| 1. |
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| 2. |
- Bellucci, Jeremy, 1984-, et al.
(author)
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A scanning ion imaging investigation into the micron-scale U-Pb systematics in a complex lunar zircon
- 2016
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In: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 438, s. 112-122
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Journal article (peer-reviewed)abstract
- The full U-Pb isotopic systematics in a complex lunar zircon ‘Pomegranate’ from lunar impact breccia 73235 have been investigated by the development of a novel Secondary Ion Mass Spectrometry (SIMS) scanning ion imaging (SII) technique. This technique offers at least a four-fold increase in analytical spatial resolution over traditional SIMS analyses in zircon. Results from this study confirm the hypothesis that the Pomegranate zircon crystallized at 4.302 ± 0.013 Ga and experienced an impact that formed, U-enriched zircon around primary zircon cores at 4.184 ± 0.007 Ga (2σ, all uncertainties). The increase in spatial resolution offered by this technique has facilitated targeting of primary zircon that was previously inaccessible to conventional spot analyses. This approach has yielded results indicating that individual grains with a diffusive distance of less than ~4 μm have been reset to the young impact age, while individual grains with a diffusive distance larger than ~6 μm have retained the old crystallization age. Assuming a broad range in cooling rate of 0.5–50 °C/year, which has been observed in a suite of similar lunar breccias, a maximum localized temperature generated by the impact that reset small prima- ry zircon and created new, high-U zircon is estimated to be between 1100 and 1280 °C.
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| 3. |
- Bellucci, Jeremy, et al.
(author)
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Direct Pb Isotopic Analysis of a Nuclear Fallout Debris Particle from the Trinity Nuclear Test
- 2017
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In: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 89, s. 1887-1891
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Journal article (peer-reviewed)abstract
- The Pb isotope composition of a nuclear fallout debris particle has been directly measured in post-detonation materials produced during the Trinity nuclear test by a secondary ion mass spectrometry (SIMS) scanning ion image technique (SII). This technique permits the visual assessment of the spatial distribution of Pb and can be used to obtain full Pb isotope compositions in user-defined regions in a 70 μm × 70 μm analytical window. In conjunction with backscattered electron (BSE) and energy-dispersive spectroscopy (EDS) mapping of the same particle, the Pb measured in this fallout particle cannot be from a major phase in the precursor arkosic sand. Similarly, the Pb isotope composition of the particle is resolvable from the surrounding glass at the 2σ uncertainty level (where σ represents the standard deviation). The Pb isotope composition measured in the particle here is in excellent agreement with that inferred from measurements of green and red trinitite, suggesting that these types of particles are responsible for the Pb isotope compositions measured in both trinitite glasses.
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| 4. |
- Bellucci, Jeremy, et al.
(author)
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Geochronology of Hadean zircon grains from the Jack Hills, Western Australia constrained by quantitative scanning ion imaging
- 2018
-
In: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 476, s. 469-480
-
Journal article (peer-reviewed)abstract
- Five Hadean (> 4 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale < 5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of < 10 μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9 Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population.
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| 5. |
- Bellucci, Jeremy, et al.
(author)
-
Pb evolution in the Martian mantle
- 2018
-
In: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 485, s. 79-87
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Journal article (peer-reviewed)abstract
- The initial Pb compositions of one enriched shergottite, one intermediate shergottite, two depleted shergottites, and Nakhla have been measured by Secondary Ion Mass Spectrometry (SIMS). These values, in addition to data from previous studies using an identical analytical method performed on three enriched shergottites, ALH 84001, and Chassigny, are used to construct a unified and internally consistent model for the differentiation history of the Martian mantle and crystallization ages for Martian meteorites. The differentiation history of the shergottites and Nakhla/Chassigny are fundamentally different, which is in agreement with short-lived radiogenic isotope systematics. The initial Pb compositions of Nakhla/Chassigny are best explained by the late addition of a Pb-enriched component with a primitive, non-radiogenic composition. In contrast, the Pb isotopic compositions of the shergottite group indicate a relatively simple evolutionary history of the Martian mantle that can be modeled based on recent results from the Sm–Nd system. The shergottites have been linked to a single mantle differentiation event at 4504 Ma. Thus, the shergottite Pb isotopic model here reflects a two-stage history 1) pre-silicate differentiation (4504 Ma) and 2) post-silicate differentiation to the age of eruption (as determined by concordant radiogenic isochron ages). The μ-values (238U/204Pb) obtained for these two different stages of Pb growth are μ1 of 1.8 and a range of μ2 from 1.4–4.7, respectively. The μ1-value of 1.8 is in broad agreement with enstatite and ordinary chondrites and that proposed for proto Earth, suggesting this is the initial μ-value for inner Solar System bodies. When plotted against other source radiogenic isotopic variables (Sri, γ187Os, ε143Nd, and ε176Hf), the second stage mantle evolution range in observed mantle μ-values display excellent linear correlations (r2 > 0.85) and represent a spectrum of Martian mantle mixing-end members (depleted, intermediate, enriched).
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| 6. |
- Bellucci, Jeremy
(author)
-
Pb-isotopic evidence for an early, enriched crust on Mars
- 2015
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In: Earth and Planetary Science Letters. - 0012-821X .- 1385-013X. ; 410, s. 34-41
-
Journal article (peer-reviewed)abstract
- Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surface measured by orbiters and landers. NWA 7533 contains monzonitic clasts that have zircon with U–Pb ages of 4.428 Ga. The Pb isotopic compositions of plagioclase and alkali feldspars, as well as U–Pb isotopic compositions of chlorapatitein the monzonitic clasts of NWA 7533 have been measured by Secondary Ion Mass Spectrometry (SIMS). The U–Pb isotopic compositions measured from the chlorapatitein NWA 7533 yield an age of 1.357 ±81Ga(2σ). The least radiogenic Pb isotopic compositions measured in plagioclase and K-feldspar lie within error of the 4.428 Ga Geochron. These data indicate that the monzonitic clasts in NWA 7533 are a product of a differentiation history that includes residence in areservoir that formed prior to 4.428 Ga with a μ-value (238U/204Pb) of at least 13.4 ±1.7 (2σ)and aκ-value (232Th/238U) of ∼4.3. This μ-value is more than three times higher than any other documented Martian reservoir. These results indicate either the Martian mantle is significantly more heterogeneous than previously thought (μ-value of 1–14 vs. 1–5) and/or the monzonitic clasts formed by the melting of Martian crust with a μ-value of at least 13.4. Therefore, NWA 7533 may contain the first isotopic evidence for an enriched, differentiated crust on Mars.
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| 7. |
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| 8. |
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| 9. |
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| 10. |
- Bouvier, Laura, et al.
(author)
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Evidence for extremely rapid magma ocean crystallization and crust formation on Mars
- 2018
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In: Nature. - : Springer Science and Business Media LLC. - 1476-4687 .- 0028-0836. ; 558, s. 586-589
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Journal article (peer-reviewed)abstract
- The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium-to-lead (U–Pb) isotopic decay system and is resistant to subsequent alteration. Moreover, given the high concentration of hafnium in zircon, the lutetium-to-hafnium (176Lu–176Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir (1,2,3) Ancient igneous zircons with crystallization ages of around 4,430 million years (Myr) have been reported in Martian meteorites that are believed to represent regolith breccias from the southern highlands of Mars (4,5) These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust4, thereby potentially providing insight into early crustal evolution on Mars. Here, we report concomitant high-precision U–Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 Martian regolith breccia. Seven zircons with mostly concordant U–Pb ages define 207Pb/206Pb dates ranging from 4,476.3 ± 0.9 Myr ago to 4,429.7 ± 1.0 Myr ago, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4,547 Myr ago. Thus, a primordial crust existed on Mars by this time and survived for around 100 Myr before it was reworked, possibly by impacts (4,5) to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars were completed less than 20 Myr after the formation of the Solar System. These timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns, resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust (6,7).
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| 11. |
- Cawood, Peter A., et al.
(author)
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Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences : The Moine Supergroup, Scottish Caledonides
- 2015
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In: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 127:3-4, s. 349-371
-
Journal article (peer-reviewed)abstract
- Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post–1000 Ma. U-Pb zircon ages for felsic and associated mafic intrusions confirm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of deformation and metamorphic pulses related to Knoydartian orogenesis of the host Moine rocks. Additional U-Pb zircon and monazite data, and 40Ar/39Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458–446 Ma and ca. 426 Ma during the Caledonian orogenic cycle.The presence of early Neoproterozoic siliciclastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America reflect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent plate-margin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession.
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| 12. |
- Ge, Rongfeng, et al.
(author)
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A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization
- 2018
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In: Geology. - : Geological Society of America. - 0091-7613 .- 1943-2682. ; 46:4, s. 303-306
-
Journal article (peer-reviewed)abstract
- Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486–4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.
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| 13. |
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| 14. |
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| 15. |
- Hewins, Rodger, et al.
(author)
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Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars
- 2016
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In: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; , s. 1-36
-
Journal article (peer-reviewed)abstract
- Northwest Africa 7533, a polymict Martian breccia, consists of fine-grained clast- laden melt particles and microcrystalline matrix. While both melt and matrix contain medium-grained noritic-monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast-laden melt rocks contain clump-like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene-plagioclase clump-aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic-monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned-pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr-bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti-bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni-poor pyroxene clasts which are from pristine rocks. Clast-laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ` 0.03 Ga (Humayun et al. 2013) and a 147Sm-143Nd isochron for NWA 7034 yielding 4.42 ` 0.07 Ga (Nyquist et al. 2016) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher D17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014). This record of protolith interaction with atmosphere- hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015), and formation of Ni-bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015).
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| 16. |
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| 17. |
- Kenny, Gavin, et al.
(author)
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Age of the Sääksjärvi impact structure, Finland: reconciling the timing of small impacts in crystalline basement with regional basin development
- 2020
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In: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 177, s. 1231-1243
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Journal article (peer-reviewed)abstract
- We report a new age for the Sääksjärvi impact structure, Finland, a 6 km diameter feature that formed in crystalline rocks of the Precambrian Baltic Shield. Two previous studies reported 40Ar/39Ar data for Sääksjärvi and suggested conflicting formation ages of ≤330 Ma or c. 560 Ma. The former age represents a possible complication for models which indicate that the region was covered by sediments of the Caledonian foreland basin throughout much of the Phanerozoic. We conducted a study combining imaging, microstructural analysis and U–Pb dating of shocked zircon from Sääksjärvi. The U–Pb dataset indicates a c. 600 Ma impact into predominantly c. 1850 Ma target rocks. A concordia age of 608 ± 8 Ma (2σ) confirms Sääksjärvi as the first known Ediacaran impact structure in the Baltic Shield and only the second worldwide. Our data indicate that the Sääksjärvi impact structure formed in exposed crystalline basement rocks of the Baltic Shield prior to the development of the Caledonian foreland basin. Given that most impact structures on Earth are relatively small features, radiometric dating of small impact structures in crystalline basement may place boundaries on the timing and spatial extent of palaeobasins that might otherwise be difficult to constrain.
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| 18. |
- Kenny, Gavin, et al.
(author)
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Recrystallization and chemical changes in apatite in response to hypervelocity impact
- 2020
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In: Geology. - 0091-7613 .- 1943-2682. ; 48:1, s. 19-23
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Journal article (peer-reviewed)abstract
- Despite the wide utility of apatite, Ca5(PO4)3(F,Cl,OH), in the geosciences, including tracing volatile abundances on the Moon and Mars, little is known about how the mineral responds to the extreme temperatures and pressures associated with hypervelocity impacts. To address this deficiency, we here present the first microstructural analysis and chemical mapping of shocked apatite from a terrestrial impact crater. Apatite grains from the Paasselkä impact structure, Finland, display intragrain crystal-plastic deformation as well as pervasive recrystallization—the first such report in terrestrial apatite. A partially recrystallized grain offers the opportunity to investigate the effect of shock recrystallization on the chemical composition of apatite. The recrystallized portion of the fluorapatite grain is depleted in Mg and Fe relative to the remnant non-recrystallized domain. Strikingly, the recrystallized region alone hosts inclusions of (Mg,Fe)2(PO4)F, wagnerite or a polymorph thereof. These are interpreted to be a product of phase separation during recrystallization and to be related to the reduced abundances of certain elements in the recrystallized domain. The shock-induced recrystallization of apatite, which we show to be related to changes in the mineral’s chemical composition, is not always readily visible in traditional imaging techniques (such as backscattered electron imaging of polished interior surfaces), thus highlighting the need for correlated microstructural, chemical, and isotopic studies of phosphates. This is particularly relevant for extraterrestrial phosphates that may have been exposed to impacts, and we urge the consideration of microstructural data in the interpretation of the primary or secondary nature of elemental abundances and isotopic compositions.
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| 19. |
- Kielman, Ross, 1991-, et al.
(author)
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A tonalitic analogue to ancient detrital zircon
- 2018
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In: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 499, s. 43-57
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Journal article (peer-reviewed)abstract
- A zircon population from an Archean tonalite sample from southern West Greenland has been used as a source analogue in order to test common methods and approaches applied to ancient detrital zircon populations. Measurements of U-Th-Pb, oxygen and Lu-Hf isotopes as well as rare earth element and Ti concentrations were made in these zircon crystals and, where possible, in multiple areas within a single grain. The population is dominated by oscillatory zoned cores aged 3.82 Ga with an isotopically and compositionally distinct rim that formed at 3.59 Ga. We demonstrate that multiple age components may be erroneously inferred from within these oscillatory zoned zircon cores, both from the total population and within individual grains. This has bearing on other zircon-hosted geochemical systems, as temporal correlations may be incorrectly assigned. Oxygen and Lu-Hf isotope compositions are relatively consistent through the population with only a small number of outliers. Ranges in rare earth element and Ti abundances are evident from the total population, from which apparent inverse cooling trends may be inferred. Additionally, we show that even with enhanced filtering of Ti concentrations using light rare earth element abundances, crystallisation temperatures derived from zircon grains of a single, hand sample sized rock can yield both wide and bimodal results. Since even simple, single “source rock” zircon populations may, without careful scrutiny, portray artificially complex results, particular care must be taken in the interpretation of complex ancient detrital zircon populations.
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| 20. |
- Kielman, Ross, 1991-
(author)
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Assessing the reliability of detrital zircon in Early-Earth provenance studies
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Our understanding of the Early Earth and the processes that have shaped its evolution have spawned predominantly from the geochemical and isotopic signatures of a small number of zircon populations around the world. Studies of trace element distributions, Hf and O isotope systematics as well as mineral inclusion chemistry in detrital zircon are combined with U-Pb chronology in order to constrain source rock characteristics. However, previous research has highlighted the potential for primary isotopic and geochemical signatures to be modified after deposition, enhanced by self-induced radiation damage and crystal-plastic deformation. Further complications arise when an unknown number of source rocks contribute to the detrital sediments, or when the source rocks are absent as is the case for Hadean detrital zircon. In this thesis, a range of analytical methods are applied to relatively uncharted ancient detrital zircon populations, in addition to a systematic investigation into the behaviour of titanium, rare earth elements, U-Th-Pb, Lu-Hf and O isotopes in a magmatic source-rock analogue to such ancient detrital zircon suites. Three localities are studied: Mt. Alfred, within the Yilgarn Craton of Western Australia (detrital study); the Saglek Block, the western-most extent of the North Atlantic Craton in northern Labrador (detrital study); and a locality south of Isua in southern West Greenland (analogue study). We have utilised a CAMECA ims 1280 Secondary Ion Mass Spectrometer (SIMS) for its high spatial resolution and small volume sampling, except for the Lu-Hf analyses which were carried out using a Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICP-MS).The main outcomes of this study are threefold. 1) Heavily discordant, ancient detrital zircon populations require extensive data filtering in order to produce reliable data for age comparison. Further, age estimations of pervasive Pb loss even in metamict detrital zircon may be achieved using intragrain discordia intercept ages. Applying this in a regional sense reveals that detrital zircon from Mt. Alfred, Western Australia have a distinct provenance in comparison to other metasedimentary units of the Youanmi Terrane, and bear resemblance to the Mt. Narryer metasediments of the Narryer Terrane. 2) Detrital zircon from metasedimentary rocks exposed to high grade metamorphism in the Saglek Block, Northern Labrador yield predominantly Mesoarchaean age signatures, along with a minor Eoarchaean aged component. Lu-Hf isotope data from these zircon reveal up to five near-chondritic populations. U-Pb-Hf data from two samples of metapelite (L1407 and L1408) suggest that a previous re-assignment of deposition age for this unit to >3.95 Ga is unsubstantiated, undermining later studies based on that interpretation. 3) Geochemical complexities in zircon from an Eoarchaean meta-tonalite, taken as a source analogue to ancient detrital zircon, challenge the assumptions and interpretations drawn from detrital zircon studies. Lu-Hf and O isotope systems display mostly homogenous compositions, despite recognised U-Pb disturbance. Petrogenetic trace element proxies such as REE and Ti yield heterogeneous results, even within individual grains. Discerning magmatic signals from detrital zircon populations can therefore, without careful scrutiny, portray artificially complex results and consequently, lead to false interpretations.
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| 21. |
- Kielman, Ross, 1991-, et al.
(author)
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U-Pb age distribution recorded in zircons from Archean quartzites in the Mt. Alfred area, Yilgarn Craton, Western Australia
- 2018
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In: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 310, s. 278-290
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Journal article (peer-reviewed)abstract
- The U-Th-Pb isotopic data from detrital zircon grains from five samples of Archean quartzite from the Mt. Alfred area of the Illaara greenstone belt in the Yilgarn Craton of Western Australia are presented in this study. The zircon grains are typically fractured and contain both irregular and oscillatory zoned internal structures as revealed by cathodoluminescence imaging. Concordant 207Pb/206Pb ages range between 3109 ± 17 and 3918 ± 16 Ma (2σ), with three main age peaks at ca. 3640, 3690 and 3760 Ma. Older 207Pb/206Pb ages up to 4067 ± 5 Ma are strongly affected by at least one recent disturbance event, however one single-grain discordia yields an upper intercept age of 4107 ± 12 (MSWD = 1.2). A further sixteen zircon grains with multiple analyses define discordia that suggest U-Pb disturbance events in the Neoarchean and the Mesozoic, the latter as a result of invasive low temperature weathering solutions. The notable lack of grains with ages less than ∼3.6 Ga in the Mt. Alfred detrital zircon population differentiates it from other quartzite samples from both the Illaara Formation and the Eoarchean zircon-bearing metasedimentary rocks of the Narryer Terrane. Also, the limited spread of zircon ages between 3640 and 3760 Ma suggests a relatively uniform and possibly local source region. However, no rocks of this age have been found in the Youanmi Terrane. This implies either the distal transport of similarly aged clastic sediments at 3.1 Ga from the Narryer Gneiss Complex (NGC) to the Mt. Alfred area, or the previous existence of NGC-like rocks near the Illaara greenstone belt that are either not currently recognised or have since been destroyed.
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| 22. |
- Kielman, Ross, 1991-
(author)
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U-Pb dating of detrital zircon from the Mt. Alfred area, Yilgarn Craton, Western Australia
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- Hadean and Paleo-Eoarchean aged detrital zircon provide a rare glimpse into the nature of the early Earth. Thus, the characterisation of new localities with rocks that host Hadean and Paleo-Eoarchean zircon provide invaluable insight into the Earth’s early crust. We report high spatial resolution SIMS U-Th-Pb data in detrital zircon from five samples of the Mt. Alfred Archean quartzite located in the Illaara Greenstone Belt, Yilgarn Craton in Western Australia. To date, only two studies have published detrital age data for this locality (Wyche et al., 2004; Thern and Nelson, 2012). A total of 487 spots in 450 zircon grains have been analysed for U-Th-Pb and the internal structure of the grains has been studied by cathodoluminescence imaging. Concordant 207Pb/206Pb ages range between 3109 ± 17 and 3918 ± 16 Ma (2σ), with three main age peaks at ca. 3640, 3690 and 3760 Ma. 207Pb/206Pb ages of up to 4067 ± 5 Ma were found, but these were highly discordant. At least two Pb-loss events are evident during the Neoarchean and Permian-Triassic. The Neoarchean event is implied from highly metamict zircon cores producing 207Pb/206Pb ages ca. 2.7-2.8 Ga, younger than a crosscutting quartz-tourmaline vein formed at 2.94 Ga. Single-grain discordia trends define the more recent Pb-loss event, with eight grains producing a weighted average lower intercept age of 224 ± 35 Ma (2σ).We propose that the similarity of ages between the Mt. Alfred detrital zircon age distribution and the Meeberrie gneiss of the Narryer Gneiss Complex (NGC) imply the distal transport of these sediments over ca. 300 km, or the previous existence of NGC-like rocks near the Illaara greenstone belt. The noticeable lack of detrital zircon aged ca. 3.34 Ga as observed in all other detrital populations of the Illaara Formation suggests a different provenance for the Mt. Alfred quartzites.
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| 23. |
- Long, Tao, et al.
(author)
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Constraining the formation and transport of lunar impact glasses using the ages and chemical compositions of Chang’e-5 glass beads
- 2022
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:39
-
Journal article (peer-reviewed)abstract
- Impact glasses found in lunar soils provide a possible window into the impact history of the inner solar system. However, their use for precise reconstruction of this history is limited by an incomplete understanding of the physical mechanisms responsible for their origin and distribution and possible relationships to local and regional geology. Here, we report U-Pb isotopic dates and chemical compositions of impact glasses from the Chang’e-5 soil and quantitative models of impact melt formation and ejection that account for the compositions of these glasses. The predominantly local provenance indicated by their compositions, which constrains transport distances to <~150 kilometers, and the age-frequency distribution are consistent with formation mainly in impact craters 1 to 5 kilometers in diameter. Based on geological mapping and impact cratering theory, we tentatively identify specific craters on the basaltic unit sampled by Chang’e-5 that may have produced these glasses and compare their ages with the impact record of the asteroid belt.
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| 24. |
- Lorand, Jean-Pierre, et al.
(author)
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The sulfur budget and sulfur isotopic composition of Martian regolith breccia NWA 7533
- 2020
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In: Meteoritics and Planetary Science. - : John Wiley & Sons, Ltd. - 1086-9379 .- 1945-5100. ; 55:9, s. 2097-2116
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Journal article (peer-reviewed)abstract
- The sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 ± 50 ppm S in agreement with LA-ICP-MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite-pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero delta-33S (-0.029 ± 0.010) signal is clearly resolved at the 2σ level in the bulk rock analyses, coupled with negative CDT-normalized δ34S (-2.54 ± 0.10 permil) and near-zero delta-36S (0.002 ± 0.09 permil). In situ analyses also yield negative delta-33S values (-0.05 to -0.30 permil) with only a few positive delta-33S up to +0.38 permil. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass-independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies - magmatic intrusions or impact-induced heat - is presently unclear. However, in situ analyses also gave negative δ34S values (+1 to -5.8 permil). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H2S/HS- aqueous fluid produced via open-system thermochemical reduction of sulfates at high temperatures (>300 °C).
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| 25. |
- Merle, Renaud E., et al.
(author)
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Pb Isotope Signature of a Low-Ό (238U/204Pb) Lunar Mantle Component
- 2024
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In: Journal of Petrology. - : Oxford University Press. - 0022-3530 .- 1460-2415. ; 65:6
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Journal article (peer-reviewed)abstract
- The chemical and isotopic characteristics of terrestrial basalts are constrained within the concept of mantle chemical geodynamics that explains the existing variety of basaltic rocks within a framework of several end-member reservoirs in Earth's mantle. In contrast, there is no comparable fully developed model explaining the isotopic composition of lunar basaltic rocks, in part owing to the lack of well-constrained age-isotope relationships in different groups of basalts identified on the Moon. Notably, the absence of agreement upon ages includes basalts from a unique group of meteorites collectively known as 'YAMM' (basalts Yamato-793169: Y-793169, Asuka-881757: A-881757, Miller Range 05035: MIL 05035 and regolith breccia Meteorite Hill 01210: MET 01210), which appear to show chemical signatures different from all other known lunar basaltic rocks. We present high-precision Pb-Pb ages and initial Pb isotopic ratios for two samples from this group, MIL 05035 and A-881757. These meteorites have Pb isotope ratios different from those of the other lunar basalts, suggesting they are derived from a distinct and depleted mantle source, with a U-238/Pb-204 ratio (mu value) lower than any other mantle source. Their depletion in rare earth elements, in conjunction with recalculated initial Nd and Sr isotopic ratios from published data and using our new age, appear to support this conclusion. The chemical and Sr-Nd-Pb isotopic characteristics of this low-mu source appear to be the opposite of those of the KREEP reservoir and many, if not all, features described in other lunar basalts (such as low- and high-Ti mare basalts) can be explained by a binary mixing of material derived from low-mu and KREEP-like reservoirs. This mixing might be the result of a slow, convection-like mantle overturn.
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| 26. |
- Merle, Renaud E., 1976-, et al.
(author)
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Sr, Nd, Pb and Os Isotope Systematics of CAMP Tholeiites from Eastern North America (ENA) : Evidence of a Subduction-enriched Mantle Source
- 2013
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In: Journal of Petrology. - : Oxford University Press (OUP). - 0022-3530 .- 1460-2415. ; 55:1, s. 133-180
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Journal article (peer-reviewed)abstract
- The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 107 km2. Here we document the geochemical characteristics of CAMP basalts from Triassic–Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr–Nd–Pb initial (t = 201 Ma) isotopic compositions (206Pb/204Pbini. = 18·155–18·691, 207Pb/204Pbini. = 15·616–15·668, 208Pb/204Pbini. = 38·160–38·616, 143Nd/144Ndini. = 0·512169–0·512499). On the basis of stratigraphy, rare earth element (REE) chemistry and Sr–Nd–Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206Pb/204Pbini. >18·5 and 143Nd/144Ndini. > 0·51238, comprises all the lastest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206Pb/204Pbini. > 18·5 and 0·51233 > 143Nd/144Ndini. > 0·51225, comprises the intermediate units. The Orange Mountain group has the highest La/Yb ratios and 143Nd/144Ndini. < 0·51235 and involves all the earliest and stratigraphically lowest units, including the entire North Mountain basalts from Nova Scotia. In this last group, three sub-groups may be distinguished: the Rapidan sill, which has 206Pb/204Pbini. higher than 18·5, the Shelburne sub-group, which has 143Nd/144Ndini. < 0·51225, and the remaining Orange Mt samples. With the exception of one sample, the Eastern North America (ENA) CAMP basalts display initial 187Os/188Os ratios in the range of mantle-derived magmas (<0·15). Simple modelling shows that the composition of the ENA CAMP basalts cannot plausibly be explained solely by crustal contamination of oceanic island basalt (OIB), mid-ocean ridge basalt (MORB) or oceanic plateau basalt (OPB) magmas. Mixing of such magma compositions with sub-continental lithospheric mantle (SCLM)-derived melts followed by crustal contamination, by either assimilation–fractional crystallization (AFC) or assimilation through turbulent ascent (ATA) processes is somewhat more successful. However, this latter scenario does not reproduce the REE and isotopic composition of the ENA CAMP in a fully satisfactory manner. Alternatively, we propose a model in which asthenospheric mantle overlying a subducted slab (i.e. mantle wedge) was enriched during Cambrian to Devonian subduction by sedimentary material, isotopically equivalent to Proterozoic–Lower Paleozoic crustal rocks. Subsequently, after subduction ceased, the isotopic composition of this mantle evolved by radioactive decay for another 170 Myr until the CAMP magmatic event. Varying amounts and compositions of the incorporated sedimentary component coupled with radiogenic ingrowth over time can account for the main geochemical characteristics of the ENA CAMP (enriched incompatible element patterns, negative Nb anomalies, enriched Sr–Nd–Pb isotopic composition) and the differences between the three chemical groups.
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| 27. |
- Merle, Renaud, et al.
(author)
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Origin and transportation history of lunar breccia 14311
- 2017
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In: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 52:5, s. 842-858
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Journal article (peer-reviewed)abstract
- In this paper, we compare the U-Pb zircon age distribution pattern of sample 14311 from the Apollo 14 landing site with those from other breccias collected at the same landing site. Zircons in breccia 14311 show major age peaks at 4340 and 4240 Ma and small peaks at 4110, 4030, and 3960 Ma. The zircon age patterns of breccia 14311 and other Apollo 14 breccias are statistically different suggesting a separate provenance and transportation history for these breccias. This interpretation is supported by different U-Pb Ca-phosphate and exposure ages for breccia 14311 (Ca-phosphate age: 3938 ± 4 Ma, exposure age: ~550–660 Ma) from the other Apollo 14 breccias (Ca-phosphate age: 3927 ± 2 Ma, compatible with the Imbrium impact, exposure age: ~25–30 Ma). Based on these observations, we consider two hypotheses for the origin and transportation history of sample 14311. (1) Breccia 14311 was formed in the Procellarum KREEP terrane by a 3938 Ma-old impact and deposited near the future site of the Imbrium basin. The breccia was integrated into the Fra Mauro Formation during the deposition of the Imbrium impact ejecta at 3927 Ma. The zircons were annealed by mare basalt flooding at 3400 Ma at Apollo 14 landing site. Eventually, at approximately 660 Ma, a small and local impact event excavated this sample and it has been at the surface of the Moon since this time. (2) Breccia 14311 was formed by a 3938 Ma-old impact. The location of the sample is not known at that time but at 3400 Ma, it was located nearby or buried by hot basaltic flows. It was transported from where it was deposited to the Apollo 14 landing site by an impact at approximately 660 Ma, possibly related to the formation of the Copernicus crater and has remained at the surface of the Moon since this event. This latter hypothesis is the simplest scenario for the formation and transportation history of the 14311 breccia.
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| 28. |
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| 29. |
- Nemchin, Alexander, et al.
(author)
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Pb-Pb ages of feldspathic clasts in two Apollo 14 breccia samples
- 2017
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 217, s. 441-461
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Journal article (peer-reviewed)abstract
- Pb-Pb isochron ages of ca. 3.92 Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determinedusing Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb systemin the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record asignificantly older crystallization age (ca. 4.33–4.35 Ga) for the rock represented by that clast. Initial Pb compositions determinedfor the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from severalApollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9 Ga Fra Mauro formation at the Apollo14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources ofthose rocks, evolved with a high 238U/204Pb (µ-value) for substantial periods of time, although the precise crystallization agesof the rocks represented by at least two of the clasts investigated here are unknown.
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| 30. |
- Shumlyanskyy, Leonid, et al.
(author)
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Eoarchean rock association in the Dniester-Bouh Domain of the Ukrainian Shield: A suite of LILE-depleted enderbites and mafic granulites
- 2020
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In: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433.
-
Journal article (peer-reviewed)abstract
- We present the results of a study of an Eoarchean rock assemblage in the Dniester-Bouh Domain of the Ukrainian Shield. This comprises granulite-facies granitoids intercalated with mafic and ultramafic granulites. Zircon U-Pb geochronology indicates enderbite crystallisation at 3786 ± 32 Ma, followed by a subsequent event at ca. 3500 Ma. Several events can be tentatively identified that affected these rocks between ca. 3000 and 2700 Ma. The last zircon growth event took place in response to granulite facies metamorphism and included two separate episodes at ca. 2000 and ca. 1900 Ma. The oldest two zircon populations in enderbites have εHf values around 0, indicating their crystallisation from a protolith with a short crustal residence time. Zircons that crystallised during the 3000–2700 Ma event(s) vary in Hf isotope systematics from εHf ~ 1 at ca. 3000 Ma to εHf ~ −14 at c. 2700 Ma. Paleoproterozoic zircons reveal even more significant variations in εHf value from +6 to –22. Such variations are indicative of juvenile input and mixing with old non-radiogenic Hf.All Eoarchean rocks are depleted in incompatible trace elements and have negative Ta-Nb, P, and Ti anomalies. Compared to the typical TTG associations, enderbites record depletion in felsic components (SiO2, Na2O, K2O, Rb, Th), and enrichment in mafic ones (TiO2, MgO, CaO, V), allowing them to be defined as “mafic” or “depleted” TTG.Geochemical data indicate that mafic and ultramafic rocks of the Dniester-Bouh Domain formed by shallow high-degree melting of the mantle, with the absence of garnet in their source, and the presence of residual Ti-bearing minerals and/or amphibole. In contrast, enderbites were formed from a mixed garnet-bearing amphibolite – eclogite source, i.e. melting over a range of pressures/depths. Our preferred model for the formation of the Eoarchean rock association involves the shallow melting of mantle and formation of basalts and accompanying ultramafic cumulates at a spreading centre, with subsequent underthrusting of one segment of oceanic crust beneath the other, and partial melting of hydrated metamorphosed (eclogitized) mafic rocks in the underthrust plate, leading to the formation of the TTG melts
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| 31. |
- Snape, Joshua, et al.
(author)
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Ancient volcanism on the Moon: Insights from Pb isotopes in the MIL 13317 and Kalahari 009 lunar meteorites
- 2018
-
In: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 502, s. 84-95
-
Journal article (peer-reviewed)abstract
- Lunar meteorites provide a potential opportunity to expand the study of ancient (>4000 Ma) basaltic volcanism on the Moon, of which there are only a few examples in the Apollo sample collection. Secondary Ion Mass Spectrometry (SIMS) was used to determine the Pb isotopic compositions of multiple mineral phases (Ca-phosphates, baddeleyite K-feldspar, K-rich glass and plagioclase) in two lunar meteorites, Miller Range (MIL) 13317 and Kalahari (Kal) 009. These data were used to calculate crystallisation ages of 4332 ± 2 Ma (95% confidence level) for basaltic clasts in MIL 13317, and 4369 ± 7 Ma (95% confidence level) for the monomict basaltic breccia Kal 009. From the analyses of the MIL 13317 basaltic clasts, it was possible to determine an initial Pb isotopic composition of the protolith from which the clasts originated, and infer a 238 U/204 Pb ratio (μ-value) of 850 ± 130 (2σ uncertainty) for the magmatic source of this basalt. This is lower than μ-values determined previously for KREEP-rich (an acronym for K, Rare Earth Elements and P) basalts, although analyses of other lithological components in the meteorite suggest the presence of a KREEP component in the regolith from which the breccia was formed and, therefore, a more probable origin for the meteorite on the lunar nearside. It was not possible to determine a similar initial Pb isotopic composition from the Kal 009 data, but previous studies of the meteorite have highlighted the very low concentrations of incompatible trace elements and proposed an origin on the farside of the Moon. Taken together, the data from these two meteorites provide more compelling evidence for widespread ancient volcanism on the Moon. Furthermore, the compositional differences between the basaltic materials in the meteorites provide evidence that this volcanism was not an isolated or localised occurrence, but happened in multiple locations on the Moon and at distinct times. In light of previous studies into early lunar magmatic evolution, these data also imply that basaltic volcanism commenced almost immediately after Lunar Magma Ocean (LMO) crystallisation, as defined by Nd, Hf and Pb model ages at about 4370 Ma.
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| 32. |
- Snape, Joshua, et al.
(author)
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Constraining the timing and sources of volcanism at the Apollo 12 landing site using new Pb isotopic compositions and crystallisation ages
- 2018
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In: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 482, s. 101-112
-
Journal article (peer-reviewed)abstract
- The basaltic suites collected at the Apollo 12 landing site have been interpreted as representing a stratigraphic sequence of volcanic flows emplaced in the Oceanus Procellarum region between approximately 3100-3300 Ma. This study presents Secondary Ion Mass Spectrometry (SIMS) Pb isotopic analyses of samples from each of the basaltic suites, which have been used to constrain precise crystallisation ages and initial Pb isotopic compositions. The new crystallisation ages are consistent with the three main basaltic suites (olivine, pigeonite and ilmenite) being emplaced over a period of approximately 60 million years, and the improved precision of these ages has made it possible to reinterpret the stratigraphic sequence of basalt flows underlying the Apollo 12 landing site. Contrary to previous studies, the three ilmenite basalts are determined as having the oldest ages (with a weighted average of 3187 +/- 6 Ma; 2 sigma) and are, therefore, interpreted as representing the lowest unit in the sequence, underlying the olivine and pigeonite basalts (with an age range constrained by the oldest and youngest pigeonite basalts; 3176 +/- 6 Ma and 3129 +/- 10 Ma; 2s). The initial Pb isotopic compositions have been compared with recalculated initial Sr and Nd isotopic compositions, and are consistent with the three main basaltic suites originating from magmatic sources that incorporated different proportions of a common primitive mafic cumulate and the residual trapped liquid fraction remaining after a majority of the lunar magma ocean had crystallised. Our data also demonstrate that the feldspathic basalt (12038) is unique, both in terms of its crystallisation age (3242 +/- 13 Ma) and its derivation from a distinct mantle reservoir.
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| 33. |
- Snape, Joshua F., et al.
(author)
-
The timing of basaltic volcanism at the Apollo landing sites
- 2019
-
In: Geochimica et Cosmochimica Acta. - 0016-7037 .- 1872-9533. ; 266, s. 29-53
-
Journal article (peer-reviewed)abstract
- Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites.
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| 34. |
- Snape, Joshua, 1986-, et al.
(author)
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Lunar basalt chronology, mantle differentiation and implications for determining the age of the Moon
- 2016
-
In: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 451, s. 149-158
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Journal article (peer-reviewed)abstract
- Despite more than 40 years of studying Apollo samples, the age and early evolution of the Moon remain contentious. Following the formation of the Moon in the aftermath of a giant impact, the resulting Lunar Magma Ocean (LMO) is predicted to have generated major geochemically distinct silicate reservoirs, including the sources of lunar basalts. Samples of these basalts, therefore, provide a unique opportunity to characterize these reservoirs. However, the precise timing and extent of geochemical fractionation is poorly constrained, not least due to the difficulty in determining accurate ages and initial Pb isotopic compositions of lunar basalts. Application of an in situ ion microprobe approach to Pb isotope analysis has allowed us to obtain precise crystallization ages from six lunar basalts, typically with an uncertainty of about +/- 10 Ma, as well as constrain their initial Pb-isotopic compositions. This has enabled construction of a two-stage model for the Pb-isotopic evolution of lunar silicate reservoirs, which necessitates the prolonged existence of high-mu reservoirs in order to explain the very radiogenic compositions of the samples. Further, once firm constraints on U and Pb partitioning behaviour are established, this model has the potential to help distinguish between conflicting estimates for the age of the Moon. Nonetheless, we are able to constrain the timing of a lunar mantle reservoir differentiation event at 4376 +/- 18 Ma, which is consistent with that derived from the Sm-Nd and Lu-Hf isotopic systems, and is interpreted as an average estimate of the time at which the high-mu, urKREEP reservoir was established and the Ferroan Anorthosite (FAN) suite was formed.
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| 35. |
- Snape, Joshua, 1986-, et al.
(author)
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Pb isotopes in the impact melt breccia 66095: Association with the Imbrium basin and the isotopic composition of lithologies at the Apollo 16 landing site
- 2017
-
In: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 466, s. 608-616
-
Journal article (peer-reviewed)abstract
- Recent in situ Secondary Ion Mass Spectrometry (SIMS) Pb isotope analyses of lunar basalts have provided precise crystallisation ages and initial Pb isotopic compositions for these samples. In this study, the same approach has been tested in the Apollo 16 impact melt breccia 66095, referred to as the “Rusty Rock” due to its enrichments in volatile elements, including Pb. Based on these analyses of the breccia, a Pb-Pb isochron age of 3909±17 Ma (at the 95% confidence level) and an initial Pb composition for 66095 have been determined. This age is interpreted as representing the time of breccia formation that, when combined with recent studies of lunar breccias, can be linked to the Imbrium basin forming impact. The directly measured initial Pb composition of the breccia from this work is similar a modelled compositions presented previously, and likely reflects an average value for the lithologies present at the Apollo 16 landing site at the time that the Imbrium ejecta was emplaced. The 66095 initial Pb isotopic composition is compared with the compositions in other lunar samples and the nature of the endmember lithologies in this mixture has been discussed within the framework of a multiple stage model of Pb isotope evolution on the Moon. This study demonstrates the effectiveness of this technique beyond its application in crystalline basalts, opening up the possibility of obtaining precise geochronological and Pb isotopic compositions from a broader sample set than was previously recognised.
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| 36. |
- Snape, Joshua, 1986-, et al.
(author)
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Phosphate ages in Apollo 14 breccias: Resolving multiple impact events with high precision U-Pb SIMS analyses
- 2016
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 174, s. 13-29
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Journal article (peer-reviewed)abstract
- The U-Pb systems of apatite and merrillite grains within four separate Apollo 14 impact melt breccia samples were analysed by secondary ion mass spectrometry. No systematic difference was identified between the 207Pb/206Pb ages of the apatites and merrillites. A combined 207Pb/206Pb age of 3927±2 Ma (95% conf.) is determined for three of these samples (14305,103: 3926±4 Ma; 14306,150: 3926±6 Ma; 14314,13: 3929±4 Ma). By combining these data with the ages previously obtained for zircons in Apollo 12 impact melt breccia fragments and the lunar meteorite SaU 169, a weighted average age of 3926±2 Ma (95% conf.) is obtained, which is attributed to the formation of the Imbrium basin. An age of 3943±5 Ma is determined for the fourth breccia (14321,134), which is similar to ages of 3946±15 Ma and 3958±19 Ma, obtained from several older phosphates in 14305,103 and 14314,13. The weighted average of these three older ages is 3944±4 Ma (95% conf.). This is indistinguishable to the age (3938±4 Ma; 2σ) obtained for a different Apollo 14 impact melt breccia in a previous study. After investigating likely sources for this older ~3940 Ma age, we conclude that the Humorum or Serenitatis basin forming events are likely candidates. The potential identification of two large impact events within ~15 Myrs has important implications for the rate of lunar bombardment around 3.95-3.92 Ga. This study demonstrates the importance of high-precision age determinations for interpreting the impact record of the Moon, as documented in lunar samples.
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| 37. |
- Thiessen, Fiona, et al.
(author)
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Impact history of the Apollo 17 landing site revealed by U-Pb SIMS ages
- 2017
-
In: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 52:4, s. 584-611
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Journal article (peer-reviewed)abstract
- Secondary ion mass spectrometry (SIMS) U-Pb ages of Ca-phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920±3 Ma, 3922±5 Ma and 3930±5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average 207Pb/206Pb age of 3930±5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca-phosphates yielding an age of 3922±5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U-Pb system preserved in some phosphate grains. Based on the currently available dataset this possibility cannot be excluded.
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| 38. |
- Thiessen, Fiona Karen, 1984-
(author)
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The evolution of lunar breccias : U-Pb geochronology of Ca-phosphates and zircon using Secondary Ion Mass Spectrometry
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Planetary bodies in our Solar System, including the Moon, were exposed to an intense asteroid bombardment between ~4.5-3.8 Ga, shaping their surfaces and leaving visible “footprints” in the form of large impact basins. The end of this period (~4.0-3.85 Ga), might have been marked by a cataclysmic increase in impacts, the so-called Late Heavy Bombardment (LHB), although this remains highly contentious. Since destructive processes, such as tectonics or erosion, have destroyed ancient (> 3.0 Ga) impact structures on Earth, studies of the early Solar System are mainly restricted to lunar samples, because impact structures are much better preserved on the Moon.In this thesis, we have therefore analysed impact breccias from three Apollo landing sites (Apollo 12, 14, and 17) with the overall aim to gain a better understanding of the lunar impact history. This endeavour included comprehensive textural and petrological analyses of the breccias and grains of interest (i.e. Ca-phosphates and zircon), as well as obtaining precise U-Pb Secondary Ion Mass Spectrometry (SIMS) ages. The U-Pb ages of Ca-phosphates obtained are consistent with the age of the Imbrium impact at ~3925 Ma, whereas an older age of ~3930 Ma yielded by Ca-phosphates in an Apollo 17 breccia might be linked to the formation of the Serenitatis basin. Furthermore, an impact event at ~3940 Ma was identified in zircon grains in Apollo 14 breccias, which is in agreement with older Ca-phosphate ages yielded in a previous study. The identification of three possible impact events within ~15 myr has important implications for the lunar bombardment history.However, there is a possibility that partial Pb loss from older grains during a relatively late event (e.g. Imbrium) might result in apparently older ages in Ca-phosphates. Incomplete resetting of the U-Pb system was recorded in zircon grains in an Apollo 12 breccia, leading to meaningless U-Pb ages which cannot be interpreted unambiguously as either magmatic or as impact events. Nevertheless, the U-Pb ages of several zircon grains occurring in lithic clasts in Apollo 14 breccias can plausibly be linked to magmatic activity, exhibiting several magmatic events between ~4286 Ma and ~4146 Ma. The data obtained in this thesis, together with previously published zircon and Ca-phosphate data, indicate several spikes in the magmatic and impact history during the first ~600 myr of lunar history. This study highlights the importance of combining high-precision age determination with thorough petrological and textural analyses in order to exclude meaningless ages and to interpret the impact and magmatic history of the Moon.
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Weis, Franz, et al.
(author)
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Water content in the Martian mantle : A Nakhla perspective
- 2017
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 212, s. 84-98
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Journal article (peer-reviewed)abstract
- Water contents of the Martian mantle have previously been investigated using Martian meteorites, with several comprehensive studies estimating the water content in the parental melts and mantle source regions of the shergottites and Chassigny. However, no detailed studies have been performed on the Nakhla meteorite. One possible way to determine the water content of a crystallizing melt is to use the water content in nominally anhydrous minerals (NAMs) such as clinopyroxene and olivine. During or after eruption on the surface of a planetary body and during residence in a degassing magma, these minerals may dehydrate. By reversing this process experimentally, original (pre-dehydration) water concentrations can be quantified. In this study, hydrothermal rehydration experiments were performed at 2 kbar and 700 degrees C on potentially dehydrated Nakhla clinopyroxene crystals. Rehydrated clinopyroxene crystals exhibit water contents of 130 +/- 26 (2 sigma) ppm and are thus similar to values observed in similar phenocrysts from terrestrial basalts. Utilizing clinopyroxene/melt partition coefficients, both the water content of the Nakhla parent melt and mantle source region were estimated. Despite previous assumptions of a relatively dry melt, the basaltic magma crystallizing Nakhla may have had up to 1.42 +/- 0.28 (2 sigma) wt.% H2O. Based on an assumed low degree of partial melting, this estimate can be used to calculate a minimum estimate of the water content for Nakhlas mantle source region of 72 +/- 16 ppm. Combining this value with values determined for other SNC mantle sources, by alternative methods, gives an average mantle value of 102 +/- 9 (2 sigma) ppm H2O for the Martian upper mantle throughout geologic time. This value is lower than the bulk water content of Earths upper mantle (similar to 250 ppm H2O) but similar to Earths MORB source (54330 ppm, average similar to 130 ppm H2O).
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