Small-volume Lu-Hf and U-Pb isotope determination of complex zircons by solution and laser ablation MC-ICP-MS

This study presents solution and laser ablation analysis methods suited to the determination of the Lu-Hf and U-Pb isotope signatures of small volumes of zircon corresponding to sub-nanogram amounts of Hf. A reduced-volume approach to laser ablation is taken with consecutive U-Pb and Lu-Hf isotopic analyses resulting in a total pit depth of 18 μm using a 25 μm laser ablation spot. This results in excavation of ~ 40 ng of zircon, equating to ~ 0.3 ng of Hf and uncertainties ~ 1 εHf (2s). The laser technique presented here is ideally suited to complex, finely-zoned zircons for which drilling to depth would intersect different zones. The possibility of reducing the total laser ablation pit depth to ~ 10 μm is demonstrated by decreasing the integration time for Hf isotope analysis without serious compromise of the uncertainty. Furthermore, the capability to determine the Hf isotope composition of the same amount of Hf using solution MC-ICP-MS is also demonstrated, as is the suitability of analyzing solutions not subject to Hf-HREE separation. For both solution and laser ablation methods, this study investigates possible methods of Yb interference correction, the potential for matrix effects, and the accurate determination of 176Lu/177Hf. Using the approaches described here, acceptable uncertainty levels are achieved to resolve complexity at the level of 25 × 18 μm (diameter × depth), and therefore this method has the potential to yield geologically meaningful results for rocks containing complexly-zoned zircons

Geochronology and structure of the eastern margin of the Tanzania Craton east of Dodoma

The precise position, nature and U-Pb zircon geochronology of the eastern margin of the Tanzania Craton has been studied in the Mpwapwa area, some 60 km east of Dodoma, central Tanzania, in a number of field transects over a ca. 45 km strike length of the craton margin. The rocks to the east of the Tanzania Craton in this area either belong to the Palaeoproterozoic Usagaran belt, or the “Western Granulite” terrane of the Neoproterozoic East African Orogen, according to different authors. The eastern part of the craton is underlain by typical Neoarchaean migmatitic grey granodioritic orthogneisses dated by ICP-MS at 2674 ± 73 Ma. There is a gradual increase in strain eastwards in these rocks, culminating in a 1 to 2 km wide, locally imbricated, ductile thrust/shear zone with mylonites indicating an oblique top-to-the-NW, transpressional sense of movement. East of the craton-edge shear zone, a series of high-grade supracrustal rocks are termed the “Mpwapwa Group”, in view of uncertain age and regional lithostratigraphic correlations. There is an apparent east-west lithological zonation of Mpwapwa Group parallel to the craton margin shear zone. In the west, immediately adjacent to the craton, the group consists of typical “shelf facies” metasediments (marbles, calc-silicates, quartzites etc.). U-Pb dating of detrital zircons from two Mpwapwa Group quartzite samples from this marginal zone contain only Archaean detritus, constraining their maximum depositional age to > ca. 2.6 Ga and suggesting that the group is Neoarchaean in age. The shelf rocks pass eastwards into garnet and kyanite-bearing semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, where the mafic amphibolite gneisses may represent meta-basalts and the felsic rocks may have meta-rhyolite, -granite or –psammite protoliths. Massive garnet-clinopyroxene amphibolite layers in the Mpwapwa Group gneisses may have been intrusive mafic sills and possibly correlate with the Palaeoproterozoic Isimani Suite, which outcrops south of the study area and includes 2 Ga eclogites. Zircons from a quartzo-feldsapthic gneiss sample from the bimodal gneisses were dated and showed it to be a probable Neoarchaean rock which underwent metamorphism during the Palaeoproterozoic Usagaran event at ca. 1950 Ma. This event was broadly coeval with subduction, closure of an ocean basin and eclogite formation further south and led to the initial juxtaposition of the two Archaean blocks. The metamorphism probably dates the tectonic event when the Archaean Mpwapwa Group rocks were juxtaposed against the orthogneissic Tanzania Craton. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1871 ± 35 Ma. Zircons in the granite have metamorphic rims dated between 550 and 650 Ma that grew during the East African orogenic event

Advances in Isotope Ratio Determination by LA–ICP–MS

LA–ICP–MS has proven to be an extremely important analytical tool within the Earth, environmental, and archaeological sciences. New developments in both instrumentation and methodology now provide the ability to extract age and isotopic tracer information in situ at a variety of scales (from nm to cm), in 2- and 3-dimensions, quickly and cost-effectively, providing considerable analytical flexibility compared to other micro-analytical techniques. Here, we review the current state of the art in laser ablation isotope ratio determination and provide some insights into future developments

Salt domes of the United Arab Emirates: evidence for late Neoproterozoic sedimentation and rift volcanism in the northern Arabian-Nubian Shield

Seven of the nine emergent salt domes of the UAE have been examined. They outcrop on the Arabian Gulf islands of Delma, Sir Bani Yas, Arzana, Qarnain, Zirku and Sir Bu N’Air, with one on-land salt dome at Jebal Dhanna. The salt domes range from about 1 km across (incomplete remnant of Qarnain) to over 6 km diameter (Delma) and form dissected hilly topography rising to about 140 m above sea-level (Zirku). The majority of the salt domes are single intrusions but two, Delma and Jebal Dhanna, appear to have multiple phases. The diapirs were emplaced penecontemporaneously with the Miocene country rocks, while evidence of recent localised halokinetic reactivation in small dome-like “salt blisters” is seen on a number of salt islands

Evaluating tools for the spatial management of fisheries

1. The ability to define the spatial dynamics of fish stocks is critical to fisheries man- agement. Combating illegal, unreported and unregulated fishing and the regula- tion of area-based management through physical patrols and port side controls are growing areas of management attention. Augmenting the existing approaches to fisheries management with forensic techniques has the potential to increase compliance and enforcement success rates. 2. We tested the accuracy of three techniques (genotyping, otolith microchemistry and morphometrics) that can be used to identify geographic origin. We used fish caught from three fishing grounds, separated by a minimum of 5 km and a maxi- mum of 60 km, to test the accuracy of these approaches at relatively small spatial scales. 3. Using nearest-neighbour analyses, morphometric analysis was the most accurate (79.5%) in assigning individual fish to their fishing ground of origin. Neither otolith microchemistry (54.0%) or genetic analyses (52.4%) had sufficient accuracy at the spatial scales we examined. 4. Synthesis and applications. The combination of accuracy and minimal resource re- quirements make morphometric analysis a promising tool for assessing compli- ance with area-based fishing restrictions at the scale of kilometres. Furthermore, this approach has promising application, in small-scale fisheries through to com- munity-based management approaches where technical and financial resources are limited

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an εNd(0) signature of –18.3 to –12.1, and is intruded by c. 800 Ma and c. 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an εNd(0) signature of –27.7 to –23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones

Changing nutrient cycling in Lake Baikal, the world's oldest lake

Lake Baikal, lying in a rift zone in southeastern Siberia, is the world's oldest, deepest, and most voluminous lake that began to form over 30 million years ago. Cited as the "most outstanding example of a freshwater ecosystem" and designated a World Heritage Site in 1996 due to its high level of endemicity, the lake and its ecosystem have become increasingly threatened by both climate change and anthropogenic disturbance. Here, we present a record of nutrient cycling in the lake, derived from the silicon isotope composition of diatoms, which dominate aquatic primary productivity. Using historical records from the region, we assess the extent to which natural and anthropogenic factors have altered biogeochemical cycling in the lake over the last 2,000 y. We show that rates of nutrient supply from deep waters to the photic zone have dramatically increased since the mid-19th century in response to changing wind dynamics, reduced ice cover, and their associated impact on limnological processes in the lake. With stressors linked to untreated sewage and catchment development also now impacting the near-shore region of Lake Baikal, the resilience of the lake's highly endemic ecosystem to ongoing and future disturbance is increasingly uncertain

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb carbonate geochronology: strategies, progress, and limitations

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data