Li, B - Behavior in Lunar Basalts During Shock and Thermal Metamorphism: Implications for H2O in Martian Magmas

2005·0.15 MB·English

by  Chaklader, Johny

Lunar and Planetary Science XXXVI (2005) 1426.pdf
Li, B - BEHAVIOR IN LUNAR BASALTS DURING SHOCK & THERMAL METAMORPHISM:
IMPLICATIONS FOR H O IN MARTIAN MAGMAS Johny Chaklader1 (jchaklad @unm.edu), C.K. Shearer1
2
([email protected]), and F. Hörz2. 1Institute of Meteoritics, Department of Earth & Planetary Sciences, University
of New Mexico, Albuquerque, NM 87131-1126. 2NASA Johnson Space Center, Houston, Texas 77058.

Introduction: The water-content of Martian Sample Description: Unshocked A-11 sample is
magmas is a topic of debate among researchers. Some a high-Ti, low-K, medium grained, subophitic basalt
Martian basalts are characterized with melt inclusions with abundant clinopyroxene, calcic plagioclase, il-
of biotite, apatite and amphibole; phases typically as- menite, minor interstitial glass and pyroxferroite. Un-
sociated with hydration reactions on Earth [1-3]. shocked A-17 sample is a high-Ti, medium-grained,
However, the H-content of melt inclusions from these subophitic basalt consisting of clinopyroxene, calcic
basalts is low, and bulk-rock H O-contents range from plagioclase, ilmenite, cristobalite, pyroxferroite, native
2
a meager 0.013 to 0.035 wt. % in Shergotty [4]. None- iron, troilite, and some mesostasis. Both basalts crys-
theless, researchers note that low present-day water tallized at f O ≈ IW -1 under extremely dry conditions
2
contents do not preclude a once hydrous past [5]. [12]. A-11 and A-17 basalts were shocked at the JSC
Since light lithophile elements (LLE), such as Li and at pressures ranging 34.8 to 78.5 GPa and 9.3 to 99.9
B, partition into aqueous fluids at T > 350 °C, workers GPa, respectively [9]. A sample of A-11 basalt was
proposed that Li-B depletions in pyroxene rims of heated for 7 days at 1000° C [10,11].
Nakhlite and Shergottite basalts reflect the loss of sev- Results & Discussion: Results show that mecha-
eral weight percent water from Martian magmas dur- nisms of coupled substitution utilized by major and
ing crystallization [6]. Since similar depletions were minor elements are important to the distribution of
observed in pyroxene rims from completely dry lunar LLE in pyroxenes. All pyroxenes express well-
basalts, it is likely that alternative mechanisms also defined compositional zoning with augitic cores and
contribute to the distribution of elements such as Li pigeonite rims in pyroxene quadrilateral components
and B [7]. Given that many Martian basalts have ex- (Figures 1, 2). A-17 pyroxenes exhibit a more exten-
perienced considerable shock pressures (15-45 GPa), it sive enrichment in Fe-content than A-11 pyroxenes.
is possible that shock and subsequent thermal meta- Shock does not affect zoning in En, Fs, and Wo-
morphism may have influenced the volatile element components. Figure 3 shows that pyroxene cores are
records of these basalts [8]. In order to better under- enriched in Ti and Al relative to rims. A-17 analyses
stand the distribution of Li and B, we are studying the plot tightly along the line of ½ slope, irrespective of
effects of crystal chemistry, shock pressure, and ther- shock pressure or distance along transect, showing a
mal metamorphism in pyroxenes from lunar basalts. clear preference for coupled substitution mechanism:
Below, we discuss results from experimentally [(Ti4+)VI+2(Al3+)IV ] ↔ [(R2+)VI+2SiIV] [13]. This
shocked and thermally metamorphosed Apollo 11, mechanism is also dominant for A-11 pyroxenes. Fur-
10017 (A-11) and Apollo 17, 75035 (A-17) basalts. thermore, results suggest that Ti-Al partitioning is
Methods: A-11 and A-17 high-Ti basalts were more important at higher Ca-contents (Figure 4). Ca is
experimentally shocked at the Johnson Space Center as a large M2-site cation (>0.10 nm S-P ionic radius) that
described by [9]. An unshocked sample of A-11 basalt likely stretches neighboring M1 and tetrahedral sites,
was heated under vacuum (~10-4 atm) following the thus facilitating substitution of the Ti-Al couple. Fig-
approach of [10,11]. Pyroxenes were imaged with a ures 5 and 6 illustrate plots of Li vs. Li/Ce and B vs.
JEOL JSM-5800 LV SEM and analyzed with a JEOL B/Ce, respectively. Ce is a light REE and behaves
JXA-8200 EMP using a 0.5 µm beam diameter, 20 kv incompatibly, increasing in concentration toward rims.
accelerating voltage and 20 nA current. Pyroxenes Pyroxene rims are depleted in Li and B, expressing
were analyzed for 7Li, 9Be, 11B, 88Sr, 140Ce, 146Nd, and lower Li/Ce and B/Ce values than cores. Late-stage Li-
174Yb using a Cameca ims 4f. Thin sections were partitioning into pyroxenes may be affected by de-
soaked in 1 % mannitol solution, rinsed in ultrapure creases in Ca-content (Figure 7). Ca influences the
(18 MΩ) water and rastered to reduce effects of B con- behavior of Al, which in turn can substitute as an octa-
tamination. Analyses were made by bombarding sam- hedrally coordinated cation that couples with Li for
ples with a 15 nA current of primary O- ions under a respective M1 and M2-site occupancy. For example,
10 kv potential focused to a 10-15 µm spot diameter. Figure 8 shows that pyroxenes with higher Li-contents
Sputtered secondary ions were energy filtered with a have greater abundances of octahedrally coordinated
sample offset voltage of 75 V and an energy window Al. These observations are not consistent with the
of ±25 V. The effect of 27Al3+ on 9Be was investigated experimental results of [14], which support limited Ca
by analyzing 9Be at a higher mass resolution. and Al affects on Li and B partitioning. Alternatively,
Lunar and Planetary Science XXXVI (2005) 1426.pdf
results of this work also document the coupled substi- Figure 4. Al/Si vs. WAopo-lcloo 1n7t Menarte pBlaosatl tf: o7r5 0A35-17 pyroxenes.
tution of Li (M2) and Cr3+ (M1) during pyroxene crys- 0.14
Cores
tallization. In these pyroxenes, loss of Cr occurs with 0.12
increasing degrees of crystallization. With decreasing 0.1
Cr-content, Li-partitioning was less significant during
raivma iflaobrmiliatyti.o nI tt hisa nno dtuewrinogrt heyar tlhieart pCerr iwodass noof tC irn-cluded Al/Si 00..0068

in the starting compositions of [14]. Higher shock 0.04 Rims
pressures can significantly redistribute B in pyroxenes 0.02
but have no notable effects on Li. It is likely that
0
shock and subsequent thermal metamorphism facilitate 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Li-B mobility among melt inclusions, pyroxene grains Ca/(Ca+Mg+Fe)
and zones of surrounding mesostasis. Measurements 9.3 GPa 20.4 GPa 30.2 GPa 40.0 GPa 60.6 GPa 66.8 GPa 74.3 GPa 85.3 GPa 99.9 GPa Unshock ed
indicate that mesostasis contains up to 50 times more
Figure 5. Li (ppm) vs. Li/Ce plot for A-17 pyroxenes.
Li and B than pyroxenes. Given the extremely dry na-

12
ture of lunar basalts, fluid activity cannot explain the Cores
Li-B rim depletions observed in this study. However, 10
Li and B distributions are influenced by changes in 8
mineral composition during crystallization, shock m)
pressure and subsequent thermal metamorphism. Li (pp 6
4

R(1e9f9e8r)e nAcme.s :M [1in]. ,J o8h3n, s9o4n2,- 9M4.6C. .[ e3t] aMl. c(S1w99e1e)n ,G HC.AY,. 5J5r., 3&4 9H-3a6rv6e. y[,2 R] .M Py. s(e1n9,9 B3.)O S.c eiet nacl.e 259, 2 Rims
1890-1892. [4] Dann, J.C. et al. (2001) MAPS, 36, 793-806. [5] McSween, H.Y. et al. 0
(2001) Nature, 409, 487-490. [6] Lentz, R.C.F. et al. (2001) GCA, 65, 4551-4565. [7] 0 1 2 3 4 5 6 7
Chaklader, J. et al. (2004) LPSC XXXV, abst. 1397. [8] Fritz, J. et al. (2003) LPSC XXXIV, Li/Ce
abst. 1335. [9] Schaal, R.B. and Horz, F. (1977) Lunar Sci. Conf. 8th, 1697-1729. [10] unshocked 93 kb 302 kb 668 kb 853 kb
Nyquist, L.E. et al. (1991) LPSC XXII, 985-986. [11] Nyquist, L.E. et al. (1991) LPSC XXII,
987-988. [12] BVSP (1981) Pergamon Press, 371-384. [13] Bence, A. E. & Papike, J. J. Figure 6. B (ppm) vs. B/Ce plot for A-11 & A-17 pyroxenes.
(1972) Lunar Sci. Conf. 3rd, 431-469. [14] Herd et al. (2004) Am. Min., 832-840. 18
Amceknntso. w Mle.d Sgpeimldeen htes:lp Le.d B woirtgh, SZE. MSh a&rp E, MR.P J aonnaelsy &se sM. .T Aheli NheAlpSeAd Mwiatrhs hFeuantidnagm eexnptearli - 16 Rims Cores
Research Program provided funding for this study. 14
12
Figure 1. Variation in quadrilateral components for A-11 pyroxenes. m) 10
D i Hd B (pp 8
Cores Apollo 11 Mare Basalt 6
unshocked 4

34.8 GPa 2
54.6 GPa 0
70.3 GPa 0 0.5 1 1.5 B/Ce 2 2.5 3 3.5
78.5 GPa unshocked 93 kb 302 kb 668 kb 853 kb A-11 heated
Rims heated

Figure 7. Li (ppm) vs. Wo-content for A-17 pyroxenes.
En Fs

Figure 2. Variation in quadrilateral componeAnptso flloor 1A7- M17a rpey Broaxseanlte s. 16

99.9 GPa 14
Di Hd 85.3 GPa 12 Cores
Cores 74.3 GPa m) 10
6660..86 GGPPaa Li (pp 8
49.4 GPa 6
40.0 GPa 4 Rims
30.2 GPa 2

20.4 GPa 0
En Rims Fs 9un.3s hGoPcaked 0 0.05 u0n.1shocke0d.159C3a k /b 0(.C2a3+0M2 gk0+b.F2e5)668 k0b.3853 k0b.35 0.4

Figure 3. Ti vs. AAplo lplol 1o7t M faorre BAas-a1lt7: 7p5y03r5oxenes. Figure 8. (Al3+)VI vs. Li (ppm) plot for A-11 & A-17 pyroxenes.
0.14

0.12 Cores 1:2 0.035

0.03
0.1
[Ti/Cation Sum]*4 000...000468 Rims 1:4 3+VI( Al ) 00..000.120552
0.01
0.02 0.005
0 0
0 0.05 0.1 0.15 0.2 0.25 0 5 10 15 20 25
[Al/Cation Sum]*4 Li (ppm)
9.3 GPa 20.4 GPa 30.2 GPa 40.0 GPa 60.6 GPa unshocked 93 kb 302 kb 668 kb 853 kb A-11 heated
66.8 GPa 74.3 GPa 85.3 GPa 99.9 GPa Unshocked
m = 1/2 m = 1/4 Linear (m = 1/2) Linear (m = 1/4)