Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey

Abstract

We examined the F, Cl, Br and I abundance of minimally retrogressed lawsonite blueschists from the Tavsanli Zone in northwest Turkey to evaluate the behaviour of halogens in subduction zones, and to determine the role coexisting high pressure minerals may play in transporting the halogens to the Earth's mantle. The blueschists contain sodic amphibole and lawsonite, with variable amounts of phengite and chlorite, and minor apatite. A positive correlation between Cl, Br and I contents in bulk rocks suggests their overall coherent behaviour in subduction zones, although high ratios of I/Cl and Br/Cl compared to altered oceanic crust indicate that Cl is preferentially lost relative to Br and I before or during blueschist metamorphism. Iodine and F are enriched relative to altered oceanic crust, suggesting incorporation from marine sediments. In situ analyses of minerals in thin sections reveal F preferentially concentrates in apatite (avg. 3.13 wt%), over phengite (482 ppm), lawsonite (avg. 413 ppm) and Na-amphibole (257 ppm). Chlorine also preferentially resides in apatite (138 ppm), followed by equal partitioning between phengite (59 ppm) and Na-amphibole (56 ppm), and lower concentrations in lawsonite (27 ppm). Upon apatite decomposition at a depth of ~200 km, F may redistribute into lawsonite and phengite in slabs, whilst Cl is likely expelled to the overlying mantle wedge. Given the stability of lawsonite and phengite to a depth of 280-300 km in cold subduction zones, they may transport F beyond subarc depths, contributing to the high F in magmas derived from the deep mantle.

Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon

“…Recent studies have suggested that up to ~90% of non-radiogenic Ar, Kr and Xe and most of the heavy halogens (Cl, Br, I) have subducted origins in the Earth's mantle (Holland and Ballentine, 2006;Holland et al, 2009;Mukhopadhyay, 2012;Caracausi et al, 2016;Kendrick et al, 2017). However, relatively few studies have investigated the combined behaviour of multiple halogens and/or noble gases during prograde metamorphism (John et al, 2011;Kendrick et al, 2015;Pagé et al, 2016;Pagé and Hattori, 2017),…”

Halogen Heterogeneity in the Lithosphere and Evolution of Mantle Halogen Abundances Inferred From Intraplate Mantle Xenoliths

“…Kendrick, Jackson, et al () accounted for the uniform and relatively low I/Cl ratio of the mantle by suggesting that addition of the hydrated oceanic lithospheric mantle with high I/Cl ratio is balanced either by subduction of Cl‐rich materials such as AOC (Chavrit et al, ; Kendrick, Honda, et al, ) or by the preferential loss of I from subducting slabs at relatively shallow depths (John et al, ; Kendrick et al, ; Pagé et al, ). However, there is support for I‐rich halogen subduction beneath and beyond sub‐arc depth.…”

The high‐pressure phase of lawsonite: A single crystal study of a key mantle hydrous phase

“…The distorted Ca site and multiple Al sites of monoclinic laswonite also have implications for the geochemical partitioning behavior of elements at depth and during dehydration. Halogen partitioning in subducting slabs is complex, and lawsonite does incorporate trace amounts of F, Cl, Br, and I [ Pagé et al ., ]. Indeed, upon apatite decomposition at ~200 km, F has been proposed to be sequestered in lawsonite and phengite [ Pagé et al ., ].…”

“…Halogen partitioning in subducting slabs is complex, and lawsonite does incorporate trace amounts of F, Cl, Br, and I [ Pagé et al ., ]. Indeed, upon apatite decomposition at ~200 km, F has been proposed to be sequestered in lawsonite and phengite [ Pagé et al ., ]. Thus, lawsonite and phengite represent two of the few minerals that may transport F to depths greater than 200 km.…”

“…Lawsonite stability in CASH system, solid black line [Newton and Kennedy, 1963;Pawley, 1994;Schmidt and Poli, 1994]. Figure modified Journal of Geophysical Research: Solid Earth 10.1002/2017JB014344 and phengite [Pagé et al, 2016]. Thus, lawsonite and phengite represent two of the few minerals that may transport F to depths greater than 200 km.…”