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V A Fernandes - One of the best experts on this subject based on the ideXlab platform.
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constraints on formation and evolution of the Lunar Crust from feldspathic granulitic breccias nwa 3163 and 4881
Geochimica et Cosmochimica Acta, 2016Co-Authors: Claire Mcleod, A D Brandon, T J Lapen, Anne H Peslier, J T Shafer, V A Fernandes, Jorg Fritz, Alan R ButcherAbstract:Abstract Lunar granulitic meteorites provide new constraints on the composition and evolution of the Lunar Crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted Lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early Lunar evolution and the bulk composition of the Lunar highlands Crust. The NWA 3163 protolith was the likely product of reworked Lunar Crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282 ± 0.000007 (2σ). A calculated source model Sr TRD age of 4.340 ± 0.057 Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360 ± 0.003 Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314 Ga) and (3) a prominent age peak in the Apollo Lunar zircon record (c. 4.345 Ga). These ages are ∼100 Myr younger than predicted timescales for complete LMO crystallization (∼10 Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during Lunar evolution associated with Crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global Crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾ 3.5 Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2 Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the Lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4 Gyr record of Lunar Crustal production, metamorphism, brecciation, impacts and eventual ejection from the Lunar surface.
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constraints on the composition and evolution of the Lunar Crust from meteorite nwa 3163
American Geophysical Union Fall Meeting 2013, 2013Co-Authors: Claire Mcleod, A D Brandon, T J Lapen, Anne H Peslier, V A Fernandes, A J IrvingAbstract:The Lunar meteorite NWA 3163 (paired with NWA 4881, 4483) is a ferroan, feldspathic granulitic breccia characterized by pigeonite, augite, olivine, maskelynite and accessory Tichromite, ilmenite and troilite. Bulk rock geochemical signatures indicate the lack of a KREEP- derived component (Eu/Eu* = 3.47), consistent with previously studied Lunar granulites and anorthosites. Bulk rock chondrite-normalized signatures are however distinct from the anorthosites and granulites sampled by Apollo missions and are relatively REE-depleted. In-situ analyses of maskelynite reveal little variation in anorthite content (average An% is 96.9 +/- 1.6, 2 sigma). Olivine is relatively ferroan and exhibits very little variation in forsterite content with mean Fo% of 57.7 +/- 2.0 (2 sigma). The majority of pyroxene is low-Ca pigeonite (En57Fs33Wo10). Augite (En46Fs21Wo33) is less common, comprising approximately 10% of analyzed pyroxene. Two pyroxene thermometry on co-existing orthopyroxene and augite yield an equilibrium temperature of 1070C which is in reasonable agreement with temperatures of 1096C estimated from pigeonite compositions. Rb-Sr isotopic systematics of separated fractions yield an average measured Sr-87/Sr-87 of 0.699282+/-0.000007 (2 sigma). Sr model ages are calculated using a modern day Sr-87/Sr-86 Basaltic Achondrite Best Initial (BABI) value of 0.70475, from an initial BABI value Sr-87/Sr-86 of 0.69891 and a corresponding Rb-87/Sr-97 of 0.08716. The Sr model Thermomechanical analysis (TMA) age, which represents the time of separation of a melt from a source reservoir having chondritic evolution, is 4.56+/-0.1 Ga. A Sr model T(sub RD) age, which is a Rb depletion age and assumes no contribution from Rb in the sample in the calculation, yields 4.34+/-0.1 Ga (i.e. a minimum age). The Ar-Ar dating of paired meteorite NWA 4881 reveals an age of c. 2 Ga, likely representing the last thermal event this meteorite experienced. An older Ar-40/Ar-39 age of c. 3.5 Ga may record the thermal event which produced the granulitic texture. Additional chronological constraints will be provided by Sm-Nd systematics. Ferroan Anorthosites like NWA 3163 have been interpreted to represent direct Lunar magma ocean (LMO) crystallization products. If this is the case, trace element concentrations in NWA 3163 primary mineral phases should be in equilibrium with residual LMO liquids present during crystallization of those phases. Results from petrogenetic modeling suggest that the NWA 3163 protolith did not form from crystallization of an initially LREE depleted LMO but rather require an initially chondritic LMO with early garnet crystallization. Furthermore, a two-stage crystallization model where plagioclase crystalized prior to pyroxene (93% vs. 99.5% of LMO crystallization) is implied.
Jeffrey G Taylor - One of the best experts on this subject based on the ideXlab platform.
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Lunar central peak mineralogy and iron content using the kaguya multiband imager reassessment of the compositional structure of the Lunar Crust
Journal of Geophysical Research, 2015Co-Authors: M Lemelin, P G Lucey, E Song, Jeffrey G TaylorAbstract:Ryder and Wood (1977) suggested that the Lunar Crust becomes more mafic with depth because the impact melts associated with the large Imbrium and Serenitatis basins are more mafic than the surface composition of the Moon. In this study, we reexamine the hypothesis that the Crust becomes more mafic with depth; we analyze the composition of crater central peaks by using recent remote sensing data and combining the best practices of previous studies. We compute the mineralogy for 34 central peaks using (1) nine-band visible and near-infrared data from the Kaguya Multiband Imager, (2) an improved version of Hapke's radiative transfer model validated with spectra of Lunar soils with well-known modal mineralogy, and (3) new Crustal thickness models from the Gravity Recovery and Interior Laboratory data to examine the variation in composition with depth. We find that there is no increase in mafic mineral abundances with proximity to the Crust/mantle boundary or with depth from the current Lunar surface and, therefore, that the Crust does not become more mafic with depth. We find that anorthosite with very low mafic abundance (“purest anorthosite” or PAN) is a minority constituent in these peaks, and there is no clear evidence of a distinct PAN-rich layer in the middle Crust as previously proposed. The composition of most of the central peaks we analyze is more mafic than classically defined anorthosites with an average noritic anorthosite composition similar to that of the Lunar surface.
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revised thickness of the Lunar Crust from grail data implications for Lunar bulk composition
44th Lunar and Planetary Science Conference, 2013Co-Authors: Jeffrey G Taylor, Roger J. Phillips, F Nimmo, M A Wieczorek, Gregory A Neumann, W S Kiefer, Jay H Melosh, S C Solomon, J C Andrewshanna, S W AsmarAbstract:High-resolution gravity data from GRAIL have yielded new estimates of the bulk density and thickness of the Lunar Crust. The bulk density of the highlands Crust is 2550 kg m-3. From a comparison with Crustal composition measured remotely, this density implies a mean porosity of 12%. With this bulk density and constraints from the Apollo seismic experiment, the average global Crustal thickness is found to lie between 34 and 43 km, a value 10 to 20 km less than several previous estimates. Crustal thickness is a central parameter in estimating bulk Lunar composition. Estimates of the concentrations of refractory elements in the Moon from heat flow, remote sensing and sample data, and geophysical data fall into two categories: those with refractory element abundances enriched by 50% or more relative to Earth, and those with abundances the same as Earth. Settling this issue has implications for processes operating during Lunar formation. The Crustal thickness resulting from analysis of GRAIL data is less than several previous estimates. We show here that a refractory-enriched Moon is not required
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ancient Lunar Crust origin composition and implications
Elements, 2009Co-Authors: Jeffrey G TaylorAbstract:Samples from the Apollo (USA) and Luna (Soviet) missions and from Lunar meteorites, coupled with remote sensing data, reveal that the ancient highlands of the Moon are compositionally diverse. The average surface material contains 80 vol% plagioclase. A major suite of rocks, the ferroan anorthosites, averages 96 vol% plagioclase. The feldspathic composition reflects plagioclase flotation in a magma ocean. Late-stage REE-rich magma pooled in the Procellarum region of the Lunar nearside. The concentration of heat-producing elements in this region triggered mantle melting and overturn of the cumulate pile, forming two more suites of chemically distinct highland rocks, the magnesian and alkali suites.
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the granulitic impactite suite impact melts and metamorphic breccias of the early Lunar Crust
Meteoritics & Planetary Science, 1999Co-Authors: Janet A Cushing, Marc D Norman, Jeffrey G Taylor, K KeilAbstract:Abstract— An important and poorly understood group of rocks found in the ancient Lunar highlands is called “feldspathic granulitic impactites.” Rocks of the granulite suite occur at most of the Apollo highlands sites as hand samples, rake samples, clasts in breccias, and soil fragments. Most Lunar granulites contain 70–80% modal plagioclase, but they can range from anorthosite to troctolite and norite. Previous studies have led to different interpretations for the thermal history of these rocks, including formation as igneous plutons, long-duration metamorphism at high temperatures, and short-duration metamorphism at low temperatures. This paper reports on a study of 24 polished thin sections of Lunar granulites from the Apollo 15, 16, and 17 missions. We identify three different textural types of granulitic breccias: poikilitic, granoblastic, and poikilitic-granoblastic breccias. These breccias have similar equilibration temperatures (1100 ± 50 °C), as well as common compositions. Crystal size distributions in two granoblastic breccias reveal that Ostwald ripening took place during metamorphism. Solid-state grain growth and diffusion calculations indicate relatively rapid cooling during metamorphism (0.5 to 50 °C/year), and thermal modeling shows that they cooled at relatively shallow depths (<200 m). In contrast, we conclude that the poikilitic rocks formed by impact melting, whereas the poikilitic-granoblastic rocks were metamorphosed and may have partially melted. These results indicate formation of Lunar granulites in relatively small craters (30–90 km in diameter), physically associated with the impact-melt breccia pile, and possibly from fine-grained fragmental precursor lithologies.
Mark A. Wieczorek - One of the best experts on this subject based on the ideXlab platform.
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Density and porosity of the Lunar Crust from gravity and topography
Journal of Geophysical Research, 2012Co-Authors: Qian Huang, Mark A. WieczorekAbstract:[1] Newly obtained gravity and topography data of the Moon, combined with a lithospheric flexure model that considers both surface and subsurface loading, are used to place constraints on the density of the upper Crust from a localized spectral admittance analysis. Subsurface loads are found to be relatively unimportant in the highlands, and when subsurface loads are neglected, the best fitting bulk densities for a number of highland regions are found to vary from 2590 to 2870 kg m−3, with a mean value of 2691 kg m−3. Crustal rock densities estimated from geochemical considerations and global iron and titanium abundances imply somewhat greater densities, which we interpret as porosity affecting the gravity-derived bulk density estimates. The average porosity in the upper few kilometers of Crust is calculated to be about 7.7%, which is consistent with porosity estimates of impact-fractured meteorites and terrestrial impact craters.
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Compositional variations of the Lunar Crust: Results from radiative transfer modeling of central peak spectra
Journal of Geophysical Research, 2009Co-Authors: Joshua T.s. Cahill, Paul G. Lucey, Mark A. WieczorekAbstract:[1] We present model mineralogy of impact crater central peaks combined with Crustal thickness and crater central peak depth of origin models to report multiple perspectives of Lunar Crustal composition with depth. Here we report the analyses of 55 impact crater central peaks and how their compositions directly relate to the Lunar highlands sample suite. A radiative transfer model is used to analyze Clementine visible plus near-infrared spectra to place compositional constraints on these central peak materials. Central peaks analyzed are dominantly magnesian- and plagioclase-poor; strong compositional similarities to Lunar Mg-suite materials are evident. Relative to Crustal thickness estimates, central peak mineralogy becomes more plagioclase-rich as the Crust thickens. Relative to the Crust-mantle boundary, the origin of peaks with dominantly mafic mineralogy are confined to the lower Crust and primarily within the South-Pole Aitken and Procellarum KREEP Terranes (PKT); additionally, central peaks with anorthositic mineralogy (>60 vol % plagioclase) are transported to the surface from all depths in the Crustal column and confined to the Feldspathic Highlands Terrane (FHT). The discovery of mafic and magnesian materials, consistent with Mg-suite rocks of the sample collection, in all Lunar terranes suggests that the process and sources that give rise to these types of rocks is not unique to the PKT and not necessarily dependent on incompatible elements for formation. The identification of ferroan and magnesian anorthositic material near the Crust-mantle boundary of the FHT is also inconsistent with an increasing mafic/feldspar ratio and Mg' with depth in the Crust.
Alan R Butcher - One of the best experts on this subject based on the ideXlab platform.
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constraints on formation and evolution of the Lunar Crust from feldspathic granulitic breccias nwa 3163 and 4881
Geochimica et Cosmochimica Acta, 2016Co-Authors: Claire Mcleod, A D Brandon, T J Lapen, Anne H Peslier, J T Shafer, V A Fernandes, Jorg Fritz, Alan R ButcherAbstract:Abstract Lunar granulitic meteorites provide new constraints on the composition and evolution of the Lunar Crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted Lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early Lunar evolution and the bulk composition of the Lunar highlands Crust. The NWA 3163 protolith was the likely product of reworked Lunar Crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282 ± 0.000007 (2σ). A calculated source model Sr TRD age of 4.340 ± 0.057 Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360 ± 0.003 Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314 Ga) and (3) a prominent age peak in the Apollo Lunar zircon record (c. 4.345 Ga). These ages are ∼100 Myr younger than predicted timescales for complete LMO crystallization (∼10 Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during Lunar evolution associated with Crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global Crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾ 3.5 Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2 Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the Lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4 Gyr record of Lunar Crustal production, metamorphism, brecciation, impacts and eventual ejection from the Lunar surface.
Lionel Wilson - One of the best experts on this subject based on the ideXlab platform.
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magmatic intrusion related processes in the upper Lunar Crust the role of country rock porosity permeability in magmatic percolation and thermal annealing and implications for gravity signatures
Planetary and Space Science, 2020Co-Authors: J W Head, Lionel WilsonAbstract:Abstract Shallow Crustal country rock on the Moon is demonstrably more fractured and porous than deeper Crustal bedrock, and Gravity Recovery and Interior Laboratory (GRAIL) mission gravity data have shown that deeper Crustal bedrock is more porous than previously thought. This raises the question of how Crustal porosity and permeability will influence the nature of magmatic dike intrusions in terms of: 1) the ability of intruding magma to inject into and occupy this pore space (shallow magmatic percolation), 2) the influence of the intruded magma on annealing of this porosity and permeability (thermal annealing) both 1 and 2 densify the country rock), and 3) the effect of Crustal porosity on favoring sill formation as a function of depth in the Lunar Crust. We analyze quantitatively the emplacement of basaltic dikes and sills on the Moon and assess these three factors in the context of the most recent data on micro- and macro-scale porosity of Lunar Crustal materials. For the range of plausible micro/macro-scale porosity and permeability determined by crack widths (mm to cm) and open crack lateral continuity (mm to tens of cm), we find that 1) rapid conductive cooling of injected magma due to the very large surface area to volume ratio restricts magmatic percolation to very limited zones (extending for at most several tens of cm) adjacent to the ascending dike or intruded sill, even in the upper several hundred meters of the Lunar Crust; 2) the conductive heat loss from intruded dikes and sills results in a thermal wave decay rate that is predicted to limit the extent of intrusion-adjacent thermal annealing to less than ~6% of the thickness of the intruded body; 3) the extremely rapid rise rate of magma in dikes originating from sources in the Lunar mantle disfavors the lateral migration of dikes to form sills in the Crust, except in specific shallow Crustal locations influenced by impact crater-related environments (e.g., floor-fractured craters). We conclude that, although magmatic percolation and thermal annealing in association with Lunar mare basalt magmatic dike and sill emplacement should be taken into consideration in interpreting gravity signatures, the effects are likely to be minor compared with the density contrast of the solidified basaltic magmatic intrusion itself.
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magnetic signature of the Lunar south pole aitken basin character origin and age
Journal of Geophysical Research, 2012Co-Authors: Michael E. Purucker, J W Head, Lionel WilsonAbstract:[1] A new magnetic map of the Moon, based on Lunar Prospector magnetometer observations, sheds light on the origin of the South Pole-Aitken basin (SPA), the largest and oldest of the recognized Lunar basins. A set of WNW-trending linear to arcuate magnetic features, evident in both the radial and scalar observations, covers much of a 1000 km wide region centered on the NW portion of SPA. The source bodies are not at the surface because the magnetic features show no first-order correspondence to any surface topographic or structural feature. Patchy mare basalts of possible late Imbrian-age are emplaced within SPA and are inferred to have been emplaced through dikes, directly from mantle sources. We infer that the magnetic features represent dike swarms that served as feeders for these mare basalts, as evident from the location of the Thomson/Mare Ingenii, Van de Graaff, and Leeuwenhoek mare basalts on the two largest magnetic features in the region. Modeling suggests that the dike zone is between 25 and 50 km wide at the surface, and dike magnetization contrasts are in the range of 0.2 A/m. We theorize that the basaltic dikes were emplaced in the Lunar Crust when a long-lived dynamo was active. Based on pressure, temperature, and stress conditions prevalent in the Lunar Crust, dikes are expected to be a dominantly subsurface phenomenon, consistent with the observations reported here.
