NOBLE GAS TRANSPORT BY AMPHIBOLES: IN SITU STRUCTURAL ANALYSIS OF NEON WITHIN FERROACTINOLITE

The Earth’s mantle is often studied in terms of noble gases, which are commonly used as tracers within terrestrial fluids. Studies describing the chemical evolution of the Earth have long treated the flux of noble gases as one-way, where noble gases are released into the atmosphere through magmatism, but not reintroduced to the Earth’s interior. Despite a readily observable signature of non-radiogenic noble gases in many mantle xenoliths, their reintroduction into the mantle is not understood. The potential for amphiboles in a subducting slab to be carriers noble gases has garnered attention, as they consistently have higher noble gas concentrations than other crustal minerals. Interestingly, a sample of ferroactinolite examined via high pressure X-ray crystallography may provide clues about this mechanism. Amphiboles such ferroactinolite, (☐{Ca₂}{Fe₅2+}(Si₈O₂₂)(OH)₂ ), are ring structure-bearing minerals where (Si,Al)O₄^-4,-5 units are joined together to form a large hexagonal vacancy. This vacancy, referred to as the A-site, may contain a cation (such as Na⁺ or Ca²⁺), but is typically vacant if charge balance is maintained. In our single-crystal synchrotron X-ray diffraction experiments, natural ferroactinolite compressed in Ne exhibited an anomaly at 2.7 GPa where the unit cell volume increased with pressure. Normal compression resumed up to 30.3 GPa, where the structure remains in the C2/m space group with no observed phase transformations. The only way in which this “negative compressibility” could be thermodynamically explained is a change in the chemical composition of the sample. Noble gases are predicted, by lattice strain theory, to preferentially occupy the neutrally-charged vacancy. We conducted detailed crystallographic analysis of the compression mechanism, with emphasis on providing evidence of the location of absorbed Ne and the amount of the Ne atoms in the unit cell. Quantifying the contribution of amphiboles to the noble gas cycle may be essential in understanding the young Earth, the creation of isotopically distinct mantle reservoirs, and correctly interpreting noble-gas isotope thermochronology. The Ne-uptake behavior of ferroactinolite may serve as a benchmark for how noble gases behave within amphiboles at pressure and temperature conditions comparable to subducting slabs.