Glasses in Mantle Xenoliths from Olmani, Tanzania

Abstract

Nineteen mantle xenoliths from Olmani, Tanzania, have deformation textures and fluid inclusions like those in olivine-rich peridotites from Hawaii. Group A comprises 7 barren harzburgites (spinel, TiO2 <. 1 wt %) and group B (spinel, TiO2.8–2.6 wt %) contains 6 clinopyroxene-dunites, 2 lherzolites, 1 wehrlite, 1 two-pyroxene-dunite, and 1 olivine-orthopyroxenite. Both groups lack garnet and porphyroclastic textures and display fingerprint intergrowths of spinel with ortho-or clinopyroxene. Many secondary inclusions of several generations within olivine are rich in Co2. Alkali-rich glasses (Na2O.4–7.3 wt %, K2O 3.3–5.1, P2O5 ∼1) in B xenoliths are unrelated chemically to the olivine-augite-phyric ankaramite host. Aphyric inter-granular glass occurs along with larger patches of residual glasses associated with euhedral olivine, clinopyroxene and spinel. Clinopyroxenes of B xenoliths show decreasing Na2O (2.1–.3 wt %) and Cr2O3 from olivine-brown cores to clear margins of larger grains, and to finer grains associated with residual glass. These properties of the xenoliths are consistent with establishment of a coarse texture during prolonged metamorphism in the mantle (∼28 kb, 1100 K?), partial melting in the presence of a CO2-rich fluid (∼26 kb, 1400 K?), transport by ankaramite magma to the Earth's surface, and rapid crystallization of some of the melts at 1300–1200 K.