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listelement.badge.dso-typeЭлемент, GARNET-BEARING XENOLITHS FROM SALT LAKE CRATER, OAHU, HAWAII: HIGH-PRESSURE FRACTIONAL CRYSTALLIZATION IN THE OCEANIC MANTLE(2007) Keshav S.; Sen G.; Presnall D.C.The focus of this study is a suite of garnet-bearing mantle xenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnet constitute the bulk of the xenoliths, and orthopyroxene is present in small amounts. Clinopyroxene has exsolved orthopyroxene, spinel, and garnet. Many xenoliths also contain spinel-cored garnets. Olivine, clinopyroxene, and garnet are in major element chemical equilibrium with each other; large, discrete orthopyroxene does not appear to be in major-element chemical equilibrium with the other minerals. Multiple compositions of orthopyroxene occur in individual xenoliths. The new data do not support the existing hypothesis that all the xenoliths formed at 1.6–2.2 GPa, and that the spinel-cored garnets formed as a consequence of almost isobaric subsolidus cooling of a spinel-bearing assemblage. The lack of olivine or pyroxenes in the spinel–garnet reaction zones and the embayed outline of spinel grains inside garnet suggest that the spinel-cored garnets grew in the presence of a melt. The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the tholeiitic and slightly silica-poor portion of the CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) system at pressures from 3.0 to 5.0 GPa. The phase relations suggest crystallization from slightly silica-poor melts (or transitional basaltic melts) in the depth range ∼110–150 km beneath Oahu. This depth estimate puts the formation of these xenoliths in the asthenosphere. On the basis of this study it is proposed that the pyroxenite xenoliths are high-pressure cumulates related to polybaric magma fractionation in the asthenosphere, thus making Oahu the only locality among the oceanic regions where such deep magmatic fractional crystallization processes have been recognized.listelement.badge.dso-typeЭлемент, MAGNETO-BIOSTRATIGRAPHY OF THE MIDDLE TO UPPER TRIASSIC TRANSITION, CENTRAL SPITSBERGEN, ARCTIC NORWAY(2007) Hounslow M.W.; Hu M.; Mørk A.; Vigran J.O.; Weitschat W.; Orchard M.J.Palaeomagnetic and biostratigraphic data were obtained for the latest Ladinian and most of the Carnian (Botneheia Formation to basal Kapp Toscana Group), from two sections in central Spitsbergen (Svalbard archipelago). Thermal and alternating field (AF) demagnetization reveals a magnetization of both normal and reversed polarities. The mean directions pass reversal and fold tests and are similar to other European Late Triassic palaeopoles. One of the sections displays 15° of vertical-axis, clockwise, tectonic rotation on a décollement in the underlying Botneheia Formation. The magnetostratigraphy is dominated by normal polarity in the uppermost Botneheia Formation and into the Tschermakfjellet and basal De Geerdalen formations. A substantial hiatus characterizes the Ladinian–Carnian boundary in central Spitsbergen, so reverse magnetozones, identified in Tethyan sections near this boundary, are absent. Magnetostratigraphic correlation, along with palynostratigraphic constraints, indicates that most of the De Geerdalen Formation is Lower Carnian. The magnetostratigraphy and palynology indicates that the Isfjorden Member (upper unit of the De Geerdalen Formation) is probably mid-Carnian in age. Change in the lithological architecture in the Isfjorden Member, compared with the underlying parts of the De Geerdalen Fm, suggests a hiatus near the base of the member, which may represent a mid-Carnian unconformity, not previously recognized on Spitsbergen.listelement.badge.dso-typeЭлемент, NATIVE Hg liq IN THE METALLIFEROUS SEDIMENTS OF THE EAST PACIFIC RISE (21°S)(2007) Dekov V.M.Native Hg droplets, documented here for the first time in mid-ocean ridge metalliferous sediments, were found in the surface sediment of a magmatically robust and ultra-fast spreading mid-ocean ridge, the East Pacific Rise. The surface sediment and Fe–Mn micronodules in it are enriched in Hg. Two different processes can produce Hg0liq in this environment: hydrothermal and biogenic. Bio-transfer of ionic Hg dissolved in the hydrothermal fluids into Hg0 in the sediment can account for the eventual native Hg occurrences in the close vicinity of the hydrothermal vents. The Hg0liq occurrences in the distal metalliferous sediments most probably have originated directly from the hydrothermal fluids and been transported by hydrothermal plumes away from the vents. Condensation of Hg0liq from Hg0vap in the vent fluids subjected to near critical phase separation seems the most plausible mode of origin of the native Hg occurrences. The high-temperature vents at the global spreading ridge chain introduce an assessed amount of 1.62–4.86 × 103 t/yr Hg0liq into the ocean. With time, the deposited Hg0liq will likely be dissolved in the overlaying seawater and will not survive in the geologic record.listelement.badge.dso-typeЭлемент, BIODIVERSITY, DISTRIBUTION AND PATTERNS OF EXTINCTION OF THE LAST ODONTOPLEUROID TRILOBITES DURING THE DEVONIAN (GIVETIAN, FRASNIAN)(2007) Feist R.; McNamara K.J.Biostratigraphical ranges and palaeogeographical distribution of mid-Givetian to end-Frasnian odontopleurids are investigated. The discovery of Leonaspisrhenohercynica sp. nov. in mid-Givetian strata extends this genus unexpectedly up to the late Middle Devonian. New material of Radiaspisradiata (Goldfuss, 1843) and the first koneprusiine in Britain, Koneprusia? sp., are described from the famous Lummaton shell-bed, Torquay, Devon. New taxa of Koneprusia, K. serrensis, K.aboussalamae, K.brevispina, and K. sp. A and K. sp. B are defined. Ceratocephala (Leonaspis) harborti Richter & Richter, 1926, is revised and reassigned to Gondwanaspis Feist, 2002. Two new species of Gondwanaspis, G.dracula and G.spinosa, plus three others left in open nomenclature, are described from the late Frasnian of Western Australia. A further species of Gondwanaspis,G.prisca, is described from the early Frasnian of Montagne Noire. Species of Gondwanaspis are shown to possess a number of paedomorphic features. A functional analysis suggests that, unlike other odontopleurids, Gondwanaspis actively fed and rested with the same cephalic orientation. The sole odontopleurid survivors of the severe terminal mid-Givetian biocrisis () belong to the koneprusiine Koneprusia in the late Givetian and Frasnian, and, of cryptogenic origin, the acidaspidine Gondwanaspis in the Frasnian. Whereas the former became extinct in the late Frasnian at the Lower Kellwasser Event, the latter disappeared, and with it the entire Odontopleuroidea, at the terminal Frasnian Upper Kellwasser global biocrisis.listelement.badge.dso-typeЭлемент, KYANITE DEFORMATION IN WHITESCHIST OF THE ULTRAHIGH-PRESSURE METAMORPHIC KOKCHETAV MASSIF, KAZAKHSTAN(2007) Beane R.J.; Field C.K.Microstructures in minerals from ultrahigh‐pressure metamorphic (UHPM) terranes are keys to understanding the rheological properties and the exhumation mechanisms of rocks from subduction zones. Kyanite‐bearing whiteschist, associated with eclogite lenses, is part of UHPM unit II located south‐west of Lake Zheltau in the Kulet region of the Kokchetav Massif. The equilibrium assemblage is kyanite + garnet + talc + phengite + coesite/quartz. Previously reported peak pressure–temperature (P–T) conditions are ∼3.5 GPa at 750 °C. A strong foliation is defined by the talc and phengite, with a corresponding weak shape preferred alignment of kyanite. Crystallographic orientation maps and analysis of kyanite blades were performed using electron backscatter diffraction methods. The data are consistent with a (100)[001] slip system for the formation of undulose extinction and kink bands in kyanite. Rotations measured across individual kink bands are 10–50° about <010>, and rotations along kyanite with undulose extinction are up to 50° about <010> with variations between adjacent points typically <2°. The undulose extinction is interpreted to have developed through crystal plastic deformation by dislocation creep. Kink bands mark the development of high‐angle grain boundaries by dislocation climb. The deformation of kyanite occurred in the fault‐bounded terrane during the exhumation of the Kokchetav Massif.