Reconstruction of magma chamber processes preserved in olivine-phlogopite micro-ijolites from the Oldoinyo Lengai, Tanzania

A detailed petrographic and mineralogical investigation of olivine-phlogopite micro-ijolite xenoliths from Oldoinyo Lengai, Tanzania indicates a complex evolutional history. These xenoliths consist of diverse textural subdomains characterized by minerals ranging from early-formed olivine, through diopside-hosted perovskite and phlogopite, to evolved aegirine-augite and titanite. Thermometry and mineral compositions in the subdomains suggest crystallization temperatures from 1070-970 degrees C to 850-700 degrees C at plutonic pressures and SiO2-activities controlled by perovskite-titanite equilibria. Double coronas are a characteristic textural feature of the olivine-phlogopite micro-ijolite, consisting of olivine cores surrounded by an inner clinopyroxene corona and an outer phlogopite corona. These double coronas might have formed during early magma chamber processes, including magma movement to a subsequent chamber resulting in dissolution of olivine with subsequent crystallization and accumulation of diopside and phlogopite. Diopside-aegirine-augite compositional zonation indicates several magma injections followed by cooling periods, during the formation of micro-ijolite groundmass. Mg# (80-83) and Ca (0.1-0.3 in wt%) contents of olivine together with the presence of primary melt inclusions in clinopyroxene, phlogopite, and nepheline indicate a magmatic origin from a possible parental olivine-nephelinite melt. There is evidence for subsolidus, or near-solidus, re-equilibration processes as indicated by the reaction of olivine with titanite forming symplectitic textures of ilmenite and diopside with minor zirconolite. Ti-exchange between phlogopite phenocrysts and other Ti-bearing minerals (perovskite, titanite, magnetite) resulted in similar to 750 degrees C equilibrium temperatures for phlogopite, which are much lower than mafic magmatic (>900 degrees C) conditions. Calculated subsolidus temperatures suggest crystallization of olivine-phlogopite micro-ijolites over a 10-20 km depth interval.

Automated summary

A detailed petrographic and mineralogical investigation of olivine-phlogopite micro-ijolite xenoliths from Oldoinyo Lengai, Tanzania reveals their complex evolutionary history and diverse textural features. The xenoliths show crystallization temperatures ranging from 1070-970 degrees C to 850-700 degrees C, controlled by perovskite-titanite equilibria under plutonic pressures. The presence of double coronas and compositional zonation in the xenoliths suggests early magma chamber processes and multiple magma injections followed by cooling periods. The study provides insights into the origin and formation processes of these micro-ijolite xenoliths.