The chemical compositions of microscopic glasses produced during meteoroid impacts on the lunar surface provide information regarding the various fractionation processes that accompany these events. To learn more about these fractionation processes, we studied the compositions of submicrometer glass spheres from two Apollo 17 sampling sites using electron microscopy. The majority of the analyzed glasses show evidence for varying degrees of impact-induced chemical fractionation. Among these are HASP glasses (high-Al, Si-poor), which are believed to represent the refractory residuum left after the loss of volatile elements (e.g., Si, Fe, Na) from the precursor material. In addition to HASP-type glasses, we also observed a group of volatile-rich, Al-poor (VRAP) glasses that represent condensates of vaporized volatile constituents, and are complementary to the HASP compositions. High-Ti glasses were also found during the course of this study, and are documented here for the first time

Keller, Lindsay P.

Mckay, David S.

Norris, John A.

English

NASA Technical Reports Server

44 Geology of the Apollo 17Landing Site
Serenitatis Basin impact event either by impact erosion or by assimi-
lation during intense magmatie 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. et el. (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 16th,
inJGR, 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) Prac. LSC6th, 641--672. [8] Lucey P. G.
and Hawke B. R. (1988) Proc.LPSC 18th, 355-363. [9] McGee J. J
(1987) Proc. LPSC 18th, 21-31. [I0] Norman M. D. et el. (1991)
GRL, 18, 2081-2084. [11] Hausen E. C. et al. (1980) Proc. LPSC
11th, 523-533. [12] McCormick K. A.et al. (1989) Proc.LPSC19th,
691--696. [13] Bersch et al. (1991) GRL, 18, 2085-2088.
IMPACT GLASSES FROM THE <20-btm FRACTION OF
APOLLO 17 SOILS 72501AND 78221. John A.Norrisl,Lindsay
P. Keller 2, and David S. MeKay 2, ZDepartment of Geology, University
of Georgia, Athens GA 30602, USA, ZCode SN,NASA Johnson Space
Center, Houston TX 77058, USA.
Introduction: The ch(maieel compositions ofmiemscopie glasses
produc_ during meteoroid impacts on the lunar surface provide
information regardingthevariousfractionationprocesses that accom-
party these events. To learn more about these fractionation processes,
we studied the compositions of submicrometer glass spheres from two
Apollo 17 sampling sites using electron microscopy. The majority of
the analyzed glasses show evidence for varying degrees of impact-
induced chemical fractionation. Among these are HASP glasses
Cnigh-/d, Si-poor), which are believed to represent the refractory
residuum left after the loss of volatile dements (e.g., Si, Fe, Na) from
the precursor material [1-3]. In addition to HASP-type glasses, we
also observed a group of VRAP glasses (volatile-rich,/d-poor) that
represent condertsates of vaporized volatile eoustituents, and are
complementary to the HASP compositions [3]. High-Ti glasses were
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
A1203 53.0 4.0 34.0 27.0 14.0 5.2
SiOa 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
TiO 2 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
* High-Si glasses contain <2 w_% each of Na20, K20, SCS, P2Os, MnO, =rid
Cr203.
la= group 1HASP; lb = group I ,high-Ti; 2= gronp 2HASP; 3 = gronp 3 HASP;
4 = basalticgl_ses,=ad5 = high-Si glasses.
Experimental: Samples from the <20-tan size fractions of two
Apollo 17 soil samples (72501 from 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 um thick) using diamond-knife ultramicrotomy. The thin
sections were analyzed using a PGT energy-dispersive X-ray 0EDX)
spectrometer with aJEOL 100CXTEM. 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 Irue diameter of any actual sphere may have
been slightly 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"Iow-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 r_,'thite (AN).
Instead, the group 1 normative mineralogy includes gelu,mite (GH) +
spinel (SP) + Ca aluminates. Group 2 HASP compositions consist of
normative GH + AN + SP + olivine + Ca ahtrninates. The group 3
O
.4-
O3
0
100
90
8O
70
6O
50
40
3O
2O
10
0
&
72501
t_
z_
4 &
z _' _' o
x _ 060 o
+
high-Ti .
0 2O 40
X Group I HASP
= Group 2 HASP
o Group 3 HASP
+ BasaRic
. High-Si
[] bulk soil
** .+...-
dP
÷
+ O0
Si02 + FeO (Wt.%)
[]
'I_ 0,= '
T
IO0
A
0
+
0
<
IO0
9O
80
70
6O
50
40
30
2O
10
0
x
x
78221
&
high-TF:"
........... .x.i =
' 2_ ' 4o' ' 6'o '
Si02 + FeO (Wt.%)
6
AA
÷
÷ g
X Group 1 HASP
,, Group 2 HASP
,, Group 3 HASP
+ Basaltic
= High-Si
[] bulk soil
80 100
Fig. 1. Compositions of submicrometer ghsses in soil 72501. Fig. 2. Compositions of submicrometer glasses in soil 78221.
https://ntrs.nasa.gov/search.jsp?R=19930009618 2020-03-17T07:48:24+00:00Z
LPI Technical Report 92-09, Part 1 45
HASP compositions are cordierite- or mullite-normative, and contain
excess Al203 and SiO2 after using all of the Ca, Na, and K to make
normative feldspar.
Results and Discussion: The corupositions of the 107 glasses
analyzed from sample 72501 and the 115 from 78221 are plotted in
Figs. 1 and 2. The compositions range from nearly 95% refi'actory
components to others composed entirely of more volatile e.omponents.
HASP compositions comprise -75% of the total glasses analyzed in
each sample. The group 1 HASP glasses comprise --6% of the total
analyses from both 72501 and 78221. The group 1 HASP composi-
tions are the most refractory and the most volatile-depleted of all the
analyzed glasses. A cluster of high-Ti ghsses were included within
group 1and plot about the origin in Fig. 1. The group 1high-Ti glasses
contain little Ca or AI, but nonetheless have undergone extreme
volatile loss in the form of nearly complete removal of Fe from what
wasonceilmenite(TableI).The othergroupImembersareprobably
derivedfrom mostlyanorthositicmaterialthathaslostmost ofits
originalSiO2content.Allbutone ofthe72501groupIHASPs are
high-Tiglasses,whereas78221containsalargerprOlXa'tionoftheCa-
andAl-richmembers ofthisgroup(TableI).
Thegroup2HASP glassespanamuch widercompositionalrange
inbothsamples,andcomprise29% oftheanalysesfrom72501and
27% ofthoseflxnn78221.Thesehaveundergonealesserdegreeof
volatileossthanthoseofgroup1,and generallyretainsignificant
amountsofSiO2andFcO,eventhoughconsiderableamountsofthese
components have been lost. The average composition of group 2
glasses (in wt%) from both samples is given in Table 1.The similarity
in both the relative proportions and the average compositions of the
group 2 HASP glasses in these two samples reflects the similarities
in bulk soil composition and soil maturity at these two sites. We note
that glasses fractionated to the extent of the group 1 and 2 HASP
compositions seem to occur only among the finest size fractions.
The group 3 HASP glasses have the least fractionated composi-
tions. The glasses of this compositional type in both samples comprise
a larger proportion of the analyzed population than does any other
single group (44% of 72501 and nearly 42% of 78221), and the
average composition of this group is very similar in both samples
(Table 1).
The high-Si glasses are comprised of the relatively volatile
elements and are cornpositionally complementary to the HASP glasses.
These compositions make up 13% of the 72501 analyses and 11% of
the 78221 population. The compositions of these glasses in both
samples span the range from nearly pure SiO 2, sometimes with other
associated volatiles such as Na, K, P, and S, to other Si-rich compo-
sitions with high F¢ concentrations (Table 1). It is believed that the
compositions of this group represent the recondensation of impact-
generated vapors [3]. As with most of the HASP glasses, these Si-rich
compositions are only found among the finest size fi-acfions of lunar
soils. This suggests that such extreme fractionations only occur at
sizes where the surface-area-to-volume ratio is high enough to allow
the degree of melting and vaporization required to produce these
unusual compositions.
In both samples, the glasses of basaltic composition constitute a
relativelysmallgroup,beingjustover8% oftheanalysesfrom72501
and nearly14% ofthosefrom 78221.Thesecompositionsdisplay
littleorno observablevolatileoss.The averagecompositionofthis
group(inwt%) from bothsamplesisgiveninTable1.
Conduslons: We foundthathemajorityoftheanalyzedglasses
inboth soilseitherhave refractorycompositionsresultingfrom
volatileloss,or arcvolatile-richcondcnsatcsof impact-generated
vapors.Inbothsamples,thethreeHASP groupscomprise-75% or
more ofthetotalnumberofanalyzedglasses.The HASP glassesarc
derived from the bulk soil and from the feldspathic component of the
soils through the loss of major amounts of Si and lesser quantifies of
Fe and alkalis. The Si-rich glasses rival the number of the unfraction-
ated basaltic glasses. The pronounced fractionations (volatilization
and condensation) that occur in the submicrometer size range result
from the large surface-area-m-volume ratio of the glasses.
This is the fLrSt report of high-Ti glasses from lunar soils.
Considering all high-Ti glasses from both samples together, a trend
is observed that begins with Fe-Ti-rich compositions and extends to
glasses that consist of nearly pare TiO 2. We conclude that these
compositions originate by the loss of Fe from ilmenite, which is the
dominant Ti-rich oxide at the Apollo 17 site. These high-Ti glasses
are one of the few types of impact glasses derived from a specific
mineral constituent of lunar soils.
References: [i]Nancy M. etal.(1976)Proc.LSC 7th,155.
[2]Vaniman D. (1990)Proc.LPSC 20th,209. [3]KeUer L.and
McKay D.(1992)Proc.LPS, Vol.22,137. /j +i
Y
ISOTOPIC AGES AND CHARACTERISTICS OF ANCIENT
(PRE-SERENITATIS) CRUSTAL ROCKS AT APOLLO 17.
W. R. Prerno and M. Tatsumoto, U.S. Geological Survey, MS 963,
Box 25046, Federal Center, Denver CO 80225, USA.
, = = . .
i =Problems with the Isotopic Systematics In Lunar Samples: ------h
Four different decay schemes, K-At, Rb-Sr, Sm-Nd, and U-Pb, have
predominantly been used for age determinations on lunar rocks.
Theseradiomctricsystemsarcparticularlyusefulbecausetheyhave
long half-lives that are on the same scale as most lunar rock ages,
which is necessary in order to obtain the most precise ages [1].
However, none of these systems is without problems when applied to
lunar samples. In order for any single radiornetric system to yield a
primary crystallization age, it must remain closed from the time of
crystallization until the present, without addition or loss of either
parent or daughter isotopes. After many years of isotopic work,
investigators have come to realize that most lunar samples (nearly all
ancient highland samples) have been metamorphosed and their
isotopic systematics disturbed [2]. Many studies _ort completely
reset and partially reset Ar-Ar ages---40Ar loss readily occurs during
impact metamorphism. However, some Ar-Ar ages agree quite well
with other radiometrie ages from the same sample, illustrating that Ar
is not lost in some lunar samples. Reports of disturbances in the Rb-
Sr and U-Pb systems [2--4] as well as the Sm-Nd system [3] are also
prevalent in the lunar literature. Disturbed Rb-Sr isochron ages are
normally explained by either mobility of both elements ormixing due
to breeciation. The Sm-Nd system appears to be the most retentive,
although some problems have also been noted. U-Pb and Th-Pb
isochron ages typically date metamorphic events [5]. Fortunately,
there are two U-Pb systems that can be compared simultaneously in
aeoncordia diagram to"look through" disturbances, This ata'ibute of
the U-Pb systems is most desirable when working with lunar samples,
and helps to identify both the age of the rock and the age of the
disturbance. A major drawback of this approseh is the necessity for
initial Pb corrections in order to calculateradiogenicPb/U ratios
[5,6]. Initial Pb compositions are typicany defined by the y-imercepts
on U-Pb and Th-Pb isochron diagrams, but because most U-Pb and
Th-Pb isochrons for lunar samples are distarbed, the initial Pb values
areundefined and thereforemust be assumed. This situation has been
confronted with norite 78235 [5]. A possible solution to the problem
is to use an age (hopefully accurate), perhaps determined using one
of the other dating techniques, and back calculate what the initial Pb
values must be in order to produce the same age with the two U-Pb


Impact glasses from the less than 20-micrometer fraction of Apollo 17 soils 72501 and 78221

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