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Unravelling the complexities in high-grade rocks using multiple techniques: the Achankovil Zone of southern India
No full textThe Achankovil Zone of southern India forms a distinct isotopic and structural boundary separating the Madurai Block to the north from the Trivandrum Block to the south. We combine isotopic and trace element geochemistry of major and accessory phases with phase equilibria modelling to provide quantitative constraints on the timing and conditions of peak metamorphism and the nature of the protoliths within the Achankovil Zone. The results suggest a clockwise pressure–temperature path with peak metamorphic temperatures of up to 950 °C at pressures of around 0.7 GPa followed by high-temperature decompression. The metamorphic peak occurred at 545–512 Ma. U–Pb and Hf isotopic analysis of detrital zircon shows that the rocks have a strong affinity with the southern part of the Madurai Block. The Achankovil Zone is interpreted as the reworked southern margin of the Madurai Block, which was metamorphosed during the final stages of the assembly of Gondwana
Relation of sortable silt grain-size to deep-sea current speeds: Calibration of the ‘Mud Current Meter’
Get PDFFine grain-size parameters have been used for inference of palaeoflow speeds of near-bottom currents in the deep-sea. The basic idea stems from observations of varying sediment size parameters on a continental margin with a gradient from slower flow speeds at shallower depths to faster at deeper. In the deep-sea, size-sorting occurs during deposition after benthic storm resuspension events. At flow speeds below 10–15 cm s−1 mean grain-size in the terrigenous non-cohesive ‘sortable silt’ range (denoted by View the MathML source, mean of 10–63 µm) is controlled by selective deposition, whereas above that range removal of finer material by winnowing is also argued to play a role.
A calibration of the View the MathML source grain-size flow speed proxy based on sediment samples taken adjacent to sites of long-term current meters set within ~100 m of the sea bed for more than a year is presented here. Grain-size has been measured by either Sedigraph or Coulter Counter, in some cases both, between which there is an excellent correlation for View the MathML source (r = 0.96). Size-speed data indicate calibration relationships with an overall sensitivity of 1.36 ± 0.19 cm s−1/μm. A calibration line comprising 12 points including 9 from the Iceland overflow region is well defined, but at least two other smaller groups (Weddell/Scotia Sea and NW Atlantic continental rise/Rockall Trough) are fitted by sub-parallel lines with a smaller constant. This suggests a possible influence of the calibre of material supplied to the site of deposition (not the initial source supply) which, if depleted in very coarse silt (31–63 µm), would limit View the MathML source to smaller values for a given speed than with a broader size-spectrum supply. Local calibrations, or a core-top grain-size and local flow speed, are thus necessary to infer absolute speeds from grain-size.
The trend of the calibrations diverges markedly from the slope of experimental critical erosion and deposition flow speeds versus grain-size, making it unlikely that the View the MathML source (or any deposit size for that matter) is simply predicted by the deposition threshold. A more probable control is the rate of deposition of the different size fractions under changing flows over several tens of years (the typical averaging period of a centimetre of deposited sediment). This suggestion is supported by a simple depositional model for which the deposited View the MathML source is calculated from measured currents with a size-varying depositional threshold. More surficial sediment samples taken near long-term current meter sites are needed to make calibrations more robust and explore regional differences
Tiny Space Magnets: X-ray Microscopy and Nanopaleomagnetism of Meteoritic Metal
Get PDFMeteorites provide a wealth of information about the early history of our solar system, the
formation and evolution of planetesimals, and their internal dynamics. One of the few di-
rect observations of internal geophysical processes within planetary bodies is the presence or
absence of a dynamo-driven magnetic field. These observations provide essential constraints
on the degree of differentiation, core solidification timescales and the driving forces for con-
vection. This dissertation focusses on the paleomagnetic information recorded by iron and
stony-iron meteorites, providing a unique view-point for the generation and variability of
core dynamo activity
Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation
No full textTectonic strain commonly overprints magmatic fabrics in AMS (Anisotropy of Magnetic Susceptibility) data for plutonic rocks produced by both compressional and extensional regimes. Mt Kinabalu, Borneo, is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts. We show that tectonic fabrics are recorded pervasively throughout the intrusion, even near contacts, and present a workflow distinguishing compressive and extensional syn-magmatic deformation. At Mt Kinabalu this reveals a pervasive tectonic fabric indicating NW-SE Miocene extension in Borneo at 7.9–7.3 Ma, later than previously recognised, oriented NW-SE at 319° ±13.1°. Comparing data from Mt Kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons. Therefore, AMS fabric can be used to identify the syn-magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn-magmatic tectonic regimes and strain orientations within temporal frameworks
From mantle to Motzfeldt: A genetic model for syenite-hosted Ta, Nb-mineralisation
No full textA genetic model for the Motzfeldt Tantalum-Niobium-rich syenite in south-west Greenland, considered to be one of the world’s largest Ta prospects, is presented. The Motzfeldt primary magma formed early in regional Gardar (1273 ± 6 Ma) rifting. Isotope signatures indicate that the Hf had multiple sources involving juvenile Gardar Hf mixed with older (Palaeoproterozoic or Archaean) Hf. We infer that other High Field Strength Elements (HFSE) similarly had multiple sources. The magma differentiated in the crust and ascended before emplacement at the regional unconformity between Ketilidian basement and Eriksfjord supracrustals. The HFSE-rich magmas crystallised Ta-rich pyrochlore which formed pyrochlore-rich crystal mushes, and it is these pyrochlore-rich horizons, rich in Ta and Nb, that are the focus of exploration. The roof zone chilled and repeated sheeting at the roof provided a complex suite of cross-cutting syenite variants, including pyrochlore microsyenite, in a ‘Hot Sheeted Roof’ model. The area was subject to hydrothermal alteration which recrystallized alkali feldspar to coarse perthite and modified the mafic minerals to hematite, creating the friable and striking pink-nature of the Motzfeldt Sø Centre. Carbon and oxygen isotope investigation of carbonate constrains fluid evolution and shows that carbonate is primarily mantle-derived but late-stage hydrothermal alteration moved the oxygen isotopes towards more positive values (up to 21‰). The hydrothermal fluid was exceptionally fluorine-rich and mobilised many elements including U and Pb but did not transport HFSE such as Ta, Hf and Nb. Although the U and Pb content of the pyrochlore was enhanced by the fluid, the HFSE contents remained unchanged and therefore Hf isotopes were unaffected by fluid interaction. While the effect on hydrothermal alteration on the visual appearance of the rock is striking, magmatic processes concentrated HFSE including Ta and the hydrothermal phase has not altered the grade. Exploration for HFSE mineralisation commonly relies on airborne radiometric surveying which is particularly sensitive to the presence of U, Th. A crucial lesson from Motzfeldt is that the best target is unaltered pyrochlore which was identified less easily by radiometric survey. Careful petrological/mineral studies are necessary before airborne survey data can be fully interpreted
Comparing dune migration measured from remote sensing with sand flux prediction based on weather data and model, a test case in Qatar
No full textThis study explores validating and calibrating the wind regime predicted by Global Circulation Models (GCM) on Earth and other planets using optical remote sensing of dune dynamics. We use Spot-5 images to track the migration of 64 Barchan dunes in Qatar using the COSI-Corr technique. We estimate the volume of the dunes using a scaling law calibrated from one particular dune, which was surveyed in the field. Using volume and migration rate, we determine the sand flux from a single dune, , and scale this estimate to the whole dune field. We compare the measured sand flux with those derived from wind velocity measurements at a local meteorological station as well as with those predicted from ERA-Interim (a Global Circulation Model). The comparison revealed that the wind velocity predicted by ERA-Interim is inappropriate to calculate the sand flux. This is due to the 6-h sampling rate and to systematic bias revealed by a comparison with the local wind data. We describe a simple procedure to correct for these effects. With the proposed correction, similar sand flux are predicted using the local and ERA-Interim data, independently of the value of the value of the shear velocity threshold,
⁎
. The predicted sand flux is about 65% of . The agreement is best assuming the value
⁎
, which is only slightly larger than the value of
⁎
estimated based in the sand granulometry measured from field samples. The influence of the dune topography on the wind velocity field could explain the underestimation. In any case, the study demonstrates the possibility of validating GCM model and calibrating aeolian sand transport laws using remote sensing measurements of dune dynamics and highlights the caveats associated to such an approach
Static and dynamic fluid-driven fracturing of adhered elastica
Get PDF© 2018 American Physical Society. The transient spreading of a viscous fluid beneath an elastic sheet adhered to the substrate is controlled by the dynamics at the tip where the divergence of viscous stresses necessitates the formation of a vapor tip separating the fluid front and fracture front. The model for elastic-plated currents is extended for an axisymmetric geometry with analysis showing that adhesion gives rise to the possibility of static, elastic droplets and to two dynamical regimes of spreading; viscosity dominant spreading controlled by flow of viscous fluid into the vapor tip, and adhesion dominant spreading. Constant flux experiments using clear, PDMS elastic sheets enable new, direct measurements of the vapor tip and confirm the existence of spreading regimes controlled by viscosity and adhesion. The theory and experiments thereby provide an important test coupling the dynamics of flow with elastic deformation and have implications in fluid-driven fracturing of elastic media more generally
Thicker shells compensate extensive dissolution in brachiopods under future ocean acidification
Get PDFOrganisms with long generation times require phenotypic plasticity to survive in changing environments until genetic adaptation can be achieved. Marine calcifiers are particularly vulnerable to ocean acidification due to dissolution and a reduction in shell-building carbonate ions. Long-term experiments assess organisms’ abilities to acclimatise or even adapt to environmental change. Here we present an unexpected compensatory response to extensive shell dissolution in a highly calcium-carbonate-dependent organism after long-term culture in predicted end-century acidification and warming conditions. Substantial shell dissolution with decreasing pH posed a threat to both a polar (Liothyrella uva) and a temperate (Calloria inconspicua) brachiopod after 7 months and 3 months exposure, respectively, with more extensive dissolution in the polar species. This impact was reflected in decreased outer primary layer thickness in the polar brachiopod. A compensatory response of increasing inner secondary layer thickness, and thereby producing a thicker shell was exhibited by the polar species. Less extensive dissolution in the temperate brachiopod did not affect shell thickness. Increased temperature did not impact shell dissolution or thickness. Brachiopod ability to produce a thicker shell when extensive shell dissolution occurs suggests this marine calcifier has great plasticity in calcification providing insights into how similar species might cope under future environmental change
Exsolved volatiles in magma reservoirs
No full textWe review our understanding of the exsolved volatile phase co-existing with magmas during pre-eruptive storage at the pressures and temperatures corresponding to crustal magma reservoirs. We explore the consequences and implications of such a volatile phase for magma and ore body petrogenesis and the fluid dynamics of magma reservoirs. We outline the geochemical constraints on the size and composition of the exsolved volatile phase that may co-exist with magmas in the crust. We distinguish between decompression-driven and crystallization-driven exsolution, and describe the implications of the volatiles for the dynamics of the magma reservoir, using key natural examples and case studies. We discuss eruptions triggered by second boiling, and the various regimes of magma mixing and magma overturn that may be induced by second boiling in a layered reservoir. We also explore the control of the volatile content of the magma on the mass erupted during an eruption episode, and compare our models to eruption datasets. We then turn to the mechanisms for magma-volatile separation, noting that in crystal-poor melts convective separation of exsolved volatiles may dominate while in crystal-rich melts, volatiles may generate channels or permeable-flow pathways through the crystal mush, thereby separating from the parent magma. We discuss the implications of the accumulation of the exsolved volatile phase at the roof zones of crystal-rich reservoirs for the large gas emissions observed during explosive eruptions, and for the development of metal-rich porphyry deposits
Experimental constraints on Li isotope fractionation during clay formation
Get PDFKnowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7‰lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was�16.6 ± 1.7‰at 20�C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are�21.5± 1.1‰,�0.2 ± 1.9‰and 15.0 ± 12.3‰for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes