Details

Autor(en) / Beteiligte

• Barth, Anna
• Newcombe, Megan
• Plank, Terry
• Gonnermann, Helge
• Hajimirza, Sahand
• Soto, Gerardo J.
• Saballos, Armando
• Hauri, Erik

Titel

Magma decompression rate correlates with explosivity at basaltic volcanoes — Constraints from water diffusion in olivine

Ist Teil von

• Journal of volcanology and geothermal research, 2019-12, Vol.387, p.106664, Article 106664

Ort / Verlag

Elsevier B.V

Erscheinungsjahr

2019

Link zum Volltext

Quelle

Elsevier ScienceDirect Journals Complete

Beschreibungen/Notizen

• Despite the strong theoretical basis for the importance of magma decompression rate in modulating the dynamics of explosive basaltic eruptions, few measurements exist. Here, we quantify magma decompression rate using the incomplete diffusive re-equilibration of water through olivine for two contrasting eruptions at Cerro Negro volcano in Nicaragua: the 1992 VEI 3 and 1995 VEI 2 eruptions. Both eruptions have the same basaltic composition (SiO2 ∼ 50 wt%) and maximum volatile concentrations (H2O ∼ 4.7 wt%). SIMS and NanoSIMS measurements of water in olivine-hosted melt inclusions (MIs) from both eruptions show a negative correlation between MI size and water loss, as predicted for diffusive re-equilibration. MIs from the 1995 eruption appear to have experienced more water loss compared to those from the 1992 eruption (up to 80–90% water loss in MIs < 50 μm), which is consistent with slower magma ascent. Quantifying the timescale for water diffusion during magma decompression requires an accurate constraint on H+ diffusivity through olivine. We perform dehydration experiments on oriented cuboids of olivine from Cerro Negro in a 1 atm. furnace at 800 and 1000 °C and at an oxygen fugacity 0.25 log units above the nickel-nickel oxide buffer (NNO + 0.25) in a sequence of heating steps. Between each heating step we measure the H+ concentration along each crystallographic direction by FTIR and model these profiles to determine the diffusivity. We find the fastest diffusion direction along a̱ and develop an Arrhenius relationship for the diffusivity along this direction: Da(m2/s)=9.6×10−6exp−125RT, where R is the gas constant 8.314 J/mol K; T is the temperature in K and the activation energy is 125 kJ/mol. These are the fastest rates ever measured for olivine dehydration, which may relate to the low forsterite content of the olivines (Fo ∼ 79). We have developed a 1D model for H+ diffusion along the fast direction since this dominates water loss (Da > 10 × Db or Dc). Using our new H+ diffusivity, we model water diffusion from MIs during magma decompression to determine ascent timescales. We find that magma from the VEI 3 eruption of 1992 ascended at a median rate of 0.007 MPa/s, which is ∼5 × faster than the median for the 1995 VEI 2 eruption, although both eruptions show a ∼ two orders of magnitude spread in decompression rate. Assuming linear decompression, this places a minimum bound of ∼6.5 h on magma ascent during the 1992 eruption and ∼35 h for the 1995 eruption. Monte Carlo simulations give a 2σ uncertainty on log 10(dP/dt) of ∼ 10% — much smaller than the order of magnitude spread in decompression rate within each eruption, which suggests that there is real variation in decompression rate for a given deposit. Our results support the hypothesis that higher decompression rates are correlated with more explosive eruptions and show that diffusion modelling of MI water loss is a powerful tool that is sensitive to changes in decompression between VEI 2 and 3 eruption intensities. •Fastest H+ diffusivity in olivine on record for dehydration, possibly linked to Fe content•Strongly anisotropic diffusivity of H+ in olivine requires care with crystal orientation.•1D diffusion models show link between eruption style and magma decompression rate.•No link between eruption style and initial magma water concentration