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Subduction zone metasomatism and its consequences for potassium-rich magmatism and deep nitrogen cycling
thesisposted on 2022-03-28, 23:30 authored 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 %.