Meteoritics B Planetary Science 34, 527-540 (1999)
8 Meteoritical Society, 1999 Pnnted in USA.
Ascent and eruption of a lunar high-titanium magma as inferred
from the petrology of the 74001/2 drill core
CATHERINE
M. WEITZIT*, MALCOLM
J. RUTHERFORDI, JAMES W.HEAD 1111 AND DAVIDS. MCKAY2
'Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
2NASA Johnson Space Center, Houston, Texas 77058-3696, USA
tpresent address: Jet Propulsion Laboratory, Pasadena, California 91 125, USA
*Correspondence author's e-mail address: [email protected]
(Received 1998 August 13; accepted in revised form 1999 February 10)
~
Abstract-An analysis of the orange glasses and crystallized beads from the 68 cm deep 7400112 core has
been conducted to understand the processes occurring during ascent and eruption of the Apollo 17 orange
glass magma. Equilibrium between melt and metal blebs (Fe85Nil4COl) within the core, along with Cr contents in olivine phenocrysts, suggest there was an oxidation of C and a reduction of the melt at an 0 fugacity
of IW-1.3 and 1320 "C to form CO gas at 200 bars or 4km depth. This was followed by development of
more oxidized conditions during ascent. Also during ascent, there was formation of euhedral, homogeneous
Fog1 olivine crystals and spinel crystals with higher Al and Mg contents than the smaller spinels in the
crystallized beads. Both the metal blebs and Al-rich spinels were trapped inside the Fog1 olivine phenocrysts
as they grew prior to eruption. The composition of the orange glasses are homogeneous throughout the core,
except for a few distinct glasses at the top that appear to have been mixed in by micrometeorite reworking. A
few glassy melt inclusions of orange glass composition trapped in the Fog1 phenocrysts contain 600 k 100 ppm
S and -50 ppm CI compared to the 200 ppm S and 50 ppm C1 in the orange glass melt when quenched. These
inclusions therefore document the addition of 400 ppm S to the CO-rich volcanic gas during the eruption.
The size and distribution of different volcanic beads in the Apollo 17 deposit indicate a mode of eruption
in which the orange glasses and partially crystallized beads formed further away from the volcanic vent where
cooling rates were faster. Progressively larger and more numerous crystals in the black beads reflect slower
cooling rates at higher optical densities in the volcanic plume. The development of a brown texture in the
orange glasses at the bottom of the core, where the black beads dominate, is interpreted to result from
devitrification by subsolidus heating either as the orange glasses fell back through the hot plume or after
deposition on the surface. The change from domination by orange glasses to black beads in the core probably
reflects a decrease in gas content over time, which consequently would increase the plume optical density and
favor slower cooling rates.
INTRODUCTION
The Apollo 17 74001/2 core is a double-drive core taken in an
orange patch of soil on the rim of the 120 m diameter impact crater
Shorty. The top 10 cm soil sample was labeled 74220 and the
68 cm deep 74001/2 core was taken just below 74220. The core is
unique because, unlike most other lunar soils that represent a collection of many types of rock fragments, the 74001/2 core is composed
completely of volcanic beads with only a minor lithic component in
the top 8 cm (McKay et al., 1978; Morris et al., 1978). About 1-3%
of the core consists of olivine phenocrysts that formed during ascent
in the conduit (Heiken and McKay, 1978). Only the upper 5.5 cm
of the core is thought to have been affected by reworking from
micrometeorites (McKay et al., 1978; Morris et al., 1978; C r o w
1978). The volcanic beads consist of high-Ti orange glasses (9 wt%
TiO2) and their Crystallized equivalents (black beads). The beads
were erupted as part of a volcanic plume. Volatiles associated with
the gas plume, such as K, S, Pb, CI, F, and Na, have been identified
on the surfaces of the beads (Butler and Meyer, 1976; Meyer et al.,
1975; Kriihenbiihl, 1980). However, the main driving volatile was
probably CO gas formed by reduction of graphite (Sato, 1979; Fogel
and Rutherford, 1995; Weitz et al., 1998).
The beads have a uniform composition throughout the core and,
therefore, represent a continuous eruption sequence (Blanchard and
Budahn, 1978). However, more recent high-precision analyses of
the 74220 beads indicate small differences (0.4 wt?? Si02) exist
between the beads, which suggests slight heterogeneities in the
magma and/or eruption variations (Delano, 1996; Hansom and
Lindstrom, 1997). Given the primitive composition, the beads are
considered the closest approximation that we have to a primary mare
magma and are therefore valuable for providing insight into the
magma source regions. Multisaturation experiments indicate that
the source of the orange glass magma was at 400-500 km depth
(Green et al., 1975); although if polybaric melting occurred, this may
represent an average depth (Longhi, 1992). In addition, the realization
that the high-Ti melts were negatively buoyant at >15-20 kbars
implies that there may have been a range of melts within an ascending diapir (Hess, 1991; Delano, 1990; Circone and Agee, 1996).
Heiken and McKay (1978) examined the 74001/2 samples and
found that near the base of the core, 93% of the beads are crystalline
whereas near the top, only 24% are crystalline black beads. In many
early papers, the black beads were referred to as devitrified glasses.
We prefer the term crystallized or black beads because the beads are
no longer glasses, and the term devitrified implies subsolidus heating,
which we will show in a later section occurred in some beads but
produced a different texture from that seen in the black beads. The
bead shapes, degree of crystallization, and olivine textures and
compositions all indicate a range of cooling rates for the beads
(Heiken and McKay, 1978). The average size of the beads is 44 p m
diameter (Heiken et al., 1974) but many beads, particularly near the
bottom of the core, are clearly fragments of larger beads (Weitz et
al., 1996). Many black beads have smaller beads attached to their
527
528
Weitz et al.
surfaces. These compound textures may have formed as beads fell
through the fountain, colliding with molten or semi-molten droplets
(Heiken et al., 1974). Heiken and McKay (1978) estimated that
10% of the beads had a compound texture near the top of the core
compared to 40% at the bottom.
Bogard and Hirsch (1978) showed that the solar exposure ages
in the core can best be modeled by an inversion of the stratigraphy
at depth by the Shorty crater impact -10 Ma ago, although Huneke
et al. (1973) found a cosmic-ray exposure age of 30 Ma. The beads
themselves have an age of 3.54 to 3.66 Ga (Huneke et al., 1973;
Schaefer and Husain, 1973), which is younger than basalt samples
collected at the Apollo 17 site. Samples of soils between 5-57 cm
depth show a nearly smooth increase in *IN, and 38Ar with
increasing depth, indicating that the original stratigraphy was not
disturbed by the Shorty impact and overturn (Bogard and Hirsch,
1978). However, the bottom 10 cm of 74001 has much higher *INe
and 38Arcontents, indicating a different depositional and irradiational
history than the upper portions of the core. If the core represents the
inverted stratigraphy exposed by the Shorty impact, then crater
excavation depths indicate that the deposit of volcanic beads is
located between 10-20 m depth (Heiken and McKay, 1978).
Unfortunately, we do not know whether the 74001/2 core represents
the beginning, middle, or end of the volcanic eruption. The beads
contained within the core were collected in the Taurus-Littrow
Valley, which is located at the eastern edge of the Taurus-Littrow
regional dark mantle deposit, also thought to be composed of the
same orange glasses and black beads as those in the 7400112 core
(Lucchitta, 1973; Pieters et al., 1974).
In this paper, we have examined thin sections taken along the
length of the 7400112 core and analyzed the different types of beads
and minerals present. Using the textures and composition of the
beads and minerals, we have determined the conditions in the
volcanic plume and during emplacement of the beads on the lunar
surface. The 0 fugacity (fO2) that existed in the magma during
ascent can also be inferred from olivine and spinel microphenocryst
compositions. These results provide new insight into processes
occurring during ascent and eruption of the orange glass magma
(Weitz et al., 1997), as well as during cooling of the beads in flight
and after deposition on the lunar surface.
a Cameca Camebax at Brown was used to determine spot major
element compositions of the glasses and minerals. Operating conditions for the Camebax were I5 KV and 30 nA with a beam diameter
of 2 pm. For the larger minerals, traverses at 4-8 p m spacing were
produced across them to search for core to rim variations. During
analysis of the orange glass compositions along the core, conditions
were 15 KV, 10 nA, and a beam diameter of 10 pm. Two to three
analyses were made for each bead to determine the variability within
each bead, and a basaltic glass standard was used to correct for drift
in the peak intensities.
DESCRIPTION OF THE 74001/2 CORE
Appearance of Thin Sections
The upper 74002 section shows clustering of the beads into
centimeter-size domains, producing a breccia-like appearance (Fig. I).
Multispectral imaging of the 74002 core before it was dissected also
showed clumping of the beads into orange and dark patches (Pieters
et al., 1980). The clumping may have occurred by compaction after
deposition onto the surface and/or during the impact and overturn by
Shorty Crater. Although there may have been small-scale mixing
within the core, the change from orange to black beads from core
top to bottom is profound and appears to be a depositional feature
according to solar exposure ages (Bogard and Hirsch, 1978). The
thin sections become progressively darker moving down the core
due to the increasing percentage of black beads. During extraction
PROCEDURE AND ANALYSIS
We have studied two sets of thin sections taken along the length
of the 74001/2 core. The top of the core (74002) extends from 10 cm
below the lunar surface to 41 cm depth, whereas the 74001 section
continues to the bottom at 78 cm depth. Each set contains 26 thin
sections, with each thin section typically 2.5 cm long and -1 cm wide.
We do not know the separation distance between the two sets of thin
sections; however, because the largest beads were over 500 p m in
diameter and we could not identify these largest beads in the other
set of corresponding thin sections, the separation of the two thin
section sets must be >500 pm.
The thin sections were studied in both reflected and transmitted
light. Spinel crystals and metal blebs were best identified in reflected
light. We also searched for the largest black bead and orange glass
in each thin section. A Cameca SX-100elemental probe at JSC was
used to produce digital elemental maps of several selected beads.
Operating conditions varied depending upon the size of the image
and the time required to produce the elemental maps. Typical beam
diameter resolution was 2 p m and probe time varied from 100-200
mdpixel. Because these digital elemental maps were not quantitative,
FIG. 1. Photomicrograph taken in transmitted light of the top of the 7400112
core. Note the clumping and fractures cutting through the section. The
orange glasses dominate over the black beads. Image is -6 mm in height.
Ascent and eruption of a lunar high-titanium magma
of the core, astronaut Harrison Schmitt suggested that there was a
sharp transition at 25 cm depth marking a change from orange glasses
to black beads (Schmitt and Cernan, 1973). However, we see no sharp
boundary along the core but rather a gradual change from orange to
black color.
Size, Texture, and Distribution of Beads
The largest black bead identified was 1.3 mm in length, and it
was found near the bottom of the 74001 core. The largest orange
glass was at 20 cm depth and it had an apparent diameter of 443 pm.
The average size of the black beads at the bottom of the core is
larger than those at the top. In addition, the large black beads at the
bottom of the core are irregular in shape and have jagged surfaces.
These beads are heavily crystallized with olivines and ilmenite, and
it is likely that breakage occurred either after deposition onto the
surface or by the Shorty Crater event (Cirlin et al., 1978). Smaller
beads that are unfractured are remarkably spherical in shape with
only a few showing slightly elongate dimensions. As the bead size
increases, particularly for the black beads, there is a tendency
towards more elongate shapes.
Many of the lunar black beads have smaller orange, brown, or
black beads attached to their surfaces, which may explain their
irregular shapes. Similar compound textures are also seen in glass
beads from fire fountain eruptions in Hawaii (Heiken, 1972). In
these compound beads, crystal growth initiated at points where the
bead surfaces joined (Heiken and McKay, 1977). Smaller orange
glasses attached to larger black beads typically display partial crystallization at this contact surface. A few lunar beads have smaller
beads completely enclosed inside of them (Fig. 2). This feature is
only possible if the larger bead was still molten during the collision,
allowing the smaller bead to be forced into the larger one before it
solidified.
Near the bottom of core, most of the orange glasses have developed brown rims or are completely altered to brown beads (Fig. 3).
The brown texture is in sharp contrast to the linear olivine and
ilmenite textures found in the black beads. Occasionally, the finegrained brown texture occurs at the margins of acicular olivines in a
partly crystallized orange glass. In fragmented beads, the brown
texture was found only on the original outer margin but not on the
FIG. 2. Reflected light photograph showing a small black bead with
dendritic olivines trapped inside a larger brown bead containing fine linear
olivines. The larger brown bead is 180pm long.
529
broken surfaces (Fig. 4), which suggests the texture results from
devitrification. Although acicular crystals of olivine are sometimes
visible in the brown texture, the crystals are generally too small to
see under highest (50x) magnification. Residual orange glass in
brown beads generally has very smalk (<2 pm) devitrification spots,
as shown in Fig. 4.
Volatiles in the Beads
Large gas bubbles (vesicles) are much more common in orange
and brown glasses at the bottom of the core compared to the top.
Heiken and McKay (1978) also noted this variation and determined
that the number of vesicles in the glasses is not continuous throughout
the core. Unfortunately, none of the vesicles contains identifiable
precipitates that could help identify the gas phase associated with
the magma. However, volatile-rich coatings are commonly found on
the surfaces of the beads. Using the SX-100 probe at Johnson Space
Center, we have produced elemental maps showing volatiles, including
FIG.3. Backscattered electron (a) and transmitted light photograph (b) of an
orange glass that has partially devitrified to brown around the edges. Notice
the irregular, orb-like crystallization fronts in the brown regions, a feature
also seen in devitrified terrestrial obsidian (Lofgren, 1971). Some linear
olivines are also visible in the brown regions. Scale bar is 50pm.
530
Weitz et al.
FIG.5. Elemental image showing Na concentration in a black bead (top) and
orange glass partially converted to brown (bottom). Sodium is homogeneously distributed in the black bead residual matrix but it is somewhat
concentrated towards the center in the residual orange glass next to the
brown (below). Therefore, the Na that was originally homogeneously
distributed in the orange glass diffused inward as the brown texture
developed at the bead edge and propagated inward. Scale bar is 50 pm.
FIG. 4. Transmitted light photographs of fragmented orange glass beads.
The brown texture is only located on the original outer edges, not on the
fragmented surfaces, suggesting that devitrification occurred in the volcanic
plume while breakage took place either when the beads landed on the lunar
surface or during the impact by Shorty. Both beads are -65 pm across.
Na, K, and S, on the surfaces and interiors of some beads. Figure 5
shows elemental maps of Na for both black and partial brown beads
from a 90-150 p m grain mount taken near the bottom of the core.
The image of the orange bead with a brown rim texture (bottom)
shows that Na diffused inward as the brown texture developed. In
contrast, the black bead in the same mount has a homogeneous
distribution of Na, except where olivine crystals have developed.
Other evidence for volatiles on the beads is shown in Fig. 6. The
black bead fragment has a higher concentration of Na, K, and S on
its unbroken outer rim and it appears that the volatiles are diffusing
inward around the olivine crystals. Quantitative electron microprobe
analysis indicates 2.3 wt%o Na and 0.6 wt%o K on the surface, with
the S-rich layer too thin to measure.
Composition of Beads
We analyzed the compositions of orange glasses along the entire
length of the core. Delano (1996) and Hansom and Lindstrom
(1997) recently noted slight compositional variations in the 74220
orange glasses. We attempted to search for similar variations along
the length of the core to determine if the source had changed during
this eruption. Ten to twenty glasses were analyzed from four
different thin sections along the core: 74002,6024 thin section was
taken at the top of the core; 74002,6030 is at 16 cm depth;
74001,6028 is at 38 cm depth; and 74001,6036 is at 58 cm depth.
Plots of major element compositions are shown in Fig. 7, and the
average glass composition at each depth is listed in Table 1. Most of
the glasses cluster around 14.4 wt% MgO, 39.0 wt% SiO2, 5.8 wt%o
A1203, 9.2 wt%o TiO2, 7.4 wt%~CaO, and 22.7 wt% FeO. Delano
(1986) determined a similar average composition of 14.9 wt% MgO,
38.5 wt% Si02, 5.8 wt% AI203, 9.1 wt% TiO2, 7.4 wt% CaO, and
22.9 wt% FeO for the 74220 orange glasses. The 0.5 wt%o difference
in Si02 between our analyses and those of Delano is likely due to
probe standard differences.
The point shown in Fig. 7 with the lowest MgO wt% (star in
Fig. 7) corresponds to orange glass in a partially crystallized bead. It
is shown to demonstrate the effect of olivine and ilmenite formation
on the composition of residual orange glass. Three glasses from the
top of the core (74220,6024) have slightly lower MgO contents
between 13.3 and 13.7 wtY0 and much higher Al contents (Fig. 7).
These compositions cannot be explained by olivine fractionation
from the orange glass magma as indicated by the olivine extraction
vectors. The compositions of these glasses are similar to the Orange I
Ascent and eruption of a lunar high-titanium magma
53 1
Orange glass melt was variably trapped during
growth of the homogeneous olivine phenocrysts. Most
melt in these inclusions experienced some crystallization
during cooling; however, a few glassy melt inclusions
were found in one FoBl olivine phenocryst (Fig. 8). A
few additional inclusions were found with glassy cores.
Analyses of these melt inclusions by electon microprobe
(EMP) indicates that they are identical to the large orange
glass beads in major element composition (Fig. 7). The S
content of these inclusions averages 600 2 100 ppm by
electron microprobe analysis; the Cl content is 50 ppm,
essentially at the background detection limit of the EMP
analysis.
K
Mineral Textures and Compositions
Olivines-Heiken and McKay (1978) identified four
types of olivine textures and compositions in the 74001/2
black beads: dendritic, tabular, acicular, and subequant.
Dendritic olivines with the lowest Mg#'s of <0.70, where
50 urn
Mg# is defined as MgO/(MgO + FeO) in moles, formed
at the highest cooling rates whereas subequant olivines
with Mg# 0.79 formed at the slowest (Heiken and
McKay, 1978). All these olivines are small compared to
large (100-200 pm) olivine phenocrysts dispersed
throughout the 74001/2 core. The phenocrysts with
Mg#'s of 0.81 are homogeneous and euhedral (Fig. 9).
Later growth during crystallization of black beads produced
a thin outer rim on some phenocrysts. Microprobe
transects across these crystals show a decrease in Mg#
content at the edges of grains, confirming late, closedFIG. 6. Elemental images of Na, K, and S for a black bead fragment located at the
system
growth of olivine phenocrysts. Because they are
bottom of the core. Notice the high concentration of the volatiles on the original outer
surface (right side of bead). It appears the volatiles diffised inward and around the unzoned and are large relative to the size of the beads and
olivine crystals in the bead. The volatiles are associated with the gas cloud in the
other olivines, the olivine phenocrysts are considered to
volcanic plume and indicate that some beads were in the plume long enough to
have
formed during ascent in the conduit or prior to the
concentrate the volatiles on their surfaces.
final ascent rather than after eruption (Weitz et al., 1997).
The dendritic olivines identified by Heiken and McKay
TABLE1. Average orange glass compositions
(1978) are not as common as the acicular olivine crystals
visible in the partially crystallized glasses. In fact,
74002,6024
74002,6030
74001,6028
74001,6036
scanning electron microscope (SEM) photos of this
39.09 f 0.28
39.10 f 0.21
39.01 ? 0.21
38.91 f 0.1 1
SiO,
dendritic olivine are more characteristic of the fibrous
14.37f 0.12
14.40 f 0.12
14.40f0.13
14.56 f 0.19
MgO
texture
seen in brown beads. Olivine crystallized from
22.62 f 0.23
22.68 f 0.24
22.73 f 0.19
22.86 f 0.19
FeO
melt in black beads range from acicular with the lowest
9.29 f 0.18
9. I 1 f 0.08
9.23 f 0.09
9.33 f 0.15
Ti02
Mg#'s to tabular and subequant.
7.44 f 0.12
7.30f0.10
7.35 f 0.10
7.39 f 0.12
CaO
Unlike the majority of large phenocrysts, which
5.74 f 0.05
5.80 f 0.07
5.82 f 0.06
5.80 f 0.05
A12°3
0.39 f 0.06
0.33 f0.03
0.35 f0.05
0.33 f0.02
Na20
typically have a euhedral shape and sharp edges, a few
large olivines have rounded surfaces and a few have large
98.91
98.93
98.92
98.97
Total
variably crystallized melt inclusions (Fig. 8). Traverses
across these olivines show no variation in Fe, Mg, and Ca
variety identified by Delano and Lindsley (1983) in an Apollo 17 composition and the Mg# is a constant 0.81 from center to edge.
This may be a simple variation of the normal olivine growth habit;
soil breccia, indicating that they are from a separate source and
alternatively, these olivines may have formed in a slightly lower
eruption. Because only the top thin section contains these three
unique glasses and it was affected by micrometeorite reworking, it is temperature region of the magma, perhaps at the edge of the conduit,
and then were pushed to regions of higher temperatures, causing
possible that they were brought to this site by impact processes. The
other three glasses that have MgO contents 4 4 wtY0 are those that
some olivine resorbtion.
have a large proportion of the brown texture in them. Their
We have found no strong correlation between olivine texture
compositions can be explained by olivine fractionation, probably
and bead size. Although the majority of the largest black beads have
occurring during development of the brown texture. In summary, euhedral olivine, others have only acicular olivine, indicating that
the orange glasses have the same compositions throughout the core,
the size of the bead could not be the only factor controlling the
except perhaps for a few anomalous glasses at the top that were
cooling rate. In addition, there are many large (>400 pm) orange
delivered to the site as a result of impact processes.
glasses near the top of the core that must have cooled rapidly despite
Na
-
S
532
Weitz et al.
12
11
13
:*
6.4
-l
14
[
-
15
.) I
16
I1
rn 74001,6028
0 74001.6036
la0 n
E
62-
U
6.0
9.5
-
9.0
-
-
58-
I
85
:*
I
0
t
L
I
0
3 a 5 1 . . . . , . . . . ~ . . . . ~ . . . ' ~ " "
11
12
13
14
15
16
FIG. 7.
Plots of orange glass compositions along the core. 74002,6024 is at
the top of the core, 74002,6030 is at 16 cm depth, 74001,6028 is at 38 cm
t
22.04
11
.
.
.
,
, . . . . , . . . .
12
13
I
14
,
.
,
,
I
15
. .
,
,
F
16
ugo (wt%)
their large size. Many of the larger black beads also show several
types of olivine textures, which suggests that they experienced a
complex cooling history in the fire fountain.
Metal Blebs-The Fe-Ni-Co blebs found in olivine phenocrysts
and volcanic beads showed that the oxidation state of the magma
changed as it ascended and then erupted (Weitz et al., 1997). We
interpret compositionally homogeneous Type I blebs (Fig. 10) to
have formed by oxidation of graphite during magma ascent. If this
is the case, the composition of the metal and glass fix t h e f 0 2 at
IW-1.3 and indicate they formed at a pressure of -200 bars (4 km
depth). They were subsequently entrapped in large homogeneous
olivine phenocrysts in the conduit and retained a homogeneous composition of 85 wtY0 Fe, 14 wt% Ni, and 1 wt% Co. Type I1 (Fig. 10)
blebs were originally Type I metal blebs that remained in contact
with the melt during ascent and experienced an oxidation that
caused Fe loss to the melt and a resulting increase in Ni content at
depth, and 74001,6036 is at 58 cm depth. Most of the glasses cluster around
14.5 wt% MgO, 39.0 wt% SO2, 5.8 wt% AI,O,, 9.3 wt% TiO,, 7.4 wt%
CaO, and 22.7 wt% FeO (see also Table 1). The point with the lowest MgO
wt% (star) corresponds to orange glass in a partially crystallized bead. The
arrows represent ilmenite and olivine extraction vectors and they demonstrate
that olivine crystallization has affected the composition of the residual orange
glass in partially crystallized beads. The olivine vector shown in the FeOMgO plot is curved to reflect the varying Fo content of the extracted olivine,
beginning at Fo,, composition. Three glasses (circled) from the top of the
core (74220,6024) have slightly lower MgO contents between 13.3 and 13.7
wt% and much higher Al contents that cannot be explained by olivine
fractionation and must indicate distinct sources. Crosses represent one
standard deviation errors.
the bleb rims. A third type of metal grain (Type 111) is found only in
black beads (Fig. 10). The Type 111 grains have an amoeba shape
and are composed of >99 wt% Fe. They are interpreted to have
formed in the volcanic plume by reduction processes occurring
during crystallization of the beads (Weitz et a[., 1997). Finally, a
few isolated metal grains (Type IV) did not fall into any of the
previous three types (Fig. 10) and could be modified Type I, 11, or
111 blebs. The composition of metal blebs from the second set of
74001/2 thin sections was also studied and found to contain more
Type IV metal fragments (16 vs. 2 for the second and first sets,
respectively) but with compositions similar to those identified in the
first set.
Spinels-Spinels that formed in the black beads are generally
small ( 4 0 pm) and have a triangular or rectangular shape. Figure
11 is a scanning electron micrograph from a bead that was ionetched (Heiken and McKay, 1977). Two spinels -10 p m in size are
Ascent and eruption of a lunar high-titanium magma
FIG. 8. Transmitted light photograph of an olivine phenocryst with four
glassy inclusions (arrows). The olivine phenocryst has unusual smooth and
rounded margins and may have experienced partial resorbtion. Scale bar is
100pm across.
visible in the center of the photograph, one with a triangle shape, the
other an irregular shape. We have measured the composition of
several spinels >10 p m in size. In addition to the ulvospinels that
formed in the black beads as they cooled, we also found three larger
spinel grains ( 1 6 2 8 pm) trapped inside olivine phenocrysts (Fig. 12).
These grains must have grown prior to or during olivine phenocryst
growth. Two of the spinel grains were partially surrounded by
crystallized glass with small associated vesicles (Fig. 12), whereas
the other spinel was surrounded only by olivine.
Figure 13 shows the compositions of the spinels in comparison
to spinels found in the black beads and in high-Ti mare basalts
analyzed by Usselman and Lofgren (1976). The compositions of the
three large spinels are higher in AI2O3 and MgO and lower in FeO
than those that grew in black beads. In addition, the Ti02 contents
are lower and the Cr203 contents are higher for two of the large
spinels, whereas the third large spinel has the same concentration of
TiOz and Cr203 as the spinels in the beads and basalts. The plots
show a generally linear trend from Cr-rich spinel trapped in olivine
to chromian ulvospinel found in the crystallized beads. The differences between spinels 2 and 3 relative to spinel I (Fig. 13) may
reflect some reequilibration with the adjacent trapped melt during
cooling. The Cr-ulvospinels in the black beads are compositionally
similar to ulvospinels found in hi-Ti02 mare basalts (Haggerty,
1978). However, with the possible exception of titanian chromites
found in 74275, those found trapped in the 74001/2 olivine
phenocrysts are much more Mg-, Al-, and Cr-rich and lower in Fe
and Ti compared to anything found in mare basalts.
Ilmenite-Ilmenite crystals occur in all the black beads but they
were too small and thin to be analyzed. The texture of the ilmenite
crystals is best seen in ion-etched SEM images. Fine (1 pm) ilmenite
crystals are commonly seen covering acicular olivine crystals,
indicating a nucleation control. As the olivine crystals change from
acicular to subequant, the ilmenite crystals increase in size and no
longer form along olivine grains but are interspersed within the glass
matrix.
533
FIG.9. Elemental image showing the Fe content in olivine phenocrysts
trapped inside a black bead. The centers of the olivines are homogeneous in
Fe content with a Fo,, composition that developed during olivine growth as
the melt ascended. However, the olivine edges are much brighter than the
centers because of the higher Fe contents in later olivine growth. The olivine
at the right has an inclusion that is partially crystallized.
DISCUSSION
Ascent History
The petrology of the volcanic beads and minerals within the
74001/2 core can be used to interpret the eruption history of the
beads. Using the metal blebs and large spinels, we have been able
to decipher the processes that apparently occurred during magma
ascent in the conduit. Weitz et al. (1997) showed that graphite in the
Apollo 17 orange glass magma (1320 "C) is unstable at pressures
<200 bars (4 km depth) and oxidizes to a CO-rich gas. Metal blebs
consisting of Fe, Ni, and Co are produced during this reaction. This
reaction and the Fe85Ni14 composition of the metal (Type 1) indicate
anfO2 of-1 1.3 (IW-1.3) at 1320 OC, the liquidus temperature of the
orange glass magma (Green et af., 1975). An oxidation of the
magma later in the ascent is required to explain the loss of Fe from
the rims of the metal blebs that remained in contact with the magma
(Weitz et af., 1997). This zonation could not have occurred by Ni
addition because the surrounding glass shows no depletion in Ni.
The effects offO2, temperature, and melt composition on the Cr
content of coexisting olivine 4 spinel and melt have also been the
subject of several experimental studies (Usselman and Lofgren,
1976; Akella et af., 1976; Huebner et af., 1976). Almost all
experimental studies show a pronounced increase in the distribution
coefficient of Cr between olivine and coexisting melt with
decreasingfO2. This change is generally attributed to an increase in
the Cr2+ in the melt with decreasing YO2, which occurs in the
vicinity of the IW 0-buffer. Among these experimental studies, the
work of Usselman and Lofgren ( I 976) is most applicable to orange
glass magma petrogenesis because they used a high-Ti mare basalt
composition (74275).
Three equant (16-28 pm) Cr-rich spinels were identified trapped
in olivine phenocrysts during their growth at depth (Fig. 12). The
534
Weitz et al.
I) Fe-Ni Blebs trapped in
Olivines (144 pm).
Core cornp:FeggNil&ol
II) Fe-Ni Blebs trapped in
Volcanic Beads.
Ni-rich rims.
111) Fe Blebs in Black Beads
<8 pm in size.
Amoeba shape.
The cluster in olivine Cr concentration at 0.36 wt%
Cr2O3 when compared to the Usselman and Lofgren
(1976) experimental data indicates an j 0 2 equal to or
slightly lower than IW-1.0 (Rutherford and Weitz, 1997).
This compares well with the estimate of IW-1.3 for the
same orange glass magma at depth, based on Fe and Ni
distribution between the melt and the Type 1 metal grains
trapped in olivine phenocrysts (Weitz et al., 1997). Type
I1 metal grains are located in the outer rims of the
euhedral olivines or in orange glasses. The metal-melt
data for these blebs indicate that the orange glass magma
underwent oxidation during and after the final stage of
olivine phenocryst growth, causing loss of Ni at the bleb
rims. One possible explanation for the trend to lower
Cr2O3 content in olivine phenocrysts is that these lowerCr grains were crystallized at successively later stages of
this oxidation. The decrease in CaO may mean the
substitution of Cr in olivine is easiest when coupled with
addition of Ca to the olivine, and the two are driven by
the oxidation state changes in the magma (Rutherford and
Weitz, 1997). Nickel contents of the olivines should
demonstrate this oxidation, but the data are not definitive.
In contrast to Rutherford and Weitz (1 997), we attribute
the trend of decreasing Cr2O3 with decreases in Fo in the
black bead olivines (Fig. 14) to a reduction relative to
earlier olivine phenocryst growth conditions. The Type
111 metal compositions indicate that a reduction was
taking place during crystallization of the black beads
(Weitz et al., 1997), possibly as a result of S degassing.
Gas Phase Origin and Development
Weitz et al. (1997) present data on metal grains
trapped in olivine phenocrysts and interpret the data as
indicating the metal formed by C reduction during
IV) Metal fragments that
formation of a CO-rich gas phase in the orange glass
magma. The compositions of the trapped FeNi metal
don't fit into other
grains and the coexisting melt in 74002/1 yield an
three types.
estimate of the 0 fugacity for the time that they were
Unusual shapes and
formed. When we compare this T-P2 estimate with P-Tcompositions.
~ 0 data
2
for the graphite-gas equilibrium (Fogel and
Rutherford, 1995), it indicates the metal and gas formed
at a pressure of 200 bars or a depth of -4 km below the
FIG. 10. Types of metal blebs identified within the 74001/2 core. Type I blebs have a
lunar surface in the orange glass magma. Metal formed
uniform composition and are interpreted to have formed during ascent in the conduit by
graphite oxidation and a corresponding reduction of the melt. Type 11 blebs were during this graphite oxidation process and remaining in
originally Type I blebs but they remained in contact with the melt during ascent and
contact with the melt (ie., not trapped in olivines)
experienced oxidation reactions that produced Ni-rich, Fe-poor rims. Type 111 blebs are became depleted in Fe and enriched in Ni as Feo became
much smaller and only composed of Fe, which suggests formation within the black beads
oxidized during magma ascent. The best explanation for
during crystallization in the volcanic plume. Type IV blebs were metal fragments that
this oxidation is a loss of various cations (e.g.,Na, Pb) to
did not fit into any of the previous three types.
the gas phase.
The new data for S and CI in melt inclusions in olivine phenocomposition of the olivine coexisting with Cr-spinel and melt procrysts help to refine and confirm estimates of the gas phase composivides a separate estimate of the orange glass magmafO2, which can
tion. The melt inclusions contain an average of 600 2 100 ppm S
be compared to that calculated by Weitz et al. (1997) from the Fe-Ni
distribution between melt and metal. The olivine phenocrysts in the
compared to an average of 200 ppm (Delano et al., 1994) in the
74001/2 core cluster at 0.36 2 0.04 wt% Cr2O3 (2460 2 200 ppm
orange glass quenched at the surface. Thus, an average of 400 ppm
olivines, but there is a small populaCr) for the Mg-rich
S was lost from the orange glass magma in the period of the eruption
tion of phenocrysts with significantly lower Cr203 (Fig. 14). The
following entrapment of glass inclusions. The fact that there was
decrease in Cr203 in the olivine phenocrysts is accompanied by a
only 600 ppm S in the orange glass magma is consistent with the
corresponding decrease in CaO from 0.3 to 0.15 wt%. The smaller
absence of S in the FeNi metal generated at the time of CO gas
olivine crystals that occur in the black beads define a trend of
formation. It is also worth noting that the transfer of 400 ppm S
decreasing Cr2O3 with decreasing forsterite content in the olivine.
from the melt to a gas phase is almost enough gas to drive a fire
Ascent and eruption of a lunar high-titanium magma
535
FIG.1 I . Ionetched SEM image of a black bead. Two spinels are visible at
the center, one shaped like a triangle and the other like a pentagon. The
bright feathery crystals are ilmenite and the dark large crystals are olivines.
Scale bar is 10pm.
fountain eruption on the Moon according to theoretical models
(Wilson and Head, 1981).
The calculations of Sat0 (1976) for S, C, Fe, 0, and Na gas
species at the f O 2 of the lunar interior indicate that the hgacities
below
atm) except for CO. The assumpare all very low (ix,
tions made by Sat0 were conservative (i.e., he assumed a higher
activity of FeS than that which we now know to be the case for the
orange glass, and his estimate of Fe activity is lower than that
required by the FeNi metal present in this magma; Weitz et al.,
1997). Both of these conservative assumptions lower the fugacities
of S gas species in equilibrium with the orange glass assemblage
relative to those calculated by Sato. Thus, the initial generation of a
gas phase in the orange glass magma could only have been by a CO
gas. Other species such as S and CI could then partition between the
gas and the melt. An interesting question is raised by the lack of CI
in glass that was trapped as melt inclusions in olivine. In contrast,
C1 is reported to be very abundant among the volatiles deposited as
surface coatings on orange glass beads (Meyer et al., 1975; Butler
and Meyer, 1976), a deposit generally considered to have formed
from the volcanic gas. This may mean that a C1-bearing gas phase
was present as the oxidation of C took place to generate the CO gas
component.
Eruption History
We now discuss the processes that appear to have occurred
during eruption of the beads in the volcanic plume. The presence of
the Type I11 metal blebs identified by Weitz et al. (1997) in the
crystallized beads indicates a reduction occurred in the volcanic
plume. The reduction produced the small Fe-rich blebs and a
decrease in Cr203 content for spinels crystallizing in the beads.
Additional loss of volatiles, such as S, Na, and K, may also have
FIG. 12. Reflected light photographs of
three large spinels (arrows)trapped
inside olivine phenocrysts. The top two spinels are partially surrounded by
crystallized glass. Scale bars are 50pm.
occurred during crystallization of the beads. The concentration of
these volatiles on the outer surfaces of the beads-but
not on
fracture surfaces-suggests that condensation of the volatiles also
was occurring in the plume. Experimental results indicate that the
black beads cooled at a rate 4 0 0 "Us, whereas both black beads
and orange glasses cooled at rates slower than those they would
have experienced under free-flight conditions (Amdt and von
Engelhardt, 1987), supporting a gas-rich plume model.
The textures within the beads suggest that location within the
plume was the dominant control on bead cooling rate, although bead
size also played a role, albeit secondary. In the following sections,
we group the beads into five representative types that formed in the
536
Weitz et al.
0.45
40
+
35
A
spinel I
spinel 2
spinel 3
om
30
* +
+
25
+
+
+
20
0.4
tI
I
1
I
1
I
I
I
I
rn
t
0.25
0.2t
0.15
74
12
I
I
I
A17 ORANGE GLASS
OLIVINES
/w*o.t
rn
rn
I
I
76
78
80
I
I
82
8 4
Fo in Olivine
14. Plot showing Cr content for the large olivine phenocrysts and
smaller olivine crystals in the black beads. The olivine phenocrysts with
Fo,,-,, composition cluster at 0.36 -C 0.04 wt% Cr20, (2460 f 200 ppm
Cr), although there is a small population of phenocrysts with significantly
lower Cr201. The decrease in Cr201 in the olivine phenocrysts is accompanied by a corresponding decrease in CaO from 0.3 to 0.15 wt%. The
smaller olivine crystals that occur in the black beads define a trend of
decreasing Cr2O1with decreasing forsterite in the olivine.
FIG.
10
8
6
4
2
12
10
a
4
22
20
+
+
%
:"
6
+
t
I
+
+
18
0
om
sr
16
+ *
14
9
12
10
fl
8
20
I
I
I
I
I
25
30
35
40
45
50
FeO (wt%)
FIG. 13. Plots of spinel compositions. The three large spinels trapped inside
olivine phenocrysts are listed as spinel 1, 2, and 3. Spinels that formed
during crystallization within the black beads are shown as solid circles.
Spinel compositions taken by Usselman and Lofgren (1976) from high-Ti
mare basalts are shown by crosses. The composition of the three large
spinels differs from those that grew in black beads; they have higher A1201
and MgO contents and a lower FeO content. The plots show a linear trend
from Cr-rich spinels trapped in olivines to chromian ulvospinel found in the
crystallized beads and mare basalts.
plume (Fig. 15a). In reality, the beads show a continuous spectrum
in crystallization history but we have chosen five types within this
spectrum to illustrate the bead textures as a function of cooling rates.
The high number of compound beads also indicates that the beads
experienced numerous collisions within the volcanic plume and,
therefore, may have experienced a complex cooling history as they
were moved from one location within the plume to another. An
illustration and location of each bead type in the volcanic plume is
shown in Fig. 15b.
Type 1: Orange Glasses-The orange glasses dominate in the
upper portion of the 74002 core. The glasses formed at the fastest
cooling rates, >I00 " U s (Amdt and von Engelhardt, 1987). The
high cooling rates may reflect their small sizes or indicate that they
cooled in the outer fringes of the volcanic plume where the optical
density of the plume is lowest and cooling rates are highest (Fig. 15b).
Because the optical density of a volcanic plume is defined as the
number of clasts in a specific region of the plume, the optical density
decreases away from the vent as the number of clasts declines (Head
and Wilson, 1989). Where the optical density is high, there are
large number of clasts in any given volume and the clasts radiate
their heat into this volume, reducing the cooling the beads experience in this region compared to locations of lower optical density.
Smaller orange glasses can also be found attached to larger beads,
indicating that the glasses were molten enough to adhere during the
collision but still small enough to cool quickly to form glass.
Type 2: Partially Crystallized Glasses-Partially crystallized
glasses are residual orange glass with only minor amounts of olivine
and ilmenite crystals visible. These beads dominate throughout the
74002 core and they must have experienced slower cooling rates than
the glasses and/or a collision with another bead caused a nucleation
site. In the case of compound beads, the texture at the collision site
has either linear olivines or a brown texture (submicroscopic crystallization). Many of the orange beads that are partially crystallized
have a nucleation site on one side of the bead and acicular olivines
emanate only from this location. Other beads show several nucleation
sites and criss-crossing acicular olivines surrounded by a thin layer
of brown texture.
Ascent and eruption of a lunar high-titanium magma
Type 3: Strongly Crystallized Beads-Acicular olivines with
compositions of Fob6 to
are interspersed in the black matrix.
Very little residual orange glass is visible (<lo% volume). Ilmenite
crystals with a feathery texture cover olivine grains. Our interpretation
is that these beads formed closer to the vent where the optical
density of the plume was high enough to inhibit cooling and allow
crystallization throughout the beads (Fig. 15b). However, it is also
possible that some of the beads cooled more slowly because of their
larger sizes, rather than their location within the plume.
Type 4: Completely Crystallized Beads-These beads contain
euhedral olivines interspersed in a black matrix (Fig. I5a). The bead
can be dominated by euhedral olivine or they can represent a mixture
of olivine types indicating a complex cooling history. Ilmenite crystals
can be long and dendritic but they do not always cover associated
olivine, particularly euhedral olivine. Spinel crystals and amoebashaped blebs of Fe are common. These beads are relatively large
and usually occur as fragments, although small unfractured beads of
this type occur and may represent the edge of originally larger beads
produced by the cutting of thin sections. The completely crystallized
beads represent the slowest cooling rates because the beads are large
enough and close enough to the vent to allow complete crystallization
and produce large crystals.
Type 5: Brown Beads-These beads have either completely or
partially altered from orange glass to a brown texture (a submicroscopic maze of fine crystals in which a few linear olivines can be
identified). They are only found in the lower part of the core where
the black beads dominate. All the orange glasses and partially
crystallized beads at the bottom of the core show some evidence of
the brown texture. The brown region always develops first at the
outer margin of an orange glass bead and then migrates inward. We
suggest that the brown texture represents devitrification of the orange
glasses by subsolidus heating. Additional support for the formation
of the brown texture by devitrification comes from the compound
beads. Smaller orange glasses adhered to the surfaces of larger
black beads have a brown texture at the contact zone, indicating that
the remaining heat within the black beads was enough to induce
devitrification at the contact. The texture in the brown beads varies
considerably, which suggests a range of cooling rates was involved.
The brown texture appears similar to the radiating cores produced by
Arndt et al. (1984) when they devitrified the Apollo 15 green
glasses. Devitrification experiments of rhyolites by Lofgren (1971)
also produced similar textures, with spherulites enclosed in orbs or
fibrous texture along a propagating devitrification front.
The occurrence of brown beads only at the bottom of the core in
association with the black beads suggests that the black beads may
have been hot enough after deposition to reheat the orange glasses
and cause devitrification in the surface deposit. A problem with this
interpretation is that broken brown beads only have the devitrification
texture on their original surface but not on broken edges (Fig. 4).
Hence, if the beads broke when they landed on the surface rather
than in-flight, then the brown texture must have developed in the
volcanic plume. Alternatively, if the beads broke during the Shorty
impact event (Cirlin et al., 1978), then the brown texture may have
developed after deposition on the lunar surface.
IMPLICATIONS FOR T H E ERUPTION AND
EMPLACEMENT OF LUNAR VOLCANIC BEADS
Hanson and Lindstrom (1997) showed that small compositional
differences exist between the crystallized beads and the orange glasses
537
in the 74220 sample. They suggested that lower Cr and higher Sm
in the crystallized beads may reflect a different time of eruption or
eruption from a different part of the fissure compared to the orange
glasses. Delano (1996) also analyzed orange glasses in the 74220
sample and claimed to be able to distinguish between high- and lowSi orange glasses, with a difference of only 0.4 wt%. He also interpreted the variations as resulting from eruptions at different parts of
the fissure or a range of melt compositions in the rising diapir. Our
results taken along the core show a few orange glasses near the top
with distinct compositions, perhaps a result of another eruption, that
were then deposited at the core location by impact reworking. The
vast majority of glasses along the core are compositionally the same
(Table 1) and support earlier observations that the deposit represents
a continuous deposit from one eruption. Because the compositional
variations identified by Hanson and Lindstrom (1997) and Delano
(1996) are from the 74220 sample, which is located at the lunar
surface, it is not surprising that some differences might exist due to
micrometeorite impact reworking and mixing with other soils. The
glasses that we analyzed show no evidence for reworking, except in
the top thin section which does have some unusual glass compositions.
In summary, the bead compositions are homogeneous throughout the
core, and we thus interpret the change from dominantly orange glasses
at the top of the core to black beads at the bottom as representing a
change in eruption activity rather than separate eruptions.
A change in the optical density of the plume is the most likely
factor that caused the change from orange to black beads over time
at the Taurus-Littrow dark mantle deposit. A change from high to
low optical density is supported by a study of metal bleb formation
in the 74001/2 core (Weitz et al., 1997). There are two possible
variations that may have caused this change in the bead crystallinity
over time. First, an increase in the volume flux would cause a
decrease in the cooling rate of the beads as more beads are confined
to the same area and can radiate heat to the gas plume, thereby
inhibiting cooling and favoring formation of the black beads.
Secondly, a decrease in the gas content over time would also concentrate more beads in a less dispersed plume and favor black bead
formation. The latter scenario is more likely because in terrestrial
eruptions, volume flux tends to decrease over time. Other support
comes from the greater number of vesicles in the glasses at the
bottom of the core compared to those at the top. The higher
proportion of vesicles at the bottom (keeping in mind that the core
represents the inverted stratigraphy at depth) implies that over time,
the melt was unable to degas as efficiently and more gas was trapped
in the melt rather than released at the vent to drive the eruption,
thereby increasing the plume optical density. Unfortunately, we
don't know if the 74001/2 core represents the beginning, middle, or
end of the eruption, and so we cannot be certain that the change in
bead crystallinity represents a minor fluctuation in plume optical
density during the course of the eruption or a significant change.
The Apollo 17 landing site is located towards the southeastern
edge of the Taurus-Littrow dark mantle deposit. Because the TaurusLittrow deposit is embayed by younger low-Ti flows of Mare
Serenitatis in the west, it is difficult to determine the original extent
and most likely location of the high-Ti source vent(s) that erupted
the beads in the dark mantle deposit. Clementine remote sensing
observations of the deposit by Weitz et al. (1 998) suggest that the
vent may be located to the northwest where the deposit appears
thickest and darkest. Wilson and Head (1981) have shown that to
eject submillimeter beads to large distances (-1 00 km), the majority
of the clasts must be larger than a few centimeters in size. The
538
Weitz et al.
FIG. 15. (a) Types of volcanic beads identified within the 74001/2 core as a function of cooling rate. The orange glasses form at the fastest cooling rates
whereas slower cooling rates allow larger and more numerous crystals to develop in the beads. The brown beads are interpreted to have formed by
devitrification of the orange glasses in the volcanic plume. (b) Illustration of the location of the different bead types defined in (a) within the volcanic plume.
In the outer portions of the plume where the cooling rates were high due to the low optical density of the plume, the orange glasses formed. Further inward,
the optical density increases and beads are able to cool slower and develop crystals. The deposit is correspondingly dominated by orange glasses at a greater
distance from the vent, whereas black beads are located in the proximal deposit. The change from orange glasses to black beads along the length of the core
must reflect a change in plume optical density with time, beginning with low density and high cooling rates and progressively increasing in density to promote
formation of black beads.
Ascent and eruption of a lunar high-titanium magma
larger clasts can decouple rapidly from the expanding gas cloud and
give added momentum to the submillimeter clasts still locked to the
cloud. Thus, lunar eruptions should resemble Hawaiian-style fire
fountain eruptions and produce a broader size distribution of clasts
emplaced over a larger area. Even though the Apollo 17 landing site
is located on the edge of the deposit and only submillimeter beads
were sampled, larger beads/clasts and a thicker deposit may exist
towards the northwest. These larger clasts will land adjacent to the
vent and, because they are still molten, they can coalesce to form
lava flows and sinuous rilles. Hence, the eruption that emplaced the
74001/2 beads in the deposit may have also produced associated
mare basalts that are now buried in Mare Serenitatis beneath the
low-Ti mare.
CONCLUSIONS
The ascent history can be summarized as: (1) FegS-Nil&ol
~ - ~ were
~
crystallizing simulmetal, Cr-spinel, and F o ~ olivine
taneously in the orange glass magma prior to its eruption; (2) both
the metal-melt equilibria and the Cr content of olivine (+spinel)
indicate m f 0 2 of IW-1.3 for the preemption magma; and (3) later
oxidation of the magma is indicated by Ni-rich metal bleb rims and
by the Cr content in the olivines. The composition of the uncrystallized orange glasses is homogeneous throughout the core and
supports a continuous eruption that emplaced the 74001/2 samples,
which later became inverted during the impact by Shorty crater that
brought the deposit to the surface.
In the volcanic plume, crystallization of the black beads, possibly
under reduced conditions, resulted in Fe-rich metal blebs. The
mineral textures in the beads reflect cooling rates in the volcanic
plume and appear to be mainly a function of location within the
plume rather than bead size. Those beads that were located in the
outer fringes of the plume where the optical density was low experienced rapid quenching to form orange glasses. Beads that formed
closer to the vent experienced slower cooling rates due to the higher
optical density of the plume. The compound nature of many of the
beads and the variety of crystal textures and compositions within
individual beads suggests that the beads experienced a complex
cooling history as they were knocked from one location within the
plume to another. The change from predominantly orange glasses to
black beads over time within the deposit could be explained by a
decrease in exsolved gas content with time, resulting in a smaller
plume and an increase in optical density in the plume. Orange
glasses that were initially ejected to the outer fringes of the plume
could devitrify as they fell back through the plume or by reheating
on the surface and produce the brown texture seen in the glasses at
the bottom of the core.
Acknowledgments-Support through the NASA Graduate Student Researchers
Program at NASA-JSC and the Zonta Foundation is greatly appreciated by
C. Weitz. NASA support for this research was provided by grant NAGS4438 for M. Rutherford and by grant NAG5-4723 for J. Head. We thank J.
Devine at Brown and V. Yang at JSC for help with probe analyses. Special
thanks to James Gardner for helpful discussions and Peter Neivert for
photographic assistance. B. Jolliff and J. Longhi provided excellent reviewer
comments that significantly improved the quality of this paper.
Edirorral handling: U. KrBenbUhl
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