& Planetary Science 39, Nr 4, 567–579 (2004) Abstract available online at http://meteoritics.org

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 85721–0092, USA *Corresponding author. E-mail: [email protected] (Received 24 June 2003; revision accepted 23 January 2004)

Abstract–The petrographic relationships in diogenites between orthopyroxene and minor phases such as chromite, , diopside, , and silica are often obscured by the intense brecciation that characterizes these . 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 .

INTRODUCTION (Bowman et al. 1997). Other common minerals include chromite and highly variable amounts of troilite and Bilanga is a diogenite breccia fall observed in Burkina (Mittlefehldt et al. 1998). Silicate phases such as Ca- Faso on October 27, 1999 (Grossman 2000). Bischoff , plagioclase, and silica are often present in minor performed the initial classification and characterization of the amounts and have been reported to occur as small breccia mineral phases, and Clayton measured the oxygen isotopes fragments, exsolution lamella, or small single inclusions in (Grossman 2000). Aspects of the petrology and trace element orthopyroxene (Mittlefehldt et al. 1998). Accessory Fe-Ni chemistry of Bilanga have previously been reported by Kolar metal and (rare) phosphates also occur in some diogenites et al. (2002), Mittlefehldt (2002), and Domanik et al. (2003). (Mittlefehldt et al. 1998). The --diogenite (HED) clan of igneous Most researchers believe that the diogenites represent meteorites is believed to be derived from orthopyroxene cumulates formed by fractional crystallization, (Drake 2001). As such, this clan contains a record of the although the nature of the parent magma is uncertain surprisingly complex differentiation of that asteroid and (Mittlefehldt 1994; Fowler et al. 1994, 1995; Shearer et al. showcases the diversity of igneous rocks produced on a small 1997; Righter and Drake 1997; Mittlefehldt et al. 1998). planetary body 4.5 Ga ago. Many of HED meteorites are Due to the intense brecciation suffered by most and contain a record of the complex impact history of diogenites, the original contacts between the minor phases the surfaces of small bodies. and cumulus orthopyroxene usually are not preserved. This Bilanga is a member of the diogenite group of the HED brecciation combined with the small size and low modal meteorites. These meteorites are typically very highly abundance of diopside, plagioclase, and silica in diogenites brecciated and contain 84–100 vol% orthopyroxene has resulted in few chemical analyses or textural descriptions

567 © , 2004. Printed in USA. 568 K. Domanik et al. of these phases being available in the literature (Mittlefehldt corrected for absorption, fluorescence, and atomic number et al. 1998). Bilanga appears to be unusual in that diopside, effects using the PAP correction method. plagioclase, and silica are often observed in place with respect to orthopyroxene and are large enough to permit electron GENERAL DESCRIPTION microprobe analysis. Although these phases are minor in abundance in Bilanga, they are ubiquitous throughout the Like most diogenites, Bilanga is a highly brecciated sample (Fig. 1). Therefore, the study of these minor minerals containing a wide range of grain sizes in Bilanga and their textural relationships with orthopyroxene (Mittlefehldt et al. 1998). Large, multi-grained regions of provide significant new information that may help to orthopyroxene ranging up to 11 × 7 mm in size are common in elucidate the magmatic and metamorphic conditions under many portions of the sample. Numerous small regions which the diogenites formed. containing an assemblage consisting of diopside + a Fe-rich phase ± plagioclase ± silica are present both between and SAMPLES AND ANALYTICAL METHODS within orthopyroxene grains. A silica-rich mesostasis also occurs in some of the triple junctions amongst orthopyroxene The sample of Bilanga (UA1911) examined in this study grains. Finally, large clasts of plagioclase-rich material are was kindly donated to the Lunar and Planetary Laboratory, present in some of the more brecciated regions of the sample. University of Arizona by Mike Farmer in June, 2000. We Major minerals present include orthopyroxene (mg# 80; examined eight thin sections of the sample in detail (sections En78, Fs21, Wo1), chromite (mg# 19, cr# 78), and troilite, (mg# M1, 2A, 2B, 3B, 3B1, 3B2, 3C, and 3C1). Major element = Mg/[Mg + Fe] cations, cr# = Cr/[Cr + Al] cations, per analysis, backscattered electron imaging, and X-ray mapping formula unit). Olivine appears to be absent. Diopside (mg# were performed using the Cameca SX 50 electron microprobe 89; En47, Fs6, Wo47), plagioclase (typically An88 but ranging at the Lunar and Planetary Laboratory, University of Arizona. down to An46), and silica are common accessory minerals. An accelerating voltage of 15 kV, a sample current of 20 nA, Minor amounts of (Fe92, Ni5, Co3), tetrataenite and 20 sec peak count times were used for most analyses. (Fe45, Ni55, Co0), (Fe6.3, Ni2.7, S8), native copper, Beam-sensitive REE-bearing phosphate minerals were light rare earth element (LREE)-bearing phosphate, analyzed using 20 kV, 10 nA, and 10 sec count times. Natural potassium feldspar, and Si, K-rich quench phases are also and synthetic standards were used, and the data were 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. Accessory silicate mineral assemblages in the Bilanga diogenite 569 kamacite, tetrataenite, and pentlandite exhibit little variation in Ca, which may represent zones of incipient exsolution of in major element composition regardless of their mode of high-Ca pyroxene. occurrence. Plagioclase compositions. on the other hand, vary widely and tend to be correlated with the textural settings in OPAQUE PHASES, PENTLANDITE, which they are found. The average compositions of these AND NATIVE COPPER phases are given in Tables 1–3. The estimated modal abundance of these phases is shown in Table 4. Opaque, Fe-rich phases such as troilite, chromite, kamacite, and tetrataenite occur in a variety of settings in ORTHOPYROXENE Bilanga. The occurrence of these minerals is strongly correlated with the presence of diopside, plagioclase, and/or The multi-grain regions of orthopyroxene in Bilanga silica and, thus, will be described in greater detail in the typically contain several large, optically distinct, following section. However, a few of the larger compound orthopyroxene crystals of varying size, separated by curved troilite/Fe-Ni metal/chromite grains in Bilanga are worth grain boundaries. Triple junctions between grains are additional comment because they sometimes contain common and are often the site of minor radial brecciation. pentlandite and, in one case, native copper. Pentlandite and The boundaries between orthopyroxene grains commonly native copper have previously been mentioned in the contain selvages and pockets of chromite and troilite as well diogenite literature (Ramdohr 1973; Gooley and Moore as lesser amounts of diopside, plagioclase, and silica. 1976), but their occurrence is not well-documented. Chromite (and to a lesser extent troilite) also occurs as small Compound troilite/chromite/Fe-Ni metal grains of the inclusions within orthopyroxene crystals. These inclusions type that contain pentlandite in Bilanga tend to be large (500– are usually, although not always, associated with small 2000 µm) and contain 100–200 µm-size inclusions of either amounts of diopside and silica. The mineral assemblages kamacite or tetrataenite (Fig. 2a). The metal grains are rimmed observed at grain boundaries and as inclusions in Bilanga are by oxidized Fe, Ni, Cu-rich material of highly variable discussed in greater detail below. composition, 5–50 µm thick, which separates them from the Although the orthopyroxene in large multigrain areas is surrounding troilite. These rims are either thinner or absent relatively unbrecciated, the individual grains contain an where chromite is adjacent to the metal grain. Oxidized abundance of curved, randomly oriented, healed, and open material in Bilanga is limited to these rims and does not appear fractures. In a thin section, most orthopyroxene grains exhibit to be the product of terrestrial weathering. In grains where mottled or patchy extinction, and a few grains display pentlandite occurs, it forms as patches at or near the boundary optically visible, discontinuous tree bark-like patterns that between the oxide rim and the surrounding troilite. The one resemble exsolution when viewed under crossed polars. observed native copper grain occurs adjacent to a large Exsolution lamellae are not observed on a scale resolvable by tetrataenite grain, just inside the oxide rim and approximately the electron microprobe. However, X-ray mapping indicates 20 µm away from a patch of pentlandite (Fig. 2b). The that some orthopyroxene grains contain inhomogeneously compositions of kamacite, tetrataenite, and pentlandite are distributed regions and diffuse bands that are slightly enriched relatively constant throughout the sample (see Table 2).

Table 1. Silicate and oxide mineral compositions. Feldspars Oxides Diopside Plagioclase Plagioclase Plagioclase K-feldspar Orthopyroxene Diopside exotic clasts type 1 type 3 exotic clasts exotic clasts Chromite wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ wt% 2σ

SiO2 55.25 0.86 54.13 0.90 54.13 0.92 SiO2 45.64 1.14 48.30 6.28 47.89 1.50 64.60 0.08 SiO2 0.09 0.10 TiO2 0.09 0.08 0.10 0.06 0.11 0.08 TiO2 0.01 0.04 0.01 0.04 0.02 0.04 0.01 0.00 TiO2 0.50 0.14 Cr2O3 0.33 0.34 0.37 0.36 0.30 0.10 Cr2O3 0.10 0.16 0.07 0.22 0.01 0.04 0.03 0.08 Cr2O3 56.24 3.34 Al2O3 0.73 0.48 0.47 0.62 0.37 0.12 Al2O3 35.20 1.20 33.47 4.24 34.00 0.86 19.65 0.38 Al2O3 9.90 2.02 FeO 13.19 0.56 3.92 0.64 3.97 0.54 FeO 0.31 0.28 0.34 0.26 0.10 0.12 0.11 0.12 FeO 27.57 0.94 MnO 0.44 0.08 0.18 0.06 0.19 0.04 MnO 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.00 MnO 0.84 0.20 MgO 29.27 0.80 17.08 0.62 16.94 0.34 MgO 0.05 0.40 0.05 0.26 0.01 0.02 0.02 0.04 MgO 3.58 0.40 CaO 0.64 0.54 23.66 1.24 23.48 0.60 CaO 17.93 0.70 15.73 4.82 16.26 0.92 0.15 0.08 CaO 0.08 0.20

Na2O 0.01 0.02 0.12 0.04 0.13 0.04 Na2O 1.33 0.42 2.49 2.70 2.29 0.42 0.08 0.02 Na2O 0.01 0.02 K2O 0.00 0.02 0.00 0.02 0.01 0.02 K2O 0.03 0.02 0.11 0.18 0.09 0.14 15.64 0.12 K2O 0.01 0.02 NiO 0.02 0.04 0.01 0.04 0.01 0.02 NiO 0.01 0.04 0.02 0.04 0.02 0.04 0.00 0.00 NiO 0.02 0.06

P2O5 0.00 0.02 0.03 0.02 0.03 0.02 P2O5 0.04 0.02 0.03 0.04 0.05 0.04 0.01 0.02 Total 99.97 1.06 100.08 1.04 99.68 1.02 Total 100.65 1.18 100.62 1.12 100.75 1.32 100.28 0.66 Total 98.88 2.20 mg# 79.8 0.9 88.6 1.5 88.4 1.4 An 88.0 3.7 77.3 24.4 79.3 4.3 0.8 0.4 mg# 18.8 2.0 En 78.3 1.3 46.9 1.6 46.9 0.3 Ab 11.8 3.6 22.1 23.4 20.2 3.6 0.7 0.1 cr# 79.2 4.2 Fs 20.5 0.9 6.3 1.0 6.5 0.9 Or 0.2 0.2 0.6 1.0 0.5 0.9 98.5 0.5 Wo 1.2 1.0 46.7 2.3 46.7 1.1 #a 163 254 19 # 64 68 26 2 # 103 a# = Number of analyses. 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 63.34 0.94 91.72 2.10 43.52 2.54 46.52 1.50 2.46 0.06 Ni 0.03 0.06 4.98 1.16 56.19 2.68 20.24 0.96 1.85 0.16 Co 0.00 0.00 3.56 0.36 0.42 0.18 0.36 0.20 0.01 0.04 S 36.56 0.50 0.01 0.02 0.01 0.02 33.66 0.32 0.05 0.04 Cr 0.05 0.16 0.19 0.64 0.16 0.46 0.01 0.04 0.01 0.02 Cu 0.04 0.08 0.03 0.10 0.19 0.32 0.05 0.06 98.24 0.34 Mn 0.02 0.04 0.01 0.02 0.02 0.04 0.01 0.02 0.02 0.06 Zn 0.04 0.12 0.03 0.10 0.04 0.12 0.04 0.08 0.02 0.00 Ti 0.01 0.02 0.00 0.02 0.00 0.02 0.01 0.02 0.01 0.02 V 0.01 0.04 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 Si 0.02 0.04 0.03 0.04 0.03 0.06 0.01 0.02 0.01 0.02 Total 100.15 1.18 100.61 1.30 100.62 1.22 100.91 0.88 102.74 0.60 #a 86 40 5 10 2 a# = Number of analyses.

Table 3. Phases associated with mesostasis. Table 4. Estimated modal abundances in the thin sections REE-bearing K-quench shown in Figs. 1a and 1b.a phosphate phase Fig. 1a Fig. 1b wt% 2σ wt% 2σ vol% vol%

CaO 38.82 1.07 SiO2 74.29 5.31 Opx 96.0 97.4 P2O5 40.44 0.94 TiO2 0.06 0.10 Cpx 1.7 1.5 FeO 0.44 0.07 Cr2O3 0.01 0.03 Plag 0.4 0.4b SiO2 0.21 0.10 Al2O3 14.29 3.38 Chr 0.5 0.4 Na2O 0.35 0.13 FeO 0.12 0.25 Troi 1.3 0.3 Y2O3 0.46 0.05 MnO 0.01 0.03 Fe-Nic t.r.d t.r. La2O3 3.05 0.19 MgO 0.02 0.05 Silica t.r. t.r. Ce O 5.43 0.32 CaO 0.88 1.93 2 3 aModal abundance estimated by image analysis of combined X-ray maps. Pr2O3 0.65 0.31 Na2O 0.39 0.81 bThe plagioclase estimate in Fig. 1b does not include plagioclase in exotic Nd2O3 1.92 0.50 K2O 9.18 5.25 clasts (see text for discussion). NiO 0.02 0.04 cFe-Ni includes kamacite, , and pentlandite. d P2O5 0.07 0.05 t.r. = trace (<0.1%). Total 91.68 0.92 Total 99.34 3.37 #a 57The minor silicate phase assemblages in Bilanga vary a# = Number of analyses. considerably in their mode of occurrence. However, the different assemblages share enough common characteristics Analyses of native copper suffered from fluorescence effects that they can be classified into five major types based on size, from an adjacent tetrataenite grain but indicate that it is at least mineral texture, mineral modal abundance, textural 98 wt% Cu with the remainder being either Ni or Fe. relationships with adjacent phases (particularly orthopyroxene), and the chemical composition of plagioclase ASSEMBLAGES CONTAINING ACCESSORY (when present). Plagioclase-rich clasts contain the same SILICATE PHASES minerals as the other assemblages but differ enough in their detailed petrology and textural setting to warrant separate Diopside, plagioclase, and silica are minor but widespread classification. The characteristics of the different types of accessory phases in Bilanga. These phases typically form assemblages and of plagioclase-rich clasts are described in assemblages consisting of diopside + a Fe-rich phase ± greater detail below. The numbering system reflects the order plagioclase ± silica. Diopside and a Fe-rich phase are generally in which the different types were recognized. present in all assemblages, while the abundance of plagioclase or silica can range from completely absent to predominant Type 1 Assemblages depending on the type of assemblage examined. The Fe-rich phase is commonly either chromite or troilite, although Type 1 assemblages are characterized by the presence of kamacite and minor tetrataenite are also present in some cases. diopside + chromite + troilite ± plagioclase ± silica. The In some instances, small (2–20 µm) patches of orthopyroxene assemblages have relatively simple textures and appear to are also present in diopside in these assemblages. have approached equilibrium during their formation (Fig. 3). Accessory silicate mineral assemblages in the Bilanga diogenite 571

Fig. 2a. Backscattered electron image of a kamacite grain enclosed in Fig. 3. Backscattered electron image of a typical type 1 assemblage. a large compound chromite-troilite grain. The kamacite is separated The assemblage occurs interstitially between two different grains of from the troilite by an oxide rim and appears to be slightly resorbed. orthopyroxene. The phases present within the assemblage include Pentlandite is observed in places along the outer edge of the oxide diopside (light gray), plagioclase (dark gray), chromite and troilite rim. The mineral abbreviations are after Kretz (1983). (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

Type 2 assemblages consist of very small (1–30 µm) diopside + chromite ± silica aggregates that are entirely included within orthopyroxene grains. Individual Fig. 2b. Backscattered electron image of a large, compound assemblages consist of minute chromite grains in contact with chromite-troilite grain. A native copper grain (white) occurs between small patches of diopside. Troilite is very rare, and Fe-Ni tetrataenite (light gray) and a rim of oxide material (black) that metal is absent. Silica, when present, is much less abundant separates the tetrataenite from the large surrounding troilite (dark gray). Small patches of pentlandite (light gray) occur in troilite near than diopside and forms as small (2–20 µm) patches within or the boundary with the oxide rim. adjacent to diopside. Plagioclase is absent in these assemblages. Kamacite is observed in a few assemblages and sometimes Although a few isolated type 2 assemblages are contains small, patchy inclusions of tetrataenite. Large type 1 observed, it is more common for them to form as arrays of assemblages (200–600 µm) typically occur along grain inclusions that are randomly distributed along the length of boundaries between different large orthopyroxene crystals healed fractures. Diopside from adjacent assemblages in these (particularly at triple junctions), contacts between arrays often coalesces into elongated patches connecting orthopyroxene and large crystals of troilite or chromite, or at individual chromite inclusions, resulting in a discontinuous, the edges of breccia fragments of orthopyroxene. web-shaped pattern of small diopside/chromite/silica Diopside occurs as relatively large (50–100 µm) anhedral inclusion trails running through the host orthopyroxene. crystals and also as aggregates of smaller (10–20 µm) lath- In a thin section, type 2 assemblages can be seen to be the shaped crystals having a tile-like appearance when viewed in surface manifestation of numerous small (1–5 µm), blebby, thin section. Plagioclase (An87 ± 2) varies from 2–300 µm in opaque, and non-opaque, crystals distributed on the surfaces size and often occurs as single crystals. Silica occurs as small, of healed fractures. In many cases, these grains form inclusion scattered (2–20 µm) patches in or near diopside. Chromite “curtains” consisting of linear columns of sub-rounded small and troilite in type 1 assemblages typically form relatively grains with the columns aligned roughly parallel to each other large 20–200 µm grains. Compound grains containing both along the fracture surfaces (Fig. 4a). phases are common. “Atoll-like” textures with chromite and Gooley and Moore (1976) reported inclusion “curtains” troilite (or, in rare cases, Fe-Ni metal) ringing a central region of metal, troilite, chromite, and silica in orthopyroxene 572 K. Domanik et al.

Fig. 4a. Plane polarized light photomicrograph of a chromite- Fig. 4b. Plane polarized light photomicrograph of two “troilite- diopside inclusion “curtain” following a healed fracture in chromite ball”-type inclusion trails. The larger trail runs horizontally orthopyroxene. The small dark gray grains are chromite. A few small across the middle of the image and consists of several black troilite grains of diopside (clear) are also present. The microscope is focused grains and a dark gray chromite grain at right. A trail of much smaller on the bottom of the thin section in this view. The curtains are balls runs east-northeast at the bottom center of the image. Neither interpreted as having formed by heterogeneous exsolution. trail follows any obvious fracture surfaces. The chromite and troilite in the balls are believed to represent material trapped during crystals from eight different diogenites that in some respects orthopyroxene growth. Associated diopside and silica may have also greatly resemble the type 2 assemblages observed in Bilanga. been trapped during crystal growth, or alternatively, the balls may have acted as defects at which diopside and silica exsolution occurred Although they noted the presence of silica in these at a later time. occurrences, they did not observe diopside to be associated with them as is seen in Bilanga. Bilanga also differs in that no distance of <20 µm. A few assemblages contain large (100– metal has been observed, and troilite is rare in these 300 µm) troilite or chromite grains. In several type 3 assemblages. assemblages, particularly those associated with veins or A much less-common type of inclusion trail, possibly of silica-rich mesostasis, chromite grains are sometimes different origin, consists of individual grains and linear arrays partially rimmed by plagioclase. of larger (10–80 µm) balls of troilite and minor chromite Mineral textures in type 3 assemblages vary greatly and (Fig. 4b). These “troilite/chromite ball” features generally do generally contain complex intergrowths of coexisting phases not lie on identifiable fracture surfaces and, in many cases, (Fig. 5). In thin section, type 3 assemblages typically appear appear to follow crystallographic planes. Single isolated cloudy due to the numerous intergrown phases present. An troilite/chromite grains with a similar spherical appearance in anastomosing texture consisting of 1–5 µm diopside and thin section are also observed. Diopside and silica are present plagioclase crystals with minor scattered silica and troilite but less abundant than in the type 2 chromite assemblages grains is particularly common. Many type 3 assemblages are described above. also surrounded by small veinlets of plagioclase (approximately 2–5 µm wide and up to 200 µm long) Type 3 Assemblages radiating outward from the assemblages into the adjacent orthopyroxene. Type 3 assemblages consist of relatively large (80– 500 µm) aggregates of diopside + chromite + troilite + Type 4 Assemblages plagioclase + silica. They differ from type 1 assemblages in texture, the relative modal abundance of the phases, the Type 4 assemblages consist of relatively thick (10– presence of plagioclase and silica in almost all occurrences, 15 µm) ribbon-like veins of chromite and occasional minor and the variable composition of the plagioclase within each troilite surrounded by an envelope (50 µm wide on average) occurrence. Type 3 assemblages are observed to form: 1) of mixed orthopyroxene and diopside, as well as smaller interstitially between large orthopyroxene crystals; 2) as patches of plagioclase and silica. These veins are inclusions completely enclosed within single orthopyroxene comparatively rare and can extend for several µm within large grains; and 3) on the periphery of zones of silica-rich orthopyroxene. They are distinguished from the much more mesostasis. Diopside, plagioclase, silica, chromite, and abundant type 2 inclusion trails by the thickness, continuity, troilite are all usually present, and any of them may be and ribbon-like nature of the chromite in the veins, by the abundant in a given occurrence. Plagioclase compositions presence of plagioclase, and by the orthopyroxene-diopside vary significantly both within the same assemblage and “envelopes” surrounding the veins. Plagioclase in type 4 between different assemblages. Large plagioclase crystals are veins varies little in composition and is typically An89. The usually strongly zoned and can vary from An89 to An48 over a envelopes adjacent to the chromite veins are primarily made Accessory silicate mineral assemblages in the Bilanga diogenite 573

Fig. 5. Backscattered electron image of a typical type 3 assemblage Fig. 6a. Backscattered electron image of a section of a type 4 exhibiting the chaotic, anastomosing texture of diopside (light gray), disrupted chromite vein with fractures separating the diopside-rich plagioclase (dark gray), and silica (very dark gray) often observed in vein envelope from the surrounding orthopyroxene. 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, Fig. 6b. Ca X-ray map of the area shown in Fig. 6a, showing patches silica, and plagioclase are particularly abundant in such areas. of diopside (light gray) apparently exsolving from orthopyroxene within the vein envelope. Even though the diopside patches appear to In a few cases, plagioclase is observed to form a partial rim be aligned parallel to a primary crystal direction in the surrounding around chromite in these veins, and small veinlets of orthopyroxene, the diopside terminates abruptly at the edge of the plagioclase extend outward from the central vein into the envelope. adjacent orthopyroxene. material. In one exceptional case, the K-rich material is Type 5 Assemblages observed to form long (~500 µm) discontinuous quench needles running parallel to each other through the silica Type 5 assemblages consist of zones of silica-rich mesostasis (Fig. 7b). The compositions of the quench crystals mesostasis that occur either interstitially between large are variable, in K, resembling non-stoichiometric SiO2-rich, pyroxene crystals or in brecciated areas. The mesostasis Al2O3-poor feldspars (Table 1). The P-rich areas in this regions consist of large (600–1000 µm) areas of silica, which mesostasis region are too small to be analyzed by electron contain numerous small diopside crystals (2–20 µm) and microprobe. However, several small grains of LREE-enriched small, sparsely distributed grains of troilite (Fig. 7a). In some phosphate (~10 wt% rare earth oxides) occur in type 3 cases, the central portion of the silica is relatively free of inclusions adjacent to this region (Fig. 7c). Mittlefehldt inclusions, with diopside and troilite being confined to the (1994) has previously described grains of a LREE-rich outer edge of the mesostasis region. Chromite is not observed phosphate mineral in the diogenite Roda. As in Bilanga, the in the mesostasis regions. However, large chromite grains are phosphate in Roda is located near a region of quenched Si-Al- common in type 3 assemblages adjacent to and partially K-Ba glass. The phosphate and K-rich glass in Roda are also incorporated in these areas. similar in composition to the phosphate and K-rich needles in X-ray mapping indicates that some mesostasis regions this study. However, in contrast to Bilanga, the phosphate contain minute grains of randomly scattered K-rich and P-rich grains in Roda are located in diopside adjacent to the glass, 574 K. Domanik et al.

Fig. 7a. Backscattered electron image of a region of type 5 silica-rich Fig. 7c. Backscattered electron image of the area outlined by the mesostasis (dark gray) occurring between large orthopyroxene grains white rectangle labeled (b) in Fig. 7a. Two grains of a highly LREE- (medium gray). The area near the large chromite grain at left (white) enriched phosphate phase occur along the edge of a chromite grain. resembles a type 3 assemblage and gradually grades into the more The phosphate grains and the entire chromite grain are surrounded by silica-rich mesostasis in the center of the image. 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

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 Fig. 7b. Backscattered electron image of the area outlined by the et al. 1998). Although examples of both of these processes white rectangle labeled (a) in Fig. 7a, which contains the main portion may be observed in Bilanga, neither mechanism can of the silica-rich mesostasis region. In the center, the region is mostly adequately explain the origin of most of the occurrences of 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 these phases. In many cases, diopside in Bilanga occurs in near the boundary with the surrounding orthopyroxene. relatively large patches, often between orthopyroxene crystals, rather than as the small (up to a few µm thick) and phases such as plagioclase, chromite, and troilite are exsolution lamella previously described in the literature. absent. Texturally, almost all plagioclase appears to be native to the rock. Finally, the common association of these phases with Plagioclase-Rich Clasts chromite, troilite, and Fe-Ni metal suggests a co-genetic origin for these minerals. One breccia region in the Bilanga sample we examined As previously noted, diogenites are believed to have contains large (1.5–2 mm) isolated clasts consisting of either undergone a complex geologic history and are generally solid plagioclase or vermicular intergrowths of plagioclase believed to be orthopyroxene cumulates formed by fractional with a fine-grained mixture of silica + diopside (Figs. 8a and crystallization (Mittlefehldt et al. 1998). By analogy with 8b). Some of the clasts appear to be relatively intact, while fractional crystallization in terrestrial layered mafic others have been plastically deformed between large complexes (Wager et al. 1960; Wagner 1968; Morse 1986, orthopyroxene grains and orthopyroxene breccia fragments. 1994), the formation of nearly monomineralic The plagioclase in these clasts has a relatively constant orthopyroxenite probably required a considerable period of composition (An79, Ab20, Or1). Where it is not too deformed, adcumulate growth of orthopyroxene and diffusive exchange the plagioclase exhibits both albite and pericline twinning. In of components between interstitial melt and melt remaining in the vermicular clasts, finger-like aggregates of 10–30 µm the main magma chamber. Slow cooling and thermal 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), Type 1 and Type 4 Assemblages diopside (light gray), and silica (black). Optical microscopy indicates that the plagioclase is part of a single crystal. The black areas around Type 1 and type 4 assemblages both appear to have the edges of the image are mostly orthopyroxene breccia. 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 Fig. 8b. Backscattered electron image of the area outlined by the orthopyroxene due to the low levels of S, Al, and Na that can white rectangle in Fig. 8a. Small rare patches of K-feldspar occur at be accommodated in this mineral. Conversely, phase equilibria the boundary between the diopside-silica intergrowths and the surrounding plagioclase. A very minor amount of troilite (white) also considerations show that clinopyroxene, plagioclase, and occurs in the diopside-silica intergrowths. silica are reasonable phases expected to crystallize after orthopyroxene during fractional crystallization of a mafic metamorphism resulted in exsolution in orthopyroxene and magma, and plagioclase and clinopyroxene are commonly re-equilibration of major elements in orthopyroxene, observed as intercumulus minerals in in chromite, diopside, and possibly plagioclase. Finally, most terrestrial layered mafic intrusions (cf., Jackson 1967; Irvine diogenites have also been subjected to shock deformation, re- 1979). Based on these observations, an origin as trapped heating, and brecciation due to impact events. In interpreting intercumulus melt appears to be highly plausible. the origin of the diopside, plagioclase, and silica-bearing We interpret type 4 veins as being similar to type 1 assemblages in Bilanga, one must account for and, in some assemblages in that they represent selvages of intercumulus cases, attempt to “see through” the overprinting produced by melt trapped by growth of orthopyroxene crystals. The phase these various processes. assemblage observed is very similar to that in type 1 Taking these factors into account, our interpretation of assemblages and includes An88 plagioclase and troilite. The type 1 assemblages is that they formed from the minor envelopes surrounding chromite in these veins are interpreted remnants of intercumulus melt trapped in the interstices to be adcumulus overgrowths that formed at the edges of the between orthopyroxene grains after crystal settling in the original orthopyroxene crystals as they fractionated from the magma chamber. Type 4 vein assemblages also appear to magma chamber. This would account for the higher have formed from trapped intercumulus melt, although in proportion of diopside, plagioclase, and silica patches found many areas, they are modified by later shock events. Type 2 in these envelopes and for the abrupt termination of diopside 576 K. Domanik et al. exsolution at the edge of the envelopes. The chromite ribbons were trapped along with the troilite and chromite. If and the remaining diopside, plagioclase, and silica in the additional data confirm that diopside and silica in these center of the veins may have been material that was trapped as occurrences do represent trapped melt, these inclusions would the overgrowths on adjacent orthopyroxene crystals gradually more properly be classified as type 1 assemblages. Although grew together. additional study is needed to precisely classify the troilite/ chromite ball features, they do appear to be distinct from the Type 2 Assemblages other type 2 chromite-bearing assemblages and appear to have had a different origin. Unlike type 1 assemblages, type 2 assemblages appear to have formed by heterogeneous exsolution on crystal defects Type 3 Assemblages within orthopyroxene grains during thermal metamorphism. The morphology of these assemblages is similar to the few We interpret type 3 assemblages as type 1, 2, and 4 descriptions and photographs of Ca-rich clinopyroxene in assemblages that have been partly re-melted and rapidly diogenites available in the literature (cf., Mittlefehldt 1994; recrystallized by late, localized shock processes. The Bowman et al. 1997), which have typically been ascribed to complex mineral textures and the heterogeneous plagioclase exsolution processes (Mittlefehldt et al. 1998). In terms of the compositions that characterize these assemblages indicate phases present and their location within orthopyroxene rapid disequilibrium crystallization. The presence of veinlets grains, type 2 assemblages bear some resemblance to the sub- of plagioclase radiating outward from these assemblages µm-sized precipitates of augite, chromite, troilite, and silica suggests that plagioclase melted and expanded into fractures located on sub-boundaries within orthopyroxene grains that in the adjacent orthopyroxene during such heating events. were observed by Mori and Takeda (1981) using transmission Zoned plagioclase crystals exhibiting large changes in electron microscopy (TEM). They bear an even stronger anorthite content over very small distances probably formed resemblance to the larger chromite/troilite/metal/silica by melting and rapid recrystallization of pre-existing “curtains” observed by Gooley and Moore (1976). Both of plagioclase. The presence of plagioclase rims on chromite in these studies attributed these features to exsolution. The some type 3 inclusions may also be indicative of high mineralogy of type 2 assemblages is relatively simple temperatures facilitating a reaction involving these phases. (chromite + diopside ± silica), and these small assemblages Some larger type 3 assemblages occurring between brecciated are almost exclusively located on healed fracture surfaces that orthopyroxene grains show evidence of flow as the result of appear to have formed well after crystallization of the host plasticity caused by partial or complete melting, while the orthopyroxene. Optically, the growth of the assemblages can adjacent orthopyroxene fragments remained rigid. Finally, the be traced from minute particles, discretely distributed on the association of some type 3 assemblages with large, quenched fracture surfaces through intermediate-sized collections of regions of silica and incompatible element-rich mesostasis grains, to larger aggregates, which intersect the sample also may be indicative of melting and rapid cooling. surface. These features suggest that nucleation of the In some cases, type 3 assemblages are only tens of mm assemblages occurred after orthopyroxene crystallization and away from type 1, 2, or 4 assemblages showing no shock that the assemblages were “frozen” in various stages of effects. Thus, if shock heating is responsible for the features growth when the temperature fell below that needed for observed in type 3 assemblages, it must have occurred on a diffusion of material to the defect sites. very localized scale. Localization of shock melt features on The “troilite/chromite ball” features that occur randomly this scale have previously been reported in ordinary or which follow crystallographic planes rather than healed by Stoeffler et al. (1991) and may be caused by fractures are more difficult to classify. Given the current differential transmission of shock energy in different parts of petrographic data, the possibility that these features also the sample. However, additional study would be required to formed by complex exsolution processes cannot be determine the detailed mechanism by which shock completely ruled out. However, based on their size, localization may have occurred in Bilanga. morphology, their presence on crystallographic planes rather than fractures, and the relative abundance of troilite, we Type 5 Assemblages believe that the troilite and chromite in these features probably formed from melt trapped during growth of the host The zones of silica-rich mesostasis in Bilanga are orthopyroxene. These trapped troilite and chromite grains generally larger than the assemblages described above, subsequently may have acted as nucleation sites for diopside contain significantly more silica, and display a marked and silica that later exsolved from the host orthopyroxene enrichment in K, P, and probably LREE. As in type 1 and, thus, would qualify as type 2 assemblages. Alternatively, assemblages, some of these regions occur in the interstices the associated diopside and silica may represent the between large orthopyroxene grains. However, like type 3 crystallized products of minor amounts of silicate melt that assemblages, they are characterized by evidence of Accessory silicate mineral assemblages in the Bilanga diogenite 577 disequilibrium rapid cooling, primarily in the form of quench sample. Plagioclase composition in the clasts is relatively crystals, inhomogeneous plagioclase compositions, and late- constant at An79, which also differs from both the An88 stage mobility of plagioclase as evidenced by plagioclase plagioclase in type 1 assemblages and the highly variable veins extending into the surrounding orthopyroxene. In compositions in type 3 assemblages found in the rest of the addition, the mesostasis regions often incorporate more Bilanga. The vermicular intergrowth of diopside-plagioclase- typical type 3 assemblages along their margins and display silica and the presence of small amounts of stoichiometric evidence of chemical exchange with these assemblages. An potassium feldspar also are observed only in these clasts and example of this is the presence of LREE phosphate grains in are not present in other parts of the sample. such type 3 assemblages, where X-ray mapping indicates that Igneous vermicular “micrographic” textures such as the P (and by inference probably the LREE) was derived from those found in “graphic granites” are generally attributed to the adjacent silica-rich mesostasis. simultaneous crystallization of phases at a eutectic point The high silica content and incompatible element (Williams et al. 1982). In the end member diopside-anorthite- enrichment of the silica-rich mesostasis regions in Bilanga silica system, at 1 bar pressure, such a eutectic point occurs at suggest that these assemblages represent melts that approximately 1222 °C (Clark et al. 1962), and this provides experienced a greater degree of fractionation than the typical a maximum estimate of the crystallization temperature of type 1 assemblages. This fractionation could have occurred if these clasts. Diogenites, of course, do not contain the these melt pockets had remained in contact with the main magnesian and calcic end members of this system, so the magma chamber after the melts that formed type 1 actual crystallization temperature must have been lower. assemblages had been cut off by adcumulate crystal growth. Even though the vermicular plagioclase clasts in Bilanga Presumably, the times at which different intercumulus melt appear to have been derived from a different source rock, pockets became cut off from contact with the main magma there are a number of similarities between these clasts and the chamber would vary somewhat due to localized differences in other diopside, plagioclase, and silica occurrences in Bilanga, adcumulus orthopyroxene growth rates, the initial size of the particularly the silica-rich mesostasis regions. Like the melt pockets, and other factors. If the composition of mesostasis areas, the vermicular clasts consist of diopside + interstitial melt in diffusive contact with the main magma silica + plagioclase + troilite, contain a relatively large chamber reflected the overall fractionation of the bulk magma, amount of silica, generally lack chromite, and display some then melt pockets that were isolated at widely separated times degree of enrichment in K. Based on these similarities, the could greatly differ in bulk composition at the time of their vermicular clasts possibly formed from a more fractionated separation from the main magma chamber. Small degrees of portion of the same magma that produced the silica-rich partial melting caused by shock may also be partly responsible mesostasis regions in Bilanga. However, considerably more for the enrichment in incompatible elements in type 5 data would be needed to confirm this. assemblages. However, shock melting alone does not appear to fully account for this enrichment, as most type 3 Igneous or Metamorphic Origin of Type 1, 3, and 4 assemblages (which also appear to have experienced shock Assemblages melting) do not contain K or P except where they are in contact with mesostasis regions. Differences in initial composition of An alternative hypothesis for the production of type 1, 3, the mesostasis regions as compared to the other types of and 4 assemblages is that they formed by exsolution of metal, assemblages in Bilanga may also be inferred by the complete chromite, diopside, and silica from orthopyroxene followed absence of chromite within the mesostasis regions. Thus, we by a series of complex metamorphic reactions during thermal conclude that the silica-rich mesostasis regions represent a metamorphism to produce troilite and plagioclase. This more evolved fraction of the diogenite parent magma that has explanation has the advantage of providing a single been altered by later partial shock re-melting. explanation for the type 1, 3, and 4 assemblages and the simpler type 2 chromite-diopside assemblages, which do Plagioclase-Rich Clasts appear to be formed by exsolution. In Bilanga, the rimming of chromite by plagioclase in some type 3 assemblages suggests Our interpretation of the large clasts of plagioclase and of that a reaction between diopside and the spinel component in plagioclase vermicularly intergrown with diopside + silica is chromite may have produced plagioclase and silica in a few that they are exotic inclusions in the Bilanga breccia. In cases. In addition, in previous work, Mori and Takeda (1981) locations where their edges have not been deformed beyond attributed sub-micron sized precipitates of augite, chromite, recognition, they appear to be entirely separate from the troilite, and silica occurring in diogenite orthopyroxene to orthopyroxene fragments that surround them. They are also exsolution processes. They recognized the difficulty posed by much larger than the adjacent orthopyroxene breccia troilite precipitation in orthopyroxene and advanced fragments. The large, well-formed, twinned plagioclase sulfidation of Fe-Ni metal as their favored explanation, observed in both the vermicular and single plagioclase clasts although they did not completely dismiss the possibility of is different in habit from any other plagioclase observed in the epitaxial growth from a melt during crystal growth. 578 K. Domanik et al.

There are several arguments against a metamorphic geothermometer of Liermann and Ganguly (2001) to selected exsolution origin of type 1, 3, and 4 assemblages in Bilanga. paired analyses of these phases, assuming a pressure of 1 Plagioclase in type 1 and in most type 3 and 4 assemblages kbar. The average temperatures obtained from these models appears to occur randomly and is not correlated with the were 823 °C and 720 °C, respectively, which are far below proximity of chromite. In some assemblages, plagioclase igneous crystallization temperatures. These temperatures fall occurs even though chromite is entirely absent. The few within the ranges observed in diogenites by Mittlefehldt type 3 assemblages containing plagioclase rims on chromite (1994) using two-pyroxene thermometry (Lindsley and all exhibit evidence of possible shock-induced melting and Andersen 1983) and the orthopyroxene-spinel contain plagioclase that is highly variable in composition, and geothermometer of Mukherjee et al. (1990). a later shock event may have been responsible for the reaction textures observed. Exsolution and metamorphic reaction also CONCLUSIONS does not account for the observation that type 1 assemblages containing plagioclase are not distributed on defects Bilanga is a complex brecciated diogenite. This throughout the orthopyroxene grains in the same manner as orthopyroxenite contains five common types of diopside + a type 2 assemblages. Presumably, if a metamorphic reaction Fe-rich phase ± plagioclase ± silica assemblages, which between diopside and chromite produced the plagioclase in appear to be native to the rock, as well as plagioclase-rich the large type 1, 3, and 4 assemblages, it would have occurred clasts, which may be fragments of a different but related rock in the small scale type 2 inclusions as well, but this is not type. Petrographic observations of the Bilanga observed. In summary, a metamorphic origin for type 1, 3, indicate that most of the diopside and plagioclase appears to and 4 would require several complex processes to occur for have crystallized from trapped intercumulus melt. Thus, which there is little petrologic evidence. The origin of type 1, plagioclase fractionated from the parent melt of Bilanga, 3, and 4 assemblages as remnant intercumulus melt and melt along with clinopyroxene, chromite, troilite, and silica, in inclusions is more consistent with the petrographic addition to the predominant orthopyroxene. As in other observations, and we strongly favor this interpretation. adcumulate rocks, most of the material in the interstitial melt that was not compatible in the growing orthopyroxene Compound Chromite-Troilite-Metal Grains crystals was presumably displaced back into the magma chamber (cf., Morse 1994). This “rejected melt” component The textures of the large compound troilite/chromite/ could have ultimately crystallized a significant amount of metal grains in Bilanga, suggest that Fe-Ni metal was initially plagioclase, clinopyroxene, and silica elsewhere in the stable but later became unstable relative to sulfide and oxide magma chamber (possibly as rock similar to the exotic materials. The strong association between chromite, troilite, vermicular clasts in Bilanga) or, alternatively, could have and Fe-Ni metal in diogenites and similar textural been erupted to the surface. relationships between these phases have previously been In Bilanga, we have recorded cumulate and adcumulate noted by Gooley and Moore (1976), which suggests that this growth, exsolution, shock, and metamorphic re-equilibration. crystallization sequence may have occurred in other Most likely, the cumulate formed from intrusions of late stage diogenites as well. The presence of pentlandite with a magma from a magma ocean into the crust of Vesta (Righter maximum thermal stability of 610 °C (Shewman and Clark and Drake 1997). Crystallization of late stage liquids, 1969) indicates that this phase formed late in the cooling subsolidus re-equilibration, and shock melting appear to history of the rock or during subsequent thermal account for all non-cumulate textures in Bilanga. The metamorphism. The nature of the opaque phases in Bilanga magmatic and metamorphic history of small asteroidal-sized and their reactions may offer clues to the crystallization bodies 4.5 Ga ago appears to be extraordinarily complex. history of diogenites and the fO2 and fS2 conditions in the diogenite magma chamber and subsequent metamorphism Acknowledgments–This manuscript was substantially and are currently the subject of additional study. improved through the thoughtful reviews of David Mittlefehldt and Cyrena Goodrich. The authors wish to thank Orthopyroxene Geothermometry Mike Farmer for donating the Bilanga sample to the Lunar and Planetary Laboratory and Bill Boynton, David Kring, and The major element compositions of orthopyroxene, Dolores Hill for making the thin sections available for this diopside, and chromite in Bilanga have re-equilibrated during study. Discussions with David Mittlefehldt and David Kring slow cooling and thermal metamorphism and, thus, do not are appreciated. This work was supported by NASA grant preserve their initial igneous compositions. This re- NAG5–12795. equilibration can be deduced from the results of applying the clinopyroxene-orthopyroxene QUILF geothermometer of Editorial Handling—Dr. Randy Korotev Andersen et al. (1993) and the orthopyroxene-chromite Accessory silicate mineral assemblages in the Bilanga diogenite 579

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