APPLICATION OF ELECTRON BACKSCATTER DIFFRACTION (EBSD) TO INVESTIGATE THE
PETROGENESIS AND SHOCK DEFORMATION HISTORY OF A PINK SPINEL ANORTHOSITE (PSA)
CLAST IN LUNAR METEORITE NORTHWEST AFRICA (NWA) 15500
D. Sheikh
1
and A. M. Ruzicka
1
,
1
Cascadia Meteorite Laboratory, Portland State University,
Department of Geology, Portland, OR 97207, USA ([email protected]).
Introduction: Geochemical investigation of pink spinel anorthosite (PSA) clasts from lunar meteorites Northwest
Africa (NWA) 15500 and 16400 [1-2] have yielded important findings supporting the petrogenesis of PSA, and pink
spinel troctolites (PST), by magma-wallrock interactions between plagioclase-undersaturated Mg-suite parental melts
and anorthositic crust [3-4]. However, evaluating the exact temporal/spatial relationship between PSA and PST is
challenging given the complexities associated with geochemical modeling of assimilation in a magma chamber [5],
and the limited sample pool of PSA/PST material available for study. In addition, the physical/chemical modification
of lunar materials by impact(s) needs to be considered, as the excavation of PSA/PST material onto the lunar surface
from depth would induce secondary shock deformation effects that may complicate interpretation. Electron backscat-
ter diffraction (EBSD) serves as a useful technique that can provide non-destructive crystallographic analysis of lithic
clasts in lunar samples and disentangle primary characteristics inherited during igneous crystallization versus those
formed by later shock deformation; previous application of EBSD to ordinary chondrites (OCs) [6-7] using inverse
pole figure (IPF, assessing mineral crystallographic orientations) maps and intragrain deformation metrics such as
grain orientation spread (GOS, the average misorientation within a grain) and crystal rotation axis (CRA, displays the
rotation axis direction for boundaries with 2-10° misorientations) have provided insight into the syn-deformational P-
T conditions and extent of shock deformation occurring on OC parent bodies. Here, we apply this technique to a PSA
clast (C5) from NWA 15500 in order to elucidate the crystalliza-
tion and shock deformation history of PSA.
Results: Indexing of spinel, plagioclase, and olivine grains
from PSA C5 resulted in a high-quality phases + band contrast
(BC) image (Fig. 1a), where non-indexing areas indicate the pres-
ence of maskelynite; mineral phase fractions are: spinel (51 %),
plagioclase + maskelynite (45 %), olivine (4 %). Spinel grains on
the IPFz chart (Fig. 1b) are randomly oriented, whereas plagio-
clase and olivine grains display noticeable lattice-preferred orien-
tations (LPO). There is a variation in GOS within and between
mineral phases (Fig. 1c); the highest GOS values are observed in
olivine (relative to spinel and plagioclase) concentrated at the rim
of PSA C5. The CRA plot for olivine (Fig. 1d) yields a concen-
tration of points at <100> indicating a predominance of C-type
slip [6-8]; calculation of R
2-10
and model deformation temperature
(T
deform
) using the approach of [6-7] yields a syn-deformation tem-
perature of 565±106 ℃.
Discussion: The IPF, texture, and phase abundances from
PSA C5 provide additional evidence favoring a genetic relation-
ship between PSA and PST. For PSA C5, only spinel is randomly oriented, indicating a crystallization sequence that
began spinel-saturated (Sp→Sp+Pl→Sp+Pl+Ol). In contrast, in a PST clast from 73002, only plagioclase (neither
olivine nor spinel) had an LPO [9], which indicates a crystallization sequence that began with olivine and spinel prior
to plagioclase. This would be consistent with an increasing degree of crustal assimilation for PSA relative to PST [3-
4]. The lower temperature of T
deform
(565±106 °C) compared with the Sp-Ol Fe-Mg geothermometer [10] equilibration
temperature (T
eq
) for PSA C5 (~1136 °C) indicates that olivine records a later, strong shock event from low ambient
temperature following the crystallization of PSA at depth. Thus, there were two distinct shock events: 1) a basin-
forming impact excavated PSA material at T
eq
~1136 °C to a cool near-surface setting, 2) a later, strong shock de-
formed all phases and incorporated the PSA clast into NWA 15500 after brief reheating to T
deform
~565 ℃.
References: [1] Sheikh D. et al. (2023) LPSC LIV, Abstract #2066. 9:941-958. [2] Sheikh D. et al. (2024) LPSC
LV, Abstract #2023. [3] Prissel T. C. et al. (2014) Earth and Planetary Science Letters 403:144-156. [4] Prissel T. C.
et al. (2016) American Mineralogist 101:1624-1635. [5] Heinonen J. S. et al. (2021) Crustal Magmatic System Evo-
lution, Chapter# 7. [6] Ruzicka A. M. and Hugo R. C. (2018) Geochimica et Cosmochimica Acta 234:115-147. [7]
Ruzicka A. M. and Hugo R. C. (2022) LPSC LIII, Abstract #1757. [8] Karato S. et al. (2008) Annual Reviews of Earth
and Planetary Sciences 36:59-95. [9] Stadermann A. C. et al. (2024) LPSC LV, Abstract #1498. [10] Jianping L. et al.
(1995) Chinese Journal of Geochemistry 14:68-77.
6468.pdf86th Annual Meeting of the Meteoritical Society 2024 (LPI Contrib. No. 3036)
