Abstract: The current Solar System architecture is aheritage of the protoplanetary disk that surrounded the young
Sun, 4.56 Gy ago. Primitive extraterrestr ial objects provide means to trace back the primordial composi-
tion and radial distribution of matter in this disk. Here, we present acombined micro-IR, Raman, chem-
ical and isotopic study of two ultracarbonaceous micrometeorites recovered from Antar ctica (UCAMMs).
This study reveals particles containing an unusually high nitrogen- and deuterium-rich organic matter
analogous to a polyaromatic hydrogenated carbon nitride, characterized by nitrogen concentration with
bulk atomic N/C ratios of 0.05 and 0.12 (locallyexceeding 0.15).We propose that such nitrogen-rich car-
bonaceous material can be formed by energetic irradiations of nitrogen-rich ices in very low temperature
regions of the Solar System. Such conditions are encountered at the surface of small objects beyond the
trans-neptunian region. UCAMMs provide unique insights on physico-chemical processes that occurred
beyond the nitrogen snow-line, revealing orga nic material from the extreme outer regions of the Solar
System that cannot be investigated by remote sensing methods.

Abstract: Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0{middle dot}3-0{middle dot}8) and high forsterite contents in olivine (averages > 91{middle dot}5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.