Abyssal Serpentinites: Transporting Halogens from Earth’s Surface to the Deep Mantle

Abstract

Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from serpentinites during the lizardite–antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for serpentinite thickness of the metamorphosed slab (500 m), antigorite serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle.

Serpentinite-derived slab fluids control the oxidation state of the subarc mantle

“…Thus, we suggest that the above correlations reflect the role of serpentinite-derived fluids in controlling the behavior of Cu in the melt by influencing the f O2 of the system and, thus, the state of sulfide saturation. This interpretation is also supported by the positive correlations of the average V/Yb of primitive arc lavas [8 to 12 wt % MgO; V/Yb (8)(9)(10)(11)(12) ], a proxy for f O2 (29,30), with the average  11 B values (r 2 = 0.77) and average B/Nb (r 2 = 0.84) for our global arc dataset (Fig. 2, C and D ), and carbon into the upper mantle at convergent margins (10,11,16).…”

“…2, A and B). Here, we used the relative Cu enrichment at 4 to 6 wt % MgO [Cu (4-6) = Cu (4-6) /Cu (8-12) − 1, where Cu (4)(5)(6) and Cu (8)(9)(10)(11)(12) are the concentrations Cu in arc lavas containing 4 to 6 wt % and 8 to 12 wt % MgO, respectively] to evaluate Cu enrichment or depletion for each arc; although this MgO range cannot represent primitive arc magmas, Cu (4-6) values show whether sulfide saturation is reached at an early stage of magma evolution and, thus, might indicate primitive magma f O2 . While magma f O2 can increase during differentiation, i.e., via the fractionation of Fe(II)-rich minerals (such as almandine), such an effect is small during basaltic to andesitic differentiation (40).…”

“…Altered oceanic crust (AOC) and sediments have long been considered as major subduction components involved in arc magma generation (8,9). However, the dehydration of subducted serpentinite has been increasingly recognized as an important source of volatiles (e.g., water, sulfur, and halogens) in the subarc mantle (10)(11)(12). Like AOC (7,13), breakdown of oxidized serpentinites can produce highly oxidizing fluids (14)(15)(16)(17)(18), which may have an important influence on the f O2 of arc magmas.…”

Halogen (F, Cl) Concentrations and Sr‐Nd‐Pb‐B Isotopes of the Basaltic Andesites From the Southern Okinawa Trough: Implications for the Recycling of Subducted Serpentinites

“…In addition, F and Cl are less influenced by magma degassing as they have higher solubilities in the melt than CO 2 and H 2 O at lower pressure (Webster, 1997; Webster et al., 1999). Therefore, the high F and Cl abundances in subduction zone magmas indicate a contribution of volatile‐rich, slab‐derived agents to the mantle (Bénard et al., 2017; Bouvier et al., 2008, 2010, 2019; Kendrick et al., 2011, 2012; Pagé & Hattori, 2019; Portnyagin et al., 2007; Rose‐Koga et al., 2012, 2014; Straub & Layne, 2003).…”

Crystal structures and high-temperature and high-pressure vibrational spectra of synthetic fluorine-bearing brucites

“…On the other hand, fluorine is the most abundant halogen element in the mantle, most of which could be carried into the deep mantle by subduction slabs (e.g. McDonough and Sun, 1995;Straub and Layne 2003;Pagé and Hattori 2019). Since, the radius of Fis quite close to that for O 2- (Shannon 1976), the OH -= Fsubstitution is very common in hydrous minerals, like serpentines, micas, as well as the humite-group minerals including chondrodite and clinohumite (e.g.…”