Do hydrogen isotopes in amphiboles record changing fO2 conditions during rapid ascent?
Abstract
Magma ascent rates are a primary control on eruption behavior, and hornblende amphibole (hbl) is a common mineral in arc magmas whose stability is sensitive to the changing P-XH2O-fO2 conditions during magma ascent. The breakdown of hbl, generalized as (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2, during ascent changes the mineral chemistry through H loss. Two different modes of H loss during magma ascent and degassingas H2O (dehydration) or as H2 (dehydrogenation)each result in opposite trajectories of H isotope fractionation with decreasing H2O. Dehydrogenation is redox sensitive because an Fe2+ is oxidized to Fe3+ to compensate for the loss of an H+ ion. We present bulk amphibole H2O and H isotope (D) data collected by rapid thermal decomposition (TC/EA) for experimental and natural samples. Preliminary hbl degassing experiments investigate the ability of hbl to record syn-eruptive shifts in magmatic fO2. Experiments were conducted in a 1 atm gas mixing furnace at 850C with hbl separates from Mt. Shasta pumices (1.5 wt% H2O and 35 D). Differences in H2O and D are observed over just 2 log units of fO2. A 5 hour-long experiment at ~NNO+1.3 yielded a bulk composition of 1.14 wt% H2O and 155 while a longer 10 hour-long experiment at ~NNO0.2 retained higher H2O (1.36 wt%) and acquired a much lower D (259). Experiments exposed to atmospheric air consistently lost most of their hydrogen (0.22-0.26 wt% H2O) in as little as 2 hours but resulted in highly variable D (115 to 59). These results suggest that bulk H2O and D compositions of hbl are sensitive to melt fO2 over a realistic range for arc magmas at timescales similar to magma ascent. Natural samples focus on explosively erupted intermediate magmas, where rapid ascent and quenching should preserve should limit the effects of slow cooling and preserve near-magmatic compositions. Hornblende separates from Cascade volcanoes generally have lower bulk H2O (1.4-1.7 wt%) than Alaskan ones (1.7-2.0 wt%) over the same D range (50 to 40). Based on the experimental data where lower H2O/higher D is achieved at higher fO2, this may suggest more oxidizing conditions are more common in Cascade volcanoes than in Alaskan systems. Future experiments will form time series at additional temperatures at these or similar fO2 conditions to further test this hypothesis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.V15F0133H