Crystallization temperatures of the oceanic carbonatites of
Fuerteventura, Canary Islands, have been determined from oxygen isotope
fractionations between calcite, silicate minerals (feldspar, pyroxene,
biotite, and zircon) and magnetite. The measured fractionations have
been interpreted in the light of late stage interactions with meteoric
and/or magmatic water. Cathodoluminescence characteristics were
investigated for the carbonatite minerals in order to determine the
extent of alteration and to select unaltered samples. Oxygen isotope
fractionations of minerals of unaltered samples yield crystallization
temperatures between 450 and 960degreesC (average 710degreesC). The
highest temperature is obtained from pyroxene-calcite pairs. The above
range is in agreement with other carbonatite thermometric Studies.
This is the first study that provides oxygen isotope data coupled with a
CL study on carbonatite-related zircon. The CL pictures revealed that
the zircon is broken and altered in the carbonatites and in associated
syenites. Regarding geological field evidences of syenite-carbonatite
relationship and the close agreement of published zircon U/Pb and whole
rock and biotite K/Ar and Ar-Ar age data, the most probable process is
early zircon crystallization from the syenite magma and late-stage
reworking during magma evolution and carbonatite segregation. The oxygen
isotope fractionations between zircon and other carbonatite minerals
(calcite and pyroxene) support the assumption that the zircon would
correspond to the early crystallization of syenite-carbonatite magmas.