The oxygen isotope composition of dark inclusions in HEDs, ordinary and carbonaceous chondrites

Introduction: Dark inclusions (DIs) are lithic fragments that form a volumetrically small, but important, component in carbonaceous chondrites [1,2]. Carbonaceous clasts similar to DIs are also found in some ordinary chondrites and HEDs [3,4]. DIs are of particular interest because they provide a record of nebular and planetary processes distinct from that of their host meteorite [1,5]. DIs may be representative of the material that delivered water and other volatiles to early Earth as a late veneer [6]. Here we focus on the oxygen isotopic composition of DIs in a variety of settings with the aim of understanding their formational history and relationship to the enclosing host meteorite. Materials and methods: DIs and related materials were obtained from the following meteorites: CV3s (Allende, NWA 2140, NWA 2364): Samples from Allende cover all categories of the four-fold classification scheme [2] (inclusion numbers analyzed in brackets). Type A clasts (1a1, 4b1, 25s1-TW1) contain chondrules, inclusions and matrix, but are somewhat finer grained than normal Allende material. Type A/B clasts (MZB) are transitional between Types A and B (Fig. 1). Type B clasts (12b1) contain opaque matrix and olivine-rich aggregates and may have experienced a hydration-dehydration cycle [2]. Type C clasts (5a1, ekpb4b1, MZ15, USNM 3876) consist of fine-grained, opaque material similar to Allende matrix. Full descriptions for of the Allende DI samples are given in [2]. DI material from NWA 2140 analyzed for this study is Type A/B and Type A for NWA 2364. HEDs: (Bholghati, PRA 04401, SCO 06040). DIs from the howardite Bholghati have not been analyzed by us for oxygen isotopes. Instead a sequence of 5-10 mg representative whole rock samples have been run to assess its carbonaceous chondrite content. PRA 04401 is an extremely coarse-grained howardite containing a high concentration (~40-50% in places) of nearly cm-sized angular carbonaceous clasts that texturally resemble CM2 material [7]. SCO 06040 is also a coarse-grained howardite breccia, contains a lower amount (~10%) of rounded, up to 2mm diameter, CM2-like clasts [7]. OCs: Sharps (H3.4) is a fragmental breccia containing accessory carbonaceous clasts up to 1 cm in diameter [8]. Oxygen isotope analysis was performed by infrared laser-assisted fluorination [9]. All analyses were obtained on untreated whole rock samples (0.5-2 mg). System precision, as determined on an internal obsidian standard is: ±0.05‰ for δ 17 O; ±0.09‰ for δ 18 O; ±0.02‰ for  17 O (2σ).