The Behaviour of Halogens (F, Cl, Br, I) and Other Fluid-Mobile Elements in Subduction Zones
Creators: name: Pagé, Lilianne
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2019
Subjects: Halogens
Subduction

description: The high abundance of fluid-mobile elements (FMEs) in arc magmas relative to the mantle attests to their recycling in subduction zones. Aqueous fluids released from the subducting slab transport these elements to the overlying mantle wedge for arc magma generation. However, some FME may be retained in the slab to the deeper mantle, incorporating into melts that form back-arc basalts, intraplate ocean island basalts (OIBs), carbonatites and kimberlites. This thesis reports the abundance and distribution of halogens (F, Cl, Br, I) and other FME (e.g., B, N) in various subducted lithologies to evaluate the behaviour of these elements during subduction, their partitioning among coexisting high pressure (HP) minerals and the role of these rocks and minerals in transporting these elements to the deeper mantle. Abyssal lizardite-serpentinites, such as those from the Dominican Republic Tertiary subduction complex, are enriched in halogens and B from hydration by seawater and sediment-modified fluids. During the subduction and subsequent lizardite-antigorite transition of serpentinites, F and B are retained or replenished, while Cl, Br and I are expelled. Halogen compositions of shallow (&lt;25 km) forearc mantle lizardite-serpentinites from the Dominican Republic suggest their hydration by fluids released from subducting altered oceanic crust, with only minor sediment contribution. This is in contrast with deep mantle wedge antigorite-serpentinites associated with the Tso Morari UHP unit, NW Himalaya where elevated F (avg. 185 ppm), Br (avg. 0.13 ppm), I (avg. 0.16 ppm), B (avg. 51 ppm) and N (avg. 23 ppm) contents and their ratios demonstrate significant contributions from subducted shallow water sediments. The latter results are significant because they demonstrate serpentinites can host FME to depths of at least 100 km in the mantle wedge, and possibly to 200 km (limit of antigorite stability) in the slab of cold subduction zones or to 300 km in phase A after antigorite decomposition. A similar decoupling of F from Cl during subduction is also observed in the metamorphosed crust. Blueschists from the Tavsanli zone, NW Turkey and eclogites from the Tso Morari UHP unit and the Sulu UHP belt, China contain low bulk Cl and Br contents relative to their presumed protoliths, where as their F contents are similar or elevated. Iodine is also enriched in the blueschists and Tso Morari eclogites, but depleted in the Sulu eclogites relative to their protolith rocks. Enrichments of F and I in the metamorphosed crust are attributed to fluids released from subducting marine sediments, suggesting geochemical exchange among subducting lithologies. Shallow water trench sediments would be especially enriched in these elements given the close proximity of continents to these subduction zones. Apatite and phengite are the major host of halogens in these rocks as demonstrated by high F in these minerals; blueschists contain up to 3.5 wt% and 500 ppm F, respectively, and eclogites contain up to 2.3 wt% and 1600 ppm F, respectively. Other halogen-bearing minerals include lawsonite, Na-amphibole +/- chlorite in the blueschists and possibly titanite +/- Na-amphibole in the eclogites. Liberated F may be subsequently incorporated into antigorite-serpentinites of the overlying mantle wedge. Bulk B contents are generally low in the subducted crust, however phengite can host up to 100 ppm B to at least blueschists facies conditions. Overall, Cl, Br and I are lost from the altered oceanic crust and lizardite-serpentinites during shallow (&lt; 50 km) subduction, while F and some B are retained. Given the stability of minerals such as antigorite, phase A (after antigorite decomposition), apatite, lawsonite and phengite to depths of 200-300 km in cold subduction zones and their accommodation of FME, they may transport significant F, some B and N and minor Cl, Br and I beyond subarc depths (~ 100 km) and contribute to the their abundances in magmas derived from the deeper mantle.