Thermal History and Physics of Melt Extraction on the Ureilite Parent Body

Introduction: Ureilites are fragments of the mantle of a small (~100 km radius) asteroid that underwent ~30% partial melting at depths corresponding to pressures in the range 30-125 bars [1,2]. The preservation of oxygen isotopic heterogeneity on the ureilite parent body (UPB), despite high-T igneous processing [1], suggests that melts were extracted very rapidly. To determine if this was possible we modelled the thermal history of the UPB, and the melt extraction process. Assumptions: The bulk composition of the silicate part of the UPB is assumed similar to that of CV chondrites [3]. This, plus the accretion time, determined the initial amount of Al. The other factors determining the thermal history are the accretion temperature and initial ice content, if any. The inferred amount of partial melting requires the UPB to have accreted soon after CAI time, and uncertainties in amounts of radioactives present other than Al have minimal influence on the time variation of the melt production rate. Heating process: Starting from a given formation time, accretion temperature, and initial ice content of the UPB, we find the amount of heat released by the decay of all radioactive species present in each of a series of small time steps. The available heat is used to raise the temperature, taking account of any latent heat involved in phase changes. In general seven stages are involved: heating of rock and ice to 273.15 K; melting of ice at 273.15 K while the latent heat of 330 kJ kg is supplied; heating of unreacted water and silicate reaction products while hydration occurs between 273.15 K and 300 K; heating of silicates until dehydration begins at 530 K; heating of dehydration products until dehydration ends at 623 K; heating of dehydration products until the onset of silicate melting at 1333 K; heating of unmelted silicates as melting progresses. During hydration (273.15 to 300 K) and dehydration (530 to 623 K), the latent heat of reaction (249 kJ kg) is added to and subtracted from, respectively, the heat released by radioactives to find the temperature rise, and it is assumed that latent heat transfer occurs uniformly in the relevant temperature interval. During silicate melting the latent heat (taken as 400 kJ kg) is absorbed uniformly across the melting temperature range. Converting available heat into temperature rise involves the specific heats at constant volume of the materials (ice, water and/or silicates) present. Both the water vapor released during dehydration and the silicate melt produced during melting would have escaped very efficiently through fractures to the surface due to the large pressures involved in their formation [4, 5] and do not contribute to the thermal budget after their formation. This is particularly important in the case of the melt, since most of the Al in the rock is contained in plagioclase, a mineral which is completely melted. Hence the main heat source is effectively removed from the asteroid interior. The sensitivity of the melting history to the input parameters is shown in Figs. 1 and 2, and Fig. 2 is used to compute the melt production rate as a function of time in Fig. 3: initial rates of nearly 40 m/s decline rapidly to less than a few m/s.