Agarwal and Alva‑Valdivia Earth, Planets and Space (2019) 71:141 https://doi.org/10.1186/s40623‑019‑1120‑9 EXPRESS LETTER Open Access Curie temperature of weakly shocked target basalts at the Lonar impact crater, India A. Agarwal1* and L. M. Alva‑Valdivia2 Abstract The study investigates Curie temperature (TC), bulk magnetic susceptibility, hysteresis, and X‑ray difraction pattern of in situ target basalts of Lonar impact crater, India. The main magnetic phase in the target basalt is low‑Ti titanomag‑ netite. This study reveals an increase in TC and decrease in magnetic susceptibility and in full width at half maxima of the 311 peaks of titanomagnetite with distance from the crater center. Changes in crystal lattice of titanomagnetite, such as straining of 311 peaks, decrease in apparent crystallite size, and grain fragmentation may be among the pos‑ sible reasons for the observed trends in TC and magnetic susceptibility. However, they both do not show any correla‑ tion between each other, indicating that diferent shock‑induced processes afect them. Keywords: Lonar impact crater, Variation in Curie temperature, Variation in bulk magnetic susceptibility, X‑ray difraction pattern Introduction 1941; Iizumi et al. 1982). At temperatures between TV Magnetite is a common ferrimagnetic mineral in the ter- and Curie temperature—TC (~ 856 K or 582.85 °C), it is restrial and extraterrestrial rocks. Magnetite and other cubic, and it is ferrimagnetic due to an anti-ferromag- ferromagnetic minerals are a source of magnetic anoma- netic coupling of Fe(III) in tetrahedral and octahedral lies associated with terrestrial and extraterrestrial impact sites and Fe(II) in octahedral sites (Néel 1948; Tarling and craters and, therefore, play an important role in their dis- Hrouda 1993). Beyond TC, it is paramagnetic (Shull et al. covery and mapping (e.g., Pilkington and Grieve 1992; 1951), due to the loss of magnetic ordering (Harrison and Plado et al. 1999; Gilder et al. 2018). Moreover, behavior Putnis 1995, 1996). of magnetite and other ferromagnetic minerals at various Titanomagnetites form a solid-solution series shock pressures has been used to identify the magnetic (Fe3-xTixO4) with magnetite (0 = x) and ulvöspinel (x = 1) mineralogy of Mars (Louzada et al. 2011). Although criti- as the two end members. Te dependence of magnetite cal for impact cratering research on Earth and elsewhere, TC upon the chemical composition is well established. the behavior of magnetite, in weakly shocked rocks, is Tere is a negative correlation between TC and the not well understood. ulvöspinel content, ‘x’ (e.g., Akimoto et al. 1957; Özdemir Magnetite has a typical inverse spinel structure with and O’Reilly 1981; Moskowitz et al. 1998; Lattard et al. Fe(II) and Fe(III) disordered in the octahedral sites and 2006). Tis correlation is owed to the change in lattice fully occupied tetrahedrons by Fe(III) cation (Verwey parameters due to replacement of Fe cation by Ti (Aki- and Haayman 1941). At temperatures below Verwey moto et al. 1957). transition (TV), about 110–120 K, magnetite has an Another factor afecting TC is hydrostatic pressure. orthorhombic crystal structure (Verwey and Haayman TC of titanomagnetite, with ulvöspinel component from 0 to 0.7, increases linearly with static pressures up to 6 GPa (Samara and Giardini 1969; Schult 1970). Under *Correspondence: amar@daad‑alumni.de 1 Albert‑Ludwigs‑Universität Freiburg, Institut für Geo‑ und hydrostatic pressure, a cubic crystal, such as that of Umweltnaturwissenschaften, Albertstraße 23b, 79104 Freiburg, Germany magnetite, results in a cubic unit cell with decreased Full list of author information is available at the end of the article interionic distances (barring any phase transformation). © The Author(s) 2019. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Agarwal and Alva‑Valdivia Earth, Planets and Space (2019) 71:141 Page 2 of 8 Tis causes increased interionic exchange and higher knowledge, this is the frst report of such correlation TC (Samara and Giardini 1969). However, a similar from weakly shocked rocks of a natural crater. direct correlation between crystallographic changes Geological setting and rock magnetic properties and TC due to dynamic shock pressure is not yet estab- lished. Tis may be partly owed to lack of sufcient Lonar crater (19° 58′ N, 76° 31′ E) in the Buldana district experimental investigations and partly due to com- of India is a hypervelocity impact crater excavated in the plex shock wave behavior (refection and refraction) ca. 65 Ma tholeiitic Deccan basalt (Hagerty and Newsom due to heterogeneity in rocks at natural craters. Shock 2003; Maloof et al. 2010). Some studies using fssion waves from natural and experimental craters may indi- track and thermoluminescence dating suggested that the rectly afect TC by fracturing the magnetic grains, thus Lonar impact occurred 50 ka (Fredriksson et al. 1973), facilitating hydrothermal alteration to increase the while other reports using 40Ar/39Ar dating argue that the proportion of non-stoichiometric magnetite (Kontny impact crater is ca. 570 ka in age (Jourdan et al. 2011). et al. 2018). Te grain fragmentation may also trans- Te ~ 1.88 km wide simple crater is 150 m deep from form multi-domain magnetite (MD) to pseudo-single the rim and is flled with a 7–10-m-deep lake. Te lake domain (PSD) or single domain (SD), thus reducing the is flled with 30–100-m-thick unconsolidated postim- magnetic susceptibility (e.g., Reznik et al. 2016). pact sediments, which are underlain by ~ 225 m of impact Tis study investigates the variation in TC, magnetic breccia (Fredriksson et al. 1973; Fudali et al. 1980; Grieve susceptibility, and XRD pattern in the weakly shocked et al. 1989). Te Lonar crater is an excellent analog of target basalts (< 3 GPa) of Lonar impact structure. Te extraterrestrial impact craters on basalt. It is relatively results show a systematic change in these properties pristine and has not undergone any postimpact tecto- with distance from the center of Lonar impact cra- nism (Maloof et al. 2010; Agarwal et al. 2016). ter, i.e., with a change in peak shock pressure. To our Te target basalt is comprised of fve very similar and homogenous Deccan basalt fows (Fig. 1), whose Fig. 1 Map of Lonar crater (after Maloof et al. 2010) with sampling sites of this study. Lava fows: Tf0, Tf1, Tf2, Tf3, Tf4, Tf5, and Tf6. Coordinates are represented in UTM—WGS84 Agarwal and Alva‑Valdivia Earth, Planets and Space (2019) 71:141 Page 3 of 8 geochemical, textural, and rock magnetic properties have magnetic susceptibility of each sample was measured at been studied in detail (e.g., Kiefer et al. 1976; Hagerty room temperature in the Kappabridge (KLY-4S). and Newsom 2003). Te TiO2 content of diferent fows Hysteresis curves were determined using Alternating of the target basalt is similar (Osae et al. 2005). Each fow Gradient Field Magnetometer Micromag apparatus in is ~ 8 to 40 m thick, separated by horizons of red and felds up to 1.4 T. Te curves allow calculating the rema- green paleosols and has vesicles flled with secondary nence of coercivity-to-coercive force (Hcr/Hc) and the materials like chlorite, zeolite, quartz, and brown limo- remanent magnetization-to-saturation remanence (Mrs/ nite, and most have a chill margin at their bottom and Ms) ratios of each sample. Tese ratios are used to esti- a heated and brecciated top (Ghosh and Bhaduri 2003). mate the relative proportions of the single domain (SD), Te fows exposed in the Lonar crater wall are chemi- multi-domain (MD), and pseudo-single domain (PSD) cally similar and are classifed as relatively low-K tholei- grains (Dunlop 2002a, b). In general, the ratio of Mrs/ itic intra-plate basalts, with minor Fe and Ca enrichment Ms is higher for more SD materials and lower for more and slight Mg and Al depletion compared to “average” MD-like materials. Agarwal et al. (2016) applied stepwise tholeiites (Osae et al. 2005). All of the shocked basalts increasing uniaxial feld on the samples to calculate the are altered, containing vesicles and veins full of carbonate saturated and non-saturated isothermal remanent mag- and clays (Hagerty and Newsom 2003). netization (IRM) acquisition curves. Tese curves were Te alteration is recorded as young (< 50 kya) viscous used to determine the median destructive feld (MDF). and/or chemical remanent magnetization in the tar- Te samples were then divided into magnetically hard, get rocks (Louzada et al. 2008; Arif et al. 2012; Agarwal soft, and intermediate groups, based on the dominant et al. 2016). Te target basalts also present an older com- magnetic domain state and the MDF. ponent similar to the Deccan direction (Louzada et al. Te XRD pattern of magnetically hard basalts (domi- 2008; Agarwal et al. 2016). Te main magnetic phase in nated by SD) was measured with a Siemens D500 difrac- the target basalt is low-Ti titanomagnetite with a mag- tometer using a Cu-Ka anode.