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The geochemistry of the opaque minerals in Apollo 14 crystalline rocks
EARTH AND PLANETARY SCIENCE LETTERS 13 (1971) 121 - 129. NORTH-HOLLAND PUBLISHING COMPANY
THE GEOCHEMISTRY IN APOLLO
OF THE OPAQUE
14 C R Y S T A L L I N E
MINERALS
ROCKS
A h m e d E L G O R E S Y , Paul R A M D O H R and Lawrence A. T A Y L O R *
Max-Planek-Institut fur Kernphysik, Postfach 1248, 69 Heidelberg, West Germany Received 2 November 1971
The opaque minerals in rock 14310, a high-alumina fine grained basalt consist of ilmenite, and optically anisotropic chromium ulv6spinel, native FeNi metal, baddeleyite, tranquillityite, schreibersite, and a Fe, Ni sulfide (pentlandite or chalcopyrrhotite?). The opaque assemblage of rock 14053, a high-Fe medium grained basalt with subophitic texture consists of ilmenite, a titanium-chromite, chromium ulv6spinel, troilite, FeNi metal, fayalite with breakdown products including pure Fe, tridymite and silica glass. In rock 14310 only one spinel was found; it is a chromium ulv6spinel which occurs in pyroxene and the groundmass and is similar in composition to the ulv6spinels formed during late-stage crystallization of Apollo 12 rocks. Spinels encountered in 14053 are a titanium aluminium rich chromite, chromium ulv6spinel and titanium picotite. Their composition is similar to spinels found in the Apollo 12 microgabbro 12018. The Fe Ni metal in 14310 contains 1.5 to 37 wt % Ni and 0.5 to 1.9 wt % Co and occurs middle to late in the paragenetic sequence. These Ni contents cannot be correlated with the mineral assemblage in which the metal phase occurs. Two coexisting iron phosphide minerals, schreibersite, one a Ni-free variety and the other containing 23 wt % Ni, were found in FeNi metal and troilite. This is the first reported occurrence of schreibersite as an indigenous mineral in the lunar rocks and casts suspicion on the criterion for meteoritic contamination used with the Apollo 11 and 12 samples. The zirconium dioxide, baddeleyite, is commonly associated with the opaque minerals, mainly ilmenite. Electron microprobe analyses indicate the presence of minor Zr in both ilmenite and ulv6spinel (0.2 up to 0.57 wt % ZrO~ in ilmenite versus 0.03 to 0.19 in ulv6spinel), i.e. Zr is preferentially concentrated in the ilmenite structure. The presence of schreibersite in 14310 and the late stage breakdown of fayalite to pure Fe + silica are strong evidences supporting crystallization under extremely low f02 .
1. I n t r o d u c t i o n O f the m a n y rocks collected during the A p o l l o 14 mission, only two weighing over 50 g are wholly basaltic in origin (i.e. rocks 14053 and 14310); the remainder consist o f breccias and u n c o n s o l i d a t e d soil samples. This places these crystalline rocks in a rather unique position among the Apollo 14 samples. The chemistry o f rock 14310 is unusual [ 1 ]. It contains high A1203, 20%; low MgO, 8%, and FeO, 7.7%; high Z r O 2 , 0 . 1 3 % ; and lower TiO2 than all A p o l l o 11 and m o s t A p o l l o 12 rocks (1.3% versus 7 - 1 2 and 1 . 2 - 5 . 1 % , respectively). R o c k 14053 has a m o r e normal c o m p o s i t i o n e x c e p t for low MgO, * Permanent address: Department of Geosciences, Purdue University, Lafayette, Indiana 47907, USA.
8.4% and TiO~, 1.5%, content. Therefore, it was considered of particular interest to see what differences there might be in the opaque mineralogy o f these crystalline rocks as o p p o s e d to the mineralogy of other Apollo 14 crystalline rocks as well as those from the earlier A p o l l o missions. R o c k 14310, which has a chemical c o m p o s i t i o n not unlike that of a high-alumina basalt [ 1 ], consists of euhedral plagioclase laths ( ~ 6 0 % ) with intersertal anhedral p y r o x e n e , the opaques, and a late-stage mesostasis of fayalite, glass, tridymite, a phosphate mineral, and opaque minerals. No forsteritic olivine was e n c o u n t e r e d . R o c k 14053 is m o r e mafic than 14310, has an ophitic t e x t u r e , and consists of 50% p y r o x e n e , 40% plagioclase, and 10% o f olivine, opaques, an unusual texture involving fayalite, as well as mesostasis.
122
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
The opaque minerals in samples 14310, 101, 14053, 2 and 14073, 1 make up only a few percent of the rock - much less than in either Apollo 11 or 12 rocks. The minerals observed are ilmenite, FeNi metal, troilite, chromium ulvOspinel, titaniferous chromite, tranquilityite, baddeleyite, chalcopyrrhotite, schreibersite, and mackinawite. No titaniferous chromite was found in the samples 14310 and 14073.
Apollo 14
]Z~
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2. Results
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in troilite in pyroxene in ilmenite in plagioclase
A I
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2.1. F e N i metal
The ubiquitous occurrence of native Fe metal in all lunar samples was encountered also in rock 143 I0 and 14053. The association of FeNi metal with troilite, that was so pronounced in Apollo 11 samples, is not common in these Apollo 14 rocks. Instead, this assemblage of FeNi metal and troilite occurs about as commonly as in the Apollo 12 sampies. However, FeNi metal is not as commonly associated with ilmenite or ulvospinel as it was in Apollo 12 rocks.
Fig. 2. Histogram of Ni concentration in metal grains in rock 14073. ship in rock 14073 (fig. 2), a crystalline rock weighing less than 50 g. Rock 14053 analyses, however, show no such relationship, indeed, just the opposite (fig. 3). The controlling factor for the partitioning of the Ni in the metal phase is not known at present; however, it may be related to the total sulfur content of the rock (Brett, 1971, personal comm.).
APOLLO 14
A p o l l o 14 [] [] • & O ® I
14310
in troilite in tridymite in plagioclase in schreibersite in pyroxene in ilmenite in mesostasis
[]
[]
illllrJJiIJrllllJllllJlJriJ~llfJlllJlrJl 0
5
10
15 20
25 30
35 40
W e i g h t P e r c e n t Nickel
Fig. 1. Histogram of Ni concentration in metal grains in rock 14310. Electron microprobe analyses were performed on several of the FeNi grains in rock 14310 (fig. 1). The results can be interpreted as demonstrating that the Ni content of the metal phase is not directly dependent upon the mineral assemblage. However, there appears to exist a slight preference for higher Ni contents for the metal phase associated with troilite. We have also found such a distinct relation-
1~]
14053,2
0
A in plagioclase [ ] in pyroxene • in troilite
1 2 3 4 Weight Percent Nickel
5
Fig. 3. Histogram of Ni concentrationin metal grains in rock
14053. 2.2. I l m e n i t e Similar to Apollo 11 and 12 rocks ilmenite is the major opaque mineral in the three rocks studied. The mineral occurs in the Fra Mauro crystalline rocks also as idiomorphic laths in the silicate groundmass or as inclusions in the pyroxene. Quantitative electron microprobe analyses of numerous grains revealed ilmenite to have a mainly FeTiO3 composition with minor MgO, MnO, Cr203, A1203, V203 and ZrO2. The geochemistry of the minor elements and their partitioning between coexisting ilmenite and spinel
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
phases is of particular interest. E1 Goresy et al. [2] reported a vanadium depletion in ilmenites compared to the coexisting spinels from the Apollo 12 crystalline rocks 12002, 12018, and 12063. Similar depletion of V was also found in ilmenites of the Fra Mauro Crystalline rocks (table 1).
123
Apollo 14 Fo
TiO2
Fe2Ti 4O 0 ~
4
~
~
~!~
Table 1 Electron microprobe of ilmenite in rock 14310 TiO 2 FeO MgO MnO Cr203 A1203 V 203 ZrO 2
51.70 46.15 0.63 0.38 0.06 0.14 0.04 0.54
Total
99.64
Atomic proportions based on 3 oxygens Ti Fe Mg Mn Cr A1 V Zr
0.9844 0.9772 0.0238 0.0082 0.0012 0.0042 0.0008 0.0066
Total
2.0064
2.3. Spinels Although several of the breccia samples collected during the Apollo 14 mission contain both chromite and ulv6spinel, as did most Apollo 12 rocks, no chromite was found in sample 14310,101 or 14073,1. The only spinel phase observed in these rocks is an optically anisotropic chromium ulv6spinel. In contrast, sample 14053,2 contains both ulvbspinel and a titaniferous chromite. The ulvbspinel is similar in optical and chemical properties to those Apollo 12 ulvbspinels which exist as distinct grains in the late-stage groundmass. The compositions of this phase from 143 t0,101, 14073,7 and 14053,2 are shown diagrammatically in the ternary compositional plot of 2+, 3+, and 4+ oxides of figs. 4, 5 and 6. In fact, 14310 and 14073 ulvbspinels have similar compositions, whereas the
"'-"'"'-.'"'- "~ 14310,101 4~0
60 800 FeCr204
v
v
\e v
\
Fig. 4. Compositional diagram displaying the composition of the spinels analyzed in sample 14310.
Apollo 14
6/ FeTi03~'
~
40/
Fe27104/~
Ti02
\ X~, FM~e~ / ~ MnO
40
X c[[o~ V203
60
FeCr20480
Fig. 5. Compositional diagram displaying the composition of the spinels analyzed in sample 14073.
Apollo 14
6/
\00
FeTi03~"
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Fe2Ti 40
T,O2 4 0 ~ ~ MnO
40
V203
6O 8O FeCr204
Fig. 6. Compositional diagram displaying the composition of the spinels analyzed in sample 14053.
A. E1 Goresy et al., Opaque minerals in Apollo 14 rocks
124
Table 2 Electron microprobe analyses of Ti-chromites and Cr-Ulvites in Apollo 14 sample 14053,2. Chromites
Ulvites
TiO2 FeO MnO MgO Cr 203 AI~03 V 2O~ ZrO 2
2.79 29.24 0.18 5.98 38.17 21.74 0.56 0.000
2.66 28.81 0.19 6.18 38.23 21.72 0.56 0.000
4.95 33.29 0.25 4.02 39.56 16.11 0.70 0.000
7.43 33.43 0.29 4.78 38.81 13.20 0.73 0.000
22.78 48.48 0.67 2.71 19.74 4.65 0.45 0.04
22.14 49.66 0.73 1.56 21.67 4.54 0.50 0.04
19.11 47.18 0.55 1.65 26.08 5.49 0.56 0.02
Total
98.66
98.35
98.88
98.67
99.52
100.84
100.64
Atomic proportions based on 32 oxygens Ti Fe Mn Mg Cr A1 V Zr
0.5498 6.4103 0.0407 2.3364 7.9118 6.7182 0.1184 0.0000
0.5251 6.3274 0.0430 2.4204 7.9390 6.7243 0.1186 0.0000
1.0119 7.5693 0.0585 1.6296 8.5039 5.1632 0.1516 0.0000
1.5316 7.6656 0.0665 1.9513 8.4126 4.2650 0.1607 0.0000
Total
24.0856
24.0978
24.0880
3+ and 4+ Cations: 2+ Cations:
15.2980 8.7876
15.3070 8.7908
14.8306 9.2574
ulv6spinels and chromites from 14053 are completely different in composition, The textural and compositional relationship of spinels in 14053 are quite similar to those in Apollo 12 rock 12018 [2]. The analyses each totaled between 98 and 101%, there are 24 cations on the basis of 32 oxygens (i.e. 3:4 ratio), and there exists overall charge balance (table 2). Therefore, we have no reason to suspect the presence of any Fe 3+ in the structure of the spinels from this rock. In addition to the Ti chromite and Cr ulviSspinel, rock 14053 also contains an assemblage of aluminium chromite, picotite and Cr-ulv6spinel (table 3). Several of the ulvbspinels show the breakdown texture according to the idealized reaction 1
Fe2TiO4 -~ FeTiO3 + Fe + ~ 02 similar to that reported from Apollo 11 and 12 rocks.
4.9120 11.6262 0.1627 1.1602 4.4767 1.5710 0.1045 0.0051
4.7530 11.8576 0.1770 0.6639 4.8902 1.5271 0.1134 0.0056
4.0915 11.2340 0.1317 0.6994 5.8713 1.8406 0.1269 0.0031
24.0533
24.0184
23.9878
23.9985
14.3699 9.6834
11.0693 12.9491
11.2893 12.6985
11.9334 12.0651
This breakdown is particularly well developed in rock 14053 which was, during formation, subjected to extremely reducing conditions. 2.4. Schreibersite Schreibersite is a mineral which has been reported exclusively from meteorites. In fact, its presence in the Apollo 11 and 12 rocks was used as a criterion for meteoritic contamination. Rock 14310, however, contains schreibersite as small rounded grains within some of the FeNi metal grains. Thus, the criterion used previously is suspect, because in this rock, the schreibersite is indigenous and not a re suit of contamination. Fig. 7 shows an example of the schreibersite occurrence in this rock. The assemblage consists of troilite, FeNi metal, and schreibersite. The metal phase contains 15.6 wt % Ni (table 4); however, as can be seen from the microprobe scans for NiKa on
125
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
Table 3 Electron microprobe analyses if coexisting sinels in Apollo 14 sample 14053. Chromite
Picotite
TiO 2 FeO MnO MgO Cr203 AI:O 3 V:O 3 Zr02
3.67 28.48 0.21 6.75 37.96 20.74 0.56
4.28 24.94 0.08 11.32 18.80 38.23 0.22
18.31 43.67 0.44 5.37 21.94 10.41 0.57 0.004
Total
98.37
97.87
100.714
Total
0.7259 6.2564 0.0471 2.6417 7.8839 6.4099 0.1174 0.0000
0.7673 4.9699 0.0153 4.0194 3.5412 10.7357 0.0421 0.0000
3.7488 9.9440 0.1006 2.1780 4.7227 3.3400 0.1249 0.0044
24.0823
24.0909
24.1634
15.0864 9.0045
11.9410 12.2224
3+ and 4+ Cations: 15.1371 2+ Cations: 8.9452
Schreibersite
Metal
Fe Ni CO P
56.07 27.96 0.46 11.43
82.21 15.55 1.11 0.09
Total
95.92
98.96
Ulvite
Atomic proportions based on 32 oxygens Ti Fe Mn Mg Cr A1 V Zr
Table 4 Electron microprobe analyses of schreibersite and coexisting metal in crystalline rock 14310.
fig. 3, the Ni content varies with respect to the two central schreibersite grains (fig. 8). Actually, two different compositions of this mineral coexist here; one is almost pure iron phosphide and the other contains 28.0 wt % Ni in addition to Fe and P (table 4). The Ni content of the metal phase immediately adjacent to the nickeliferous schreibersite is considerably lower ( ~ 5 - 7 wt % Ni) than throughout the remainder of this phase. The FeNi metal grain also contains only a trace (i.e. 0.09 wt %) of P. The physical-chemical significance of the two coexisting schreibersites and of the Ni depletion in the metal adjacent to the Ni-rich schreibersite are unknown at present.
Fig. 7. Schreibersite (light grey) inclusions in FeNi metal (white). Dark grey is troilite. Specimen 14310,101.
126
A. E1 Goresy et al., Opaque minerals in Apollo 14 rocks
I
I0 microns
i
Fig. 8. X-ray probe scans for Fe, Ni. Co, P, and S of the schreibersite, FeNi, troilite assemblage shown in fig. 7.
127
A. El Goresy et aL, Opaque minerals in Apollo 14 rocks
2.5. Baddeleyite Rock 14310 contains one of the highest amounts of ZrO2 (i.e. 0.13%) reported by the LSPET [1]. This ZrO2 content is contributed mainly by the mineral baddeleyite. The amount of this mineral in rock 14310 is far greater than in any sample from Apollo 11 or 12. It most commonly occurs with ilmenite as a thin border and as both rounded and bladed inclusions; however, it sometimes occurs alone or with troilite or ulvbspinel. Table 5 Electron microprobe analyses of baddeleyite in crystalline rock 14310. 1
2
ZrOa HfO= MgO
97.39 1.65 0.07
98.23 1.70 0.06
Total
99.11
99.99
Zr/Hf ratio
51.63
50.52
The analyses of two representative baddeleyite grains are given in table 5. These grains contain similar amounts of Hf which lead to Zr/Hf ratios of ~51. This value is very close to that reported by Morrison et al. [3] for Apollo 11 coarse-grained rocks and is about the same as the ratio calculated from the cosmic abundance value. No zircon was found in this rock 14310 and it would appear that most Zr and Hf are present in the baddeleyite. That is, there appears to be little preferential incorporation of Hf into any other phase. The presence of baddeleyite in a rock containing free SiO2 (i.e. tridymite) is of significance, particularly since no zircon was found. This limits the fugacity of oxygen to values below the univariant curve associated with the reaction ZrO2 + SiO2 = ZrSiO4. The The ilmenites and ulv~3spinels associated with baddeleyite were microprobed for Zr content. As shown in fig. 9, the ZrOz contents of the ilmenites range from about 0.2 up to 0.57% ZrO> whereas the ulv6spinels contain up to 0.19% ZrO2. The present investigation indicates a strong geochemical preference of Zr for the ilmenite structure. On the other hand, this is the first reported occurrence of Zr as a minor ele-
Apollo 14
14310,101
•
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0
u,vosp,oo,
[] Ilmenite
••
I
I
I
I
01
0.2
0.3
0.4
[3G []
[
05
0.6
Weight Percent ZrO 2
Fig. 9. ZrO~ distribution in coexisting ilmenite and ulvOspinel in sample 14310,101.
ment in ulv6spinel. These values may represent the degree of substitution of Zr 4+ for Ti 4+ in ilmenite and ulv6spinel. 2.6. Fayalite breakdown Rock 14053 was found to contain an unusual texture (fig. 10) which is interpreted as resulting from the breakdown of fayalite rich olivine. The association consists of SiO2 in the form of tridymite, Fe metal grains, glass consisting mainly MgO and SiO2, and the fayalite. All degrees of this breakdown were found from incipient to complete. The composition of several of the fayalites associated with this texture were determined by electron microprobe analyses. These olivines range between 86 and 96 mole percent of the fayalite molecule (Fa). The associated Fe phase was also analyzed and found to always contain less than 0.02 wt % Ni in contrast to the FeNi grains associated with other portions of the same sample (fig. 3). This is the first reported occurrence of this breakdown texture in terrestrial or extra-terrestrial rocks and is of particular significance, As can be seen from fig. 11, the f02 required for this breakdown is very low indeed - lower than present during the formation of Apollo 11 or 12 rocks. 2.7. Tranquillityite Two optically and compositionally distinct varieties of tranquillityite were found in rock 14310. The one is similar in most respe.cts to the tranquillityite originally described from the Apollo 11 rocks. The composition of tranquillityite will be given elsewhere.
128
A. El Goresy et aL, Opaque minerals in Apollo 14 rocks
I
'
300
microns
I
[
Fig. 10. A fayalite grain with typical sieve texture with partial breakdown to pure spongy Fe + SiO2 . 2.8. O t h e r o p a q u e minerals Troilite, chalcopyrrhotite, and mackinawite were also identified in these rocks. The troilite is stoichiometric FeS and always contains ~ 0 . 5 % impurities.
-10
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Chalcopyrrhotite and mackinawite were also identified within troilite grains. The chalcopyrrhotite was too small for quantitative microprobe analysis; however, qualitative examination confirmed the presence of Cu. The mackinawite was also too small for analysis, but was identified based on its distinctive optical properties. As with the Apollo 12 rocks, the presence of mackinawite is evidence that subsolidus reequilibration occurred to temperatures not far removed from the ambient conditions (i.e. ~150°C).
3. S u m m a r y
-22 - 2 4 -,oo" I
8oo. I
10
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9ooI
9
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Fig. 1 l. Plot of -log fo, versus temperature of the buffer curves t:% 04 + 'FeO', 'FeO' + Fe, and Fe 2 SiO, * SiO2 .
(1) FeNi metal possesses higher Ni c o n t e n t when associated with troilite than was found in other assemblages. (2) C h r o m i u m ulvbspinel is the only spinel phase found in 14310 and 14075 and is similar in optical (anisotropic) and chemical composition to the end
A. El Goresy et al., Opaque minerals in Apollo 14 rocks stage ulvbspinels from Apollo 12 rocks. Rock 14053 contains Ti chromite, A1Cr spinel in addition to ulv6spinel. These ulvbspinels have a different composition from those found in the other two rocks. (3) A pure Fe schreibersite and one containing 28% Ni occur in FeNi metal. This find of indigenous schreibersite in the lunar rocks invalidates the criterion used previously for meteoritic contamination. (4) Baddeleyite is common and is usually associated with ilmenite. The Zn/Hf ratio of this mineral is ~51, approximately equal to the cosmic abundance. No zircon was found thus, placing an upper limit on the fugacity of oxygen present during formation. (5) Rock 14053 contains the first reported occurrence of the breakdown of fayalite to SiO2, Fe, and glass. (6) The presence of schreibersite, baddeleyite and the fayalite breakdown texture all indicate that the Apollo 14 crystalline rocks formed under more reducing conditions than either Apollo 11 or 12 rocks.
129
Acknowledgements The authors would like to express their thanks to Dr. Robin Brett for his helpful suggestions concerning this manuscript. One of us (L.A.T.) would also like to thank Professor Gentner, Director of the MaxPlanck Institut fur Kernphysik at Heidelberg, for financial support during this study.
References [ 11 Lunar Sample Preliminary Examination Team, Preliminary Examination of Lunar Samples, in: NASA Manned Spacecraft Center, Apollo 14 Preliminary Science Report SP-272 (1971) 109. [21 A. El Goresy, P. Ramdohr and L.A. Taylor, The opaque minerals in the lunar rocks from Oceanus Procellarum, Proc. Second Lunar Science Conf., Vol. 1 (M.I.T. Press, 1971) 219. [3] G.ll. Morrison, J.T. Gerard, A.T. Kashuba, E.V. Gangadharan, A.M. Rothenberg, N,M. Potter and G.B. Miller, Multielement analysis of lunar soil and rocks, Science 167 (1970) 505.
THE GEOCHEMISTRY IN APOLLO
OF THE OPAQUE
14 C R Y S T A L L I N E
MINERALS
ROCKS
A h m e d E L G O R E S Y , Paul R A M D O H R and Lawrence A. T A Y L O R *
Max-Planek-Institut fur Kernphysik, Postfach 1248, 69 Heidelberg, West Germany Received 2 November 1971
The opaque minerals in rock 14310, a high-alumina fine grained basalt consist of ilmenite, and optically anisotropic chromium ulv6spinel, native FeNi metal, baddeleyite, tranquillityite, schreibersite, and a Fe, Ni sulfide (pentlandite or chalcopyrrhotite?). The opaque assemblage of rock 14053, a high-Fe medium grained basalt with subophitic texture consists of ilmenite, a titanium-chromite, chromium ulv6spinel, troilite, FeNi metal, fayalite with breakdown products including pure Fe, tridymite and silica glass. In rock 14310 only one spinel was found; it is a chromium ulv6spinel which occurs in pyroxene and the groundmass and is similar in composition to the ulv6spinels formed during late-stage crystallization of Apollo 12 rocks. Spinels encountered in 14053 are a titanium aluminium rich chromite, chromium ulv6spinel and titanium picotite. Their composition is similar to spinels found in the Apollo 12 microgabbro 12018. The Fe Ni metal in 14310 contains 1.5 to 37 wt % Ni and 0.5 to 1.9 wt % Co and occurs middle to late in the paragenetic sequence. These Ni contents cannot be correlated with the mineral assemblage in which the metal phase occurs. Two coexisting iron phosphide minerals, schreibersite, one a Ni-free variety and the other containing 23 wt % Ni, were found in FeNi metal and troilite. This is the first reported occurrence of schreibersite as an indigenous mineral in the lunar rocks and casts suspicion on the criterion for meteoritic contamination used with the Apollo 11 and 12 samples. The zirconium dioxide, baddeleyite, is commonly associated with the opaque minerals, mainly ilmenite. Electron microprobe analyses indicate the presence of minor Zr in both ilmenite and ulv6spinel (0.2 up to 0.57 wt % ZrO~ in ilmenite versus 0.03 to 0.19 in ulv6spinel), i.e. Zr is preferentially concentrated in the ilmenite structure. The presence of schreibersite in 14310 and the late stage breakdown of fayalite to pure Fe + silica are strong evidences supporting crystallization under extremely low f02 .
1. I n t r o d u c t i o n O f the m a n y rocks collected during the A p o l l o 14 mission, only two weighing over 50 g are wholly basaltic in origin (i.e. rocks 14053 and 14310); the remainder consist o f breccias and u n c o n s o l i d a t e d soil samples. This places these crystalline rocks in a rather unique position among the Apollo 14 samples. The chemistry o f rock 14310 is unusual [ 1 ]. It contains high A1203, 20%; low MgO, 8%, and FeO, 7.7%; high Z r O 2 , 0 . 1 3 % ; and lower TiO2 than all A p o l l o 11 and m o s t A p o l l o 12 rocks (1.3% versus 7 - 1 2 and 1 . 2 - 5 . 1 % , respectively). R o c k 14053 has a m o r e normal c o m p o s i t i o n e x c e p t for low MgO, * Permanent address: Department of Geosciences, Purdue University, Lafayette, Indiana 47907, USA.
8.4% and TiO~, 1.5%, content. Therefore, it was considered of particular interest to see what differences there might be in the opaque mineralogy o f these crystalline rocks as o p p o s e d to the mineralogy of other Apollo 14 crystalline rocks as well as those from the earlier A p o l l o missions. R o c k 14310, which has a chemical c o m p o s i t i o n not unlike that of a high-alumina basalt [ 1 ], consists of euhedral plagioclase laths ( ~ 6 0 % ) with intersertal anhedral p y r o x e n e , the opaques, and a late-stage mesostasis of fayalite, glass, tridymite, a phosphate mineral, and opaque minerals. No forsteritic olivine was e n c o u n t e r e d . R o c k 14053 is m o r e mafic than 14310, has an ophitic t e x t u r e , and consists of 50% p y r o x e n e , 40% plagioclase, and 10% o f olivine, opaques, an unusual texture involving fayalite, as well as mesostasis.
122
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
The opaque minerals in samples 14310, 101, 14053, 2 and 14073, 1 make up only a few percent of the rock - much less than in either Apollo 11 or 12 rocks. The minerals observed are ilmenite, FeNi metal, troilite, chromium ulvOspinel, titaniferous chromite, tranquilityite, baddeleyite, chalcopyrrhotite, schreibersite, and mackinawite. No titaniferous chromite was found in the samples 14310 and 14073.
Apollo 14
]Z~
E]E][] I
2. Results
• [] • A
"--~--~44U/.-~, 7
0
I
4
I
I
in troilite in pyroxene in ilmenite in plagioclase
A I
I
I
I
I
I
I
8 12 16 20 Weight Percent Nickel
I
24
2.1. F e N i metal
The ubiquitous occurrence of native Fe metal in all lunar samples was encountered also in rock 143 I0 and 14053. The association of FeNi metal with troilite, that was so pronounced in Apollo 11 samples, is not common in these Apollo 14 rocks. Instead, this assemblage of FeNi metal and troilite occurs about as commonly as in the Apollo 12 sampies. However, FeNi metal is not as commonly associated with ilmenite or ulvospinel as it was in Apollo 12 rocks.
Fig. 2. Histogram of Ni concentration in metal grains in rock 14073. ship in rock 14073 (fig. 2), a crystalline rock weighing less than 50 g. Rock 14053 analyses, however, show no such relationship, indeed, just the opposite (fig. 3). The controlling factor for the partitioning of the Ni in the metal phase is not known at present; however, it may be related to the total sulfur content of the rock (Brett, 1971, personal comm.).
APOLLO 14
A p o l l o 14 [] [] • & O ® I
14310
in troilite in tridymite in plagioclase in schreibersite in pyroxene in ilmenite in mesostasis
[]
[]
illllrJJiIJrllllJllllJlJriJ~llfJlllJlrJl 0
5
10
15 20
25 30
35 40
W e i g h t P e r c e n t Nickel
Fig. 1. Histogram of Ni concentration in metal grains in rock 14310. Electron microprobe analyses were performed on several of the FeNi grains in rock 14310 (fig. 1). The results can be interpreted as demonstrating that the Ni content of the metal phase is not directly dependent upon the mineral assemblage. However, there appears to exist a slight preference for higher Ni contents for the metal phase associated with troilite. We have also found such a distinct relation-
1~]
14053,2
0
A in plagioclase [ ] in pyroxene • in troilite
1 2 3 4 Weight Percent Nickel
5
Fig. 3. Histogram of Ni concentrationin metal grains in rock
14053. 2.2. I l m e n i t e Similar to Apollo 11 and 12 rocks ilmenite is the major opaque mineral in the three rocks studied. The mineral occurs in the Fra Mauro crystalline rocks also as idiomorphic laths in the silicate groundmass or as inclusions in the pyroxene. Quantitative electron microprobe analyses of numerous grains revealed ilmenite to have a mainly FeTiO3 composition with minor MgO, MnO, Cr203, A1203, V203 and ZrO2. The geochemistry of the minor elements and their partitioning between coexisting ilmenite and spinel
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
phases is of particular interest. E1 Goresy et al. [2] reported a vanadium depletion in ilmenites compared to the coexisting spinels from the Apollo 12 crystalline rocks 12002, 12018, and 12063. Similar depletion of V was also found in ilmenites of the Fra Mauro Crystalline rocks (table 1).
123
Apollo 14 Fo
TiO2
Fe2Ti 4O 0 ~
4
~
~
~!~
Table 1 Electron microprobe of ilmenite in rock 14310 TiO 2 FeO MgO MnO Cr203 A1203 V 203 ZrO 2
51.70 46.15 0.63 0.38 0.06 0.14 0.04 0.54
Total
99.64
Atomic proportions based on 3 oxygens Ti Fe Mg Mn Cr A1 V Zr
0.9844 0.9772 0.0238 0.0082 0.0012 0.0042 0.0008 0.0066
Total
2.0064
2.3. Spinels Although several of the breccia samples collected during the Apollo 14 mission contain both chromite and ulv6spinel, as did most Apollo 12 rocks, no chromite was found in sample 14310,101 or 14073,1. The only spinel phase observed in these rocks is an optically anisotropic chromium ulv6spinel. In contrast, sample 14053,2 contains both ulvbspinel and a titaniferous chromite. The ulvbspinel is similar in optical and chemical properties to those Apollo 12 ulvbspinels which exist as distinct grains in the late-stage groundmass. The compositions of this phase from 143 t0,101, 14073,7 and 14053,2 are shown diagrammatically in the ternary compositional plot of 2+, 3+, and 4+ oxides of figs. 4, 5 and 6. In fact, 14310 and 14073 ulvbspinels have similar compositions, whereas the
"'-"'"'-.'"'- "~ 14310,101 4~0
60 800 FeCr204
v
v
\e v
\
Fig. 4. Compositional diagram displaying the composition of the spinels analyzed in sample 14310.
Apollo 14
6/ FeTi03~'
~
40/
Fe27104/~
Ti02
\ X~, FM~e~ / ~ MnO
40
X c[[o~ V203
60
FeCr20480
Fig. 5. Compositional diagram displaying the composition of the spinels analyzed in sample 14073.
Apollo 14
6/
\00
FeTi03~"
\,
Fe2Ti 40
T,O2 4 0 ~ ~ MnO
40
V203
6O 8O FeCr204
Fig. 6. Compositional diagram displaying the composition of the spinels analyzed in sample 14053.
A. E1 Goresy et al., Opaque minerals in Apollo 14 rocks
124
Table 2 Electron microprobe analyses of Ti-chromites and Cr-Ulvites in Apollo 14 sample 14053,2. Chromites
Ulvites
TiO2 FeO MnO MgO Cr 203 AI~03 V 2O~ ZrO 2
2.79 29.24 0.18 5.98 38.17 21.74 0.56 0.000
2.66 28.81 0.19 6.18 38.23 21.72 0.56 0.000
4.95 33.29 0.25 4.02 39.56 16.11 0.70 0.000
7.43 33.43 0.29 4.78 38.81 13.20 0.73 0.000
22.78 48.48 0.67 2.71 19.74 4.65 0.45 0.04
22.14 49.66 0.73 1.56 21.67 4.54 0.50 0.04
19.11 47.18 0.55 1.65 26.08 5.49 0.56 0.02
Total
98.66
98.35
98.88
98.67
99.52
100.84
100.64
Atomic proportions based on 32 oxygens Ti Fe Mn Mg Cr A1 V Zr
0.5498 6.4103 0.0407 2.3364 7.9118 6.7182 0.1184 0.0000
0.5251 6.3274 0.0430 2.4204 7.9390 6.7243 0.1186 0.0000
1.0119 7.5693 0.0585 1.6296 8.5039 5.1632 0.1516 0.0000
1.5316 7.6656 0.0665 1.9513 8.4126 4.2650 0.1607 0.0000
Total
24.0856
24.0978
24.0880
3+ and 4+ Cations: 2+ Cations:
15.2980 8.7876
15.3070 8.7908
14.8306 9.2574
ulv6spinels and chromites from 14053 are completely different in composition, The textural and compositional relationship of spinels in 14053 are quite similar to those in Apollo 12 rock 12018 [2]. The analyses each totaled between 98 and 101%, there are 24 cations on the basis of 32 oxygens (i.e. 3:4 ratio), and there exists overall charge balance (table 2). Therefore, we have no reason to suspect the presence of any Fe 3+ in the structure of the spinels from this rock. In addition to the Ti chromite and Cr ulviSspinel, rock 14053 also contains an assemblage of aluminium chromite, picotite and Cr-ulv6spinel (table 3). Several of the ulvbspinels show the breakdown texture according to the idealized reaction 1
Fe2TiO4 -~ FeTiO3 + Fe + ~ 02 similar to that reported from Apollo 11 and 12 rocks.
4.9120 11.6262 0.1627 1.1602 4.4767 1.5710 0.1045 0.0051
4.7530 11.8576 0.1770 0.6639 4.8902 1.5271 0.1134 0.0056
4.0915 11.2340 0.1317 0.6994 5.8713 1.8406 0.1269 0.0031
24.0533
24.0184
23.9878
23.9985
14.3699 9.6834
11.0693 12.9491
11.2893 12.6985
11.9334 12.0651
This breakdown is particularly well developed in rock 14053 which was, during formation, subjected to extremely reducing conditions. 2.4. Schreibersite Schreibersite is a mineral which has been reported exclusively from meteorites. In fact, its presence in the Apollo 11 and 12 rocks was used as a criterion for meteoritic contamination. Rock 14310, however, contains schreibersite as small rounded grains within some of the FeNi metal grains. Thus, the criterion used previously is suspect, because in this rock, the schreibersite is indigenous and not a re suit of contamination. Fig. 7 shows an example of the schreibersite occurrence in this rock. The assemblage consists of troilite, FeNi metal, and schreibersite. The metal phase contains 15.6 wt % Ni (table 4); however, as can be seen from the microprobe scans for NiKa on
125
A. El Goresy et al., Opaque minerals in Apollo 14 rocks
Table 3 Electron microprobe analyses if coexisting sinels in Apollo 14 sample 14053. Chromite
Picotite
TiO 2 FeO MnO MgO Cr203 AI:O 3 V:O 3 Zr02
3.67 28.48 0.21 6.75 37.96 20.74 0.56
4.28 24.94 0.08 11.32 18.80 38.23 0.22
18.31 43.67 0.44 5.37 21.94 10.41 0.57 0.004
Total
98.37
97.87
100.714
Total
0.7259 6.2564 0.0471 2.6417 7.8839 6.4099 0.1174 0.0000
0.7673 4.9699 0.0153 4.0194 3.5412 10.7357 0.0421 0.0000
3.7488 9.9440 0.1006 2.1780 4.7227 3.3400 0.1249 0.0044
24.0823
24.0909
24.1634
15.0864 9.0045
11.9410 12.2224
3+ and 4+ Cations: 15.1371 2+ Cations: 8.9452
Schreibersite
Metal
Fe Ni CO P
56.07 27.96 0.46 11.43
82.21 15.55 1.11 0.09
Total
95.92
98.96
Ulvite
Atomic proportions based on 32 oxygens Ti Fe Mn Mg Cr A1 V Zr
Table 4 Electron microprobe analyses of schreibersite and coexisting metal in crystalline rock 14310.
fig. 3, the Ni content varies with respect to the two central schreibersite grains (fig. 8). Actually, two different compositions of this mineral coexist here; one is almost pure iron phosphide and the other contains 28.0 wt % Ni in addition to Fe and P (table 4). The Ni content of the metal phase immediately adjacent to the nickeliferous schreibersite is considerably lower ( ~ 5 - 7 wt % Ni) than throughout the remainder of this phase. The FeNi metal grain also contains only a trace (i.e. 0.09 wt %) of P. The physical-chemical significance of the two coexisting schreibersites and of the Ni depletion in the metal adjacent to the Ni-rich schreibersite are unknown at present.
Fig. 7. Schreibersite (light grey) inclusions in FeNi metal (white). Dark grey is troilite. Specimen 14310,101.
126
A. E1 Goresy et al., Opaque minerals in Apollo 14 rocks
I
I0 microns
i
Fig. 8. X-ray probe scans for Fe, Ni. Co, P, and S of the schreibersite, FeNi, troilite assemblage shown in fig. 7.
127
A. El Goresy et aL, Opaque minerals in Apollo 14 rocks
2.5. Baddeleyite Rock 14310 contains one of the highest amounts of ZrO2 (i.e. 0.13%) reported by the LSPET [1]. This ZrO2 content is contributed mainly by the mineral baddeleyite. The amount of this mineral in rock 14310 is far greater than in any sample from Apollo 11 or 12. It most commonly occurs with ilmenite as a thin border and as both rounded and bladed inclusions; however, it sometimes occurs alone or with troilite or ulvbspinel. Table 5 Electron microprobe analyses of baddeleyite in crystalline rock 14310. 1
2
ZrOa HfO= MgO
97.39 1.65 0.07
98.23 1.70 0.06
Total
99.11
99.99
Zr/Hf ratio
51.63
50.52
The analyses of two representative baddeleyite grains are given in table 5. These grains contain similar amounts of Hf which lead to Zr/Hf ratios of ~51. This value is very close to that reported by Morrison et al. [3] for Apollo 11 coarse-grained rocks and is about the same as the ratio calculated from the cosmic abundance value. No zircon was found in this rock 14310 and it would appear that most Zr and Hf are present in the baddeleyite. That is, there appears to be little preferential incorporation of Hf into any other phase. The presence of baddeleyite in a rock containing free SiO2 (i.e. tridymite) is of significance, particularly since no zircon was found. This limits the fugacity of oxygen to values below the univariant curve associated with the reaction ZrO2 + SiO2 = ZrSiO4. The The ilmenites and ulv~3spinels associated with baddeleyite were microprobed for Zr content. As shown in fig. 9, the ZrOz contents of the ilmenites range from about 0.2 up to 0.57% ZrO> whereas the ulv6spinels contain up to 0.19% ZrO2. The present investigation indicates a strong geochemical preference of Zr for the ilmenite structure. On the other hand, this is the first reported occurrence of Zr as a minor ele-
Apollo 14
14310,101
•
@ •
0
u,vosp,oo,
[] Ilmenite
••
I
I
I
I
01
0.2
0.3
0.4
[3G []
[
05
0.6
Weight Percent ZrO 2
Fig. 9. ZrO~ distribution in coexisting ilmenite and ulvOspinel in sample 14310,101.
ment in ulv6spinel. These values may represent the degree of substitution of Zr 4+ for Ti 4+ in ilmenite and ulv6spinel. 2.6. Fayalite breakdown Rock 14053 was found to contain an unusual texture (fig. 10) which is interpreted as resulting from the breakdown of fayalite rich olivine. The association consists of SiO2 in the form of tridymite, Fe metal grains, glass consisting mainly MgO and SiO2, and the fayalite. All degrees of this breakdown were found from incipient to complete. The composition of several of the fayalites associated with this texture were determined by electron microprobe analyses. These olivines range between 86 and 96 mole percent of the fayalite molecule (Fa). The associated Fe phase was also analyzed and found to always contain less than 0.02 wt % Ni in contrast to the FeNi grains associated with other portions of the same sample (fig. 3). This is the first reported occurrence of this breakdown texture in terrestrial or extra-terrestrial rocks and is of particular significance, As can be seen from fig. 11, the f02 required for this breakdown is very low indeed - lower than present during the formation of Apollo 11 or 12 rocks. 2.7. Tranquillityite Two optically and compositionally distinct varieties of tranquillityite were found in rock 14310. The one is similar in most respe.cts to the tranquillityite originally described from the Apollo 11 rocks. The composition of tranquillityite will be given elsewhere.
128
A. El Goresy et aL, Opaque minerals in Apollo 14 rocks
I
'
300
microns
I
[
Fig. 10. A fayalite grain with typical sieve texture with partial breakdown to pure spongy Fe + SiO2 . 2.8. O t h e r o p a q u e minerals Troilite, chalcopyrrhotite, and mackinawite were also identified in these rocks. The troilite is stoichiometric FeS and always contains ~ 0 . 5 % impurities.
-10
I
I
I
~
~
I
I
I
L
I
-14 e, -16
o --
-18
O
-20
d
, J , ozd:
Chalcopyrrhotite and mackinawite were also identified within troilite grains. The chalcopyrrhotite was too small for quantitative microprobe analysis; however, qualitative examination confirmed the presence of Cu. The mackinawite was also too small for analysis, but was identified based on its distinctive optical properties. As with the Apollo 12 rocks, the presence of mackinawite is evidence that subsolidus reequilibration occurred to temperatures not far removed from the ambient conditions (i.e. ~150°C).
3. S u m m a r y
-22 - 2 4 -,oo" I
8oo. I
10
I
9ooI
9
I
,ooo. ,,o?. ,27, I
I
8 l i T x 10 4
I
7
Fig. 1 l. Plot of -log fo, versus temperature of the buffer curves t:% 04 + 'FeO', 'FeO' + Fe, and Fe 2 SiO, * SiO2 .
(1) FeNi metal possesses higher Ni c o n t e n t when associated with troilite than was found in other assemblages. (2) C h r o m i u m ulvbspinel is the only spinel phase found in 14310 and 14075 and is similar in optical (anisotropic) and chemical composition to the end
A. El Goresy et al., Opaque minerals in Apollo 14 rocks stage ulvbspinels from Apollo 12 rocks. Rock 14053 contains Ti chromite, A1Cr spinel in addition to ulv6spinel. These ulvbspinels have a different composition from those found in the other two rocks. (3) A pure Fe schreibersite and one containing 28% Ni occur in FeNi metal. This find of indigenous schreibersite in the lunar rocks invalidates the criterion used previously for meteoritic contamination. (4) Baddeleyite is common and is usually associated with ilmenite. The Zn/Hf ratio of this mineral is ~51, approximately equal to the cosmic abundance. No zircon was found thus, placing an upper limit on the fugacity of oxygen present during formation. (5) Rock 14053 contains the first reported occurrence of the breakdown of fayalite to SiO2, Fe, and glass. (6) The presence of schreibersite, baddeleyite and the fayalite breakdown texture all indicate that the Apollo 14 crystalline rocks formed under more reducing conditions than either Apollo 11 or 12 rocks.
129
Acknowledgements The authors would like to express their thanks to Dr. Robin Brett for his helpful suggestions concerning this manuscript. One of us (L.A.T.) would also like to thank Professor Gentner, Director of the MaxPlanck Institut fur Kernphysik at Heidelberg, for financial support during this study.
References [ 11 Lunar Sample Preliminary Examination Team, Preliminary Examination of Lunar Samples, in: NASA Manned Spacecraft Center, Apollo 14 Preliminary Science Report SP-272 (1971) 109. [21 A. El Goresy, P. Ramdohr and L.A. Taylor, The opaque minerals in the lunar rocks from Oceanus Procellarum, Proc. Second Lunar Science Conf., Vol. 1 (M.I.T. Press, 1971) 219. [3] G.ll. Morrison, J.T. Gerard, A.T. Kashuba, E.V. Gangadharan, A.M. Rothenberg, N,M. Potter and G.B. Miller, Multielement analysis of lunar soil and rocks, Science 167 (1970) 505.