The stability of antigorite in subduction zones revisited: the effect of F on antigorite stability and its breakdown reactions at high pressures and high temperatures, with implications for the geochemical cycles of halogens

Abstract

We present new experimental data on the effect of F on the stability of antigorite and its breakdown products at high pressures (2–6 GPa) and high temperatures (570–850 °C). The experiments show that F does not affect the stability of antigorite, but addition of F to the system affects which minerals are formed when antigorite breaks down. In a F-free system and in a system with intermediate F contents (2 wt% F), antigorite breaks down to olivine and orthopyroxene, but in a F-rich system (5 wt% F), antigorite breaks down to other hydrous and F-bearing mineral assemblages which include chlorite, clinohumite and humite-group minerals (HGM). Since the latter mineral phases are stable at higher pressures and temperatures, and contain more F than antigorite, significant amounts of F and potentially other halogens can be retained in the subducting slab and transported deep into the mantle and possibly even into the Earth’s transition zone.

Dense hydrated Mg-silicates in diamond: Implications for transport of H 2 O into the mantle

“…F was measured only for hydroxylchondrodite and mica; the oxygen equivalent of fluorine (O═F) was subtracted. inherited from serpentinite protoliths is the enrichment in halogens (41)(42)(43)(44), consistent with the presence of F-and Cl-bearing minerals as inclusions in the diamond in this study.…”

“…Beyond the antigorite stability field, the remaining water budget will be hosted by dense hydrous magnesium phases ( 38 – 40 ), namely, the humite-group minerals ( 12 ). Another characteristic inherited from serpentinite protoliths is the enrichment in halogens ( 41 – 44 ), consistent with the presence of F- and Cl-bearing minerals as inclusions in the diamond in this study.…”

“…Besides the large intrinsic uncertainties surrounding the phase equilibria calculated in the MSH system with THERMOCALC, two additional observations need to be considered: (i) the fluorine content (2.8 to 4.7 wt %) of the hydroxylchondrodite inclusions, which is known to have a strong effect on P - T stability ( 41 , 77 ), and (ii) the isolated presence of hydroxylclinohumite within the same diamond.…”

Raman spectroscopy for crystallochemical analysis of Mg‐rich layered silicates: Serpentine and talc

“…Accordingly, the dataset of sample Srp20, a lizardite-dominant polygonal/polyhedral serpentine 53 was not further considered in the construction of correlation trends. Moreover, even for F-containing serpentinegroup minerals and talc, where F content will not exceed 1 [60][61][62] and 1.5 24 wt.%, respectively, total oxides above 88 and 98 wt.%, respectively, will not be realistic and were not taken into consideration, as they may refer to additional mineral phases at the measurement point or to inclusions. The chemical formulae of the serpentine and talc samples calculated from the EMPA data are given in Tables 1 and 2, correspondingly.…”

Single-crystal elasticity of humite-group minerals by Brillouin scattering

“…Subduction is a crucial process in redistributing volatile elements between the Earth's surface and interior. Despite the fact that most volatiles return to the surface at relatively shallow depths during slab sinking, some minerals, including serpentine, lawsonite, aragonite, and magnesite, are capable of transporting water and carbon into the deeper mantle (Dasgupta and Hirschmann, 2010;Dixon et al, 2002;Flemetakis et al, 2022;Kaminsky, 2012;Kobayashi et al, 2017;Ohtani et al, 2018;Thompson, 1992;Wirth et al, 2009). Among volatile-rich minerals, the humite-group minerals [nM 2 SiO 4 ·M 1-x Ti x (F, OH) 2-2x O 2x ] have long been considered as significant water and fluorine carriers in subduction slabs (Figure 1) (Akimoto et al, 1977;Engi and Lindsley, 1980;Rice, 1980).…”