Niocalite-cuspidine solid solution and manganoan monticellite from natrocarbonatite, Oldoinyo Lengai, Tanzania
Niocalite-cuspidine solid solution and manganoan monticellite from natrocarbonatite, Oldoinyo Lengai, Tanzania
复制标题
来自坦桑尼亚奥多因约伦盖的钠碳酸岩的镍钙石-尖峰固溶体和锰蒙锂石
作者：
R. Mitchell;F. Belton
摘要2003年8月从坦桑尼亚Oldoinyo Lengai的T56 B锥中挤出的钠质碳酸岩的火山砾和飞溅物异常富含磷灰石、硅酸盐、氧化物和硫化物矿物。这些矿物主要存在于灰榴辉岩中淬冷的钠碳酸岩包裹体和Nyeregoryte灰榴辉岩共生体中的裂隙中。硅酸盐包括钠质尖晶石-钠质尖晶石-铌钙石固溶体系列和锰质钙镁橄榄石。氧化物是以镁铁酸盐-铁锰矿-磁铁矿固溶体系列的成员为代表。按丰度递减顺序出现的硫化物包括：亚铁硫铁矿、锰铁闪锌矿、方铅矿和磁黄铁矿。对这些矿物的岩相学和成分数据的解释表明，它们都是高温（<900-650ºC）的早期结晶相，来自相对未演化的钠碳酸岩岩浆，硫化物在硅酸盐之前形成，后者在钙铝榴石之前形成。在硫化物-氧化物组合形成过程中，碳酸盐岩岩浆的硫逸度和氧逸度分别降低和升高。尖晶石、钙镁橄榄石和含硅磷灰石的结晶作用迅速消耗了初始低硅含量的钠碳酸岩岩浆，阻止了后期基质硅酸盐的结晶作用。
Abstract Lapilli and spatter of natrocarbonatite extruded from the T56B cone of Oldoinyo Lengai (Tanzania) in August 2003 are unusuallyrich in apatite, silicate, oxide and sulphide minerals. These minerals occur primarily within inclusions of quenched natrocarbonatite in gregoryite and fractures in nyerereitegregoryite intergrowths. Silicates include members of the sodian cuspidine-niobian cuspidine-niocalite solid-solution series and manganoan monticellite. Oxides are represented bymembers of the magnesioferrite-jacobsite-magnetite solid-solution series. Sulphides occurring in decreasing order of abundance include: ferroan alabandite, manganoan ferroan sphalerite, galena and pyrrhotite. Petrographic and compositional data for these minerals are interpreted to indicate that all are hightemperature (<900-650ºC), early-crystallizing phases from relatively-unevolved natrocarbonatite magma, with the sulphides forming prior to the silicates, and the latter before gregoryite. Sulphur fugacityand oxygen fugacityof natrocarbonatite magma are considered to decrease and increase, respectively, during the formation of the sulphide-oxide assemblage. Crystallization of cuspidineniocalite, monticellite and Si-bearing apatite rapidlydepletes natrocarbonatite magma of its initial low silica content, preventing crystallization of late-stage groundmass silicates.