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- 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
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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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| 2. |
- 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
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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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| 5. |
- 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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| 7. |
- 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
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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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| 8. |
- Snape, Joshua F., et al.
(author)
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The timing of basaltic volcanism at the Apollo landing sites
- 2019
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In: Geochimica et Cosmochimica Acta. - 0016-7037 .- 1872-9533. ; 266, s. 29-53
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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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| 9. |
- 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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