| 1. |
- Bellucci, J. J., et al.
(författare)
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A Pb isotopic resolution to the Martian meteorite age paradox
- 2016
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 433, s. 241-248
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Tidskriftsartikel (refereegranskat)abstract
- Determining the chronology and quantifying various geochemical reservoirs on planetary bodies is fundamental to understanding planetary accretion, differentiation, and global mass transfer. The Pb isotope compositions of individual minerals in the Martian meteorite Chassigny have been measured by Secondary Ion Mass Spectrometry (SIMS). These measurements indicate that Chassigny has mixed with a Martian reservoir that evolved with a long-term U-238/Pb-204 (mu) value similar to two times higher than those inferred from studies of all other Martian meteorites except 4.428 Ga clasts in NWA7533. Any significant mixing between this and an unradiogenic reservoir produces ambiguous trends in Pb isotope variation diagrams. The trend defined by our new Chassigny data can be used to calculate a crystallization age for Chassigny of 4.526 +/- 0.027 Ga (2 sigma) that is clearly in error as it conflicts with all other isotope systems, which yield a widely accepted age of 1.39 Ga. Similar, trends have also been observed in the Shergottites and have been used to calculate a >4 Ga age or, alternatively, attributed to terrestrial contamination. Our new Chassigny data, however, argue that the radiogenic component is Martian, mixing occurred on the surface of Mars, and is therefore likely present in virtually every Martian meteorite. The presence of this radiogenic reservoir on Mars resolves the paradox between Pb isotope data and all other radiogenic isotope systems in Martian meteorites. Importantly, Chassigny and the Shergottites are likely derived from the northern hemisphere of Mars, while NWA 7533 originated from the Southern hemisphere, implying that the U-rich reservoir, which most likely represents some form of crust, must be widespread. The significant age difference between SNC meteorites and NWA 7533 is also consistent with an absence of tectonic recycling throughout Martian history.
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| 2. |
- Bellucci, J. J., et al.
(författare)
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Tracing martian surface interactions with the triple O isotope compositions of meteoritic phosphates
- 2020
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 531
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Tidskriftsartikel (refereegranskat)abstract
- The triple oxygen isotope compositions of phosphate grains in six martian meteorites have been measured by Secondary Ion Mass Spectrometry (SIMS) and combined together with their chlorine isotope and halogen concentrations have been used to constrain hydrosphere-lithosphere interactions on Mars. These samples include three enriched shergottites (Zagami, Roberts Massif 04262 and Larkman Nunatak 12011), one depleted shergottite (Tissint), an orthopyroxenite (Allan Hills 84001), and a regolith breccia (Northwest Africa 7533). The phosphates measured here have a range in δ18O [(18O/16O)sample/(18O/16O)Standard-1] × 103] from +1.0 to +6.8‰ and could be a result of indigenous mantle values, mixing with martian water, or replacement reactions taking place on the surface of Mars. Three samples have a Δ17O [δ17O-1000(1 + δ18O /1000)0.528-1] in equilibrium with the martian mantle (ALH 84001, Tissint, and Zagami), while three samples (LAR 12011, RBT 04262, and NWA 7533) have an elevated positive Δ17O outside of analytical uncertainty of the martian fractionation line (MFL). The phosphates in the latter group also have positive and negative δ37Cl [(37Cl/35Cl)sample/(37Cl/35Cl)standard – 1] × 103] and enrichments in halogens not seen in the rest of the sample suite. Perchlorate formation on Earth fractionates Cl in both positive and negative directions and generates a correlated positive Δ17O. Further, perchlorate has been detected in wt% amounts on the martian surface. Thus, these results strongly suggest the presence of multiple Cl isotope reservoirs on the martian surface that have interacted with the samples studied here over the last ca. 2 Ga of geologic time. The weighted average of Δ17O measurements from phosphate grains (n = 13) in NWA 7533, which are the explicit result of exchange reactions on the martian surface, yields a statistically robust mean value of 1.39 ± 0.19‰ (2σ, MSWD = 1.5, p = 0.13). This value likely represents an accurate estimate for an oxidized surface reservoir on Mars.
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| 3. |
- Cawood, Peter A., et al.
(författare)
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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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Ingår i: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 127:3-4, s. 349-371
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Cawood, P. A., et al.
(författare)
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Orogenesis without collision : Stabilizing the Terra Australis accretionary orogen, eastern Australia
- 2011
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Ingår i: Geological Society of America Bulletin. - 0016-7606 .- 1943-2674. ; 123:11-12, s. 2240-2255
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Tidskriftsartikel (refereegranskat)abstract
- The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis.At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting.The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis.Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.
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| 5. |
- Joy, K. H., et al.
(författare)
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Timing of geological events in the lunar highlands recorded in shocked zircon-bearing clasts from Apollo 16
- 2020
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Ingår i: Royal Society Open Science. - : Royal Society. - 2054-5703. ; 7:6
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Tidskriftsartikel (refereegranskat)abstract
- Apollo 16 soil-like regolith breccia 65745,7 contains two zircon-bearing clasts. One of these clasts is a thermally annealed silica-rich rock, which mineralogically has affinities with the High Alkali Suite (Clast 1), and yields zircon dates ranging from 4.08 to 3.38 Ga. The other clast is a KREEP-rich impact melt breccia (Clast 2) and yields zircon dates ranging from 3.97 to 3.91 Ga. The crystalline cores of both grains, which yield dates ofca3.9 Ga, have undergone shock pressure modification at less than 20 GPa. We interpret that the U-Pb chronometer in these zircon grains has been partially reset by the Imbrium basin-forming event when the clasts were incorporated into the Cayley Plains ejecta blanket deposit. The zircon grains in Clast 1 have been partially decomposed, resulting in a breakdown polymineralic texture, with elevated U, Pb and Th abundances compared with those in the crystalline zircon. These decomposed areas exhibit younger dates around 3.4 Ga, suggesting a secondary high-pressure, high-temperature event, probably caused by an impact in the local Apollo 16 highlands area.
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| 6. |
- Menneken, M., et al.
(författare)
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CO2 fluid inclusions in Jack Hills zircons.
- 2017
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 172
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO2 inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO2 densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO2 inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO2 prior to analysis. This indicates a close relationship between CO2 and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO2-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.
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| 7. |
- Nemchin, A.A., et al.
(författare)
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Record of the ancient martian hydrosphere and atmosphere preserved in zircon from a martian meteorite
- 2014
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Ingår i: Nature Geoscience. - 1752-0894 .- 1752-0908. ; 7, s. 638-642
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Tidskriftsartikel (refereegranskat)abstract
- Mars exhibits ample evidence for an ancient surfacehydrosphere. The oxygen isotope compositions of carbonateminerals and alteration products in martian meteoritessuggest that this ancient hydrosphere was not in isotopicequilibrium with the martian lithosphere1–4. Martian meteoriteNWA 7533 is composed of regolith breccia from the heavilycratered terrains of ancient Mars and contains zircon grainsfor which U–Pb ages have been reported5. Here we reportvariations between the oxygen isotopic compositions offour zircon grains from NWA 7533. We propose that thesevariations can be explained if the mantle melts from whichthe zircon crystallized approximately 4.43Gyr ago hadassimiliated 17O-enriched regolith materials, and that someof the zircon grains, while in a metamict state, were lateraltered by low-temperature fluids near the surface less than1.7Gyr ago. Enrichment of the martian regolith in 17O beforethe zircon crystallized, presumably through exchange withthe 17O-enriched atmosphere or hydrosphere during surfacealteration, suggests that the thick primary atmosphere ofMars was lost within the first 120Myr after accretion. Weconclude that the observed variation of 17O anomalies in zirconfrom NWA 7533 points to prolonged interaction between themartian regolith, atmosphere and hydrosphere.
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| 8. |
- Nemchin, A. A., et al.
(författare)
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U-Pb isotope systematics and impact ages recorded by a chemically diverse population of glasses from an Apollo 14 lunar soil
- 2022
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533. ; 321, s. 206-243
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Tidskriftsartikel (refereegranskat)abstract
- Glass beads formed by ejection of impact-melted lunar rocks and soils are an important component of lunar soils. These glasses range from 100s of microns to up to a few cm in diameter and contain variable, but usually relatively low (several hundred ppb to a few ppm), quantities of U. Because Pb is a volatile element, it tends to be lost from the melts, so individual impact glasses can be dated by the U-Th-Pb isotopic systems. The presence of two additional Pb components in lunar glasses, likely linked to addition of lunar Pb to the beads during their residence on the lunar surface and from terrestrial laboratory contamination, require corrections to the data before accurate formation ages of the glasses can be determined. Here we report a U-Th-Pb isotopic and geochemical study of impact glasses from the Apollo 14 soil 14163, which documents multiple impacts into chemically diverse targets that can be linked to the main groups of rocks found on the Moon, i.e., mare basalts, highlands plagioclase-rich rocks, and KREEP (from high contents of K, REE and P) enriched rocks. The impact ages show a bimodal distribution with peaks at ~3500-3700 Ma and < 1000 Ma, similar to that obtained previously by Ar-40-Ar-39 dating of other suites of lunar regolith glasses. Our data suggest two predominant age peaks at ~100 Ma and ~500 Ma, with other statistically definable clusters of ages also possible. As Pb is relatively resistant to subsolidus diffusive loss in these glasses, the age clusters probably represent primary formation ages during impact events, although processes such as preferential preservation of young glasses and impact conditions necessary for production of regolith glasses need further quantification. (C) 2021 The Authors. Published by Elsevier Ltd.
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| 9. |
- Bellucci, J. J., et al.
(författare)
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Insights into the chemical diversity of the martian mantle from the Pb isotope systematics of shergottite Northwest Africa 8159
- 2020
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 545
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Tidskriftsartikel (refereegranskat)abstract
- Shergottite Northwest Africa (NWA) 8159 is a basaltic rock derived from a mantle source with chemical characteristics that are unique in the martian meteorite suite. To further investigate this source reservoir, the Pb isotope compositions of plagioclase/maskelynite, pyroxene, phosphates, and shock melt-glass in NWA 8159 have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Due to the limited spread in Pb isotope data, these Pb isotope compositions have been used to calculate an imprecise PbPb isochron age of 3.4 ± 2.1 Ga (2σ), which is broadly consistent with the crystallization age of 2.37 ± 0.25 Ga determined previously by 147Sm143Nd. The lack of radiogenic in-growth within individual minerals since 2.4 Ga means that this sample is depleted in U, which is in agreement with NWA 8159's positive initial ε143Nd. An initial Pb composition was calculated using an x-y weighted average of the least radiogenic Pb isotope population measured in the sample. This initial Pb composition is not consistent with the model for Pb growth in the shergottite mantle at 2.4 Ga. This composition is, however, consistent with the model for the Nakhla-Chassigny mantle. Using the latter model, a source μ (238U/204Pb) of 2.6 ± 0.6 has been calculated. This μ-value is in contrast with the other depleted shergottites (1.4-1.5) and falls significantly off the array of source ε143Nd vs. μ defined by the rest of the martian meteorite suite and thus, necessitates a differentiation history distinct from the other martian meteorites. Sequestering Pb in sulphides during differentiation is the only mechanism to fractionate U from Pb and create a low-μ reservoir. Consequently, the relatively high μ-value of the source of NWA 8159 is in contrast with the positive initial ε143Nd and indicates that its mantle source region likely lacked significant sulphur. This is consistent with the lack of sulphides in the sample itself and could have played a role in its complicated oxidation history.
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| 10. |
- Connelly, J. N., et al.
(författare)
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Calibrating volatile loss from the Moon using the U-Pb system
- 2022
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533. ; 324, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- Previous isotope studies of lunar samples have demonstrated that volatile loss was an important part of the early history of the Moon. The radiogenic U-Pb system, where Pb has a significantly lower T50% condensation temperature than U, has the capacity to both recognize and calibrate the extent of volatile loss but this approach has been hindered by terrestrial Pb contamination of samples. We employ a novel method that integrates analyses of individual samples by Ion Microprobe and Thermal Ionization mass spectrometry to correct for ubiquitous common Pb contamination, a method that results in significantly higher estimates for mu-values (238U/204Pb) than previously reported. Using this method, six of seven samples of low-Ti basaltic meteorites return mu-values between 1900 and 9600, values that are consistent with a re-evaluation of published results that return mu-values of 510-2900 for both low-and high-Ti basalts. While some degree of fractionation during partial melting may increase mu-values, we infer that the source region(s) for the basalts must also have had elevated mu-values by the time the lunar magma ocean solidified. Models to account for the available initial Pb isotopic compositions of lunar basalts in light of timing constraints from thermal modelling imply that their source regions had a mu-value of at least 280, consistent with the elevated mu-values of lunar basalts and that inferred for their sources. Such high mu-values are attributed to the preferential loss of more than 99% of the Pb over U relative to a precursor with a Mars-like composition in the aftermath of the giant impact. Such an extensive loss of Pb is consistent with previously reported losses of other elements with comparable volatility, namely Zn, Rb, Ga and CrO2. Finally, our modelling of highly-radiogenic lunar Pb isotopes assuming crystallization of the lunar magma ocean over 100s of millions of years shows that the elevated mu-values allows for, but does not require, a young Moon formation age.
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