Long-lived zircon growth in trapped eclogite
Authors: Hand Martin , Tamblyn Renee , Zivak Diana , Raimondo Tom 
Published: March 2020

&lt;p&gt;The residence time of rocks within subduction channels provides a narrative on the physical processes that reflect the interplay between subduction rate and angle, coupling between the lower and upper plate and hydration of the mantle wedge.&amp;#160; In oceanic subduction systems, it is now recognised that rocks can reside within subduction channels for 10&amp;#8217;s of millions of years.&amp;#160;&amp;#160; These apparently long-lived durations of entrainment in the subduction channel probably require circulatory motions that recover material from terminal subduction and simple one-cycle exhumation. &amp;#160;In turn, these residence times can plausibly be used to deduce geodynamic variables that control the subduction system.&lt;/p&gt;&lt;p&gt;Establishing the duration a rock has been stored within a subduction environment typically requires application of multi-mineral geochronology coupled with considerations of closure systematics. &amp;#160;However because subduction environments are commonly fluid-rich, a mineral with great potential to reveal durations rocks can reside within subduction channels is zircon.&amp;#160; In subduction environments, several studies have documented apparently long-lived records of zircon growth, but seemingly have not recognised the potential for zircon to extract information on the duration a rock experienced subduction channel metamorphism.&lt;/p&gt;&lt;p&gt;Lawsonite-bearing eclogite in eastern Australia has a remarkable microstructural record of zircon growth.&amp;#160; Thin section-scale 1-3 micron resolution synchrotron mapping by X-ray Fluorescence (XFM) reveals the presence of 1000&amp;#8217;s of micron-sized zircons which occasionally range up to 15 microns in size.&amp;#160; Zircon: (1) defines inclusion trails in garnets, (2) is a foliation defining matrix mineral and (3) occurs in retrograde chlorite-bearing veins that formed during post-eclogite blueschist paragenesis.&amp;#160;&amp;#160; In-situ U-Pb geochronology shows that zircon growth occurred over the interval c. 520-400 Ma.&amp;#160; The zircons have hydrothermal characteristics with elevated LREE and simple tetragonal morphologies.&amp;#160; The apparently long duration of zircon growth is generally consistent with other geochronology from the eclogite: garnet Sm-Nd and Lu-Hf ages between 530-490 Ma, matrix foliation titanite U-Pb c. 450 Ma, and matrix foliation phengite Ar-Ar and Rb-Sr ages of 460-450 Ma.&amp;#160; &amp;#160;&lt;/p&gt;&lt;p&gt;The small size of the zircons means they cannot be readily extracted using bulk rock methods.&amp;#160; Instead, fast, high-resolution imaging methods such as synchrotron XFM mapping coupled with spatially precise U-Pb-trace element analysis reveal a long history of HFSE element mobility resulting in microstructurally organised zircon growth that allows rock residence time in a subduction channel to be determined.&lt;/p&gt;&lt;p&gt;If lawsonite eclogite from eastern Australia records more than 100 Ma of zircon growth at eclogite-blueschist facies conditions, the single eclogite sample reflects around 5000-7000 km of consumption of the palaeo-pacific plate under the east Gondwana margin while remaining trapped in the subduction channel.&lt;/p&gt;