Element partitioning between coexisting carbonate and silicate liquids

A number of experimental studies have shown that the first melt of carbonated mantle peridotite Is carbonatitic in character. This first melt will contain a sizeable portion of the mantle budget for Incompatible trace elements and, owing to its physical properties, it is likely to be very mobile. If no Anther melting occurs, the carbonatitic melt will migrate through the mantle, scavenging incompatible elements, before freezing in the lithosphere. Alternatively, melting can progress, resulting In the incorporation of this precursor melt In more voluminous silicate melt. In any case, carbonate melt silicate mantle equilibria are likely to influence the trace-element characteristics of material produced. One way of constraining the geochemical characteristics of mantle-derived, carbonate melt is by investigation of the partitioning behaviour between coexisting carbonate and silicate liquids for trace elements of Interest. This has been achieved by a series of experimental runs under a range of temperatures and pressures (0.2-2.0 OPa, 1100-1400°C), involving both natural rock compositions (phonolites, nephelinites and melilitites), and synthetic charges in the system Si02-Al203-CaO-Na20-0O2. Whilst showing that carbonate and silicate melts are likely to be completely miscible under mantle conditions, these experiments have also shown that a wide range of Ca-Mg-Na-Fe carbonate liquids can be experimentally produced by liquid immiscibility at crustal pressures. This range incorporates the vast majority of carbonatite compositions seen at the Earth's surface. Coexisting carbonate and silicate liquids in the experimental charges produced were characterised by electron probe microanalysis. Melt compositions were found to be accurately reproducible, and subject to small but consistent variations with pressure and temperature. The charges were studied optically to ensure that the coexisting liquids were in equilibrium, to investigate quenching processes occurring in the liquids on cooling, and to analyse the two-liquid textures obtained. Carbonate and silicate liquids produced in these experiments were analysed for a range of trace elements using ion microprobe techniques. The trace elements studied include examples of large-ion lithophile (LIL), high field-strength (LIPS), and rare-earth elements (REE). The carbonate liquid was found to be enriched relative to the silicate liquid in LILE (e.g. Rb, Cs, Ba2 , S, Pb and 1,REE3+) and in extreme HFSE (e.g. M0 6 , P5k, V5 and Nb5 ). The silicate liquid was most strongly enriched in ions of moderate size and charge (e.g. Ti", S& A13 , Th4 , and Hø+). Variations in pressure, temperature, and composition have little effect on partitioning in the synthetic experiments, but seem to exert more influence on partitioning in experiments on natural rock compositions. The major chemical controls on partitioning are Ionic size, and ionic charge. In general, partitioning of an ion can be predicted from these characteristics. Exceptions to this rule include the ion pairs U 4 Th4 , Zr41. Hø, and Nb-Ta5 whose partitioning is significantly different despite their similar ionic radii. Partition coefficients (L 1'-'D1) obtained from these experimentally-produced liquids give an insight into the geochemical differences between carbonate and silicate melts, and the expected trace-element characteristics resulting from carbonate involvement. Mantle carbonate liquid is expected to have high ratios of LILE/HFSE, extreme LREWHREE enrichment, low uIVU, high Zr/Hf, high Nb)Ta and high concentrations of P t V and Mo. With time, carbonate liquid, or material influenced by carbonate involvement, will also evolve distinctive Sr-, Nd-, and Pb-isotopic characteristics. These trace-element and isotopic predictions are tested against geochemical data from a wide variety of mantle-derived material in an attempt to constrain the role of carbonate liquid in mantle petrogenesis.