“Poseidic” Explosive Eruptions at Loihi Seamount, Hawaii
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Downloaded from geology.gsapubs.org on October 5, 2010 “Poseidic” explosive eruptions at Loihi Seamount, Hawaii C. Ian Schipper*1, James D.L. White1, Bruce F. Houghton2, Nobumichi Shimizu3, and Robert B. Stewart4 1Geology Department, University of Otago, PO Box 56, Leith Street, Dunedin 9016, New Zealand 2School of Ocean and Earth Science and Technology (SOEST), University of Hawai’ i at Ma¯noa, 1680 East-West Road, Honolulu, Hawaii 98622, USA 3Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 4Soil and Earth Sciences, Institute of Natural Resources (INR), Massey University, PB 11-222, Palmerston North 4474, New Zealand ABSTRACT (A.D. 1996) of Loihi’s ~400 ka history (Moore Much remains unknown about submarine explosive eruptions. Their deposits are found et al. 1982; Garcia et al. 2006). to great depths in all the world’s oceans, but eruptions are typically described by analogy Here we describe the southern cone on the to a subaerial nomenclature that ignores the substantial and inevitable infl uences of hydro- southeast summit plateau of Loihi (18°54′N, static pressure and magma-water interaction at submerged edifi ces. Here we explore mag- 155°15′W), examined in October 2006 with matic volatile exsolution and magma-water interaction for a pyroclastic cone-forming erup- the Hawaiian Undersea Research Laboratory’s tion at ~1 km depth on Loihi Seamount, Hawaii. We examine vesicle textures in lapilli—the Pisces IV submersible. The cone is ~60 m high, physical manifestation of degassing; dissolved volatiles in matrix glasses and olivine-hosted 4 × 106 m3 in volume, with a faintly discernable glass inclusions—the geochemical record of ascent and volatile exsolution; and fi ne ash summit rim we interpret as the edge of a partly morphology—the evidence for if and how external water assisted in fragmentation. This infi lled crater at ~1080 mbsl (Fig. 1C). We col- approach allows a submarine explosive eruption style to be defi ned: the magma achieved lected fi ve large samples of lapilli (2–64 mm)– ~40% vesicularity through almost perfectly closed-system volatile exsolution from ~3 km dominated material from ~1292–1100 mbsl on below the vent, which accelerated and weakened the melt, allowing it to be fragmented by the cone fl anks (Fig. 1C), on which occasional explosive magma-water interaction. We introduce the name “Poseidic” for this end-member subspherical bombs as large as 40 cm were style of submarine basalt explosivity. Poseidic eruptions are identifi able from measurable observed, but not collected intact. Our several- features in pyroclasts, and are possible at all subaqueous basaltic volcanoes. kilogram samples are unique in their volume, grain-size range, and fi eld characterization, INTRODUCTION sive eruption style, here termed “Poseidic,” is and allow us to use measurable features from Explosive submarine eruptions have been the fi rst to be analytically defi ned. a range of the pyroclasts to rigorously interpret described by analogy to subaerial eruption eruption style. styles for which deposit dispersal is a defi n- LOCATION, DEPOSIT, AND METHODS The bulk densities of 225 lapilli were mea- ing characteristic (Walker, 1973), with later Loihi Seamount is the youngest Hawaiian sured following the technique of Houghton and modifi cations to the subaerial classifi cation volcano. Located ~35 km southeast of the island Wilson (1989), who included error estimates scheme incorporating varying mechanisms of Hawaii (Fig. 1A), it rises ~3–5 km from the for such measurements. Vesicle textures were of magma ascent and degassing (Wilson and seafl oor, to an ~12 km2 summit plateau at ~1200 examined on standard polished thin sections. Head, 1981; Vergniolle and Jaupart, 1986; m below sea level (mbsl) (Fig. 1B). The summit Major element glass compositions of lapilli, Parfi tt, 2004; Houghton and Gonnermann, plateau has several conical landforms that reach ash (glass selvages on olivine crystals sieved 2008). Most researchers have interpreted sub- just under 1 km depth, and three pit craters, one directly from the samples), and olivine-hosted marine explosive eruptions as Strombolian of which formed in the most recent eruption glass inclusions, were determined by quantita- (Davis and Clague, 2006; Clague et al., 2008; tive energy-dispersive electron microprobe. H2O Sohn et al., 2008), driven by the accumulation and CO2 contents were determined by transmis- and escape of volatiles at unknown rates, from sion Fourier transform infrared spectroscopy 158oW 156oW N S N magma bodies of unconstrained sizes, at inde- A B 0 (FTIR) on lapilli glasses, and by secondary ion Hawaii terminate depths. Such scenarios are diffi cult, 21oN mass spectroscopy (SIMS) on ash and glass Kilauea 4 Loihi if not impossible, to test. Deposits have been Pacific inclusions. FTIR and SIMS have been shown Ocean Mauna Mauna km below sea level Loa characterized from small, almost exclusively o Loihi 8 to yield comparable results for volatile analysis 19 N Loa Pacific ocean crust fi ne-grained samples (Clague et al., 2003, C Summit plateau -155.15.0 (Hauri et al., 2002). Analytical operating con- 2008; Davis and Clague, 2006; Sohn et al., ditions are given in the GSA Data Repository.l 2008), and are often from unidentifi ed vents. -1200 Fine ash particles were examined and character- -1100 These provide little textural, and no dispersal, 18.54.5 ized using scanning electron microscopy. evidence to support analogies with subaerial eruptions of any type. Here we show that at Fault? VESICLE TEXTURES Loihi Seamount, Hawaii, early closed-system Southern cone lapilli have modal bulk vesicu- -1100 degassing into small vesicles that remained cone larity of ~42%, higher than typical Loihi lavas Southern mechanically coupled to their host melt (vol- 100 m atile-coupled degassing) facilitated later inter- action of the ascending melt with seawater to Figure 1. A: Hawaiian Islands, with Mauna 1GSA Data Repository item 2010081, supplemen- drive violent eruptions. This submarine explo- Loa, Kilauea, and Loihi volcanoes. B: Loihi tary information and Tables DR1–DR3, is available cross section (Garcia et al., 2006), 2× vertical online at www.geosociety.org/pubs/ft2010.htm, or exaggeration. C: Southern cone bathymetry on request from [email protected] or Docu- (created by J.R. Smith), 20 m contours. Stars ments Secretary, GSA, P.O. Box 9140, Boulder, CO *E-mail: [email protected]. mark sample locations. 80301, USA. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2010; 2010 v. 38; no. 4; p. 291–294; doi: 10.1130/G30351.1; 4 fi gures; Data Repository item 2010081. 291 Downloaded from geology.gsapubs.org on October 5, 2010 (mostly 0%–20%, reaching ~40% only in rare mon in subaerial Strombolian ejecta (e.g., Lau- (Figs. 3A and 3B). Because the chemistry pillow lavas; Moore et al., 1982; Garcia et al., tze and Houghton, 2005; Polacci et al., 2006). of lapilli and crystals sieved directly from the 2006), and overlapping the range for subaerial deposit does not vary with sample site, the exso- Hawaiian (45%–95%; Cashman and Mangan, VOLATILE EXSOLUTION PROCESSES lution paths captured represent volatile system- 1994) and Strombolian (40%–76%; Lautze and The ~1 km depth of Loihi’s summit is of atics in the southern cone magma as a whole, not Houghton, 2005; Polacci et al., 2006) pyro- interest for volatile exsolution dynamics. At just in an isolated parcel of magma preserved as clasts (Fig. 2A). Quenched sideromelane rims this ~10 MPa pressure, CO2 has limited solu- a single olivine-bearing clast or lava sample. on the Loihi clasts have ~34% subspherical to bility in basalt (Dixon and Stolper, 1995), but A plot of CO2 versus H2O (Fig. 3C) shows polylobate vesicles. Tachylite interiors have H2O solubility is controlled in large part by the that the matrix glasses and glass inclusions fi t a ~45% vesicles, similarly shaped, but with an CO2-H2O-magma system evolution. If exsolved closed-system, volatile-coupled exsolution path additional subpopulation of small subspherical CO2-rich fl uids remain in contact with the melt, from the most CO2-rich inclusion measured vesicles (Fig. 2A). No signifi cant vesicle coales- they thermodynamically stabilize CO2+H2O fl u- (Holloway, 1976; Dixon and Stolper, 1995; cence is apparent. Vesicularity is much less than ids, facilitating subsequent exsolution of water Newman and Lowenstern, 2002); but not an 70%, and vesicles are not signifi cantly intercon- (Holloway, 1976; Dixon and Stolper, 1995; open-system, volatile-decoupled path. The spe- nected, indicating that the magma had limited Dixon et al., 1995; Dixon and Clague, 2001; cial case of volatile decoupling, with CO2-rich permeability (Polacci et al., 2006; Namiki Newman and Lowenstern, 2002), so that vola- magmatic fl uids fl uxing through the magma to and Manga, 2008). Vesicle textures in Loihi tile coupling greatly infl uences eruption style. induce H2O exsolution without the development lapilli resemble those in scoria from the 1959 Lapilli and ash matrix glasses are geochemi- of vesicles (Rust et al., 2004; Spilliaert et al., Kilauea Iki Hawaiian fi re fountains (Fig. 2B). cally identical (Fig. 3A), indicating that the 2006), is unlikely for the southern cone melt Observed episodicity of Kilauea Iki fountains southern cone was derived from a single batch because degassing trends in Figure 3C crosscut implies some degassing variability (Houghton of magma. Southern cone glass inclusions are vapor isopleths, and vesicle textures suggest that and Gonnermann, 2008), but late-stage exsolu- subspherical, hosted in slightly zoned (Fo86–85) the magma permeability required for such fl ux- tion of magmatic H2O in this classic Hawaiian olivine crystals. Matrix glass compositions can ing would have been negligible (Polacci et al., fountaining eruption was melt-coupled (Parfi tt, be obtained by <15% olivine fractionation from 2006; Namiki and Manga, 2008).