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Subduction zone metasomatism and its consequences for potassium-rich magmatism and deep nitrogen cycling

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posted on 28.03.2022, 23:30 by Michael W. Förster
In total, subduction zones span 40,000 km across Earth's surface and recycle an average thickness of 500 m of sediment. During burial and heating these sediments eventually start melting at T >675 °C, following which Si‐rich hydrous melts infiltrate the peridotites of the mantle wedge above the subducting slab. In this thesis, a high‐pressure experimental approach is used to examine the reaction of sediments and peridotites at 2‐6 GPa in subduction zones and its consequences on the generation of K‐rich magmatism and on deep nitrogen cycling. All experiments are conducted in a layered arrangement, where the depleted peridotite is placed above the sediments in a 1:1 ratio. At 2‐3 GPa, the reaction of melts of sediment with depleted peridotite, simulating the fore‐arc of a subduction zone, leads to the formation of layered phlogopite pyroxenites and selective incorporation of major and trace elements in these metasomatic layers. Partial melting of these phlogopite pyroxenites produces melts rich in K2O (>9 wt%) with K/Na >>2 and a trace element pattern comparable to "orogenic lamproites". At similar pressures, the reaction of hydrous mantle melts with depleted peridotites produces metasomatic layers that show K/Na ~1 and a trace element pattern that closely resembles "anorogenic lamproites". In both cases, K‐enrichment is facilitated by the crystallization of an eclogitic residue rich in Na, poor in K, and consequently with low K/Na. At 4‐6 GPa, the reaction of melts of sediment with depleted peridotite does not produce mica, instead resulting in alkali chlorides with K/Na ratios similar to saline fluid inclusions in diamonds. Besides the chlorides, magnesite also crystallises in the peridotite. Both phases are important ingredients for the generation of salty kimberlites such as Udachnaya East. The change in metasomatic style from mica‐ to chloride formation between 3 to 4 GPa corresponds to the depth of the mid‐lithospheric discontinuity, a zone of low seismic velocities that is found intermittently beneath all continents at a depth of 80-100 km. The subduction of sediment is the main mechanism that recycles nitrogen back to Earth's mantle. The partitioning of nitrogen between fluid and melt (DN(Fluid/Melt)) and fluid and bulk residue (melt+mica) (DN(Fluid/Bulk)) was found to increase linearly with temperature normalized to pressure. Using the new partition coefficients, the amount of N recycled to Earth's mantle since the onset of subduction is calculated as 50 ±6 %.

History

Table of Contents

I. Introduction -- Chapter 1. Sediment‐peridotite reaction c ontrols fore‐arc metasomatism and arc‐magma geochemical signatures -- Chapter 2. Melting of sediments in the deep mantle produce saline fluid inclusions in diamonds -- Chapter 3. Two‐stage formation of K‐enrichment in ultrapotassic magmatism confirmed by a novel experimental approach -- Chapter 4. An experimental study of the role of partial melts of sediments versus mantle melts in the sources of potassic magmatism -- Chapter 5. Melting phlogopite‐rich MARID: lamproites and the role of alkalis in olivine‐liquid Ni‐partitioning -- Chapter 6. Partitioning of nitrogen during slab‐melting and recycling in subduction zones -- II. Thesis conclusion -- References -- Appendices.

Notes

Empirical thesis. "ARC Centre of Excellence of Core to Crust Fluid Systems and Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia" -- title page. Bibliography: pages 140-160

Awarding Institution

Macquarie University

Degree Type

Thesis PhD

Degree

PhD, Macquarie University, Faculty of Science and Engineering, Department of Earth and Planetary Sciences

Department, Centre or School

Department of Earth and Planetary Sciences

Year of Award

2019

Principal Supervisor

Stephen F. Foley

Additional Supervisor 1

Nathan Daczko

Additional Supervisor 2

Simon Clark

Rights

Copyright Michael W. Förster 2019. Copyright disclaimer: http://mq.edu.au/library/copyright

Language

English

Extent

1 online resource (222 pages) illustrations

Former Identifiers

mq:72161 http://hdl.handle.net/1959.14/1282003