Kinetics of deuteration in andradite and garnet

Abstract

The hydrogen mobility in andradite single crystals from an iron-skarn deposit was investigated through H-D and D-H exchange experiments. Thin slices were annealed in a horizontal furnace flushed with a gas mixture of Ar/D 2(10%) and Ar/H 2(10%) at ambient pressure between 400 and 700 °C. FTIR analyses were performed before and after each annealing run. Between 15 and 35% of the original OH content remained in the crystal structure at the end of the deuteration experiments. This contrasts with the results of similar experiments performed in other NAMs, where all hydrogen atoms are replaced by deuterium in the structure. However, because a steady state was reached at the end of the experiments, the diffusion law for the exchange process was determined as: D H-D = D 0 exp[-(96 ± 11) kJ/mol/RT], with logD 0 = -5.9 ± 0.7 (in m 2 /s). The activation energy is similar to that for hydrogen diffusion in grossular, but H diffusivity is more than two orders of magnitude faster. Our results demonstrate that, because major element composition has a major effect on H-D diffusion laws, it must be considered in any discussion of δD signatures in garnets. In andradite-rich garnets, hydrogen isotope data can only be used to record short, low-grade metamorphic or metasomatic events, at temperatures lower than 400 °C.

Experimental constraints on hydrogen diffusion in garnet

“…3), significantly lower than an Ea of 200/350 kJ mol − 1 determined in pyrope and grossular-andradite garnets (Blanchard and Ingrin 2004a;Dai et al 2012;Kurka et al 2005) but is close to that of the almandine-rich ZT garnet of Kurka (2005) (E a = 180 kJ mol − 1 ). Deuteration in grossular and andradite shows slightly lower activation energies [102 kJ mol − 1 , GRO, (Kurka et al 2005); 96 kJ mol − 1 , An99, (Zhang et al 2015)]. This low Ea for H + diffusion is likely associated with iron oxidation, i.e., proton-polaron exchange (as shown for H + diffusion in ferromagnesian olivine (Kohlstedt and Mackwell 1998;Mackwell and Kohlstedt 1990) E a ≈ 130 kJ mol − 1 ).…”

“…This method has been used to aid in allocation of peaks from pyroxene (Ferriss et al 2016) and olivine (Padron-Navarta et al 2014). Hydrogen extraction and deuteration has been studied in pyrope (Blanchard and Ingrin 2004a, b;Wang et al 1996) and grossular-andradite (Kurka 2005;Kurka et al 2005;Phichaikamjornwut et al 2012;Zhang et al 2015). Blanchard and Ingrin (2004a) demonstrated that pyrope-rich garnet from Dora Maira (Italy) showed different diffusion rates for hydrogen associated with different peaks, interpreted as representing two point defects with different stability with respect to hydrogen loss.…”

Kinetic D/H fractionation during hydration and dehydration of silicate glasses, melts and nominally anhydrous minerals

“…Now, the possible mechanisms controlling hydrogen migration through the garnet structure is less well understood than for other NAMs. A few studies suggest nonetheless that processes operating for garnets are similar to other NAMs (Blanchard and Ingrin, 2004;Kurka et al, 2005;Zhang et al 2015). In ferromagnesian olivines, hydrogen diffusion is charge compensated either by a fast flux of polarons or by a slower coupling to metal vacancies diffusion (Mackwell and Kohlstedt, 1990;Kohlstedt and Mackwell, 1998).…”

Structural and Magnetic Studies of Zn Doped LaRh_1-2xCu_2xO₃

“…is the ionic sphere volume where = 1 2 34 2 [15] and 4 is the ionic raduis of each ion. ( ) is the number of mole of Cu and Zn ions.…”