Out-burst flood (lahar) triggered by retrogressive landsliding, 18 March 2007 at Mt Ruapehu,New Zealand—a successful early warning

The summit crater of Mt Ruapehu volcano normally hosts a 15.4-ha warm lake, whose water has been repeatedly wholly or partly ejected by explosive and extrusive eruptions. Some of the larger eruptions have modified the lake outlet by burying it under unconsolidated tephra (volcanic ash and blocks), creating a dam-break flood hazard independently of the occurrence of an eruption. Eruptions in 1995 and 1996 followed this sequence; a break-out flood was anticipated and a warning system was installed to mitigate the risk from this event and subsequent lahars in the same catchment. The 11-year filling time allowed much planning and rehearsal. The warning system involved manual inspections of dam integrity, and seepage and lake-level monitoring to constrain the likely failure window, and telemetered instruments including a tripwire and geophones to detect breaching of the dam and propagation of the outbreak flood. The dam-collapse sequence, captured by a time-lapse camera, involved a series of retrogressing landslides initiated and accelerated by seepage forces and toe scour when the lake was 1.1 m below overtopping. The barrier failed in two phases on 18th March, 2007, beginning at 09:55 (NZST), with rapid retreat of one of the erosion scarps on the downstream slope of the eastern barrier, initiated by internal erosion. Headward retrogression of the scarp into the barrier formed an initial breach in the dam, after which increasing outflow led to erosion and undercutting of the wider downstream toe of the western barrier. A final, larger dam breach occurred between 11:21 and 11:22 as slope instability caused retrogressive failure of the remaining barrier. Five-hundred meters downstream of the dam, a large landslide was reactivated by toe scour during the flood, contributing about a million cubic meters of solid material to the volumetric bulking of the outflow, which reached the coast, 215 km away, 17 h later. The success of the planning and warning system allowed the whole event to occur with little damage to infrastructure and without causing injury.     

Heavy Metal Concentrations in European Mosses: 2000/2001 Survey

The heavy metals in mosses survey was originally established in 1980 as a joint Danish–Swedish initiative under the leadership of Åke Rühling, Sweden and has, since then, been repeated at five-yearly intervals with an increasing number of countries and individuals participating. Twenty-eight European countries, almost 7000 sites and about 100 individuals have been involved in the most recent survey in 2000/2001. The survey provides data on concentrations of 10 heavy metals (arsenic, cadmium, chromium, copper, iron, lead, mercury, nickel, vanadium, zinc) in naturally growing mosses throughout Europe. The technique of moss analysis provides a surrogate measure of the spatial patterns of heavy metal deposition from the atmosphere to terrestrial systems, and is easier and cheaper than conventional precipitation analysis. The aims of the survey are to determine patterns of variation in the heavy metal concentration of mosses across Europe, identify the main polluted areas, produce regional maps and further develop the understanding of long-range transboundary pollution.As in previous surveys, there was an east/west decrease in heavy metal concentrations in mosses, related in particular to industrial emissions. Former industrial sites and historic mines accounted for the location of some high concentrations in areas without contemporary industries. Long-range transboundary transport appears to account for elevated concentrations of heavy metals in areas without emission sources, such as lead in southern Scandinavia (presumably from emission sources elsewhere in Europe).     

Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA

Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in southern Washington represent fragments of mantle lithosphere from the back-arc side of the Cascade arc front. Previous studies have shown that metasomatism by either silica-rich fluids or hydrous melts crystallized phlogopite, imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and more radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re–Os, Sm–Nd, Sr, and U–Th–Pb isotopic data shed further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb=18.63–19.55, ²⁰⁷Pb/²⁰⁴Pb=15.56–15.63, ²⁰⁸Pb/²⁰⁴Pb=38.22–38.87). The most radiogenic Pb isotopic compositions extend beyond the most radiogenic Pb isotopic compositions for the Cascade arc lavas and display a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at the high ²⁰⁶Pb/²⁰⁴Pb end of this array. Therefore, models for rapid transfer of components from the slab to the mantle lithosphere are not viable in this case. Instead, a multi-stage model is preferred. In the first stage, the slab component is transferred via fluid or melt into, and reacts with the hanging wall mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting of the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures. In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compositions of the clinopyroxenes, and the presence of high alkali glasses in the xenoliths, are consistent with the metasomatic agent derived from the hybridized hanging wall being alkali-rich, and possibly similar to potassic-rich lavas found in arc and back-arc settings. These data therefore demonstrate the importance of the hybridized hanging wall mantle above slabs as a source for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at least tens of million years.     

Source contributions to Devonian granite magmatism near the Laurentian border, New Hampshire and Western Maine, USA

Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; ε_Nd (at 370 Ma)=−2.7 to −0.7; initial ²⁰⁷Pb/²⁰⁴Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; ε_Nd (at 370 Ma)=−7.0 to −0.6; initial ²⁰⁷Pb/²⁰⁴Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; ε_Nd (at 370 Ma)=−2.6 to +0.3; initial ²⁰⁷Pb/²⁰⁴Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures.

Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust.

New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma.