Zircon survival in shallow asthenosphere and deep lithosphere

Abstract

Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature 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) at high 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 ZrO2 and 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, and carbonatite 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.

Jizerka Gemstone Placer—Possible Links to the Timing of Cenozoic Alkali Basalt Volcanism in Jizera Mountains, Czech Republic

“…For example, spherical zircon grains about 1 cm in diameter will be completely dissolved at 1300 • C and 0.5 GPa in a tholeiitic melt within ~11 years. A 100-micron zircon will dissolve in 9.7 h, a zircon sphere of 50 microns in 2.6 h and a 10-micron zircon in 0.2 h [44]. Paquette and Margoil-Daniel [3] also report such dissolution reactions of zircons during their residence in basaltic magma.…”

“…Borisova et al [44] tested thermobarometric effects of hot basaltic melt on zircon xenoliths. Their data suggest high solubility of zircon in basaltic magma and very fast congruent dissolution of zircon in basaltic melt at pressures of 0.2 to 0.7 GPa.…”

Desilicification Rims of Zircon Xenocrysts Record the Timing of Kimberlite Emplacement

“…These rims can start growing shortly after kimberlite emplacement (Kamenetsky et al., 2014) and/or during limited magma transport within the crust. The crustal depths of their formation may range from at least 0.7 GPa (Borisova et al., 2020; Gervasoni et al., 2017) to the near‐surface (Kamenetsky et al., 2014). Nevertheless, the DSRs record the timing of kimberlite emplacement, given the growth of these rims at exclusively crustal levels, as suggested in the present study, and the rapid (hours to days) ascent of such magmas (Canil & Fedortchouk, 1999; Jollands et al., 2018; Peslier et al., 2008; Sparks et al., 2006).…”

Geochronology and Petrogenesis of Early Pleistocene Dikes in the Changbai Mountain Volcanic Field (NE China) Based on Geochemistry and Sr-Nd-Pb-Hf Isotopic Compositions

“…Although the zircon crystals of the two dikes could be interpreted as antecrysts, however, their petrographic features do not support such hypothesis. The growth of zircons in poorly evolved magmas may be due to different processes including (Borisova et al, 2020) the partial melting of hydrated peridotite, the felsic crustal involvement/recycling or zircon sealing, total zircon dissolution, and reprecipitation and/or a contribution of metasomatic Si-F-rich fluid. Our data do not allow us to better constrain the origin of the CMVF zircons studied here.…”