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ItemCryogenian iron formations: glaciation and oxygenationLechte, Maxwell ( 2018)The Cryogenian Period (720–635 Ma) experienced extreme glaciations broadly coincident with a transformation of the Earth’s surface oxidation state, supercontinent breakup, and the evolution of complex animal multicellularity. However, the cause-and-effect relationships of these events are unresolved. The Cryogenian ice ages, known as ‘Snowball Earth' events, would have placed important constraints on the biosphere, and it remains unclear what role global refrigeration played in setting the stage for eukaryotic diversification and the origin of animals. The Cryogenian also experienced the deposition of iron-rich marine chemical sediments (iron formations), representing the first episode of global iron formation deposition in over one billion years. This shift in iron cycling highlights complexities in seawater chemistry and oxidation state during this time, and these iron formations offer valuable insights into Cryogenian palaeoenvironments. Iron formations from Cryogenian glacial successions in Namibia, USA and Australia were studied in order to investigate Cryogenian iron formation genesis and elucidate the relationships between glaciation, ocean chemistry, oxygenation and biotic evolution. In-depth sedimentology, stratigraphy and petrography reveals that these iron formations are intimately associated with Sturtian glacial sediments and are interpreted have been deposited in a range of glaciomarine environments. Geochemical analysis of these chemical sediments permits the reconstruction of Cryogenian ocean chemistry and the synglacial palaeoredox landscape. Multiple geochemical proxies, including rare earth element and iron isotope systematics, indicate widespread marine anoxia with increasing seawater oxidation with proximity to the ice shelf grounding line. A genetic model is proposed whereby the mixing of oxygenated glacial fluids with ferruginous seawater led to the deposition of iron formations in glacial successions during the Cryogenian. Atmospheric oxygen trapped in glacial ice was likely an important oxidant source, delivered to Cryogenian glaciomarine environments via subglacial meltwater outwash. This meltwater supply may have been crucial in establishing oxygenated marine habitats for eukaryotes, including early animals, during Snowball Earth. Multi-million-year oxidation of the oceans via this mechanism may have also set the stage for a Neoproterozoic marine oxygenation event.
ItemIsotopic disequilibrium in granitic systems: the origins of heterogeneity in granites and implications for partial melting in the crust and petrogenetic modelsIles, Kieran Anthony ( 2017)Unravelling the processes involved in granite magmatism is essential to our understanding of the continental crust, its formation and evolution. Radiogenic isotope systems are commonly employed to this end, but the behaviour of these systems may not be as simple as is often assumed. Understanding the causes of isotopic complexity in granite suites is the aim of this research. By modelling the disequilibrium partial melting of isotopically heterogeneous protoliths the different compositions of the melt, source and restite for a range of hypothetical scenarios have been calculated. Results demonstrate that the melt produced may have Sr, Nd and Hf isotope compositions distinct from both the protolith and restite. A corollary is that restite-bearing magmas may exhibit different isotope compositions than their melts, a feature which should be preserved as a difference between the Hf isotope compositions of bulk-rock samples and their magmatic zircon populations. The same modelling also suggests that a single source rock can produce melts with diverse isotope compositions. The predictions of this modelling have been tested by analysing S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, including iconic examples of restite-bearing rocks. Comparisons of Hf isotope compositions between bulk-rocks and their magmatic zircons reveal discrepancies (ΔεHfbulk-zircon) ranging from -0.6 to +2.5 ε units for I-type granites. This intra-sample Hf isotopic heterogeneity is interpreted to represent disequilibrium between the melt and restite assemblage. The ΔεHfbulk-zircon values are consistent with calculated ΔεHfmagma-melt values (from -4.2 to +7.4) based on the disequilibrium amphibole dehydration melting of 0.5-1.0 Ga meta-igneous protoliths. S-type granites also record differences between their bulk-rock and magmatic zircon Hf isotope compositions; however, the disparity is more subtle. Both positive and small negative ΔεHfbulk-zircon values are observed, consistent with modelling the partial melting of isotopically heterogeneous meta-sedimentary protoliths. In addition to low-temperature granites, case studies of two high-temperature I-type granitoid suites (Boggy Plain and Wallundry) have also been conducted. Both display a weak coupling between geochemical parameters that have been interpreted previously to indicate the involvement of assimilation and fractional crystallisation (AFC) processes. Positive ΔεHfbulk-zircon values obtained in the Boggy Plain Suite support the existing petrogenetic model in which basaltic melt becomes variously contaminated by material derived from the continental crust. The positive value is explained by retention of earlier-crystallised, more radiogenic phases in isotopically evolved, more felsic samples. In contrast, the Wallundry Suite is characterised by negative ΔεHfbulk-zircon values caused by the presence of unmelted components of its contaminant. A complex interplay of contamination, crystallisation, melt segregation and interaction between magma batches is required to account for the Wallundry Suite isotope data. The results of this study indicate that disequilibrium partial melting can produce within-suite isotopic variability without recourse to assimilation or mixing processes (1) in mafic to felsic samples caused by the progressive separation of melt from its isotopically distinct restite assemblage; and (2) via the extraction of multiple batches of isotopically distinct melts produced from a single source as anatexis proceeds. Furthermore, the isotope variation resulting from restite unmixing may be distinguished from magma mixing by decoupling of the Rb-Sr, Sm-Nd and Lu-Hf isotope systems. Importantly, the isotopic discrepancy between bulk-rock granite samples and their magmatic zircon populations suggests that the most mafic bulk-rock granite samples of a given suite, not magmatic zircon, provide the most accurate estimate of source rock Hf isotope compositions. This raises concerns regarding the ubiquitous use of zircon Hf isotope data to constrain crustal growth models.
ItemThe geology and geochemistry of the Agnew Intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in Central Ontario, CanadaVogel, Derek Christian ( 1996-07)The Early Proterozoic Agnew Intrusion is a well-preserved leucogabbronoritic to gabbronoritic layered intrusion that is a member of the East Bull Lake suite of layered intrusions (ca. 2490-2470 Ma) occurring in central Ontario. These intrusions are related to the development of the Huronian Rift Zone, which may be part of a much more widespread rifting event that involved the Fennoscandian Shield. Structural data suggest that these intrusions have been subjected to ductile deformation and are erosional remnants of one or more sill-like bodies originally emplaced along the contact between Archaean granitic rocks of the Superior Province and an Early Proterozoic Huronian continental flood basalt sequence in the Southern Province.
ItemGeochemistry and mineralisation of primary and secondary platinum-group elements in the ultramafic "Alaskan-type" Owendale complex and laterites in the Fifield Region, New South Wales, AustraliaShi, Bielin ( 1995)The Owendale Complex belongs to a family of ultramafic-mafic intrusions that is characterised by a zonal, nonstratiform arrangement of the principal ultramafic units. The ultramafic rocks of the Owendale Complex are virtually identical to many of the Alaskan-type intrusions, however the associated gabbroic rocks (wehrlites) are K-rich and Si-undersaturated, in contrast to the tholeiitic gabbroic rocks of the Alaskan examples. The intrusion history of the Owendale Complex is thought to have involved emplacement of a gabbroic intrusion that was invaded by an ultrabasic magma, possibly while the former was still only partly solidified. Emplacement of both magmas probably occurred during Late Devonian tectonism and deformation synchronous with emplacement and crystallisation is necessary to explain the present non-stratiform arrangement of the rock units. The most obvious linkage factor between the two proposed parent magmas (gabbroic and ultrabasic) of the Owendale suites is their mutual affinity with tholeiitic basalt magmas and the similarities of their products with intrusions of alkalic basalt derivation. This suggests the possibility that the Owendale Complex rocks and those of other tholeiitic intrusions of the regions are comagmatic products of an ancestral magma that may have also produced the widespread assemblage of complexes. Viewed from this perspective, the ultramafic rocks of Owendale Complex would thus represent a very minor product of a period of regional magmatic activity. Most alloys, erlichmanite, cooperite and some grains with exclusion texture of Pt-Os-Ir-Pd-Rh are considered to represent a primary high-temperature paragenesis. Concentration of PGE in pegmatoidal units of dunite-wehrlite is explained by the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex. The high-temperature PGM segregate directly from a silicate melt and were not generated by exsolution from spinels or magmatic sulphides. These suggest that fS2 was generally low (subordinate sulphide formation) and, after some influence at the beginning, has given way to rising fO2 (chromite, olivine and Pt-Fe-Cu-Ni alloys formation). After lithification, the ultramafic rocks become subject to "reducing" conditions, i.e., conditions of lower O2 and S2 activities. Ni-Fe alloys, native Fe and Bi formed in cracks which filled the serpentine matrixes. The former PGM (erlichmanite, cooperite and Pt-Fe alloys) were exposed to the reducing conditions via cracks were desulphurated to form porous cooperite with Pt-Fe alloys and multiphase textural Os-Ir-Ni, Pt-Ir aggregates. It is plausible that the veinlets and aggregates of unnamed Rh-Sb-S, (Pt, Ir)2(Fc, Cu)3(S, Sb, AS)3 in the dunites may also have been formed by reduction of Ni-rich sulphides and erlichmanite, Pt-Fe alloys or cooperite. Late PGM are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. A hydrothermal origin is also indicated for native Fe, native Bi and awaruite (NiFe) and the base-metal sulphides (pentlandite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and some Ni-Co-Fe sulfide). The cause of the reducing conditions may have been related to H2 production accompanying hydrous alteration of the dunites and clinopyroxenites. The laterites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and well-preserved weathering profiles. Field, textural and geochemical data all support a chemical weathering origin for the profiles and compatible with meteoric and ground water origins. Meteoric water with intermediate Eh and pH and negligible dissolved species sinks into the laterite where these parameters are modified. The Eh rises and pH decreases to the conditions typical of lateritic soils and the concentration of dissolved species increases. In this state the water is able to take PGE and Au into solution from a finely disseminated form in the bedrock as a part of the process of lateritisation. When the soil solution transports the PGE and Au towards a transitional interface must exist between the ferruginous and saprolite zones with lower Eh, neutral pH and lower concentration of dissolved salts. At this transitional region, deposition of the PGE and Au occurred. The presence of magnetic Pt-Fe-Cu-Ni alloys suggests that hydrothermal solutions play a later role in the Fifield region, and the alloys have grown in situ in a lateritic soil by a process involving laterite water solution in the high Eh, low pH conditions prevalent in such soil, followed by deposition when the conditions become less extreme. Some examples of the Pt-Fe alloys from such an environment become frequently strongly magnetic with larger size. It is assumed that the temperature of the hydrothermal solution is in the range of 300° - 500° C (Bowles, 1990). PGE mineralisation in the primary rocks and laterite in this region has demonstrated a good example of multi-stage process mineralisation including primary high temperature magmatic formation; low temperature postmagmatic hydrothermal alteration and residual lateritic enrichment.