Fire and ice: unravelling the climatic and volcanic history of James Ross Island, Antarctic Peninsula

Back in the mid-nineteenth century British explorer James Clark Ross took his ships, HMS Terror and HMS Erebus , farther south than anyone else had been. He now lends his name to James Ross Island, a part-volcanic edifice that rises out of the sea off the north-east tip of the Antarctic Peninsula. The island records a geological history dating back to the Cretaceous, though its great peaks are volcanic. The most recent rocks of the island record a monumental struggle between fire and ice, the volcanoes, and the ice sheets that cover them. The glacigenic sediments that are interspersed with the volcanic rocks contain rich fossil assemblages which suggest that at times, the climate was warmer, with the ice retreating. Their study may help us to delimit the patterns of climate change in the Antarctic Peninsula region as Earth's global climate warms.     

Magmatic evolution of the South Shetland Islands,Antarctica, and implications for continental crust formation

Lavas from the South Shetland Islands volcanic arc (northern Antarctic Peninsula) have been investigated in order to determine the age, petrogenesis and compositional evolution of a long-lived volcanic arc constructed on 32-km-thick crust, a thickness comparable with average continental crust. New ⁴⁰Ar–³⁹Ar ages for the volcanism range between 135 and 47 Ma and, together with published younger ages, confirm a broad geographical trend of decreasing ages for the volcanism from southwest to northeast. The migration pattern breaks down in Palaeogene time, with Eocene magmatism present on both Livingston and King George islands, which may be due to a change in both subduction direction and velocity after c. 60 Ma. The lavas range from tholeiitic to calc-alkaline, but there is no systematic change with age or geographic location. The compositions of lavas from the north-eastern islands indicate magma generation in a depleted mantle wedge with relatively low Sr and high Nd isotopic compositions and low U/Nb, Th/Nd and Ba/Nb ratios that was metasomatized by hydrous fluids from subducted basaltic oceanic crust. Lavas from the south-western islands show an additional sedimentary influence most likely due to fluid release from subducted sediments into the mantle wedge. Although magmatic activity in the South Shetland arc extended over c. 100 m.y., there is no evolution towards more enriched or evolved magmas with time. Few South Shetland arc lavas are sufficiently enriched with incompatible elements to provide a potential protolith for the generation of average continental crust. We conclude that even long-established subduction zones with magmatic systems founded on relatively thick crust do not necessarily form continental crustal building blocks. They probably represent only the juvenile stages of continental crust formation, and additional re-working, for example during subsequent arc-continental margin collision, is required before they can evolve into average continental crust.     

Late Miocene volcanic sequences in northern Victoria Land,Antarctica: products of glaciovolcanic eruptions under different thermal regimes

Late Miocene (c. 13–5 Ma) volcanic sequences of the Hallett Volcanic Province (HVP) crop out along >250 km of western Ross Sea coast in northern Victoria Land. Eight primary volcanic and six sedimentary lithofacies have been identified, and they are organised into at least five different sequence architectures as a consequence of different combinations of eruptive and/or depositional conditions. The volcanoes were erupted in association with a Miocene glacial cover and the sequences are overwhelmingly glaciovolcanic. The commonest and most representative are products of mafic aa lava-fed deltas, a type of glaciovolcanic sequence that has not been described before. It is distinguished by (1) a subaerially emplaced relatively thin caprock of aa lavas lying on and passing down-dip into (2) a thicker association of chaotic to crudely bedded hyaloclastite breccias, water-chilled lava sheets and irregular lava masses, collectively called lobe-hyaloclastite. A second distinctive sequence type present is characterised by water-cooled lavas and associated sedimentary lithofacies (diamictite (probably glacigenic) and fluvial sands and gravels) similar to some mafic glaciovolcanic sheet-like sequences (see Smellie, Earth-Science Reviews, 74, 241–268, 2008), but including (for the first time) examples of likely sheet-like sequences with felsic compositions. Other sequence types in the HVP are minor and include tuff cones, cinder cones and a single ice-marginal lacustrine sequence. The glacial thermal regime varied from polar, characterised by sequences lacking glacial erosion, glacigenic sediments or evidence for free water, to temperate or sub-polar for sequences in which all of these features are conspicuously developed.     

Fluorine and boron geochemistry of an ensialic marginal basin volcano: Deception Island, Bransfield Strait, Antarctica

We present the initial results of a quantitative investigation of the volatile geochemistry of Deception Island, an active volcano situated near the spreading axis of a Quaternary ensialic marginal basin (Bransfield Strait, northern Antarctic Peninsula). Fluorine contents in Deception Island magmas (112–461 ppm) are comparable with lavas from a range of tectonic environments but F-K₂O relationships most closely compare with continental flood basalts and lavas from island arcs and some marginal basins. Boron contents are high (4.3–16.3 ppm) and the values overlap with those of arc lavas; they provide strong support for the presence of a mantle source component derived from the slab subducted at the coeval trench (by melting at the slab/wedge interface and/or during slab dehydration). Both F and B acted incompatibly in Deception Island magmas but there is significant variation in incompatible-element ratios such as K/F, K/B, P/F, P/B, which strongly suggests that the magmatic system was open to some or all of these elements during differentation. The variations in these ratios also provide evidence for the presence of at least two stages in the magmatic evolution of the volcano. During pre-caldera times, mafic magma was emplaced into the upper crust where it evolved and may have reacted with the crustal envelope, thus changing the contents of some or all of the elements F, B, K and P and their inter-element ratios. A later, large influx of hot, mafic magma into the chamber may have been responsible for a major eruption that ultimately led to the formation of the caldera. K/F, K/B, etc, ratios in the magma chamber were “reset” and subsequently continued to change, possibly by further crustal interaction during melt evolution in post-caldera times.     

Explosive rhyolite tuya formation: classic examples from Kerlingarfjöll,Iceland

Rhyolite eruptions in Iceland mostly take place at long-lived central volcanoes, examples of which are found associated with each of the present-day rift-zone ice caps. Subglacial eruptions at Kerlingarfjöll central volcano produced rhyolite tuyas that are notable for their exposures of early-erupted pyroclastic material. Observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation. Eruptions begin with violent phreatomagmatic explosions that generate massive tuff (mT), but the influence of water quickly declines, leading to the formation of massive lapilli-tuffs (mLT) containing magmatically-fragmented vesicular pumice and ash. These are deposited rapidly near the vent, probably by moist pyroclastic density currents, confined by ice but not within a meltwater lake. The explosive-effusive transition is controlled by the ascent rate and gas content of the magma. An unusual obsidian-rich massive lapilli-tuff lithofacies (omLT) is identified and interpreted as pyroclastic material that was intruded into gas-fluidised deposits at the explosive-effusive transition. The effusive phase of eruption involves the emplacement of intrusions and lava caps. Intrusions of lava into the early-erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite. Intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores. Large lava bodies with columnar-jointed margins cap the tuyas and have intrusive basal contacts with the tephras. The main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region. This is in contrast to subglacial basaltic tuya-forming eruptions, which are characterised by the formation of meltwater lakes, phreatomagmatic fragmentation and subaqueous deposition. The lack of meltwater storage may reduce the potential for large jökulhlaups.     

Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery

Monitoring of the remote South Sandwich Islands volcanic arc, using advanced very high resolution radiometer (AVHRR) data, has identified a radiant pixel on channels 3 (3.55–3.93 μm) and (rarely) 4 (10.3–11.3 μm). The position of the pixel is coincident with Mount Michael, Saunders Island, the active summit crater of a snow-covered basaltic stratovolcano with a persistent steam plume. The radiant pixel was continuously present in successive AVHRR images acquired during intervals of several months in the period examined (March 1995–February 1998), although apparently disappearing for similar time intervals. More than 5000 images were examined during this study. The radiant pixel is interpreted to indicate that the crater has contained a lava lake, the first to be recorded in the South Sandwich Islands. The lake appears to persist in the crater for several months at a time, at least. It may have drained completely at times and was probably absent when the crater was viewed briefly during an overflight in January 1997. Persistent or recurring lava lakes are very uncommon world-wide and that at Mount Michael is one of only two recorded in the Antarctic region.     

Petrogenesis of boninites in the Ordovician Ballantrae Complex ophiolite, southwestern Scotland

Primitive lava and hyaloclastite with unusual, highly refractory compositions, form part of the Early Ordovician Balcreuchan Group within the ophiolitic Ballantrae Complex, southwestern Scotland. They are identified as likely high-Ca boninites on the basis of new XRF and INAA results and are the first unambiguous boninites to be discovered in the British Isles. The boninites are interbedded with low-Ti tholeiitic lavas with which they share some distinctive geochemical characteristics suggestive of a close petrogenetic relationship. The low-Ti tholeiite lavas have been interpreted as island-arc tholeiites but they also resemble back-arc basin basalts. The newly discovered boninites confirm an intra-oceanic environment of eruption; their distinctive features include relatively high SiO₂, MgO, Cr and Ni but low Al₂O₃ and HFSE abundances, U-shaped REE patterns, low Ti/Zr and high Zr/Hf ratios. Bulk geochemical trends are indicative of low-temperature, seawater-dominated alteration of the lavas but these alteration conditions apparently had little effect on the distribution of critical diagnostic elements such as Zr, Ti, Sc, Ta and the mid-heavy rare earths. We suggest that the Ballantrae boninites and low-Ti tholeiites represent different batch melts derived from a common, depleted mantle source region variably modified compositionally (i.e., made “streaky”) by fluids and/or melts during slab interaction (subduction metasomatism). A contribution from slab-derived pelagic sediments and/or a carbonatite melt is necessary to account for the fractionated, non-chondritic Zr/Hf ratios in the boninites. In view of the close compositional similarity of the Ballantrae lavas to Cenozoic boninite suites, we believe that these interpretations may have wider application to the petrogenesis of boninites in general.