The Fate of Zircon and Other Accessories in Deeply Subducted Terrigeneous Sediments

Although volumetrically insignificant, the mineral zircon and its fellow accessory minerals monazite and rutile exert strong control over the distribution of several groups of geochemically- and strategically-important elements that are concentrated in the Earth’s crust: the high-field strength elements (Zr and Hf in zircon; Ti, Nb and Ta in rutile), the radiogenic and heat-producing elements U and Th (in zircon and monazite), and the rare-earth element group, La-Lu (in zircon and monazite). Isotopic analyses of these accessory minerals have been extensively used for decades to date rocks of the continental crust, but it has only been with experimental study of the behavior of these accessory minerals in the laboratory, under conditions where high-temperature fluids or partial melts (magmas) are present, that geochemists have been able to make more broad-ranging insights into the nature of crustal tectonics and the extent of oceans and emergent continents on the early Earth.

The low solubility of these minerals in crustal fluids and melts suggests that they are likely to survive ultra-metamorphism in continent-continent collision zones (e.g., the Himalayas), or in any continental-derived sediments that are thought to be carried down deeper into the mantle in subduction zones, into the slab graveyards of the lower mantle at depths of 700 km or more. These are where mantle plumes that feed volcanism on ocean island chains found in the middle of the world’s oceans may originate.

A geochemical signature attributable to this deeply recycled (subducted) continental component is seen in the lavas erupted on the Pitcairn and Samoan archipelagos in the middle of the SW Pacific Ocean. How that continental signature comes to be in the source region for these lavas deep in the Earth’s lower mantle, and the role that the accessory minerals zircon, monazite, and rutile play in conveying that signature, is the subject of this experimental study.

The experiments may also lead to the discovery of new crystal structures and novel ceramics that may be of practical use in the long-term storage of high-level radioactive waste.

An experimental study of the behavior of zircon, monazite and rutile in a lithologic context appropriate to subduction of hydrous continental sediments from the base of the lithosphere (~5- 7 GPa) to the top of the lower mantle (~23-25 GPa) is proposed. The stability of these minerals with respect to dehydration reactions involving major mineral phases (e.g., phengite and hydrous aluminous minerals), and their solubility in the fluids these reactions give rise to, will be ascertained under conditions appropriate to variable slab geotherms extending from the crust to the lower mantle.

The concentrations of the high field strength (HFSE), heat-producing (HPE), and rare Earth elements (REE) in the accessory minerals and in the sediment-derived fluids with which they are in equilibrium will be determined by electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). These analyses will be used to assess the effects of pressure on the partitioning systematics of the HFSEs, HPEs, and REEs between hydrous fluids and zircon, monazite and rutile (or their high- pressure polymorphs) in subducted terrigenous sediments.

The potential for additional fractionation of the geochemical pairs Zr-Hf and Nb-Ta, and fractionation of the light REEs from the heavy REEs, will also be assessed. The research would provide empirical constraints on the nature of the “continental” isotopic and geochemical signature in the plume source for ‘enriched-mantle’ ocean island basalts.

A better understanding of the role that the minerals zircon, monazite and rutile play in conveying a crustal signature into the lower mantle, in terms of the HFSEs, HPEs and REEs, is envisioned. New crystal structures appropriate for the storage of HLRW may emerge from the proposed experimental studies, and the development of new experimental techniques that increase the efficiency of multi-anvil experiments and new analytical techniques for measuring trace elements in coexisting fluids, melts and minerals are additional benefits expected to come from this research.

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