Subsolidus hydrogen partitioning between nominally anhydrous minerals in garnet-bearing peridotite

Abstract

International audienceHydrogen distribution between nominally anhydrous minerals (NAMs) of a garnet-lherzolite under subsolidus conditions has been investigated. Separated NAMs from a garnet-peridotite from Patagonia (Chile) are annealed together (olivine, orthopyroxene, clinopyroxene, and garnet) using a piston-cylinder at 3 GPa and 1100 °C using talc-pyrex cell assembly for 10, 25, and 100 h. The talc-pyrex assembly provides enough hydrogen in the system to re-equilibrate the hydrogen concentrations at high pressure. The three coexisting nominally anhydrous minerals (NAMs, i.e., olivine, orthopyroxene, and clinopyroxene) were successfully analyzed using FTIR. The resulting hydrogen concentrations exceed significantly the initial hydrogen concentration by a factor of 13 for olivine and a factor of 3 for both pyroxenes. Once mineral-specific infrared calibrations are applied, the average concentrations in NAMs are 115 ± 12 ppm wt H2O for olivine, 635 ± 75 ppm wt H2O for orthopyroxene, and 1214 ± 137 ppm wt H2O for clinopyroxene, garnet grains are dry. Since local equilibrium seems achieved over time (for 100 h), the calculated concentration ratios are interpreted as mineral-to-mineral hydrogen partition coefficients (i.e., Nernst’s law) for a garnet-peridotite assemblage. It yields, based on mineral-specific infrared calibrations, DOpx/Ol = 5 ± 1, DCpx/Ol = 10 ± 2, and DCpx/Opx = 1.9 ± 0.4. While DCpx/Opx is in agreement (within error) with previous results from experimental studies and concentration ratios observed in mantle-derived peridotites, the DPx/Ol from this study are significantly lower than the values reported from mantle-derived xenoliths and also at odd with several previous experimental studies where melt and/or hydrous minerals co-exists with NAMs. The results confirm the sensitivity of hydrogen incorporation in olivine regarding the amount of water-derived species (H) in the system and/or the amount of water in the coexisting silicate melt. The results are in agreement with an important but incomplete dehydration of mantle-derived olivine occurring at depth, during transport by the host magma or during slow lava flow cooling at the surface. The rapid concentration modification in mantle pyroxenes also points out that pyroxenes might not be a hydrogen recorder as reliable as previously thought

A Subsolidus Olivine Water Solubility Equation for the Earth's Upper Mantle

“…These bands are similar to those described by Kovács et al () using natural orthopyroxene sensor material (despite the higher Al 2 O 3 content in their starting sensor orthopyroxene (2.83 and 6.30 wt %) compared to this study, <0.80 wt %) and those described by Férot and Bolfan‐Casanova () for the FMASH system below 5 GPa. Similar results were found by Demouchy et al (), during annealing at subsolidus conditions. Interestingly at P ≥ 4 GPa and 950°C a new peak appears at ~3,685 cm −1 (Figure S2 in the supporting information, black triangles) that might correspond to high clinoenstatite (e.g., Withers & Hirschmann, ).…”

“…In the first case, the maximum water content in olivine is constrained at the conditions where the hottest dehydration reactions intersect the pressure-temperature (P-T) trajectories of the hydrated subducting slab releasing aqueous fluid. Chlorite dehydration is roughly parallel to the isopleth of 50 ± 20 ppm wt H 2 O (dashed blue line in Figure 9); this implies a bulk water content retained in the slab mantle of approximately 90 ± 20 ppm wt H 2 O (assuming a model harzburgite from Schmidt & Poli, 1998;200 and 6 ppm wt H 2 O in orthopyroxene and garnet, respectively, and a partition coefficient Kd cpx/opx of 2.1, Demouchy & Bolfan-Casanova, 2016, Demouchy et al, 2017. This value is roughly independent on the specific P-T trajectory followed by the slab (Figure 9) and is significantly lower than previous estimates (400 ppm wt H 2 O, Arcay et al, 2005).…”

“…Interestingly at P ≥ 4 GPa and 950°C a new peak appears at~3,685 cm À1 ( Figure S2 in the supporting information, black triangles) that might correspond to high clinoenstatite (e.g., Withers & Hirschmann, 2007). Clinopyroxene shows a prominent peak at 3,640 cm À1 and two broad minor peaks at~3,450 and~3,360 cm À1 equivalent to those described by Kovács et al (2012) and Demouchy et al (2017).…”

Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

“…Figure 3 visualizes the results of experimental studies concerning the H 2 O content in relation to pressure and Al 2 O 3 in various systems (pure enstatite, single-element-doped and multipleelement-doped enstatite, natural orthopyroxene). In a recent study, Demouchy et al (2017) determined 716 ppm H 2 O at 30 kbar in natural orthopyroxene (3.72% Al 2 O 3 , 6.12% FeO, and 0.5% Cr 2 O 3 ; composition according to in coexistence with a garnet peridotite assemblage. The latter strongly enhances H 2 O storage in orthopyroxene ( Figure 4) but Cr seems to counterbalance this (Stalder et al, 2005).…”

“…However, more recent experiments indicate that the D Peridotite=melt H2O could be as high as 0.02 (range: 0.017-0.023; Bindeman et al, 2012). In addition, recent experiments with natural or, at least, chemically complex systems (Demouchy et al, 2017;Stalder et al, 2015) indicate that orthopyroxene may actually store such high amounts of water. These values are still relatively high but not entirely unreasonable.…”