A new interpretation of decomposition products of serpentine under shock compression Youjun Zhang 1 , Toshimori sekine 1, * and hongliang he 2 1 Department of Earth and Planetary Systems Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan 2 National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, P.O. Box 919-111, Mianyang 621900, China absTracT Dense hydrous magnesium silicates (DHMSs) may play an important role in water transport dur- ing planetary accretion and as water reservoirs in the Earth’s deep mantle. We show that the dynamic decomposition products of antigorite, Mg 3 Si 2 O 5 (OH) 4 , can be interpreted as containing the newly discovered, dense hydrous silicate, phase H (MgSiO 4 H 2 ). The Hugoniot for phase H was calculated based on the Hugoniots for its constituent oxides and equation of state data derived from irst-principles calculations. The measured antigorite Hugoniot, previously suggested to decompose into high-pressure phases without generating luid H 2 O, was compared with those derived from calculations involving phase H. Sound velocity data were also compared to conirm that the dynamic breakdown product of antigorite at pressures above ∼40 GPa is most likely phase H plus MgO without formation of luid H 2 O. Keywords: Dense hydrous magnesium silicates, phase H, high pressure, Hugoniot, decomposi- tion, serpentine inTroducTion Due to their stabilities at high pressures, the dense hydrous magnesium silicates (DHMSs) may provide important insights into deep-focus earthquakes, water sources for the Earth’s interior, and formation of the primitive atmosphere and oceans (e.g., Tyburczy et al. 1990; Meade and Jeanloz 1991; Ulmer and Trommsdorff 1995; Peacock 2001; Drake 2005; Kawakatsu and Watada 2007; Sekine et al. 2012). Phase D (MgSi 2 O 6 H 2 ) previ- ously was thought to be the only possible dense hydrous magne- sium silicate present in the lower mantle (Irifune and Tsuchiya 2007). It has a wide stability field up to 40–50 GPa in pressure, at temperatures to ∼1800 K, dehydrating to form an assemblage containing perovskite (Pv) and magnesiowustite (Mw) at higher temperatures (Shieh et al. 1998). Recently, using first-principles methods Tsuchiya (2013) predicted a new high-pressure hydrous phase with composition MgSiO 4 H 2 as a product of a high- pressure phase transition of phase D. Subsequently, Nishi et al. (2014) observed this phase (designated phase H) experimentally at ∼50 GPa and 920 °C in both quench experiments and in situ X-ray diffraction measurements using multi-anvil apparatus. At 0 K, phase H is theoretically predicted to be stable up to ∼52 GPa before it dissociates into Pv plus H 2 O (ice VIII) (Tsuchiya 2013). Phase H forms a solid solution with δ-AlOOH, and the stability field of the resulting aluminous phase H (Al) expands to higher pressures and temperatures, extending to those that characterize the lower mantle at depths of up to ∼2000 km (Nishi et al. 2014). Shock wave experiments play an important role in understanding the dynamic behavior of hydrous minerals and their stability during impact process and can be used to study their potential ability to be a water carrier. meThod From theoretical calculations (Tsuchiya 2013), phase H is characterized by the zero-pressure density, bulk modulus, and its first derivative of ρ 0 = 3.412 g/cm 3 , K 0 = 185.8 GPa, K′ 0 = 4.20, respectively, although hydrogen-bond symmetrization can be expected to occur above ∼30 GPa. A Hugoniot (U s = C 0 + su p , U s shock veloc- ity, u p particle velocity, constants of C 0 and s) for phase H can be estimated using the equations of 0 0 0 / C K ρ = = 7.38 km/s and s = (K′ 0 + 1)/4 = 1.30, respectively. Moreover, phase H is compositionally a mixture of the phases brucite Mg(OH) 2 (Br) plus stishovite SiO 2 (St), which are stable at our pressures of interest. Therefore, the Hugoniot for phase H can also be calculated based on the known Hugoniots of Br and St using the additive volume law. This approch is applicable to estimate Hugo- niots for an isochemical mixture of minerals with known Hugoniots (Al’tshuler and Sharipdzhanov 1971; Kalashnikov et al. 1973; Telegin et al. 1980). At pressure P, the specific volume of the mineral mixture V(P) can be computed by means of the relation: (1) Here, α i is the weight fraction of mineral i, and . V i (P) is the specific volume of mineral i, and can be described as (2) Therein, A i = s i 2 , B i = 2s i + C 2 i,0 /(V i,0 P), V i,0 , C i,0 , and s i are the initial specific volume, the bulk sound velocity at zero pressure, and the slope of the shock Hugoniot of mineral i, respectively. Hugoniot parameters calculated for phase H based on both the results of first-principles calculations and the additive volume law, are listed in Table 1. The calculated Hugoniots are very close to each other for P > ∼40 GPa. A predicted Hugoniot for phase D (ρ 0 = 3.49 g/cm 3 ) is also calculated from theoretical equation of state data (Tsuchiya et al. 2005), and listed in Table 1. resulTs The Hugoniot for natural antigorite, Mg 3 Si 2 O 5 (OH) 4 , contain- ing small amounts of Al 2 O 3 , FeO, and Fe 2 O 3 , has been determined American Mineralogist, Volume 99, pages 2374–2377, 2014 0003-004X/14/1112–2374$05.00/DOI: http://dx.doi.org/10.2138/am-2014-5021 2374 * E-mail: toshimori-sekine@hiroshima-u.ac.jp