Implicit in current understanding of the location of terrestrial enriched and depleted reservoirs is the notion that they are spatially separated. The depleted reservoir on Earth is situated in the upper mantle, and the complementary enriched reservoir is located in the crust. However, Earth reservoirs are continually being modified by recycling driven by mantle convection. The Moon is demonstrably different from Earth in that its evolution was arrested relatively early - effectively with 1.5 Ga of its formation. It is possible that crystallized trapped liquids (from the late stages of a magma ocean) have been preserved as LILE-enriched portions of the lunar mantle. This would lead to depleted (cumulate) and enriched (magma ocean residual liquid) reservoirs in the lunar upper mantle. There is no evidence for significant recycling from the highland crust back into the mantle. Therefore, reservoirs created at the Moon's inception may have remained intact for over 4.0 Ga. The topics discussed include the following: (1) radiogenic isotopes in high-Ti mare basalts; (2) formation of cogenetic depleted and enriched reservoirs; and (3) melting of the source to achieve high-Ti mare basalts

Halliday, Alex N.

Snyder, Gregory A.

Taylor, Lawrence A.

English

NASA Technical Reports Server

LPI Technical Report 92-09, Part I 53
floor, and as the scarp cuts the valley wall of North Massif, its trend
cuts sharply to the west. The slope of North Massif approaches 20°,
therefore if the scarp is the expression of awes twardly dipping reverse
fault, the dip must exceed 20°. The problem, however, is that the
morphology of wrinkle ridge systems is highly variable and permits
a spectrum of tectonic styles to be involved. Within Serenitatis alone,
there are complex systems of ridges (Fig. 1), some of which have
distinct vertical offsets, while others do not (Fig. 2). Two models have
been proposed to account for the vertical offsets: thrust faulting [25]
and nearly vertical faulting [24]. There does not appear to be any
correspondence between the vertical offset across aridge element and
its topographic relief. Furthermore, apparent offsets across some
ridges are tm_duced because the ridge is developed on a sloping mare
surface; when the regional trend is removed, so is the offset in several
cases. This suggests that wrinkle ridges include a variety of compres-
sional tectonic structures, perhaps ranging from simple thrust faults
to nearly vertical reverse faults m complex zones of buckling [22].
There does not, however, appear to be any indication of substantial
ov_ting. " / t:F_'
- Nature and Timing of Tectonic P_iHe g0rmatto_lf" Concen-
trically oriented tectomcnlles deform the flanks of many of the large
mare basins including Serenitatis. These smactures have been attrib-
uted to the extensional deformation associated with mascon loading
[1]. Golornbek [25] proposed that these graben were produced
through simple extension, that the bounding faults intersected at a
depth of a few kilometers, and that the faulted layer corresponded to
the"megaregolith." In that analysis, graben formation due to bending
was dismissed because under the assumed conditions, the mare
surface would slope away from the graben by up to 10°. However,
Golombek apparently did not consider the effects of increasing the
thickness of the faulted !ayer. Figure 3 shows the surface slope that
would result from layer bending to produce the size of lunar tines
observed (50-150 m deep, 2-4 kra wide; [25]). If the depth to the
neutral surface were half the thickness of the elastic lithosphere
(-100 kin; [1]), then the slopes induced by bending would be <-1°, in
excellent agreement with measurements of the slopes on mare
surfaces containing linear rilles [4]. This analysis indicates that
graben arom_d lunar basins can be accounted for solely by bending of
the elastic lithosphere.
The slxatigraphic relationships between tectonic rilles and the
major volcanic units exposed in mare basins provide dues to the
timing of basin deformation. It has long been recognized that rille
formation ended prior to 3.4 Ga [26]. In addition, assessment of
southeastern Serenitatis shows that the oldest votcanic unit (unit I;
Plenius basalts equivalent; [3] ) was emplaced prior to the onset ofrille
deformation. There are no cases where unit I clearly floods or embays
any rifles nor is there any indication that rifles are truncated at the
boundary between this unit and the highlands. In constrast, rilles that
intersect the younger unit II surface are consistently truncated and
embayed by these lavas. Elongate collapse feamxes, indicative of
buried rilles, are observed on unit II in the southeastern portion of the
basin, but no such features are evident on exposed unit I surfaces.
Consequently, it seems that rille formation in southeastern Serenltstis
began after unit I emplacement and culminated before unit IT.
Samples returned from Apollo 17 indicate that the unit I basalts
were deposited over arange of-150 Ma from -3.84 Ga to-3.69 [27].
Thus the post-unit-I onset of rille formation appears to signal a
relatively slow response of the lithosphere to the increasing volcanic
load in the Serenitatis Basin. The most likely explanation for this
involves the isostatic state of the Serenitatis Basin during early mare
emplacement. It is conceivable that appreciable quantities of the early
volcanics would be required to offset the mass deficiency created
during the impact basin formation. If this is the case, volcanic in.filling
of the southern portion of Mare Screnitatis did not reach supedsostatic
levels until the majority of unit I was emplaced.
References: [1] SolomonS.C. andHeadJ.W. (1979)JGR,84,
1667-1682. [2] Bratt S. R. et al. (1985) JGR, 90, 3049-3064.
[3] Howard K. A. et al. (1973) In Apollo 17 Preliminary Science
Report, NASA SP-330, 29-1 to 29-35• [4] Sharpton V. L. and Head
J. W. (1982) JGR, 87, 10983-10998. [5] Phillips R. J. et al. (1973)
Proc. LSC 4th, 2821-2831. [6] Peeples W. J. et al. (1978) JGR, 83,
3459-3470. [7] Cooper M. R. et al. (1974) Rev. Geophys. Space
Phys., 12, 291-308. [8] Lucchitta B. K. (1972) U.S. Geol. Surv. Misc.
Geol. Inv. Map 1-800, sheet 2. [9] Apollo Lunar Geology Investiga-
tion Team, U.S. Geological Survey (1972) Astrogeology, 69, 62 pp.
[10] Wolfe E. W. et al. (1975) Proc. LSC 6th, 2463-2482.
[11] Muehlberger W. R. et al. (1973) In Apollo I7 Preliminary
Science Report, NASA SP-330, 6-1 to 6-91. [12] Wilhelms D. E.
(1987) US. Geol. Surv. Prof. Paper 1348, 302 pp. [13] Head J. W.
(1976) Moon, 15,445-462. [14] Balley N. G. and Ulrich G. E (1975)
Apollo 17 Voice Transcript Pertaining to the Geology of the Landing
Site, USGS-GD-74-031,361 pp. [15] Schmitt H. H. and Ccman E. A.
(1973) In Apollo 17 Preliminary Science Report, NASA SP-330,
5-1 to 5-21. [16] Suxnn R. G. (1972) Moon, 187-215. [17] Young
R. A. et al. (1973) In Apollo 17 Preliminary Science Report, NASA
SP-330, 31-1 to 31-11. [18] Hodges C. A. (1973) In Apollo 17
Preliminary Science Report, NASA SP-330, 31-12 to 3i-21.
[19] Howard K. A. and Muchlberger W. R. (1973) In Apollo 17
Preliminary Science Report, NASA SP-330, 31-22 to 31-25. [20] Scott
D. H. (1973) In Apollo 17 Prelinu'nary Science Report, NASA SP-
330, 31-25 to 31-29. [21] Bryan W. B. (1973) Proc.LSC4th, 93-106.
[22] Sharpton V. L. and Head L W. (1987) Proc. LPSC 18th,
307-317. [23] Tjia H. D. (1970) Geol. Soc.Am. Bull., 81,3095-3100.
[24] Lucchitta B. K. (1976) Proc. LSC 6th, 2761-2782. [25] Plescia
J.B. andGolombek M. P. (1986) Geol. Sac.Am. Bull, 97,1289-1299.
[25] Golombek M. P. (1979) JGR, 84, 4657-4666. [26] Lucchitta
B. K. and Watklns J. A. (1978)Proc.LPSC9th, 3459-3472. [27] Ba-
saltic Volcanism Study Group (1981) Basaltic Volcanism on the
Terrestrial Planets, Pergamon, 1286 pp. /3 7
N 8
MELTING OF COGENETIC DEPLETED AND ENRICHED
RESERVOIRS AND THE PRODUCTION OF I-HGH-TI MARE
BASAL'IS. GregoryA. Snyderl,LawremceA.Taylorl,andAlexN.
Halliday 2, 1Department of Geological Sciences, University of
Tennessee, Knoxville TN 37996, USA, 2Deparunent of Geological
Sciences, University of Michigan, Ann Arbor M148109, USA.
_ Implicit in current understanding of the location of terrestrial
-,_'iched and depleted reservoirs is the notion that they are spatially
separated. The depicted reservoir on Earth is situated in the upper
mantle, and the complementary enriched reservoir is located in the
crust. However, Earth reservoirs are continually being modified by
recycling driven by mantle convection. The Moon is demonstrably
different from Earth in that its evolution was arrested relatively
early--effectively within 1.5 Gaofits formation [1]. It is possible that
crystallized trapped liquids (from the late stages of amagma ocean)
have been preserved as LILE-entlched portions of the lunar mantle.
This would lead to depleted (cumulate) and enriched (magma ocean
residual liquid) reservoirs in the lunar upper mantle. There is no
evidence for significant recycling from the highland crust back into
the mantle. Therefore, reservoks created at the Moon's inception may
have remained intact for over 4.0 Ga.
https://ntrs.nasa.gov/search.jsp?R=19930009623 2020-03-17T07:48:52+00:00Z
54 Geology of the Apollo 17Landing Site
6
=_
4
HIGHoTi MARE BASALT$
n-8_ 17_1 tT-A
7-B2
_,J 1 I i
0.69910 0.69915 0.6992 0.69925 0.6993 0.69935
STSr/S_Sr initial
!0/ ... \
........_. _.. High-Ti Mare Basaits
81 "'_'. "_:_,_/ CI'L= Trapped Liquid)
6f........-.....°"\....
= 2 ................? '_/_":8_"1:6%T_"....
-..\%
......;'6:" "''_ ""'- O
,,, __._ "--....."-.,_.,_
-. -...............-_
2 , Nd _ 0.165
3.4 3.6 3.8 4 4.2 4.4 4.6
Time (Ga)
Fig.L _ - vlg.3.
: \ _ __
! f RidlogenlcI_top_ in mgh-Xl Mare 13_H: ._ide,,_ of =t°wer_e, anda d__ m_gm_o_ [61.Th_ ou_
Heterogeneity? I_)ata fTom all high-Ti basalts (Apollo 11 an_I-_magmaocean, ormagmasphare,progressivelycrystallizedtoformthe
Apollo 17"landing_tes, as well as intermediate-Ti ilrnenite b_alts : upper mantle of the Moon and, once plagioclase became a Ikluidus
flora Apollo 12)-display a broad scatter in initial Nd and Sr isoiopic
: _mposition as shown in Fig. 1 [2-5]. Basalts from Apollo 11
: generally plot with less radiogenic Nd compared with those from
Apollo 17. Although Apollo 11 group B 1 basalts h_v_ _Nd values that
aresirnilartoApollo]7 _k: B 1, and C basalts, _ese groups are distinct
in Sr isotopic composition. These variatkms in initial bid and Sr
isotopic compositions have lex_previous_workers to postulate that the
basalts were derived from separate-soL_:es and have been used as
: evidence of heterogeneity in the lunar uI_peunantie [4]. Though
interpretation seems quke adequate, it is difficult_ reconcile with the
view that the lonaruppermantle was a simple system that crystallized
early and has _t been subject to later mixing and recycling processes.
- Furtheamaore: this interpretation, which includes myriad different
sources. _ces not explain the homogeneity in mineralogy and major-
elemcat chemistry for these rocks.
_' ]formation of.Cbgenetic Depleted and Enriched Reservoirs(
_is generally believed that for less than the fast 0.2 Ga of its history_
the Moon differentiated into a small Fe-rich core, an olivine-rich
phase, its anorthositic crust. The crystallization of the magmasphere
probably led to layering of the upper mantle and included phases, such
as iimenite,late in its differentiation. The residualliquidmagmasphere
became more evolved with time, leading to enrichment in the LILE.
This LILE-enriched liquid could have been trapped in variable, yet
small, proportions and effectively "metasomatized" the relatively
LILE-depleted crystallizing marie cumulate. In this way, adjacent
regions or layers of the mantlecoald maintain mineralogic and major-
element homogeneity while exhibiting heterogeneities in their trace
elements.
The LILE-enriched, trapped liquid end member of the source is
represented by refidua] liquid after 95% crystallization of the LMO
(as per Snyder et al. [7]). The isotopic and u-ace-element composition
of this KREE_ liquid comtxment may be repre.sented by KREEP
basalt sampIe 15382 (Rb= 16ppm,Sr= 195 ppm,_Sr_SSr(3.84 Ga) =
0.70115; Sm= 31 ppm, Nd = 112 ppm, _Sd(3.84) = --3; [8,9]). The
cumulate portion of the source is modeled as a cpx-pigeomte-
ilmenite -olivine perfect adcurauiate (Rb = 0.01 ppm, Sr = 4.16 ppm,
2O
•_ 10
"O
4D
L_
Vm[
L_
2
0.5
Upper Mantle Marie Adcumulate +
KREEPy Trapped Liquld_
t t i i / i t t I I I i i t I t i ) i i
Rb Ba La Eu Tb Lu Hf
f;r Ce Nd Sm Gd Dy El" Yb
0.32
=Z O.28
_ 0.24
°I
0.16
0
X6.0"_
- "%_.0g..
IG_EEPy Trap_ A
• ! i !
0.01 0.02 0.03
STRbf_Sr
Fig. 2. Fig. 4.
LPI Technical Report 92-09, Part 1 55
Sm -- 0.631 ppm, Nd = 1.2 ppm, again as per [7]; Figs. 2 and 3) that
has an extremely elevated _Hd (+8 at 3.84 Ga) and the]owest Sr initial
ratio (0.69910) at 3.84 Ga. However, it isnot likely that these two
components remained distinct over a period of 500 Ma, when the
interior of the Moon vtas s_ll hot [10]. Recrystallization of the
curnulate-trapped liq_d pile could have occurred, yielding a source
that was heterogeneous in trace elements on the scale of meters. Due
to the low S'rn/Nd ratio of the trapped liquid relative to the cmnulate,
those Ifortious of the mantle that contained a larger proportion of this
coinponent would evolve with more enriched isotopic signatm¢._
__--i_ _- Meltin_ ofthe Source toA_:hieveJSIlgh.T| Mare Basalts?-_,_fter =
an extended period of evolution (e.g., >0.5 Ga), earliest melting of the
trapped liquid-cumulate pair would probably affect regions that were
relatively enriched in the LILE (containing heat-producing elements
U, Th, and K). Therefore, those regions that trapped the largest
proportion of residual LMO liquid would melt first. Melts of these
regions would exhibit relatively enriched isotopic signatures. Later
melting would tap regions with less trapped liquid and would yield
more isotopicaliy depleted melts. Obviously, the degrees of enrich-
ment and depletion of the melts arc highly dependent upon the
proportion of trapped liquid and the extent of melting of the cttmu-
late + trapped-liquid pile. However, the trapped-liquld component is
LILE-enfiched (generally by at least an order of magnitude over the
marie cumulate) and would have originally consisted of low-tempera-
rare melting phases that would readily remelt. Therefore, even a small
proportion (e.g., 1%) in the cumulate pile will greatly affect the
isotopic signature of initial derived melts. However, because of its
small proportion, the trapped liquid would have a lesser effect
(inversely proportional to the degree o f melting) on the major -element
composition of the melt.
The low 147Sm/lUNd of this KREEPy trapped liquid, in concert
with the relatively high abundances of Sm and Nd, obviates a large
proportion of trapped liquid in the source (Fig. 2). This is illustrated
in Fig. 2, where small _ous (<_5% ) of trapped liquid have been
added to a model high-Ti adcumulate. This addition of trapped liquid
has the effect of lowering the Sm/Nd ratio, yet increasing the
abundances of Sm and Nd, thereby leading to a viable cumulate source
region. Again, only 2-3% of trapped liquid is required in the source
for modal melting, and less if the trapped liquid is an early melting
component (if the source was not recrystallized).
This can be further appreciated by looking at the parent/daughter
ratios of the high-Ti basalts (Fig. 4, diagonally hatched areas). Simple
two-component mixing of a high-Ti"adcumulate" source with vary.
ing percentages of KREEPy trapped liquid (where sample 15382
represents the KREEPy liquid) yields a curve of chemical mixtures.
The compositions of the high-Ti basalts fix_n both Apollo 11 and
Apollo 17 lie along this curve. Any source that contains residual mafic
minerals, such as pigeonite, clinopyroxene, and olivine, would be
more depleted (lower in s'rRb/_SSr and higher in 14_Sm/144Nd) than
the basalts from which it was generated. Therefore, the field of
permissible sources (shaded) indicates <1.5% trapped liquid. Al-
though this trapped KREF..Py liquid is minor in volume, it controls the
radiogenic isotope signature of the derived melt.
Similar calculafiom to discern the proportion of KREEP in these
basalts were performed by Hughes et al. [ 11] and Paces et al. [4]. Both
groups concluded that small percentages (generally <1%) of a Rb-,
St-, and REE-em'iched componenL with high Rb/Sr and low Sm/Nd
ratios, are required to explain the compositions of parentalmagmas
for the high-Ti basalts. However, both groups envisioned this corapo-
nent as distal to the cumulate source and added to the source prior to
its fusion, but not cogenetic with its inception. Paces et al. [4] pointed
out the problems inherent in such a scenario, but neither group
explored the possibility of an ancient KREEPy reservoir that was
spatially associated with the cumulate source_as trapped inter-
cumulus liquid from the late stage of evolution of the LMO--since its
inception over 4.4 Ga.
Summary: The interpretation of cogenetic depleted and en-
riched reservoirs in the Moon is the consequence of events unique to
the Moon. First, the late-stage LREE- and Rb-enriched residual liquid
from a crystalllzing LMO was trapped in variable and small prepor-
floats in the depleted upper mantle cumulates. A lack of recycling in
the_unar environment would allow these reservoirs to diverge along
sq_arate isotopic evolutionary paths. This portion of the mantie would
remain undisturbed for _>0.5 Ga, prior to being melted to form the
oldesthigh-Ti mare basalts.The isotopicharacterofthemeltswould
be controlled by the degree of melting, as the least radiogenic
reservoir would be melted Fn'st, i.e., that portion of the cumulate
containing the greatest proportion of trapped liquid would melt first.
The range in Sr and Nd isotopic ratios seen in basalts from Mare
Tranqml"litatis (Apollo 11) is due to melting of a clinopyroxene-
pigeonite-ilmenite-olivine cur_ulate layer with variable proportions
of trapped intercumulus liquid. Types B2 and B3 basalts were melted
fi-om a portion of the cumulate layer with intermediate amounts of
Irapped KREEPy liquid. Type B 1 basalts from both Apollo 11 and 17
are melted from a "near-perfect" adeumulate portion of this layer.
Apollo 12 ilmenite basalts represent the final known melting of this
cumulate source, aftez it had been nearly exhausted of its ilmenite and
trapped-liquid components. Type A basalts were probably exmuied
from a vent or vents near the Apollo 17 landing site [12] and could,
therefore, represent melting of a similar source, albeit with the added
complexity of neoKREF_.P assimilation.
References: [1] Nyquist L. E. and Shill C.-Y. (1992) GCA,.56,
2213-2234. [2] Papanastassiou D. A. et al. (1977) Proc. I.SC 8th,
1639-1672. [3] Onruh D. M. et al. (1984) Proc. LPSC 14th, in JGR,
89, B459-B477. [4] Paces J. B. et al. (1991) GCA, 55, 2025-2043.
[5] Snyder G. A. et al. (1993) EPSL, subraitted. [6] Agee C. B. and
Longki J., eds. (1992) LPITech. Rpt. 92-03, 79 pp. [7] Snyder G. A.
et al. (1992) GCA, 56, in press. [8] Nyquist L. E. (1977) Phys. Chem.
Earth, 10, 103-142. [9] Lugntair G. W. and Marti K. (1978) EPSL,
39, 349-357. [10] Warren P. H. et al. (1991) JGR, 96, 5909-5923.
[1 I]Hughes S. S. et al. (1989) Proc. LPSC 19th, 175-188. [12] Jerde
E. A. et al. (1992) EPSL, submitted. .o
8.8>t !3 / 7 s
BASALTIC IMPACT MELTS IN TI-IE APOLLO COLLEC-
TIONS: HOW MANY IMPACTS AND WHICH EVENTS
ARE RECORDED? Paul D. Spudis, Lunar and Planetary Insti-
tute, Houston "IX 77058, USA.
Many of the rocks in the Apollo collections from the lunar
highlands are impact melt breccias of basaltic bulk composition
[1-3]. They are known by a variety of names, including "low-K Fra
Mauro basalt" [1], "VHA basalt" [2], and "basaltic impact melts" [3].
These rocks have been studied to understand the compositional nature
of the lunar crust [ 1,4], to decipher the processes of large body impact
[4], and to comprehend the record of impact bombardment of the
Moon [51.
Study of tearestrial craters has led to a model for impact melt
generation (e.g., [6]) whereby diverse target lithologies are totally,
not partially, melted during impact. The impact melt makes up a few
percent of the total volume of crater material; superheated silicate
liquids of the impact melt have extremely low viscosities and mix
intimately. This mixing thoroughly homogenizes the melt chemically
during the excavation of the crater. Colder, unmelted debris is


Melting of cognetic depleted and enriched reservoirs and the production of high Ti Mare basalts

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