Nagaoka et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-0324-8(2020) 7:12

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Formation of ferroan dacite by lunar silicic
volcanism recorded in a meteorite from theMoon
Hiroshi Nagaoka1,2* , Timothy J. Fagan3, Masahiro Kayama4,5,6, Yuzuru Karouji7, Nobuyuki Hasebe1,8 and
Mitsuru Ebihara3

Abstract
An igneous clast from the Northwest Africa 773 (NWA 773) clan of lunar meteorites formed by silicic volcanism on
the Moon. The clast was identified in Northwest Africa 2727 (NWA 2727), which is included in the NWA 773 clan.
Over 80 mode% of the clast consists of silica + plagioclase + K-Ba-feldspar. The silica phases cristobalite, tridymite,and quartz are all present in the clast, indicating rapid cooling at low pressure in agreement with a volcanic setting.
This clast is characterized as a dacite on the basis of mineral modes and whole-rock chemical composition. Olivine
and pyroxene in the clast have high Fe/Mg ratios (olivine Fe# > 99, pyroxene Fe# > 99; with Fe# = molar Fe/(Fe +Mg) × 100), suggesting crystallization from a residual liquid after fractionation of more magnesian silicates. The clast
is similar in some respects to ferroan gabbro alkaline-phase-ferroan (FG/ARFe) clasts that are inferred to be comagmatic with olivine gabbro (OG) and other mafic lithologies of the NWA 773 clan. However, the high silica
concentration and dominance of apatite as the main Ca-phosphate (no merrillite was identified) are distinct fromthe FG/ARFe clasts. Thus, the dacite clast probably crystallized in a magmatic setting that was independent of the
OG and FG/ARFe lithologies. The mafic major element composition, young age, and high KREEP-content of theNWA 773 clan have been used previously to infer an origin from the Procellarum-KREEP terrane (PKT) on thenearside of the Moon. Several candidates for silicic volcanism/plutonism have been identified in the PKT (e.g.,
Gruithuisen Domes, Hansteen alpha, Lassell Massif). The presence of the dacite lithology provides additional support
for an origin in or around PKT for meteorites of the NWA 773 clan, and for complex igneous activity in PKT region.
Keywords: Moon, Mineralogy, Petrology, Volcanology, Silicic volcanism

Introduction
On Earth, igneous processes in diverse geochemical settings
have resulted in wide compositional ranges of mafic to silicic
rocks which form the crust. In contrast, the surface of theMoon is dominated by anorthositic rocks in the highlands
and basaltic rocks in the lowlands. Anorthosites of the lunar
highlands are interpreted as flotation cumulates from thelunar magma ocean (Warren 1985). Lunar basalts form by
volcanic eruptions from the interior after partial melting in
the mantle and are concentrated on the nearside of the* Correspondence: hiroshi-nagaoka@asagi.waseda.jp
1
Research Institute for Science and Engineering, Waseda University, 3-4-1
Okubo, Shinjuku, Tokyo 169-8555, Japan
2
Institute of Space and Astronautical Science, Japan Aerospace Exploration
Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan
Full list of author information is available at the end of the article

Moon, in and near the Procellarum-KREEP Terrane (PKT,with KREEP indicating enrichment in K, rare earth elements,
and P) (Jolliff et al. 2000; Grove and Krawczynski 2009).
Though the absolute age (or ages) of anorthosites in thehighlands has been a topic of recent research (e.g., Borg et al.
2011; Barboni et al. 2017), it has been well established bothfrom crater frequency counting and isotopic ages that basaltsof the lunar lowlands (maria) are younger than the highlands
anorthosites (e.g., Nyquist and Shih 1992; Hiesinger et al.
2003, 2010; Morota et al. 2011; Snape et al. 2016). For themost part, the anorthosites have crystallization ages prior to
4.3 Ga, whereas the main pulse of mare volcanism occurredfrom 3.9 to 3.2 Ga. Isotopic crystallization ages for a few
lunar basalts are as young as 2.9–3.0 Ga (Borg et al. 2009;
Elardo et al. 2014), and crater counts indicate that basaltic© The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

(2020) 7:12

Page 2 of 18

volcanism continued in the PKT to at least 1.5 Ga (Hiesinger
et al. 2003; Morota et al. 2011).
In contrast to basalts and anorthosites, granitic rocks
are rare on the Moon, comprising well under 0.1% bymass of the Apollo collection as reported by Seddio
et al. (2015). Nonetheless, the presence of granitic rocks
is an indicator of compositional evolution by igneous
processes in the Moon. Some candidates for localities
where the lunar silicic rocks occur have been identified
from remote sensing data sets that combine infrared
spectroscopy, gamma-ray spectroscopy, and topography.
Gruithuisen Domes, Hansteen alpha, and Lassell Massif
have infrared spectra indicating silica-rich compositions(Glotch et al. 2010) and are located in the PKT, which as
described above, has a long-term history of volcanismon the Moon. Gruithuisen domes and Hansteen alpha
are topographic highs, indicating that they may haveformed from rhyolitic volcanism, whereas intrusive granites may be exposed at Lassell and parts of Aristarchus
crater (see Head and McCord 1978 and Hawke et al.
2003, in addition to Glotch et al. 2010).
Some of the processes that have been linked to the origin of silicic rocks on the Moon include (1) underplatingof crust by basaltic magma and low-degree partial melting of crustal rocks (e.g., Hagerty et al. 2006; Glotch
et al. 2010); (2) silicate liquid immiscibility resulting inco-existence of FeO-rich mafic and SiO2-alkali-rich liquids (e.g., Rutherford et al. 1976; Fagan et al. 2014); and(3) fractional crystallization of basaltic liquid (e.g., Seddio et al. 2013). Similar processes play roles during granite formation on the Earth, but plate tectonics,
terrestrial weathering and sedimentation, and high-waterpressures in the crust have dramatic effects on formationof terrestrial granitic rocks (e.g., Tatsumi 2000; Weinberg and Hasalová 2015). In the case of Earth, granite
formation is often associated with wet conditions during
various stages of melting and crystallization (Campbell
and Taylor 1983; Brown 2013). However, terrestrial Atype or ferroan granitoids are characterized by more anhydrous conditions (Frost and Frost 2011), and sharesome similarities in mineralogy, chemistry, and possibly
origin with granitic rocks on differentiated bodies otherthan the Earth (Bonin 2012). Based on the andesitic meteorites Graves Nunatak 06128/9 (GRA 06128/9; Dayet al. 2009), Northwest Africa 11119 (NWA 11119; Srinivasan et al. 2018), and trachyandesite ALM-A from the
Almahata Sitta ureilite strewn field (Bischoff et al. 2014),
it is known that silica-rich volcanism occurred in theearly solar system, within a few million years of Ca-Alrich inclusions. From study of meteorites from asteroid
4 Vesta, dacite formation occurred on Vesta by crustal
partial melting (Hahn Jr. et al. 2017). Therefore, silicic
igneous rocks formed by conditions and processes distinct from the settings typical of Earth. Petrologic

description of samples of lunar silicic rocks is an essential step toward evaluating the igneous processes that led
to granite formation on the Moon and other celestialbodies.
Here, we have found a silicic clast in the lunar meteorite breccia Northwest Africa 2727 (NWA 2727), which isincluded in the NWA 773 clan of lunar meteorites (Jolliff et al. 2007; Nagaoka et al. 2019). NWA 773 clan meteorites share a distinctive olivine gabbro (OG) lithology(Fagan et al. 2003; Jolliff et al. 2003; Zhang et al. 2011;
Valencia et al. 2019). In addition to the OG lithology,
the NWA 773 clan contains a variety of basaltic clasts,
feldspar + pyroxene gabbro, olivine-pyroxene-silica
symplectites, and silica-bearing alkaline-phase-ferroan(ARFe) clasts. Several of these clast-types have been
linked together to form during a common igneous differentiation suite (Fagan et al. 2014; Shaulis et al. 2017;
Valencia et al. 2019). The likely parental material of thedifferentiation suite had a very low-Ti mafic and
KREEP-bearing composition similar to green picritic
glass from Apollo 14 site (Jolliff et al. 2003; Valencia
et al. 2019). As discussed above, several candidates forsilicic magmatism have been identified in the PKT(Glotch et al. 2010 and references therein). In this study,
we describe the mineralogical and petrological characteristics of the silicic clast to constrain its origin. Furthermore,
we compare the silicic clast with other silica-bearing lithic
clasts of the NWA 773 clan and other lunar and terrestrial
samples to consider implications of the clast for the originof silica-bearing igneous rocks in general.

Nagaoka et al. Progress in Earth and Planetary Science

Methods/Experimental
NWA 2727, a member of the NWA 773 clan of lunar
meteorites

Lunar meteorite NWA 2727 was found in Morocco or
Algeria in 2005 (Connolly et al. 2006), and is a polymict,mafic breccia belonging to the NWA 773 clan (Bunch et al.
2006; Zeigler et al. 2007; Kayama et al. 2018; Valencia et al.
2019). As a whole, the NWA 773 clan consists of a coarsegrained regolith breccia with variable abundances of gabbroic
and basaltic lithologies (Valencia et al. 2019). The gabbroic lithologies were roughly divided into three types (Fagan et al.
2003, 2014; Jolliff et al. 2003; Zhang et al. 2011; Valencia
et al. 2019): olivine gabbro (OG, an olivine-pyroxeneplagioclase-rich rock); anorthositic gabbro (AG, aplagioclase-rich rock with some pyroxene); and ferroan gabbro (FG, a group of lithologies all having high Fe# = molar
Fe/[Fe + Mg]; see below). OG clasts in NWA 773 clan havebeen well characterized by previous studies (Fagan et al.
2003; Jolliff et al. 2003; Zhang et al. 2011; Nagaoka et al.
2015; Valencia et al. 2019). Isotopic Sm-Nd isochron ages of
the OG lithology have been analyzed by Borg et al. (2009),
who reported an age of 2.99 ± 0.03 Ga, and by Nyquist et al.
(2009), reporting an age of 3.10 ± 0.05 Ga. Isotopic analyses

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

of baddeleyite crystals in the OG lithology and in other typesof clasts yield Pb-Pb ages near 3.1 Ga (Zhang et al. 2011;
Shaulis et al. 2017). In contrast, a zircon crystal from theNWA 773 breccia has a U-Pb age of 3.958 ± 0.018 Ga (Shaulis et al. 2017). The AG lithology consists mostly of plagioclase and pyroxene. Anorthositic gabbro clasts havecumulate texture consisting of coarse crystals of anhedral
plagioclase intergrown with pyroxene (Valencia et al. 2019).
Although FG is variable in mineral assemblage, mineral zoning, and texture, the ferroan composition of mafic minerals
is common in the FG lithology. The following ferroan lithologies are included in FG group (Valencia et al. 2019): pyroxene gabbro, symplectite clasts, and alkaline-phase-ferroan(ARFe) clasts (Fagan et al. 2003, 2014; Jolliff et al. 2003). The
pyroxene gabbro is dominated by zoned pyroxene and usually contains plagioclase feldspar as well. Symplectite clastsconsist of fine, curved intergrown fayalite, hedenbergite, andsilica. Even though silica occurs as a characteristic phase of
the symplectite lithology, the symplectite also contains fayalite and is not SiO2-rich in its bulk composition. Fayalite andsilica also occur together in the ARFe lithology, but they
form coarser crystals instead of symplectic intergrowths; inaddition, the ARFe clasts contain K-Ba-feldspar and/or feldspathic glass along with merrillite, ± apatite, ± ilmenite, and± hedenbergitic pyroxene.
These three gabbros (OG, AG, FG) formed from a comagmatic sequence of crystallization (Fagan et al. 2014;
Shaulis et al. 2017; Valencia et al. 2019). Furthermore,olivine phyric basalt (OPB) is a clast-type included inseveral NWA 773 clan meteorites, and is interpreted asco-magmatic with the three gabbros (Valencia et al.
2019). NWA 2727 includes clasts of all of the four lithologies described above (OG, AG, FG, OPB), as reported
by Valencia et al. (2019).
In addition to the OPB, other types of basaltic clasts
with pyroxene phenocrysts have been found in theNWA 773 clan. Basalts with pyroxene phenocrysts infine pyroxene+ feldspar groundmass occur in NWA 773(Fagan et al. 2003, see their “straw-textured clast”) andin NWA 2727 (Kayama et al. 2018). Anhedral quartz
single crystals (10 to 20 μm in radius) are present inclinopyroxene-phyric basalt (CPB) clasts in NWA 2727
(Kayama et al. 2018). The presence of multiple types ofbasalts indicates that some clasts of the NWA 773 clan
formed initially from variable types of volcanism.
The NWA 773 clan shares (1) mafic minerals andmajor element composition; (2) isotopic ages of 3.0 to
3.1 Ga for the most part, though some clasts in the breccia are older; and (3) KREEP-rich minor element composition (Fagan et al. 2003; Jolliff et al. 2003; Zhang et al.
2011; Valencia et al. 2019). On the basis of mafic composition, geochemical KREEP signature, and young
crystallization age, NWA 773 clan is considered to originate from PKT (Jolliff et al. 2003).

Page 3 of 18

Analytical methods for mineralogy and petrology

The polished thin section (PTS) of NWA 2727 wasinvestigated by a combination of petrographic andchemical microanalyses including electron microprobe
spectroscopy and scanning electron microscope imaging
and mapping. Thin-section scale back-scattered electron(BSE) images and X-ray elemental maps were collectedby an electron probe micro-analyzer (EPMA) (JEOL
JXA-8900) at Kobe University. Clast-scale BSE images
and X-ray elemental maps of the silicic clast were captured using the Hitachi S-3000 N scanning electron
microscope (SEM) and JEOL JXA-8900 EPMA at
Waseda University. These X-ray elemental images were
adjusted and combined using an imaging analytical tool
(Adobe Photoshop®). Photoshop was also used to estimate the area fraction (mode) of minerals from elemental maps covering approximately 3 mm2 in the silicic
clast by counting pixels of each phase, which is taken to
be proportional to the volume fraction. Mineral phases
and modes were identified and determined based on the
combined X-ray element maps.
Quantitative analyses of minerals in the silicic clast
were performed by wavelength dispersive spectroscopyusing a JEOL JXA-8900 EPMA at Waseda University
with (1) an accelerating voltage of 15 kV and a probe
current of 2.0 × 10−8 A for feldspar, silica, pyroxene, olivine, and Fe-oxide, and (2) 7 kV and 2.0 × 10−8 A for Caphosphate. Quantitative major element analyses were determined for the silicates and Fe-oxide; the main goal of
the Ca-phosphate analyses was to use Ca/P ratios to distinguish apatite from merrillite. During electron irradiation using EPMA, alkali feldspar crystals showed blue
cathodoluminescence that faded during exposure to theelectron beam. Well-characterized oxides, silicates, andphosphates were used as standards. Approximate lower
limits of detection for the 15 kV analyses are as follows(in wt%): Na2O = 0.02; MgO = 0.02; Al2O3 = 0.03; SiO2 =0.04; P2O5 = 0.03; K2O = 0.01; CaO = 0.02; TiO2 = 0.05;
Cr2O3 = 0.04; MnO = 0.05; FeO = 0.06; BaO = 0.03. Only
data with analytical totals of 98–102 wt% with close fits tothe mineral stoichiometries were selected in this paper.
Laser micro-Raman spectroscopy was carried out toidentify minerals using Thermo Electron (Nicolet Almega
XR) with the 532 nm excitation line of the Nd:YAG laser
at Okayama University of Science. The source laser power
was set at 20 mW on the sample with a ~ 2 μm spot
diameter. Raman spectra of minerals were collected withthe wavenumber range from 150 to 800 cm−1 with step
size of 1 cm−1. Raman intensity mapping was measured
along the X and Y directions with 30 μm step sizes. In the
Raman maps, the integral intensities of the main bands foreach mineral were extracted from the spectral data of eachmeasuring point to observe distributions of minerals. The
Raman data were collected in five accumulations of 60 s

(2020) 7:12

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for the spectra and three accumulations of 30 s exposure
time for mapping. All the spectral data were calibrated bymonitoring the position of the O-Si-O bending vibration(464 cm−1) in reference standard quartz of high opticalgrade before and after each measurement. Detailed information on the Raman spectroscopy is shown in Kayama
et al. (2018).

Pyroxene (dark green in Fig. 2b; 27 vol%) forms elongate
crystals between coarser plagioclase. Olivine (light bluein Fig. 2b; 36 vol%) shows some euhedral crystal faces,with grain sizes ranging from 0.3 to 2 mm across. Silica
minerals were not identified. The mineral assemblage ofthis clast is similar to that of the OG, but the mineraltextures and modal abundances are different. Plagioclasein this clast is much more abundant than in OG clasts.
This clast has coarse laths of anhedral plagioclase intergrown with pyroxene similar to the cumulate texture of
AG as described by Valencia et al. (2019). Therefore, theplagioclase-rich clast is categorized as AG, although thepresence of olivine was not reported in the AG clasts described by Valencia et al. (2019).
The FG group includes variable types of ferroan lithologies (described above; also see Fagan et al. 2014; Valencia
et al. 2019). One clast of pyroxene gabbro (part of the FG
group) in the PTS is approximately 7.8 × 5.2 mm across inthis PTS (Fig. 2c). The clast consists mostly of plagioclase
and pyroxene, which are in subequal proportions. Plagioclase (red in Fig. 2c) occurs as both small and large crystals,
ranging up to 1.6 mm across. The pyroxene crystals are up
to 2.0 mm across and are zoned to Fe-rich compositions(from dark green to blue color in Fig. 2c), which are characteristic of the pyroxene gabbro lithology as defined by
Fagan et al. (2014). No silica minerals were identified in theinternal texture of this clast, although silica minerals (e.g.,
tridymite) were found along the boundary between the FG
clast and the breccia matrix, and in veins that cut throughthe clast (Kayama et al. 2018). In addition to the large pyroxene gabbro clast, other FG lithologies are present as
smaller clasts in the PTS and in other specimens of NWA
2727 (Bunch et al. 2006; Valencia et al. 2019).
One large basaltic clast, covering an area of 5.6 ×
3.6 mm is present in the PTS (Fig. 2d). Olivine phenocrysts (light blue in Fig. 2d) are surrounded by a finegrained groundmass of pyroxene (dark blue in Fig. 2d)and plagioclase (red in Fig. 2d). The grain sizes of olivine
phenocrysts are from 0.3 × 0.1 mm to 0.6 × 0.3 mm
across. The presence of olivine phenocrysts and finegrained groundmass consisting of pyroxene and plagioclase of this basaltic clast are similar to the olivinephyric basalt (OPB) described by Valencia et al. (2019).
Plane-polarized light, BSE, and elemental RGB images of the silicic clast are presented in Fig. 3. The
silicic clast consists mostly of plagioclase, alkali
feldspar, and silica, which comprise > 80 vol% of themode as exposed in our PTS (Table 1; Fig. 3). The silicic clast is 2.2 × 1.3 mm in size in this PTS. Olivine
and high-Ca pyroxene also occur in the clast, and Caphosphate, ilmenite, chromite, and troilite are present
as minor minerals. All of the Ca-phosphate analyzed
here by EPMA yielded apatite stoichiometry; merrillite
was not identified.

Nagaoka et al. Progress in Earth and Planetary Science

Results
Petrography

Figure 1 shows false-color elemental mapping of Al(red), Mg (green), and Fe (blue) Kα x-rays, and distinguishes the main minerals of NWA 2727, as shown in thefollowing colors: light green and blue colors indicate olivine; dark green and blue colors indicate pyroxene; redcolor indicates plagioclase. The PTS contains several typesof clasts identified as mafic lithologies including OG, AG,
FG (pyroxene gabbro), OPB, and the silicic clast (Fig. 1).
The diversity of clast-types in our polished thin section(PTS) is consistent with previous work on NWA 2727, except for the silicic clast, which has not been described previously (Bunch et al. 2006; Zeigler et al. 2007; Valencia
et al. 2019). Petrological descriptions of each lithology in
the PTS are summarized below.
The OG lithology consists mostly of olivine and pyroxene with plagioclase. Large and small varieties of OG
clasts are included in the PTS. The largest OG clast in this
PTS is 8.0 × 6.2 mm in size (Fig. 2a). Olivine (light green
grains in Fig. 2a; 57 vol%) is the most abundant mineral inthis OG clast. Olivine occurs as subhedral to euhedral
crystals ranging from 100 μm up to 1.8 mm across. Several
grains of olivine are over 1 mm across. Pyroxene (darkgreen grains in Fig. 2a; 25 vol%) is the second most abundant mineral. Pyroxene crystals in the OG also are coarse,
with grain sizes up to 1 mm across. Anhedral crystals ofplagioclase (red in Fig. 2a; 16 vol%) typically occur between coarse olivine and pyroxene. Plagioclase often appears elongate in the OG clast, with grain sizes up to
1.3 × 0.3 mm. Large olivine crystals with pyroxene andplagioclase filling interstices indicate the cumulate texture.
Fe-oxides occur in trace abundances in the OG clast, andare located between coarse minerals. Silica minerals are
not found in the OG clast. These petrological features are
similar to other OG clasts in the NWA 773 clan (Fagan
et al. 2003; Jolliff et al. 2003; Zhang et al. 2011; Nagaoka
et al. 2015; Valencia et al. 2019).
The AG has a cumulate texture and is dominated byplagioclase and pyroxene (Valencia et al. 2019). One
plagioclase-rich clast with texture similar to the AG was
identified in the PTS. The clast is 4.4 × 4.3 mm in size inthis PTS (Fig. 2b). Plagioclase (red in Fig. 2b; 35 vol%)occurs as elongate-appearing crystals, with grain sizes up
to 2.1 × 0.5 mm. Plagioclase in the clast (35 vol%) ismore abundant than in the OG lithology (16 vol%).

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Fig. 1 False-color X-ray elemental image of NWA 2727 breccia
modified after Kayama et al. (2018). Red, green, and blue (RGB)
represent Al, Mg, and Fe, respectively. Kα X-rays were used for all
elements. Clast name abbreviations: OG olivine gabbro, AG
anorthositic gabbro, OPB olivine-phyric basalt

Page 5 of 18

Plagioclase is present in the center of the clast andtends to be in contact with silica (Fig. 3c, d). Silica
occurs as (1) massive to elongate grains up to 550 μm inlong diameter (labeled Si(1) in Fig. 3d); (2) narrowdomains enveloping thin plagioclase feldspar laths(~ 50 μm across) (labeled Si(2) in Fig. 3d); and (3) elongate
crystals intergrown with K-Ba-feldspar (labeled Si(3) inFig. 3d). K-Ba-feldspar is not as abundant as plagioclase orsilica, but comprises approximately 15 vol% of the mode in
the clast and appears to have co-crystallized with silica.
Most of the mafic minerals occur in a cluster of olivine(310 μm × 260 μm), pyroxene, and apatite near the left
margin of the clast as shown in Fig. 3c, d. In this part of theclast, fine pyroxene crystals (< 100 μm; Px fine1 in Fig. 3c)occur adjacent to apatite. Pyroxene also occurs as fine clusters of crystals disseminated among silica and feldspar near
the right margin of the clast (< 100 μm; Px fine2 in Fig. 3c),and one coarser crystal (210 μm × 140 μm) occurs near thebottom of the clast (Px crs in Fig. 3c).
Silica is abundant in the clast, constituting nearly 40%of the mode (Table 1). Silica polymorphs were determined using micro-Raman spectroscopy in digital mapsof three localities within the clast (Fig. 3e–h). Most spectra showed weak bands indicative of poor crystallinity,

Fig. 2 False-color X-ray elemental images of mafic lithologies of NWA 2727. Detailed explanations of each figure are as follows: a olivine gabbro(OG); b anorthositic gabbro (AG); c pyroxene gabbro; d olivine-phyric basalt (OPB). RGB elemental map with R = Al, G = Mg, B = Fe(2020) 7:12

Page 6 of 18

but some points yielded spectral patterns characteristicof tridymite, cristobalite, and quartz, all present within
the clast (Fig. 4). Quartz was identified only where silica
is intergrown with K-Ba-feldspar, whereas cristobalite
and tridymite were found where the silica has a more
massive texture (Fig. 3e–h).
Mafic mineral crystals in the silicic clast are muchfiner-grained (0.3 mm in maximum dimension) thanthose of three gabbros (OG, AG, FG), and similar in size
to olivine phenocrysts in the OPB. The finer-grained
texture of the silicic clast indicates more rapid coolinghistory than those of the three gabbros. Silica minerals
with abundant plagioclase feldspar were not identified in
OG, AG, and FG lithologies in this PTS. The silicic clast
occurs isolated in breccia, and has no spatial or zoning
relationships with those gabbros. Some of FG clasts(symplectite and ARFe) in other portions of NWA 773
clan include silica minerals (Fagan et al. 2014; Valencia
et al. 2019), but the modal abundances of silica are lessand grain sizes are smaller than silica of the silicic clast.

1991), and the FeO content in plagioclase of the silicic
clast is as high as those of the Apollo very-low-Ti marebasalt (0.90–1.34 wt% in Papike et al. 1991). Minor K2O(Or~ 2) was detected in all analyses of plagioclase and BaO
was detected in some analyses, though only in minor concentrations (< 0.2 wt% BaO). Alkali feldspar is enriched inK, with minor Na, detectable Ca, and variable Ba (Ab3An2Or95Cn0–Ab4An2Or81Cn13). The BaO concentration in
the K-Ba-feldspar is zoned (0.1–6.7 wt%). Analyses of silica are mostly SiO2, with minor TiO2 (mostly 0.1 to
0.2 wt%); K2O and Al2O3 were detected during analyses,
possibly due to beam overlap with adjacent feldspar.
However, as noted by Seddio et al. (2015), the hightemperature polymorphs of silica have more open structures than quartz and thus, may accommodate elevatedconcentrations of minor elements. The silicic clast formedat or near the lunar surface at a cooling rate fast enoughto preserve cristobalite and tridymite. Zoning of BaOconcentration in K-Ba-feldspar is consistent with a fastcooling rate, as are the fine grain sizes of mafic minerals
in the silicic clast. These features indicate that this clast is
not plutonic rock, but rather derived from volcanic or
near-surface origin. On the IUGS classification of volcanic
rocks based on modal abundances of silica, alkali feldspar,and plagioclase, this clast falls on the boundary between
rhyolite and dacite (Fig. 7).

Nagaoka et al. Progress in Earth and Planetary Science

Mineral compositions in the silicic clast

Olivine and ilmenite in the silicic clast have Fe# (molar Fe/[Fe + Mg] × 100) of 99–100 (Table 2) corresponding tolate-stage magmatic crystallization. All of the pyroxene analyses indicate hedenbergitic compositions with Fe# and Ti#
both ≥ 99 (Ti# = molar Ti/[Ti + Cr] × 100) (Table 2; Figs. 5
and 6). Fine pyroxene crystals that are interwoven withapatite in the mafic cluster and the coarse pyroxene haveessentially identical Wo-En-Fs compositions; the fine pyroxene near the upper right margin (Px fine2 in Fig. 3) has
slightly lower Wo-content (Wo36–38 in Fig. 5). All pyroxenes in the silicic clast are more ferroan than those in the
ARFe clasts analyzed by Fagan et al. (2014) (Figs. 5 and 6).
All analyzed pyroxenes from the silicic clast have a Ti/
Al ratio ranging from 0.4 to 0.6 with an average valueof 0.5 as shown in Fig. 6a. In lunar basaltic rocks,
low Ti/Al (~ 0.25) is associated with pyroxene thatcrystallized prior to feldspar, whereas higher Ti/Al(~ 0.5) indicates pyroxene that crystallized after
feldspar crystallization had started (Bence et al. 1970;
Fagan et al. 2014). Thus, it is likely that all of thepyroxene from the clast crystallized after the onset offeldspar crystallization. The high Fe# and Ti# are
consistent with late-stage crystallization in an igneous
system (Fig. 6b).
Plagioclase has chemical compositions near An87 withlimited Ab-An zoning (An82–91; Table 3). The An value
is higher than those of plagioclase in the Apollo granitic
samples (An80–50; Wieczorek et al. 2006). The FeO content in plagioclase (1.15 wt%) is higher than those in the
Apollo granite/felsite (0.06–0.79 wt%: Warren et al.
1983; Jolliff 1991; Seddio et al. 2013, 2015) and KREEP
basalt (0.16–0.71 wt%: Simon et al. 1988; Papike et al.

Comparison of major element composition of the silicic
clast with other lithologies in NWA 773 clan

The whole rock composition of the silicic clast was determined by modal recombination (Table 4) using modal
abundances, densities of minerals, and their elemental
concentrations. All Fe is calculated as Fe2+. And thedensity of all SiO2 is used as 2.2 g cm−3, which is comparable to the density of cristobalite (Holland and Powell2011). Based on bulk SiO2 and Na2O + K2O contents,
the silicic clast falls along andesite-dacite boundary on
the total alkali-silica (TAS) diagram of Le Bas et al.
(1992); Fig. 8). This clast is characterized as a dacite
on the basis of the low-pressure origin indicated bycristobalite and tridymite combined with the bulk
composition.
The bulk composition of the dacite clast is comparedwith those of other clast lithologies (OG, AG, FG, OPB) in
NWA 773 clan and the Apollo quartz monzodiorite(QMD), granite, and felsite in Table 4. The dacite clast ismore enriched in SiO2 and alkali elements (Na, K, Ba)
than other lithologies of NWA 773 clan. Furthermore, thebulk composition of the dacite (Mg# < 1) is much more
ferroan than other NWA 773 clan lithologies that wereanalyzed by Valencia et al. (Mg#51–70, see Table 4). From
the above results, the dacite clast appears to be a distinct
lithology from the existing lithologies in the NWA 773
clan. On the other hand, the concentrations of SiO2 andNagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 7 of 18

Fig. 3 Images of silicic clast and Raman maps showing silica polymorphs. Detailed explanations of each figure are as follows: a photomicrograph in plane
polarized light; b back-scattered electrons (BSE); c RGB elemental map with R = Al, G = Ca, B = Fe, d RGB image with R = K, G = Si, B = P. Silica occurs as: (1)massive to elongate grains; (2) narrow domains enveloping thin plagioclase feldspar laths; and (3) elongate crystals intercalated with K-Ba-feldspar. Areas
where Raman peaks were mapped are color-coded in panel b, with maps e, f enclosed in the yellow box, map g in the blue box, and map h in the
purple box. The Raman peak intensity maps show the following peaks: e peak intensity at 351 cm−1 (tridymite); f peak intensity at 411 cm−1 (cristobalite); g
peak intensity at 411 cm−1 (cristobalite); h peak intensity at 464 cm−1 (quartz). Warm colors (red) indicate high peak intensities and dark (blue) colors
denote low intensities. Mineral abbreviations: Apa apatite, Fa fayalite, Ilm ilmenite, Kfs K-feldspar, Plg plagioclase, Px pyroxene, Si silica

alkali elements in the dacite are not as high as those in the
Apollo granite and felsite. The bulk P2O5 content(1.3 wt%) in the dacite clast is higher than those (0.086–
0.34 wt%) of other lithologies in NWA 773 clan andApollo granite/felsite (0.05–0.20 wt%: Quick et al. 1977;
Seddio et al. 2013), but falls in the range of P2O5
concentrations of QMD (0.25–1.72 wt%: Taylor et al.
1980; Ryder and Martinez 1991; Jolliff et al. 1993;
Fagan et al. 2014).

Discussion
Comparisons of the dacite clast with other lunar silicic rocks

The high Fe# of mafic silicates combined with the abundance of silica in the dacite clast can be explained by
crystallization from a residual liquid after fractional
crystallization. Some of the silica in the clast is intergrown with feldspars, suggesting co-crystallization of silica with plagioclase and alkali feldspars during stages ofcrystallization from liquid. The interlocking textures ofNagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 8 of 18

Table 1 Modal abundances (vol%) of minerals in the NWA 2727 silicic clast, Apollo granite, QMD
Sample nameNWA 2727

12033,634-34

12023,14-10

12032,366-19

15405,145

Clast

Silicic clast

GraniteGraniteGranite

QMD

Refer.

this work

Seddio et al. (2015)

Seddio et al. (2014)

Seddio et al. (2013)

Fagan et al. (2014)

Ol

4.9

7.1

0.8

2.8

-

LCP

-

8.7

2.2

5.5

HCP

5.9

Plg

31.4

30.3

14.2

6.3

20.2

Kfs

14.9

21.1

45.7

52.3

24.5

Si

39.4

24.0

36.7

32.0

23.8

Ox

<1

1.0

<1

<2

2.3

Ap

2.6

4.0

Tr

Tr

0.3

Mer

-

3.9

-

Tr

2.8

12.1
13.7

Ol olivine, LCP low-Ca pyroxene (pigeonite + orthopyroxene), HCP high-Ca pyroxene (augite), Plg plagioclase, Kfs K-feldspar, Si silica, Ox oxide phases, Ap apatite,
Mer merrillite, Tr trace amount. The unit of modal abundances in Seddio et al. (2013, 2014) are converted into “vol%” from “wt%”Fig. 4 Representative Raman spectra of a tridymite and cristobalite
and b quartz in the silicic clast. Raman bands of tridymite at 207,
301, 351, and 432 cm−1, cristobalite at 228 and 411 cm−1, and quartz
at 199 and 464 cm−1 (e.g., Etchepare et al. 1974; Ling et al. 2011;
Seddio et al. 2015) were detected

the dacite clast are distinct from granulitic textures(mainly minerals having rounded shapes) associated withsilicic rocks formed by impact, and no shock melt
pockets were observed (Takeda 1986; Barrat et al. 2009;
Takeda et al. 2015). Moganite, a monoclinic SiO2 polymorph associated with carbonaceous chondrite-derived
lunar alkaline fluid that becomes a source of subsurface
ice deposits, has been identified along with the highpressure polymorphs coesite in matrix grains and stishovite in some shock veins elsewhere in NWA 2727
(Kayama et al. 2018). Some portions of cristobalite andtridymite also occur in silica micrograins in the shock
veins, which apparently transformed from moganite during post-shock decompression. However, the coarse
grain sizes and intergrown feldspar texture of the dacite
clast are distinct from the shock-related occurrences ofsilica identified by Kayama et al. (2018). Therefore, theformation of tridymite and cristobalite in the dacite clast
by unloading after shock impact can be excluded; ashock melt origin of the dacite clast is unlikely. We inferthat this clast crystallized from silicic melt that formedby endogenic igneous processes on the Moon.
The occurrence of three low-pressure silica polymorphs indicates rapid, near-surface cooling, and disequilibrium. Cristobalite and tridymite observed in theclast are thought to have been quenched from conditions of low pressures (< 1 GPa) and high temperatures(~ 870 to ~ 1500 °C) (e.g., Presnall 1995; Koike et al.
2013). These silica polymorphs are also found in silicic
volcanic settings in terrestrial settings (e.g., Baxter et al.
1999; Hamasaki 2002; Horwell et al. 2010), and the presence of tridymite on the surface of Mars has been interpreted as an indicator of silicic volcanism (Morris et al.
2016). The presence of cristobalite provides evidence of
higher crystallization temperature (Koike et al. 2013)

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 9 of 18

Table 2 Average compositions of olivine, pyroxene and ilmenite in the silicic clast
OlivinePyroxene

Ilmenite

n

4

10

2

wt%

mean

s.d.

mean

s.d.

mean

s.d.

SiO2

30.53

0.27

46.48

0.71

0.06

0.04

TiO2

0.06

0.02

Al2O3

<0.03

0.99

0.28

51.29

0.23

1.32

0.37

0.06

0.02

Cr2O3

<0.04

FeO

68.91

0.58

32.87

2.16

47.32

0.70

MnO

0.81

0.11

0.34

0.04

0.43

0.15

MgO

0.15

0.03

0.12

0.06

0.04

0.04
0.02

<0.04

<0.04

CaO

0.63

0.11

18.54

1.75

0.07

Na2O

0.05

0.02

0.04

0.02

<0.02

K2O

<0.01

P2O5

0.06

0.03

<0.01

0.01

<0.03

<0.03
0.04

0.00

BaO

na

0.07

Total

101.2

100.8

99.3

na

at. per oxy at.

4

6

3

Si

1.013

0.006

1.927

0.025

0.002

0.001

Ti

0.001

0.001

0.031

0.009

0.985

0.001

0.018

0.002

0.001

Al

b.d.

0.065

Cr

b.d.

b.d.

Fe

1.912

0.015

1.140

0.077

1.011

0.010

Mn

0.023

0.003

0.012

0.001

0.009

0.003

Mg

0.007

0.001

0.008

0.003

0.002

0.002

Ca

0.022

0.004

0.823

0.076

0.002

0.000

Na

0.004

0.001

0.003

0.002

K

b.d.

P

0.002

Ba

na

0.001

total

2.984

4.010

2.013

Fe#

99.8

99.3

99.8

b.d.

b.d.
0.001

b.d.

Wo

41.8

En

0.4

Fs

57.8

Ti#

99.9

Ti/Al

0.48

b.d.
0.000

0.000

b.d.
0.001

na

na

Note: Our values in “s.d.” column are the standard deviations (1σ) of the individual mineral compositions and reflect the compositional variation of each mineralin the silicic clast. The word of “n” means number of analyses, “b.d.” means below the detection limit, “n.a.” means no analysis

than typical lunar granitic samples (T < 1000 °C) (Jolliff
et al. 1999; Gullikson et al. 2016). The occurrence of
three low-pressure silica polymorphs in the same rock is
unusual; however, Seddio et al. (2015) observed hackle
fracture patterns in quartz in Apollo samples of lunar
granites, and inferred that the fractures formed by a volume reduction during recrystallization from cristobalite

or tridymite to quartz. The phase transition to quartz
would depend on cooling rate. The dacite clast probably crystallized at a relatively fast cooling rate that
resulted in incomplete transformation of the high-T
polymorphs to quartz. The poor crystallinity of muchof the silica in the clast is likely due to rapid cooling
during crystallization.

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 10 of 18

Fig. 5 Pyroxene quadrilateral compositions from the silicic clast (this work). Compositions reported by Fagan et al. (2014) for pyroxenes fromNWA 773 ARFe clasts and 15405 QMD (56, 145), are also shown

Fig. 6 a Al versus Ti and b Fe# versus Ti# in pyroxenes from thesilicic clast (this work). Compositions of ARFe clasts in NWA 773(Fagan et al. 2014), NWA 2977 OG (Nagaoka et al. 2015), and 15405
QMD (56, 145) (Fagan et al. 2014) are also shown

Other silica-bearing igneous clasts in NWA 773 clan
include clinopyroxene-phyric basalt (CPB) clasts, symplectite, and alkaline-phase-ferroan (ARFe) clasts (Fagan
et al. 2014; Kayama et al. 2018). The symplectite and
ARFe classifications of Fagan et al. (2014) are grouped
together as variations within ferroan gabbro (FG) by Valencia et al. (2019), but both groups of workers (Fagan
et al. 2014; Valencia et al. 2019) report similar texturesand mineral compositions. The CPB clasts have anhedral
quartz single crystals on the order of ≤ 40 μm across andhave abundant pyroxene + feldspar-rich groundmass,
distinct from the dacite clast (Kayama et al. 2018). The
symplectite (one variety of FG of Valencia et al. 2019) iscomposed of fayalite, silica, and hedenbergitic pyroxene
with curved, fine intermingled textures (Fagan et al.
2003, 2014; Jolliff et al. 2003). Fayalite, silica, and hedenbergitic pyroxene also occur in the dacite clast, but feldspars also are abundant in the clast, and the texturesand formation processes are distinct. Whereas the symplectites formed dominantly by pyroxferroite breakdown(Fagan et al. 2003, 2014; Jolliff et al. 2003; Valencia et al.
2019), textures in the dacite clast are consistent with formation by igneous crystallization from melt.
Igneous textures, such as elongate K-feldspar intergrown with silica, also occur in some ARFe clasts of
NWA 773 (Fagan et al. 2014; approximately equivalent
to silica-K-feldspar granophyre-bearing clasts of Jolliff
et al. 2003, and grouped within the FG lithology of Valencia et al. 2019, see their Figs. 9–11 of FG-A, FG-B,
FG-C, FG-D, FG-F, FG-I). The presence of ferroan mafic
silicates, silica, feldspar, and Ca-phosphates are common
to the FG/ARFe lithology (combining the classification
schemes of Fagan et al. 2014 and Valencia et al. 2019)
and the dacite clast, and textures in FG/ARFe rocks anddacite clast indicate igneous origin. In detail, however,
the two lithologies are distinct. In the FG/ARFe clasts,

(2020) 7:12

Nagaoka et al. Progress in Earth and Planetary Science

Page 11 of 18

Table 3 Average compositions of plagioclase, K,Ba-feldspar and silica in the silicic clast
Plagioclase

K,Ba-feldspar

n

13

6

Silica

wt%

mean

s.d.

mean

s.d.

mean

s.d.

SiO2

47.29

1.13

62.81

1.86

99.61

0.78

TiO2

<0.05

0.18

0.12

0.16

0.03

Al2O3

31.91

1.14

19.57

0.60

0.55

0.18

Cr2O3

<0.04

FeO

1.15

MnO

<0.04

<0.04

<0.04

MgO

<0.02

<0.02

<0.02

11

<0.04
0.29

0.14

<0.04
0.05

0.11

0.08

CaO

18.14

0.79

0.31

0.12

0.05

0.03

Na2O

1.26

0.25

0.42

0.04

0.09

0.04

K2O

0.33

0.14

14.61

1.00

0.28

0.14

P2O5

<0.03

BaO

0.09

Total

100.17101.49100.88atoms per oxy at.

8

8

2

Si

2.187

Ti

b.d.

<0.03
0.05

0.051

0.058

3.45

<0.03
2.30

0.03

0.02

2.919

0.050

0.992

0.002

0.007

0.004

0.001

0.000

1.073

0.046

0.006

0.002

Al

1.740

Cr

b.d.

Fe

0.045

Mn

b.d.

Mg

b.d

Ca

0.899

0.038

0.016

0.006

0.001

0.000

Na

0.113

0.023

0.038

0.004

0.002

0.001

K

0.020

0.008

0.866

0.049

0.003

0.002

b.d.
0.011

0.005

b.d.
0.002

b.d.
b.d.

P

b.d

Ba

0.002

total

5.006

4.987

Ab

10.9

3.9

0.063

An

87.0

1.6

Or

1.9

88.1

Cn

0.2

6.4

0.001

b.d.

b.d.
0.001

0.001
b.d.

b.d.
0.043

0.000

0.000

1.006

Note: Our values in “s.d.” column are the standard deviations (1σ) of the individual mineral compositions and reflect the compositional variation of each mineralin the silicic clast. The number of analyses is indicated by “n”, “b.d.” means below the detection limit. The BaO content was determined in 11 of the 13 analyses ofplagioclase feldspar

K-feldspar is more abundant than plagioclase, but the reverse is true for the dacite clast. The main Ca-phosphatein the FG/ARFe lithology is merrillite, whereas only
apatite has been identified in the dacite clast. Finally, although K-feldspar and silica are characteristic of the FG/
ARFe clasts, they typically occur in minor modal abundances compared to mafic silicates. In contrast, feldspars
and silica are much more abundant than mafic phases inthe dacite clast. The differences in mineral modes reflect

the high bulk SiO2 content of the dacite compared to
the FG/ARFe clasts (Fagan et al. 2014; Valencia et al.
2019), suggesting differences in origin.
Therefore, although the dacite clast shares several features of the FG/ARFe lithology described by Fagan et al.
(2014) and Valencia et al. (2019), the dacite clast appears
to be a new lithology in the NWA 773 clan, which has
not been described previously. While we cannot rule out
the possibility that the dacite clast is an unusual variety

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Fig. 7 IUGS Classification of volcanic rocks (Streckeisen 1979). White
circle plot represents the silicic clast (this work)of the FG/ARFe lithology, the high SiO2-content, highratio of plagioclase/K-feldspar and occurrence of apatite
without merrillite all point toward a distinct origin forthe dacite clast (see “Petrogenesis of the dacite clast”
section for further discussion).
The whole-rock composition distinguishes the dacite
clast from other lunar silicic rocks such as QMD and granite. In Fig. 9, the bulk composition of the dacite clast iscompared with lunar evolved rocks returned by Apollo

Page 12 of 18

missions (KREEP basalt, QMD, granite/felsite, granitic
breccia) (Hubbard et al. 1973; Rhodes and Hubbard 1973;
Murali et al. 1977; Quick et al. 1977; Warren and Wasson
1978; Warren et al. 1978; Taylor et al. 1980; Warren et al.
1983; Salpas et al. 1987; Lindstrom et al. 1988; Simon
et al. 1988; Morris et al. 1990; Jolliff et al. 1991; Marvin
et al. 1991; Ryder and Martinez 1991; Snyder et al. 1992;
Jolliff et al. 1993; Seddio et al. 2013; Fagan et al. 2014).
Lunar sample return missions collected no dacite or andesite samples (that have yet been identified), so several
analyses of terrestrial ferroan (Mg# < 50) andesite and dacite (Shimazu et al. 1985; Gust and Arculus 1986; Imaiet al. 1995; Togashi et al. 2000; Sensarma and Palme 2013)
are plotted in Fig. 9 for the comparison.
The dacite clast plots between QMD and lunar granitic rocks in terms of SiO2 versus MgO + FeO contents(Fig. 9a), and is distinct from the clusters of lunar
KREEP basalts, QMD, and granitic rocks in terms of
SiO2 versus Al2O3 (Fig. 9b). In both cases, the clast issimilar to some of the terrestrial ferroan andesite/dacite
samples. However, the dacite clast has lower Na2O +K2O contents at comparable SiO2 and is more ferroan
than both the terrestrial andesite/dacites and lunar granitic rocks (Fig. 9c, d). The difference in alkalis reflects
the lower mode of K-feldspar and the low albite content(Ab11) of plagioclase in comparison with the Apollo
granitic samples and in the terrestrial andesite/dacite.
The reported whole-rock Na2O and K2O (and SiO2)
concentrations of the dacite in NWA 773 clan depend ofcourse on the mode of the clast, which is likely to vary

Table 4 Major element abundances of the silicic clast (this work), other lithologies (OG, AG, FG, OPB) in NWA 773 clan, and Apollo
evolved samples
NWA 2727

NWA 773 clan

Silicic clast

OG

15405,145

12032, 366-19

12013, light

QMD

GraniteFelsiteOxide (wt%)

This work

Valencia et al. 2019

Fagan et al. 2014

Seddio et al. 2013

Warren et al. 1983

SiO2

63.0

43.47

47.3

47.52

TiO2

0.89

0.36

0.17

1.07

45.39

60.3

70.1

73

0.85

2.15

1.07

0.6

Al2O3

13.5

5.02

19.5

Cr2O3

<0.1

0.42

0.28

6.91

8.01

10.3

13.5

11.9

0.44

0.52

0.08

-

-

FeO

9.3

19.08

MnO

<0.1

0.234

9.58

19.83

21.72

11.0

4.98

0.90

0.17

0.279

0.284

0.17

0.07

-

MgO

<0.1

CaO

9.2

24.65

7.05

12.92

12.71

3.18

0.14

0.7

7.04

15.1

10.83

9.59

7.6

3.04

1.4

Na2O
K2O

0.51

0.122

0.38

0.157

0.163

0.76

2.47

1.40

2.3

0.029

0.04

0.150

0.018

3.42

4.58

6.80

AG

FG

OPB

P2O5

1.3

0.086

-

0.34

0.094

1.42

0.052

-

BaO

0.54

0.01

<0.07

0.03

0.01

-

0.68

0.65

Total

100.5

100.5

99.6

100.5

99.4

100.4

100.7

97.4

Mg#

<1

70

57

54

51

34

5

58

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 13 of 18

Fig. 8 Total alkali-silica (TAS) diagram (Le Bas et al. 1992). White circle represents the silicic clast (this work)at least somewhat from the mode of the original lithology on the Moon. If the clast has disproportionately
high modes of silica or low modes of feldspars, then thecalculated Na2O + K2O vs. SiO2 content could fall along
the lunar variation trend shown in Fig. 9c. However, thelow Ab-content of plagioclase and high Fe# of mafic silicates of the dacite clast are distinct from the terrestrial
and most of the lunar samples plotted in Fig. 9.

Petrogenesis of the dacite clast

Although there are mineralogical and compositional differences between the dacite and FG/ARFe lithology describedby Fagan et al. (2014) and Valencia et al. (2019), the originof the dacite clast is broadly similar to the origin of the FG/
ARFe lithology. Both lithologies crystallized from igneous
liquids that were depleted in MgO and enriched in SiO2
and alkalis. Composite clasts with zoned pyroxene ± feldspar attached to FG/ARFe domains indicate that the FG/
ARFe lithology formed after Fe-enrichment (Mg-depletion)
by fractional crystallization (Fagan et al. 2014; Valencia
et al. 2019). Rounded inclusions in fayalite in the FG/ARFe
lithology contain igneous feldspathic glass or alkali feldspar+ silica, and formed by immiscibility of alkali-silica-richliquid from Fe-rich silicate liquid (see Rutherford et al.
1974, 1976). Spatial continuity of variations within the FG
lithology (ARFe rocks, symplectites, and pyroxene gabbro),combined with mineral compositions in the OG lithology
and similar U-Pb ages of baddeleyite are strong evidence
that these gabbroic rocks—OG, AG, FG lithologies—

formed from a co-magmatic sequence of crystallization(Fagan et al. 2014; Shaulis et al. 2017; Valencia et al. 2019).
Whether the dacite clast also formed as part of thesame NWA 773 clan co-magmatic sequence that started
with OG is problematic. As described above, the highFe# of mafic silicates and the general mineral assemblageof the dacite are similar to the glass and feldspar + silica-bearing immiscible inclusions in FG/ARFe rocks. If
the FG/ARFe immiscible inclusions coalesced, they might
have formed a more coarse-grained rock similar to the dacite. However, the immiscible inclusions are dominated byalkali feldspar rather than plagioclase, and the main Caphosphate of the ARFe lithology is merrillite rather thanapatite. Therefore, if the dacite formed from the alkalisilica-rich inclusions in the ARFe rocks, simple coalescence of inclusions alone is not a likely origin of the dacite. Some additional mixing with plagioclase materials or
additional petrologic steps would be required to make thedacite from the ARFe inclusions. Based on the evidence
currently available, we favor the interpretation that the dacite clast formed in a system independent of the olivine
gabbro → anorthositic gabbro → ferroan gabbro (pyroxene gabbro + symplectite + ARFe) trend.
The presence of apatite without merrillite in the dacite
clast is somewhat unusual because both Ca-phosphateminerals commonly occur together in felsic rocks from
the Moon (Jolliff et al. 1993; Seddio et al. 2013, 2015;
Fagan et al. 2014). However, apatite-bearing felsic rocks inwhich no merrillite was identified have been identified inlunar samples. Jolliff et al. (1993) reported apatite but noNagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

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Fig. 9 Comparisons of bulk compositions between the silicic clast (Dacite; this work), Apollo evolved samples and several terrestrial andesite/
dacites (see text): a SiO2 vs. MgO + FeO, b SiO2 vs. Al2O3, c SiO2 vs. Na2O + K2O, d SiO2 vs. Mg#. Rocks shown in the plot include: the silicic clast(dacite; this work); KREEP basalt (Hubbard et al. 1973; Rhodes and Hubbard 1973; Murali et al. 1977; Warren and Wasson 1978; Warren et al. 1978;
Salpas et al. 1987; Simon et al. 1988); QMD (Taylor et al. 1980; Lindstrom et al. 1988; Marvin et al. 1991; Ryder and Martinez 1991; Jolliff et al. 1993;
Fagan et al. 2014); granite/felsite (Quick et al. 1977; Warren et al. 1983; Morris et al. 1990; Jolliff et al. 1991; Snyder et al. 1992; Seddio et al. 2013);
granitic breccia (Morris et al. 1990); terrestrial ferroan (TF) andesite/dacite (Shimazu et al. 1985; Gust and Arculus 1986; Imai et al. 1995; Togashi
et al. 2000; Sensarma and Palme 2013)

merrillite in Apollo felsite 14161,7269, and likewise, Warren et al. (1983) identified apatite without merrillite in the
granitic sample 14303,204. Factors controlling apatite vs.
merrillite are in all likelihood connected to the relative
abundances of volatile elements, which are concentrated
in apatite, and of rare earth elements, which are concentrated in merrillite. Thus, the dacite clast, like otherapatite-rich/merrillite-poor felsic rocks, probably formedin setting that was relatively volatile-rich on the Moon.
Another possible factor in the origin of the dacite is silicate
liquid immiscibility (SLI). Immiscibility has been discussed asa possible explanation for the formation of lunar granites
and the absence of intermediate lithologies (e.g., Jolliff 1991;
Seddio et al. 2013). The occurrence of SLI has been found as
micro-scale inclusions in a variety of lunar samples (e.g.,
Roedder and Weiblen 1970; Shearer et al. 2001) in addition
to the evidence from the NWA 773 clan described above.
SLI might have played a role in the origin of the dacite clast.

However, no direct textural evidence of SLI has been identified in the dacite. The dacite clast is isolated in breccia: there
are no spatial or zoning relationships that show a genetic relationship with lithologies of the NWA 773 clan OG magmatic system. On the other hand, the possibility that the
dacite might have formed by immiscibility of other magmatic
system involving large-scale gravity separation of silica-richmelt from a parent melt remains plausible in view of the factthat the dacite composition plots on the edge of the miscibility gap of Roedder (1951) (see Appendix). The partitioningof Ba between Fe-rich and Si-alkali-rich immiscible liquids
depends on the molar (Na2O + K2O + CaO + BaO)/Al2O3
ratio in the granitic immiscible melt. If the ratio is below 1,
Ba is partitioned into the granitic melt and if the ratio is over
1, Ba is partitioned into the basic melt (Neal and Taylor1989). The “alkali”/Al2O3 ratio of the dacite clast (1.5) is over
1, and the value is within the range of “alkali”/Al2O3 valuesof most lunar granites/felsites (1.0–1.7 from Warren et al.

(2020) 7:12

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1983; Neal and Taylor 1989; Morris et al. 1990; Snyder et al.
1992; Seddio et al. 2013; except for 73255c, 0.84 from Warren et al. 1983). The presence of BaO in zoned alkali feldsparof the dacite clast shows that some BaO was present in theigneous melt at the time of crystallization. It is possible thatBaO was rapidly depleted from the melt by alkali feldspar
crystallization. A potential immiscible pair of thedacite clast has not been identified, so partitioning of
BaO between immiscible liquids cannot be assessed indetail. However, similar values of “alkalis”/Al2O3 ofthe dacite clast and several granitic samples from theMoon suggest general similarities in compositional
evolution of BaO in lunar felsic rocks.
Previous work (Gullikson et al. 2016) supports the crustal
melting model for the production of lunar granite fromcrustal protoliths by partial melting at temperatures below
1000 °C. This model is consistent with the observation thatquartz is the only SiO2 polymorph found in lunar granitic
samples (Warren et al. 1983; Ling et al. 2011). However, thislow-temperature crystallization model does not fit the dacite
clast from NWA 2727 because the higher-temperature silica
polymorphs cristobalite and tridymite occur along withquartz in the dacite.
The presence of a silica-rich feldspathic glass in the
lunar meteorite Yamato 983885 (Y-983885) also requires
silica-enrichment followed by rapid cooling (Arai et al.
2005). According to the model of Arai et al. (2005), theSi, Al, K-rich glass was derived from rapid cooling of an
immiscible felsic liquid that separated from parent liquid
before the crystallization of silica (Arai et al. 2005). The
glass in Y-983885 apparently represents a case of silicaenrichment followed by rapid cooling distinct from theorigin of the dacite clast in NWA 2727. No glass was identified in the dacite clast, suggesting that it cooled more
slowly than the Y-983885 glass, but still fast enough topreserve the three low-pressure silica polymorphs.
The high Fe#99–100 of mafic silicates in dacite clast
can be explained by crystallization from residualliquid after Mg-depletion by fractional crystallization.
The Apollo granite (12032,366-19) studied by Seddio
et al. (2013) has a ferroan composition with fayalite
and hedenbergite similar to the dacite clast and isinterpreted as a product of extended fractional
crystallization. Thus, it also has a ferroan composition
like the dacite. The coexistence of multiple polymorphsof silica in the NWA 2727 dacite clast distinguishes thedacite from the Apollo 12032,366-19 granite, though thequartz in the Apollo 12 granite has a hackle patternsuggestive of contraction after crystallization and may haverecrystallized from a higher-temperature silica polymorph(Seddio et al. 2013, 2015).
Calzada-Diaz et al. (2015) investigated the launch site of
the NWA 773 using bulk composition, lunar global element maps (FeO, TiO2, and Th), and age constraints. Theysuggested that Mare Serenitatis, Crisium, or the west sideof Oceanus Procellarum are plausible candidates of thelaunch site of NWA 773. Most proposed sites of silicic
volcanism identified by remote sensing occur in the PKT,
located on lunar nearside (Glotch et al. 2010). The presence of the dacite clast is, thus, consistent with a PKT origin for meteorites of the NWA 773 clan, as are the mafic
composition, geochemical KREEP signature, and young
crystallization age. Thus, among the possibilities discussed
by Calzada-Diaz et al. (2015), we prefer the Procellarumand Serenitatis launch sites, as Crisium is farther from thePKT (also see Jolliff et al. 2003). In future work, we will investigate the launch site of NWA 773 clan by comparing
the geochemical and petrological data with imaging datawith higher spatial resolution.

Nagaoka et al. Progress in Earth and Planetary Science

Conclusions
NWA 773 clan of lunar meteorites includes a diversity of lithologies: olivine gabbro, anorthositic gabbro, ferroan gabbro(pyroxene gabbro, ferroan symplectite, alkaline-phase ferroan(FG/ARFe)), olivine- and pyroxene-phyric basalts, and, as
shown in this study, at least one clast of a silicic lithology.
Based on its petrological and geochemical composition, thesilicic clast is a dacite. The dacite clast has silica polymorphsof cristobalite, tridymite, and quartz, indicating rapid coolinghistory of a silicic magma at or near the surface of the Moon.
The high silica content, high Fe# and Ti# of pyroxene in theclast are consistent with formation at late-stages of magmaticdifferentiation. Its highly ferroan (Mg# < 1) and intermediate
SiO2 composition (63 wt%) distinguish the dacite clast fromother evolved lunar rocks.
The dacite clast differs from the NWA 773 clan FG/ARFe
clasts in the following: (1) plagioclase is more abundant thanK-feldspar; (2) only apatite has been identified in the dacite
clast, whereas the main Ca-phosphate in the ARFe lithology
is merrillite; (3) feldspars and silica are much more abundant
than mafic minerals in the dacite clast. The NWA 773 FG/
ARFe clasts formed in a co-magmatic system that also produced the OG and AG lithologies, pyroxene gabbro, andsymplectite. The FG/ARFe clasts have curved inclusions ofsilica-rich feldspathic glass and/or silica + K-feldspar intergrowths. Both types of inclusions in the FG/ARFe clasts
formed by silicate liquid immiscibility after Fe-enrichment.
However, the dacite clast shows no direct textural evidenceof immiscibility associated with the OG magmatic system.
The highly ferroan and silica-rich composition of dacite clast
formed at least in part by crystallization from residual liquid
after Mg-depletion by fractional crystallization of a parental
melt. It is possible that immiscibility followed fractional
crystallization. The presence of the dacite clast, taken
together with mafic composition, geochemical KREEPrich signature and young crystallization age, suggest
an origin in or around PKT for meteorites of theNWA 773 clan.

Nagaoka et al. Progress in Earth and Planetary Science(2020) 7:12

Page 16 of 18

Appendix

Fig. 10 Ternary diagram among SiO2, FeO + MgO + CaO, Na2O + K2O + Al2O3 (Seddio et al. 2013). Black dotted line encloses the field of SLI(Roedder 1951). White circle plot represents the dacite clast (this work)

Abbreviations
ARFe: Alkaline Ferroan; BSE: Back-Scattered Electron; EPMA: Electron probe
micro-analyzer; KREEP: Potassium (K); OC: Olivine cumulate gabbro;
P: Phosphorus; PKT: Procellarum KREEP Terrane; PTS: Polished thin section;
QMD: Quartz monzodiorite; REE: Rare earth element; RGB: Red, Green, Blue;SEM: Scanning electron microscope; SLI: Silicate liquid immiscibility;
TAS: Total alkali-silica
Acknowledgments
We are grateful for loans of thin section from NASA/JSC (R.A. Zeigler, curator)
for Apollo 15405 QMD. We acknowledge the helpful comments from two
anonymous reviewers and editorial handling by Noriko Kita.
Authors’ contributions
HN proposed the topic, conceived and designed the study. HN and TF
carried out the SEM and EPMA analyses. MK carried out Raman spectroscopy.
HN, TF, and MK analyzed the data and helped in their interpretation. All
authors discussed the manuscript. All authors read and approved the final
manuscript.
Funding
This work was supported in part by Research Fellowships for Young
Scientists from the Japan Society for the Promotion of Science (JSPS) 18
J01786 (PI: Hiroshi Nagaoka) and by JSPS KAKENHI Grant Number 18H01298(PI: Masahiro Kayama).
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated oranalyzed during the current study. Please contact author for data requests.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Research Institute for Science and Engineering, Waseda University, 3-4-1
Okubo, Shinjuku, Tokyo 169-8555, Japan. 2Institute of Space andAstronautical Science, Japan Aerospace Exploration Agency, 3-1-1
Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan. 3Department of Earth
Science, School of Education, Waseda University, 1-6-1 Nishiwaseda, Shinjuku,
Tokyo 169-8050, Japan. 4Department of Earth and Planetary Materials
Science, Graduate School of Science, Tohoku University, Aramaki aza Aoba
6-3, Aoba-ku, Sendai 980-8578, Japan. 5Creative Interdisciplinary Research

Division, Frontier Research Institute for Interdisciplinary Sciences, Tohoku
University, Aramaki aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan.
6
Department of General Systems Studies, Graduate School of Arts andSciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902,
Japan. 7JAXA Space Exploration Center, Japan Aerospace Exploration Agency,
3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan. 8School Advanced
Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo
169-8555, Japan.
Received: 2 July 2019 Accepted: 5 February 2020

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