Experimental studies on three potassium-rich ultramafic rocks from Damodar Valley, East India

Summary An experimental study on the phase relationships of three potassium-rich ultramafic rocks from the Damodar Valley, Gondawana basins, has been performed under upper mantle P–T conditions (1.0–2.5 GPa, 700–1200 °C). The Mohanpur lamproite and Satyanarayanpur minette, both from the Raniganj basins, have been investigated with the addition of 15 wt% H₂O. No water was added in the experiments done on an olivine minette from the Jarangdih coal mine, Bokaro Basin, which originally contains 15 wt% CO₂ and 2.86 wt% H₂O. In all cases, olivine is the liquidus phase followed by phlogopite. The subsolidus assemblage for the three rocks is a phlogopite-bearing harzburgite, associated with apatite, Mg-ilmenite and carbonates for the Jarangdih rock; apatite, chromian spinel and carbonates and priderite (only between 1.0 and 1.2 GPa) in the case of the Mohanpur lamproite, and finally apatite, chromian spinel, rutile, and carbonate in the Satyanarayanpur sample. Although orthopyroxene is absent in the natural potassium-rich ultramafic rocks, its presence in the run products of the Jarangdih rock is possibly related to a reaction between olivine and a CO₂-bearing fluid phase. The presence of orthopyroxene in the run products of Mohanpur and Satyanarayanpur rocks may be due to a reaction between K-feldspar, olivine and a vapour phase to produce phlogopite and orthopyroxene. On the basis of present experimental investigation and isotopic studies made by previous investigators, it has been suggested that these K-rich rocks have crystallized from melts derived by vein-plus-wall-rock melting of a phlogopite-bearing harzburgite source rock. Received December 15, 1999; revised version accepted June 17, 2001