Interpreting the Carbon Isotopic Composition of Ureilites

Ureilites and carbonaceous chondrites: The origin of ureilite meteorites has been consistently regarded as problematic [1]. A distinctive feature of ureilites is that they contain carbonaceous matter up to 6 wt. %, particularly in the form of graphite in veinlets between silicate (mostly olivine) crystals, and along their cleavage planes. The carbonaceous matter within them suggests a relationship with carbonaceous chondrites, so that a range of models developed in which carbonaceous chondrites were either a precursor to the ureilite parent body or had impacted into the parent body [2, 3, 4]. Current models still assume at least partial derivation of ureilites from carbonaceous chondrite-like material [5, 6, 7]. Support for this lies in trace element composition [8], oxygen isotope composition [9], noble gas composition [10] and silicate petrography [11]. However, a perceived problem with this origin is the carbon isotopic composition of ureilites, which is mostly in the range δCPDB –10 to –2 ‰ [12, 13], while the composition of carbonaceous chondrites is typically –21 to –11 ‰ [14]. This difference has been interpreted to indicate that ureilites could not have been derived from carbonaceous chondrites [12]. We re-evaluate this by study of a terrestrial analogue for derivation of vein graphite from a carbonaceous precursor. Analogue for ureilite carbon: For an analogue study, the requirement was a setting where a hot body of magma interacted with carbon-rich rocks at a geologically rapid rate. Our choice is a set of Ordovician gabbroic intrusions in Neoproterozoic graphitic pelites in northeast Scotland. The intrusions are the so-called ‘Newer Gabbros’ [15]. The pelites are the Easdale Subgroup of the Dalradian Supergroup [16]. This is a good analogue because of numerous similarities to what is observed in ureilites: 1. The intrusions have ultramafic to basic mineralogy, from peridotites (olivine cumulates) to troctolites (olivine-plagioclase cumulates) to gabbros (pyroxenefeldspar cumulates) [15]. 2. Partial melting occurred in the pelitic country rocks, evident in both the aureoles of the intrusions and in xenoliths within the intrusions [17]. 3. Intermingling occurred between the intrusions and the country rocks. 4. A mobile carbonaceous phase was generated from the country rock to precipitate graphite in fractures. 5. The rocks containing fracture-bound graphite have a carbon content greater than the source rocks. 6. The carbonaceous matter in the country rocks was already refractory (graphitic) at the time of intrusion, as the intrusions overprinted a pre-existing metamorphic mineralogy [18]. 7. The temperature reached up to 850 ̊C in the inner aureole [19]. This approaches the temperatures interpreted to explain the reduction features in ureilites [6, 20]. The pressure in the aureole was up to 5000 bars, substantially higher than in the ureilites, but in both cases the fluid pressure of the graphiteprecipitating fluid exceeded the confining pressure. Data: Vein graphite occurs within the magmatic rocks at several localities close to the boundary between the magmatic bodies and their country rocks (Fig. 1). Graphite was sampled at four of these localities for determination of carbon isotopic composition. Samples were also taken from the country rock pelites at a coastal exposure at Portsoy (Fig. 1): Vein graphite Knock -18.2 ‰ Rothiemay -15.5 ‰ Huntley -14.1 ‰ Bodiebae -19.1 ‰ Graphitic pelites Portsoy A -26.4 ‰ Portsoy B -26.6 ‰ Portsoy C -26.5 ‰