Zircon survival in shallow asthenosphere and deep lithosphere

Borisova, A.Y.; Bindeman, I.N.; Toplis, M.J.; Zagrdenov, N.R.; Guignard, J.; Safonov, O.G.; Bychkov, A.Y.; Shcheka, S.; Melnik, O.E.; Marchelli, M.; Fehrenbach, J.

Авторы: Borisova A.Y., Bindeman I.N., Toplis M.J., Zagrdenov N.R., Guignard J., Safonov O.G., Bychkov A.Y., Shcheka S., Melnik O.E., Marchelli M., Fehrenbach J.
Журнал: American Mineralogist
Том: 105
Год издания: 2020
Издательство: Mineralogical Society of America
Местоположение издательства: United States
Первая страница: 1662
Последняя страница: 1671
DOI: 10.2138/am-2020-7402
Аннотация: Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrialprocesses. To shed light on question of zircon survival in the Earth’s shallow asthenosphere, hightemperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) athigh current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, andcarbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.
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Zircon survival in shallow asthenosphere and deep lithosphereстатья

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