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- Title
A Novel Approach to Identifying Mantle‐Equilibrated Zircon by Using Trace Element Chemistry.
- Authors
Ni, Ziqin; Arevalo, Ricardo; Piccoli, Philip; Reno, Barry L.
- Abstract
One of the requirements for inferring mantle redox states via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry is to select mantle‐equilibrated zircon derived from melts that have not interacted with the hydrosphere nor assimilated exogenous materials. 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. This study proposes a simple filter scheme for selecting mantle‐equilibrated zircon using trace element systematics alone (P, Ti, Y, Nb, rare earth elements [REE], Hf, Th, and U) to provide the geological context of zircon from three perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and (3) zircon saturation context. The filter scheme presented here, comprising of 13 criteria, is established based on detailed classifications of 2,173 zircon analyses from 30 independent references and is shown to distinguish between nonmagmatic zircon, magmatic zircon with significant inclusions and/or sourced from highly enriched source melts, and mantle‐equilibrated zircon. The filter scheme is validated with known, well‐characterized mantle‐equilibrated Jack Hills zircon (n = 53) and derives an average mantle redox state of ΔFMQ1.5±1.3 (sd) circa 4,400 Ma, in agreement with previously published results. Further application of the filter scheme to zircon dataset (n = 76) from three metasedimentary rocks successfully reveals the mantle‐equilibrated zircon in the dataset, and implies a mantle redox state of ΔFMQ−1.5±1.3 (sd) circa 2,950 Ma. This filter scheme can be applied to studies of out‐of‐context detrital and/or xenocrystic zircon in the future. Plain Language Summary: Zircon is a robust accessory mineral commonly found in felsic rocks. The Ce/Ce* and Ti content in zircon can be used to infer the redox states of local magmatic environments from which they crystallized. Importantly, only mantle‐equilibrated zircon (i.e., unaltered magmatic zircon that crystallized from melts without interaction to the hydrosphere nor assimilated exogenous materials) record the chemistry of mantle source, and provide insights of mantle redox state via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry. Traditionally, the identification of mantle‐equilibrated samples has relied on comprehensive examination of zircon and their host rocks (i.e., whole rock geochemistry, zircon morphology, texture, and trace element and isotopic measurements) in order to obtain information for zircon saturation context, source melt petrogenesis, and postmagmatic history. This study, however, presents a simple filter scheme that facilitates the selection of mantle‐equilibrated zircon and provides insights about zircon petrogenesis using trace element systematics alone (P, Ti, Y, Nb, REE, Hf, Th, and U). Key Points: This study has established a multidimensional filter scheme to select mantle‐equilibrated zircon using trace element chemistry aloneThe local melt redox states have been estimated from zircon using Ti‐in‐zircon thermometry and Ce/Ce* oxybarometryThis proposed filter scheme has been validated with external datasets and derived mantle redox states around ΔFMQ since the early Earth
- Subjects
TRACE elements; HYDROSPHERE (Earth); SEISMOLOGY; TOPOGRAPHY; LITHOSPHERE
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2020, Vol 21, Issue 11, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2020GC009230