SE Australia Research Articles

On the long-distance transport of Ferrar magmas

Abstract The distribution and geochemical relationships of the Early Jurassic Ferrar large igneous province (LIP) are examined and it is concluded that they support the lateral flow model for the emplacement of the province, with a source along the strongly magmatic Early Jurassic Antarctica–Africa rifted margin. Published data and new analyses from the Pensacola Range are used to show that the dominant magma type in the Ferrar, the Mount Fazio chemical type (MFCT), occurs in the Theron Mountains, Shackleton Range, Whichaway Nunataks, Pensacola Mountains (all Antarctica) and South Africa, as well as well-known outcrops in Victoria Land, Antarctica, SE Australia and New Zealand. Chemical compositions are shown to be somewhat varied, but similar enough for them to be considered as representing closely related magmas. Examination of geochemical trends with distance from the interpreted magma source indicates that Mg# and MgO abundances decline with distance travelled, and it is argued that this is consistent with the lateral flow model. The Scarab Peak chemical type (SPCT), which occurs as sills in the Theron Mountains and Whichaway Nunataks, and as lavas in Victoria Land, is geochemically very homogeneous. Despite this, Mg#, MgO, Ti/Y and Ti/Zr all fall with distance from the interpreted source, consistent with fractional crystallization occurring during the lateral flow of the magmas. Flow took place in dykes or (more likely) sills. No feeder dyke swarm has been identified. The distances flowed, at least 4100 km for MFCT and 3700 km for SPCT, are the longest interpreted lateral magma flows on Earth.

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U–Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U–Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age ( T DM C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone. Zircons from the Ellisras Basin are aged 2700–2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite–greenstone terrane of the Kaapvaal Craton. The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160–880 Ma, host magma mafic granitoid (< 65% SiO 2) derived from juvenile depleted mantle sources ( ε Hf positive) at 1.65 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock–carbonatite), derived from mixed crustal and juvenile depleted mantle sources ( ε Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga. Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700–500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth–Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.

Structural and isotopic constraints on fluid flow regimes and fluid pathways during upper crustal deformation: An example from the Taemas area of the Lachlan Orogen, SE Australia

Structural and stable isotope studies of calcite veins and host rocks in a 1000 m thick, Devonian limestone sequence in the Taemas area of the Lachlan Orogen in SE Australia indicate that externally derived fluids migrated through the sequence over a protracted interval during folding and associated reverse faulting at temperatures of ∼200°C. The evolution of fluid pathways was governed by growth of fold‐related and fault‐related fracture networks at transiently supralithostatic fluid pressures. The internal structures of veins indicate growth during repeated cycles of permeability enhancement and sealing. Systematic increase inδ18O of vein calcite upward through the limestone sequence, and the presence of an O isotope alteration front in the vein system, is related to buffering of fluid compositions by fluid‐rock reaction during upward migration of overpressured, lowδ18O fluids. Repeated switching of fractures between high‐permeability and low‐permeability states promoted episodic flow and changes in flow paths during progressive deformation. A lack of significant migration of the isotopic front during the flow history is consistent with fluid‐rock interaction being associated with numerous, separate pulses of fluid ascending through the percolation network. Irregular variations in fluidδ18O with time during vein growth are interpreted to be driven by repeated changes in reactive path lengths, reaction rates, and flow rates in a discontinuous flow regime. Systematic upward changes in veinδ18O over an area of 20 km2requires that on a time‐averaged basis, most of the vein system was hydraulically well connected to the external fluid reservoir and that growth of the fracture network occurred by fluid‐driven, invasion percolation processes. Repeated failure and sealing events in the fracture system are interpreted to have been driven by episodic migration of fluid pressure waves up through the fracture network, possibly in response to deeper level fault rupture events repeatedly breaching an overpressured fluid reservoir.

The hydrochemical response of cave drip waters to sub-annual and inter-annual climate variability, Wombeyan Caves, SE Australia

A thorough understanding of cave seepage waters is necessary to interpret geochemical variations in speleothem calcite in terms of changing surface climatic conditions at a particular site. Here we present the hydrochemistry of ten cave drip waters from a karst system in SE Australia based on up to 5.5 years of monitoring. Discharge was continuously measured at six sites and manually at the other sites. Dripwater samples were analysed for pH, electrical conductivity, cations and anions at all sites at monthly or more frequent intervals. Each drip possesses a unique chemistry, and not all drip waters responded to antecedent short-term hydrological variations. For example, the hydrochemical behaviour of three adjacent drips at a bedrock depth of 45 m was completely different to that of shallower sites, and was apparently un-related to surface hydrology throughout the investigation. Based on modelled calcite precipitation vectors, prior calcite precipitation was demonstrated at several sites but can only be linked directly to changes in surface recharge at the shallowest sites. At extremely low flow, shallow drip waters accessed a high Mg, Sr and Ba source, thought to be the overlying soil. High-frequency sample collection allows for the calculation of predicted Mg/Ca calcite and Sr/Ca calcite values, highlighting that the sites with the greatest potential to record high-resolution palaeohydrological records are those situated at shallow depth. Longer temporal-resolution palaeohydrological records may be recorded at deeper sites but longer-term monitoring is required to identify probable time scales. Inherent system non-linearities, dissolution of secondary calcite in pore spaces of the aquifer, changes in the source of trace elements, and the presence of multiple reservoirs confirm the need for the use of multiple speleothems and a multi-proxy approach to gain accurate palaeohydrological records from this site.

A comparative U–Th–Pb (zircon–monazite) and 40Ar–39Ar (muscovite–biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen

AbstractMylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U–Th–Pb system in zircon and monazite and of the 40Ar–39Ar system in micas during ductile deformation. Meso‐ and micro‐structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top‐to‐the‐SW and ‐NE, respectively) and developed under upper greenschist facies conditions. In situ U–Pb dating by laser‐ablation inductively coupled plasma‐mass spectrometry of zircon areas with well‐preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross–Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U–Th–Pb ages of c. 440 Ma, in agreement with in‐situ Ar laserprobe ages of syn‐shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal‐chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U–Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ‘ghost’ structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U–Pb ages of c. 450 and 410 Ma. The c. 450‐Ma date matches within errors most monazite U–Th–Pb ages and in‐situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician–Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear‐zone system formed in response to ∼NE–SW‐directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician–Late Devonian Lachlan Fold Belt.

The geomorphic and ecological effectiveness of habitat rehabilitation works: Continuous measurement of scour and fill around large logs in sand-bed streams

Geomorphologists, ecologists and engineers have all contributed to stream rehabilitation projects by predicting the physical effect of habitat restoration structures. In this study we report the results of a stream rehabilitation project on the Snowy River, SE Australia; that aims to improve fish habitat and facilitate migration associated with scour holes around large wood in the streambed. Whilst engineering models allow us to predict maximum scour, the key management issue here was not the maximum scour depth but whether the holes persisted at a range of flows, and if they were present when fish actually required them. This led to the development of a new method to continuously monitor scour in a sand-bed, using a buried pressure transducer. In this study we monitored fluctuations in the bed level below three large logs (1 m diameter) on the Snowy River. Each log had a different scour mechanism: a plunge pool, a horseshoe vortex (analogous to a bridge pier), and a submerged jet beneath the log. The continuous monitoring demonstrated a complex relationship between discharge and pool scour. The horseshoe vortex pool maintained a constant level, whilst, contrary to expectations, both the plunge pool and the submerged jet pool gradually filled over the 12 months. Filling was associated with the average rise in flows in winter, and occurred despite several freshes and discharge spikes. The plunge pool showed the most variation, with bed levels fluctuating by over 1 m. A key factor in pool scour here may not be the local water depth at the log, but the position of the log in relation to larger scale movements of sand-waves in the stream. These results question assumptions on the relative importance of small floods or channel-maintenance flows that lead to beneficial scour around large wood in sand-bed streams. Further, the continuous measurement of scour and fill around the logs suggested the presence of pool scour holes would have met critical requirements for Australian bass ( Macquaria novemaculeata) during the migration period, whereas less-frequent monitoring typical of rehabilitation trials would have suggested the contrary. The results of this study have demonstrated that geomorphic effectiveness is not always synonymous with biological effectiveness. Whilst physical models emphasise extreme changes, such as maximum scour, the key biological issue is whether scour occurs at the critical time of the life cycle. Continuous measurement of sand levels is an example of a geomorphic technique that will help to develop models that predict biologically meaningful processes, not just extremes.

Predicting the spatial patterns of hillslope sediment delivery to river channels in the Murrumbidgee catchment, Australia

Sediment yield data derived from long-term sedimentation rates in 26 small farm dams in SE Australia were used to calibrate a spatially distributed soil erosion and sediment delivery model (WATEM/SEDEM) that takes into account contribution from gully erosion in areas of concentrated flow. For three different land use categories (poor, moderate and good vegetative cover), a sediment transport capacity coefficient was calibrated. All other parameters being equal, it was found that sediment transport capacity for cropland is 2 times higher than for degraded pasture and 20 times higher than for native forest and good pasture. Model efficiencies for the prediction of specific and total sediment yield are 0.56 and 0.89, respectively. These model efficiencies are much higher compared to those obtained when intense erosion in concentrated flow areas is not considered explicitly. Several of the catchments that were used for the calibration have very high sediment yield rates, which are attributed to the presence of gullies. The good performance of WATEM/SEDEM to all catchments suggests that the model accounts well for gully erosion. Next, the calibrated WATEM/SEDEM was applied to the Murrumbidgee River basin (30,000 km2). A mean annual sediment input into the river channels from the hillslopes of 478,000 t was predicted. The spatial pattern of hillslope-derived suspended sediment delivery in the Murrumbidgee indicates that most of the sediment originates from a few tributaries downstream of Burrinjuck Reservoir. Given the fact that high-resolution datasets (including digital elevation models) are becoming available at reasonable cost, WATEM/SEDEM provides a powerful tool to predict hillslope sediment delivery under different environments including the spatial patterns of hillslope generated sediment fluxes.

Late Pleistocene and Holocene climate of SE Australia reconstructed from dust and river loads deposited offshore the River Murray Mouth

The terrigenous component of deep-sea sediment core MD03-2611, offshore South Australia, has been examined for mineralogical, geochemical and isotopic tracers to reconstruct the deglacial and Holocene climate on the adjacent southeastern Australian continent. The provenance of the terrigenous component is constrained by using a combination of clay minerals, conservative elements like rare earths (REEs) and Sr- and Nd isotopic ratios. From 17 to 5 ka BP, the sediment signature is dominated by alluvial sediments from the Murray–Darling Basin (MDB). Starting at 8 ka and culminating from 5 ka to the present, enrichment of REEs and kaolinite content, increasing grain size, high 87Sr/ 86Sr ratios and strongly negative εNd(0) values are attributed to an influx of aeolian dust, probably sourced from older crustal terranes in central and/or western Australia, although the exact provenance of the dust could not be assessed. The input of fluvial material from a common MDB source ceased at 13.5 ka BP, but 2 periods of increased fluvial input, originating mainly in the Murray catchment, are indicated by clay contents, K and Rb concentrations, and Sr and Nd isotope ratios. These fluvial pulses probably reflect periods of increased rainfall in southeastern Australia. The first fluvial pulse from 13.5 to 11.5 ka had not been recorded in the region previously, while the second pulse from 9.5 to 7.5 ka BP corresponds to periods of increased rainfall already documented in records from 2 lakes in the region. The supply of fluvial versus aeolian material in core MD03-2611 mirrors patterns of humidity and aridity inferred from other continental records in SE Australia, although our study provides information for the Late Pleistocene/Holocene transition not recorded elsewhere.

Rare earth element and Pb isotope variations in a 52 kyr peat core from Lynch’s Crater (NE Queensland, Australia): Proxy development and application to paleoclimate in the Southern Hemisphere

Accurate prediction of future climate scenarios is contingent on our understanding of past and present climate mechanisms. This is done in part through the reconstruction of historical climate changes using proxy records from terrestrial and marine archives. Terrestrial archives covering the Holocene and late Pleistocene are limited, most acutely in the Southern Hemisphere. Here, Rare earth elements (REE) and Pb isotopes are developed as inorganic geochemical proxies of mineral dust source changes and, by extension, climate change. Using a peat core from Lynch’s Crater in NE Queensland, Australia, we present the first long-term (c. 52 kyr) terrestrial record of atmospheric REE and Pb deposition (with the exception of four wet events which represent periods of erosion from the crater itself) in the Southern Hemisphere covering both glacial and interglacial times. Based on a combination of correlation analyses, Al and Ti normalised profiles and elemental patterns, we establish REE are immobile within the peat deposit and not subject to significant post depositional diagenetic changes (important particularly for Ce). This is vital as REE can be mobile under acid and organic rich conditions like those that can occur during the development of a peat deposit. The volcanic provinces of eastern Australia have characteristic Eu anomaly signatures, which allowed their use in a novel way to detect changes in dust source to Lynch’s Crater. Between 41,095 and 52,505 BP the deposit was under the influence of dust carried by long distance transport (>1500 km) from SE Australia. From 8525 to 40,815 BP regional sources (100–1500 km) dominated the deposited signals while between 1740 and 8390 BP the dust signal was controlled by local sources (<100 km). These findings were also confirmed by Pb isotope data. Changepoint modelling refined the timing of these changes in dust source, recognizing concurrent shifts in our tracing tools ((Eu/Eu ∗) PAAS and 206Pb/ 207Pb). These changepoints were then compared to other palaeoenvironmental records (pollen, lake levels and dune building) from eastern Australia and found to be similar. Our results demonstrate that REE and Pb isotopes are effective tools for tracing past changes in atmospheric dust sources and to the study of climate change using minerotrophic peat deposits.

Improved predictions of groundwater salinisation rates in the Tintinara Highlands area of South Australia - the role of geology and geophysics

In many areas of South Eastern Australia, saline soil-water in the unsaturated zone overlies fresher groundwater. In areas where remnant vegetation has been removed and has been replaced with pasture/cropping, enhanced drainage has resulted which, after a lag time, flushes the saline soil-water into the underlying aquifer. For an area near Tintinara in SE Australia, spatial simulations of groundwater salinisation under dryland and irrigation scenarios were developed for a 200 year timeframe using a distributed parameter recharge model. Inputs to the model include spatial patterns of near surface clay materials determined from the constrained inversion of helicopter frequency domain EM data. The clay significantly affects the lag time and the salt flux to the aquifer. Under dryland agriculture leaching of the salt to the aquifer takes between 50 and 200 years, whereas under irrigation leaching takes between 20 and 50 years. Under irrigation the salinity of the groundwater is expected to increase from 1000 ? 6000mg/L over the next 25-40 years. While there are no obvious economic measures available to prevent salinisation, the modelling suggests that efficient irrigation on appropriate soils could slow the process.

Tsunami and palaeotsunami depositional signatures and their potential value in understanding the late-Holocene tsunami record

In recent years, much research on modern and palaeotsunami deposits has been published. From these studies, a range of signature types has been identified. Identifying and dating such deposits is an important element in understanding late-Holocene tsunami hazard and risk. However, important questions such as, ‘do modern and palaeotsunami leave similar or dissimilar traces?’ ‘do tsunami leave the same signatures all around the world or are there significant variations?’ and ‘what is the actual record of tsunami in different parts of the world?’ still remain. Answering these questions is not an easy task but examining megatsunami flood deposits should shed some light on these questions because such high-magnitude events should leave very clear and detailed traces within the coastal landscape. The coast of SE Australia is reported to have been affected by numerous palaeomegatsunami in the late Holocene. As such, the coast of New South Wales offers an important natural laboratory to examine in detail deposits associated with such events. Here, we summarize the published characteristics of modern and palaeotsunami deposits globally and within Australia. We briefly outline the tsunami risk to Australia before examining a site called Minnamurra Point on the coastline of SE Australia (south of Sydney) that has previously been described as containing evidence for a palaeomegatsunami of an unknown age. We describe the results of a detailed coastal survey, field stratigraphic investigation and various standard laboratory analyses. Surprisingly, we are unable to replicate the previously reported findings of tsunami deposits. Whilst we prefer the interpretation that the sequence is an in situ soil (the sediment sequence examined contains none of the usually reported lines of evidence to demonstrate tsunami provenance), we recognize and discuss the significance and difficulty of identifying tsunami deposits in the field and consider the implications of our findings to the wider debate about the preservation of tsunami-deposited sediments.

Secondary U-phosphates—possible links to Quaternary pluvial periods in SE Australia

Secondary U-phosphates—possible links to Quaternary pluvial periods in SE Australia

The relationship between gabbros and I-type granites: An example from the Lachlan Fold Belt, SE Australia

The relationship between gabbros and I-type granites: An example from the Lachlan Fold Belt, SE Australia

A 500 Ma Rb–Sr age from the Siluro-Devonian Hawksview granite, Lachlan Fold Belt, SE Australia: Mixing artifact or source isochron?

A 500 Ma Rb–Sr age from the Siluro-Devonian Hawksview granite, Lachlan Fold Belt, SE Australia: Mixing artifact or source isochron?

3D Mapping of alteration zones: An example from Cohen’s Reef, Walhalla, SE Australia

3D Mapping of alteration zones: An example from Cohen’s Reef, Walhalla, SE Australia

Regional structural controls of gold mineralisation, Bendigo and Castlemaine goldfields, Central Victoria, Australia

The Bendigo and Castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the Bendigo Zone of central Victoria, SE Australia. Detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold-quartz mineralisation. Mineralised quartz veins are hosted by the Castlemaine Group, an Early-to-Middle Ordovician turbidite succession at least 3,000 m thick. Gold deposits are controlled by low-displacement faults that are clustered into several belts (the goldfields) indicating a regional structural control. The timing of mineralisation overlapped with that of the major period of deformation including folding, cleavage development and regional faulting. The Bendigo and Castlemaine goldfields are located in an area termed the Whitelaw thrust sheet bounded by two unmineralised, high-displacement, regional-scale faults. Mapping has revealed an interrelationship between the regional-scale faults, regional structural style and goldfield location. The goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub-horizontal to synclinal enveloping surfaces, relatively low co-axial strains and moderate cleavage development. The non-gold-bearing areas immediately east of each goldfield correspond with the lower part of the Whitelaw thrust sheet and are characterised by higher non-coaxial strains, stronger cleavage and folds with wide west-dipping limbs giving rise to easterly vergent sections and steeply west-dipping enveloping surfaces. That mineralisation was an integral part of the thin-skinned style of deformation in the central Bendigo Zone is indicated by timing relationships and the interrelationship between local-scale mineralised structures and regional-scale features such as large-displacement unmineralised faults, regional variations in fold style and overall thrust sheet geometry. The work supports previous models that suggest mineralised fluids were focussed along a linked system of deep-seated faults. The primary conduits may have been major regional-scale ‘intrazone’ faults, which are inferred to sole into detachments near the base of the Castlemaine Group. It is proposed that these structures linked with minor intrazone faults and then with networks of low-displacement mineralised faults that were strongly controlled by folds. The location of minor intrazone faults was probably controlled by internal thrust sheet geometry. The distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low-displacement faults.

Uranium-series isotopes in colloids and suspended sediments: Timescale for sediment production and transport in the Murray–Darling River system

We have measured 238U– 234U– 230Th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the Murray–Darling River basin (SE Australia). Continuous compositional trends of composition are observed for ( 234U/ 238U) and ( 230Th/ 238U) activity ratios over the range of size fractions studied. They are explained by the variable contribution of two components: detrital alumino-silicates and organic matter. Using relationships between activity ratios and the loss-on-ignition, it is possible to estimate the U-series isotopic composition of both detrital and organic end-members, which provide the best estimate for the composition of the true detrital phase (i.e. controlled only by the removal of elements through weathering) and the true dissolved phase (i.e. colloid-free), respectively. When performing a single filtration at 0.2 μm, it appears that size fractions > and < 0.2 μm underestimate the disequilibrium in both the true detrital and dissolved phases, respectively. We show that erosion does not currently operate in steady state in the Murray–Darling basin and catchment soils are currently being degraded. This could be a consequence of the intensification of agriculture and deforestation over the past 100 yr. The residence time of sediments in the different sub-basins is calculated by computing the composition of the detrital phase with a continuous weathering model. It yields 11 ± 2 kyr for the Darling River basin, only 2–3 ± 1 kyr for the Murray River and 7 ± 2 kyr for the Goulburn River. These values are correlated with the age of changes in river dynamics, in turn probably controlled by climate fluctuation, and indicate the age of the modern river system for each sub-basin.

Lithospheric structure of Tasmania from a novel form of teleseismic tomography

In 2001 and 2002, a temporary array of 72 seismic recorders was deployed across northern Tasmania (SE Australia), with the aim of imaging the underlying crust and upper mantle using three‐dimensional (3‐D) teleseismic tomography. Using a recently developed adaptive stacking technique, 6520 relative P wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period. A novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method, a grid‐based eikonal solver, is used to map the residual patterns as P wave velocity anomalies. The new scheme proves to be both fast and robust, making it well suited to large data sets and the reconstruction of complex anomalies. The resultant tomographic images of Tasmania exhibit significant lateral perturbations in P wave velocity structure (≤5%) from a 1‐D reference model. A marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern Tasmania is underlain by dense rocks with an oceanic crustal affinity, contrasting with the continentally derived siliciclastic core of western Tasmania. Significantly, the Tamar Fracture System does not overlie the narrow transition from relatively fast to slow velocities, which suggests that it may be a near‐surface feature rather than a manifestation of deeper crustal‐scale suturing as previously thought. Farther west, an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group and Arthur Lineament may be related to remnant subduction of oceanic lithosphere associated with the mid‐Cambrian Delamerian Orogeny.

Colonisation of Introduced Timber by Algae and Invertebrates, and its Potential Role in Aquatic Ecosystem Restoration

As part of a habitat restoration experiment wood substrates (red gum) were introduced to two lowland streams of SE Australia in which habitat has been severely degraded by deposition of sand eroded from higher in the catchment. We monitored net primary production (NPP) and community respiration (CR), nutrient concentrations and the succession of algae and invertebrates (abundance and species richness), sampling at 2, 4, 8, 12, 16 and 20 weeks. Colonisation by algae was rapid, and there were distinct changes in the assemblages over the first 4 weeks. Thereafter, changes were much less marked. There were also differences in nutrient concentrations and some measures of algal abundance between the two creeks. As with the algae, invertebrates colonised these substrates extremely rapidly, peaking in abundance and richness in week 8. Invertebrate abundances closely tracked changes in the abundance of algae. By the end of the study both algal and invertebrate communities were in apparent decline, with sharp decreases in invertebrate and algal abundance and invertebrate species richness. Rates of GPP also declined toward the end of the experiment, and this coincided with the detachment of large mats of filamentous algae and the recession of flows over the summer months. However, in both streams the added timber quickly created habitat with high levels of primary production in an otherwise strongly heterotrophic stream system. These hotspots of autotrophic production were quickly colonised by high numbers of macroinvertebrates indicating timber addition may provide an effective means of augmenting habitat for algae and invertebrates in sanded streams.

The isotopic composition of magnesium in mantle olivine: Records of depletion and metasomatism

We have investigated detailed Mg isotopic variations in the lithospheric mantle by analysing olivine in mantle-derived peridotite xenoliths and megacrysts. High-precision in situ analyses of 26Mg/ 24Mg and 25Mg/ 24Mg are made using a laser-ablation microprobe and MC-ICPMS. Measurements are done using a standard-sample bracketing technique with an in-house olivine standard. Replicate analyses of this standard give a precision of 0.20‰ (2sd) for δ 26Mg (= [( 26Mg/ 24Mg) sample / ( 26Mg/ 24Mg) standard − 1] × 1000) and 0.12‰ (2sd) for δ 25Mg. The analysed olivine grains represent the lithospheric mantle beneath Archean cratons (Siberia, Kaapvaal, Slave) and Phanerozoic fold belts (SE Australia). Results from olivines show significant heterogeneity in the lithospheric mantle: δ 26Mg ranges from − 3.01‰ to + 1.03‰ and δ 25Mg from − 1.59‰ to + 0.51‰, relative to the magnesium isotopic standard DSM-3. There is a broad trend from lighter Mg isotopic compositions in depleted xenoliths from Archean mantle to heavier Mg in the less depleted Phanerozoic samples. Samples with petrographic evidence of refertilisation (including modal metasomatism) show large ranges in δMg values within samples. Sheared peridotite xenoliths from the Kaapvaal and Slave Cratons show a shift to higher δMg associated with the introduction of fluids with an ‘asthenospheric’ signature. Olivine in the least metasomatised peridotites from SE Australia has isotopically light Mg, whereas olivine in cryptically and modally metasomatised peridotites (amphibole ± apatite-bearing) becomes progressively heavier, both absolutely and relative to pyroxene and amphibole, with increasing degrees of metasomatism. The heterogeneity measured in individual samples suggests that Mg isotopic fractionations produced by processes of mantle metasomatism are preserved on the intra-grain scale, and the magnitude of the observed fractionations indicates that diffusion-related (kinetic) processes are important in controlling isotope fractionation at high temperatures. The in situ measurement of Mg isotopes provides a powerful new method for investigating processes in the mantle.