Accessory silicate mineral assemblages in the Bilanga diogenite: A petrographic study

Transcription

1 Meteoritics & Planetary Science 39, Nr 4, (2004) Abstract available online at Accessory silicate mineral assemblages in the Bilanga diogenite: A petrographic study Kenneth DOMANIK, * Serena KOLAR, Donald MUSSELWHITE, and Michael J. DRAKE Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona , USA * Corresponding author. domanik@lpl.arizona.edu (Received 24 June 2003; revision accepted 23 January 2004) Abstract The petrographic relationships in diogenites between orthopyroxene and minor phases such as chromite, troilite, diopside, plagioclase, and silica are often obscured by the intense brecciation that characterizes these meteorites. Although brecciated, Bilanga preserves numerous clasts displaying primary textural relations between orthopyroxene and these minor phases that are large enough to analyze by electron microprobe. In this study, we focus on the distribution, composition, and origin of the minor phases in Bilanga to provide new insights into the crystallization and metamorphic history of these rocks. The samples examined consist mainly of orthopyroxene grains plus five types of assemblages containing diopside + a Fe-rich phase (chromite, troilite, and/or Fe-Ni metal) ± plagioclase ± silica. We interpret type 1 assemblages as being remnants of intercumulus melt trapped in the interstices between orthopyroxene grains after crystal settling in a magma chamber. Type 2 assemblages appear to have formed by heterogeneous exsolution during thermal metamorphism. Type 3 assemblages are believed to be remnants of other assemblages that have been shocked, melted, and rapidly recrystallized by impact events. Type 4 assemblages consist of veins that also appear to have formed from trapped intercumulus melt. Regions of silica-rich mesostasis (type 5) appear to be larger patches of more evolved intercumulus melt that have been significantly affected by late-stage impact melting. Finally, large clasts containing plagioclase ± diopside are interpreted to be exotic fragments of a different but possibly related rock type incorporated in the Bilanga breccia. INTRODUCTION Bilanga is a diogenite breccia fall observed in Burkina Faso on October 27, 1999 (Grossman 2000). Bischoff performed the initial classification and characterization of the mineral phases, and Clayton measured the oxygen isotopes (Grossman 2000). Aspects of the petrology and trace element chemistry of Bilanga have previously been reported by Kolar et al. (2002), Mittlefehldt (2002), and Domanik et al. (2003). The howardite-eucrite-diogenite (HED) clan of igneous meteorites is believed to be derived from asteroid 4 Vesta (Drake 2001). As such, this clan contains a record of the surprisingly complex differentiation of that asteroid and showcases the diversity of igneous rocks produced on a small planetary body 4.5 Ga ago. Many of HED meteorites are breccias and contain a record of the complex impact history of the surfaces of small bodies. Bilanga is a member of the diogenite group of the HED meteorites. These meteorites are typically very highly brecciated and contain vol% orthopyroxene (Bowman et al. 1997). Other common minerals include chromite and highly variable amounts of troilite and olivine (Mittlefehldt et al. 1998). Silicate phases such as Capyroxene, plagioclase, and silica are often present in minor amounts and have been reported to occur as small breccia fragments, exsolution lamella, or small single inclusions in orthopyroxene (Mittlefehldt et al. 1998). Accessory Fe-Ni metal and (rare) phosphates also occur in some diogenites (Mittlefehldt et al. 1998). Most researchers believe that the diogenites represent orthopyroxene cumulates formed by fractional crystallization, although the nature of the parent magma is uncertain (Mittlefehldt 1994; Fowler et al. 1994, 1995; Shearer et al. 1997; Righter and Drake 1997; Mittlefehldt et al. 1998). Due to the intense brecciation suffered by most diogenites, the original contacts between the minor phases and cumulus orthopyroxene usually are not preserved. This brecciation combined with the small size and low modal abundance of diopside, plagioclase, and silica in diogenites has resulted in few chemical analyses or textural descriptions 567 Meteoritical Society, Printed in USA.

2 568 K. Domanik et al. of these phases being available in the literature (Mittlefehldt et al. 1998). Bilanga appears to be unusual in that diopside, plagioclase, and silica are often observed in place with respect to orthopyroxene and are large enough to permit electron microprobe analysis. Although these phases are minor in abundance in Bilanga, they are ubiquitous throughout the sample (Fig. 1). Therefore, the study of these minor minerals in Bilanga and their textural relationships with orthopyroxene provide significant new information that may help to elucidate the magmatic and metamorphic conditions under which the diogenites formed. SAMPLES AND ANALYTICAL METHODS The sample of Bilanga (UA1911) examined in this study was kindly donated to the Lunar and Planetary Laboratory, University of Arizona by Mike Farmer in June, We examined eight thin sections of the sample in detail (sections M1, 2A, 2B, 3B, 3B1, 3B2, 3C, and 3C1). Major element analysis, backscattered electron imaging, and X-ray mapping were performed using the Cameca SX 50 electron microprobe at the Lunar and Planetary Laboratory, University of Arizona. An accelerating voltage of 15 kv, a sample current of 20 na, and 20 sec peak count times were used for most analyses. Beam-sensitive REE-bearing phosphate minerals were analyzed using 20 kv, 10 na, and 10 sec count times. Natural and synthetic standards were used, and the data were corrected for absorption, fluorescence, and atomic number effects using the PAP correction method. GENERAL DESCRIPTION Like most diogenites, Bilanga is a highly brecciated orthopyroxenite containing a wide range of grain sizes (Mittlefehldt et al. 1998). Large, multi-grained regions of orthopyroxene ranging up to 11 7 mm in size are common in many portions of the sample. Numerous small regions containing an assemblage consisting of diopside + a Fe-rich phase ± plagioclase ± silica are present both between and within orthopyroxene grains. A silica-rich mesostasis also occurs in some of the triple junctions amongst orthopyroxene grains. Finally, large clasts of plagioclase-rich material are present in some of the more brecciated regions of the sample. Major minerals present include orthopyroxene (mg# 80; En78, Fs21, Wo1), chromite (mg# 19, cr# 78), and troilite, (mg# = Mg/[Mg + Fe] cations, cr# = Cr/[Cr + Al] cations, per formula unit). Olivine appears to be absent. Diopside (mg# 89; En47, Fs6, Wo47), plagioclase (typically An88 but ranging down to An46), and silica are common accessory minerals. Minor amounts of kamacite (Fe92, Ni5, Co3), tetrataenite (Fe45, Ni55, Co0), pentlandite (Fe6.3, Ni2.7, S8), native copper, light rare earth element (LREE)-bearing phosphate, potassium feldspar, and Si, K-rich quench phases are also observed. Orthopyroxene, diopside, chromite, troilite, Fig. 1. Superimposed backscattered electron images and Ca X-ray maps of two Bilanga thin sections (thin section M1 [a] and thin section 2B [b]) showing the distribution of the Ca-rich phases diopside and plagioclase (red areas) in the samples. The association of diopside and plagioclase with large chromite, troilite, and metal grains (bright white areas) is particularly evident in the upper portion of (a) but is more readily observed at higher magnifications. The large Ca-rich areas in the lower part of (b) are plagioclase-rich exotic clasts set in orthopyroxene breccia (see text for discussion). Although plagioclase and diopside are not abundant compared to orthopyroxene in either section, they are widely distributed throughout both the brecciated and non-brecciated portions of the samples.

3 Accessory silicate mineral assemblages in the Bilanga diogenite 569 kamacite, tetrataenite, and pentlandite exhibit little variation in major element composition regardless of their mode of occurrence. Plagioclase compositions. on the other hand, vary widely and tend to be correlated with the textural settings in which they are found. The average compositions of these phases are given in Tables 1 3. The estimated modal abundance of these phases is shown in Table 4. ORTHOPYROXENE The multi-grain regions of orthopyroxene in Bilanga typically contain several large, optically distinct, orthopyroxene crystals of varying size, separated by curved grain boundaries. Triple junctions between grains are common and are often the site of minor radial brecciation. The boundaries between orthopyroxene grains commonly contain selvages and pockets of chromite and troilite as well as lesser amounts of diopside, plagioclase, and silica. Chromite (and to a lesser extent troilite) also occurs as small inclusions within orthopyroxene crystals. These inclusions are usually, although not always, associated with small amounts of diopside and silica. The mineral assemblages observed at grain boundaries and as inclusions in Bilanga are discussed in greater detail below. Although the orthopyroxene in large multigrain areas is relatively unbrecciated, the individual grains contain an abundance of curved, randomly oriented, healed, and open fractures. In a thin section, most orthopyroxene grains exhibit mottled or patchy extinction, and a few grains display optically visible, discontinuous tree bark-like patterns that resemble exsolution when viewed under crossed polars. Exsolution lamellae are not observed on a scale resolvable by the electron microprobe. However, X-ray mapping indicates that some orthopyroxene grains contain inhomogeneously distributed regions and diffuse bands that are slightly enriched in Ca, which may represent zones of incipient exsolution of high-ca pyroxene. OPAQUE PHASES, PENTLANDITE, AND NATIVE COPPER Opaque, Fe-rich phases such as troilite, chromite, kamacite, and tetrataenite occur in a variety of settings in Bilanga. The occurrence of these minerals is strongly correlated with the presence of diopside, plagioclase, and/or silica and, thus, will be described in greater detail in the following section. However, a few of the larger compound troilite/fe-ni metal/chromite grains in Bilanga are worth additional comment because they sometimes contain pentlandite and, in one case, native copper. Pentlandite and native copper have previously been mentioned in the diogenite literature (Ramdohr 1973; Gooley and Moore 1976), but their occurrence is not well-documented. Compound troilite/chromite/fe-ni metal grains of the type that contain pentlandite in Bilanga tend to be large ( µm) and contain µm-size inclusions of either kamacite or tetrataenite (Fig. 2a). The metal grains are rimmed by oxidized Fe, Ni, Cu-rich material of highly variable composition, 5 50 µm thick, which separates them from the surrounding troilite. These rims are either thinner or absent where chromite is adjacent to the metal grain. Oxidized material in Bilanga is limited to these rims and does not appear to be the product of terrestrial weathering. In grains where pentlandite occurs, it forms as patches at or near the boundary between the oxide rim and the surrounding troilite. The one observed native copper grain occurs adjacent to a large tetrataenite grain, just inside the oxide rim and approximately 20 µm away from a patch of pentlandite (Fig. 2b). The compositions of kamacite, tetrataenite, and pentlandite are relatively constant throughout the sample (see Table 2). Table 1. Silicate and oxide mineral compositions. Pyroxenes Feldspars Oxides Orthopyroxene Diopside Diopside exotic clasts Plagioclase type 1 Plagioclase type 3 Plagioclase exotic clasts K-feldspar exotic clasts Chromite wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ SiO SiO SiO TiO TiO TiO Cr 2 O Cr 2 O Cr 2 O Al 2 O Al 2 O Al 2 O FeO FeO FeO MnO MnO MnO MgO MgO MgO CaO CaO CaO Na 2 O Na 2 O Na 2 O K 2 O K 2 O K 2 O NiO NiO NiO P 2 O P 2 O Total Total Total mg# An mg# En Ab cr# Fs Or Wo # a # # 103 a # = Number of analyses.

4 570 K. Domanik et al. Table 2. Sulfides and metals. Troilite Kamacite Tetrataenite Pentlandite Copper wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ Fe Ni Co S Cr Cu Mn Zn Ti V Si Total # a a # = Number of analyses. Table 3. Phases associated with mesostasis. REE-bearing phosphate K-quench phase wt% 2σ wt% 2σ CaO SiO P 2 O TiO FeO Cr 2 O SiO Al 2 O Na 2 O FeO Y 2 O MnO La 2 O MgO Ce 2 O CaO Pr 2 O Na 2 O Nd 2 O K 2 O NiO P 2 O Total Total # a 5 7 a # = Number of analyses. Analyses of native copper suffered from fluorescence effects from an adjacent tetrataenite grain but indicate that it is at least 98 wt% Cu with the remainder being either Ni or Fe. ASSEMBLAGES CONTAINING ACCESSORY SILICATE PHASES Diopside, plagioclase, and silica are minor but widespread accessory phases in Bilanga. These phases typically form assemblages consisting of diopside + a Fe-rich phase ± plagioclase ± silica. Diopside and a Fe-rich phase are generally present in all assemblages, while the abundance of plagioclase or silica can range from completely absent to predominant depending on the type of assemblage examined. The Fe-rich phase is commonly either chromite or troilite, although kamacite and minor tetrataenite are also present in some cases. In some instances, small (2 20 µm) patches of orthopyroxene are also present in diopside in these assemblages. Table 4. Estimated modal abundances in the thin sections shown in Figs. 1a and 1b. a Fig. 1a Fig. 1b vol% vol% Opx Cpx Plag b Chr Troi Fe-Ni c t.r. d t.r. Silica t.r. t.r. a Modal abundance estimated by image analysis of combined X-ray maps. b The plagioclase estimate in Fig. 1b does not include plagioclase in exotic clasts (see text for discussion). c Fe-Ni includes kamacite, taenite, and pentlandite. d t.r. = trace (<0.1%). The minor silicate phase assemblages in Bilanga vary considerably in their mode of occurrence. However, the different assemblages share enough common characteristics that they can be classified into five major types based on size, mineral texture, mineral modal abundance, textural relationships with adjacent phases (particularly orthopyroxene), and the chemical composition of plagioclase (when present). Plagioclase-rich clasts contain the same minerals as the other assemblages but differ enough in their detailed petrology and textural setting to warrant separate classification. The characteristics of the different types of assemblages and of plagioclase-rich clasts are described in greater detail below. The numbering system reflects the order in which the different types were recognized. Type 1 Assemblages Type 1 assemblages are characterized by the presence of diopside + chromite + troilite ± plagioclase ± silica. The assemblages have relatively simple textures and appear to have approached equilibrium during their formation (Fig. 3).

5 Accessory silicate mineral assemblages in the Bilanga diogenite 571 Fig. 2a. Backscattered electron image of a kamacite grain enclosed in a large compound chromite-troilite grain. The kamacite is separated from the troilite by an oxide rim and appears to be slightly resorbed. Pentlandite is observed in places along the outer edge of the oxide rim. The mineral abbreviations are after Kretz (1983). Fig. 3. Backscattered electron image of a typical type 1 assemblage. The assemblage occurs interstitially between two different grains of orthopyroxene. The phases present within the assemblage include diopside (light gray), plagioclase (dark gray), chromite and troilite (white), and small inclusions of orthopyroxene within the diopside (medium gray). of diopside, plagioclase, and patchy orthopyroxene are commonly observed. The previously described large compound troilite/chromite grains containing kamacite, tetrataenite, pentlandite, and copper are also typically associated with minor diopside, silica, and plagioclase and appear to fit into the type 1 classification. Type 2 Assemblages Fig. 2b. Backscattered electron image of a large, compound chromite-troilite grain. A native copper grain (white) occurs between tetrataenite (light gray) and a rim of oxide material (black) that separates the tetrataenite from the large surrounding troilite (dark gray). Small patches of pentlandite (light gray) occur in troilite near the boundary with the oxide rim. Kamacite is observed in a few assemblages and sometimes contains small, patchy inclusions of tetrataenite. Large type 1 assemblages ( µm) typically occur along grain boundaries between different large orthopyroxene crystals (particularly at triple junctions), contacts between orthopyroxene and large crystals of troilite or chromite, or at the edges of breccia fragments of orthopyroxene. Diopside occurs as relatively large ( µm) anhedral crystals and also as aggregates of smaller (10 20 µm) lathshaped crystals having a tile-like appearance when viewed in thin section. Plagioclase (An 87 ± 2 ) varies from µm in size and often occurs as single crystals. Silica occurs as small, scattered (2 20 µm) patches in or near diopside. Chromite and troilite in type 1 assemblages typically form relatively large µm grains. Compound grains containing both phases are common. Atoll-like textures with chromite and troilite (or, in rare cases, Fe-Ni metal) ringing a central region Type 2 assemblages consist of very small (1 30 µm) diopside + chromite ± silica aggregates that are entirely included within orthopyroxene grains. Individual assemblages consist of minute chromite grains in contact with small patches of diopside. Troilite is very rare, and Fe-Ni metal is absent. Silica, when present, is much less abundant than diopside and forms as small (2 20 µm) patches within or adjacent to diopside. Plagioclase is absent in these assemblages. Although a few isolated type 2 assemblages are observed, it is more common for them to form as arrays of inclusions that are randomly distributed along the length of healed fractures. Diopside from adjacent assemblages in these arrays often coalesces into elongated patches connecting individual chromite inclusions, resulting in a discontinuous, web-shaped pattern of small diopside/chromite/silica inclusion trails running through the host orthopyroxene. In a thin section, type 2 assemblages can be seen to be the surface manifestation of numerous small (1 5 µm), blebby, opaque, and non-opaque, crystals distributed on the surfaces of healed fractures. In many cases, these grains form inclusion curtains consisting of linear columns of sub-rounded small grains with the columns aligned roughly parallel to each other along the fracture surfaces (Fig. 4a). Gooley and Moore (1976) reported inclusion curtains of metal, troilite, chromite, and silica in orthopyroxene

6 572 K. Domanik et al. Fig. 4a. Plane polarized light photomicrograph of a chromitediopside inclusion curtain following a healed fracture in orthopyroxene. The small dark gray grains are chromite. A few small grains of diopside (clear) are also present. The microscope is focused on the bottom of the thin section in this view. The curtains are interpreted as having formed by heterogeneous exsolution. crystals from eight different diogenites that in some respects greatly resemble the type 2 assemblages observed in Bilanga. Although they noted the presence of silica in these occurrences, they did not observe diopside to be associated with them as is seen in Bilanga. Bilanga also differs in that no metal has been observed, and troilite is rare in these assemblages. A much less-common type of inclusion trail, possibly of different origin, consists of individual grains and linear arrays of larger (10 80 µm) balls of troilite and minor chromite (Fig. 4b). These troilite/chromite ball features generally do not lie on identifiable fracture surfaces and, in many cases, appear to follow crystallographic planes. Single isolated troilite/chromite grains with a similar spherical appearance in thin section are also observed. Diopside and silica are present but less abundant than in the type 2 chromite assemblages described above. Type 3 Assemblages Type 3 assemblages consist of relatively large ( µm) aggregates of diopside + chromite + troilite + plagioclase + silica. They differ from type 1 assemblages in texture, the relative modal abundance of the phases, the presence of plagioclase and silica in almost all occurrences, and the variable composition of the plagioclase within each occurrence. Type 3 assemblages are observed to form: 1) interstitially between large orthopyroxene crystals; 2) as inclusions completely enclosed within single orthopyroxene grains; and 3) on the periphery of zones of silica-rich mesostasis. Diopside, plagioclase, silica, chromite, and troilite are all usually present, and any of them may be abundant in a given occurrence. Plagioclase compositions vary significantly both within the same assemblage and between different assemblages. Large plagioclase crystals are usually strongly zoned and can vary from An 89 to An 48 over a Fig. 4b. Plane polarized light photomicrograph of two troilitechromite ball -type inclusion trails. The larger trail runs horizontally across the middle of the image and consists of several black troilite grains and a dark gray chromite grain at right. A trail of much smaller balls runs east-northeast at the bottom center of the image. Neither trail follows any obvious fracture surfaces. The chromite and troilite in the balls are believed to represent material trapped during orthopyroxene growth. Associated diopside and silica may have also been trapped during crystal growth, or alternatively, the balls may have acted as defects at which diopside and silica exsolution occurred at a later time. distance of <20 µm. A few assemblages contain large ( µm) troilite or chromite grains. In several type 3 assemblages, particularly those associated with veins or silica-rich mesostasis, chromite grains are sometimes partially rimmed by plagioclase. Mineral textures in type 3 assemblages vary greatly and generally contain complex intergrowths of coexisting phases (Fig. 5). In thin section, type 3 assemblages typically appear cloudy due to the numerous intergrown phases present. An anastomosing texture consisting of 1 5 µm diopside and plagioclase crystals with minor scattered silica and troilite grains is particularly common. Many type 3 assemblages are also surrounded by small veinlets of plagioclase (approximately 2 5 µm wide and up to 200 µm long) radiating outward from the assemblages into the adjacent orthopyroxene. Type 4 Assemblages Type 4 assemblages consist of relatively thick (10 15 µm) ribbon-like veins of chromite and occasional minor troilite surrounded by an envelope (50 µm wide on average) of mixed orthopyroxene and diopside, as well as smaller patches of plagioclase and silica. These veins are comparatively rare and can extend for several µm within large orthopyroxene. They are distinguished from the much more abundant type 2 inclusion trails by the thickness, continuity, and ribbon-like nature of the chromite in the veins, by the presence of plagioclase, and by the orthopyroxene-diopside envelopes surrounding the veins. Plagioclase in type 4 veins varies little in composition and is typically An 89. The envelopes adjacent to the chromite veins are primarily made

7 Accessory silicate mineral assemblages in the Bilanga diogenite 573 Fig. 5. Backscattered electron image of a typical type 3 assemblage exhibiting the chaotic, anastomosing texture of diopside (light gray), plagioclase (dark gray), and silica (very dark gray) often observed in these assemblages. This particular assemblage is entirely included in a much larger orthopyroxene crystal (medium gray). A large troilite grain (white) occurs at right, and smaller troilite grains are scattered throughout the assemblage. Note the zoning in the large plagioclase grain at right and also the plagioclase veinlets at top following fractures in the host orthopyroxene. up of orthopyroxene but contain a significant amount of diopside, often displaying apparent exsolution textures. The boundary between the vein envelope and the host orthopyroxene is compositionally abrupt, with diopside terminating sharply at its edge (Figs. 6a and 6b), and is also sometimes marked by a fracture surface. In places, the veins have been disrupted by deformation of the host orthopyroxene, and the chromite is distorted into discreet hammer head shapes in which chromite has been pushed into cleavage planes in the host orthopyroxene. Diopside, silica, and plagioclase are particularly abundant in such areas. In a few cases, plagioclase is observed to form a partial rim around chromite in these veins, and small veinlets of plagioclase extend outward from the central vein into the adjacent orthopyroxene. Type 5 Assemblages Type 5 assemblages consist of zones of silica-rich mesostasis that occur either interstitially between large pyroxene crystals or in brecciated areas. The mesostasis regions consist of large ( µm) areas of silica, which contain numerous small diopside crystals (2 20 µm) and small, sparsely distributed grains of troilite (Fig. 7a). In some cases, the central portion of the silica is relatively free of inclusions, with diopside and troilite being confined to the outer edge of the mesostasis region. Chromite is not observed in the mesostasis regions. However, large chromite grains are common in type 3 assemblages adjacent to and partially incorporated in these areas. X-ray mapping indicates that some mesostasis regions contain minute grains of randomly scattered K-rich and P-rich Fig. 6a. Backscattered electron image of a section of a type 4 disrupted chromite vein with fractures separating the diopside-rich vein envelope from the surrounding orthopyroxene. Fig. 6b. Ca X-ray map of the area shown in Fig. 6a, showing patches of diopside (light gray) apparently exsolving from orthopyroxene within the vein envelope. Even though the diopside patches appear to be aligned parallel to a primary crystal direction in the surrounding orthopyroxene, the diopside terminates abruptly at the edge of the envelope. material. In one exceptional case, the K-rich material is observed to form long (~500 µm) discontinuous quench needles running parallel to each other through the silica mesostasis (Fig. 7b). The compositions of the quench crystals are variable, in K, resembling non-stoichiometric SiO 2 -rich, Al 2 O 3 -poor feldspars (Table 1). The P-rich areas in this mesostasis region are too small to be analyzed by electron microprobe. However, several small grains of LREE-enriched phosphate (~10 wt% rare earth oxides) occur in type 3 inclusions adjacent to this region (Fig. 7c). Mittlefehldt (1994) has previously described grains of a LREE-rich phosphate mineral in the diogenite Roda. As in Bilanga, the phosphate in Roda is located near a region of quenched Si-Al- K-Ba glass. The phosphate and K-rich glass in Roda are also similar in composition to the phosphate and K-rich needles in this study. However, in contrast to Bilanga, the phosphate grains in Roda are located in diopside adjacent to the glass,

8 574 K. Domanik et al. Fig. 7a. Backscattered electron image of a region of type 5 silica-rich mesostasis (dark gray) occurring between large orthopyroxene grains (medium gray). The area near the large chromite grain at left (white) resembles a type 3 assemblage and gradually grades into the more silica-rich mesostasis in the center of the image. Fig. 7c. Backscattered electron image of the area outlined by the white rectangle labeled (b) in Fig. 7a. Two grains of a highly LREEenriched phosphate phase occur along the edge of a chromite grain. The phosphate grains and the entire chromite grain are surrounded by a continuous rim of plagioclase. Outside the plagioclase rim, the groundmass consists of complexly intergrown diopside, orthopyroxene, and silica cut by veins of plagioclase. diopside and silica crystals along with small (5 10 µm) troilite grains are complexly intergrown within large single crystals of plagioclase. A few small crystals of almost pure potassium feldspar occur in scattered locations along the boundary between plagioclase and the diopside/silica aggregates. DISCUSSION Fig. 7b. Backscattered electron image of the area outlined by the white rectangle labeled (a) in Fig. 7a, which contains the main portion of the silica-rich mesostasis region. In the center, the region is mostly silica cut by parallel needles of a K-rich, feldspar-like material. Dense clusters of diopside with minor troilite occur in the silica farther out near the boundary with the surrounding orthopyroxene. and phases such as plagioclase, chromite, and troilite are absent. Plagioclase-Rich Clasts One breccia region in the Bilanga sample we examined contains large (1.5 2 mm) isolated clasts consisting of either solid plagioclase or vermicular intergrowths of plagioclase with a fine-grained mixture of silica + diopside (Figs. 8a and 8b). Some of the clasts appear to be relatively intact, while others have been plastically deformed between large orthopyroxene grains and orthopyroxene breccia fragments. The plagioclase in these clasts has a relatively constant composition (An 79, Ab 20, Or 1 ). Where it is not too deformed, the plagioclase exhibits both albite and pericline twinning. In the vermicular clasts, finger-like aggregates of µm In general, previous studies have concluded that diopside in diogenites is formed exclusively by subsolidus exsolution and that plagioclase is most likely present only as exotic breccia fragments derived from other rock types (Mittlefehldt et al. 1998). Although examples of both of these processes may be observed in Bilanga, neither mechanism can adequately explain the origin of most of the occurrences of these phases. In many cases, diopside in Bilanga occurs in relatively large patches, often between orthopyroxene crystals, rather than as the small (up to a few µm thick) exsolution lamella previously described in the literature. Texturally, almost all plagioclase appears to be native to the rock. Finally, the common association of these phases with chromite, troilite, and Fe-Ni metal suggests a co-genetic origin for these minerals. As previously noted, diogenites are believed to have undergone a complex geologic history and are generally believed to be orthopyroxene cumulates formed by fractional crystallization (Mittlefehldt et al. 1998). By analogy with fractional crystallization in terrestrial layered mafic complexes (Wager et al. 1960; Wagner 1968; Morse 1986, 1994), the formation of nearly monomineralic orthopyroxenite probably required a considerable period of adcumulate growth of orthopyroxene and diffusive exchange of components between interstitial melt and melt remaining in the main magma chamber. Slow cooling and thermal

9 Accessory silicate mineral assemblages in the Bilanga diogenite 575 assemblages appear to have formed primarily by heterogeneous exsolution within orthopyroxene grains during thermal metamorphism. Type 3 assemblages are believed to be the remnants of other assemblages that have been shocked, melted, and rapidly recrystallized during late impact events. Type 5 regions of silica-rich mesostasis appear to be larger patches of more evolved intercumulus melt that have also been significantly affected by late-stage impact melting. Plagioclase-rich clasts are interpreted to be exotic fragments of a different but possibly related rock type that have been incorporated in the Bilanga breccia. The bases for these interpretations are explained in greater detail below. Fig. 8a. Ca X-ray map of a plagioclase-rich clast containing vermicular, diopside-silica intergrowths, plagioclase (dark gray), diopside (light gray), and silica (black). Optical microscopy indicates that the plagioclase is part of a single crystal. The black areas around the edges of the image are mostly orthopyroxene breccia. Fig. 8b. Backscattered electron image of the area outlined by the white rectangle in Fig. 8a. Small rare patches of K-feldspar occur at the boundary between the diopside-silica intergrowths and the surrounding plagioclase. A very minor amount of troilite (white) also occurs in the diopside-silica intergrowths. metamorphism resulted in exsolution in orthopyroxene and re-equilibration of major elements in orthopyroxene, chromite, diopside, and possibly plagioclase. Finally, most diogenites have also been subjected to shock deformation, reheating, and brecciation due to impact events. In interpreting the origin of the diopside, plagioclase, and silica-bearing assemblages in Bilanga, one must account for and, in some cases, attempt to see through the overprinting produced by these various processes. Taking these factors into account, our interpretation of type 1 assemblages is that they formed from the minor remnants of intercumulus melt trapped in the interstices between orthopyroxene grains after crystal settling in the magma chamber. Type 4 vein assemblages also appear to have formed from trapped intercumulus melt, although in many areas, they are modified by later shock events. Type 2 Type 1 and Type 4 Assemblages Type 1 and type 4 assemblages both appear to have formed from melt trapped between growing cumulus orthopyroxene grains. In addition, the type 1 assemblages, and most portions of the type 4 veins, have been relatively unaltered by later shock events. On the basis of the apparent similarity in their origins, these two types of assemblages are discussed together in this section. There are several reasons for interpreting type 1 assemblages as trapped intercumulus melt. In the larger orthopyroxene aggregates in Bilanga, type 1 assemblages are primarily located at triple junctions and along grain boundaries between orthopyroxene crystals. This habit is consistent with an origin as intercumulus liquid trapped during adcumulus orthopyroxene growth. In addition, type 1 assemblages almost invariably contain plagioclase and troilite, sometimes as quite large crystals. Unlike diopside or chromite, neither of these phases could be expected to exsolve directly from orthopyroxene due to the low levels of S, Al, and Na that can be accommodated in this mineral. Conversely, phase equilibria considerations show that clinopyroxene, plagioclase, and silica are reasonable phases expected to crystallize after orthopyroxene during fractional crystallization of a mafic magma, and plagioclase and clinopyroxene are commonly observed as intercumulus minerals in orthopyroxenites in terrestrial layered mafic intrusions (cf., Jackson 1967; Irvine 1979). Based on these observations, an origin as trapped intercumulus melt appears to be highly plausible. We interpret type 4 veins as being similar to type 1 assemblages in that they represent selvages of intercumulus melt trapped by growth of orthopyroxene crystals. The phase assemblage observed is very similar to that in type 1 assemblages and includes An 88 plagioclase and troilite. The envelopes surrounding chromite in these veins are interpreted to be adcumulus overgrowths that formed at the edges of the original orthopyroxene crystals as they fractionated from the magma chamber. This would account for the higher proportion of diopside, plagioclase, and silica patches found in these envelopes and for the abrupt termination of diopside

10 576 K. Domanik et al. exsolution at the edge of the envelopes. The chromite ribbons and the remaining diopside, plagioclase, and silica in the center of the veins may have been material that was trapped as the overgrowths on adjacent orthopyroxene crystals gradually grew together. Type 2 Assemblages Unlike type 1 assemblages, type 2 assemblages appear to have formed by heterogeneous exsolution on crystal defects within orthopyroxene grains during thermal metamorphism. The morphology of these assemblages is similar to the few descriptions and photographs of Ca-rich clinopyroxene in diogenites available in the literature (cf., Mittlefehldt 1994; Bowman et al. 1997), which have typically been ascribed to exsolution processes (Mittlefehldt et al. 1998). In terms of the phases present and their location within orthopyroxene grains, type 2 assemblages bear some resemblance to the sub- µm-sized precipitates of augite, chromite, troilite, and silica located on sub-boundaries within orthopyroxene grains that were observed by Mori and Takeda (1981) using transmission electron microscopy (TEM). They bear an even stronger resemblance to the larger chromite/troilite/metal/silica curtains observed by Gooley and Moore (1976). Both of these studies attributed these features to exsolution. The mineralogy of type 2 assemblages is relatively simple (chromite + diopside ± silica), and these small assemblages are almost exclusively located on healed fracture surfaces that appear to have formed well after crystallization of the host orthopyroxene. Optically, the growth of the assemblages can be traced from minute particles, discretely distributed on the fracture surfaces through intermediate-sized collections of grains, to larger aggregates, which intersect the sample surface. These features suggest that nucleation of the assemblages occurred after orthopyroxene crystallization and that the assemblages were frozen in various stages of growth when the temperature fell below that needed for diffusion of material to the defect sites. The troilite/chromite ball features that occur randomly or which follow crystallographic planes rather than healed fractures are more difficult to classify. Given the current petrographic data, the possibility that these features also formed by complex exsolution processes cannot be completely ruled out. However, based on their size, morphology, their presence on crystallographic planes rather than fractures, and the relative abundance of troilite, we believe that the troilite and chromite in these features probably formed from melt trapped during growth of the host orthopyroxene. These trapped troilite and chromite grains subsequently may have acted as nucleation sites for diopside and silica that later exsolved from the host orthopyroxene and, thus, would qualify as type 2 assemblages. Alternatively, the associated diopside and silica may represent the crystallized products of minor amounts of silicate melt that were trapped along with the troilite and chromite. If additional data confirm that diopside and silica in these occurrences do represent trapped melt, these inclusions would more properly be classified as type 1 assemblages. Although additional study is needed to precisely classify the troilite/ chromite ball features, they do appear to be distinct from the other type 2 chromite-bearing assemblages and appear to have had a different origin. Type 3 Assemblages We interpret type 3 assemblages as type 1, 2, and 4 assemblages that have been partly re-melted and rapidly recrystallized by late, localized shock processes. The complex mineral textures and the heterogeneous plagioclase compositions that characterize these assemblages indicate rapid disequilibrium crystallization. The presence of veinlets of plagioclase radiating outward from these assemblages suggests that plagioclase melted and expanded into fractures in the adjacent orthopyroxene during such heating events. Zoned plagioclase crystals exhibiting large changes in anorthite content over very small distances probably formed by melting and rapid recrystallization of pre-existing plagioclase. The presence of plagioclase rims on chromite in some type 3 inclusions may also be indicative of high temperatures facilitating a reaction involving these phases. Some larger type 3 assemblages occurring between brecciated orthopyroxene grains show evidence of flow as the result of plasticity caused by partial or complete melting, while the adjacent orthopyroxene fragments remained rigid. Finally, the association of some type 3 assemblages with large, quenched regions of silica and incompatible element-rich mesostasis also may be indicative of melting and rapid cooling. In some cases, type 3 assemblages are only tens of mm away from type 1, 2, or 4 assemblages showing no shock effects. Thus, if shock heating is responsible for the features observed in type 3 assemblages, it must have occurred on a very localized scale. Localization of shock melt features on this scale have previously been reported in ordinary chondrites by Stoeffler et al. (1991) and may be caused by differential transmission of shock energy in different parts of the sample. However, additional study would be required to determine the detailed mechanism by which shock localization may have occurred in Bilanga. Type 5 Assemblages The zones of silica-rich mesostasis in Bilanga are generally larger than the assemblages described above, contain significantly more silica, and display a marked enrichment in K, P, and probably LREE. As in type 1 assemblages, some of these regions occur in the interstices between large orthopyroxene grains. However, like type 3 assemblages, they are characterized by evidence of

11 Accessory silicate mineral assemblages in the Bilanga diogenite 577 disequilibrium rapid cooling, primarily in the form of quench crystals, inhomogeneous plagioclase compositions, and latestage mobility of plagioclase as evidenced by plagioclase veins extending into the surrounding orthopyroxene. In addition, the mesostasis regions often incorporate more typical type 3 assemblages along their margins and display evidence of chemical exchange with these assemblages. An example of this is the presence of LREE phosphate grains in such type 3 assemblages, where X-ray mapping indicates that the P (and by inference probably the LREE) was derived from the adjacent silica-rich mesostasis. The high silica content and incompatible element enrichment of the silica-rich mesostasis regions in Bilanga suggest that these assemblages represent melts that experienced a greater degree of fractionation than the typical type 1 assemblages. This fractionation could have occurred if these melt pockets had remained in contact with the main magma chamber after the melts that formed type 1 assemblages had been cut off by adcumulate crystal growth. Presumably, the times at which different intercumulus melt pockets became cut off from contact with the main magma chamber would vary somewhat due to localized differences in adcumulus orthopyroxene growth rates, the initial size of the melt pockets, and other factors. If the composition of interstitial melt in diffusive contact with the main magma chamber reflected the overall fractionation of the bulk magma, then melt pockets that were isolated at widely separated times could greatly differ in bulk composition at the time of their separation from the main magma chamber. Small degrees of partial melting caused by shock may also be partly responsible for the enrichment in incompatible elements in type 5 assemblages. However, shock melting alone does not appear to fully account for this enrichment, as most type 3 assemblages (which also appear to have experienced shock melting) do not contain K or P except where they are in contact with mesostasis regions. Differences in initial composition of the mesostasis regions as compared to the other types of assemblages in Bilanga may also be inferred by the complete absence of chromite within the mesostasis regions. Thus, we conclude that the silica-rich mesostasis regions represent a more evolved fraction of the diogenite parent magma that has been altered by later partial shock re-melting. Plagioclase-Rich Clasts Our interpretation of the large clasts of plagioclase and of plagioclase vermicularly intergrown with diopside + silica is that they are exotic inclusions in the Bilanga breccia. In locations where their edges have not been deformed beyond recognition, they appear to be entirely separate from the orthopyroxene fragments that surround them. They are also much larger than the adjacent orthopyroxene breccia fragments. The large, well-formed, twinned plagioclase observed in both the vermicular and single plagioclase clasts is different in habit from any other plagioclase observed in the sample. Plagioclase composition in the clasts is relatively constant at An 79, which also differs from both the An 88 plagioclase in type 1 assemblages and the highly variable compositions in type 3 assemblages found in the rest of the Bilanga. The vermicular intergrowth of diopside-plagioclasesilica and the presence of small amounts of stoichiometric potassium feldspar also are observed only in these clasts and are not present in other parts of the sample. Igneous vermicular micrographic textures such as those found in graphic granites are generally attributed to simultaneous crystallization of phases at a eutectic point (Williams et al. 1982). In the end member diopside-anorthitesilica system, at 1 bar pressure, such a eutectic point occurs at approximately 1222 C (Clark et al. 1962), and this provides a maximum estimate of the crystallization temperature of these clasts. Diogenites, of course, do not contain the magnesian and calcic end members of this system, so the actual crystallization temperature must have been lower. Even though the vermicular plagioclase clasts in Bilanga appear to have been derived from a different source rock, there are a number of similarities between these clasts and the other diopside, plagioclase, and silica occurrences in Bilanga, particularly the silica-rich mesostasis regions. Like the mesostasis areas, the vermicular clasts consist of diopside + silica + plagioclase + troilite, contain a relatively large amount of silica, generally lack chromite, and display some degree of enrichment in K. Based on these similarities, the vermicular clasts possibly formed from a more fractionated portion of the same magma that produced the silica-rich mesostasis regions in Bilanga. However, considerably more data would be needed to confirm this. Igneous or Metamorphic Origin of Type 1, 3, and 4 Assemblages An alternative hypothesis for the production of type 1, 3, and 4 assemblages is that they formed by exsolution of metal, chromite, diopside, and silica from orthopyroxene followed by a series of complex metamorphic reactions during thermal metamorphism to produce troilite and plagioclase. This explanation has the advantage of providing a single explanation for the type 1, 3, and 4 assemblages and the simpler type 2 chromite-diopside assemblages, which do appear to be formed by exsolution. In Bilanga, the rimming of chromite by plagioclase in some type 3 assemblages suggests that a reaction between diopside and the spinel component in chromite may have produced plagioclase and silica in a few cases. In addition, in previous work, Mori and Takeda (1981) attributed sub-micron sized precipitates of augite, chromite, troilite, and silica occurring in diogenite orthopyroxene to exsolution processes. They recognized the difficulty posed by troilite precipitation in orthopyroxene and advanced sulfidation of Fe-Ni metal as their favored explanation, although they did not completely dismiss the possibility of epitaxial growth from a melt during crystal growth.

12 578 K. Domanik et al. There are several arguments against a metamorphic exsolution origin of type 1, 3, and 4 assemblages in Bilanga. Plagioclase in type 1 and in most type 3 and 4 assemblages appears to occur randomly and is not correlated with the proximity of chromite. In some assemblages, plagioclase occurs even though chromite is entirely absent. The few type 3 assemblages containing plagioclase rims on chromite all exhibit evidence of possible shock-induced melting and contain plagioclase that is highly variable in composition, and a later shock event may have been responsible for the reaction textures observed. Exsolution and metamorphic reaction also does not account for the observation that type 1 assemblages containing plagioclase are not distributed on defects throughout the orthopyroxene grains in the same manner as type 2 assemblages. Presumably, if a metamorphic reaction between diopside and chromite produced the plagioclase in the large type 1, 3, and 4 assemblages, it would have occurred in the small scale type 2 inclusions as well, but this is not observed. In summary, a metamorphic origin for type 1, 3, and 4 would require several complex processes to occur for which there is little petrologic evidence. The origin of type 1, 3, and 4 assemblages as remnant intercumulus melt and melt inclusions is more consistent with the petrographic observations, and we strongly favor this interpretation. Compound Chromite-Troilite-Metal Grains The textures of the large compound troilite/chromite/ metal grains in Bilanga, suggest that Fe-Ni metal was initially stable but later became unstable relative to sulfide and oxide materials. The strong association between chromite, troilite, and Fe-Ni metal in diogenites and similar textural relationships between these phases have previously been noted by Gooley and Moore (1976), which suggests that this crystallization sequence may have occurred in other diogenites as well. The presence of pentlandite with a maximum thermal stability of 610 C (Shewman and Clark 1969) indicates that this phase formed late in the cooling history of the rock or during subsequent thermal metamorphism. The nature of the opaque phases in Bilanga and their reactions may offer clues to the crystallization history of diogenites and the fo 2 and fs 2 conditions in the diogenite magma chamber and subsequent metamorphism and are currently the subject of additional study. Orthopyroxene Geothermometry The major element compositions of orthopyroxene, diopside, and chromite in Bilanga have re-equilibrated during slow cooling and thermal metamorphism and, thus, do not preserve their initial igneous compositions. This reequilibration can be deduced from the results of applying the clinopyroxene-orthopyroxene QUILF geothermometer of Andersen et al. (1993) and the orthopyroxene-chromite geothermometer of Liermann and Ganguly (2001) to selected paired analyses of these phases, assuming a pressure of 1 kbar. The average temperatures obtained from these models were 823 C and 720 C, respectively, which are far below igneous crystallization temperatures. These temperatures fall within the ranges observed in diogenites by Mittlefehldt (1994) using two-pyroxene thermometry (Lindsley and Andersen 1983) and the orthopyroxene-spinel geothermometer of Mukherjee et al. (1990). CONCLUSIONS Bilanga is a complex brecciated diogenite. This orthopyroxenite contains five common types of diopside + a Fe-rich phase ± plagioclase ± silica assemblages, which appear to be native to the rock, as well as plagioclase-rich clasts, which may be fragments of a different but related rock type. Petrographic observations of the Bilanga meteorite indicate that most of the diopside and plagioclase appears to have crystallized from trapped intercumulus melt. Thus, plagioclase fractionated from the parent melt of Bilanga, along with clinopyroxene, chromite, troilite, and silica, in addition to the predominant orthopyroxene. As in other adcumulate rocks, most of the material in the interstitial melt that was not compatible in the growing orthopyroxene crystals was presumably displaced back into the magma chamber (cf., Morse 1994). This rejected melt component could have ultimately crystallized a significant amount of plagioclase, clinopyroxene, and silica elsewhere in the magma chamber (possibly as rock similar to the exotic vermicular clasts in Bilanga) or, alternatively, could have been erupted to the surface. In Bilanga, we have recorded cumulate and adcumulate growth, exsolution, shock, and metamorphic re-equilibration. Most likely, the cumulate formed from intrusions of late stage magma from a magma ocean into the crust of Vesta (Righter and Drake 1997). Crystallization of late stage liquids, subsolidus re-equilibration, and shock melting appear to account for all non-cumulate textures in Bilanga. The magmatic and metamorphic history of small asteroidal-sized bodies 4.5 Ga ago appears to be extraordinarily complex. Acknowledgments This manuscript was substantially improved through the thoughtful reviews of David Mittlefehldt and Cyrena Goodrich. The authors wish to thank Mike Farmer for donating the Bilanga sample to the Lunar and Planetary Laboratory and Bill Boynton, David Kring, and Dolores Hill for making the thin sections available for this study. Discussions with David Mittlefehldt and David Kring are appreciated. This work was supported by NASA grant NAG Editorial Handling Dr. Randy Korotev