Noreen J. Evans

The zircon ‘matrix effect’: evidence for an ablation rate control on the accuracy of U–Pb age determinations by LA-ICP-MS

Many studies now acknowledge the occurrence of systematic discrepancies between U–Pb ages determined in zircons in situ by LA-ICP-MS and the benchmark analytical method ID-TIMS. In this study, we present detailed investigations into the ablation characteristics of zircons that suggest an underlying mechanism responsible for these age biases relative to ID-TIMS. Confocal laser scanning microscopy of laser ablation pits reveals that there are small but significant differences in the amount of material removed by the laser between different zircons. Based on numerous pit depth and LA-ICP-MS 206Pb/238U ratio measurements of a suite of natural zircon reference materials and samples, we demonstrate that a systematic age bias is strongly correlated with the offset in ablation rates between the primary reference material and sample zircons. We offer further insights concerning the effects of thermal annealing on the ablation behaviour of zircons and demonstrate that, although there is a change in laser ablation rates for annealed zircons, the variations between different zircons are not eliminated. Finally, we show that slight variations in laser focus also influence the ablation behaviour of zircons and may further degrade the accuracy of U–Pb age determinations.

Seeing is believing: Visualization of He distribution in zircon and implications for thermal history reconstruction on single crystals

Inverse modeling of intracrystal (U-Th)/(Pb-He) abundances in individual zircons constrains the thermal history of Earth’s crust. Zircon (U-Th)/He thermochronometry is an established radiometric dating technique used to place temporal constraints on a range of thermally sensitive geological events, such as crustal exhumation, volcanism, meteorite impact, and ore genesis. Isotopic, crystallographic, and/or mineralogical heterogeneities within analyzed grains can result in dispersed or anomalous (U-Th)/He ages. Understanding the effect of these grain-scale phenomena on the distribution of He in analyzed minerals should lead to improvements in data interpretation. We combine laser ablation microsampling and noble gas and trace element mass spectrometry to provide the first two-dimensional, grain-scale zircon He “maps” and quantify intragrain He distribution. These maps illustrate the complexity of intracrystalline He distribution in natural zircon and, combined with a correlated quantification of parent nuclide (U and Th) distribution, provide an opportunity to assess a number of crystal chemistry processes that can generate anomalous zircon (U-Th)/He ages. The technique provides new insights into fluid inclusions as potential traps of radiogenic He and confirms the effect of heterogeneity in parent-daughter isotope abundances and metamictization on (U-Th)/He systematics. Finally, we present a new inversion method where the He, U, and Th mapping data can be used to constrain the high- and low-temperature history of a single zircon crystal.

Survival of ancient landforms in a collisional setting as revealed by combined fission track and (U-Th)/He thermochronometry: A case study from Corsica (France)

The age of high-elevation planation surfaces in Corsica is constrained using new apatite (U-Th)/He data, field observations, and published work (zircon fission track, apatite fission track [AFT] data and landform/stratigraphical analysis). Thermal modeling results based on AFT and (U-Th)/He data, and the Eocene sediments uncomformably overlapping the Variscan crystalline basement indicate that present-day elevated planation surfaces in Corsica are the remnants of an erosion surface formed on the basement between ∼120 and ∼60 Ma. During the Alpine collision in the Paleocene-Eocene, the Variscan crystalline basement was buried beneath a westward-thinning wedge of flysch, and the eastern portion was overridden by the Alpine nappes. Resetting of the apatite fission track thermochronometer suggests an overburden thickness of >4 km covering Variscan Corsica. Protected by soft sediment, the planation surface was preserved. In the latest Oligocene to Miocene times, the surface was re-exposed and offset by reactivated faults, with individual basement blocks differentially uplifted in several phases to elevations of, in some cases, >2 km. Currently the planation surface remnants occur at different altitudes and with variable tilt. This Corsican example demonstrates that under favorable conditions, paleolandforms typical of tectonically inactive areas can survive in tectonically active settings such as at collisional plate margins. The results of some samples also reveal some discrepancies in thermal histories modeled from combined AFT and (U-Th)/He data. In some cases, models could not find a cooling path that fit both data sets, while in other instances, the modeled cooling paths suggest isothermal holding at temperature levels just below the apatite partial annealing zone followed by final late Neogene cooling. This result appears to be an artifact of the modeling algorithm as it is in conflict with independent geological constraints. Caution should be used when cross-validating the AFT and (U-Th)/He systems both in the case extremely old terrains and in the case of rocks with a relatively simple, young cooling history.

Scheelite elemental and isotopic signatures: Implications for the genesis of skarn-type W-Mo deposits in the Chizhou Area, Anhui Province, Eastern China

Abstract Scheelite is well developed in hydrothermal deposits, providing a window into genetic processes and facilitating comparative studies, however, few studies have focused on characterizing scheelite in skarn-type W-Mo deposits. The primary ore mineral in the Jitoushan and Baizhangyan skarn-type W-Mo deposits (Anhui Province, Eastern China), scheelite was analyzed for major, trace, and rare earth element (REE) abundance and for Sr-Nd isotopes. The analysis revealed two unique geochemical characteristics that distinguish the scheelite from skarn-type W-Mo deposits to that from vein-type Au-W and porphyry-type W-Mo deposits: higher Mo content with a negative correlation between MoO3 and WO3 and a strong HREE depletion. Skarn-type scheelite mainly inherited REE signatures from ore-forming fluids, and the early precipitation of skarn minerals (e.g., garnet, diopside, and amphibole) has most likely resulted in the observed strong HREE depletion in scheelite and the decoupling of LREEs and HREEs. Of the numerous substitution mechanisms suggested by previous workers, 3Ca2+ = 2REE3+ + □Ca (where □Ca is a Ca-site vacancy) is preferred for the substitution of REE3+ for Ca2+ and in this study, particularly given the low salinity of ore fluids. As the scheelite Eu anomalies were inherited from ore-forming fluids with variable redox conditions and pH, the complex δEu/Mo correlation indicates that Mo increasingly entered the scheelite under oxidizing conditions and reached a maxim at δEu values of 0.8 to 1. In contrast, under reducing conditions, Mo contents in scheelite decrease gradually and Mo is precipitated as molybdenite as a result of the change in dominant valence state. Unlike the Sr-Nd isotope compositions of scheelite from vein-type Au-(W) and W-(Sb-Au) deposits, the scheelite from skarn-type W-Mo deposits has low (143Nd/144Nd)(t) (most <0.5125) and intermediate (87Sr/86Sr)(t) values (most between 0.708 and 0.715). The εNd(t) values of the scheelite varied from -16 to -12.3 in the Baizhangyan deposit and from -9.5 to -9.1 in the Jitoushan deposit, indicating that the ore-forming materials in the two W-Mo deposits were mainly derived from crustal sources.

An 40Ar/39Ar geochronology on a mid-Eocene igneous event on the Barton and Weaver peninsulas: Implications for the dynamic setting of the Antarctic Peninsula

The genesis of basaltic to andesitic lavas, mafic dikes, and granitoid plutons composing the subaerial cover on the Barton and Weaver peninsulas, Antarctica, is related to arc formation and subduction processes. Precise dating of these polar rocks using conventional 40Ar/39Ar techniques is compromised by the high degree of alteration (with loss on ignition as high as 8%). In order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching, acetone washing, and hand picking, followed by an overnight bake at 250°C. After this procedure, groundmass samples can yield accurate age plateaus consisting of 70%–100% of the total 39Ark released using high-resolution heating schedules. The different rock types studied on the Barton and Weaver peninsulas yielded almost coeval ages, suggesting a giant igneous event in the Weaver and Barton peninsulas at 44.5 Ma. A compilation of newly published ages indicate that this event took place throughout the whole South Shetland Islands, suggesting a dynamic incident occurred at this stage during the arc evolution history. We related this igneous event to a mantle delamination mechanism during Eocene times. The delamination process began at ∼52 Ma, and the resultant upwelling of asthenosphere baffled the subduction of Phoenix plate, causing an abrupt decrease in convergence rate. Then multiple magmatic sources were triggered, resulting in a culminating igneous activity during 50–40 Ma with a peak at ∼45 Ma along the archipelago. The delamination also caused the extension regime indicated by the dike swarm, plugs and sills all over the archipelago, and the uplift of Smith metamorphic complex and Livingston Island. Delamination process may have finished at some time during 40–30 Ma, leaving a weak igneous activity at that stage and thereafter. The convergence rate then recovered gradually, as indicated by the magnetic anomaly identifications. This model is supported by seismic observation of deep velocity anomalies beneath the Antarctic Peninsula.

Assessing the magmatic affinity and petrogenesis of granitoids at the giant Aktogai porphyry Cu deposit, Central Kazakhstan

Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with high Sr/Y rocks, while barren or weakly mineralized granitoids show typical low Sr/Y features. The Aktogai giant porphyry Cu deposit occurs in the Koldar pluton and provides a good natural laboratory in which to investigate this relationship, while determining the petrogenesis of the pluton and its mineralization. Zircon U-Pb dating, mineral chemistry, whole rock geochemistry and Sr-Nd-Pb and zircon Hf-O isotopic analyses were carried out on the pre-ore granodiorite (the major component of the Koldar pluton) and on the mineralized granodiorite porphyry. Zircon U-Pb ages indicate that the pre-ore granodiorite and mineralized granodiorite porphyries were emplaced at 345 and 328 to 331 Ma, respectively. Distinctly higher apatite SO3 contents in the granodiorite porphyry relative to the granodiorite suggest an increase in fO2 during the petrogenesis of the mineralized porphyries (>NNO+1). Although all rocks share similar geochemical characteristics (calc-alkaline, strong depletion in Nb, Ta and Ti, and enrichment in LREE and LILE), the pre-ore Koldar pluton has normal arc related magmatic features [low Sr/Y and (La/Yb)N, high Y and YbN], while the granodiorite porphyries and diorite (trace component of Koldar pluton) exhibit high Sr/Y and (La/Yb)N, low Y and YbN features. All samples show similar Sr-Nd-Pb-Hf-O isotopic compositions [(87Sr/86Sr)i = 0.70369 to 0.70413, εNd (t) = + 3.6 to + 5.6, (206Pb/204Pb)i = 18.16 to 19.32, zircon εHf (t) = + 11.8 to + 15.9, and δ18O = + 3.8 to + 5.9 ‰], and very young whole rock T2DM (Nd) (640 – 680 Ma) and zircon TDMC (Hf) (320 – 590 Ma) values, suggesting that they were probably derived from partial melting of juvenile lower crust. Geochemical patterns and partial melt modeling indicate that the high Sr/Y rocks were probably formed by partial melting of eclogitized, thickened lower crust, while the Koldar pluton formed by partial melting of normal thick lower crust. We propose that pre-ore low Sr/Y rocks were probably generated earlier via subduction of Junggar-Balkhash oceanic crust, and that the high Sr/Y rocks were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. High oxygen fugacity and the high melting temperature of the high Sr/Y rocks ensured that all sulfide was dissolved in the magma, which intruded the previously emplaced low Sr/Y pluton and resulted in significant mineralization.

Eo-Alpine metamorphism and the ‘mid-Miocene thermal event’ in the Western Carpathians (Slovakia): new evidence from multiple thermochronology

A combination of zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th)/He (AHe) dating methods is applied to constrain the metamorphic and exhumation history of the Tatric part of the Branisko Mountains in the Western Carpathians. ZHe ages from the basement samples prove the basement experienced a very low-grade to low-grade Eo-Alpine metamorphic overprint in mid-Cretaceous times. Miocene AFT and AHe ages found in the basement and in the Palaeogene sediments conclusively demonstrate that the Branisko Mts experienced a ‘mid-Miocene thermal event’. This thermal event had a regional character and was related to magmatic and/or burial heating that exposed the sediment and basement samples to ~ 120–130°C and ~ 100–190°C, respectively.

An in situ technique for (U–Th–Sm)/He and U–Pb double dating

We report on a new laser-based technique for rapid, quantitative and automated in situ double dating (U–Pb and (U–Th–Sm)/He) of minerals, for applications in geochronology, thermochronology and geochemistry. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of spatial resolution, productivity, and safety. This new approach/methodology utilizes an interoperable and integrated suite of analytical instruments including a 193 nm ArF excimer laser system, quadrupole ICP-MS, quadrupole helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. We describe the analytical protocols for zircon analysis including grain mounting in Teflon, parameters for parent and daughter isotopic measurement, and standard development, and provide a freeware application for determining (U–Th–Sm)/He ‘pairwise’ ages from analytical data. The in situ double dating method described is applied to the Ellendale lamproite pipe and country rocks, Western Australia and successfully replicates conventional U–Pb and (U–Th–Sm)/He age variations determined previously by conventional techniques.

The tetrad effect and geochemistry of apatite from the Altay Koktokay No. 3 pegmatite, Xinjiang, China: implications for pegmatite petrogenesis

In order to better constrain the evolution and petrogenesis of pegmatite, geochemical analysis was conducted on a suite of apatite crystals from the Altay Koktokay No. 3 pegmatite, Xinjiang, China and from the granitic and amphibolitic wall rocks. Apatite samples derived from pegmatite zones show convex tetrad effects in their REE patterns, extremely negative Eu anomalies and non-chondritic Y/Ho ratios. In contrast, chondritic Y/Ho ratios and convex tetrad effects are observed in the muscovite granite suggesting that different processes caused non-chondritic Y/Ho ratios and lanthanide tetrad effects. Based on the occurrence of convex tetrad effects in the host rocks and their associated minerals, we propose that the tetrad effects are likely produced from immiscible fluoride and silicate melts. This is in contrast to previous explanations of the tetrad effect; i.e. surface weathering, fractional crystallization and/or fluid-rock interaction. Additionally, we put forward that extreme negative Eu and non-chondritic Y/Ho in apatite are likely caused by the large amount of hydrothermal fluid exsolved from the pegmatite melts. Evolution of melt composition was found to be the primary cause of inter and intra-crystal major and trace element variations in apatite. Mn entering into apatite via substitution of Ca is supported by the positive correlation between CaO and MnO. Different evolution trends in apatite composition imply different crystallization environments between wall rocks and pegmatite zones. Based on the geochemistry of apatite samples, it is likely that there is a genetic relationship between the source of muscovite granite and the source of the pegmatite.

Early Cambrian metamorphic zircon in the northern Pinjarra Orogen: Implications for the structure of the West Australian Craton margin

The Mesoproterozoic Pinjarra Orogen formed when present-day India and Australia amalgamated to form Gondwana. Outcrop of the Pinjarra Orogen is limited to the Leeuwin, Mullingarra, and Northampton Complexes, which are exposed as basement inliers in the Paleozoic to Mesozoic Perth and Southern Carnarvon Basins. We used U-Pb zircon geochronology to date Pinjarra Orogen basement rocks from the Wendy-1 drill core, which intersects the Paleozoic Tumblagooda Sandstone and its underlying paragneiss basement east of the Northampton Complex. Our results suggest an Early Cambrian (526.3 ± 12 Ma) metamorphic age for this basement domain, which is uncharacteristic for the nearby Northampton Complex, but correlates well with the much more distant Leeuwin Complex. Detrital ages between 1120 Ma, 1210 Ma, and 1530 Ma dominate the zircon cargo of this basement sample, which may have been sourced from the Albany-Fraser Orogen to the south and east. An Archean detrital zircon component is also identified from one concordant analysis, and from radiogenic Pb-loss modeling. These results have important implications for the crustal architecture of the western margin of the West Australian craton and for correlating domains of the Mesoproterozoic Pinjarra Orogen in reconstructions of Gondwana. Our data suggest that the basement below the Perth Basin is more segmented than previously assumed. Evidence for a common Indian-Australian tectonometamorphic event in the Late Neoproterozoic to Early Cambrian is not limited to the Leeuwin Complex in the southwest corner of present-day Western Australia but also now identified in basement rocks in the Northampton area. These results confirm the in situ formation of Pinjarra Orogen basement complexes in the Mesoproterozoic with a metamorphic reactivation in the Neoproterozoic–Cambrian during the collision with present-day India.