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Chemical Fingerprinting of Pleistocene / Holocene Volcanoes Around The XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S12_006 Chemical fingerprints from Pleistocene / Holocene volcanoes around the northern Antarctic Peninsula Kraus, S.1 (1) Instituto Antártico Chileno, Plaza Muñoz Gamero 1055, Punta Arenas, Chile. [email protected] Introduction A significant goal in reconstructing the timing of past climatic events is constrain the age of prehistorical events. One possibility is to establish time markers that would allow comparison of climate signals from different regions based on relative stratigraphy. Vol- canic ash layers are among these proven time markers. In order to determine the source of volcanic ash layers found in ice or sediment cores, it is necessary to collect data from the local volcanic centers that might be considered to be the source of the respective layer. It is necessary to generate new quantitative data for a tephra database that contains petrological, geochemical, isotopic and age data from vol- canic centers that might have affected the investigation area. Analyses of the tephra parti- cles found in ice or sediment cores and subsequent comparison/correlation with the data- base then helps to identify the source volcano. Geological background Known active volcanism in Antarctica is restricted to Marie Byrd Land, parts of the Ross Sea and the Antarctic Peninsula. This study focuses on the Antarctic Peninsula, where active volcanic centers owe their existence to a highly complex and unique geotectonic setting, leaving fingerprints in the chemical compositions of the resulting magmas. Crustal extension and rifting processes opened the Bransfield Strait between the Antarctic Peninsula and the South Shetland Islands not earlier than 4 Ma ago [1]. Similar exten- sional processes on the eastern side of the Antarctic Peninsula are responsible for the vol- canism along Larsen Rift that stretches from the Seal Nunataks in the south to Cape Pur- vis and Paulet Island volcanoes in the north. In the northern Antarctic Peninsula area, there are at least 11 volcanic centers with known or suspected Late Pleistocene / Holocene explosive activity (Fig. 1). Among all these Holocene volcanic centers, so far only Deception Island has been recognized as a source for tephra particles or sulfate identified in a number of Antarctic ice cores [e.g. 2; 3] and lake records [4]. 1 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Previous work In most cases the chemical composition of tephras (particularly the glass shard compo- nent) from northern Antarctic Peninsula volcanoes is currently unpublished or undeter- mined [5]. Whereas Deception Island has been investigated in detail and numerous ana- lytical data are available (though almost exclusively for lava, not for tephra), the data re- cords for the other volcanic centers around the northern Antarctic Peninsula exhibit sub- stantial gaps, and do not include detailed and comprehensive tephra studies. Results from fieldwork Fieldwork has been carried out during austral summers 2007/2008 and 2008/2009 around the northern Antarctic Peninsula. Visited volcanoes include the islands Deception, Pen- guin, Bridgeman and Paulet, moreover Melville Peak on King George Island and Rezen Peak on Livingston Island. Of special importance is the second ever reported visit and sampling at Sail Rock (after E. Godoy approx. 25 years ago). It is a tiny 30 m high stack located about 35 km SW of Deception Island (Fig. 1). It marks the top of a volcano the rest of which is submerged below sea level. The age of Sail Rock remains unknown, and the stack is composed of visible layers of pyroclastic breccia and tuffs alternating with lava flows (Fig. 2). Another fieldwork location deserving special mention was the 550 m high Cape Purvis Volcano, located in the southern part of Dundee Island (eastern Antarctic Sound, Fig. 1). On February 4th, 2008, the outcrops at the northern slopes of Cape Purvis Volcano were visited. The sampling site was located at about 360 m a.s.l. and composed primarily of pyroclastic rocks resembling ground surge deposits. The top of Cape Purvis Volcano, suspected by [6] to represent the youngest phase of activity, had never been visited be- fore. A short investigation of the outcrops at the summit of the volcano yielded the first samples ever taken from these units. Results and Conclusions Following standard petrographical work, the samples were prepared at Universidad de Chile (Santiago) for ICP-MS geochemical analyses using an agate mortar. ICP-MS analyses were then carried out at Activation Laboratories (Ontario, Canada). The new bulk tephra geochemical data, specifically the trace element data, provide a reli- able framework to distinguish the individual volcanic centers around the northern Antarc- tic Peninsula. Based on their Mg-number, Melville Peak and Penguin Island seem to rep- resent the most primitive magma source. Sail Rock and Bridgeman Island exhibit high Al contents, while Paulet Island shows extremely high Sr levels. Nb/Y ratios higher than 0.67 (Fig. 3) in combination with elevated Th/Yb and Ta/Yb ra- tios and strongly enriched LREE seem to be diagnostic to distinguish the volcanoes lo- cated along the Larsen Rift (Cape Purvis and Paulet Island) from those associated with Bransfield Rift (Sail Rock, Deception Island, Penguin Island, Melville Peak and Bridge- man Island). Sr/Y ratios, on the other hand, might be used to discriminate within the Lar- 2 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 sen Rift volcanoes, Paulet Island showing considerably higher values than Cape Purvis volcano (Fig. 4). Among the Bransfield Rift volcanoes, Bridgeman Island and Melville Peak show notably lower Nb/Y (Fig. 3) and much higher Th/Nb than Deception Island, Penguin Island and Sail Rock. Sail Rock displays almost double the Th/Yb ratio as compared to Deception Island, and also much higher LREE enrichment but extraordinarily low Ba/Th, the latter discriminating it clearly from Penguin Island. Such extremely low Ba/Th ratios are also typical for Melville Peak, but for none of the other volcanoes. Penguin Island has almost double the Ba/Th and Sr/Y ratios higher than any other of the investigated volcanic cen- ters (Fig. 4). Whereas the volcanoes located in the northern part of Bransfield Strait (Penguin Island, Melville Peak and Bridgeman Island) have Zr/Hf ratios lower than N-MORB, all other volcanic centers including the Larsen Rift volcanoes display Zr/Hf higher than N-MORB (Fig. 4). It is expected that the correlation of the generated data with published data from tephra layers identified in ice, lake and marine sediment cores will contribute to a better con- strained timing of individual climatic events identified in ice and lake sediment cores col- lected in the northern Antarctic Peninsula area. Acknowledgements I want to thank the Chilean Navy and INACH for supporting the fieldwork. César Ar- riagada and Cristián Rodrigo were valuable companions in the field. References [1] Larter, R.D. & Barker, P.F. (1991) Effects of ridge-crest trench interaction on Antarc- tic-Phoenix Spreading: Forces on a young subducting plate. J. Geophys. Res., 96, 19583- 19607. [2] Aristarain, A.J. & Delmas, R.J. (1998) Ice record of a large eruption of Deception Is- land Volcano (Antarctica) in the XVIIth century. J. Volc. Geoth. Res., 80, 17-25. [3] Delmas, R.J., Kirchner, S., Palais, J.M. & Petit, J.-R. (1992) 1000 years of explosive volcanism recorded at the South Pole. Tellus, 44(B), 335-350. [4] Björck, S., Sandgren, P. & Zale, R. (1991) Late Holocene tephrochronology of the northern Antarctic Peninsula. Quat. Res., 36, 322-328. [5] Smellie, J.L. (1999) The upper Cenozoic tephra record in the South Polar region: a review. Global and Planetary Change, 21, 51-70. [6] Smellie, J.L., McIntosh, W.C., Esser, R. & Fretwell, P. (2006) The Cape Purvis vol- cano, Dundee Island (northern Antarctic Peninsula): late Pleistocene age, eruptive proc- esses and implications for a glacial palaeoenvironment. Ant. Sci., 18 (3), 399-408. [7] Winchester, J.A. & Floyd, P.A. (1977) Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chem. Geol., 20, 325-343. 3 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Fig. 1: Map of the northern Antarctic Peninsula showing potential centers Fig. 2: Sail Rock, an approx. 30 m high stack of Late Pleistocene / Holocene explosive volcanic eruptions and terres- representing the uppermost part of a sub- trial and marine tephra localities [modified after 5]. merged volcanic edifice. The stack is com- posed of pyroclastic rocks (lighter, reddish parts) and lava flows (dark brown parts). Pho- Fig. 3: Determination of magma series and rock type classification using SiO2 and immobile elements. Diagram modified after [7]. Except for the Fig. 4: Trace element ratios reflecting differences dacite/rhyolite-boundary, all original, horizontal (SiO2-content defined) between the magma sources of the individual boundaries delimiting the fields of the subalkali-series were adjusted to volcanic centers. The high Sr/Y ratios dis- the now generally accepted SiO2-boundaries used in the total alkali- played by the Penguin Island volcano dis- silica-diagram (TAS). The (sub-) vertical boundaries and the boundaries criminate it clearly from the other volcanic delimiting the fields of the alkali-series correspond to the original dia- centers. Note the geographical vicinity of the gram published by [7]. Note the geographical separation between east- low-Zr/Hf-ratio centers (see Fig. 1). Note that ern (alkali series) and western (subalkali series) Antarctic Peninsula. the different volcanic centers plot in well de- fined and well separated clusters. 4.
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