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New Eucrite Dar Al Gani 872: Petrography, Chemical Composition, and Evolution

New Eucrite Dar Al Gani 872: Petrography, Chemical Composition, and Evolution

& Planetary Science 38, Nr 5, 783–794 (2003) Abstract available online at http://meteoritics.org

New eucrite 872: Petrography, chemical composition, and evolution

Andrea PATZER,* Dolores H. HILL, and William V. BOYNTON

Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA *Corresponding author. E-mail: [email protected] (Received 16 May 2002; revision accepted 5 March 2003)

Abstract–Dar al Gani 872 (DaG 872) is a new from Libya that we classified by means of Instrumental Neutron Activation Analysis (INAA), electron microprobe, and optical microscopy. According to our results, DaG 872 is a Mg-rich main group eucrite, i.e., a monomict noncumulate basaltic eucrite displaying a predominant coarse-grained relict subophitic and a fine-grained granulitic texture. The meteorite also shows pockets of late-stage mesostasis and is penetrated by several calcite veins due to terrestrial weathering. Finally, it exhibits shock phenomena of stage 1–2 including heavily fractured mineral components, undulose extinction of , kinked lamellae, and mosaicism in corresponding to peak pressures of ~20 GPa. In view of petrographic criteria as well as compositional and exsolution characteristics of its pyroxenes, the sample represents a metamorphic type 5 eucrite. Assuming the metamorphic type to be a function of burial depth on the and taking into account the relatively high shock stage, the excavation of DaG 872 was likely induced by a major . Prior to this point, DaG 872 apparently underwent a 4-stage geological evolution that is reflected by intricate textural and mineralogical features.

INTRODUCTION subsequent geochemical investigations. Detailed studies of siderophile trace element systematics in eucrites (Newsom Eucrites are basaltic and share a common and Drake 1982; Newsom 1985) indicated that much higher provenance with and (e.g., Tschermak degrees of melting of the source rock than previously believed 1885; Clayton and Mayeda 1996; Papike 1998). The may have been necessary and initiated a geochemical -eucrite- (HED) suite can be, as no other reassessment of ’s evolution (Righter and Drake 1996, meteoritic class, almost certainly linked to a specific , 1997). On account of the new findings, 4 Vesta appears to namely 4 Vesta. The connection is based on reflectance have sustained an early magma ocean from which equilibrium spectra as well as mineralogical and chemical evidence. crystallization occurred (see also Ruzicka et al. 1997). Dynamical simulations, impact studies, experimental As distinct from this picture, Warren and Jerde (1987) petrology, and geochemical modeling principally appear to advocate a fractional crystallization origin for most main support this hypothesis (Stolper 1977; Consolmagno and group eucrites and those of the Nuevo Laredo Trend. They Drake 1977; Drake 1979; Melosh 1984; Wetherill 1978, have based their conclusions on geochemical calculations that 1987; Cruikshank et al. 1991; Binzel and Xu 1993; Asphaug involve the mg# (molar MgO/[MgO + FeO]) and 1997; Binzel et al. 1997; Gaffey 1997; Righter and Drake incompatible element contents of eucrites (see also Delaney 1997; Drake 2001). 1986; Hewins 1987; Takeda 1997; Warren 1997). The Nuevo Based on experimental studies of Stolper (1975, 1977), Laredo Trend and main group eucrites are 2 of 3 subtypes most eucrites were originally believed to be primary partial among the monomict noncumulate or ordinary (Takeda 1997) melts of a primitive asteroid showing a composition similar to eucrites defined in dependence on their mg# and Ti content that of ordinary (see also Wänke and Palme 1974). (BVSP 1981). The Nuevo Laredo trend displays intermediate Subsequent geochemical work in connection with lithophile Ti abundances in connection with a more ferroan trace elements pursued this idea and led to a model suggesting composition, while the main group eucrites are characterized that the main group eucrites formed by 4–15% equilibrium by relatively low Ti concentrations and high mg#. The third melting while other, more evolved eucrites represent either subtype, Trend, exhibits intermediate mg# along cumulates or fractional crystallization products of that melt with high Ti values. (Consolmagno and Drake 1977; Drake 1979). This The idea of diogenites and eucrites originating from the hypothesis, however, had to be modified soon in view of same parent body and having formed through fractional

783 © , 2003. Printed in USA. 784 A. Patzer et al. crystallization and simple partial melting, respectively, has examined by optical microscope and electron microprobe. been critically illuminated by Jurewicz et al. (1997). These The micro analyzer installed at the University of Arizona authors conclude on the grounds of experimental work that, Lunar and Planetary Laboratory is a Cameca SX-50. The presuming eucrites are primary partial melt products, procedure applied for the investigation of DaG 872 has been diogenites and eucrites may have a common origin only if described by Kring et al. (1996). Details on the parameters their parent body was chemically heterogeneous (see also and techniques involved with the INAA have been given by Shearer et al. 1997). Hill et al. (1991; see also Patzer et al. 2001). Parts of the data Further details of the history of 4 Vesta have been on DaG 872 reported here have been previously published as discussed in the framework of metamorphism and impact a conference abstract (Patzer et al. 2002). heating of eucrites (e.g., Nyquist et al. 1986, 1997; Takeda and Graham 1991; Metzler et al. 1995; Bogard 1995; Hsu and RESULTS AND DISCUSSION Crozaz 1996; Yamaguchi et al. 1996, 1997a, b; Takeda et al. 1997; Arai et al. 1998). Interestingly, almost all eucritic Petrographic Description samples show signs of thermal overprinting, and many are (monomict) formed by impact mixing. In fact, In transmitted light, and plagioclase can be Bogard (1995) demonstrated that the bombardment history of easily recognized. The sample appears fresh and lacks the HED parent body equals the moon’s in complexity. One limonitic staining (owing to the virtual absence of original model developed in this context, therefore, addresses the metal) but shows numerous small calcite veins and one major phenomenon of eucritic brecciation and metamorphism by fracture filled with calcite cutting deep into the sample. In proposing a setting involving contact zones of crater walls addition, the meteorite was substantially shocked leading to and hot impact melt sheets (Nyquist et al. 1986; Metzler et al. the manifestation of undulose extinction in feldspars, heavily 1995). fractured mineral phases, kinked lamellae, and mosaicism in Another theory explains the variable degrees of many pyroxenes. In reflected light, DaG 872 displays 3 metamorphism in terms of a long-lasting global heating and opaque phases: ilmenite, chromite, and . In addition, 2 burial of crustal to different depths (Yamaguchi et al. tiny blebs of Fe, Ni-metal were observed. 1996, 1997a, b). The most recent work of Yamaguchi et al. The main portion of the thin section (UA1902) consists (2001) refines earlier conclusions and presents a 4-stage of relatively coarse-grained pyroxene and plagioclase (crystal model on the thermal history of parts of 4 Vesta’s . It sizes from about 0.1 to 1 mm) with a relict igneous, includes the initial crystallization of eucritic rocks from lava subophitic texture (Fig. 1). About one quarter of UA1902 is flows or dikes on or near the surface followed by some composed of a heterogeneous, partly crushed, and highly brecciation, burial due to overflowing lava, and a second short deformed area that appears dark gray in contrast to the light reheating event. The existence of layered intrusions and the gray colored main rock. We also observed a few finer-grained occurrence of extensive metamorphism on the HED parent zones of granulitic texture and patches of mesostasis (Fig. 2). body have also been proposed by Takeda et al. (1997). In their No sharp boundaries exist between the subophitic and view, the thermal environment of some eucrites was granulitic lithologies. Instead, they are often bordered by comparable to that of cumulate eucrites in the sense that they large plagioclase laths that appear to form a transition zone to experienced a similarly extensive period of recrystallization. Subsequent cooling, however, took place at a higher rate. This study gives a petrographic description of the new eucrite DaG 872, including the first constraints on its thermal history. The main focus, however, concerns its classification inferred from chemical data that were obtained by electron microprobe and INAA.

SAMPLE AND ANALYTICAL METHODS

DaG 872 was found in the Libyan Sahara in 2001 weighing 885 g and being completely covered with fusion crust. Macroscopically, it exhibits a fresh appearance. For our investigation, a thin section was made (UA1902) and 3 individual chips were prepared for INAA. Two of these samples (187.15 and 186.21 mg) were taken from the light Fig. 1. Secondary electron image of the coarse-grained, relict subophitic lithology in DaG 872 (dark gray: Ca-rich plagioclase, gray colored main rock, a third 45.86 mg piece was broken off light gray: pigeonite with augite lamellae). of a dark gray colored melt region. The thin section was New eucrite Dar al Gani 872 785

Fig. 3. Chemical composition of pigeonite and augite in DaG 872 and other noncumulate monomict eucrites (En = enstatite Mg2Si2O6, Fs = ferrosilite Fe2Si2O6, Hd = hedenbergite CaFeSi2O6, Di = diopside CaMgSi2O6; data of eucrites taken from Kitts and Lodders [1998]). The average composition of DaG 872 was calculated by assuming Fig. 2. Secondary electron image of the fine-grained, granulitic that 65% of the pyroxene shows pigeonitic composition and 35% lithology in DaG 872 (scale bar = 100 µm). In this particular comprises augite. The resulting mg# (42.6) is similar to the one example, the pyroxene phase is augite (light gray). Exsolution determined from bulk Fe and Mg concentrations (43.5). lamellae of pigeonite composition are rare. Plagioclase (medium gray) forms coarse laths. The dark gray areas represent SiO2-rich mesostasis. The only opaque phases (white) visible in this area are inclusions mostly show the shape of rounded blebs and are ilmenite and chromite. often arranged in clouds and chains (see Harlow and Klimentidis 1980). In addition, they occur as needles that the adjacent subophitic mineral phases. The feldspar of DaG display orientation in 2, almost perpendicular directions. 872 often still reveals a lathy shape, while the pyroxenes Apart from the large feldspar laths, we also observed small mostly display anhedral to subhedral crystals. Both minerals plagioclase crystals that were embedded in the pyroxene. In often share curvy boundaries. In the granulitic areas, irregular two cases (small grains of <50 µm), we detected almost pure grain boundaries are predominant. However, 120° triple orthoclase (Or99Ab0.4–0.5). junctions that are indicative of extensive recrystallization Within the relict subophitic framework of pyroxene and have also been found. feldspar crystals, and in addition to the fine-grained granular The pyroxene is ferroan pigeonite that displays a clasts, distinct patches of late-stage mesostasis have been homogeneous composition (En36–37Wo1.6–3.0Fs60–63; Fig. 3; found (Fig. 2; Table 1). They either occur as pockets of pure Table 1). It is penetrated by variably distributed (up to 50 µm silica or as an amalgamation of very fine-grained or apart) and wide (up to 30 µm thick) augite exsolution lamellae cryptocrystalline SiO2 and a phase that shows a pigeonitic (En30–31Wo42–44Fs25–28; Fig. 3; Table 1) sometimes showing a composition. Both phases can also be found in the matrix of herringbone texture (twinning). The pigeonite also exhibits the granular regions. The mesostasis pockets often reveal abundant inclusions like chromite and ilmenite. Embedded distinctively curved, irregular, and partly amoeboid feldspar and troilite drops are less common. Inclusion sizes boundaries. Occasionally, they exhibit tiny blebs of troilite. vary from 2 to 15 µm. In a few cases, we identified discrete, Opaque minerals have been observed in only minor <100 µm-sized augite crystals. This is particularly true for the abundances. Two of these phases, ilmenite and chromite, are granulitic fractions of the rock. The pigeonite present in the ubiquitous. Ilmenite shows a homogeneous composition, granulitic clasts reveals only rare exsolution. while chromite includes Ti-rich and Ti-poor varieties yielding Unlike the pigeonite, plagioclase of DaG 872 proved to rather heterogeneous distributions of major elements (Table 1; have a rather heterogeneous, bytownitic to anorthitic Fig. 5). Both types span an array of Usp41.8–48.6Chm33.4–35.2 composition with contents spanning a range of 79– and Usp30.8–37.1Chm38.2–35.9, respectively (note that Usp is 99 (average values are An87.5 ± 2.4Ab12.1 ± 2.3; Table 1). Zoning mol% ulvöspinel Fe2TiO4 and Chm is mol% chromite was not observed. However, twin lamellae of various widths FeCr2O4). The oxides in DaG 872 are generally associated have been identified. The chemical heterogeneity of the with pigeonite and either occur along grain boundaries feldspar may be partly due to the high abundance and adjacent to the pyroxene or grew as nm- to µm-sized crystals diversity of inclusions that we found (Fig. 4). They comprise and aggregates within it. The grain size of individual ilmenite phosphates, troilite, ilmenite, chromite, and orthopyroxene crystals typically ranges between 20–50 µm but can reach up forming individual crystals of 10–20 µm in size. Among the to 150 µm. The spinels are generally smaller and exhibit 10– ubiquitous embedded phases of only 1–2 µm (and less) are 30 µm sizes. In some cases, both phases grew epitactically in mainly pigeonite and augite as well as, occasionally, a single grain. Larger individual ilmenite crystals of up to 150 orthopyroxene and troilite. The tiny clinopyroxenitic µm as well as a single intergrowth (100 µm) of ilmenite, 786 A. Patzer et al.

Table 1. Chemical composition of mineral phases in DaG 872 (average wt%). Si-rich Pigeonitic Plagioclase Pigeonite Augite mesostasis mesostasis Chromitea, c Chromiteb, c Ilmenite

An87.5 ± 2.4 En36.3 ± 0.5 En30.1 ± 0.5 Usp31 – 37 Usp42 – 49 Ab12.1 ± 2.3 Fs61.8 ± 0.6 Wo43.3 ± 0.7 Chm38 – 36 Chm33 – 35 Or~0.5 Wo~2 Fs26.6 ± 0.7 Si 21.4 ± 0.5 23.1 ± 0.1 23.8 ± 0.2 46.4 ± 0.3 23.2 ± 0.3 0.12 ± 0.13 0.09 ± 0.09 0.08 ± 0.13 Al 18.5 ± 0.5 0.08 ± 0.03 0.30 ± 0.06 0.08 ± 0.10 0.18 ± 0.08 4.02 ± 0.36 1.86 ± 0.26 b.d.d Ca 12.6 ± 0.5 0.62 ± 0.10 14.6 ± 0.4 0.06 ± 0.06 1.74 ± 1.18 0.14 ± 0.05 0.21 ± 0.12 0.17 ± 0.11 Na 1.00 ± 0.19 b.d. 0.05 ± 0.01 b.d. 0.04 ± 0.08 b.d b.d. b.d. Mg 0.26 ± 0.38 7.11 ± 0.11 6.17 ± 0.11 0.06 ± 0.06 7.04 ± 0.67 0.23 ± 0.08 0.27 ± 0.06 0.29 ± 0.07 Fe 0.33 ± 0.17 27.8 ± 0.4 12.6 ± 0.5 0.36 ± 0.15 25.9 ± 2.0 27.6 ± 1.0 34.4 ± 1.2 34.6 ± 0.3 K 0.06 ± 0.02 b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cr b.d. b.d. 0.13 ± 0.03 b.d. b.d. 32.7 ± 1.7 21.8 ± 1.9 0.10 ± 0.15 Mn b.d. 0.87 ± 0.02 0.41 ± 0.02 b.d. 0.82 ± 0.06 0.64 ± 0.03 0.67 ± 0.02 0.74 ± 0.02 Ti b.d. b.d. 0.16 ± 0.07 b.d. b.d. 2.35 ± 0.80 8.81 ± 1.04 30.2 ± 0.6 aLow Ti modification. bHigh Ti modification. c Usp = mol% ulvöspinel Fe2TiO4; Chm mol% = chromite FeCr2O4. db.d. = below detection limits.

Fig. 4. Secondary electron image of feldspar in DaG 872. As distinct from pigeonite, plagioclase has not been thoroughly homogenized. Fig. 5. Chemical composition of spinel in DaG 872 in mol% of 2Ti, However, we did not detect primary igneous zoning that could Al, and Cr calculated on the basis of oxides and normalized to 100. account for the heterogeneous composition. Instead, we found The diagram reveals a bimodal composition of chromite in this ubiquitous, often only nm-sized inclusions to be responsible for the sample and illustrates the presence of Ti-rich and Ti-poor varieties. chemical variability. The grade of impurity of plagioclase was Both types exhibit rather heterogeneous elemental abundances. generally high. We detected inclusions of phosphate, pigeonite, augite, sulfide, ilmenite, and chromite. Pyroxenitic inclusions were small interstitial crystals, often associated with mesostasis partly arranged in chains or clouds. regions. Within the granulitic zones, troilite appears to be more homogeneously distributed. apatite, plagioclase, and mesostasis are associated with the Finally, we identified several zircons of 10–20 µm. The granular areas of DaG 872 (Fig. 6). While forming abundant presence of zircon in eucrites is not uncommon (e.g., EET inclusions in pigeonite, ilmenite and, occasionally, chromite 90020: Yamaguchi et al. 2001). A modal analysis of DaG 872 have also been detected as tiny elongated crystals within could distinguish 8 phases and quantified them as 42.7 vol% augite lamellae. pyroxene, 41.2% plagioclase, 4.0% calcite veins, 3.3% Troilite was observed as another minor opaque ilmenite, 5.9% mesostasis (Si-rich phases), 1.8% chromite, component. As distinct from the oxides, the sulfide tends to and 1.6% troilite (apatite and zircon total less than 0.5 vol%). occur in loosely arranged clusters of µm-sized droplets and Accordingly, the sample is relatively depleted in pyroxene yet New eucrite Dar al Gani 872 787

abundance of plagioclase and pyroxene is indicative of its differentiated character. The particular texture of the feldspar- pyroxene network reveals that pyroxene crystallized first and then split and drifted apart to make room for the expanding plagioclase laths (Fig. 7). These textural details invoke turbulent crystallization conditions. Subsequent to the extrusion and cooling of the DaG 872 lava flow, metamorphism took place. The high temperature event induced the thorough homogenization of the pyroxene before exsolution. The redistribution of cations (Fe, Mg, and Ca) in pyroxene most likely occurred before the formation of augite lamellae at temperatures above the two-pyroxene solvus, as the distribution of augite bands indicates no variation in Ca concentrations. The uneven width of the exsolution lamellae may be attributed to a nonsynchronous formation of augite with the coarser lamellae growing at higher temperatures than the finer ones (Boctor et al. 1994). The co-existence of two pigeonite grains as depicted in Fig. 7, however, strongly Fig. 6. This secondary electron image depicts a granulitic portion of suggests to us different crystal orientations and the two- DaG 872 that contains a relatively large, exceptional intergrowth of dimensional character of the thin section to be the simple ilmenite (white), apatite (light gray), plagioclase (medium gray), and reason for the variability in widths of augite lamellae. SiO2 (dark gray). Ilmenites of up to 150 µm in diameter are often The homogenization of pyroxene before the exsolution of found in the granulitic areas. They represent late-stage products of Ca-rich augite must have taken place at temperatures above the igneous process and tend to be associated with mesostasis. the two-pyroxene solvus. The melting temperature of eucrites has been determined to be ~1060°C (Stolper 1977), relatively rich in oxides and troilite when compared with constraining the temperature interval of the metamorphic other eucrites (e.g., Delaney et al. 1984). process to ~1000°–1060°C (Yamaguchi et al. 1996). The The dark gray portion (“melt region”) of our thin section event involved a long period of slow cooling that triggered the still displays basically a relict subophitic assembly of feldspar exsolution of augite from the pigeonite host. The thermal and pyroxene. However, a considerable part of it appears overprinting and subsequent phase of annealing may have crushed into many tiny fragments. A polycrystalline shock been due to the DaG 872 rock being exposed to hot impact melt vein with an average thickness of 100 µm is also present. melt sheets or hot impact in a contact zone, according In addition, patches of Si-rich components (mesostasis) are to the scenario described by Nyquist et al. (1986) and Metzler abundant. The plagioclase again contains innumerable et al. (1995). As an alternative, metamorphism may have inclusions, chiefly of augite and pigeonite. Like in the light taken place in a gradually growing crust and under insulating gray part of the sample, the pyroxene is mostly low in Ca regolith or impact melt and hot impact breccias as elucidated (pigeonite) but displays Ca-rich exsolution lamellae (augite) by Takeda and Graham (1991) or due to burial under and exhibits rounded inclusions of anorthitic composition. successive lava flows (Yamaguchi et al. 1996). According to Other inclusions show ilmenite and chromite compositions. the classification scheme describing the degree of Exsolution of augite in the pigeonite host proves to be even equilibration of eucritic pyroxenes by Takeda and Graham more variable than in the remainder of the meteorite. We (1991), we call DaG 872 a type 5 eucrite. could not only observe various abundances and sizes of augite Unlike the pigeonite, plagioclase in DaG 872 reveals a lamellae but also individual augite crystals and augite grains heterogeneous composition. However, igneous zoning, which showing occasional fine lamellae of pigeonite. Deformation usually accounts for the heterogeneity of plagioclase in of pyroxene, like fracturing and kinking within twinned eucrites (e.g., Yamaguchi et al. 1996, 1997a), could not be crystals, is omnipresent. Additional mineral phases in the found. Apparently, primary zoning was either never dark gray portion are some interstitial ilmenite and minor established as a result of the turbulent crystallization occurrences of troilite. conditions or thermally extinguished. In view of the relatively high metamorphic temperatures inferred from the Interpretation of Petrographic Findings homogenization of pyroxene (~1000°–1060°C), the thermal event, which equilibrated the pyroxenes, may have yielded The relict subophitic texture of DaG 872 testifies to the temperatures high and persistent enough to homogenize the igneous provenance and relatively rapid cooling of this feldspar as well. However, as documented by Morse (1984), meteorite (e.g., Mittlefehldt et al. 1998), while the high feldspar apparently reacts markedly resistant to metamorphic 788 A. Patzer et al.

temperature annealing. According to those authors, chromite starts precipitating in pyroxene and produces clouding as a consequence of shock or thermal metamorphism, provided that subsequent cooling is slow (see also Harlow and Klimentidis 1980; Mori and Takeda 1985). Prolonged annealing then forces the opaque phases to finally migrate completely out of the pyroxene and grow discrete crystals at grain boundaries. DaG 872 probably displays an incomplete, either intermediate or complex state of this process since clouding in some pyroxenes is still present while it is extant in others. Alternatively, the variable Ti contents of spinels may simply reflect their crystallization at different times during the formation of the rock (El Goresy, personal communication). Chromite crystals can grow over a broad range of temperatures and are richer in Ti the later they are disseminated. Possibly, the Ti-rich chromites in DaG 872 have been recruited from both late-stage primary Fig. 7. Secondary electron image that illustrates how pyroxene crystallization and secondary metamorphic precipitation. The crystals drifted apart while plagioclase laths kept growing. Most set of Ti-poor spinels may entirely be a primary product. likely, the movement was due to turbulent cooling conditions. It also Like many other eucrites, DaG 872 reveals granulitic displays variable widths and distribution of augite exsolution regions that display no sharp boundaries to the relict lamellae in pigeonite of DaG 872. Judging from this image that shows subophitic texture. They probably represent matrix clasts that 2 directly adjacent pigeonite crystals with very different lamellae textures, we speculate that the variation is only caused by the were introduced by an impact and brecciation event. In fact, difference in orientation of individual pigeonite grains and the two- the occurrence of fine-and coarse-grained lithologies in one dimensional character of the thin section. and the same eucrite appears to be a common feature of these (Yamaguchi et al. 2001). The granular homogenization. We, therefore, suggest that the chemical (recrystallized) character was most likely established during variability of plagioclase in DaG 872 is related to a turbulent the prolonged metamorphic period, along with the crystallization environment as well as to primary impurities homogenization of pyroxene. Possibly, the large feldspar and ubiquitous, often only nm-sized inclusions that are of laths that often frame the granular mesostasis-rich areas also primary origin, which means that they grew epitactically with recrystallized from mesostasis material or increased their size the feldspar. in response to the ongoing period of high temperatures. Among the opaque phases in eucrites, ilmenite is known DaG 872 certainly belongs to the more severely shocked as a common oxide (e.g., Delaney et al. 1984). It represents a eucrites displaying similar features as A-87272 (Yamaguchi primary late stage crystallization product owing to the et al. 1997a). The occurrence of undulose extinction in incompatibility of Ti and is, therefore, most abundant in the plagioclase, heavily fractured silicates, and kinked lamellae mesostasis. In DaG 872, the patches of mesostasis are either suggests equilibrium peak pressures of ~20 GPa and is made of pure SiO2 or a complex mixture of silica and attributable to stage 1 of shock metamorphosed basaltic pyroxene and only partly exhibit accessory troilite but never achondrites (Stöffler et al. 1986, 1988). Mosaicism in mafic ilmenite. However, we detected large (up to 150 µm) ilmenite silicates and a polycrystalline shock vein (in the dark-gray crystals as well as an intergrowth of ilmenite and other late- portion of the rock), which are also counted among the stage igneous products, including apatite, feldspar, and observed shock phenomena, are considered to be established mesostasis associated with the granulitic areas (Fig. 6). only at pressures above 20 GPa (stage 2; see also Chen et al. Within the relict subophitic portion, ilmenite seems to be 1996). Other products of this latter shock stage comprise distributed rather homogeneously. Its origin in these isotropic plagioclase and mechanical twinning in pyroxene nongranular parts of the meteorite may be related to and ilmenite. Maskelynitization of plagioclase may have metamorphism in analogue to the formation of secondary Ti- taken place, but no traces of this process are left in the thin rich chromite (see below). The rather small grain size (20–50 section studied. If isotropic plagioclase was present, it µm) of ilmenites unrelated to the granulitic, mesostasis-rich undoubtedly recrystallized in the course of the post-igneous zones is consistent with this hypothesis. metamorphic event (Ostertag and Stöffler 1982). Pyroxene As opposed to ilmenite, chromite chiefly coexists with and ilmenite in DaG 872, however, reveal no signs of pigeonite and exhibits 2 species clearly distinguished by their mechanical twinning, leading us to suggest shock stage 1–2 Ti content. Recent experiments by Arai et al. (1998) brought for this meteorite. The more severe deformation and to light that Ti-rich spinels might emerge from high alteration of the dark-gray portion of DaG 872 indicate a New eucrite Dar al Gani 872 789 significant gradient in shock energy. The formation of the still very fine crystalline shock melt vein must have taken place relatively late in the meteorite’s history, after metamorphism. We, therefore, envisage the more severely shocked dark gray area to be the result of the impact event that produced the DaG 872 . The dissemination of fine inclusions (clouding) in pyroxene and plagioclase of DaG 872 may also be partly related to shock. Harlow and Klimentidis (1980) concluded that shock induced fractures act as preferable nucleation sites of incipient chromite precipitation. We often observed especially chromite and ilmenite inclusions in pyroxene and innumerable tiny pyroxene droplets in plagioclase of DaG 872 that are arranged in chains. Mori and Takeda (1985) promote that the nucleation and growth of chromite on possibly impact- induced micro fractures in pigeonite requires extensive thermal annealing. Thus, it seems conceivable that the chromite and Fig. 8. Fe/Mn versus Co/Cr in lunar and eucritic samples (data of ilmenite inclusions in pyroxene of DaG 872 partly formed in lunar meteorites from MetBase; data of eucrites from Kitts and Lodders [1998] and MetBase). Some elemental bulk ratios like Fe/ this manner and preferentially crystallized on shock-generated Mn and Co/Cr are petrologic fingerprints and serve well to discontinuities. Analogously, we suppose that the tiny rounded discriminate rocks of different planetary origin. DaG 872 perfectly pyroxene inclusions may have precipitated on micro cracks in matches the eucritic range. impure plagioclase. Provided this hypothesis is valid, DaG 872 exhibits 2 generations of inclusions. have been taken from the literature. The REE signature of DaG 872 does not necessarily discriminate this meteorite INAA Data of DaG 872: Discrimination from Lunar from lunar samples that display a broad range of REE Meteorites and Relationship to Other Eucrites normalized abundances and patterns. But, it agrees well with that of the majority of main group eucrites (type example: In this section, we will at first focus on the chemical Juvinas; see Consolmagno and Drake 1977) and, hence, is composition of DaG 872 as gained from INAA and its consistent with the overall classification. discrimination from lunar meteorites. A specific examination The primary carriers of REE in eucrites are phosphates for chemical characteristics of Moon rocks made sense as the accounting for about 90 to 95% of observed abundances as finders of DaG 872 believed they had recovered a new lunar well as pyroxene and plagioclase (e.g., Hsu and Crozaz 1996). sample. Their assumption was, among others, based on the Phosphates, however, while significantly influencing the macroscopic resemblance of DaG 872 to DaG 626 and the whole rock signature, are very heterogeneously distributed in fact that both meteorites were located only 8 km apart. Their eucrites and were detected in only accessory amounts in our assumption also received support from the first analysis of the thin section of DaG 872. In detail, we observed one apatite meteorite’s oxygen isotopic composition (Sipiera, personal inclusion of ~10 µm in plagioclase and the exceptional communication). intergrowth of ilmenite and apatite in the granulitic region According to our INAA data (Table 2), however, DaG 872 depicted in Fig. 6. Possibly, more phosphates existed in the clearly shows typical eucritic elemental ratios (Fig. 8). specimen that we used for INAA. While phosphates in Elemental ratios including Fe/Mn and Co/Cr are diagnostic for eucrites generally reveal pronounced negative Eu anomalies different rock types, meaning they represent characteristic accompanied by slight enrichments in LREE, the pattern of fingerprints of planetary bodies and have been applied to plagioclase is usually characterized by a considerable confirm the classification of lunar rocks (e.g., Warren and overabundance of Eu. Pyroxenes yield negative relative Eu Kallemeyn 1989, 1993; Lindstrom et al. 1994; see also Papike concentrations but, as opposed to the phosphates, small 1998). In the case of DaG 872, they provide the opposite enrichments in HREE (Hsu and Crozaz 1996). In DaG 872, evidence and clearly confirm its genetic relation to the eucrites. the presence of phosphates certainly influenced the total REE The rare earth elements (REE) in DaG 872 also support a budget significantly and, in combination with the REE grouping with the eucrites. We found a typical eucritic pattern inventories of pyroxene and plagioclase, led to a flat, elevated with a flat slope and displaying abundances of about 10 to 15 pattern. × CI (Fig. 9). The relatively high concentrations of the REE From their investigation of REE in noncumulate eucrites, correspond to the expected signature of a differentiated Hsu and Crozaz (1996) concluded that these meteorites were . An anomalous trend for Eu is not detectable derived from a chondritic source. This was confirmed by the (within uncertainties). Data of other eucrites plotted in Fig. 9 model developed by Stolper (1977). Hsu and Crozaz (1996) 790 A. Patzer et al.

Table 2. Elemental concentrations of DaG 872 and of soil at the Dar al Gani site in Libya.a DaG 872 main rock DaG 872 melt region Soilb 187.1 mg 186.2 mg 45.86 mg Na 3370 3360 3420 2920 Mg 4.60 4.70 4.70 – Al 5.85 6.04 7.02 – K 498 451 534 9090 Ca 6.61 6.78 7.08 7.00 Sc 27.6 30.7 29.1 5.6 Ti 4500 3900 b.d. – V 67.0 63.0 64.0 – Cr 1921 2166 1969 – Mn 4065 3965 3995 – Fe 13.3 15.08 13.96 1.6 Co 5.13 5.53 6.65 – As 0.423 0.408 0.35 3.45 Br 0.35 0.36 0.61 0.92 Sr 105 113 130 130 Zr 63.0 52 72.0 – Ba b.d. 84.0 102 320 La 2.81 2.78 3.50 21.6 Ce 6.50 7.00 8.50 – Nd 5.10 6.00 6.80 – Sm 1.554 1.825 2.182 – Eu 0.576 0.641 0.618 – Tb 0.46 0.476 0.58 – Ho 0.72 0.491 0.65 – Tm 0.25 0.27 0.28 – Yb 1.81 1.932 2.06 2.14 Lu 0.266 0.30 0.291 – Hf 1.27 1.32 1.54 – Ta 0.243 0.188 0.26 – Au b.d. 0.001 b.d. – Th 0.306 0.338 0.45 7.55 U 0.237 0.171 0.24 2.36 aConcentrations are in µg/g except for Mg, Al, Fe, Ca (%). Uncertainties are 1–5% for Sc, Fe, Cr, Sm, Na, La, Ca, Al, Eu, Mn, Co, Hf, K, Lu, and Yb; 5–10% for Ce, V, Th, Sr, Dy, Mg, As, Br, Tb, Ho (except for “melt:” 17%), and Nd (except for “melt:” 22%); 10–20% for Zr, Tm, Ta, U, and Ti. bData from Dreibus et al. (2000, 2001). agree with Stolper (1977) that the Stannern Trend and main significantly contributed to the high abundances of these group eucrites may have formed by different degrees of nuclides. Hf, in close association with Zr, is mainly partial melting. (Stolper [1977] also suggested that, in concentrated in the tiny zircon crystals we detected. The contrast to main group eucrites and the Stannern Trend, the elevated Ti content in DaG 872 can be chiefly attributed to Nuevo Laredo Trend eucrites were produced by a fractional ilmenite that accounts for ~ 3.3 vol% of the meteorite, while crystallization process.) The REE signature we determined Mn represents an element commonly harbored by mafic for DaG 872 principally follows the trend of the main group silicates (pyroxene). eucrites at generally high normalized abundances (~12–15 × The lithophile nuclide Mg and especially siderophile CI) and appears to fit into the partial melting picture drawn by elements like Co typically display depleted concentrations. Hsu and Crozaz (1996). The relatively strong depletion of Co (~0.01 × CI) is Like the REE signature, the overall distribution pattern of consistent with the virtual lack of metallic phases and goes the 32 elements determined for DaG 872 (Table 2) meets the along with low concentrations of Re, Os, and Ir that are, in trends observed in other eucrites (Fig. 10). In detail, and as in fact, undetectable in DaG 872. Owing to their pronounced other eucritic samples, we see relative enrichments of ~10 to siderophile tendency, Re, Os, and Ir accumulate in the solid 20 × CI for the lithophiles Al, Ca, Sc, the REE, Hf, and Ti. Mn metal phase, which is basically extant in DaG 872, during is present in a concentration of ~2.5 × CI. Al, Ca, and the REE igneous fractionation (Rambaldi and Cendales 1980). Co are elements typically accommodated by plagioclase, which behaves less siderophile than Re, Os, and Ir but still follows comprises over 40 vol% of the rock and, by inference, the same geochemical trend. New eucrite Dar al Gani 872 791

Unlike other monomict eucrites, DaG 872 turns out to contain relatively high amounts of Ba, U, Br, and As (Table 2). Arsenic, for instance, is known to be preferentially accommodated by the metal phase of meteorites (97% of As in L chondrites are associated with the metal phase: Li [2000]) and, therefore, should exhibit a clearly subchondritic abundance. The most straightforward explanation for the discrepancies is alteration. The occurrence of ~4 vol% (secondary) calcite testifies to the influence weathering has taken on the rock. With respect to elemental concentrations, the CI-normalized La/U ratio of only ~0.4 reliably indicates terrestrial contamination (Dreibus, personal communication). For monomict eucrite falls like Lakangaon and Sioux County, this ratio plots close to 1. The anomalies detected for the trace elements Ba, U, Br, and As can be directly related to the high Fig. 9. REE pattern of DaG 872 in comparison to the signatures of contents of these elements in soil at the Dar al Gani site in Libya other eucrites (data taken from MetBase). In principal, DaG 872 follows the flat trend shown by the so-called main group eucrites. (Table 2; Dreibus et al. 2000; see also Dreibus et al. 2001).

One characteristic feature of noncumulate eucrites is CONCLUSIONS relatively low molar Mg/(Mg + Fe) ratios, while cumulate eucrites show distinctly higher mg# (BVSP 1981). When Overall, we group DaG 872 with the monomict plotting the mg# as a function of the incompatible element Sm noncumulate basaltic (ordinary) eucrites displaying a (in µg/g) (Fig. 11), the data point of DaG 872 obviously falls predominant coarse-grained, relict subophitic and a fine- close to other main group eucrites, which are supposed to grained, granulitic lithology. The relict subophitic texture, scatter around the convergence point of the Stannern and like that observed for most eucrites, implies relatively rapid Nuevo Laredo Trends. Within the range of the depicted mg#, cooling from a melt. The granulitic areas are unlikely to have DaG 872 apparently represents the Mg-richest main group formed in situ but probably represent matrix clasts introduced eucrite plotting just beyond the high end of mg# (~0.425) of by impact brecciation. Both textures in DaG 872 have been the trend discussed by Warren (1997). According to the data subsequently altered by thermal overprinting and impact of Warren (1997), most noncumulate eucrites show deformation. According to petrographic criteria, as well as intermediate mg# between ~0.36 and ~0.40. His model, compositional and exsolution characteristics of its pyroxenes, NERD (noncumulate eucrites as residua of diogenites), is the sample represents a metamorphic type 5 eucrite, which, in consistent with these observations. It predicts high mg# turn, has been suggested to be a function of burial depth on (>0.50) only for cumulate eucrites. Pomozdino, representing the parent body (Yamaguchi et al. 1996). Metamorphic a unique case, has been classified as an anomalous, high Mg temperatures reached at least ~1000°C. As distinct from all yet REE-rich eucrite (Warren et al. 1990) that may contain an other noncumulate eucrites known so far, DaG 872 turned out additional cumulate component (Hsu and Crozaz 1997; to be slightly richer in Mg. Warren 1997; Kitts and Lodders 1998). The relatively high Taking into account the various textural and mg# in DaG 872 (0.433), however, compares significantly mineralogical features of DaG 872, as well as its relatively better to the mg# of other noncumulate eucrites than to that of high shock stage, the evolution of this meteorite was Pomozdino (0.485) and, therefore, suggests that DaG 872 is a apparently complex. We see evidence for 4 main stages, noncumulate eucrite with a relatively high Mg content. similar to the evolutionary history of most eucrites. Our In the opinion of Warren and Jerde (1987), Fig. 11 hypothesis on the formation of DaG 872 supports models that implies that DaG 872 (as a main group eucrite) was generated invoke extensive metamorphism taking place within the crust from a residual liquid in the course of a fractional of the eucrite parent body (e.g., Nyquist et al. 1986; Metzler et crystallization process, i.e., was not derived from a primary al. 1995; Yamaguchi et al. 1996; Takeda et al. 1997): melt (see also Jurewicz et al. 1997; Ruzicka et al. 1997; 1. Extrusion on the eucrite parent body and relatively rapid Shearer et al. 1997; Takeda 1997; Warren 1997). Warren et al. crystallization led to the manifestation of a subophitic (1990) favor the same hypothesis, based on their investigation texture in DaG 872. A first generation of inclusions, of Pomozdino. They advocate that Pomozdino formed as a namely feldspar, formed in pyroxenes while phosphates, partial cumulate from a primary parent melt with relatively sulfides, and other embedded phases epitactically grew low mg# (Stannern type). The main group eucrites and the with the feldspar laths. Nuevo Laredo Trend are also derivates of a fractional 2. Impact(s) into the crust of the HED asteroid at the DaG crystallization process. 872 site caused brecciation and deformation. The latter 792 A. Patzer et al.

Fig. 10. Elemental abundance patterns of DaG 872 and other eucrites (data taken from MetBase; see also Kitts and Lodders 1998). “DaG 872 main rock” refers to the light gray portion of the sample we analyzed, while “DaG 872 melt region” shows a dark gray color in hand specimen and a crushed, highly shocked texture.

Fig. 11. Illustration of the bulk Sm content dependant on mg# (molar Mg/[Mg + Fe] calculated from bulk Fe and Mg concentrations). Basically, two trends are represented among eucrites: the Nuevo Laredo trend (fractional crystallization) and the Stannern trend (partial melting) (Warren and Jerde 1987; data of eucrites taken from MetBase and Kitts and Lodders [1998]). The main group eucrites scatter around the convergence point of both trends. DaG 872 plots close to this latter group but reveals a slightly higher mg#. The higher bulk concentration of Mg is probably due to a lower degree of oxidation and demonstrates that this parameter varied among the main group eucrites.

effect paved the way for the precipitation of a second homogenization of the pigeonite host, which was generation of dusty chromite and ilmenite inclusions in followed by the development of augite exsolution pyroxene and possibly pigeonite inclusions in lamellae and recrystallization of mesostasis pockets, as plagioclase (stage 3). well as matrix clasts, as cooling slowly proceeded. In 3. As a consequence of ongoing extrusions and impacts, addition, this phase of extensive annealing yielded the the source rock layer of DaG 872 was gradually buried required geological environment for the precipitation of under subsequent lava flows and/or regolith breccias tiny inclusions (clouding) within pyroxene, their and/or impact melt sheets and experienced reheating. migration towards the crystal rims, and finally, the The elevation of temperatures just above the pigeonite/ growth of individual chromites and ilmenites on grain augite solvus (~1000°–1060°C) initiated the boundaries. New eucrite Dar al Gani 872 793

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