A stochastic sampling approach to zircon eruption age interpretation

The accessory mineral zircon is widely used to constrain the timing of igneous processes such as magma crystallisation or eruption. However, zircon U-Pb ages record zircon crystallisation, which is not an instantaneous process. Zircon saturation calculations link zircon crystallisation, temperature, and melt fraction, allowing for the estimation of zircon crystallisation distributions as a function of time or temperature. Such distributions provide valuable prior information, enabling Bayesian estimates of magma eruption time and allowing for comparison of the relative accuracy of common weighted-mean and youngest-zircon age interpretations with synthetic datasets. We find that both traditional interpretations carry a risk of underestimating the uncertainty in eruption age; a low mean square of weighted deviates (MSWD) does not guarantee the accuracy of weighted-mean interpretations. In the absence of independent confirmation that crystallisation timescale is short relative to analytical uncertainties, a Bayesian approach frequently provides the most accurate results and is least likely to underestimate uncertainty. Finally, since U-Pb zircon studies now routinely resolve geological age dispersion due to increasing analytical precision, such considerations are increasingly critical to future progress in resolving rates and dates of Earth processes

Volcanic-plutonic parity and the differentiation of the continental crust

The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks – volcanic and plutonic rocks – and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global dataset of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings, but divergent in continental rifts. Offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation

A red bole zircon record of cryptic silicic volcanism in the Deccan Traps, India

Abstract Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP.ISSN:0091-7613ISSN:1943-268

Magma Emplacement, Differentiation and Cooling in the Middle Crust: Integrated Zircon Geochronological–Geochemical Constraints from the Bergell Intrusion, Central Alps

U–Th–Pb zircon geochronology is an essential tool for quantifying the emplacement, differentiation and thermal evolution of crustal magmatic systems. However, the power of U–Pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry, as this permits more detailed interpretations of geochronological datasets than conventionally achieved. Our approach to better relating zircon dates and geological processes consists of a multi-method analytical workflow, including cathodoluminescence imaging (CL), in situ LA-ICPMS/EPMA zircon geochemistry, U–Pb zircon ID-TIMS geochronology, and solution ICPMS zircon Trace Element Analysis (U–Pb TIMS-TEA). These methods are here applied to zircon from the Bergell Intrusion, a composite Alpine pluton preserving a ~10 km mid-crustal transect. Hand samples of tonalite, granodiorite and hybridized granitoid each record 250–700 kyr of autocrystic zircon growth. Bergell zircons are ubiquitously zoned with ca. 104 –106 yr growth histories, as evidenced by ID-TIMS analysis of microsampled fragments from single crystals. U–Pb TIMS-TEA data exhibit compositional trends on multiple spatiotemporal scales, including the handsample-scale, representing in situ differentiation at the emplacement level (e.g., Th/U); lithology-scale, defining trajectories corresponding to the production of tonalitic versus granodioritic magmas (Lu/Hf); and pluton-scale, indicating increasingly-evolved melts over ~1.6 Myr of pluton assembly (Zr/Hf). These absolute TIMS-TEA temporal trends are corroborated by relative LA-ICPMS/EPMA core-to-rim geochemistry. We compare records of trace element evolution from TIMS-TEA, Bergell whole-rock geochemistry, and a global compilation of whole-rock geochemical data. These findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales, and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust

Insights into (U)HP metamorphism of the Western Gneiss Region, Norway: A high-spatial resolution and high-precision zircon study

Combining high-spatial resolution and high-precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh-pressure (UHP) metamorphism resulting from the collision of Baltica–Avalonia and Laurentia during the Scandian orogeny in the Western Gneiss Region of Norway. Zircons were extracted from a layered eclogite in the Saltaneset region (southern UHP domain) and from an eclogite in the Ulsteinvik region (central UHP domain). Zircons were first analyzed for U–Pb and trace element compositions by laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICP-MS), followed by analysis of those same zircons that yielded Scandian dates by integrated U–Pb isotope dilution–thermal ionization mass spectrometry and Trace Element Analysis (TIMS–TEA). LASS results from a garnet–quartz layer within the Saltaneset eclogite give Scandian dates of ca. 413–397 Ma, with subsequent ID–TIMS analyses ranging from 408.9 ± 0.4 Ma to 401.4 ± 0.2 Ma (2σ). An omphacite-rich layer from the same eclogite yields LASS dates of ca. 414–398 Ma and a single ID–TIMS date of 396.7 ± 1.4 Ma. In comparison, the Ulsteinvik eclogite LASS results give dates spanning ca. 413–397 Ma, whereas ID–TIMS analyses range from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma. ID–TIMS zircon data from the eclogites reveals two age populations: 1) ca. 409–407 Ma and 2) ca. 402 Ma. Both in situ and solution trace element data show a distinct pattern for Scandian zircons, with strongly-depleted HREE and weakly-negative Eu anomalies (Eu/Eu*), whereas inherited zircon REE patterns are distinguished by steep HREE slopes and marked negative Eu/Eu*. When coupled with partition coefficients calculated for zircon and garnet, these REE patterns indicate that zircon (re)crystallized during eclogite-facies metamorphism at ca. 409–407 Ma and ca. 402 Ma at two widely separated UHP localities

U-Pb constraints on pulsed eruption of the Deccan Traps across the end-Cretaceous mass extinction

Temporal correlation between some continental flood basalt eruptions and mass extinctions has been proposed to indicate causality, with eruptive volatile release driving environmental degradation and extinction. We tested this model for the Deccan Traps flood basalt province, which, along with the Chicxulub bolide impact, is implicated in the Cretaceous-Paleogene (K-Pg) extinction approximately 66 million years ago. We estimated Deccan eruption rates with uranium-lead (U-Pb) zircon geochronology and resolved four high-volume eruptive periods. According to this model, maximum eruption rates occurred before and after the K-Pg extinction, with one such pulse initiating tens of thousands of years prior to both the bolide impact and extinction. These findings support extinction models that incorporate both catastrophic events as drivers of environmental deterioration associated with the K-Pg extinction and its aftermath

Using Eclogite Retrogression to Track the Rapid Exhumation of the Pliocene Papua New Guinea UHP Terrane

The D’Entrecasteaux Islands of eastern Papua New Guinea (PNG) host the youngest known ultrahigh-pressure terrane on Earth and represent the only location where ultrahigh-pressure (UHP) rocks have been exhumed in an active rift. The PNG (U)HP rocks, consisting of Pliocene eclogites, garnet amphibolites and migmatitic gneisses, are exposed in five domal structures across the Islands. Zirconium-in-rutile thermometry records peak temperatures of ∼780°C from the eastern Oiatabu and nearby central Mailolo Domes, and hotter temperatures of ∼825–865°C within the western Goodenough Dome. Uranium–lead (U–Pb) and trace element zircon compositions from a suite of eclogite, host gneiss, felsic dikes and pegmatite from three domes document the rapid exhumation history of the PNG UHP terrane. High-spatial resolution laser-ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analyses of select eclogite zircons exhibit no resolvable age zoning within single crystals. The same eclogite zircons, combined with separate zircons extracted from additional eclogite, host gneiss and felsic intrusions, were subsequently analysed by high-precision U–Pb chemical-abrasion isotope-dilution thermal ionization mass spectrometry and solution ICP-MS trace element analysis (TIMS-TEA). The results record discrete tectonic events across the three domes at sub-million year timescales: (1) (re)crystallization of host gneiss within the lower crust exposed in the eastern Oiatabu Dome from c.5·7–4·5 Ma; (2) initial retrogression and local decompression melting of eclogites from the Oiatabu and Mailolo Domes at c.4·6–4·3 Ma; (3) melt crystallization of weakly deformed felsic dikes of the Oiatabu Dome at c.3·0–2·9 Ma; and (4) retrogression and melt crystallization within eclogite–amphibolite-facies rocks in the western Goodenough Dome at c.2·9–2·6 Ma. In comparison to Zr-in-rutile peak temperature estimates, Ti-in-zircon temperatures >800°C may reflect increased temperatures during exhumation that resulted in partial melting of the eclogites. Inclusions of crystallized hydrous melt consisting of Na-rich plagioclase ± K-feldspar + quartz within eclogite zircons document this process. The elevated temperatures and the presence of the polyphase inclusions are the first documentation of partial melting of the (U)HP eclogites within PNG during initial retrogression from c.4·6–4·3 Ma. Overall, U–Pb zircon geochronology and geochemistry track both the timing of retrogressive overprinting within the lower-to-middle crust and final upper crustal emplacement over a relatively short span of ∼2 Myr during the rapid (≥2·3 cm/yr) exhumation of the youngest known (U)HP eclogites