Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O-Hf isotopes

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ18O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ϵHft range from −8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ϵHft (∼−7 to −4) and elevated δ18O values (∼9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ϵHft (∼−9 to −6) but more subdued δ18O values (∼7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ϵHf and mantle-like δ18O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (ϵHft as high as +8.5, δ18O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (ϵHft as high as −0.5, δ18O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two

Low-temperature thermochronology of francolite: Insights into timing of Dead Sea Transform motion

Cambrian siliciclastic sequences along the Dead Sea Transform (DST) margin in southern Israel and southern Jordan host both detrital fluorapatite [D‐apatite] and U‐rich authigenic carbonate‐fluorapatite (francolite) [A‐apatite]. D‐apatite and underlying Neoproterozoic basement apatite yield fission track (FT) data reflecting Paleozoic‐Mesozoic sedimentary cycles and epeirogenic events, and dispersed (U‐Th‐Sm)/He (AHe) ages. A‐apatite, which may partially or completely replace D‐apatite, yields an early Miocene FT age suggesting formation by fracturing, hydrothermal fluid ascent and intra‐strata recrystallisation, linked to early DST motion. The DST, separating the African and Arabian plates, records ~105 km of sinistral strike‐slip displacement, but became more transtensional post‐5 Ma. Helium diffusion measurements on A‐apatite are consistent with thermally activated volume diffusion, indicating Tc ~52‐56±10°C (cooling rate 10°C/Myr). A‐apatite AHe data record Pliocene cooling (~35‐40°C) during the transtensional phase of movement. This suggests that timing of important milestones in DST motion can be discerned using A‐apatite low‐temperature thermochronology data alone

Low-temperature thermochronology of francolite: Insights into timing of Dead Sea Transform motion

Cambrian siliciclastic sequences along the Dead Sea Transform (DST) margin in southern Israel and southern Jordan host both detrital fluorapatite [D‐apatite] and U‐rich authigenic carbonate‐fluorapatite (francolite) [A‐apatite]. D‐apatite and underlying Neoproterozoic basement apatite yield fission track (FT) data reflecting Paleozoic‐Mesozoic sedimentary cycles and epeirogenic events, and dispersed (U‐Th‐Sm)/He (AHe) ages. A‐apatite, which may partially or completely replace D‐apatite, yields an early Miocene FT age suggesting formation by fracturing, hydrothermal fluid ascent and intra‐strata recrystallisation, linked to early DST motion. The DST, separating the African and Arabian plates, records ~105 km of sinistral strike‐slip displacement, but became more transtensional post‐5 Ma. Helium diffusion measurements on A‐apatite are consistent with thermally activated volume diffusion, indicating Tc ~52‐56±10°C (cooling rate 10°C/Myr). A‐apatite AHe data record Pliocene cooling (~35‐40°C) during the transtensional phase of movement. This suggests that timing of important milestones in DST motion can be discerned using A‐apatite low‐temperature thermochronology data alone

Oxygen isotopic heterogeneity in the Temora-2 reference zircon

For the past decade and a half, Geoscience Australia has distributed zircon from a portion of the Middledale Gabbroic Diorite under the label “Temora-2”. This reference zircon was originally developed as a reference material for use in ion microprobe U-Th-Pb geochronological analyses. As ion probe capability has increased to allow the measurements of other isotopic systems at geologically useful precision and accuracy, the Temora-2 zircon has remained a convenient reference material to use for those systems. However, the suitability of this material for non-geochronological applications must be continuously reassessed. This study demonstrates that some (but not all) aliquots of the Temora-2 zircon, distributed by Geoscience Australia to analytical laboratories worldwide, have δ18O values up to 1‰ lower than the reference laser fluorination δ18O value quoted in Black et al. (2004). Although the long and complex collection history of this material makes it difficult to pinpoint the cause of this discrepancy, we suspect it relates to material from two or more boulders from the Temora-2 site, with different δ18O values, being sampled and mixed together in the field prior to storage at Geoscience Australia. Therefore, oxygen isotope measurements on SIMS mounts where Temora is the only reference zircon may be biased towards heavier values by up to 1‰, unless there are additional constrains on the actual δ18O value of the specific aliquot of Temora-2 zircons placed on that particular ion probe mount. We recommend that future oxygen isotope work should use a reference zircon other than Temora-2, until Geoscience Australia can replace the current stock of heterogeneous Temora-2 material with zircon which has a uniform δ18O value

Timing and duration of syn-magmatic deformation in the Mabel Downs Tonalite, northern Australia

Detailed outcrop mapping combined with microstructural and U–Pb SHRIMP zircon data indicate that emplacement of the Mabel Downs Tonalite spanned progressive regional D₃ deformation in the Palaeoproterozoic Halls Creek Orogen of northern Australia, and that the duration of magmatism exceeded the crystallisation time of the pluton had its entire volume been emplaced instantaneously (∼10⁵ y). The pluton comprises several compositionally distinct phases, which show (i) a regional solid-state S3a foliation-forming event, predated by a strongly deformed porphyritic monzogranite with a U–Pb SHRIMP zircon age of 1837.3±6.0 Ma (95% confidence level); and (ii) overprinting by the localised S3b Ord River Shear Zone, which crosscuts a 1831.9±3.3 Ma foliated granodiorite and contains 1826.6±7.3 Ma undeformed felsic veins, providing a younger age limit for D3 deformation.\ud \ud The protracted nature of deformation and magmatism during regional D3 deformation is significant in the context of the evolution of Halls Creek Orogen, which is characterised by a prolonged thermal event spanning three regional deformation events (D2, D3 and D4) within a 30–40 million-year interval. The accumulated finite strain is more probably the product of relatively long-lived events (of the order of several millions of years) with low average crustal strain rates, rather than high crustal strain rates during short-lived deformation episodes (=10⁵ y). Thus the partitioning of strain accumulation into discrete deformation events during the rapid development of the Halls Creek Orogen was probably not as pronounced as in orogenic belts characterised by higher accumulated strain or longer intervals between deformation events

Best of both worlds: combining SHRIMP and CA-TIMS methods in refining geochronological determinations for timescale calibration

The accurate and precise calibration of the geological timescale is a fundamental aspect of geoscience. Recent work focussed on understanding mass extinctions and climate change across the critical Late Permian-Early Triassic boundary in Australia has provided an opportunity to test collaborative approaches to using two commonly used isotopic analytical geochronology methods. Samples - largely assumed to be airfall tuffs - were prepared for in-situ ion probe analysis on a Sensitive High Resolution Ion Microprobe (SHRIMP) in a standard fashion including cathodoluminesence imaging. Images and acquired isotopic data were then used to guide the removal of individual grains from the ion probe mounts for chemical abrasion thermal ionisation mass spectrometry (CA-TIMS). This approach was able to characterise samples relatively rapidly to identify any inherited components thereby providing better targeting and resolution of high-precision ages for timescale calibration. The first-pass SHRIMP ages compare well with CA-TIMS with typicall

Rapidity of orogenesis in the Paleoproterozoic Halls Creek Orogen, northern Australia; evidence from SHRIMP zircon data, CL zircon images, and mixture modeling studies

Combining U-Pb SHRIMP zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely-spaced geological events important for understanding tectonothermal processes in the Paleoproterozoic Halls Creek Orogen of northern Australia. The youngest detrital zircon grains from a low-grade quartz-muscovite psammite of the Tickalara Metamorphics have a 207 Pb/ 206 Pb SHRIMP age of 1864+ or -4 Ma, defining a maximum depositional age for the unit. Zircon crystals from a high-grade garnet-biotite metapelite ( approximately 5-10 volume percent leucosome) from the same sequence are considerably more complex, and SHRIMP analyses form a single, large concordant group in the range approximately 1885 to 1830 Ma. The zircon crystals contain three distinct CL zoning patterns, and individual SHRIMP spots show a corresponding variation in 207 Pb/ 206 Pb ages. Concentric oscillatory-zoned zircon dominates pre-1850 Ma ages and is interpreted as detritus from igneous source rocks. Narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning. These rims comprise most of the post-1850 Ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism, reflecting the interaction of a robust pre-existing detrital zircon suite with a very limited melt volume. In addition, some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 Ma. Dividing the data into "detrital" and "metamorphic" suites solely on the basis of CL imaging yields an older group of 18 analyses ( 207 Pb/ 206 Pb age = 1867+ or -4 Ma) and a younger group of 10 analyses ( 207 Pb/ 206 Pb age = 1843+ or -4 Ma), not including five SHRIMP spots within areas of unzoned zircon. Mixture modeling of all 33 analyses in the post-1900 Ma data set resulted in a best-fit solution composed of two distinct components: (1) an older group of 19 analyses with an age of 1867+ or -4 Ma, and (2) a younger group of 14 analyses with an age of 1845+ or -4 Ma. These results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores. Importantly, the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of CL zoning patterns. These data imply that high-temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source. Such rapid rates of erosion, deposition, and burial have rarely been proposed for Proterozoic rocks, despite evidence for analogous orogenic processes in the Mesozoic and Cainozoic on a comparable timescale. Careful evaluation of geological and geochronological data in Proterozoic provinces elsewhere may reveal similar patterns, with potential implications for the possible rates of Proterozoic orogenesis and crustal evolution

Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

An igneous zircon reference material (OG1) was characterised for U-Pb isotopes by ID-TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes (207 Pb*/206Pb*). The TIMS207Pb*/206Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high207Pb* (∼ 30 μg g-1), negligible common Pb, and isotopic homogeneity permitted precise (± 1-2‰)207Pb*/206Pb* measurements within the analytical sessions. External reproducibility of mean207Pb*/206Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean207Pb*/206Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to -2.4 ± 1.3‰, and a grand mean IMF value (twenty-six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated207Pb*/206Pb*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb+ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve207Pb*/206Pb* accuracy and external reproducibility

Geological setting of Earth's oldest fossils in the ca. 3.5Ga Dresser Formation, Pilbara Craton, Western Australia

International audienceDetailed stratigraphic, petrographic, and zircon U–Pb geochronological data are provided for surface outcrops and newly obtained, unweathered drillcore intersections of Earth's oldest fossiliferous sedimentary rocks, the lowermost chert–barite unit of the ca. 3.5 Ga Dresser Formation, Warrawoona Group, Pilbara Craton.Results show that the ∼8 m thick unit at the drilling locality consists of thinly bedded, originally micritic carbonates deposited under quiet water conditions that are interbedded with volcaniclastic conglomerates and coarse polymict conglomerates (diamictites) deposited during periods of tectonic instability. The presence of rapid vertical and lateral facies changes, tilted bedding in some members, and internal erosional unconformities, combined with analysis of fault offsets, indicate that tectonically unstable periods were caused by growth faulting. Intense hydrothermal fluid flow accompanied episodes of growth faulting and resulted in pulsed, repeated precipitation of silica ± barite ± sphalerite that alternated with precipitation of pyrite. Clasts of hydrothermal minerals in sandstone and coarse, polymict conglomerate beds at several levels within the unit highlight the repeated nature of hydrothermal fluid circulation during sediment accumulation. Hydrothermal fluid circulation caused widespread acid–sulfate alteration of the footwall, extensive replacement of the newly deposited carbonate sediments by hydrothermal precipitates, and crystallization of hydrothermal chert–barite–pyrite in veins perpendicular to bedding in the footwall and parallel to bedding in the sedimentary unit. The combined evidence points to deposition of the chert–barite unit within an active volcanic caldera. A 10 cm thick bed of felsic volcaniclastic tuff within finely bedded carbonates near the top of the unit has yielded a maximum age of deposition of 3481.0 ± 3.6 Ma (2σ uncertainty), confirming earlier Pb–Pb age data for the antiquity of these rocks.Putative signs of life are present as stratiform, columnar, domical, and coniform stromatolitic laminates at various levels throughout the unit. Petrographic observations show that red- and black-weathering stromatolitic laminates on the surface consist of pyrite in unweathered drillcore material. Observation of local relics of carbonate between pyrite crystals in these laminates indicates a carbonate protolith prior to replacement by hydrothermal pyrite, which provides support for a biological origin of stromatolitic laminates. Further support is provided by clasts of laminated carbonaceous material in thinly bedded, primary micritic carbonates.Textural analysis of jaspilitic “cherts” near the top of the unit reveal haematite as tiny crystals within recrystallized siderite/dolomite rhombs in carbonate beds affected by hydrothermal silica alteration. The presence of unaltered diagenetic pyrite crystals in the haematite-altered siderite indicates that alteration did not result from oxidizing fluids. Rather, haematite alteration is interpreted as the result of an increase in pH during diagenetic alteration by mildly reducing, silica-rich fluids associated with eruption of overlying basalts, possibly with the influence of microbial activity. This has important implications for the origin of jaspilitic cherts throughout the early Archean record and for atmospheric conditions of early Earth

Portrait of a reference material: zircon production in the Middledale Gabbroic Diorite, Australia, and its implications for the TEMORA standard

The Middledale Gabbroic Diorite (MGD) in New South Wales, Australia, is host to the internationally distributed TEMORA 2 zircon reference material and its prototype, TEMORA 1. The original characterisation of the source of the reference zircons revealed that the more altered TEMORA 2 host rock contains an order of magnitude more zircon than the TEMORA 1 host, despite similar bulk-rock Zr concentrations. Although TEMORA 1 and 2 preserve the same U-Pb age, they are variable in oxygen isotope composition.In this study, petrographic observations in concert with bulk-rock and mineral geochemistry and zircon U-Pb geochronology have been applied to new samples of the MGD to investigate the link between alteration and zircon abundance. Trace element maps reveal that the products of late-stage, deuteric alteration (particularly actinolite after hornblende) are depleted in Zr, and many other trace elements, relative to the unaltered mafic, magmatic phases. It is posited that the conversion of hornblende to secondary amphiboles in the latter stages of magma solidification liberated Zr, permitting the crystallisation of additional zircon.New high resolution SIMS U-Pb determinations on four samples confirm the age homogeneity of the zircon across the pluton and reaffirm the value of TEMORA 2 as a valuable geochronological reference material. Zircon oxygen isotope data have been acquired for these same samples and the mean δ18O values encompass the accepted values for TEMORA 1 and 2. Likewise, the Hf-isotope determinations are similar to the accepted TEMORA 2 composition. Together with petrographic observations, these data reveal the TEMORA 2 zircon and its host to be broadly reflective of the relatively coarse-grained portions of the MGD, and that the isotopically less evolved compositions (i.e. the lower δ18O of TEMORA 1 and low δ18O, high 176Hf/177Hf of one sample from this study) are associated with a relatively fine-grained, marginal lithology. Given δ18O values greater than typical mantle-derived zircon and the broad correlation between O- and Hf-isotopic compositions, the data imply the pluton evolved by crustal contamination of a primitive magma