Ascent and eruption of a lunar high-titanium magma as inferred from the petrology of the 74001/2 drill core

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1 Meteoritics B Planetary Science 34, (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 CATHERNE M. WETZT*, MALCOLM J. RUTHERFORD, JAMES W. HEAD 1111 AND DAVD S. MCKAY2 'Department of Geological Sciences, Brown University, Providence, Rhode sland , USA 2NASA Johnson Space Center, Houston, Texas , USA tpresent address: Jet Propulsion Laboratory, Pasadena, California , USA *Correspondence author's address: cweitz@jpl.nasa.gov (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 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 W-1.3 and 1320 "C to form CO gas at 200 bars or 4 km 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 C 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. NTRODUCTON The Apollo /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 and the 68 cm deep 74001/2 core was taken just below 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 row 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, C, 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 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 km depth (Green et al., 1975); although if polybaric melting occurred, this may represent an average depth (Longhi, 1992). n 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. n 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 pm 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

2 528 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 *N, 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 has much higher *Ne and 38Ar contents, indicating a different depositional and irradiational history than the upper portions of the core. f 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 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 core (Lucchitta, 1973; Pieters et al., 1974). n this paper, we have examined thin sections taken along the length of the 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. PROCEDURE AND ANALYSS 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 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 pm 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-100 elemental 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 pm and probe time varied from mdpixel. Because these digital elemental maps were not quantitative, a Cameca Camebax at Brown was used to determine spot major element compositions of the glasses and minerals. Operating conditions for the Camebax were 5 KV and 30 na with a beam diameter of 2 pm. For the larger minerals, traverses at 4-8 pm 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. DESCRPTON OF THE 74001/2 CORE Appearance of Thin Sections The upper section shows clustering of the beads into centimeter-size domains, producing a breccia-like appearance (Fig. ). Multispectral imaging of the 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 FG. 1. Photomicrograph taken in transmitted light of the top of the core. Note the clumping and fractures cutting through the section. The orange glasses dominate over the black beads. mage is -6 mm in height.

3 Ascent and eruption of a lunar high-titanium magma 529 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 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. n 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). n 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. n fragmented beads, the brown texture was found only on the original outer margin but not on the 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 FG. 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. FG. 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.

4 530 Weitz et al. FG. 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. FG. 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 pm 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. n 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 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% A203, 9.1 wt% TiO2, 7.4 wt% CaO, and 22.9 wt% FeO for the 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. t 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

5 Ascent and eruption of a lunar high-titanium magma 53 1 Na S K - 50 urn FG. 6. Elemental images of Na, K, and S for a black bead fragment located at the bottom of the core. Notice the high concentration of the volatiles on the original outer surface (right side of bead). t appears the volatiles diffised inward and around the olivine crystals in the bead. The volatiles are associated with the gas cloud in the volcanic plume and indicate that some beads were in the plume long enough to concentrate the volatiles on their surfaces. TABLE 1. Average orange glass compositions SiO, MgO FeO Ti02 CaO A12 3 Na20 Total 74002, , , , f f f f f f f f f f f f f f f f f f f f f variety identified by Delano and Lindsley (1983) in an Apollo 17 soil breccia, indicating that they are from a separate source and eruption. Because only the top thin section contains these three unique glasses and it was affected by micrometeorite reworking, it is 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 have a large proportion of the brown texture in them. Their compositions can be explained by olivine fractionation, probably occurring during development of the brown texture. n summary, the orange glasses have the same compositions throughout the core, except perhaps for a few anomalous glasses at the top that were delivered to the site as a result of impact processes ? f f f f f f 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 ppm by electron microprobe analysis; the Cl content is 50 ppm, essentially at the background detection limit of the EMP analysis. 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 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 ( 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, closedsystem growth of olivine phenocrysts. Because they are unzoned and are large relative to the size of the beads and other olivines, the olivine phenocrysts are considered to have formed during ascent in the conduit or prior to the final ascent rather than after eruption (Weitz et al., 1997). The dendritic olivines identified by Heiken and McKay (1978) are not as common as the acicular olivine crystals visible in the partially crystallized glasses. n fact, scanning electron microscope (SEM) photos of this dendritic olivine are more characteristic of the fibrous texture seen in brown beads. Olivine crystallized from melt in black beads range from acicular with the lowest Mg#'s to tabular and subequant. Unlike the majority of large phenocrysts, which typically have a euhedral shape and sharp edges, a few large olivines have rounded surfaces and a few have large variably crystallized melt inclusions (Fig. 8). Traverses across these olivines show no variation in Fe, Mg, and Ca 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; alternatively, these olivines may have formed in a slightly lower temperature region of the magma, perhaps at the edge of the conduit, and then were pushed to regions of higher temperatures, causing some olivine resorbtion. We have found no strong correlation between olivine texture and bead size. Although the majority of the largest black beads have euhedral olivine, others have only acicular olivine, indicating that the size of the bead could not be the only factor controlling the cooling rate. n addition, there are many large (>400 pm) orange glasses near the top of the core that must have cooled rapidly despite

6 532 Weitz et al :* 6.4 -l [.) rn 74001, la0 - n E U :* 0 t L a ,.... ~.... ~... ' ~ " " t,,..,, F ,,....,...., 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 blebs (Fig. 10) to have formed by oxidation of graphite during magma ascent. f this is the case, the composition of the metal and glass fix thef02 at W-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 1 (Fig. 10) blebs were originally Type 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 FG. 7. Plots of orange glass compositions along the core ,6024 is 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. Most of the glasses cluster around 14.5 wt% MgO, 39.0 wt% SO2, 5.8 wt% A,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 FeO- MgO 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 V) did not fall into any of the previous three types (Fig. 10) and could be modified Type, 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 V 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 ( 40 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 pm in size are

7 Ascent and eruption of a lunar high-titanium magma 533 FG. 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 pm in size. n addition to the ulvospinels that formed in the black beads as they cooled, we also found three larger spinel grains (1628 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 A2O3 and MgO and lower in FeO than those that grew in black beads. n 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 (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. lmenite-lmenite 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. FG. 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. Ascent History DSCUSSON 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 (W-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 W 0-buffer. Among these experimental studies, the work of Usselman and Lofgren ( 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

8 534 Weitz et al. ) Fe-Ni Blebs trapped in Olivines (144 pm). Core cornp:feggnil&ol ) Fe-Ni Blebs trapped in Volcanic Beads. Ni-rich rims. 111) Fe Blebs in Black Beads <8 pm in size. Amoeba shape. V) Metal fragments that don't fit into other three types. Unusual shapes and compositions. FG. 10. Types of metal blebs identified within the 74001/2 core. Type blebs have a 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 originally Type blebs but they remained in contact with the melt during ascent and experienced oxidation reactions that produced Ni-rich, Fe-poor rims. Type 111 blebs are much smaller and only composed of Fe, which suggests formation within the black beads during crystallization in the volcanic plume. Type V blebs were metal fragments that did not fit into any of the previous three types. composition of the olivine coexisting with Cr-spinel and melt provides a separate estimate of the orange glass magmafo2, which can be compared to that calculated by Weitz et al. (1997) from the Fe-Ni distribution between melt and metal. The olivine phenocrysts in the 74001/2 core cluster at wt% Cr2O3 ( ppm Cr) for the Mg-rich olivines, but there is a small population of phenocrysts with significantly lower Cr203 (Fig. 14). The decrease in Cr203 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 Cr2O3 with decreasing forsterite content in the olivine. The cluster in olivine Cr concentration at 0.36 wt% Cr2O3 when compared to the Usselman and Lofgren (1976) experimental data indicates an j02 equal to or slightly lower than W-1.0 (Rutherford and Weitz, 1997). This compares well with the estimate of W-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 1 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 lower- Cr 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. n contrast to Rutherford and Weitz (1997), 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 formation of a CO-rich gas phase in the orange glass magma. The compositions of the trapped FeNi metal grains and the coexisting melt in 74002/1 yield an estimate of the 0 fugacity for the time that they were formed. When we compare this T-P2 estimate with P-T- ~ 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 lunar surface in the orange glass magma. Metal formed during this graphite oxidation process and remaining in contact with the melt (ie., not trapped in olivines) became depleted in Fe and enriched in Ni as Feo became oxidized during magma ascent. The best explanation for this oxidation is a loss of various cations (e.g., Na, Pb) to the gas phase. The new data for S and C in melt inclusions in olivine phenocrysts help to refine and confirm estimates of the gas phase composition. The melt inclusions contain an average of ppm S compared to an average of 200 ppm (Delano et al., 1994) in the orange glass quenched at the surface. Thus, an average of 400 ppm S was lost from the orange glass magma in the period of the eruption following entrapment of glass inclusions. The fact that there was only 600 ppm S in the orange glass magma is consistent with the absence of S in the FeNi metal generated at the time of CO gas formation. t is also worth noting that the transfer of 400 ppm S from the melt to a gas phase is almost enough gas to drive a fire

9 Ascent and eruption of a lunar high-titanium magma 535 FG. 1. onetched 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 fo2 of the lunar interior indicate that the hgacities are all very low (ix, below atm) except for CO. The assumptions 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 C could then partition between the gas and the melt. An interesting question is raised by the lack of C in glass that was trapped as melt inclusions in olivine. n 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 11 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 FG. 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 400 "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. n the following sections, we group the beads into five representative types that formed in the

10 536 Weitz et al spinel + spinel 2 A spinel A17 ORANGE GLASS 0.41 OLVNES 1 t rn a om * :%" * sr om 10 fl FeO (wt%) FG. 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. + t t t /w*o.t rn rn Fo in Olivine FG. 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 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 Cr2O1 with decreasing forsterite in the olivine. plume (Fig. 15a). n 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 core. The glasses formed at the fastest cooling rates, >00 "Us (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 core and they must have experienced slower cooling rates than the glasses and/or a collision with another bead caused a nucleation site. n 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.

11 Ascent and eruption of a lunar high-titanium magma 537 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). lmenite 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. 5a). The bead can be dominated by euhedral olivine or they can represent a mixture of olivine types indicating a complex cooling history. lmenite 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. MPLCATONS FOR THE ERUPTON AND EMPLACEMENT OF LUNAR VOLCANC BEADS Hanson and Lindstrom (1997) showed that small compositional differences exist between the crystallized beads and the orange glasses in the 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 sample and claimed to be able to distinguish between high- and low- Si 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 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. n 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 Taurus- Littrow 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. (1998) 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

12 538 Weitz et al. FG. 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) llustration of the location of the different bead types defined in (a) within the volcanic plume. n 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.

13 Ascent and eruption of a lunar high-titanium magma 539 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. CONCLUSONS The ascent history can be summarized as: (1) FegS-Nil&ol metal, Cr-spinel, and F o ~ olivine ~ - ~ were ~ crystallizing simultaneously in the orange glass magma prior to its eruption; (2) both the metal-melt equilibria and the Cr content of olivine (+spinel) indicate mf02 of W-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. n 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 NAGS for M. Rutherford and by grant NAG 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 REFERENCES AKELLA J., WLLAMS R. J. AND MULLNS 0. (1976) Solubility of Cr, Ti, and A1 in co-existing olivine, spinel, and liquid at 1 atm. Proc. Lunar Planet. Scr. Conj 7th, ARNDT J. AND ENCELHARDT W. VON (1987) Formation of Apollo 17 orange and black glass beads. Proc. Lunar Planet. Scr. Conz 17th; J. Geophys. Res. Suppl. 92, E372-E376. ARNDT J., ENGELHARDT W. VON, GONZALEZ-CABEZA. AND MEER B. 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