Green Lake Landslide and other giant and very large postglacial landslides in Fiordland,New Zealand

Green Lake Landslide is an ancient giant rock slide in gneiss and granodiorite located in the deeply glaciated Fiordland region of New Zealand. The landslide covers an area of 45 km² and has a volume of about 27 km³. It is believed to be New Zealand's largest landslide, and possibly the largest landslide of its type on Earth. It is one of 39 known very large (10⁶–10⁷ m³) and giant (≥10⁸ m³) postglacial landslides in Fiordland discussed in the paper. Green Lake Landslide resulted in the collapse of a 9 km segment of the southern Hunter Mountains. Slide debris moved up to 2.5 km laterally and 700 m vertically, and formed a landslide dam about 800 m high, impounding a lake about 11 km long that was eventually infilled with sediments. Geomorphic evidence supported by radiocarbon dating indicates that Green Lake Landslide probably occurred 12 000–13 000 years ago, near the end of the last (Otira) glaciation. The landslide is described, and its geomorphic significance, age, failure mechanism, cause, and relevance in the region are discussed, in relation to other large landslides and recent earthquake-induced landslides in Fiordland. The slope failure occurred on a low-angle fault zone undercut by glacial erosion, and was probably triggered by strong shaking (MM IX–X) associated with a large (≥ M 7.5–8) earthquake, on the Alpine Fault c. 80 km to the northwest. Geology was a major factor that controlled the style and size of Green Lake landslide, and in that respect it is significantly different from most other gigantic landslides. Future large earthquakes on the Alpine Fault in Fiordland are likely to trigger more very large and giant landslides across the region, causing ground damage and devastation on a scale that has not occurred during the last 160 years, with potentially disastrous effects on towns, tourist centres, roads, and infrastructure. The probability of such an event occurring within the next 50 years may be as high as 45%.     

Book reviews

Marine decapod Crustacea of southern Australia: a guide to identification. By Gary C. B. Poore. Published in 2004 by CSIRO Publishing, P.O. Box 1139, Collingwood, VIC 3066, Australia. 608 p., hardcover. ISBN: 0–643–06906–2. Price: AU$180.00 (orders: publishing.sales@csiro.au).

Marine mammals: fisheries, tourism and management issues. Edited by Nicholas J. Gales, Mark A. Hindell and Roger Kirkwood. Published in 2003 by CSIRO Publishing, P.O. Box 1139, Collingwood, VIC 3066, Australia. 446 p., hardcover. ISBN: 0–643–06953–4. Price: AU$116 (orders: publishing.sales@csiro.au).

Freshwater fishes of north‐eastern Australia. By Brad Pusey, Mark Kennard, and Angela Arthington. Published in 2004 by CSIRO Publishing, P.O. Box 1139, Collingwood, VIC 3066, Australia. 700 p., hardcover. ISBN: 0–643–06966–6. Price: au$89.00 (orders: publishing.sales@csiro.au).     

Amino acid racemization dating of fossil bones,I. inter-laboratory comparison of racemization measurements

Enantiomeric measurements for aspartic acid, glutamic acid, and alanine in twenty-one different fossil bone samples have been carried out by three different laboratories using different analytical methods. These inter-laboratory comparisons demonstrate that D/L aspartic acid measurements are highly reproducible, whereas the enantiomeric measurements for the other amino acids show a wide variation between the three laboratories. At present, aspartic acid measurements are the most suitable for racemization dating of bone because of their superior analytical precision.     

Origins of subcalcic garnets and their relation to diamond forming fluids—Case studies from Ekati (NWT-Canada) and Murowa (Zimbabwe)

Subcalcic, high-Cr (G10) garnets are found as inclusions within diamonds and in peridotitic xenoliths. The strong spatial associations between G10 garnets and diamond make them an important tool in the investigation of diamond genesis. We present an integrated study of the major and trace element composition and oxygen-Sr-Nd-Hf isotopic ratios of eight G10 garnets from the Ekati mine (NWT-Canada) and four from the Murowa mine (Zimbabwe) in an attempt to determine their petrogenetic evolution and to further examine a possible relationship between the metasomatic agents responsible for G10 garnet signatures and diamond forming fluids.All garnets display sinusoidal to mildly sinusoidal REE patterns and have negative Ti, Sr and positive U anomalies. They have variably radiogenic ⁸⁷Sr/⁸⁶Sr (0.703261-0.731191) and non-radiogenic ε_Nd values (−8.1 to −27.1), except for one sample from Murowa that has a positive ε_Nd of 2.5. One Ekati sample has an extremely low ε_Hf value of −61.6. The Ekati garnets we have studied all appear to come from a single depth in the Slave lithospheric mantle. On the base of Cr-Ca relations they have crystallized at 4.9 GPa and display dunitic Ca intercept values. Their δ¹⁸O values range between +5.23‰ and +5.42‰.The Ekati G10 garnets record a complex, multi-stage metasomatic history involving the interaction of several components during their genesis. One metasomatic agent was enriched in HFSE, LREE, Sr, and depleted in Nb. This agent had the least radiogenic Sr. Another metasomatic agent had highly radiogenic Sr, and was enriched in LREE, Sr, Nb, Th and U.The G10 garnets have very low ε_Nd and ε_Hf values combined with radiogenic Sr, thus, they require an early lithospheric mantle enrichment event at some stage during their genesis or during the evolution of any precursor material that they formed from. The only Hf isotope composition measurable from the Ekati suite is so unradiogenic (ε_Hf = −61) that it yields a Lu/Hf model age of 3521 Ma. This indicates that the lithospheric enrichment event seen by the Ekati garnets or their precursors may have occurred in the early stages of the craton stabilization, during the diamond forming event [Westerlund K., Shirey S., Richardson S., Carlson R., Gurney J. and Harris J. (2006) A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re-Os isotope systematics. Contrib. Mineral. Petrol.152(3), 275-294]. Although our data cannot unequivocally discriminate between a variety of models for the genesis of subcalcic garnets it is clear that the host peridotite originated via melting at shallow depths followed by subduction and that the observed geochemical fingerprint of the garnets is strongly influenced by diamond forming fluids. Diamond forming fluids sampled from fibrous diamonds, have steep REE patterns, negative Ti and Sr anomalies and very low Sm/Nd ratios that are very similar to G10 garnet characteristics. These diamond forming fluids have been recently shown to have extreme Sr and Nd isotopic compositions [Klein-BenDavid O., Pearson D. G., Nowell G. M. and Cantigny P. (2008) Origins of diamond forming fluids—constraints from a coupled Sr-Nd isotope and trace element approach. Extended abstracts to the 9th International Kimberlite Conference, Frankfurt, Germany, 9IKC-A-00118.] that are closely concordant with G10 garnets. The fluids are also rich in LREE, P, K and water, sharing these features with mica-rich metasomes. These similarities suggest that ancient lithospheric metasomes could either provide a source region for, or be a product of diamond forming fluids. Diamond forming fluids appear to be intimately involved in the evolution of G10 garnets in the lithospheric mantle, either acting as a metasomatic agent, or being integral to triggering or enhancing garnet growth in a Cr-rich protolith. Such a link explains the strong association between G10 garnets and diamonds.     

Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ¹⁸O_olivine = 5.17 ± 0.08‰; ⁸⁷Sr/⁸⁶Sr = 0.7029 to 0.7031; ε_Nd = +5.7 to +7.1; ¹⁸⁷Os/¹⁸⁸Os = 0.1481 to 0.1750; ²⁰⁶Pb/²⁰⁴Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ¹⁸O_olivine = 4.87 ± 0.18‰; ⁸⁷Sr/⁸⁶Sr = 0.7031 to 0.7032; ε_Nd = +5.0 to +6.4; ¹⁸⁷Os/¹⁸⁸Os = 0.1421 to 0.1460; ²⁰⁶Pb/²⁰⁴Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ¹⁸O_olivine and high ²⁰⁶Pb/²⁰⁴Pb for La Palma, and elevated ¹⁸⁷Os/¹⁸⁸Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic ²⁰⁶Pb/²⁰⁴Pb, ¹⁸⁷Os/¹⁸⁸Os, elevated Re/Os and Pt/Os, and low-δ¹⁸O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).     

Effects of temperature on strength and compressibility of sand-bentonite buffer

Dense sand-bentonite buffer (γ_d = 1.67Mg/m³) has been proposed in Canada as one of several barriers for isolating nuclear fuel waste. The buffer will be required to function under conditions of high total pressures and elevated temperatures approaching 100°C. Summary results are presented from two test programs: (1) isothermal consolidated undrained triaxial (CIU¯) tests; and (2) isothermal drained constant-p′ (CID) triaxial tests. Specimens were consolidated at effective stresses up to 9.0 MPa and temperatures up to 100°C.

The results indicate parallel hardening lines at systematically lower values of specific volume at elevated temperatures. In shear, increased temperatures produced lower values of maximum deviator stressq_f, and higher pore water pressure changesΔu_f. The net result is curved peak strength envelops in plots ofq_f versusp_f′ that are higher at elevated temperatures, even though the strengths,q_f of individual specimens are lower. The critical state strength envelope is curved inq, p′-planes.

The effect of drained heating on buffer to 100°C is not marked. Compressibilities, stiffnesses, strengths, and pore water pressure generation are all affected, but none of the changes are great.     

The effect of methanogenesis on the geochemistry of low temperature water–Fe0–basalt reactions

Hydrogen gas produced in the subsurface from the hydration of mafic rocks is known to be a major energy source for chemolithotrophic life in extreme environments such as hydrothermal vents. The possibility that in situ anaerobic microorganisms present in the deep subsurface are sustained by low temperature H₂-generating water–rock reactions taking place around them is being investigated. Whether the growth and activity of H₂-utilizing microbes directly influences aqueous geochemistry, rates of mineral dissolution, and the chemical composition of the alteration products is also being quantitatively evaluated.To explore how microorganisms are affected by water–rock reactions, and how their activity may in turn affect reaction progress, laboratory experiments have been conducted to monitor the growth of a methanogenic Archaea in the presence of H_2(g) produced from low temperature water–Fe⁰–basalt reactions. In these systems, the conversion of Fe(II) to Fe(III) and subsequent hydrolysis of water is responsible for the production of H_2(g). To characterize key components of the geochemical system, time series measurements of H₂ and CH₄ gas concentrations, Fe and Si aqueous concentrations, and spatially resolved synchrotron-based analyses of microscale Fe distribution and speciation were conducted. Culture experiments were compared with an abiotic control to document changes in the geochemistry both in the presence and absence of the methanogen.In the control abiotic batch experiment, H₂ was continuously produced, until the headspace became saturated, while in the biotic experiments, microbial consumption of H₂ for methanogenesis draws H₂ down and produces CH₄. Purging the headspace gas reinitiates H₂ and CH₄ production in abiotic and culture experiments, respectively. Mass balance analysis of the amount of CH₄ produced suggests that the total H₂ production in microbial experiments does not exceed the abiotic experiment. Soluble Si concentrations, while buffered to relatively constant values, were higher in culture experiments than the abiotic control.Iron_(aq) concentrations appear to respond to perturbations of H₂ and CH₄ gas concentrations in both culture experiments and the abiotic control. A pulse of Fe preceded the rise in either H₂ or CH₄ production, and as the gas concentrations increased the Fe_(aq) decreased. Iron-bearing mineral assemblages change with increasing reaction time and mineral assemblages vary between culture experiments and the abiotic control. These geochemical trends suggest that there are different reaction paths between the culture experiments and the abiotic control.The hydration of mafic rocks is a common geologic reaction and one that has taken place on Earth for the majority of its history and is postulated to occur on Mars. These reactions are important because of their effect on the rheology and geochemistry of the ocean crust. While most often studied at temperatures of ～250 °C, this work suggests that at lower temperatures microorganisms may have a profound effect on what has long been thought to be solely an abiotic reaction, and may produce diagnostic mineral assemblages that will be preserved in the geological record.     

Petrology of the 1979 eruption of Soufriere volcano,St. Vincent,Lesser Antilles

Major element, trace element, and Sr isotope data are used to study the temporal variation in the chemistry of the ejecta from the 1979 eruption of Soufriere volcano, St. Vincent, and to compare the compositions of the 1979 and 1971/2 magmas. Both the 1971/2 and 1979 products were basaltic andesites almost identical in petrography. A small temporal variation in chemistry is apparent in the 1979 samples but these cannot be related to the 1971/2 lava by fractional crystallisation of phenocryst phases, and the two eruptions may therefore have sampled different batches of magma. ⁸⁷Sr/⁸⁶Sr ratios of the two magmas were identical within analytical error.Microprobe analyses of phenocryst phases and glasses from the 1979 ejecta are presented. Clinopyroxene phenocryst cores with very high Mg/Fe ratios indicate that the basaltic andesites are products of fractionation of magnesian parents. Such magmas are represented by lavas on St. Vincent similar to the microphyric alkali picrites found to the south in Grenada. A common origin for the basaltic andesites of both islands by fractional crystallisation of picritic magmas is suggested. Dacitic glass is abundant in the groundmass of scoria blocks from the eruption. It does not represent the liquid originally in equilibrium with the phenocryst phases, but rather this liquid modified by quench crystallisation. Published interpretations suggesting that dacitic glass compositions in tephra from eruptions of the Soufriere are evidence of mixed-magma eruptions are therefore rejected.     

Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals,and stable isotope exchange in metamorphic rocks

Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H₂O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.