GEOCHEMISTRY
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exclusive licensee American Association for
the Advancement of Science. Distributed
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NonCommercial License 4.0 (CC BY-NC).
10.1126/sciadv.1500380
The chlorine isotope fingerprint of the lunar
magma ocean
Jeremy W. Boyce,
1,2
* Allan H. Treiman,
3
Yunbin Guan,
1
Chi Ma,
1
John M. Eiler,
1
Juliane Gross,
4
†
James P. Greenwood,
5
Edward M. Stolper
1
The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an
observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free
(
“
dry
”
) Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find
little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because
37
Cl/
35
Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-
outgassing hypothesis. Instead,
37
Cl/
35
Cl correlates positively with Cl abundance in apatite, as well as with whole-
rock Th abundances and La/Lu ratios, suggesting that the high
37
Cl/
35
Cl in lunar basalts is inherited from urKREEP,
the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar
basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the
Moon
’
s history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an im-
portant stage in the formation of terrestrial planets.
INTRODUCTION
The Earth and Moon are nonidentical twins, born in the same giant
impact (
1
). Chemically dissimilar from th
e volatile-rich rocks of Earth,
rocks from the Moon are depleted in the volatile elements. This
difference expresses across the entire periodic table (
2
), but more no-
ticeably in the absence in lunar rocks of common terrestrial hydrous
minerals such as biotite and amphibole. In contrast to those differences
in elemental abundances, rocks from the Moon and Earth have very
similar isotopic ratios for many of the most abundant elements (
3
–
10
),
which strongly ties the two bodies together with respect to their genesis.
One of the most significant exceptions to this is lunar chlorine. It re-
cently has been shown that lunar basalts, glasses, and apatite (a common
accessory mineral, Ca
5
[PO
4
]
3
[F,Cl,OH], which can contain abundant Cl)
have
d
37
Cl values [reported as
d
37
Cl relative to standard mean ocean
chlorine (SMOC)] that range from typical terrestrial values of 0 ± 1
‰
to +81
‰
(
11
,
12
), far greater than observed in any other solid materials
in our Solar System (
11
). These elevated
d
37
Cl values were interpreted as
having been produced by isotopic fr
actionation during extensive out-
gassing of Cl, probably from eruptions of basalt that were sufficiently
poor in H such that HCl was not
an important gas species (
11
). This
“
lava-outgassing
”
scenario is consistent with
observations that bulk lu-
nar rocks are generally poor in H relativ
e to their terrestrial counterparts
(
13
–
15
), but is at odds with data suggesting that many lunar magmas
contained significant H before eruption (
16
–
19
), and still other data
suggesting that some lunar magmas apparently had H abundances
and D/H ratios similar to their terrestrial counterparts (
20
,
21
).
It has been difficult to reconcile interpretations based on H abun-
dances and isotopic ratios with those based on Cl abundances and
isotopic ratios, in part because there have been limited data on H and
Cl abundances and isotopic composi
tions for the same materials. Here,
we present measurements of H and Cl abundances as well as
37
Cl/
35
Cl
and D/H ratios in apatite crystals from a range of lunar basalts, where
measurements are from the same th
in sections and, when possible, in
the same apatite crystals (for example, Fig. 1). We use these data to place
constraints on the origin of the unusual chlorine isotopic composition
of the Moon and to place that chlorine within the larger framework of
lunar volatiles.
RESULTS
Chlorine isotope ratios in the samples analyzed here range from
d
37
Cl =
−
4
‰
in Miller Range (MIL) 05035 to +18
‰
in 12039 (Fig. 2 and
Table 1) and are consistent with the few previous analyses of
d
37
Cl
in apatite from mare basalts, with the exception of MIL 05035: The
d
37
Cl for this sample (
−
4±2
‰
) is among the lowest measured in any
natural material from anywhere in the Solar System. Lunar apatites
also display a wide range of D/H (Fig. 3), with
d
D values [reported as
d
D
relative to Vienna standard mean ocean water (VSMOW)] of multispot
grain averages ranging from
−
150 to +970
‰
. Of the basalts, 10044 has
the greatest range in
d
37
Cl (+2 to +15
‰
), as well as in
d
D, from +540 to
+950
‰
(
n
= 6), where the latter are consistent with other analyses of
10044 (Fig. 3).
Chlorine abundances in apatites from low-Ti basalts (Fig. 2A)
range from 480 to 16,000 ppm. Chlorine abundances in seven of eight
apatites in high-Ti basalt 10044
are between 220 and 480 ppm, with one
outlier at 4000 ppm Cl. Apatite grains in low-Ti mare basalts have
hydrogen concentrations (always reported here as the oxide H
2
Oequiv-
alent) from below 100 ppm H
2
O in sample 12040 to 2920 ppm H
2
Oin
12039, with data from the literature extending this to more than 5000 ppm
(Fig. 2B). Apatites from high-Ti mare basalts (10044 and 75055) vary
from 420 to 1430 ppm H
2
O.
Lunar rocks are remarkable for their high
37
Cl/
35
Cl values, which im-
ply a distinctive fractionation event in lunar history that is not preserved
in rocks of other planetary bodies. The lava-outgassing hypothesis of
1
Division of Geological and Planetary Sciences, Caltech, 1200 East California Boulevard,
Pasadena, CA 91125, USA.
2
Department of Earth, Planetary, and Space Sciences, University
of California, Los Angeles, Los Angeles, CA 90095
–
1567, USA.
3
Lunar and Planetary Institute,
3600 Bay Area Boulevard, Houston, TX 77058, USA.
4
Department of Earth and Planetary
Sciences, American Museum of Natural History, Central Park West at 79th Street, New York,
NY 10024, USA.
5
Department of Earth and Environmental Sciences, Wesleyan University,
Middletown, CT 06459, USA.
*Corresponding author. E-mail: jwboyce@alum.mit.edu
†
Present address: Department of Earth and Planetary Sciences, Rutgers University, 610
Taylor Road, Piscataway, NJ 08854, USA.
RESEARCH ARTICLE
Boyce
et al
. Sci. Adv. 2015;1:e1500380 25 September 2015
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Sharp
et al
.(
11
) requires that the basalt magmas contain minimal hydro-
gen (otherwise, degassing would be predicted to be of HCl gas, which
would cause little fractionation of
37
Cl from
35
Cl). Independent of
the mechanism of fractionating chlorine, the timing of the chlorine
loss resulting in the fractionation must also be constrained: The lava-
outgassing model suggests that this loss and fraction occur circa
eruption.
DISCUSSION
The data reported here allow three tests of the lava-outgassing hy-
pothesis, two based on abundances and one on isotope ratios:
(i) Elevated
37
Cl/
35
Cl ratios should be inversely proportional to
Cl abundance.
(ii) Elevated
37
Cl/
35
Cl ratios should not be observed in samples
with abundant H.
(iii) Elevated
37
Cl/
35
Cl ratios should be observed only in samples
that have enrichments in D/H.
Although the quantitative significance of measured abundances
of H and Cl in apatite has been called into question (
22
), H and Cl
concentrations
—
or even simply their presence or absence
—
are still
valuable constraints on models used to explain observed H and Cl
isotopes in lunar basalts. This is especially true of Cl, which does
not experience the same order of magnitude of increase in apatite
due to fractionation of fluorine.
On the basis of the lava-outgassing hypothesis, one would predict
that high
d
37
Cl values should be found preferentially in samples with
low abundances of Cl, because elevated
d
37
Cl is thought to develop
only through extensive Cl loss (
11
). We find, to the contrary, that
apatites with high
d
37
Cl are among the most Cl-rich, with Cl abun-
dances positively correlated with
d
37
Cl values for the same samples
(Fig. 2A). If the mechanism for increasing
d
37
Cl involves preferential
loss of
35
Cl (which seems likely, relative to
37
Cl) and is occurring syn-
and post-eruption, there must have been a subsequent process that
concentrated Cl (with elevated
d
37
Cl) relative to F and OH. This im-
plies that the fractionation of Cl did not occur in the degassing ac-
companying eruption and cooling at the surface, because there is no
subsequent event after eruption that can preferentially enrich high-
d
37
ClsamplesintheirClabundancemorethanthatsameprocessen-
riches low-
d
37
Cl samples. This is especially problematic given that the
samples that are more fractionated (more elevated
d
37
Cl) likely have
A
B
Fig. 1. Photomicrographs of two basalts.
Photomicrographs of two
basalts analyzed in this study, with yellow crosses marking the locations
of analyses. (
A
) Subhedral apatite grains (outlined in yellow for clarity)
forming a cluster in Apollo 12039,42. (
B
) Euhedral to subhedral apatite
grains in Apollo 12040,211 that show skeletal growth with hollow centers.
H
2
O (ppm)
0
1000
2000
3000
4000
5000
δ
37
Cl (per mil)
+10
+5
+0
+15 +20 +25 +30
Cl (ppm)
10
2
10
3
10
4
10
5
KREEP
10044
75055
10058
Hi-Ti
Low-Ti basalts
12039
12040
MIL 05035
15555
NWA 2977
12064
NWA 4472
A
B
72275
Fig. 2. Plots of Cl and H
2
O versus
d
37
Cl.
(
A
and
B
) Plots of Cl (A) and
H
2
O (B) concentrations versus
d
37
Cl values in apatite from the Moon. Each
data point represents a single apatite grain in a rock, including data from
the literature plotted as smaller symbols (
11
,
12
,
33
); magenta symbols are
high-Ti basalts; blue symbols are low-Ti basalts; gray symbols are KREEP
basalts. There are no statistically significant correlations observed for any
of the sample types studied here. Elevated
d
37
Cl values are associated
with both high H
2
O and high Cl concentrations: The former is inconsistent
with the idea that elevated
d
37
Cl values are only associated with anhy-
drous conditions, and the latter is inconsistent with a simple, single-stage
fractionation model.
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experienced greater degrees of fractional loss to achieve higher
37
Cl/
35
Cl
values
—
but they must finish with higher Cl abundances.
A second prediction of the lava-outgassing model is that magmas
with high
d
37
Cl should be nearly anhydrous (
11
), and thus, the apatite
crystals would also be anhydrous. The negative relationship between
H abundance and
d
37
Cl in apatites from some lunar basalts (for exam-
ple, NWA 2977) could be interpreted as support for this prediction
(Fig. 2B). However, nearly all lunar apatites have high
d
37
Cl relative
to the range of terrestrial samples (0 ± 1
‰
), and many of them are rich
in H, some exceeding 5000 ppm H
2
O. Thus, although it is possible that
loss of transition metal chlorides (
11
) contributes to elevated
d
37
Cl, it
does not seem possible that this signature is entirely generated by de-
gassing from H-free melts. Whether or not this scenario is possible,
there is experimental evidence suggesting that it is not necessary: evap-
oration of HCl at standard temperat
ure and pressure yields an increase
in +9
‰
in
d
37
Cl in the residue after 90% evaporation (
23
). Therefore,
it is not impossible to achieve fractionations of the same order of mag-
nitude as those observed in mare basalts from phase transformations
of HCl.
Third, we note that if the H-poor magmas required for generation
of
d
37
Cl anomalies were themselves generated by extensive degassing
of H
2
(favoring the lighter isotope), on
e should expect that mare basalts
with elevated
d
37
Cl would also have relatively elevated
d
D. This is not
only because
—
in the context of the lava-outgassing hypothesis
—
extensive
loss of hydrogen is required to gene
rate conditions under which metal
chloride degassing domi
nates over HCl degassing, but also (more gen-
erally) because magmas that have outgassed Cl should also have had the
opportunity to outgas H. If the buil
ding blocks of Earth and its Moon
were H-bearing, and the lava-outgassing hypothesis requires the mag-
ma to be H-free when Cl loss is initiated syn-eruption, then hydrogen
must have been lost before Cl, resulting in an elevated D/H ratio in the
residue. This expectation is not met: I
nsteadofthepositivecorrelation
one might expect, lunar basalts actua
lly define two separate populations
with no apparent relationship between
d
Dand
d
37
Cl: Some basalts with
very high
d
37
Cl (for example, samples 12040 and NWA 2977, which
range up to +20
‰
)havelow
d
D(
≤
0
‰
)(Fig.4).
It should also be noted that neither the relationship between
d
D
and H abundance (Fig. 3) nor the presence of apatite with strongly
Table 1. New data generated for this study.
Sample
Section Grain H
2
O (ppm) H
2
O2
s
Cl (ppm) Cl 2
sd
37
Cl
d
37
Cl 2
sd
D
d
D2
s
Category
10044
12
10
1105
29
365
104
+9
3
+933
31
High Ti basalt
10044
12
1a
728
27
292
11
+6
3
+954
27
High Ti basalt
10044
12
1b
————
+6
4
——
High Ti basalt
10044
12
1c
1220
30
300
11
+2
4
+781
23
High Ti basalt
10044
644
2
877
26
250
10
+11
3
+536
57
High Ti basalt
10044
644
8
421
22
3976
122
+15
3
——
High Ti basalt
10044
644
4a
826
25
219
10
+12
3
+606
33
High Ti basalt
10044
644
4b
1155
29
479
16
+12
3
+702
28
High Ti basalt
75055
55
1
604
23
398
14
+8
3
+621
35
High Ti basalt
75055
55
2
1225
35
485
16
+6
3
+794
26
High Ti basalt
75055
55
3
1241
30
334
12
+14
3
+968
27
High Ti basalt
75055
55
4
1430
34
410
14
+6
3
+794
26
High Ti basalt
75055
55
101
————
+5
3
——
High Ti basalt
12039
42
4
1996
43
928
29
+18
3
+720
30
Low Ti basalt
12039
42
6
2379
47
477
16
+17
3
+830
31
Low Ti basalt
12039
42
10
————
+16
3
——
Low Ti basalt
12039
42
11
————
+17
3
——
Low Ti basalt
12039
42
17a
2784
95
1157
36
+17
3
+729
28
Low Ti basalt
12039
42
17b
2916
57
665
21
+16
3
+698
28
Low Ti basalt
12040
211
1
105
21
15884
489
+17
3
+9
163
Low Ti basalt
12040
211
4
3
20
2689
83
+13
3
+14
84
Low Ti basalt
12040
211
5
16
20
3388
104
+14
4
−
150
26
Low Ti basalt
MIL 05035
—
1
————
−
42
——
Low Ti basalt
RESEARCH ARTICLE
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negative
d
D can be explained by extensive losses of H
2
from a chondritic
reservoir (
24
,
25
). Given the presence of implanted solar wind, spallo-
genic D, and H-rich chondritic mate
rials gardened into the upper lunar
regolith through repeated impacts, it is plausible that the regolith serves
as an exchangeable reservoir of H for adjacent magmatic dikes, sills, and
lava flows that can modify the D/H ratio of magmas (
26
,
27
). This form
of
“
crustal contamination
”
[as previously suggested by Hui
et al
.(
28
)to
explain major and trace element var
iability in Apollo 14 basalts] may
exert significant leverage on D/H ratios, especially in magmas with low
intrinsic H abundances. Apatite grains in individual basalt samples
show wide ranges of
d
D values, with individual samples having ranges
as large as 800
‰
(10044), which is also consiste
nt with partial late-stage
reequilibration with extrinsic hydrogen.
We conclude that none of the three tests of the lava-outgassing hy-
pothesis are consistent with this new
data set, and therefore, we develop
other explanations, focusing on processes that are implied by correla-
tions between d37Cl and other geochemical properties: The
d
37
Cl values
of apatite from lunar basalts are correlated with Cl content, which is
known to be enriched in KREEP. KREEP is the name given to a chem-
ical component
—
possibly derived from the urKREEP, the last vestige of
the lunar magma ocean
—
rich in potassium (K), the rare earths (REE),
and phosphorus (P) and other elemen
ts that are incompatible during
the crystallization of magmas (
29
–
31
). Incompatible elements are those
that are preferentially excluded f
rom crystals growing from a melt and
therefore concentrated in that residual melt
—
in this case, the lunar
magma ocean. The lunar magma ocean first crystallizes from below,
then from both above and below, resulting in a
“
sandwich horizon
”
of melt with decreasing volume and increasing abundances of incom-
patible elements (
31
). KREEP (and the proposed urKREEP source) have
long been implicated in the trace element variations of lunar basalts
[for example, (
32
)], with different units having different contributions
of KREEP (sometimes referred to as
“
KREEPiness
”
). More KREEP-rich
(or
“
KREEPy
”
) samples are observed to have higher abundances of trace
elements such as Th, Cl, as well as the major and trace elements that
make up the acronym KREEP. Model compositions of the urKREEP
(
29
) also indicate that the urKREEP may h
ave elevated La/Lu ratios, rel-
ative to the mare basalt source. Thus, trace element abundances and ra-
tios can be used to determine the magnitude of KREEP contamination/
addition. The
d
37
Cl values of mare basalt apatites are also positively
correlated with abundances and ratios in their host rocks of the ele-
ments that are enriched in the lunar KREEP component, notably bulk
Th abundance and La/Lu abundance ratios derived from the literature
(Fig. 5) (
29
,
31
).
The correlations between Cl, Th, La/Lu, and
d
37
Cl suggest that Cl
abundances and
d
37
Cl of mare basalts are controlled
—
at least in part
—
bytheabundanceoftheKREEPcomponent.Thehighestvaluesof
d
37
Cl
found in previous studies for apatite from lunar basalt were from a
KREEPy clast in the highlands breccia 72275 (
11
), whereas the highest
values observed by (
33
) were also in a KREEPy clast. The trends in Fig. 5
are consistent with the existence of a background lunar mantle reservoir
containing Cl with a
d
37
ClvaluesimilartothebulkEarth(~0
‰
)anda
KREEP reservoir such as urKREEP co
ntaining more abundant Cl with a
d
37
Cl of
≥
+30
‰
(
34
). The lunar mantle end member might be recording
an undegassed lunar source, a partially degassed source that did not have
the special conditions required for fractionation of Cl isotopes, or meteor-
itic material that could have a
d
37
Cl as low as
−
4
‰
(
35
). Chlorine degassed
from a mare basalt at any stage of lunar evolution might be expected to
have low
d
37
Cl relative to its source and could return to the Moon as ad-
sorbates on regolith grains (
11
), providing another possible explanation
for the negative
d
37
Cl values observed for lunar apatite in MIL 05035.
H
2
O (ppm)
10
1
10
2
10
3
10
4
δ
D (per mil)
−300
0
300
600
900
1200
Hi-Ti
10044
75055
10058
Low-Ti basalts
12039
12040
MIL 05035
15555
72275
NWA 2977
12064
15058
LAP 04841
15386
Hi-Al
14053
KREEP
NWA 4472
Fig. 3.
d
D versus H
2
O.
d
D versus H
2
O for new data (large symbols) and
data from the literature (small symbols). Hollow symbols are multiple hy-
drogen abundance measurements plotted with bulk
d
D values. Data have
been previously interpreted as representing mixing between a high-
d
D,
high-H
2
O end member (possibly comets), and a low-
d
D, low-H
2
O end mem-
ber (possibly solar wind), or evolution from or contamination with a material
of chondritic composition (
16
,
24
,
25
,
33
), though neither explanation ex-
plains all the data. Note the large number of analyses with
d
D values in
the range for bulk chondrites (+750 to
−
200
‰
)(
58
).
δ
37
Cl (‰ SMOC)
–5
+10
+50
+15
+20
+25
+30
δ
D (‰ VSMOW)
–200
0
+200
+400
+600
+800
+1000
KREEP
10044
75055
72275
Hi-Ti
Low-Ti basalts
12039
12040
MIL 05035
NWA 2977
NWA 4472
Fig. 4. Plotof
d
D versus
d
37
Cl.
Plot of
d
Dversus
d
37
Cl for data generated in
this study and those from previous studies (plotted as smaller symbols).
Hollow symbols indicate in situ
d
37
Cl measurements plotted versus bulk or
mean apatite
d
D from the literature. The presence of high
d
37
Cl apatite with
low
d
Dvaluesisinconsistentwitht
he prediction that elevated
d
37
Cl would
accompany elevated
d
D, because both isotopes should shift toward higher
d
values during degassing. The
lack of correlation between
d
Dand
d
37
Cl sug-
gests that the two isotope systems are at least partially decoupled and that
multiple processes may be at work.
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We interpret the high
d
37
Cl of urKREEP to have formed during
vapor phase loss of Cl from the mag
ma ocean, perhaps by a mechanism
akin to that of Sharp
et al
.(
11
): loss of transition metal chlorides from a
hydrogen-poor melt. However, it is
also possible that the mechanism is
that of HCl loss, which was also put forth by Sharp
et al
.(
23
). Either
mechanism of Cl loss is consistent with recent interpretations of Cl,
Zn, and K isotopes from lunar materi
als, subsets of which have similar
patternsoffractionations(
36
). Chlorine that survives the degassing of
the lunar magma ocean is enriched in
37
Cl relative to
35
Cl, and is then
concentrated into the urKREEP, because Cl is incompatible in all of the
major lunar mantle phases and will choose to stay in the ever-shrinking
remnants of the lunar magma ocean. This results in a reservoir that is en-
riched in Cl and Th, has elevated La/Lu, and still has the fingerprint of the
magma ocean degassing preserved in the elevated isotopic ratio of Cl.
Basalts may acquire this isotopic s
ignature via incorporation of KREEP-
rich materials in the lunar mantle or as the magmas are traversing
—
and
assimilating
—
crust or regolith materials that have a KREEP signature.
Other planets are inferred to have had magma oceans early in their
histories
—
including Earth
—
which shows no evidence of elevated
d
37
Cl
(
37
). It is of course possible that the budget of Cl on Earth is dominated
by late delivery of volatile-rich materials (
38
), but this is inconsistent
with the argument that the terrestri
al planets received their hydrogen
early (
39
). Even if Earth
’
s chlorine is primordial, the pressure, tempera-
ture, and chemistry of the atmosphere above a magma ocean most likely
control
—
at least in part
—
the extent to which loss can change the iso-
tope ratio of the residuum (
40
,
41
).Totheextentthatfractionationdue
to Cl loss is governed by parent body size
—
with smaller bodies predicted
to have less atmosphere and therefore larger potential fractionations
—
the magnitudes (if not the directions) of the
d
37
Cl anomalies for rocks
from Earth, Mars, and the Moon are consistent with the relative size
and expected atmospheric pressures of those bodies during their mag-
ma ocean phases. This model predicts elevated
d
37
Cl for other small
bodies that had magma oceans, as is inferred for 4Vesta (
42
). Thus, chlo-
rine isotopes may provide a unique tool for exploring the existence,
extent, and conditions during the magma ocean phase that is
thought to be a significant stage in the formation of terrestrial plan-
etary bodies.
MATERIALS AND METHODS
Experimental design
The purpose of this study was to investigate the relationship between
chlorine and other elements (as well as their isotope ratios) in lunar
basalts. We set out to test the hypothesis that
d
37
Cl variations in lunar
basalts are due to anhydrous degassing of basalts during eruption (
11
).
Three predictions following from that hypothesis were tested:
(i) Elevated
37
Cl/
35
Cl ratios should be inversely proportional to Cl
abundance.
(ii) Elevated
37
Cl/
35
Cl ratios should not be observed in samples with
abundant H.
(iii) Elevated
37
Cl/
35
Cl ratios should be reserved for samples that
have enrichments in D/H.
ThesehypothesesweretestedbymakingtheH,Cl,D/H,and
37
Cl/
35
Cl
measurements in apatite crystals from the same samples and, when possi-
ble, in the same crystals. After these
predictions were tested, additional
hypotheses were formulated, including the hypothesis that the degassing
of the lunar magma ocean had generated the high
37
Cl/
35
Cl ratios ob-
served in the samples. This hypothesis makes three additional predictions:
(i) Elevated
37
Cl/
35
Cl ratios should be positively correlated to Cl
abundance.
(ii) Elevated
37
Cl/
35
Cl ratios should be positively correlated to Th
abundance, which is elevated in urKREEP.
(iii) Elevated
37
Cl/
35
Cl ratios should be positively correlated to La/Lu
ratio, which is elevated in urKREEP.
These predictions were tested by direct measurement of
d
37
Cl and Cl
abundance (above), and by comparis
on with literature values for bulk
Th content and La/Lu ratio.
Materials
Six thin sections of five samples were a
nalyzed by ion microprobe for this
study: two high-Ti basalts (10044 and 75055) and three low-Ti basalts
(12039, 12040, and MIL 05035).
Th (ppm)
0
0.5
1.0
1.5
δ
37
Cl (‰ SMOC)
+10
+5
0
+15
+20
+25
+30
La/Lu (ppm/ppm)
0
5
10
15
20
25
30
5.5
6.0
6.5
Fig. 5. Bulk Th and La/Lu versus
d
37
Cl.
(
A
and
B
) Measures of KREEP
contribution, bulk rock Th abundance (A), and bulk rock La/Lu (B), as a
function of
d
37
Cl, with trace element data from (
43
,
54
,
59
). Symbols are
the same as those in previous figures. For all basalts,
d
37
Cl is strongly
correlated with Th and La/Lu, suggesting that pure urKREEP would have
d
37
Cl
≥
+30
‰
.
RESEARCH ARTICLE
Boyce
et al
. Sci. Adv. 2015;1:e1500380 25 September 2015
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Apollo sample 10044.
This is a low-K ilmenite basalt that has a Ti
content lower than typical ilmenite basalts from Apollo 11 (
43
). On the
basis of its texture, it was classified as
coarse-grained po
rphyritic basalt
(
44
) and microgabbro [for example, (
45
)], which consists of subhedral
to anhedral pyroxene, set in a matrix o
f plagioclase, anhedral pyroxene,
and ilmenite, with minor apatite, spinel, silica, and symplectitic inter-
growth. For a more detailed descrip
tion of the petrography of 10044, see
the Lunar Sample Compendium (
43
). Here, apatites from two thin
sections were examined, 10044,12
and 10044,644, which appear mostly
as inclusions in pyroxene and oxides. The grain sizes range from <5 to
200
m
m. Most grains are euhedral to subhedral in shape, with some
grains appearing as irregular-shaped aggregates. For this study, five ap-
atite grains were analyzed (31 analys
es total) with ranges in grain size
from 20 to 200
m
m.
Apollo sample 75055.
This is a medium-grained ilmenite basalt
that is more aluminous and less Ti-rich than other Apollo 17 basalts
(
46
). On the basis of its texture, the sample has been described as sub-
ophitic (
44
,
47
) with tabular plagioclase intergrown with subhedral to
anhedral pyroxene and ilmenite l
aths. For further petrographic
information, see the Lunar Sample Compendium (
43
). Here, Apollo
sample 75055,55 was analyzed. Apatite in this sample appears as in-
clusions mainly in pyroxene and oxides, but some can be found in
plagioclase. Apatite ranges in grain size from <5 to 110
m
m and are
mostly subhedral in shape. Some grains show skeletal growth with
hollow centers similar to apatite from Apollo 12040 (Fig. 1). Five ap-
atite grains have been analyzed for this study (13 analyses total), none
smaller than 10
m
m in size.
Apollo sample 12039.
This is a medium-grained pigeonite basalt/
microgabbro (
48
,
49
) and one of the most Fe-rich and Mg-poor Apollo
12 igneous rocks (
50
). Texturally, it ranges from porphyritic (
44
), to
subophitic, to granular (
50
). Petrographically, it is mainly composed
of plagioclase and pyroxene with long needles of ilmenite and tridymite
cutting across the plagioclase and pyroxene. Minor troilite, tranquilli-
tyite, chromite, metal, apatite, and sy
mplectitic intergrowths are present
(
43
). Here, apatite in thin section 12039,
42 was analyzed. Apatite is sub-
hedral to anhedral, sometimes presen
t as long needles, with grain sizes
ranging from <10 to 400
m
m long. Some grains occur as clusters, and
some grains show skeletal growth (Fig. 1). Seventeen points in five dif-
ferent grains were analyzed.
Apollo sample 12040.
This is a coarse-grained olivine basalt
with a high proportion of mafic minerals. Texturally, it is equigran-
ular with an average grain size of 1 mm (
51
). It is mainly composed of
olivine and pyroxene with minor plagi
oclase, ilmenite, chromite, tro-
ilite, metal, phosphates, and alkali feldspar (
52
,
53
). Here, three apatite
grains were analyzed (14 spots total) in thin section 12040,211. Apa-
tite is mostly subhedral to anhedral, and grain sizes range from <5 to
30
m
m and occurs in pyroxene and plagioclase. Small, subhedral to
euhedral apatite grains show skeletal growth with hollow centers
(Fig. 1).
Lunar meteorite MIL 05035.
MIL 05035 is a coarse-grained lunar
gabbroic meteorite (
54
). It mainly consists of py
roxene(54to69volume
%) with grain sizes up to 6 mm and plagioclase (17 to 36 volume %) with
grain sizes up to 4 mm. It contains minor fayalitic olivine, ilmenite,
spinel, FeS, apatite, and silica, which represent crystallized products
of its residual melt (
54
,
55
). It also contains symplectic intergrowth
that is composed of silica, fayalitic olivine, and hedenbergitic pyrox-
ene (
54
). Here, a single apatite grain in the thin section MIL 05035,6
was analyzed (two points total).
Methods
Measurements of H, Cl, F, as well as D/H and
37
Cl/
35
Cl were all made in
the Caltech Center for Microan
alysis using the Cameca 7f-GEO
secondary ion mass spectrometer. Standards used for abundance mea-
surements are those described in (
56
) with the slightly revised values of
(
57
). Isotopic measurements are reported relative to Durango apatite at
d
37
Cl = +0.40
‰
SMOC and
d
D=
−
120 ± 5
‰
VSMOW (2
s
)(
16
).
Abundances of OH, F, and Cl.
Measurements of volatile abun-
dances in lunar apatite are made by measuring
16
O
1
H,
18
O(reference
element),
19
F,
31
P (secondary reference element),
32
S, and
35
Cl using a
−
0.5-nA, 10-keV Cs
+
beam at a mass resolving power of ~5500, suffi-
cient to separate peaks of interest from all known interferences (
18
). Pre-
analytical sputtering was performed with a 2-nA beam and 25-
m
m×25-
m
m
raster for 300 s, except in cases where abundance measurements followed
isotopic measurements, in which case preanalytical sputtering was re-
duced to 20 s. This was followed by 10 to 30 cycles of measurement with
a2-
m
m×2-
m
m raster. Secondary ions were accelerated to
−
9keV,and
those passing through a ~100-
m
m field aperture were measured with dy-
namic transfer via electron multiplier, except for F and Cl, that in some
cases were measured with a Faraday cup. An 80% electronic gating was
applied to further reduce contamination. Measurements of less than
100 ppm H
2
O are very conservatively assumed to be within error of
the blank for epoxy-bearing thin sections such as those studied here.
Cl isotopes.
Measurements of Cl isotopes were made in two ses-
sions, both normalized to
Durango apatite at +0.4
‰
SMOC. In the first
session, samples were presputtered for 300 s at ~2 nA with a 25-
m
m×
25-
m
m raster. Data were collected for
35
Cl (1 s) and
37
Cl (1 s) using a
lower primary beam current (~0.5 nA), rastered over a smaller area
(2
m
m×2
m
m) for 300 cycles. Data were collected with an electron mul-
tiplier (dead time = 44 ns) at low mas
s resolving power (~2000). Addi-
tional settings were as follows: field aperture (400
m
m), contrast aperture
(150
m
m), no dynamic transfer, no E-gating, 45-eV energy window. The
second session used similar settings,
except that a Faraday cup detector
was used instead of an electron multiplier, with a higher primary current
(~3 to 3.5 nA for the presputter, ~1.5 to 2 nA), and shorter analysis times
(120-s presputter, 30 cycles of measurement). No difference was observed
in the reproducibility of the reference materials between the two sessions.
H isotopes.
Measurements of H isotopes were made at low mass
resolution [mass resolution power
(MRP) ~800] with the electron mul-
tiplier, with 50 to 100 cycles of ~3-nA, 2-
m
m×2-
m
m rastered beam
sputtering, preceded by 180 s of ~3-nA, 25-
m
m×25-
m
m raster pre-
sputtering. Durango apatite was used as a
d
D standard, with a value
of
−
120
‰
VSMOW (
16
). Additional settings include the following:
field aperture of 100
m
m, no dynamic transfer, electronic gating of
80% (64% by area).
Statistical analysis
All uncertainties are reported as 2 SE
Ms. For single analyses, uncertain-
ties are reported as 2 SEMs, which cons
ists of the analytical uncertainty.
For multiple analyses of the same sa
mple, it consists of either the ana-
lytical uncertainty (2 SEMs) or two standard deviations of the mean of
the measured values for that sample
, whichever is larger. For measure-
mentsofhydrogenabundance(alwaysreportedasH
2
O, regardless of
speciation), all values below 100 ppm H
2
O are assumed to be within error
of zero, because all of our samples are epoxy-bearing thin sections, which
are not ideal for low-blank H
2
O measurements by ion microprobe. No
outliers were discarded: The only data not reported here are due to
operator error on the instrument and are noted before data reduction.
RESEARCH ARTICLE
Boyce
et al
. Sci. Adv. 2015;1:e1500380 25 September 2015
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Acknowledgments:
We are grateful to the institutions and people who made samples available
for this research, including the Lunar Sample and Meteorite curators at the Johnson Space
Center, and T. Bunch of Northern Arizona University. We are also grateful to F. McCubbin and
RESEARCH ARTICLE
Boyce
et al
. Sci. Adv. 2015;1:e1500380 25 September 2015
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J. Mosenfelder, who graciously provided apatite and other mineral standard materials, and
M. Le Voyer, who was instrumental in developing Cl isotope capabilities at Caltech. The first
author would like to acknowledge the support that he received from his coauthors and col-
leagues during the duration of this project, which was considerably lengthened by a near-fatal
illness. The manuscript benefitted from detailed, thoughtful, and constructive criticism from the
editor, two anonymous reviewers, and C. Neal - who is perhaps thanked twice in this sentence.
Funding:
This research was supported by NASA Early Career Fellowships to J.W.B.
(NNX13AG40G) and J.G. (NNX13AF54G), as well as NASA grants to A.H.T. (NNX12AH64G) and
J.P.G. (NNX11AB29G).
Author contributions:
J.W.B., A.H.T., J.M.E., and E.M.S. conceived the project.
C.M., A.H.T., and J.G. performed the petrographic and petrologic descriptions. Y.G., C.M., A.H.T.,
and J.W.B. performed the analyses. All authors participated in the data reduction and writing of
the manuscript.
Competing interests:
The authors declare that they have no competing in-
terests.
Data and materials availability:
All new data generated for this paper are found in
Table 1. Samples analyzed were borrowed from several sources (see above) and must be re-
quested directly to those sources.
Submitted 23 March 2015
Accepted 31 July 2015
Published 25 September 2015
10.1126/sciadv.1500380
Citation:
J. W. Boyce, A. H. Treiman, Y. Guan, C. Ma, J. M. Eiler, J. Gross, J. P. Greenwood,
E. M. Stolper, The chlorine isotope fingerprint of the lunar magma ocean.
Sci. Adv.
1
,
e1500380 (2015).
RESEARCH ARTICLE
Boyce
et al
. Sci. Adv. 2015;1:e1500380 25 September 2015
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Sci Adv
Stolper (September 25, 2015)
M. Eiler, Juliane Gross, James P. Greenwood and Edward M.
Jeremy W. Boyce, Allan H. Treiman, Yunbin Guan, Chi Ma, John
The chlorine isotope fingerprint of the lunar magma ocean
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