Meteoritics & Planetary Science 38, Nr 5, 783–794 (2003) Abstract available online at http://meteoritics.org New eucrite Dar al Gani 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 meteorite 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 plagioclase, kinked lamellae, and mosaicism in pyroxenes 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 parent body and taking into account the relatively high shock stage, the excavation of DaG 872 was likely induced by a major impact event. 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 achondrites and share a common and Drake 1982; Newsom 1985) indicated that much higher provenance with howardites and diogenites (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 howardite-eucrite-diogenite (HED) suite can be, as no other reassessment of 4 Vesta’s evolution (Righter and Drake 1996, meteoritic class, almost certainly linked to a specific asteroid, 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 chondrites (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, Stannern 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 © Meteoritical Society, 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) breccias formed by impact mixing. In fact, In transmitted light, pyroxene 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 troilite. In addition, 2 burial of crustal basalts 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 crust. 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