The role of trace elements in controlling H incorporation in San Carlos olivine

Abstract

We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ + 1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10 − 11 m 2 /s. Total H as H 2 O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H 2 O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H 2 O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm − 1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm − 1 , correlate negatively with Na + , but are positively correlated with the difference between molar Cr 3+ and Na + . This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements.

Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry

“…This offset is difficult to explain as one would expect D gt/ol Ti to be relatively constant, with only a small dependence on temperature and garnet composition. A change in Ti substitution from the octahedral to the tetrahedral site was reported in wet versus dry mantle olivine, and the presence of Na appears to enhance uptake of Ti in olivine (Tollan et al 2018). As Na contents in Akwatia olivines inclusions are relatively low, we cannot exclude a crystal-chemical control on Ti partitioning.…”

“…Recent work has also provided data for supra-subduction zone xenoliths, an important reservoir (Ionov 2010;Pirard et al 2013;Tollan et al 2017) missing from previous compilations. The increase in interest of the behaviour of H in nominally anhydrous minerals has led to many studies about the interaction between trace element uptake in olivine with hydrogen (e.g., Berry et al 2005Berry et al , 2007Tollan et al 2018), showing that trace element partitioning can be strongly influenced by the chemical environment.…”

Reduced methane-bearing fluids as a source for diamond

“…Considering the redox conditions of our experiments, reactions 2 and 3 should be more relevant where diamond forms upon removal of H 2 . One way for this to happen is for H to become sequestered in orthopyroxene and olivine via the equilibria proposed by Tollan and Herman 44 for orthopyroxene: and by Tollan et al 45 for olivine: The [] in equilibrium (4) denotes a lattice vacancy in orthopyroxene. The formation of OH in olivine during the experiments is documented by the FTIR spectra presented in Fig.…”

Hydrogen incorporation in plagioclase

“…For these reasons, a full thermodynamic treatment of the data is beyond the scope of this work. Future experiments employing a two-stage approach (Bai and Kohlstedt, 1993;Tollan et al, 2018) to control Fe 3+ /Fe tot by preannealing at fixed f O 2 prior to high-pressure annealing would go a long way towards addressing this shortcoming of the study. Yang (2012a) documented an increase in H solubility in Pl with increasing f O 2 between the NNO and HM buffers and derived an m exponent (Eq.…”