Excavation Depths as Indications of Magnesium Spinel Formation via Impact Melting GARNIER, Mikala, ESCHENFELDER, Jonas, FINTEL, Alysa, Kickapoo High School, 3710 S. Jefferson Ave, Springfield, MO 65807 We’re gonna have to science the ‘poo out of this We’re gonna have to science the ‘poo out of this Introduction Research and Data Conclusions Since the launch of Chandrayaan-1 on October Drawing from our results, we have found that 22, 2008, data from the Moon Mineralogy Mapper three of the four proposed hypotheses are not (M3) has provided extensive insight into the consistent with our data. composition of the lunar surface. The latest discovery was that of the pink-spinel anorthosite There was no visual evidence of volcanic (PSA). This new rock has a very unique features or structures in the majority of the composition which consists of anorthosite, 20- impact structures studied. Two craters

30% Mg-spinel (MgAl O ), and less than 5% of Figure 2: Joliot crater Figure 3: Eudoxus crater 2 4 Location: 25.9 93.4 Location: 44.1 16.6 demonstrate fracturing within the crater floor but Excavation Depth: 12.3 km Excavation Depth: 5.7km mafic materials [10]. The Mg-spinel in this Crustal thickness: 25-35 km Crustal thickness: 30-40 km there is no visible sign of volcanic material being Although this crater exceeds the 10 km boundary and could have This crater represents a crater with a shallow excavation depth. possibly been excavated, the crater is closely associated with The excavation depth is less than the required 10 km, proving that anorthosite requires certain conditions to form, impact melt. This indicates that there is a direct relationship spinels can form higher in the crust from the impactor providing extruded from them. All identified PSA deposits between the impact and the extreme pressures and heat that is the necessary enthalpy rather than excavation. such as high pressures and high temperatures. needed for spinel formation. [6] have less than 5% mafic materials [10] – a feat The spinel’s composition can only be achieved that would be exceedingly difficult to accomplish by an interaction of a basaltic mix with the by volcanism through a crust replete with mafic anorthositic crust. Such interactions have been material. Figure 8: This graph depicts the 30 craters in which the spinels can be found in the central peaks. Prissel and his hypothesized in four theories; volcanism, colleagues concluded that 10 km is the boundary for magnesium spinels to be able to form by intruding plutons. The reasoning behind this is that the conditions below this 10 km is a suitable forming environment due to the correct impactor remnants, excavation, and impact temperatures (1300º C)and pressures (>.5kb). All but two of the craters we analyzed excavated materials closer to the surface meaning that the magnesium spinels could not have been uplifted from that depth. [8,9]. According to Pieters et al. [6], no known spinel melting [6,7.9,12]. The purpose of this research deposits are located in ejecta materials nor along was to determine a plausible explanation for the the crater floor. Mg-spinel deposits are locally

Figure 4: Tycho Crater Figure 5: Copernicus Crater origin of these Mg-spinels by testing these four Location: -43.3 -11.1 Location: 8.9 -19.5 distributed and in small percentages in central Excavation Depth: 6.7km Excavation Depth: 7.5km hypotheses. Understanding the origin of Mg- Crustal thickness: 25-35 km Crustal thickness: 25-35 Tycho is also closely associated with impact melt, which Copernicus has one single spinel located on a knob off the peaks and specific locales along the wall within indicates great heat and pressure from the impactor thus central peak. This crater is a large impact with a final spinels can provide a more accurate explanation reheating the basaltic dike and creating the specific chemistry diameter of 96.07 kilometers. necessary for spinel formation. There is a single exposure of the crater structure. of the origins and formation of this newly spinel in the central peak of this crater. identified rock type and a better understanding 33 of the 36 impact structures studied, did not of the construction of the lunar crust. excavate material from depths of 10 kilometers or Methodology greater (Figures 8-9). 10 km is the depth We analyzed 36 spinel-bearing craters [cf, 6] using necessary to attain the temperatures and high-resolution images from the Lunar pressures that would form Mg-spinels by igneous Reconnaissance Orbiter Camera Narrow Angle or metamorphic activity.

Camera. All 36 craters were analyzed to determine if Figure 6: Dalton Crater Figure 7: Ball Crater Location: 17.1 -84.5 Location: -39.9 -8.4 Figure 9: This image depicts the intrusion of basaltic dike swarms in the subsurface of the lunar crust. It depicts the Excavation Depth: 5 km Excavation Depth: 3.6 km 10 km boundary necessary for spinel formation due to adequate temperatures and pressures. As the basaltic magma Based upon our results and those of Treiman the spinel deposits were in close proximity to any Crustal thickness: 30-40 km Crustal thickness: 25-35 km reacts with the crustal chemistry, the extensive process of creating the magnesium spinels begin. [8] The Ball crater is a small spinel-bearing craters located in the southern hemisphere. The spinel is only located in the central volcanic structures or features. We then analyzed peak in this crater. [12], we propose that the Mg-spinel deposits are the ejecta material and nearby regions to determine a result of an impact event into an anorthositic if any spinel deposits were located in the ejecta crust with a near-surface mafic intrusion (i.e dike blanket. The diameters of all 36 impact structures Results swarm) (Fig. 9). The impact energy provides the analyzed in this study were obtained from the necessary pressure (>0.5kbar), temperature 36 of the analyzed spinel-bearing impact structures showed no evidence of volcanic structures Lunar Impact Crater Database (1300ºC) needed to melt the anorthositic crust or features such as pyroclastic deposits, volcanoes, rilles, or domes within the crater walls, ( http://www.lpi.usra.edu/resources/). We then and the mafic intrusion, and thereby for spinel. crater floor, or central peak . Two structures (Dalton and Pitatus) demonstrate evidence calculated the transient crater diameters (Dtc) using This spinel is then brought up to the lunar 1/1.18 of fracturing in the crater floor but no evidence of volcanic materials being extruded from them. the formula Dtc≈[DrDsc0.18] [, where Dr is the surface through the central peak during the uplift final (measured) crater diameter and D is the stage of crater formation. sc No Mg-spinel deposits were found in impact ejecta [14] or nearby regions of any of the 36 diameter of the simple-to-complex transition (= References 1.87x106 cm) [15]. We calculated the excavation structures analyzed. [1] Cintala, Mark, Richard A.F. Grieve. (1998). Scaling Impact melting and crater dimensions: Implications for the depths (d ) using the formula d =D *0.1 [1]. We lunar cratering record. Meteoritics & Planetary Science Volume 33, Issue 4, pages 889–912. e e tc [2] Dhingra, Deepak, Carle M. Pieters, and James W. Head. (2014). Nature and distribution of olivine at 26 of the studied impact structures with a central peak (30), excavated to depths ranging Copernicus Crater: new insights about origin from integrated high resolution mineralogy and imaging. Lunar then compared our excavation depths to the depths and Planetary Science Conference, 45, n. pag. between 3.5-9.0 kilometers. Four impact structures excavated at depths ranging from 9.7 -12.5 [3] Lal, D., et al. (2011). Identification of spinel group of minerals on central peak of crater Theophilus. Lunar and needed for the formation of Mg-spinel by deep- Planetary Science Conference, 42, n. pag. kilometers (Figure 8). [4] Martel, Linda M. V. and G. Jeffrey Taylor. (2014). Moon’s Pink Mineral. Planetary Science Research seated plutons [8,9]. Discoveries, n. pag. [5] Pieters, C. M. et al. (2011). Mg-spinel lithology: A new rock type on the lunar farside. Journal of Geophysical Acknowledgements Research, 116, 1-14. Region of Study [6] Pieters, Carle M, et al. (2014). The distribution of Mg-spinel across the Moon and constraints on crustal origin. American Mineralogist. 99, 1893-1910. Dr. Georgiana Kramer for her student mentoring on scientific analysis and critique of the methods of [7] Prissel, T.C., et. al. (2014). Pink Moon: The petrogenesis of pink spinel anorthosites and implications concerning mg-suite magmatism. Earth and Planetary Science Letters, 144-156. science. [8] Prissel, T.C., et al. (2013). An uncollected member of the mg-suite: Mg-Al Pink spinel anorthosites and their place on the Moon. Lunar and Planetary Science Conference, 44, n. pag. [9] Prissel, T. C. et al. (2012). Melt-wallrock reactions on the Moon: Experimental constraints on the formation of Dagmar Eschenfelder for her contributions on the understanding of the fundamentals and applications newly discovered Mg-spinel anorthosites. Lunar and Planetary Science Conference, 43, n. pag. [10] Taylor, L.A. and C. M. Pieters. (2013) Pink-spinel anorthosite formation: considerations for a feasible of chemistry to the formation of magnesium spinels in the laboratory. petrogenesis. Lunar and Planetary Science Conference, 44, n. pag. [11] Sun, Y. et al. (2013). Detection of Mg-spinel bearing central peaks using M3 images. Lunar and Planetary Science Conference, 44, n. pag. Dr. Oliver Stratmann and Dr. Stephan Will for their contributions on the physics and chemistry of [12] Treiman, A. H., et al. (2015). Lunar rocks rich in Mg-Al spinel: Enthalpy constraints suggest origins by impact melting. Lunar and Planetary Science Conference, 46, n. pag. [13] Wieczorek, Mark A. and Maria T. Zuber. (2001). The composition and origin of the lunar crust: constraints magnesium spinel formation and their review of scientific concepts and theories proposed in this from central peaks and crustal thinkness modeling. Geophysical Research Letters, 28(21), 4023-4026. (14) Yue, Z. et al. (2013). Projectile remnants in central peaks of lunar impact craters. Nature Geoscience pg. 1-3. research. (15) Croft, S.K. (1980), Cratering Flow Fields; Implications for the excavation and transient expansion stages of Figure 1: Distribution of craters studied in this research on the lunar nearside (left) and farside (right) crater formation. Lunar and Planetary Science Conf. 11th p. 2347-2378