Magmatic to hydrothermal zircons: Textural and chemical evolution
Résumé
Zircon is undoubtedly the most sought-after mineral for
geochemical studies, for its ability to provide information on
its host rock, spanning from geochronology, tracing of source
and processes, geothermometry, and, recently, redox
conditions [1]. However, it is crucial to ascertain its primary
origin, and in the past two decades there has been increasing
evidence of its crystallization from hydrothermal fluids [2].
Two of the main characteristics that are widely used to
ascertain the magmatic or hydrothermal origin of zircon are
texture and trace-element chemistry. However, most of these
data are contradictory and can be similarly attributed to a
primary and secondary origin [3], resulting in a poor
understanding of hydrothermal zircon characteristics.
We present data on a suite of zircons from the
Ambohimirahavavy alkaline complex, Madagascar, that
display impressive textural, morphological and compositional
variations, strongly suggesting a span in origin from magmatic
to hydrothermal (Fig. 1). Clearly magmatic zircons yield ages
of 20.40 ± 0.16 and 21.21 ± 0.44 Ma. Hydrothermal zircon
yields a similar age of 20.64 ± 0.48 Ma. Evidence for
hydrothermal origin includes its occurrence with quartz in
pseudomorphs after primary minerals, as botryoidal crystals
filling cavities, and precipitation in exoskarn [4]. Strong
variations in the amounts and distribution of trace element also
occur among different sectors in zoned crystals.