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American Mineralogist, Volume 91, pages 15531564, 2006
0003-004X/06/00101553$05.00/DOI: 10.2138/am.2006.2088 1553
INTRODUCTION
The amount of water in martian magmas has significant
implications for the evolution of Mars through geologic time.
Volatile species in martian magmas could potentially influence
the planets atmosphere-hydrosphere cycle during volcanic
outgassing (McSween and Harvey 1993). The abundance of
volatile phases in extruded basalts affects the development of
acid-sulfate and neutral-chloride hydrothermal systems with
possibly significant consequences on prebiotic chemistry (Hu-
ber and Wchtershuser 1998). The abundance of magmatic
water affects physical properties of magmas, such as density
and viscosity, which in turn shape mantle dynamics and styles
of planetary volcanism (Mysen et al. 1998). Furthermore, the
extent of water in martian magmas can affect the compositionof the martian crust (Mysen et al. 1998). Basaltic magmas with
low water contents typically do not produce a broad spectrum of
diverse magmas through fractional crystallization; they instead
generate predominantly basaltic crustal compositions. Alterna-
tively, basaltic magmas with high water contents can evolve
to greater extents and produce rocks of greater compositional
variety, including more SiO2-rich, andesitic crust (Morse 1994;Hess 1989; Minitti and Rutherford 2000).
The amount and distribution of water in martian basalts is
a topic of debate among researchers (Dann et al. 2001; Foley
et al. 2003a, 2003b; Johnson et al. 1991; McSween et al. 2001,
2003; McSween and Harvey 1993; Mysen et al. 1998). Data from
orbital missions, such as Pathfinder and Global Surveyor, suggest
that andesites or basaltic andesites may exist on Mars, thereby
implying an important role for magmatic water (Minitti and Ruth-
erford 2000; Foley et al. 2003a, 2003b; McSween et al. 2003).
However, spectral signatures attributed to andesites or basaltic
andesites may instead reflect weathering of basalts (McSween et
al. 2003). Some martian basalts are characterized by melt inclu-
sions containing biotite, apatite, and amphibolephases typi-
cally associated with hydrous magmas on Earth (Johnson et al.
1991; Mysen et al. 1998; McSween and Harvey 1993). However,
the H contents of melt inclusions from these basalts are low, the
individual phases are generally anhydrous, and bulk rock water
contents are a meager 0.013 to 0.035 wt% in Shergotty (Dann
et al. 2001). Righter et al. (1997) determined that there was 0.1
wt% H2O in glass in the melt inclusions in Chassigny. Watson
et al. (1994) determined that there was approximately 0.5 wt%
H2O in biotite from melt inclusions in Chassigny. Nonetheless,
researchers note that low present-day water contents do not* Present address: 104-22 112thStreet Richmond Hill, New York11419, U.S.A. E-mail: [email protected]
The behavior of Li and B in lunar mare basalts during crystallization, shock, and thermal
metamorphism: Implications for volatile element contents of martian basalts
J. CHAKLADER,* C.K. SHEARER, ANDL.E. BORG
Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1126, U.S.A.
ABSTRACT
Late-stage rims of magmatic pyroxenes from some martian basalts show decreases in Li and B
contents relative to earlier-formed pyroxene cores. This behavior is different than expected from their
documented incompatible element behavior. Previous workers interpreted such depletions to reflect
the loss of several wt% magmatic water during basalt crystallization. This interpretation has profound
implications for the nature of the martian mantle and recent exchange of volatiles between the martian
mantle and atmosphere. To assess alternative mechanisms that may influence the behavior of Li and B
in the absence of aqueous fluid activity, the effects of changing pyroxene composition during crystal-
lization, shock pressure, and shock-associated thermal metamorphism were studied. Lithium and B
depletions are documented in late-stage rims of pyroxenes from anhydrous lunar basalts indicating that
mechanisms other than aqueous fluid activity must have influenced Li and B partitioning in these py-roxenes. Depletions of Li and B are most likely associated with changing pyroxene composition during
crystallization, and occur in lunar and martian pyroxenes with late-stage Fe-enrichment. It is interesting
that pyroxenes without late-stage Fe-enrichment show no concomitant Li and B increases. Lithium
loss may occur during breakdown of metastable pyroxferroite. Additionally, changes in Cr content
may influence the substitution mechanism involved for incorporating Li. Shock does not redistribute
Li or B but may facilitate subsequent thermally driven diffusion by the introduction of mechanical
defects in grains. Thermally metamorphosed pyroxenes exhibit higher Li and lower B contents rela-
tive to unheated pyroxenes. It is likely, therefore, that Li and B are redistributed through interactions
between pyroxenes and surrounding zones of mesostasis during thermal metamorphism.
Keywords: Lithium, boron, lunar mare basalts, martian basalts, shock pressure, thermal metamor-
phism, crystal chemistry, pyroxenes
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preclude a once hydrous past (McSween et al. 2001).
It is possible that pre-eruptive martian magmas contained
significant amounts of water, which were subsequently lost
through extensive volcanic outgassing (Dann et al. 2001; Johnson
et al. 1991; McSween and Harvey 1993). Geochemical evidence
of aqueous fluid loss from parent magmas may be preserved in
silicate minerals that crystallized from those magmas (McSween
et al. 2001; Lentz et al. 2001). Although volatile element behavior
under hydrous magmatic conditions is poorly constrained, work-
ers have suggested that Li and B depletions in pyroxene rims
of several martian basalts reflect removal of volatile elements
by escaping aqueous fluids from magmas during crystallization
(McSween et al. 2001; Lentz et al. 2001). Alternatively, given
that many martian basalts have experienced considerable shock
pressures (1545 GPa), it is possible that shock and subsequent
thermal metamorphism influenced the volatile element records of
these basalts (Fritz et al. 2003; Langenhorst et al. 1991; Stffler
et al. 1986). Previous experiments indicate that shock may affect
the extent of volatile-phase mobility and water loss from silicate
phases (Boslough et al. 1980; Minitti et al. 2003; Monkawa et al.
2003). Heating experiments on lunar basalts show that significant
redistribution and loss of volatile elements, such as Rb, occurs
from individual minerals (Nyquist et al. 1991a, 1991b). It has also
been demonstrated that pyroxene composition has a significant
effect on the partitioning behavior of elements (McKay et al.
1990; Shearer et al. 1989). Whether pyroxene composition has
an effect on Li and B behavior has not yet been documented.
To understand better the behavior of Li and B in pyroxenes
unaffected by aqueous fluid activity, this study was designed to
examine the effects of changing pyroxene composition, shock
pressure, and thermal metamorphism in anhydrous Apollo 17
and 11 lunar basalts.
SAMPLEDESCRIPTIONS
Apollo 17 Basalt 75035
Sample 75035 is a low-K, high-Ti basalt that was collected
from the Taurus-Littrow Valley along the eastern rim of the
Serenitatis impact basin at Station 5, near Camelot Crater (Fig.
1a). It is a medium-grained, subophitic basalt with subhedral
plagioclase laths (An8089, 0.10.3 mm 1 mm in size, 33%
in modal abundance) that occur in an interlocking network of
anhedral clinopyroxene (CPX) grains (continuously zoned from
early formed augitic to later-formed ferro-pigeonite composi-
tions, 0.250.5 mm in size, 44%) and irregular ilmenite laths
(0.53.0 mm in size, 15%). Anhedral to subhedral cristobalite
(5%), pyroxferroite (2%), and minor (
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CHAKLADER ET AL.: THE BEHAVIOR OF Li AND B IN LUNAR MARE BASALTS 1555
the 0.10200 g bulk sample. The test tube containing the sample was flame sealed
at ~104atm using an oxygen gas torch and a vacuum line equipped with a rotary
vein pump. Liquid nitrogen cooled the test tube during flame sealing to prevent
premature volatilization of the sample. The Apollo 11 basalt was heated for 168
hours at 1000 C, and prepared for imaging and microbeam analyses.
Pyroxenes in all samples initially were documented by back-scattered electron
(BSE) images and X-ray maps using a JEOL JSM-5800 LV scanning electron
microscope (SEM) at the Institute of Meteoritics. The BSE images and X-ray maps
of Si, Ti, Al, Mg, Fe, Mn, Ca, Na, and Cr were made with a 20 nA beam current,
20 kv accelerating potential, and
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by analyzing 9Be at a higher mass resolution (~600). The effect of matrix on Li
and B measurements is discussed in detail in previous works including Ottolini
et al. (1993), Hervig (1996), Herd et al. (2004a, 2005), and Grew et al. (1997).
Secondary ions were detected with an electron multiplier in pulse-counting mode.
All concentrations were calculated from empirical relationships between intensity
ratios against Si [(IX/ISi) SiO2, whereIXis intensity of the trace ion of interest and
ISiis the intensity of Si30]. Concentrations were obtained from well-documented
standards. Accuracy and precision are within approximately 15% for REE data
and within 5% for all other elements.
ANALYTICALRESULTS
Major- and minor-element characteristics of Apollo 17 and
11 pyroxenes
The major- and minor-element zoning characteristics of
pyroxenes from low-Ti basalts have been described exten-
sively elsewhere (Bence et al. 1970, 1971; Bence and Papike
1972). Zoning characteristics of pyroxenes in high-Ti basalts
have been discussed by Papike et al. (1976) and Longhi et al.
(1974). Representative EMP and SIMS analyses of unshocked
pyroxenes from Apollo 17 (75035) and 11 (10017) basalts areshown in Table 1.
The major- and minor-element distribution shows the follow-
ing characteristics. Pyroxenes in 75035 define a single crystalliza-
tion trend of Ca-depletion and Fe-enrichment (Fig. 3a). Samples
do not have hopper crystals, so we interpret crystal cores as early
and rims as relatively late. Most pyroxenes are zoned from augitic
(Wo43En42Fs15) cores to rims with composition of approximately
Wo9En4Fs87. Pyroxferroite is a late-stage mineral and plots at the
end of the pyroxene trajectory. In contrast, pyroxenes in 10017
show a more restricted Ca-depletion, Fe-enrichment trend (Fig.
3b). The very Fe-rich point in Figure 3b appears atypical. In
the present study, ~8.5 times as much major and minor element
data were collected for shocked 10017 pyroxenes relative to
unshocked or heated 10017 pyroxenes. The Fs content across
shocked, unshocked and heated samples is expected to be statis-
tically uniform if additional analyses of unshocked and heated
10017 samples are conducted. The 75035 data represent a more
equal distribution between shocked and unshocked samples and
are characterized by greater uniformity in Fs content. Early 10017
pyroxenes have augitic cores (Wo40En44Fs16) whereas late-stage
rims have a composition of approximately Wo11En21Fs68, and
pyroxferroite does not appear. These zoning trends contrast with
those observed within the low-Ti basalts. For example, pyroxenes
in the Apollo 12 and 15 pigeonite basaltsfirst exhibit an increase
in Ca (with limited Fe-enrichment) followed by an increase in
Fe (with limited variation) in Ca. In part, differences in zoning
between Apollo 12, 15, and 17 pyroxenes are a function of differ-
ences in crystallization sequence with plagioclase crystallization
occurring substantially after pyroxene in the Apollo 12 and 15pigeonite basalts.
Plots of Ti vs. Al + Cr provide insight on the role of octahe-
drally coordinated Cr substitution in addition to that of Ti and Al.
Cores of pyroxene crystals in 75035 have higher Ti and Al+Cr
abundances than their rims (Fig. 4a). No sharp compositional
discontinuities are observed in the trend of 75035 pyroxenes in
Figure 4a. The data define a slope of ~0.49 with a correlation
coefficient of ~0.96. The constant slope indicates early plagio-
clase saturation and concurrent crystallization in which Al must
enter plagioclase as well as pyroxene. Similarly, early pyroxene
FIGURE2.Chart summarizing variation in shock features as a function of pressure (GPa) in Apollo 17 and 11 basalts, based on the observations
of this study and a comprehensive review by Schaal and Hrz (1977).
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FIGURE3.Pyroxene quadrilaterals showing results for (a) unshocked 75035, (b) unshocked 10017, (c) shocked 75035, and (d) shocked and heated
10017 pyroxenes. Early refers to crystallographic cores, and late refers to crystallographic rims. Traverses are approximately 400 m in length.TABLE1A. Representative EMP analyses of unshocked pyroxenes from Apollo 17 and 11 basalts with listed oxide weight percents and cation
totals calculated per 6 O anions
Apollo 17 (75035)Unshocked Apollo 11 (10017)Unshocked
A17-01 A17-02 A17-03 A17-04 A17-05 A11-01 A11-02 A11-03 A11-04 A11-05
SiO2 51.48 51.31 50.19 48.41 45.84 51.32 49.49 51.27 50.28 50.61TiO2 1.59 1.30 1.06 0.82 0.65 1.98 1.87 1.20 1.01 0.67Al2O3 1.77 1.38 1.03 1.03 1.95 2.11 2.09 1.42 1.19 0.79MgO 15.70 15.36 10.90 5.76 3.39 17.03 14.25 13.90 13.01 12.99FeO 11.97 14.90 21.77 31.32 41.24 10.61 14.64 20.74 24.20 28.80MnO 0.25 0.32 0.40 0.53 0.68 0.21 0.25 0.37 0.38 0.47CaO 16.93 15.14 15.15 12.09 6.47 16.54 16.12 11.53 9.69 6.21Na2O 0.09 0.06 0.08 0.03 0.02 0.09 0.06 0.05 0.05 0.04Cr2O3 0.30 0.23 0.15 0.09 0.08 0.59 0.55 0.32 0.37 0.19 Total 100.08 99.99 100.73 100.09 100.32 100.49 99.31 100.80 100.18 100.76Si 1.92 1.93 1.94 1.96 1.92 1.90 1.89 1.95 1.95 1.97Ti 0.04 0.04 0.03 0.03 0.02 0.06 0.05 0.03 0.03 0.02Al 0.08 0.06 0.05 0.05 0.10 0.09 0.09 0.06 0.05 0.04Mg 0.87 0.86 0.63 0.35 0.21 0.94 0.81 0.79 0.75 0.75Fe 0.37 0.47 0.70 1.06 1.44 0.33 0.47 0.66 0.78 0.94Mn 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.02Ca 0.68 0.61 0.63 0.52 0.29 0.66 0.66 0.47 0.40 0.26Na 0.01 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00Cr 0.01 0.01 0.00 0.00 0.00 0.02 0.02 0.01 0.01 0.01 Total 3.99 4.00 4 .00 3.99 4.01 4.00 4.00 3.98 3.99 3.99
TABLE1B.Distribution of cations across the Tetrahedral, M1, and M2 pyroxene sites for the same EMP analyses
Tetrahedral Site M1 Site M2 Site
Si Al Total Al Mg Cr Ti Mn Fe Total Fe Mn Mg Ca Na Total
A17-01 1.92 0.08 2.00 0.00 0.87 0.01 0.04 0.01 0.06 1.00 0.31 0.00 0.00 0.68 0.01 0.99A17-02 1.93 0.06 2.00 0.00 0.86 0.01 0.04 0.01 0.08 1.00 0.39 0.00 0.00 0.61 0.00 1.00A17-03 1.94 0.05 1.99 0.00 0.63 0.00 0.03 0.01 0.32 1.00 0.38 0.00 0.00 0.63 0.01 1.02A17-04 1.96 0.04 2.00 0.01 0.35 0.00 0.03 0.02 0.60 1.00 0.46 0.00 0.00 0.52 0.00 0.99A17-05 1.92 0.08 2.00 0.02 0.21 0.00 0.02 0.02 0.73 1.00 0.72 0.00 0.00 0.29 0.00 1.01A11-01 1.90 0.09 1.99 0.00 0.93 0.02 0.06 0.00 0.00 1.00 0.33 0.01 0.01 0.66 0.01 1.01A11-02 1.89 0.09 1.98 0.00 0.81 0.02 0.05 0.01 0.11 1.00 0.36 0.00 0.00 0.66 0.01 1.02
A11-03 1.95 0.05 2.00 0.01 0.79 0.01 0.03 0.01 0.15 1.00 0.51 0.00 0.00 0.47 0.00 0.98A11-04 1.95 0.05 2.00 0.00 0.75 0.01 0.03 0.01 0.20 1.00 0.59 0.00 0.00 0.40 0.00 0.99A11-05 1.97 0.03 2.00 0.00 0.75 0.01 0.02 0.02 0.20 1.00 0.73 0.00 0.00 0.26 0.00 0.99
TABLE1C.Representative SIMS analyses of unshocked pyroxenes from Apollo 17 and 11 basalts
Apollo 17 (75035)Unshocked Apollo 11 (10017)Unshocked
A17-01 A17-02 A17-03 A17-04 A17-05 A11-01 A11-02 A11-03 A11-04 A11-05
Li (ppm) 8.1 8.4 6.3 4.7 4.2 14.0 14.8 14.8 11.4 13.1B (ppm) 0.6 0.8 0.3 0.4 0.2 7.1 6.4 3.9 2.9 2.0Ce (ppm) 2.1 2.4 3.0 6.1 7.5 5.1 5.9 19.1 28.3 86.2Yb (ppm) 6.2 7.0 6.4 23.7 44.8 5.9 6.8 18.2 22.5 50.8Li/Yb 1.3 1.2 1.0 0.2 0.1 2.4 2.2 0.8 0.5 0.3B/Ce 0.3 0.3 0.1 0.1 0.0 1.4 1.1 0.2 0.1 0.0
Note:Trace elements were measured at the same spots as major and minor elements but with a larger beam diameter (10 m vs.
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in 10017 has higher Ti and Al+Cr values with respect to their rims
(Fig. 4b). Apollo 11 pyroxenes also show no sharp discontinuities
in their trend, and the constant slope of ~0.46 with a correlation
coefficient of ~0.89 also indicates early plagioclase saturation
and concurrent crystallization. Thus, Ti and Al behavior reflect
co-crystallization of plagioclase and pyroxene in both Apollo
17 and 11 basalts. This is not in agreement with the Apollo 11
crystallization sequence proposed by Bence and Papike (1972),
in which plagioclase crystallizes later. Additionally, the constant
slopes on plots of Ti vs. Al+Cr for pyroxenes from Apollo 17 and
11 high Ti basalts also contrast with slopes for analogous data
for pyroxenes from low-Ti basalts. For example, pyroxenes in
the Apollo 12 and 15 pigeonite basalts show a sharp change in
slope in Ti/Al from
0.25 to
0.5 (Bence and Papike 1972). Thisdiscontinuity reflects the onset of plagioclase crystallization and
is not seen in the high-Ti basalts where early plagioclase growth
was concurrent with that of pyroxene.
The trends on the pyroxene quadrilateral diagrams illustrate
that the properties of Wo-En-Fs components do not vary with
changes in shock pressure or thermal metamorphism for Apollo
17 and 11 pyroxenes (Figs. 3c and 3d). Given the significantly
greater number of analyses of shocked proxenes compared with
unshocked or heated proxenes, the apparent differences in Fs
content in 10017 pyroxenes are attributed to non-representative
sampling. Additionally, no significant change is seen in Ti or Al
+ Cr with changes in shock pressure or thermal metamorphism
(Fig. 4). However, heated pyroxenes have higher Na2O contents
(by ~15%) than unheated pyroxenes in the Apollo 11 basalt,
which likely reflects thermally driven Na redistribution between
pyroxenes and surrounding phases.
Trace-element characteristics of Apollo 17 and 11 pyroxenes
Within the Ca-depletion and Fe-enrichment trajectory for
pyroxenes in 75035, the concentrations of incompatible trace
elements, such as Ce, a light rare-earth element (REE), and Yb, a
heavy REE, increase from cores to rims. The partition coefficients
of REE in pyroxenes are dependent on Ca content, which expands
the distance between Si tetrahedra in pyroxenes and facilitates
partitioning of REE into the M2 site (Shearer et al. 1989). As Ca
contents decrease in pyroxenes, REE contents are expected to
decrease as well. The observation that Ca decreases with increas-
ing REE toward pyroxene rims implies that REE concentrations
in melt increased strongly enough to overcome the decrease
of REE partition coefficients. Increasing Ce and Yb contents
from pyroxene cores to rims is consistent with experimentally
determined partition coefficients for Ce and Yb between CPX
and basaltic melt of about 0.06 and 0.3, respectively (McKay
et al. 1990). Similar Ce and Yb increases are seen in the late-
stage rims of pyroxenes in 10017. These observations are also
consistent with late-stage Ce and Yb increases in pyroxenes from
the Apollo 12 and 15 low Ti basalts (Shearer et al. 1989). Both
Ce and Yb occur in greater abundance in Apollo 11 pyroxenes
compared to Apollo 17 pyroxenes. In part, this may reflect dif-
ferences in the overall Ce and Yb abundances in the bulk 75035
and 10017 basalts.
Although Li generally behaves as an incompatible element
in pyroxenes (with the exception of spodumene, LiAlSi2O6) and
has an average experimentally determined partition coeffi
cientof ~0.2, similar to that of Yb, Li distribution is unique in the
pyroxenes of this study (Brenan et al. 1998; Herd et al. 2002).
Lithium depletions are observed in late-stage, Yb-rich rims rela-
tive to early formed, Yb-poor cores of Apollo 17 pyroxenes (Fig.
5a). In contrast, Apollo 11 pyroxenes exhibit relatively constant
Li values from their cores to their rims (Fig. 5b).
Lithium distribution varies with compositional changes in
75035 pyroxenes. Low Li abundances correspond to regions of
high Fe, low Ca, and low octahedrally coordinated Cr3+in Apollo
17 pyroxenes (Fig. 6). In contrast, Apollo 11 pyroxenes show no
correlations between Li and Fe, Ca, or octahedrally coordinated
Cr3+. Boron depletions occur in rims relative to cores in both
75035 and 10017 pyroxenes (Fig. 7).
Trace-element behavior does not appear to be affected byshock pressure. However, experimentally heated pyroxenes
in 10017 are characterized by greater Li contents and lower B
contents than unheated pyroxenes. Boron contents in plagioclase
range from 0.4 to 0.7 ppm and are less than or equal to those of
pyroxene. Plagioclase that surrounds pyroxene has about 2 5
the Li content of pyroxenes, ranging from 18 to 20 ppm. In ad-
dition to pyroxferroite, plagioclase, and oxides, the mesostasis
of the lunar basalts contain basaltic glass and Si-rich glass, both
of which exhibit up to 50 the Li and B contents of pyroxenes,
ranging from 35 to 55 ppm Li and 25 to 60 ppm B.
y = x
y = 0.5x
y = 0.4916x
R2= 0.9579
0
0.05
0.1
0.15
0.2
0.25
0.3
0 0.05 0.1 0.15 0.2 0.25 0.3
[(Al+Cr)/Cation Sum]*4
[Ti/Catio
nSum]*4
y = x
y = 0.5x
y = 0.4572x
R2= 0.8941
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.1 0.2 0.3 0.4
[(Al+Cr)/Cation Sum]*4
(Ti/Catio
nSum)*4
(A)
(B)
Late
Early
Early
Late
75035 Pyroxenes
10017 Pyroxenes
y = 0.49xR
2= 0.96
y = 0.46xR
2= 0.89
FIGURE 4. Plots of Ti vs. (Al + Cr), normalized to cation sum
for (a) Apollo 17 pyroxenes and (b) Apollo 11 pyroxenes. Symbols
are identical to those used in Figure 3. Early to late distinctions are
based on the decreasing Ca and increasing Fe trends observed in the
pyroxene quadrilaterals of Figure 3. Traverses are approximately 400
m in length.
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DISCUSSION
Crystal chemistry of Li and B in pyroxene
The pyroxene structure has been described comprehensively
in previous works (Deer et al. 1992; Cameron and Papike 1981).
Cation partitioning into the tetrahedral, octahedral (M1), and
polyhedral (M2) sites of pyroxenes occurs through a balance
of charge and ionic radius (Fig. 8). Substitution of minor ele-
mentsincluding Ti, Cr, and Alinto the pyroxene structure
has been documented previously by Bence at al. (1970), Papike(1980), and Robinson (1980), whereas Shearer et al. (1989)
discussed potential substitution mechanisms for selected trace
elements.
The distribution of Ti, Cr, and Al in pyroxenes in 75035 and
10017 reflects magmatic changes during basalt crystallization
and has not been disturbed through shock or heating events (Fig.
4). Pyroxenes from both basalts have a slope of ~ on plots of
Ti vs. Al + Cr cations per six oxygen anions, thus [VI(Ti4+)+ 2
IV(Al3+)] [VI(R2+)+ IV2Si] was the dominant mechanism for
Al and Ti substitution. Negative deviations from the slope
FIGURE5.Plots ofLi (ppm) vs. Li/Yb for pyroxenes from (a) Apollo
17 and (b) Apollo 11 basalts. Symbols are identical to those used in
Figure 3. Early to late distinctions are based on the decreasing Ca and
increasing Fe trends observed in the pyroxene quadrilaterals of Figure3. Apollo 17 pyroxenes display greater extents of core-to-rim decreases
in Ca and increases in Fe, compared to Apollo 11 pyroxenes. Traverses
are approximately 400 m in length.
FIGURE6. Plot ofoctahedrally coordinated Cr3+vs. Li (ppm) for
pyroxenes from Apollo 17 basalt. Symbols are identical to those used
in Figure 3. Early to late distinctions are based on the decreasing Ca and
increasing Fe trends observed in the pyroxene quadrilaterals of Figures
3a and 3c. Traverses are approximately 400 m in length.
FIGURE7. Plots ofB (ppm) vs. B/Ce for pyroxenes from (a) Apollo 17
(75035) and (b) Apollo 11 (10017) basalts. Symbols are identical to those
used in Figure 3. Early to late distinctions are based on the decreasing
Ca and increasing Fe trends observed in the pyroxene quadrilaterals of
Figure 3. Traverses are approximately 400 m in length.
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imply incorporation of Cr through [VI(Cr3+)+ IV(Al3+)][VI(R2+)
+ IVSi] into pyroxenes. Alternatively, Al may have partitioned
into pyroxenes through [VI(Al3+)+ IV(Al3+)] [VI(R2+)+ IVSi].
Slopes of unity and represent 100% incorporation of Ti3+or
Ti4+, respectively in Figure 4. Positive deviations from a slope
of 1/2 have been interpreted as indicating the presence of a
minor Ti3+ component through [VI(Ti3+)+ IV(Al3+)] [VI(R2+)+IVSi] in Apollo 17 and 11 pyroxenes, which is consistent with
the low oxygen fugacity in which these basalts crystallized, as
well as the general lack of Fe3+in lunar minerals (Papike et al.
1998). Late-stage decreases in Ti content of pyroxenes may
reflect the appearance of ilmenite on the liquidus (Stimac and
Hickmott 1994).
As is the case for Ti, Cr, and Al, the partitioning behavior of
the REE and Li1+(0.076 nm ionic radius) into the M2-site and
B3+(0.011 nm ionic radius) into the tetrahedral site of pyroxenes
requires charge balancing and will be influenced by the avail-
ability of cations suitable for coupled substitutions (Fig. 8; Bloss
1994; Deer et al. 1992; Shearer et al. 1989). Trivalent REE, such
as Ce and Yb, occur in pyroxenes as M2(REE)3+M1R2+IV(Si,Al)O6,
where R2+is Fe or Mg (Shearer et al. 1989). Lithium (M2) may
couple with trivalent Al (M1) through [VI(Li1+)+ VI(Al3+)][2
VI(R2+)] or Cr (M1) through [VI(Li1+)+ VI(Cr3+)] [2VI(R2+)],
whereas B (tetrahedral) may couple with Al (M1) through [IV(B3+)
+ VI(Al3+)][VI(R2+)+ IVSi] during substitution.
Partitioning of Li is correlated with that of Cr in Apollo 17
pyroxenes (Fig. 6). Depletions in Li occur in late-stage rims,
which also exhibit depletions in Cr. These observations may
reflect the possibility that Li1+ (M2) and Cr3+ (M1) substitute
together into pyroxene through [VI(Li1+)+ VI(Cr3+)][VI2(R2+)].
With the appearance of spinel on the liquidus, partitioning of Cr3+
into the M1 site may be inhibited. Consequently, Li may lose
its coupled cation and may not be incorporated into late-stage
pyroxene rims as efficiently as in early cores.
Previous experimental work determined partition coefficients
(D-values) for the volatile elements, Li and B, between CPX and
basaltic melt (Brenan et al. 1998; Herd et al. 2002). D-values
average 0.2 and 0.02 for Li and B, respectively. As CPX crystals
grow in a closed basaltic system, concentrations of Li and B in
the melt increase. During final stages of pyroxene crystalliza-
tion, higher abundances of Li and B in the residual melt result in
higher concentrations of these elements in CPX. Thus, analyses
of pyroxene are expected to show increasing Li and B concentra-
tions from cores to rims (McSween et al. 2001). This behavior is
similar to that of other non-volatile, incompatible elements. The
D-value for Li closely matches that of Yb, whereas theD-value
for B is similar to that of Ce (McKay et al. 1990).
However, Li and B behavior does not resemble that of Yb
and Ce in pyroxenes of this study. Figure 5 compares Li (ppm)
against Li/Yb for pyroxenes from the Apollo 17 and 11 basalts.
Ytterbium behaves incompatibly, and its concentration increases
toward crystal rims. With increasing Yb contents, Li/Yb ratios
decrease such that rims are characterized by lower Li/Yb values
than cores. Figure 5a demonstrates that clear Li depletions (on
the order of ~10 ppm) exist in rims relative to cores in Apollo
17 pyroxenes. In comparison, B is lower in pyroxene rims rela-
tive to cores in both basalts by ~16 ppm, as shown in Figure 7,
which plots B (ppm) against B/Ce for pyroxenes from the Apollo
17 and 11 basalts. Cerium behaves incompatibly, increasing in
amount toward rims. Thus, Li and B do not correlate with REE
abundances, indicating that these elements are not behaving in
a manner predicted from measured Li, B, and REE partition
coefficients between CPX and basaltic melt.
Experimental studies on Li and B partitioning have yielded
FIGURE8. Plot of pyroxene crystallographic sites and associated cations (Shannon and Prewitt 1969). Despite the similarity in experimentally
determined partition coefficients between Li and Yb, mechanisms of Li incorporation into pyroxenes from basaltic melt are very different from
those of Yb, due largely to contrasts between ionic radii and valence. Ionic radii contrasts are particularly noteworthy for B and Ce, which also
have similar experimentally determinedD-values between pyroxene and basaltic melt.
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CHAKLADER ET AL.: THE BEHAVIOR OF Li AND B IN LUNAR MARE BASALTS 1561
conflicting results. Herd et al. (2004b) suggested that the parti-
tion coefficients for Li and B between pyroxene and anhydrous
basaltic melt are not influenced by changing Ca (M2) or Al (tetra-
hedral, M1) contents of pyroxenes. However, Herd et al. (2004b)
did not study the effects of either Cr or Fe enrichment. Brenan et
al. (1998) examined the influence of changing CPX composition
on partitioning of Li and B between CPX and aqueousfluid. Their
results show that Li and B partitioning into aqueous fluid relative
to CPX decreases with decreasing Al/Si ratios in CPX (Brenan
et al. 1998). It is possible that D-values of trace elements vary
with the availability of coupled cations during substitution. For
the Apollo 17 basalt, Li couples with Cr during partitioning into
pyroxenes. During early stages of pyroxene growth, DLimay
approach unity with ready availability of Cr. Subsequently,DLi
decreases with decreasing magmatic Cr in 75035 pyroxenes.
The present study contends that coupled substitution can play a
significant role within the realm of Henrys law behavior.
Lithium-chromium coupling is seen in Apollo 17 pyroxenes
(Fig. 6), and Li partitioning into the M2-site of pyroxene is in-
fluenced by the availability of Cr, which enters the M1-site. The
REE partition into the M2-site coupled with divalent M1 cations,
typically Fe and Mg. Boron partitions into the tetrahedral site
with the octahedrally coordinated M1 cation, Al. However, such
coupled substitution was not observed and is not a satisfactory
explanation for late-stage B decreases in pyroxenes. Coupled sub-
stitution during early crystallization would have to increaseDB
between pyroxene and basaltic melt by two orders of magnitude
(from 0.02 to unity) for B to behave as a compatible element,
then decrease with diminishing Al availability during late-stages
of crystallization (Herd et al. 2004b). Such a dramatic increase
inDBwith Al has not been observed in previous experimental
studies (Herd et al. 2004b).
Abundances of incompatible element pairs with similar parti-
tion coeffi
cients in pyroxenes, such as Li-Yb and B-Ce, shouldincrease from cores to rims. However, contents of Li in the Apollo
17 basalt and B in both basalts decrease at rims of unshocked
pyroxenes. The depletion trends of Li and B in pyroxenes from
these lunar basalts are consistent with observations of McSween
et al. (2001) and Lentz et al. (2001) on pyroxenes from martian
basalts. Extrapolating the interpretations of McSween et al.
(2001) and Lentz et al. (2001), pyroxenes from high-Ti lunar
mare basalts would have had to exsolve greater than 4 wt%
magmatic water to achieve such Li and B depletions. However,
given the completely anhydrous nature of the lunar basalts,
alternative explanations are necessary.
Li and B partitioning into a volatile phase
As shown above, Li is infl
uenced more than B by changingpyroxene composition during crystallization of lunar basalts.
However, it is possible that partitioning into volatile phases also
may give rise to late-stage decreases. Analyses of pyroxenes from
several martian meteorites (QUE 94201, NWA 480, Shergotty,
Zagami) show that earlier-formed, augitic cores have higher Li
and B contents than later-formed rims of pigeonite composition
(McSween et al. 2001; Lentz et al. 2001, 2004; Beck et al. 2004).
This decrease in Li and B toward later-formed regions of py-
roxenes has been interpreted as indicating loss of several weight
percent magmatic water from mantle-derived, highly evolved
melt during crystallization (McSween et al. 2001; Lentz et al.
2001, 2004; Beck et al. 2004). Although lunar basalts have no
water, they do have vugs and vesicles, indicative of CO (Fogel
and Rutherford 1995; Weitz et al. 1999).
Recent analyses of Apollo 17 orange volcanic glasses suggest
that oxidation of C and reduction of multivalent cations, such as
Fe and Cr, likely formed CO that drove fire fountaining (Weitz et
al. 1999). Although experimental work on the partitioning of Li
and B between basalts and CO is lacking, observations of Li and
B volatility during gas-charged lunarfire-fountaining have been
discussed (Delano 1986; Meyer and Schonfeld 1977; Shearer et
al. 1994). Loss of volatile elements, such as S, during eruption has
been documented (e.g., Shearer et al. 1994). Delano (1986) and
Meyer and Schonfeld (1977) have suggested that B also may have
been lost during eruption. Alternatively, although S is depleted in
glasses relative to crystalline mare basalts, Li/Be and B/Be ratios
between the two are similar (Shearer et al. 1994). In fact, Li is
somewhat more enriched in some fire-fountain glasses relative
to crystalline mare basalts at the same Be content, whereas some
loss of B has been documented (Shearer et al. 1994). Significant
loss of Li to a volatile (gaseous) phase has not been inferred for
lunar fire-fountain glasses, and therefore may not have been an
important process in lunar basalt evolution. The observation from
the present study, that experimentally heated pyroxenes have
lower B contents than unheated pyroxenes, may lend support to
gas-phase losses of B during eruption. However, the extent of
such B loss has yet to be examined in detail.
Trace element behavior during shock
Shock effects in martian meteorites have been described in
detail by Stffler et al. (1986), and include complete transforma-
tion of plagioclase to diaplectic glass (maskelynite), formation
of planar deformation features, and mechanical twinning and
fi
ne-grained crystalline textures following localized melting atgrain boundaries.
The results of shock-recovery experiments on hydrous miner-
als suggest that variation in shock pressure influences the extent
of water mobility and dehydration (Boslough et al. 1980). Also,
recent work suggests that water-poor amphibole found in martian
meteorites did not crystallize at depth as a primary phase but
instead formed by impact at pressures ranging from 38 to 50 GPa
(Monkawa et al. 2003). Furthermore, during a series of heating
experiments on shocked lunar basalts, significant redistribution
and loss of volatile elements, such as Rb, was observed in pla-
gioclase and pyroxene mineral separates (Nyquist et al. 1991a).
Although workers have attributed volatile-element depletions in
pyroxene rims to outgassing-induced water loss on Mars, Li and
B rim depletions have been observed predominantly in basalticShergottites that show pervasive shock features in pyroxenes and
plagioclase (McSween et al. 2001; Lentz et al. 2001, 2004; Beck
et al. 2004). In contrast, the relatively unshocked Nakhlites do
not show extensive Li and B rim depletions.
The results of this study suggest that shock does not in-
duce loss and redistribution of volatile elements in pyroxenes,
plagioclase, or glass. As shown in Figure 3, pyroxene major-
element compositions are not affected by shock pressure. Also,
pyroxenes exhibit no loss or redistribution of Li or B. Sodium,
Li, and B contents in plagioclase remain the same in shocked
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CHAKLADER ET AL.: THE BEHAVIOR OF Li AND B IN LUNAR MARE BASALTS1562
and unshocked basalts. Mesostasis, although remelted at high
pressure, does not appear to have lost Li and B. The textural and
petrographic observations of shocked samples in this study are
consistent with those from previous work and illustrate that shock
pressures generate mechanical defects, microfracturing, and
deformation in pyroxenes (Schaal and Hrz 1977; Kieffer et al.
1976). Such mechanical distortions of pyroxenes may facilitate
the redistribution of Li and B during subsequent shock-associated
thermal metamorphism.
Trace-element behavior during shock-associated thermal
metamorphism
Thorough discussions of diffusion mechanisms operational
during shock and thermal metamorphism are provided in Freer
(1981) and Manning (1974). Pyroxenes from all basalts that un-
derwent thermal metamorphism in the laboratory have increased
Li and Na contents relative to pyroxenes from unheated samples.
Plagioclase has 25the Li content and 100the Na concen-
tration of pyroxenes. Therefore, plagioclase may have been a
Li and Na contributor. Additionally, mesostasis surrounding
pyroxene has up to 50the Li content of pyroxenes. Mesostasis
may have re-melted during heating experiments and may be the
source for heightened Li contents in thermally metamorphosed
pyroxenes. The solidus temperature for high-Ti basalts exceeds
1100 C (Longhi 1992), whereas the heating experiments of this
study were conducted at 1000 C. The presence of even small
amounts of melt offers volatile elements like Li a free surface and
direction of rapid melt-mineral diffusion (Freer 1981). Recent
experimental work demonstrates that Li diffusivity in molten
silicates is extraordinarily high, with a diffusion coefficient (8
105cm2/s), similar to these of dissolved molecular water and
He at 7 105cm2/s and 5 105cm2/s, respectively (Richter
et al. 2003).
Experimentally heated pyroxenes show lower B contents thanunheated pyroxenes. Because mesostasis contains up to 50 the
B content of pyroxenes, it is not a likely sink for B lost from
heated pyroxenes. Plagioclase grains surrounding pyroxenes in
the heated sample show no increases in B content relative to
unheated samples and also are not viable sinks. It is possible that
B diffused into surrounding regions that were not analyzed. Con-
versely, efficient loss of B may suggest that the Apollo 11 basalt
behaved as an open system during heating. However, zones of
mesostasis do not show significant loss of B with heating. The re-
sults of this work demonstrate that the gradual diffusion effects of
thermal metamorphism are more significant than the immediate
effects of shock pressure in redistributing Li and B in anhydrous
basaltic pyroxenes. However, the textural variations caused by
shock likely facilitate the process of high-temperature Li and Bdiffusion between pyroxenes and surrounding phases.
Relevance to Li and B zoning in martian basalts
Analyses of pyroxenes from several martian meteorites
(QUE 94201, NWA 480, Shergotty, Zagami) show that Li and B
contents generally decrease from cores to rims (McSween et al.
2001; Lentz et al. 2001, 2004; Beck et al. 2004). The majority
of late-stage pigeonite rims of these pyroxenes exhibit extensive
Fe-enrichment (up to 30 wt% FeO) similar to those of pyroxenes
from lunar mare basalt 75035. Martian basalts with late-stage Li
and B decreases in pyroxene rims all contain pyroxferroite. Late-
stage Apollo 17 pyroxene also has significant Li and B decreases,
and pyroxferroite is present. The formation and breakdown of
pyroxferroite requires a net loss of Ca, during which Li loss also
may occur. Pyroxenes from the Apollo 11 basalt are more similar
to the majority of pyroxenes from Nakhla, in which late-stage Fe-
enrichments are not as extensive as those in 75035, QUE 94201,
NWA 480, Shergotty, and Zagami. No late-stage pyroxferroite or
Li decreases are observed in the Apollo 11 or Nakhla basalts. An
additional control on Li distribution could be Fe3+, which occurs
in martian pyroxenes. Karner et al. (2006) estimated that the
Fe3+/(Fe3++ Fe2+) in martian pyroxenes ranged up to 3%. Dyar
and Delaney (2000) estimated that this ratio could be as high as
20%. In either case, a coupled substitution involving trivalent
Fe and Li could be important. Until Fe3+variations in pyroxene
are systematically compared to those of Li, this relationship
cannot be confirmed.
CONCLUDINGREMARKS
McSween et al. (2001) and Lentz et al. (2001) suggested
that Li and B depletions in later-formed regions of pyroxenes
reflect the loss of several weight percent magmatic water during
crystallization and eruption of martian basalts. The results of the
present work show that clear Li and B depletions also occur in
later-formed regions of pyroxenes from anhydrous lunar mare
basalts. Thus, mechanisms other than fluid processes can give
rise to Li and B depletions in pyroxenes. Possible magmatic
processes that can cause Li and B decreases in the Apollo 17
and 11 pyroxenes of this study are mentioned below and sum-
marized in Figure 9. The availability of a cation-couple during
substitution can influence the compatibility of an element during
crystallization (e.g., octahedrally coordinated M2-Cr3+for M1-
Li in pyroxenes). Cation compatibility may also be influenced
by changes in mineral composition. Lithium concentrationsdecrease in pyroxenes with extreme Fe enrichment. Such high
Fe contents permit the formation of metastable pyroxferroite.
During pyroxferroite breakdown, a net loss of Ca is structurally
FIGURE9. Summary of magmatic processes that can give rise to
late-stage Li and B decreases in Apollo 17 and 11 pyroxene rim.
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CHAKLADER ET AL.: THE BEHAVIOR OF Li AND B IN LUNAR MARE BASALTS 1563
required and Li may be excluded in the process. Alternatively,
DLiduring pyroxene crystallization may exceedDLiduring py-
roxferroite crystallization. Volatile loss from lunar basalts into
CO during gas-charged fire-fountaining appears unlikely to in-
fluence Li but may affect B; pyroclastic glasses have somewhat
lower bulk B-contents than crystalline mare basalts (Shearer
et al. 1994). Additionally, experimentally heated pyroxenes in
this study exhibit lower B contents and higher Li contents than
unheated ones. Open-system loss and closed-system redistribu-
tion through interactions between pyroxenes and surrounding
zones of plagioclase and mesostasis have likely occurred during
heating in the laboratory. Although high shock pressures do not
redistribute Li or B in pyroxenes, shock likely facilitates later
thermal diffusion processes through the introduction of mechani-
cal defects in grains.
ACKNOWLEDGMENTSThanks go to Mike Spilde and Jana Berlin for assistance with EMP analyses,
to Dave Draper, Rhian Jones, Zachary Sharp, and Abdul-Mehdi Ali for assistancewith experimental set-up and sample preparation, and to Dave Vaniman, SteveSimon, and Kevin Righter for their diligent reviews. This research was partially
supported by the Institute of Meteoritics and NASA Mars Fundamental Researchgrant no. NAG5-12783 to C. K. Shearer. Lunar thin sections and bulk sampleswere provided by the NASA Johnson Space Center, where shock experimentswere carried out by Fred Hrz.
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