Kirsten Nicolaysen

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

Origin of continental components in Indian Ocean basalts: evidence from Elan Bank (Kerguelen Plateau, ODP Leg 183, Site 1137)

Basalts forming the floor of the Indian Ocean are geochemically distinct from those of the Atlantic and Pacific Oceans. These differences have been attributed to a deeply recycled continental component or to widespread dispersion of plume-type mantle within the Indian Ocean asthenosphere. The discovery of Proterozoic continental crustal rocks within the uppermost basaltic basement of Elan Bank, part of the Kerguelen Plateau, shows instead that direct shallow-level contamination of basaltic magmas by isolated continental crust fragments may have produced the anomalous isotopic ratios of some Indian Ocean basalts.

Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean

At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and that the breakup of Gondwana dispersed continental fragments into the nascent Indian Ocean lithosphere.

Temporal geochemical trends in Kerguelen Archipelago basalts: evidence for decreasing magma supply from the Kerguelen Plume

Abstract The Kerguelen Archipelago, a ∼39 Ma to recent volcanic–plutonic complex, is interpreted to be a manifestation of the Kerguelen Plume. Most, ∼85%, of the surface area is covered by flood basalts ranging in age from ∼29 to 25 Ma. The youngest (∼25 Ma) studied flood basalts are in the Southeast (SE) Province of the archipelago. A composite 460 m section of this southeast flood basalt dominantly consists of evolved (3 to 6% MgO) alkalic basalt and trachybasalt with a few interbedded highly evolved lavas (trachyandesites), a 40–70 m conglomerate which contains lignite beds, and a trachytic breccia/tuff unit. All of the lavas in this composite section have Sr and Nd isotopic ratios that are typical of the Kerguelen Plume; e.g., >80% of the 115 analyzed archipelago lavas with >2.3% MgO have ( 87 Sr / 86 Sr ) i =0.70515±12 and ( 143 Nd / 144 Nd ) i =0.51259±5. These ranges include the southeast flood basalts. Pb isotopes, however, are more variable; these 25 Ma lavas have high 206 Pb / 204 Pb at ∼18.4 to 18.6, relative to other archipelago lavas. The temporal trend of the archipelago flood basalt from older, ∼29 Ma, transitional basalts to younger, ∼25 Ma, alkalic basalt with an increasing proportion of highly evolved lavas and intra-bedded sediments in the relatively young southeast section indicates: (a) a temporal decrease in extent of melting and (b) a decreasing supply of magma from the plume to the crust. These temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge-centered plume at ∼43 Ma to its intraplate setting. Supporting evidence for this interpretation is: (a) the absence of a MORB geochemical signature in these 25 Ma lavas; and (b) the relatively low abundances of heavy rare-earth elements in these southeast lavas which reflect partial melting within the garnet stability field.

Boomerang Seamount: the active expression of the Amsterdam–St. Paul hotspot, Southeast Indian Ridge

During a survey of the axis of the Southeast Indian Ridge (SEIR), we discovered a 1100 m tall, volcanically active submarine volcano, Boomerang Seamount, near the spreading center on the Amsterdam–St. Paul (ASP) Plateau. The summit of the volcano is 650 m below sea level and has a 200-m-deep, 2-km-wide circular caldera. Samples of very fresh volcanic glass, dated by the 210Po–210Pb technique at ∼5 months old, were collected from the floor of the caldera in March 1996. The volcano is 18 km northeast of Amsterdam Island near the intersection of a long spreading segment and the Boomerang Transform Fault. It is built on the stationary Antarctic Plate, where widely scattered volcanic activity thickens the crust, continuing to build the plateau. Water column profiles reveal a 1.7°C temperature anomaly and a 0.3 V stepped nephelometer (water column turbidity) anomaly within the caldera, nearly an order of magnitude larger than other hydrothermal plume anomalies we measured. These anomalies suggest hydrothermal activity within the caldera. Volcanic glass compositions at two sample sites on the volcano summit are similar to each other and to Amsterdam and St. Paul Island basalts, but have some important differences as well. K2O/TiO2 ratios in Boomerang Seamount glasses are similar to St. Paul Island samples, but differ significantly from Amsterdam Island samples. Rare earth element patterns in lavas from Boomerang, Amsterdam, and St. Paul are similar. Sr, Nd, and Pb isotope ratios in samples from the Boomerang Caldera floor are similar to samples from Amsterdam Island. However, another sample from Boomerang Seamount deviates from a SEIR–St. Paul–Amsterdam mixing trend and shows evidence of mixing with a Kerguelen-like source component. The geochemical complexity of these three closely spaced volcanic edifices on the ASP Plateau suggests that the Boomerang Seamount source is heterogeneous on a very small scale.

Palaeomagnetic study of Oligocene (24-30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean): directional analysis and magnetostratigraphy

We report palaeomagnetic determinations in order to estimate the palaeosecular variation of the geomagnetic field for the upper Oligocene period from a southern site. We measured the anisotropy of magnetic susceptibility (AMS) of basalts from five sections to check the reliability of applying tectonic correction and concluded that no stratigraphic corrections were needed. A positive reversal test implies that secondary components were sufficiently removed from the characteristic remanent magnetization. Also, the sampling was sufficiently random to accurately reflect upper Oligocene palaeosecular variation. Calculations of the between-flow angular standard deviation ( SB) from the geographic pole show values between 21.2 and 21.8 ◦ . This palaeosecular variation estimate agrees with Model G for palaeosecular variation [J. Geophys. Res. 96 (B3) (1991) 3923], which is axisymmetric and assumes an equatorial symmetry of the geomagnetic field. The virtual geomagnetic poles (VGPs) from the Mont des Ruches section seem to exhibit two polarity changes: reverse–normal (R–N) first, and in more detail a second normal–reverse (N–R) one. Finally, assuming that no polarity chrons are missed by any of the sections, we compare the magnetostratigraphy we defined to the global geomagnetic polarity scale of [Geophys. J. Int. 129 (1) (1997) 176] using previously obtained radiometric dates to correlate the sections [J. Petrol. 39 (4) (1998) 711; Earth Planet. Sci. Lett. 174 (2000) 313; J. Petrol. 43 (7) (2002) 1341]. The precise age constraints furnished by these correlations allow an estimation of eruption rates from 0.30 to at least 1.31 km/my, with wide variations from section to section and an increase of eruption rate for younger sections. However, due to the non-uniqueness of the correlations, the age limits of the sections and consequently the extrusion rate estimations can vary. Moreover, assuming the Mont des Ruches section that has probably recorded all the magnetic polarity intervals, the Mont de la Rabouillere and Mont de la Tourmente intermediate VGP directions may indicate unrecorded polarity chrons. The eruption rates calculated can thus be overestimated.

Statistical properties of paleomagnetic directions in Kerguelen lava flows: Implications for the late Oligocene paleomagnetic field

( 40 Ar/ 39 Ar) ages are reported to aid in the calibration of the paleomagnetic results. The primary contribution of this report, however, is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the Kerguelen flood basalts. In total, 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 Ma window: from 25 to 30 Ma. The composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale. Our approach is to investigate the average normal and reversed polarity field directions over this 5 Ma window. We calculated a paleomagnetic pole found to be located at l = 85.5N, f = 189.3 E( A95 =2 .3, K = 16.5, N = 233). This pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the Indo-Atlantic hot spot reference frame. In the statistical analysis we tackle the specific question: Is the secular variation isotropic? This question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles, which is still a matter of debate among paleomagnetists. By means of statistical tests we show that the paleomagnetic data from Kerguelen agree with an isotropic model for paleosecular

Clinker, Pumice, Scoria, or Paralava? Vesicular Artifacts of the Lower Missouri Basin

Abstract Abrading artifacts made of vesicular (porous) rock are not uncommon at archaeological sites along the Missouri River and adjacent areas. Various terms have been used to describe this material including pumice, scoria, clinker, and floatstone. Each of these terms implies different geologic origins (volcanic vs. non-volcanic) and affects interpretation of the potential modes of transport. Identification of the source area of these materials may provide significant information regarding past human movements and activities. This study focuses on vesicular artifacts in the central Plains and in particular from the Leary site (25RH1) in the southeastern corner of Nebraska. Scanning Electron Microscopy (SEM) was used to identify the chemical compositions of a subset of the Leary artifacts and comparative geologic samples of volcanic and metasedimentary origin. The results imply that the Leary (and likely many other) vesicular artifacts from the central Plains are non-volcanic in origin. The raw material from which these artifacts were made is more properly termed “paralava” and derives from outcrops in the northern Plains. Historical documents suggest that this buoyant material was transported naturally by the Missouri River as “floatstone”.

Great Plains Workshop held to prepare for USArray deployment

Relative to most parts of North America, the Great Plains region, which is bordered by the Rocky Mountain Front on the west and the Mississippi River on the east, has been understudied in terms of the structure, formation, and evolution of the underlying crust, mantle, and core. The anticipated arrival of the USArray portable seismic stations, which will cover the entire United States regardless of surface geology and tectonic activities, and the deployment of the accompanying flexible array stations and the permanent seismic stations in this area, will fill this gap and address numerous problems related to the structure and dynamics of the Earth. Detailed information about USArraycan be found at http://www.earthscope.org/usarray/. To maximize the effectiveness of the upcoming USArray, formulate cooperative studies, and identify geologic targets for detailed studies using the flexible array stations of USArray, a pre-EarthScope Great Plains workshop was recently hosted by Kansas State Universitys Department of Geology The workshop brought together about 40 geoscientists with interests ranging from surface processes to mantle dynamics, from about 25 institutions. Participants discussed scientific objectives related to USArrays Great Plains coverage, with an emphasis on future collaborations to maximize our understanding of the geology of the Great Plains region, from the Earths surface to the core-mantle boundary. This will lead to a better understanding of the geologic development of cratonic regions, and provide valuable data for integrated studies of continental lithosphere and deep Earth structure over a wide range of scales.