Retention of radiation damage in zircon xenocrysts from kimberlites, Northern Yakutia

Abstract

We have studied zircon xenocrysts from Mesozoic kimberlites from the Kuoika and Ary-Mastakh fields in Northern Yakutia. Zircon xenocrysts are assumed to originate from crustal rocks. Our SHRIMP (Sensitive High mass Resolution Ion MicroProbe) analyses yielded predominantly concordant U-Th-Pb ages (up to~3570 Ma; Paleoarchean) that clearly predate kimberlite formation. The general U-Th-Pb concordance observed excludes notable disturbance of the zircon xenocrysts U-Th-Pb isotope system during kimberlite ascent and emplacement. In addition, zircon xenocrysts were found to be significantly more radiation-damaged than would correspond to damage accumulation only since the time of kimberlite formation. This observation first indicates that zircon crystals were sampled by the kimberlite magma at comparably shallow depths not exceeding 10-12 km. If, in contrast, zircon crystals originated from deeper levels of the Earth's crust, they would have been exposed to temperatures of 250-300°C or more. This in turn would have caused long-term thermal annealing of the radiation damage, which was however not observed in our study. Second, our observation contradicts the hypothesis that high temperatures experienced by zircon xenocrysts during kimberlite ascent will cause notable structural reconstitution by short-term thermal annealing. Consequently, zircon crystals cannot have spent more than a few hours at temperatures exceeding ca. 700-800°C, or more than a few days at temperatures exceeding ca. 500-600°C. This in turn suggests that (i) temperatures of the ascending kimberlite magmas were rather moderate, and (ii) kimberlite ascent is a comparably short process followed by rapid cooling.

Metasomatic and magmatic processes in the mantle lithosphere of the Birekte terrain of the Siberian craton and their effect on the lithosphere evolution

“…The average chemical composition of the rocks in this series is close to that of the Al-undepleted komatiites [Solov'eva et al, 1994]. For four crystals of zircon from kimberlites of the Rubin pipe (Kuoika filed), ages determined by the SHRIMP method fall into two groups, 1.8-2.1 (~1.95 Ga in average) and 2.3-2.6 Ga (~2.4 Ga in average) [Nasdala et al, 2014]. In the opinion of the authors, the above-mentioned ranges reflect the main evolution stages of the crust in the Birekte block.…”

“…Возраст метасоматического флогопита (1640-1800 млн лет) несколько ниже возраста присоединения Биректинского террейна к Сибирскому кратону (1.8-1.9 млн лет [Rosen, 2003]), что, возможно, объясняется частичной потерей 40 Ar в анализируемой слюде. С другой стороны, это значение близко интервалу позднего эпизода формирования коры в Биректинском блоке 1.8-2.1 млрд лет [Nasdala et al, 2014] и соответствует времени привноса радиогенного осмия в мантийную литосферу из зоны субдукции (1.7-2.2 лет [Pernet-Fisher et al, 2015]). Геохимия изотопов кислорода в породах перидотит-пироксенитовой серии из трубки Обнаженная также свидетельствует о присутствии в них древней субдукционной компоненты (рис.…”

Spectroscopic study of inclusions in gem corundum from Mercaderes, Cauca, Colombia

“…There is of course the possibility that the dark inclusions consist of zircon and a highly radioactive phase, but no indication of the latter was detected in our Raman study. Another possible explanation is that such zircon inclusions are protogenetic in nature, and the zircon was already radiation damaged at the time of the entrapment and has not experienced complete structural recovery upon corundum formation (as shown for zircon inclusions in kimberlite; Nasdala et al 2014).…”

Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt

“…In some cases, the ages of zircon xenocrysts have been utilized to elucidate the nature of unexposed ancient fragments of continental lithosphere (Qiu et al, 2000;Hargrove et al, 2006;Iizuka et al, 2006;Smyth et al, 2007;Zhang et al, 2012a;Gaschnig et al, 2013;Torsvik et al, 2013;Reimink et al, 2014), and/or provide evidence of pre-existing continental crust (Siebel et al, 2009;Gao et al, 2011). In general, detailed studies of zircon xenocrysts, e.g., deep-seated xenocrysts from volcanic rocks such as lamproites (Zheng et al, 2006), kimberlite (Valley et al, 1998;Griffin et al, 2000;Nasdala et al, 2014;Ashchepkov et al, 2014) and mantle-derived magmas such as ultrapotassic rocks , xenoliths (Liati et al, 2004;Zheng et al, 2008;Pan et al, 2014;Tang et al, 2014), oceanic gabbros (Pilot et al, 1998) as well as granitic magmas (Smithies et al, 2001;Bea et al, 2007;Demoux et al, 2009b;Buys et al, 2014;Jeon et al, 2014) have extended our knowledge of unexposed parts of the crust through which the host magmas ascended. To better understand the significance of zircon xenocrysts in juvenile igneous rocks, Stern et al (2010) summarized the distribution of ancient zircons in juvenile crust, and explained the occurrence of compositional dependence of igneous rocks.…”