LKFM basaltic impact melts are abundant among Apollo lunar samples, especially those from Apollo 15, 16, and 17. They are generally basaltic in composition, but are found exclusively as impact melts. They seem to be related to basins and so could represent the composition of the lower lunar crust. They contain lithic clasts that cannot be mixed in any proportion to produce the composition of the melt matrix; components rich in transition elements (Ti, Cr, Sc) and REE are not considered. To search for the mysterious cryptic component, we previously investigated the mineral clast population in two Apollo 14 LKFM basaltic impact melts, 15445 and 15455. The cryptic component was not present in the mineral clast assemblage of these breccias either, but some olivine and pyroxene grains appeared to be from lithologies not represented among identified igneous rocks from the lunar highlands. In addition, none of the mineral clasts could be unambiguously assigned to a ferroan anorthosite source. We have now extended this study to Apollo 17, starting with two LKFM impact melt breccias (76295 and 76315) from the Apollo 17 station 6 boulder. The results from the study are presented

Norman, Marc D.

Ryder, Graham

Spudis, Paul

Taylor, G. Jeffrey

English

NASA Technical Reports Server

42 Geology of the Apollo 17 Landing Site _3
,o
LITHOLOGIES CONTRIBUTING TO THE CLAST POP-
ULATION IN APOLLO 17 LKFM BASALTIC IMPACT
MELTS. Marc D. Norman 1, G. Jeffrey Taylor 1, Paul Spudis 2, and
Graham Ryder 2, IPlanetary Geosciences, Department of Geology and
Geophysics, University of Hawaii, Honolulu I-H96822, USA, 2Lunar
and Planetary Institute, 3600 Bay Area Blvd., Houston "IX 77058,
USA.
LKFM basaltic impact melts are abundant among Apollo lunar
samples, especially those from Apollo 15, 16, and 17. They are
generally basaltic in composition, but are found exclusively as impact
melts [11. They seem to be related to basins and so could represent the
composition of the lower lunar crust [2---4]. They contain lithic clasts
that cannot be mixed in any proportion to produce the composition of
the melt matrix; components rich in transition elements (Ti, Cr, Sc)
and REE are not accounted for __]. To search for the mysterious cryptic
component, we previously investigated the mineral clast population
in two Apollo 14 LKFM basaltic impact melts, 15445 and 15455 [6].
The cryptic component was not present in the mineral clast assem-
blage of these breccias either, but some olivine and pyroxene grains
appeared to be from lithologies not represented among identified
igneous rocks from the lunarhighlands. In addition, none of the
mineral cl.asts could be unambiguously assigned to a ferroan anortho-
site source. We have now extended this study to Apollo 17, sta_,_g
with two LKFM impact melt broccias (76295 and 76315) from the
Apollo 17 station 6 boulder. The results |how that mineral clasts in
these rocks arc dominated by Mg-suite s_urces, but that once again
lithologiesnot recognized as pristine rocks are represented in the
mineral clasts. The compositions and relative abundances ofmineral
clastssuggesta significantcon_bution from olivinegabbronorites
similarto lherT_zflite67667. Little,ifany, ferroananorthositewas
detected. The cryptic comlxment remains enigmatic.
Rocks 76295 and 76315 are fme-grained impact melt breccias that
display little reaction between clasts and the melt matrices [7]. We
analyzed virtually all olivine, pyroxene, and plagioclase clasts larger
than 50 pm in one thin section of each rock. Data were obtained with
anew Cameca SXS0 electron microprobe using prtw_ures designed
to obtain precise analyses for minor elements. The two rocks differ in
the relative abundances of pyroxene to olivine, mafics to plagioclase,
and mineral compositions (Fig. 1); these results are generally consis-
tent with published data [7]. For both rocks, plagioclase ranges from
An72 to An97 with a mode around An94-96 and a tail to sodic values
(Fig. 1). Plagioclase >_.An95 is abundant in 76315 and comparatively
uncommon in 76295. Olivine and pyroxene clasts have diverse
compositions (Figs. 1-3). 76295 contains less magnesian olivine than
does 76315, and the olivine/pyroxene ratio is 0.85 in 76295 compared
to 5,2 in 76315. In both sam#es, the raft o of high-Ca pyroxene to low-
Ca pyroxene issurprisinglyhigh,about 0-_.
Minor-element abundances in both olivine and pyroxene (Figs. 2
and 3) are unlike those found in rocks of the ferroan anorthosite suite
(FAS), suggesting at most a small contribution from FAS rocks.
Ferroan noritic anorthosites such as those found in 67215 and 67016
[9,10] are especially ruled out; any ferroan anorthosite component in
the clast population must have been nearly pure anorthosite contrib-
5O
4O
30
g 20
Apollo 17 LKFM clasts
_ 76295
76315
Plagioclase
70 75 80 85 90 95 100
An content
o"
It
12 Olivine
8
6
,t
2
0
65 70 75 80 85 90 95
Fo content
3
g, 2
e-
®
8- 1
.e
It 0
Low-Ca pyroxene
70 72 74 76 78 80 82 84
En content
Fig. 1. Histograms of plagioclase, olivine, and low-Ca pyroxene clast compositions in 76295 and 76315.
https://ntrs.nasa.gov/search.jsp?R=19930009617 2020-03-17T07:48:19+00:00Z
LPI Technical Report 92-og, Part 1 43
1.0
0.8
Cr203
0.6
0.4
0.2
0.0 "
0.0
• t " I O I
! • !
0.1 0.2 0.7
' I t I
Low-Ca pyr0xene
• ! . I . l , m |
0.3 0.4 0.5 0.6
MnO
1 _0 ! ! /
High-Ca pyroxene . J
o.,
0.2 AA * l
0.0 _ ' , I , /
0 1 2 3
AI203
Cr203
Cr203
0.3
0.2
0.1
0.0
0.0
0.3
0.2
0
0
° o
o
o
Olivine
• 76295
o 76315
O0 0 _ •
o ='%"...
0.1 0.2 0.3 0.4 0.5
MnO
o
0
0 o
o
o
I • !
Olivine
• 76295 Io 76315
o o
0 o
o.//,,o o .° o00
0.02 0.04 0.06 0.08 0.10 0.12
"1102
0.6
Fig. 2. Minor-dement abundances in pyroxene clasts from 76295 and 76315
compa_.d to fields repzesenting o0mposidons observed in laistine highland rocks
(data from [13,11D. F = ferroan anorthosite suite, T = Mg-suite troctolites, N =
Mg-mite norkes, G = gablaxa_rites, AA = alkali anorthosites.
Fig.3. Minor-dementdmndaneesinolivineclastsfrom76295 and 76315
cornpare.dtofields_:m_mdng compodfionsobservedinpristinehighlandrocks.
Source of pristine rock data _d abbreviations same as Fig. 2 except that "I'D=
tmctolites and dunite.
uting only plagioclase. Low-Ca pyroxenes in both 76295 and 76315
have an affinity with Mg-sulte norites or gabbronorites, whereas
several of the high-Ca pyroxene clasts have minor-element cornposi-
fions outside the known ranges for pristine highland rocks. The
abundance of high-Ca pyroxene clasts, together with their unusual
compositions, suggests the pre,sencc of poorly sampled gabbroic
lithologies in the target stratigraphy of these Apollo 17 LKF/vl impact
melts. That gabbroic rock types may be more abundant in the lunar
highlands than is apparent in the sample collection has been sug-
gested by remote sensing studies [8]. Many of the olivine clasts have
Cr and Ti contents considerably higher than those observed in pristine
highland rocks, including 67667, which has an average divine
composition with 0.049% "1302 and 0.13% Cr203.
Plagioclase compositions are ambiguous with regard to distin-
guishing their source lithologies, but considering that marie phases
tend to be dissolved preferentially when incorporated into _-
heated impact melts, the relatively high abundance of divine and
pyroxene suggests that the dast assemblage is dominated by Mg-suite
rocks. The abundance of high-Ca pyroxene combined with the
relatively Fe-rieh olivine and pyroxene compositions and sodic
plagioclase suggest the presence of one or more evolved olivine
gabbronorite eomlxments with overall characteristics generally simi-
lar to lherzolite 67667, but with distinct minor-element abundances
in the marie phases. Although some minor-clement abundances are
outside the ranges measured for minerals in identified pristine rocks,
none have compositions suitable for the cryptic component rich in
transition metals.
The clast populations in the two Apollo 17 LKFM melt rocks
studied so far are dominated by Mg-suite norites, troctolites, and
gabbronorites. Rock 76295 appears to contain a greater proportion of
the norite or gabbronorite comIxment whereas mineral clasts in 76315
were derived predominantly from ¢octolites. Differences in the
pmtx_ous of clast types is not unusual for impact melt breceias from
the same crater [ 12]. A dearth of ferroan anorthosite was also found
in Apollo 15 LKFM breccias [6] and suggests either (I) it was never
present in the basin target, (2) the FAS crust was removed before the
44 Geology of the Apollo 17Landing Site
Serenitails B ash impact event either by impact erosion or by assimi-
lation during intense magmafic activity, or (3) impact dynamics
during basin formation prevented upper crustal rocks from being
included in the excavated impact melt.
References: [1] Reid A. M. etal. (1977) Proc. LSC 8th,
2321-2338. [2] Ryder G. and Wood J. A. (1977) Proc. LSC 8th,
655-668. [3] Spudis P. D. (1984) Proc. LPSC 15th, in JGR, 89,
C95=-C 107. [4] Spudis P. D. and Davis P. A. (1986) Proc.LPSC ]6th,
in JGR, 91, D84-D90. [5] Ryder G. (1979) Proc.LPSC lOth, 561-
581. [6] Spudis P. D. et al. (1991) Proc. LPS, Vol. 21, 151-165.
[7] Simonds C. H. (1975) Proc. LSC 6th, 641-672. [8] I.aw,ey P. G.
and Hawke B. R. (1988) Proc.LPSC 18th, 355-363. [9] McGee J. J
(1987) Proc. LP$C ]Sth, 21-31. [10] Norman M. D. etal. (1991)
GRL, 18, 2081-2084. [11] Hansen E. C. et al. (1980) Proc. LPSC
]lth, 523-533. [12] McCormick K. A. etal. (1989)Proc.LPSC 19th,
691-696. [13] Borsch et al. (1991) GRL, 18, 2085-2088.
N a 8 -/18 7
IMPACT GLASSES FROM THE <20-p.m FRACTION OF
APOLLO 17 SOILS 72501AND 78221. JohnA.Norrisl,Lindsay
P. Keller2, and David S. MeKay 2,1Department ofC_mology, University
of Georgia, Athens GA 30602, USA, 2Code SN,NASA JohnsonSpace
Center, Houston TX 77058, USA.
Introduction: The chemical compositions ofmi_c glasses
ixeduced during meteoroid impacts on the lunar surface provide
information regarding the various fractionationprocesses that accom-
pany these events. To learn more about these fxacdonadon processes,
we studied the compositions of submicrometer glass spheres from two
Apollo 17 sampling sites using elecoron microscopy. The majority of
the analyzed glasses show evidence for varying degrees of impact-
induced chemical fracfionation. Among these are HASP glasses
(high-A1, Si-poor), which are believed to represent the refractory
residuum left afterthe loss of volatile elements (e.g., S i, Fe, Ha) from
the precursor material [1-3]. In addition to HASP-type glasses, we
also observed a group of VRAP glasses (volatile-rich, Al-poor) that
represent condensates of vaporized volatile constituents, and are
complememary to the HASP compositions [3]. High-Ti glasses we_-e
aLso found during the course of this study, and are documented here
for the first time.
TABLE 1. Average EDX analyses for subgroups of
impact glasses in soils 72501 and 78221.
la lb 2 3 4 5*
MgO 7.0 4.0 9.0 6.0 14.0 3.1
A120 a 53.0 4.0 34.0 27.0 14.0 5.2
SiO2 10.0 7.5 30.0 49.0 46.0 65.0
CaO 23.0 1.5 18.0 11.0 9.0 2.1
TiO2 3.3 64.0 3.0 1.0 3.0 0.6
FeO 3.7 19.0 6.0 6.0 14.0 20.0
* HiglvSi glasses contain <2 wt.% each of NazO. K20, SO3, PzOs, MnO, and
Cr20-,.
la = group l HASP; l b= gro_p l,high.Tt; 2 = group 2 HASP; 3 = group 3 HASP;
4 = basaldc glasses, md 5 = high-Si glasses.
Experimentah Samples fi'om the <20-ttm size fiactious of two
Apollo 17 soil samples (72501 flom station 2 at the base of the South
Massif, and 78221 from station 8 at the base of the Sculptured Hills)
were embedded in low viscosity epoxy and cut into thin sections
(-80-100 nm thick) using diamond-lmife ulwamicrotomy. The thin
sections were analyzed using a PGT energy-dispersive X-ray (EDX)
spectrometer with a JEOL 100CX TEM. EDX analyses were obtained
for 107 spheres from 72501 and 115 from 78221. The apparent
diameters of these spheres in thin section were typically between 100
and 400 rim, although the true diameter of any actual sphere may have
been slighdy larger. The relative errors associated with the EDX
analyses were estimated by analyzing a grossular standard. The
relative errors for AI, Si, Ca, and Fe are -5%. These relative errors
increase significantly for concentrations <5 wt%.
The glass compositions were initially divided into a "high-Si"
group (SiO2 > 60 wt%) or a"low-Si" group (SiO 2< 60 wt%). For the
"low-Si" compositions a standard CIPW norm was calculated. How-
ever, many of these glasses (e.g., HASP) contain insufficient Si to be
used with this method. For these Si-poor compositions, a new
normalization scheme was developed using three groups of progres-
sively Si-deficient normative minerals. Group 1 HASP compositions
contain insufficient Si to calculate any normative rx-',hite (AN).
Instead, the group 1 normative mineralogy includes geba,mit¢ (GH) +
spinel (SP) + Ca aluminates. Group 2 HASP compositions consist of
normative GH + AN + SP + olivine + Ca aluminates. The group 3
o
O3
o
+
o3
o
oJ
<
100
90
8O
70
60
5O
40
3O
2O
10
0
&
72501
6
6 /,
+
X Group 1 HASP
_, Group 2 HASP
o Group 3 HASP
+ Basaltic
= High-Si
[] bulk soil
i_ltl
"_ .+r. "--
#
+o
high-Ti -,
......... x :
o 2o ,o 6; 8;
Si02 + FeO (Wt.%)
Ii
_olj
o
o
+
<
100
9O
8O
7O
6O
5O
40
3O
2O
-10
0
x
xx
78221 X Group 1 HASP
, Group 2 HASP
o Group3 HASP
+ BasaRio
a = High-Si
&A
&
_ [] bulk soil
Ct_ °
&O CO "
& &&
oo
o'_ od= o
oo +
.............. x'. o
high.Ti i x + =
: x : ,0
40 6O
Si02 + FeO (Wt.%)
8O
= ?=
IO0
Flg. l. Comlx_ilionsofsubmicrometerghssesinsoi172501. Fig. 2. Compositions of submicrometer glasses in soi178221.


Lithologies contributing to the clast population in Apollo 17 LKFM basaltic impact melts

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