Ulrike Martin

Compositional variation during monogenetic volcano growth and its implications for magma supply to continental volcanic fields

Individual volcanoes of continental monogenetic volcanic fields are generally presumed to erupt single magma batches during brief eruptions. Nevertheless, in two unrelated volcanic fields (the Waipiata volcanic field, New Zealand, and the Miocene–Pliocene volcanic field in western Hungary), we have identified pronounced and systematic compositional differences among products of individual volcanoes. We infer that this indicates a two-stage process of magma supply for these volcanoes. Each volcano records: (1) intrusion of a basanitic parent magma to lower- to mid-crustal levels and its subsequent fractionation to form a tephritic residual melt; (2) subsequent transection of this reservoir by a second batch of basanitic melt, with tephrite rising to the surface at the head of the propagating basanite dyke. Eruption at the surface then yields initial tephrite, typically erupted as pyroclasts, followed by eruption and shallow intrusion of basanite from deeper in the dyke. By analogy with similar tephrite–basanite eruptions along rift zones of intraplate ocean-island volcanoes, we infer that fractionation to tephrite would have required decades to centuries. We conclude that the two studied continental monogenetic volcanic fields demonstrate a consistent history of early magmatic injections that fail to reach the surface, followed by capture and partial eruption of their evolved residues in the course of separate and significantly later injections of basanite that extend to the surface and erupt. This systematic behaviour probably reflects the difficulty of bringing small volumes of dense, primitive magma to the surface from mantle source regions. Ascent through continental crust is aided by the presence in the dyke head of buoyant tephrite captured during transection of the earlier-emplaced melt bodies.

Shallow-marine phreatomagmatic eruptions through a semi-solidified carbonate platform (ODP Leg 144, Site 878, Early Cretaceous, MIT Guyot, West Pacific)

Abstract Leg 144 (1995) of the Ocean Drilling Program recovered basalts and volcaniclastic material from the volcanic basement of several guyots in the northwest Pacific, including MIT Guyot in site 878. Tectonic reconstruction suggests that this seamount originated as an intraplate volcano on the Pacific Superswell in the Early Cretaceous. Tephra emplaced by mass flow within the sedimentary cap is the subject of this study. The tephra unit consists of palagonitized, moderately to strongly vesicular basaltic clasts of apparent phreatomagmatic origin, carbonate mud- to grainstone clasts, and armoured lapilli. Armoured lapilli are inferred to have been formed during eruption within a steam envelope that developed above the vent due to magma–water interaction and high gas content of the magma. Another distinctive feature of the tephra unit are abundant truncation surfaces that cut the lithotypes due to subsidence, and are juxtaposed to the same lithotypes. These truncation surfaces are inferred to represent intra-eruption slip surfaces, which formed during eruption of the volcano that generated the tephra pile. The resulting slumps initiated grainflows and dilute turbidity currents, whose deposits are interspersed with those of high-concentration density currents. A subaqueous setting of deposition and eruption is indicated by the abundance of sideromelane glass shards, cauliflower-chilled clasts, the underlying marine carbonates, the lack of tachylite within the deposits, and sedimentary structures within the breccia.

Hyaloclastites, peperites and soft-sediment deformation textures of a shallow subaqueous Miocene rhyolitic dome-cryptodome complex, Pálháza, Hungary

Abstract The NE Tokaj Mountains at Pálháza in NE Hungary are made up of a complex association of Miocene rhyolitic shallow intrusions, cryptodomes and endogenous lava domes emplaced into and onto soft, wet pelitic sediment in a shallow submarine environment. The intrusive–extrusive complex shows a range of interaction textures with the host muddy sediment, ranging from blocky peperites, formed on a 0.1 m-scale, through to irregular contacts closely resembling globular mega-peperites, on a >10 m-scale. The over 200 m-thick igneous succession is interpreted to result from the pulsatory growth of shallow cryptodomes through muddy saturated host sediment. The intrusions eventually breached the sedimentary cover to build up thick in situ hyaloclastite piles in the shallow subaqueous environment. The coherent rhyolitic cryptodome facies is surrounded by intrusive hyaloclastite in the contact zone to the pelitic host sediment. In the upper level of the complex, rhyolitic dome rock is capped and surrounded by hyaloclastite formed due to quench fragmentation upon contact of the lava surface with sea water.

Peperitic lava lake-fed sills at Ság-hegy, western Hungary: A complex interaction of a wet tephra ring and lava

Abstract Ság-hegy is the remnant of a complex volcano consisting of several phreatomagmatic pyroclastic sequences preserved in immediate contact with a thick (c. 50 m) coherent lava body. Due to the intensive quarrying, the inner part of the lava has been removed, leaving behind a castle-like architecture of pyroclastic rocks. The outcrop walls thus demonstrate the irregular morphology of the lava, which was emplaced in a NW-SE-trending ellipsoidal vent zone in a phreatomagmatic volcano. Pyroclastic beds in the quarry wall are cross-cut by dykes and sills, inferred to have been fed from a central magma zone. Thin (<10 cm) strongly chilled, black, angularly jointed aphanitic basaltic lava mantles the pyroclastic sequence, and has a corrugated margin as a consequence of sudden chilling against the cold and wet phreatomagmatic tephra in the inner wall of the tuff ring crater. These corrugated zones are inferred to be a characteristic textural feature, indicating extensive mixing of lava and host tephra which led to peperite formation along the outer rim lava lake. A spectrum of peperite formed along the lake margin, and fluid oscillation, due to fluidization of the wet tephra, disrupted a steam envelope formed around the lava, causing basaltic magma to invade and mix with the phreatomagmatic tephra. The presence of unconformities in the tephra ring facilitated the formation of sills fed from the central lava body.

Melting and mingling of phonolitic pumice deposits with intruding dykes: an example from the Otago Peninsula, New Zealand

Abstract A mound of bedded phonolitic pumice lapilli ash was deposited on the marine shelf (modern Otago Peninsula, New Zealand) during a shallow subaqueous eruption in the Miocene. Basaltic dykes subsequently intruded the unconsolidated pumice deposit and formed an unusual peperite. Dyke–sediment interaction began with weak steam-driven mixing, accompanied by fluidisation at the contact zone that produced a metres-broad zone in which bulbous tongues (up to 3 m) of the dykes extended into, and locally engulfed, host sediment. Irregular, sinuous contacts of dykes with the pumice are marked by black glass that formed by partial melting, shearing and remoulding of the glassy pumice. Such melting and remoulding of host sediment is not a typical peperite feature. Temperature evolution, volume of water vapour, load pressure and time, as well as initial contact temperature, control the welding and remoulding of pumice and glass shards. A slow cooling rate of the dykes at the dyke contacts is inferred from remoulding textures, and is attributed to continuous feeding of the dyke. Seawater may have promoted softening and remoulding of the pumice because Na and K, if partially taken into the glass structure, can decrease the melting point of glass dramatically and reduce the viscosity of the shards. Remoulding may have occurred by continuous conversion of seawater in the peperite to steam, which is inferred to have enhanced transfer of Na and K from seawater to pumice glass.

Eruptive mechanism of phreatomagmatic volcanoes from the Pinacate Volcanic Field: comparison between Crater Elegante and Cerro Colorado, Mexico [Eruptionsmechanismen phreatomagmatischer Vulkane des Pinacate Vulkanfelds: Ein Vergleich zwischen Krater Elegante und Cerro Colorado, Mexiko]

The Pinacate Volcanic Field is located just near the northern end of the Gulf of California (Sea of Cortez) in Sonora, Mexico. Extensive lava flows cover an area similar to 2000 km2 which is cut through by more than 400 vents, predominantly scoria cones. Eight of the vents are maars that erupted immediately after emplacement of pre-maar lava flows, which are exposed in the crater wall. Two of the phreatomagmatic vents are especially spectacular by their size and volume, and their contrasting architecture; Crater Elegante and Cerro Colorado. Crater Elegante is about 1600 m across with a crater that is about 250 m deep, which is surrounded by a few tens of metres complete crater rim. Its age is inferred to be 0.15 Ma, and its pyroclastic deposits are dispersed more than a km away from the crater rim. They form a gentle sloping blanket over pre-maar lava flows exhibiting plastering effect over obstacles such as pressures ridges and lava blisters of the pre-maar lavas. Pyroclastic units are predominantly lapilli tuffs that are rich in fine silt, sand and angular, non-vesicular sideromelane glass shards tephritic in composition. The fine lapilli tuff and tuff units about a few hundred metres away from the rim are especially rich in angular quartz fragments that are loosely packed. Scour fills in the pyroclastic succession are calcite cemented. There is a notable trend in a quick reduction in the volume of large volcanic accidental lithic fragments derived from the various pre-maar lava flows in the lapilli tuffs from the crater rim toward distal areas. Bedding characteristics of the pyroclastic succession are predominantly massive to well bedded in near-vent settings that quickly change to dune bedded successions with dunes having a few dm amplitude over metres wavelength. These are characteristics of deposition from sudden blast triggered base surges. The Cerro Colorado is just ~8-km to the NE from Crater Elegante and forms a ~ 100 m positive volcanic landform. The crater floor is just a few tens of metres below the inferred synvolcanic surface and forms a ~600-m wide, flat depression. The crater inner wall is mantled by collapsed blocks of tuff breccia and lapilli tuff that feed small reworked volcaniclastic fans. The pyroclastic succession of Cerro Colorado is significantly coarser grained, and thicker bedded than the Crater Elegante succession. Lapilli tuffs and tuffs are rich in amoeboid shaped moderately-to-highly vesicular sideromelane lapilli and coarse ash tephritic in composition that are rimmed by gel palagonite. Intact gravel/pebble of mud and/or silt are the main accidental lithic fragments. The distinct differences in size, vesicularity, shape and alteration effect of the sideromelane shards, the ratio between juvenile to accidental lithic fragments reflect the difference of the depth of fragmentation and/or source of water interacted with the magma. Crater Elegante