49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1047.pdf

Does have a Lopsided Asthenosphere? Insights from Katla’s Magma Plumbing System, Iceland. Steven M. Battaglia1,2, 1Univ. of Illinois, Dept. of Geol., Natural History Bldg, 1301 W. Green St., Champaign, IL, 61801; 2Northern Illinois Univ., Dept. of Geology, 312 Davis Hall, Normal Rd., DeKalb, IL, 60115 (email: [email protected])

Introduction: Io’s asthenosphere, or “magma ocean,” feeds melts to the hundreds of active hotspots observed on the surface. The diverse volcanism that is produced from these centers include lava lakes, lava flows, basaltic to ultramafic eruptions, and -rich plumes [1-5]. Differences in volcanic resurfacing mecha- nisms suggest contrasting magma plumbing systems for Io’s ascending melts. To explore these subsurface struc- tures, a sulfur solubility model [6] is used with previously identified magma compositions from recent eruptions of the Icelandic Katla [7-9]. Comparisons are made of sulfur solubility transitions in Katla’s inferred magma plumbing systems that correlate to diverse cooling scenar- ios for Io melts. Assuming lithosphere thickness, or dura- tion to ascend to the surface, as a primary cooling method for asthenospheric melts, it is proposed that Io’s magma ocean may be lopsided, favoring the leading hemisphere, based on volcanic resurfacing characteristics.

Fig. 1: Sulfur solubilities for Katla’s magma plumbing system. The Katla: Iceland, situated on the Mid-Atlantic Ridge solubilities are segregated into intervals within each cyclic peri- and atop of a hotspot, is a very volcanically active territo- od. Transitions (T1, T2… etc.), represent changes from one ry [10-11]. The high eruption rate throughout the Holo- plumbing system interval to the next. The diagram depicts the plumbing system changes for each interval in the inferred cycle. cene, including at the Katla volcano in southern Iceland

[12-13], was likely from additional melting as an outcome of isostatic rebound following deglaciation at the end of Sulfur Solubility Changes in Katla Magma Supply: the last ice age [14-15]. Since Io has an excessive heat Fig. 1 shows the sulfur solubilities and their transitions flux and a continuous sulfur-lithosphere resurfacing pro- from one plumbing system to the next. Intervals with a cess, Katla, with its recent activity, makes for a favorable lower sulfur solubility (e.g., VII, IV) are associated with a candidate to find analogies with Io’s magma dynamics. plumbing structure of sills and dikes while those with a Compositions of Katla’s basaltic eruptions were pre- higher solubility contain either a defined magma chamber viously determined by an analysis of Icelandic tephra (e.g., VI, III?, II?) or a simple system of vertical tubes layers [8]. The K2O concentrations were found to occur in that feed melts to the surface (e.g., VIII, V, I). A decrease a cyclic pattern and interpreted to be a result from chang- in sulfur solubility occurs for sill and dike systems that es in three magmatic plumbing system types: (1) a simple are formed following a simple structure. The solubility system, (2) a series of sills and dikes, and (3) a defined then increases as the series of sills and dikes transition magma chamber (Table 1). To compare Katla’s plumbing into a defined magma chamber. The sulfur solubility of structures and eruptive activity with Io’s sulfur-rich vol- melts may have supplementary increases if the magma canism, the sulfur solubility is determined for each chamber progresses into a simple plumbing system. plumbing system interval. Katla’s eruption frequencies are related to the magma supply’s plumbing structure and sulfur solubility. Sill and Table 1: Intervals of Katla’s magma plumbing systems. Time is dike systems, of which have low solubilities, are correlat- rounded to the nearest century. Eruption frequencies are relative ed to increased melt cooling and crystal fractionation that to the plumbing system types. Plumbing cycles are separated in- to periods of system changes from simple à sill and dike à can result in higher eruption rates and increased explosiv- magma chamber à simple (see [8] for reference, data). ity. The regularity of eruptions decreases as the sill and dike series evolve into a magma chamber, and then per- Interval Time Plumbing Eruption Plumbing haps into a simple system, from decreased melt cooling (Age) (ka) System Frequency Cycle and increased sulfur solubility. Melts among the magma VIII 8400 simple low cycle 1 chamber, however, can have increased crustal contamina- VII 7700 sill+dike high cycle 1 tion that may lead to a higher explosive index for erup- VI 6900 chamber medium cycle 1 V 6300 simple low cycle 1, 2 tions compared to a simple system. The simple structures IV 4500 sill+dike high cycle 2 that feed melts directly to the surface are expected to have III 3700 chamber? medium cycle 2, 3? the least explosive eruptions (in comparison to the other II 1900 chamber? medium cycle 3? two plumbing systems) from a further decrease in melt I 1000 cimple? low cycle 3? fractionation and increase in sulfur solubility. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1047.pdf

Discussion: Katla’s sulfur solubility changes occur thickness of Io’s lithosphere is greater on the trailing primarily from an increase or decrease in the melt cooling hemisphere that slows melt cooling, decreases sulfur sol- scenarios that correspond to the magma plumbing struc- ubility, and forms sill and dike systems with large sulfu- ture. A series of sills and dikes increases crystal fractiona- rous degassing at the surface and a higher index of explo- tion from more melt cooling with decreases in sulfur sol- sivity (e.g. , Babbar, Pillan). On the other hand, melts ubility. In addition, a magma chamber or simple plumb- in Io’s leading hemisphere have short cooling durations in ing system undergoes less melt cooling and a decrease in simple systems that ascend through a thin lithosphere and crystal fractionation with a progression to a higher sulfur resurface mostly in the form of lava flows with a lower solubility. On Io, the volcanism is driven by extensive index of explosivity (e.g. , Masubi, Kanehekili). interior heating [16] and a continuous lithosphere-sulfur The melts in the jovian and anti-jovian regions therefore burial process [5] that can be assumed to occur over the contain a mixture of these two end-spectrums (i.e. lava entire satellite. Using these criterion, a primary cooling lakes with periodic eruptions, some sulfur-plumes, and method for ascending Io melts is the thickness of the lith- lava flows) that likely develop from the formation of a osphere, or depth to the asthenosphere, in which the defined magma chamber beneath the hotspot (e.g. Prome- plumbing systems that form are thus dependent on the theus, Surt, Loki, , Tvashtar). Furthermore, if previ- duration for Io magmas to reach the near-surface. ous analyses of Io’s crustal thickness variations is as- The transitions in sulfur solubility throughout the sumed to be between 30-50 km [2,4], then the depth to the Holocene for Katla’s plumbing system can relate to Io’s magma ocean on the leading hemisphere is implied to be diverse volcanism. For instance, Pele, located on Io’s near the 30 km parameter while the trailing hemisphere is trailing hemisphere, sustains an active lava lake, high near the 50 km extent. eruption rates, and produces a large, degassed, sulfurous Io’s resurfacing is derived by excess interior heat plume. These characteristics are consistent with Katla’s from the orbital tides with , Europa, and Gany- sill and dike system interval that incorporates increased mede. It has been shown that a model of a tidally heated melt cooling and decreased sulfur solubility. The Amirani Io with both solid (deep mantle) and fluid (magma ocean) volcano on Io’s leading hemisphere demonstrates Katla’s influences can better explain the volcanic heat flux, geo- more simplified periods of vertical tubes that directly feed graphic centers of hotspots, and the topography [16]. A melts to the surface with effusive eruptions and generous model that therefore includes a lopsided asthenosphere, in lava flows from high sulfur solubility and decreased melt which the layer favors Io’s leading hemisphere, may fur- cooling. The patera located on Io’s anti- ther improve these results. jovian hemisphere near the equator has volcanic attributes of both effusive eruptions (e.g., Amirani) and explosive- References: [1] Radebaugh et al. (2001), J. Geophys. Res. style volcanism (e.g., Pele) yet exhibit these resurfacing 106, 33005-33020. [2] Lopes et al. (2004), Icarus 169, 140-174. mechanisms on smaller scales similar to Katla’s intervals [3] Spencer et al. (2007), Science 318, 240-243. [4] Khurana et of a defined magma chamber. al. (2011), Science 332, 1186-1189. [5] Battaglia et al. (2014), Icarus 235, 123-129. [6] Li & Ripley (2009), Econ. Geol. 104, A Lopsided Magma Ocean?: Using the observed 405-412. [7] Larsen (2000), Jokull. 49, 1-28. [8] Oladottir et al. (2008), Bull. Volcanol. 70, 475-493. [9] Larsen (2010), Dev. volcanoes and their resurfacing characteristics to identify Quat. Sci. 13, 23-49. [10] Saemundson (1978), Jokull. 29, 7-29. differences in magma plumbing dynamics, while also [11] Wolfe et al. (1997), Nature 385, 245-247. [12] Howell et considering depth to the asthenosphere as a primary cool- al. (2014), Earth Planet. Sci. Lett. 392, 143-153. [13] Budd et ing method for melts, suggests that Io’s interior may have al. (2016), Geochem. Geophys. Geosyst. 17, 966-980. [14] an asymmetric magma ocean, or a lopsided asthenosphere Thordarson et al. (2003), In: Volcanic degassing, Geol. Soc. (Fig. 2). London, 103-121. [15] Sturkell et al. (2010), Dev. Quat. Sci. 13, Within this model of a lopsided magma ocean, the 5-21. [16] Tyler et al. (2015), APJ Supp. Series 218 (22), 17pp.

Fig. 2: (Right Panel) A sketch of Io with a lopsided asthenosphere. The orange represents the magma ocean that favors the leading hemi- sphere and is offset by ~30° longitude per heat flux and tidal heating models [16]. (Lower Panel) An estimation map of changes in litho- sphere thickness from Io’s surface. The purple region represents areas with greater depths (~50 km) to the asthenosphere on the trailing hemisphere. The blue region represents areas where Io’s crust is thin- ner (~30 km) on the leading hemisphere.