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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Zircon age-temperature-compositional spectra in plutonic rocks

We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat ) and rock solidification (T solid ). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid . We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional change over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.

58 GEOSCIENCES↗
DOE OSTI2019

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (<0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ~50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ~0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.

Brenhin Keller, C. (ORCID:0000000174009428)↗
NASA NTRS1988

Role of zircon in tracing crustal growth and recycling

Single crystal ion probe ages of zircons is discussed, which allow much better time resolution compared to other geochronological methods, although the technique is not without problems. Rocks from two areas that contain composite zircon populations, including true magmatic zircons as well as a variety of xenocrystic types are described. It is often difficult to distinguish these; xenocrystic zircons, for example, cannot always be identified on the basis of morphology alone. Additional evidence is needed before making age interpretations. Evidence is also presented of zircon growth long after the original time of crystallization, in some cases apparently at temperatures less than 300 C. The spectacular discovery of 4.1 to 4.2 Ga detrital zircons in metaquartzites from the Mount Narryer area of Western Australia is described. Similar zircons with ages as old as 4276 Ma have been found in the nearby Jack Hills area. The source areas or parent lithologies of these zircons have not yet been determined, but the author expects that they may be unrecognized or buried antecedents of the K rich Narryer gneisses. U or Th concentrations of zircon cannot be used to discriminate between felsic and mafic source rocks.

Compston, W.↗
NASA NTRS1992

U-Pb isotopic results for single shocked and polycrystalline zircons record 550-65.5-Ma ages for a K-T target site and 2700-1850-Ma ages for the Sudbury impact event

The refractory mineral zircon develops distinct morphological features during shock metamorphism and retains these features under conditions that would anneal them in other minerals. In addition, weakly shocked zircon grains give primary ages for the impact site, while highly reconstituted (polycrystalline) single grains give ages that approach the age of the impact event. Data for a series of originally coeval grains will define a mixing line that gives both of these ages providing that no subsequent geological disturbances have overprinted the isotopic systematics. In this study, we have shown that the three zircon grain types described by Bohor, from both K-T distal ejecta (Fireball layer, Raton Basin, Colorado) and the Onaping Formation, represent a progressive increase in impact-related morphological change that coincides with a progressive increase in isotopic resetting in zircons from the ejecta and basement rocks. Unshocked grains are least affected by isotopic resetting while polycrystalline grains are most affected. U-Pb isotopic results for 12 of 14 single zircon grains from the Fireball layer plot on or close to a line recording a primary age of 550 +/- 10 Ma and a secondary age of 65.5 +/- 3 Ma. Data for the least and most shocked grains plot closest to the primary and secondary ages respectively. The two other grains each give ages between 300 and 350 Ma. This implies that the target ejecta was dominated by 550-Ma rocks and that the recrystallization features of the zircon were superimposed during the impact event at 65.5 Ma. A predominant age of 550 Ma for zircons from the Fireball layer provides an excellent opportunity to identify the impact site and to test the hypothesis that multiple impacts occurred at this time. A volcanic origin for the Fireball layer is ruled out by shock-related morphological changes in zircon and the fact that the least shocked grains are old. Basement Levack gneisses north of the Sudbury structure have a primary age of 2711 Ma. Data for three single zircons from this rock, which record a progressive increase in shock features, are displaced 24, 36, and 45 percent along a Pb-loss line toward the 1850 +/- 1 Ma minimum age for the impact as defined by the age of the norite. Southeast of the structure three shocked grains from the Murray granite record a primary age of 2468 Ma and are displaced 24, 41, and 56 percent toward the 1853 +/- 4 Ma even as defined by coexisting titanite.

Krogh, T. E.↗
DOE OSTI2018

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.

58 GEOSCIENCES↗
DOE OSTI2020

The internal structure and geodynamics of Mars inferred from a 4.2-Gyr zircon record

Combining U–Pb ages with Lu–Hf data in zircon provides insights into the magmatic history of rocky planets. The Northwest Africa (NWA) 7034/7533 meteorites are samples of the southern highlands of Mars containing zircon with ages as old as 4476.3 ± 0.9 Ma, interpreted to reflect reworking of the primordial Martian crust by impacts. We extracted a statistically significant zircon population (n= 57) from NWA 7533 that defines a temporal record spanning 4.2 Gyr. Ancient zircons record ages from 4485.5 ± 2.2 Ma to 4331.0 ± 1.4 Ma, defining a bimodal distribution with groupings at 4474 ± 10 Ma and 4442 ± 17 Ma. We interpret these to represent intense bombardment episodes at the planet’s surface, possibly triggered by the early migration of gas giant planets. The unradiogenic initial Hf-isotope composition of these zircons establishes that Mars’s igneous activity prior to ~4.3 Ga was limited to impact-related reworking of a chemically enriched, primordial crust. A group of younger detrital zircons record ages from 1548.0 ± 8.8 Ma to 299.5 ± 0.6 Ma. The only plausible sources for these grains are the temporally associated Elysium and Tharsis volcanic provinces that are the expressions of deep-seated mantle plumes. The chondritic-like Hf-isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir, indicating that Mars has been in a stagnant-lid tectonic regime for most of its history. Our results imply that zircon is ubiquitous on the Martian surface, providing a faithful record of the planet’s magmatic history.

58 GEOSCIENCES↗
NASA NTRSDate not supplied

Zircon (U-TH)/He Impact Crater Thermochronometry and the Effects of Shock Microstructures on Helium Diffusion Kinetics

Accurate age determinations of hyper-velocity impact and cratering events remains difficult and often controversial, while less than half of all known impact craters are regarded as accurately and precisely dated. Besides 40Ar/39Ar and U-Pb methods, zircon (U-Th)/He dating of impactites is a burgeoning technique to date large- to medium-sized impact structures. Zircon (U-Th)/He ages can be fully reset in minutes at 1000°C, T commonly reached in and directly adjacent to impact melt domains, whereas complete resetting of zircon (U-Th)/He at <300°C, which might be encountered near the crater margins or persist in post-impact hydrothermal systems, may take >103-4 years. However, there is a critical need to test the reliability of (U-Th)/He impact dating in shock deformed zircon, and to quantify helium diffusion kinetics in well-characterized grains with a broad spectrum of shock-induced defect substructures. For this purpose, we investigated samples from two impact structures, the 66 Ma Chicxulub multi-ring basin and the 15 Ma Ries complex crater, to compare zircon diffusion kinetics from impact structures with varying parameters, including size, age, and hydrothermal system longevity. Shock microstructures were characterized by backscattered-electron (BSE) imaging prior to determination of diffusion step-heating fractional release experiments using light-bulb furnace with prograde and retrograde incrementally 10°C steps from 300°C to 600°C. We find that zircon with low-level shock microstructures exhibit no significant deviation from helium diffusion kinetics of undamaged zircon. In contrast, zircon grains with planar deformation features and granular textures classified by SEM are characterized by a dramatic decrease in helium retentivity, similar to radiation damage, due to the reduction in the effective domain size and the introduction of fast diffusion pathways. This likely renders shocked grains more susceptible to impact-induced hydrothermal resetting. Hence, characterization of impact microstructure is critical for determining accurate impact ages, but also offers the opportunity to determine the magnitude and duration of post-impact hydrothermal circulation.

Catherine Ross↗
DOE OSTI2025

Temperature dependence of uranium and thorium partitioning in igneous zircons

Zircon is a key mineral in geochronology because of its chemical and physical durability and tendency to incorporate radioactive trace elements such as U and Th. Quantifying the partitioning of the actinide elements is critical to constrain initial non-secular equilibrium amounts of 230 Th in zircon. An excess or deficit of 206 Pb will be produced from such an initial excess/deficit of 230 Th from the secular equilibrium condition, which influences the calculated 206 Pb/ 238 U age (Schärer, 1984, Mattinson, 1973). However, there is no standard way to calculate Th/U partitioning ratios when applying age corrections to young igneous zircon, making uncertainties hard to estimate. To better understand U and Th partitioning of zircon/melt that is the cornerstone of the secular disequilibrium correction, zircon was synthesized in one-atmosphere experiments using basaltic andesite, andesite, and rhyolite starting materials, doped with Zr, U, and Th. Different experimental temperatures and oxygen fugacity conditions (ΔQFM−4 to ΔQFM+4) were explored to examine their effects on U and Th partitioning. In addition, we specifically quantify the effects of sector zoning, fractional crystallization, and melt composition on U and Th partitioning. By combining experimental and natural zircon data, we find that temperature has the primary control on the partitioning of U and Th in the zircon and calibrate an inverse relationship between these partition coefficients and crystallization temperatures. The calibrated equation can be applied to the 230 Th correction for an improvement in the accuracy of Th-corrected 206 Pb/ 238 U dates when the zircon crystallization temperature is known.

206Pb/238U dating↗
NASA NTRSDate not supplied

Zircon (U-Th)/He Impact Crater Thermochronometry and the Effects of Shock Microstructures on Helium Diffusion Kinetics

Absolute age determination of impact cratering events remains difficult and often controversial; a challenge that has resulted in < 50% of known impact craters regarded as accurately and precisely dated. Besides conventional 40Ar/39Ar and U-Pb methods, zircon (U-Th)/He (ZHe) dating of impactites has been applied to large- to medium-sized impact structures. ZHe dates can be fully reset in minutes at 1000°C, which is commonly reached in central sections of the melt sheet, whereas resetting of ZHe at <300°C, which might be encountered near the crater margins or persist in post-impact hydrothermal systems, may take >103-6 years. There is a critical need to quantify the effects of shock-induced microstructures and impact metamorphism on helium diffusion kinetics in well-characterized, variably shocked zircon to further establish the reliability of (U-Th)/He for dating impacts. For this purpose, we investigated suevite and impact melt samples from two impact structures, the Chicxulub multi-ring basin and the Ries complex crater, which enables us to compare zircon helium diffusion kinetics from impact structures with differing sizes, ages, and hydrothermal system longevities. Shock microstructures were characterized by backscattered-electron (BSE) imaging prior to diffusion step-heating fractional release experiments using light-bulb furnace with prograde and retrograde incremental 10°C steps from 250°C to 600°C. Afterward, we characterize the diffusion domain sizes and their interconnectivity within the shocked zircon grains using electron backscatter diffraction (EBSD). We find that zircon with few shock microstructures exhibit no significant deviation from helium diffusion kinetics of undamaged zircon. In contrast, zircon grains with planar deformation features (PDFs) and granular textures classified by BSE and EBSD are characterized by a dramatic decrease in helium retentivity, similar to radiation damaged grains, due to a reduction in the effective domain size and the introduction of interconnected fast diffusion pathways created by shock microstructures. A subset of grains were dated by ZHe after the external morphology of the grains was determined by BSE imaging. The euhedral grains yielded a weighted mean age within the uncertainty of the accepted impact ages, whereas the grains with PDFs or granular textures exhibited younger ages. Thus, these diffusion experiments and ZHe dates suggest that the dramatic decrease in domain size likely renders shocked grains more susceptible to impact-induced hydrothermal resetting and subsequent overprinting. Hence, characterization of shock microstructures is critical for determining accurate impact ages using ZHe methods especially when applied to previously unconstrained craters. The thermochronometer also offers the opportunity to determine the magnitude and duration of post-impact hydrothermal circulation.

zircon↗
DOE OSTI2021

Oxygen isotope ratios in zircon and garnet: A record of assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small <100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.

Geochemistry & Geophysics↗
NASA NTRS2006

Heavy Isotope Composition of Oxygen in Zircon from Soil Sample 14163: Lunar Perspective of an Early Ocean on the Earth

Thirty oxygen analyses of a large (sub-millimetre) zircon grain from the lunar soil sample 14163 have been determined using CAMECA 1270 ion microprobe. The sample 14163 was returned form the Fra Mauro region by Apollo 14 mission. Zircon grain of 0.6-0.8 mm in size extracted from the sample was imaged using CL detector fitted to the Philips Electron Microscope in order to reveal internal structure. Oxygen isotopes have been analysed during two sessions. The first set of data was collected using the original mount where the grain was set in the resin attached to the glass slide. This resulted in the two complications: (i) standard zircon has to be analysed from the separate mount and (ii) the lunar zircon grain was rased in the holder compared to the standard. In order to investigate, if the elevated oxygen compositions observed during this session could have resulted from this difference in geometric configuration during the standard and sample analyses, the lunar zircon was extracted from the original mount, remounted with the standard chip in the new resin disk and reanalysed during the second session. All analyses made during the first session show delta O-18 values heavier than 6.0%. The second set of data has a wider spread of delta O-18 values with some values as low as 5.6%. Nevertheless, a half of observed delta O-18 values in this set is also higher than 6.0%. Slightly lighter oxygen compositions observed during the second session indicate possible dependence of measured delta O-18 values on the geometry of analysed samples. Presence of zircons with similar heavy oxygen isotope compositions on the Moon, which neither had liquid water or felic crust similar to that on the Earth nor ever developed regime similar to plate tectonics, suggests that other mechanisms can be responsible for elevated delta O-18 values in zircons. This implies that there is no support for the presence of an ocean on the surface of the early Earth and as the ocean appears to be an essential ingredient for the plate tectonics, there is no basis for belief that this mechanism was operating in the early history of the planet.

Nemchin, A. A.↗
DOE OSTI2024

Zircon-to-reidite phase transition enhanced by minor radiation damage: Implications for hypervelocity impacts

Reidite, a high-pressure phase of zircon, is increasingly identified at terrestrial impact sites. Despite its growing recognition, the potential applications for estimating minimum impact pressure face impediments due to existing discrepancies in the condition of zircon-reidite transformation, controversial models governing the transformation mechanism, and unclear effects of pre-existing radiation damage on reidite formation. Here, we show enhanced reidite formation by synchrotron X-ray diffraction, Raman spectroscopy, and transmission electron microscopy analyses of zircon grains that have experienced different alpha-decay doses from U and Th impurities and subsequent pressurization in diamond anvil cells. Below ~1 × 10 18 α-decay events/g, the α-decay-induced isolated point defects in the still crystalline zircon facilitate the minor atomic readjustments required for reidite formation. However, above this dose, the loss of long-range periodicity in severely damaged or even metamict zircon inhibits the transformation. The enhanced reidite formation by minor radiation damage coincides with the more common occurrence of reidite at impact sites for which the precursor zircon has a relatively lower alphadecay- event dose before the impact event. In addition, the detailed atomic-scale structures of twinned reidite provide unambiguous evidence for a characteristic internal stress-induced martensitic transition. Furthermore, these findings have important implications for interpreting the formation conditions of natural reidite due to the convergence of pressure from the static, shockwave, and natural reidite samples.

58 GEOSCIENCES↗
DOE OSTI2012

Theoretical investigations of the physical properties of zircon-type YVO{sub 4}

The crystal structure, electronic properties, elastic properties, hardness and thermodynamic properties of the laser host material zircon-type YVO{sub 4} are studied using the pseudopotential plane wave method within the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated ground state values such as lattice parameter, bulk modulus and its pressure derivative, the band structure and densities of states were in favorable agreement with previous works and the existed experimental data. The elastic constants C{sub ij}, the aggregate elastic moduli (B, G, E), Poisson's ratio and elastic anisotropy have been investigated. In YVO{sub 4}, V-O bonds with shorter bond length and larger Mulliken population make great contribution to hardness than Y-O bonds. Using quasi-harmonic Debye model considering the phonon effects, bulk modulus, heat capacity and thermal expansion coefficient of YVO{sub 4} are calculated within a range of 0-6 GPa and 0-1200 K. - Graphical Abstract: (a) Directional dependence of Young's modulus in zircon-type YVO{sub 4} and (b) projections of the directional dependent Young's modulus in different planes for zircon-type YVO{sub 4}. The units are in GPa. Highlights: Black-Right-Pointing-Pointer This paper systematically studied the physical properties of zircon-type YVO{sub 4} from first-principles calculations. Black-Right-Pointing-Pointer Zircon-type YVO{sub 4} is mechanically stable and it is ductile for B/G>1.75 and v>0.26. Black-Right-Pointing-Pointer Universal elastic anisotropy index A{sup U} for zircon-type YVO{sub 4} is 2.41, so YVO{sub 4} is anisotropic. Black-Right-Pointing-Pointer V-O bonds with shorter bond length and larger Mulliken population make greater contribution to the hardness of YVO{sub 4}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Synchronistic preparation of fibre-like SiC and cubic-ZrO{sub 2}/SiC composite from zircon via carbothermal reduction process

Graphical abstract: Display Omitted Highlights: ► Zircon carbothermal reduction was carried out in a tailor-made device at high-temperature air atmosphere. ► Fibre-like SiC and cubic-ZrO{sub 2}/SiC composite were obtained synchronically. ► Zirconium and silicon in zircon ore was initial separated. ► [SiO{sub 4}] was mutated to fibre-like SiC, while [ZrO{sub 8}] was transformed to cubic ZrO{sub 2}. ► The SiC were surprisingly enriched in the reducing atmosphere charred coal particles layers by gas–solid reaction. -- Abstract: Fibre-like SiC and cubic-ZrO{sub 2}/SiC composite were prepared respectively from zircon with yttrium oxide addition via carbothermal reduction process at 1600 °C for 4 h in an air atmosphere furnace, where the green samples were immerged in charred coal particles inside a high-temperature enclosed corundum crucible. The reaction products were characterized by XRD, XRF, XPS and SEM. The results indicate that ZrO{sub 2} in the products was mainly existed in the form of cubic phase. The reacted samples mainly contain cubic ZrO{sub 2}, β-SiC and trace amounts of zircon, with the SiC accounting for 14.8 wt%. Furthermore, a large quantity of fibre-like SiC was surprisingly found to concentrate in the charred coal particles layers around the samples. This study obtains fibre-like SiC and cubic-ZrO{sub 2}/SiC composite synchronically from zircon via carbothermal reduction process, which also bring a value-added high-performance application for natural zircon.

36 MATERIALS SCIENCE↗
NASA NTRS2019

Mechanisms and Consequences of Intra-Crystalline Enrichment of Ancient Radiogenic Pb in Detrital Hadean Zircons from the Jack Hills, Western Australia

The recent discovery of Pb* (radiogenic lead) -enriched domains (PEDs) in zircon using high-resolution scanning ion imaging, atom probe tomography and transmission electron microscopy indicates that the U-Pb isotopic system in zircon can be affected not only by Pb* loss but also by intra-crystalline Pb* enrichment. However, the formation mechanism of PEDs and the consequences for in-situ U-Pb dating remain elusive. To further understand these issues, scanning ion imaging and U-Pb dating were carried out on 51 Hadean detrital zircon grains from the Jack Hills, Western Australia. Of these, 8 grains were found to contain micrometer-scale PEDs with elevated Pb-207 and Pb-206 in the ion images. Isotope profiles across these PEDs confirm that they represent unsupported Pb* resulting from intra-grain mobilization and local enrichment of Pb* during ancient overprinting events. The PEDs are rare in most grains, with only 1 – 3 Pb* hotspots present in any ~70 μm × 70 μm imaged area, and are located either in dark-CL, high-U growth zones, or associated with bright-CL, low-Th recrystallized domains, suggesting that they were likely formed by local-scale processes. The PEDs located in dark-CL, high-U growth zones likely resulted from Pb* trapped in local, interconnected pathways produced by radiation damage during thermal overprinting events. Those associated with bright-CL, low-Th domains are interpreted as Pb* concentrated in nanoscale pores formed during fluid-present recrystallization of non-metamict zircon. Our interpretations therefore imply that intra-crystalline enrichment of Pb* in zircon can occur at various scales and through diverse mechanisms. Age calculations based on the ion images demonstrate that incorporation of PEDs in conventional in-situ SIMS spots produces spuriously old ages that are either reversely discordant or concordant, depending on the time of Pb* mobilization and the analytical precision. The implication is that careful investigation of intra-grain distribution of U-Th-Pb isotopes is required in order to better understand the U-Pb isotopic system and to obtain reliable crystallization ages for ancient zircons affected by complex Pb* mobilization and redistribution.

Unsupported radiogenic lead↗
DOE OSTI2023

Chemical abrasion: the mechanics of zircon dissolution

Chemical abrasion is a technique that combines thermal annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U–Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. Acid frequently reaches crystal interiors via fractures spatially associated with radiation damage zoning and inclusions to dissolve soluble high-U zones, some inclusions, and material around fractures, leaving behind a more crystalline zircon residue. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically controlled with dissolution proceeding almost exclusively along the c axis. Increasing the leaching temperature from 180 to 210 °C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. How a grain dissolves strongly depends on its initial radiation damage content and defect distribution as well as the size and position of inclusions. As such, the effectiveness of any chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely sample-dependent. We also briefly discuss the implications of our findings for deep-time (U-Th)/He thermochronology.

58 GEOSCIENCES↗
NASA NTRS1993

Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta

U-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

Krogh, T. E.↗
NASA NTRS2005

Hadean Crustal Processes Revealed from Oxygen Isotopes and U-Th-Pb Depth Profiling of Pre-4.0 Ga Detrital Zircons from Western Australia

Because physical and chemical processes of the past are determined from analysis of a preserved geologic record, little is known about terrestrial crustal processes of the first 500 Ma during the so-called Hadean Eon. What is known from direct measurements has been derived almost exclusively from the study of greater than 4.0 Ga detrital zircons from the Jack Hills, Western Australia. The geochemistry of these zircons has direct application to understanding the origin and evolution of the rocks during the Hadean because: (i) U-Th-Pb age determinations by ion microprobe suggests the presence of crust as early as 4.37 Ga, or shortly after lunar formation; (ii) high-resolution U-Th-Pb zircon depth profiles reported here reveal several episodes of zircon growth in the Hadean previously unrecognized; (iii) core regions of pre-4.0 Ga zircons with igneous compositions are enriched in O-18 and contain metaluminous and peraluminous mineral inclusions, both features indicative of S-type grainitod protoliths. Study of these ancient zircons provides a unique window into the first half billion years that permits assessment of the potential of the Hadean Earth to host an emergent biosphere.

Trail, D.↗