Experimentally determined phase and melting relations of carbonate eclogite from 9 to 20 GPa

G.M. Yaxley1, E. Kiseeva1,2, K. Litasov2

1.Research School of Earth Sciences, The Australian National University 2. Earth and Planetary Material Physics Research Group, Tohoku University

There is now general acceptance that carbonate in altered oceanic basalt can survive subduction into the deeper mantle. Recently, a number of studies have used high-pressure experimental techniques to examine the petrological and melting behaviour of the high pressure forms of this subducted material, carbonate eclogite, up to 10 GPa.
       We now have preliminary results of an investigation from 9 – 20 GPa, of the phase and melting relations of a natural, carbonated (but anhydrous) average, altered MORB composition (GA1) to which 10 wt% calcite was added. GA1 contains 3.5 wt% Na2O and 0.4 wt% K2O. At 9 GPa, the sub-solidus assemblage was garnet + clinopyroxene (cpx) + rutile + magnesite. The solidus was located at ≈1200°C, and with increasing temperature a carbonatitic melt formed. At 13 GPa the subsolidus assemblage is garnet + cpx + stishovite + magnesite. The solidus is between 1200 and 1300°C and is again marked by the appearance of carbonatite melt. At 17 GPa the subsolidus assemblage is garnet + calcite + magnesite + K-hollandite + accessory stishovite, and again the solidus lies between 1200 and 1300°C. Cpx is absent at P≥17 GPa due to complete dissolution in garnet to form majoritic garnet. At 20 GPa, the experiments contain abundant majoritic garnet + Ca-rich carbonate + K-hollandite + Ca perovskite ± stishovite. The solidus lies between 1200 and 1300°C. Garnets crystallised at all pressures are sodium and phosphorous-rich, with Na content increasing systematically with pressure. Low pressure melts are generally low in Na/K, whereas high pressure melts have high Na/K values.
       A striking feature of the new data is the relatively constant solidus temperature over a very large pressure interval. This may partly relate to changes in the relative incompatibility of Na2O and K2O with increasing pressure. For example, in the lower pressure runs, Na behaves compatibly due to the relative stability of the jadeite component in clinopyroxene. With increasing pressure progressive dissolution of clinopyroxene into majoritic garnet may lead to Na becoming incompatible and it may flux melting at a sodic carbonate solidus.
       Nevertheless, this solidus lies well above estimated subducted slab geotherms and well below the temperatures of estimated mantle adiabats. The depth at which deeply subducted, carbonate-bearing, average MORB begins to melt to yield carbonatitic liquids will therefore depend on the rate of subduction and the rate at which the relatively cold slab heats up conductively due to surrounding hotter ambient mantle. If carbonatitic partial melts of carbonate eclogite form and segregate from the slab material, they may metasomatise the surrounding peridotite mantle and may form sources suitable for yielding deeply derived, geochemically enriched magmas such as kimberlites and related rocks.