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Lunar bulk chemical
composition: a post-Gravity
Recovery and Interior
Laboratory reassessment
rsta.royalsocietypublishing.org
G. Jeffrey Taylor1 and Mark A. Wieczorek2
1 Hawai‘i Institute of Geophysics and Planetology, University of
Research
Cite this article: Taylor GJ, Wieczorek MA.
2014 Lunar bulk chemical composition: a
post-Gravity Recovery and Interior Laboratory
reassessment. Phil. Trans. R. Soc. A 372:
20130242.
http://dx.doi.org/10.1098/rsta.2013.0242
One contribution of 19 to a Discussion Meeting
Issue ‘Origin of the Moon’.
Subject Areas:
geochemistry, geophysics
Keywords:
refractory elements, volatiles,
bulk composition, crust, mantle, Gravity
Recovery and Interior Laboratory
Author for correspondence:
G. Jeffrey Taylor
e-mail: [email protected]
Hawai‘i, Honolulu, HI 96822, USA
2 Institut de Physique du Globe de Paris, Sorbonne Paris Cité,
Université Paris Diderot, Case 7071, Lamarck A, 35 rue Hélène Brion,
Paris Cedex 13 75205, France
New estimates of the thickness of the lunar highlands
crust based on data from the Gravity Recovery and
Interior Laboratory mission, allow us to reassess
the abundances of refractory elements in the Moon.
Previous estimates of the Moon fall into two distinct
groups: earthlike and a 50% enrichment in the
Moon compared with the Earth. Revised crustal
thicknesses and compositional information from
remote sensing and lunar samples indicate that the
crust contributes 1.13–1.85 wt% Al2 O3 to the bulk
Moon abundance. Mare basalt Al2 O3 concentrations
(8–10 wt%) and Al2 O3 partitioning behaviour between
melt and pyroxene during partial melting indicate
mantle Al2 O3 concentration in the range 1.3–3.1 wt%,
depending on the relative amounts of pyroxene and
olivine. Using crustal and mantle mass fractions,
we show that that the Moon and the Earth most
likely have the same (within 20%) concentrations of
refractory elements. This allows us to use correlations
between pairs of refractory and volatile elements to
confirm that lunar abundances of moderately volatile
elements such as K, Rb and Cs are depleted by 75%
in the Moon compared with the Earth and that highly
volatile elements, such as Tl and Cd, are depleted
by 99%. The earthlike refractory abundances and
depleted volatile abundances are strong constraints on
lunar formation processes.
2014 The Author(s) Published by the Royal Society. All rights reserved.
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1. Introduction
The thickness of the lunar crust has been revised several times since the first seismic data
were analysed during the Apollo missions. These revisions were in part a result of using
improved computational approaches to analyse the seismic data, but also the collection
of improved topographic and gravitational data from subsequent orbital missions. In this section,
we review the history of how the thickness of the lunar crust has been estimated, and then provide
mass fractions of the Moon’s major geochemical reservoirs using the most recent GRAIL-derived
crustal thickness models.
The initial analyses of the Apollo seismic data implied that the crust was about 55–60 km thick
in the vicinity of the Apollo 12 and 14 landing sites. Toksöz et al. [14] reported a sharp seismic
discontinuity in the P-wave velocity profile at 55 km depth, and a later independent analysis by
Nakamura [15] obtained a concordant value of 58 ± 8 km. A study by Goins et al. [16] provided
tentative evidence for a crustal thickness of 75 km in the highlands at the Apollo 16 site. These
three seismic analyses were the basis of numerous bulk Moon compositional studies that were
published over two decades. A summary of how measurements of the seismically determined
and average crustal thickness of the Moon have evolved with time is given in figure 1.
Laser altimeter data were collected along the equatorial ground tracks of the Apollo 15 and 17
command and service modules, showing that the elevations over the nearside hemisphere were
about 4 km lower than those over the farside [17]. As these large-scale variations in topography
were not associated with any significant gravity anomaly, the crust should be largely isostatically
compensated at these scales. If the variations in surface elevation were compensated by a crustal
root (Airy compensation), the farside crust would be thicker than that of the nearside and the
.........................................................
2. Thickness of the crust
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130242
A firm knowledge of the bulk chemical composition of the Moon is important for unravelling the
details of lunar origin. For example, it is well known that the Moon is depleted in volatile elements
compared with the Earth, suggesting loss during lunar origin. Researchers have debated whether
the Moon is enriched in refractory elements (Th, U, Zr, rare earth elements, etc.) compared with
the Earth, with some authors arguing for very similar abundances [1–5] and others suggesting
that the Moon is enriched in refractory elements by at least 50% compared with the Earth
[5–11]. (In this paper, ‘refractory’ will be used to denote elements that condense in a gas of
solar composition at temperatures greater than 1300 K at a pressure of 10−4 bar [12]. Note that
condensation temperatures are only a guide to volatility as they apply to equilibrium in a gas
of solar composition at low pressure, conditions vastly different from those in the hypothetical
protolunar disc.)
These two drastically different views of lunar refractory abundances were driven largely by
estimates of the thickness of the feldspathic crust (see §2). However, data from heat flow probes
installed during the Apollo 15 and 17 missions also seemed to indicate high concentrations of
U and Th, at least locally, and these measurements were used by some to also favour a global
enrichment in refractory elements. Global γ-ray data from the Lunar Prospector mission helped
identify the confines of the trace-element-rich province that has been named the Procellarum
KREEP Terrane (PKT [9]). One of the heat-flow measurements was made in this province, with
the other just exterior to it, calling into question the idea that the Moon was enriched globally in
U and Th. Indeed, the asymmetric distribution of Th, as well as its unknown depth dependence,
makes it difficult to determine its concentration in the crust and mantle, as seen by variations in
estimates of the bulk Moon Th abundance [5,9,11,13].
In this paper, we reassess the abundances of refractory elements on the basis of revised
estimates of crustal thickness derived from gravity data obtained from the Gravity Recovery and
Interior Laboratory (GRAIL) mission. We conclude that the Moon is not enriched in refractory
elements in comparison to the Earth. We then use the revised lunar refractory abundances and
element correlations to determine the abundances of volatile elements in the bulk Moon.
2
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80
average crustal thickness
3
70
crustal thickness (km)
40
30
seismic crustal thickness
A16
70
60
A12/14
Apollo zone
50
Apollo zone
40
A16
A15
A12
A14
Apollo zone
30
Apollo zone
1970
1975
1980
1985
1990 1995 2000
publication date
2005
2010
Figure 1. Seismically determined crustal thickness estimates (bottom) and average crustal thickness estimates as a function of
publication date. As analyses of the Apollo seismic data have improved with time, and as orbital missions have improved our
knowledge of the lunar gravitational field and topography, estimates of the Moon’s crustal thickness have decreased by almost
a factor of two over a 40 year time period.
average crustal thickness of the Moon would be even greater than that obtained from the seismic
analyses [17]. After accounting for lateral variations in crustal density, as estimated from orbital
γ-ray spectrometer data, Haines & Metzger [18] showed that the farside crust should be on
average 22 km thicker than the nearside crust. In using the seismic constraint of Goins et al. [16],
they reported an average crustal thickness of 73 km.
The Clementine and Lunar Prospector missions, launched respectively in 1994 and 1998,
obtained the first near-global topographic maps of the Moon [19] and allowed for the construction
of a vastly improved gravitation field, especially over the nearside hemisphere of the Moon [20].
With these data, it was possible to create the first global crustal thickness models by assuming
densities for the crust and mantle, combined with knowledge of the crustal thickness at one locale.
Neumann et al. [21] obtained an average crustal thickness of 61 km by requiring their models
to match the seismically determined 55 km thickness at the Apollo 12 and 14 sites. Using an
improved inversion approach, Wieczorek & Phillips [22] obtained an average thickness of 66 km
with a 60 km constraint at the Apollo 12 and 14 sites. However, if the crust of the Moon were
isostatically compensated by crustal thickness variations, Wieczorek & Phillips [23] showed that
the relation between the lunar geoid and topography implied an average crustal thickness of
43 ± 20 km (revised to 49 ± 16 km [24]). At the low elevations of the Apollo 12 and 14 sites, the
crustal thickness should have been about 28 km. Rather than call into question the accuracy of
the Apollo seismic constraints, these authors instead interpreted this finding to mean that the
lunar crust was stratified in density, with a low-density anorthositic layer overlying a denser
noritic layer.
Two independent re-analyses of the Apollo seismic data were performed following the
Lunar Prospector mission, with both suggesting that the lunar crust was considerably thinner
than previously thought. Khan et al. [25] suggested that the crustal thickness was 45 ± 5 km
in the Apollo zone (which is most representative of the Apollo 12 and 14 landing sites given
the distribution of seismic events used in their analysis). In a subsequent analysis that tested the
hypothesis of having a crust thinner than the Apollo-era analyses, they showed that models with
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With the GRAIL crustal thickness constraints, we calculate the mass fractions of the major lunar
geochemical terranes. We report results that make use of the 30 km seismic constraint [10] with
a crustal porosity of 12% and provide results for three other GRAIL models in table 1. If the
radius of the core is 380 km with a density of 5200 kg m−3 [32], then the mass fractions of the
core, mantle and crust are 1.6%, 94% and 4.4%, respectively. Based on orbital geochemical data,
as well as analyses of lunar samples, the composition of the crust is known to vary among
three major provinces that have distinct geologic evolutions. Following Jolliff et al. [9], these
correspond to the Procellarum KREEP Terrane, the Feldspathic Highlands Terrane (FHT) and
the interior of the South Pole–Aitken (SPA) impact basin. The FHT was originally divided into
an inner anorthositic region and an outer, somewhat more mafic region, but we ignore this
distinction here. The PKT is distinguished by having elevated abundances of the geochemical
component called KREEP (for potassium, rare earth elements and phosphorous), which includes
the important heat-producing elements uranium, thorium and potassium. The FHT is composed
largely of anorthositic materials that are believed to represent the Moon’s primary crust that
formed by flotation in a magma ocean. Finally, the interior of the SPA impact basin possesses
a mafic geochemical anomaly, along with moderate enhancements in incompatible elements. The
composition of this basin’s floor may represent either lower crustal materials, or a differentiated
impact-melt pool [33].
.........................................................
3. Mass fractions of major geochemical terranes
4
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thicker crusts had unrealistic P-wave velocity profiles and that a crustal thickness of 38 ± 3 was
preferred [26]. Lognonné et al. [10] performed an independent reprocessing of the Apollo seismic
data, including the picking of new arrival times, and suggested that the crustal thickness in the
Apollo zone was 30 ± 2.5 km.
Several global crustal thickness models were constructed following the revision of the Apollo
seismic results. Wieczorek et al. [24] obtained an average crustal thickness of 53 km using the
45 km seismic constraint at the Apollo 12 and 14 landing sites. From a joint seismic and global
crustal thickness inversion, Chenet et al. [27] found the average thickness of the crust to lie
between 32 and 45 km, with a best fit value of 40 km. This study also provided seismically
determined crustal thickness estimates at numerous meteoroid impact locations and provided
revised crustal thicknesses at the Apollo 12, 14, 15 and 16 sites of 33 ± 5, 31 ± 7, 35 ± 8 and
38 ± 7 km, respectively. In an inversion by Hikida & Wieczorek [28], they required only that
the minimum crustal thickness be less than 1 km and obtained an average crustal thickness of
43 km, with a thickness of about 40 km near the Apollo 12 and 14 landing sites. Using a similar
approach, but with improved global topography and gravity models from the Kaguya mission,
Ishihara et al. [29] obtained an average crustal thickness of 53 km, with a thickness of about 47 km
near the Apollo 12 and 14 sites.
Almost all of the global crustal thickness models discussed above assumed that the density of
the crust was somewhere between 2800 and 2900 kg m−3 , which corresponds to the grain density
of typical anorthositic rocks. High-resolution gravity data from the GRAIL mission, however,
have shown that the bulk density of the lunar crust is considerably lower, 2550 kg m−3 [30],
corresponding to a porosity of about 12%. As discussed in Wieczorek et al. [30], this porosity
probably extends into the uppermost mantle. Taking into account lateral variations in crustal
density as implied by remote sensing data, Wieczorek et al. [30] varied the mantle density in order
to find a global crustal thickness model that fit the recent seismic constraints and that possessed
minimum crustal thicknesses close to zero. The thinnest crust was found to occur beneath the
Crisium and Moscoviense impact basins, precisely where remote sensing data suggest that the
mantle may have been excavated [31]. When using the seismic crustal thickness constraints of 30
and 38 km at the Apollo 12 and 14 landing sites with 12% porosity in the crust, the average crustal
thickness was found to be 34 and 43 km, respectively. Two additional models were constructed
using a 7% porosity for the entire crust that yielded nearly identical results, but with somewhat
different mantle densities.
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Table 1. Crustal thickness models used in assessing crustal composition.
model 2
model 3
model 4
surface area (%)
77.53
77.53
77.53
77.53
..........................................................................................................................................................................................................
mean elevation (km)
0.58
0.58
0.58
0.58
volume (m3 )
1.06 × 1018
1.09 × 1018
1.31 × 1018
1.32 × 1018
crustal thickness (km)
36.66
37.81
45.87
45.99
mass (kg)
2.71 × 10
2.95 × 10
3.37 × 10
3.57 × 1021
3.75
4.08
4.66
4.94
2566
2711
2566
2712
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
21
21
21
..........................................................................................................................................................................................................
silicate mass fraction (%)
..........................................................................................................................................................................................................
−3
average density (kg m )
..........................................................................................................................................................................................................
PKT
..........................................................................................................................................................................................................
surface area (%)
16.56
mean elevation (km)
−1.49
16.56
16.56
16.56
..........................................................................................................................................................................................................
−1.49
−1.49
−1.49
..........................................................................................................................................................................................................
volume (m )
1.63 × 10
crustal thickness (km)
26.36
mass (kg)
4.22 × 10
4.57 × 10
5.53 × 10
5.80 × 1020
silicate mass fraction (%)
0.58
0.63
0.77
0.80
−3
2594
2742
2594
2742
3
17
1.67 × 10
17
2.13 × 10
17
2.12 × 1017
..........................................................................................................................................................................................................
27.01
34.75
34.46
..........................................................................................................................................................................................................
20
20
20
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
average density (kg m )
..........................................................................................................................................................................................................
SPA
..........................................................................................................................................................................................................
surface area (%)
5.91
mean elevation (km)
−3.37
5.91
5.91
5.91
..........................................................................................................................................................................................................
−3.37
−3.37
−3.37
..........................................................................................................................................................................................................
volume (m )
4.51 × 10
4.54 × 10
6.26 × 10
6.08 × 1016
crustal thickness (km)
20.48
20.59
28.52
27.70
mass (kg)
1.18 × 10
1.26 × 10
1.64 × 10
1.68 × 1020
silicate mass fraction (%)
0.16
0.17
0.23
0.23
−3
2618
2766
2619
2767
3
16
16
16
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
20
20
20
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
average density (kg m )
..........................................................................................................................................................................................................
The areas spanned by the three major crustal provinces, shown in figure 2, are estimated using
thorium and iron abundances obtained from the Lunar Prospector γ-ray spectrometer. The PKT
is here defined by those regions having more than 4 ppm Th as determined from the 2◦ lowaltitude Th map [34]. As some regions within this province have been covered by thin layers of
low thorium basalts, we also include FeO-rich regions (more than 13 wt%) where the thorium
abundances are greater than 2.75 ppm. The interior of the SPA basin is defined by those regions
having more than 6.5 wt% FeO using the 5◦ data [35]. As shown in figure 2 and table 1, the FHT,
PKT and SPA basin comprise, respectively, 77.5%, 16.6% and 5.9% of the Moon’s surface area. For
GRAIL model 1 (thin crust, high porosity), the average crustal thicknesses of these three provinces
are 36.7, 27.9 and 20.5 km, which correspond to bulk Moon silicate mass fractions of 3.75%, 0.58%
and 0.16%, respectively.
4. Al2 O3 concentration in the bulk silicate Moon
The most straightforward way to assess the bulk abundance of refractory elements in the Moon
is to determine the contributions to the bulk lunar Al2 O3 abundance from the crust and mantle.
We make the common cosmochemical assumption that refractory elements are not fractionated
.........................................................
highlands
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model 1
5
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(a)
nearside
Feldsp
eldspathic
thic
Highlands
Ter
errane
ane
South
outh
Pol
ole-Ait
itken
en
Ter
errane
ane
Figure 2. Distribution of the three prominent lunar terranes: the FHT (white), the PKT (yellow) and the SPA Terrane (blue). This
image is shown in two Lambert azimuthal equal-area projections centred over the nearside (a) and farside (b) hemispheres.
The distribution of mare basalts is shown by thin black lines.
relative to each other. Taylor [36] discusses this concept and shows that it is reasonable in the case
of Mars. Determining the bulk lunar Al2 O3 requires knowing the masses of crust and mantle,
which we report above (table 1), and the mean concentrations in the crust and mantle, which we
evaluate in this section.
(a) Crust
The total abundance of Al2 O3 in the crust depends on the relative sizes of the three major crustal
terranes shown in figure 2, and on their Al2 O3 contents. We divide each of the three terranes
into three layers, though not all are necessarily present, and in some cases might be mixed. We list
thorium concentrations, a refractory trace element, in each crustal column in figure 3 for reference,
but do not try to compute bulk Th concentrations because the depth dependence of this element
in each terrane is not easily constrained.
(i) Upper layer
A somewhat-mafic layer is exposed on the surface virtually everywhere in the highlands. Lunar
meteorites [37] and orbital spectral observations [31] indicate that the uppermost crust is less
feldspathic than the underlying anorthosite layer. Meteorite regolith breccias from the lunar
highlands (defined as those with Al2 O3 more than 25 wt%) average 28 ± 1 wt% (compilation
of published bulk meteorite analyses taken from [38]). Global spectral reflectance data for the
highlands are in the range 4–6 wt% FeO, which corresponds to 24–28 wt% Al2 O3 using the linear
inverse correlation between FeO and Al2 O3 in lunar rocks [39]. We adopt an Al2 O3 content of
approximately 28 wt% for the upper layer. A somewhat lower value might be suitable, but this
upper value maximizes Al2 O3 in the bulk crust to test the idea of lunar refractory enrichment.
Its thickness over the FHT is approximately 10 km [31], but the revised, lower crustal thickness
suggests a thinner value, which we assume to be approximately 5 km. We use a thickness of
between 5 and 10 km in our mass-balance calculations. Our analysis of published [40,41] remote
FeO measurements suggests that it is much thinner, and probably absent, in the PKT and SPA
regions. In our mass-balance calculations of Al2 O3 , we allow for up to a 5 km thickness in the
PKT and SPA terranes.
6
.........................................................
eldspathic
thic
Feldsp
Highlands
Ter
errane
ane
farside
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Procellarum
ellarum
KREEP
Ter
errane
ane
(b)
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highlands
0–5 km
SPA
21.7–
31.7 km
0–5 km
17.9–
27.9 km
0–5 km
crustal thickness (km)
area of moon (%)
volume of moon (%)
silicate mass fraction (%)
36.7
77.5
4.80
3.75
27.9
16.6
0.74
0.58
0–5 km
upper layer
0–5 km
ferroan anorthosite
10.5–
20.5 km
LKFM
20.5
5.9
0.21
0.16
Figure 3. Key parameters for the three crustal terranes. Crustal thicknesses are derived from the GRAIL crustal thickness model
1. Thicknesses and compositions of layers within each stratigraphic column are based on assessments of remote sensing and
sample data. Parameters for different crustal thicknesses are given in table 1.
(ii) Anorthosite layer
The presence of almost pure anorthosite in basin rings [31,42] suggests that the upper somewhatmafic layer is underlain by a substantial thickness of anorthosite. Spectral data indicate that
the anorthosite contains less than 2 wt% FeO, corresponding to more than 31 wt% Al2 O3 , and
contains no detectable mafic silicates. We assume an Al2 O3 content of 34 wt% to account for the
presence of at least some olivine and pyroxene in the anorthosites as shown by Apollo samples
[43]; for comparison, pure anorthite contains approximately 36 wt%. Anorthosite occupies most
of the crustal column in the feldspathic highlands (figure 3), with its thickness depending on
the thickness of layers above and below it, and on the total crustal thickness. Sample studies
suggest that anorthosite is rare, but not absent in the PKT (anorthosites were collected at the
Apollo 15 site), so we allow for an anorthosite layer up to 5 km thick in that terrane. Remote
sensing observations do not unequivocally identify anorthosite in the SPA except in its outermost
regions, but we allow for a layer of 5 km thick, recognizing that it could be essentially absent.
(iii) Mafic lower layer
It is plausible that a more mafic layer resides below the anorthosite layer, as first argued by
Ryder & Wood [44] on the basis of the compositions of mafic impact melt breccias of the type
‘LKFM basalt’ (low-K Fra Mauro [45]) collected at the Apollo 15 and 17 landing sites. Using LKFM
impact melt rocks as a guide, we estimate that the layer contains approximately 19 wt% Al2 O3 .
Based on the abundance of LKFM at the landing sites in the PKT, including in ejecta from small
and large craters, it seems likely that most of the crustal column in the PKT is composed of rock
with such basaltic compositions [9]. We assume that mafic rock dominates the crust in the SPA
basin, too, though with different abundances of incompatible elements.
We wish to calculate the total contribution the crust makes to the bulk lunar composition of
refractory elements. The four models shown in table 1 (corresponding to the four GRAIL crustal
thickness models [30]), combined with the three layers possible in each terrane (figure 3 and above
discussion) result in a large combination of possible Al2 O3 concentrations, so we present the
lowest and highest values. The lowest (1.13 wt%) is obtained for the case of model 1 (thinnest crust
.........................................................
PKT
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5–10 km
Al2O3 (wt%) Th (ppm)
28
0.5
34
0.05
19
5
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3.5
8
maximum Al2O3 in mantle source
2.5
10 wt% Al2O3 in mare basalt
10% partial melting
2.0
9 wt% Al2O3 in mare basalt
5% partial melting
1.5
minimum Al2O3 in mantle source
1.0
0.2
0.3
0.4
0.5
pyroxene/(pyroxene + olivine)
0.6
0.7
Figure 4. Estimate of Al2 O3 in the mantle mare basalt source regions. Curves are shown for mare basalt magmas with 9 and
10 wt% Al2 O3 and for 5% and 10% partial melting. The horizontal axis shows the ratio of pyroxene to total mafic silicates. The
more pyroxene in the source, the higher the Al2 O3 because it partitions into pyroxene in preference to olivine. Using the lowest
and highest Al2 O3 values on the lines, we infer that the mantle contributes between 1.3 and 3.1 wt% to the bulk lunar Al2 O3
content.
with the lowest mass fraction of total lunar silicate), with no upper layer or anorthosite in the PKT
and SPA terrane, and a 10 km upper layer and 5-km lower layer in the feldspathic highlands. The
highest crustal Al2 O3 contribution (1.85 wt%) is for model 4 (thickest crust with the highest silicate
mass fraction), with no lower mafic layer and a 5-km upper layer in the highlands, and 5-km
thick layer of anorthosite in the SPA terrane and PKT. This range in crustal Al2 O3 contribution is
drastically different from other previous estimates that used a thicker crust (hence higher crustal
mass fractions), which ranged as high as 3.3 wt% [11]. Considering that the Earth has a bulk Al2 O3
of 4.0 wt% (e.g. average of estimates in [46–48]), having more than 3 wt% Al2 O3 in the crust would
make it unavoidable that the bulk Moon is enriched in refractory elements.
(b) Mantle
We determined the Al2 O3 concentration in the mantle from the Al2 O3 contents in mare basaltic
lavas, the Al2 O3 contents in residual pyroxene obtained from multiple-saturation partial melting
experiments ([49] and references cited therein) and estimates of the olivine/pyroxene ratio in
mare basalt mantle source regions. Residual pyroxene in the multiple-saturation experiments
average 3 ± 1 wt% Al2 O3 (1-σ ) with no apparent trend with pressure. From the range of Al2 O3 in
mare basalts (8–10 wt%), assuming 5–10% melting, and that pyroxene constitutes 30–70% of the
residual mafic assemblage (a range consistent with some interpretations of geophysical data [50]),
we estimate that the Al2 O3 in their mantle sources range from 1.3 (for 8 wt% in the magma and
5% partial melting) to 3.1 (for 10 wt% in the magma and 10% partial melting), as summarized in
figure 4. Strictly speaking, these Al2 O3 concentrations apply only to the portion of the mantle that
melted to produce mare basalts. We have essentially no data on the mantle source regions of the
Mg-suite and KREEP basalts, which contain more Al2 O3 than do mare basalts.
An additional uncertainty in the composition of the mantle is that our data are based on
mare basalts, which have been estimated to have formed at depths ranging from 150 to 500 km
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3.0
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Al2O3 of mantle source (wt%)
mare basalt source regions in mantle
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9
4
3
bulk M
o
on Al O
2 3 1.5
times E
a
rth
0.9
range in mantle
2
bulk M
oon Al
2 O3 same
as Earth
1
range in crust
0
0.5
1.0
1.5
2.0
2.5
Al2O3 (wt%) crust contribution
Figure 5. Contributions to bulk Moon abundance of Al2 O3 as a function of Al2 O3 in the mantle and crust. Crustal contributions
from consideration of models 1–4 (table 1) and thicknesses of lithologic layers (figure 3) range from 1.13 to 1.85 wt% Al2 O3 .
Mantle concentrations are from figure 4. Dark lines represent 1.0 and 1.5 times the Earth abundance, with light lines showing
increments of 0.1. The full range of possibilities for crustal and mantle concentrations of Al2 O3 straddle the line for which bulk
lunar Al2 O3 is the same as in the bulk Earth. Strong enrichment relative to the Earth is ruled out, although enrichments of 10–20%
are possible.
[49] based on multiple-saturation experiments. These depths may be minima due to possible
formation at a range of pressures and to fractional crystallization during migration to the lunar
surface [51], so the mare basalt source regions could perhaps be representative deeper parts of the
mantle. Some analyses of the Apollo seismic data are consistent with the mantle not having any
seismic discontinuities [50,52], suggesting a uniform composition. Nevertheless, the composition
of the lower mantle is uncertain, so it is worth examining how a reasonable range in Al2 O3
concentrations would affect the bulk mantle, and hence bulk Moon, composition.
If the core is 380 km in radius, the crust is 34 km thick and the upper mantle extends to 500 km
beneath the base of the crust, the lower mantle would constitute 35% of the total mantle. If
the lower mantle contains 6 wt% Al2 O3 (the bulk Moon abundance if the Moon is enriched in
refractory elements by 50%), the mantle contribution to the bulk Moon would be 2.9–4.1 wt%
for an upper mantle Al2 O3 of 1.3–3.1 wt%, respectively. As can be inferred from figure 5, the
upper part of this range could indicate a refractory-enriched Moon, although not enriched by
50%. In the more likely case that the lower mantle contains 4 wt% Al2 O3 , the amount in the bulk
silicate Earth [46–48], the total mantle contribution changes from 1.3 to 3.1 wt% to 2.2 to 3.4 wt%,
not significantly different from the ranges shown in figure 5. Finally, the lower mantle could be
depleted in Al2 O3 if a large fraction of the initial, deep cumulates from the magma ocean did not
rise to higher levels during mantle overturn. If the lower mantle contains only 1 wt% Al2 O3 , the
mantle contribution would drop to 1.2–2.4 wt%, making a firmer case for no enrichment relative
to the Earth.
(c) Bulk silicate Moon
We combine the crustal and mantle contributions in figure 5. The intersection of the mantle and
crust ranges defines the probable lunar composition. The negative-sloping heavy lines represent
the expected relations for bulk compositions exactly like that of Earth and 1.5 times terrestrial
.........................................................
1.5
1.4
1.3
1.2
1.1
1.0
5
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Al2O3 (wt%) mantle contribution
6
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Volatile elements are informative about planetary formation and the source materials for
the planets. In the case of the Moon, they might hold especially useful information about
the processes involved in formation of the Moon. As noted above, refractory elements have
condensation temperatures greater than 1300 K (at a pressure of 10−4 bar [12]). The condensation
temperature is really the temperature at which half the mass of an element is in the solid and
is properly called the 50% condensation temperature. Moderately volatile elements (e.g. K and
Rb) have 50% condensation temperatures in the range 1230–800 K; highly volatile elements (e.g.
Tl and Cd) have condensation temperatures in the range 750 K (Bi) to 530 K (Tl). (Hg has a 50%
condensation temperature of only 250 K, but far fewer measurements of Hg concentrations are
available than for the other highly volatile elements.)
If our model is correct and the Moon has terrestrial abundances of refractory elements, then
we can determine the absolute abundances of volatile elements. We do this by searching for
correlations between two elements with very similar geochemical behaviour, one refractory
and the other volatile. For example, we can establish the K concentration in the Moon by its
correlation with an incompatible refractory element such as La or Th. If the correlation is strong,
we multiply the ratio of the volatile to the refractory element (e.g. K/La) by the abundance of
the refractory, assuming that the lunar refractory abundance is the same as in the Earth. For
terrestrial values, we have used those determined by McDonough & Sun [47]. For comparisons
between refractory and volatile incompatible elements (these concentrate in magma in preference
to major minerals), it is useful to use as wide a range in element concentrations as possible. Thus,
we use concentrations in mare basalts (mostly low concentrations) and KREEP basalts (high
concentrations) to define the volatile/refractory element ratio most reliably. Although ferroan
anorthosites and Mg-suite rocks contain useful information as well, their cumulate nature masks
the incompatible behaviour of trace elements. In addition, far fewer highland rocks have been
analysed for volatile trace elements.
The alkali elements K, Rb and Cs in mare and KREEP basalts are plotted against La in figure 6.
In spite of the scatter at low concentrations, the linear fits are impressive because of the large range
in concentrations and large number of data points. The correlations and the terrestrial bulk La
value (0.648 ppm) allow us to estimate the bulk concentrations in the Moon (table 2). Uncertainties
are calculated from the quality of the fit using the York method [54]. The 2-σ uncertainties for the
Moon/Earth ratios incorporate the uncertainties in the terrestrial values [47]. Other moderately
volatile elements do not have strong correlations with a refractory element.
The highly volatile elements Tl (figure 7) and Cd are incompatible and correlate with the
incompatible refractory element La (R2 , the square of the correlation coefficient, is 0.9 and
0.6, respectively). Br correlates adequately with Tl. It would be preferable to determine the
Br abundance from a refractory element rather than Tl, which is already an estimate from its
correlation with La, but no such correlations were observed. Bi is an incompatible element, but
it does not correlate well (R2 < 0.5) with any refractory element. We estimate its abundance
by determining its ratio with Th, using the mean abundance in rocks where both have been
.........................................................
5. Absolute abundances of volatile elements
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values; lighter lines represent increments 0.1 times terrestrial abundances. The line representing
no enrichment relative to the Earth roughly bisects the box defined by our estimates of Al2 O3
in the crust and mantle, indicating that it is likely that the Moon is not enriched in Al and by
inference other refractory elements compared with the Earth. Nevertheless, the data allow for
a modest enrichment or depletions of up to ±20% (relative), which we take as an estimate of
the uncertainty of the estimate. When the crust was thought to be thicker and more aluminous
than our analysis indicates (more than 2.5 wt% Al2 O3 ), enrichment relative to the Earth was far
more likely.
The similar aluminium abundances in the Earth and the Moon indicate that processes during
formation of the Moon did not cause discernible enrichment of refractory elements in the Moon.
This is not the case for volatile elements as shown below.
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105
11
concentration (ppm)
K
102
y = 0.24x
R2 = 0.94
10
1
y = 0.009x
R2 = 0.878
Rb
10–1
Cs
10–2
1
10
100
1000
La (ppm)
Figure 6. Correlations of incompatible volatile alkali elements with incompatible refractory La. Knowing that refractory
elements are similar in the Earth and the Moon allows us to determine K, Rb and Cs concentrations in the bulk Moon. The
quality of the linear fits to the data (R2 values) decrease with decreasing concentration, perhaps indicating greater analytical
uncertainty, particularly for Cs. Data from [53].
Table 2. Concentrations of moderately volatile and volatile elements in the bulk silicate Moon.
element
temp. (K)a
conc.
moderately volatile elements
2-σ
methodb
ratioc
K (ppm)
1006
36.9
0.9
S
K/La
0.205
0.01
0.068
Rb (ppm)
800
0.13
0.005
S
Rb/La
0.24
0.02
0.056
Cs (ppb)
799
5.0
0.3
S
Cs/La
0.28
0.03
0.026
Moon/Earth
2-σ
Moon/CI
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
highly volatile elements
..........................................................................................................................................................................................................
Sb (ppb)
0.02
0.01
S
Sb/Tl
0.003
0.003
0.00013
..........................................................................................................................................................................................................
Bi (ppb)
746
0.02
0.006
A
Bi/Th
0.008
0.007
0.00018
Zn (ppm)
726
0.40
0.15
A
Zn/Sc
0.007
0.004
0.0013
Cd (ppb)
652
0.31
0.14
S
Cd/La
0.009
0.004
0.00055
Br (ppb)
546
0.79
0.05
S
Br/Tl
0.019
0.01
0.00274
Tl (ppb)
532
0.030
0.004
S
Tl/La
0.010
0.005
0.00025
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
a 50% condensation temperature at 1 × 10−4 bars [12].
b S indicates use of slope of correlation line. A indicates average of ratio of abundances to another element.
c Ratio indicates elements used to determine volatile element abundance.
measured. Zn is a weakly incompatible to compatible element with behaviour similar to Sc (in
the absence of sulfide phases), so we estimate its abundance by taking the ratio of the average
concentrations of Zn and Sc.
Lunar abundances of selected refractory and volatile elements are shown in figure 8 and
table 2. We normalize them to terrestrial values as this is most informative about lunar formation
by a giant impact. Though normalization to CI chondrites could be useful, planets do not
necessarily have CI abundances of volatile elements, making it difficult to track changes in
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y = 67.60x
R2 = 0.95
104
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6
12
y = 0.057x
R2 = 0.87
Tl (ppb)
3
KREEP basalts
2
1
mare basalts
0
0
10
20
30
40
La (ppm)
50
60
70
80
Figure 7. Incompatible highly volatile element Tl plotted against incompatible refractory element La in lunar mare and KREEP
basalts. La from [53]; Tl from [55–58]. R2 values are high, but driven by the highest values on the plot.
1
Moon/Earth
0.1
0.01
0.001
lunar abundances normalized
to Earth abundances
0.0001
Sr
Ba Be Eu Mg
K
Rb Cs Sb
Bi
Zn Cd Br
Tl
Figure 8. Abundances of volatile elements in the Moon, normalized to terrestrial abundances. Refractory elements shown for
reference and assumed to be the same as in the Earth. Lunar depletions compared with the Earth are 0.24 for moderately volatile
K, Rb and Cs, and 0.01 for highly volatile elements (values are geometric means). Uncertainties for refractory and moderately
volatile elements are within the symbols.
their abundances during formation of the Moon. As has been known since early analysis of
Apollo samples, lunar abundances of volatiles are depleted relative to the Earth abundances. The
moderately volatile elements are curiously inversely related to their condensation temperatures,
but not strongly so (figure 8). Their mean abundance is 0.24 times the Earth. The alkalis are
progressively depleted in order of condensation temperature when normalized to CI chondrites
(table 2).
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5
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On the basis of revised crustal thicknesses from GRAIL, our model calculations indicate that
refractory elements are not enriched in the Moon compared with their abundances in the bulk
silicate Earth. The similarity in abundances of refractory elements implies that processes in
the protolunar disc did not lead to relative fractionation of refractory elements or to a general
enrichment during condensation and partial crystallization of the disc before the Moon accreted
from it. Chemical models of disc processes [59–61] need to account for the approximate similarity
with terrestrial abundances of refractory elements.
The volatile elements might allow us to constrain processes in the protolunar disc after a giant
impact, ignoring the possibility that volatiles could have been lost from the magma ocean after
the Moon formed. The moderately volatile elements have lunar concentrations only about 25%
(table 2 and figure 8) of their concentrations in the Earth. Assuming that most of the material
making up the Moon derived from the Earth, then 75% of the initial concentrations of moderately
volatile elements were lost during the Moon-forming impact and subsequent evolution of the
protolunar disc. About 99% of the highly volatile elements were lost during lunar formation,
with little fractionation among them. Thus, there appears to be a rough correlation of percentage
loss with volatility (moderately volatile versus highly volatile elements), but losses within each
group were accompanied by only limited fractionation of one element from another. Preliminary
results [62,63] address volatile loss from the protolunar disc through a combination of mixing
with the proto-Earth (similar to that proposed by Pahlevan & Stevenson [59]) and subsequent
hydrodynamic escape of the volatile-rich vapour portion of the disc. The concentrations of
volatiles in the Moon would depend on their solubility in the magma portion of the protolunar
disc (assuming minimal loss from the magma ocean).
The discovery of H components (H2 , OH and H2 O) in the lunar interior [64] adds another
dimension to the picture of lunar volatiles. H components are even more volatile than the
highly volatile elements, yet may be present in higher abundance than the volatile elements.
For example, estimates for the concentrations of H components (expressed as equivalent H2 O)
in the source regions of pyroclastic glasses [65] and mare basalts [66] are approximately 100 ppm.
These are similar to terrestrial Mid-Ocean Ridge basalt (MORB) source regions, 80–180 ppm [67],
and about 20% of minimum bulk Earth concentration [68]. (Enriched MORB mantle sources have
higher H2 O concentrations, 200–950 ppm [67].)
Volatile elements and H2 O are not well correlated. In spite of the overall strong depletion of
volatile elements in the Moon, pyroclastic glasses have concentrations of highly volatile elements
that are close to those in MORBs. By contrast, low-Ti mare basalts, many of which appear to have
similar H2 O concentrations as do the pyroclastic glasses, have only 1% of terrestrial abundances
of highly volatile elements. It appears that H components and highly volatile elements are
decoupled. Whether the lack of correlation between H components and volatile elements is due
to the nature of the impactor (e.g. ice-rich), early additions to the Moon, or processes inside the
complicated and evolving protolunar disc is not yet known. An additional complication is that H
concentrations do not appear to be uniform inside the Moon, as shown by the significantly lower
.........................................................
6. Implications for lunar origin
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The highly volatile elements are severely depleted [55]. The geometric mean of their Earthnormalized abundances is 0.01. Although the uncertainties are high, there is a hint of an inverse
relation with condensation order, as observed for the alkali elements. Additional analyses of lunar
samples are needed to establish whether this weak trend is statistically significant or not. It is
possible that the highly volatile elements are uniformly depleted relative to the Earth. Wolf &
Anders [55] and Taylor et al. [11] drew attention to the uniformity in CI-normalized concentrations
in a large suite of low-Ti mare basalts, but did not determine abundances by element correlations.
Our results indicate that CI-normalized abundances among the highly volatile elements differ
by close to a factor of 10. By contrast, Earth-normalized abundances are similar to each other,
although present at only 1% of terrestrial abundances.
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Brad Jolliff and an anonymous reviewer were helpful, constructive and insightful.
Funding statement. Support was provided by NASA grant no. NNX12L10G (G.J.T.). M.A.W. was supported by a
grant from the French space agency, CNES.
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