V13C-2126
Volcanology and Petrology of Clasts and Tephra in the AND-2A core, ANDRILL Southern McMurdo Sound Project, Antarctica
Volcanic material is persistent throughout the AND-2A core and include, in order of relative abundance, volcanic sediment, pyroclasts and lava. Volcanic clasts range from <1-13 cm and consist of lava, breccia, and rounded scoria and pumice granules and pebbles. Clasts of lava vary in texture from glassy to fine- grained pilotaxitic to coarsely porphyritic, and in composition from mafic (clinopyroxene (Mg-rich), olivine, plagioclase) to intermediate (plagioclase, clinopyroxene (Fe-rich), amphibole) to felsic (K-feldspar, clinopyroxene (aegirine), amphibole). The relationships between silica and alkali contents indicate an overall alkaline affinity with significant compositional differences. Volcanic clasts from the top of the core belong to a strongly alkaline lineage (basanite to phonolite) while samples deeper in the core belong to a moderately alkaline linage (alkali basalt to trachyte). The moderately alkaline lineage does not exist in the McMurdo Ice Shelf core (AND-1B), which was drilled ~50 km to the east. The distinction may signify an important spatial and temporal change in volcanism within the region. In the AND-2A core, scoria and pumice are considered to be primary or minimally reworked; windblown and/or transported atop or within ice. Several primary to gently reworked tephra layers are found within sandstone and siltstone in the early to mid-Miocene interval (1093 to 640 mbsf). A primary six cm thick layer of lapilli tuff occurs at 640 mbsf and several clast-supported accumulations of pumice, up to 3.5 cm thick, occur in ripple cross-laminated sands at 709 mbsf. Pumice consists of highly vesiculated colourless glass showing tubular vesicles with delicate bubble walls, which in most cases, are filled by authigenic minerals or completely replaced. The pumice contain rare phenocrysts of anorthoclase that are also found as separate dispersed crystals in the tephra horizons. The pumice glass is highly altered, having very low alkali, magnesium and iron contents. Shards of brown glass are relatively fresh and range from basanite to mugearite. Apart from the in-situ volcanic deposits recovered at the top of the core, which are considered to be from a previously unknown proximal vent, the specific point sources for the volcanics are not known. The potential sources are numerous and include many large volcanoes that surround the SMS drill site and range in age from ~19 Ma to currently active.
V13C-2127
40Ar/39Ar Geochronology of Minna Bluff, Antarctica: Timing of Mid-Miocene Glacial Erosional Events Within the Ross Embayment
40Ar/39Ar dating of forty-eight samples of volcanic rock from the eastern end of Minna Bluff provides a framework to interpret volcanic and glacial history in the region. Minna Bluff is an east-west trending volcanic peninsula extending 45 km from Mt. Discovery eastward into the Ross Embayment. Since its formation Minna Bluff has acted as an impediment to the flow of the north-flowing Ross Ice Sheet during intervals of expanded glaciation. Concurrent over-riding of Minna Bluff by the Ross Ice Sheet is recorded in laterally extensive glacial unconformities exposed in cliffs at the eastern end. These unconformities can be compared to the ANDRILL MIS-1 submarine sediment core to assess regionally important episodes of glaciation. New 40Ar/39Ar results from sanidine and amphibole mineral separates and groundmass concentrates from Minna Bluff lavas show two series of closely spaced eruptions. The first, between 11.86 ± 0.06 Ma to 11.20 ± 0.10 Ma, was followed by a second from 10.5 ± 0.3 to 8.18 ± 0.10 Ma. The oldest ages are concentrated along the Northern end of Minna Hook (the eastern end of the bluff) and the youngest ages are associated with a series of lava domes capping Minna Bluff. The ages of glacial erosional events at Minna Bluff are bracketed by dating of multiple pairs of samples collected from above and below the glacial unconformities. The data support at least two major glacial erosional events (Using errors at 2-sigma), the first between 10.46 to 10.31 Ma and the second between 9.81 to 9.46 Ma. These periods of mid-Miocene glacial erosion should correspond to surfaces of glacial erosion in the ANDRILL MIS-1 sediment core. The chronostratigraphy of the MIS-1 core is not yet fully resolved for the Miocene, but current estimates suggest approximately 100 m of core corresponds to 1 my. Using this estimate, the older Minna Bluff unconformity may correspond to core unconformities at 1048 or 1053 m depth and the younger Minna Bluff unconformity may correspond to core unconformities at 951 or 978m depth.
V13C-2128
Evidence for Complex Mixing Processes Controlling the Composition of a Wide Range of Alkaline Volcanic Rocks at Minna Bluff, Antarctica.
Minna Bluff, a 45-km-long chain of coalesced volcanic centers formed by a wide range of alkaline magmatic compositions. Compositions between basanite and phonolite are represented and these form volcanic features ranging from small, primitive, cinder cones to large, evolved domes. Abundant stacked lava flows (subaerial to subglacial), feeder dikes, and vent complexes are exposed in cliffs up to ~1000 m in height. A notable field feature of many of the volcanic rocks at Minna Bluff is the presence of large (up to 5 cm) amphibole and feldspar megacrysts, which are found in rocks of a range of compositions and eruptive styles. Microprobe analysis shows that many lavas exhibit strikingly common disequilibrium textures, consisting primarily of phenocrystic kaersutitic to ferro-kaersutitic amphibole that is either partially, or fully, broken down to a combination of plagioclase, pyroxene and magnetite. The extent to which breakdown has occurred varies from crystal to crystal. In some cases, a breakdown rim of only a few 10s of microns thickness is present, whereas in other parts of the sample, mm-scale kaersutites are almost completely reacted. Although individual phenocrysts are compositionally uniform, a range of kaersutite compositions are present (even within a single sample) with FeO and MgO contents between 9.5-18 and 7.5-13.5 wt. percent respectively. Approximately the same compositional range of kaersutite (as well as the same disequilibrium textures) is observed in samples with a range of bulk compositions. Some of the same samples contain feldspar phenocrysts with relatively evolved plagioclase (andesine) cores and more primitive (labradorite) rims. Amphibole-related disequilibrium textures are observed in basanitic lavas containing olivine with Mg- rich cores (Fo87). The textural and compositional characteristics of the Minna Bluff volcanic rocks suggest that, for many, mixing between one or more magmatic sources, shortly before the eruption process, controls the final bulk magmatic composition. Two possible mixing endmembers would be a hydrous, evolved, kaersutite-rich magma and a second, hotter, more primitive melt, and the relative abundance of the two endmembers may influence the final composition of erupted lava. These observations suggest that the mixing processes outlined by Scanlan et al., (this volume) for a single example of inclusion-rich lava at Minna Bluff may be important over a much larger part of the volcanic complex.
V13C-2129
Petrology of Inclusion-Rich Lavas at Minna Bluff, Antarctica: Implications for Magma Origin, Differentiation and Eruption Dynamics
Inclusion-bearing lava, dome and cinder cone deposits are distributed along the top of Minna Bluff, a 45 km long volcanic peninsula located in the southern Ross Sea. One deposit, informally named Xeno Ridge, consists of phonolitic lavas that host a diversity of inclusion types that vary in size (<1 to 25 cm), shape (angular to teardrop), texture (porphyritic to granular) and composition (mafic to felsic). The dominate types include salt and peppered colored enclaves (nepheline-bearing syenite and kaersutite-rich diorite) that have granular textures and sharp contact margins with the host lava indicating that they were fully solidified when entrained. Others are dark, highly vesicular, amphibole-rich phonotephrite inclusions with fluidal forms and crenulate contact margins with the host lava indicating magma comingling. All amphibole is kaersutite and phenocrysts within the host lava are often mantled by thick fine-grained rims of diopside, plagioclase and magnetite formed by devolatilization and reaction with the surrounding melt at shallow depth. Semi- quantitative thermobarometric results for kaersutite and clinopyroxene indicate P-T conditions for crystallization of hydrous magmas within the lower crust and upper mantle (5-9 kbars, ~ 15-27 km depth; >= 1000°C). High water contents caused early crystallization of amphibole and clinopyroxene and suppression of plagioclase, which crystallized later with magnetite at lower pressures. The magmatic history of Xeno Ridge is summarized as follows: first, phonolitic magmas rise into the upper crust, stagnate and devolatilize; second, replenishing mafic magmas mix with phonolite and trigger eruption; third, the mixed magmas entrain crystalline selvages from conduit walls on ascent to the surface.
V13C-2130
Ar-Ar Ages of Glacially Derived Detrital Hornblende Grains along the West Antarctic Margin
Recent collapse of large West Antarctic ice shelves and associated increase in ice stream activity has raised awareness about the vulnerability of the West Antarctic Ice Sheet. The progression of changes in ice shelf stability along the West Antarctic margin in the past may provide important insights into the behavior of the ice sheet under changing climate conditions. Conway et al. (1999, Science, p. 280-283) found that the past grounding line retreat of the West Antarctic Ice Sheet appears to have progressed from North to South through the termination of the Last Glacial Maximum. Such progressions might also be determined for the past in marine sediment cores if the variations in the sub-glacial geology along the margin are sufficiently systematic. Sedimentary provenance studies can provide powerful constraints on sub-glacial geology, and previous work has showed that there is a very strong ~500 Ma Ar-Ar hornblende population from glaciogenic sediments from around much of the East Antarctic perimeter, and older Mesoproterozoic and Paleoproterozoic ages along the Wilkes Land margin. (Roy et al., 2007, Chemical Geology, p. 507-519). Roy et al. also found a clear distinction between West and East Antarctic sources based on Ar-Ar hornblende ages. Here we present a closer survey of Ar-Ar ages from detrital hornblende grains taken from glaciogenic sediments along the West Antarctic margin. This is part of a larger effort that will include petrographic examination, major and trace element geochemistry and radiogenic isotope compositions. The variations in the Ar-Ar ages along the West Antarctic margin are remarkably systematic, with a general progression from younger ages in northern locations along the Antarctic Peninsula to older in southern locations. This systematic variation is particularly useful as it should allow testing the latitudinal progression of ice sheet events along the margin in the past.
V13C-2131
Geochemical, lithology, and age results from Marie Byrd Land and Larsen-B Ice Shelf sediments: Implications for provenance tracing
We present new sediment provenance data from Marie Byrd Land (MBL), and the Larsen-B (LIS-B) embayment on the northeast Antarctic Peninsula. The bedrock in these areas have characteristic lithologic, geochemical, and radiometric age signatures that are preserved in glacially transported and deposited sediment. These signatures may allow us to identify ice rafted debris and fine-grained sediment that originate from these two areas, which have undergone recent ice shelf retreat and possibly past episodes of ice sheet collapse, respectively. This study involves the determination of characteristic features of near-shore glacial- marine sediment from each source area that are representative of the sub-ice bedrock, which is largely inaccessible. Bulk sediment elemental abundances including major, minor, and trace elements including heavy metals and rare earth elements (REEs) were measured from the < 63 μm fraction. Preliminary geochemical data from Marie Byrd Land yields Al/Ti values of 15-21 suggesting an average continental crust source or possibly a mixture of upper continental crust and mafic intrusions. εNd values from samples within Pine Island Bay and near the Thwaites Ice Tongue range from -2.3 to -8.3, which bracket the offshore εNd values reported by Roy et al., 2007. 40Ar/39Ar dates from individual hornblende crystals picked from the > 250 μm fraction are concentrated near 80-120 Ma for samples from near the Getz Ice Shelf and inner Pine Island Bay, with a small population of ages at 220-280 Ma for a sample from outer Pine Island Bay near the Abbot Ice Shelf. Samples from the Larsen Ice Shelf Embayment yields Al/Ti values of 21-30, suggesting an upper continental crust source. εNd values range from -1.9 to -4.9, which are distinctly different from both the high negative values observed from the central Weddell Sea to the east, and from the low positive values from the northwestern Antarctic Peninsula (Roy et al., 2007). Thin section grain mounts of the 500-2000 μm fraction used for lithic clast identification in conjunction with geochemical and 40Ar/39Ar ages allow us to assign source area fingerprints for both MBL and LIS-B.
V13C-2132
Gold-Bearing Veining in Transcrustal Fault Zone in the Transantarctic Mountains (Northern Victoria Land, Antarctica): Implications for the Paleo-Pacific Margin of Gonwana
Paleozoic gold mineralizations occur all along the former Pacific margin of Gondwana, for instance in eastern Australia, the South Island of New Zealand and southern South America. Here we describe syntectonic gold- bearing veins in northern Victoria Land (Antarctica) that was part of this margin during the Paleozoic: this is the first gold occurrence signalled in the Transantarctic Mountains of Antarctica. Northern Victoria Land can be subdivided into three NW-SE trending domains known as the Wilson, Bowers and Robertson Bay terranes. The terrane arrangement is increasingly interpreted as a fossil arc-trench system resulting from a westward-directed subduction at the paleo-Pacific margin of Gondwana during the Early Paleozoic Ross Orogeny and therefore the three terranes should represent the continental magmatic arc, the forearc/back- arc and the trench sedimentary sequence, respectively. The mineralized veins occur inside the Bowers terrane, not far from the contact with the outboard Robertson Bay terrane. The veins are hosted primarily by greenschist to low-greenschist metabasalts with interlayers of metasandstones of Middle Cambrian age. Gold occurs as coarse-grained native gold, associated with silver, arsenopyrite and an ironarsenic compound. Regional structural setting is characterized by a NW-SE-trending fold belt, cut by NW-SE and N-S fault systems linked to a transpressional regime of deformation. Quartz-carbonate veins occurs in a brittle-ductile high strain zone superimposed on the earlier regional metamorphic foliation and folds. The high strain zone is characterized by foliated fault rocks with S-C structures, widespread veining, and hydrothermal alteration of the host rocks. The vein network is surrounded by a zoned alteration halo approximately up to 500 m wide. Chlorite thermometry provides temperature estimates ranging from 270 to 280° C in the chlorite-altered metabasalts and from 290 to 310° C in the more altered samples. 39Ar-40Ar dating on sericites provides Carboniferous ages for the mineralization event.
V13C-2133
The Vestfjella Dike Swarm, Western Dronning Maud Land, Antarctica: an Archive of Mantle Heterogeneity in the Karoo LIP
The Vestfjella dike swarm crosscuts Karoo-related flood basalts at the rifted continental margin of western Dronning Maud Land, Antarctica. The dikes range in age from ~180 Ma to ~150 Ma, include rocks that lack geochemical evidence of contamination, and thus provide insights into the evolution of Gondwana breakup magmatism and the mantle sources involved. The Vestfjella dike swarm is mainly composed of subalkaline tholeiitic picrites, basalts, and basaltic andesites (MgO 3-25 wt.%) that show affinities to low-Ti and high-Ti magmas (TiO2 1.2-4.8 wt.%). The initial epsilon Nd values vary from -12 to +8 and Nb/Nb* from 0.3 to 1.2, suggesting involvement of lithospheric and asthenospheric magma sources. Dikes with positive epsilon Nd values, Nb/Nb* >0.8, (Th/Ta)N <0.8 and Nb/Zr <0.06 show geochemical affinities to oceanic island picrites (e.g. Hawaii) and MORB. Based on their mildly enriched or depleted immobile incompatible element (IE) signatures, they were derived from depleted mantle and are designated as D-type dikes. Isotopic and immobile IE ratios of Karoo-related lavas and dikes from Antarctica and southern Africa define two principal trends which extend from the field of D-type dikes towards light REE- enriched, high-Zr and low-Zr end-members, possibly representing lithospheric mantle-derived (lamproite-like) and crust-derived contaminants, respectively. These variations are compatible with a broad 3-component mixing system for the Karoo magmas, with the D-type dikes representing a depleted mantle source component. The majority of D-type dikes exhibit marked positive Ba, K, and Sr anomalies and many show a distinctive positive V anomaly. These anomalies are remarkably similar to those of gabbroic cumulates; it is tempting to interpret this affinity as evidence for recycled oceanic lower crust in the mantle source. On the other hand, similar LILE-enrichments in modern mid-ocean ridge basalts in the southern Atlantic and southwestern Indian ridges have been ascribed to subduction-related enrichment of the upper mantle prior to Gondwana breakup. It is quite possible that both kinds of subduction-related sources have been involved in the petrogenesis of the Vestfjella dike swarm. In fact, the overall geochemical ranges of the D-type dikes can be modeled with binary mixing between depleted mantle plume and LILE-enriched upper mantle. A subset of the Vestfjella dikes is characterized by positive epsilon Nd values, Nb/Nb* >0.8, and (Th/Ta)N <0.7 combined with Nb/Zr >0.1; these E-type dikes show geochemical affinities to ocean island basalts and probably represent a sublithospheric enriched mantle source component. They plot apart from the main field of Karoo magmas in IE diagrams, however, implying that enriched mantle sources may have played a minor role in the Karoo LIP. Overall, geochronological and geochemical data for the Vestfjella dike swarm and the wider Karoo province indicate that the lithospheric overprinting of magmas decreased in the course of time. Depleted mantle sources, possibly influenced by deep and shallow recycling of subducted oceanic crust, were contemporaneously tapped during early (~180 Ma) and late (~165 Ma) stages of Karoo magmatism and may be linked to mantle heterogeneity recorded by modern oceanic lavas associated with Gondwana breakup.
V13C-2134
The Role of Magma Replenishment in the Construction of the Lower 500m of the Layered Mafic Dufek Intrusion, Antarctica
The Jurassic age Dufek Intrusion is arguably one of the largest layered mafic intrusions on Earth, yet it remains relatively little studied due to the remoteness of its location. During the austral summer of 2006/07, we conducted detailed sampling and logging of exposures in the lower ~500m of the Dufek Massif section of the intrusion. We collected 630 oriented cores over this interval from the Walker Anorthosite and portions of the overlying Aughenbaugh Gabbro along two vertical transects 5 km apart - a spur of Neuburg Peak and at Walker Peak. Cryptic mineral chemistry variations have been determined by electron microprobe analysis of both the cores and the rims of cumulus and intercumulus plagioclase, orthopyroxene (inverted pigeonite), and clinopyroxene from ~50 thin sections, with an average sample spacing of 10 m over the lower 500m of the Dufek Massif. The mineral composition data reveal a dramatic change of 74 to 56 in orthopyroxene Mg# and An83 to An61 in plagioclase anorthite content with increasing stratigraphic height. The change of 22 An# occurs over an interval of 500m compared to thicknesses of 3-4 km required for a similar change in the Bushveld and Stillwater intrusions. This rapid change is consistent with these lower cumulates forming from a relatively thin magma body. Field observations and trends in the mineral chemistry are interpreted to indicate three units within the lower 500m of the intrusion. The change from three phase to two-phase cumulates and the change in the slope of plagioclase An#, FeO and K2O content, and orthopyroxene Mg# with height across the boundary between the Walker Anorthosite and the overlying Aughenbaugh Gabbro suggests crystallization from two batches of magma with different compositions. Also, the occurrence of the sharp-bottomed, several meter- thick Neuburg Pyroxenite at ~210 m above the Walker-Aughenbaugh contact together with changes in plagioclase, orthopyroxene and clinopyroxene composition and an influx of meta-sedimentary and igneous xenoliths suggest that the pyroxenite and overlying gabbro might also have crystallized from a new batch of magma. Thus, we suggest that the lowermost exposed 500m section might reflect construction by 3 separate magma replenishment events. The presence of numerous, 20 cm-10 m thick, sharp bottomed pyroxenites throughout the rest of the Dufek Intrusion may suggest that replenishment events were common.
V13C-2135
Structural Analysis of the Vanda Dike Swarm, McMurdo Dry Valleys, Southern Victoria Land, Antarctica
The 472 -467 Ma (Grunow, 1995) Vanda Dike Swarm of mafic to felsic compositions is thought to have intruded during the transition from subduction to extension at the end of the Ross orogeny. Field examination of 609 dikes, which crosscut orogenic plutonic rocks and gneissic basement foliation, exhibit a strong NNE strike. Average strike of mafic dikes (55% of total dikes) is 28°. The felsic dikes (30% of dikes) strike 32°, and the intermediate dikes (15% of dikes) strike 41°. The dikes dip to the east with the magnitudes ranging widely (<5°-89°) as a function of rock composition and position along strike of the dike swarm. Paleomagnetic data (Grunow, 1995) and the apparent absence of listric faults show that the dikes could not have been rotated significantly, therefore these are primary dips. Dike widths vary greatly (<1 m to 49.4 m), with mafic dikes routinely being thinner (1.5 m average) than the intermediate (7.4 m average) and felsic (12 m average) dikes. The emplacement depth of the dikes is unlikely to be shallow based on the lack of vesicles in all dikes, including those that contain hydrous minerals. Mafic dikes tend to be discontinuous with numerous splays and right stepping en echelon offsets. In contrast, felsic and intermediate dikes are continuous for several kilometers. There are no obvious, visible cracks or faults in the area that could have been exploited by the mafic dikes to produce the observed echelon pattern. The right stepping echelon pattern in the mafic dikes could indicate a counter-clockwise rotation of the stress field with decreasing depth about a vertical axis during the time of dike emplacement. The combination of a dominant NNE strike with a broad range of dips for dikes in the Lake Vanda dike swarm is atypical of the dike patterns observed both in rift and arc volcano settings. Thus, the tectonic setting for emplacement of the Vanda dikes is unclear and may represent a hybrid tectonic situation or a transition from one tectonic regime to another. Grunow, A.M., 1995, JGR, 100, 12589-603
V13C-2136
Amphibole Content and Composition of the Vanda Dike Swarm: Dry Valleys, Southern Victoria Land, Antarctica
The Ordovician Vanda Dike swarm is composed of hundreds of dikes in the Dry Valleys of Antarctica. We examined over 600 dikes across Bull Pass, Taylor, Victoria, and Wright Valleys. A previous age determination on one of the dikes yields 470 +- 7 Ma. The dikes are mostly emplaced in pre-tectonic and syn-tectonic granites that range in age from 589 to 490 Ma. A few dikes are found in coarse-grained post- tectonic granites that are 486 to 477 Ma. The dikes occur in concentrated clusters, and the different clusters are characterized by limited compositional ranges. For example, in Wright Valley near Lake Vanda, dikes are predominantly mafic, whereas felsic porphyry is common at the mouth of Bull Pass. Most of the dikes and all of the granites contain amphibole, which is being applied as a thermobarometer to understand the conditions of emplacement and uplift history of the terrain. The temperatures of felsic dikes obtained from apatite-zircon saturation thermometry range from 791 to 826° C for zircon and for apatite, 921 to 946° C. Most of the Vanda dikes have non-vertical dips (typically 40 to 80°), irregular margins, and served as ductile shear zones, all of which point to mid-crustal emplacement. Thus, the dike swarms were likely emplaced during regional uplift at the end of the Ross orogeny.
V13C-2137
Building on Decades of Research on the McMurdo Volcanic Group, Antarctica: A Geologic Field Guide to Observation Hill
Based on more than four decades of research on the rocks of the Erebus Volcanic Province of the McMurdo
Volcanic Group, a geologic field guide to the Observation Hill walking tracks near McMurdo Station,
Antarctica has been developed. The geologic field guide was an outcome of questions generated by: (1)
Teachers participating in the Andrill Research Immersion for Science Educators (ARISE) program; (2)
McMurdo Station support staff, as well as (3) Geoscientists with specialties outside volcanology and
petrology. Whilst these individuals are acutely aware of the more than a century of references to Observation
Hill in exploration literature, there was little in the way of easily-accessible information about the geologic
history of Hut Point and Observation Hill, as well as other nearby volcanoes (e.g. Mt. Erebus, White and Black
Islands) and larger scale geologic features (e.g. Transantarctic Mountains) that can be seen from the
vantage point of Observation Hill. Questions also focused on smaller scale features of the landscape (e.g.
patterned ground) and textures and minerals observed in volcanic rocks exposed on the trails. In order to
encompass the wide-ranging background of the audience and facilitate access, the field guide will be
available in three formats: (1) A downloadable MP3 file, which includes the general information and stop-by-
stop information; (2) A double-sided paper brochure that provides a relatively simple, easier-to-digest guide
to views and geologic features; (3) A Google Earth Layer that includes access to the MP3 files and the paper
brochure, as well as additional geologic information. Links to the field guide can be found at
http://www.andrill.org/education.
http://www.andrill.org
V13C-2138
Multiple Sulfur Isotopic Composition of Sulfate in the Fresh Water, Deception Island, South Shetland Islands, Antarctica
Isotopic compositions of sulfur (δ33S, δ34S, δ36S) from sulfate of the fresh water in Deception island were measured to provide the information on the sources of sulfate in the surface water and to check the possibility of mass independent fractionation of sulfur in this area. Most part of the Deception Island is covered by volcanic rocks from the recent activities not exceeding 200 ka. To south and north of the Deception Island, plutonic rocks of granitic composition ranging from Mesozoic to Cenozoic are widely distributed. Because of the recent volcanic activities in Deception Island (most recent eruptions in 1970), sulfur containing aerosols produced in the stratosphere might have been added and could contribute the mass independent signature to the hydrologic system. The δ34S values of sulfate extracted from water samples at Deception Island range from 8.1 to 17.3 per mil. The Δ33S values of sulfate extracted from water samples at Deception island range from 0.000 to 0.046 per mil. Δ36S values of sulfate extracted from water samples range from -0.257 to 0.186 per mil. These waters represent the concentration from Antarctic snow and ice. In Antarctic region the natural source of sulfate dissolved in water could be originated from marine biogenic source (DMS), sea-salt, volcanic source, or other continental sources. The δ34S values of water sulfate at Deception Island well support the dominance of marine biogenic origin for the source of sulfur. Mass independent sulfur isotope anomalies are known to be produced through photochemical reactions and have been reported in Precambrian rock samples, recent atmospheric aerosols, and ice cores containing the volcanic erupted ashes piercing through stratosphere. Isotopic composition of sulfate in fresh water indicates that only mass-dependent fractionation was prevailing for sulfur isotopic system at Deception Island.
V13C-2139
Microstructural & AMS Constraints on the Origin of Cumulates From the Dufek Range, Antarctica
The Jurassic (180Ma) Dufek Complex in the Pensacola Mountains of Antarctica is one of the largest layered mafic intrusions in the world, with an estimated volume up to 175,000km3. It is largely buried beneath the Antarctic ice sheet, but is exposed in two parallel mountain ranges; the Dufek Massif and the Forrestal Range. The lowermost exposed section is in the Dufek Massif, consisting of the Walker Anorthosite, overlain by the Aughenbaugh Gabbro. Here we present microstructural data from anorthosite, gabbronorite and pyroxenite samples recovered from the lower 600m of this section, to constrain the origin of the cumulates within the Dufek magma chamber. Petrographic study shows that plagioclase and pigeonite were cumulus phases and clinopyroxene was an oikocryst forming intercumulus phase. Original pigeonite grains coalesced during inversion to form 3- >30mm orthopyroxene oikocrysts enclosing plagioclase chadocrysts. Moderate to weak mineral foliations are present in most samples. Representative samples were analyzed for shape preferred orientation (SPO) and alignment factor (AF); crystal size distribution (CSD); and spatial distribution pattern (SDP). Crystal lattice preferred orientation (LPO) data were determined using Electron Back-Scatter Diffraction. Plagioclase crystals define the mineral foliation and have AF's of 15 to 37, with 100 being perfect alignment, in sections cut perpendicular to the foliation. No statistically significant mineral lineations are observed in sections cut parallel to the foliation. Similarly, the LPO data display a foliation consistent with SPO data and no measurable lineation, suggesting that the foliation was not formed by diffusion driven compaction. Spotted anorthosites contain 3-8mm orthopyroxene and clinopyroxene oikocrysts, which in turn contain smaller plagioclase chadocrysts that reflect the initial structure of the plagioclase crystal framework, prior to densification. The chadocrysts display a similar SPO and AF to the host rock, suggesting that the plagioclase foliation formed early in the history of cumulate formation, perhaps during the initial accumulation of the crystals and/or by mechanical compaction of an initially random oriented crystal pile. SDP data for the chadocrysts and the host rock suggest that the host rock SPO developed by a combination of overgrowth during porous media convection and mechanical compaction. Plagioclase CSD's are log-linear with slopes ranging from -3.42 to -0.62mm-1. Some of the CSDs are convex upward at small crystal sizes, suggestive of post-cumulus growth or Ostwald ripening or winnowing of the small grain-size fraction before accumulation. The eigenvectors of the LPO data in some, but not all, cases are reflected in the orientation of magnetic fabrics determined by anisotropy of magnetic susceptibility (AMS) and anisotropy of thermal remanence (ATRM). The minimum eigenvectors of both AMS and ATRM are generally subparallel to the pole to the silicate foliation. Both magnetic fabric estimates reveal a statistically distinct lineation that is not evident in the LPO data; we are further investigating this observation.
V13C-2140
Sulfur and Oxygen Isotopic Composition of Sulfate in the Fresh Water, King Sejong Station, King George Island, Antarctica
Isotopic compositions of sulfur (δ34S) and oxygen (δ18O) were measured for the sulfate of the fresh water near the King Sejong Station, King George Island, Antarctica. Sejong station is located in the Barton peninsular of the King George Island. The geology around King Sejong station mainly composed of basalt-andesite, quart monzodiorite, and granodiorite. Lapilli tuff, conglomerate, sandstone, and siltstone occur along the southern and eastern shore of the Barton peninsula. Lapilli tuff also occurs on the highland located on southeastern part of the Barton peninsula. The δ34S values of sulfate extracted from fresh water samples at King Sejong Station range from 13.7 to 16.3 per mil excluding 1 sample. These sulfur values are very narrow in their range compared with those from anthropogenic sources. These sulfur values are 5 to 7 per mil lower than those of typical present seawater. Considering the rocks occurring near the King Sejong station, these sulfur isotopic values do not seem to be related to any evaporites of certain age. In Antarctic region the natural source of sulfate dissolved in water could be originated from marine biogenic source (DMS), sea-salt, volcanic source, or other continental sources. Most of the δ34S values of sulfate at King Sejong station seems to indicate the dominance of marine biogenic origin for the source of sulfur. The δ18O values of sulfate extracted from fresh water samples at King Sejong Station range from 1.9 to 6.4 per mil excluding 1 sample. These oxygen isotope values are lower than those of the sulfate in the present seawater by 6 per mil. However, both sulfur and oxygen isotope values strongly represent the influence of the seawater sulfate. One sample have 2.6 and -1.1 per mil in its δ34S and δ18O values, respectively, that are quite different from the isotopic values of other samples. This sample was collected in the highland far from the King Sejong station. Therefore this sample might reflect the composition of rather pure precipitation not affected by seawater sulfate. The atmospheric deposition might have been the major source of dissolved sulfate but it is not clear whether the source materials are from natural and/or anthropogenic origin.