Since the long standing paradigm of an anhydrous Moon was challenged there has been a renewed focus on investigating volatiles in a variety of lunar samples. However, the current models for the Moons formation have yet to fully account for its thermal evolution in the presence of H2O and other volatiles. When compared to chondritic meteorites and terrestrial rocks, lunar samples have exotic chlorine isotope compositions, which are difficult to explain in light of the abundance and isotopic composition of other volatile species, especially H, and the current estimates for chlorine and H2O in the bulk silicate Moon. In order to better understand the processes involved in giving rise to the heavy chlorine isotope compositions of lunar samples, we have performed a comprehensive in situ high precision study of chlorine isotopes, using NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry) of lunar apatite from a suite of Apollo samples covering a range of geochemical characteristics and petrologic types

Anand, M.

Barnes, J. J.

Franchi, I. A.

McCubbin, F. M.

Neal, C. R.

Tartese, R.

Since the long standing paradigm of an anhydrous Moon was challenged there has been a renewed focus on investigating volatiles in a variety of lunar samples. Numerous studies have examined the abundances and isotopic compositions of volatiles in lunar apatite, Ca5(PO4)3(F,Cl,OH). In particular, apatite has been used as a tool for assessing the sources of H2O in the lunar interior. However, current models for the Moon's formation have yet to fully account for its thermal evolution in the presence of H2O and other volatiles. For ex-ample, in the context of the lunar magma ocean (LMO) model, it is anticipated that chlorine (and other volatiles) should have been concentrated in the late-stage LMO residual melts (i.e., the dregs enriched in incompatible elements such as K, REEs (Rare Earth Elements), and P, collectively called KREEP, and in its primitive form - urKREEP,  given its incompatibility in mafic minerals like olivine and pyroxene, which were the dominant phases that crystallized early in the cumulate pile of the LMO. When compared to chondritic meteorites and terrestrial rocks, lunar samples have exotic chlorine isotope compositions, which are difficult to explain in light of the abundance and isotopic composition of other volatile species, especially H, and the current estimates for chlorine and H2O in the bulk silicate Moon (BSM). In order to better understand the processes involved in giving rise to the heavy chlorine isotope compositions of lunar samples, we have performed a comprehensive in situ high precision study of chlorine isotopes in lunar apatite from a suite of Apollo samples covering a range of geochemical characteristics and petrologic types

NASA Technical Reports Server

THE CHLORINE ISOTOPIC COMPOSITION OF LUNAR URKREEP. J. J. Barnes1,*, R. Tartèse1,2, M. 
Anand1,3, F. M. McCubbin4, C. R. Neal5, and I. A. Franchi1, 1 Planetary and Space Sciences, The Open University, 
Milton Keynes, MK7 6AA, UK, *Jessica.barnes@open.ac.uk, 2IMPMC, Muséum National d'Histoire Naturelle, Par-
is, 75005, France, 3Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK, 4NASA John-
son Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, USA, 5Department of Civil & Envi-
ronmental Engineering & Earth Science, University of Notre Dame, IN, 46556, USA.  
 
 
Introduction:  Since the long standing paradigm of 
an anhydrous Moon [1] was challenged there has been 
a renewed focus on investigating volatiles in a variety 
of lunar samples (e.g., [2-9]). Numerous studies have 
examined the abundances and isotopic compositions of 
volatiles in lunar apatite, Ca5(PO4)3(F,Cl,OH) [3-7]. In 
particular, apatite has been used as a tool for assessing 
the sources of H2O in the lunar interior (e.g., [3-5]). 
However, current models for the Moon’s formation 
have yet to fully account for its thermal evolution in the 
presence of H2O and other volatiles [10-11]. For ex-
ample, in the context of the lunar magma ocean (LMO) 
model, it is anticipated that chlorine (and other vola-
tiles) should have been concentrated in the late-stage 
LMO residual melts (i.e., the dregs enriched in incom-
patible elements such as K, REEs, and P, collectively 
called KREEP, and in its primitive form – urKREEP, 
[12]), given its incompatibility in mafic minerals like 
olivine and pyroxene, which were the dominant phases 
that crystallized early in the cumulate pile of the LMO 
(e.g., [13]). When compared to chondritic meteorites 
and terrestrial rocks (e.g., [14-15]), lunar samples have 
exotic chlorine isotope compositions (Figure 1, [7,16-
19]), which are difficult to explain in light of the abun-
dance and isotopic composition of other volatile spe-
cies, especially H, and the current estimates for chlo-
rine and H2O in the bulk silicate Moon (BSM) [2,20]. 
In order to better understand the processes involved 
in giving rise to the heavy chlorine isotope composi-
tions of lunar samples, we have performed a compre-
hensive in situ high precision study of chlorine isotopes 
in lunar apatite from a suite of Apollo samples cover-
ing a range of geochemical characteristics and petro-
logic types.  
Samples:  The polished thin-sections of Apollo lu-
nar samples investigated in this study were 10044, 
10058, 14304, 15386, 15555, 70035, 76535, 78235 
and 79215. Twenty nine apatite grains were identified 
as being suitable for analysis by ion probe. Our sample 
set includes low-Ti, high-Ti, KREEP and very high 
potassium (VHK) basalts, and selected plutonic high-
lands rocks from the magnesian suite. 
Methods:  The Cl content and Cl isotopic composi-
tion of lunar apatite grains were measured using the 
Cameca NanoSIMS 50L at the Open University fol-
lowing a protocol modified after Tartèse et al. [18]. A 
primary Cs+ beam of ~50 pA was used. Negative sec-
ondary ions of 16O1H, 18O, 35Cl, and 37Cl were collected 
simultaneously on electron multipliers, and 40Ca19F was 
used to locate apatite using real time isotope imaging. 
Typically, areas of ~5 × 5 μm were analyzed and elec-
tronic gating was applied to collect data from the inner 
25% of each analysis area.  
Results and discussion:  Figure 1 shows the results 
obtained during this study [19]. The Cl isotopic com-
position of apatite from low- and high-Ti mare basalts 
are consistent with previous studies [7,16], with δ37Cl 
values from ~+2 to +18 ‰ and Cl contents from ~0.01 
to 0.4 wt.% Cl. 
Figure 1. The chlorine isotopic compositions of different 
objects in the Solar System. All of the data available for 
lunar samples [7,16-19] have been plotted together with 
their individual uncertainties. The other data are from 
Sharp et al. [14-15]. *data from the in situ analyses of 
apatite and **data included from both in situ analyses of 
apatite and bulk Cl measurements. 
In contrast, apatite from KREEP-rich basalts such 
as KREEP basalt 72275 [16] and very high potassium 
(VHK) basalt 14304 have distinctly heavier Cl isotopic 
compositions than apatite found in mare basalts (Figure 
1). Similarly apatite from highlands samples display 
very heavy Cl isotopic compositions (>+20 ‰) and 
have >0.5 wt.% Cl.   
Magmatic degassing.  H2O in apatite from mare 
basalts is generally characterized by H-isotopic compo-
sitions that are elevated (>+500 ‰) relative to Earth 
(e.g., [3-4]). One of the explanations put forward for 
this is the degassing of H2 from mare magmas which 
had starting H isotopic compositions similar to Earth’s 
1439.pdf47th Lunar and Planetary Science Conference (2016)
https://ntrs.nasa.gov/search.jsp?R=20160003146 2019-08-31T04:09:06+00:00Z
mantle (~0 ‰, [4]). Degassing has also been proposed 
to explain the heavy Cl isotope values of lunar sam-
ples, provided that low H/Cl ratios prevailed in the 
parent magma to permit degassing of metal chlorides 
[16,21]. However, it is difficult to reconcile both the H 
& Cl isotopic compositions and OH-rich nature of apa-
tite in some mare basalts with a Cl degassing model 
[16] given the partitioning relationships for volatiles 
between apatite and melt [22]. Samples 72275 and 
78235 are considered to have escaped degassing and 
their apatite contain H2O with lower H isotopic compo-
sitions [5-6], which are simultaneously characterized 
by elevated Cl isotopic compositions (Figure 1). There-
fore, these rocks are particularly good candidates with 
which to understand the primordial signatures of H and 
Cl in the Moon.  
Relating Cl-isotopes to KREEP assimilation.  We 
investigated whether the heavy Cl isotope compositions 
of lunar rocks could be related to the proportion of 
KREEP component they contain, by comparing the Cl 
isotope compositions of apatite with bulk-rock incom-
patible trace element data. Figure 2 shows that there is 
a strong positive correlation between apatite δ37Cl val-
ues and La/Sm ratios, which strongly indicates mixing 
between a mantle source with low Cl isotopic composi-
tion (~0 ‰) and a KREEP-rich component character-
ized by a δ37Cl value ~+30 ‰.          
                                                                    
Figure 2. New Cl isotope data for lunar apatite (aver-
ages) compared to carbonaceous chondrite-normalized 
La/Sm bulk-rock ratios from the literature. Uncertainties 
represent the standard deviation in the δ37Cl values of 
apatite for each sample. The Cl isotope data for 12039 
and 12040 are from Boyce et al. [7], and 72275 is from 
Sharp et al. [16]. 
How did urKREEP obtain a heavy Cl isotopic 
composition?  The internal differentiation of the Moon 
via a LMO predicts a volatile-rich urKREEP layer 
dominated by Cl, containing at least 1350 ppm Cl [2]. 
Boyce et al. [7] have proposed that degassing of Cl 
from the LMO would account for the fractionation of 
Cl isotopes and δ37Cl values ~+30 ‰ in the residual 
urKREEP layer. Whilst the LMO model provides an 
elegant mechanism for concentrating Cl in the Moon, 
the solubility of Cl in basaltic silicate liquids is high 
(e.g., [23]) and the confining pressure beneath the 30-
40 km of lunar crust [24] would be sufficient to prevent 
the loss of Cl by degassing.  
Therefore, in order to explain the fractionated Cl 
isotopic composition of urKREEP, we envisage a sce-
nario in which, during the latter stages of LMO crystal-
lization (>95 %), a large bolide punctured the lunar 
crust [25] to a depth sufficient to expose the urKREEP 
layer to the vacuum of space, which drastically de-
creased the solubility of Cl in the residual LMO mag-
matic liquids and enabled degassing of metal chlorides 
[16] into a vacuum, leading to the fractionation of Cl 
isotopes. If such an event was restricted to the nearside 
of the Moon, i.e., the Procellarum KREEP Terrane, 
then one would expect rocks from outside of this re-
gion to have relatively unfractionated Cl isotope com-
positions. Additionally, important insights into the pro-
cesses operating to fractionate chlorine isotopes could 
be gleaned from studies of other reduced, airless, and 
differentiated bodies [26]. 
References: [1] Taylor S. R. et al. (2006) Rev. Min. 
Geochem., 60, 657-704. [2] McCubbin F. M. et al. (2015) 
Am. Mineral., 100, 1668-1707. [3] Greenwood J. P. et al. 
(2011) Nat. Geosci., 4, 79–82. [4] Tartèse R. et al. (2013) 
GCA, 122, 58–74. [5] Barnes J. J. et al. (2014) EPSL, 390, 
244–252. [6] Tartèse R. et al. (2014) Geology 42, 363–366. 
[7] Boyce J. W. et al. (2015) Sci. Adv., 1, e1500380. [8] Saal 
A. E. et al. (2013) Science 340, 1317–1320. [9] Füri E. et al. 
(2014) Icarus 229, 109–120. [10] Canup R. M. (2012) Sci-
ence 338, 1052-1055. [11] Ćuk M. and Stewart S. T. (2012) 
Science 338, 1047-1052. [12] Warren P. H. and Wasson J. T. 
(1979) Rev. Geophys. Sp. Phys., 17, 73-88. [13] Elkins Tan-
ton L. T. et al. (2002) EPSL, 196, 239–249. [14] Sharp Z. D. 
et al. (2007) Nature 446, 1062-1065. [15] Sharp Z. D. et al. 
(2013) GCA, 107, 189–204. [16] Sharp Z. D. et al. (2010) 
Science 329, 1050–1053. [17] Treiman A. H. et al. (2014) 
Am. Mineral., 99, 1860–1870. [18] Tartèse R. et al. (2014) 
Meteoritics & Planet. Sci., 49, 2266–2289. [19] Barnes J. J. 
et al. (In review, EPSL). [20] Hauri E. H. et al. (2015) EPSL, 
409, 252–264. [21] Ustunisik G. et al. (2015) Am. Mineral., 
100, 1717-1727. [22] Boyce J. W. et al. (2014) Science 344, 
400-402. [23] Webster J. D. et al. (1999) GCA, 63, 729-738. 
[24] Wieczorek M. A. et al. (2013) Science 339, 671-675. 
[25] Kamata S. et al. (2015) Icarus 250, 492-503. [26] Bar-
rett T. J. et al. (2016) LPSC, this volume. 
Acknowledgements:  We thank NASA CAPTEM 
for allocation of lunar samples. This research was sup-
ported by a grant from STFC, UK (grant # ST/L000776/1 
to M.A. and I.A.F). 
1439.pdf47th Lunar and Planetary Science Conference (2016)


The Chlorine Isotopic Composition Of Lunar UrKREEP

THE CHLORINE ISOTOPIC COMPOSITION OF LUNAR URKREEP. J. J. Barnes1,*, R. Tartèse1,2, M. 
Anand1,3, F. M. McCubbin4, C. R. Neal5, and I. A. Franchi1, 1 Planetary and Space Sciences, The Open University, 
Milton Keynes, MK7 6AA, UK, *Jessica.barnes@open.ac.uk, 2IMPMC, Muséum National d'Histoire Naturelle, Par-
is, 75005, France, 3Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK, 4NASA John-
son Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, USA, 5Department of Civil & Envi-
ronmental Engineering & Earth Science, University of Notre Dame, IN, 46556, USA.  
 
 
Introduction:  Since the long standing paradigm of 
an anhydrous Moon [1] was challenged there has been 
a renewed focus on investigating volatiles in a variety 
of lunar samples (e.g., [2-9]). However, the current 
models for the Moon’s formation have yet to fully ac-
count for its thermal evolution in the presence of H2O 
and other volatiles [10-11]. When compared to chon-
dritic meteorites and terrestrial rocks (e.g., [12-13]), 
lunar samples have exotic chlorine isotope composi-
tions [7,14-17], which are difficult to explain in light of 
the abundance and isotopic composition of other vola-
tile species, especially H, and the current estimates for 
chlorine and H2O in the bulk silicate Moon [2,18].  
In order to better understand the processes involved 
in giving rise to the heavy chlorine isotope composi-
tions of lunar samples, we have performed a compre-
hensive in situ high precision study of chlorine iso-
topes, using NanoSIMS, of lunar apatite from a suite of 
Apollo samples covering a range of geochemical char-
acteristics and petrologic types.  
Results and discussion:  We show that the Cl iso-
topic composition of apatite from low- and high-Ti 
mare basalts are consistent with previous studies 
[7,14], with δ37Cl values from ~+2 to +18 ‰. In con-
trast, apatite from KREEP-rich basalts such as KREEP 
basalt 72275 [14] and very high potassium (VHK) bas-
alt 14304 have distinctly heavier Cl isotopic composi-
tions than apatite found in mare basalts. Similarly apa-
tite from highlands samples display very heavy Cl iso-
topic compositions (>+20 ‰). 
We investigated whether the heavy δ37Cl values of 
lunar rocks could be related to the proportion of 
KREEP component they contain, by comparing the Cl 
isotope compositions of apatite with bulk-rock incom-
patible trace element data. Our results strongly indicate 
mixing between a mantle source with low Cl isotopic 
composition (~0 ‰) and a KREEP-rich component 
characterized by a δ37Cl value ~+30 ‰.  
The internal differentiation of the Moon via a LMO 
predicts a volatile-rich urKREEP layer dominated by 
Cl [19], containing at least 1350 ppm Cl [2]. Boyce et 
al. [7] proposed that degassing of Cl from the LMO 
would account for the fractionation of Cl isotopes and 
δ37Cl values ~+30 ‰ in the residual urKREEP layer. 
Whilst the LMO model provides an elegant mechanism 
for concentrating Cl in the Moon, the solubility of Cl in 
basaltic silicate liquids is high (e.g., [20]) and the con-
fining pressure beneath the 30-40 km of lunar crust 
[21] would be sufficient to prevent the loss of Cl by 
degassing.  
Therefore, in order to explain the fractionated Cl 
isotopic composition of urKREEP, we envisage a sce-
nario in which, during the latter stages of LMO crystal-
lization (>95 %), a large bolide(s) punctured the lunar 
crust [22] to a depth sufficient to bring KREEP-rich 
material to lower pressures, drastically decreasing the 
solubility of Cl in the residual LMO magmatic liquids 
and enabled degassing of metal chlorides [14, 23], 
leading to the fractionation of Cl isotopes. If such an 
event was restricted to the nearside of the Moon, i.e., 
the Procellarum KREEP Terrane, then one would ex-
pect rocks from outside of this region to have relatively 
unfractionated Cl isotope compositions.  
References: [1] Taylor S. R. et al. (2006) Rev. Min. 
Geochem., 60, 657-704. [2] McCubbin F. M. et al. (2015) 
Am. Mineral., 100, 1668-1707. [3] Greenwood J. P. et al. 
(2011) Nat. Geosci., 4, 79–82. [4] Tartèse R. et al. (2013) 
GCA, 122, 58–74. [5] Barnes J. J. et al. (2014) EPSL, 390, 
244–252. [6] Tartèse R. et al. (2014) Geology 42, 363–366. 
[7] Boyce J. W. et al. (2015) Sci. Adv., 1, e1500380. [8] Saal 
A. E. et al. (2013) Science 340, 1317–1320. [9] Füri E. et al. 
(2014) Icarus 229, 109–120. [10] Canup R. M. (2012) Sci-
ence 338, 1052-1055. [11] Ćuk M. and Stewart S. T. (2012) 
Science 338, 1047-1052. [12] Sharp Z. D. et al. (2007) Na-
ture 446, 1062-1065. [13] Sharp Z. D. et al. (2013) GCA, 
107, 189–204. [14] Sharp Z. D. et al. (2010) Science 329, 
1050–1053. [15] Treiman A. H. et al. (2014) Am. Mineral., 
99, 1860–1870. [16] Tartèse R. et al. (2014) Meteoritics & 
Planet. Sci., 49, 2266–2289. [17] Barnes J. J. et al. (In re-
view, EPSL). [18] Hauri E. H. et al. (2015) EPSL, 409, 252–
264. [19] Elkins Tanton L. T. et al. (2002) EPSL, 196, 239–
249. [20] Webster J. D. et al. (1999) GCA, 63, 729-738. [21] 
Wieczorek M. A. et al. (2013) Science 339, 671-675. [22] 
Kamata S. et al. (2015) Icarus 250, 492-503. [23] Ustunisik 
G. et al. (2015) Am. Mineral., 100, 1717-1727. 
Acknowledgements:  We thank NASA CAPTEM 
for allocation of lunar samples. This research was sup-
ported by a grant from STFC, UK (grant # ST/L000776/1 
to M.A. and I.A.F). 
 
 
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The Chlorine Isotopic Composition Of Lunar UrKREEP

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