Assessing the mechanisms of common Pb incorporation into titanite

Common Pb, the portion of non-radiogenic Pb within a U bearing mineral, needs to be accurately accounted for in order to subtract its effect on U-Pb isotopic ratios so that meaningful ages can be calculated. The propensity to accommodate common Pb during crystallization, or later, is different across the range of U bearing minerals used for geochronology. Titanite frequently accommodates significant amounts of common Pb. However, the most appropriate method to correct for this requires knowledge on the mechanism and timing of common Pb incorporation; information that is commonly difficult to extract. In this study, the spatial and compositional distribution of trace elements (including Pb) in metamorphic titanites from a Greenland amphibolite is investigated on the grain- to nano-scale. Titanites have an isotopically similar signature for both common and radiogenic-Pb in all grains but significantly different quantities of the non-radiogenic component. Microstructural and compositional examination of these grains reveals undeformed, but high common Pb (F207%) titanites have homogeneous element distributions on the atomic scale suggesting common Pb is incorporated into titanite during its growth and not during later processes. In contrast, deformed titanite comprising low-angle boundaries, formed by subgrain rotation recrystallization, comprise networks of dislocations that are enriched in Mg, Al, K and Fe. Smaller cations may migrate due to elastic strain in the vicinity of the dislocation network, yet the larger K cations more likely reflect the mobility of externally-derived K along the orien tation interface. The absence of Pb enrichment along the boundary indicates that either Pb was too large to fit into migrating lattice dislocations or static low-angle boundaries and/or that there was no external Pb available to diffuse along the grain boundary. As the common Pb composition is distinctly different to regional Pb models, the metamorphic titanite grew in a homogeneous Pb reservoir dominated by the break-down of precursor U-bearing phases. The different quantity of common Pb in the titanite grains indicates a mineral-driven element partitioning in an isotopically homogeneous metamorphic reservoir, consistent with low U, low total REE and flat LREE signatures in high F207% analyses. These results have implications for the selection of appropriate common Pb corrections in titanite and other accessory phases

Hypogene Calcitization: Evaporite Diagenesis in the Western Delaware Basin

Evaporite calcitization within the Castile Formation of the Delaware Basin is more widespread and diverse than originally recognized. Coupled field and GIS studies have identified more than 1000 individual occurrences of calcitization within the Castile Formation outcrop area, which includes both calcitized masses (limestone buttes) and laterally extensive calcitized horizons (limestone sheets). Both limestone buttes and sheets commonly contain a central brecciated zone that we attribute to hypogene dissolution. Lithologic fabric of calcitized zones ranges from little alteration of original varved laminae to fabrics showing extensive laminae distortion as well as extensive vuggy and open cavernous porosity. Calcitization is most abundant in the western portion of the Castile outcrop region where surface denudation has been greatest. Calcitization often forms linear trends, indicating fluid migration along fractures, but also occurs as dense clusters indicating focused, ascending, hydrocarbon-rich fluids. Native sulfur, secondary tabular gypsum (i.e. selenite) and hypogene caves are commonly associated with clusters of calcitization. This assemblage suggests that calcium sulfate diagenesis within the Castile Formation is dominated by hypogene speleogemesis

Isotopic insight into the Proterozoic crustal evolution of the Rudall Province, Western Australia

© 2018 The Authors The Proterozoic assembly of Australia involved the convergence of three main Archean cratonic entities: the North, West and South Australian Cratons, and is recorded in the Proterozoic orogenic belts surrounding these continental nuclei. The Rudall Province of northern Western Australia is the sole exposure of a Paleo- to Mesoproterozoic orogen lying between the North and West Australian Cratons, and may record the effects of their amalgamation. We present new zircon O, U–Pb and Lu–Hf isotope data from magmatic rocks across the Rudall Province, to which we add existing isotope data to yield a crustal evolution overview. Hf evolution trends for the ca. 1804–1762 Ma Kalkan Supersuite, the ca. 1589–1549 Ma Krackatinny Supersuite and the ca. 1310–1286 Ma Camel Suite, indicate a significant input of Archean East Pilbara Basement material, albeit as a mix with more juvenile material, including a possible ca. 1900 Ma component. Zircon d18O data suggest a contribution from supracrustal material into the magmatic source of the Kalkan Supersuite, which may have been emplaced in an extensional setting. In contrast, the Krackatinny Supersuite and Camel Suite have mantle-like d18O which may reflect partial melting of deeper Archean sources. Geochemical data for the Krackatinny Supersuite shows geochemical trends implying that melting of thickened mafic crust progressed from deeper to shallower levels, possibly in a rift setting. Camel Suite K-rich leucogranites may also have been emplaced in an extensional setting towards the end of high-P metamorphism. All terranes of the Rudall Province are para-autochthonous with respect to the Pilbara Craton, with no requirement for arc-related magmatism. We outline two potential scenarios for the Paleo- to Mesoproterozoic geodynamic evolution of the Rudall Province: an early cratonic amalgamation between the West and North Australian Cratons ca. 1680 Ma followed by Mesoproterozoic intraplate events; or a later assembly ca. 1377–1275 Ma. We lean towards this later amalgamation scenario

Geochronology of metasedimentary and igneous rocks in the Lamboo Province, Kimberley region: reassessing collisional geodynamic models

ISBN : 978-1-74168-964-8 / ISSN : 1834-2280U–Pb and Lu–Hf isotope data for detrital zircons from metasedimentary rocks of the Lamboo Province have been used to test collisional and intraplate geodynamic models for the Paleoproterozoic development of the Kimberley region. The 1870–1840 Ma turbiditic metasedimentary rocks deposited across the Western, Central and Eastern Zones of the Lamboo Province have remarkably consistent detrital zircon age signatures, with a dominant 1875–1860 Ma age component and a subsidiary c. 2500 Ma age component. These age components have similar Lu–Hf isotope values across all three zones, consistent with a common source for the sedimentary protoliths. The close similarities in provenance suggest that all three zones of the Lamboo Province developed in a continental intraplate setting prior to the 1837–1808 Ma Halls Creek Orogeny, which has previously been considered to represent a collision between an exotic Kimberley Craton and the proto- North Australian Craton. Comparable provenance signatures of coeval metasedimentary rocks across the broader North Australian Craton suggest that assembly of the main craton elements was complete prior to c. 1885 Ma

Geochronology of metasedimentary and igneous rocks in the Lamboo Province, Kimberley region: reassessing collisional geodynamic models

ISBN : 978-1-74168-964-8 / ISSN : 1834-2280U–Pb and Lu–Hf isotope data for detrital zircons from metasedimentary rocks of the Lamboo Province have been used to test collisional and intraplate geodynamic models for the Paleoproterozoic development of the Kimberley region. The 1870–1840 Ma turbiditic metasedimentary rocks deposited across the Western, Central and Eastern Zones of the Lamboo Province have remarkably consistent detrital zircon age signatures, with a dominant 1875–1860 Ma age component and a subsidiary c. 2500 Ma age component. These age components have similar Lu–Hf isotope values across all three zones, consistent with a common source for the sedimentary protoliths. The close similarities in provenance suggest that all three zones of the Lamboo Province developed in a continental intraplate setting prior to the 1837–1808 Ma Halls Creek Orogeny, which has previously been considered to represent a collision between an exotic Kimberley Craton and the proto- North Australian Craton. Comparable provenance signatures of coeval metasedimentary rocks across the broader North Australian Craton suggest that assembly of the main craton elements was complete prior to c. 1885 Ma

Mechanical twinning of monazite expels radiogenic lead

International audienceMechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208 Pb/ 232 Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element-oxygen (REE-O) bonds in the large [REE]O 9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases