and og-2 via VolumeGraphics StudioMax 2.2 software are shown in Figures 2a and 2c. Subsequently, wedges taken from slabs cut along equatorial planes were used for scanning electron microscopy (SEM) and magnetic force microscopy (MFM; Figure 4).

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Figure 4. SEM backscattered electron micrographs of (a) unshocked og-1 and (b) shocked og-1 from zone IV (see Figure 2 for zonation labeling). (c, d) MFM topographic images of unshocked and shocked og-2, respectively. (e, g) Corresponding MFM magnetic images taken over exactly the same area as Figures 4c and 4d. The magnetic state of these images corresponds to NRM. Labeling: ‘‘Ilm’’ designates ilmenite and ‘‘Tmt’’ designates titanomagnetite.

MFM imagery enables visualization of the grain structure and magnetic domain patterns of unshocked and shocked samples. MFM images of og-2 reveal the structure of exsolved magnetite and ilmenite lamellae (Figures 4c and 4e) with complex magnetization patterns. Magnetic domains are small relative to the size of

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the grain and their orientation generally perpendicular to lamellae boundaries can be attributed to internal stress. SEM and MFM images for both og-1 (Figure 4b) and og-2 (Figure 4d) also reveal numerous shock- induced cracks in the shocked specimens. The X-ray microtomography images reveal that the shocked sample og-1 has a central cavity with an aver- age diameter of 9 mm, whereas the central region of sample og-2 contains numerous bubbles of up to few mm in size (Figure 2). We suggest that these bubbles are remnants of a central cavity that has been partially filled with shock melt. Petrographic shock effects observed with transmitted light microscopy confirm that the intensity of shock metamorphism decreases from the center to the margins of the shocked spheres. Accordingly, we define four concentric petrographic shock zones in og-1 and og-2 (Table S2): I—total melt zone; II—zone of partial melting; III—zone of total conversion of plagioclase to diaplectic glass; IV—zone of solid-state shock features in minerals. Zone I features opaque bands containing newly formed magnetite and olivine (Figure S6a). Zones II and III are characterized by plagioclase recrystallization (Figure S6b) and total conversion to glass (Figures S6c and S6d), respectively. Plagioclase crystals in Zone IV contain planar deformation features (Figure S6e and see section S4 in Supporting Information for further details), and typical shock-induced grain fracturing of Fe-Ti- oxides in Zone IV is shown in Figures 4b and 4d. Zone IV of og-2 can be further divided into subzones IVa, IVb, and IVc that correspond to variable development of shock effects from strong to very weak.

3.2. Rock Magnetic Effects of Shock Specimens for rock magnetic analyses were cut from og-1 and og-2 spheres along radial transects using a diamond wire saw (Figures 2b and 2d). These specimens cover all shock zones, and have been used, togeth- er with unshocked sister specimens, to document the nature and extent of shock-induced changes of magnetic minerals. 3.2.1. Ferromagnetic Mineralogy and Verwey Transition Changes Inferred From Low-Temperature Data Stoichiometric magnetite is characterized by a crystal symmetry transition from cubic to monoclinic upon

cooling through the Verwey transition temperature Tv 120K[Verwey, 1939]. This transition affects many physical properties [e.g., Honig, 1995], including magnetocrystalline anisotropy [Abe et al., 1976], so that

cooling and warming cycles of remanent magnetization across Tv are characterized by an inﬂection in rema-

nent magnetization [Feinberg et al., 2015]. Accordingly, Tv is conventionally deﬁned as the peak on the ﬁrst

derivative of the warming curve of a zero-ﬁeld-cooled (ZFC) remanent magnetization acquired below Tv [Ozdemir€ and Dunlop, 1999] (Table 1). Another method for determining the Verwey transition is to deter- mine the median demagnetization temperature of ZFC remanence during zero-ﬁeld warming [Carporzen

and Gilder, 2010], referred to as Td [Sato et al., 2016] (Table 1).

Carporzen and Gilder [2010] observed a static pressure-induced increase of Td in stoichiometric magnetite specimens after pressure release, which was found to be 1 K/GPa for pressures up to 5 GPa. This increase was attributed to internal stress associated with crystal defects caused by the nonhydrostatic component of applied pressures, since stress is known to affect the Verwey transition temperature and the choice of a magnetic easy axis [Coe at al., 2012]. Reznik et al. [2016] reported a shock-induced increase in Verwey transi-

tion temperature Tv* (determined from the maximum in the ﬁrst derivative of temperature dependence of low-ﬁeld magnetic susceptibility acquired in 83–300 K range) by 6 K for all shocked samples of magnetite

with regard to their unshocked analogue; however, Tv* remained essentially constant within the samples

shocked to 5, 10, 20, and 30 GPa. The authors attribute such changes in Tv* to a combination of grain fracturing, plastic deformation mechanisms, and amorphization but argue that in the 5–30 GPa dynamic pres-

sure range Tv* cannot be used as a geobarometer because strain memory saturation occurs by 5 GPa. In order to assess possible shock-induced changes in Verwey transition temperature in magnetite-bearing og-1 samples, we imparted a saturation magnetization at 5 K and measured magnetization as they warmed to 300 K. In the field-cooled experiments (FC), the sample was cooled from 300 to 5 K in a 2.5T field. In the ZFC experiments the sample was cooled to 5 K in near-zero field and then pulsed with a 2.5T field. Low-temperature cycling of room temperature saturation magnetization (RT-SIRM) experiments were also conducted. These experiments were conducted with a Quantum Design Magnetic Property Measurement System (MPMS)22 at the Institute for Rock Magnetism (IRM) and a Quantum Design MPMS-XL5 at Kochi University. ZFC warming curves are presented in Figure 5a.

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Table 1. Main Rock Magnetic Parameters of Unshocked and Spherically Shocked og-1 and og-2 Specimens (See Figure 2 for Specimen Identiﬁcation)a Shock Memory b b Specimen D (mm) Zone P (GPa) T (8C) Bc (mT) Bcr (mT) Bcr/Bc Mrs/Ms Td/Tv (K) Ratio (%) Unshocked og-1 og-1un (13) 25 7.2 6 0.9 21 6 2 2.9 6 0.1 0.082 6 0.008 104.4/1063c 58 Shocked og-1 og-1s_a 6.4 II1III 53 574 13.7 34 2.5 0.134 107.6/108.3 54 og-1s_b 10.0 IV 23 278 12.8 34 2.6 0.120 107.6/108.0 54 og-1s_c 14.8 IV 12 216 10.9 31 2.8 0.110 107.0/107.4 54 og-1s_d 19.4 IV 11 242 10.9 31 2.9 0.102 107.5/107.9 57 og-1s_e 23.2 IV 12 265 10.9 31 2.8 0.105 107.5/107.7 57 Unshocked og-2 og-2un (15) 25 15.6 6 0.3 29 6 0.3 1.9 0.164 6 0.003 63 Shocked og-2 og-2s_a 2.6 I >160 >1500 29.6 47 1.6 0.290 95 og-2s_b 8.4 II 97 974 41.3 74 1.8 0.258 94 og-2s_c 12.9 III 44 620 31.4 61 2.0 0.212 85 og-2s_d 16.0 IV 38 548 27.6 54 2.0 0.202 og-2s_e1 19.1 IV 33 541 26.9 54 2.0 0.201 og-2s_e2 19.7 IV 32 540 26.6 53 2.0 0.204 og-2s_f2 23.4 IV 25 532 28.4 55 1.9 0.214 73d

a D is distance of specimen center to sphere center; P is peak shock pressure; T is postshock temperature; Bc is coercive field; Bcr is remanent coercivity (i.e., field required to demagnetize a saturation remanent magnetization); Ms is saturation magnetization; Mrs is saturation remanent magnetization; Td is Verwey transition temperature determined from the median of ZFC warming curves (also referred to as median demagnetization temperature of ZFC); Tv is Verwey transition temperature determined from the peak on the first derivative of ZFC warming curves. Numbers in brackets in the first column after og-1un and og-2un designate the number of unshocked specimens used for measurements (mean values with standard deviations are presented). bErrors in P and T estimates are within 65% and 610%, respectively. cMeasured on a single specimen. dDetermined for a sister sample og-2s_f1 from the same position on the radius.

Low-temperature remanence experiments reveal shock-induced increases in Td and Tv (Figure 5 and Table 1).

Td is 104 K for the unshocked og-1 specimen and increases in the shocked specimens og-1s_a-e to 107–

108 K (Table 1). These data reveal shock-induced increases of Td by 3–4 K from specimens that only have solid-state shock effects. As in Carporzen and Gilder [2010], we see an increase in Td following shock although in our experiment shock pressures reach far above 5 GPa (10–53 GPa for the solid-state specimens). Similar

to Reznik et al.[2016]weobservethatTd and Tv increase in the shocked specimens with regard to unshocked analogs, but are similar for samples that experienced shock from 11 to 53 GPa (Figure 5b). Memory ratio is deﬁned as the fraction of remanence remaining after a RT-SIRM cooling-warming cycle (Figures S3c and S3d), divided by the initial RT-SIRM before low-temperature cycling. For sample og-1, the memory ratio after shock is similar but slightly less than that in the unshocked samples: the memory ratio

Figure 5. (a) ZFC warming curves (dots) with corresponding model ﬁts (lines) for unshocked and shocked og-1 specimens (see Figure 2 and Table 1), which include a contribution repre- senting the magnetization loss across the Verwey transition (dashed lines). (b) Verwey transition temperatures estimated from 50% loss (Td or median, lower limit) and maximum slope (Tv or mode, upper limit) of the dashed curves in Figure 5a. The shaded region is a guide for the eye.

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for the unshocked specimen og-1un_a sample is 58% while it is 57% for shocked specimens og-1s_d and og-1s_e and 54% for og-1s_a-c. The data for the og-2 sample show a signiﬁcant shock-induced increase in memory ratio from 63% for the unshocked specimen og-2un_a up to 73–95% for the shocked specimens og-2s_f2, c, b, and a. Given that the remanence that demagnetizes through low-temperature cycling is associated with multidomain grains, this memory ratio increase indicates a shock-induced change in domain state. 3.2.2. Changes in Bulk Domain State/Domain Wall Pinning as Inferred by Hysteresis and First-Order Reversal Curves As a means to assess shock-induced changes in bulk domain state, we ran major hysteresis loops and back- ﬁeld remanence demagnetization experiments using Princeton Micromag Vibrating Sample Magnetometers (VSMs) at the Institute for Rock Magnetism and at Kochi University. Domain state changes are inferred from

bulk hysteresis parameters Ms (saturation magnetization), Mrs (saturation remanence), Bc (coercivity), and Bcr (coercivity of remanence) [Day et al., 1977; Dunlop, 2002] (Table 1 and Figure 6).

Shock produced increases in Mrs/Ms and in Bcr relative to the unshocked sister specimens (Figure 6). The net effect of shock is equivalent to a shift of the bulk magnetic properties toward the single-domain end-member, as seen in the Day plot (Figure 6). This trend away from end-member multidomain behavior is likely related to domain wall pinning and fracturing of multidomain ferromagnetic grains. For the innermost specimen of og-2, changes in rock magnetic properties are attributed to melting and crystallization of new ferromagnetic phases in the form of dendritic (titano) magnetite grains (Figure 7). Such dendrites are typical for basaltic glass samples in natural lava flows [Shaar and Feinberg, 2013]. We acquired room-temperature first-order reversal curves (FORCs) for all unshocked (untreated), shocked, and vacuum-heated og-1 and og-2 specimens to further fully characterize domain state and interactions (Figures 8, S7, and S8). First-order reversal curve distributions provide information about the distribution of microscopic coercivities (represented along the horizontal axis of a FORC diagram) and the magnetic interactions between domains or particles within magnetic mineral assemblages (which are represented as a bias field along the vertical axis). Coercivity and bias field distributions allow for better discrimination of domain states and their contribution to the bulk magnetization with respect to bulk magnetic measurements [Roberts et al., 2000]. Stacked, high-resolution FORC measurements using field steps of 0.5–0.7 mT were performed on selected samples and processed with the VARIFORC software package [Egli, 2013; http://www.conrad-observatory.at/ zamg/de] (Figure 8). Coercivity distributions have been calculated with VARIFORC from the first derivative of the backfield or DC-demagnetization curve, which coincides with the set of remanent magnetizations obtained from the FORC measurements. FORC diagrams (Figure 8) are dominated by the typical signature of pseudo-single-domain (PSD) magnetite

particles, consisting of triangular-shaped contour lines with maximum vertical spread along Bc 5 0[Roberts et al., 2000; Muxworthy and Dunlop, 2002; Carvallo et al., 2006; Ludwig et al., 2013]. Additional features appear in addition to this general trend over the lower quadrant, breaking the Preisach-type of symmetry

about Bu 5 0 seen in synthetic PSD particle assemblages [Muxworthy and Dunlop, 2002].

The most evident feature is a small-amplitude, negative peak located between the Bu 52Bc diagonal and

the Bu 5 0 axis. Its appearance in Figures 8a and 8b is enhanced by a nonlinear color scale; nevertheless, it is highly signiﬁcant, having a signal-to-noise ratio >50 [Heslop and Roberts, 2012; Egli, 2013]. In addition, the

central peak of all FORC diagrams is slightly offset toward positive Bu values. These features are slightly rem- iniscent of the ‘‘wishbone’’ structure seen in FORC diagrams of single-domain particle arrays affected by a strong negative mean interaction ﬁeld [Pike et al., 2005]. In presence of a mean interaction ﬁeld, the effec-

tive ﬁeld experienced by the magnetic carriers is given by Beff 5 B 2 a M/Ms, where B is the ﬁeld applied dur-

ing the measurements, M is the corresponding magnetization, Ms is the saturation magnetization, and a is a constant expressing the maximum amplitude of the mean internal (i.e., interaction) ﬁeld. The correct value

of a can be empirically found by replacing the FORC measurement ﬁelds with Beff, until the symmetry between lower and upper quadrant is restored as far as possible. This type of correction has been performed with VARIFORC by choosing the value of a for which the central peak offset is eliminated (Figure 8c). This correction reduces the extent of the asymmetry, but does not eliminate it completely because basalt is a heterogeneous material for which different internal ﬁelds might be required to describe different

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 8 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

Figure 6. Summary of hysteresis parameters associated with unshocked and shocked specimens of og-1 basalt and og-2 diabase. All specimens are labeled accordingly (see Table 1 and

Figure 2). Bc is coercivity, Bcr is coercivity of remanence, Ms is saturation magnetization, Mrs is saturation remanent magnetization. Horizontal lines in Figures 6a, 6b, 6c, and 6d depict the mean value and range of data from the unshocked specimens. In the (e) Day plot, the labels SD, PSD, and MD are for single domain, pseudo-single-domain and multidomain, respectively. A theoretical SD-MD mixing curve for magnetite after [Dunlop, 2002] is also shown for reference.

parts of the specimen. Hence, a should be regarded as a statistical distribution, rather than the single value used for the corrections. In any case, the untreated and the shocked specimens required the same mean field correction given by a 5 25 mT, while the heated specimen has been corrected with a 5 10 mT. The mean fields of og-2 samples are too large to be explained by a demagnetizing field associated with the bulk magnetization and we tentatively attribute them to the interaction between two phases in ilmenite- titanomagnetite intergrowths (Figures 4c and 4e) (see Supporting Information). The effect of shock on the FORC properties of og-2 is equivalent to a widening of the FORC distribution

over both the Bc and Bu axes, whereby the distribution shape itself does not change signiﬁcantly (Figure

Figure 7. MFM (a) topographic and (b) magnetic images from the shock melt zone (zone I) of the og-2 diabase dike sample. Notice the dendritic texture of ferrimagnetic grains recrystallized from the shock melt.

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8b). A better assessment of magnetic property changes induced by shock and heat may be based on a quantitative comparison of the FORC distributions obtained after mean ﬁeld correction (Figures 8c and 8d). This comparison is based on the superposition of the contours of one distribution to the reference distribu-

tion of the untreated sample, after rescaling the Bc and Bu axes for best contour match over the upper

Figure 8.

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quadrant. The upper quadrant has been chosen because it is less affected by reversible magnetizations due to unremoved mean ﬁeld contributions. As seen in Figure 8c, a good match between untreated and

shocked specimens is obtained if Bc and Bu of the latter are scaled by a factor 1/1.65 and 1/1.35, respectively. If Bc is identiﬁed with coercivity in the framework of a Preisach interpretation of the FORC diagram [Preisach, 1935; Pike et al., 1999], this means that the shocked specimen is 65% harder than the untreated specimen.

This result compares well with the observed increase of Bc and Bcr values listed in Table 1. On the other hand, the vertical spread of the FORC distribution increases only by 35%, so that the overall aspect ratio of the distribution is decreased. This change in aspect ratio is equivalent to an apparent PSD grain size decrease, as seen in synthetic samples [Muxworthy and Dunlop, 2002]. With respect to coercivity, the opposite trend is obtained with the heated specimen (Figure 8d). In this case, the FORC distribution appears to be fundamentally differ-

ent from that of the untreated specimen, with differences related to (1) a rounding of the contours near Bc 5 0

in the upper quadrant (labeled as 1 in Figure 8d), (2) enhanced high-coercivity contributions just below Bu 5 0 (labeled as 2 in Figure 8d), and (3) an upward shift of all contours near Bc 5 0 in the lower quadrant (labeled as 3 in Figure 8d). Interestingly, the same differences (albeit in a much reduced form) are responsible for the residual mismatches between the FORC diagrams of shocked and untreated specimens (Figure 8c). In summary, the FORC distribution of the shocked specimen may be considered to be a rescaled version of the unshocked material with changes due to increase in coercivity and decreases in effective grain size and with small residual differences being similar to the changes observed upon heating. These conclusions are supported by the backfield coercivity distributions of unshocked, shocked, and heated specimens (Figures 8e and 8f). The distributions of unshocked and shocked specimens become identical when calculated from mean-field-corrected data, after rescaling fields and magnetization values such that the distribution peaks coincide. This result cannot be obtained without mean field correction. On the other hand, the coercivity distribution of the vacuum-heated specimen appears fundamentally different, as no match is obtained upon rescaling. Fundamental differences in the shape of a coercivity distribution can be introduced by the addition of a new magnetic component [Egli, 2003], as suggested by the appearance of a central ridge toward the high-coercivity end of the FORC diagram (Figure 8d). An important result of the detailed FORC analysis is that the existence of a mean internal field alters the measured magnetic properties and introduces differences between unshocked and shocked specimens, which do not reflect intrinsic changes of the magnetic particles. This can be explained by the fact that the mean field is controlled by the total magnetization, which includes the reversible component, while coercivity distributions and the upper quadrant of the FORC diagram are governed by irreversible processes [Pike, 2003; Carvallo et al., 2005]. The overall conclusion from this analysis is that the experimental shock did not alter the intrinsic shape of coercivity and FORC distributions while heating experiment did—likely due to the growth and chemical modifica- tion of magnetic particles. Possible causes for the observed shock hardening and apparent grain size decrease through shock include the introduction of crystal defects and cracking [Reznik et al., 2016]. As far as magnetic properties are concerned, the formation of cracks is not equivalent to the partition of the crystals into smaller particles. For instance, vortex structures originally occupying the whole crystal will still be able to form across the cracks, as seen in finely exsolved submicron magnetite blocks [Harrison et al., 2002]. Therefore, the apparent grain size decrease is likely related to the larger fields required to nucleate magnetic vortex states.

Figure 8. FORC analyses of sample og-2. FORC distributions of (a) an untreated og-2 specimen and (b) shocked specimen og-2s_d (collected from shock zone IV, see Table 1). Color scales and ﬁeld ranges are identical for each sample for ease of comparison. Note that the cen-

tral peak of both distributions is slightly offset toward positive Bu values. (c) FORC distribution of an untreated og-2 specimen (color map) after a mean field correction with a/Ms 5 25 mT, used to bring the central peak on Bu 5 0. This distribution is compared with that of og-2s_d with the same mean field correction (blue contours). The Bc and Bu axes have been rescaled to obtain the best match of the two FORC distributions over the upper quadrant. Circled numbers refer to differences between the two distributions as explained in the text. (d) Same as Figure 8c for the comparison of the untreated specimen with a specimen heated to 7008C (fast heating and cooling in vacuum as described in section S3.3). (e) Backfield coercivity distributions derived from the original FORC measurements for the untreated,

shocked, and heated og-2 specimens, respectively. All curves are normalized by the specimen’s Ms, so that the unit of the vertical axis is T21. The insert shows the same curves, after the horizontal and vertical axes have been rescaled by the ﬁeld and coercivity distribution value corresponding to the curve maximum. (f) Same as the inset of Figure 8e for backﬁeld coercivity distributions calculated from FORC

measurements after mean ﬁeld corrections with a/Ms 5 25 mT (untreated and shocked specimens) and a/Ms 5 10 mT (heated specimen). All diagrams have been generated using VARIFORC built-in functions [Egli, 2013].

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 11 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

Finally, standard-resolution (i.e., 5 mT ﬁeld step) FORC distributions acquired for all shocked specimens of og-1 (Figure S7 in Supporting Information) and og-2 (Figure S8) demonstrate progressive shock-induced coercivity hardening and changes toward more single-domain (SD) like behavior with increasing peak shock

pressure toward the sphere center (as the distributions spread out from the origin along the Bc axis). 3.2.3. Magnetic Remanence All shocked specimens were stepwise demagnetized by alternating magnetic fields (AF) prior to other rock magnetic experiments (Figure 9). All (mutually oriented) specimens from both og-1 and og-2 contained a low-coercivity overprint that was unidirectional within each parent sample and that was removed at AF levels <20 mT, which we interpret to be an isothermal remanent magnetization (IRM) (Figure 9). Application of REM0 method [Gattacceca and Rochette, 2004] (REM0(AF) 5 DNRM(AF)/DSIRM(AF)) indicates for this low- coercivity component a REM0 10–20%, which is typical of IRM. Such IRM could be acquired between the time of shock experiment and the time of magnetic measurements as a result of magnetic contamination and complicates interpretation of the magnetic remanence. Above AF levels of 20 mT, we observed two different classes of remanence behavior. Specimens og-2s_a, og-2s_b, and og-2s_c, which were sampled from the interior of the og-2 sphere (Figure 2d), have a unidirectional high-coercivity component that is likely a thermoremanent magnetization (TRM) acquired as a result of shock-induced heating to temperatures in excess of the (titano)magnetite Curie temperature (531–5808C, see section S2 and Figure S2 in Supporting Information) and subsequent cooling in the ambient field. The remanence of specimens in the shock melt zone (e.g., og-2s_a in Figure 9) must be a TRM and therefore the fact that the same direction is observed in the interior-most unmelted specimens (e.g., og-2s_c in Figure 9) is best explained by that remanence being a full TRM as well. This interpretation is consistent with the estimated postshock temperatures (Table 1) and the corresponding REM0 values of 2–3% for the high- coercivity component, consistent with TRM acquired in the Earth’s magnetic field. In contrast, specimens from the outer portion of the sphere (e.g., og-2s_e and og-2s_f) do not appear to contain this shock-induced TRM as the remanence direction isolated after removal of the IRM overprint is distinct from that of the impact melt (e.g., og2s_e2 in Figure 9). This interpreted absence of a preserved shock-related TRM or partial TRM (pTRM) in the outer specimens suggests that shock heating did not reach far into the range of blocking

Figure 9. Zijderveld diagrams for the mutually oriented shocked specimens of og-2 (Figure 2d): (a) og-2s_e2 (zone IV), (b) og-2s_c (zone III), (c) og-2s_a (zone I) (see Table 1). Note that the Zijderveld diagram for the least shocked sample og-2s_f2 is almost identical to (a) presented here.

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 12 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

temperatures for the ferromagnetic grains. For example, if specimen og-2s_e2 (Figure 9a) was heated to 4508C then there would be little remanence acquired by the heating as the unblocking temperature spec- trum of the og-2 sample reveals (un)blocking temperatures dominantly above 4508C (Figure S1). This interpretation suggests that the calculated temperatures of shock-related heating (estimated to be 5408C for og- 2s_e2 with an uncertainty of 610%) associated with the experiments are at the low end, or below, the reported uncertainty of the calculations. Temperature measurements on basalt samples experimentally shocked to pressures of 20–30 GPa reveal peak temperatures between 330 and 4708C[Stewart et al., 2007]. If peak shock temperatures experienced by samples such as og-2s_e2 were 470–4858C or below, pTRM acquired during the spherical shock experiment would have been more readily overprinted by the observed IRM. The unidirectional magnetizations observed at high AF demagnetization levels in shocked og-2 d-f2 specimens are likely preshock remanence. Magnetizations at AF demagnetization levels of 20–40 mT are 40% weaker in the shocked specimens than in unshocked equivalents. The weaker magnetization intensities observed in the high coercivity fraction of the shocked samples are likely the result of shock Table 2. Shock Effects in og-1 Basalt and og-2 Diabase With Shock Pressure Calibration demagnetization. It is unlikely that this Shock Effect Shock Pressure (GPa) remanence is a result of shock remanent 0 Shock Wave Barometry for og-1 Basalt/og-2 Diabase magnetization (SRM). REM values for this Zone IV (Partial Overlap With 13–39/40–41 high-coercivity component in og-2 speci- a Classes 1 and 2 mens d-f2 areconsistentwiththoseforthe Plagioclase Rare planar deformation unshocked samples with NRM and are like- features (PDFs) ly incompatible with SRM, because Mid- Partial transformation continent Rift basaltic dikes within the into diaplectic glass Pyroxene Slate Islands have low SRM acquisition effi- Mechanical twinning ciencies (1% of TRM acquired in the Weak to moderate mosaicism same magnetic field), at least for shock Irregular and planar fractures Magnetite/Titanomagnetite pressures 2GPa[Tikoo et al., 2015]. This Irregular fractures low SRM acquisition efficiency may be Zone III (Class 3a) 39–44/41–63 Plagioclase attributed to the fact that these samples Total transformation into 39/41 have relatively high coercivity magnetic diaplectic glass carriers in the PSD size range, whereas SRM Pyroxene Mechanical twinning is acquired most easily by multidomain Planar fractures grains with low coercivities (often <20 mT) Strong mosaicism [Gattacceca et al., 2007; Tikoo et al., 2015]. Magnetite/Titanomagnetite Strong irregular fracturing SRM does not appear to be preserved in Zone II (Classes 4–5a) 44–70/63–158 the remanence of samples including those Plagioclase that were not thermally overprinted (e.g., Incipient melting Pyroxene shock levels of 35 GPa). However, SRM Mechanical twinning could have been acquired by low coercivity Numerous planar fractures grains and subsequently overprinted by Strong mosaicism Incipient melting TRM and IRM. Magnetite/titanomagnetite Fragmentation Incipient melting 4. Discussion Zone Ib 70–101c {>29}/>158 {>42} Whole-rock melting >70c/>158 4.1. Petrographic Effects in Melt glass with pores Comparison to Shock Magnitude and bubbles Petrographic observations of the shocked lm-dendrites of olivine and magnetite samples show the progressive development of shock effects with increasing aClassification after Kieffer et al. [1976], Schaal and Horz€ [1977], and Schaal et al. [1979]. peak shock pressure from the edges bZone I: peak pressure for the incoming spherically convergent shock of the og-1 and og-2 spheres to their wave is also indicated in brackets {} (first peak pressure in Figure 3). c centers. The observed shock effects in Indicated pressure values may be underestimated as the material was likely shocked to higher pressures in the center and then moved to its final comparison to shock magnitude are sum- position on the radius. marized in Table 2.

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 13 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

Pyroxene grains in marginal parts of the samples display brittle and weak plastic deformation features, including irregular fractures, planar features, mosaicism, and mechanical polysynthetic twinning. Closer to the sphere centers, plagioclase crystals lose the birefringence and have planar deformation features (PDFs) while secondary minerals do not show any pleochroism, and intensity of shock damages in pyroxene increases. Further pressure increases resulted in total conversion of plagioclase to diaplectic glass, decom- position of secondary minerals, partial melting, and total melting of the rocks. These shock effects are roughly consistent with previously described petrographic effects [see Stoffler€ et al., 2006, Table 5.3] and five shock metamorphic classes [Kieffer et al., 1976; Schaal and Horz€ , 1977; Schaal et al., 1979] (see section S4 in Supporting Information for details). Due to the specific geometry of shock waves in spherical shock experiments, our samples experienced mechanical damage to a lesser extent than in planar shock experiments, which makes our experiments more analogous to natural conditions of giant impacts where shocked rocks are supported by surrounding matrices. This difference may be the source of the observed discrepancy in shock magnitude, needed for the total conversion of plagioclase into diaplectic glass between this work and previous studies (other possible reasons are discussed in section S4 of Supporting Information).

4.2. Changes in Rock Magnetic Properties Two opposed shock effects have been reported in the literature in rocks with a range of magnetic mineralo- gies: shock-induced magnetic hardening (i.e., an increase in bulk coercivity) [Cisowski and Fuller, 1983; Peso- nen et al., 1997; Langenhorst et al., 1999; Gattacceca et al., 2007; Louzada et al., 2007; Bezaeva et al., 2011; Mang et al., 2013; Reznik et al., 2016] and shock-induced magnetic softening (i.e., a decrease in bulk coercivity) [Bezaeva et al., 2010; Kohout et al., 2012]. Halls [1979] observed that target rocks from the most shocked central zone of the Slate Islands impact structure had somewhat higher remanent coercivities than target rocks from the periphery of the exposed central uplift (see Figure 13c of their paper). Shock-induced magnetic hardening may be observable in target rock samples from other impact craters as well. Shock-induced decreases in bulk coercivity are less often observed and takes place as a result of either magnetic mineral phase transformations, such as the disordering of tetrataenite into taenite [Bezaeva et al., 2010] or as the result of shock wave propagation in magnetite-bearing porous targets [Kohout et al., 2012]. Kohout et al. [2012], who studied rocks with preshock bulk densities of 2.1 g/cm3, argue that their observed shock- induced magnetic softening is likely due to higher initial porosity, causing higher shock temperatures (as a result of pore crushing) accompanied by partial or localized total melting of the sample, melt migration, and possibly recrystallization of magnetite aggregates into larger grains. With the exception of magnetite in the innermost melt, no magnetic mineral phase transformations were observed in our experiments and our targets were not particularly porous (our bulk densities fall between 2.80 and 2.84 g/cm3 range, as indicated above in section 2.2). In this work, peak shock pressures P between 10 and

>160 GPa produced increases in Bc and Bcr within all specimens of og-1andog-2 (Table 1 and Figures 6 and 8), up to total melting. For the central glass specimen og-2s_a, the increase in coercivity is attributed to the creation of new ferromagnetic minerals during cooling of the shock-induced melt (Figure 7). For other specimens that did not contain melt glass, coercivity hardening is likely caused by irreversible solid-state changes such as fracturing, domain wall pinning, and introduction of planar defects and dislocations within ferromagnetic grains, and the development of share bands and twins [Gattacceca et al., 2007; Reznik et al., 2016], which are evident in MFM and SEM images of the shocked samples and their unshocked precursors (Figure 4). Magnetic hardening is accompanied by an increase in domain stability as indicated by the shift of bulk hysteresis parameters of shocked specimens toward the SD ﬁeld on the Day plot (Figure 6e) and changes in the FORC distribution (Figure 8). We did not observe shock-induced formation of superparamagnetic grains in our shocked specimens, as seen by the lack of major frequency-dependent in-phase and out-of-phase magnetic susceptibility compo- nents between 5 and 300 K (Figure S9 in Supporting Information). This result is consistent with [Reznik et al., 2016], but contrasts with the ﬁndings of Van de Moorte`le et al. [2007].

4.3. Discrimination of Thermal Versus Mechanical Effects of Shock on Rock Magnetic Properties As is evident from the presence of shock melt and thermal resetting of magnetic remanence near the center of the og-1 and og-2 sample spheres, high levels of shock can produce substantial shock-induced heating (Figure 3). Hysteresis data and backﬁeld remanence curve measurements from unshocked og-1

specimens after heating under Ar and He atmospheres revealed an increase of Bcr with respect to the

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 14 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

unheated material, so that the discrimination of shock and heat-induced magnetic hardening effects is not straightforward (Figure 10a). To test whether the shock-induced hardening in og-1 and og-2 is predominantly due to mechanical shock-related changes such as microfracturing of ferromagnetic grains (Figure 4), rather than shock-induced heating, we conducted stepwise vacuum-heating experiments with unshocked og-1 and og-2 specimens using fast (>608C/min) and slow (108C/min) heating-cooling rates (see section S3 in Supporting Information for details). The cooling rate in the heating experiments under vacuum was similar to the cooling timescale associated with the spherical shock experiments. With an exception of specimen

og-1s_a, which partially contains shock melt, Bcr values for all shocked og-1 specimens are distinctly higher

than Bcr values of heated unshocked equivalents (Figure 10a). For og-2, temperature-induced changes in Bcr are far below Bcr values for shocked samples (Figure 10b). We conclude that with exception of material that underwent melting and associated crystallization of new ferromagnetic grains, the shock-induced coercivity hardening in our shocked samples is predominantly due to solid-state, mechanical effects of shock rather than alteration associated with shock heating. This conclusion is supported by the analyses of high-resolution FORCs and visual inspection of their contours (see

Figure 10. Results of heating experiments: remanent coercivity versus postshock temperature for (a) og-1 and (b) og-2. Data labeled as ‘‘untreated’’ correspond to unshocked/unheated specimens. Slow-heating experiments (108C/min, black circles) were conducted stepwise on the same specimens. Heating under argon and helium (red circles) were conducted on separate specimens. Fast-heating experiments (>608C/min, red crosses) were conducted on sister specimens (one specimen per temperature step). Shocked specimens (yellow squares)

are shown for comparison. Atmospheric composition does not signiﬁcantly impact Bcr changes in og-1 and og-2.

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 15 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006583

above, section 3.2.2). Quantitative comparison of high-resolution FORC diagrams for shocked and heated og-2 specimens against their untreated analog demonstrate that shock and heating effects are fundamentally different: shock does not alter the intrinsic shape of coercivity distributions and of the FORC function (apart from ﬁeld scaling consistent with the observed magnetic hardening) while heating does as seen from the alteration of the FORC contours. Overall, (i) conducting heating experiments at temperatures resembling those experienced during high- pressure shock events on untreated equivalents of shocked rocks and (ii) quantitative comparison of high- resolution FORC diagrams for shocked, heated, and untreated specimens represent novel, independent methods to discriminate between thermal and mechanical effects of shock on rock magnetic properties at their simultaneous action.

4.4. Remanence Properties SRM is acquired during shock experiments in ambient magnetic field as a result of shock wave propagation at shock pressures starting at <0.045–0.25 GPa [Nagata, 1971; Pohl et al., 1975; Gattacceca et al., 2010; Tikoo et al., 2015]. Above several tens of GPa the postshock temperature increase is high enough to exceed the corresponding Curie points of ferromagnetic minerals in host rocks [Stoffler€ et al., 1991; Nyquist et al., 2001; Fritz et al., 2005], such that the impact-acquired remanence is a full TRM (and thus no more SRM acquisition is observed) [Cisowski and Fuller, 1978]. In these experiments, samples that reached pressures >40 GPa experienced a full thermal resetting of the Acknowledgments Permission from Ontario Provincial initial remanence (indicative of heating above the Curie temperatures of low-Ti titanomagnetite-bearing Parks to conduct fieldwork in the Lake og-2) (Figures 9b and 9c). Thus, shock-induced thermal remagnetization is fully developed at >40 GPa. We Superior Archipelago Conversation interpret the remanence of (titano)magnetite-bearing rocks that experienced the pressure range 11–38 GPa Area is gratefully acknowledged. Research was supported by National (Figure 9a and Table 1) to preserve a preexperiment remanence after removal of a pervasive secondary IRM Science Foundation (NSF) grant EAR- observed in all of our shocked specimens. This preexperiment remanence is weaker than in unshocked sam- 1316395 awarded to N. L. Swanson- ples likely as a result of shock demagnetization. Any shock-induced partial thermal overprint or shock rema- Hysell, Institute for Rock Magnetism (IRM) visiting fellowships to N. S. nent magnetization that was acquired in the experiment may have resided in low-coercivity grains (<20 mT) Bezaeva and N. L. Swanson-Hysell, and that were overprinted by the IRM. Overall, in the pressure range >10 GPa, occurrence of shock demagnetiza- an IRM visiting fellowship to S. M. tion versus thermal remagnetization depends on peak shock pressure as well as associated postshock tem- Tikoo. The IRM was supported by the NSF EAR Instruments and Facilities peratures and their relation to the Curie temperature of the dominant remanence carriers in the host rock. program. The work was supported by Act 211 Government of the Russian Federation, agreement 02.A03.21.0006 5. Conclusions and is performed according to the Russian Government Program of The abundance of large, complex craters on terrestrial bodies within our solar system such as Earth, Mars, and Competitive Growth of Kazan Federal the Moon indicates that large regions of planetary crusts have experienced shock. Unraveling the magnetic University. SEM work was carried out in the Characterization Facility, characteristics of planetary materials such as meteorites thereby requires an understanding of the effects of University of Minnesota, which shock. These effects are intertwined with the effects of temperature as seen in the remanence of the samples receives partial support from NSF subjected to the highest pressures (>40 GPa), which we attribute to a full thermal remagnetization. Following through the MRSEC program. B. E. Strauss was funded by the University our spherical shock experiments to pressures ranging between 10 and 160 GPa, we observed increases in of Minnesota Doctoral Dissertation bulk coercivity and the coercivity of remanence in the shocked specimens. Through stepwise heating experi- Fellowship. Experimental contributions by E. A. Kozlov, A. A. Degtyarev, A. V. ments of unshocked samples and quantitative comparison of high-resolution FORCs for shocked, heated, and Olkhovsky, and D. A. Varfolomeev are untreated specimens, we demonstrate that these changes are due to the mechanical effects of shock rather gratefully acknowledged. We are than the accompanying shock-induced heating. Therefore, these rock magnetic changes can primarily be grateful to Neva Technology Ltd. and Nikon Metrology for their work on the ascribed to grain fracturing, domain wall pinning, and the introduction of crystallographic defects within ferro- microtomography. We thank D. magnetic grains. Microscopy reveals pervasive fracturing of the FeTi-oxide grains, which is consistent with this Bilardello (IRM), M. Jackson (IRM), and mechanism. The observed increase in coercivity suggests that rocks that have been previously shocked to P. Solheid (IRM) for help and assistance with experiments. We are thankful to pressures ranging between 10 and 160 GPa will be more resistant to acquiring subsequent low coercivity S. Banerjee (IRM) and J. M. Feinberg overprints such as shock remanent magnetization than unshocked analogs. (IRM) for helpful discussions. We thank Agnes Kontny, Roger Fu, and Jer ome^ Gattacceca for constructive reviews. References Supporting data are included as six sections with an overall of nine figures Abe, K., Y. Miyamoto, and S. Chikazumi (1976), Magnetocrystalline anisotropy of low temperature phase of magnetite, J. Phys. Soc. Jpn., 41, and two tables in Supporting 1894–1902. Information; any additional data may Bezaeva, N. S., D. D. Badjukov, P. Rochette, J. Gattacceca, V. I. Trukhin, E. A. Kozlov, and M. Uehara (2010), Experimental shock metamor- be obtained from the corresponding phism of the L4 ordinary chondrite Saratov induced by spherical shock waves up to 400 GPa, Meteoritics Planet. Sci., 45, 1007–1020, doi: author. 10.1111/j.1945-5100.2010.01069.x.

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