Magma Ascent Rates Malcolm J Rutherford Dept. of Geological Sciences Brown University Providence RI (Presentation for MSA short Course Dec. 13, 2008)

1. Why study magma ascent rates? - Better understanding of subsurface magmas and processes. - Prediction of eruption style changes, e.g., explosive vs. effusive. 2. How can we determine ascent rates? - Melt degassing produced by ascent-induced decompression. - Crystallization driven by decompression and degassing. - Crystal-melt reactions “ “. - Seismicity changes associated with magma ascent. - Theoretical modeling of magma ascent. 3. Caveats and problems involved? - Determining where magma ascent began. - Determining the exact nature of reactions involved. - Involvement of magma convection and mixing. - The size of the conduit. Outline

Phase equilibria of MSH dacite magma showing amphibole stability limit; inset Shows internal zoning in the amphibole and Opx that requires two cycles of convection from 300 to 120 MPa prior to final ascent (after Rutherford and Devine, 2008). Amph Opx 4 km -

Using diffusion data for water, Humphries et al., (2008) calculate time required to develop measured profile in tubular melt channels. Assuming 4.6 wt% H 2 O initially, L= 5 km, C.R. = 25 m, v = m/s for MSH May 18, Method similar to that of Anderson (1996) & Liu et al., 2007.

Conduit Radius (effective) and Magma ascent velocity changes derived from magma mass eruption rates at Mount St. Helens in eruptions assuming single phase flow (after Geschwind and Rutherford, 1995). - Ave. Ascent rate for 1980 explosive = 3 m/s for 8 km (C.R. = 33 m). - Theoretical modeling V = m/s (C.R.; 17-33m) Papale & Dobran(1994). - Humphries et al., (2008) V = m/s. * Conduit radius and depth both critical in determining rate of ascent.

I I I I 0.01 Ascent rates (m/s) Ascent rates based on crystallization induced by decompression in MSH dacite magma (after Geschwind and Rutherford, 1995). Assume 8 km starting depth and conduit radius from mass eruption rate data (Swanson et al., 1987).

Phase equilibria of MSH dacite magma showing amphibole stability limit; inset shows amphibole from a 2005 sample with a reaction rim (after Rutherford and Devine, 2008). SH308 Amph

Amphibole reaction rim-width growth in dacitic and andesitic magmas as a function of decompression time (constant rate decompression) from experiments at o C (after Rutherford and Hill, 1993; Rutherford and Devine, 2003).

Images showing reaction rims on amphibole phenocryst in the Black Butte (CA) dacite; also illustrated is the contrast in texture of the phenocrysts relative to the lineated matrix. After McCanta et al., (2007). - Rim widths (34  m) in each of 4 lobes of BB lava dome erupted at 890 o C, yield an ave. magma ascent rate of m/s for ascent from 200 MPa (~8 km) to the surface. - ** Single stage ascent and no mixing indicated for these magmas.

New amphibole reaction rim growth rate study show how the rate varies with P at a 840 o C (Brown and Gardner, 2006). 840 o C

Temperature affect of decompression-induced crystallization in MSH dacite Modified after Blundy et al., (2006)

F-rich amphibole rims develop in very slow ascending Mount St. Helens dacite magma preventing rim growth (DeJesus and Rutherford, 2008) F wt % A B

- Zoning developed in Mt Unzen Ti- magnetite. (Nakamura, 1995). Profile reflects time following magma mixing, and yields a minimum ascent time assuming that mixing accompanied onset of ascent. - Ascent of Mt. Unzen mixed magma from 7 km was at a min. rate of m/s ( da) based on 20  m profile.

Montserrat: TiO 2 profiles for natural and experimental Ti- magnetite (after Devine et al., 2003) o C andesitic magma reached surface from 5 km depth ~20 days after basaltic andesite influx.

Real Time Seismic Amplitude counts build-up at site near Mount St. Helens in 1986 along with focal depth shows magma rising from 1.4 km to surface over 12 hours; this corresponds to an ave magma ascent rate of 0.32 m/s (Endo et al., 1996)

Xenolith-melt reactions. Two scales of reaction in olivine- rich xenoliths carried up in basalt at La Palma, Canary Islands. (1) Long diffusion profiles in Olv = yrs storage at 7-15 km; (2) Melt bearing fractures in Olv have zoning that indicates 0-4 days origin and ascent rates of > 0.06 m/s (Klugel et al., 1998) assuming cracks began with ascent. Formation at 7-15 km (1) (2)

Garnet-melt reaction in kimberlite (Canil and Fedortchouk, 1999). Garnet dissolution features (~25  m) interpreted using experimental data yield exposure times; minimum ascent rates depending on depth where garnet is exposed and T.

Water loss profiles in olivine in garnet peridotite xenolith carried in alk basalt (Demouchy et al., 2006; Peslier and Luhr, 2006). Assuming 40 and km depth for inclusion enclosure in basalt respectively, initial water = 300 ppm, and 1200 o C, D et al., calculate 6.3 m/s ascent; P & L ascent rate = m/s.

Kimberlite Magma Ascent Rates from theoretical flow modeling 1.Calculation is for buoyancy-driven dike flow (Stage 2), does not consider effect of gas expansion that would be particularly important in Stage 3 (1- 0 km). 2.Calculation agrees well with estimates from garnet dissolution and with xenolith transport requirements (Spera, 1984).

Dissolved volatiles in Min2 shoshonite and F.R latite olv- and cpx-hosted melt inclusions (FTIR) after Mangiacapra et al., 2008, GRL.

Lobe 1, SHM-22, < 50um Slope: , intercept: um, slope: , intercept: Plagioclase Phenocrysts and microlites in Black Butte CA dacite. CSD for the two phases of crystallization gives growth rate

1. Why study magma ascent rates? - Better general understanding of subsurface volcanic processes. - Prediction of eruption style, e.g., explosive vs. effusive. 2.How can we determine ascent rates? - Magma degassing rates from melt phase. - Crystallization driven by decompression and degassing. - Crystal-melt reactions “ “. - Seismicity associated with magma ascent. - Zoning developed in crystals by decompression. - Theoretical modeling of magma ascent. 3. Caveats and problems involved? - determining where magma ascent began, the nature of reaction observed, the parameters controlling the reaction, involvement of magma convection and mixing, the size of the conduit. Objectives

Magma storage zone at Mount Pinatubo in 1991 based on seismicity and petrological phase equilibria of the phenocryst - melt assemblage (after Pallister et. al., 1996 and Hammer, 2003).