Depositional record of historic lahars in the upper Whangaehu Valley, Mt. Ruapehu, New Zealand: implications for trigger mechanisms, flow dynamics and lahar hazards

Mt. Ruapehu, in the central North Island of New Zealand, is one of the most lahar-prone volcanoes in the world. Since historic observations began in 1861 AD, more than 50 individual lahars have been recorded in the Whangaehu valley alone, the natural outlet to the summit Crater Lake. These lahars have been triggered by a variety of mechanisms, including explosive eruptions that displaced Crater Lake water over the outlet or ejected it onto the snow-clad summit area of the volcano; rain-remobilisation of tephra deposits on steep slopes; displacement over the outlet as a result of syn-eruptive changes in lake bathymetry; and lake break-outs from Crater Lake following impoundment of excess water behind temporary barriers of tephra and/or ice emplaced over the outlet. However, only 9 lahar deposits can be distinguished in the upper Whangaehu valley on sedimentological, stratigraphic, geomorphic and petrological grounds, and these are skewed towards either the largest or the most recent flows. In some cases magnitude can be reconstructed from deposit geometry, with the largest lahars producing the highest level terraces, the coarsest deposits, and crossing drainage divides into normally inactive channels. This under-representation of historic events reflects the low preservation potential of unconsolidated deposits in a steep alpine environment, and the overprinting and recycling effect of large magnitude lahars that rework material down to bedrock and effectively reset the stratigraphic record. Development of magnitude-frequency relationships for Ruapehu lahars therefore requires the identification of lahar deposits in proximal, medial and distal settings in order to ensure that the full range of events is represented.     

Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

ToF-SIMS and SEM study on the preferential oxidation of chalcopyrite

Chalcopyrite is known to be slow reacting mineral in hydrometallurgical systems and is considered one of the most inert sulphide minerals with respect to leaching. Such character of chalcopyrite seems to be linked to a formation of a passive layer on its surface. This work reports that freshly fractured chalcopyrite surfaces exhibit highly selective reactivity depending on the exposed fracture planes. ToF-SIMS was used to qualitatively characterize various fracture planes in freshly fractured chalcopyrite particles, prior to and after hydrometallurgical treatment. It was found that, prior to treatment, certain areas exhibited pronounced contamination from atmospheric hydrocarbons; whereas, others were highly unreactive and remarkably free from adventitious hydrocarbon contamination. The positive ion spectra recorded from these areas were found to be dominated by peaks from Fe- and Cu-elements and related compounds. The negative ion spectra for the reactive areas on the other hand showed a high content of oxidized (sulphur) species.The differences between the areas of low and high reactivity, as detected after leaching, were more subtle than prior to leaching; whereas, SEM analysis showed clear evidence for selective attack of ferric sulphate to specific planes. Furthermore, it was shown that, when chalcopyrite is in intimate contact with pyrite, it experiences an enhanced oxidation compared to when there is no electric contact with pyrite.Attempts were made to explain the preferential oxidation observed based on the different chemistry of the fracture surfaces.     

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist,New Zealand

The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.     

Small-scale fluctuations in acoustic and physical properties in surficial carbonate sediments and their relationship to bioturbation

 Measurements of physical and acoustic properties within the top 40 cm of sediment indicate that there are three classes of parameter variability relevant to acoustic bottom scattering at the Marquesas Keys and Dry Tortugas sites. The three classes of variability, spatially and temporally different in terms of scale, are generated by biological and physical processes acting on sediments. Interplay of bioturbation, trawling and storm events create fine laminations of sand–silt–clay and high gravel-size mollusk shell content at the Marquesas Keys site; the predominance of biological processes creates strong surficial gradients and lateral variability in sediment properties at the Dry Tortugas site.     

Crustal structure of the deepwater west Niger Delta passive margin from the interpretation of seismic reflection data

The interpretation of 2D and 3D seismic reflection data complemented with gravity data allows the crustal architecture of the deepwater west Niger Delta passive margin to be defined. The data show that the area is underlain by oceanic crust that is characterised by a thickness of 5–7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Some of the dipping reflections can be traced up to the top of the basement where they offset it across a series of minor to major thrust faults. Other internal reflections are attributed to extensional shear zones and possibly due to intrusions in the lower crust. The Moho can be correlated as a discrete reflection over >70% of the study area. It is generally smooth, but localised relief of up to 1 km is observed. The southeastern part of the study area is dominated by a zone of SW–NE striking basement thrusts.     

Excavating eurypterids,giant arthropods of the Palaeozoic

The eurypterids of the Silurian Bertie Group of North America are some of the most striking fossils known, with their dark organic cuticle outlined against a background of light coloured dolomite. Eurypterus is the most abundant genus but the pterygotids with their long grasping claws have fired our imagination as giant extinct predators. Large complete specimens are rarely found—the specimens are dominated by moulted exoskeletons that often disarticulate—but a giant that emerged from the rock more than 50 years ago is being prepared to go on display.