THE FIELD OF STABILITY OF BLUE JADEITE: A NEW OCCURRENCE OF JADEITITE AT SORKHAN, IRAN, AS A CASE STUDY

摘要

A new occurrence of "blue jade" is described. The jadeitite occurs as metasomatic veins in magnesite bodies within meta-ultramafic rocks in the Sorkhan area of southeastern Iran. The veins are composed of almost pure jadeite, 90 to 99.5 mol.percent Jd, contain minor amounts of Ba-bearing K-feldspar, lawsonite and katophoritic amphibole, but unlike other occurrences of "blue" or "lavender jade", do not contain high amounts of Ti. The jadeitite veins formed at low-temperature - high-pressure conditions, around 1.6 GPa and 420 deg C. Such P-T conditions are characteristic of zones of cold subduction in which lawsonite blueschists to lawsonite eclogites typically form. Thermodynamic studies show that the mineral assemblage within the blue jade is strongly pressure- and temperature-dependent. Jadeitites containing two clinopyroxenes (jadeite and omphacite) are stable at high pres-sure (>=0.8 GPa) and low temperature (<=430 deg C) conditions, whereas blue jade with only one clinopyroxene (jadeite) forms at higher temperature or lower pressure. On the basis of these new calculations, P-T conditions of formation are re-examined for all occurrences of blue jade.