Tungsten Isotope Constraints on Archean Geodynamics

Geodynamics on Earth are active since more than 4 billion years and have continuously lead to differentiation and homogenization of mantle and crust. However, evidence from short-lived nuclide decay systems (e.g. 129I – 129Xe and 146Sm – 142Nd) suggest that primordial heterogeneities, which formed in the early Hadean Eon (i.e. during the first ca. 100 million years after formation of the Earth), survived for a long period of time – in some cases for Eons and until present day. Studying rocks that display variations in the decay products of short-lived nuclide decay systems offer two intriguing perspectives: (1) The cause for their formation refers to processes that must have had operated at a time when no other witnesses preserved in the geological rock record. (2) Understanding the mechanisms that allowed for their long-term preservation provides important insights into the temporal evolution of the bulk silicate Earth (BSE) and into the processes that lead to the present-day state of the mantle, including the onset of plate tectonic processes on Earth. One of the short-lived decay series that has increasingly been applied in geochemistry is the 182Hf-182W decay system with a half-life of 8.9 million years. Over the past decade, several studies investigated W isotope systematics in terrestrial rocks and found differences in the relative abundance of 182W. While Archean rocks were found to exhibit predominantly elevated 182W isotope compositions compared to the modern depleted mantle, modern oceanic island basalts (OIBs) and one Archean komatiites system (the ca. 3.55 Ga Schapenburg komatiite suite from the Kaapvaal Craton, southern Africa) were shown to display negative anomalies. These findings were interpreted as evidence for the preservation of early-formed heterogeneities in the sources of Archean rocks and taken as evidence for the presence of primordial mantle components that participate to the provenance of modern igneous reservoirs. However, the processes that lead to the formation of these reservoirs remained ambiguous. These processes include (1) incomplete equilibration of the mantle source with late accretionary material (late accretion hypothesis), (2) early fractionation of Hf from W by silicate crystal-liquid fractionation, e.g., in an early magma ocean, or (3) core-mantle interaction. Matters are further complicated because secondary processes (fluid-mediated alteration) often obscured primary W budgets of metamorphosed Archean rock assemblages and the analytical standards to obtain high-precision 182W isotope measurements turned out to be challenging. In this study, an analytical protocol is presented to obtain high-precision 182W isotope measurements on samples with low bulk-rock W concentrations (several ng/g). In three chapters we report high-precision 182W isotope data for Archean rocks from the ca. 3.9-3.6 Ga Itsaq Gneiss Complex of southern West Greenland (chapter 1), the ca. 3.6-3.2 Ga Pilbara Craton, NW Australia (chapter 2), and the ca. 3.6-3.2 Ga Kaapvaal Craton, southern Africa (chapter 3). In all studies, we combine 182W isotope analysis with high-precision isotope dilution measurements for high field strength element (HFSE), U, and Th abundances, to assess the elemental W systematics in our samples. This allows us to obtain a precise understanding of the primary and secondary processes that modified the W abundances and isotope compositions. As we demonstrate, the elemental W budgets of many mantle-derived rocks are dominated by metasomatic agents that mix reservoirs of variable 182W isotope compositions and obscure primary signatures (chapters 1 and 2). If not taken into consideration, this can lead to ambiguous interpretations of 182W isotope compositions observed in Archean lithostratigraphic successions. Our studies on rocks from different Archean cratons reveal that several processes are responsible for the origin of 182W isotope anomalies. Excesses of 182W in rocks from the Pilbara Craton (chapter 2) are best explained by missing late accreted additions in their mantle sources. Anomalies in rocks from the Itsaq Gneiss Complex (chapter 1) and the Kaapvaal Craton (chapter 3) instead were inherited from mantle sources that underwent early silicate differentiation during the lifetime of 182Hf (i.e. in the first ca. 60 million years after Solar System formation). Our results demonstrate that these Hadean signatures remained isolated in the mantle for several hundred million years. Understanding the evolution of 182W isotope systematics in the BSE through time requires comprehensive studies of lithostratigraphic successions that cover relatively long time frames of Archean geodynamic evolution, as shown by our study on the geological rock record of the Pilbara Craton (chapter 2). We further demonstrate that information about the temporal evolution of 182W isotope systematics of individual cratons is archived in Archean shales, which provide an average of the 182W isotope composition of the upper crust (chapter 2). These findings allow for observational constraints, which have important implications for understanding timescales of geodynamic processes on the early Earth (e.g. mantle stirring rates). As reported in chapter 3, rocks from the Kaapvaal Craton display correlations between 182W isotope compositions and initial εNd(t) and εHf(t) values. To our knowledge, this is the very first co-variation observed between 182W isotope systematics and long-lived radiogenic nuclides (147Sm-143Nd and 176Lu-176Hf systematics). The only plausible model to explain these patterns is the presence of recycled mafic restites from Hadean protocrust in the ancient mantle beneath the Kaapvaal Craton. As further demonstrated by our model, the striking isotopic similarity between recycled restites from Hadean protocrust and the low 182W endmember of modern OIBs might also be the missing link bridging 182W isotope systematics in Archean and young mantle-derived rocks. This finding offers important constraints on the geodynamic evolution of Earth’s mantle through time, indicating inefficient homogenization of Hadean silicate reservoirs

Terrain Diffusion Network: Climatic-Aware Terrain Generation with Geological Sketch Guidance

Sketch-based terrain generation seeks to create realistic landscapes for virtual environments in various applications such as computer games, animation and virtual reality. Recently, deep learning based terrain generation has emerged, notably the ones based on generative adversarial networks (GAN). However, these methods often struggle to fulfill the requirements of flexible user control and maintain generative diversity for realistic terrain. Therefore, we propose a novel diffusion-based method, namely terrain diffusion network (TDN), which actively incorporates user guidance for enhanced controllability, taking into account terrain features like rivers, ridges, basins, and peaks. Instead of adhering to a conventional monolithic denoising process, which often compromises the fidelity of terrain details or the alignment with user control, a multi-level denoising scheme is proposed to generate more realistic terrains by taking into account fine-grained details, particularly those related to climatic patterns influenced by erosion and tectonic activities. Specifically, three terrain synthesisers are designed for structural, intermediate, and fine-grained level denoising purposes, which allow each synthesiser concentrate on a distinct terrain aspect. Moreover, to maximise the efficiency of our TDN, we further introduce terrain and sketch latent spaces for the synthesizers with pre-trained terrain autoencoders. Comprehensive experiments on a new dataset constructed from NASA Topology Images clearly demonstrate the effectiveness of our proposed method, achieving the state-of-the-art performance. Our code and dataset will be publicly available

Sublithospheric diamond ages and the supercontinent cycle.

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300-700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb-Sr, Sm-Nd, U-Pb and Re-Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic-Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth-with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust-could have enhanced supercontinent stability

Exobiology and Future Mars Missions

Scientific questions associated with exobiology on Mars were considered and how these questions should be addressed on future Mars missions was determined. The mission that provided a focus for discussions was the Mars Rover/Sample Return Mission

Data analysis of the U–Pb geochronology and Lu–Hf system in zircon and whole-rock Sr, Sm–Nd and Pb isotopic systems for the granitoids of Thailand

© 2018 This data article provides zircon U–Pb and Lu–Hf isotopic information along with whole-rock Sm–Nd, Sr and Pb isotopic geochemistry from granitoids in Thailand. The U–Pb ages are described and the classification of crystallisation and inherited ages are explained. The petrography of the granitoid samples is detailed. The data presented in this article are interpreted and discussed in the research article entitled “Probing into Thailand's basement: New insights from U–Pb geochronology, Sr, Sm–Nd, Pb and Lu–Hf isotopic systems from granitoids” (Dew et al., 2018)

Procedural Generation: An Algorithmic Analysis of Video Game Design and Level Creation

Procedural generation is a method for generating mass quantities of data algorithmically rather than manually. One perfect example of this is the recently famous No Man’s Sky, a video game where the entire marketing scheme was structured around its procedurally generated universe. The game’s trailer and advertisements promised its players 18,446,744,073,709,551,616 unique planets[1], all of which were procedurally generated. In other words, the developers did not create exclusive profiles for every single planet, but instead programmed the game in such a way that the planets were built from the code. This method of content creation is the essence of procedural generation. Procedural generation has a broader application; it is a solution to many problems. It can take many different forms, and there are many different algorithms that have been written to accomplish many scenarios. The purpose of this analysis was to choose four common and interesting procedural generation algorithm archetypes, analyze each of them in the same way, and then demonstrate a practical application for each of them. Procedural generation techniques are some of the more convoluted programming techniques in existence; therefore, extra emphasis is placed on writing, demonstrating, and applying each algorithm. [1] How 4 Designers Built A Game With 18.4 Quintillion Unique Planets. Accessed April 16, 201

Tungsten isotope composition of the Acasta Gneiss Complex

AbstractHigh-precision tungsten (182W/184W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε182W values between +0.06 to +0.15. Two ca. 3600 Ma felsic samples from this terrane have ε182W ∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε182W data are correlated with ε142Nd (Roth et al., 2014) and we attribute this variability to incomplete metamorphic homogenisation of the 3960 Ma protolith with more recent material in a 3370 Ma tectono-thermal event. Notably, the value of the positive ε182W anomalies seen in the 3960 Ma AGC samples that are least affected by metamorphic homogenisation is comparable to that observed in other early Archean rocks (Isua Supracrustal Belt, Greenland; Nuvvuagittuq Supracrustal Belt, Canada) and the late Archean Kostomuksha komatiites (Karelia). This demonstrates a globally constant signature. We infer that the presence of a pre-late veneer mantle represents the most straightforward interpretation of a uniform distribution of εW182∼+0.15 value in Archean rocks of different ages. We show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra-mafic Archean mantle-derived rocks. The absence of anomalous ε182W and ε142Nd so far measured in samples younger than ca. 2800 Ma suggests progressive convective homogenisation of silicate reservoirs. The downward mixing of an upper mantle rich in late-delivered meteoritic material might account for these combined observations

Formation of Paleo- to Meso-Archean continental crust in the western Dharwar Craton, India: Constraints from U Pb zircon ages and Hf-Pb-Sr isotopes of granitoids and sedimentary rocks

The combination of U–Pb zircon ages with Hf-Sr-Pb isotopes of different intrusive and extrusive felsic and sedimentary rocks provides constraints on the petrogenetic evolution of the continental crust in the western Dharwar Craton, India. The oldest detrital zircon preserved formed at ∼3.6 Ga and represents a relic of the oldest felsic crustal material in the region. The dominant granitoid units of the western Dharwar Craton contain zircon grains with magmatic ages between 3.4 Ga and 3.0 Ga that indicate the formation of major felsic continental crust during this interval. Trace element abundances of the granitoids indicate that the oldest members of the intermediate to felsic suite derived by partial melting of mafic material at ∼3.6–3.4 Ga. The initial bulk rock Hf isotope compositions of these granitoids are consistent with their formation by melting of even older mafic material that was slightly enriched relative to the depleted mantle composition. This mafic and slightly enriched material formed by mantle melting at ∼ < 3.8 Ga. The Hf isotope compositions of individual zircon grains, obtained by two different analytical techniques (in-situ and complete dissolution followed by chromatographic separation) give evidence for the presence of such older mafic material (<3.8 Ga) that formed the immediate precursor of their granitoid host rocks. Such a mafic source for the granitoids is consistent with Pb–Sr isotope systematics of these that shows no indication of Eoarchean enriched/evolved material in the western Dharwar Craton. The mafic source material of the granitoids thus represents an intermediate stage of crust formation that started after 3.8 Ga with the formation of mafic crust by mantle melting. The combined geochronological and isotopic constraints suggest that the Mesoarchean felsic crust of the Dharwar Craton formed by differentiation of melts derived from an amphibolite/eclogite source rock and included increasing contributions of reprocessed crustal material with time from ∼3.6 to 3.0 Ga. The major interval of growth of felsic continental crust was from 3.4 to 3.0 Ga. The younger generation of granitoids formed mostly by reworking of older intermediate to felsic crust. These different felsic magmatic bodies with distinct petrogeneses and sources, that include the depleted mantle, older mafic crust and the evolved continental crust, became essential elements of the stable continental crust of the western Dharwar Craton, the majority of which was generated from 3.4 to 3.0 Ga.ISSN:0009-2541ISSN:1872-683

Activities at the Lunar and Planetary Institute

The activities of the Lunar and Planetary Institute for the period July to December 1984 are discussed. Functions of its departments and projects are summarized. These include: planetary image center; library information center; computer center; production services; scientific staff; visitors program; scientific projects; conferences; workshops; seminars; publications and communications; panels, teams, committees and working groups; NASA-AMES vertical gun range (AVGR); and lunar and planetary science council