Abstract
Liquidus and sub-liquidus phase relationships are reported for melts formed from an aphanitic kimberlite composition crystallized at 5–12 GPa and 900–1400 °C. The liquidus phase over the pressure range investigated is forsteritic olivine. This is followed with decreasing temperature by olivine plus garnet as the initial sub-liquidus solid phase assemblage. Supra-solidus assemblages consist of olivine+garnet+clinopyroxene+Mg-ilmenite+liquid at 5–7 GPa or olivine+garnet+clinopyroxene+hematite–ilmenite solid solutions (+/−perovskite)+liquid at 8–12 GPa. Phlogopite forms as a near-solidus phase only at 900 °C and 6 GPa. Orthopyroxene does not form at any temperature and pressure. All garnets formed at 6–7 GPa are Ti-rich almandine–grossular–pyrope solid solutions and not Cr-pyrope, whereas garnets formed above 8 GPa are Ti- and Fe 3+ -rich and have no natural counterparts. Quenched liquids are represented by magnesite at 10–12 GPa and Mg–Ca-carbonates at lower pressures. In addition to forming discrete crystals, Mg-ilmenite and hematite–ilmenite solid solutions occur as lamellar intergrowths that are identical in texture to naturally occurring intergrowths. Mg-ilmenite compositions at 6–7 GPa are similar to those of the natural occurrences, whereas clinopyroxenes are richer in Ca. The effects of graphite versus platinum capsules on the oxygen fugacity of the experimental charges and the composition of the olivine, clinopyroxene, Fe–Ti-oxides and garnets formed are described. These experimental data are interpreted to indicate that kimberlite magmas are unlikely to be formed by very small degrees of partial melting of a simple homogeneous carbonated garnet lherzolite mantle. It is proposed that kimberlite magmas form by extensive partial melting of metasomatized mantle, i.e. mineralogically complex carbonate-bearing veins in a lherzolitic/harzburgitic substrate, and that lamellar ilmenite–clinopyroxene intergrowths represent the products of non-equilibrium growth in kimberlite magma.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.