Accepted Manuscript

Estimating the lunar mantle water budget from phosphates: Complications as-

sociated with silicate-liquid-immiscibility

J.F. Pernet-Fisher, G.H. Howarth, Y. Liu, Y. Chen, L.A. Taylor

PII: S0016-7037(14)00550-X

DOI: http://dx.doi.org/10.1016/j.gca.2014.09.004

Reference: GCA 8967

To appear in: Geochimica et Cosmochimica Acta

Received Date: 11 June 2014

Accepted Date: 1 September 2014

Please cite this article as: Pernet-Fisher, J.F., Howarth, G.H., Liu, Y., Chen, Y., Taylor, L.A., Estimating the lunar

mantle water budget from phosphates: Complications associated with silicate-liquid-immiscibility, Geochimica et

Cosmochimica Acta (2014), doi: http://dx.doi.org/10.1016/j.gca.2014.09.004

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Estimating the lunar mantle water budget from phosphates: Complications associated with

silicate-liquid-immiscibility

J. F. Pernet-Fisher1*, G. H. Howarth1, Y. Liu1, 2, Y. Chen2, and L. A. Taylor1

1 Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of

Tennessee, Knoxville, TN 37996, USA.

2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.

*corresponding author: email: jpf@utk.edu

Abstract

The discovery of water within the lunar mantle has broad implications for the formation

of the Earth-Moon system, differentiation of the Moon, and the magmatic evolution of lunar

basalts, as well as the highland rocks. Recently, there has been considerable interest in using

combined water abundances and H-isotope systematics of lunar apatites from mare basalts to

quantify the origin and extent of water within the Moon’s mantle. However, the petrologic and

geochemical conditions that govern apatite crystallization are not well-constrained, especially for

high-FeO basaltic melts that crystallize at fO2 values below the iron-wüstite buffer. Apatites are

typically located within the late-stage interstitial regions. In this contribution, we present

detailed textural descriptions of late-stage inter-cumulus, residual-liquid pockets (i.e., mesostasis

pockets), in order to understand the petrogenesis of lunar apatite. Results from five mare basalts

demonstrate that the majority of the residual liquids in mesostasis regions have undergone

silicate-liquid immiscibility (SLI) splitting into Si-K-rich (felsic) and Fe-rich (Fe-basaltic)

conjugate liquids. We demonstrate the complexity of these residual liquids by documenting a

wide range of water contents for apatites in several mesostasis pockets within a single mare

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basalt, a complexity common to many basalts. These data illustrate that the individual apatite-

hosting mesostasis pockets behave as independent sub-systems, even within a single rock.

Furthermore, we present water concentrations for another phosphate phase, merrillite, indicating

additional uncertainties during considerations of water partitioning into apatite.

Fractional crystallization trends have been used in order to assess the conditions under which

magmas are likely to undergo SLI. Predicted liquid lines of descent indicate that it is the late-

stage residual liquids of lunar basalts with relatively low-Mg# (e.g., <~50) that intersect the two-

liquid field during crystallization, forming conjugate liquids that are chemically and

mineralogically distinct. However, residual liquids for basaltic magmas with high-Mg#

(e.g.,>~50) may not intersect the two-liquid field depending upon the fractionation trends in the

late-stage mesostasis pockets. For samples that undergo SLI, the apatite/melt-partition

coefficients required for back-calculating water abundances of the parental melts are

compromised by the generation of two populations of apatites-merrillites from conjugate

immiscible liquids. This process highlights an important complexity inherent to all water back-

calculations that use apatite, as this requires an additional set of partition coefficients. We

emphasize that the complex petrologic nature and common development of SLI of apatite-

bearing, late-stage mesostasis pockets have not been considered in published apatite-volatile

data. These factors, in additional to other considerations, illustrate why water back-calculations

to model the primary melts from such data must be viewed with caution.

1. Introduction

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Over the past decade, apatites crystallizing from late-stage, residual-melt pockets in lunar

mare basalts have been used to determine the lunar mantle volatile budget. This has resulted in a

large database of lunar apatite water (‘water’ is used here to include both OH and H2O

throughout) concentrations and D/H values (e.g., McCubbin et al., 2007; McCubbin et al., 2010;

Boyce et al., 2010; Greenwood et al., 2011; Barnes et al., 2013; Tartèse et al., 2013, 2014;

Barnes et al., 2014; Pernet-Fisher et al., 2014; Robinson and Taylor, 2014; Boyce et al., 2014).

Water abundances and D/H values have been analyzed on a wide range of lunar lithologies.

However large ranges, particularly for water abundances, have often been reported for individual

high-Ti mare basalt sample (Fig. 1). For example, within one section of mare basalt 12039 water

concentrations vary by a factor of ~6, from ~1,000 to >6,000 ppm OH and δD values vary by a

factor of 2.5, from 391 ‰ to 1010 ‰ (Greenwood et al., 2011). As such, this range has resulted

in considerable debate regarding both the quantity and origin of lunar mantle water. Estimates

based on apatite water contents range from relatively dry values (64 ppb), to values comparable

to the Earth’s upper mantle (>750 ppm; McCubbin et al., 2010; Boyce et al., 2010). However,

recently the value of using apatites in order to estimate the volatile budget of the moon has been

questioned; i.e., the apatite paradox (Anand, 2014; Boyce et al., 2014). In addition, various

potential water-origin mechanisms have been proposed, ranging from post-Moon-forming

cometary impacts (Greenwood et al., 2011) to a shared origin with the Earth, pre-dating Moon

formation (Barnes et al., 2014).

Fundamentally, a better understanding of the petrology of residual-liquid pockets

(mesostasis pockets), in which apatites crystallize, is crucial for interpreting the large disparities

in water abundances and hydrogen isotopic ratios. Here, we emphasize the role of Silicate-

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Liquid-Immiscibility (SLI) in these interstitial late-stage mesostasis pockets. It is the

combination of high-FeO basaltic magma compositions and low fO2 values during melt

crystallization on the Moon that result in the relatively common development of SLI. The role of

SLI is an important, yet commonly unappreciated, magmatic feature of ferro-basalts on Earth,

typically developing after significant crystal fractionation (>90 %) and Fe-enrichment (e.g.,

Philpotts 1982). In basaltic rocks, SLI is generally observed on small-scales (50-300 μm), within

melt inclusions and late-stage residual melt pockets in both terrestrial and lunar basalts (e.g.,

Roedder and Weiblen 1970; Taylor et al., 1971; Rutherford et al., 1974; Hess et al., 1975;

Philpotts 1979; Longhi 1998; Shearer et al., 2006; Veksler et al., 2007; Taylor, 2011). These

residual liquids have been overlooked in previous petrologic descriptions of lunar rocks, due to

their small size and difficulty of mineral identification with transmitted-light optical microscopy

(e.g., Fig. 2). Recently, there has been renewed interest in large-scale SLI relating to the

emplacement of granitic rocks on Earth (e.g., Charlier et al., 2011; Zhou et al., 2013) and the

Moon (e.g., Neal and Taylor 1989; Jolliff 1998). However, understanding the evolution of SLI

on a small-scale, such as within mesostasis pockets, is important in light of the recent

investigations into lunar apatites (e.g., Taylor, 2011; Carmody et al., 2013; Pernet-Fisher et al.,

2014). It is only by considering the additional complications of SLI, the origin and scale of the

indigenous lunar-water budget can be accurately estimated. In order to illustrate the variability

of these pockets between different lunar mare basalts, we present textural descriptions of

mesostasis pockets, from a suite of four lunar basalts that display variable Fe-enrichment (i.e.,

Mg# 57 to 38). In addition to these samples, we present new water analyses for Apollo 14 basalt

14053, in order to highlight heterogeneities between mesostasis pockets within an individual

rock, as well as between apatite and merrillite.

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2. Samples and Methods

2.1 Lunar Mare Basalts

Lunar basalts are typically Fe-rich (15-25 wt.% FeO) relative to terrestrial ferro-basalts

(<15 wt.%). Thin sections from four Apollo mare basalts were selected to illustrate mesostasis

variability. These samples comprise two high-Ti basalts (Apollo 11 rock 10003; Apollo 17 rock

70035) and two low-Ti basalts (Apollo 12 rocks 12063 and 12018). All four samples are

characterized by medium-grained, sub-ophitic textures. The more primitive basalt bulk-rock

compositions are for samples 12018 (Mg# 57) and 70035 (Mg# 48), whereas 12063 (Mg# 44)

and 10003 (Mg# 38) are more Fe-rich. The Fe-enrichment of 12063 and 10003 is strongly

reflected in their pyroxene compositions, extending virtually to the ferrosilite-hedenbergite join

on the pyroxene quadrilateral (SOM 1; Hollister et al., 1971; Carter et al., 1971; McGee et al.,

1977; Beaty and Albee, 1978; Gamble et al., 1978). In contrast, pyroxenes from 12018 show

only minor Fe-enrichment, whereas pyroxenes for 70035 trends to greater Fe-enrichment, but

does not reach high-ferro-pyroxene compositions (SOM 1; Papike et al., 1974; Kushiro et al.,

1971). Sample 12018 displays evidence for accumulation of pyroxene and olivine (~70 % modal

pyroxene and olivine); this is likely to account for the high Mg# of this sample. In addition to

the above-mentioned four samples, one chip of Apollo 14 basalt 14053 (Mg# 48) was selected to

investigate inter-mesostasis water variations. This sample is a low-Ti (2.9 wt. % TiO2) ophitic

basalt and contains a high modal proportion of large (>300 μm) mesostasis pockets, which

contain sufficiently large phosphates upon which to conduct SIMS analyses on multiple grains

per pocket. This is the same basalt sample studied by Boyce et al. (2010). In contrast to the

other mare-basalts selected here, breccia material found on one side of this sample suggests that

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this basalt may have been a clast within a boulder (Meyer, 2011). Further, this sample contains

distinct reduction textures (Fig. 7; Taylor et al., 2004; Greenwood et al., 2011), interpreted to

have been the result of implanted by solar-wind protons on the lunar surface, followed by

reheating during an impact event. This results in sub-solidus reactions with the implanted

protons, creating extremely reducing conditions. The effects of this are seen within the sample

by textures such as intergrowths of Fe0 and Si , resulting from the brake down of Fe2SiO4

(fayalite; Taylor et al., 2004).

Before assessing the nature of the late-stage mesostasis pockets, it is necessary to define the

extent of these pockets. As depicted on the SiO2-Ol-Plag ternary (Fig. 3), during the end stages

of fractional crystallization, a basaltic liquid will reach a pseudo-eutectic. Whereas this ternary

is used to consider crystallization of mare basalts, the pseudo-eutectic involves more than the

three phases identified (clinopyroxene, plagioclase, and silica); experiments as well as textural

observations indicate that for the high-Ti basalts, ilmenite also co-crystalizes at this point (e.g.,

Hess et al., 1978). The resultant residual liquid consists of an extremely FeO-enriched melt near

saturation with incompatible and volatile elements (i.e., KREEP = Potassium-Rare Earth

Elements-Phosphorous). It is this residual liquid, interstitial to the pseudo-eutectic minerals (i.e.,

pyroxene, plagioclase, silica), that define the extent of the mesostasis pocket. Furthermore, it is

this late-stage KREEP-rich liquid that with further crystallization intersects the two-liquid field

undergoing SLI, thereby producing conjugate felsic and ferroan liquids (Fig. 3).

2.2 Analytical Methods

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2.2.1 Microprobe Analysis

Minerals within the different mesostasis pockets were analyzed for major- and minor-

element compositions with wave-length dispersive spectrometers (WDS) on a Cameca SX-100

electron microprobe at the University of Tennessee, using an accelerating potential of 15 keV,

beam current of 20 nA, 1 μm diameter beam, and standard PAP corrections. Counting times

were 20 s for Si, Mg, Fe, Na, and Al; 30 s for Ca, Cr, K, and Mn; and 40 s for P. A beam current

of 10 nA and 10 μm beam was used for apatite analyses. 40 s count times were used for F and

Cl. It has been previously noted that F and Cl can suffer from variable count rate (e.g., Stormer

et al., 1993). This was not found to be a problem for Cl, achieving reproducibility of 0.04 wt. %.

However these effects resulted in an reproducibility of 0.2 wt. % for F. The instrument was

calibrated daily using both natural and synthetic standards. Backscattered-electron (BSE)

images and elemental X-ray maps for Ti, Ca, Fe, K, F, P, and Ba were collected for ~ 6

mesostasis pockets in each sample, using a 15 keV, 19 nA beam, and a step size of 2 μm.

Counting times were 50 ms per step. Modal abundances of phases, and the total area of the

mesostasis pocket were obtained with ImageJ manipulations of combinations of X-ray maps.

The bulk composition of each mesostasis pocket was estimated from the modal analyses and the

EMP phase compositions.

2.2.2 SIMS OH and δD Analyses of 14053 Phosphates

Volatile abundances (H, Cl, F, S), and H isotopes (2H and 1H) of phosphates in sample

14053 were analyzed using a Cameca ims-7f GEO secondary ion mass spectrometer (SIMS) at

Caltech. One chip was mounted in Indium for SIMS analysis. Negative ions were generated with

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a Cs+ primary beam of 5 nA and 20 keV and charge balanced at the sample with a normal-

incidence electron gun with a 20 keV impact energy. The areas of interest were rastered with a

~20 μm beam over an area of 20 μm by 20 μm for ~3-5 minute to remove the conductive coating

(AuPd in this case). During analysis, a field aperture of 5 μm by 5 μm was used to sample only

the central ~10 μm of the beam, which produced an effective 15 by15 μm analyzed area. An

electron gating of ~60% was applied to this area. . Subsequently ion imaging using 12C- and

16O1H- was performed at each potential site, and then the position of the beam was adjusted to

avoid hot spots of 12C- and 16O1H- (e.g., cracks, pits in the sample). Negative ions, 12C-, 16O1H-,

18O-, 19F-, 31P-, and 32S-, were measured for 10 cycles, each with a 2 s counting time for 12C- and 1

s for others. The MRP was 5000 during measurements of volatile abundances. Detection limits

were determined using the method of Mosenfelder et al. (2001). The known dry olivine

(GRR1017) was analyzed yielding a 16O1H/18O of 0.0037, translating to 6 ppw H2O. This vaule

has been subtracted from the measured apatite data.

The δD measurements were conducted on the same spots where H abundances were

obtained. The Cs+ beam conditions are similar to that for H analysis at ~20 keV and ~5 nA. The

1H- and 2H- ions were differentiated with a MRP of ~800 counted for 33-100 cycles, for 1s and

10s, respectively. An electron gate of 80 % was used to limit the contamination from the

surrounding area of the rastered area. Ion imaging of 1H- was used to locate the previous spot

and to avoid any hot spots. Natural apatite standards (AP003 to AP005) of OH ranging from

0.03 to 0.57 wt.% H2O from Boyce et al. (2010) and McCubbin et al. (2010), and Boyce et al.

(2012) were used for determining OH contents in 14053 phosphates. These standards were

mounted in Indium. The mean error between the standard values and the calibration line gave an

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average error of 10 %. The Durango apatite with δD of -120± 5 ‰ VSMOW (Greenwood et al.,

2011 was used to examine the instrument mass fractionation (IMF) on D/H. All data presented

here have been corrected for the IMF, which was +150 ‰ over the period of analysis. The

counting statistic errors on the D/H values correspond to about 10 ‰, whereas the standard

reproducibility is about 30 per mil. The overall reproducibility of the D/H ratios measured on the

reference apatites gave an average of 50 ‰ (2 σ). Data was examined cycle by cycle for each

measurement. The volatile and D/H compositions of apatite did not change significantly during

each analysis, which lasted about 20 min for volatile concentrations and 1-2 hours for D/H

measurements. We find no evidence for the effect of volatilization resulting from previous

SIMS sputtering.

3. Results

3.1 Mesostasis Mineralogy and Textures

Mesostasis pockets in lunar mare basalts represent late-stage residual liquids interstitial to the

pseudo –eutectic minerals. The proportions of these areas in a given thin-section were estimated

using ImageJ on reflected light photomicrograpahs of the entire thin-section. Where zonation in

the pseudo –eutectic minerals were observed, the rims were also incorporated into volume

estimates. The pockets are present in low abundances within the sections studied here (i.e., <3-8

vol.%), displaying a wide variety of minerals abundances and textures. Four to six mesostasis

regions were investigated within each basalt. Textures can be divided into two distinct groups,

which correspond to the most primitive samples from this study (high-Mg# group) and the most

evolved samples from this study (low-Mg# group). Representative false-color composite X-ray

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elemental maps (Si-Ca-K-Fe; Fig. 4) are presented in Figs. 5 and 6 in order to clearly highlight

the typical mineralogy and textures of mesostasis pockets. In addition, representative mesostasis

mineral analyses are presented in Table 1.

3.1.1 Low-Mg# mesostasis pockets

Mesostasis pockets within the low-Mg# basalts range in size from 50 to 500 μm in length,

with an average of ~250 μm (Fig. 5c, d). The typical mineral assemblage consists of a Si-K-Ba-

rich feldspathic phase (e.g., 73 wt.% SiO2; 6 wt.% K2O; 0.8 wt.% Na2O; 1.4 wt.% BaO; and 13

wt.% Al2O3) and fayalite (Fo <10); other minor (2-10 vol.%) phases include: silica (tridymite),

phosphates (apatite and merrillite), sanidine, ilmenite, FeNi metal, and troilite. Both merrillite

and apatite were identified; merrillite relative to apatite was identified based on lower CaO

content and the absence of detectable F-Cl during EMP analyses (Table 1). Mesostasis pockets

are surrounded by coarse pyroxene, plagioclase, and silica grains (>100 μm). Pyroxenes are

typically zoned, ranging from augite cores to FeO-rich rims, whereas the plagioclases are zoned

from CaO-rich cores to Na2O rich-rims. In some cases, the zoning is sharp (orange lines

highlight pyroxene Fe-rich rims, Fig. 5d), thus, mesostasis boundaries can be visually judged by

inspecting BSE and X-ray maps; however, in some cases, this zonation is diffuse; as such, it is

difficult to distinguish the boundaries of individual pockets from the surrounding pyroxene and

plagioclase. In these cases, the zonation can make up a significant proportion of the mesostasis

region (up to 30 vol.%; Table 2).

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Mesostasis pockets within these samples are characterized by distinct ‘sieve’ textures, which

comprise of Si-K-Ba-rich feldspathic glassy spheroidal textures (i.e., isotropic under crossed-

polarized light, hereafter referred to as K-glass) and silica globules, enclosed by fayalite. This

‘sieve’ texture is interpreted to represent distinct Fe-rich (i.e., fayalite) and Si-rich (i.e., K-glass

and silica) conjugate immiscible liquids (i.e., SLI), which form during late-stage crystallization

(e.g., Rutherford et al., 1974; Hess et al., 1975). The modal proportion of fayalite relative to K-

glass within the mesostasis pockets ranges from 50:50 to 70:40 in both samples. In one extreme

case, fayalite dominates the ‘sieve’ texture 90:10 (Fig. 5f). Modal silica globules within the

fayalite for both samples are <5 %. This range in modal proportions of fayalite relative to K-

glass is evident in sample 12063 Fig. 5 d-f. Despite this range, the globules of K-rich glass are

fine-grained and evenly distributed. In contrast, sample 10003 displays a similar range of K-

glass proportions; however, the globules are coarser-grained and irregular in morphology. It is

important to keep in mind that the modal proportions estimated from these samples are subject to

large uncertainties, as the X-ray images presented here represent 2D sections of a 3D body.

Thus, depending on the cut of the sample, the modal abundance may not fully represent the

accurate modal proportions of the sample in question.

Silica is observed as two distinct textural associations: 1) rare globules contained within

fayalite and 2) coarse-grains surrounding the mesostasis pockets or contained within pockets.

The former represents a component of the SLI texture; however, the latter represents earlier-

formed silica (i.e., pseudo-eutectic silica, not to be confused with minor silica crystallizing from

the mesostasis melt). The timing of silica formation is discussed in detail in section 4.1). This

distinction can be challenging within the coarser-grained mesostasis pockets.

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3.1.2 High-Mg# mesostasis pockets

Mesostasis pockets within the high-Mg# basalts are smaller, ranging in length from 30 to 300

μm (Fig. 6). These pockets comprise dominantly of a Si-K-Ba-rich feldspathic glass (K-glass),

silica, sanidine; and minor amounts of apatite, merrillite, ilmenite, FeNi metal, and troilite. In

general, modal proportions do not vary between mesostasis pockets in a single sample as

significantly compared to the low-Mg# samples (12018, Table 2). Several key differences are

observed relative to the low-Mg# mesostasis pockets. The main differences based on

observations from samples 12018 and 70035 are the absence of fayalite and ‘sieve’ textures. The

absence of ‘sieve’ textures suggests that SLI may not have occurred in these cases. Furthermore,

sanidine occurs as distinct laths (Fig. 6 b and c) associated with interstitial K- glass, which is not

a common feature of the low-Mg# samples. As with the low-Mg# mesostasis, all pockets are

surrounded by plagioclase, pyroxene, and silica. However, unlike the low-Mg# basalts, the

pyroxene and plagioclase do not show extensive zoning at the mesostasis boundaries. Another

important difference from the low-Mg# basalt is the occurrence of very-coarse (~400 μm) silica

grains associated with mesostasis regions (Fig. 6a and c). Furthermore, in sample 12018, the

mesostasis regions commonly also contain coarse ilmenite crystals (~150 μm). As highlighted

previously, sample 12018 is coarse-grained in nature, and thus may represent a deeper part of a

flow or sill. The lack of evidence for SLI may result from factors such as a lack of degassing,

thus retaining more water. Higher water contents are reportedly able to suppress the occurrence

of SLI (e.g., Charlier and Grove, 2012).In summary, it should be appreciated that mesostasis

textures distinct types of textures can exist in the sample, depending upon the isolated nature of

these sub-system chemistries during the fractional crystallization process. As such, it is

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important to note that the absence of SLI within these samples cannot be unequivocally stated at

present.

3.2 Phosphate OH Abundances in Apollo 14 Rock 14053

Lunar basalt 14053 is characterized by mesostasis pockets with distinct ‘sieve’ textures (Fig.

7). The texture and mineralogy is similar to that described for samples 10003 and 12063,

indicating late-stage SLI has occurred in the basalt. This sample was chosen due to a high

abundance of mesostasis and coarse-grained phosphates; furthermore, it was also the same

sample from which Liu et al. (2010) and Boyce et al. (2010) have previously reported on for

water values in apatite. Where possible, apatites from the same mesostasis pocket were analyzed

by SIMS, in order to determine possible variations of water abundances and D/H values, both

within individual pockets and between different pockets within this single rock-chip. As

highlighted earlier, this sample has been suggested to have undergone sub-solidus reactions with

implanted solar-wind protons. It is these reactions that have been suggested to account for the

low D/H values previously reported for this samples (Boyce et al., 2010; Greenwood et al.,

2011). Whereas measured D/H vaules may reflect these processes, the effect on water

concentration through this process is considered to be relatively small (<20 ppm H2O; Saal et al.,

2013), particularly considering the young exposure ages of 14053 (~25 Ma; Stettler et al., 1973).

Thus, variations in measured water abundances are likely to retain original signatures in each

mesostasis.

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In total, five mesostasis pockets were selected from one polished surface of a chip of 14053;

one to three apatites from each pocket were analyzed, displaying distinct ranges in water

(reported as H2O) (Table 3 and Fig. 1). Apatites were analyzed from four pockets and merrillite

was analyzed in the fifth. Merrillite relative to apatite was identified based on lower CaO

content, higher Ce (REE) content, and the absence of detectable F-Cl. These variations are

evident in X-ray compositional maps (Ca-P-Ce; Fig. 8). Apatite water contents within individual

pockets range from 217 to 1000 ppm H2O, and the overall variation between pockets ranges from

217 to 2409 ppm H2O, a variance of >10. The merrillite analyzed in pocket 5 yielded a water

content of 218 ppm H2O, suggesting that lunar merrillite possesses a solid-solution with

whitlockite (e.g., Hughes et al., 2008). In this case, the water abundance of the merrillite

suggests that it contains a small (~2.5 %) whitlockite component. All apatite-water display a

limited range of δD values, lying within analytical uncertainty of each other, and yielding an

average value of 8 ‰. Our analyses for 14053 are higher in δD than those reported by

Greenwood et al. (2011), however, it is not uncommon for apatite grains to display

heterogeneities. For example, Greenwood et al., (2011) reported apatite grains which display a

few hundred per mil variation in δD values for the sample as reported here.

4. Discussion

4.1 Complications defining bulk mesostasis compositions

We previously highlighted that during the final stages of fractionation, basaltic liquids will

reach a pseudo-eutectic. In the case of Fig. 3, the pseudo-eutectic is projected onto the SiO2-Ol-

Plag plane, illustrating that clinopyroxene, plagioclase, and silica co-crystallize. This is

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consistent with our observation of the typical mineral assemblage that surrounds and

occasionally intrudes into the mesostasis pockets. However, in some cases, the surrounding

pseudo-eutectic minerals can be hard to distinguish from the mesostasis mineral assemblage,

particularly in the coarser-grained mesostasis pockets. Consequently, silica has commonly been

interpreted to be part of the mesostasis assemblage (e.g., Potts et al., 2014); however, the coarse-

grained nature of many silica grains observed in this study suggests that they crystallized prior to

mesostasis formation as part of the pseudo-eutectic assemblage (Fig. 3). Thus, if these coarse

silica grains are included within mesostasis bulk-composition estimates, the resultant bulk-

composition calculations will significantly overestimate the SiO2 content, yielding more granitic

compositions. In addition, as previously highlighted, some pockets display strong zoning in the

pyroxene and plagioclase that surround the mesostasis pockets (Fig. 5d). These zoned rims may

represent equilibration with mesostasis liquids and pre-existing eutectic grains, adding

difficulties in identifying mesostasis pocket boundaries. Thus, the calculation of mesostasis

bulk-compositions could be highly biased depending if these rims are included or excluded (e.g.,

Griffiths et al., 2014; Potts et al., 2014).

4.2 Mesostasis textural maturity

The general concept that we want to develop here is the extent of “textural maturity”

observed within the mesostasis pockets of both the high- and low-Mg# basalts. Within the low-

Mg# basalts, the extent of textural maturity relates chiefly to the development of obvious SLI

textures. The experimental study of Rutherford et al. (1976) has shown that immiscible liquids

will strongly fractionate during small-degrees of cooling (10-30 oC) after SLI formation,

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resulting in variable textures; this is reflected in the textures observed in the basalts studied.

Mesostasis textural variations are most notable in sample 12063 (Fig. 5 d-f). Within this sample,

SLI is recognized by the occurrence of a distinct ‘sieve’ texture characterized by fine-sized K-

glass droplets within host fayalite (Fig. 5 d). This texture is produced by rapid cooling shortly

after the formation of the conjugate silicate liquids, preserving Si-K-Ba-droplets trapped within

fayalite; classified as an Immature texture. With slower cooling, the Si-K-Ba-droplets coalesce

into larger blebs, forming large patches of glass rather than small globules (Fig. 5e, f); classified

as a Mature texture. Overall, we note a a gradational change from Immature to Mature;

therefore, those mesostasis pockets showing intermediate textures are classified here as Sub-

mature textures – an intermediate state. In the case of Mature textures where distinct ‘sieve’

textures are not observed, it is not always clear whether they have undergone SLI. A textural

gradation can also be observed in the high-Mg# basalts. Rapid cooling of these mesostasis

pockets results in glassy Si-K-Ba-rich feldspathic phases - an Immature texture (Fig. 6 f).

However, numerous mesostasis pockets contain distinct euhedral sanidine laths, indicating

slower cooling of the pocket, resulting in the crystallization of sanidine (Fig. 6 b, c) - a Mature

texture. Despite both the high-Mg# mesostasis pockets and the Mature low-Mg# pockets

showing similar textures (i.e., coarse-crystalline textures, lacking ‘sieve’ patterns), our

observations indicate that it is the dominance of fayalite within mesostasis pockets that is one of

the main characteristics of the low-Mg# mesostasis pockets relative to the high-Mg# mesostasis.

4.3 Residual-liquid evolution and development of mesostasis pockets

In order to investigate the compositional conditions at which late-stage basaltic melts

potentially undergo SLI, crystallization modeling was undertaken for whole-rock, major-element

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compositions of samples from this study. In particular, attention was paid to samples from both

the high Mg# and low Mg# group from our suite (10003, Mg# 38; and 70035, Mg# 48). Liquid

lines of descents were calculated using the ‘Simulating Planetary Igneous Crystallization

Environments’ package (SPICEs; Davenport et al., 2013). The SPICEs code integrates the

algorithms and calibrations of MAGFOX and FXMOTR (Longhi et al., 1991; Longhi, 2006),

which were specifically developed to model fractionation under conditions relevant to the Moon;

i.e., log fO2 = iron-wüstite (IW) -1. Slater et al. (2003) suggested that the MAGFOX (i.e.,

SPICEs) algorithms were the most appropriate for simulating crystallization at lunar conditions

and have been widely used in lunar basaltic crystallization modeling (e.g., McCallum and

Schwartz, 2001; Righter et al., 2005; Sun et al., 2013).

Average bulk-compositions for samples 10003 and 70035 from the compilation of Meyer

(2011) were used as the starting compositions. A crystallization pressure of 1 bar was used, and

liquidus temperatures were calculated by SPICEs. In the case of high-Mg# basalt 70035, the

SPICEs algorithm was only able to model crystallization up to 85 % fractionation. In contrast,

for low-Mg# basalt 10003, the SPICEs algorithm was able to model crystallization up to 95 %

fractionation (Fig. 9). Whereas the SPICEs algorithm does not directly calculate whether a melt

will enter a two-liquid field or not, by plotting the resulting calculated liquid lines of descent

onto multi-component ternary plots (for which the immiscible field is constrained), we are able

to estimate the likelihood that a melt will intersect the two-liquid field (e.g., Fig. 9).

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Immiscible melts are typically illustrated within a projections of the leucite-fayalite-SiO2

system (e.g., Roedder, 1978; Shearer et al, 2001; Charlier et al., 2011; Charlier and Grove,

2012). However, due to the complex multi-component nature of this 2D pseudo-ternary (SiO2 –

[Na2O + K2O + Al2O3] – [MgO FeO + TiO2 + CaO + P2O5], compositional differences in lunar

basalts and fractionation paths are not easily discernible. Thus, by considering both the

‘Roedder’ ternary (Fig. 9a) and the AFM plots (Fig. 9b), this enables liquid-lines of descent and

compositional differences in Mg# to be clearly distinguished. The two-liquid fields in Fig. 9 are

based on: 1) glass compositions from lunar immiscible-melt inclusions from Shearer et al.

(2006); 2) fayalite and K-glass analyses from basalt 10003 (Table 1); and 3) experimental

studies at appropriate lunar conditions (e.g., Rutherford et al., 1974). It is important to note that

the two-liquid field is readily altered by many factors, such as the temperature, pressure, and

oxygen fugacity conditions of the crystalizing basalt (e.g., Charlier and Grove, 2012). For

instance, studies have shown that at lower equilibration temperatures, such as on the moon, the

SLI field will expand with respect to the terrestrial field (Veksler et al., 2010).

Due to the low oxygen fugacities during their crystallization (e.g., IW -1), lunar basalts

undergo extreme Fenner-Trend FeO-enrichment, which is illustrated by the sharp fractionation

trends in both samples, towards the FeO apex in Fig. 9b. Thus, in a system at low oxygen

fugacities, the liquid line of descent will more readily interest the SLI field (e.g., Charlier and

Grove, 2012). The SPICEs results suggest that the low-Mg# basalt (10003) melt evolves along

a liquid-line of descent that intersects the field of two-liquid immiscibility after ~95 %

crystallization, when plotted on the AFM ternary (Fig. 9). These modelling results are consistent

with the textural observations for this rock, in addition to the bulk-mesostasis compositions.

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Based on the EMP analyses and modal abundances for basalt 10003, the bulk mesostasis

composition for the five mesostasis pockets studied were calculated Each pockets lies within the

SLI field, highlighting the immiscible nature of the mesostasis pockets. Large scatter is observed

within the bulk-compositions, which is likely to reflect error in accurately defining the boundary

of mesostasis pockets, together with a lack of constraint of the mesostasis pockets in the third

dimension. This is particularly the case for minerals such as apatite, which controls the bulk P2O5

budget of pockets. This mineral may appear to be very irregularly distributed, particular when

considering mesostasis pockets from one polished surface to another.

In contrast, SPICEs results suggest that, whereas the higher Mg# basalt (i.e., 70035) also

undergoes significant Fe-enrichment, it is unclear whether the residual liquid reaches the two-

liquid field. At ~80 % crystallization for sample 70035, SPICEs predicts that ilmenite dominates

the crystallizing assemblage, causing the liquid-line of descent to move away from the FeO apex

towards the alkali apex. However, after 85 % crystallization, SPICEs is unable to predict the

residual liquids composition for 70035. Thus, based on these trends, it is unclear if the residual

liquid will intersect the two-liquid field (Fig. 9b). If sufficient fractionation of Fe-rich minerals

(e.g., ilmenite) takes place (i.e., Fe-depletion in the residual liquid), then the residual liquid will

follow line (1) in Fig. 9b. This will cause the liquid-line of descent to move toward the alkali

apex, possibly without intersecting the two-liquid field. However, if insufficient fractionation of

Fe-rich minerals takes place (i.e., continued Fe-enrichment of the melt), then the liquid-line of

descent may continue toward the two-liquid field (line 2, Fig. 9b). This has been noted by

Charlier and Grove (2012), who highlighted that although it is difficult to assess whether higher-

Mg# basalts have undergone SLI, we suggest that in this case, the liquid line of descent for the

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high-Mg# basalts may not intersect the two-liquid field based on three observations: 1) the

absence of ‘sieve’ textures in late-stage mesostasis pockets of the most primitive basalts

investigated within this study (i.e., 12018 and 70035, Fig. 7); 2) the occurrence of ilmenite

within the mesostasis region of these samples, in addition to the apparent lack of fayalite; and 3)

the absence of zoned plagioclase and pyroxene at the margins of the mesostasis areas; i.e., no re-

equilibration of pyroxene towards pyroxferroite, indicating an absence of a late-stage Fe-rich

residual liquid. For these reasons, we suggest that the high-Mg# basalts analyzed in this study

likely do not undergo late-stage SLI.

4.4 Complications associated with estimating lunar water budgets

Our detailed investigation into mesostasis petrology has highlighted a number of

complicating factors that pertain to lunar mantle-water back-calculations. The key issues

include: 1) differing partition coefficients introduced due to SLI; 2) heterogeneity of mesostasis

pocket compositions reflected by varying water contents in apatites; 3) the presence of additional

hydrous phases, such as merrillite; and 4) other factors affecting apatite OH KDs.

4.4.1 Effects of SLI on apatite-melt partition coefficients

At present, there is a lack of relevant partition coefficients for magmas at lunar conditions

in order to accurately estimate apatite water abundances in equilibrium with the host melt (e.g.,

McCubbin et al., 2014a). Instead, most studies (e.g., Boyce et al., 2010; McCubbin et al., 2010;

Barnes et al., 2014) model water back-calculations using partition coefficients for basaltic melts

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determined at either terrestrial (Mathez and Webster, 2005) or martian (Vander Kaaden et al.,

2012) conditions. However, it is important to note that apatite/melt partition coefficients are

strongly dependent on melt composition (e.g., Mathez and Webster, 2005; Webster et al., 2009),

which results in differing partition coefficients for basaltic versus felsic liquids. In this

contribution, we have demonstrated that apatites in basalts are commonly associated with

mesostasis pockets that have undergone late-stage SLI. Whereas phosphorous generally

partitions into the Fe-rich immiscible liquid, a small portion is also present in the Si-rich

conjugate liquid; therefore, apatites may crystallize from both the Fe-rich and Si-rich immiscible

liquids. Thus, apatites which have crystallized within the Si-rich versus Fe-rich liquids would

partition water differently. Considerable future experimental work will be required to determine

these values at relevant lunar conditions (e.g., McCubbin et al., 2014a). Furthermore, the

development of SLI requires an additional set of partition coefficients to account for the

partitioning of water between the conjugate liquids of the SLI during its formation (i.e.,

Dbasalt/felsite; Fig. 3). However, at present, few experimental studies have investigated water-

partitioning behavior during SLI, and future work is needed to constrain these values. Based on

partitioning studies on other volatiles (e.g., F-Cl) in terrestrial samples, it is likely that water will

partition preferentially into the felsic fraction (Lester et al., 2013a). However, further

complicating the issue of water partitioning into conjugate immiscible liquids is volatile

diffusion. Given sufficient time, some degree of equilibrium will be attained between the two

liquids. Potentially, this is a significant complication for the mesostasis pockets that show the

most mature textures, as these are the pockets that are likely to have cooled the slowest, and thus,

are the most likely to have attained water equilibrium.

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Furthermore, identifying from which immiscible fraction an apatite has crystallized from

remains challenging, as there is no simple chemical means to distinguish apatites that have

crystallized from melts of different compositions. Recent experimental work has demonstrated

that 18O is preferentially partitioned into felsic melts relative to 16O during immiscibility,

resulting in immiscible liquids with ~1 ‰ δ18O disparities (Lester et al., 2013b). This finding

presents a tantalizing possibility that with additional stable-isotopic analysis, phosphates that

evolved from co-existing immiscible melts could be distinguished.

4.4.2 Mesostasis maturity and apatite water heterogeneity

The concept of melts within late-stage mesostasis pockets evolving to different degrees of

crystallization can be seen within a single rock (i.e., mesostasis maturity, section 4.2). This may

be part of the explanation for the large disparity in water contents in apatites between the several

mesostasis pockets in lunar rock 14053. Indeed, evidence for variations in maturities of

mesostasis pockets within a single basalt sample is reflected by the large range of water

abundances of apatites, as demonstrated herein, and by similar large ranges that have been

reported for other mare basalts; e.g., low-Ti basalt 12039 (Greenwood et al., 2011). We interpret

that the variations in textures to represent different stages of maturity, reflecting crystallization

processes occurring within mesostasis pockets. This can result in a range of mineral chemistry

within the most ‘mature’ pockets, as these represent the most ‘evolved’ or slowest cooled

mesostasis pocket. The role of fractional crystallization has recently been highlighted by Boyce

et al. (2014), who presented correlations between non-volatile trace-elements in apatites,

illustrating that apatite trace-element chemistry is controlled by fractional crystallization

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processes. It is perhaps pertinent to note, that the largest range of textures within mesostasis

pockets are observed mainly within samples that have undergone SLI; thus, we predict that such

samples will contain the greatest range in apatite chemistry.

In addition to the role of fractional crystallization and mesostasis ‘maturity’, the timing of

degassing in relation to apatite crystallization is also an important factor to consider when

account for variations in D/H. Degassing has been shown to be an important control on volatile

systematics (i.e., all H-bearing species such as H2, CH4, HCl, in addition to OH) of lunar

magmas (e.g., Tartèse et al., 2013). Tartèse and Anand (2013) estimated that >95 % loss of H2

would increase δD by over 500 ‰. In a study of lunar magmatic degassing, Tartèse et al. (2013)

identified two distinct degassing ‘end-member’ styles, which are interpreted to reflect the

relative timing of the degassing events. For example, high-Ti basalts display a limited range of

OH, but large variations of δD; this is interpreted to reflect coeval apatite crystallization and

degassing. In contrast, low-Ti basalts typically display a large range of OH concentrations, but at

narrower range of associated δD values. This is interpreted to reflect degassing prior to apatite

crystallization. The reasons for differences in degassing styles remain poorly constrained (e.g.,

Tartèse et al., 2013); however, it is clear that the timing of degassing, particularly, during coeval

apatite crystallization, may also play an important role in generating large ranges in D/H values

observed. Petrologic observations may place important constraints on degassing processes. The

late-stage nature of apatite crystallization requires that degassing plays an important role once

lavas are on the lunar surface. Magmas with such high extents of crystallization by the time

apatite begins to crystalize are unlikely of being capable of eruption (e.g., Marsh, 2002).

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4.4.3 Phosphate mineralogy

It is not uncommon for apatite and merrillite to co-exist (e.g., Taylor et al., 2004;

McCubbin et al., 2011). However, in the present study, we have identified both phases to be

water-bearing (Table 3, Fig. 8). This is consistent with reports from Martian basalts; McCubbin

et al. (2014b) reported merrillite with comparable minor whitlockite components (< 4.5%) co-

existing with OH-rich apatites. The presence of a whitlockite component within merrillite has

been proposed to result from a solid-solution between these two minerals (Hughes et al., 2008).

As highlighted by McCubbin et al., (2014b), whitlockite is not stable in high-temperature

systems, dehydrating fully to merrillite at 1050 oC. Thus, it is not expected that significant

whitlockite components will be found within lunar merrillites. Evidently, the low water

abundance reported for the merrillite analyzed in this study will not impact greatly on the overall

water budget for basalts that contain high-OH apatites, such as reported for sample 14053.

However, the occurrence of ~200 ppm H2O within merrillite may be more significant for

interpreting the water budget of the Mg-suite basalts and more evolved lithologies (i.e., quartz

monzogabbros and felsites), which display much lower apatite water abundances (< 200 ppm

H2O; Robinson et al. 2014). This is particularly pertinent considering the intergrowths of apatite

and merrillite that commonly occur within lunar samples, such as observed in basalt 14053 (Fig.

8). Considering the observation of such textures, it is clear that it is critical to fully characterize

apatite analytical points before SIMS analysis. Water is likely to partition differently between

different phosphate phases; however, at present, apatite/merrillite partitioning for water is

unconstrained. Whereas the variation in apatite water content was ascribed to varying degrees of

fractional crystallization within individual mesostasis pockets (above), an additional

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complicating factor may be that water partitioning between different phosphate minerals may be

analogous to REE behavior, which is dependent on the crystallizing relationship between

merrillite and apatite (i.e., whether merrillite crystallization preceded apatite or co-crystallized,

Jolliff et al., 1993). As highlighted by Patiño-Douce and Roden (2006), the presence of

merrillite within basalts, may reflect the ratio of P to F+Cl content of the crystalizing magma,

suggesting either enrichment in P or depletion in halogens. In either case, it is clear that

phosphate mineralogy plays an important role in controlling the bulk, incompatible-element

chemistry of mesostasis melts (e.g., Jolliff et al., 1993). As such, in order to place better

constraints on the crystallization history between co-existing apatite and merrillite, each

mesostasis pocket should be considered as individual ‘magmatic systems’.

Despite only one lunar merrillite grain having been identified as containing a minor

whitlockite component, the occurrence of such a merrillite co-existing with OH-rich apatites on

the Moon, and on Mars (McCubbin et al., 2014b), challenges the long held view that merrillite

only crystalizes within magmatic systems that are water poor (i.e., by Patiño-Douce and Roden,

2006; Ionov et al., 2006; Hughes et al., 2008). Overall, the occurrence of merrillite with minor

whitlockite components, together with the identification of water contained within the olivine

and plagioclase that surround mesostasis pockets (i.e., NAMs, Liu et al., 2012) suggests that the

assumption that all mesostasis water is contained within apatite is incorrect. Thus, crystallization

of merrillite and NAMs within mesostasis pockets must also be carefully considered or resultant

back-calculations will likely under-estimate the overall water budget for mare basalts.

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4.4.4 Other Considerations

A new slant on this apatite water problem has been fostered recently by Boyce et al.

(2014); i.e., “the apatite paradox”. In particular, it has been suggested that high water abundance

in apatites may not reflect elevated water within the host melt, but instead, simply reflects how

F-Cl-OH partitions into apatite (i.e., F-Cl-OH partition into apatite analogous to Mg-Fe

partitioning into olivine; Boyce et al., 2014). Early crystallizing apatites will preferentially

incorporate F and Cl, resulting in apatites that become increasingly OH-enriched as fractionation

proceeds. Therefore, in order to accurately back-calculate magmatic water abundances, it may

be more pertinent to use apatites with the lowest water concentrations, coupled with apatite non-

volatile, trace-element contents (i.e., SiO2, FeO) in order to establish which apatites are the first

to crystallize within mesostasis pockets (Boyce et al., 2014). Back-calculations of such apatites

may more closely reflect the initial water concentrations of the host melt. However, if indeed

such values are used within back-calculations, this assumes that the lowest apatite with the

lowest water abundance has been sampled within the period of study. This may not be the case,

and as such, by assuming this, incorrect back-calculations may still result.

An important point to further consider is when apatite crystallization begins. At present,

this is not well constrained; however, it is late in the fractional crystallization of basalt. For

example, if apatite crystallization begins prior to the onset of SLI, then the varied water contents

of apatites between individual pockets may simply reflect the isolation of pockets at different

times during fractional crystallization (i.e., Boyce et al., 2014) or the degassing history of the

lava. Despite the occurrence of SLI textures associated with each pocket, it is not necessarily

clear if SLI preceded apatite crystallization. Regardless of whether apatite crystallization

preceded or followed SLI, the differences in apatite water contents between pockets from our

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analyses of 14053 illustrate the complications associated with selecting appropriate apatites for

back-calculations (Fig. 1). For example, pocket 3 contains apatites with significantly higher

water contents than pockets 1, 2, and 4. Therefore, irrespective of whether the apatites with the

highest or lowest water contents within individual pockets are used, back-calculations of water

based on 14053 pocket 3 versus pockets 1, 2, and 4 will give dramatically different results. This

variation simply reflects large-scale water heterogeneity within mesostasis pockets, highlighting

that individual pockets may not be representative of the overall water concentration of the bulk-

rock.

5. Summary

Substantial water variations in apatite within and between individual mesostasis pockets in

lunar mare basalts reflect a complex interplay between the development of SLI and the

heterogeneity in bulk mesostasis pocket compositions, which is driven by fractionation of these

late-stage liquids. For apatite that has crystallized in the presence of immiscible silicate melts,

understanding the effects of the SLI process is critical in order to accurately back-calculate the

water content for mare basalts, and ultimately the water content of the lunar mantle. In

particular, we emphasize the occurrence of SLI and the presence of merrillite in late-stage

mesostasis pockets, which greatly complicate the back-calculations of volatile contents from

lunar apatites. Such issues, however, have been overlooked within the literature. Furthermore,

the large variation in apatite water abundances between pockets within the same sample suggests

heterogeneity between individual pocket bulk compositions, raising the problem of which pocket

is representative and which composition of apatite should be used for water back-calculations.

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From this present study, our observation helps to explain the large spread of apatite water

abundances within the literature. However, it is clear that further experimentation is required in

order to understand: 1) the nature of OH-partitioning coefficients for the apatites and merrillites

formed during SLI; 2) the relationships of the individual mesostasis pockets to the bulk-

chemistry of the basalt; and 3) partitioning of OH, relative to F and Cl during lunar SLI

conditions. Only then can robust interpretations of lunar volatile budgets can be modelled from

the existing database.

Acknowledgments

This paper has received considerable input and encouragement from many of our planetary

colleagues, for which we are collectively grateful. In particular, we wish to thank Romain

Tartèse, Malcolm Rutherford, and one anonymous reviewer for their helpful comments and

suggestions. We also wish to thank Allan Patchen, who provided assistance with the EMP

analyses, and Yunbin Guan for his guidance of SIMS analysis. This work was supported by

NASA Cosmochemistry grant to LAT (NNX11AG58G) and funds from the Planetary

Geosciences Institute at UT. YL and YC are supported by funds at JPL, which is managed by

California Institute of Technology under a contract with NASA.

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Figure and Table Captions

Figure 1: Plot of δD (‰) vs. OH (as ppm H2O) for lunar apatites. Stars represent apatites

measured from basalt 14053. Analyses are divided by mesostasis pocket; each pocket contains

distinct ranges in H2O ppm. Grey symbols for sample from 12039. (Greenwood et al., 2011).

Data for High-Ti basalt field from: Greenwood et al. (2011); Greenwood et al. (2012). Data for

Mg-Suite and Evolved rocks field from: Robinson et al. (2013); Barnes et al. (2013); Robinson et

al. (2014). Data for noritie field from: Barnes et al. (2014). Terrestrial range after Lécuyer et al.

(1998).

Figure 2: Images of mesostasis pocket from basalt 12063 investigated in this study. a)

Mesostasis pocket under reflected light and b) under transmitted light. Plag (Plagioclase); Ilm

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(Ilmenite) and; Meso (mesostasis). This pockets is made up of predominantly K-rich glass (dark,

rounded grains) set within fayalite. Lighter areas are likely to be sanidine or silica.

Figure 3: Ol-SiO2-Plag ternary illustrating crystallization sequence for a typical lunar basalt. At

the pseudo-eutectic the liquid will co-crystallize clinopyroxene, plagioclase, and SiO2. It is the

residual liquid, interstitial to these minerals, that defines the mesostasis pockets. This liquid is

then able to undergo SLI into conjugate Fe-rich and Si-rich liquids. The typical minerals

associated with each liquid fraction are listed. Water (OH) partition coefficients for apt/melt are

adapted from Boyce et al., 2014).

Figure 4: BSE, K (red), Si (blue), Ca (green), Fe (grey), and composite false-color X-ray maps

for a mesostasis pockets from sample 10003. The composite image was created by combining

the colorized images for K, Si, Ca, and Fe. Apt (Apatite); Fa (Fayalite); Plag (Plagioclase); Px

(Pyroxene); Kfs (K-feldspar); Ilm (Ilmenite) and; SiO2 (silica).

Figure 5: False-color composite (Si – blue; Ca – green; K- red; Fe - grey) X-ray maps for

representative mesostasis pockets for a-c) basalt 10003; and d-f) basalt 12063. All samples show

immature to sub-mature mesostasis textures, displaying SLI ‘sieve’ textures. Apt (Apatite); Fa

(Fayalite); Plag (Plagioclase); Px (Pyroxene); Kfs (K-feldspar); Ilm (Ilmenite); and; SiO2 (silica).

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Figure 6: False-color composite (Si – blue; Ca – green; K- red; Fe - grey) X-ray maps for

representative mesostasis pockets for a-c) basalt 70035; and d-f) basalt 12018. All samples

contain mature mesostasis textures, lacking evidence for SLI. Apt (Apatite); Fa (Fayalite); Plag

(Plagioclase); Px (Pyroxene); Kfs (K-feldspar); sa (sanidine); Ilm (Ilmenite); Tro (troilite) and;

SiO2 (silica).

Figure 7: a-c) False-color composite (Si – blue; Ca – green; K- red; Fe - grey) X-ray maps for

sample 14053. Pockets contain SLI ‘sieve’ textures; in addition to Fa brake-down into Fe-metal

and SiO2. Apt (Apatite); Fa (Fayalite); Plag (Plagioclase); Px (Pyroxene); Kfs (K-feldspar); sa

(sanidine); Ilm (Ilmenite); Tro (troilite) and; SiO2 (silica).

Figure 8: a) BSE image and b) false color (Ca, P, Ce) X-ray maps for mesostasis pocket 2 of

basalt 14053. Apatite and Merrilite are resolved from this combination of elements, highlighting

the complex intergrowth of phosphates in some grains.

Figure 9: a) (SiO2 – [Na2O + K2O + Al2O3] – [MgO FeO + TiO2 + CaO + P2O5]) ternary. b)

AMF ([Na2O + K2O] – MgO – FeO). Colored lines on both plots illustrate the liquid lines of

descent, as indicated by SPICEs crystallization modeling.. Stars represent the starting

compositions. The fractionation trend for basalt 10003 (green) intersects the two-liquid field

after ~95% crystallization. SPICEs crystallization modeling was only able to accurately model

crystallization up to 85% for sample 12018 (red). Within plot b) lLine 1) and 2) represent the

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potential liquid lines of descent for the residual liquid of sample 12018. The field of two liquids

(grey shaded area) are sensitive to P, T, fO2 conditions of the crystalizing lava. The fields were

estimated for appropriate lunar conditions from Shearer et al., (2001), Charlier et al., (2011), and

Charlier and Grove, (2012) . Data points for bulk-mesostasis compositions were calculated from

the EMP analyses of mesostasis minerals from 10003 (Table 1) and mineral modes (Table 2).

All reconstructed compositions fall within the two liquid field, suggesting that mesostasis melts

consisted on two conjugate liquids.

SOM 1: Pyroxene quadrilaterals for samples high Mg# samples (10003, 12063) and low Mg#

samples (12018, 70035). Low Mg# sample extend to ferroan compositions extending to the Hd-

Fs join. In contrast high Mg# sample do not extent to as ferroan compositions. Data from

Hollister et al. (1971), Carter et al. (1971), Kushiro et al. (1971), Papike et al. (1974), McGee et

al. (1977), Beaty and Albee, (1978), and Gamble et al. (1978).

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Table 1: Average electron microprobe analyses of mesostasis minerals in basalt 10003.

Fa SiO2 K-glass Px Ilm Plag Apt Mer no. analyses 31 6 10 26 37 10 5 4 P2O5 (wt. %) - - 0.42 (30) - - - 39.6 (1) 41.3 (5)

SiO2 29.9 (3)* 98.8 (120) 73.1 (100) 48.9 (119) 1.5 (1) 49.3 (14) 0.6 (4) 1.8 (5) TiO2 - 0.22 (4) 0.28 (16) 1.10 (1) 47.8 (4) - - - Al2O3 - 0.13 (8) 13.6 (40) 1.89 (31) 0.56 (12) 31.7 (15) - - Cr2O3 - - - 0.18 (5) 0.30 (9) - - MgO 2.72 (20) - < 0.03 4.85 (389) 0.28 (14) 0.94 (15) CaO 0.47 (10) 0.31 (3) 2.58 (88) 14.77 (97) 0.18 (15) 15.9 (13) 52.7 (10) 38.1 (2) MnO 0.76 (3) - < 0.03 0.39 (4) 0.42 (3) - - - FeO 65.9 (40) 0.54 (6) 1.05 (96) 26.97 (455) 47.6 (4) 0.83 (17) 4.74 (4) BaO - - 1.37 (299) - - - - Na2O - 0.11 (9) 0.88 (16) 0.09 (2) 2.13 (57) 0.06 (4) 0.05 (3) K2O - - 5.99 (200) - - 0.34 (18) - - F 3.09 Cl 0.28

Total 99.7 100.1 99.3 99.2 98.7 100.2 96.3 86.9 * The value in parentheses represents the 1 sigma variance in the average as expressed by the last-digit cited.

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Table 2: Modal mineral abundances for 5 mesostasis pockets from basalt 12018 and 10003.

Pocket 1 Pocket 2 Pocket 3 Pocket 4 Pocket 5 12018 Plag (vol. %) 32 34 18 24 58 K-glass + sa 42 49 49 48 58 Apt 1 4 3 7 7 Ilm 2 5 13 3 12 Silica 22 8 16 17 7 10003 Px 33 32 10 17 14 Na-plag 20 39 28 36 44 K-glass 8 5 13 6 2 Fa 27 10 26 23 7 Apt 6 2 6 5 10 Silica 3 9 15 13 23 Ilm 3 3 2 < 1 < 1

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Table 3. H2O concentrations and δD of phosphates in mesostasis pockets of sample 14053.

a Calculated using VSMOW standard. b. uncertainties quoted are 2sd.

Pocket 1 Pocket 2 Pocket 3 Pocket 4 Pocket 5 H2O

(ppm) D (�a) H2O D H2O D H2O D H2O

Apt 1 679 � 10b -15 ��� 692 ���� -23 � � 2409 � �� -86 � �� 217 ����� - - Apt 2 577 ���� - 527 ���� - 2244 ���� - 721 ���� 53 � �� - Apt 3 1026 ���� - 1447 ���� -14 ���� - Apt 4 407 ���� - - Merr 218 ����

H2O ppm

δD (‰

)

-1000

100200300400500600700800

0 2000 4000 6000

Terrestrial Range

Pocket 1Pocket 2Pocket 3Pocket 4

14053:

14053

Fig. 1

Norities

Mg-Suite & Evolved rocks

Low-Ti BasaltsHigh-Ti Basalts

12039

Figure 1

Fig. 2

plag

IlmIlm

150 μm

Meso

a) 12063 - Reflected light b) 12063 - Transmitted light

Meso

150 μm

plag

Figure 2

Fig. 3

*

SiO2

PlagOl

En

*SiO2

Plag

Spol

cpxopx

K-

K-fraction-K-glass-Apt-SiO2-K-spar

Cpx+ Plag + SiO2 + KREEP

fraction REEP-fraction

SLI

(Fenner Trend)[Low fO2 – Fe Enrichment]

DOH basalt/felsite

REEP-frac-Fa-Ilm-Apt

Ol

Opx +Cpx

Cpx + Plag

(F-Cl) H OKD apt/felsite =

(F-Clapt )(H2Ofelsite )2

(F-Clfelsite )(H2Oapt )(F-Cl) H OKD apt/basalt

=(F-Clapt )(H2Obasalt )

2(F-Clbasalt )(H2Oapt )

Starting point

Pseudo-eutectic

P

PPeritectic point

Figure 3

100 μm 100 μm

100 μm

BSE - 10003 X-Ray composite

Ilm

Plag

Px

SiO2

Fa

kfs

Px

Plag

Plag

Fa

apt

K Kα Si Kα

K-glass

SiO2

kfs

100 μm100 μm

Ca Kα

Phos

Ilm

Px

PlagPlag

100 μm

Ca Kα

apt

Ilm

Fig. 4

Fa Fa

Fa

Figure 4

200 μm

200 μm

Fa

12063K-glass

Px

Px

Fa

kfs

Plag

200 μm

Plag

Pxapt

d) 12063 Mg# 44

Ilm

Ilm

SiO2

Fa

K-glass

a) 10003 Mg# 38 b) 10003 Mg# 38

Fig. 5

e) 12063 Mg# 44 f ) 12063 Mg# 44

c) 10003 Mg# 38

200 μm 200 μm

200 μm200 μm

Plag

Px

Px

Px

Plag

Px

Plag

Fa

Fa

kfs

apt

SiO2

SiO2

SiO2

SiO2

Fa Fa

Figure 5

150 μm

Plag

Plag

200 μm

200 μm

SiO2

Px

Px

kfs

apt

apt

Ilm

Ilm

d) 12018 Mg# 57

a) 70035 Mg# 48

e) 12018 Mg# 57 f ) 12018 Mg# 57

200 μm 200 μm

b) 70035 Mg# 48

PxPlag

kfs

PlagPx

apt K-glass

apt

Px

Ilm

Ilm

K-glass

apt

c) 70035 Mg# 48

200 μm 200 μm

Fig. 6

kfs

Plag

Px

Plag

Tro Tro

Plag

kfs + SiO2

kfs + SiO2

SiO2

SiO2

SiO2

Figure 6

Fig. 7

b) 14053 Mg# 48 c) 14053 Mg# 48

200 μm

Na-richplag

px

Px SiO2apt

apt

px

a) 14053 Mg# 48

k-glassfa

kfs

px

Ca-rich plag

kfs

k-glass

Ilm

200 μm 200 μm

Figure 7

apt

mer

mer

SIMS spot

a) BSE -14053 b) X-ray map: Ca-P-Ce - 14053

plag

Ilm

K-glass

Px

fa

Phos

Fa break-down

(SiO2 + Fe)

SIMS spot 100 μm

apt

badd

mer apt

Figure 8

100 μm

Figure 8

FeO

40

20

60

80

2040

6080

Na 2O

+ K

2OM

gO

2)Re

sidua

l liq

uids

1)??

Fig.

9

Na 2O

+ K

2O +

Al 2O

3

FeO

+ M

gO

+

TiO

2 + K

2O +

P2O

5

SiO

2

20

40

60

2040

6080

2 liq

uid

field

1000

3 M

g# 3

870

035

Mg#

48

Bulk-m

esos

tasi

s

liqui

d lin

e of

desce

nt

a)b)

Fig

ure

9