Reconciling the discrepancy between the dehydration rates in mantle olivine and pyroxene during xenolith emplacement

Abstract

Hydrogen concentration profiles through olivine and pyroxene in peridotite xenoliths carried in rift basalts from northern Tanzania (Lashaine, Eledoi, and Kisite localities) show bell-shaped distributions, indicating that diffusive hydrogen loss has occurred in all minerals. Homogeneous major element concentrations and equilibration of hydrogen between the cores of olivine and coexisting pyroxene suggest that hydrogen loss resulted from diffusive degassing during host magma emplacement. For these samples, hydrogen diffusivities in olivine and coexisting pyroxene must be within the same order of magnitude, similar to experimentally determined diffusivities, but in contrast to previous observations made on xenolithic peridotites. We demonstrate here, for the first time using natural samples, that significant differences in activation energy is likely the primary parameter that causes the discrepancy between hydrogen diffusion in olivine and pyroxene observed in different suites of mantle xenoliths. Because hydrogen diffuses faster in olivine than in pyroxene as temperature increases, hydrogen loss in the Tanzanian mantle xenoliths must have occurred at relatively low temperatures ($750 to $900°C), whereas hydrogen loss observed in previous xenolith studies likely occurred at higher temperatures ($950 to >1200°C). Thus, the diffusive loss of hydrogen in the Tanzanian mantle xenoliths may have occurred at shallow depths or at the Earth's surface.

The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals

“…This is particularly relevant for our lower model temperatures, where the 100% re-equilibration timescales require melt flow for a few weeks to over a year for the different mineral phases ( Figure 5). Dis-equilibrium water contents between NAM phases could thus reflect differences in activation energy between Arrhenius relationships at a given temperature (Xu et al 2019) and H + diffusion may be useful for deciphering the timescales of this process.…”

Behavior and origin of hydrogen defects in natural orthopyroxene during high-temperature processes

“…interesting evolutions from core to rim of hydrogen defects in orthopyroxene from a garnet lherzolite have also been presented by Xu et al (2019). It shows that the intensity ratio of the 3546 cm -1 band to 3520 cm -1 band increases from core to rim.…”

“…The 3546 cm -1 band is not always observed for natural orthopyroxenes. Several studies have reported this band in mantle orthopyroxenes and its height variation from core to rim (Grant et al 2007;Tollan and Hermann 2019;Xu et al 2019). In contrast to the 3546 cm -1 band, the 3460 cm -1 band is scarcely observed in natural mantle orthopyroxenes.…”

“…Indeed, based on the observation on the synthetic orthopyroxene doped with Al and Fe, the Fe 3+ -related 3460 cm -1 band increases when the dehydrated sample is annealed in H 2 (Tollan and Hermann 2019). However, the 3546 cm -1 band has been reported in some Al-bearing natural mantle orthopyroxenes (e.g., Grant et al 2007;Tollan and Hermann 2019;Xu et al 2019). Anyway, the dehydration experiments in our study in M vacancies appeared accompanying loss of other hydrogen defects (Skogby and Rossman 1989;Ferriss et al 2016;Yang et al 2019a).…”

“…DOI: https://doi.org/10.2138/am-2021-7741. http://www.minsocam.org/ experienced outgassing during their transport to the surface (e.g., Tian et al 2017;Xu et al 2019). Furthermore, previous dehydration experiment for synthetic orthopyroxene shows that the cm -1 band intensity increases during the initial annealing at lower temperatures , indicating that hydrogen can be re-introduced into the relevant M site at appropriate conditions.…”