Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.Thanks are due to A. Lemon, A. Zabula, C. Mcheka, I. Nkukumila (Mkango Resources Ltd.), É. Deady (BGS) and P. Armitage (Paul Armitage Consulting Ltd.) for logistical support and enthusiastic discussions in the field. This contribution benefitted from reviews by Jindřich Kynický and Ray Macdonald, as well as anonymous reviewers, who we thank for their time and insightful comments. This work was funded by a NERC BGS studentship to SBF (NEE/J50318/1; S208), the NERC SoS RARE consortium (NE/M011429/1) and by Mkango Resources Ltd. AGG publishes with the permission of the Executive Director of the British Geological Survey (NERC)

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

Comments on Richards et al., Journal of Archaeological Science 35, 2008 "Strontium isotope evidence of Neanderthal mobility at the site of Lakonis, Greece using laser-ablation PIMMS"

We present an evaluation of the laser ablation Sr isotope data reported by Richards et al. (2008) for a Neanderthal tooth recovered from a site in Greece. Based on an alternative and analytically more robust method of correcting for isobaric interferences present during the analysis the tooth appears to be isotopically homogenous and within uncertainty of the value for modern seawater. If this is the case then contrary to the migration model proposed by Richards et al. (2008) the Neanderthal individual may have actually been a coastal dweller and lived within the vicinity of the find site

Metamorphism, melting, and channel flow in the Greater Himalayan Sequence and Makalu leucogranite: Constraints from thermobarometry, metamorphic modeling, and U-Pb geochronology

The Makalu leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation-parallel sheets along the top of the Greater Himalayan Sequence. It is part of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms Bt leucogranites but, unlike most other Himalayan granites, also locally contains coarse-grained cordierite. The cordierite-bearing leucogranite intrudes through and overlies lower sheets of "normal" tourmaline granites and represents the most recent phase of magmatism. Cross-cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting (∼700°C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in leucogranites record the last low-pressure decompression phase of melting. P-T determinations detail peak sillimanite grade metamorphism at 713°C/5.9 kbar, with a secondary cordierite overprint at 618°C/2.1 kbar; this P-T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different leucogranites. The main phase of leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite leucogranite and the migmatitic Barun gneisses at 15.6 ± 0.2 and 16.0 ± 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700°C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene. Copyright 2010 by the American Geophysical Union

Burial and exhumation history of a Lesser Himalayan schist: Recording the formation of an inverted metamorphic sequence in NW India

Coupled analysis of the pressure–temperature (PT) evolution and accessory phase geochronology of a single sample reveals the burial-uplift history of part of the Lesser Himalaya during the Middle Miocene. Phase-equilibria calculations indicate that a peak temperature of 600–640 °C followed burial to approximately 25 km depth. Laser-ablation monazite geochronology yields a weighted mean 206Pb/238U age of 11.1 ± 2.0 Ma and a Tera-Wasserburg Concordia intercept age of 10.6 ± 0.9 Ma, with no distinguishable age difference between matrix and inclusion grains. Considerations of the likelihood of excess 206Pb further suggest that the crystallization age lies in the range 9–10 Ma. Textural analysis suggests that monazite grew during prograde metamorphism. Peak metamorphic conditions were followed by exhumation and cooling, forming a distinctively tight PT path closure. Both the shape of this path and its relatively young prograde phase distinguish Lesser Himalayan evolution from that typically inferred for the High Himalaya, and allow exploration of the thermal mechanisms that operated in the western Himalaya during the interval ca. 23–6 Ma. The PTt history is characteristic of footwall heating due to rapid overthrusting of hot rock (the Higher Himalaya), followed by incorporation into a thrust sheet that exhumed the sequence rapidly enough to preserve an inverted metamorphic gradient

U-Pb zircon ages for Yarlung Tsangpo suture zone ophiolites, southwestern Tibet and their tectonic implications

Ophiolite complexes preserved along the Yarlung Tsangpo suture zone (YTSZ) and obducted onto the northern continental margin of India in southern Tibet represent the remnants of the once extensive Permian-Mesozoic Neo-Tethyan Ocean that separated India from Asia. Complete ophiolite successions are preserved near Xigaze, whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks. U-Pb zircon LA-MC-ICP-MS geochronology on two gabbro samples from the Luobusa ophiolite yielded concordant data with mean 206Pb/238U ages of 149.9 ± 1.4 (2σ) Ma and 150.0 ± 5.0 Ma. These ages are in contrast to a younger age of 131.8 ± 1.0 Ma obtained from a pegmatitic gabbro in Xigaze. Five U-Pb zircon TIMS ages from gabbroic samples in the western portion of the ophiolite belt reveal that the Dangxiong ophiolite formed between 126.7 ± 0.4 Ma and 123.4 ± 0.8 Ma. Zircons from the Jungbwa ophiolite have similar ages of 123.4 ± 0.8 Ma and 123.9 ± 0.9 Ma. A single zircon analysed from a gabbro in Kiogar has an age of 159.7 ± 0.5 Ma. Geochronological data reported here show YTSZ ophiolites formed in association with intra-oceanic subduction zone systems and are related a significant tectonic episode within the Tethyan Ocean during Late Jurassic to Early to mid Cretaceous times. © 2013 International Association for Gondwana Research

Geochemical evidence of Milankovitch cycles in Atlantic Ocean ferromanganese crusts

Hydrogenetic ferromanganese crusts are considered a faithful record of the isotopic composition of seawater influenced by weathering processes of continental masses. Given their ubiquitous presence in all oceans of the planet at depths of 400–7000 meters, they form one of the most well-distributed and accessible records of water-mass mixing and climate. However, their slow accumulation rate and poor age constraints have to date limited their use to explore 100 ka paleoclimatic phenomena. Here it is shown how the Pb isotope signature and major element content of a Fe-Mn crust from the north-east Atlantic responded to changes in the intensity and geographic extent of monsoonal rainfall over West Africa, as controlled by climatic precession during the Paleocene. The studied high-spatial resolution (4 μm) laser-ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) Pb isotope data is a nearly 2 order of magnitude improvement in spatial and temporal resolution compared to micro-drill subsamples. The record demonstrates cyclicity of the 206Pb/204Pb and 208, 207Pb/206Pb ratios at the scale of single Fe-Mn oxide laminae, in conjunction with variations in the Fe/Mn ratio, Al, Si and Ti content. Time-frequency analysis and astronomical tuning of the Pb isotope data demonstrates the imprint of climatic precession (∼20 ka) modulated by eccentricity (∼100 and 405 ka), yielding growth rates of 1.5–3.5 mm/Ma consistent with previous chemostratigraphic age models. In this context, boreal summer at the perihelion causes stronger insolation over West Africa, resulting in more intense and geographically extended monsoonal rainfalls compared to aphelion boreal summer conditions. This, in turn, influences the balance between the weathering endmembers feeding the north-east Atlantic basin. These results provide a new approach for calibrating Fe-Mn crust records to astronomical solutions, and allow their isotopic and chemical archive to be exploited with an improved temporal resolution of 1000–5000 years.</p

Monazite geochronology and petrology of kyanite- and sillimanite-grade migmatites from the northwestern flank of the eastern Himalayan syntaxis

A combined geochronological and petrological study of pelitic migmatites from the northwestern flank of the eastern Himalayan syntaxis has constrained the timing and P-T conditions of two high-grade metamorphic events that affected the south Lhasa block (Asian margin) and provides new insight into the tectonothermal evolution of the India-Asia collision. U(-Th)-Pb dating of in situ monazite shows that upper amphibolite-facies sillimanite-grade metamorphism and consequent partial melting occurred between c. 71 and 50 Ma at P-T conditions above 6.3 ± 1.2 kbar and 750 ± 30 °C. Further partial melting at upper amphibolite-facies kyanite-grade conditions occurred between c. 44 and 33 Ma at minimum P-T conditions of 10.4 ± 1.0 kbar and 698 ± 20 °C. These data are interpreted to record a south Lhasa block mid-crustal sillimanite-grade melting event in the Late Cretaceous to Early Eocene related to regional heat advection caused by coeval and prolonged emplacement of Gangdese batholith units. This was followed by a higher pressure and lower temperature kyanite-grade melting event during the Middle Eocene to Early Oligocene associated with deformation and crustal thickening in the south Lhasa block, coeval with kyanite-grade metamorphism along the Himalaya, as a result of the on-going India-Asia collision. These partially-melted crustal lithologies offer potential sources (or otherwise analogs for sources) for the Miocene emplacement of adakitic intrusions previously documented in the eastern Himalayan syntaxis region. © 2013 International Association for Gondwana Research

Antecedents and consequences of Internet channel performance

The LA-ICP-MS U-(Th-)Pb geochronology international community has defined new standards for the determination of U-(Th-)Pb ages. A new workflow defines the appropriate propagation of uncertainties for these data, identifying random and systematic components. Only data with uncertainties relating to random error should be used in weighted mean calculations of population ages; uncertainty components for systematic errors are propagated after this stage, preventing their erroneous reduction. Following this improved uncertainty propagation protocol, data can be compared at different uncertainty levels to better resolve age differences. New reference values for commonly used zircon, monazite and titanite reference materials are defined (based on ID-TIMS) after removing corrections for common lead and the effects of excess 230Th. These values more accurately reflect the material sampled during the determination of calibration factors by LA-ICP-MS analysis. Recommendations are made to graphically represent data only with uncertainty ellipses at 2s and to submit or cite validation data with sample data when submitting data for publication. New data-reporting standards are defined to help improve the peer-review process. With these improvements, LA-ICP-MS U-(Th-)Pb data can be considered more robust, accurate, better documented and quantified, directly contributing to their improved scientific interpretation