REACTIONS BETWEEN ECLOGITE AND PERIDOTITE - MANTLE REFERTILIZATION BYSUBDUCTION OF OCEANIC-CRUST

An experimental approach has been used to explore and constrain recent suggestions that the incorporation of previously subducted eclogite i n upwelling mantle plumes results in distinctive magmatic provinces of high melt volume and high silica and iron content. We have studied th e melting behaviour of coesite eclogite, crystallised from average alt ered oceanic crust composition (GA1), at a pressure of 3.5 GPa. This p ressure was chosen as representing much thicker lithosphere than in mi d-ocean ridge petrogenesis, and conditions well within the eclogite st ability field. The coesite eclogite solidus is at T less than or equal to 1250 degrees C and its liquidus is at T approximate to 1500 degree s C, compared with the MORE pyrolite (representing modern upper mantle ) solidus at approximate to 1550 degrees C. Low degree partial melts a re dacitic (> 63 wt% SiO2, > 7 wt% alkalies) and co-exist with coesite eclogite residue. Higher temperature melts (T greater than or equal t o 1350 degrees C) move along the garnet-clinopyroxene cotectic, remain ing andesitic to basaltic andesitic in composition and producing liqui dus phases which converge towards compositions typical of garnet and c linopyroxene of mantle peridotite. Interactions between siliceous part ial melts of eclogite and peridotite were investigated by sandwiching a layer of GA1 against MORE pyrolite and conducting experiments at 3.5 GPa between the coesite eclogite solidus and the pyrolite melting int erval. Experiments at 1200-1400 degrees C became melt free; the silice ous melt from the eclogite layer producing orthopyroxene enrichment in neighbouring Iherzolite and leaving sub-solidus refractory garnet and clinopyroxene in the former eclogite layer. At higher temperatures (g reater than or equal to 1425 degrees C) a nepheline-normative picritic melt is formed, representing melting at the olivine-orthopyroxene-cli nopyroxene-garnet minimum in an enriched Iherzolite composition. At hi gh pressures, a thermal divide represented by pyrope-rich garnet, omph acite and orthopyroxene, divides the SiO2-rich liquids from eclogite s ources, from nepheline-normative picritic liquids in alkali-rich, lher zolitic sources. Liquids cannot mix through this thermal divide. The p rocess of re-fertilisation of mantle by admixing of subducted crust is achieved by ephemeral melts from eclogite sources effecting local min eralogical change, particularly in orthopyroxene : olivine ratio. Resi dual phases from eclogite and metasomatised phases in Iherzolite conve rge in composition, but modal inhomogeneity (bands, lenses and schlier en) is enhanced. Heating or adiabatic upwelling of this modally hetero geneous mantle produces nepheline-normative picritic liquids at the so lidus, or tholeiitic picritic liquids at higher temperatures. Such liq uids will be marginally higher than normal MORE in Fe/Mg at near solid us conditions, but are silica-poor and olivine-rich. Trace and minor e lement, and isotopic characteristics will reflect the pre-history of t he admired materials, the time-integrated variations reflecting modal variations and the proportions of the admired material and '' normal m antle ''.