Degassing at Anatahan volcano during the May 2003 eruption: Implications from petrology, ash leachates, and SO 2 emissions

Abstract

On 10 May 2003, Anatahan volcano (located at 16°21′ N 145°40′ E on the Mariana arc) entered its first historical eruptive episode, sending ash to > 12 km into the atmosphere. Abundant accretionary lapilli, quenched pumice textures, and hydrothermal minerals in the earliest eruptive deposits indicate hydromagmatic interaction and active mining of the pre-eruptive hydrothermal system. Whole-rock compositions of the products erupted within the first week are chemically homogenous, with SiO 2 ∼61%, MgO ∼2.1%, K 2O ∼1.4%, Na 2O ∼4.1% and Fe 2O 3 ∼9.1%. The products are classified as medium-K andesites with tholeiitic affinity. Slightly more silicic matrix glass compositions (up to 63% SiO 2 in microlite-rich matrices) overlap with whole rock, suggesting limited crystal fractionation with microlite crystallization responsible for the more evolved residual melt. Decreasing corrected LOI values (2.3–1.4 wt.%) upsection are consistent with waning hydrothermal mineral contributions as the eruption progressed. Oxygen fugacity calculations based on the ferric to ferrous iron ratio of bulk samples indicate an oxidized magma with ΔNNO ∼+1. Two-pyroxene equilibrium thermometry suggests magmatic temperatures of 1050–1100 °C. Matrix glass volatile contents show a degassed residual melt, with < 0.5 wt.% H 2O, 1000–2000 ppm Cl, 480–780 ppm F, 50–150 ppm S, and < 5 ppm CO 2. A magmatic SO 2 flux of 3–4.5 kt/day was measured by COSPEC on 21 May. Ash leachate data indicate a decreasing S/Cl ratio (3.3–0.7) in the eruptive plume between 10 and 21 May, with a relatively constant Cl concentration. Assuming a constant Cl flux, an SO 2 flux of 14–22 kt/day is calculated for 10 May. The average S concentration from ash leachates (1230 mg/kg) suggests that at least 25% of the SO 2 (∼60 kt) erupted from Anatahan between 10 and 21 May was removed from the plume by the precipitation of sulphate salts in the eruption column, adsorbtion onto ash particles and subsequent deposition. Molar ratios in ash leachates elucidate CaSO 4 and NaCl as the most likely soluble salts formed in the plume. Total element abundances, molar S/Ca > 1 and Ca, Mg, Na, and K ratios in the leachates suggest a hydrothermal fluid contribution to elements present as water soluble salts adsorbed onto ash. Sulfur budget calculations based on estimates of pre-eruptive magmatic and residual melt S contents, mass of erupted magma, and total SO 2 output fluxes require an additional source of S other than the erupted magma. Multiple lines of evidence, including high SO 2 emissions early in the eruption, the presence of accretionary lapilli and hydrothermal minerals in the early eruptive deposits, quenched pumice textures, and cation and anion ratios and abundances in ash leachates suggest that a S-rich free volatile phase exsolved from a large magma body. Magmatic volatiles were stored as components of the hydrothermal system (pressurized gases, hydrothermal fluids, and/or hydrothermal minerals) to be remobilized early in the eruption to contribute to the total SO 2 output.