CARBONATE GRAINS IN ANHYDROUS IDPs

Introduction: Carbonate is a minor phase in hydrous meteorites and hydrous interplanetary dust particles (IDPs), where it is believed to have formed by aqueous processing on asteroidal parent bodies [1]. Carbonate has been reported in the dust shells around evolved stars [2] and in other interstellar and circumstel-lar environments [3] where liquid water is unlikely to have been present. Toppani et al. [4] demonstrated that carbonates can be formed in these environments by condensation from a H 2 O-CO 2-rich vapour. However, carbonate formation is not predicted by equilibrium models of grain condensation in the cooling of a nebula of Solar composition [5], thus carbonate in our Solar System is expected only as a product of later aqueous alteration. We previously reported carbonate in an anhydrous IDP [6] but were not able to establish that the carbonate was indigenous. Carbonate Search: The chondritic porous (CP) IDPs are the most primitive, i.e. least altered by aqueous or thermal processing , extraterrestrial materials available for laboratory analysis [7]. They are believed to preserve the original building blocks of the Solar System. We used the Scanning Transmission X-ray Microscope on Beamline X1A at the National Synchrotron Light Source to map, with an ~35 nm beamspot, ultramicrome sections for grains exhibiting the CO 3 absorption feature of carbonate, and identified rare, sub-micron carbonate grains in five CP IDPs. Results: Individual grains in CP IDPs are generally not in direct contact. Each grain is coated with an ~50 to 200 nm thick layer of organic matter. This structure indicates a three-step sequence for formation of primitive dust. First, individual grains condensed, then the grain surfaces were coated by a refractory organic, and finally the grains collided forming the first dust of the Solar System, the CP IDPs. The grains that acquired this organic coating are the earliest grains present in the Solar Nebula. In two cases the carbonate grains were large enough that we could obtain good carbon X-ray Absorption Near-Edge Structure (XANES) spectra of the coating. These carbonate grains are coated with organic matter that gives a carbon XANES spectrum indistinguishable from the other grains in the IDP. Conclusions: Our results indicate some carbonate was present in the Solar Nebula before the era when the grains collided, forming the initial dust aggregates, the CP IDPs. Thus, some car-bonate grains formed prior to parent-body aqueous processing, which is consistent with non-equilibrium grain condensation in the …