Drawing from our results, we have found that
three of the four proposed hypotheses are not
consistent with our data.
There was no visual evidence of volcanic
features or structures in the majority of the
impact structures studied. Two craters
demonstrate fracturing within the crater floor but
there is no visible sign of volcanic material being
extruded from them. All identified PSA deposits
[6] have less than 5% mafic materials [10] – a feat
that would be exceedingly difficult to accomplish
by volcanism through a crust replete with mafic
material.
According to Pieters et al. [6], no known spinel
deposits are located in ejecta materials nor along
the crater floor. Mg-spinel deposits are locally
distributed and in small percentages in central
peaks and specific locales along the wall within
the crater structure.
33 of the 36 impact structures studied, did not
excavate material from depths of 10 kilometers or
greater (Figures 8-9). 10 km is the depth
necessary to attain the temperatures and
pressures that would form Mg-spinels by igneous
or metamorphic activity.
Based upon our results and those of Treiman
[12], we propose that the Mg-spinel deposits are
a result of an impact event into an anorthositic
crust with a near-surface mafic intrusion (i.e dike
swarm) (Fig. 9). The impact energy provides the
necessary pressure (>0.5kbar), temperature
(1300ºC) needed to melt the anorthositic crust
and the mafic intrusion, and thereby for spinel.
This spinel is then brought up to the lunar
surface through the central peak during the uplift
stage of crater formation.
36 of the analyzed spinel-bearing impact structures showed no evidence of volcanic structures
or features such as pyroclastic deposits, volcanoes, rilles, or domes within the crater walls,
crater floor, or central peak . Two structures (Dalton and Pitatus) demonstrate evidence
of fracturing in the crater floor but no evidence of volcanic materials being extruded from them.
No Mg-spinel deposits were found in impact ejecta [14] or nearby regions of any of the 36
structures analyzed.
26 of the studied impact structures with a central peak (30), excavated to depths ranging
between 3.5-9.0 kilometers. Four impact structures excavated at depths ranging from 9.7 -12.5
kilometers (Figure 8).
Excavation Depths as Indications of Magnesium
Spinel Formation via Impact MeltingGARNIER, Mikala, ESCHENFELDER, Jonas, FINTEL, Alysa,
Kickapoo High School, 3710 S. Jefferson Ave, Springfield, MO 65807
Introduction Research and Data Conclusions
References
AcknowledgementsRegion of StudyDr. Georgiana Kramer for her student mentoring on scientific analysis and critique of the methods of
science.
Dagmar Eschenfelder for her contributions on the understanding of the fundamentals and applications
of chemistry to the formation of magnesium spinels in the laboratory.
Dr. Oliver Stratmann and Dr. Stephan Will for their contributions on the physics and chemistry of
magnesium spinel formation and their review of scientific concepts and theories proposed in this
research.
Since the launch of Chandrayaan-1 on October
22, 2008, data from the Moon Mineralogy Mapper
(M3) has provided extensive insight into the
composition of the lunar surface. The latest
discovery was that of the pink-spinel anorthosite
(PSA). This new rock has a very unique
composition which consists of anorthosite, 20-
30% Mg-spinel (MgAl2O4), and less than 5% of
mafic materials [10]. The Mg-spinel in this
anorthosite requires certain conditions to form,
such as high pressures and high temperatures.
The spinel’s composition can only be achieved
by an interaction of a basaltic mix with the
anorthositic crust. Such interactions have been
hypothesized in four theories; volcanism,
impactor remnants, excavation, and impact
melting [6,7.9,12]. The purpose of this research
was to determine a plausible explanation for the
origin of these Mg-spinels by testing these four
hypotheses. Understanding the origin of Mg-
spinels can provide a more accurate explanation
of the origins and formation of this newly
identified rock type and a better understanding
of the construction of the lunar crust.
[1] Cintala, Mark, Richard A.F. Grieve. (1998). Scaling Impact melting and crater dimensions: Implications for the
lunar cratering record. Meteoritics & Planetary Science Volume 33, Issue 4, pages 889–912.
[2] Dhingra, Deepak, Carle M. Pieters, and James W. Head. (2014). Nature and distribution of olivine at
Copernicus Crater: new insights about origin from integrated high resolution mineralogy and imaging. Lunar
and Planetary Science Conference, 45, n. pag.
[3] Lal, D., et al. (2011). Identification of spinel group of minerals on central peak of crater Theophilus. Lunar and
Planetary Science Conference, 42, n. pag.
[4] Martel, Linda M. V. and G. Jeffrey Taylor. (2014). Moon’s Pink Mineral. Planetary Science Research
Discoveries, n. pag.
[5] Pieters, C. M. et al. (2011). Mg-spinel lithology: A new rock type on the lunar farside. Journal of Geophysical
Research, 116, 1-14.
[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.
[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.
[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
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
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.
[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
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.
(15) Croft, S.K. (1980), Cratering Flow Fields; Implications for the excavation and transient expansion stages of
crater formation. Lunar and Planetary Science Conf. 11th p. 2347-2378
Methodology
We analyzed 36 spinel-bearing craters [cf, 6] using
high-resolution images from the Lunar
Reconnaissance Orbiter Camera Narrow Angle
Camera. All 36 craters were analyzed to determine if
the spinel deposits were in close proximity to any
volcanic structures or features. We then analyzed
the ejecta material and nearby regions to determine
if any spinel deposits were located in the ejecta
blanket. The diameters of all 36 impact structures
analyzed in this study were obtained from the
Lunar Impact Crater Database
( http://www.lpi.usra.edu/resources/). We then
calculated the transient crater diameters (Dtc) using
the formula Dtc≈[DrDsc0.18]1/1.18 [, where Dr is the
final (measured) crater diameter and Dsc is the
diameter of the simple-to-complex transition (=
1.87x106 cm) [15]. We calculated the excavation
depths (de) using the formula de=Dtc*0.1 [1]. We
then compared our excavation depths to the depths
needed for the formation of Mg-spinel by deep-
seated plutons [8,9].
Figure 2: Joliot crater
Location: 25.9 93.4
Excavation Depth: 12.3 km
Crustal thickness: 25-35 km
Although this crater exceeds the 10 km boundary and could have
possibly been excavated, the crater is closely associated with
impact melt. This indicates that there is a direct relationship
between the impact and the extreme pressures and heat that is
needed for spinel formation.
Figure 3: Eudoxus crater
Location: 44.1 16.6
Excavation Depth: 5.7km
Crustal thickness: 30-40 km
This crater represents a crater with a shallow excavation depth.
The excavation depth is less than the required 10 km, proving that
spinels can form higher in the crust from the impactor providing
the necessary enthalpy rather than excavation.
Figure 4: Tycho Crater
Location: -43.3 -11.1
Excavation Depth: 6.7km
Crustal thickness: 25-35 km
Tycho is also closely associated with impact melt, which
indicates great heat and pressure from the impactor thus
reheating the basaltic dike and creating the specific chemistry
necessary for spinel formation. There is a single exposure of
spinel in the central peak of this crater.
Figure 9: This image depicts the intrusion of basaltic dike swarms in the subsurface of the lunar crust. It depicts the
10 km boundary necessary for spinel formation due to adequate temperatures and pressures. As the basaltic magma
reacts with the crustal chemistry, the extensive process of creating the magnesium spinels begin. [8]
Figure 5: Copernicus Crater
Location: 8.9 -19.5
Excavation Depth: 7.5km
Crustal thickness: 25-35
Copernicus has one single spinel located on a knob off the
central peak. This crater is a large impact with a final
diameter of 96.07 kilometers.
Figure 8: This graph depicts the 30 craters in which the spinels can be found in the central peaks. Prissel and his
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
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].
Results
Figure 1: Distribution of craters studied in this research on the lunar nearside (left) and farside (right)
We’re gonna have to science the ‘poo out of this We’re gonna have to science the ‘poo out of this
Figure 6: Dalton Crater
Location: 17.1 -84.5
Excavation Depth: 5 km
Crustal thickness: 30-40 km
The
Figure 7: Ball Crater
Location: -39.9 -8.4
Excavation Depth: 3.6 km
Crustal thickness: 25-35 km
Ball crater is a small spinel-bearing craters located in the
southern hemisphere. The spinel is only located in the central
peak in this crater.