Zircons - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 127

Zircons

Zircon is one of the most prevailing accessory minerals of magmatic, metamorphic
and some metasomatic formations of different composition. Due to its mechanical
strength, during the disintegration of indigenous sources the zircons sometimes accu-
mulate in the sedimentary rocks up to the formation of placer deposits. This mineral is
presented in high amounts in the granites, diorites, nepheline syenites, lamproites, car-
bonatites and alkaline metasomatic rocks, more rarely it occurs in the gabbros, eclog-
ites, kimberlites, minettes, lamproites, carbonatites, gneisses, amphibolites and other
metamorphic rocks. Main regularities of distribution of REE in zircons from many
types of magmatic and metamorphic rocks of different composition and genesis of the
series of articles discussed in the topic edited by Hancher & Hoskin [Zircon., 2003].
Recently, the zircons were found in ultramafites from some mantle xenoliths in alkali
basalts in several provinces of China and Russia [Zheng et al ., 2006, Saltykova et al .,
2008], in ore chromitites occurring among phlogopite-bearing peridotites of Finero
massif (Southern Alps) [Grieco et al ., 2001], as well as in dunites and ore chromitites
of Voykar-Syn'insky massif (Polar Urals, Russia) [Savel'eva et al ., 2006, 2007]. In the
latter case, as the authors assume, zircons crystallized from the residual portions of
fractionated basic melts that infiltrated into peridotites. Zircons with Archean U-Pb
isotopic age have recently been identified in the dunites of the Nizhnetagilsky massif,
which is a part of Platinum-bearing belt of the Urals (Russia) [Malich et al ., 2009].
Note that in some gabbroid rocks the crystals of accessory ilmenites were identified
surrounded by narrow (1-100 microns) rims that consist of microcrystals of zircon.
The formation of the latter, as suggested by the researchers, was due to a later infil-
tration of high-temperature interstitial fluids enriched with Zr [Morisset & Scoates,
2008].
According to its crystal structure and chemical composition the zircons are ortho-
silicates. Their chemical formula is usually written as follows: ZrSiO 4 . The average
SiO 2 content in them is 67 wt%. The main trace elements

in zircons might be REE, Hf, Y, Ta, U, Th, Pb, Mo, Fe, Ti, Ca, P, the total amount of
them can reach first weight percents. In recent decades, the zircons have been widely
used for isotope dating of magmatic and metamorphic rocks using U-Pb-method,
which considerably increased interest in a more detailed geochemical study of this min-
eral, including the study of REE distribution in it. Note that previously we presented
a brief review of materials published in recent decades on REE distribution in zircons
from rocks of different composition and origin [Lesnov, 2005], as well as on some
features of their isomorphism in this mineral [Lesnov, 2011].

∼ 33 wt%, ZrO 2 content is ∼