SILICATE-CARBONATE LIQUID IMMISCIBILITY IN UPPER-MANTLE ECLOGITES - IMPLICATIONS FOR NATROSILICIC AND CARBONATITIC CONJUGATE MELTS

Two eclogites from the Jagersfontein kimberlite, South Africa, contain silicate-carbonate immiscible liquids, a very rare feature in upper-mantle xenoliths. The two eclogites have unusual chemistries relative to other eclogites from Jagersfontein; JEC 90-2 contains 5.7 wt% Na2O and JEC 91-11 contains 1.2 wt% BaO, and both eclogites belong to a group of four Jagersfontein eclogites which all contain natrocarbonatitic mineral assemblages (NMA) defined by carbonate, hydrous Na-Al silicate (natrolite), barite, and pectolite. Immiscible liquid-bearing eclogites display veined metasomatism, whereas the two other eclogites (JEC 91-15 and 91-17) are pervasively metasomatized. Garnets in these four eclogites average Py50Alm38Gr12; however, ''mantled'' garnets in JEC 90-2 have rims ranging from Py61 to Py65. Pyroxenes average Di58Jd42 in composition. Amphiboles in these eclogites range in composition from pargasite to kaersutite. Pectolite occurs in patches of calcite or as minute needle inclusions in Na-Al silicate ocelli. Calcite and natrolite occur in patches or as ocelli in Na-rich (approximately 13 wt%) or K-rich (approximately 6 wt%) devitrified glasses. Textural evidence for liquid immiscibility includes sharp ocelli boundaries, kidney- and teardrop-shaped ocelli, and ocelli trapped in the process of pinching off. Silicate and carbonate immiscible liquids in the two eclogites demonstrate good correlation with experimental and other natural silicate-carbonate conjugate liquids, and calculated bulk liquid compositions for these eclogite-hosted immiscible liquids fall within the two-liquid field as determined by KJARSGAARD and HAMILTON (1989). The silicate liquid in JEC 91-11 is similar in composition to a melilitite or melanephelinite, whereas the silicate liquid from JEC 90-2 is comparable to a phonolite. Strongly zoned garnets imply a multi-stage history for these eclogites: equilibration in the upper mantle, followed by a higher temperature melting episode and reprecipitation in the uppermost mantle or lower crust. A model is presented for the generation of these immiscible liquids that involves partial melting of omphacitic pyroxene (+/-phlogopite), the release of Na and K, and injection of a CO2-H2O rich ''fluid'' or melt. The unmixed carbonate liquid is immiscible in the alkaline silicate fraction, and two melts are produced. Occurrence on a large scale could generate conjugate silicate (melilititic to phonolitic) and carbonate magmas (sovites) typical of natural carbonatite complexes.