A novel approach to identifying mantle-equilibrated zircon by using
trace element chemistry
Abstract
One of the requirements for inferring local mantle redox states via
zircon Ce/Ce* oxybarometry and Ti-in-zircon thermometry is to select
mantle-equilibrated zircon (i.e., unaltered magmatic zircon without
interaction to hydrosphere). Traditional protocols for identifying
mantle-equilibrated zircon require a comprehensive examination of whole
rock geochemistry in addition to zircon morphology, texture, and trace
element and isotopic measurements to obtain information for zircon
saturation context, source melt petrogenesis, and post-magmatic history.
This study proposes a simple filter scheme for selecting
mantle-equilibrated zircon using trace element systematics alone (P, Ti,
Y, Nb, REE, Hf, Th, U). A total of 13 filtering criteria are synthesized
from previous studies based on simulations, experiments, and compiled
global datasets, all of which help to provide geological context and
ultimately constrain the inferred melt redox state from three
perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and,
(3) zircon saturation context. The filter scheme presented here, which
is based on detailed classifications of zircon morphology/texture and
host rock compositions of 2317 zircon analyses from 30 independent
references, is shown to distinguish non-magmatic zircon (Group III),
magmatic zircon with significant inclusions and/or sourced from highly
enriched source melt (Group II), and mantle-equilibrated zircon (Group
I). A case study of Group I mantle-equilibrated zircon from Greenland
suggests that their respective mantle source has reached FMQ -0.5 since
ca. 2950 Ma. Future applications of this filter scheme include studies
of out-of-context detrital and/or xenocrystic zircon.