Biotite Reference Materials for Secondary Ion Mass Spectrometry 18O/16O Measurements

Five new biotite reference materials were calibrated at the SwissSIMS laboratory (University of Lausanne) for oxygen isotope determination by secondary ion mass spectrometry (SIMS) and are available to the scientific community. The oxygen isotope composition of the biotites, UNIL_B1 to B5, was determined by laser‐heating fluorination to be 11.4 ± 0.11‰, 8.6 ± 0.15‰, 6.1 ± 0.04‰, 7.1 ± 0.05‰ and 7.6 ± 0.04‰, respectively. SIMS analyses on spots smaller than 20 μm gave a measurement repeatability of 0.3‰ (2 standard deviation, 2s). The matrix effect due to solid solution in natural biotite could be expressed as a linear function of X_Mg and X_F for biotite. No effect was found for different crystallographic orientations. SIMS analysis allows the oxygen isotope composition of biotite to be measured with a measurement uncertainty of 0.3–0.4‰ (2s) for biotites with similar major element compositions. A measurement uncertainty of 0.5‰ (2s) is realistic when F poor biotites (lower than 0.2% m/m oxides) within the compositional range of X_Mg of 0.3–0.9 were compared from different sessions. The linear correlation with F content offers a reasonable working curve for F‐rich biotites, but additional reference materials are needed to confirm the model.     

Layered mantle structure beneath the western Guyana Shield, Venezuela: Evidence from diamonds and xenocrysts in Guaniamo kimberlites

Mantle xenoliths and xenocrysts from Guaniamo, Venezuela kimberlites record equilibration conditions corresponding to a limited range of sampling in the lithosphere (100-150 km). Within this small range, however, compositions vary considerably, but regularly, defining a strongly layered mantle sequence. Major and trace element compositions suggest the following lithologic sequence: highly depleted lherzolite from 100 to 115 km, mixed ultra-depleted harzburgite and lherzolite from 115 to 120 km, relatively fertile lherzolite from 120 to 135 km, and mixed depleted harzburgite and relatively fertile lherzolite from 135 to 150 km. Based on comparison with well-documented mantle peridotites and xenocrysts from elsewhere, we conclude that the Meso-proterozoic Cuchivero Province (host to the Guaniamo kimberlites) is underlain by depleted and ultra-depleted shallow Archean mantle that was underplated, and uplifted, by Proterozoic subduction, perhaps more than once. These Proterozoic subduction events introduced less-depleted oceanic lithosphere beneath the Archean section, which remains there and is the source of the abundant Guaniamo eclogite-suite diamonds that have ocean-floor geochemical signatures. Although diamond-indicative low-Ca Cr-pyrope garnets are abundant, they are derived primarily from the shallow depleted layer within the field of graphite stability, and the rare peridotite-suite diamonds are either metastably preserved at these shallow depths, or were derived from the small amount of depleted lithosphere sampled by these kimberlites that remains within the diamond stability field (the mixture of Archean and Proterozoic mantle in the depth range 135-150 km).     

Depth habitats and seasonal distributions of recent planktic foraminifers in the Canary Islands region (29°N) based on oxygen isotopes

Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ¹⁸O_shell) of five planktic foraminiferal species. Both the δ¹⁸O_shell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ¹⁸O_shell of G. ruber (w) from surface sediments is similar to the δ¹⁸O_shell values measured from the sediment-trap samples during winter. However, the δ¹⁸O_shell of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ¹⁸O_shell values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (～180 m) and deep surface water (～350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ¹⁸O_shell.     

On A☆ search with stopover areas

Today's map navigation software offers more and more functionality. For example, it cannot only route from a start to a destination address, but the user can also specify a number of via destinations that are to be visited along the route. What is not commonly found in the software is the handling of so-called stopover areas. Here the user specifies a sequence of geographic areas and wants the route to lead into or through each of these areas (but he does not actually care where exactly). A modification of the well-known A^☆ algorithm that considers convex stopover areas is presented. Algorithmic variants and implementation issues are discussed. Example results on a real-world data set are presented for the case of axis parallel rectangular stopover areas.     

Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt% MgO = 12.8-13.8; wt% Cr₂O₃ = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt% CaO = 3.3-4.3) but are very Cr-rich (wt% Cr₂O₃ = 9.7-16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ¹³C_PDB = −6.2‰) but with variable nitrogen abundance (1296-2550 ppm). Carbon isotopes in eclogitic stones range from “normal” for the upper mantle (δ¹³C_PDB = −5.5‰) to somewhat low (δ¹³C_PDB = −10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2-779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction.     

Synchronous pattern of moisture availability on the southern Tibetan Plateau since 17.5 cal. ka BP – the Tangra Yumco lake sediment record

A possible asynchronicity of the spatial and temporal moisture availability on the Tibetan Plateau has been a controversial subject of discussion in recent years. Here we present the first attempt to systematically investigate possible spatial and temporal variations in moisture availability by examining two lakes, Tangra Yumco and Nam Co, on an east–west transect on the southern Tibetan Plateau using identical proxies for palaeoenvironmental reconstruction. In this study, an independent record from Tangra Yumco was analysed applying a multi‐proxy approach to reconstruct variations in moisture availability since the Lateglacial. Results were subsequently compared with previously published records from Nam Co and additional records from Tso Moriri (northwestern Himalaya) and Naleng Co (southeastern Tibetan Plateau). Our results show that Tangra Yumco was at least partially ice covered prior to 17.1 cal. ka BP. A temperature rise after 17.1 cal. ka BP probably resulted in thawing of the permafrost. At 16.0 cal. ka BP moisture availability increased, representing an initial monsoonal intensification. Warmer conditions between 13.0 and 12.4 cal. ka BP and cooler conditions between 12.4 cal. ka BP and the onset of the Holocene reflect the Bølling‐Allerød and Younger Dryas. At the onset of the Holocene moisture availability rapidly increased, with moisture highest prior to 8.5 cal. ka BP when temperatures were also highest. After 8.5 cal. ka BP the moisture availability gradually decreased and showed only minor amplitude variations. These findings are consistent with the records from large lakes like Nam Co, Tso Moriri, and Naleng Co, revealing a synchronous pattern of moisture availability on the southern Tibetan Plateau.     

Trace element geochemistry of coesite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal craton

Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ¹⁸O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.     

Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge

The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid–solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (>500 Hz) body waves and slow, intermediate frequency (<20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.