Peridotite melting at 1 center dot 0 and 1 center dot 5 GPa: an experimental evaluation of techniques using diamond aggregates and mineral mixes for determination of near-solidus melts

The experimental determination of liquid compositions in lherzolite as func tions of pressure and temperature provides constraints on mantle dynamics a nd magma genesis. In this paper, we present a detailed evaluation of the us e of natural mineral mixes as starting material in peridotite melting studi es at 1.0 GPa. As an example we have chosen to test the data obtained by Ba ker & Stolper (1994, Geochimica et Cosmochimica Acta 58, 2811-2827) on a lh erzolite composition (MM-3) presented as a potential source for mid-ocean r idge basalts (MORB). That study is the most fully documented published melt ing study using natural mineral mixes. We have tested the Baker & Stolper d ata in three ways: (1) we have defined the liquidus phases and conditions o f the partial melt compositions obtained by Baker & Stolper; (2) we have re acted these pal tial melt compositions with a fine-grained synthetic starti ng mix of MM-3 composition; (3) we have performed additional melting experi ments at 1.0 and 1.5 GPa using the synthetic mix of peridotite MM-3. Our re sults demonstrate that only the highest temperature experiment of Baker & S tolper, performed at 1390 degrees C, approached an equilibrium melt of peri dotite MM-3 composition and that lower temperature experiments have not rea ched equilibrium, retaining residual unreacted minerals and metastable melt compositions. The degree of disequilibrium increases progressively with lo wer temperature. Disequilibrium is attributed to the lack of reaction of th e natural mineral mix and to disequilibrium melting reactions of the metast able, relatively coarse-grained mineral mix. Other contributing factors inc lude disequilibrium caused by the use of a diamond aggregate trap. We also present peridotite melting experiments using the mineral mix KLB-1 at 1.0 G Pa. Our results demonstrate that the mineral mix KLB-1 fails to equilibrate even after similar to 340 h at temperatures of 1280-1300 degrees C. We pre sent reversals of the 1.0 GPa peridotite melting experiments of Hirose & Ku shiro (1993, Earth and Planetary Science Letters 114, 477-489). Our reversa ls demonstrate that the mineral mix-diamond aggregate trap technique used b y Hirose & Kushiro has also failed to produce equilibrium melts of a mantle peridotite composition. It is recommended that data from peridotite meltin g studies utilizing natural mineral mi,res be used with reservation and tha t natural mineral mixes are not a suitable starting material for such studi es. The use of diamond aggregate for separation and trapping of the melt ph ase compounds rather than solves the problems inherent in the use of natura l mineral mixes.