Desilicification Rims of Zircon Xenocrysts Record the Timing of Kimberlite Emplacement

Abstract

Kimberlites are deep and relatively rare mantle‐derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle‐derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U–Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70–2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb–Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology.

A craton‐like subcontinental lithospheric mantle beneath the Trans‐North China Orogen revealed by the ca. 1.54 Ga kimberlites

“…A xenocrystic origin is thus preferential for the baddeleyites from the Yingxian kimberlites. Because the U–Pb isotopic system commonly remains open under the high temperature of mantle, and the U–Pb isotopic clock of baddeleyite was triggered while being caught by the mantle‐derived magmas, the baddeleyite from the kimberlites could record the emplacement age of magma (Heaman & LeCheminant, 1993, 2001; Melnik et al, 2022; Schärer et al, 1997). The baddeleyites from this work give the consistent age of 1537 ± 5 Ma (Figure 2), which therefore represents the emplacement age of the Yingxian kimberlites.…”

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

“…This information is in agreement with U-Pb ages of zircon and baddeleyite from the Jizerka blue sapphire. Although there is a difference between their ages with respect to the relative scale of their respective experimental errors, they may be combined to a mean According to Melnik et al [39], the desilicification rims of zircon xenoliths represented by baddeleyite form in the crust shortly before or soon after the emplacement of kimberlites and thus record the timing of kimberlite emplacement. This indicates that the baddeleyite reaction rim in the studied material is a good target for deciphering the emplacement age of the alkali basalt hosting gemstone minerals (zircon and sapphire) in the Jizerka Quaternary placer.…”

“…Baddeleyite (monoclinic ZrO 2 ) and quartz symplectitic mixtures document quick thermal cooling (Figure 6). According to Melnik et al [39], desilification rims of zircon xenoliths represented by baddeleyite record the timing of kimberlite emplacement [40]. It indicates that such baddeleyite reaction rims can also be used for constraining the timing of emplacement of the alkaline-basalt-hosting gemstone minerals (zircon and sapphire).…”