/30 Budget of shallow magma plumbing system at Asama volcano, Japan, revealed by ground deformation and volcanic gas studies Speaker: Ryunosuke Kazahaya Colleagues: Yosuke AOKI, Hiroshi SHINOHARA 1

/30 Magma budget OUTPUT: Emission from volcano ： Erupted magma, Ash, volcanic gas INPUT: Supply of magma from depths; lateral/vertical movement of magma INPUT > OUTPUT Volcano inflates INPUT < OUTPUT Volcano Deflates Deformation = “INPUT -OUTPUT” 2

/30 3 Brief explanation of Asama volcano One of most active volcanoes in Japan (andesitic) Major eruption in 2004; minor ones in 2008,2009, 2015… Volcanic gas and geodetic observations were conducted

/30 Observations / Data collection 4 SO 2 emission rates measurements (Ohwada et al., 2013) Volcanic gas composition measurements (Shinohara, 2013) GPS measurements (Aoki et al., 2005; 2013) Petrologic data collection (Shimano et al., 2005; Miyake et al., 2005; Ohta et al., 2007)

/30 [Asama volcano] ground deformation vs. volcanic gas emission 5 Inflations and deflations were observed recursively. High SO 2 emission rate periods coincided with the Inflations [Activity and sync.] Optimal for calculating magma budget Inflation periods: Jul to Mar Jun to Dec Deflation periods: Feb to May 2004 May 2005 to Jun. 2008

/30 Way to link degassing and deformation? 6 To calculate magma budget, magma pathway and degassing mechanism are needed to be assumed. Volume change due to volcanic gas discharge depends on degassing mechanism Permeable flow -> removal of gas phase Magma convection -> Melt shrinkage (exsolution) To link degassing and deformation, we need to know - a main source of deformation? - Magma property at deformation source Shinohara, 2008 Volcano deflates by degassing

/30 Magma pathway and degassing mechanism of Mt. Asama 7 Shinohara, 2008 Magma convective degassing proposed by Ohwada et al. [2013] for Asama volcano Kazahaya et al. [2015] Degassing mechanism:Magma pathway: Conduit, Dike, and Mid-crustal magma reservoir. Dike is a main source of deformation [Aoki et al. 2013].

/30 Volumetric balance of the dike 8 Two volume flows at DIKE are considered: Between conduit and dike (Shallow) Flow s = magma convection (non-degassed/degassed magma moving) Between dike and magma reservoir (Deep) Flow d = supply from deep magma reservoir Volume decrease by volcanic gas discharge: Flow s (observable) Volume change of dike was derived by GPS (ΔV gps ): ΔV gps = Flow s + Flow d (observable) F Flow s Flow d Thus, supply from deep magma reservoir is expressed as: Flow d = Flow s - ΔV gps estimate

/30 Magma shrinkage by gas discharge Calculation of (Flow s ) Magma shrinks by: 1)removal of gas phase 2)Exsolution of volatiles 9 Approximate Ideal gas; equation of state Given by experiment At P=75 Mpa; T= 1050 ℃ H 2 O molar volume in gas phase = 15×10 -5 m 3 /mole H 2 O molar volume in the melt = 2.3×10 -5 m 3 /mole (about 6 times difference) Ochs and Lange, 1997 H 2 O is most dominant species of volcanic gas; here we consider only H 2 O emission PV = nRT

/30 Magma shrinkage by gas discharge Calculation of (Flow s ) 10 The volumes of H 2 O in the gas phase and dissolved in the melt at the dike (Flow s ) were calculated to estimate the magma shrinkage for each period Dike source Pressure (P) : 75 MPa Temp. (T) : 1323 K (from Shimano et al and Aoki et al. 2013) Gas volume fraction: % (from Papale et al and Soave, 1972) Volatile components in melt SO 2 concentration: ppm H 2 O concentration: 2-4 wt.% CO 2 concentration: ppm (from Ohta et al and Shinohara, 2013) SO 2 emission rates ton/day (Ohwada et al. 2013; Mori et al. 2013) H 2 O/SO 2 ratio of 30 (Shinohara, 2013) H 2 O molar volumes in gas phase and melt Net volumetric H 2 O emission rates + + Gas Observation data Calculation parameters calculate

/30 Magma budget calculation results 11 e.g. Inflation period ( ) Volume change by volcanic gas discharge (Flow s ): m 3 Ejecta volume of eruption (Nakada et al. 2005) : m 3 Supply from Deeper source (Flow d ) might be at least two times larger than ΔV gps Main source of OUTPUT from the volcano is volcanic gas discharge Volume change of the dike (GPS obs.; ΔV gps ): 10 7 m 3 comparable Diff. 1 mag. order More details, see Kazahaya et al. [2015] Flow d = Flow s - ΔV gps

/30 Key points Volume decrease of magma in a reservoir by volcanic gas discharge is estimated The volume decrease is larger than deformation of reservoir inferred by geodesy Volcanic gas discharge is one of the main mechanisms to cause ground deformation The content of this study has been published as Kazahaya et al. [2015] on JGR. 12 Kazahaya, R., Aoki, Y., and Shinohara, H. (2015), Budget of shallow magma plumbing system at Asama Volcano, Japan, revealed by ground deformation and volcanic gas studies, J. Geophys. Res. Solid Earth, 120, doi: / 2014JB

/30 Thanks for listening 13

/30 14

/30 Magma Budget of the dike (a)During inflation periods, the amount of magma supplied to the dike (I m +O m ) is likely to be at least 2 times and up to 1 order of magnitude larger than the volume change of the dike (ΔV gps ) (b)During deflation periods, the volume change of the dike (ΔV gps ) could be caused solely by the volcanic gas discharge (I s +O s ); however, taking the degassed magma volume into account, I m is unlikely to be zero and I m ≈ -O m (i.e. I m +O m ≈0) 11

/30 Results Period February 2003 to May 2004 July 2004 to March 2005 May 2005 to June 2008 June 2008 to December 2008 TypeDeflationInflationDeflationInflation ΔV GPS (=I m + O m + I s +O s ): Volume change of the dike (10 6 m 3 ) Volume change by volcanic gas discharge ~ I s +O s (10 6 m 3 ) a -7.5 – – – – -4.4 Ejecta volume of eruption (10 6 m 3 ) b I m + O m : Magma exchange between the dike and mid-crustal magma reservoir (10 6 m 3 ) -3.1 – – – – 10.7 Volume of degassed magma (10 6 m 3 ) c 22.3– – – – a Calculated using the molar volume of H 2 O in the gas phase, the partial molar volume of H 2 O in the melt [Ochs and Lange, 1997], and the gas volume fraction of 3.7–27.4% derived using the solubility model [Papale et al., 2006] and the equation of state [Soave, 1972]. b Data from Nakada et al. [2005] and calculated using data from Maeno et al. [2010] and the melt density reported by Ohwada et al. [2013]. c Calculated using the SO 2 emission rates reported by Ohwada et al. [2013] and the SO 2 concentrations in melt of 2400 – 4800 ppm reported by Ohta et al. [2007].

/30 Results Period February 2003 to May 2004 July 2004 to March 2005 May 2005 to June 2008 June 2008 to December 2008 TypeDeflationInflationDeflationInflation ΔV GPS (=I m + O m + I s +O s ): Volume change of the dike (10 6 m 3 ) Volume change by volcanic gas discharge ~ I s +O s (10 6 m 3 ) a -7.5 – – – – -4.4 Ejecta volume of eruption (10 6 m 3 ) b I m + O m : Magma exchange between the dike and mid-crustal magma reservoir (10 6 m 3 ) -3.1 – – – – 10.7 Volume of degassed magma (10 6 m 3 ) c 22.3– – – – The ejecta volume of the eruptions was minor compared to the volume decrease by gas discharge (I s +O s ) → Main source of OUTPUT from the volcano is volcanic gas discharge

/30 Results Period February 2003 to May 2004 July 2004 to March 2005 May 2005 to June 2008 June 2008 to December 2008 TypeDeflationInflationDeflationInflation ΔV GPS (=I m + O m + I s +O s ): Volume change of the dike (10 6 m 3 ) Volume change by volcanic gas discharge ~ I s +O s (10 6 m 3 ) a -7.5 – – – – -4.4 Ejecta volume of eruption (10 6 m 3 ) b I m + O m : Magma exchange between the dike and mid-crustal magma reservoir (10 6 m 3 ) -3.1 – – – – 10.7 Volume of degassed magma (10 6 m 3 ) c 22.3– – – – The volume of degassed magma was an order of magnitude larger than ΔV gps → Significant amount of magma up to one order of magnitude larger than ΔV gps may have been supplied to the shallow depths.

/30 Results Period February 2003 to May 2004 July 2004 to March 2005 May 2005 to June 2008 June 2008 to December 2008 TypeDeflationInflationDeflationInflation ΔV GPS (=I m + O m + I s +O s ): Volume change of the dike (10 6 m 3 ) Volume change by volcanic gas discharge ~ I s +O s (10 6 m 3 ) a -7.5 – – – – -4.4 Ejecta volume of eruption (10 6 m 3 ) b I m + O m : Magma exchange between the dike and mid-crustal magma reservoir (10 6 m 3 ) -3.1 – – – – 10.7 Volume of degassed magma (10 6 m 3 ) c 22.3– – – – Volume decrease by gas discharge (I s +O s ) was almost the same to ΔV gps → During inflation periods, I m +O m might be at least two times larger than ΔV gps → During deflation periods, ΔV gps could be solely explained by volcanic gas discharge

/30 Degassing and amount of degassed magma produced 20 SO 2 emission + SO 2 content in magma From SO 2 emission rates and SO 2 content in magma, degassed magma produced within the volcano can be calculated. [e.g., Ohwada et al. 2013] calculation Data are from Mori et al and Ohwada et al Degassed magma ( ): 4–9×10 8 m 3 SO 2 content in magma from 2400 to 4800 ppm was derived by melt inclusion analyses [Ohta et al. 2007]

/ eruption 21 Change in baseline Proxy of pressurization of dike

/ eruption 22 The dike inflated a month before the eruption. SO 2 emission rates increased with the eruption. Proxy of pressurization of dike Change in baseline

/ eruption 23 Contraction of KAHG-AVOG baseline -> Shear traction of the conduit Stretch of KVCO-TASH baseline -> dike pressurization

/ eruption 24 Contraction of KAHG-AVOG baseline -> Shear traction of the conduit Stretch of KVCO-TASH baseline -> dike pressurization

/ eruption 25 Shear traction in the conduit increased after the eruption Dike pressurized a month before the eruption SO 2 emission rates increased with the eruption