Supra-subduction Zone Pyroxenites from San Jorge and Santa Isabel (Solomon Islands)

Peridotites associated with pyroxenites (with rare olivine andspinel) are exposed on the islands of San Jorge and Santa Isabelin the Solomon Islands. Orthopyroxenite occurs in large outcrops(100 m²) whereas websterite and clinopyroxenite occur as layersand veins/dykes in peridotites. The bulk compositions of thepyroxenites are characterized by high Mg²⁺/(Mg²⁺ + Fe²⁺) (0·78–0·91)and low Al₂O₃ (<2·7 wt %). Low rare earth elementabundances are coupled with large ion lithophile element enrichmentsand positive Sr and Pb anomalies (primitive mantle-normalized)relative to adjacent rare earths. Temperatures of equilibrationfor the pyroxenites are between 950 and 1050°C. These relativelylow temperatures, combined with the occurrence of primary fluidinclusions, suggest that the pyroxenites formed by interactionof peridotite protoliths with an aqueous fluid. Bulk-rock andmineral compositions of the orthopyroxenites are similar tothose of mantle-derived pyroxenites, whereas the websteriteshave closer chemical affinity with crustal arc cumulates. Nevertheless,field relationships plus petrological, textural and geochemicalevidence are consistent with formation of all pyroxenite typesin supra-subduction zone mantle, resulting from metasomatismof peridotite by subducted Pacific Plate-derived fluid. Sucha setting for pyroxenite has not previously been reported indetail. We propose that these processes produce mantle pyroxenitewith compositions similar to crustal pyroxenite. KEY WORDS: mantle metasomatism; pyroxenite; supra-subduction zone     

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.     

Paper I – Optically stimulated luminescence (OSL) dating of perennially frozen deposits in north‐central Siberia: OSL characteristics of quartz grains and methodological considerations regarding their suitability for dating

Arnold, L. J. & Roberts, R. G. 2011: Paper I – Optically stimulated luminescence (OSL) dating of perennially frozen deposits in north?central Siberia: OSL characteristics of quartz grains and methodological considerations regarding their suitability for dating. Boreas, Vol. 40, pp. 389–416. 10.1111/j.1502‐3885.2011.00209.x. ISSN 0300?9483. Optically stimulated luminescence (OSL) dating of coarse‐grained quartz is increasingly being used as the main chronological tool in late Quaternary palaeoenvironmental reconstructions of Siberia. However, relatively little information has been published on the suitability of OSL dating for the various types of perennially frozen sediments found in this region. Here we provide a systematic examination of the quartz luminescence characteristics of 21 perennially frozen samples from the Taimyr Peninsula and adjacent coastal lowlands of north‐central Siberia, and discuss their implications for the reliability of single‐grain and multi‐grain OSL chronologies in such contexts. The results of this study suggest that the quartz luminescence characteristics of these samples are, in principle, favourable for OSL dating but, in practice, require that a series of validation checks are made of the chosen experimental conditions. If these tests are satisfied, then reliable OSL chronologies should be obtained for sedimentary deposits in this region. Importantly, however, the single‐grain and multi‐grain aliquot equivalent dose (D_e) distribution characteristics for our samples reveal that there are advantages in targeting certain types of depositional settings for OSL dating studies of Siberian sediments. We also show that samples from the same depositional settings, and even from the same sites, do not necessarily display similar D_e distribution characteristics. The latter complication favours the use of single‐grain analysis to unravel the bleaching and burial histories of young (mid‐ to late Holocene) sediments in these Arctic environments.     

Paper II – Dirt,dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia,and their implications for sedimentary ancient DNA studies

Arnold, L. J., Roberts, R. G., MacPhee, R. D. E., Haile, J. S., Brock, F., Möller, P., Froese, D. G., Tikhonov, A. N., Chivas, A. R., Gilbert, M. T. P. & Willerslev, E. 2010: Paper II – Dirt, dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia, and their implications for sedimentary ancient DNA studies. Boreas, Vol. 40, pp. 417–445. 10.1111/j.1502‐3885.2010.00181.x. ISSN 0300‐9483 The sedimentary ancient DNA (sedaDNA) technique offers a potentially invaluable means of investigating species evolution and extinction dynamics in high‐latitude environments. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by postdepositional mixing. In this paper we investigate whether these fundamental conditions are upheld at seven perennially frozen wetland sites across the Taimyr Peninsula and coastal lowlands of north‐central Siberia. Optically stimulated luminescence (OSL) and radiocarbon (¹⁴C) dating are used to constrain the ages of both the inorganic and organic fractions of perennially frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and ¹⁴C age/depth profiles, as well as single‐grain equivalent dose (D_e) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and postdepositional processes that have affected specific sedimentary facies. The results of this study demonstrate that even though DNA preservation and stratigraphic integrity are commonly superior in perennially frozen settings, this does not, in itself, guarantee the suitability of the sedaDNA approach. The combined OSL and ¹⁴C chronologies reveal that certain perennially frozen sites may be poorly suited for sedaDNA analysis, and that careful site selection is paramount to ensuring the accuracy of any sedaDNA study – particularly for ‘latest appearance date’ estimates of extinct taxa.     

Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany

Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well-bedded plane parallel to cross-bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross-bedded strata. With some reservations these may be compared with the high flow-regime alluvial bedforms produced experimentally in flumes. Most of the cross-bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low-dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross-bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune-like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions.     

Low-O18 Igneous Rocks from the Intrusive Complexes of Skye, Mull, and Ardnamurchan, Western Scotland

Oxygen isotope analyses have been obtained on rocks and coexistingminerals from the Tertiary stocks, ring-dikes, and cone-sheetsin Ardnamurchan (18 samples), Skye (41 samples), and Mull (18samples); these include a few samples of the plateau basaltcountry rocks. Almost all of the rocks in the vicinity of thecentral ring complexes (within 2 mi. of an intrusive contact)are strongly depleted in O¹⁸ relative to ‘normal’igneous rocks from other areas. The rocks in Skye ( 150 sq.mi.) and Mull ( 150 sq. mi.) have suffered an overall O¹⁸ depletionof about 6 to 7 per mil, and those in Ardnamurchan ( 30 sq.mi.) about 3 to 6 per mil. These data indicate that very large hydrothermal convectionsystems involving heated low. O¹⁸ meteoric ground waters wereestablished in these areas at the time of igneous intrusion.The heated ground waters exchanged with the gabbros, granities,and basaltic lavas, locaclly lowering their O¹⁸/ O¹⁶ rationsby at least 12 per mil. Much, if not all, of this exchange occurredafter crystallization of a given igneous rock was essentiallycomplete, inasmuch as feldspars invariably have undergone muchmore depletion in O¹⁸ than has coexisting quartz or pyroxen.The meteoric-hydrothermal process and the presence of an aqueousgas phase may possibly be responsible for the widespread epidote-chloritealteration, the turbid or cloudy feldspars, the abundant feldspathicveins, the felsitic and granophyric textures, the miaroliticcavities, and much of the explosive volcanic activity and brecciationthat are found in these areas.     

Sequence of till deposition and erosion in drumlins

Whittecar, G.R. & Mickelson, D. M. 1977 06 01: Sequence of till deposition and erosion in drumlins.
Extensive sand and gravel workings have exposed structural and compositional features of 17 gravel-cored drumlins of late Wisconsin age in eastern Wisconsin. The drumlins are blanketed by 3 m of sandy basal till which truncates lower tills of earlier advanccs, outwash gravels, and an overlying till which is conformable to the gravel bedding and indistinguishable in composition from the surface till. Sands and gravels in the interior of some drumlins are deformed into large overturned folds, and into elastic dikes of fine sand and silt which penetrate to the top of the drumlin and warp overlying gravels. Both the folds and horizontal bedding are truncated by either the drumlin edge or the till blanket.
We interpret the conformable, truncated, and in some cases folded, till as a basal till deposited during glacial advance. The capping, truncating till is viewed as a basal till left by retreating ice.
The following sequence of events is suggested: (1) advance of ice over outwash, and deposition of till in a zone mar the margin; (2) thickening of the ice and erosion of the drumlin shapes; (3) local folding of the gravels and continued erosion; (4) retreat of ice and deposition of basal till under thin ice; (5) deposition of localized ablation till and stratified deposits.     

Cenozoic Magmatism of the North-Eastern Eurasian Margin: The Role of Lithosphere Versus Asthenosphere

Sikhote-Alin and Sakhalin are located in the Russian Far Eastflank of the northernmost part of the Sea of Japan. Magmatismin this region preceded, was concurrent with, and continuedafter the extension and sea-floor spreading (25–18 Ma)that formed the Sea of Japan. Among the Sikhote-Alin and Sakhalinvolcanic suites, Eocene–Oligocene (55–24 Ma) lavasare characterized by greater large ion lithophile element andrare earth element enrichments compared with Early–Mid-Miocene(23–15 Ma) tholeiites, and also show a depletion in highfield strength elements (HFSE). The geochemical characteristicsof the Eocene–Oligocene and Early–Mid-Miocene basaltsare consistent with migration of the locus of magma generationbeneath the Sikhote-Alin and Sakhalin areas from subduction-modifiedlithospheric mantle into mid-ocean ridge basalt (MORB)-sourceasthenosphere as spreading in the Sea of Japan progressed. Mid-Miocene–Pliocene(14–5 Ma) lavas, erupted following the opening of theSea of Japan, include alkaline and sub-alkaline basalts withwide ranges in trace-element abundances, varying between twodistinct end-members: (1) volumetrically minor alkaline basaltswith Zr–Nb and Sr–Nb–Pb isotope compositionssimilar to asthenosphere-derived, intra-plate–hotspotbasalts from eastern China; (2) more abundant, lithosphere-derived,low-alkali tholeiites depleted in HFSE. The similarity of isotopicsignatures coupled with systematically different rare earthelement (REE) abundances in the Mid-Miocene–Pliocene andChinese basalts are best modeled by similar extents of meltingof spinel lherzolite and garnet lherzolite, respectively. TheMid-Miocene–Pliocene alkali basalts were generated bysmall degrees of partial melting of hot asthenosphere beneatha thin lithospheric lid; the thin lithospheric mantle beneaththe Sikhote-Alin and Sakhalin region resulted from heating andextension associated with the opening of the Sea of Japan. KEY WORDS: north-eastern Eurasian margin; Sikhote-Alin–Sakhalin; Japan Sea opening; subcontinental lithosphere; asthenosphere