Detrital zircon geochronology can help address stratigraphic- to lithospheric-scale geological questions. The approach is reliant on statistically robust, representative age distributions that ...fingerprint source areas. However, there is a range of biases that may influence any detrital age signature. Despite being a fundamental and controllable source of bias, handpicking of zircon grains has received surprisingly little attention. Here, we show statistically significant differences in age distributions between bulk-mounted and handpicked fractions from an unconsolidated heavy mineral sand deposit. Although there is no significant size difference between bulk-mounted and handpicked grains, there are significant differences in their aspect ratio, circularity and colour, which indicate inadvertent preferential visual selection of euhedral and coloured zircon grains. Grain colour comparisons between dated and bulk zircon fractions help quantify bias. Bulk-mounting is the preferred method to avoid human-induced selection bias in detrital zircon geochronology.
Abstract
The Kalkarindji Large Igneous Province (LIP) is a Middle Cambrian (511 Ma) continental flood basalt (CFB) province located in northern and central–west Australia that has been linked to an ...extinction event at the Early–Middle Cambrian boundary. The extent of this LIP has been estimated at about 2·1 x 106 km2, with exposures in Western Australia, Northern Territory, Queensland and South Australia. Major and trace element datasets reveal geochemical characteristics typical for continental flood basalts (CFBs) including: tholeiitic affinity; an enrichment in incompatible elements, in particular, large-ion lithophile elements (LILE); enrichment of light rare earth elements (LREE) compared to heavy rare earth elements (HREE) relative to N-MORB; negative Nb and Ta anomalies in normalized extended element patterns. Here we present the first comprehensive geochemical investigation of the Kalkarindji CFB province. The Kalkarindji CFBs are geochemically homogeneous, low-Ti basaltic andesites, with a nearly complete lack of basalts as defined using a total-alkalis vs silica diagram. All of the rocks analysed for Sr, Nd, Pb isotopic ratios display enriched initial (t = 511 Ma) isotopic compositions (143Nd/144Ni = 0·511928–0·511981; 87Sr/86Sri = 0·70917–0·71029; 206Pb/204Pbi = 18·105–18·843; 207Pb/204Pbi = 15·726–15·805; 208Pb/204Pbi = 38·374–39·208). Crustal assimilation models are interpreted to suggest that the geochemical characteristics, as well as the homogenous composition across the entire province, cannot be explained by continental crust contamination. Therefore, the enriched isotopic ratios (particularly the extremely high 207Pb/204Pbi and elevated 208Pb/204Pbi for moderate 206Pb/204Pbi), coupled with relative depletions in Nb and Ta concentrations, indicate the involvement of an ancient enriched lithospheric-like component in the genesis of the Kalkarindji CFB. We propose a model in which the source region was affected by an enrichment event at around 2·5 Ga (possibly through the addition of subducted sediments). Decompression melting and mantle warming (focused by edge driven convection) allowed melting of the fertile mantle to generate the Kalkarindji CFB province at c. 511 Ma.
Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different ...geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far.
In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8–0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool.
Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology.
Re–Os radiometric dating of crude oil can be used to constrain the timing of hydrocarbon generation, migration or charge. This approach has been successfully applied to marine petroleum systems; ...however, this study reports on its application to lacustrine-sourced natural crude oils. Oil samples from multiple wells producing from the Cretaceous Nantun Formation in the Wuerxun-Beier depression of the Hailar Basin in NE China were analysed. Subsets of the Re–Os data are compatible with a Cretaceous hydrocarbon generation event (131.1 ± 8.4 Ma) occurring within 10 Myr of deposition of the Nantun Formation source rocks. In addition, two younger age trends of 54 ± 12 Ma and 1.28 ± 0.69 Ma can be regressed from the Re–Os data, which may reflect the timing of subsequent hydrocarbon generation events. The Re–Os geochronometer, when combined with complementary age dating techniques, can provide direct temporal constraints on the evolution of petroleum system in a terrestrial basin.
One of the most impassioned topics in large igneous province (LIP) research is how prolonged the duration of these large-scale magmatic events are, as LIP magmatism has considerable impact on models ...of associated reconstructions, of climate variability or tectonic events. High-precision geochronology is pivotal to LIP basalt emplacement rate, and thus to unravel the role these enormous magmatic events have throughout Earths geological and environmental history. Four high-precision 40Ar/39Ar plagioclase plateau ages for the Tasmanian dolerites (Ferrar) indicate ∼1.6 ± 0.4 Ma of resolvable, continuous magmatic activity; 184.27 ± 0.24 to 182.69 ± 0.54 Ma (2σ). The 40Ar/39Ar results provide evidence of distinctly older intrusions and a more prolonged duration than the observed 182.4-182.9 Ma age range and duration indicated by the main zircon record. Moreover, the precision of our 40Ar/39Ar results coupled with secondary electron microscopy analyses provide evidence of plagioclase crystal inheritance from slightly older magmatism entrained into younger magmatic pulses by exploiting pre-existing conduits. Numerical diffusion models, calculated for a theoretical age spectrum resulting from two slightly different plagioclase ages, provide an excellent match for measured data. Coupling geochemical data to the new age data indicates a silica and incompatible element evolution of the Ferrar magmatic system through time. The older generation of intrusions (ca. Zr: 92 ppm, SiO2: 53.67 wt.%) are seemingly less enriched in incompatible elements and silica than the youngest generation (ca. Zr: 147 ppm, SiO2: 56.5 wt.%). Here, we suggest that the magma chambers differentiated to more incompatible/silica-rich compositions saturating zircon only at evolved magmatic stages. This implies that plagioclase dates the full duration of magmatic Ferrar LIP activity of ca. 1.6 Myrs whilst zircon ages might be naturally biased and restricted to post-Zr saturation stage. The extended duration of Ferrar magmatism indicates that it is coeval with the Pliensbachian-Toarcian boundary. Therefore, we speculate that Ferrar (±Karoo) magmatism triggered the Pliensbachian-Toarcian extinction event and contributed to the Toarcian oceanic anoxic event, from which the environment did not begin to recover until only after the waning and cessation of Ferrar magmatic activity at ∼182 Ma, with zircon crystals recording the final flux of magma.
•High-precision 40Ar/39Ar plagioclase geochronology indicate ∼1.6 Ma duration.•Potential plagioclase crystal inheritance revealed within 40Ar/39Ar age spectrum.•Extended Ferrar duration indicates coeval magmatism with the Pl/To extinction event.
Re‐Os isotope‐dilution geochronology has been widely used to date the timing of molybdenite, pyrite and chalcopyrite formation across a variety of geological settings. However, in situ methods have ...been impeded by the isobaric interference of 187Re on 187Os. In situ Re‐Os geochronology using LA‐ICP‐MS/MS has been shown to be a useful technique to chemically separate Os from Re, as Os reacts with CH4 to create higher‐mass reaction products, which can then be measured with minimised interference of 187Re. However, application of the method requires matrix‐matched primary reference materials, e.g., age‐homogenous molybdenite amenable to laser ablation. Here, we characterise and present two new molybdenite mineral reference materials for in situ Re‐Os geochronology by LA‐ICP‐MS/MS, verified by ID‐TIMS Re‐Os measurements. We also present case studies from molybdenite samples with varying Re mass fractions and Re‐Os age mapping. The method provides accurate and precise age data, with excellent precision for high Re samples. The benefits of the LA‐ICP‐MS/MS approach include: (1) simple sample preparation, (2) rapid data acquisition, (3) targeting of specific textural domains including growth zones and (4) the ability to simultaneously collect trace elements used to link the timing and conditions of ore‐formation.
Key Points
Methodology for in situ Re‐Os LA‐ICP‐MS/MS includes separating Os from Re with methane.
Two new matrix‐matched in situ molybdenite reference materials have been developed.
Uncertainties on weighted mean Re‐Os ages can approach ∼1.4%, if the data forms one age population.
Large Igneous Provinces mark important and consequential events that span almost the entirety of the Earth's history. These punctuated geologically significant events that include earth processes ...from the core to the atmosphere have been linked to climate changes, mass extinctions, and even large-scale tectonic shifts. To begin to understand the role LIPs play in these important moments across Earth's lifespan, a thorough understanding of how they form in the first place is imperative. The ∼180 Ma Karoo Continental Flood Basalt (CFB) Province has been heavily studied with one of the largest geochemical and geochronological databases available for a CFB; providing an excellent avenue to study in detail the petrogenesis of these large collections of magma. However, despite this large dataset, the origin of the Karoo CFBs is still debated, with many different and conflicting models proposed. Disparities in these models can often be traced down to the degree of influence a plume and/or the sub-continental lithospheric mantle (SCLM) had during the genesis of this magmatic province. Nine sills intruded into the Western Cape Province of the Karoo Basin were investigated using 40Ar/39Ar geochronology, whole rock geochemistry, and Sr, Nd, Pb isotopes. Five of these sills are peculiar as they contain hydrated minerals (biotite and hornblende), an unusual feature for Karoo basaltic rocks. Biotite and/or hornblende separates yielded statistically indistinguishable 40Ar/39Ar ages as plagioclase separates; indicating that the biotite and hornblende found within these samples are primary features requiring that water was present in the magma during the crystallization of those sills. Major and trace element geochemistry place these rocks into the low-Ti suite of the Karoo CFBs. Trace elements indicate the involvement of fluids over sediment within the source region for these Karoo magmas. All samples from the Western Cape Province display enriched Sr–Nd–Pb initial values with large variations in elemental concentrations for a restricted range of isotopic ratios. Assimilation models indicate these enriched isotopic contents and observed trends are not the result of crustal contamination and are representative of the composition of the source. The trace element and isotopic geochemistry as well as the primary hydrated minerals indicate that water present in the system at the time of formation came directly from one of the mantle sources, supporting the hypothesis that the SCLM was metasomatized by fluids plausibly as a result of the Proterozoic Kibaran Orogeny and/or the Permian Cape Orogeny. This first direct evidence of water within the mantle sources of tholeiitic CFBs, until now only theorized and modeled, potentially changes the fundamentals in how we approach our considerations of CFB generation as a whole.
•Primary hydrous minerals provide first evidence of hydrated material within the Karoo magmas.•The major, trace, and isotopic geochemistry of these hydrous dolerites indicate a hydrated source for the Karoo magmas.•These results support the hypothesis that the SCLM was metasomatized by fluids from ancient subduction.
•The mineralisation at the Nimbus deposit is syn-volcanic at approximately 2.7 Ga.•This study validates the 2.7 Ga stratigraphy as fertile for VHMS deposits.•A younger ca. 2.63 Ga event overprints ...VHMS alteration ages at the Nimbus deposit.•A multi-disciplinary approach makes up for otherwise inconclusive isotopic studies.•4D evolutionary models of ore deposits is a valuable tool for exploration geology.
Due to its unique mineralogy in the Archaean Yilgarn Craton, the origin of the high-grade Nimbus Ag-Zn-(Au) deposit has been contentious for a number of years. Recent interpretations of the deposit as a shallow water and low temperature volcanic-hosted massive sulphide (VHMS) deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), were based on detailed studies of its volcanology, mineralogy, hydrothermal alteration assemblages, geochemistry, multiple S isotopes and trace element content of sulphide. However, this model has been questioned in favour of a late, epithermal fault-hosted system. Effective greenfields exploration for similar deposits elsewhere requires a robust deposit model, and well understood timing of ore formation. We present a comprehensive multi-disciplinary study to further constrain the processes involved in the evolution of Nimbus and generate a 4D evolutionary model of the system.
Host rock dacite formation is well constrained by SHRIMP U-Pb zircon geochronology to c. 2703 Ma. Re-Os ages for the first sulphide phase, gives an imprecise maximum age to the ore formation of 2682 ± 110 Ma. High precision Pb-isotope variations in ore-stage and late galenas track changes in the sulphides over time, complemented by new geochronology. The Pb-isotope data indicates that the polymetallic ore precipitation happened at the same time as the volcanism, with the second generation indicating minor, local remobilisation of Pb in late quartz-carbonate veins at c. 2630 Ma. 40Ar/39Ar dating of sericite-altered plagioclase from foliated dacite, and U-Pb SHRIMP dating of syn- to post-deformation monazites have yielded ages of ~2630 Ma. The late event at c. 2630 Ma is most likely related to widespread late low-Ca granite emplacement across the Eastern Goldfields.
This study provides arguments that suggest that Nimbus is a syn-volcanic deposit. The ore timing is consistent with the model of replacement-type VHMS deposit, with the age of the main mineralisation similar to the age of the dacitic host-rocks at 2703 Ma. The late event modifies the pre-existing ore and overprints the previous ore-related alteration ages from the altered plagioclase. High precision, double spiked, Pb isotope constraints from late quartz-carbonate veins provides an opportunity to vector to hidden deposits.
Hypervelocity impacts throughout Earth's history have profoundly affected the evolution of the continental crust. Accessory minerals like zircon are typically used to date impact events and ...rock-forming minerals like quartz are routinely used as shock barometers. However, feldspar group minerals – a major constituent of most crustal rocks – are generally underutilized in the documentation of impact-induced deformation and alteration. Alkali feldspar contains appreciable amounts of Pb and analysis of Pb isotopes in feldspar may offer the opportunity to identify impact-related isotopic modifications of shocked crustal target rocks and estimate their timing. Here, we apply a combination of laser ablation inductively coupled plasma (LA-ICP) and thermal ionization mass spectrometry (TIMS) Pb isotope analysis with imaging techniques, including electron backscatter diffraction (EBSD), cathodoluminescence (CL), and time of flight secondary ion mass spectrometry (ToF-SIMS), to shocked alkali feldspar from monzogranite in the oldest confirmed terrestrial impact structure (2229 ± 5 Ma) at Yarrabubba, Western Australia. Alkali feldspar preserves microstructures such as sub-planar and irregular fractures, sets of planar deformation bands that accommodate misorientations of up to ∼20°, sets of damage lamellae, and broad domains of lattice damage that can be linked to impact-related deformation. The Pb isotope compositions in alkali feldspar correlate with variations in electron diffraction band contrast – a proxy for crystallinity – and also the degree of misorientation and CL response. Less damaged alkali feldspar yields Pb model ages similar to the igneous zircon U–Pb crystallization age of the host monzogranite (∼2650 Ma), whereas younger Pb model ages correspond to zones of damage (high relative misorientation, low crystallinity, weak CL response). The observed Pb isotope behaviour implies radiogenic ingrowth of Pb, from decay of U and Th within damaged alkali feldspar, and therefore mixing with a grain-scale Pb reservoir that formed at the time of impact. The U and Th zonation in some shock-deformed alkali feldspar is broadly similar and follows the orientation of sub-planar fractures and damage lamellae. Detailed imaging reveals the zones of U and Th enrichment are associated with trains of monazite micro-inclusions, in conjunction with magnetite and/or hematite in places, which are inferred to have precipitated during impact-induced hydrothermal circulation. Hence, the Pb isotopic data record grain-scale hydrothermal alteration in superficially weakly altered monzogranite target rocks.
•Pb isotopic signatures correlate to crystallinity of shocked alkali feldspar.•Radiogenic ingrowth is evident in Pb signatures from damaged alkali feldspar.•Shock microstructures in alkali feldspar contain secondary mineral growth.•Monazite and Fe oxides precipitated from grain-scale hydrothermal circulation.•Impact-driven oxidizing alteration in the Paleoproterozoic is conceivable.
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