Inherited Continental Zircons in Oceanic Rocks – True or False

Abstract

Many studies have reported U-Pb dates of zircons that are older than the rocks that contain them, and they are therefore thought to be inherited from older rock complexes. Their presence has profound geodynamic implications and has been used to hypothesize about concealed micro-continents, continental crust beneath ocean islands, and recycling of continental material in the mantle beneath mid-ocean ridges. However, there is skepticism in the scientific community whether these zircons are truly inherited from crustal fragments or represent contaminants from sample preparation and processing. In this study, we combine single zircon U-Pb dates with structural radiation damage determined by Raman spectroscopy from a Pliocene mid-ocean ridge gabbro and show that the thermal history of Precambrian zircons found in the sample contradicts an inherited origin. To confirm that our approach allows distinction between contamination and true inherited zircons, we investigated igneous rocks that formed during the opening of the Atlantic Ocean and find that the calculated radiation damage age of old inherited grains corresponds with the crystallization dates of the young magmatic zircons.

Zircon xenocrysts from Easter Island (Rapa Nui) reveal hotspot activity since the middle Jurassic

“…Pilot et al (1998) was the first to report Paleozoic and older zircons from the Central Atlantic mid-ocean ridge. Since this first finding, diverse populations of Paleozoic and older zircons have also again been collected from the Central Atlantic Ridge and reported from the North Atlantic and South Atlantic ridges (Bea et al, 2020;Bjerga & Pedersen, 2021;Bortnikov et al, 2008;Skublov et al, 2022), and recently from intra-oceanic, remote hotspot islands of Hawai'i (Greenough et al, 2021) and Galápagos . These ancient zircons cannot have formed from crystallizing magma at the young mid-ocean ridges, or in the oceanic crust below the hotspot volcanoes, and were consequently interpreted as xenocrysts.…”